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Use of Two Distillation Columns in Systems with Maximum Temperature Limitations Rebecca H. Masel, Dalton W. Smith, and William L. Luyben* Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States ABSTRACT: Maximum temperature limitations are encountered in distillation columns in which thermally sensitive materials are being separated. Fixing the temperature and composition of the bottoms product fixes the column base pressure and, hence, the condenser pressure. The distillate composition at this pressure sets the condenser temperature. In many systems, this temperature is lower than that attainable with cooling water, so expensive refrigeration must be used. Instead of using a single column with a refrigerated condenser, it is sometimes more economical to use two columns. In the first column, pressure is established to permit the use of cooling water (322 K reflux-drum temperature), and a portion of the light component is produced as the distillate stream. The maximum base temperature limitation is achieved by taking a bottoms stream that is a mixture of the light and heavy components. Then this stream is separated in a second column that uses a refrigerated condenser. The rest of the light component is produced as the distillate and the heavy component is produced as the bottoms. This paper presents a quantitative comparison of the two alternative flowsheets. Results show that the two-column process is more economical when two conditions exist: (1) there is a large difference in the normal boiling points between the light and heavy components, and (2) the composition of the feed stream has a significant amount of the light component.

1. INTRODUCTION The design and operating pressures in the majority of distillation columns are set by the desire to use inexpensive cooling water in the condenser. A minimum reflux-drum temperature of about 322 K is usually used to permit a reasonable temperature differential between the cooling water and the process. However, in some distillation columns the pressure must be selected so that a maximum temperature limitation is not violated. If the heavy component is thermally sensitive, the temperature in the base of the column must be kept below some maximum to prevent polymerization, detonation, degradation, etc. The conventional design procedure assumes that the compositions of the distillate and bottoms are given. The base pressure is fixed by a bubble-point calculation using the known bottoms composition and known maximum temperature. The condenser pressure is somewhat lower than the base pressure by the pressure drop through the trays or packing. Then the reflux-drum temperature is determined by a bubblepoint calculation using the known distillate composition and the known reflux-drum pressure. If this reflux-drum temperature is greater than 322 K, inexpensive heat removal by cooling water can be used. However, if a lower temperature is calculated, refrigeration must be used. The above-described procedure applies to a single distillation column that produces a distillate product rich in the light component and a bottoms product rich in the heavy component. An alternative flowsheet uses two distillation columns in series. The pressure in the first column is set so that the reflux-drum temperature is 322 K, and a portion of the light component is removed in the distillate. The composition of the bottoms is adjusted to give a base temperature at this pressure that does not exceed the maximum temperature limitation by dropping some of the light component down into the bottoms. © 2013 American Chemical Society

Thus the bottoms stream is a mixture of light and heavy components, which requires further separation. A second column makes this separation at a lower pressure so that the maximum base temperature limitation is not exceeded when the bottoms is rich in the heavy component. At this low pressure, refrigeration must be used in the condenser to produce a distillate rich in the light component. But the refrigeration load on the second condenser is smaller than in the one-column design. So the higher capital cost of a twocolumn flowsheet is offset by smaller refrigeration costs. Many distillation textbooks discuss vacuum operation. For example, Kister1 gives a qualitative discussion of the effects of varying pressure on process parameters such as base temperature, heating-medium temperature, condenser temperature, coolant temperature, condenser size, column diameter, etc. However, we have not found any references that discuss the alternative two-column flowsheet in systems with maximum base temperatures. This paper studies the two alternative flowsheets to establish under what conditions a two-column process is more economical than a conventional one-column process in distillation systems with maximum temperature limitations.

2. MOTIVATING EXAMPLE An example of a typical system is the separation of a heavy component, toluene diamine [TDA; normal boiling point (NBP) = 557.15 K] from two light components (methanol, NBP = 337.85 K, and water, NBP = 373.15 K). This mixture occurs in the process in which dinitrotoluene is hydrogenated Received: Revised: Accepted: Published: 5172

December 7, 2012 February 14, 2013 March 13, 2013 March 13, 2013 dx.doi.org/10.1021/ie3033735 | Ind. Eng. Chem. Res. 2013, 52, 5172−5176

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Table 1 gives economic information about reboiler energy cost (high-pressure steam) and refrigeration cost (253 K

to form TDA and water in a methanol solvent. The ternary mixture is assumed to be 40 mol % methanol, 30 mol % TDA, and 30 mol % water. Aspen Plus simulations with UNIQUAC physical properties are used for the design calculations. The separation is very easy, so columns with 10 theoretical trays are used and very high product purities are specified: 10 ppm (molar) TDA in the distillate product (methanol and water) and 10 ppm (molar) water in the bottoms TDA product. Feed tray locations are optimized to minimize reboiler duty. A maximum temperature limitation of 480 K is assumed to prevent the detonation of any unreacted dinitrotoluene that could enter the column during upset conditions in the upstream reactor. 2.1. One-Column Process. With a 480 K base temperature and a bottoms composition of essentially pure TDA, the column base pressure must be 0.1128 atm. With 10 trays and a pressure drop of 0.1 psi/tray, the condenser pressure is 0.0448 atm (34 mmHg). At this pressure and a distillate composition of 57.14 mol % methanol and 42.86 mol % water, the refluxdrum temperature is 281.5 K. This is well below the 322 K required for the use of cooling water in the condenser, so refrigeration is required. Using information from Turton et al.,2 we assume a refrigerant at 253 K is used at a cost of $7.89/GJ. The base temperature of 480 K requires the use of high-pressure steam at 527 K and a cost of $9.88/GJ. Figure 1 gives the flowsheet with process conditions. The refrigeration load in the condenser is 0.8296 MW. The reboiler

Table 1. Process and Economic Resultsa one column P (atm) QR (MW) TR (K) QC (MW) TC (K) i.d. (m) shell capital (106 $) HX capital (106 $) total capital (106 $) reboiler energy (106 $/yr) refrig energy (106 $/yr) capital (106 $) energy + refrig (106 $/yr) TAC (106 $/yr) a

two columns, C1

0.0448 0.400 1.011 1.131 480 480 0.8296 0.7826 281.5 322.8 1.425 0.7102 0.1269 0.0604 0.1543 0.1993 0.2812 0.2597 0.3430 0.3524 0.2064 0 System Totals 0.2812 0.3454 0.5494 0.3962 0.6432 0.5114

two columns, C2 0.0042 0.07753 480 0.079 11 304.3 0.7102 0.0604 0.025 32 0.085 74 0.024 16 0.019 68

TDA/methanol/water, Tmax = 480 K.

refrigeration). Capital investment includes the column shell, reboiler, and condenser. The total cost of steam and refrigeration in this one-column design is $549 400/year. Total capital investment is $281 200. Total annual cost (TAC), for a payback period of 3 years, is $643 200/year. 2.2. Two-Column Process. Now we consider the use of two columns instead of one. Figure 2 gives the flowsheet. The pressure in the first column is set at 0.40 atm, which gives a reflux-drum temperature of 322.8 K with distillate composition of 57.46 mol % methanol, 42.54 mol % water, and 10 ppm (molar) TDA (the design specification). Cooling water can be used in the condenser. The duty is 0.7826 MW but is essentially zero cost since inexpensive cooling water is used. A 480 K base temperature is achieved at this higher pressure by making the bottoms composition 5.2 ppm (molar) methanol and 1.264 mol % water instead of high-purity TDA. The column base pressure is 0.468 atm with 10 trays and a pressure drop of 0.1 psi/tray. Note that only a small amount of water is needed to keep the base temperature below the high limit because of the large difference between the boiling point of water and TDA. Energy cost in this column comes only from the high-pressure steam used in the reboiler (1.131 MW), which is only slightly higher than that required in the column in the one-column process. A very small reflux ratio (0.0083) is required. Only a very small amount of water goes out in the bottoms, so the distillate stream contains almost all the methanol and water coming in with the feed. The bottoms of the first column is fed to a second column that operates at 0.045 02 atm in the reflux drum such that the base temperature (with high-purity TDA) is 480 K. The resulting reflux-drum temperature is 304.3 K with the distillate composition of 0.04 mol % methanol, 99.96 mol % water, and 10 ppm (molar) TDA. The flow rate of this distillate is quite small (0.3843 kmol/h). Refrigeration is required to remove the 0.079 11 MW of energy in the condenser. Notice that this refrigeration duty is significantly lower than the 0.8296 MW required in the one-column process. Table 1 give economic information about reboiler energy costs (high-pressure steam) in both columns C1 and C2 and

Figure 1. One-column process: TDA/water/methanol, Tmax = 480 K.

duty is 1.101 MW. The reflux ratio is 0.0042. The column diameter is 1.425 m. Condenser and reboiler areas are calculated with overall heat-transfer coefficients of 0.852 and 0.568 kW·m−2·K−1, respectively. The capital costs of pressure vessels and heat exchangers are taken from Douglas.3 A pressure factor of 1.15 (200 psia), carbon steel construction, and an M/S factor of 1216 are assumed. pressure vessel installed cost = 17640(D1.066)(L0.802)

heat exchanger installed cost = 7296(area)0.65

where D = vessel diameter (meters), L = vessel length (meters), and area = heat-transfer area (square meters). 5173

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Figure 2. Two-column process: TDA/water/methanol, Tmax = 480 K.

refrigeration cost (253 K refrigeration) in C2 only. Capital investment includes the column shells, reboilers, and condensers. The total cost of steam and refrigeration in this two-column design is $396 200/year, which is lower than the $549 400/year in the one-column design. Total capital investment is higher ($345 400) than in the one-column design ($281 200), but TAC is smaller ($511 400/year versus $643 200/year). Therefore, the two-column design is economically superior to the one-column design in this system. Note that in this system the difference in normal boiling points between heavy TDA (557.15 K) and the next component water (373.15 K) is very large. In addition, there is a significant amount of the light components in the feed. In the next sections we explore the effects of these two parameters (boiling point difference and feed composition) on the two alternative flowsheets.

Table 2. Normal Boiling Points and Vapor Pressures at 322 K component

NBP (K)

pressure at 322 K (atm)

nC4 nC5 nC6 nC7 nC8

272.65 309.22 341.88 371.56 398.83

4.754 1.517 0.5138 0.1771 0.062 68

so we would expect the two-column process to be more economical. The first column in Table 3 gives results for the C6/TDA separation in one- and two-column flowsheets. In the onecolumn design, the pressure must be 0.044 92 atm to keep the base temperature at the high limit of 480 K with high-purity TDA bottoms. The resulting condenser temperature (TC) is 268.1 K, so refrigeration is required. Reboiler duty is 0.944 MW, which costs $294 100/year. Refrigeration cost is $126 800/year. Capital investment is $272 800, giving a TAC of $511 800/year. In the two-column design, the first column pressure is 0.5138 atm, which gives a reflux-drum temperature of 322 K for the nC6 component going overhead. The pressure in the second column is 0.044 92 atm, giving a reflux-drum temperature of 268.1 K and requiring refrigeration. The condenser duty in the second column is only 0.0850 MW, so refrigeration cost is only $21 150/year, which is a reduction of $105 650/year compared to the one-column design. Total energy cost in two reboilers is $346 200/year, which is somewhat higher than in the onecolumn design. Capital investment in the two columns is $306 900, which is slightly higher than in the one-column design. Because of the very large reduction of refrigeration costs, the TAC of the two-column design ($469 700/year) is lower than the TAC of the one-column design ($511 800/year). So the two-column design is economically better in the C6/TDA separation, which features a large difference in boiling points between the components.

3. EFFECT OF BOILING-POINT DIFFERENCE The difference in the boiling points between the light and heavy components is an important parameter. The bigger the NBP difference, the bigger the difference between the condenser and reboiler temperatures, which increases the probability that refrigeration is required in the condenser. To explore this issue, binary mixtures of TDA with various paraffinic hydrocarbons are studied. As we move up in carbon number from n-hexane to n-octane, the normal boiling points increase, so the difference between the light component boiling point and the heavy component boiling point becomes smaller. The smaller this difference, the less likely refrigeration is required. Table 2 gives normal boiling points of various normal paraffin hydrocarbons. Also shown is the vapor pressure of the light component at 322 K, which will be the pressure in the column that is producing a distillate stream that is rich in this component and uses cooling water in the condenser. 3.1. nC6 Case. An equimolar mixture of TDA and n-hexane (nC6) is the feed to the system. A maximum temperature limitation of 480 K in the reboiler is assumed. The difference in boiling points is large in this system (557.18 − 341.88 = 215.3 K), which is even larger than that in the TDA/water separation, 5174

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separation even though the difference in boiling points between the components is not a large as in the C6 system. 3.3. nC8 Case. An equimolar mixture of TDA and n-octane (nC8) is the feed to the system. A maximum temperature limitation of 480 K in the reboiler is assumed. The difference in boiling points is smaller in this system (557.18 − 398.83 = 158.4 K). The third column in Table 3 gives results for the C8/TDA separation in one- and two-column flowsheets. In the onecolumn design, the pressure is 0.04476 atm. The resulting condenser temperature (TC) is 315.0 K, which is higher than in the C7 system, but refrigeration is still required. Reboiler duty is 1.181 MW, which costs $455 000/year. Refrigeration cost is smaller ($87 010/year) than in the other systems because the condenser duty (QC) is lower due to the higher reflux-drum temperature. In addition, with a 315 K reflux-drum temperature, the cost of refrigeration is smaller ($4.43/GJ at 285 K) than with the lighter hydrocarbon components. Capital investment is $282 800, giving a TAC of $549 300/year. In the two-column design, the first column pressure is now lower at 0.062 68 atm, which gives a reflux-drum temperature of 322 K for the nC8 component going overhead. The pressure in the second column is 0.044 76 atm, giving a reflux-drum temperature of 315.0 K and still requiring a small amount of refrigeration. The condenser duty in the second column is 0.1184 MW, so refrigeration cost is $16 540/year. Total energy cost in two reboilers is $409 300/year. Capital investment in the two columns is $401 500. In this case the TAC of the two-column design ($559 600/ year) is higher than the TAC of the one-column design ($549 900/year). So the one-column design is economically better in the C8/TDA case. The results of these cases demonstrate that the two-column design is economically better than the one-column design when there is a large difference in the boiling points of the two components. In the TDA−hydrocarbon systems, the difference must be greater than 150 K.

Table 3. Effect of Carbon Number light component nC6 P (atm) QR (MW) TR (K) QC (MW) TC (K) total capital (106 $) energy (106 $/yr) refrig (106 $/yr) TAC (106 $/yr) P (atm) QR (MW) TR (K) xB1(light) QC (MW) TC (K) P (atm) QR (MW) TR (K) QC (MW) TC (K) total capital (106 $) energy (106 $/yr) refrig (106 $/yr) TAC (106 $/y)

nC7

One Column 0.044 92 0.044 82 0.9440 1.056 480 480 0.5096 0.5664 268.1 292.5 0.2728 0.2539 0.2941 0.3290 0.1268 0.1384 0.5118 0.5521 Two Columns, C1 0.5138 0.1771 1.043 1.118 480 480 0.021 27 0.010 87 0.4459 0.5267 322 322 Two Columns, C2 0.044 92 0.044 82 0.0629 0.0604 480 480 0.0850 0.068 64 268.1 292.5 Two Columns 0.3069 0.3344 0.3462 0.3671 0.021 15 0.017 08 0.4697 0.4957

nC8 0.044 76 1.181 480 0.6228 315.0 0.2828 0.3680 0.087 01 0.5493 0.062 68 1.197 480 0.002 65 0.6113 322 0.044 76 0.1164 480 0.1184 315.0 0.4015 0.4093 0.016 54 0.5596

3.2. nC7 Case. An equimolar mixture of TDA and nheptane (nC7) is the feed to the system. A maximum temperature limitation of 480 K in the reboiler is assumed. The difference in boiling points is not as large in this system (557.18 − 371.56 = 185.6 K) as that in the C6/TDA system, so we would expect less of a difference between the one- and twocolumn designs. The second column in Table 3 gives results for the C7/TDA separation in one- and two-column flowsheets. In the onecolumn design, the pressure must be 0.044 82 atm to keep the base temperature at the high limit of 480 K with high-purity TDA bottoms. The resulting condenser temperature (TC) is 292.5 K, which is higher than in the C6 system, but refrigeration is still required. Reboiler duty is 1.056 MW, which costs $329 000/year. Refrigeration cost is $138 400/year. Capital investment is $253 900, giving a TAC of $552 100/year. In the two-column design, the first column pressure is 0.1771 atm, which gives a reflux-drum temperature of 322 K for the nC7 component going overhead. The pressure in the second column is 0.044 82 atm, giving a reflux-drum temperature of 292.5 K and requiring refrigeration. The condenser duty in the second column is only 0.068 64 MW, so refrigeration cost is only $17 080 per year. Total energy cost in two reboilers is $367 100/year. Capital investment in the two columns is $334 400. Because of the very large reduction of refrigeration costs, the TAC of the two-column design ($495 700/year) is lower than the TAC of the one-column design ($552 100/year). So the two-column design is economically better in the C7/TDA

4. EFFECT OF FEED COMPOSITION In all the cases considered above, the feed was a 50/50 molar mixture of light and heavy components. So the amount of light component to be removed as distillate was large. If there is only a small amount of the light component in the feed, we would expect that the one-column design could be more economical. This issue is explored in this section. Feed streams consisting of nC7 and TDA with differing feed compositions are studied, and the economics of the one- and two-column designs are compared. Table 4 gives results for feed compositions (z) ranging from 10 to 50 mol % nC7. As the amount of nC7 in the feed is reduced, the refrigerated condenser duty QC in the onecolumn design decreases quite significantly. With a feed composition of 50 mol % nC7, QC is 0.5664 MW. With a feed composition of 10 mol % nC7, QC is only 0.1242 MW. Energy cost increases slightly as feed composition decreases, but capital investment decreases. Capital investment in the two column system is higher than in the one-column systems for all feed compositions. Refrigeration costs are only slightly smaller except with a 10 mol % feed, where the refrigeration cost of the two-column system is somewhat higher than that of the one-column system. The TAC of the one-column design is smaller than that of the two-column design for feed compositions less than 30 mol 5175

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Table 4. Effect of Feed Composition

Table 5. Effect of Low-Pressure-Drop Packing for nC7 Case z (C7)

0.1 QR (MW) QC (MW) total capital (106 $) energy (106 $/yr) refrig (106 $/yr) TAC (106 $/yr) QR1 (MW) QC1 (MW) QR2 (MW) QC2 (MW) total capital (106 $) energy (106 $/yr) refrig (106 $/yr) TAC (106 $/y)

0.2

One Column 1.156 1.138 0.1242 0.2418 0.2378 0.2448 0.3602 0.3546 0.0309 0.0616 0.4703 0.4964 Two Columns 1.168 1.162 0.1048 0.2168 0.1087 0.096 64 0.1234 0.1098 0.2780 0.2954 0.3978 0.3921 0.033 54 0.027 32 0.5212 0.5180

pressure drop per theoretical tray (psi) 0.3

0.5

1.112 0.3513 0.2522 0.3465 0.087 41 0.5179

1.056 0.5664 0.2539 0.3290 0.1384 0.5521

1.151 0.3238 0.0933 0.1048 0.3092 0.3877 0.026 08 0.5168

1.118 0.5267 0.0604 0.068 64 0.3344 0.3671 0.017 08 0.4957

0.02 P (atm) QR (MW) TR (K) QC (MW) TC (K) Trefrig (K) total capital (106 $) energy (106 $/yr) refrig (106 $/yr) TAC (106 $/yr) P (atm) QR (MW) TR (K) xB1(light) QC (MW) TC (K)

% nC7. These results illustrate that the two-column design is superior only in those cases where significant amounts of light component are present in the feed.

P (atm) QR (MW) TR (K) QC (MW) TC (K)

5. EFFECT OF LOW-PRESSURE-DROP PACKING In all the cases considered above, the pressure drop per theoretical tray was assumed to be 0.1 psi, which is typical for trays. Lower pressure drops could be achieved by using packed columns. A smaller pressure drop through the column means that the condenser pressure is higher because the base pressure is fixed by the maximum temperature limitation and the bottoms composition. The higher condenser pressure raises the reflux-drum temperature, which means less expensive refrigeration is required. The result could be a smaller difference between the one- and two-column processes in terms of economics. To explore this effect, the nC7 case is run with a quite small tray pressure drop of 0.02 psi/tray. In the one-column process, the condenser pressure increases from 0.044 82 atm with 0.1 psi pressure drop to 0.099 25 atm with 0.02 psi pressure drop. The reflux drum temperature increases from 292.5 to 308.7 K. Instead of using 255 K refrigerant at $7.89/GJ, a refrigerant at 288 K can be used (at $4.43/GJ). In the first column C1 of the two-column process, the top temperature (322 K) is fixed, which fixes the pressure (0.1771 atm) for the specified distillate composition. The lower pressure drop through the column means a lower base pressure, so less nC7 needs to be dropped out in the bottoms to keep the base temperature at the 480 K limit. The bottoms composition changes from 1.087 to 0.6394 mol % nC7. In the second column C2, the condenser pressure is higher (0.099 25 atm), which gives a higher reflux-drum temperature (308.7 K) and permits the use of less expensive 288 K refrigerant. Table 5 gives a detailed comparison of the 0.1 and 0.02 psi pressure-drop cases. Refrigeration costs are smaller in the 0.02 psi case. However, the TAC of the two-column process is still smaller than that of the one-column process. The difference in TAC between the two flowsheets is smaller ($33 900/year versus $56 400/year), but the two-column configuration still looks attractive.

total capital (106 $) energy (106 $/yr) refrig (106 $/yr) TAC (106 $/yr)

One Column 0.099 25 1.079 480 0.5386 308.7 288 0.2796 0.3362 0.075 24 0.5046 Two Columns, C1 0.1771 1.112 480 0.006 394 0.5238 322 Two Columns, C2 0.099 25 0.036 64 480 0.041 13 308.7 Two Columns 0.3213 0.3579 0.005 746 0.4707

0.1 0.044 82 1.056 480 0.5664 292.5 253 0.2539 0.3290 0.1384 0.5521 0.1771 1.118 480 0.010 87 0.5267 322 0.044 82 0.0604 480 0.068 64 292.5 0.3344 0.3671 0.017 08 0.4957

6. CONCLUSION The alternative designs of one- and two-column flowsheets have been explored for a number of situations. Results demonstrate that the two-column design is more economical when the difference between the boiling points of the light and heavy components is large (>150 K) and when the feed composition is moderately rich in the light component (>25%).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 610-758-4256. Fax: 610758-5057. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kister, H. Z. Distillation Design; McGraw-Hill: New York, 1992; p 97. (2) Turton, R.; Bailie, R. C.; Whiting, W. B.; Shaelwitz, J. A. Analysis, Synthesis and Design of Chemical Processes, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 2003. (3) Douglas, J. M. Conceptual Design of Chemical Processes; McGrawHill: New York, 1988.

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