distillation calculation (for talal)

8/6/2019 Distillation Calculation (for Talal)

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University of JordanFaculty of Science & Technology

Department of Chemical Engineering

Chemical Engineering Laboratory III

"Distillation Column"

Date of

Performing : 4/4/2005

Date of

submission : 19/4/2005

"This report is made due to the request of chemical Engineering Department to study the

effects of varying the reflux ratio on the Distillation Column efficiency"


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Table of Contents :

1- Summary

3

2- Introduction.. 4

3- Theory..

6

4- Equipment11

5- Procedure.13

6- Results.. 15

7- Discussion of Results 20

8- Conclusion 22

9- Nomenclature 23

10- References 24

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11- Appendices 25

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Summary:

The Distillation Column is an apparatus which separates two

substances with difference in their relative volatilities. This separation

process is studied by varying the reflux ratio between the reflux stream

and the product stream, and so the composition of the distillate coming

out also varies. Also the number of stages loaded inside the column

affects the performance of the separation process, and so the efficiencyof each plate or stage loaded, which is called "murphee plate

efficiency".

Equilibrium relations besides two methods: McCabe & Thiele andPanchon & Savarit methods; were used to obtain the Theoretical

Number of stages required.

As a main results, The Distillate composition (with highervolatility material) rises with the reflux ratio. Theoretical number of

stages was unable to be calculated with neither of the two methods

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Introduction:

Distillation is an operation whereby the vaporization of a liquidmixture yields a vapor phase containing more than one component, and

it is desired to recover one or more of these components in a pure state.Or in another way, it's a process in which a liquid or vapor mixture of

two or more substances is separated into its component fractions of

desired purity, by the application and removal of heat.

Distillation columns are classified on the basis of how they'reoperated, so they classify into:Batch columns & Continuous columns.

Our experiment is carried under Continuous column section,

which can be further classified according to:

the nature of the feed that they are processing,

binary column - feed contains only two components

multi-component column - feed contains more than two components

the number of product streams they have

multi-product column - column has more than two product streams

where the extra feed exits when it is used to help with theseparation,

extractive distillation - where the extra feed appears in the bottomproduct stream

azeotropic distillation - where the extra feed appears at the top product

stream

the type of column internals

tray column - where trays of various designs are used to hold up the

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liquid to provide better contact between vapor and liquid, hence better

separation

packed column - where instead of trays, 'packings' are used to enhance

contact between vapor and liquid

This experiment is done to:

1. Demonstrate the effect of variation of reflux ratio upon

distillate composition, which is the composition of thedesired product.

2. Determine the number of theoretical plates within thecolumn using the methods of McCabe-Thiele & Panchon-

Savarit.

3. Find the murphee plate efficiency of one plate,

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Theory:

Separation of components from a liquid mixture via distillation

depends on the differences in boiling points of the individualcomponents. Also, depending on the concentrations of the components

present, the liquid mixture will have different boiling point

characteristics.

Mathematical-graphical methods for determining the number of

theoretical trays or stages needed for a given separation of a binary

mixture of A and B has been developed by:

1) McCabe-Thiele method:

The main assumption made in this method is that there must

be equimolar overflow through the tower between the feed inletand the top tray and the feed inlet and bottom tray. A total

material balance gives:

Vn+1 + Ln-1 = Vn + Ln

A component balance on one material (A) gives:

Vn+1 yn+1 + Ln-1 xn+1 = Vnyn + Lnxn

As the feed is from the bottom of the tower, then the whole

sections is considered to be enriching section, and followingequation are derived:

An overall material balance around the entire column statesthat the entering feed of (F) must equal the distillate (D) plus the

bottoms (W) in:

F = D + W

A total material balance on component (A) gives:

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F xF = D xD + W xW

The vapor form the top tray having a composition y1 passes

to the condenser, where it's condensed so that the resulting liquidis at the boiling point. The reflux stream L and distillate D have

the same composition, so y1 = xD. Since equimolal overflow is

assumed, L1 = L2 = Ln and V1 = V2 = Vn = Vn+1.

Making a total material balance around the upper three stages:

Vn+1 = Ln + D

Making a balance on component (A):

Vn+1 yn+1 = Ln xn + D xD

Solving for yn+1, the enriching section operating line is:

11

1

++

+

+=

n

D

n

n

n

nV

xDx

V

Ly

Since DLV nn +=+1 ,1

1+

=

+RR

VL

n

n

with the upper equation gives:

111

+

+

+

=+

R

xx

R

Ry Dnn

2) Ponchon-Savarit Method:

This method, it obviates the need for the constant phase-ratio

flow assumptions.

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Start numbering the stages from the top as n, stage n-1 of the

column shown in fig (a) is a mixing device where streams Ln,Vn-2enter and the equilibrated streams Vn-1, Ln-1 leave.

The vaporVn-2 and liquid Ln are mixed to give overall

composition z, which then separates into two equilibrium vapor

and liquid phases Vn-1, Ln-1 that are connected by a tie line

through z, as shown in fig (b).

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The ponchon diagram embodies both enthalpy and material

balance relationships as well as phase equilibrium conditions.

Since it is unnecessary to assume constant molal overflow, the

calculations can be done on per mole or per pound basis.

By making material and enthalpy balances about the

portion of the enriching section of the column in fig (a) enclosedwith the dotted line:

For the more volatile component:

Yn-2 Vn-2 = xn-1 Ln-1 + DxD

Total material balance:Vn-2 = Ln-1 +D

And the enthalpy balance:

qD D + Hn-2 Vn-2 = hn-1 Ln-1 + hDD

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By solving the material balance equations for Ln-1/D:

12

21

=

nn

nDn

xy

yx

D

L

And by simultaneous solution of the upper two equations:

( )

12

21

=

nn

nDDn

hH

Hqh

D

L

Which these two equations represent the operating line for two passingstreams Vn-2 and Ln-1 by the equation:

( )

12

12

2

2

=

nn

nn

nD

nDD

xy

hH

yx

Hqh

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Equipments:

As shown on fig(C) :

Bubble cap column: 1500mm x 80mm dia fitted with 8 type 316SS bubble cap trays

Condenser: 100mm dia x 0.5ft.2

Product cooler: 40mm dia x 0.2ft.2

Boiler: 150mm dia x 0.5ft.2 (steam)

Reflux control: Variable area flow meters infinitely variable

Safety: Graphite rupture discs, element temperature sensor, zener

barriers (intrinsically safe electricals)

Water: Control valve, flowmeter, pressure gauge, twotemperature indicators

Vacuum: Control valve, pressure gauge

Steam: Reducer, control valve, pressure gauge

Process: Reflux controller, fifteen temperature indicators

The major components with their duties are:

1. A verticalshellwhere the separation of liquid components is

carried out.

2. Column internals trays/plates which are used to enhance

component separations.

3. a re-boilerto provide the necessary vaporization for the

distillation process

4. a condenserto cool and condense the vapor leaving the top of the

column

5. a reflux drum to hold the condensed vapor from the top of the

column so that liquid (reflux) can be recycled back to the column

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Fig(C)

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Procedure:

Starting the cooling water pump and slowly adjusting the

flowrate of cooling water to about 4L/min on flow meter.

The pressure should not exceed 2 bars

Open slowly the steam inlet valve and also the (steam trap by-

pass).

The pressure of steam should not exceed 1.5 bar.

Vapor generated in the re-boiler rises through the column and

is

condensed in a Vertical water-cooled condenser.

When distillate liquid is seen on top of the column, open the

valve that leads to the flowmeters.

The condensed product leaves the column and passes into an

infinitely variable reflux ratio controller incorporating variable

area flowmeters.

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Calibration of the flow rate should be done by the flowmeter

controllers.

When done, adjusting the flowrate of the Product stream.

Taking the temperature readings across the column, and the

flowrate of the reflux stream.

Reflux is returned to the column and product passes through a

cooler and graduated pipe section and can be passed either to a

receiving vessel, which allows product removal while operating

under vacuum, or back to the boiler.

Vapor and liquid compositions throughout are determined by

temperature measurement.

Repeating the experiment for different reflux ratios.

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Results:

Table (4): Rotameter reading and the actual reading.

Reflux Product

Rotameter readingActual reading

rotameter readingActual reading

ml/min ml/min

3.75 65.35 1.00 27.403

3.20 57.76 1.20 30.1628

2.20 43.96 1.60 35.6824

2.00 41.20 2.00 41.202

1.70 37.06 2.40 46.7216

Table(5): reflux ratio vs. composition of distillate

reflux ratio xD

2.385 0.980

1.915 0.960

1.232 0.915

1.000 0.835

0.793 0.790

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Discussion of Results:

It's apparent that the relation between the rotameter scale

readings and the actual flowrate is a direct linear relation, andof course it should be that, because the utility of the rotameter

will be much better and easier for the user to calculate byinterpolation the flowrate of the streams; than if it was non

linear.

In fig (5), the relation of the Reflux Ratio and the distillate

composition is direct. And that is logically correct, because, when

the reflux ratio is high; it means that the reflux flowrate that goes

back to the column will go for further separation through the

stages, and so for further concentrating for the distillate and thebottom streams, which is desirable and more profitable with the

product distillate stream to be high concentrated. And that is the

target of the experiment.

when using the McCabe-Thiele method to get the number of

theoretical stages, the result was infinity; which isunbelievable, but the only convincing reason for that is: when

first the steam pressure if so high that cause a large amount of

vapor to pass through the stages, and the following effectsmay occur:

o Foaming

Foaming refers to the expansion of liquid due to

passage of vapor or gas. Although it provides high

interfacial liquid-vapor contact, excessive foaming often

leads to liquid buildup on trays. In some cases, foaming

may be so bad that the foam mixes with liquid on the tray

above. Whether foaming will occur depends primarily on

physical properties of the liquid mixtures, but is sometimesdue to tray designs and condition. Whatever the cause,

separation efficiency is always reduced.

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o Entrainment

Entrainment refers to the liquid carried by vapor up tothe tray above and is again caused by high vapor flow rates.

It is detrimental because tray efficiency is reduced: lowervolatile material is carried to a plate holding liquid of

higher volatility. It could also contaminate high purity

distillate. Excessive entrainment can lead to flooding.

o Flooding

Flooding is brought about by excessive vapor flow,

causing liquid to be entrained in the vapor up the column.

The increased pressure from excessive vapor also backs upthe liquid in the downcomer, causing an increase in liquid

holdup on the plate above. Depending on the degree of

flooding, the maximum capacity of the column may be

severely reduced. Flooding is detected by sharp increases in

column differential pressure and significant decrease in

separation efficiency.

Also the effects will hit the Panchon-Savarit method whencalculating the theoretical number of stages in the same way.

Due to these effect the efficiency of the plate will by

undetermined, because the temperature of the liquid and the

vapor above the stage will be the same, and the liquid of stage

n-1 due to flooding or any other effect will approach the stage

n and so differs it's really efficiency.

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Recommendation:

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Nomenclature:

Em Murphee plate efficiency in vapor terms

R Reflux Ratio

T Temperature Co

xD composition of distillate @ top of column %

y mole fraction of methanol in vapor phase %

x mole fraction of methanol in liquid phase %

x rota rotameter reading

y Act Actual flow rate ml/min

Xn composition of vapor at stage n %

Yn composition of liquid at stage n %

XB the composition of the bottom vapor in the tower %

y*n Composition in the hypothetical vapor phase that

would be in equilibrium with liquid composition

leaving the actual stage %

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References:

1. " Equilibrium-Stage Separation Operations in Chemical Engineering"_

Ernest J.Henley & J.D.seader ,unknown edition, John Wiley

2. " Transport Processes and Unit Operations" _ Christie J.Geankoplis,Third Edition, 1993, PTR PH

3. " Unit Operations of Chemical Engineering" _ McCabe & Smith &

Harriott, sixth edition, 2001, McGraw-Hill

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Appendices: _

Sample of Calculations:

Trial no. 3 will be chosen as a sample:

From the given data, table no.(1) of rotameter, reading its actual

flow rate, fig(1) is drawn; with an equation:

( ) 604.13799.13 += xy

By interpolation, the actual flow rates of obtained rotameter

readings are gathered.

Rotameter reading for Reflux = 2.2 = x rota (1)Rotameter reading for product = 1.6 = x rota (2)

Actual flow rate for Reflux = y Act (1) = 13.799(2.2) + 13.604

= 43.96 minml

Actual flow rate for Product = y Act (2) = 13.799(1.6) + 13.604

= 35.68 minml

And so fig (1') is generated

The reflux ratio:

rateflowproduct

rateflowrefluxR =

232.1

35.68

43.96

=

=

R

R

from the T-x-y diagram , fig(2), arbitrarily X (mole fraction of

methanol in liquid) values are chosen from the x-axis, going

vertically from there, till hitting the liquid saturation curve, then

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Starting drawing horizontally from 45o line @ XD till the

equilibrium line @ (Vn, Ln) ; then going vertically till the

operating line @ (Ln, Vn-1), and so on in the same way till

reaching XB or step off it. The number of triangles madebetween the equilibrium line & the operating line are the

Theoretical no. of stages required.

As shown in fig (3); the theoretical no. of stages is infinity

.

o Using Panchon - Savarit method:

From the given data in table (4); the enthalpy

diagram for methanol-water system is drawn; fig (4).

To get the Theoretical no. of stages ; locate the

composition of the distillate & bottom @ the x-axis, going

upward vertically from XD; the distance between the

saturated liquid & vapor curves is [a]; the distance from the

saturated vapor till an unknown point [ D] is [b]. From

lever Rule; the distance ratio a/b = R = L/D, R is 1.232, [a]

is measured by a ruler to be 4.4cm, so b = a*R =

4.4*1.232= 4.928cm. Locating [ D] from the saturated

vapor curve by a distance 4.928cm.

[a]: represents the distillate flow rate at the top of the

column.

[b]: represents the liquid flow rate.

Starting at point (Vn) on fig(4) and using theequilibrium curve fig(2), we get Ln @ Xn=0.72 on the

saturated liquid curve; from this pint going on a straight

line toward [ D]; when crossing the saturated vapor

curve; Vn-1 is located @ Yn-1.

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Completing in the same way to get a number of tie

lines (lines that represents the equilibrium between the two

outgoing phases from a stage); these tie lines indicate the

Theoretical no. of stages required.

It's obviously seen that also the Theoretical number

of stages is .

o The Murphee plate efficiency:

%100*

1

*

1

=

nn

nnv

yy

yyE

= %100*217.0217.0

217.0195.0

=

Given Data:

Table(1): Calibration curve data:

RI/(B+1A)flow rate

mL/min

0.8 25.7

1.1 28

1.2 30.5

1.5 35.1

1.9 37.2

2.6 50.1

3 55.6

Table(2): Equilibrium data

xM yM0 0

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0.1 0.26

0.2 0.45

0.3 0.62

0.4 0.72

0.5 0.803

0.6 0.8710.7 0.916

0.8 0.95

0.9 0.984

1 1

Table(3): Enthalpy data for methanol/water mixture

HL x y HVKJ/g.mole KJ/g.mole

6.061 0.00 0.000 47.625

5.267 0.05 0.273 45.144

4.723 0.10 0.418 43.681

4.389 0.15 0.517 42.678

4.138 0.20 0.579 42.009

3.929 0.30 0.665 41.089

3.846 0.40 0.729 40.379

3.804 0.50 0.779 39.835

3.804 0.60 0.825 39.3343.804 0.70 0.870 38.832

3.804 0.80 0.915 38.331

3.816 0.90 0.958 37.871

3.887 0.95 0.979 37.620

3.954 1.00 1.000 37.453

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distillation calculation (for talal)

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