Prediction of Distillation Column Performance for Surface Tension Positive and Negative Systems The effect of wetting. assessed by measurement of the contact angle, under conditions simulating distillation a t total reflux, upon column performance has been investigated for the binary systems methanol-water, n-heptane-toluene, ethanol-water,
n-heptane-methyl
n-propanol-water,
cyclohexane, acetone-water,
benzene-n-heptane, a n d benzene-
cyclohexane. A direct relationship between contact angle and column efficiency was observed for the surface tension positive systems. The negative systems exhibited interfacial instability which resulted in higher efficiencies than would be expected from film area changes produced by mass transfer.
B o t h changes in composition and temperature affect the wetted area of a packing. In distillation it is observed that wetting is promoted by increasing temperatures because of the corresponding reduction in surface tension of the liquid phase. However, when considering a binary liquid, the interchange of components between the liquid and vapor may produce a higher or lower surface tension and thus will either increase or decrease the wetting of the packing. The influence of these surface tension changes has been studied by Zuiderweg and Harmens (1968) who defined systems as positive when the less volatile component of a binary mixture had the higher surface tension, and negative when this component had the lower surface tension. From their experiments they concluded that positive systems gave high degrees of wetting, while for negative systems the liquid films were unstable and rivulet flow occurred in the columns studied. They accounted for this behavior in terms of the Marangoni effect. Inadequate data have been published on column efficiencies even at atmospheric pressure and particularly for systems which exhibit large changes in surface tension for small changes in composition. Norman (1961) has pointed out the difficulty experienced when designing a column using negative systems and it is the purpose of this communication t o illustrate how easily measured contact angles obtained under the appropriate distillation conditions may be used to predict the effect of concentration on efficiency. Values of contact angle have been measured by Ponter et al. (1967a,b) under conditions simulating total reflux (and equilibrium) for the systems methanol-water and n-propanol-water on a carbon surface and recently by Boyes and Ponter (1970) for the systems methanol-water, n-heptane-methyl cyclohexane, acetone-water, n-heptanetoluene, ethanol-water, n-propanol-water, benzene-nheptane and benzene-cyclohexane on a copper surface using the apparatus shown in Figure 1. Figure 2 compares the contact angles obtained for the system n-propanol-water a t total reflux using graphite 140
Ind. Eng. Chem.
Process Des. Develop., Vol. 10, No. 1, 1971
and copper surfaces. The results indicate that the wetting properties of the system are not appreciably affected by the nature of the solid surface provided that the surface energies are high. This is borne out in practice where
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distillation units made of different metals and indeed containing heavy coatings of foreign matter give very similar efficiencies for the same systems. Of course, the geometry of the equipment is important and Figure 3 illustrates how breakdown on a wetted wall column (glass) and disk column (carbon) differ because of the different flow patterns and yet maintain basically the same form. Comparison between the contact angles measured by Boyes and Ponter (1970) and reported column efficiencies by Norman and Binns (1960), Norman et al. (1963), Duncan et al. (1942), Zuiderweg and Harmens (1958), Qureshi and Smith (1958), Sawistowski et al. (1964), and Ponter et al. (1967a,b) of different geometrical systems with high energy surfaces viz random packed, wetted wall and disk columns are made in Figures 4-11. For all the surface tension positive systems examined
,
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it is demonstrated that column efficiencies and the corresponding contact angles are directly related to the liquid composition so that for any two reported values of efficiency at given concentrations the total range may be computed. For surface tension negative systems this simple procedure breaks down because spray formation occurs, as observed by Zuiderweg and Harmens (1958), Sawistowski et al. (1964), and Haselden and Sutherland (1960). Hence in surface tension negative systems the surface area of the liquid film is reduced with increase in contact angle but the effect of the spray formation outweighs this reduction in area. Until this increased area produced by interfacial instability with resulting spray formation can be assessed for a particular negative system there appears little chance of formulating a successful design procedure using these components.
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Figure 2. Comparison of contact angles on high energy surfaces
Figure 4. Methanol-water system (positive) x Contact angles A Norman et al. (1963) (disk column) reflux rote, 0 77 cm'/sec
X Contact angles on copper surface; Boyes and Ponter (1970)
0 Contact angles on graphite surface; Ponter et al. (19670)
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Duncan et 01. (1942) (packed column)
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Figure 5 . n-Heptane-methyl cyclohexane (positive) Figure 3. Minimum wetting rates measured at total reflux for the system n-propanol-water on carbon and glass X
Ponter et ai. (1967b)
0 Norman and Binns (1960)
Contact angles 0 HETP values; Zuiderweg and Harmens (1958) (Vigreux column) 0 HETP values; Zuiderweg ond Harmens (1958) (packed column) A HOG values; Qureshi and Smith (1958) (wetted wall column) X
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
141
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Figure 6. Acetone-water system (positive) x Contact angles 0 Duncan et 01. (1942)
Figure
9.
Ethanol-water
system (positive below azeotrope)
Contact angles
X
0 Duncan et al. (1942) (packed column)
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i:
Figure 7. n-Heptane-toluene system (positive)
x Contact angles A HOG values; Norman et 01. (1963) (disk column) 0 HETP values; Zuiderweg and Harmens (1958) (Vigreux column)
0 HETP values; Zuiderweg and Harmens (1958) (packed column)
Figure 10. n-Propanol-water system (positive below azeotrope, negative above azeotrope) X
Contact angles
0 HOG values; Norman et 01. (1942) (disk column) 0 Minimum wetting rates; Ponter ef ai. 1967b (disk column)
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Figure 1 1. Benzene-n-heptane system (negative) Figure 8. Benzene-cyclohexane system (positive above azeotrope, negative below azeotrope) X Contact angles
0 Sawistowski et a / . (1964)
142
Ind. Eng. Chem. Process Des. Develop., Vo!. 10, No. 1, 1971
Contact angles 0 HOG values; Norman et 01. (1942) (disk column) A HETP values; Zuiderweg ond Harmens (1958) (Vigreux column) 0 HETP values; Zuiderweg and Harmens (1958) (packed column) X
100
literature Cited
Boyes, A. P., Ponter, A. B., J. Chem. Eng. Data, 15, 235 (1970). Duncan, D. W., Koffolt, J. H., Withrow, J. R., Trans. Amer. Inst. Chem. Eng., 38, 259 (1942). Haselden, G. G., Sutherland, J. P., Proc. Int. Symp. Distill., p 27, Brighton, England, 1960. Norman, W. S., “Absorption, Distillation & Cooling Towers,” p 246, Longmans, London, 1961. Norman, W. S., Binns, D. T., Trans. Inst. Chem. Eng., 38, 294 (1960). Norman, W. S., Cakaloy, T., Fresko, A. Z., Sutcliffe, D. H., ibid., 41, 61, (1963). Ponter, A. B., Davies, G. A., Beaton, W. I., Ross, T. K., Int. J . Heat Mass Transfer, 10, 733 (1967a). Ponter, A. B., Davies, G. A., Beaton, W. I., Ross, T. K., Trans. Inst. Chem. Eng., 45, T345 (1967b).
Qureshi, A. R., Smith, W. J., Petrol. Reo. Inst. Petrol., 44, 137 (1958). Sawistowski, H., Smith, W., Ind. Eng. Chem., 51, 915 (1959). Sawistowski, H., Bainbridge, G. S., Stacey, M. J., Theobald, A., Proc. Symp. Distill., p 143, London, 1964. Zuiderweg, F. J., Harmens, A., Chem. Eng. Sci.. 9, 89 (1958). ADRIAN P. BOYES ANTHONY B. PONTER’ Department of Chemical Engineering,
University of New Brunswick, Fredericton, N.B., Canada
’ T o whom correspondence should be addressed. RECEIVED for review October 6, 1969 RESUBMITTED July 20, 1970 ACCEPTED September 14, 1970
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
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