Distillation Column Design in Separating Ethanol-Water Mixture

7/28/2019 Distillation Column Design in Separating Ethanol-Water Mixture

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University of California,

Los Angeles

Winter 2000

Distillation Column Design In Separating

Ethanol-Water Mixture

Marie Dang

Sandy Lao

Hang-Tam Nguyen

ChE 108A Project

Professor Choi


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Table of Contents

Introduction 3

Procedure 4

Calculation 5

Result 9

Discussion 11

Conclusion 13

Contribution 14

Index 15


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Introduction

A conventional azeotropic distillation uses entrainer such as benzene to purify products.

Thus a distillation process without entrainer will cost more and one would need to adjust

the process variables to minimize the cost. This project focuses on designing a system

consisting of two distillation columns to obtain 99.9 wt% ethanol from a feed stream that

composed of 40 wt% ethanol, 60 wt% water, at a total flow rate of 100kg/hr. The feed

enters the first column at 25 C and 1 atm. For the basic case design, the first column will

contain 60 stages, with a feed stage at 58 and the recycle-in stage is 10. The top pressure

is 0.10 atm and the bottom pressure is 0.12 atm. We use total condenser with a distillate

rate of 410 kg/hr with a reflux ratio of 25. The pumps output pressure is 1.1 atm. As for

the second distillation column, the number of stages is 90 and the feed stage is at tray 10.

The top pressure is 1.0 atm and the bottom pressure is 1.1 atm. Again, we use the total

condenser with a distillate rate of 370 kg/hr and a reflux ratio of 25.

With the above parameters in mind, we utilize PRO/II with the NRTL thermodynamic

model to design and simulate the base case design. By adjusting the following variables,

we can come up with the best separation process design.

1. Number of trays of column 12. Number of trays of column 23. Position of the feed tray for column 14. Position of the feed tray for column 25. Reflux ratio of column 1

6. Reflux ratio of column 27. Position of the recycle-in stage for column 18. Flow rate of the recycle stream

We then came up with three different designs in which we minimize the material cost.

The ultimate goal is to obtain a final design, which is economically the best, or at the


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very least, has significant improvement from the basic design. The first design, which is

denoted as Optimal 2, uses 15 and 39 trays for the two columns with a saving in cost of

13.6% in comparison to the base cost. The second design, Hang1, requires 37 and 40

trays for the two columns and saving us 83% less in comparison to the Base Run cost.

Our last run, Sandy2, which uses 32 and 26 trays, saves us 28% less to theBase Run case.

Procedure

For the Basic run, which involves two distillation columns, ProII is utilized to quickly

calculate the features of each tray of the distillation column and generate a report for each

run. First the basic run design is schematically drawn with ProII. The conditions are

then entered into each column and initial estimates are provided. Note that the initial

estimates for each stream coming in and out of the distillation reflect the overall

component mass balance around the each unit. Once the basic run is generated, we can

adjust the variables to come up with the better designs, which we could evaluate based on

the economic analysis. By comparing the different cases we would be able to select

which of the potential candidate designs would be the best. We note that this may not be

the most optimal design but it is certainly presents improvement from the base design and

that its set of costs are within reasonable tolerances. Thus by minimizing the number of

trays of each column, the reflux ratio of the condensers, and the recycle flow rate, we can

reduce the expense considerably. The choice of a design is based on the total annualized

costs which would consists of both capital costs and operating costs, and the balanced

minimum of the two would lead to the optimal design.


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Calculation

Sample Calculation for Determination of Column Size (Case Sandy2 Column 1)

From the generated report for PROII simulation run, we first locate the vapor and liquid

flow rates for tray #2 and the bottom tray. We picked tray #2 as our top tray because this

is the actual first tray in the column. We also find the density (Rho) for both vapor and

liquid at the top and bottom. Summarizing these results provided by PROII:

L'(kg/hr) V' (kg/hr)

RhoL(kg/m^3)

RhoV(kg/m^3)

Top (Tray #2) 10554 10660 783.806 0.17735

Bottom 60.09 4187 986.644 0.07937

Then we determine Flv, which is defined by the following relation:

5.0

'

'

=

L

g

lvV

LF

For the top tray:

)(014893.0

806.783

17735.0

10660

10554)(

5.0

unitlesstopFlv =

=

)(000129.0644.986

07937.0

4187

09.60)(

5.0

unitlessbottomFlv =

=

Now we pick 24 tray spacing and turn to Figure 4.4 in out textbook Systematic Methods

of Chemical Process Design and find Csb in ft/s.

s

fttopCsb 39.0)( = and s

ftbottomCsb 4.0)( =

Updating our table:

L'(kg/hr) V' (kg/hr)

RhoL(kg/m^3)

RhoV(kg/m^3)

Flv (nounit) Csb (ft/s)

Top (Tray #2) 10554 10660 783.806 0.17735 0.014893 0.39


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Bottom 60.09 4187 986.644 0.07937 0.000129 0.4

We now calculate the flooding velocity Unfgiven by the expression:

2.0

5.0

20

=

g

gL

nf CsbU

Where is the surface tension in dynes/cm. For the first column, we use the surface

tension of ethanol at the top since it is mostly ethanol, and the surface tension of water at

the bottom. For the second column, we use the surface tension of ethanol for both top

and bottom. Note here that the ethanol surface tension used, is at 50C, as opposed to

about 80C, which is the actual temperature of ethanol at the streams since this is the

highest temperature that we can find. And for water, we used the surface tension at 30C

while the actual stream is at 29C.

( )cm

dynesCC ethanolethanol 475.88)50(80 =

According to CRC Handbook of Chemistry, 80th

edition,1999-2000.

s

m

ft

m

s

ftTopUnf 869.5

3048.0

475.88

20

17735.0

17735.0806.78339.0)(

2.05.0

=

=

s

m

ft

m

s

ftBottomUnf 544.10

3048.0

2.71

20

07937.0

07937.0644.98640.0)(

2.05.0

=

=

Now we assume that we want to operate the column at 80% flooding, then the diameter

of the column is given by the expression:

))((8.0

'4

gnfU

VD

=

( ) ( ) cmdynes

CC waterwater 2.712930 =


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Where is the fraction of the area available for vapor flow and since we picked the

cheaper sieve tray, is 0.75.

m

m

kg

s

m

s

hr

hr

kg

TopD 457.2

)17735.0)(75.0()869.5)(14.3(8.0

3600)10660(4

)(

3

=

=

m

m

kg

s

m

s

hr

hr

kg

BottomD 717.1

)07937.0)(75.0()544.10)(14.3(8.0

3600)4187(4

)(

3

=

=

mtopDbottomDD .457.2))(),(max((max) ==

And we take this to be the diameter of our column. Summarizing the results:

L'(kg/hr)

V'(kg/hr)

RhoL(kg/m^3)

RhoV(kg/m^3) Flv (no unit) Csb (ft/s) Unf (m/s) D (m)

Top 10554 10660 783.806 0.17735 0.014893 0.39 5.869 2.457

Bottom 60.09 4187 986.644 0.07937 0.000129 0.4 10.544 1.717D(max)= 2.457 meters

We perform the same calculation for column 2 as well, except that for column 2, we use

the surface tension of ethanol for both top and bottom trays.

To determine the column height, we use a rough approximation of the tray spacing of 0.6

meter. So the total tray stack height would be:

mnstacktrayH 6.0*)1()( =

where n = number of trays, so for column 1:

mmCH 6.186.0*)132()1( ==


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Adding this to the extra feed space (1.5mX2 feed stages for column1), Disengagement

space 1.5m), and skirt height (1.5m), gives the total height of the column.

)()()()()( skirtHentDisengagemHspacefeedHstacktrayHcolumnH +++=

Thus for column 1,

mmmmmcolumnH 6.245.15.10.36.18)1( =+++=


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Results

Data Result

As can see from the generated graph of the basic run, Figure 1 attached on the next page,

we notice the excess number of trays in both columns one and two were used to obtain a

100% pure ethanol. Thus this is a significant source of waste posed by the extra trays.

The dimensions are also quite large. The diameters are 2.47 and 1.45 meters for column

one and two ofBasic Run respectively. The lengths are 42.9 and 59.4 meters for column

one and two. The number of trays is 60 and 90 for columns one and two. The feed enters

at stage 58 for the first column, and at stage 10 for the entering recycle stream. For the

second column, the feed enters at stage 10. The final product purity is 100% ethanol.

However, we only need 99.9% ethanol, therefore we can reduce the amount of trays and

reflux ratio.

Foroptimal 2 case, the number of tray is reduced to 20 for the first column and 39 for the

second column. The reflux ratio is also decreased to 25 and 18. With such a drastic cut

in the tray number, the length column went down to 18.9 and 28.8 meters respectively.

The diameter stays relatively the same, 2.47 and 1.24 meters. Here the feed enters at

stage 18 and the recycle stream was introduced in stage 2. For the second column, the

feed enters at stage 5.

For the Hang1 case, the number of tray is 37 and 40 for the two columns. The reflux

ratio is 4 and 2.5. As for the dimensions, the diameters are 1.08 and 0.52, however, the

column heights reduced to 15.9 and 30.6 meters. Thus, the first column is only almost

half of the second column in diameter. The feed enters at stage 25 and 2 for the entering

recycle stream for the first column. For column two, the stream enters at second stage.


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Since we have looked at the two extremes of number of tray and the lowest reflux ratio,

now on the final optimal case, we try to even out the number of tray in both columns with

the lowest reflux ratio possible to see if this would lower our overall cost.

For the last case, Sandy2 case, the tray numbers are 32 and 26 for the two columns. The

diameters are 2.46 and 0.95 meters. The heights are 26.10 and 21.0 meters respectively

for the two columns. The reflux ratios are 25 and 10. For the entering streams of the first

column, the feed enters at stage 22 and the recycle stream at 2. As of the second column,

the stream enters at stage 2. Thus the following Table1 summarizes four different trials to

provide a quick comparison between different runs.

Runs Columns Num. OfTrays

Diameter(m)

Height(m)

RefluxRatio

Condenser HeatDuty (M*KJ/HR)

Reboiler HeatDuty (M*KJ/HR)

Pump Work(KW)

Basic Run Column1 60 2.47 42.9 25 -10.258 10.2103 0.0142

Column2 90 1.45 59.4 25 -8.6674 8.7227

Optimal 2 Column1 20 2.47 18.9 25 -10.2793 10.2357 0.0142

Column2 39 1.24 28.8 18 -6.3485 6.4039

Hang1 Column1 37 1.08 29.1 4 -1.9542 1.9109 0.0142

Column2 40 0.52 29.4 2.5 -1.1541 1.2093

Sandy2 Column1 32 2.46 26.1 25 -10.0178 9.9747 0.0142

Column2 26 0.95 21.0 10 -3.5678 3.6229

Cost Analysis Results

This section focuses on the economic factor in designing a separation process. According

to the Basic Run, which would cost roughly 16 million dollars to purify 40% ethanol to

99.9% pure. Comparing this cost value to the optimal runs, we see a significant

improvement. For Optimal2 case, the NPV(cost) is only 13.8million dollars. Yet for

Hang1 case, the cost is now only 2.7 million dollars. Thus we have saved around 82.9%

of theBasic Run. Table 2 below summarizes the different types of cost for each run.


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Runs Total

Capital

Cost ($)

Total

Annualize

Utility Cost

($)

NPV (Cost)

($)

% Saved NPV

(Cost)

Basic Run 1254360 4918508 15980036 0

Optimal2 615332 4353813 13802432 13.62702812

Hang1 322485 817055 2733632 82.89345531

Sandy2 621785 3597620 11483816 28.1364823

We can see that the costs of all optimal runs are significantly less than that of the Basic

Run. However, each of the trials has its own advantage and disadvantage as will be

discussed in detail in the next section.

The following Table 3 presents a rough calculation of the profit we would have obtained

if the designs were to implement. Note that this represents a very crude calculation of the

profit just so we would have an idea if this is actually profitable investment. We see that

all the trials seem to yield reasonable gain. Even theBasic Run, which costs much higher

than the other three optimal runs, brings 35 fold profits for a 10 years period. This

indicates that either the retail-selling price is too high ($30/L of ethanol) or that the

process does bring considerable gain. Either way, this evaluation confirms that the

Hang1 run is still the best in term of economic factor.

Profit Evaluation

Basic Run Optimal2 Hang1 Sandy2

Fixed Capital 1254360 615332 322485 621785

Working Capital (0.20 f.c) 250872 123066.4 64497 124357

Fixed and Working Capital 1505232 738398.4 386982 746142

Product Rate (lb) 699031.77 699031.77 699031.77 699031.77

Raw Material ($0.08/lb prod) 55922.54 55922.54 55922.54 55922.54

Utilities ($0.012/lb prod) 8388.381 8388.381 8388.381 8388.381

Labor ($0.015/lb prod) 0.015 10485.48 10485.48 10485.48

Maintenance (0.06yr f.c.) 75261.6 36919.92 19349.1 37307.1


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Supplies (0.02yr f.c.) 25087.2 12306.64 6449.7 12435.7

Depreciation (straightline over ~10yrs) 125436 61533.2 32248.5 62178.5

Taxes, insurance (0.03/year) 37630.8 18459.96 9674.55 18653.55

Total Manufacturing Cost ($0.131/lb) 91573.16 91573.16 91573.16 91573.16

Gross Sales 11982762 11982762 11982762 11982762

Gross Profit (GS-TM) 11891189 11891189 11891189 11891189

SARE Expenses (0.10sales tax) 1198276 1198276 1198276 1198276

Net Profit Before Taxes (GP-SARE) 10692913 10692913 10692913 10692913

Taxes (0.50 net profit) 5346456 5346456 5346456 5346456

Net Profit after Taxes 5346456 5346456 5346456 5346456

Return on Investment (ROI) (net income/ f&w cap) 35519% 72406% 138158% 71655%

Payout Time (total cap./net annual profit) 0.1391369 0.0686598 0.0360817 0.0693757

*Assume ethanol costs $30/Liter From Sigma

Discussion

Data Discussion

As can see from Figure 1, the purity of ethanol actually reaches 100% long before the

tray number reaches 60 trays for the first column of the Basic Run. Thus this indicates

that there are significant number of excess trays in the first column. The extra number of

tray would cost us an additional cost to operate this design. In order to reduce the cost

yet at the same time achieving the ultimate goal, of producing 99.9% ethanol, the stage

number can be cut down to the minimum amount. However, if we push for the border

line amount of tray number, the ethanol purity might not reach 99.9%, thus adding an

extra 5% of tray number would serve our purpose adequately. The lowest number of tray

would give the lowest design dimensions, thus would lower the construction cost of such

a design. For the optimized runs, we not only push for the lowest number of tray number

but also minimizing the reflux ratio as well as the dimensions of the design. The same

purpose would serve for having the lowest reflux ratio, this would give a lower cost for


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the feeding steam entering the column. Also the feed tray number plays an important role

in maximizing the design. The recycle stream and the stream from the first column to

second column need to feed in at the top tray. This makes sense because the ethanol

concentration in these two stream are saturated with ethanol, thus having them fed in the

top trays would separate the water out more efficiently. With all these considerations in

mind, we eventually derive the three optimal runs. The first run, Optimal 2, aims for the

lowest possible distillation tray-number for the second column, yet still produces 99.9%

ethanol. Since the number of tray for the second column is too high for theBasic Run,

minimizing this would considerably lower the cost of building such a tall column.

Having 20 trays in the first column and 39 trays in the second column results in a 99.9%

ethanol release forOptimal2. However, as ProII iterates through the design, the system

converges significantly slower than theBasic Run, this could be due to the high number

of cycle the recycle stream has to reverse to the first column in order to obtain the desired

purity. However, this trial was not considered to be a good design because the reflux

ratio was quite high, causing a large heat duty amount in the reboiler and condenser, thus

the cost of the feeding steam will be expensive.

For the second run,Hang1, we minimize the reflux ratio with an intention that this would

lower the utility cost of feeding steam into the columns. The reflux ratios are 4 and 2.5

for the two columns. This design leads to only 1.9542 and 1.1541 MJ in heat duty of

the condenser. Comparing this heat duty with that of the Basic Run, which is 10.258

and 8.6674 MJ for the heat duty of the condenser. We see almost a 10 fold decrease in

the heat duty. Thus the annualize utility cost of feeding steam is only $2,733,632, which

is 83.39% less than the Basic Run for the NPV(cost). So far this design seems very


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attractive in term of operating cost. However, there are drawbacks in having such a low

reflux ratio design, the columns height is much higher in comparison to the previous

runs. Now it requires more stage number to separate the mixtures to 99.9% ethanol. This

however, is compromisable if our intention is to minimize the operation cost.

As for our last design, Sandy2, we aim for the lowest number of trays in both columns,

thus this would give us a relatively the same number of tray for both column. For this

case the number of stage for column one is 32 while we only need 26 on the second

column. This design might considered to be more advantageous over the previous two

designs in term of space design, because the numbers of trays for both columns are close

to each other. This offers a better design in the sense that construction would be much

easier. The heights of both columns are not too tall or not too short in comparison to all

the other runs. However, the reflux ratio is still high leading to a high utility cost. Once

again, this demonstrates the need of priority when it comes to process design. If the

intention is to save space and building columns that would fit in a designated area, this

design would be more superior to the other two.

Conclusion

As can see from the three trials, low tray number does not necessarily mean that it is the

better design, there are several other factors involve that can significantly affect the

capacity of a design. The reflux ratio seems to dominate over all the other factors in term

of cost. Thus the lower the reflux ratio, the lower the cost would be. However, too low

of a reflux ratio would require higher distillation stage number. Thus when designing a

separation process, one would need to consider how the space and location of where the

columns are to be built and from there to determine the priorities in designing the


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process. For our purpose, we try to obtain the lowest cost operation yet with a relatively

not too high number of trays, thus, Hang1 run seems to serve our purpose. This design

saves us $13 million in comparison to the Basic Run case. Thus it is important to have as

low reflux ratio as possible yet with reasonable column height in order to maximize profit

of a design.


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Contribution

Marie Dang: Runs column, designs an optimal case and analyzes that particular case as

well as contributing in writing the report.

Sandy Lao: Runs column, designs an optimal case, analyzes that particular case and

participate in writing the report.

Hang-Tam Nguyen: Runs the column, designs an optimal case, analyzes that particular

case and writing the report.

Table of Index

1. First Report: Base Case Run Report

2. Second Report: Optimal 2 Run Report

3. Third Report: Optimal 3 Run Report

4. Fourth Report: Sandy2 Run Report

5. Dimensional Analysis Report For All Runs

6. Cost Analysis Report For All Runs

Documents

Distillation Column Design in Separating Ethanol-Water Mixture