the production of acetylsalicylic acid

8/13/2019 The Production of Acetylsalicylic Acid

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specific the Crystallizer. Some of the major components in the P&ID of the crystallizer are

basically the design specifications as the installation of the instrumental and control, the flow

of the process, various components of the machine which are important in the process of

accomplishing the requirement for the processing.

Table of Contents

Summary 2

6

INTRODUCTION 6

1.0 Project Brief 6

1.1 Project Objectives 6

1.2 Project Plan 8

1.3 Technical Objectives 8

1.4 Personal Objectives 9

1.5 Techniques to Accomplish the Objectives 10

2 CRYSTALLIZER DESIGN 10

Design of a batch crystallizer 11

Chemical engineering design 12

Retention time 12

Heat transfer area 13

Vaporization surface 14

Elements of a batch crystallizer 14

Pipe diameter 14
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Wall thickness 15

Stirrer size 15

Diameter 16

Height 16

Baffle height and width 17

Power 18

Design Parameters 19

Calculation of volume 19

Calculation of baffle width 20

Calculation of baffle height 20

Calculation of wall thickness 20

Design 23

Mass and energy balance in the crystallizer 23

Energy balance around the crystallizer: 23

Q for Crystallizer Inlet: 24

Acetic acid (by product): 25

Acetic anhydride: 25

Sulphuric acid catalyst: 26

Q for Crystalliser outlet: 27

Energy balance 28

Mechanical design 32

Design Consideration 32

Habits of the crystals 32
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Defects in the crystals 33

Characteristics of the particles formed 33

Mixing in the crystallizer 35

Simulation of the fluid mechanics in the crystallizer 36

Mechanism Models 38

Scale up 39

Summary 39

HAZOP (Hazards and Operations Analysis) 40

HAZOP process 42

Piping and Instrumentation Diagram (P&ID) 44

47

Control and instrumentation 48

Control diagram 48

Batch crystallizer control and instrumentation 48

Introduction 48

Control 49

Operating at Maximum control 50

Control and Instrumentation 51

Feedback controllers 52

Economic Appraisal of Process 52

References 58

APPENDIXES 62

A.Cost Index for some equipment. 62
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B. General characteristics for various equipment types(Coulson 2011). 64

INTRODUCTION1.0 Project Brief

This individual design project entails the development of a process to produce 5000 tonnes of

acetylsalicylic acid (Aspirin) per year. The plant will be located in the UK. The feedstock to theplant are; solid salicylic acid, acetic anhydride and sulphuric acid which act as a catalyst. The

process main equipments include the feedstock storage tanks, reactor, filter, crystalliser,

slurry tank, centrifuge, distillation column, washer, drier and Grinder. The individual

assignment was to design and develop the crystalliser. The crystallisation process is a batch

process with the main inputs being Aspirin, acetic acid, acetic anhydride and sulphuric acid.

These inputs appear as solids at the end of the crystallisation process.

1.1 Project Objectives

The general objective of this project is to design a manufacturing facility that has a capacity

of producing 5000 tonnes of aspirin per year. The main objective of the individual project is

to design and evaluate the performance of the crystalliser in the aforementioned

manufacturing facility.

In the design and evaluation of the crystalliser for the manufacturing facility, the following

main factors need to be taken into consideration;
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Evaluation of the crystallisation process

First I will study the crystallisation process and the factors that affect the process

Evaluation of the different crystallisation units used in conventional manufacturing facilities.Next, I will study the different crystallisation units that are currently available in the market.

A detailed study of these crystallisation units, their capacity, limits and demerits will be done.

This will be done through a thorough literature review of the existing crystallisation

technologies and equipment required.

Selection of the best crystallisation facility: After a detailed examination of the existingtechnology, I will select the best crystalliser to use. The selection of the crystalliser will be

based on costs, its efficiency, resulting quality of the product and compatibility with other

processes in the manufacturing line.

Evaluation of the chemical engineering design of the crystalliser: After selecting thecrystalliser, a detailed description of the crystalliser will be done. The different equipments

and the chemical changes that occur in this process will be elaborated. The design

methodology of the crystalliser is described in detail. A datasheet with all the design data

and specification will then be prepared. A concept drawing of the crystalliser will then be

drawn.

Design of the control and instrumentation systems for the crystalliser: The control andinstrumentation of the crystalliser will then be described. All the systems, instruments

functions as well as the control strategy will be elaborated.

Design and drafting of the piping and instrumentation diagram for the crystalliser: This willbe a design of all the piping, equipments and instrumentation diagram for the crystalliser. A

legend of all the symbols used will then be drawn.

Conduct a hazard and operable study (HAZOP) for the crystalliser:The hazard andoperability study of one of the major line in the acetylsalicylic acid manufacturing process

will be done. This part will be done as a group activity. Perform an economic evaluation of the whole process: An estimate of the overall capital

cost and operation and maintenance cost for the whole acetylsalicylic acid manufacturing

plant will be carried out.

Prepare a project report for the selected process: A detailed report will then be prepared Reference all the work properly

1.2 Project Plan


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In order to archive the set objectives, the researcher prepared a Gnatt chart detailing all

activities and time schedule they will take. This enabled the researcher perform all activities

comprehensively and in time. The Gnatt chart is attached at appendices section.

1.3 Technical Objectives

The main technical objectives are;

To evaluate the aspirin manufacturing process To develop a project plan and draw a gnat chart To evaluate the different crystalliser units To design and draw the crystalliser unit To calculate the mass balance for the crystalliser To calculate the energy balance for the crystalliser To design the control and instrumentation system for the crystalliser To design and draft the piping and instrumentation diagram for the crystalliser

1.4 Personal Objectives

Research: for the design to be appropriate, a detailed research of the manufacturingprocess is necessary. The researcher will conduct a detailed research which involves

extensive reading of academic journals, books, company catalogue and other materials.

Organisation: conducting this research against a fixed timeline require a lot of organisationand time scheduling. During this research, I will ensure that I adhere strictly to the gnatt

chart.

Enhance knowledge and skills: During the course of the research, design and drawing, I willgain an in depth understanding of this process as well as a general understanding of the

chemical engineering processes. My expertise in the field of chemical engineering will be

greatly improved.

Presentation: After the final design, an oral presentation and a report will be written. Myskills in writing and presenting the finding must be improved in order to succeed.

Enhance computer skills: this project requires that I use several computer programs fordifferent functions. for the project to be successful, I must understand how to use the

flowing programs

Microsoft word: this program will be used for writing the final report Microsoft excel: this program will be used to perform all computations and for drawing

graphs and for drawing the gnat chart.

Microsoft Visio: for drawing PFDs


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Microsoft outlook: for communication with group members Microsoft project; Internet explorer: for searching materials

1.5 Techniques to Accomplish the Objectives

In order to archive the set objectives, the researcher will;

Ensure that the timeline set in the gannt are strictly adhered to Conduct extensive literature review on aspirin manufacturing process Engage in teamwork and group discussions Consult with the course instructor regularly

Ensure that am well versed with the IT knowledge and software packages to be used

2 CRYSTALLIZER DESIGN

When considering the design of a crystallizer, there are a number of things to consider.

Solubility of the components in the solution, the supersaturation and the growth of the

components are crucial in the overall design of the crystallizer. During the design of the

crystallizer, it is important to identify the activities such as the growth of the crystals. The

growth of the crystals only occurs at the supersaturated state(Crundwell 2008). The growth is

usually started with the nuclear formation and the gradual growth hence follows. In order to

make a proper design of the crystallizer for the Acetylsalicylic acid, the crystal growth rate

should be determined in the laboratory. This is where the residence time is

determined(Letcher 2004). The rate of the crystal growth is also determined in the process

which also corresponds to the time spent in the in the crystallizer. For the batch process,

crystallization can be speeded up by seeding. The overall crystals weight can be estimated by

the McCabe Delta-Law where the original crystals grow to the same size(Seliger Khraisheh, &

Jawahir 2011). Therefore the overall size can quote in terms of the size increment which can

be denoted by L(Silla 2003).

Determination of the number of crystals formed per a batch, or per unit volume can be

calculated from the rate of nucleation and the degree of supersaturation. Xo

Where the Xo can be determined by


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Xo=X-Xs

Design of a batch crystallizer

In the design of a batch crystallizer, it is important to put in mind the importance of correctly

placing the equipment to ensure optimum operations are maintained. The agitator should

always be well placed to ensure the mixing is done in a proper and enabling manner. The

optimum operating conditions are obtained at the supersaturated and metastable conditions.

The growth of the crystals follows the increased growth which is based on the McCabe Delta-

Law. The law highlights that as the size of the crystals increases, the mass also increase

therefore posing a threat of the suspension settling at the bottom of the crystallizer(Silla

2003). To overcome the challenge, the agitator is maintained in circulation in order throw

away any suspension settling at the bottom. That is mainly why the agitator is situated at the

bottom of the crystallizer. The increase in mass of the crystals results in the increase in the

length. The relationship between the increase in mass and the length can be illustrated is

basically based on a computer program (Rangaiah & Kariwala 2011).

Chemical engineering

designThe objective of the project is to design a crystallizer to crystallize aspirin from a batch

reactor designed in the earlier project. The temperature of the reactor is 90 degrees Celsius

meaning that the exit temperature of the products is at the same. This temperature must be

reduced by cooling in order to form crystals of required size in an effective and economical

manner. According to the scale of operation a batch crystallizer is fit since it has small

capacity, simple instrumentation and is self cleaning as indicated in the table of general

characteristics below.

Retention time

Earlier in this project we designed a batch reactor to be used to produce 5000 tonnes of

aspirin per annum(Crundwell 2008). It was assumed therefore that the plant is to run for

47.143 weeks or 330 days in a year including day and night. This translates to7920 hours

time of running annually. The remaining time which is about 35 days takes care of

emergencies, cleaning, repair and maintenance(Rangaiah & Kariwala 2011).


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The time for a batch of the reactor was earlier assumed to be five hours. Therefore retention

time in the crystallizer is related to the overall time in the crystallizer. A time of three hours

apposite for cooling and the remaining two hours for feeding and unloading for every batch

and removal of dirt before feeding the next batch. Cooling time is enough thus energy can be

saved since pumps which could have been used for forced circulation are not necessary(Tung

2009). Optimum crystallization is achieved when batch time is long thus ensuring maximum

heat transfer(Coulson 2011).

Heat transfer area

Heat transfer area can be calculated from the dimensions of the crystallizer. The curved

surface of the vessel has water (coolant) jackets and is thus the most effective surface for

heat transfer (Sam Mannan 2005). Though the top and bottom are also heat transfer

surfaces they may be ignored since their contribution to the overall heat transfer is not

significant compared to the curved surface. Baffles are good conductors of heat and are fixed

on the curved surface (Silla 2003). They conduct heat from the vessel wall pass it to the

vessel contents. The total heat transfer area is therefore the sum of curved surface and the

area of the baffles.

The diameter and height of the crystallizer is =1.627 m and = 1.843 m respectively. Curved

surface area is given by 2

=2 (1.627) (1.843)

=18.84 m2

Surface area of the baffles

The height and width of the baffle are 1.659 m and 0.1627 m respectively. They are four and

distributed uniformly on the wall. Each baffle has two opposite surfaces(Letcher 2004).

=2(4) (1.659) (0.1627)

= 2.16 m2

Total heat transfer area =18.84+2.16

=20.1 m2

Vaporization surface


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Surface area for vaporization of water is determined from diameter of the vessel. For a

diameter of 1.627 m the vaporization area is given by;

Area, A= r2

= (1.627/2)2

= 2.08 m2

Elements of a batch crystallizer

There are three major considerations in the design of a crystallizer namely policy of

operation, performance and size of vessel. First, the capacity of the crystallizer must first be

known. It is determined by the amount of contents from the batch reactor and the time spentin the crystallizer. Particle size and suspension density determines the size of the agitator,

power requirement and the speed of the pump(Theodore & Ricci 2011).

Secondly,performanceis measured based on quality of crystals and the rate of production in

terms of mass. There are aspects of design such as level of maximum super saturation which

set up constraints(Crawley, Preston, & Tyler, 2008).

Policy of operation, the suitable cooling curve, rate of evaporation must be specified. For

production of crystals of defined mass and size a seeding policy needs to be known and a

suitable batch time should be particular so that the final size of the particle is attained.Therefore performance and policy of operation and size of vessel are correlated(Crundwell

2008).

Pipe diameter

The sizing of a pipe leading to and from the crystallizer is simply based on the diameter of

the mixer (Coulson 2011). Therefore to ensure smooth delivery and discharge, the mixer,

entry and exit diameter should be of the same size. It is important note that this feature is

not as important for a batch crystallizer as it is in the continuous crystallizer (Silla 2003).

Wall thickness

Wall thickness is reliant on a number of factors considered in design of a crystallizer. There

are several functions a wall executes in every vessel. These includes holding vessels contents

in position, gives the necessary strength to encounter stresses caused by pressure column,

provides room for coolant jacket, serves as a conductor of heat and acts as heat transfer

surface area for the coolant and mixer contents(Crawley, Preston, & Tyler, 2008).


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Material must be selected based on heat transfer consideration. Good conductors of heat are

essential for optimal heat transfer to be attained. From engineering standards, an allowance

of 0.01 m for the water jacket gives a range up to 0.02m wall thickness sufficient to hold the

vessel contents enhancing endurance to both thermal stresses and pressure due to column of

the mixer contents(Seliger Khraisheh, & Jawahir 2011).

Stirrer size

For effective cooling maximum heat transfer should occur between the coolant and the

crystals(Crundwell 2008). A thorough mixing in the crystals should hence be enhanced

through bringing into play a mixer. Good mixing occurs when turbulent flow transpires in the

vessel warranting uniform distribution of heat gain from the baffle and wall.

Optimum mixing is also dependent on the location of the impeller. The impeller should be in a

position that will give the best possible mixing for the batch. While power is transmitted from

the motor to the agitator shaft, there exists an associated torque as a resultant resistance at

its blades due to viscosity and frictional resistance of the fluid.

Shear stresses are hence induced on the agitator shaft and for design purposes a

consideration for this stresses must made so as to avoid material yield and strain. Shear

stresses are distributed from zero at the neutral point (the center line) to the maximum at

the outer surface(Silla 2003). Stresses and strains affect the diameter of the shaft as well as

the ratio of shaft length to the shaft diameter(Sam Mannan 2005). The choice of size, length

and material type is hence based on the tensile strength(Coulson 2011).

Figure: diagram showing the diameter and height of the agitator.

Diameter

Stirrer blades must be designed in a warped manner such that they can move the contents

against gravity without twisting or flattening(Seliger Khraisheh, & Jawahir 2011). The agitator

shaft bearing should be firm enough to give enough reaction due to fluid acceleration. The

length of the blades is constrained by the radius of the vessel(Wan 2005). In favor of

optimum fluid mixing there is a length for optimum mixing to be achieved and is usually a

point where the ratio of impeller diameter to the vessel diameter 2:5.

Height


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The principle of an impeller is to muddle up the filling in the container. The impeller (also

known as the agitator) has blades usually rotating at lower portion of container. This is the

position for optimum mixing in the container(Silla 2003). Given the height of the crystallizer if

defined then the agitator height starts from the top most point where the motor is mounted

and runs down to the bottom of the vessel leaving just a sufficient clearance for fluid sweep

without formation of vortices(Crundwell 2008).

Baffle height and width

Baffle heightis determined by the overall height of the vessel starting from the tangent line

at the bottom to the top. Since the stirrer blades must be located at the lowest point in the

crystallizer, the baffle width may be modified at the bottom to give room for the

impeller(Ende 2011). An adequate clearance is considered to avoid the creation of vortexes

at the ends. A ratio of 10:9 (ratio of vessel height to baffle height) is apposite for sufficient

turbulence to be achieved. Heat transfer can also be enhanced by the baffles since they

increase the heat transfer surface area(Tung 2009).

Figure, vessel height, baffle height and width

The figure is half section of the crystallizer showing the key dimensions. For this design the

overall height is 1.843 m and the baffle width and height are 1.659 m and 0.1627 m

respectively(Coulson 2011).

In baffle widthdesign, viscosity is a vital factor for concern as it determines the type of flow

of the vessel fluid. If we are to achieve cooling effect through heat transfer as well as uniform

crystal distribution, then contents must be kept flowing in a turbulent state failure to which

sedimentation will occur especially in the case of laminar flow. In this design we are aiming at

attaining turbulent flow. When considering the width of the baffle one must keep in mind the

dimensional constraints due to the geometrical nature of the vessel. A baffle is normally fixed

on the vessel wall and protrudes towards the center of the mixer. Effective mixing i.e.

maximum mixing occurs at some distance between the agitator arm and the wall of the

mixer(Silla 2003). This can be done experimentally for fluids with different specific gravities,

viscosities and choosing various ratios of height to diameter(Crundwell 2008). Standard

ratios for different types of crystallizers have been determined and tabulated for quick

reference. In case of batch type crystallizer we refer to the Metric standard where the baffle

width to mixer diameter ratio is 1:10(Chianese & Kramer 2012).

Power

Without energy no work can be done. Work is said to be done when a force is moved through

a distance(Theodore & Ricci 2011). The crystals in the crystallizer have mass, thus due to


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then a thickness of 0.02m would be sufficient to meet all vessel design requirements(Coulson

2011).

Stirrer/ impeller diameter

The function of the impeller is to mix the contents in the vessel. Assuming high viscosity anddensity for our case, four blades will ensure a good mixing is done in the vessel (Couper

2005). Let the impeller diameter be d and the diameter of the vessel is D(Ende 2011).

Optimum mixing to be achieved when ratio of impeller diameter to the vessel diameter

2:5(Sam Mannan 2005).

Vessel diameter D = 1.627 m

Stirrer diameter d = 2D/5

=21.627/5

= 0.6508m

Hence impeller diameter of 0.6508m will ensure event and optimum mixing in the vessel.

The stirrer thickness is about 19 percent of the stirrer diameter.

0.6508=0.1264 m.

Power calculations

In order to run the run the agitator the power rrequire3d is given

P=

Where D is the stirrer diameter and N is the speed of rotation

NPis power number sg is the specific gravity of the slurry

P=

Suppose we want to run the agitator at a speed of 20 rpm and a power number of 1E13

number is given by;

P =

=661 watts


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DesignPerformance measure of crystallization is measured based on the CSD and crystal yield(Tung

2009). The parameters being measured are constrains on the system which is mainly the

supersaturation level and the size of the vessels(Silla 2003). The two parameters are related

to each other (Coulson 2011).

Mass and energy balance in the crystallizer

Energy balance around the crystallizer:

CRYSTALLISER

1 atm

273 K

Aspirin (Liquid)

Acetic acid (Liquid)

Acetic anhydride (Liquid)

Sulphuric acid(Liquid)

Aspirin (Solid)

Acetic acid (Liquid)

Acetic anhydride (Liquid)

Sulphuric acid (Liquid)


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= 244.66 J / mol.K

Q = 17.449 kmol/batch x 244.66 J / mol.K x (363 273K) x 1000 mol/kmol

= 384216.5 KJ / batch

Acetic acid (by product):

Q = to be calculated

M = 1046.928 kg/ batch =17.449 kmol/batch. We will use the molar value since we want the

units to cancel to KJ / batch

Cp = calculated using the equation given

We will use a Treference value of 0C (273K) for calculating the change in temperature. The inlet

temperature of the crystalliser is 90C

Therefore T = 363K 273K

Cp at temperature: 363K

-18.994 + (1.0971E+00 x 363K) + (-2.892E-03 x 3632K) + (2.9275E-06 x 3633K)

= 138.206 J / mol.K


= 217040.08 KJ / batch

Acetic anhydride:



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M = 468.3624 kg/ batch =4.592 kmol/batch. We will use the molar value since we want the






Cp at temperature: 363K

71.831 + (8.888E-01 x 363K) + (-2.653E-03 x 3632K) + (3.3501E-06 x 3633K)

= 205.125 J / mol.K


= 84774.06 KJ / batch

Sulphuric acid catalyst:


M = 281.6296 kg/ batch = 2.873 kmol/batch. We will use the molar value since we want the






26.004 + (7.03E-01 x 363K) + (-1.39E-03 x 3632K) + (1.03E-06 x 3633K)

Cp at temperature: 363K= 147.301 J / mol.K


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=339904.76 KJ/batch

Net heat transfer is given by; 384216.5 339904.76

=44311.74 KJ/batch

Temperature is to be dropped from 900to 00and 44311.74 KJ of heat is to be removed.

Mass and energy balance in the chemical engineering design for the crystallizer is calculated

from the mass transfer; mainly the crystal yield and the heat loaded to the

crystallizer(Crundwell 2008).

The overall mass balance is therefore written as

Mlo= M1f+ Mcf+ Mg+ VR

Mlois the initial mass of the seeds crystals

M1fmass of the feed

Mcfmass of the crystals

Mgmass of the by products

VRis the volume

The overall energy balance is given by

)

217040.08 KJ / batch

)

is enthalpy of the cooling

hc is the enthalpy of crystallization

Cp specific heat capacity of the solution

The calculations are on the condition of constant specific heat(Wan 2005).


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For the cooling process and in the formation of the crystals, the following formula was applied

Mcf =MnR(Co-Cf)

1-Cf (R-1)

R is the universal gas constant which is 8.315Jmol-1K-1

Therefore

)

)

From the equations the crystal yield can be estimated if the solubility of the components is

known(Ende 2011). The equation does not consider the effect of nucleation, primary and the

secondary. If the nucleation is considered then the CSD can be estimated in

someway(Nikrityuk 2011).

CSD prediction(Coulson 2011).

Prediction of the CSD in a batch crystallizer reactor can be done by the Mcabes law; L

law. Analysis by L law is based on a critical assumption of the behavior of the crystals

formation feature(Shioiri, Izawa, & Konoike 2010 ). It is assumed that the crystals have thesame shape throughout. The crystal growth in the process is invariant and that the

crystallizer size does not affect the structure of the crystal formed; the primary nucleation or

rather the classification of the size(Crundwell 2008). The relative velocity of the liquor and

the crystal is assumed to be constant(Vu, 2007).

Basically the calculations are based on the crystal size and the number of seed put in the

crystallizer. Ns and Ls dNp and Lp

L is the increment in then crystals which can then be integrated to obtain

Ls is the nuclear size

M1 is the mass of the feed

L and CSD can be evaluated if the Mp is known. The evaluation can be done the

temperature range of 90 to 0 degrees Celsius(Rao, 2009).


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Nucleation can happen at any supersaturation level even at the even that the supersaturation

is low the secondary nucleation still occur.

This implies that

Where Mcf is the crystal yield

==0.04717, the amount secondary nucleation.

A reasonable size of crystal in the yield must be accounted for. The number of crystals

formed can be obtained with the mass crystallizer reducing the crystal size.

Population balance

CSD is calculated from the Kinetic data though the crystal breakage and the agglomeration

cannot be considered in balancing the population(Sam Mannan 2005).

The population mass balance can be calculated from

Generally the nucleation depends on supersaturation whereas temperature is of little

importance in the growth of the crystals(Azadani 2007).

Mechanical design

.High strength material (steel) 109 MPa is enough.

Good conductor of heat and electricity is appropriate for water jacket as well as the inner

heat transfer area(Couper 2005).

Design Consideration

In the design of the crystallizer, there are a number of important elements which are

considered to ensure the design is of high quality. The size of the crystals in the crystallizer is

one of the major elements being considered. The crystals being manufactured are meant for

a pharmaceutical application thus some conditions over specification is considered(Rangaiah

& Kariwala 2011). The crystals formed rarely resemble each other mainly in symmetry about

the centre, the plane and the axis geometry(Richardson, Coulson, Harker, & Backhurst

2002). The crystals adopt some faces which can be described in the x, y and z axes.

Generally they form a characteristic feature which is crucial in the design of the

crystallizer(Barbosa-Pvoa, Matos, & Matos 2004).

Habits of the crystals


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Crystals develops some shape which is proportional to the shape being formed on each side

of the crystal. The idea of heterogeneous development of the crystals has been applied in

Chemical engineering design mainly on the eve of growing the design. It was suggested that

the shapes formed due to equilibrium were basically due to the free energies the crystal

areas(Crundwell 2008). This was offered in the BFDH theory. Growth of crystals using the

crystal modeling where prediction the shape of the crystals is more sophisticated than it was

before. Substances exhibit some different forms which are then essential in determining the

growth of the crystals. The habits which the crystals exhibit are affected by the type of

solvent in place and classified with the way the crystals grow.

In the manufacture of the pharmaceutical products, it is essential to apply crystallization

process from the simmerisation(Coulson 2011). The enantiomers are separated by

crystallization process. The chemical structure of the crystals applied in the process is

essential in the manufacture of high quality products. Crystals in the pharmaceutical sector

have been essential in the manufacture of the pharmaceuticals. The crystallization method of

separation of the crystals is enantiomer pairs. The enantiomer pairs involves some case of

the mechanical development of the crystals which mainly the conglomerates. The structures

given should be considered as being separated in the physical means in which racemecrystal

structure are formed.

Defects in the crystals

The crystals formed at a temperature which is above the absolute temperature have some

defects in the way the structure is presented(Silla 2003). The lattices are occupied by a

regular organization of the crystals(Nikrityuk 2011). Basically the organization results in the

formation of atomic structure leaving a vacant position for the impurities to occupy thus the

purity of the crystals is then put to question. The line defects which are essentially the major

force behind development of defects in the crystals can be classified as the screw dislocation

and edge dislocation. The defects are essential in development of secondary nucleation

process(Couper 2005).

Characteristics of the particles formed

A solid appearance of the products is usually affected by the crystals structure. Basically the

crystals formed in the crystallization process are broken down in other physical process or the

surface development is done to ensure sizeable tablets are manufactured. The objective of

the crystallization is not only the separation process but also the formation of the crystals

which have specific characteristics that are essential in the final production of the aspirin as a

finished product(Barbosa-Pvoa, Matos, & Matos 2004). The shape and other characteristics

of the crystal are the basis for the design of the crystallizer mainly affecting the volume, thetime taken in the crystallizer and the products(Crundwell 2008).


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Shape of the crystals can be used in the calculation of the volume of the crystals. Basically

the shape factor aid in the calculation. The shape factor gives the relationship between the

surface area of the crystal and the volume of the crystal. The characteristics of the

dimensions relating to the shape factor are specific(Shioiri, Izawa, & Konoike 2010 ). The

surface shape factor is the relationship between the outer areas in comparison with the

dimensions. It relates with various shapes of the crystals. The volume factor relates with the

shape of the crystal in relation to its volume(Silla 2003).

Particle size distribution

The crystals obtained industrially have different size and range within some limits however

some sizes are dominant over the other sizes. The size distribution follows the normal

distribution curve and can be calculated using the statistical procedure and method(Ende

2011). The mean size of the particles can be calculated by assuming two numbersrepresenting the whole population of crystals in the analysis(Vu, 2007). Basically

classification of the groups can be done in two ways; the property of the mean size (weight is

defined and the mean calculated from the population while the mean property size is based

on fiction about the size of particles to be calculated) (Shioiri, Izawa, & Konoike 2010 ). All

the essential properties such as the size, the mean volume, the surface areas and variation

are all calculated in the same way to extract the exact qualities of the crystals being

processed(Coulson 2011).

For this design, it is considered that the external properties does not exit the internal balanceas it requires more calculation if the external features has some effects on the internal

design(Theodore & Ricci 2011).

The effects of the size distribution can be witnessed in the separation of the solids and the

liquids formed in the crystallization process. A larger distribution of the particles enables a

wide packing of the particles thus enabling the materials handling more effective. The

challenge in estimating the size is basically the size distribution which is mainly because the

sizes of the crystals formed may not be of the same shape through out thus estimating the

void may lead to errors(Silla 2003).

Generally in the design of the crystallizer, the factor of the crystal shape and size has been

considered as one of the most important factors which have been taken into account in

consideration of the overall volume design and the design of a control system(Seliger

Khraisheh, & Jawahir 2011).

Mixing in the crystallizer

The solid formed in the process of crystallization tends to settle in the bottom of the

crystallizer(Woo 2007). The solid settling at the bottom of the crystallizer prevents effective


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heat transfer from taking place and also the quality of the crystals formed may be

compromised in the process. An agitator is essentially important in ensuring the solids do not

settle at the bottom of the crystallizer and also the heat transfer to the fluid in the crystallizer

is evenly. During the mixing, it is possible that the mixture formed a vortex therefore

introducing swirling of the fluids. Swirling does not allow the fluids to mix well therefore, not

required in mixing. To solve the problem, the baffles are included in the design. Baffles are

designed to interrupt the formation of swirling which result in vortex formation. Mixing will be

evenly and the vortex formation will happen as well as settling of the crystals at the bottom

of the crystallizer.

In operating the agitator, it is necessary to use an agitator at a certain speed and with the

ability to create enough suspension of the solids. The speed however should not cause a

collision in the walls of the crystallizer which may result in the wearing of the machine. A

relative average speed would be essential in achieving optimum mixing of the slurry

considering more solids are formed as the mixing continues(Woo 2007).

Mixing in the crystallizer is not expected to be turbulent thus there is no expectation of the

eddy currents or other factors being displayed by the turbulence flow.

Simulation of the fluid mechanics in the crystallizer

The flow of the fluid in the crystallizer depends on the shape of the vessel. The dynamics of

the fluids in the crystallizer may not be understood easily without the aid of Computational

Fluid Dynamics. CFD gives both the dynamics of running the flow in the process and after the

process analysis. The data obtained from one process can be used to develop another process

or rather be used in the correction of error in the control of the crystallizer. Basically the

operations within the crystallizer especially when stirring is very important in the analysis of

the process thus in doing so, using the modeling process, it creates an important

development that would ensure a more precise production process is followed(Couper 2005).

There are a number of simulations that can be done to obtain an important data to impose on

the process(Crundwell 2008). These are some of the common modeling used in the

simulating process. There is the momentum resource modeling and the slide modeling(Vu,

2007). Among the two modeling, the most important one is the slide modeling basically

because it presents a more detailed and precise patterns of flow in the crystallizer. The

simulation doesnt cater for the impurities in the Aspirin crystals which about 5 percent of the

total solids formed. The flow pattern and the solids formation is also an important feature

being accounted for in the simulator. Since the flow patterns are known to be difficult to

study in a reactor, it is necessary to use the chemical engineering models which are more

precise in the outcome. The macromixing and the micromixing are studied all through with

the aim of coming up with one of the most important models in the design and control of the

process of crystallization(Silla 2003).


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The macromixing is the study of the process in consideration of the ideal performance of the

crystallizer. Basically it would be difficult if the process operations within the crystallizer

would be estimated using the residence time and the distribution of the elements of the fluid

dynamics in the system(Coulson 2011). The study using the model helps in development of

the mathematical models which are essential in the design of the control system for the

crystallization process(Nikrityuk 2011). Basically the modeling considers the feed to the

system and the output. From the information obtained from the mixing inside and the flow is

important in developing the overall process of modeling(Azadani 2007).

In modeling the micromixing, it is possible that the process undergoes modeling which

essentially important in the reduction of the eddy current developed during the mixing

process(Crundwell 2008). Basically the micromixing model is in operation with the fluid

entering the system being the main focus whereas the rest of the operation is of little

importance in the simulation using the micromixing model. Some of the parameters being

viewed in the process though are not considered in the micromixing. Basically the aim of the

modeling involving mixing is mainly to study the impact of mixing in the reaction or rather

the crystallization process(Richardson, Coulson, Harker, & Backhurst 2002).

Mechanism Models

There are a variety of models which have been used for simulating the crystallizer both the

physical models and the mechanism models. The physical models are responsible

development of a more sophisticated process which considers the whole process of mixing.

The major assumption in conducting simulation is mainly achievement of evenly mixed

mixture. Through the mass balance, the population count and the mass balance can be

obtained. The numerical figures pertaining formation of the crystals can be obtained from the

balance of population and the probabilities of the crystal growth using different parameters.

Basically the simulation is essential is providing operation of the crystallizer in a more precise

mode which is essential in the design of an optimum operating crystallizer. From the

simationation, various parameters under control can be programmed using the values

obtained from simulations. The crystallization of aspirin is a simple separation process where

the process has to be conducted in a moderate mixing with less turbulence. The crystals

formed from the process are 95% with some form of acetylene and the rest aspirin.

Therefore the conditions for the operation depend mostly on the simulation for the system to

be programmed and ensure the control system is precise. The design of the crystallizer itself

is also important in developing an optimum condition. For instance the effect of hard mixing

or improper placement of the mixer still affects the performance of the crystallizer and the

quality of the crystals produced at end of the process.

Scale up


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Various processes in the chemical engineering design require a scale up in order to have a

maximum operation. Some of the major scale up processes is the constant speed of the

stirring in mixing, the speed of the tip of the mixer is also essential in the performance of the

crystallizer. It ensures a constant shear is obtained in the process basically it is expected to

offer an optimum mixing which in turn cool the slurry in a faster rate. This is done in the

estimating the effect of the shear force on the fluid but there is direct measurement of the

effect. The constant power input volume is another parameter which can be used in ensuring

that maximum operation is reached(Silla 2003). The scale up patterns in mixing can be

difficult sometimes especially when the stirring needs the conventional scale up(Crundwell

2008). The process has been tried practically but a convention on it have not been proven

rather the convention on various parameters have not been of importance in the process. The

difference between the time taken by various parameters when the constant power input

volume is applied complicate the scale since without the conventional definition of the scale,

it becomes difficult for the engineers to use the scale up. Basically the microbehavior patterns

in the crystallizer are essential in developing the major outcome of the process thus it is

important to consider the little effects of the convention failure and other parameters that

would adversely affect the output of the system(Sangwal 2007). Thus a good scale process

results in a production of quality crystals(Crawley, Preston, & Tyler, 2008).

Summary

The design of the batch crystallizer is mainly based on equations which are developed from

the cooling process. All the performance equations for the batch crystallizer start with the

McCabe L law. The L law is essential in predicting the crystal size and in development of

the nuclear which is essentially used in developing other equations for use in development of

system equations(Barbosa-Pvoa, Matos, & Matos 2004). The population balance can be

obtained using the mass balance and the prediction of the kinetics of the crystallization. The

kinetics can be obtained through the use of the controlled parameter and the application of

seeding which basically essential during the crystal growth control. For the crystallizer to

form crystals of high quality, the supersaturating has to be maintained in the metastable

condition throughout the process(Kletz 2006). This is achieved by application of vortexcooling or rather the control of the cooling patterns such as controlling the rate of cooling of

the crystal and sometimes destruction of the crystals to ensure proper crystal size is

achieved.


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HAZOP (Hazards and

Operations Analysis)Hazards and operations analysis is a result of the theory that the problems in the operations

failure is mainly caused by the failure of the process design and the inability of the system to

cope with the challenges during the operations(Coulson 2011). Basically these problems can

be identified from the control system of the Piping and the instrumentation diagram. In

exploration of the problems that might have been caused by the faults in the design or any

other problem developed during the processing of the materials can be obtained mainly from

the human analysis of the process. Basically it is important for the engineers operating the

crystallizer to have an assessment of the fault in the system in order to come up with the

main challenge causing the fault in the process. The major techniques that can be used in the

analysis are brainstorming and the qualitative risk assessment tool. Other method includes

prediction of the faulty of the system based on the experiences obtained in the past while

operating the process(Chianese & Kramer 2012). The engineers ought to come up with a

solution at the end to ensure a proper engineering process is precise and good quality

products are process to the final products(Silla 2003).

The HAZOP is mainly used in the assessment of any crystallizer to ensure that the design

specifications are up to the standards stated by the manufacturer and according to the safety

standards set by the bodies involved in standards for instance the ISO standards. The

operating conditions of the crystallizer are also analyzed to ensure the conditions are suitable

for a safer environment during working. In addition, the machines have to be maintained to

ensure a proper procedure is duly followed(Letcher 2004).

A part from the effects of the machine failure in the crystallization process, the control of the

machine might be the failure itself. Through the brainstorming process, it is necessary to

consider the instrumentation and ensure that it is well coordinated. The operational modes

such as the starts of the control, the normal operation and the emergency response of the

control system.

HAZOP has a wider importance of ensuring the degree of safety in the industry is high. Errors

resulting from human activity in a plant can be recognized easily. Measurement and other

procedure in solving problems can be avoided(Vu, 2007). Instances such as the production of

poor quality crystals may not be identified through measurement but the effect of

brainstorming brings out the important aspect of theory of the behavior of crystal

formation(Branan 2005). A comprehensive activity on the performance of the crystallizer is

then analyzed based on the brainstorming that the tools ready in place for the use in the

analysis of the process(Sangwal 2007).


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The set back in using the HAZOP is the lack of a strategic means of directly measuring or

conducting the system analysis(Chianese & Kramer 2012). There is no particular process

procedure involving the works and the control of the team conducting the brainstorming

process of the use of HAZOP thus the proposed improvement by the team have no direct

solid basis for the assurance of the process(Houson 2011).

HAZOP process

HAZOP process can be divided into four phase(Coulson 2011). The first phase is the definition

of the problem, the objective of HAZOP and selecting of the team capable of realizing the

objective of the group.

Preparation of the process involves the procedural plan of the areas of interest, how the data

is collected, the time taken to complete the assignment and the schedule to be followed.

After the examination, it is crucial to concentrate on the next phase which is the examination

phase.

In the examination process, the problem to be solved is defined and mentioned. Various

design intentions are sorted out to allow the process of reviewing the optimum operating

conditions which are then essential in the process of HAZOP. The elements selected are

received stepwise with guidance to ensure the problem is identified correctly. The problems

caused by the operations resulting from the faults in the machine and the environment(Silla

2003). The areas under investigation must be reviewed to find if they have a significant effect

on the whole process(Jones 2002). In the course of examination, a strategy is taken mainly

on the protection that can be offered to the machine, future detection of such faults and the

characteristics of a system undergoing the same problem. In the course of the analysis, a

remedy strategy can be put in place to ensure the whole process of may not be necessary

stop in the account of trying to solve a fault. The remedial strategy heavily depends on the

nature of the challenge and the magnitude. Once all the strategies are set, it is important

that a consensus is set out to ensure that everyone agrees with the proposed changes on the

element. The same can be done for all the elements under investigation.

After the investigation on the process, the findings of the investigation are taken into

account. The activity involves the documentation of the findings. The compiled document is

then signed off and documented. The products of the investigation are put into the practical

during the follow up process. In the case of unsatisfactory outcome during the follow up, it is

necessary to rectify the fault in the process and come up with the conclusion of the whole

activity, where a report is finally published.

In order to conduct a successful HAZOP design, there are some words which are essential in

the brainstorming process. Words that quantify a situation are commonly used to ensure a

thorough analysis is taken into account.


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In summary the objective of the HAZOP is mainly to apply the brainstorming method in

solving the industrial challenge but with the use of professional engineers in the course.

Engineers have some process to follow when solving technical problems occurring in a plant

but in the case of HAZOP, the more radical analysis is carried to ensure the process is

completely taken into the account of being the part of the solution of the industrial

challenges(Branan 2005). The major parts of the HAZOP process are the process of

evaluating the various parameters which are essential in the analysis and then taking a

proper use of the guiding words with the parameters(Crundwell 2008). Some of the words

commonly used for instance are: No, Less, More, Other Than As Well As, Reverse among

other essential words in the design(Silla 2003).

Piping andInstrumentation Diagram

(P&ID)The P&ID is an important diagram during the design and when applied in the field. Basically

the diagram is essential in showing the piping system and the control of the plant and in

specific the Crystallizer(Coulson 2011). Some of the major components in the P&ID of the

crystallizer are basically the design specifications as the installation of the instrumental and

control, the flow of the process, various components of the machine which are important in

the process of accomplishing the requirement for the processing. The use of the valves, the

level of quality, the subsystems the sequence of the plan and other important physical

appearance of the process. P&ID is useful in ensuring the orientation to the plant operation is

useful to the users and the newcomers. Basically an overview of the system will be simplified

and the person might be in a position to come up with a better understanding of the

system(Kletz2006).

The major focus in the design of the P&ID is to ensure the major parts of a system are

displayed in a better and simpler manner that can be used for the interpretation of the overall

control and management of the plant. Location of some of the components of the plant is

easier(Sam Mannan 2005). Also in the maintenance process, it is easier to tackle the problem

faced

The major components in the crystallizer P&ID are the valves, the control systems, and the

essential pumps and the power to the crystallizer. The process and the flow of cooling fluid on

the crystallizer and other major parts(Silla 2003). Since the design of a chemical plant may

not be in a position to mention most parts of the words, universal symbols which are widely


33/45

used in engineering. Some of the examples are the pipes, the mixing vessels the pump, a

mixer with a jacket, valves the control and any other essential part that may be essential in

developing the system. It can be noted that the most important components of the design

are mainly displayed in the form of symbols which are universally acceptable by the

engineers and can be acceptable everyone using the instruments in the design (Coulson

2011). Basically the difficulty is developing the P&ID is in the design and simulation of the

flow and the use of various parameters which are essential in the crystallizer for instance

(Theodore & Ricci 2011).

Figure 2 The piping and instrumentation diagram for the feedstock and product in thecrystallizer

Figure 3 showing the piping and instrumentation diagram for the crystallisation control

system

The legend is shown below


34/45

Control andinstrumentationControl diagram

The figure is a control diagram for the crystallizer.

A is crystallizing cylinder/vessel indicating the mixture and the baffle; B, heater; C, heater;D, thermometer; F, recording control; G, relay; I, coolant pump; J, coolant tank; L, coolant

exit; K coolant entry point; H, temperature controller.

Controlling the the crystallizer involves the use of an immersion heater labeled B as well as

the circulating water cooling the system. A computer is mainly used and a program that

ensures all the optimum conditions are considered as approached in the design.

Maintaining the crystallizer contents in the transient supersaturation is basically the most

crucial activity being monitored by these control parameters.

Batch crystallizer control and instrumentation

Introduction

The design of a batch crystallizer is mainly a consideration of the size of the equipment, the

operating conditions and measurement of the performance. The size of crystallizer is

determined by time spend and the cooling rate (occasioned by suspension of solids).

Operating conditions depends on the cooling curves formations and times spent in the batch.

Basically time is important since it ensure the correct size of crystals is reached. The quality

of the products is usually measured on the amount of crystals formed and Crystal Size

Distribution (CSD). A part from the ideal design, other factors such as the supersaturation

affects the design of the equipment.

The major parameters in the control of the crystallizer are the feed that is the feed for the

batch process should be in the ratio stated in the design. The operating condition for the feed

also has a hand in control. The feed is expected to reach the crystallizer a temperature of 90

degrees Celsius from the batch reactor. The temperature has to be reduced to zero degrees

Celsius. The mixing process should be moderate to avoid turbulence which might disturb the


35/45

crystal from forming(Houson 2011). All the conditions affect the most important parameter

which under the control of the engineer. The quality of the crystal is considered the output.

From the chemical engineering design, the crystals formed should have some specific shape

and size from them to be considered quality. From modeling of various mathematical models,

the control of the parameters in the crystallizer design can be obtained and have an effective

control of the crystallization process(Silla 2003).

Mass and energy balance in the crystallizer includes: yielding of crystals and the heat load

(Crundwell 2008). The mass balance is based on the population distribution. Among other

factors the mass balance can be obtained through the nucleation rate calculations. The mass

balance of the population is important in enhancing the control of the parameters in the

crystallizer(Richardson, Coulson, Harker, & Backhurst 2002).

Control

The most important parameter to consider in the crystallizer control design is the

supersaturation. The parameter is responsible is responsible for the scaling of the heat

transfer areas(Letcher 2004). It is also responsible for the nucleation process and the CSD.

When the natural cooling is considered in cooling slurry in the crystallizer, the level of

supersaturation is rapidly increased thus slurry reaches metastable state in the early stage of

crystallization. Basically the resulting crystals formed are finer and of poor quality(Branan

2005).

When seeding a controlled cooling is applied, the desired crystal size is then obtained. In this

case the cooling rate is under control therefore a metastable state is maintained throughout

the process of nucleation. Also the CSD and large particles are formed with the effects of

fouling in the heat transfer areas being catered for. Basically the cooling in the crystallizer is

an important factor and has been subjected to mathematic modeling to ensure a good

controlled system is established. Secondary nucleation can occur at metastable state

therefore a mass balance for the crystallization can be obtained by taking into account the

control parameters(Woo 2007).

= . +

From the mass balance equation, a correct graph equation can be obtained. In comparison

with the natural cooling, the graph obtained from the equation forms a convex curve which is

a contrast to the system under natural cooling.

Operating at Maximum control


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Though the main aim of nucleation controlling cooling in crystallizer is to obtained large size

crystals, natural cooling forms the largest crystals overall(Coulson 2011). The problem is

because the formation is uneven. Nucleation happens at some point in the process. Basically

that complicates the process since the crystal growth integral cannot be easily estimated with

the time taken in the batch. Batch process can only be defined in terms operating at optimum

over a period of time and not an instant performance.

In the case where the graph of the cooling rate between the natural cooling and the

controlled cooling is drawn, a natural cooling shows a rapid temperature drop whereas

controlled cooling evenly and slowly drop(Jones 2002). The principle of maximum cooling is

used.

The variables for the cooling rate can be stated in the principle of maximum cooling. The

principle is used in the design of a control system for the crystallizer.

Where H is the Hamiltonian function, x is the vector and u is the control variable. The

variables are mainly the size of the crystals while the control parameter is the temperature of

the solution. Since the aim of the process is to maximize the size of the crystals, an ordinary

deferential equation is obtained for the process(Barbosa-Pvoa, Matos, & Matos 2004).

From a graph dawn from the formula, it can be deduced that maintaining a constant

temperature throughout the operation in the crystallizer maximizes the final outcome of the

particles size and the CSD. It prevents an early nucleation and maintains an optimumoperating condition(Silla 2003). Optimization is not ideal however the process has maximum

operating conditions.

Control and Instrumentation

Control in the crystallizer is mainly done by the immersion heater as well as the circulating

water cooling the system. Water circulates in the crystallizer through a draft-tube in the walls

of the crystallizer and a controller which is mainly computer programmed to ensure the set

point is not exceeded. Cooling curves are observed and any deviation from the predicted

curve is considered a fault in the process.

The data obtained from experiment done by researchers shows the importance of the

controlled cooling as compared the cooling process as result of natural cooling. Maintaining

the slurry in the transient supersaturation is basically the most crucial activity being

monitored by the control parameters. In a situation where the transient supersaturation is

kept low, the crystal size is likely to be below the average required size. Therefore it is

important to consider the shapes the graphs drawn.


37

the production of acetylsalicylic acid

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