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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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7017
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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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