Ind. Eng. Chem. Res. 2004, 43, 1279-1283
1279
Simulation of Salt-Containing Extractive Distillation for the System of Ethanol/Water/Ethanediol/KAc. 2. Simulation of Salt-Containing Extractive Distillation Jiquan Fu† Center of Chemical Engineering, Beijing Key Laboratory, Beijing Institute of Clothing Technology, Beijing 100029, P.R. China
The salt-containing extractive distillation for the ethanol/water/ethanediol/KAc system of an industrial plant (2300T/Y) was simulated by SCLCM and an improved Rose relaxation method. The simulation results agreed well with industrial data. The design of the extractive distillation column was optimized under the conditions of different total stage numbers, different feeding locations, and different amounts of mixed agent. The results showed that 20 stages, a feeding location at 9 stages, and an amount of mixed agent of 2.0 kmol/h may be the optimum operation conditions. The simulation method of salt-containing extractive distillation is simple and effective for the system. Introduction Research on the simulation method of salt-containing extractive distillation is important for technology process analysis and the optimum design of the distillation tower. Simulation calaculation can be used conveniently to investigate the influence of various factors on distillation operations. While the experimental data of saltcontaining distillation are important for testing of the simulation method, industrial device operation data can be more convincing. Unfortunately, very limited reports in this field are found so far.1,2 Lei et al.2 reported in detail industrial operation data of salt-containing extractive distillation for the system of tert-butanol/water/ethanediol/KAc. Fu3 reported a simulation method of salt-containing extractive distillation, which has been used successfully in computer simulation for industrial columns of salt-containing distillation for the above system. Duan et al.2 reported the industral operition results of their studies on the system of ethanol/water/ethanediol/KAc; although reported data were inadequate, the comparison of deviation between industrial and simulation data was used to test the simulation method, and an optimum design for salt-containing extractive distillation column I was made in order to analyze the technology conditions of the column. The present simulation work is based on the previous research of a salt-containing vapor-liquid equilibrium (VLE).4 Technology Process of Salt-Containing Extractive Distillation The process of salt-containing extractive distillation is similar to that of conventional extractive distillation, as illustrated in Figure 1. The system here is ethanol (1)/water (2)/ethandiol (3)/KAc (4). Column I is a salt-containing extractive distillation column. A mixture with an azeotrope (ethanol and water) was fed at location F, and the mixed extractive † Tel.: 0086-010-64288287. E-mail:
[email protected].
Fax:
0086-010-64288287.
Figure 1. Industrial process for salt-containing extraction distillation.
agent (ethanediol and KAc) was charged at point S. The pure light component (ethanol) was separated from the mixture with a solvent (ethanediol) by several trays from point S to the top, and the liquid at the bottom (water + ethanediol + KAc) is fed to column II, the extractive agent restoring column. The heavy component of the system with an azeotrope was removed from the top of column II, and the mixed agent (ethanediol and KAc) at the bottom was fed back to column I again. The salt solution of ethanediol and KAc at the bottom of column II can be used again by transporting it in the liquid state after the water was removed. Thus, the difficulty of the transportation and restoration of the solid salt in conventional salt-containing distillation was overcome. Simulation Method The simulation method of conventional multicomponent distillation has been widely reported.5,6 The main equilibrium stage methods are the matrix method, trayby-tray calculation, and the nonstable equation method. The improved Rose relaxation method, a nonstable equation method,6 was adopted because of the existence of salt and the large variance in the boiling points between the components, and it was used in the success
10.1021/ie0308700 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/29/2004
1280 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004
simulation of extractive distillation for the system of tert-butanol/water/ethanediol/KAc.3 The operating equation for the simulation is (K) (K+1) x(K+1) ) x(K) + µj[Vj+1yj+1 + Lj-1xj-1 j j
Vjy(K) - Ljx(K) j j ] (1) However, the operating equations for the condenser, feeding stage, and reboiler are slightly different from eq 1. They are shown as eqs 2-4.
The condenser: (K) (K) (K) ) x(K) x(K+1) 1 2 + µ1[V2y2 - V1y1 - L1x1 ]
(2)
The reboiler: (K+1) (K) (K) ) x(K) x(K+1) N N + µN[LN-1XN-1 - VNyn - LNxN ] (3)
The feeding stage: (K) ) x(K) x(K+1) f f + µf[FZ + Vf+1yf+1 + (K+1) (K) Lf-1xf-1 - Vfy(K) f - Lfxf ] (4)
The flow rates of the vapor and liquid phases at each stage were also obtained by calculating the mixing enthalpy. The enthalpies of the vapor and liquid phase mixture were computed by the ideal solution mixing enthalpies in eqs 5 and 6: C
HVmj )
∑i HVij yVij
HLmj )
∑i HLij xLij
(5)
C
(6)
To calculate the enthalpy by the Cp ∼ t relationship, a simplified salt processing is used: the heat capacity Cp of the salt in the vapor phase is zero; Cp of the salt in the liquid phase was replaced by Cp of the solid salt. The convergence criteria are C N
∑ ∑ i)1 j)2 and N
∑ j)3
|
[
]
x(K+1) - x(K) ij ij x(K+1) ij
|
2
e x
Optimum Design of Column I
V(K+1) - V(K) ij j V(K+1) j
(7)
Simulation of Salt-Containing Extractive Column I. The system is ethanol (1)/water (2)/ethanediol (3)/KAc (4). Column I was simulated under the conditions as reported by ref 1. The height of packing equivalent to one theoretical stage (HEPT) adopted in this work is 0.20 m.7 The initial value of x of each stage is the concentration of the feed, and the initial values of temperature are the linear distribution of the temperature at the top and bottom of the column. The initial values of the flow rates of the vapor-liquid phase are constant molar flow rates. The relaxation factor is U ) 0.10/(FS + F), and the convergence accuracies are x ) 5 × 10-6 and V ) 10-2. The simulation results are shown in Table 1, and the comparison with industrial data for distillation column I is shown in Table 2. The comparison shows that the simulation result agreed with the actual value. This confirms that SCLCM is appliable to the simulation of the distillation column for this quaternary system, and the simplified processing of the enthalpy calculation of salt is convenient. Thus, we can analyze the operating conditions for column I and optimize them by the simulation method. For example, from Table 1 we can see that there is a constant concentration zone within the column where the stages in the zone have no separation effect; therefore, the optimum numbers of the stage may need to be redetermined. Simulation of Extractive Agent Restoring Column II. The system is ethanol (1)/water (2)/ethanediol (3)/KAc (4). The actual column is a packed column, and HEPT adopted in this work is 0.50 m.7 The simulation method and equilibrium model are the same as those previously presented, and the convergence criteria are x e 6.6 × 10-6 and V ) 10-2. The simulation results are shown in Table 3. The comparison with all of the industrial data for distillation column II is shown in Table 4. The comparison of the simulation values with the industrial data shows satisfactory results. Because the simulations of the two columns were carried out independently, there may be a matching problem of column I with column II (that is, the bottom composition of column II is slightly different from the mixed agent feeding composition of column I in the simulation process). Several simulations of the columns were required to adjust the composition, and the simulation calculations of the columns were stopped when ∆x < 0.0001.
e V
(8)
The software was designed and written primarily for simulating classical multicomponent distillation. It was used here directly to simulate the salt-containing distillation columns in this work. Because the salt was regarded as a “solvent” and was nonvolatilizable in the SCLCM method, the input Antoine constants of the salt were A ) -20.0, B ) 0.0, and C ) 0.0 and the capacity of the salt is input using the solid salt’s value; therefore, it was not necessary to change the software.
The results of the previous simulation show that the simulation method is effective for the system; therefore, the optimum design for the columns can be done by the method. As an example, the total stage number, feeding location, and mixed agent amount were investigated using the simulation software. Influence of Different Stage Numbers on the Composition of the Top and Bottom Products in Column I. The feeding stage location was determined by a ratio of the feeding stage number and total stage number in an industrial device (21/28 ) 0.75). Simulation calculations were carried out at different total stage numbers of 36, 32, 28, 24, 20, 16, and 12. Other operation conditions were the same as those in Table
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1281 Table 1. Simulation Results of the Distillation Column Ia stage no.
T (°C)
P (kPa)
V (kmol/h)
L (kmol/h)
x1
x2
x3
x4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
50.00 79.90 90.43 90.70 90.98 91.25 91.52 91.78 92.05 92.31 92.57 92.83 93.08 93.34 93.59 93.84 94.09 94.36 94.67 94.95 90.76 91.97 95.15 102.54 114.50 112.13 160.89 188.85
101.330 106.397 107.531 108.666 109.801 110.926 112.061 113.196 114.331 115.476 116.600 117.725 118.860 119.995 121.130 122.265 123.400 124.542 125.659 126.794 127.929 129.064 130.119 131.334 132.459 133.593 134.728 135.863
0.0 3.64 3.78 3.82 3.82 3.83 3.83 3.83 3.84 3.84 3.84 3.85 3.85 3.85 3.86 3.86 3.86 3.87 3.87 3.87 3.92 3.88 3.86 3.83 3.82 3.86 3.49 2.91
2.03 1.94 4.00 4.00 4.00 4.01 4.01 4.01 4.02 4.02 4.02 4.03 4.03 4.03 4.03 4.04 4.04 4.04 4.05 4.09 5.92 5.90 5.87 5.86 5.90 5.55 5.01 2.25
0.9991 0.9854 0.4924 0.4928 0.4932 0.4935 0.4939 0.4943 0.4946 0.4950 0.4953 0.4957 0.4960 0.4964 0.4967 0.4970 0.4972 0.4967 0.4938 0.4868 0.5908 0.5320 0.3957 0.1942 0.0545 0.0104 0.0011 0.0001
0.0003 0.0003 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0013 0.0015 0.0022 0.0058 0.0214 0.0696 0.1273 0.2607 0.4585 0.5887 0.5210 0.2578 0.1122
0.0006 0.0143 0.4550 0.4547 0.4543 0.4540 0.4537 0.4534 0.4530 0.4527 0.4524 0.4521 0.4518 0.4515 0.4512 0.4509 0.4506 0.4504 0.4498 0.4417 0.3050 0.3059 0.3087 0.3124 0.3220 0.4317 0.7003 0.7966
0.0000 0.0000 0.0513 0.0512 0.0512 0.0512 0.0511 0.0511 0.0510 0.0510 0.0510 0.0509 0.0509 0.0508 0.0508 0.0508 0.0507 0.0507 0.0507 0.0501 0.0346 0.0347 0.0349 0.0350 0.0347 0.0369 0.0409 0.0911
a C ) 4, N ) 28, N ) 21, N ) 3, R ) 1.25, F ) 1.87 kmol/h, T ) 356.15 K, Z F S F F,1 ) 0.8811, ZF,2 ) 0.1189, FS ) 2.00 kmol/h, TS ) 366.15 K, ZS,1 ) 0.0, ZS,2 ) 0.0023, ZS,3 ) 0.8952, ZS,4 ) 0.1025 (for the initial value of xi, Z1 ) 0.4250, Z2 ) 0.0581, Z3 ) 0.4642, and Z4 ) 0.1025), Q1 ) 1.6386 × 105 kJ/h, Qn ) 2.0064 × 105 kJ/h, and U ) 5.5 × 10-3.
Table 2. Comparison of the Simulation Result and Industrial Data for the Distillation Column I T (°C)
composition of the top product
composition of the bottom product
value
top
bottom
x1
x2
x3
x4
x1
x2
x3
x4
obsd calcd
50.00
188.85
0.9950 0.9991
0.0010 0.0003
0.0040 0.0006
0.0000 0.0000
0.0001
0.1122
0.7966
0.0911
Table 3. Simulation Result for Distillation Column IIa stage no.
T (°C)
P (kPa)
V (kmol/h)
L (kmol/h)
x1
x2
x3
x4
1 2 3 4 5 6 7 8 9 10 11 12
30.00 69.68 118.12 128.83 134.22 137.09 138.73 139.78 140.53 141.11 141.19 141.27
13.355 16.000 16.162 16.334 16.497 16.669 16.831 17.003 17.165 17.332 17.500 17.662
0.00 0.88 0.69 0.62 0.87 0.87 0.86 0.86 0.85 0.85 0.85 0.85
0.63 0.44 0.37 2.87 2.87 2.86 2.86 2.85 2.85 2.85 2.85 2.00
0.0009 0.0001 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000
0.9876 0.5311 0.0621 0.0850 0.0536 0.0351 0.0236 0.0162 0.0111 0.0074 0.0046 0.0023
0.0115 0.4688 0.9378 0.8437 0.8749 0.8933 0.9046 0.9120 0.9171 0.9208 0.9236 0.8952
0.0000 0.0000 0.0000 0.0713 0.0715 0.0716 0.0717 0.0718 0.0719 0.0719 0.0719 0.1025
a C ) 4, N ) 12, N ) 4, R ) 2.5, F ) 2.25 kmol/h, T ) 353.15 K, Z F F F,1 ) 0.0001, ZF,2 ) 0.1122, ZF,3 ) 0.7966, ZF,4 ) 0.0911 (the initial value of xi is the same as that of ZF,i), Q1 ) 0.4300 × 105 kJ/h, Qn ) 0.6733 × 105 kJ/h, and U ) 2.222 × 10-2.
Table 4. Comparison of the Simulation Result and Industrial Data for the Distillation Column II T (°C) value obsd calcd
top
bottom
30.00
145.0 141.27
composition of the top product x1 0.0009
x2 0.9876
x3 0.0115
1. Comparisons of the top and bottom products were shown in Table 5. The x1 value of the top and bottom compositions in a 24-stage column agreed with that in a 28-stage column (industrial column). The x1 value in the bottom begins to increase from 20 to 12 stages. From Table 1, as in the above discussion, we can see that there is a constant concentration zone within column I. It is well-known that the constant concentration zone has
composition of the bottom product x4 0.0000
x1
x2
x3
x4
0.0000
0.0050 0.0023
0.8952
0.1025
no separation effect, and maybe 20 stages is enough for the entire column if the feeding location is correct. Influence of Different Feeding Locations on the Composition of the Top and Bottom Products in Column I. Simulation calculations were carried out at different feeding locations of 6, 9, 12, 15, and 18 for a 20-stage column; other operation conditions were the same as those in Table 1. Comparisons of the top and
1282 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 Table 5. Change of the Top and Bottom Compositions at Different Total Stage Numbers top composition
bottom composition
total stage
feed stage
x1
x2
x3
x4
x1
x2
x3
x4
36 32 28 24 20 16 12
27 24 21 18 15 12 9
0.9991 0.9991 0.9991 0.9991 0.9991 0.9991 0.9991
0.0003 0.0003 0.0003 0.0003 0.0003 0.0003 0.0003
0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006
0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0001 0.0001 0.0004 0.0010 0.0026
0.1133 0.1135 0.1122 0.1108 0.1105 0.1105 0.1091
0.7956 0.7953 0.7966 0.7979 0.7980 0.7974 0.7972
0.0911 0.0911 0.0911 0.0911 0.0911 0.0911 0.0911
Table 6. Change of the Top and Bottom Compositions at Different Feeding Stage Locations top composition
bottom composition
total stage
feed stage
x1
x2
x3
x4
x1
x2
x3
x4
20 20 20 20 20
6 9 12 15 18
0.9991 0.9991 0.9991 0.9991 0.9991
0.0003 0.0003 0.0003 0.0003 0.0003
0.0006 0.0006 0.0006 0.0006 0.0006
0.0 0.0 0.0 0.0 0.0
0.0001 0.0001 0.0002 0.0004 0.0015
0.1129 0.1113 0.1100 0.1105 0.1095
0.7959 0.7975 0.7987 0.7980 0.7978
0.0911 0.0911 0.0911 0.0911 0.0911
Table 7. Change of the Top and Bottom Compositions at Different Mixed Agent Amounts (N ) 20 and NF ) 9) top composition
bottom composition
FS (kmol/h)
x1
x2
x3
x4
x1
x2
x3
x4
0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.9987 0.9994 0.9993 0.9991 0.9989 0.9988 0.9986
0.0011 0.0002 0.0002 0.0003 0.0003 0.0003 0.0003
0.0003 0.0004 0.0005 0.0006 0.0008 0.0009 0.0010
0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0001 0.0024 0.0016 0.0001 0.0026 0.0043 0.0054
0.3353 0.1626 0.1322 0.1113 0.0889 0.0732 0.0624
0.5963 0.7525 0.7781 0.7975 0.8153 0.8278 0.8365
0.0683 0.0825 0.0880 0.0911 0.0932 0.0946 0.0957
bottom product compositions were shown in Table 6. The x1 values of the top composition in different feeding locations show no changes, while the x1 values of the bottom composition are increased with the number of the feeding stage. Because ethanol is a key composition, optimum design standards are x1 g 0.9991 in the top composition and x1 e 0.0001 in the bottom composition (see Table 2). The standards are based on the simulation value versus the industrial value [x1 ) 0.9991 (simulation) vs x1 ) 0.9950 (industrial) in the top and x1 ) 0.0001 (simulation) vs x1 ) 0.0 (industrial) in the bottom]. The x1 values of the top and bottom compositions in the 20-stage column (the feeding location is at the ninth stage) agreed with those in the 28-stage column (industrial column). Thus, we believe that a 20stage column (the feeding location is at the ninth stage) is a better optimized design if the mixed agent amount is suitable. Influence of Different Mixed Agent Amounts on the Composition of the Top and Bottom Products in Column I. Operation conditions are the same as those in Table 1, but the stage number is 20 and the feeding stage is at the ninth stage. Simulation results under the conditions of FS ) 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 kmol/h are shown in Table 7. As shown in Table 7, satisfactory products can be obtained from column I when FS ) 2.0 kmol/h. Excessive amounts of mixed agent showed no better effect. In industrial operation, FS ) 2.0 kmol/h, and it is a suitable amount of the mixed agent. To summarize, it is very convenient to use the simulation software to carry out calculations and to analyze the effects of the various factors on the compositions of the top and bottom products. Conclusion The SCLCM method is suitable for the calculation of the VLE and simulation of the salt-containing extractive
distillation system presented in this paper. The simplification for calculating the salt ethalpy may be feasible for simulating the distillation column. Some softwares for calculating the phase equilibrium of the miscible systems and those for simulating the distillation column can be directly used in salt-containing extractive distillation. The simulation method of salt-containing extractive distillation is simple and effective for optimum design. Notation C ) number of components D ) total flow rate of the overhead product F ) flow rate of the feed L ) flow rate of the liquid phase N ) number of equilibrium stages P ) pressure Q ) flow rate of heat R ) actual reflux T ) temperature U ) relaxation factor x ) liquid-phase mole fraction y ) vapor-phase mole fraction X ) liquid-phase mole fraction at KAc, or ethanediol, and/ or KAc + ethanediol free Y ) vapor-phase mole fraction at KAc, or ethanediol, and/ or KAc + ethanediol free Z ) feed mole fraction Superscripts and Subscripts L ) liquid phase V ) vapor phase S ) agent F ) feed
Literature Cited (1) Duan, Z. T.; Lei, L. H.; Zhou, R. Q.; Yeng, J. H.; Qiang, W. H.; Ji, S. F.; Jiang, W. J. Study on Salt-containing Extractive Distillation (I) Preparation of Anhydrous Alcohol Using Ethanediol and KAc. Petrochem. Technol. (China) 1980, 9, 350. (2) Lei, L. H.; Duan, Z. T.; Xu, Y. F.; Qian, W. C.; Zhou, R. Q.; Ji, S. F. Study of Salt-Containing Extractive Distillation (II) Development of Purificatory Process for tert-Butanol. Petrochem. Technol. (China) 1986, 11 (6), 404. (3) Fu, J. Q. Salt-Containing Simulation of Salt-Containing Extractive Distillation. AIChE J. 1996, 42, 3364.
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1283 (4) Fu, J. Q. Simulation of Salt-Containing Extractive Distillation for the System of Ethanol/Water/Ethanediol/KAc. 1. Calculation of the Vapor-Liquid Equilibrium for Salt-Containing System. Ind. Eng. Chem. Res. 2004, 43, 1274-1278. (5) Henley, E. J.; Seader, J. D. Equilibrium-Stage Separation Operations in Chemical Engineering; John Wiley and Sons: New York, 1981. (6) Gao, T. M. Vapor-Liquid Multicomponent Phase Equilibrium and Distillation; Chem. Ind. House: Beijing, 1983.
(7) Editing Committee of Chemical Engineering Handbook. Chemical Engineering Handbook (China); Chem. Ind. House; Beijing, 1991; Part 3.
Received for review August 15, 2003 Accepted December 5, 2003 IE0308700
