complete removal of 02. At still higher temperatures, for ruthenium and platinum with 2500 ppm 02,the remaining CO (200 ppm) after complete removal of O2 is free to react with NO. This is shown b y the subsequent decrease in the X O outlet concentration. For 5000 ppm 0 2 the noble metals exhibited maxima in NO2 production a t temperatures somewhat above the minima in the XO concentration. For 2500 ppm 0 2 , ruthenium and platinum showed similar behavior; however, the NO output concentration of rhodium was negligible above 200"C, and formation of NO2 commenced a t 300°C. The experiments with added 0 2 were usually conducted sequentially in steps of increasing O2 a t a given temperature. With platinum and ruthenium the S O output l e ~ remained l for a short period a t the steady-state value of the previous run; then a gradual decrease in NO conversion occurred. In the temperature range where NO? was formed, a gradual increase in SO2 concentration occurred simultaneously with the decay in NO conversion. I n summary, the results of this study with copper-based catalysts are in general accord with those of Roth and Doerr (1961) mho showed little change in N O reduction by CO a t 0 2 concentration up to nearly the stoichiometric requirement for CO oxidation. With ruthenium, however, a near stoichiometric quantity of 0 2 resulted in a sharp decrease in NO conversion. Nitric oxide conversion on platinum a t 0 2 concentrations of stoichiometric proportions and above, and a t temperatuies below 350"C, is, in a large part, due to other oxides of nitrogen. On the other hand, KO conversion on rhodium
was not affected by 02 concentration u p to the stoichiometric amount. Based on these results, it appears that all the catalysts tested, except for platinum and ruthenium, can be used effectively a t O2 concentration u p to the stoichiometric level. Platinum and ruthenium appear to be limited to operation a t about half the stoichiometric 0 2 level. In addition, to minimize formation of Y20, operation with all catalysts would be most advantageous above 300°C. literature Cited
Baker, R. A., Doerr, R. C., Air Pollzit. Contr. Ass. J., 14, 409 (1964). Rnkpr, R. A., Doerr, R. C., Ind. Eng. Chem. Process Des. Develop., .. 4, 188 (1965). Bernstein, L. S., Raman, A. K. S., Wigg, E. E., presentation to Technical Session, "Engines and Emissions," Central States Section, The Combustion Institute, Ann Arbor, Mich., 1971. Jones, J. H., Kummer, J. T., Otto, K., Shelef, K., Weaver, E. E., Presentation to Joint Meeting of AIChE and Instituto Mexican0 de Ingenieros Quimicos, Control of Nitrogen Oxides Pollution Session, Denver, Colo., 1970. Roth, J. F., Doerr, R. C., Ind. Eng.Chem., 53,293 (1961). Shelef, M., Otto, K., Atmos. Enuiron., 3 , 107 (1969). Shelef, RI., Otto, K., Gandhi, H . , J . Catal., 1 2 , 361 (1968). Souririjan, S.,Blumenthal, J. L., Int. J . Air Water Pollut. (Oxford), 5 , 24 (1961). Stroupe, J. D., J . Amer. Chem. Soc., 71, 569 (1949). Taylor, F. R., Air Pollution Foundation Report S o . 28 (1959). RECEIVED for review July 23, 1971 ACCEPTED December 13, 1971 Work supported by grant no. A P 00913, Air Pollution Control Office, Environmental Protection Agency.
Separation of Benzene and Hexane with Liquid Membrane Technique Navin D. Shah' and Thomas C. Owens2 Department of Chemical Engineering, University of North Dakota, Grand Forks, N.D. 58201
The separation of a mixture of benzene and hexane was studied with a liquid membrane technique. Sparkleen detergent was used as the surfactant, and heavy mineral oil as the solvent to receive the permeates. The separation and permeation mechanisms are discussed with respect to composition of feed, contact time with solvent, ratio of volume of solvent to volume of hydrocarbon feed, ratio of volume of surfactant sobtion to volume of hydrocarbon feed, and concentration of surfactant. The high values of separation factors suggest that the technique could b e more effective than fractional distillation for separating a mixture of benzeneandhexane.
T h e separation and purification of materials are important in many aspects of modern technology. There are continuing efforts to improve the methods or to develop new methods for producing high purity materials. Today, there is heightPresent address, Chemical and Petroleum Refining Engineer-
ing Department, Colorado School of Mines, Golden, Colo. 80401.
T o whom correspondence should be addressed.
58
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1, 1972
ened interest in the separation techniques based on the preferential pasage of one component of a mixture through a membrane. Water purification based on reverre osmosis (Agrawal and Sourirajan, 1969) and the concentration and separation of ions b y Donnan dialysis (Wallace, 1967) have received considerable attentio,l from theoretical as well as from experimentaI points of view.
The discovery of a liquid membrane is remarkable in the history of membrane technology. h liquid membrane is a film formed a t an oil/water interface by a surfactant solution (Li, 1971a). Such films are formed by dispersing t h e solution to be separated in the form of droplet,s in a surfactant solution. The droplets covered with liquid membranes are then cotitacted with an organic solverit phase to receive the permeates. During this process one of the components of the mixiure transfers from the droplets through the liquid membrane aiid into the organic solvent at a faster rate than the other. The organic solverit becomes rich in the more permeable component,, atid t,he droplets become rich in the less permeable component, thus achieving a separation of the components. I n the present, study benzene and hexane were separated with an aqueous i;olutioii of Sparkleen (registered trademark of Calgoti C'orp.) detergent as the surfactant solution and heavy mineral oil as the organic solvent to receive t h e permeates. The normal boiling points of benzene and hexane are 80.1OC and 69OC, respectively, making t,he sel~nrationof these components relatively difficult by fractional distillation. The vapor-liquid eqiiilibrium diagram of the benzene-hexane system is given in Figure 1. The relatively small deviation between the equilibrium curve and the diagonal also indicates that the separation by distillation is relatively difficult. The present study was undertaken t'o investigate the effectiveiies of the liquid membrane technique t'o separate a mixture of benzene and hexane as compared to fractional distillation. I n addition, the effects on separation factor of the several process variables, such as composition of feed, contact time wilh solvent, ratio of volume of solvent to volume of hydrocarbon feed, ratio of volume of surfactant solution to volume of hydrocarbon feed, and concent,ration of surfactant, were studied. When the effect of one variable was studied, all other variables ryere kept constant. Experimental
Reagents. T h e benzene, hexane, slid Sparkleeti surfactant were obtained from Fisher Scietit'ific Co. The benzene was Thiophene free and met, ASTM specificabions. The hexane contained a mixture of isomers wit'h formula weight of 86.11 . The Sparkleen detergent was developed by Hall Laboratories in conjunction with the Mellon Institute of Industrial Research in Pittsburgh, P a , I t cotitained all sodium cations with approximately 50% sodium hexametaphosphate, about 10% anionic sulphonate (40y0 active), and approximately 0.5% nonionic surfactant. It also contained sodium carbonate and sodium bicarbonate as pH buffering agents. The heavy mineral oil wai; of high purity and distributed b y Rexall Drug Co. Procedure. Mixtures of benzene aiid hexane were prepared by mixing the required volumes of the components b y use of a, buret to measure volumes. The mixer for emulsification of this mixture waJ a cylindrical vessel equipped with four baffles. A variable steady speed impeller with four blades was used for mixing. Initially, the mixture of benzene and hexane was stirred a t low speed while the required quantity of surfactant solution was slowly poured into the mixer. The stirrer speed was then increased to 1500 rpm, aiid the mixture stirred a t this speed for about 10 min. The emulsion formed was contacted with the solvent, heavy mineral oil, for t'he desired length of time. During this time the mixture was gently stirred with a hand stirrer a t frequent intervals to insure uniform contact between the emulsion and the solvent. After contacting with solvent, the solution was transferred
I .o
4 *
I0 . 8
0.4 a
6 02
/--C o n t a c t time
..
: 5 min. Curve based on Y = i+
Vopor liquid equilibrium curve
0
02
04
0 6
08
10
M o l e f r o c t l o n o f benzene I n p r o d u c t f r o m e m u l s i o n
Figure 1. Relationship between mole fraction of benzene in solvent and mole fraction of benzene in emulsion
to a separatory funnel where it was allowed to sep:irate for about 5 miii. The mixture separated into three l:iyer.; i n tlie separat~oryfunnel. T h e bottom layer, which was coiisisted of excess aqueous surfactant .solutioii not entered the emulsion phaqe or surfactant solution resulting from droplet breakup. The middle layer ma. coinpohed of the solvelit, heavy mineral oil? contailling the permeated mixture of benzene and hexane. The upper layer coiitnitied hhe emulsion coniposed of the surfactaiit solution and that portion of the misture of benzene and hexane which did not permeate into t,he solvent,. The relative volume. of the three layers depended on esperimeiital condition
