Ind. Eng. Chem. Process Des. Dev. 1903, 22, 80-86
00
Alternative MstMation ConlNgurations for Energy Conservation in Four-Component Separations Abolghasem Elaahl and William L. Luyben' Deparhnent of Chemical Engineedng, Lehigh University, Bethlehem, Pennsylvania 180 15
number of alternative configurations of distillation columns for the separation of fourcomponentmixtures were evaluated on the basis of energy consumption. Sequences of conventional single-feed,two-product columns were compared with nonconventlonal, complex configurations using sequences of prefractionators and sidestream columns. Reductions in energy consumption of 20-40% were obtained from some of the complex configurations compared to the best conventional configurations. The best conflguration was one in which thermodynamic reversibility was more closely approached: preliminary separatlons were made in two prefractionators and the four final-product streams were removed from the last column, which produced two sidestreams, distillate product and bottoms product. A
Introduction The rapid increase of energy prices in the past decade has given motivation to many efforts to conserve energy. Distillation columns are the major energy-consumingunits in the chemical and petrochemical industries. Mix et al. (1978) reported figures regarding the contribution of distillation process in energy consumption. The synthesis of multicomponent distillation systems has been studied by many workers. Most of the work deals with simple configurations (sequences of conventional one-feed, two-product columns). There has been little exploration of more complex configurations (sequences of columns with a t least one column having more than one feed and/or more than two products). To separate N components by distillation, a sequence of N - 1conventional columns is required. These columns can be put together in a number of ways. The required separation energy can be minimized by arranging the columns in an optimized sequence. Three techniques have been developed for synthesis of simple configurations: (1) heuristics or "rules of thumb" which have been established mainly to reduce the number of schemes to be studied, (2) algorithmic techniques in which optimization methods are used, and (3) evolutionary strategies where both heuristics and algorithmic techniques are combined. King (1971) reported four general heuristics. Nishimura and Hiraizumi (1971) proposed a fifth heuristic. The heuristics are as follows: (1)Separations where the relative volatility of the key components is close to unity should be performed in the absence of non-key Components. (2) Sequences which remove the components one by one in column overheads should be favored. (3) Sequences which give a more nearly equimolal division of the feed between the distillate and the bottoms product should be favored. (4) Separations involving very high recoveries should be performed last in a sequence. (5) Separation of a component which is contained in excess in the feed should be performed first in a sequence. Freshwater and Henry (1975) studied these heuristics for several systems of three, four, and five components. They showed that in many cases the optimal configuration could be justified in terms of one of the heuristics. Due to the serial structure of the sequence of distillation columns without heat integration, dynamic programming can be used to optimize the sequences. Hendry and Hughes (1972) used dynamic programming as an algor0196-43O5183I1122-0O80$01.50/0
ithmic technique. Westerberg and Stephanopouloe (1975), Rodrigo and Seader (1975), Gomez and Seader (1976), Seader and Westerberg (1977), and Nath and Motard (1981) are others who developed techniques for the synthesis of simple configurations. To conserve more energy, the idea of heat integration has also received some attention. Because of the feedback of information, the heat exchanger network has a nonserial structure. Rathore et al. (1974a,b) used a combined decomposition and dynamic programming technique to find the optimal network. They gave guidelines for optimal heat stream matching. Freshwater and Ziogou (1976) expanded the work of Rathore et al. to a range of four- and five-component feeds. Morari and Faith (1980) also developed an algorithmic technique to optimize this nonserial problem. Petyluk et al. (1965) were among the first to study complex configurations. They presented several thermodynamically reversible schemes for ternary separations. One of their schemes was discussed by Stupin and Lockhart (1972). Tedder and Rudd (1978) studied in detail both simple and complex configurations for ternary separations. They presented their results in ternary charts which give the expected regions of optimality for different configurations. Doukas and Luyben (1978) studied both simple and complex configurations for BTX (benzene toluene-xylene) separation. For most cases of feed compositions they found either a single sidestream column or a prefractionator/sidestreamcolumn, both complex configurations, to be more economical than the conventional confiiations. They also reported that the dominant part of the total annualcost is the cost of utilities. Stephenson and Anderson (1980) discussed some methods to conserve energy in distillation. They formulated thermodynamic efficiency and lost work in distillation columns and showed that more reversible systems are less energy consumptive. They suggested the use of intermediate condensers and reboilers for energy conservation. Present Study The purpose of this paper is to extend the work of Doukas and Luyben (1978) to four-component systems. Energy consumption of both simple and complex configurations for four-component separations are explored. There are five possible "simple" configurations for a four-component separation, shown in Figure 1. Many "complex" configurations may be proposed for such a 0 1982 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983 81 Configuration 2
Table I. Five Feed Composition Cases Used In Preliminary Studies feed composition
A,B>$
component
case 1
case 2
case3
case4
case 5
A
0.25 0.25 0.25 0.25
0.7 0.1 0.1 0.1
0.1 0.7 0.1 0.1
0.1 0.1
0.1 0.1 0.1
B C D
Ccnfiguration 5
Configuration 4
f l RfiB
PAP
Figure 1. Simple configurations for four-componentseparations. Configuration 7
Configuration 6 r
r
A
W
Configuration 9
4BF
A
I
-LConfiguration D 10 ApfiB
Figure 2. Proposed complex configurations for four-component separations.
separation. Five complex schemes were selected for study on the basis that they appeared to offer advantages over simple schemes due to an increase in thermodynamic reversibility. These schemes are shown in Figure 2. The study was done in two parts: preliminary scouting studies over a range of parameters (particular feed composition) and final study of a specific system.
I. Preliminary Studies A. Basis and Methods. Simplifying assumptions such as equimolar overflow, constant relative volatilities, and a fixed column pressure in all configurations were made in these studies. Four Components, A, B, C, and D, with relative volatilities of 8:4:2:1, respectively, were chosen. Purities of 95% for product streams A and D and 90% for product streams B and C were assumed. A saturated liquid feed of 1814.37 kg-mol/h was sent to the first column of all configurations. Five feed composition cases, given in Table I, were studied. A general steady-state multicomponent distillation program using a modified Wang-Henke (1966) method was written to simulate a column with multiple feeds and multiple sidestreams. For faster convergence, the “theta” method of Holland and Pendon (1974) was used instead
0.7 0.1
0.7
of the normalization method used in the original WangHenke method. The conventional two-product columns were optimally designed using the “short-cut” method. The value 1.1was used for the optimum ratio of actual to minimum reflux ratio. The optimal design of nonconventional multiplefeeds, multiple-products columns was a more complex task. Initially, the sections between each two products were assumed as separate columns. The *short-cut” method modified for the approximate correlation of NT = 2N,, was used to determine the number of trays and feed tray location for each section. The total number of trays, feed tray locations, and sidestream tray locations of each column were then determined by adding the different sections of the column. The final optimum tray numbers and locations were determined by trial-and-error using the rigorous mathematical model of the columns to evaluate several values of tray numbers and locations close to those found by the “short-cut” method. The other major task in the simulation of complex configurations was to determine the optimum values for flow rates and compositions of intermediate streams such that the lowest overall energy consumption was achieved. As an example, consider configuration 8. In the first column of this scheme we have to determine flow rates and compositions of the bottoms product and the overhead product. This gives eight unknowns. However, only four equations of overall material balance can be written, which gives us four degrees of freedom or simply four values to assign to flow rates and/or some compositions. The complexity is even more pronounced in the second column where we have 12 unknown flow rates and compositions and only four equations. This gives a total of 12 parameters for the entire system whose optimum values must be found such that the total energy consumption for the entire system is minimized. Many sets of intermediate flow rates and compositions were tried. The basic objective of the first column in configuration 8 was to separate components A and D. Therefore, it seemed natural to search for an optimum impurities specification of these two components in the two product streams. For the case of equimolal feed composition, for example, a nearly equimolal split of the feed in the first column between distillate and bottoms product with component specification of XD(D) = XB(A) = 0.02 appeared to be the best. The optimum split of feeds in the second column, for this case, turned out to be 37.5% of total feeds flow rate as bottoms product, 37.5% as overhead product, and the remaining 25% of the total feeds as the sidestream flow rate. In the third column, a fourdimensional Newton-Raphson method of convergence was used to converge for four products purities by changing the distillate flow rate, the reflux ratio, and flow rates of two sideatreams. For all other complex configurationsand for different feed compositions, the same approach of trial-and-error was implemented to find the near optimum flow distributions and minimum energy consumptions. Column diameters were determined by assuming a maximum superficial vapor velocity of 0.762 m/s. Overall
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Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1 , 1983
Table 11. Total Heat Duties (GJ/h) of Configurations for Different Feed Composition Cases feed composition configuration case 1 case 2 case 3 case 4 case 5 1 57.05 42.58 49.79 51.39 36.66 2 61.03 59.52 49.08 48.39 3 5.76 3 72.38 80.01 70.93 50.98 35.14 4 67.28 60.7 7 69.97 54.08 32.82 5 62.15 43.50 68.40 52.55 34.13 41.94 33.02 39.27 35.29 27.60 6 7 55.48 40.13 45.14 3 8.94 31.44 38.55 33.94 36.62 27.66 26.37 8 9 63.00 40.34 36.96 35.63 29.61 10 160.3 75.78 37.84 49.21 44.82
D Y
Q
m
*co
I
heat transfer coefficients of 1.533 and 2.044 MJ/h m2 K (75 and 100 Btu/h ft2 OF) were assumed for all condensers and reboilers, respectively. B. Results. 1. Simple Configurations. The total number of trays (20-30 trays for each column), total areas of reboilers and condensers, and column diameters turned out about the same for all configurations. Based on this evidence and according to the results given by Doukas (1976) regarding the dominant contribution of utilities cost in total cost, we decided to make our comparison of schemes based upon only the heat duties of reboilers. Table 11shows the results on total heat inputs of all simple configurations for the five-feed composition cases. Configuration 1 turned out to consume the least energy for three cases of feed composition, favoring heuristics 2 and 5. However, as the composition of heavier components is increased in the feed, configurations 3, 4, and 5 become comparable to configuration 1. For feed composition 5 where heaviest component D is in excess, these configurations are even better than configuration 1 (favoring heuristic 5). Configuration 2, which is the most reversible simple scheme, shows promising results for four-feed composition cases. This led us to explore reversible, complex configurations. 2. Complex Confmtions. All the proposed complex configurations were designed and simulated for the five feed composition cases. Partial condensers were used in most intermediate columns to reduce the heat duties of downstream columns. For example, the heat duty of the second column in confiiation 8, for feed composition case 1,was dropped by 59% when a partial condenser was used in the first column. Three alternatives of configuration 9, depending on which product of the second column was fed to the third column, were also investigated 9a designates the scheme where distillate of the second column is fed to the third column; 9b is the scheme where the sidestream from the second column is fed to the third column; 9c is the scheme where the bottoms product of the second column is fed to the third column. For each feed composition case the three alternatives were tried and the most energy conservative scheme was determined. The same treatment was made for scheme 10 where different products of the fiist column were fed to the second column in loa, 10b and 1Oc. Tables 111-VI1 show the final results of design parameters, flow r a m , and heat duties of complex confiiations for five feed composition cases. Here again the results for column sizes, condenser and reboiler areas are about the same for all configurations (except configuration 10). The major difference among the schemes is energy consumption. Table 11summarizes the heat duties for the five feed compoaition cases. Configuration 8 turned out to be the moat energy conservative scheme for almost all cases. Configuration 6 was the second best for four feed com-
8
d N
8 2
8
rl I
8
h
m
m
m
8 4
3:
53 ri 3
A
Ind.
[email protected]. Process Des. Dev., Vol. 22, No. 1, 1983
m
3
0
m
I
m
4
IN
N
W
83
84
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983
position cases. Special attention should be paid to configurations 6 and 8. They are the only configurations in which final producta are taken only from the last column. The f i t two columns of these configurationshave a lower number of trays and lower reflux ratios, and consequently, lower separation and less heat inputs than the first two columns of other configurations. The third column in these two configurations has more trays, higher reflux ratio, and higher heat duty than those of the third column in the other schemes. However, the total energy consumption of each of these two configurations is less than the energy consumptions of other configurations for most feed composition cases. These results indicate that the optimum sequence for multicomponent distillation columns is that in which components are partially separated in the initial columns, and the final separation of all components into pure product streams is left for the last column. The results given by Doukas and Luyben (1978) for ternary systems are consistent with this conclusion. Configuration 10 does not seem promising except for feed composition case 3, where its lower capital cost makes it the most economical. Table I1 illustrates that for most cases of feed compositions, complex configurations are more energy conservative than simple configurations. The complex configurations may have higher capital cost, but they are well justified by their energy savings. Configuration 8 has savings in the range of 20-40% compared to configuration 1 for different feed compositions. 11. Study of a Specific System
A more definitive comparison was made between configurations 1and 8, the most energy conservative schemes in the two groups, using a specific four-component system: propane, isobutane, normal butane, and isopentane. A feed containing 27.53% propane, 20.34% isobutane, 21.46% n-butane, and 30.67% isopentane was chosen. Feed flow rate was 2000 kg-mol/h (=30000barrels/day) and feed temperature was 322 K. Product purities were the same aa those in the preliminary studies, namely: 95% propane, 90% isobutane, 90% n-butane, and 95% isopentane. Configurations 2 and 5 were included in this study because they had shown close results of energy consumption with configuration 1for this caw of feed composition. Also, configuration 5 could be less energy consumptive than configuration 1 for these particular feed components (heuristic 1). All the simplifying assumptions made in the preliminary studies were removed. Schemes were optimally designed using the same procedures discussed in preliminary studies. But this time changes of liquid flow rates, vapor flow rates, temperature, and pressure across the column were taken into account in the mathematical models of the columns. Liquid enthalpies, vapor enthalpies, and vapor pressure data were all taken from the work of Maxwell (1950) and were curve-fit by appropriate polynomials. Column pressures were set by the bubble point of the distillate at 322 K (120 O F ) if the column was using a water-cooled overhead condenser to produce reflux and a liquid product. If the column was sending a vapor product to a downstream column, the pressure was set at 1.36 atm higher than the downstream column. Configuration 8 happened to have the highest column pressures (all columns over 15 atm). Other schemes also had high-pressure first columns (over 15 atm). These high pressures are undesirable because relative volatilities of hydrocarbons decrease with increasing pressure, making the separation more difficult. Another disadvantage of these high pressure columns is their rather high bottoms temperatures which require
Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983 85
Table VII. Results for Configuration 10 -
~
~
feed composition case 1 (lob)
col. 2
col. 1 QR, GJ/h Qcr GJ/h
AR, ma A,, m3
NT NF NS DIST, kg-mol/h SS,kg-mol/h
RR
-
diameter, m total QR, GJ/h
142.4 17.9 142.4 17.9 2507.9 315.2 1044.9 131.3 40 20 9,31 10 20 463.7 463.9 906.2 21.004 1.765 4.75 1.68 160.3
case 3 (1Oc)
case 2 (loa) col. 1
col. 2
col. 1
55.27 20.51 35.06 40.72 973.4 361.2 257.3 298.8 37 17 25 15 17 1448.1 1324.3 191.3 1.735 1.203 2.96 2.54 75.78
col. 2
32.2 5.64 32.2 5.64 567.1 99.4 236.3 41.4 37 17 15 7 21 118.0 117.5 1404.6 18.556 2.441 2.26 0.94 37.84
case 4 (lob) col. 1 col. 2 36.91 12.30 36.91 12.30 650 216.5 270.8 90.2 37 17 10 5 18 153.2 113.8 1509.9 16.267 6.746 2.42 1.39 49.21
case 5 (lob) col. 1
col. 2
37.29 7.53 37.29 7.53 656.7 132.6 273.6 55.3 37 17 13 9 22 175.3 178.8 302.9 14.245 2.017 2.43 1.09 44.82
0.6998 MPa
20:T
0 , 4 7 3 4 MPo
9 25.23 GJ/hr
54.76 GJlhr
1
Y
&
Figure 3. Final design of configuration 1 (the numbers above streamlines with no units designate flow rates in kg-mol/h and numbers inside the columns designate tray numbers).
Figure 4. Final design of configuration 8 (the numbers above streamlines with no units designate flow rates in kg-mol/h and numbers inside the columns designate tray numbers).
higher pressure steam. One point worthy of mention, however, is the relation between vapor enthalpy and pressure. Vapor enthalpy of hydrocarbons decreases with increasing presaure introducing lower heats of vaporization at higher pressures. This gain of energy trades off with just a small part of the loss of energy due to lower relative
volatilities at high pressures. Different sets of vapor enthalpy data were used for high- and low-pressure columns to find the effect of high pressure heat of vaporization on heat duties of the columns. The heat duty of the first column in configuration 1, for example, was reduced by about 2 GJ/h when high-pressure enthalpy data were used.
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Ind. Eng. Chem. Process Des. Dev., Vol. 22, No. 1, 1983
Table VIII. Results for the Specific Four-Component System configuration 1 configuration 2 QR, GJ/h
GJ/h AR, mz A,, m 2 Qc,
APH, mz NT NF NS RR
diameter. m TB,K TD,
I'D, atm
total QR, GJ/h
col. 1
col. 2
col. 3
col. 1
col. 2
col. 3
31.67 18.71 533.0 1054.2 102.0 20 11
54.76 64.55 473.7 4408.7
25.23 27.34 21 5.3 1972.5
9.28 23.36 89.7 1331.6
18.74 30.29 159.7 1745.5
50 20
20 8
75.12 46.95 1004.2 76 2 136.4 65 30
18 9
19
1.377 1.25 382.4 322.0 15.89
8.041 1.78 354.9 319.5 6.90 111.66
2.169 1.20 354.2 320.6 4.66
3.337 1.89 400.9 347.6 17.25
1.901 1.05 360.9 322.0 15.89 103.14
The feed at 322 K was subcooled for the high-pressure first columns, causing higher heat duties in their reboilers. This problem was overcome to some extent by sending some of the product streams to feed preheaters. In configuration 1, the bottoms product of the third column was sent to the feed preheater. In configurations 2 and 5, bottoms products of the second and third columns were sent to feed preheaters. In configuration 8, the bottoms product and the two sidestreams were sent to feed preheaters providing the highest rise to the feed temperature. See Figures 3 and 4. Table VI11 shows the final results of this study. Sufficient information is provided in this table for those who may want to make economic evaluations of these schemes using their cost evaluation technique and data. The energy consumptions of configurations 2 and 5 are close and both are less than the energy consumption of configuration 1 for this particular feed. Configuration 5 has a relatively low third column base temperature that could make it quite interesting if this scheme was to be part of an integrated plant where a hot stream available in the plant could be used as its heat source. The big energy consumption difference, however, is between configurations 1 and 8 where a 27.5% savings in energy makes the latter remain attractive for this real four-component system as well. Figures 3 and 4 show the final design of configurations 1 and 8. One disadvantage of configuration 8 over 1, however, is its rather higher base temperatures which require higher pressure steam. This fact does reduce the net savings of energy in configuration 8 to some extent, but the scheme remains attractive especially if steam is available for all schemes at 6.8 atm (100 psia). This specific case study showed the same result as preliminary studies regarding the lower energy consumption in configuration 8. However, it revealed something that could not have been observed in preliminary studies: the effect of high pressure on energy consumption. This could happen to be of such magnitude that one decides to choose an optimal, simple configuration in an integrated plant and consume the least energy. Our results, however, are in favor of configuration 8. Our next step in exploring this scheme would be a study of its dynamics and controllability. F u t u r e Work We obviously have explored only a very small portion of an enormous field. Our immediate plans are to study the effects of relative volatilities and product purities on
configuration 5 col. 1 col. 2 col. 3 31.38 23.01 474.8 532.7
44.59 60.87 296.5 4123.0
8
29.74 i9.70 501.2 1110.0 125.3 20 11
20 9
47 22
2.565 1.20 354.1 322.0 4.66
1.499 1.25 382.5 322.0 15.89
1.471 1.20 379.2 338.6 8.26 105.73
7.524 1.64 337.9 319.6 6.90
configuration 8 col. 1 col. 2 col. 3 28.42 8.52 368.4 123.0 256.6 20 9
21.77 24.31 355.4 441.8
30.69 43.10 643.7 2438.2
38 6, 30
79 5, 38, 73 18 15,64 2.0 4.160 0.6 1.23 1.05 1.35 399.8 407.5 414.2 363.5 343.3 322.1 18.64 17.28 15.92 80.88
these results. Relative volatilities may have only a slight effect if the findings of Doukas and Luyben (1978) in three-component systems also apply in four-component systems. Nomenclature A, = condenser area, m2 ApH = total area of feed preheaters, m2 AR = reboiler area, m2 DIST = distillate flow rate, kg-mol/h NF = feed tray N,h = minimum number of trays N S = sidestream tray N T = total number of trays PD = distillate pressure (pressure at the condenser), atm Q, = heat duty of the condenser, GJ/h QR = heat duty of the reboiler, GJ/h RR = reflux ratio SS = sidestream flow rate, kg-mol/h TB = bottoms product temperature, K TD = distillate temperature, K XB(D) = mole fraction of component D in the bottoms product X D ( B ) = mole fraction of component B in the distillate Literature Cited Doukas. N. M. S.Thesis, Lehigh University, Bethlehem, PA, 1976. Doukas, N.; Luyben, W. L. Ind. Eng. Chem. Process Des. D e v . 1978, 17, 272. Freshwater, D. C.; Henry, 8 . D. The C t " . Eng. 1975, 301, 533. Freshwater, D. C.; Ziogou, E. Chem. Eng. J . 1976, 11, 215. Gomez, A.; Seeder, J. D. AI&€ J . 1978, 2 2 , 970. Hendry, J. E.; Hughes, R. R. Chem. Eng. Prog. 1972, 68(6), 71. Holland, C. D.; Pendon, G. P. h)dvcarbon Process. 1974, 5 3 , 148. King, C. J. "Separation Processes"; McGraw-Hill: New York, 1971: Chapter 13. Maxwell, J. B. "Data Book on Hydrocarbons"; D. Van Nostrand Co., Inc.: Prlnceton, 1950. Mix, T. J.; Dweck, J. S.:Welnberg, M.; Armstrong, R. C. Chem. Eng. Prog. 1978, 74(4),49. Morarl, M.; Falth, D. C., 111 AIChE J. 1980, 2 6 , 916. Nath, R.; Motard, R. L. AIChE J. 1981, 2 7 , 578. Nlshlmura, H.; Hlraizumi, Y. Int. Chem. Eng. 1971, 1 1 , 168. Petlyuk, F. V.; Platonov, V. M.; Slavlnski, D. M. Int. Chem. Eng. 1985, 5 . 555. Rathore. R. N. S.;Van Wormer, K. A,: Powers, G. J. AIChE J . 1974a, 20, 491. Rathore, R. N. S.;Van Wormer, K. A,; Powers, G. J. AIChE J . 1974b. 2 0 , 940. Rodrigo, F. R.; Seader, J. D. A I C M J . 1975. 21, 885. SMder, J. D.; Westerberg, A. W. AIChE J. 1977, 2 3 , 951. Stephenson, R. M.; Anderson, T. F. Chem. Eng. Bog. 1980, 76(8),68. Stupln, W. J.; Lockhart, F. J. Chem. €ng. Prog. 1972, 68(10), 71. Tedder, D. W.; Rudd, D. F. AIChE J. 1978, 2 4 , 303. Wang. J. C.; Henke, G. E. m r b o n process. 1968, 45(8), 155. Westerberg, A. W.; Stephanopouios, G. Chem. Eng. Sci. 1975, 3 0 , 963.
Receiued for reuiew November 30, 1981 Revised manuscript received June 3, 1982 Accepted July 8, 1982
