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Ind. Eng. Chem. Res. 2001, 40, 314-318
Separation of a Ternary System in a Packed Distillation Column Samuel O. Fasesan* and Elijah A. Taiwo Department of Chemical Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria
A ternary system of methanol-ethanol-water was distilled in a packed distillation column having an internal diameter of 0.10 m. The data obtained indicated that an increase in the intermediate component concentration in the feed mixture reduces the component efficiency of the most volatile component. The data also indicated that the influence of intermolecular interaction on the separation efficiency of the component in the mixture is significant. The results compare favorably with few reliable data in the literature. Introduction There is a growing body of multicomponent distillation work in the literature. Yet, no satisfactory prediction models have emerged. Qureshi and Smith1 reported a breakdown in the theory relating to the equality of the component efficiency when applied to ternary systems, which is not the case for the binary systems. However, for a total reflux condition and for a condition where the concentration change of the three components through the column is small, the number of transfer units for a ternary mixture was presented as
NTUoyi )
YiT - YiB Yi,lm
(1)
where
Yi,lm )
(Y*iT - YiT) - (Y*iB - YiB) Y*iT - YiT ln Y*iB - YiB
{
}
(2)
For a binary system, however, the distillation efficiency in a packed column has been expressed as
NOG )
∫yByTy*dy- y )
KOGaZ ug
(3)
The point value for the performance of a packed column
EOG )
KOGa dy/dz ) y* - y ug
(4)
That is, from eqs 3 and 4
EOG ) NOG/Z
(5)
Toor and Burchard,2 in contrast to Qureshi and Smith, mentioned that (1) a highly efficient plate will tend to yield multicomponent efficiencies equal to each other and equal to the binary efficiencies, (2) as the fraction of the total transfer resistance in the liquid phase increases, the difference among the multicomponent * To whom all correspondence should be addressed.
component efficiencies and the binary efficiency should tend to diminish, and (3) if the molecular diffusion in the gas phase plays a significant part in the transfer process, then the component efficiency will be different for each component and the difference will increase as the difference between diffusion coefficients of the binary pairs increases. If all of the diffusion coefficients are equal, there will be no gas-side interaction. However, if equimolal countercurrent transfer takes place and no liquid-side interaction takes place, then all of the component efficiencies will be equal and equal to the binary efficiency. Medina et al.3-5 commented that plate efficiencies are, in principle, different for the different components in multicomponent systems. Their report states that the mole fraction of the intermediate components can reach a maximum or minimum at intermediate stages of a distillation column. Although this last statement of Medina et al. is in variance with the result earlier published by Chernykh et al.6 showing a consistent drop in concentrations of the three components of a ternary system from the column top section to the column bottom section, the comments of the former authors are generally accepted to be correct for ideal stage contact. Diffusional interaction is recognized by some of the earlier workers2,7,8 as being responsible for the deviation between binary and ternary transfer. Krishna and Standart7 approached the prediction of multicomponent efficiency through the binary efficiency
d ln Vm(I) dYb ) NTU′oy(Yb - Y*b) {Yb - Y*b} dYi dYi
(6)
where Yb is the column vector for the vapor-phase composition, Y*b is the column vector for the vaporphase composition that will be in equilibrium with the corresponding bulk liquid composition, (NTU)′oy is the nonzero rate overall vapor-phase number of the transfer unit, I is the identity matrix, and Vm is the molar volume flow rate. Prausnitz et al.9 published tables of results of intermolecular forces between two identical molecules and between those which are not identical. These tables of results show that the H bonding of the separating species decreases with an increase in the C-C bonds of the molecules. The present study attempts to introduce more information on the effects of diffusional and intermolecular
10.1021/ie9904605 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/02/2000
Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 315 Table 1. NTU Values charge
Qureshi and Smith NTU
pseudobinary NTU′
run
XMe
XEt
NTUoy1
NTUoy2
NTUoy3
NTUoy1
NTUoy2
NTUoy3
1 2 3 4 5 6 7 8 9
0.051 0.051 0.051 0.052 0.052 0.052 0.063 0.063 0.063
0.035 0.035 0.035 0.060 0.060 0.060 0.028 0.028 0.028
1.0044 0.3587 0.8996 0.1820 0.1920 0.3690 0.3380 0.4342 0.5693
0.2814 0.2427 0.1072 0.0580 0.1025 0.0498 0.2325 0.1998 0.3039
0.5501 0.31433 0.3976 0.1045 2.6902 0.1449 0.2276 0.2247 0.2940
1.0041 0.3586 0.8986 0.1811 0.1917 0.3692 0.3380 0.4343 0.5692
0.2814 0.2432 0.1070 0.0880 0.1027 0.0498 0.2353 0.1998 0.3038
0.4459 0.2241 0.3978 0.1050 0.1139 0.1450 0.2280 0.2247 0.2938
refractive index and density composition measurement techniques. A Carl Zeiss refractometer was employed for the refractive index measurements. The prism’s temperature was controlled to within (0.025 °C by water from a constant-temperature bath. The accuracy of the instrument was reported to be 2.0 × 10-5 refractive index units. Graduated lipkin pycnometers of 5 mL nominal volume were employed for the density measurement in a thermostatic control water bath. The accuracy obtained from this method of analysis is on the order of (0.0005 in mole fraction units. All relevant data are listed in Tables 1 and 2. Results and Discussion
Figure 1. Packed distillation column.
interactions on the components in multicomponent systems. A ternary system of methanol-ethanol-water was examined for its component efficiencies, and the extent of the deviation of these efficiencies from equality is explained with respect to their intermolecular forces. Experimental Section A quickfit visible-flow packed distillation column, a product of Coming Process Plant Engineering, Staffordshire, England, was employed in the study. The equipment was designed to have an overall height of 5.70 m, and it occupied a ground space of approximately 1.70 m2. The column inside diameter was 0.l m and was packed with borosilicate Raschig rings of 8 mm nominal diameter to a height of 1.7 m. The reboiler system is made of a boiler-type heat exchanger, fitted externally to a spherical vessel of nominal capacity of about 20 L, in a thermosiphon loop. With the present setup of equipment, steady-state operation was attained within 30 min of operation. A diagrammatic representation of the experimental rig is shown in Figure 1. The analysis of the samples of vapor and liquid taken from the column for their compositions was obtained by
The ternary distillation efficiency for the methanolethanol-water system was determined according to Qureshi and Smith1 and also according to Krishna and Standart7 approaches. The data obtained through the two independent approaches are tabulated in Table 1. The data reveal strong interaction with the components in the separating liquid systems. No significant differences are noted between the data obtained through the two different approaches. This gives credence to the earlier observation of Fasesan.8 In the ternary system examined, however, an increase in the component efficiency of the most volatile component was recorded for a corresponding decrease in the intermediate component concentration in the charge. This is considered to be due to the increase in interaction between the system components. This phenomenon is further examined with respect to the variation of the individual component efficiency of the ternary system with the average concentration of the respective component. Figure 2 illustrates the variation of the component efficiency of methanol with the average methanol composition in the distilling ternary system with the charge composition as the parameter. When the methanol concentration in the charge is 0.051 mole fraction, the component efficiency, EOG, of methanol was found to form a minimum at an average composition of methanol of 0.375 mole fraction, plot 1. When the intermediate component concentration in the charge mixture was almost double its initial concentration, such that it is slightly higher than the methanol concentration in the charge, the component efficiency of methanol described a sharp drop with the average methanol composition in the column, as shown in plot 2. However, when the methanol component concentration in the charge mixture was significantly increased while the intermediate component concentration was substantially reduced, the magnitude of the component of methanol in the distilling mixture shows a notable monotonic rise with the average methanol concentration in the distilling mixture
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Table 2. System Compositions and Flow Rates liquid composition bottom
vapor composition
distillate
bottom
average composition
top
liquid
104 kg s-1
vapor
run
XMe
XEt
XMe
XEt
yMe
yEt
yMe
yEt
XMe
XEt
yMe
yEt
L
V
1 2 3 4 5 6 7 8 9
0.272 0.28 0.30 0.42 0.40 0.39 0.41 0.40 0.39
0.14 0.16 0.172 0.231 0.290 0.224 0.11 0.09 0.08
0.46 0.48 0.392 0.60 0.63 0.58 0.53 0.50 0.48
0.32 0.22 0.298 0.29 0.30 0.296 0.17 0.15 0.13
0.042 0.516 0.290 0.590 0.520 0.540 0.520 0.500 0.490
0.280 0.260 0.230 0.290 0.301 0.270 0.250 0.252 0.250
0.580 0.660 0.480 0.670 0.675 0.680 0.660 0.670 0.690
0.340 0.320 0.250 0.315 0.315 0.280 0.305 0.295 0.305
0.366 0.380 0.346 0.510 0.515 0.485 0.470 0.45 0.435
0.230 0.190 0.235 0.261 0.295 0.260 0.140 0.120 0.105
0.500 0.588 0.355 0.630 0.598 0.610 0.590 0.585 0.590
0.310 0.290 0.241 0.303 0.308 0.275 0.278 0.274 0.278
5.27 6.11 6.93 5.99 6.31 6.91 5.27 6.60 7.98
6.34 7.58 7.69 6.10 6.63 7.05 6.39 6.48 8.11
Figure 2. Variation of the methanol component efficiency with the average column composition of methanol for a methanolethanol-water system.
in the column, plot 3 of Figure 2. These results show that an increase in the concentration of the intermediate component in the distilling ternary mixture reduces the separability of the most volatile component in the mixture. The plots of the EOG magnitude of ethanol with respect to the ethanol composition in the distilling mixture are presented in Figure 3. When the ethanol composition in the charge is less than 70% of the methanol composition present in the charge, the EOG magnitude of ethanol with respect to the ethanol composition in the distilling mixture is reasonably high, plot 1. Keeping the methanol composition in the feed fairly constant and increasing the ethanol composition by 71% of the initial composition in the charge, the EOG magnitude of ethanol with respect to the average ethanol composition in the distilling mixture is drastically reduced by about 62.50% of the highest value recorded for plot 1 and forms a maximum, plot 2. However, when the initial ethanol concentration in the charge was drastically reduced to less than half while the methanol concentration was increased, separability of ethanol from the distilling mixture described a minimum with the average concentration of ethanol in
Figure 3. Variation of the ethanol component efficiency with the average column composition of ethanol for a methanol-ethanolwater system.
the column, plot 3. This lends credence to the earlier observation of Medina et al. on the behavior of the mole fraction of intermediate components of the ternary system in a distillation column. The position of the minimum displayed in plot 1 of Figure 2 with respect to the methanol component in the distilling mixture suggests less severe interaction between the molecules of methanol and ethanol at the prevailing average methanol concentration in the column. Furthermore, in plot 3 of Figure 3, another condition of the minimum is exhibited. In both cases, the concentrations of ethanol in the initial charges for these runs were much lower than methanol concentrations. However, when the ethanol concentration was made higher (almost twice the value of the latter concentrations), the position of the maximum is described. This is depicted in plot 2 of Figure 3 as the ethanol separability with respect to the average ethanol concentration in the column. This result tends to confirm the presence of severe interaction between the molecules of methanol and ethanol in the distilling mixture at the prevailing concentration. These observations are in-line with the recorded values of the surface of interaction for methanol and ethanol, given by
Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 317
molecular interaction within the system. However, when the ternary system contains water as one of the components, a much larger surface is presented for the intermolecular interaction than that available when no water molecule is present. Hence, a marked reduction in the separability of the components in the system is eminent. Conclusion Data from the present investigation have shown that an increase in the charge concentration of the intermediate component reduces the component efficiency of the most volatile component in the system. The thermodynamic properties of the liquid system have been further demonstrated to exhibit s significant influence on the separability of the component in the multicomponent systems. It is also highlighted that although diffusional interaction may be significant in the gaseous state of the separating system, intermolecular interaction within the separating multicomponent system is equally significant and therefore contributes immensely to the reduction in the component efficiency of the systems. Acknowledgment Figure 4. Variation of the water component efficiency with the average column composition of water for a methanol-ethanolwater system.
Prausnitz et al.9 The value of the surface of interaction recorded for methanol is higher than that recorded for ethanol. It is obvious that thermodynamic properties of any pure substance are determined by intermolecular forces which operate between the molecules of that substance. Similarly, thermodynamic properties of a mixture depend on intermolecular forces which operate between the molecules of the mixture. In the latter case, however, intermolecular interaction exists not only between molecules of the same component but also between dissimilar molecules. Hence, the reduction in the separation efficiency of the more volatile component with an increase in the charge concentration of the intermediate component is to be expected, because such an increase in the component charge concentration will only enhance the intermolecular interaction between the dissimilar molecules. Toor and Burchard2 made a simplifying assumption that diffusional interaction is less severe in liquid. These workers overlooked the fact that if diffusional interaction is less severe in liquid, intermolecular interaction is significant. Prausnitz et al.9 reported that intermolecular attractions in alcohols are dominated by the -OH group (hydrogen bonding). For completeness however, in Figure 4, the water separation efficiency was examined with its average composition in the column. Plots 1 and 2 in this figure indicate conditions of maxima, which suggest the presence of severe interaction between the molecules of water and the alcohols. These results also offer support to the earlier report of Prausnitz et al.9 on intermolecular attractions in alcohols. It is therefore not surprising that the present results show marked deviation from the earlier binary system results, where no water molecule was present in the system. The fact that the distilling mixture is a ternary system causes the observed increase in the
This experimental study has been supported by a fund from University Research Committee of Obafemi Awolowo University, Ile-Ife, Nigeria, for which the authors are grateful. Nomenclature I ) identity matrix NTU ) number of transfer units NTU′ ) pseudobinary number of transfer units X ) liquid-phase concentration Y ) vapor-phase concentration lm ) log mean Vm ) molar vapor flow rate Superscripts * ) equilibrium condition ′ ) psuedobinary Subscripts B ) column bottom condition b ) bulk phase i ) i component y ) vapor phase oy ) overall vapor-phase condition T ) column top condition
Literature Cited (1) Qureshi, A. K.; Smith, W. The Distillation of Binary and Ternary mixtures. J. Inst. Pet. 1958, 44, 137. (2) Toor, H. L.; Burchard, J. K. Plate Efficiencies in Multicomponent Distillation. AIChE J. 1960, 6, 202. (3) Medina, A. G.; Ashton, N.; McDermott. C. Murphree and vaporisation Efficiencies in Multicomponent Distillation. Chem. Eng. Sci. 1978, 33, 331. (4) Medina, A. G.; McDermott, C.; Ashton, N. Prediction of Multicomponent Distillation Efficiencies. Chem. Eng. Sci. 1979, 34, 861. (5) Medina, A. G.; Ashton, N.; McDermott, C.; Hansen and Murphree Efficiencies in Binary and Multicomponent Distillation. Chem. Eng. Sci. 1979, 34, 1105. (6) Chernykh, G. N.; Malyusov, V. A.; Malafeev, N. A. Calculation of the kinetics of Fractionation of a Three-component Mixture. Theor. Found. Chem. Eng. 1971, 5, 281.
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(7) Krishna R.; Standart, G. L. A multicomponent film Model incorporating a General matrix Method of Solution to the MaxwellStefan Equations. AIChE J. 1976, 22, 383. (8) Fasesan, S. O. Multicomponent Distillation of Non-Ideal Mixtures. J. Niger. Soc. Chem. Eng. 1985, 4, 67. (9) Prausnitz, J. M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria; PrenticeHall: Englewood Cliffs, NJ, 1986.
(10) Fasesan, S. O.; Sanni, S. A.; Idem, R. O. Effect of Liquid System Properties of Distillation Performance of Binary Systems. J. Sep. Process Technol. 1988, 9, 21.
Received for review June 25, 1999 Accepted July 22, 2000 IE9904605
