Ind. Eng. Chem. Res. 2007, 46, 8813-8826
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Balancing Diameters of Distillation Column with Vapor Feeds Phillip C. Wankat* School of Chemical Engineering, 480 Stadium Mall DriVe, Purdue UniVersity, West Lafayette, Indiana 47907-2100
Methods for balancing distillation vapor loads to achieve smaller, more-uniform diameters are developed for vapor feeds and the largest calculated diameter in the enriching section. The diameter can be reduced by (i) cooling the entire feed, (ii) cooling or condensing part of the feed while leaving the other part vapor (twoenthalpy feed), or (iii) condensing vapor inside the column with an intermediate condenser. For distillation of a saturated vapor feed of 10 mol % ethanol and 90 mol % water, the use of a two-enthalpy feed resulted in a 58% reduction in column volume with the same purity and no increase in energy use. Examples are also shown for methanol-water and n-butane-propane distillations. The column diameter for distillation is usually designed to be a set percentage of flooding, typically in the range of 60%80%. Flooding is calculated using procedures detailed in standard textbooks1,2 and handbooks3 and included in commercial simulators. The diameter calculated at different locations usually varies. Typically, the column is designed at the location with the largest column diameter and built with a constant diameter, unless the calculated diameters at different locations differ by a large amount. When this design procedure is followed, the percentage of flooding will be less, often significantly less, than the nominal design percent of flooding everywhere in the column except for the location with the maximum calculated diameter. In cases with extreme variations in calculated diameter, the column is built in two sections with different diameters. When there is a vapor feed, the calculated diameter is usually largest in the enriching section. For example, distillation of ethanol-water, methanol-water, and n-butanepropane show this behavior. The most commonly used methods to increase column capacity are to modify standard trays and downcomers,4 to use high-capacity trays,4 or to use high-capacity packings.1,2 Although these methods increase capacity, the approach to flooding in different parts of the column still varies considerably. If this variation is large, neither the trays nor the packing will operate at optimum levels in parts of the column where the vapor velocity is much less than the design vapor velocity. Although not used in many applications where they would be useful, the following closely related processing methods are known to allow one to adjust the liquid and vapor flow rates in the enriching section of the column. (1) Use an intermediate condenser5,6 (Figure 1). For example, a vapor stream is withdrawn from the column, condensed, and then returned to the column as a liquid. Cascades of intermediate condensers or circulating reflux are commonly used in petroleum refineries to allow for use of energy at a variety of temperature levels and to balance vapor flow rates, which can help to reduce the maximum calculated diameter.6 (2) Remove the vapor and use a heat pump to condense it.5 This is similar to using an intermediate condenser, but the method of obtaining the liquid is slightly different. (3) Use liquid pump-around, which withdraws liquid, cools it, and returns it to the column.5,7 The effect is similar to an * Tel.: 765-494-0814. Fax: 765-494-0805. E-mail address: wankat@ ecn.purdue.edu.
Figure 1. Intermediate condenser distillation system.
intermediate condenser and the main application has been for energy recovery and control of liquid flow rates,7 rather than for diameter balancing. Because these methods have similar effects, we will explore only intermediate condensers among these previously known methods. (4) A commonly used method in industry is to condition the feed by cooling the entire feed stream to control calculated diameter in the enriching section or for debottlenecking.8,9 With all of these systems, there will usually be an energy penalty in an increase in overall heating and cooling loads, but the column diameter will be decreased, resulting in capital cost savings. While studying the known methods for diameter balancing,5-9 we discovered that a new methods(5) cooling or condensing a part of the feed while leaving the other part vapor with a twoenthalpy feed systemscan also be effective. The basic idea for a two-enthalpy feed when the original feed is a vapor is shown in Figure 210. In normal operation, the entire feed is cooled and will be partially or totally condensed. In a two-enthalpy feed operation, the feed is split into a vapor component (F2) and a component that is condensed in the heat exchanger (F1). It is not necessary to totally condense stream F1. What is of critical importance is to separate the feed into two parts before one part is cooled. The distillation column is then designed in the same fashion as a normal two-feed column, with the only difference being that, in this case, the two feeds have the same
10.1021/ie0705554 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007
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distillation systems than comparing differences in the predicted vapor mole fractions.
error in (yMVC - xMVC) (%) ) (y - x)predicted - (y - x)data (1) 100 (y - x)data
[
Figure 2. Two-enthalpy feed distillation system with feed that is initially vapor.
composition.10 King and co-workers11 developed a similar multifeed method for reducing energy costs in cyrogenic distillation, but they did not use it to reduce column diameter. The driving philosophy behind this research is to consider a large difference in the calculated diameters as potential to either reduce the column volume or as potential to process more material in a retrofitted column. If the calculated diameter is high in the enriching section, the vapor flow rate is reduced in this section. To do this, we reduce the reflux ratio from the condenser and do additional condensing either with an intermediate condenser or with a condenser on all or part of the feed. Reduction of the diameters in columns where the calculated diameter is largest in the stripping section, which is common with liquid feeds, is considered elsewhere.12 Base Cases for Standard Distillation Systems To illustrate the differences in calculated diameters that are possible, consider three base cases for sieve tray columns: an ethanol-water vapor feed, a methanol-water vapor feed, and a methanol-water liquid feed. Ethanol (10 mol %)-Water (90 mol %) Vapor Feed Base Case. The feed for this base case is 1000 kmol/h of a saturated vapor stream at 2.5 atm. A 10 mol % feed is relatively typical of the product from the beer still in a bioethanol refinery. Because, in bioethanol refineries, the distillate is usually a vapor that is sent to a pressure swing-adsorption unit, a vapor distillate product from a partial condenser with yD ) 79.01 mol % ethanol and a liquid bottoms product with 99.86 mol % water were chosen. Details of the base case are given in Table 1. The column has 34 equilibrium stages plus a partial condenser (contact 1) and a kettle reboiler (contact N ) 36 in AspenPlus notation). For all simulations in this paper, the optimum feed stage was determined as the feed stage that gave the largest separation, with the other parameters kept constant. If the column has an overall efficiency of ∼70%, this represents a column with 50 actual stages, which is fairly typical. Calculations were performed with the AspenPlus simulator, using the NRTL VLE correlation. The fit with the vapor-liquid equilbrium (VLE) data in Perry’s Handbook7 was measured as the percentage error in measured and predicted values of the vapor minus the liquid mole fractions (y - x) of the morevolatile component (MVC) at equilibrium. The value y - x, which represents the difference between equilibrium and the total reflux operating line, is a more-sensitive measure for binary
]
At low ethanol mole fractions (xE ) 0.07) the error is 660.8 kmol/h, the pinch point will be at the liquid feed pinch point. For example, increasing the amount of feed condensed to FL ) 800 kmol/h results in lines 6, 7, and 8. If all of the feed is condensed, we have the usual liquid feed case, using lines 6 and 9. Note that these values agree with the simulations, which showed that, with FL ) 650 kmol/h, the vapor pinch was operative (Table 6). Thus, the McCabe-Thiele analysis can be used to estimate the highest value of FL that can be used without changing the pinch point. In the usual method of conditioning the feed by cooling the entire feed, the pinch point changes as soon as any fraction of the feed is condensed. For example, if half of the feed is condensed, the two-phase feed line would go through point yE ) xE ) zE ) 0.2 with a slope ) -FL/FV ) -1, as shown in Figure 3. Because the pinch point changes, there is no guarantee
that the desired purities can be obtained with the same value of QR and more stages for even modest changes in FL. Processes for Vapor Feed: 20% Ethanol-80% Water Because feed concentration can have a large effect on distillation, a vapor feed that is 20 mol % ethanol, which is also a reasonable concentration in ethanol refineries, was simulated. Feed rate and pressures for the base case were the same as the base case for 10% ethanol. Lower purities were arbitrarily chosen for this case, (yD,E ) 0.6929 and xB,w ) 0.9965). Conditions and results are given in Table 6. The diameter was largest at the top of the column (2.36 m, area ) 4.36 m2), slightly smaller immediately above the feed stage (2.02 m), and significantly smaller immediately below the feed stage (0.70 m) and at the bottom of the column (0.70 m). A two-enthalpy feed system was designed with the same parameters as the base case, except 650 kmol/h of feed was condensed and sent to the optimum feed stage. This column produced distillate (yD,E ) 0.6929) and bottoms (xB,w ) 0.9965) products that match the base case. Results are presented in Table 6. The pinch point at the vapor feed allows the two-enthalpy feed system to obtain the same separation with the same energy requirements and the same number of stages as the base case. Retrofitting Columns. Suppose that we had already built the base case system for the 20% ethanol feed and wanted to retrofit it to increase the feed rate by 50% to 1500 kmol/h. What modifications are required? By setting the feed rate to 1500 and trying different values for the amount of feed to condense in the two-enthalpy feed system, we find that by condensing 540 kmol/h of feed while keeping QR/F constant, we obtain the same product purities as the base case. The calculated diameter is 2.35 m. Because the calculated diameter for the base case was 2.36 m, the flooding velocity will be very slightly less than the nominal 80% at tray 2. The calculated column diameters are as follows: stage 2 ) 2.35 m, stage 17 ) 2.10 m, stage 27 ) 2.08 m, stage 29 ) 0.86 m, and stage 35 ) 0.85 m. Results are listed in Table 6. This system requires a heat exchanger with a cooling load of -6150 kW for the condensing portion of the feed and a partial condenser with Qc ) -7300 kW. The value of Qc, total /F ) -8.97 kW/(kmol/h) is the same as that for the base case. The column must have a splitter and a heat exchanger added to the feed line, and it needs the addition of a liquid feed port on stage 17 and a vapor feed port for stage 27. The arrangements for removal of a second liquid layer caused by a buildup of fusel oils must be checked to be sure they remain adequate. Additional capacity is needed for the column reboiler, but the column condenser now has extra capacity, because much of the cooling load is done by condensing a portion of the feed. If the original column was built in one piece with a constant diameter of 2.4 m, then no other changes are required in the column. If it was constructed
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Table 7. Results for the Distillation of a 20% Ethanol-80% Water Vapor Feed in a Packed Beda FL
hF,Vb
0 (base) stage:e
13.4 (24)
650 stage:e
15.2 (27)
550 stage:e
14.6 (26)
hF,Lc
height of packing, h (m)
diameter, dia (m)
area, A (m2)
Vol (m3)
Volp
locationd
New Ethanol-Water Base Case 2.20 48.5 45.5 0 (2)
QR,total
Qc
920
-8970
Qc,total
decrease in Vcol (%)
-8970
59.0
20.7 (N ) 35)
1.67
8.5 (16)
20.7 (35)
1.06
9.1 (17)
Retrofit of Two-Enthalpy Feed with F ) 1500 kmol/h, D ) 427.5 in the Base Case Column 20.1 1.67 2.20 48.5 44.2 0 1390 -13500 (34) (2)
Two-Enthalpy Feed (FL ) 650 kmol/h): 0.88 19.9 18.3 0 (2)
920
increase in QR (%)
a Conditions are the same as for Table 5, except a packed bed is used with a HETP of 2.0 ft (0.6096 m). Distillate product contains a 0.6929 mole fraction of ethanol, and bottoms product contains a 0.9965 mole fraction of water. b Height of the packing below the condenser to the vapor feed location. c Height of the packing below the condenser to the liquid feed location. d The location where the largest diameter is calculated. e The listed stage shows the equivalent location, based on a staged analysis.
in two pieces, of diameter 2.4 and 0.7 m, then the top section will work well, but the bottom stripping section will not, and retrofitting will require an additional small stripping column. Packed-Bed Operation. The procedures for balancing column diameters are also applicable to packed beds. The three cases from Table 6 were simulated for 2 in. metal Pall rings (see Table 7). This example is for illustrative purposes, because if fusel oils are present, the portion of the column where the fusel oils are withdrawn would normally use trays. The column volume (Vol) is given as
Vol ) A[h + (2 + number of feeds + number of withdrawals) × (spacing for disengagement and feeds)] (5) where the spacing for disengagement and feeds is 18 in. (0.4572 m) for each feed or disengagement region. A is the crosssectional area of the column, and h is the height of the packing. The base case has one feed while both of the two-enthalpy feed systems have two feeds. The base case column is operating at 80% of flooding at the top of the enriching section and at 55.1% of flooding at the bottom of the enriching section. The stripping section ranges from 7.0% of flooding at the top of the section to 6.8% of flooding at the bottom of the section. This would probably require building the column in two sections. The decrease in column volume with the two-enthalpy feed with FL ) 650 kmol/h (59.0%) is very similar to the result obtained in Table 6 for the equivalent run with a tray column (58.5%). The top of the enriching section operates at 80% of flooding and the bottom of the enriching section at 66.0% of flooding. The top of the intermediate packed section (immediately below the liquid feed) is at 77.5% of flooding, while the bottom of this section is at 59.5% of flooding. The stripping section ranges from 17.5% at the top of the section to 17.1% of flood at the bottom of the section. The retrofit of the base case column to a feed rate of F ) 1500 kmol/h was also simulated (see Table 7). In Table 7, the total column volume is the same as the base case column, but the packing volume is less. This occurs because enough packing was removed from the base case column to make room for the liquid feed stage in the two-enthalpy feed system, thus; the number of stages was reduced by one. Because the space required for a feed is less than the HETP, there is a small amount of extra space available. This is assumed to be part of the disengagement spaces. The results are very similar to Table 6, except an FL value of 550 kmol/h was required to match the diameter of the base case. The value of Qc, total/F (-8.97 kW/ (kmol/h)) is the same as that for the base case. The top of the enriching section is operating at 80% of flooding and the bottom
of the enriching section is at 57.5% of flooding. The top of the intermediate packed section (immediately below the liquid feed) is at 58.0% of flooding, whereas the bottom of this section is at 56.0% of flooding. The stripping section ranges from 10.6% of flooding at the top of the section to 10.3% of flooding at the bottom of the section. If the original base case was built as two separate sections of different diameters, the enriching section can be retrofitted without difficulty, but the stripping section will probably be above its nominal flooding velocity. This will require either an additional small stripping section or repacking the stripping section with a higher capacity packing. Ethanol-Water Distillation with Liquid Feed Because several variations of ethanol biorefineries are possible, the feed to the distillation column could be a liquid instead of a vapor. The 10 mol % feed case is identical to the limiting case of condensing all the vapor (see Table 3), but without the large energy load to condense the feed. Because the variation in diameter is small (see Table 4), particularly between the feed stage and stage 2, it is difficult to use the processes of this paper to decrease the diameter further. We tried using a two-enthalpy feed system with one feed being a saturated liquid and the other being subcooled to 30 °C. After the value of QR was increased to match the purity of the liquid base case, there was no decrease in the calculated column area at stage 2. The conclusion was similar for an intermediate condenser removing vapor from stage 31 and returning it above the feed stage. We also tried a 3 mol % ethanol feed with a saturated liquid feed. In this case, the maximum diameter is at the feed stage, and it is very close to the diameter at the top of the column. Thus, there is no room for improvement. The conclusion from these runs is that, with a liquid feed, the use of diameter balancing methods do not reduce the calculated diameter for ethanol-water separations. However, if there are both liquid and vapor feeds, then the use of a two-enthalpy feed for the vapor feed portion will probably be advantageous. Vapor Feed for Methanol-Water Distillation The base cases for both vapor and liquid feeds are given in Table 1. Table 2 showed that with a vapor feed the calculated diameter is much larger at the top of the column. With a liquid feed, the calculated diameters for a dilute methanol-water system are well-balanced throughout the column. Thus, we expect to be able to drastically reduce the column diameter for vapor feeds but not for liquid feeds. Two-Enthalpy Feed System for Methanol (5%)-Water (95%), Vapor Feed. Half of the feed (500 kmol/h) was
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8821 Table 8. Results for Distillation of a 5 mol % Methanol, 95 mol % Water Vapor Feeda FL
NF,V
0 (base)
10
NF,L
500 11 500 12 600 12 750 13 750 16 1000 (all liquid)
6 6 6 6 7 9
N
diameter, dia (m3)
area, A (m3)
20
2.84
6.33
20 22 20 20 26 20
2.08 2.07 1.89 1.56 1.56 1.20
tray with maximum diameter
Vol (m3)
Base Case 55.0 Two-Enthalpy Feed 29.4 32.5 24.3 16.7 21.8 9.8
3.38 3.38 2.80 1.92 1.91 1.13
QR
Qc,total
2
1065
-12330
2 2 2 2 2 2
1070 1065 1070 1090 1065 2415
-12340 -12330 -12340 -12360 -12330 -13680
decrease in Vol (%)
increase in QR (%)
46.5 40.9 55.8 69.6 60.3 82.1
0.5 0 0.5 2.3 0 127
Intermediate Condenser FWithdr 300 450
NF,V
NV,with
NL,ret
N
diameter, dia (m3)
11 11
10 10
6 6
20 20
2.41 2.22
area, A (m3)
Vol (m3)
tray with maximum diameter
QR
Qc,total
decrease in Vol (%)
increase in QR (%)
4.56 3.88
39.6 33.7
2 10/11
1067 1067
-12340 -12340
28.0 38.6
0.2 0.2
Two-Enthalpy Feed (FL ) 600 kmol/h) Plus One Intermediate Condenser Fwithdr 100 80
NF,V
NF,L
NV,with
NL,ret
N
diameter, dia (m3)
area, A (m3)
Vol (m3)
tray with maximum diameter
QR,total
Qc
decrease in Vol (%)
increase in QR (%)
12 12
6 6
5 13
5 6
20 20
1.69 1.72
2.23 2.33
19.4 20.2
2 2
1073 1079
-12340 -12345
64.7 63.2
0.8 1.3
Two-Enthalpy Feed (FL) 600 kmol/h, NF,V ) 12, NF,L ) 6, N ) 20) Plus Two Intermediate Condensers Fwthd1
NVwth1
NLret1
Fwthd2
NVwth2
NLret2
diameter, dia (m3)
area, A (m3)
Vol (m3)
tray with maximum diameter
QR,total
Qc
decrease in Vol (%)
increase in QR (%)
100
5
4
80
13
8
1.50
1.77
15.4
2/6
1081
-12350
72.1
1.6
Two-Enthalpy Feed (FL ) 680 kmol/h) Plus One Intermediate Condenser Fwithdr 100
NF,V
NF,L
NV,with
NL,ret
N
diameter, dia (m3)
area, A (m3)
Vol (m3)
tray with maximum diameter
QR,total
Qc
decrease in Vol (%)
increase in QR (%)
12
6
5
5
20
1.51
1.79
15.5
6
1081
-12,350
71.7
1.6
Distillate contains a 0.9543 mole fraction of methanol, and bottoms contains a 0.9976 mole fraction of water. Tray spacing ) 18 in. (0.4572 m). Notation is the same as that in Table 3. Base case conditions are listed in Tables 1 and 2. When two trays are listed, they have the same diameters. The decrease in volume and increase in QR are compared to the base case. a
condensed before being fed to the optimum feed stage. By increasing the reboiler duty to 1070 kW, the desired distillate and bottoms products were obtained. The condenser duty becomes -6630 kW, and the duty for condensing the feed is -5710 kW for a total cooling requirement of Qc,total ) -12 340 kW, which is an increase in the absolute value of Qc of 0.08%. Alternately, the desired distillate and bottoms products can be obtained with constant QR and Qc,,total values by increasing the number of stages (see Table 8). By increasing the amount of condensation to 600 kmol/h, the reduction in volume is increased to 55.8% while the increase in QR remains ∼0.5% (Table 8). With a further increase in FL to 750 kmol/h, the desired distillate and bottoms products are obtained by increasing the energy loads or the number of stages (Table 8). The decrease in column volume is >60% in both cases; however, QR or N starts to increase considerably. The limiting case when all 1000 kmol/h of feed is condensed (FL ) 1000 kmol/h and FV ) 0 kmol/h) is identical to the liquid feed base case plus the cooling load to condense all of the feed (-9530 kW; see Tables 8 and 9). The stripping section of this totally liquid feed system has much larger calculated diameters and vapor flow rates than the vapor base case and the twoenthalpy feed runs. Condensing all the feed results in the smallest column volume, but the absolute value of the cooling load increases by 10.9% and the heating load increases by 127%. Intermediate Condenser for Methanol (5%)-Water (95%), Vapor Feed. Starting with the base case for this vapor feed, first withdraw vapor from stage 11 (at FWithdr ) 300 kmol/h), condense it, and return it to stage 10. With the same value of N, the same distillate rate (50 kmol/h), and the same value of
QR (1065 kW), xD,M ) 0.9541, which is slightly below the value for the base case. Increasing the reboiler load to QR ) 1067 kW, we match the vapor base case (xD,M ) 0.9543, xB,W ) 0.9976) shown in Table 8. If Fwithdr is increased to 450 kmol/h with the original value of QR (1065 kW), xD,M ) 0.9541. Increasing QR to 1067 kW, xD,M ) 0.9543 and xB,W ) 0.9976, which match the purities of the vapor base case. The absolute value of the total cooling load was increased by 0.02%. Diameters for both values of FWithdr are reported in Table 9. For FWithdr ) 450 kmol/h, the calculated diameter of feed at stage 11 is greater than the calculated diameter at tray 2. Further increases in the vapor withdrawal rate (Fwithdr) will have little effect on the required column diameter. In this case, a twoenthalpy feed process can produce larger changes in the column area than an intermediate condenser. Two-Enthalpy Feed Plus Intermediate Condensers for Methanol (5%)-Water (95%), Vapor Feed. For the ethanolwater system, there was no advantage in using both a twoenthalpy feed and an intermediate condenser. Because the methanol-water system has a more-favorable equilibrium, this combined process will be reconsidered. Because the twoenthalpy feed with FL ) 600 kmol/h had a considerable reduction in column volume but the energy requirements had not increased much, this will be the starting point (see Tables 8 and 9). To further reduce the diameter of stage 2, a vapor is withdrawn near the top of the column, condensed, and returned as liquid (see Tables 8 and 9, FL ) 600 kmol/h, Fwithdr ) 100 kmol/h). This intermediate condenser causes a considerable reduction in the calculated column diameter at stage 2 with no increase in QR, compared to the starting point. The column
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Table 9. Calculated Column Diameters and Vapor Flow Rates for Selected Systems from Table 7a diafeed_stb (m)
tray location for diafeed_st
2.23
2.22
10
1.61
Two-Enthalpy Feed System, FL ) 600 kmol/h 1.60 12 494 0.84
dia 2 (m)
V2 (kmol/h)
dia 6 (m)
2.84
1250
1.89 1.20 2.41 2.16
557 230 903
V6 (kmol/h)
487
Vfeed_st (kmol/h)
diabelow_feedb (m)
tray location for diabelow_feed
Vbelow_feed (kmol/h)
dia 19 (m)
V19 (kmol/h)
0.84
11
93.0
0.84
93.4
13
93.7
0.84
94.3
Base Case 1093
0.99
Two-Enthalpy Feed System, FL ) 1000 kmol/h (Entirely Liquid Feed) 1.18 9 205 1.18 10
1.95
2.22
Intermediate Condenser, Fwithdr ) 300 11 1093 0.84
12
93.2
0.84
93.8
2.22
Intermediate Condenser, Fwithdr ) 450 11 1093 0.84
12
786
1.15
211
730
1.79
93.2
0.84
93.9
447
Two-Enthalpy Feed (FL ) 600 kmol/h) Plus Intermediate Condenser (Fwithdr ) 100, NV,with ) 5) 1.61 487 1.60 12 494 0.84 13 93.7
0.84
94.3
465
Two-Enthalpy Feed (FL ) 600 kmol/h) Plus Intermediate Condenser (Fwithdr ) 80, NV,with ) 13) 1.51 408 1.49 12 414 0.84 13 14.2
0.84
94.9
1.50
355
1.50
Two-Enthalpy Feed (FL ) 600 kmol/h) Plus Two Intermediate Condensers 408 1.49 12 414 0.84 13
14.4
0.84
95.1
1.50
355
1.51
Two-Enthalpy Feed (FL ) 680 kmol/h) Plus Intermediate Condenser (Fwithdr ) 100, NV,with ) 5) 408 1.49 12 414 0.84 13 94.3
0.84
95.0
1.69 1.72
a
636
205
b
Plate spacing was 18 in. (0.4572 m). Tray locations are denoted in parentheses.
Table 10. Results for Vapor and Two-Phase Feeds (30% Methanol-70% Water)a F2-phase
FL
0 (base)
NF,V
NF,L
12
NF,2-phase
diameter (m)
area, A (m2)
2.77
6.02
Vol (m3) Vapor Feed 52.3
tray with maximum diameter
QR
Qc,total
2
1215
-12190
Cool Entire Vapor Feed to Two-Phase Mixture (50% Vapor and 50% Liquid) 12 2.17 3.70 32.1 2 2455 -13420
1000
increase in QR (%)
38.5
102
-13420
35.0
102
-12345
47.2
13.2
Originally Two-Phase Feed, VF/F ) 0.5 Processed in Two-Enthalpy Feed Column 12 2.17 3.70 32.1 2 2455 -7700 7 13 2.00 3.13 27.2 2 2475 -7720 9 16 1.85 2.68 23.3 2 2735 -7980 11 17 1.82 2.61 22.7 2 3150 -8400
15.4 27.5 29.5
0.8 11.4 28.3
500
14
11
500
14
11
0 (base) 200 400 500
decrease in Vol (%)
Use Two-Enthalpy Feed 2.23 3.91 34.0 2 2455 High purity: xD,M ) 0.9943 and xB,w ) 0.9976. 2.01 3.18 27.6 2 1375 Purity matches the base case: xD,M ) 0.9514 and xB,W ) 0.9792.
a Distillation of a feed with F ) 1000 kmol/h, D ) 300, N ) 20. Plate spacing is 18 in. (0.4572 m), and pressure is 1.0 atm. Unless noted otherwise, all systems produce xD,M ) 0.9514 and xB,W ) 0.9792. Vapor feed systems are compared to the vapor feed base case. Two-phase feed systems are compared to two-phase feed base case. Notation is the same as that in Table 3.
diameter on stage 2 can also be reduced by withdrawing vapor for the intermediate condenser below the vapor feed stage (see Tables 8 and 9, FL ) 600 kmol/h, Fwithdr ) 80 kmol/h). The withdrawal rate is limited by the low vapor rate in the stripping section, and a withdrawal rate of 80 kmol/h reduces vapor flow rate to V13 ) 14.2 kmol/h (see Table 9), which may cause operating difficulties. Withdrawal below the feed stage reduces the calculated column diameter for all of the stages in the enriching section. Because the column pinch point is at the vapor feed stage, this withdrawal reduces the purity significantly, unless the energy requirements are increased (see Table 8). Next, two-enthalpy feed plus both of the intermediate condensers was simulated. Although fairly complicated, this system has a very attractive combination of large reduction in column volume and small increase in energy requirements (Table 8), but it again has a very low vapor flow rate immediately below the vapor feed stage (V13 ) 14.4 kmol/h) (see Table 9). This potential difficulty can be eliminated by removing the withdrawal for the intermediate condenser below the feed tray and increasing the amount of feed in the twoenthalpy feed system that is condensed to 680 kmol/h (the last
line in both Tables 8 and 9). The results of this design are very similar to the two-enthalpy feed system with two intermediate condensers, but it is simpler and eliminates the very low vapor flow rate immediately below the vapor feed stage (see Table 8). For the last two processes, the enriching section has an almost constant diameter, as does the stripping section. Further reductions in the column area will probably require significant increases in energy requirements. Methanol (30%)-Water(70%), Vapor Feed Distillation. For the ethanol-water vapor feed, it was better to use a twoenthalpy feed system and condense only a portion of the feed than to use the normal feed conditioning process that uses the same amount of energy to cool the entire feed to a two-phase feed. Here, this question is investigated again for a more concentrated methanol-water vapor feed. The base case is a vapor feed that is 30 mol % methanol and 70 mol % water (Table 10). The purities for this base case are arbitrarily set at xD,M ) 0.9514 and xB,W ) 0.9792. First, the entire feed is conditioned by cooling it to a two-phase feed with VF/F ) 0.5 (Table 10) and the value of QR is increased to match the base case purities (xD,M ) 0.9514 and xB,W ) 0.9792). The results in Table 10 show a significant decrease in volume and a significant
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8823 Table 11. Results for the Distillation of Methanol and Water with Two Feeds of Different Compositiona FL,1
FL,2
0 (base) 100 200 250
NF,V1
NF,V2
14
7
0 0 0
14 14 14
7 7 7
0 0 0
250 450 600
14 14 14
8 8
100 150
600 600
14 14
NF,L1
NF,L2
11 11 11 7 7 7 11 11
7 7
diameter, dia (m)
area, A (m2)
2.77
6.02
decrease in Vol (%)
increase in QR (%)
-11980 -12020 -12110
9.8 19.2 23.4
0.5 4.9 14.8
915 915 915
-11980 -11980 -11980
24.2 35.2 58.3
1215 1760
-12270 -12820
64.9 64.9
tray with maximum diameter
QR
Qc,total
Base Case 52.3
2
915
-11980
2.63 2.49 2.42
Two-Enthalpy Feed for F1 5.43 47.2 4.86 42.2 4.61 40.0
2 2 2
920 960 1050
2.41 2.08 1.79
Two-Enthalpy Feed for F2 4.56 39.6 3.39 29.5 2.51 21.8
2 2 2
Vol (m3)
Two-Enthalpy Feed for Feed F1 and Condense All of F2: 1.64 2.11 18.3 7 1.64 2.11 18.3 7
32.8 92.4
a For all systems: F L,total ) 1000 kmol/h, F1 (5% methanol) ) 400 kmol/h, F2 (30% methanol) ) 600 kmol/h, D ) 207.5 kmol/h, N ) 20, xD,M ) 0.9514, xB,W ) 0.9792, sieve tray spacing ) 18 in. (0.4572 m). Operation is at 80% of flooding, and p ) 1.0 atm.
increase in energy use. If the same QR value is used with the two-enthalpy feed, the reduction in column volume is similar, but the product purities are much higher. Reducing the value of QR in the two-enthalpy feed system to match the base case purities, we obtain a 47.2% decrease in volume and a relatively modest increase in the value of QR, compared to the base case. Compared to conditioning the entire feed to a two-phase feed, the column volume is less and the energy use is significantly lower with the two-enthalpy feed. A related question is, does the two-enthalpy feed system help reduce the diameter for a feed that is initially a two-phase feed? The base case conditions and results for this two-phase feed are also shown in Table 10. Note that this base case is the same as cooling a vapor to a two-phase mixture, except that, because the feed is initially a two-phase mixture, the -5630 kW duty needed to condense the mixture is no longer required. Calculated column diameters are as follows: stage 2 ) 2.17 m (area ) 3.70 m2), stage 12 ) 1.97 m, stage 13 ) 1.20 m, and stage 19 ) 1.15 m. Initially, a two-enthalpy feed system is designed with a constant reboiler heat duty. The optimum feed stages for the liquid portion of the feed and for the remaining two-phase part of the feed were determined. The reboiler heat duty (QR) then was increased until the base case separation was matched (see Table 10). When more than ∼400 kmol/h of feed is condensed, it becomes increasingly difficult to reach the desired purities and the additional energy required becomes large. Application of Two-Enthalpy Feed for a Column with Two Feeds of Different Composition. Consider a distillation column with a total condenser and a kettle reboiler that is distilling two saturated vapor feeds. The first feed is F1 ) 400 kmol/h and is composed of 5 mol % methanol and 95 mol % water, and the second feed is F2 ) 600 kmol/h and is composed of 30 mol % methanol and 70 mol % water. The base case for this system is delineated in Table 11. The distillate and bottoms purities were matched to the values obtained for the base case for a 5% methanol feed (xD,M ) 0.9545 and xB,W ) 0.9976) by varying the distillate flow rate. With the optimum feed stages, the final value was D ) 207.5 kmol/h. Calculated column diameters for the two-feed base case are as follows: stage 2 ) 2.77 m, stage 7 ) 2.24 m, stage 8 ) 1.56 m, stage 14 ) 1.55 m, stage 15 ) 0.77 m, and stage 19 ) 0.77 m. Because there is a significant change in calculated column diameter, the methods for controlling vapor velocity can be used to balance the calculated column diameter. The diameter can be decreased by condensing part or all of either feed F1, F2, or both. First, two-enthalpy feed was applied to feed F1 (see Table 11). This results in reasonable decreases
in volume, but with increasing values for energy loads. Note that the percentage increase in QR is from a very small base. Second, two-enthalpy feed was applied to feed F2 (Table 11). Because the flow rate of this feed is higher, more feed can be condensed with a resulting larger decrease in volume. In this case, no increase in energy loads is required, even when all of feed F2 (600 kmol/h) is condensed. Two-enthalpy feed then was used for both of the feeds with all of feed F2 condensed (Table 11). This initially shows some further decrease in calculated column volume, but with an increase in energy loads. Increasing FL,1 to 150 kmol/h increases the energy loads significantly but has no effect on the calculated column area. Because it has a significant reduction in column size and is simple, the best solution for this particular case is probably to condense all of feed 2 (FL,2 ) 600 kmol/h) and leave feed 1 as a vapor. Vapor Feed of Propane and n-Butane Of course, these procedures can be applied to the distillation of mixtures other than alcohols and water. The application of methods to reduce the diameter of columns separating a vapor mixture of propane and n-butane represents a nice contrast to the ethanol-water and methanol-water systems. If the feed is a liquid, the calculated diameter for this system is largest at the bottom of the column. With a vapor feed, the calculated diameter becomes largest at the top plate, although there is little variation in the enriching section. This system is also of interest, because it shows the application of these procedures when the changes in the calculated column diameter is much more modest. Propane, n-Butane Vapor Feed Base Case. A distillation column is separating 1000 kmol/h of a saturated vapor feed that is 50 mol % propane and 50 mol % n-butane. The column has 28 stages plus a partial reboiler and a total condenser (N ) 30). Calculations were performed with the AspenPlus simulator, using the Peng-Robinson VLE correlation. For an external reflux ratio of 2.5, the distillate was 98.96 mol % propane and the bottoms was 98.96 mol % n-butane. Other information is listed in Table 12. The calculated diameters and vapor flow rates are as follows: stage 2 ) 2.57 m (area ) 5.17 m2), V2 ) 1750 kmol/h; stage 13 ) 2.54 m, V13 ) 1520 kmol/h; stage 14 ) 1.74 m, V14 ) 524 kmol/h; and stage 29 ) 1.86 m, V29 ) 546 kmol/h. The variation in column diameter is significant, although not as large as with the alcohol-water systems. Two-Enthalpy Feed and Intermediate Condenser for Propane, n-Butane Vapor Feed. Increasing values of FL were tried for the two-enthalpy feed system (see Table 12). Above a
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Table 12. Results for the Base Case, Two-Enthalpy Feed, and Intermediate Condenser for 50% Propane-50% n-Butane Vapor Feedsa FL
FWithdr
0 (base)
NF,V
NF,L
NV,with
NL,ret
13
200 400 500 600
14 16 18 21 200 400 500 600
diameter, dia (m) 2.57
7 8 8 9
13 14 15 15
2.44 2.37 2.33 2.31 14 15 16 16
7 8 7 7
2.44 2.37 2.33
area, A (m2)
Vol (m3)
Base Case 5.17 91.4 Two-Enthalpy Feeds 4.69 82.9 4.41 78.0 4.26 75.3 4.17 73.7
decrease in Vol (%)
increase in QR (%)
-7705 -7770 -7870 -8105
9.2 14.7 17.6 19.3
0.9 3.2 6.7 14.8
-7710 -7770 -7870
9.2 14.7 17.8
1.1 3.0 6.7
tray with maximum diameter
QR
Qc,total
2
2840
-7685
7 8 8 9
2865 2930 3030 3260 2870 2925 3030
Intermediate Condensers 4.69 82.9 7 4.41 78.0 8 4.25 75.1 7 not possible; stage 16 has no vapor
a Distillate is 98.96% propane, and bottoms is 98.96% n-butane. Pressure is 7.0 atm, tray spacing ) 24 in. (0.6096 m), N ) 30. Notation is the same as that in Table 3.
value of FL ≈ 400 kmol/h, the energy requirements start to increase much more rapidly than the calculated column area decreases. To decrease the column diameter with an intermediate condenser, the liquid withdrawal must be below the feed stage. Because the calculated diameters at the feed stage and at the top of the column are almost identical for the base case, changing the calculated diameter of only stage 2 is not very helpful. The optimum stage for return of the liquid is not absolutely clear. For example, at FL ) 500 kmol/h, stage 7 produces a lower column diameter, but also a lower purity than using stage 8; thus, some judgment was used to select the stage that seemed to be the best compromise. The results for the twoenthalpy feed and the intermediate condenser are almost identical, as long as there is sufficient vapor in the stripping section for the intermediate condenser. The differences in the operation of the intermediate condenser system between the propane-n-butane system and the ethanolwater system for vapor feeds can be explained. For the ethanolwater system, because there is very little vapor below the feed stage, the vapor for the intermediate condenser cannot be withdrawn at a rate of >74 kmol/h below the feed stage. However, drawing vapor from above the feed is still useful for the ethanol-water system (but not as useful as using a twoenthalpy feed), because the calculated diameter at the top is significantly greater than that at the feed stage. Preliminary Cost Estimates To obtain some idea of the savings possible with a twoenthalpy feed, a preliminary cost estimate was made for the base case and for a two-enthalpy feed (FL ) 600 kmol/h) for a plant processing 1000 kmol/h of a 10 mol % ethanol feed (see Table 3). This comparison was chosen because the energy requirements are identical for the two processes and the scale (∼12 million gallons/yr of pure ethanol product for 330 operating days/yr) is appropriate for ethanol mini-plants. For the cost estimate, heat exchangers were designed with assumed values of U ) 300 Btu/(h ft2 °F) ) 1.703 kW/(m2 K) and 150 Btu/(h ft2 °F) ) 0.852 kW/(m2 K) for the reboiler and condensers, respectively (lowest values reported by Greenkorn and Kessler13). The reboilers operate at 2.03 atm and 393.9 K. We arbitrarily assumed that the temperature of the saturated steam was 420 K. For QR ) 902 kW (Table 3), the resulting area is 20.3 m2. The estimated bare module cost of a kettletype reboiler, obtained from standard charts14 with September 2001 data, for both the base case and the two-enthalpy feed systems, was $87 000.
The top condensers were operated at a pressure of 1.796 atm and a temperature of 367.1 K. Cooling water was arbitrarily assumed to be available at 25 °C and was assumed to exit at 50 °C. For the base case (Qc ) -10 700 kW), the required area is 223 m2, and the bare module cost was $169 000 with copper tubes. For the two-enthalpy feed system (Qc ) -3832 kW), the required area is 79.7 m2, and the bare module cost was $127 000. The feed condenser for the two-enthalpy feed system (Qcondense_feed ) -6848 kW) was operated at 2.0 atm and 378.5 K. The resulting area was 118.5 m2, and the bare module cost was $112 000. Because the driving force for heat transfer is larger for the feed condenser, less area is needed per kilowatt. Thus, the total heat transfer area for the two-enthalpy feed system (198.2 m2) is less than the condenser area for the base case. However, because the cost per area is significantly larger for smaller heat exchangers, the total condenser bare module cost for the two-enthalpy feed system ($239 000) is greater than the cost of the base case condenser. To simplify the distillation cost calculation, each column was assumed to be constructed with a constant diameter. The base case and the two enthalpy feed system (FL ) 600 kmol/h) in Table 3 have 34 equilibrium stages. With an estimated overall efficiency of 0.7, this translates to 49 real trays. The estimated costs of the column shell and the sieve trays were obtained from standard charts14 based on September 2001 costs. For the base case, the bare module cost of the vessel was $391 000 and of the sieve trays was $184 000. For the two-enthalpy feed, the bare module cost of the vessel was $180 000 and of the sieve trays was $82 000. The total bare module cost for the column, trays, and heat exchangers for the base case was $588 000 and for the twoenthalpy feed system, this cost was $624 000. These costs are not updated to current costs and do not include installation costs. Assuming that the cost indices and the relative installation costs for the two systems are the same, we took the ratio of the two cost estimates. The cost of the distillation system for the twoenthalpy feed system was 71% of the cost of the base case distillation system. The net result is a substantial savings from the costs related to using the two-enthalpy feed system for new construction. Because dividing the two cost estimates will also cause some of the errors in the cost estimates to cancel, the percentage savings is probably reasonably accurate. Because the variations in cost per volume of the tower and the cost per area of the sieve trays (or cost per volume of packing) and the cost per area of the reboiler becomes significantly less as the column diameter increases, larger ethanol plants will show larger percentage savings than the
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8825
example cost estimate for a mini-ethanol plant. For example, the preliminary estimated cost of a two-enthalpy feed distillation system (including all heat exchangers) for a plant that produces, nominally, 100 million gallons/yr was 57% of the cost of the base distillation system. Discussion and Conclusions The distillation column designed for the ethanol-water separation is considerably simpler than the units used in bioethanol plants. Although a final decision on what distillation system is best will require a detailed economic analysis for the actual column configuration, the projected savings are large enough to justify these additional calculations. Because diameter balancing is a processing technique for distillation, it is generally applicable to any type of equipment used for distillation. Thus, the methods developed in this paper can be applied to different types of trays and different random and structured packings. Although the calculated diameters in different types of equipment will be different, if the conditions of the distillation are the same (feed, L/D, product purities, and so forth), then the ratio of the diameter at location 1 to the diameter at location 2 will be approximately the same for different types of equipment. Thus, the percentage decrease in column volume obtained by applying the diameter balancing techniques will be approximately the same for different types of trays and approximately the same for different types of packing. Because withdrawal and feed lines are installed between packing sections but, in tray systems, are usually installed within the normal tray spacing, the percentage volume reduction may be somewhat less when diameter balancing is used with packed systems than with tray systems. Retrofitting to use a two-enthalpy feed or an intermediate condenser is probably easier with trays than with packed systems. Because successful diameter balancing results in a decrease in column diameter, the liquid loads in the column will increase. After the final design has been determined, a tray rating or packing rating analysis should be done to ensure proper hydraulic behavior. Downcomer backup or liquid holdup in the packing did not seem to be excessive in the runs conducted here; however, minor adjustments in design might be required to accommodate the higher liquid loads. It is interesting to consider the possible causes of changes in the calculated diameter. The most common cause is probably an increase in vapor flow rate. This was illustrated for systems with vapor feed such as that at the vapor feed stage (stage 23) for ethanol-water separation (Table 2) and the further increase in both vapor flow rate and diameter up to stage 2. Increases in liquid flow rate can also increase the calculated diameter, although the effect is usually smaller. This is illustrated for a liquid feed of methanol and water on the feed stage (stage 9) in Table 2, and by the increase in calculated diameter at the liquid feed stage for the two-enthalpy feed systems in Table 4. For example, for the two-enthalpy feed system with liquid flow rate of FL ) 815 kmol/h and the original QR (Table 4), the liquid flow rate increases from 122.4 kmol/h (stage 17) to 935.2 kmol/h (stage 18, the feed stage) while the vapor flow rate is constant at 247.4 kmol/h. The corresponding calculated diameters are 1.00 m (stage 17) and 1.19 m (stage 18), and stage 18 has the largest calculated diameter in this column. Variations in the physical properties can also cause significant changes in the calculated diameter, even when there are no increases in vapor or liquid flow rates.12
If the condenser operates at a sufficiently low temperature, the two-enthalpy feed and intermediate condenser systems may have an additional advantage. Because these systems both condense at higher temperatures than the distillate condenser, it might be possible to use a higher temperature and lessexpensive coolant. If this is possible, the temperature driving forces and heat-transfer areas would be almost the same for all three systems, but the two-enthalpy feed and intermediate condenser systems would save operating costs. Acknowledgment Discussions with Profs. Rakesh Agrawal, Michael Ladisch, and William Luyben were very helpful. Nomenclature A ) column cross-sectional area (m2) D ) distillate flow rate (kmol/h) dia ) calculated column diameter (m) F ) feed rate (kmol/h) FL ) feed rate of liquid in 2-enthalpy feed column (kmol/h) FV ) feed rate of vapor in 2-enthalpy feed column (kmol/h) Fwithdr ) withdrawal rate of vapor stream sent to intermediate condenser (kmol/h) h ) height of packing (m) hF,L ) height of packing for liquid feed (m) hF,V ) height of packing for vapor feed (m) Lj ) liquid flow rate leaving stage j (kmol/h) L1/D ) external reflux ratio (L/D)min ) minimum external reflux ratio L h ) liquid flow rate in stripping section (kmol/h) L′ ) liquid flow rate in middle section of 2-enthalpy feed column (kmol/h) N ) number of trays + condenser + reboiler Nfeed ) optimum feed stage with condenser labeled as stage 1 NF,L ) optimum feed locations for the liquid portion of the feed NF,V ) optimum feed locations for the vapor portion of the feed NL,ret ) optimum location to return liquid from intermediate condenser NV,with ) optimum withdrawal plate for the vapor FWithdr pcond ) condenser pressure (atm) Qc ) condenser duty (kW) Qcondense_feed ) duty to condense all feed (kW) Qc,total ) total condenser duties including intermediate and feed condensers (kW) QR ) reboiler duty (kW) U ) overall heat transfer coefficient (kW/(m2 K)) Vj ) vapor flow rate leaving stage j (kmol/h) V h ) vapor flow rate in stripping section (kmol/h) V′ ) vapor flow rate in middle section of 2-enthalpy feed column (kmol/h) VF/F ) vapor fraction of feed for 2-phase feed Vol ) column volume (m3) Volp ) packing volume (m3) xB,W ) mole fraction water in bottoms product xD,M ) mole fraction methanol in distillate x* E ) liquid mole fraction ethanol in equilibrium with yE ) zE yD,E ) mole fraction ethanol in vapor distillate y* E ) vapor mole fraction ethanol in equilibrium with xE ) zE zE ) mole fraction of ethanol in the feed Greek Symbols ∆p ) pressure drop (psi/tray)
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Subscripts feed_st ) feed stage E ) ethanol M ) methanol W ) water Literature Cited (1) Seader, J. D.; Henley, E. J. Separation Process Principles, 2nd Edition; Wiley: New York, 2006. (2) Wankat, P. C. Separation Process Engineering, 2nd Edition; Prentice Hall PTR: Upper Saddle River, NJ, 2007. (3) Fair, J. R. Gas Absorption and Gas-Liquid System Design. In Perry’s Chemical Engineers’ Handbook, 7th Edition; Perry, R. H., Green, D. W., Eds.; McGraw-Hill: New York, 1997; pp 13-23-13-38. (4) Sloley, A. W. Should You Switch to High Capacity Trays? Chem. Eng. Progress 1999, 94 (1), 23. (5) King, C. J. Separation Processes, 2nd Edition; McGraw-Hill: New York, 1981; pp 704-707. (6) Bannon, R. P.; Marple, S., Jr. Heat Recovery in Hydrocarbon Distillation. Chem. Eng. Progress 1978, 74 (7), 41. (7) Seader, J. D. Distillation. In Perry’s Chemical Engineers’ Handbook, 7th Edition; Perry, R. H., Green, D. W., Eds.; McGraw-Hill: New York, 1997; Section 13.
(8) Liebert, T. Distillation Feed PreheatsIs It Energy Efficient? Hydrocarbon Process. 1993, 72 (10), 37. (9) Ognisty, T. P. Distillation/Energy Management. In Encyclopedia of Separation Science, Vol. 3; Wilson, I. D. (Editor in Chief), Cooke, M., Poole, C. F., Eds.; Academic Press: New York, 1999; pp 1005-1012. (10) Wankat, P. C.; Kessler, D. P. Two-Feed Distillation: Same Composition Feeds with Different Enthalpies. Ind. Eng. Chem. Res. 1993, 32 (12), 3061-3067. (11) King, C. J.; Gantz, D. W. Gantz; Barnes, F. J. Systematic Evolutionary Process Synthesis. Ind. Eng. Chem. Process Des. DeV. 1972, 11, 271. (12) Wankat, P. C. Reducing Diameters of Distillation Columns with Largest Calculated Diameter at the Bottom. Ind. Eng. Chem. Res., in press. (13) Greenkorn, R. A.; Kessler, D. P. Transfer Operations; McGrawHill: New York, 1972. (14) Turton, R.; Baillie, R. C.; Whiting, W. B.; Shaeiwitz, J. A. Analysis, Synthesis and Design of Chemical Processes, 2nd Edition; Prentice Hall PTR: Upper Saddle River, NJ, 2003; Appendix A.
ReceiVed for reView April 20, 2007 ReVised manuscript receiVed August 21, 2007 Accepted September 7, 2007 IE0705554
