distillation column report

CE 3003 Advanced Process Design – Individual Project

EXECUTIVE SUMMARY:

The main objective of this project was to carry out a design on the Propane distillation column for a

process that produces 1,3 Butadiene. The raw materials used are Butane, Oxygen and Water and

the process capacity is 100 000 tonnes per annum nominal. In the previous work it was stated that

the plant is located in China. The main location of the plant is in the capital city of the Gansu

Provence, Lanzhou. This was decided as the location over other provinces due to the excellent

transport links and the availability of raw materials and cheap labour. The close proximity to

suppliers, customers and skilled labour all make Lanzhou the ideal location for the plant to be

located.

The design of the distillation column consists of the packed type. The distillation is a multi-

component distillation involving separation of Propane from the other components at very high

pressure. The number of theoretical stages chosen was 16 stages based on a reflux ratio of 0.608.

The column diameter calculated was 0.61 at maximum pressure gradient possible for distillation

columns and 0.66m for minimum pressure gradient, which was then scaled up to 0.65m. This was a

reasonable decision as it may be helpful when the plant capacity increases. The column has a height

of 8m. Feed location for the arrangement of packing is at stage 3 from the top of the column. Pall

rings constructed out of stainless steel with a size of 25mm is used for packing. The vessel thickness

of the column is around 5mm, constructed out of stainless steel also to avoid corrosion.

A preliminary design on the condenser for this distillation column was also carried out. The type of

condenser is a fixed plate with a 1 shell and 1 tube pass. The tubes are stainless steel, 119 in

number, 2.44m in length, and with a square pitch arrangement. The overall heat transfer coefficient

was also calculated as 486.026W/m2C. The pressure drop on the tube side was 7.306 N/m2 (almost

negligible) as only one tube pass is used with a very short length. The pressure drop on the shell

side was 46.64 N/m2

The next section details on the Piping and Instrumentation over the column. The control system

used for better efficiency of the units is also featured in the same P and ID.. The control mainly used

for this, is the cascade control for the temperature and product composition, also in the varying of

the bottom product flow rate to control the level at the bottom of the column. The variables

Richie Gandhi SUN-075909279


controlled are the top and bottom temperatures including product composition and the reflux drum

and also the level at the base of the column.

There were two group tasks performed in this term along with the individual design. The first group

task performed was the HAZOP on the feed line of the propane distillation column. The aim of the

HAZOP was to evaluate the selected process line and identify the possible deviations which could

result into a hazard; the possible causes to the deviations, consequences and actions needed to

avoid the consequences were also discussed. An improvised P and ID was then produced after

Hazop was done. The Hazop was done with Dr. Titiloye as the Study Leader. The deviations

discussed include no flow, less flow, less temperature due to weather conditions, more temperature

likely to occur as a result of external fire and extra phase in an event of poor separation, presence of

heat transfer fluids, presence of off gases, corrosion inside the pipe catalyst pellets in the pipe and

excess residue due which leads ultimately to contamination.

The second group work is the economic appraisal, which was useful in concluding that if this project

were to be operational; the payback period would be two years which would leave 13 years of net

profit. So, from a financial aspect, the project was found to be very viable and attractive.

Both the group works are attached in the Appendix VI and VII



Contents

1.0 Project Brief:...................................................................................................................................5

1.1 Introduction to the product:.......................................................................................................5

1.2 Production:................................................................................................................................. 6

1.3 Location [4]...................................................................................................................................7

2.0 Project Plan and Objectives:.........................................................................................................10

2.1Technical Objectives:.................................................................................................................10

2.2 Personal Objectives:................................................................................................................. 12

2.3 Schedule................................................................................................................................... 13

2.3.1Project Schedule.................................................................................................................13

3 The Revised Process:........................................................................................................................15

4 The Chemical Design:.......................................................................................................................17

4.1 Calculations:..............................................................................................................................20

4.1.1 Reflux ratio and the number of Theoretical Stages............................................................21

4.1.2 Determination of the Diameter of the Column:.................................................................28

4.1.3Feed Location:.....................................................................................................................31

4.2 Choice of Plates and Packing:...................................................................................................33

4.2.1 Types of Packing:...............................................................................................................34

4.3 Column Internals [21][22]:..............................................................................................................43

5.0 CONDENSER:.................................................................................................................................48

5.1 Energy Balance Over the condenser:........................................................................................49

5.2 Shell Side Coefficient:............................................................................................................... 54

5.3 Tube Side Coefficient:...............................................................................................................55

5.4 Overall Heat transfer Coefficient U;..........................................................................................56

5.5 Shell side Pressure Drop:..........................................................................................................59

5.6Tube Side Pressure Drop:...........................................................................................................63

6.0 REBOILER:..................................................................................................................................... 65

6.1 Heat Duty over the Reboiler:....................................................................................................66

6.2 Choice of type of reboiler used with the Propane Distillation:[28].............................................67

7.0 Overall Energy Balance over the Distillation Column:...................................................................68

8.0 Piping and Instrumentation Diagram............................................................................................69



8.1 Control Loop............................................................................................................................. 72

8.2 Control Systems over the Distillation Column:.........................................................................73

9.0 Hazard and Operability Studies HAZOP........................................................................................76

10.0 Conclusion:................................................................................................................................. 77

11.0 References:.................................................................................................................................78

12.0 APPENDIX:...................................................................................................................................81

Appendix I.......................................................................................................................................81

Appendix II......................................................................................................................................81

Appendix III.....................................................................................................................................81

Appendix IV.....................................................................................................................................81

Appendix V......................................................................................................................................81

Appendix VI.....................................................................................................................................81

Appendix VII....................................................................................................................................81



1.0 Project Brief:

1.1 Introduction to the product:

Butadiene is a simple conjugated diene. It is an

important industrial chemical used as a monomer in

the production of synthetic rubber. Butadiene at most

of the times refers to 1,3-butadiene. 1,2-butadiene,

which is a cumulated diene is an isomer which is

difficult to prepare and has no industrial significance.[1]

1, 3 Butadiene (CH2=CH-CH=CH2), is a colourless gas

with mild aromatic odour. Butadiene is soluble in

alcohol and ether, insoluble in water and polymerizes readily, particularly if oxygen is present. It is

non-corrosive and has a molecular formula of 54.09. Its boiling point is -4.4C and its vapour pressure

is 1,790 mm Hg (239kPa) at 20◦C. It is easily liquefied, with a density of 0.6211 g/ml at 20C. It is

soluble with ethanol, diethyl ester, and organic solvents and very slightly soluble in water. 1, 3

Butadiene has a flash point of -76C and can slowly be dimerised and may form peroxides upon

exposure to air. Because 1, 3 Butadiene is a highly volatile gas, it is expected to partition in the

atmosphere and then undergo rapid destruction by photo-initiated reactions. [1]

A table of properties can be seen in Appendix I section (a)

In 1863, a French chemist isolated a previously unknown hydrocarbon from the pyrolysis of amyl

alcohol.[1] This hydrocarbon was identified as butadiene in 1886, after Henry Edward

Armstrong isolated it from amongst the pyrolysis products of petroleum.[1] In 1910,

the Russian chemist Sergei Lebedev polymerized butadiene, and obtained a material with rubber-

like properties. This polymer discovered was too soft to replace the natural rubber in many of its

uses, especially automobile tires.

The butadiene industry originated during the World War II. Many of the belligerent nations realized

that in the event of war, they could be cut off from rubber plantations controlled by the British

Empire, and sought to remove their dependence on natural rubber.


http://en.wikipedia.org/wiki/British_Empire

http://en.wikipedia.org/wiki/British_Empire

http://en.wikipedia.org/wiki/World_War_II

http://en.wikipedia.org/wiki/Sergei_Vasilyevich_Lebedev

http://en.wikipedia.org/wiki/Russia

http://en.wikipedia.org/wiki/1,3-Butadiene#cite_note-Armstrong-1

http://en.wikipedia.org/wiki/Petroleum

http://en.wikipedia.org/wiki/Henry_Edward_Armstrong

http://en.wikipedia.org/wiki/Henry_Edward_Armstrong

http://en.wikipedia.org/wiki/1,3-Butadiene#cite_note-Caventou-0

http://en.wikipedia.org/wiki/Amyl_alcohol

http://en.wikipedia.org/wiki/Amyl_alcohol

http://en.wikipedia.org/wiki/Pyrolysis

http://en.wikipedia.org/wiki/Diene

http://en.wikipedia.org/wiki/Synthetic_rubber

http://en.wikipedia.org/wiki/Monomer

http://en.wikipedia.org/wiki/Diene

http://en.wikipedia.org/wiki/Conjugated_system


In 1929,Eduard Tschunker and Walter Bock, working for IG Farben in Germany, made a copolymer

of styrene and butadiene that could be used in automobile tires. Worldwide production quickly

ensued, with butadiene being produced from grain alcohol in the Soviet Union and the United

States and from coal-derived acetylene in Germany.[1]

Butadiene is the raw material used [2] in the making of various synthetic rubbers and polymer resins

as well as a few chemical intermediates.

It is mainly used to make styrene butadiene rubber (SBR) which is used to make automobile tyres. It

is also used in adhesives, sealants, coatings and rubber article such as shoe soles. SBR is has a high

molecular weight, as it has excellent resistance to abrasion, it is widely used in the automobile tyre

industry.

Various other uses of butadiene are detailed in the table attached in Appendix I Section (b).

Figure 2: Chart Showing Various Uses of different forms of 1,3

Butadienehttp://www.sriconsulting.com/WP/Public/Reports/pie_charts/Butadiene.gif

1.2 Production: Butadiene is produced commercially by three main processes:[3]

Steam Cracking of Paraffinic Hydrocarbons: In this process, butadiene is a co-product in the

manufacture of ethylene (the ethylene co-product process).

Catalytic Dehydrogenation of n-Butane and n-Butene (the Houdry process).

Oxidative Dehydrogenation of n-Butene (the Oxo-D or O-X-D process).


http://www.sriconsulting.com/WP/Public/Reports/pie_charts/Butadiene.gif

http://en.wikipedia.org/wiki/Germany

http://en.wikipedia.org/wiki/Acetylene

http://en.wikipedia.org/wiki/Coal

http://en.wikipedia.org/wiki/United_States


http://en.wikipedia.org/wiki/Soviet_Union

http://en.wikipedia.org/wiki/Ethanol

http://en.wikipedia.org/wiki/Tire

http://en.wikipedia.org/wiki/Automobile

http://en.wikipedia.org/wiki/Styrene

http://en.wikipedia.org/wiki/Germany

http://en.wikipedia.org/wiki/IG_Farben

http://en.wikipedia.org/w/index.php?title=Walter_Bock&action=edit&redlink=1

http://en.wikipedia.org/w/index.php?title=Eduard_Tschunker&action=edit&redlink=1


Each of these processes produces a stream commonly referred to as crude butadiene that is rich in

1,3-butadiene.

In the United States, western Europe, and Japan, butadiene is produced as a by-product of

the steam cracking process which is used to produce ethylene and other olefins. The quantity of

butadiene produced depends on the hydrocarbons used as feed. Light feeds, such as ethane, give

primarily ethylene when cracked, but heavier hydrocarbons favour the formation of heavier olefins,

butadiene, and aromatic hydrocarbons.

Butadiene can also be produced by the catalytic dehydrogenation of normal butane. The first

commercial plant, producing 65,000 tons per year of butadiene, began operations in 1957

in Houston, Texas.

In the previous project it was decided that Oxydehydrogenation Process would be employed to

produce butadiene for the plant to be built in China, which uses Butene as the main raw material.

Due to the easy availability of Butane from the nearby industries, it was decided to integrate the

process, where butane was first decided to be catalytically dehydrogenated to Butnene and the O-

X-O D Process follows after that. The specification was not clearly illustrated in the project earlier,

and hence it’s now detailed further in this project.

1.3 Location [4]

Since 2002, the Global Butadiene industry has maintained relatively fast growth especially in Asia

due to its variety of uses. The global annual butadiene output increased from 8.08 million tons in

2002 to 10.15 million tons in 2007, with an average annual growth rate of 4.68%.

China is one of the fastest developing Asian countries in the world. Along with the rapid

development of china butadiene industry, the national output of butadiene increased from 725,000

tons in 2001 to 1.36 million tons in 2007, with an average annual growth of 9.4%. Even after

increase in production, China has to import huge quantities of butadiene from different countries.

Korea is the biggest supplier of butadiene to china.


http://en.wikipedia.org/wiki/Houston,_Texas

http://en.wikipedia.org/wiki/Ton

http://en.wikipedia.org/wiki/Dehydrogenation

http://en.wikipedia.org/wiki/Aromatic_hydrocarbons

http://en.wikipedia.org/wiki/Ethylene

http://en.wikipedia.org/wiki/Ethane

http://en.wikipedia.org/wiki/Olefin

http://en.wikipedia.org/wiki/Ethylene

http://en.wikipedia.org/wiki/Steam_cracking

http://en.wikipedia.org/wiki/Japan

http://en.wikipedia.org/wiki/Europe



Figure 3 : graph to show Production Capacity and Output of Butadiene in China, 2001-2008 (Unit:

10,000 tons)http://www.researchinchina.com/UpLoads/Article/2008112601.gif

It was decided that the best place locate the butadiene plant in China which can satisfy the

demands of its market in near future is in Lanzhou, the capital of the Gansu province which is in

north western china.

Figure 4: Map of Gansu Province in China

http://janetong.com/image_gallery/data/agansubest.jpg i

While deciding the plant location several factors were taken into consideration:

Productivity - Since 1949 Lanzhou has been transformed from the capital of a poverty-stricken

province into the centre of a major industrial area. The GDP per capita of Lanzhou was 25,566 Yuan

(RMB) (US$3,681) in 2008, ranked no. 134 among 659 Chinese cities.


http://janetong.com/image_gallery/data/agansubest.jpg

http://www.researchinchina.com/UpLoads/Article/2008112601.gif


River - Lanzhou is situated at the upper course of the Yellow River. The river can provide water to

the plant as the process requires large quantities of water for scrubbing and cooling. Also the river

provides hydropower to the industries and cities in Gansu. A large multipurpose dam has been built

in the Liujia Gorge on the Yellow River above Lanzhou. The river also helps in transportation.

Natural Resources - Lanzhou has many natural resources which include coal, gold, silver, nickel,

manganese, clay and dolomite. The Coal is obtained from Qinghai which provides thermal power.

The catalyst for our process is nickel which will be cheaper to buy and will be available in large

quantities.

Cheap Labour - Due to the presence of the Yellow river the site is a residential area for more than

3.3 million people. Cheap labour is available for the plant. There are many top ranked universities in

this province which means qualified staff will be available at the site.

Industrial area - Gansu has one of the largest oil refineries in the country and Lanzhou itself is the

centre of the province's petrochemical industry. The main industries include rubber, petrochemical,

oil refinery and machinery industry. Butane which is the raw material for our process is readily

available and also Butadiene (product) can be sold to the neighbouring industries. Also, the

machinery parts will be available from the neighbouring industries which will reduce the cost of

transportation and energy and time.

Transportation - Lanzhou is very well interconnected to various cities and provinces through

highways, railways and airlines. There are 3 major national highways namely China National

Highways 212,213 and312, connecting Lanzhou to different provinces of the country.

Transportation is cheap and easy. The Lanzhou Sustainable Urban Transport Project in China aims at

improving the transport infrastructure and urban road networks. The total Asian Development Bank

contribution is estimated at US$150 million. This project is under planning and is expected to get its

approval by 11th Dec; 2010.This will greatly enhance the transportation of our product to different

provinces when our plant is ready for production.



2.0 Project Plan and Objectives:

The Second part of this project aims at carrying out a design study on a selected process section for

the production of 100 000 tonnes per year of 1, 3 Butadiene from Butane for a plant to be built in

China.

Project Supervisor: Dr. James Titiloye

Project Author: Richie Gandhi

The process development project aims to give third year Chemical Engineering students a chance to

experience what working on a project in an industry might be like. Working in groups, they must

cover all the major areas involved in designing a process. The main process unit which has been

discussed in this project is a Distillation Column which separates Propane from 1,3 butadiene.

Within this task other objectives have been set to ensure that the goal is met which are outlined

below:

2.1Technical Objectives:

Chemical engineering design to achieve the following outcomes is to be done

Understanding solving a complex engineering design problem

Design a distillation column according to the engineering standards

To design a distillation column to meet the desired criteria

Gain an understanding of chemical engineering unit operations

Description of a control system for the distillation column for best possible results is to be

done

To study concepts of process control including principles of feedback and feed forward,

apply these concepts to the design of instrumentation and control system for a

distillation column.

Propose a control system which will execute the desires of the process functionality

Piping and Instrumentation Diagram for the distillation column is obligatory

Exhibit an understanding of engineering codes, standards and regulations

Develop skills through use of computer software such as Microsoft Visio

Carry out a group study on the HAZOP for any one of the individual selected processes to

achieve the following outcomes



To be able to investigate how the chemical plant might deviate from the design intent

To be able to identify scenarios that would lead to the release of hazardous and

flammable materials to the environment and also to determine whether a particular

deviation would result into an hazard

Work effectively in a team to achieve the project goal

Economic appraisal (group work) for the full process on production of 1,3 Butadiene in order

to gain the following outcomes

Work effectively in the group to enhance team work skills

To estimate the costs and benefits of production of products using the desired process

route

To study the economic feasibility of the production of products

Written report preparation based on the selected design to achieve the following

Assemble and use relevant background information

Provide Complete Referencing

Appendices containing lengthy derivations, calculations, large drawing, computer

printouts etc.



2.2 Personal Objectives: Understanding how chemical process work: Upon completion of this project, a better

understanding of the Butadiene process will be gained. Also technical skills in carrying out a

detailed distillation column design study will be improved, which will help towards future

projects as a chemical engineer.

Enhance organisational skills and keeping to deadlines: The tasks would be scheduled at

regular meeting with the supervisor where the deadlines will be decided which have to be

met for a successful project.

To utilize our initiative: Many decision making processes will be used in the undertaking a

couple sections of this project. Each member will need to use their initiative to come to a

sensible and reasonable conclusion, which will benefit the entire group.

Develop Communication Skills: Communication skills are enhanced both by meetings with

the supervisor and the group meetings. This would provide and individual a platform to build

his confidence and propose his ideas.

Enhance Team-working Skills: HAZOP and Economic Appraisal have to be carried out as a

group. The group members are required to co-operate with each other, share and listen to

ideas and take constructive criticism from group members. This will encourage each member

to provide advice and feedback.

Professional report writing: At the end of our project I wish to able to deliver a professionally

laid out report.



2.3 ScheduleTo support me in achieving these objectives a schedule has been devised to plan the project and

ensure that all required tasks are undertaken, within the time period set for the study to be

completed. This can be seen below. A Gantt chart is also attached for assistance in the Section 3(b)

in the Appendix II. This is the revised Gantt Chat according to with the schedule has actually been

carried out. A Gantt chart which was made earlier to aid in maintain time is attached in Section 3(a)

in the Appendix II

2.3.1Project ScheduleSerial

No. Objective Start Date End Date

1

Get feedback on report/presentation and

mass and energy balance from Dr. Titiloye

Submit project plan. 19/01/10 22/01/10

2 Tutor meeting and discussing the topic.

Gathering relevant information from 1st

term project

25/01/10 29/01/10

3 Discuss findings with tutor

Put together a list of all the equations

required to design a Distillation Column for

the process

Start Designing the Distillation Column in

the Butadiene production process.

01/02/10 07/02/10

4 Calculating diameter and size of Distillation

Column along with the flow though them

thereby completing the design

Review the mass balance and calculating

other dimensions

08/02/10

22/02/10

19/02/10

5/03/10



5 Piping and instrumentation diagram

Control and instrumentation research

Calculating cost and profit

1/03/09 10/03/10

6 HAZOP 10/03/10 19/03/10

7 Calculating cost and profit

Economic appraisal

01/03/10 15/03/10

8 Completing referencing

Executive summary

25/01/10

13/03/10

17/03/10

17/03/10

9 Ensuring completion of all section

Submit report

Review with tutor

Make changes

20/03/10 21/03/10

10 Proof read

Submit report

22/03/09 26/03/09

To complete the objectives of the project various sources would be utilised for research work. The

main source of information used will be university library, Birmingham city library, Aston

University’s access to online journals, internet and contacting companies in relation to scope of the

project. Discussions would be carried out with Dr. Titiloye to check the track of the work and to

ensure correct research is carried out.

3 The Revised Process:



Phillips OXO-Process [3]

The n-butene, steam and air react at 590-650°C on a fixed bed heterogeneous catalyst (probably a

ferrite (iron or iron alloy) with Zn, Mn or Mg). Addition of steam controls the selectivity. With

butane conversions between 75-80%, the butadiene selectivity reaches

roughly 88-92%.

Magnesium ferrite

1 bar / 590-650°C

It is a one step process, where Butene gets oxidised straight away, without the need of butane

dehydrogenation. Since our plant is close to an oil refinery plant, where butane can be obtained at a

cheaper price in abundance, a decision to integrate the conversion of Butane to Butene in our

process was made. This reaction would take place in the first reactor and then the produced butene

is fed into the second reactor for further processing. The catalytic dehydrogenation of Butane to

Butene is not a part of our OXO process which we had considered in the previous report.[3] The

same method was carried out in the first project, but it was not stated clearly stated in literature.

Equation (1) gives 100% completion; higher conversions at lower temperatures are possible with

fewer side reactions and improved yields. It is also easier to remove the hydrogen as it is now

present as water vapor and can be condensed out. It is an exothermic reaction; heat input to the

reactor is thus eliminated as heat is recovered to generate high pressure steam. By feeding stream

and air to the reactor it is possible to carry out continuous in-situ catalyst regeneration. The steam

also absorbs some heat released by a small amount of hydrocarbon oxidation and has a beneficial

effect on butene selectivity to butadiene. In general, the equipment used is smaller and utilities

usage is reduced for a given throughout compared with the conventional dehydrogenation process.

A typical flow goes from the feed stream, with a composition of butene feedstock, 90% n-butene,

5% butane, 2% of small amounts of Hydrocarbon and 1% Butadiene. Air is compressed and mixed

with steam before heating in a furnace at 480-590◦C. This mixture is then blended with butenes and

passed over the oxidative dehydrogenation catalyst bed in the reactor. Three reactors are used in


C4H8 + ½ O2 C4H6 + H2O..................................(1)

C4H10 + O2 C4H8 + H2O.................................(2)Tubular Rector

1Atms/5900CCromina-Alumina


parallel (multitubular reactor) with a fixed bed of mixed oxide catalysts, feeding into a single stream

recovery and purification train. The products from the reactor are cooled. Heat is recovered from

reactor effluent with water sprays and can be used to generate process stream. After quenching,

Stream is cooled to about 400◦C, by direct contact with cold water. Cooling the stream causes

condensation, the condensate being removed by a simple phase separation. Washing and scrubbing

occurs in order to remove all water-soluble impurities, the C4 fraction is recovered in an oil

absorption section. The mineral oil absorber has the ability to absorb the hydrocarbon components

of the stream, in this case the butadiene whilst rejecting gases such as nitrogen, carbon dioxide. The

fat oil is stripped and crude butadiene is transferred to the final purification step. The product is

then obtained from the tailing column which is then fed into the recovery unit. Small amounts of

oxygenated compounds are also produced and these are separated and taken to a waste disposal

unit. Water sprays are used to reduce fouling in the plant.

The advantages for this process are that the catalyst life is long which lasts for around 1000 hours.

The capital costs compared to the other oxidative dehydrogenation process routes are more

favorable because of the low steam requirements for the mixture and the relatively high

concentration of butadiene leaving the reactor. Reactors are set up to de-coke the catalyst and

restore its activity, and to allow more effluent output of butadiene. At high temperatures (up to

6000C), oxygen acts to oxidatively regenerate the catalyst. Also there are a class of metal Vanadate

catalysts that are newly being used, which has highly selectivity and high conversions and the

formation of oxygenated by-products is suppressed. The feedstock raw materials are inexpensive in

the United States; a major factor causing this is the trend towards greater usage of feedstock such

as natural gas liquids (ethane, propane). However, there is a growing shortage of these raw

materials, encouraging the use of heavy feedstock such as ethylene. During this process, various by

products are produced. These by-products must be removed to produce butadiene, so it can be

polymerised. One major by product is vinyl acetylene, which is a highly unsaturated compound, a

poison for the catalysts that polymerise butadiene. Therefore there is a need to produce a

purification process by providing a stream with distillation steps. This allows the process to be

energy-conserving and simple.



A revised Flow Sheet is attached in Appendix III. A compressor is added after the extractive

distillation column, in the new flow sheet that increases the pressure of the process before it can be

sent to the Propane Distillation Column for further purification.

I assume the mass balance to be correct as it was based on many assumptions which were

acceptable. A Copy of the Mass Balance done in the previous project is attached in Appendix IV.

4 The Chemical Design:

The design chosen for this project is a continuous distillation column. The separation of the liquid

mixture by distillation depends on differences in volatility between the components. The separation

becomes easier if the differences in the volatilities between the components are massive. Figure 5

shows a basic distillation column. The liquid mixture is heated up and routed into the distillation

column. The feed when enters the column flows down. Heat is vaporise the volatile components is

provided by the reboiler. The components with lower boiling points vaporise and rise to the top of

the distillation column. Vapour flows up and the liquid flows counter currently down the column.

These two phase come in contact with each other on a surface which can be trays or packing. The

composition of the vapour above the liquid differs from the liquid composition. The vapour is then

separated and condensed into a liquid; it becomes richer in the lower boiling component(s) of the

original mixture.

Part of the condensate from the condenser is returned to the top of the column which provides

liquid flow above the feed point and part of the liquid from the base of the column is vaporised in

the reboiler and returned to provide vapour floe rate. [5]


http://en.wikipedia.org/wiki/Condensation


Figure 5: Basic Distillation Column

http://wpcontent.answers.com/wikipedia/commons/e/e0/Distillation_Column.png

The highlighted distillation column in the flow sheet in Appendix III is the Propane/Butadiene

distillation column which is considered to be designed. The main objective of this unit is to get rid of

propane from the process. The distillation is a multi-component distillation thereby it involves more

than two components. It operates at very high pressure of 13.5 bars and consists of three streams.

The feed enters the distillation column at a temperature of about 350C. On distillation, the Propane

vapour leaves the top of the column to the condenser and collected in the reflux drum. Partial

amounts of 1,3 Butadiene is also lost at the top of the column. The bottom product majorly

comprises of 1, 3-Butadiene, 1, 2-Butadiene and Pentane.

The table below shows the preliminary Mass Balance over the Distillation Column


http://wpcontent.answers.com/wikipedia/commons/e/e0/Distillation_Column.png


Table 1 (a): FEED:

Components Flow rate (kg/h) Kmols/hr Kmols fraction

Propane 424.23 9.64 0.04

1,3 butadiene 11904.76 220.46 0.91

1,2 butadiene 500.00 9.26 0.038

Pentane 138.84 1.93 0.008

Total 12967.83 241.29 1

Table 1 (b): Distillate:


Propane 424.23 9.64 0.687

1,3 Butadiene 237.6 4.4 0.313

Total 661.83 14.04 1

Table 1 (c): Bottom:


1,3 Butadiene 11667.16 216.06 0.951

1,2 Butadiene 500.00 9.26 0.041

Pentane 138.84 1.93 0.0085

Total 12306.00 227.25 1

The boiling points of the feed components at 1 atmospheric pressure are as below:

Propane: −42.09 °C

1,3 Butadiene: -4.4 °C

1,2 Butadiene: 10.8 °C

Pentane: 36.1 °C

Looking at the temperature differences it can be said that the components can be separated easily

by reducing the temperatures but since in tangible situations, it is very expensive to reduce down

the temperatures in the plant for continuous process we have to increase the pressure inside the

unit to separate it at reasonable costs. This is done by using compressors or pumps before the feed

is fed into the unit.



It is assumed that the distillate is at a temperature of 400C which is a reasonable assumption as

chilled water can be used to cool down the components in the distillate. Taking this as our basis we

can calculate the pressure and temperature for rest of our unit using Antoine’s Equation. Detailed

calculations are attached in Appendix V; Section (a)

4.1 Calculations:Basis: 1hr

Units: 1 Kg

Propane boiling point: 40°C/313K @ 13.5 bars

Specifications

Feed condition: Cold Liquid under pressure

Feed inlet temperature @35°C/308K

Temperature of Top of distillation column @ 45°C/318K

Temperature of Bottom of distillation column @ 95°C/368K

Therefore column Temperature will be taken as the average @ 70°C/343K

The detail distillation design was performed by using the main steps below

- Determination of reflux ratio and number of stages required for the distillation

- Calculation of the column diameter and determining the type of column to be used, Packed

or trays

- Column design in detail



4.1.1 Reflux ratio and the number of Theoretical StagesThe vapour reaching the top of the column is totally condensed and the resulting liquid is divided

into two parts. One part, L (reflux), is returned to the column and the other part, D (distillate), is

withdrawn as product. The reflux ratio is the ratio of L to D, that is R = L/D. [6]

Smaller values of reflux ratios means less the number of theoretical stages in the distillation column

increases, which reduces the energy costs as most of the vapour is condensed as distillate and only

partial amounts of reflux is sent back to the column.

The minimum reflux ratio and the infinite reflux ratio place a constraint on the range of separation

operation. Any reflux ratio between Rmin and Total R will produce the desired separation, with the

corresponding number of theoretical stages varying from infinity at Rmin to the minimum number

(Nmin at Total R). The relationship between R and N is shown in the Figure below. The choice of

reflux ratio to use is governed by cost considerations [7]

Figure 6: Generalised graph between number of theoretical stages V/S Reflux Ratio

http://www.separationprocesses.com/Distillation/DT_Chp04n.htm

Calculations:

The minimum reflux ratio is calculated by using Underwood method. [8]

αA x fAαA−θ

+αB x fBαB−θ

+αC x fCαC−θ

+αD x fDαD−θ

=1−q………….(1)

αA xdAαA−θ

+αB xdBαB−θ

+αC xdCαC−θ

+αD xdDαD−θ

=1+Rmin

……(2)



http://www.separationprocesses.com/Distillation/DT_Chp04n3.htm

http://www.separationprocesses.com/Distillation/Fig062.htm


Since the minimum reflux is calculated for a distillation column which consists of four components,

four variables are used in the above equation. For a multi-component mixture to be split into two

streams (distillate and bottoms) by distillation, it is common to specify the separation in terms of

two ‘’key components’’ of the mixture. Hence in this way multi components can be reduced to

equivalent binary systems.

Light Key: Most Volatile component in the Bottom Product

Heavy Key: Least Volatile component in the Top Product. [8]

Hence, from the table 1 (b) and (c);

Heavy Key (HK) – 1,3 Butadiene

Light Key (LK) – 1,3 Butadiene

This will cause a problem in the calculation, since both the HK and LK are the same component

therefore an unreasonable reflux ratio will be obtained. Therefore to trounce this problem, a small

amount of Propane was added to the bottom product, making Propane the LK.

I assume that 99% mole of propane and 2% mole of 1,3 Butadiene is now in the Distillate and the

remaining are the bottom product.

Revised Balance over the distillation column is as follows:

Table 2 (a); FEED

Feed

Components

Flow rate

(kg/h)

Kmols/hr Kmol

Fraction

Propane 424.23 9.64 0.04

1,3

butadiene 11904.76

220.46 0.91

1,2

butadiene 500.00

9.26 0.04

Pentane 138.84 1.93 0.01

Total 12967.83 241.29 1

Table (b); Distillate and Bottoms



Distillate Bottoms

Components Kmols/hr Kmol Fraction Kmols/hr Kmol Fraction

Propane 9.544 0.684 0.096 0.0005

1,3

butadiene

4.41 0.316 216.05 0.95

1,2

butadiene

- - 9.26 0.041

Pentane - - 1.93 0.0085

Total 13.954 1 227.336 1

Therefore new key components are:

HK – 1,3 Butadiene

LK – Propane

The next step is to find the relative volatilities of each component with respect to the HK which in

this case is 1, 3 Butadiene. The calculations are attached in the Appendix V, Section (b) and are

tabulated below:

Table 3:

Components α

Propane α =2.724

1,3 Butadiene α = 1

1,2 Butadiene α = .66

Pentane α = .31

Using the UNDERWOOD EQUATION:

αPropane x fPropane

αPropane−θ+α1,3Butadiene x f 1,3 Butadiene

α1,3 Butadiene−θ+α 1,2Butadiene x f 1,2 Butadiene

α1,3 Butadiene−θ+αPentane x fPentaneαPentane−θ

=1−q……………………………… ..(1)

Where q ≠ 1 as the feed is cold liquid @ 35°C.

Calculation of feed condition q [9] :



q=Heat ¿vapourside 1mol of feed ¿molar latent heat of t he feed

…………….(3)

Therefore,

q=mc p∆T +mH vapuorisation

mH vapuorisation

………………………(4)

Table 4: Latent Heat of the Components inside the Column [9]

Components Latent Heat in

KJ/Kg

Relative

Molecular

Mass

Calculation Latent Heat in

KJ/Kmol

Propane 229.93 44 229.93*44 10116.92

1,3 Butadiene 335.55 54 335.55*54 18119.7

1,2 Butadiene 387.38 54 387.38*54 20918.52

Pentane 333.24 72 333.24*72 23993.28

Latent Heat = (10116.92* 0.04) + (18119.7* 0.91) + (20918.52* 0.038) + (23993.28* 0.008)

Latent Heat = 17880.454 J/mol or KJ/Kmol

Table 5: Specific Heat of the Components inside the Column [9]

Components Specific Heat in

KJ/KmolK

Propane 81.84

1,3 Butadiene 138.78

1,2 Butadiene 136.08

Pentane 182.88

Specific Heat to Vaporize the 1 mol of feed = mcp∆T + mHvapuorisation



=> {[(81.84*0.04) + (138.78*0.91) + (136.08*0.038) + (182.88* 0.008)] * (70-35)] } + 17880.454

J/mol

Specific heat to Vaporise 1 mol of feed to 70oC = 4766.79 J/mol + 17880.454 J/mol

Therefore, Specific Heat = 22647.244 J/mol

q=22647.22417880.454

=1 .267

q = 1.267

Using the above value of “q” in the Underwood Equation (1), θ is obtained;

2.724∗0.042.724−θ

+ 1∗0.911−θ

+ 0.66∗0.040.66−θ

+ 0.31∗0.01.31−θ

=1−1.267

Therefore; by using excel spread sheet:

Θ= 2.67; which is acceptable as the value for Θ should lie between the values obtained for relative

volatility of heavy key and light key.

Next, proceed to calculate Rmin.

αPropane x fPropane

αPropane−θ+α1,3Butadiene x f 1,3 Butadiene

α1,3 Butadiene−θ=1+R

min

2.724∗0.042.724−2.67

+ 1∗0.911−2.67

=1+Rmin

Rmin +1=1.468, therefore Rmin = 0.468.

Reflux ratio is generally between 1.1 -1.5 times the minimum reflux ratio based on practical values

but there is no relationship between Rmin and the optimum value.[8]

R = 1.1*0.468= 0.515

To find the number of theoretical stages using FENSKE EQUATION [11]

N m=log

x LKx HK d

∗x HKx LK w

log α LK…………………… ..(5)



N m=log

0.6840.316 d

∗ 0.950.0005 w

log 2.724=¿8.30 (approximately8)

Therefore,

RR+1

= 0.5150.515+1

=0 .34…………… ...… ..(6)

RminR min+1

= 0.4680.468+1

=0 .319……………….(7)

From CHART – Erbar-maddox correlation in the figure below we have,

Graph 1: Ebbar Maddox Correlation

N mN

=0.4…………… ..…… ...(8)

8N

=0.4

N= 20 theoretical stages



Table 6: Number of theoretical stages at different Reflux Ratios

R R/R+1 Nm/N N

1.1*0.468 = 0.515 0.34 0.4 20

1.2*0.468 = 0.562 0.36 0.42 19

1.3*0.468 = 0.608 0.39 0.49 16

1.4*0.468 = 0.655 0.4 0.53 15

1.5*0.468 = 0.702 0.41 0.55 15

1.6*0.468 = 0.749 0.43 0.58 14

From the above observations it is clear that increasing the reflux ratio reduces the number of

stages. But it is seen that increasing the minimum reflux ratios by 1.4 to 1.5 does not changes the

number of theoretical stages.

There are two options available to decide which reflux ratios could be used in our design. Either a

line can be drwan from the origin of the graph as shown below which means that the best available

technique is a mid way between the number of stages and the reflux ratio or this can be debated on

the basis of economic costs.

Figure 8: Selection of Reflux ratio and the number of stages


The increase in the reflux ratio reduces the number of stages in a distillation column. But energy

costs shoot up which has to be considered as well. Since the Distillation column is almost running

for (350 days* 15 years) approximately, it is not a good option to run the column at high reflux

ratios. But if very low reflux ratios are opted, the metal costs to build the column go up. The reflux




ratio chosen for the process is 1.3 × Rmin. I have decided to use a reflux ratio of 0.608 which requires

reasonable costs and the number of stages are also acceptable. [8]

If the Feed is assumed to enter the column at its boiling point there is a huge difference in the reflux

ratios

4.1.2 Determination of the Diameter of the Column:

FAIR CORELLATION (1961)[10] is used to calculate the diameter of the column. To find the diameter,

velocity of the fluids has to be calculated. The calculations are detailed below:

Internal Traffic [11]:

L =RD ; 0.608*661.83 = 402.39kg/hr

L’ = L + qF ; 402.39+(1.267*12967.83) = 16832.63 kg/hr

V = L+D ; 402.39+661.83 = 1063.22 kg/hr

V’ = V – (1-q)F ; 1063.22 – (1-1.267)* 12967.83= 4525.63 kg/hr

FLV = L/G (ρV/ρL)0.5 [18] where;

L = Liquid mass flow rate, kg/s

G = Vapour mass flow rate, kg/s

Vapour density, kg/m3 = ρV

Liquid density, kg/m3 = ρL

The calculation of ρL and ρv (see section (C) of Appendix V)

ρL = 599.69 Kg/m3 and ρv = 30.324 kg/m3

The Flooding correlation is calculated at both the sides of the column because it can be a stepped

column.

Therefore, the flooding correlation above the feed

FLV=LG

(ρ v

ρL

)0.5

………………………..(9)

FLV=402.39

1063.22( 30.324

599.69)

0.5

=0.085



Using the CHART –flooding velocity, sieve plates, in Graph 2 and taking a generalized plate spacing

of 0.6m:

Graph 2: Flooding Velocity Sieve plates

K1 (top) = 0.1

The velocity can now be calculated by;

u f=K1(top)∗[(ρL−ρV )

ρV]0.5

………………….(10)

Hence,

u f=0.1∗[(599.69−30.324 )

30.324]0.5

=0 .4333m /sec

Taking percentage flood @ 80% based on good design considerations, therefore velocity @

flooding:

uf (top) = 0.8*0.4333 => 0.34664 m/sec



Maximum Volumetric Flow Rate:

Top= 1063.22Kg /hr(30.324Kg /m3∗3600 sec /hr )

=9 .74∗10−3m3/sec

Net Area Required

A=Volumetric Flow RateVelocity

………………….(11)

A=9.79∗10−3m3/sec0.34664m/ sec

=0 .028m2

Allowing 10% for down comer and 10% for support rings;

There total area = 0.028*1.1*1.1 = 0.034 m2

Hence column diameter above the feed point;

d=√ A∗4π

=√ 0.034∗4π

=0 .21m

Similarly for the lower section of the column;

FLV=16832.634525.63

( 30.324599.69

)0.5

=0.84

Using the CHART –flooding velocity, sieve plates in Graph 2; taking a generalized plate spacing of

0.6m:

K1 (bottom) = 5*10-2

The velocity can now be calculated by;

u f=0.05∗[(599.69−30.324)

30.324]

0.5

=0 .21665m /sec



Taking percentage flood @ 80% based on good design considerations, therefore velocity @

flooding:

uf (bottom) = 0.8*0.21665 => 0.17332 m/sec

Maximum Volumetric Flow Rate:

Bottom= 4525.63Kg /hr(30.324Kg /m3∗3600 sec /hr )

=0 .041m3/sec

Net Area Required

A= 0.041m3/sec0.17332m /sec

=0 .237m2

Allowing 10% for down comer and 10% for support rings;

There total area = 0.237*1.1*1.1 = 0.288 m2

Hence column diameter below the feed point;

d=√ A∗4π

=√ 0.292∗4π

=0 .61m

4.1.3Feed Location:The number of stages is greatly affected by the subsequent position of the feed into the column.

The feed should enter the column at a point that matches best with the composition of the feed. It

is always a wise decision to provide two or three feed point nozzles around the predicted feed point

which will account for the changes in the feed compositions after the start up.

The propane Distillation column is operating under high pressure. Hence, the feed is either

compressed or pumped into the column. It can be made to compress to a slightly higher pressure

into the feed pipe, approximately 14 bars, so that when it enters the column which is slightly at a

lower pressure than the pipe, some of the propane from the components will immediately expand



and undergo a partial flash vaporisation resulting in vapour liquid separation as it enters the

distillation column.

4.1.3 Calculation of the Feed Position:

Krickbride(1994)[13] can a practical equation to calculate the feed point location

logN r

N s

=0.206 log [ BD∗x fHK

x fLK∗xbLKxdHK

2]………………… (12)

Where,

Nr = number of stages above the feed, including any partial condenser

Ns = number of stages below the feed, including the reboiler

B = Molar Flow Bottom Product

D = Molar Flow Top Product

XfHK = concentration of the heavy key in the feed

XfLK = concentration of the light key in the feed

XbLK = concentration of the light key in the bottom product

XdHK = concentration of the heavy key in the Distillate

Therefore, by substituting the values in the above equation;

logN r

N s

=0.206 log [ 277.33613.954

∗0.91

0.04∗0.0005

0.316

2]log

N r

N s

=−0.607

N r

N s

=0.2472

This can also be rewritten as;



N r=0.2472N s ......................(A)

Since the total number of stages being used is 16,

Therefore;

N r+N s=16…………….. (B)

Substitution the value of Nr from Equation (A) in equation (B),

We get, Ns = 12.8 (≈ 13) which means, there are 13 stages below the feed position which includes

the reboiler as well.

Since, the feed location is almost at the top of the column leaving 3 stages, the design of a stepped

column is pointless. It is made with an overall diameter of 0.6m.

4.2 Choice of Plates and Packing:

The calculated value for the distillation column is approximately 0.6 m. The choice between packing

and trays was made on the basis of advantages and disadvantages of each type, and it is concluded

that a PACKING would be best for such small diameters. This decision was made because: [14]

Plate columns are generally designed to handle wider range of vapour and liquid flow rates

than packed columns. Since the gas flow rates in the top of the column is minor, plate

installation would not be a good idea.

Packed columns are not suitable for very low liquid flow rates.

The pressure drop per equilibrium stage (HETP) is lower for packing than plates. Since, the

column is operated at high pressures; it would be a better option to choose packed column

rather than plates to avoid complications.

Packing should always be considered for smaller diameters, generally for column diameters

>0.9m as it is very expensive and difficult to install plates in such columns.

After the decision is made, the design over the packed column has to be done. This includes the

following procedures: [14]

The type and packing size has to selected corresponding to the diameter of the column

Determining the height of the column required for the desired separation

Calculation of the column diameter (capacity), to handle the liquid and vapour flow rates.



Selection of column internal features like; packing support, liquid distributor, redistributors.

4.2.1 Types of Packing:

A packing should always fulfil the following requirements: [14]

Provide a large interfacial area between the gas and the liquid.

Should have an open structure for low resistance to gas flow.

Should provide consistent liquid distribution on the packing surface

Promote unvarying vapour gas flow across the column cross section

Hence, keeping the above requirements into consideration, the packing is broadly classified as

Structured Packing: These are packing with standard geometry made of wire mesh or

perforated metal sheets which are folded and arranged together. Some metal structured

packing commonly used in distillation columns are shown in the figure 10. They have low

height of equivalent theoretical plates (HETP) and low pressure drops. They are generally

used in difficult separations which require many stages, like separating isotopes or in high

vacuumed distillation columns. Using structured packing can also used to increase the

capacity and reduce the reflux ratio requirements.

Figure 10: Structured Packing

http://img.alibaba.com/photo/50678452/Ceramic_Metal_Plastic_Structured_Packing.jpg

Random Packing: These are variously shaped in rings, saddles etc, and are dumped into the

distillation column to take up a random shape. These structures can be made up of plastics,


http://img.alibaba.com/photo/50678452/Ceramic_Metal_Plastic_Structured_Packing.jpg


ceramics and metals. The various types of random packing are shown in figure 11 below. The

packing manufacturers should be consulted for details for many special types of packing that

are available for special applications.

Figure 11: Various kind of random packing

http://img.alibaba.com/photo/50553772/Ceramic_Random_Packing.jpg

Raschig rings are the oldest types of random packing whose length is equal to the diameter. These

are cheaper compared to any of the random packing structures but not as efficient as others.

These were then modified to Pall rings which have openings made by folding strips of the surface

into the ring. This increases the liquid distribution by increasing the surface area.

Other types of random packing are, Berl Saddle Ceramic, Intalox Saddle Ceramic, Metal Hypac and

Super Intalox Ceramics.


http://img.alibaba.com/photo/50553772/Ceramic_Random_Packing.jpg


Selection of Packing:

Type and Material [15]:From the above information it would be cheaper to use Random Packing with Metallic (Carbon-

Steel) Pall Rings. This is because the column is operating at high pressures and has a very small

diameter. Also the components inside the column are non-corrosive. It would require too much

effort and capital to install structured packing inside the column which would separate the same

amount of propane. Also Ceramic pall rings can be considered, but they are more expensive than

the metallic ones.

Figure 12: Metallic Pall rings

http://www.pall-ring.com/images/product/Chempack_metal_pall_ring_02.jpg

Packing Size [15]: The largest size of packing suitable for any size of column should not exceed more than 50mm. This

is because the larger packing sizes would cause flooding due to poor liquid distribution. Very small

sizes are also available but are much more expensive than the larger ones.

The sizes recommended for different column diameters are:

Table 7: Packing size depending on the Column Diameter

Column Diameter Packing Size

< 0.3m < 25mm (1inch)

0.3 m to 0.9m 25mm to 38mm (1inch to 1.5 inches)

> 0.9m 50mm to 75mm (2 inch to 3 inches)

Hence, for the Propane distillation column Metallic Pall rings with a size of 25mm is used.


http://www.pall-ring.com/images/product/Chempack_metal_pall_ring_02.jpg


Column Diameter (capacity)[16]: A distillation column is always designed to operate at height economical pressure drop. This ensures

good liquid and gas distribution. For random packing the pressure drop normally does not exceed

more than 80mm of H2O/m of the packing column.

A distillation column’s design values for pressure gradients are between 40 -80 mm Hg/m packing. [17]These values are almost halved when the liquid starts to foam. The proposed design has a

diameter of 0.61m with trays. Since random packing is now considered a new diameter has to be

calculated.

Pressure gradient over the enriching section using column diameter as 0.61m

A generalised pressure gradient/flooding correlation graph can be seen in Graph 3 below:

Graph 3:

Graph

to

determine the Flooding Line

Values for “x” and “y” axis can be calculated and the flooding line can be plotted in the figure above.



“ X”axis :LG √ ρg

ρL

……………………… (13)

Where,

L = Liquid Rate per Area

G= Gas Rate per Area

Since the area remains same; the ratio can just be taken as the normal flow rates of vapours and

liquids.

Hence;

“ X”axis :16832.224525.63 √ 30.524

599.69=0 .839

Now,

Y axis :G2∗F p∗φ∗μL

0.2

ρv∗ρL∗g………………… (14)

Where,

Fp = Wet Packing Factor; Appendix V; section (d)

Ψ = Ratio; (density of water/Density of Liquid inside the column)

μL = Liquid Viscosity of 1,3 butadiene is considered in the calculations, as major fraction of the liquid

comprises of 1,3 Butadiene

g = Gravitational constant

Hence;

Y axis :¿¿

From the Graph 3; the flooding line is calculated at 6, which is 81 mm Hg/meter. This result is

acceptable since the pressure gradient calculated above, 6mm Hg, it is concluded that the system is

expected to run at very extreme pressure gradients.

The values of pressure gradients for the lower range over the distillation column can be used to

calculate a new diameter. Any of the values between the two ranges can be used for the design.



From the same graph above a new value of “Y” is plotted at 40mmHg/m is now calculated.

Hence the Flooding line would be,

40mmHg13.6mmHg

=2.9≈3…………………… ..(15)

As 13.6 mm Hg is the specific gravity of mercury.

“Y” axis in now 0.015.

Again using Equation on the Y axis in Graph 3; a new gas flow rate per area is calculated which

would give the new diameter.

G=√ 0.015∗30.324∗599.69∗9.81160∗1000

599.69∗0.250.2

=3 .64

Since,

G=VA

Therefore,

New Area (A )=VG

= 4525.633600∗3.64

=0 .345m2…………… ..(16)

Since the gas flow rate per area in decreased; the diameter has to be increased.

Hence,

D=√ A∗4π

=√ 0.345∗4π

=0.662m≈0 .66m

Since, the change in diameter is not drastic the packing used can still be suitable for this diameter.

The capacity of the column can be anything between 0.61m – 0.66m depending on the flooding line.

Hence, it has been concluded that the capacity will be 0.65m; which can also be helpful when the

production capacity of the plant increases. The flooding like can be changed accordingly.

Packed Bed Height:[18]

The heights of equivalent equilibrium stage or HTEP is the height of a single packing which gives the

same amount of separation as an equilibrium stage in a trayed column. The HTEP is for a specific

type of packing is usually constant and is independent of the physical properties of the components.



This provides good liquid distribution and does not let the pressure drop fall below 17mm H2O/m of

the packing height.

Table 8: The recommended HETP’s for Pall rings

Size of the Pall Ring in mm HETP in m

25mm 0.4m – 0.5m

38mm 0.6m – 0.75m

50mm 0.75m – 1.0

Hence, HETP for the propane distillation column is 0.5m as the size for pall rings used is 25mm.

This means, one stage is 0.5m since there are 16 stages calculated earlier, total height of the column

= 0.5*16 = 8m

Flooding is prevented due to the tall height of the column.

Since the feed stream is put after 3 stages; the position of the feed stream is (0.5m*3 stages) 1.5 m

at the top and the length below it is 6.5 m.

Choice of material used for construction: The column is decided to be made up of stainless steel (18Cr/8Ni, Ti stabilised)(321). Steel is a very

strong metal with high ductility and high malleability which can withstand high stress. Since the

column has to with stand high pressures and high temperatures, poor choice of material should not

be used. Also the column is very tall. Also stainless steel does not corrode. It is more expensive than

carbon steel, but it one time investment is affordable. Also the column is required for purification

purposes; hence any corroded metal in the product would not be appreciated.



Thickness of the metal: It is assumed that the column is cylindrical in shape. The welds are fully radio-graphed. This column

is operating at very high pressure of 13.5 bars. Hence, a pressure has to be designed in such a way

that the column resists the increase in pressure due to equipment failure.

Internal pressure is designed at 10% above operating pressure.

= (13.5 – 1)* 1.1 => 13.75 bars (1.375N/mm2)

The thickness of the cylindrical section of the column can be calculated by a formula from British

standards PD 5500 :

e=P i∗d i

(2∗σ s )−Pi

……………….(17)

Where,

Pi = Internal pressure

σs = Design Stress for stainless steel @ 1000C; Appendix V; section (e)

e = minimum thickness of the material

Hence,

e=1.375∗0.65∗1000(2∗150 )−1.375

=2.99mm≈3mm

Allow 1mm for corrosion;

Hence the thickness of the cylindrical section should be 4mm.

The thickness of the Domed Section of the column can be calculated by:

A standard dished head (torisphere) is assumed;

Crown Radius, Rc = Di = 0.65 m



Knuckle radius = 6%of Rc = 0.039m

C s=14 (3+√ Rc

Rk)=1

4 (3+√ 0.650.039 )=1 .77

A head of this size would be formed by pressing: no joints, hence, J = 1

e=P iRcC s

2 fJ+Pi(C s−0.2)………………………(18)

Substituting, correct values in the above equation;

e= 1.375∗0.65∗1000∗1.77(2∗150∗1)+1.375(1.77−0.2)

=5 .24mm

A tank diameter of less than 15m should have a minimum allowable thickness of 5mm [16]. The

thickness obtained from calculation would hold the weight of content in the vessel, but would not

stand depression or dent. Therefore, a 5mm minimum allowable thickness was given for any vessel

less than 1m because this can withstand depression. Hence, the results obtained are acceptable.

Diameter and Thickness of the Pipes:It is decided to use Pipes made of Stainless Less because they have high tensile strength and are

ductile. These twp parameters are matter much as the system is operating at high pressures. The

diameter is calculated by equation 19[20] and the thickness is calculated from equation 20[20]

For Stainless steel;

d ,optimum=260G0.52 ρ−.37……………….(19)

The thickness of the pipe; t

t=P do

20σd+P………………… (20)



The Feed Pipe :

d ,optimum=260 (12967.833600

)0.52

599.69−.37=47 .48mm

And;

t= 13.5∗47.48(20∗145)+13.5

=0 .22mm

Pipe to the Condenser:

d ,optimum=260¿

And,

t= 13.5∗39.02(20∗145)+13.5

=0 .18mm

Pipe to the Reboiler:

d ,optimum=260 (16832.633600

)0.52

599.69−.37=54 .38mm

And,

t= 13.5∗54.38(20∗145)+13.5

=0 .252mm

4.3 Column Internals [21][22]:The internal packing in a packed distillation column should be designed carefully, to ensure better

performance.

Generally, the standard fittings are developed by the packing manufacturers and its uses are

specified. Some of the typical designs are discussed as follows.



Packing Support: The support plate

carries the weight of the random

structures inside the column. A poorly

designed of the support plates causes

flooding or very high pressure drops

inside the column. A simple Grid or a

perforated plate support can be used.

The only disadvantage with such kind of

support systems is that the liquid and

the gas have to vie through the same

openings. Hence, wide spaced grids are

used to avoid this problem and increase

the flow area. The larger sizes packing are dumped into the column first if wider grids are used,

which support the smaller random packing. (As shown in Figure 13)

The best design for packing support is the

gas injection type of support. The gas

inlets are provided above the level where

the liquid flows from the bed. The

problem of local flooding and high

pressure drops is solved with the

application of these support rings. These

supports are available in a variety of sizes

and materials, like plastics, ceramics and metals.

Figure 14; shows design for the gas injection supports and Figure 15 shows the Principle of gas

injection packing support



Liquid Distributors:

Uniform flow of liquid throughout the

column and good initial liquid distribution

is a very essential factor for good

performance of a column. A central open

feed pipe or a pipe with a spray nozzle is

best suited for proper distribution of

liquid in small diameter columns. But for

larger diameters, more elaborate designs

are required. The two most common type

of distributors used are orifice type and

weir type.

The orifice type has holes in the plate

which allows passage for the liquid and it has short stand pipes, through which the gas flows. Sizing

of gas pipes are compulsory as it provides sufficient area for the gas flow without letting the

pressure to drop significantly. The holes should be of an appropriate size, neither too small nor too

big so that at lowest liquid flow rate, there should be a level of liquid always present on the plate

and at highest flow rates, the liquid distributor does not over flows. Figure 16 shows the C.S and the

T.S of an orifice distributor.

In the Weir types the liquid flows over the notched weirs in the gas stand-pipes. These stand an

upper grade over the orifice type in handling wider range of liquid flow rates. The column designed



uses Orifice type of distributor. A pipe manifold distributor shown in figure 17 is used when the

liquid is fed to the column under pressure and the flow rate is reasonably constant. The distribution

pipes and orifices are accurately sized for equal uniform distribution.

Orifice

type is exploited in the propane distillation column.

Liquid Redistributors:

The main purposes of redistributors

are to collect liquid that has moved

to the column walls and redistribute

it evenly over the packing also

redistributors will even out any

maldistribution that has occurred

within the packing.

A full distributor shown in figure 18

combines the function of packing support and liquid distributor.

The maximum bed height that should be used without liquid redistribution lies solely on the type of

packing and the process. The wall-wiper type is mainly used in small columns of less than 0.6m in

diameter. This operates in a mechanism were a ring collects liquid from the column wall and

redirects it into the centre packing.



Hold Down Plates:

Fluidisation of the top layer of packing can

occurs due to high gas flow rates or surging

effects due to mis-operation. The ceramic

packing crushes during such circumstances but

the metal and plastic packing can be wafted

out of the column. Bed-Limiters are used for

plastic and metal packing to prevent to

prevent expansion of bed at higher pressure

drops.

These are similar to Hold Down plates. Such

plates should have smaller grids to retain the

packing but not too small to restrict the fluid flow.

Installing Packing:

The column is filled with water and the packing is dumped into the water ensuring the level of

water always being above the packing at all times. Metal and Ceramic packing are generally

deserted into the column “WET”. This ensures random distribution and prevents any damage to the

packing.

In case of “DRY” packing the packing should be lowered into the columns in buckets or other

containers.

Concept Drawing:

The Concept Drawing Details the application of column internals more in detail. Bit-Map is used to

create the column showing every possible detail and dimension. The non calculated values are

assumed to British standards.



5.0 CONDENSER:

Condensers are units similar to heat exchangers, which are used to condense the vapours to their

liquid state. Condensers vary in size ranging from hand handled units to large industrial scale units.

For condensers used in distillation columns, the process fluid generally flows through the shell side

and the cooling fluid is made to flow through the tube side. The two fluids never come in contact

with each other. The diagram below shoes a one tube pass condenser.

Figure 20: One pass tube side condenser

http://upload.wikimedia.org/wikipedia/commons/c/cd/Straight-tube_heat_exchanger_1-

pass.PNG

Four condenser configurations are possible:[23]

Horizontal, with condensation in the shell, and the cooling medium in the tubes

Horizontal, with condensation in tubes

Vertical, with condensation in shell

Vertical, with condensation in the tubes

Horizontal shell side and vertical tube side is widely used as condensers. The other two are usually

used in arrangements for heaters and vaporisers using steam as the heating medium.


http://upload.wikimedia.org/wikipedia/commons/c/cd/Straight-tube_heat_exchanger_1-pass.PNG

http://upload.wikimedia.org/wikipedia/commons/c/cd/Straight-tube_heat_exchanger_1-pass.PNG


Assumptions:

It is assumed that a horizontal shell side condenser is used to cool the propane vapours.

It is a complete condenser. Hence all the vapours are cooled down.

There is one shell and one tube pass inside the condenser and the flow is counter current.

Since the top product mainly comprises of the propane, the physical properties of propane

are used for all the calculations.

The cooling fluid chosen is chilled water that enters the tube side at 150C and leaves at 450C.

The tube layout for this particular design is arranged in a squared pitch.

Basis: 1 hour

Flow rate entering the condenser; V

V = L+D; 402.39 + 661.83 = 1063.22 kg/hr

5.1 Energy Balance Over the condenser:Q = (Vapour flow rate * latent heat of vaporisation of methanol) + mCp∆T

Q=mL+mC p∆T ………………….(21)

“mL” due to phase change, since it is a complete condenser where phase change occurs; and

“mCp∆T” due to increase in the energy per degree rise in temperature.

Vapour Flow Rate;

m=1063.22

Kghr

∗1

3600 s=0.295Kg / s

Latent heat of Vaporisation of Propane (1,3 Butadiene is ignored as it would not bring a vast

difference)= L = 322.76 kJ/Kg

Therefore,

mL=0.295kg /s∗322.76 kJ /Kg=¿95 .214KJ /s∨95 .214KW

mC p∆T=

661.83Kgsec

∗1.75KJ

KgK∗10K=3.22KJ /sec

OverallQ=95.214KW +3.22KW=98 .434KW



Since the Heat lost by the condenser is calculated the flow rate of water required can also be

calculated as relevant from the following formula;

Q=(mC p∆T )1=(mC p∆T )2………………(22)

It is assumed earlier that the chilled water enters the condenser @ 150C and leaves @ 450C. Hence

∆T = 300C

Hence,

(mC p∆T )1=98.434KW=

X Kgsec

∗4.2KJ

KgK∗30K=0 .781Kg /sec

Water flows at a flow rate of 0.781Kg/sec inside the tubes.

Figure 21: My Diagram for better understanding of ∆TLM

From the diagram above it now becomes easier to ∆TLM. The Log mean temperature difference is

given by:

∆T LM=(T ¿−tOUT )−(T OUT−t¿)

ln(T¿−tOUTTOUT−t ¿

)………………… ..(23)

Where,

∆TLM = log mean temperature



TIN = inlet temperature of process fluid

TOUT = outlet temperature of process fluid

tIN = inlet temperature of cooling water

tOUT = outlet temperature of cooling water

∆T LM=(45−40 )−(35−15)

ln( 45−4035−15

)=10.820C

To begin with the design, we assume an approximate overall coefficient. This is done in aid with

Graph 4; where the fluid in the process side are classified as Paraffin and the fluid in the service side

is taken as River, Well or Sea Water.

Hence, the assumed overall coefficient = 600 W/m2 0C

Graph 4: Overall Coefficients (join process side to the service side and read U values from the

centre line)



Trial Area:

Q=UA ∆T LM………………(24)

Where,

Q= Heat Transfer per unit Time

U= Heat Transfer Coefficient

A= Area

∆TLM = Log Mean Temperature difference

A= QU ∆T LM

= 98.434∗1000W

600W /m2K∗10.82K=15 .16m2

Layout and Tube Size [24] :

Usually the plant standards require tubes of:

The outer diameter is 20mm

The inner diameter is 16 mm

The length is taken to be 2.44 m (8 ft)

The tubes are made of arbitrary brass.

The arrangement is a squared pattern.

Hence, the surface area can now be calculated from the above data specified.

Surface Area (A )=τdl=3.14∗20∗10−3∗2.44

Surface Area (A )=0 .1533m2

Therefore,

Number of tubes= Total Trial AreaSuraface Area of one tube

Number of tubes= 15.16m2

0.1533m2 =99 tubes



Since there is slight condensation occurring, hence the number of tubes calculated is so less.

Since, it was decided to use a Square Pitch;

Pt=1.25∗do=1.25∗20mm=¿25mm

Tube Bundle Diameter; Db

Db=do ¿

Where,

Nt = number of tubes

K1 and n = constants obtained from table 9[25]

Do = Outer diameter

Table 9: K and n constants for square arrangements:

Square pitch, Pt = 1.25 do

Number of Passes 1 2 4 6

K1 0.215 0.156 0.158 0.0402

n1 2.207 2.291 2.263 2.617

Hence,

Db=20¿

Number of tubes in the centre row:

Tubes∈t hecentre row=Db

Pt

……………… ..(26)

Tubes∈t hecentre row=321.91mm25mm

=12 .87 tubes 13 tubes

We take it as 12 tubes as the number of tubes should be even and divisible by 4 as the arrangement

is a square pitch.



5.2 Shell Side Coefficient:The Tube Wall temperature, Tw can be estimated if the condensing coefficient is assumed. The value

for the condensing coefficient is assumed to be 1500W/m0C

Mean temperature:

Shell side=35+452

=400C

Tube side=15+402

=27.50C

( 40−T w ) 1500=( 40−27.5 )600

Hence, the wall temperature is 350C.

Table 10: Physical Properties of Propane @ 350C[26]

Liquid Density (ρL) @ 350C 514.13 Kg/m3

Liquid Viscosity (μL) @ 350C 0.11321 centi-poise

Liquid Thermal Conductivity (kL) @ 350C 0.095W/m K

Vapour Density @ 40 0C mean vapour temperature (ρV) = 28.6 Kg/m3

Using Kern’s Method [27], the mean transfer coefficient for the tube bundle is given by:

¿

Where,

Гh=W c

L N t

Wc = Flow rate of the distillate in Kg/sec

L = Length of the tubes

Nt = Number of tubes

N r=23∗number of tubes∈t he centrerow…………… ..(28)

N r=23∗12=8



Hence,

Гh=0.295

2.44∗99=1 .22∗10−3 Kg /msec

Substituting the values in the Kerns Equation:

¿

¿

The value obtained is close enough to the value assumed (1500W/m2K), hence the estimated Tw is

acceptable.

5.3 Tube Side Coefficient:

Eagle and Ferguson (1930) adapted a much reliable formula for the heat transfer coefficient for

water used in the tube side. This is stated as below:

hi=4200(1.35+0.02 t)ut

0.8

d i0.2 ……………………(29)

Where,

hi = inside coefficient for water

t = mean water temperature

ut = water velocity

di = tube inside diameter

ut can be calculated from the internal diameter of the tube

Tube Cross- Sectional Area;

( A )=π∗(.016)2∗994

=0 .0199m2



Density of water at 27.50C (mean water temperature in the tubes) = 995.65 Kg/m3

Tube Side Velocity, ut

ut=m

A∗ρL………………… ..(30)

ut=0.781

995.65∗0.0199=0 .0394m /sec

Hence, substituting the values of velocity, the heat transfer coefficient of water inside the tube is;

hi=4200(1.35+0.02∗27.5)0.03940.8

(.016)0.2 =1375 .49W /m2 0C

5.4 Overall Heat transfer Coefficient U;

1U

= 1ho

+ 1hod

+do ln (

do

d i

)

2kw+

do

d i

∗1

hid+

do

d i

∗1

hi………………(31)

Where,

U = the overall coefficient based on the outside area of the tube, W/m 2 0C

ho = outside fluid film coefficient, W/m 2 0C

hi = inside fluid film coefficient, W/m 2 0C

hod = outside dirt coefficient; fouling factor, W/m 2 0C

hid = inside dirt coefficient; fouling factor, W/m 2 0C

kw = thermal conductivity of the tube wall material, W/m0C

di = tube inside diameter, m

do = tube outside diameter, m

Fouling Factors: As neither of the fluid is heavily fouling, the dirt coefficient at both the sides has

been assumed to be 6000 W/m 2 0C and kw is assumed to be 50 W/m0C

Substituting all the values in the equation 31;



1U

=1

1646.979+

16000

+(20∗10−3) ln ( 20

16)

2∗50+

2016

∗1

6000+

2016

∗1

1375.49

1U

=1.9354∗10−3

Hence, Overall Heat transfer coefficient;

U=516 .689W /m2 0C

The value obtained is significantly lower than the assumed value of 600 W/m2 0C

The calculation has to be repeated using different values of Overall heat transfer coefficient by trial

and error method; and the closest possible answer firms up the final design.

The calculations are redone with a new value of 500 W/m2 0C

A= QU ∆T LM

= 98.434∗1000W

500W /m2 K∗10.82K=18 .19m2

Surface Area (A )=τdl=3.14∗20∗10−3∗2.44

Surface Area (A )=0 .1533m2

Hence, the number of tubes now changes,

Number of tubes= Total Trial AreaSuraface Area of one tube

= 18.19m2

0.1533m2=119 tubes

Tube Bundle Diameter; Db

Db=do ¿



Tubes∈t hecentre row=Db

Pt

=349.89925

=14 tubes

Shell Side Coefficient:

As already known,

¿

Where,

Гh=0.295

2.44∗119=1 .009∗10−3Kg /msec

And

N r=23∗number of tubes∈t hecentre row=2

3∗14≈9 tubes

Therefore, substituting the values of Гh and Nr in equation 32

¿

¿

Tube Side Coefficient:

Again using Eagle and Ferguson’s (1930) formula tube side coefficient is calculated. But the velocity

of the water in the tubes changes;

New Cross sectional Area

( A )=π∗(.016)2∗1194

=0 .0239m2

Hence, the tube velocity,



ut=0.781

995.65∗0.0239=0 .033m /sec

Hence, the new calculated value for the Tube Side Coefficient;

hi=4200(1.35+0.02∗27.5)0.0330.8

(.016)0.2 =1220 .06W /m20C

Overall Heat transfer Coefficient;

1U

=1

1630.35+

16000

+(20∗10−3) ln ( 20

16)

2∗50+

2016

∗1

6000+

2016

∗1

1220.06

1U

=2.0575∗10−3

Hence, Overall Heat transfer coefficient;

U=486 .026W /m20C

The results are satisfactory. The values obtained are close enough to the values assumed hence, the

design is confirmed.

5.5 Shell side Pressure Drop:

Specifications:

Pull through Floating head is used, no close clearance is required.

35% cut of the Baffle Spacing

A condenser has wider baffle spacing, lB ≈ Ds

Pressure Drop is given by;



∆ P s=8

jf∗D s

de

∗L

lb∗ρus

2

2( μμw

)−.14

………………….(32)

Where,

Jf = Friction factor

L = Length of the tubes

Lb = baffle Spacing

All the unknowns are calculated below.

Shell internal Diameter; Ds

Ds=Db+clearance

Clearance is = 89mm estimated from the graph 5

Graph 5: Shell- Bundle Clearance

Hence,

Ds=349.899mm+89mm=438 .899mm



Cross Sectional Area As for the hypothetical row of tubes at the Shell Equator

A s=( pt−d0)D slB

pt…………………………(33)

Where,

Pt = Tube Pitch

Ds = shell inside diameter

LB = baffle spacing

A s=(25−20 ) 438.899∗438.899∗10−6

25=0 .039m2

Shell Side mass Velocity Gs

Gs=W s

A s

…………………….(34)

Where,

Ws = Fluid Flow Rate on the shell side in kg/sec

Gs=0.2950.039

=7 .56Kg /m2 sec

Linear Velocity us

us=G s

ρ= 7.56

29.16=0 .26m /sec

Shell Side Equivalent Diameter; de

The equivalent diameter for a square pitch arrangement is given by;

de=4 (

p t2−π do

2

4)

π do

=¿ 1.27do

( p t2−0.785 d0

2 )………………(35)

de=1.2720

( 252−0.785¿202 )=19 .8mm



Friction Factor; jf

Friction factor is estimated from a graph in FIG; which is a correlation between Reynolds’s number

and baffle cut.

Hence,

Re=Gs∗de

μv

=7.56∗19.8∗10−3

0.008∗10−3 =18 ,711

Graph 6: Shell side Friction Factor; Segmental Baffles

From the graph Friction Factor is determined to be 4.8 *10-2

Substituting all the above calculated values in the pressure drop equation;

∆ P s=

8∗4.8∗10−2∗438.89919.8

∗2.44

0.438899∗29.16∗0.262

2( μμw

)−.14



Ignoring the viscosity correlation;

∆ P s=46 .64N /m2

The Pressure drop is negligible; hence more sophistication calculation is not justified.

5.6Tube Side Pressure Drop:

The pressure Drop on the Tube side is given by,

∆ Pt=N p [8 jf∗Ld i

∗( μμw)−m

+2.5] ρut2

2………………(36)

The viscosity correlation is neglected;

Jf ; Friction Factor is given by a graph 7

Where,

Reynolds number Re=ut∗d i∗ρ

μ…………………(37)

Hence,

Re=0.033∗16∗10−3∗995.65

0.6¿10−3 =876 .172



Graph 7: Tube side Friction Factor

Hence from the graph below,

Jf = 9*10-3

Substituting the values in the pressure drop equation;

∆ Pt=1[ 8∗9∗10−3∗2.4416∗10−3 +2.5 ] 995.65¿0.0332

2

∆ Pt=7 .306N /m2

The pressure drop is almost negligible due to the following reasons:

The flow is laminar as Re < 2000

The velocity of the cooling water is very low

Only one tube pass is used, hence there is a straight pipe through which the pipe flows.



6.0 REBOILER:

Reboilers are used with the distillation column to vaporise a fraction of the bottom product.

There are three different kind of reboilers used: [28]

Forced Circulation; where the fluid is pumped through the exchanger. The vapour formed is

separated and sent back to the bottom of the distillation column and rest is removed.

Figure 22: Forced Circulation Reboiler

http://upload.wikimedia.org/wikipedia/commons/a/a7/ForcedCirculation.png

Thermosyphon, unlike forced circulation it has natural circulation. The liquid circulation is

maintained by the difference in densities of the two-phase mixtures of vapour and liquid.

These kinds of Reboilers can have vaporisation on the tube side; when they are vertically

placed or the vaporisation occurs on the shell side, when placed horizontally.

Figure 23: Thermosyphon

http://upload.wikimedia.org/wikipedia/commons/5/57/Thermosyphon_Reboiler.png


http://upload.wikimedia.org/wikipedia/commons/5/57/Thermosyphon_Reboiler.png

http://upload.wikimedia.org/wikipedia/commons/a/a7/ForcedCirculation.png


Kettle Reboilers, also called submerged bundle reboilers; where boiling takes place on the

tubes immersed in a pool of liquid. The liquid does not circulate through the heat exchanger.

These are the most expensive kind of Reboilers.

Figure 24: Kettle Reboiler

http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/2/13/Fig_2_13_5.gif

6.1 Heat Duty over the Reboiler:Q=V '∗∆ H∅

vapourisation…………………….(38)

In the above equation, the ∆H® (Heat of Vaporisation) is considered only for 1, 3 Butadiene as major

fraction of the component entering the Reboiler comprises of 1, 3 Butadiene. Minor fractions of

propane, pentane and 1, 2 Butadiene is ignored. This would not affect the result drastically as the

Heat of vaporisation of the other components somewhere lies in the same range.

Hence,

Assuming; that the bottom product is liquid at boiling point which enters the Reboiler at 368K.

Q=

4525.63Kghr

∗1hr

3600 sec∗335.55KJ

Kg=421 .826KW

Amount of Steam Required:

Heat obtained by the process fluid is the heat lost by the steam entering the Reboiler.


http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/2/13/Fig_2_13_5.gif


Hence the “Q” remains same. Assuming steam entering the Reboiler at 2.6 bars,

Hence, ∆H® for steam at 2.6 bars = 2177.42 KJ/Kg

S= Q

∆H ∅vapourisation

= 421.826KW2177.42KJ /kg

=0 .194Kg / sec

6.2 Choice of type of reboiler used with the Propane Distillation:[28]

The choice of the kind of reboiler used with a distillation column solely depends on the following:

The viscosity and the propensity to fouling of the process fluid

The operating pressure

The headroom available to layout the equipment.

After careful judgement, it is concluded that a Horizontal Thermosyphon Reboiler would be used

with this distillation column, since the horizontal reboilers require less headroom. These are most

economical type of reboilers for most application and are not suitable for high viscous fluids. The

fluids in the column have very low viscosity.

Also these reboilers are suitable for high temperatures and high pressure which fulfil the demands

for this unit.

The only disadvantage with this kind of reboiler is that a hydrostatic head is required for the

thermosyphon effect which requires elevation of the column base. This problem can be overcome

by using supporters at the distillation column is not huge.



7.0 Overall Energy Balance over the Distillation Column:

The Energy Balance over the column could not be calculated in the first part of the assignment as

the Reflux Ratios were unknown.

The Heat Duty over the Condenser and the Reboiler is calculated. It is summarised in the table

below.

Units Energy Required Energy Released

Condenser - 98.434 KW

Reboiler - 421.826KW -

Total -323.392 KW

The Negative Sign shows that the Energy is required to run the unit.


8.0 Piping and Instrumentation Diagram

The Piping and Instrument diagram (P and ID) shows the engineering details of the

equipment, instruments, piping, valves and fittings and their arrangements which is not

detailed in the process Flow sheet. It should include the following. [29] [30]

All process equipment identified by an equipment number. The equipment should be

drawn roughly in proportion, and the location of nozzles shown.

All pipes, identified by a line number. The pipe size and material of construction

should be shown. The material can be included as part of the line identification

number.

All valves control and block valves, with an identification number. The type and size

should be shown. The type may be shown by the symbol used for the valve or

included in the code used for the valve number.

Pumps identified by a suitable code number.

All control loops and instruments, with an identification number.

For simple processes, the utility (service) lines can be shown on the P and I diagram.

For complex processes, separate diagrams should be used to show the service lines,

so the information can be shown clearly, without cluttering up the diagram.

Since the Process of Butadiene production is very complex which includes many units, the P

and ID is only done over the Propane Distillation Column which is attached in Appendix, 6

(issue number 002) The basic units and symbols used in the P and ID are briefly described

below:

Control Valves [31]: The parameters such as temperature, pressure, flow and level are

controlled by the control valve. Control Valves can be manual or automatic which are

partially or fully open or closed. The action of the control valves are in response to the

signals received from their respective controllers that compare the set-point to the

process variables.


Figure25: Detailed Diagram of a Control Valvehttp://www.isa.org/Images/InTech/2005/April/20050415-3.gif

Gate Valve [32]: These valves are opens by lifting a round or rectangular wedge out of

the path of the fluid. Gate valves are used when a straight-line flow of fluid and

minimum restriction is desired. These valves should never be used as control valves

unless those are specifically designed for it. Typical gate valves are designed to be

fully opened or closed. There is very low frictional loss when the valve is fully open

due to negligible obstruction. Gate valves are present before and after every control

valve and pump which offers effortless maintenance during equipment failure.

Figure 26: Detailed diagram of a Gate Valvehttp://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/12/1/fig_12_1_1.gif

Transmitters: These are sensors which monitor the changes in the process variables

and simultaneously send signals to the controller.

Richie Gandhi Page 69SUN - 075909279

http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/12/1/fig_12_1_1.gif

http://en.wikipedia.org/wiki/Fluid

http://www.isa.org/Images/InTech/2005/April/20050415-3.gif


They are named as FT (flow Transmitter), PT (Pressure Transmitter), TT (Temperature

Transmitter) and LT (Level Transmitter) respectively. They are Locally Mounted which

means that the controller and the display are located in the plant where the sensing

instrument is placed.

Drain Valve: As the name suggests, these valves are used to drain the components

from the unit or pipelines. These are generally closed and are in operation usually

during site or unit maintenance.

Relief Valve: The relief valve is a type of valve used to control or limit the pressure in a

system or vessel which can build up by a process upset, instrument or equipment

failure, or fire. The pressure is relieved by allowing the pressurised fluid to flow from

an auxiliary passage out of the system. Since the distillation Column taken into

consideration is operating under high pressures, a relief valve can act as a

precautionary measure which would help to get rid out the extra pressure built up

into the unit. It is built on the reflux drum, as building it over the column can lose its

components in the vapour phase that is released. As it is the last stage the purified

butadiene cannot be afforded to be wasted.

Pneumatic Lines: The signals transmitted from a controller to the valve are generally

shown by pneumatic lines on a P and ID.

Electric Lines: The dotted lines show the transmission of electric signal in the P and ID.

The electric signal is send by the transmitter to the controller.


http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/Valve


High and Low Level Alarms: Alarms alert the operators of serious and potentially

hazardous deviations in the process conditions. Key instruments are fitted with

switches. If there is a delay or lack of response by the operator a hazardous situation

can occur in a short span of time. Often drills are practiced to overcome such

catastrophic situations. When this system turns on, a trip system automatically comes

into action which includes, shutting down of pumps, closing valves, operating

emergency systems etc.

8.1 Control LoopA chemical plant is composed of several processing units; these cannot be treated as simple

systems working independently. They are integrated to form a systematic, rational and

controlled process. This process must then satisfy many requirements in order to achieve its

objective of converting raw material to the desired products. Control systems are employed

to successfully satisfy the requirements of safety, product specifications, taking into account,

operational constraints, plant economics and environmental regulations. The control system

is composed of an arrangement of equipment such as temperature / pressure indicators,

pneumatic valves, process controllers linked with computers (CPU). The requirements of

control are stated below:[33]

Safety [33]

A principal prerequisite of the plant is to operate safely for the well being of plant

stakeholders and to continue satisfying the economic goal. To achieve this, temperature,

pressures, flow rates etc need to be controlled within allowable limits.

Operational Constraints [33]

Different process equipment have variable process constraints i.e. tanks should not overfill,

distillation columns should not be flooded and centrifugal pumps must maintain a certain net

positive suction head, these need to be considered when determining the control system.

Environmental Constraints [33]

Involves ensuring discharges are within acceptable limits e.g. as prescribed under IPC.



Economics [33]

Plant operating conditions need to be controlled to ensure that the plant is operating at its

optimum, thus ensuring maximum profitability.

Although control is essential, it is known in industry that trying to tightly control absolutely

every stream in the process is very costly and unrealistic and can result in total inflexibility

and even destabilisation of the plant, (control loops may start to interfere with each other’s

actions). Similarly measurement sensors and control valves are expensive, so unnecessary

measurement is a loss of money. Therefore these factors will need to be taken into account

when deciding upon the control.

8.2 Control Systems over the Distillation Column:

Various control measures have been taken into account to meet the objectives of the plant

such as safety, operation, environmental constraints and economics. Sensors have been

installed at various process lines and equipments to make the operator aware of the

situation with the equipment and the process. The sensors transfer signals to the controller

which then send back commands to the respective valves to carry out the action.

Control system of the process will be explained as we go along the PID from left to right.[34][35]

FC 1: the feed pipe is installed with a flow controller, which helps to control the flow

throughout the pipe. The flow rate is received as input to the FT-1, which then sends output

signals to the flow controller FC, which then sends on information to the automated control

valve CV-1. This ensures a steady state and control flow rate of feed to the distillation

column.

PC 1: The distillation column is operating under high pressures. The pressure is a very crucial

variable and has to be controlled to avoid flooding. The pressure is read by a pressure gauge

and this input is received by a PT1 which sends signal to the PC 1 which then forwards the

information to the control valve on the condenser. The condenser would reduce the flow

rate of water if the pressure drops; this intern increases the temperature of the distillate

entering the reflux drum as desired heat exchange was not possible; which increases the

pressure back into the column. The reverse action is carried out if the pressure inside the



column increases. There is a relief valve on the reflux drum which also operates when the

pressure inside the column increases drastically.

LC 1: For desired separation to take place, it is important to maintain the ratio of the distillate

to the reflux Liquid. Hence it is vital to maintain the level in the reflux drum. The level is read

by a level indicator which sends the information to the LT 1 which further forwards the signal

to the control valve on the pipe back to the condenser. If the level in the drum falls, the

controller sends signal to close the valve which results in maintaining the level in the drum.

TC 1 and FC 2: The temperature should be well maintained inside the column to provide

vaporisation of the volatile fluids for desired separation. Heat is provided by the steam flow

in the reboiler. Hence a controller valve is installed on the pipe which carries steam to the

reboiler. This controller receives signal from the flow transmitter (FT2) or the temperature

transmitter (TT1). If the temperature inside the column reduces, the controller sends signal

to the control valve to open slightly. This increases the flow rate of steam into the reboiler

increasing the temperature of the column.

LC 2: Flooding is a problem commonly faced by distillation columns if the design is poor.

Sometimes blockage in the support systems inside the column can cause obstruction to the

passage of the liquid and the gases. This can increase the level in the column resulting in

spillage of the valuable product. A level controller is installed in the pipe leaving the column

at the bottom. This control valve receives signals from the level controller which gets its input

from the level transmitter. If the level in the column falls down, the valve closes slightly to

maintain the level and prevent it from becoming dry. Also when the level falls down too low

or rises to high, the alarms blow up which would alert the operators to take serious actions

immediately like evacuating the plant or shutting all the valves and stopping the flow rates.

Indicators:



There are three main indicators used in the P and ID. These are mainly to provide

information of the operating conditions. There are two sample indicators SI (1-1) and SI (1-2),

which provide information about the purity of the fluids entering and leaving the column.

Since, the column’s top temperature is different from the bottom one, two temperature

indicators are required.

A combination of automated valves, flow and temperature indicators along with controllers

are used to control the process. Investment in these instruments and the control system is

required and is necessary. The control system along with the PID is then checked by senior

manager and engineers over a HAZOP meeting. Changes are made to the control and the PID

before the system is installed, in such a way money is saved and at the same time safety of

the operator is ensured.

Since Hazop was carried out on this section of the process, there were certain changes made

from the previously designed control loop system. Issue 001 in Appendix VI shows the draft

copy of the previously designed P and ID which was then improvised to Issue 002



9.0 Hazard and Operability Studies HAZOP

HAZOP is a procedure for critical, systematic examination of the operability of a process.

Procedure of a Hazop involves taking a full description of a process, generally using a PID, and

systematically questioning every part of it. This helps to establish deviations from the design

intent. When the deviations are identified assessments are made on the consequences of the

deviation and the required action which should be taken to prevent or control the hazard

from taking place.

The analysis of Hazop is carried out in a structured way by following the guidlines attached; it

completely relies on the realising and imagination to discover the credible causes of the

deviations by the Hazop team. In practise many of the causes will be very obvious such as

pump failure. But putting a Hazop meeting encourages highlighting many causes which might

be obvious in first consideration. In this way the study is carried out and becomes more

rigorous and encourages the involvement of all the team members. The result of the Hazop

there is a good chance of identifying and recording relevant actions to be taken against

failures and problems which have not been experienced before. More over a documented

file of Hazop is required by legislation to confirm that preventive measures had been taken to

ensure the safe working condition of the operators and the environment.

In our study we carried out a Hazop with Dr. Titiloye on the Feed pipe entering the Propane

Distillation column. (As highlighted”Green” in Figure attached in Appendix VII). General

consequence of a Hazop leads to an updated version of the PID to be made after including all

the safety features discussed during the meeting. Hence the slight deviation in the final P and

ID (ISSUE no. 2) is due to Hazop done on the pipe.

In practise Hazop should be carried out over the whole process, but due to time constraints

and requirements of the project only one line of the process is looked at.

Hazop discussions have also been added, this includes all the causes and their consequences

looked at and also preventive actions have been highlighted. The Hazop is attached in

Appendix VII.



10.0 Conclusion:

The design project has been successful in its aims of providing a detailed knowledge a

chemical process unit. This project was a learning process where different challenges were

faced. At this stage, such a project is aimed to increase general design knowledge of a given

operating unit in this case; a packed bed distillation column. As this was just a preliminary

design, the amount of detail of the design and the accuracy of the final calculations might be

questionable. However, it remains a fact that this project targets more than just design but

also gives an idea of how to make reasonable assumptions and where data is not available,

the use of values which can be used to make reasonable estimates. Overall the project

enhances one’s ability to play around with available data in order reach a reasonable

estimate.

After calculating all the values over my distillation column I conclude that one distillation

column can be used to separate propane and recover 1,3 butadiene. A stepped column can

be used as the diameter for the pentane distillation column, (the unit where the 1,3

butadiene is pumped to for further purification.) is 2 m as suggested my group mate. Since

the diameter for the propane distillation column is 0.65m the top part of the column can

have this diameter and the lower part of the column should have a diameter of 2m. This can

reduce the operating costs and also a significant amount of raw material to build the column

would be reduced.

Also a separator can be used instead of a distillation column, but the temperature has to be

reduced too low, to obtain such separation. This can be more expensive than providing high

pressure inside the column.



11.0 References:

[1]: http://www.wakefield.gov.uk/NR/rdonlyres/F456AD74-5BC2-4697-9414-55C32FCBE9E2/0/s1_13Butadiene.pdf,( (Part 1: Group Project on1,3 Butadiene Production) accessed date 28/11/09

[2]: http://www.icis.com/v2/chemicals/9075172/butadiene/uses.html; accessed date 07/02/10

[3]: http://www.dow.com/productsafety/pdfs/butadiene_guide.pdf accessed date 15/03/10

[4]: http://www.gansu.gov.cn/en/BasicDetail.asp?CID=50 (Part 1: Group Project on1,3 Butadiene Production)

[5]: http://www.britannica.com/EBchecked/topic/166112/distillation-column; accessed date 13/02/10

[6]: http://www.websters-online-dictionary.org/re/reflux+ratio.html accessed date 13/02/10

[7]: http://www.separationprocesses.com/Distillation/DT_Chp04n.htm; accessed date 23/02/10

[8]: Dr. Bruce, 3rd year Chemical Engineering Advanced Separation Class Notes, Aston University, 2010; Lecture 1

[9]: http://www.processglobe.com/Liquid_Specific_Heat.aspx; accessed date 24/02/10

[10]: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 567-569,

[11]: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 579



[14] : Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 588-593


[16]: Dr. Bruce, 3rd year Chemical Engineering Advanced Separation Class Notes, Aston University, 2010; Lecture 3


http://www.processglobe.com/Liquid_Specific_Heat.aspx


http://www.websters-online-dictionary.org/re/reflux+ratio.html

http://www.britannica.com/EBchecked/topic/166112/distillation-column

http://www.gansu.gov.cn/en/BasicDetail.asp?CID=50

http://www.dow.com/productsafety/pdfs/butadiene_guide.pdf

http://www.icis.com/v2/chemicals/9075172/butadiene/uses.html

http://www.wakefield.gov.uk/NR/rdonlyres/F456AD74-5BC2-4697-9414-55C32FCBE9E2/0/s1_13Butadiene.pdf

http://www.wakefield.gov.uk/NR/rdonlyres/F456AD74-5BC2-4697-9414-55C32FCBE9E2/0/s1_13Butadiene.pdf


[17]: http://www.tower-packing.com/metal/metal_pall_ring_tower_packing.htm, accessed date 29/02/10



[20]: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 216 and 221

[21]: http://www.zehua-chem.com/

[22]: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 609-616


[24]: http://www.alibaba.com/showroom/condenser-tube.html; assessed on 26/02/10

[25]: Dr. John Brammer, 2nd Year Chemical Engineering Heat transfers Notes, Aston University.

[26] : http://www.processglobe.com/Liquid_Specific_Heat.aspx



[29]: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 195-199

[30]: http://www.azom.com/Details.asp?ArticleID=863; accessed date 09/03/10

[31]: http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf ; accessed date 14/03/10

[32]: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 197 -198

[33]: Dr. Fletcher, 2nd Year Chemical Engineering Control Process Notes, Aston University[34]:Page S. Buckley, William L. Luyben and Joseph P. Shunta ‘’Design of Distillation Column Control Systems’’


http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

http://www.azom.com/Details.asp?ArticleID=863

http://www.processglobe.com/Liquid_Specific_Heat.aspx

http://www.alibaba.com/showroom/condenser-tube.html

http://www.zehua-chem.com/

http://www.tower-packing.com/metal/metal_pall_ring_tower_packing.htm


[35] :http://www.see.ed.ac.uk/~jwp/control06/controlcourse/restricted/course/fourth/course/module3-1.html

[36]: Dr. Drahun, 2nd Year Chemical Engineering Advanced Design Process, Aston University.

[37]: Dr Brammer’s, 3rd Year Chemical Engineering Process Economics lecture notes, Section C, [38]: Dr Brammer’s, 3rd Year Chemical Engineering Process Economics lecture notes

[39]: (ref: www.informaworld.com/index/778741608.pdfl); accessed date 24/03/10

[40]: Dr Brammer’s, 3rd Year Chemical Engineering Process Economics lecture notes, Section 4(a)

[41]: Dr Brammer’s, 3rd Year Chemical Engineering Process Economics lecture notes, Section 4(b)Graphs and Correlations:

Graph 1 : Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 524

Grapgh 2: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 568

Graph 3: Dr, Bruce Davies; 3 year Chemical Engineering advanced Separation notes; Lecture 3Graph 4: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 639

Graph 5: Coulson J.M & Richardson J.F ‘’Chemical Engineering Design’’, 4th ed., vol. 6, Pergamon, Oxford, 2005 page 646




http://english.chem99.com/news/165180.html

http://www.see.ed.ac.uk/~jwp/control06/controlcourse/restricted/course/fourth/course/module3-1.html

http://www.see.ed.ac.uk/~jwp/control06/controlcourse/restricted/course/fourth/course/module3-1.html

distillation column report

Documents

propane distillation

column diameter

individual design

distillation columns

process capacity

preliminary design

multicomponent distillation

stainless steel