Scott, J. W . . Bridge, A. G . , Advan. Chem. Ser., No. 103 (1971). Scott, J . W . , Patterson, N . J . , World Petrol. Congr., Proc.. 7th. 4, 97 (1967). Stangeland. B . E., Kittrell, J. R.. lnd. Eng. Chem., Process Des. Develo p . , 11, 16 (1972). Usami, H., Hydrocarbon Process.. 103 (1972). Weekman, V , w , , J ~ , lnd, , them,, Process Des, 385 (1969) Weekrnan, V . W., J r . , Nace. D. M . , A l C h E J . . 16, 397 (1970).
Zhorov, Yu. M., e t a / . . Int. Chem. Eng.. 11, 256 (1971).
Received f o r recieu April 27, 1973 Accepted October 9, 1973
Presented a t the 164th National Meeting of the American Chemical Society, Division of Petroleum Chemistry. Kew York, N. Y., Aug 1972.
Separation of Sulfur Dioxide and Nitrogen by Permeation through a Sulfolane Plasticized Vinylidene Fluoride Film Dennis R. Seibel and
F. P. McCandless*
Department of Chemical Engineering, Montana State University, Bozeman, Montana 59715
Vinylidene fluoride plasticized with sulfolane (tetrahydrothiophene 1 , l -dioxide) was found to be an effective membrane for the separation of SO2 from a binary mixture with NP. Depending on permeator conditions, actual separation factors varying from about 30 to 100 and fluxes varying from 0.02 to 1.86 scfd/ft2 were observed for the optimum membrane composition containing about 8.2 wt % sulfolane. Feed gas pressures of 100 to 500 psig were investigated. The membrane selectivity is highly dependent on feed and membrane composition, and on permeator pressure.
Introduction and Background The permeability coefficient of-a gas in a polymeric film is considered to be a function of both solubility and diffusion coefficients. Thus, membrane selectivity may be greatly influenced by the relative solubilities of the components to be separated in the film. In view of this it seemed appropriate to investigate the inclusion of an SO2 solvent as a plasticizer in a relatively impermeable polymer in a study of the separation of SO2 from other gases by gas permeation. A similar technique was recently used t o make a permeation membrane selective for aromatics in the separation of aromatic and naphthenic hydrocarbons by vapor permeation (McCandless, 1973).
Experimental Section The permeation cell and the apparatus for permeability measurement have been described in detail (McCandless, 1972) although the flux measurement procedure was modified somewhat to facilitate the measurement of the permeate which contained u p to about 90% S 0 2 . The permeating gas was allowed to vent to the atmosphere through a l/B-in. nylon tube and through an oil seal while the system reached steady state. To measure the permeation rate, oil was drawn into the calibrated vent tube and the position of the oil-gas interface was timed. Sampling for gas analysis was done using a gas-tight syringe through a silicon rubber septum in the vent line. During a test a feed gas rate 'of about 1.5 scfd was maintained through the high-pressure side of the cell. This rate kept the feed gas composition nearly constant for most runs although a t some conditions the flux and separation factor were such that about 30% of the feed SO2 was removed through the membrane. The feed gas mixtures were made by pressurizing a cylinder from commercial grade (99.9%j gases (Matheson 76
Ind. Eng. Chem., Process
Des. Develop.,Vol. 13, No. 1, 1974
Gas Products). Analysis of the feed and permeate was accomplished with a thermal conductivity gas chromatograph using a Proapak Q-S column (Waters Associates, Inc.). The chromatograph was previously calibrated using known gas mixtures. The accuracy of permeation measurements using a similar apparatus has been reported to be of the order of *5Yc (Stern, et al., 1963).
Membrane Manufacture The membranes were made as follows. A casting solution was prepared by dissolving appropriate amounts of vinylidene fluoride resin (Kynar, Grade 301, Pennwalt Corp.) and sulfolane (Phillips Petroleum Co.) in dimethylformamide. A ratio of dimethylformamide/resin of 5.7 ccjg was used throughout the study. Gentle heating at about 100" aided dissolution. The films were then cast on a 9.5 x 5 x 3/16 in. glass plate between three thicknesses of masking tape by pouring the mixture on the glass plate and distributing it evenly by drawing a glass rod down the plate with the rod resting on the masking tape. This was then placed in an electrically heated oven held a t about 105" for 20 min to evaporate the solvent. The plate was cooled to room temperature before stripping the film from the plate for mounting in the test cell. The resulting films were about 1 mil thick.
Results Vinylidene fluoride resin was chosen as the film material because of its relative impermeability. Hence, it was felt that the membrane selectivity and permeation rate would largely be controlled by the properties of the plasticizing agent because of the high solubility of SO2 (0.65 lb/lb of sulfolane at 20" and 1 atmj in the material. It has
I CELLTEMP
23 .C
'-
i
5m PS
rm
PSI
3w Rl
20
GEVBRANE Figure 1. Preliminary data on S O Z - K ~separation through a sulfolane membrane. Effect of membrane composition on separation and flux. WT
SULF~LALEIN
40 60 CELL TEYlp., "C
80
133
Figure 2. Effect of cell temperature on separation and flux.
been used to recover SO2 from dilute gas streams through absorption (Kirk and Othmer, 1969). Membrane performance was measured in terms of separation factor and permeate flux. The separation factor, analogous to relative volatility in distillation, was defined as
where y = fraction SO2 in permeate and x = fraction SO2 in feed stream. Preliminary work showed that, indeed, a membrane selective for SO2 could be made by including sulfolane as a plasticizer in the vinylidene fluoride film. However, as research proceeded it became evident that reproducibility was rather poor, there being a significant variation in flux and a small variation in separation factor for membranes made under identical conditions. This indicated that small, uncontrollable deviations in casting technique or solvent evaporation procedures made a sometimes significant difference in membrane performance. This problem was never fully resolved although reproducibility became better as technique developed. Nevertheless, the response due to changing variables using individual membranes were consistent and the trends noted in using different membranes were felt to be meaningful. Initial tests were made using a feed gas containing 6% SOz. This was chosen because it represents the approximate composition of flue gas streams available in nonferrous smelters.
Membrane Composition Figure 1 shows the effect of the composition of the casting mixture on a solvent free basis on the separation factor and flux for different cell pressures using a feed gas containing 690 SOz. The exact composition of the resulting film was not determined although it should be close to that of the casting solution considering the low vapor
100
40 20
w
l 3
6
%
6
: 8 .s,.
PERCEhT SO, IN FEED
Figure 3. Effect of feed gas composition on separation and flux
pressure of sulfolane. It is evident from this figure that the membrane performance is highly dependent on both the membrane composition and permeator pressure. A rapid increase in the separation factor was observed a t a composition of about 8% sulfolane especially for the higher cell pressures. Increasing the amount of sulfolane to 10.8% and then to 12.8% resulted in a rapid decline in the separation factor. There was also a rapid increase in the flux a t about 8% sulfolane which tended to level off as the amount of sulfolane was increased to 12.8%. Increasing the sulfolane to 15.2% resulted in a further decline in the separation factor and a rapid increase in the flux. No sepInd. Eng. Chem., Process Des. Develop., Vol. 13, No. 1 , 1974
77
aration was obtained using a membrane containing 20% sulfolane. The membrane behavior in the lower sulfolane concentration range probably results from a complicated SO2 solubility effect which changes with pressure and the amount of sulfolane in the membrane in a nonlinear manner. The decreased membrane selectivity and high flux a t sulfolane compositions greater than 12.8% probably is due to over-plasticization which causes pin-hole leaks. An addition level of 8.2% sulfolane was chosen for further parameter studies. Effect of Permeator Temperature Figure 2 shows the effect of cell temperature on membrane selectivity and flux using a membrane containing 8.2% sulfolane and a feed containing 6% SOz. For the runs made at 14" a container of ice was placed in the constant temperature enclosure to reduce the temperature below room temperature. As can be seen, the separation factor decreased rapidly with increasing temperature while there was a nearly linear increase in flux. This response is probably due to a decrease in the solubility of SO2 in the plasticizing medium with increasing temperature coupled with an increase in the diffusion coefficient. Effect of Feed Composition The effect of feed composition on flux. permeate composition, and separation factor is shown in Figure 3. These data were taken at room temperature using membranes containing 8.2% sulfolane. Data were not obtained using the 12.5% SO2 feed material and 500 psig cell pressure because the gas mixture becomes saturated in SO2 below this pressure. As can be seen, the permeation rate and separation factor are highly dependent on feed composition and this probably indicates a nonlinearity in the concentration dependence of the permeability coefficient
Discussion The behavior of the sulfolane plasticized vinylidene fluoride membrane can probably be adequately explained by the high solubility of SO2 in sulfolane. It would be interesting to compare the premeation data with quantitative data on the solubility of's02 and Nz in sulfolane a t various temperatures and pressures but unfortunately these data are not available. The membrane behavior appears to be very similar to the permeability of organic vapors through plastic films which show a complicated dependence on pressure and concentration owing to a strong interaction between the solute and membrane (Li, et al., 1965). This is in contrast to permanent gases which have permeation coefficients independent of pressure. Thus, a high flux and separation factor results at high pressures and high SO2 concentrations where saturation of the gas with SO2 is approached. Since the diluent is a permanent gas the behavior of the system is probably more complicated than vapor permeation, however. The long-term performance of the plasticized membrane has not been determined and probably deterioration with a decrease in selectivity and flux with time could be expected. From this standpoint the lower temperature would probably be better or. it may be possible to use a heavier sulfone as a plasticizer to advantage. Literature Cited Kirk, R. E., Othmer, D. F., Ed., "Encyclopedia of Chemical Technology," Vol. 19, 2nd ed, Interscience, New York, N. Y., 1969, p 252. Li, N. N., Long, R. B..Henley, E. J., lnd. Eng. Chem.. 57, 18 (1965). McCandless, F. P., lnd. Eng. Chem.. Process Des. Develop., 11, 470 (1972). McCandless, F. P., Ind. Eng. Chem., Process Des. Develop.. 12, 354 (1973). Stern, S. A. Gareis, P. J., Sinclair, T. F., Mohr, P. H.. J. Appl. Polym. Sci.. 7, 2035 (1963).
Received for recierc: June 6, 1973 Accepted September 19, 1973
Drop Size Distributions Produced by Turbulent Pipe Flow of Immiscible Fluids through a Static Mixer Stanley Middleman Chemical Engineering Department. University of Massachusetts. Amherst, Massachusetts 01002
Data are presented for six organic liquids of viscosities ranging from 0.6 to 26 CP and interfacial tensions ranging from 5 to 46 dyn/cm dispersed in water as the continuous phase. The effect of mixer pitch and number of mixing elements is illustrated. Some aspects of Kolmogoroff's theory provide a basis for correlation of the data.
When a mixture of two immiscible fluids is subjected to pipe flow, a dispersion is created which can be characterized by a "drop-size distribution function," @@). From +(D) it is possible to calculate various average drop sizes. For example, the Sauter mean diameter may be defined as -
D32 =
78
1-P
@(D) d D / J m D 2 W D ) d D
(1)
0
Ind. Eng. Chem., P r o c e s s D e s . Develop., Vol. 13,
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The Sauter mean is a particularly useful average since the interfacial area per unit volume can be obtained directly from
A,
=
WID32
(2)
where 4 is the volume fraction of dispersed phase. The drop-size distributions produced by turbulent pipe flow have been the subject of theoretical and experimental
