SI
= segment length. in. f , = gas-liquid interfacial temperature.
Literature Cited
"F
t , = inside tube wall temperature, "F L', = overall heat transfer coefficient between liquid film and the cooling water, Btu/hr ft2 "F LTx = overall heat transfer coefficient used to calculate the inside wall temperature, Btu/hr ft2 O F W , = gas flow rate, Ih/hr X = gas mass fraction X = mole per cent conversion Y,O3 = mole fraction of SO3 in gas stream tridecylbenzene viscosity, CP pp tridecylbenzenesulfonic acid viscosity, c P pm = a mixture of p1 and p z , c P PI, = liquid film viscosity. c P
Danckwerts, P. V., "Gas-Liquid Reactions," McGraw-Hill, New York, N. Y., 1970, p 148. Davis, E. J.. David, M. M . , Ind. Eng. Chem., fundarn., 3, 11 1 (1964). Derrig, M. J., Gulf Oil Company Sales Department Brochure, March 3, 1967. Gilbert. E. E., Veldhuis. B., Carlson, E. J., Giolito, S. L., Ind. Eng. Chem.. 45,2070 (1953). Gilliland, E. R., Sherwood, T. K., Ind. Eng. Chem., 26, 516 (1934). Hurjbert. R. C., Knott, R. F., Cheney, H. A , , Soap Chem. Spec., 122, 248 (May 1967a). Hurlbert, R . C.. Knott, R. F., Cheney. H. A . , Soap Chem. Spec., 88, 100 (June 1967b). Jacobsen, R. L., Ohren, T. H., U.S. Patent, 3,531,518 (Sept29, 1970). Johnson, G. R., M.S. Thesis, School of Chemical Engineering, Oklahoma State University, July, 1971 Knaggs, E. A , , etal., U.S. Patent, 3,169,142 (Feb 9, 1965).
Su hscripts
1 = inlet to a segment 2 = outlet from a segment
Received for review September 25, 1972 Accepted August 6 , 1973
Simultaneous Absorption and Chemical Reaction of Butenes Daniel L. Shaffer, Jennings H. Jones, and Thomas E. Daubert" Departmenl of Chemical Engineering, The Pennsylvania State University. University Park, Pennsylvania 16802
Absorption of gaseous isobutylene and 1-butene in 70% trifluoroacetic acid was studied at conditions of industrial significance in small stirred cell and packed column absorbers. Isobutylene absorption rates in the aqueous perfluoro acid conformed to film and penetration theory predictions for absorption accompanied by fast, pseudo-first-order chemical reaction. Depending upon temperature, solute partial pressure, and liquid agitation the presence of this chemical reaction yielded an absorption rate enhancement factor of 30 to 100 relative to the nonreactive 1-butene. Stirred cell experiments yielded specific absorption rates which, when applied to packed column absorption measurements, produced the important design parameter of interfacial contact area effective for mass transfer.
1nt.roduction Operations such as thermal and catalytic cracking produce quantities of isobutylene mixed with other closeboiling Cq olefins and paraffins. The separation of isobutylene from these complex mixtures is normally accomplished by a n absorption-chemical reaction scheme, frequently liquid-liquid extraction with aqueous sulfuric acid. Nonreactive paraffins and secondary olefins are present in the extract only a t very dilute concentrations because of their low solubility in the acid phase. Sulfuric acid processes generally require dilution of the extract phase before isobutylene regeneration to avoid polymerization and subsequently require reconcentration of the acid for recycle to the extractor. Previous work (Fenske and Jones, 1956; Robin, 1967) showed that 70% trifluoroacetic acid reacts preferentially with gaseous isobutylene in a Cq mixture to yield the ester trrt-butyl trifluoroacetate and tert-butyl alcohol. Isobutylene could be regenerated from the acid with heat, requiring no dilution to suppress polymerization. These gasliquid absorptions were performed batch-wise with no attempt at a continuous contacting scheme. Knowledge of the absorption-reaction mechanism and effects of critical processing variables must be obtained before a continuous process can be evaluated and designed. The goal of the present work (Shaffer. 1971) was the experimental confirmation of a quantitative model of absorption and reaction 14
Ind. Eng. Chem., Process Des. Develop., Vol. 13, No. 1, 1974
of gaseous isobutylene in 70% trifluoroacetic acid that incorporates the combined effects of critical system variables and establishes a basis for process design.
Absorption-Reaction Model The carbonium ion type reaction of isobutylene and 70 wt 90 aqueous trifluoroacetic acid to form tert-butyl trifluoroacetate can be considered to be essentially irreversible at the 25" temperature used throughout this study. Above 50" the equilibrium shifts and this assumption cannot be made. The fundamental relations governing simultaneous diffusion and chemical reaction of a dissolved species have been reviewed by Danckwerts (1970). For one-dimensional diffusion of a single species with diffusivity independent of concentration $a
da
D.4= ax? at
+ r.A
The reaction rate term r , is generally a function of solute concentration and of one or more liquid reactant concentrations. If these reactant concentrations vary appreciably, continuity equations for each reactant must be solved simultaneously with eq 1 to obtain the solute concentration profile. However, when the liquid reactants are present in great excess relative to a sparingly soluble dissolving solute, their concentrations can be "lumped" with the second-order reaction rate constant to yield a pseudo-first-
order rate constant k l . The reaction rate term in eq 1 for a n irreversible reaction then becomes
r A= kla
(2)
For this case the film and penetration absorption models predict the same specific absorption rates for fast reactions
(3) Requirements for eq 3 are that there is no gas-phase mass transfer resistance and t h a t the following inequalities are satisfied
D d 0 iZ A * D A > 3 ( m / k d
(4)
The enhancement factor for absorption with chemical reaction is defined as the ratio of the rate of absorption with chemical reaction, to the rate of absorption without chemical reaction ( h , ) for zero initial concentration of solute (isobutylene). If inequalities 4 and 5 are satisfied, this factor approaches ( D , & l / k ~ ) ~ ' ~ . Equation 3 states that the specific absorption rate of the reactive solute is independent of liquid-phase hydrodynamics and depends only upon nonflow physical and chemical system properties. A derivation of this equation for very general flow conditions has been given by Stewart, et al (1970).
Experimental Section Solubility Measurements. Solubilities of isobutylene and 1-butene in 70% trifluoroacetic acid were determined a t 25 and 35" in a bucket pump recirculating liquid-gas contactor immersed in a constant temperature bath. Intimate gas-liquid contacting was achieved by recirculating the aqueous acid through stainless steel wire packing in the gas volume above the liquid level. The sealed contactor was fitted with a thermocouple probe, pressure tap. and gas and liquid charging and sampling valves. The bath temperature was controlled by a mercury thermoregulator having a sensitivity of 0.01"F. For both 1-butene and isobutylene solubility measurements, 75-80 ml of 70% acid was charged to the equilibrium cell immersed in the temperature bath. Solute gas was added to the cell in amounts required to give desired liquid concentrations. Following solute gas addition, acid was recirculated until a constant system pressure was attained indicating equilibrium. For the nonreacting l - b u tene, equilibration times ranged between 1 and 3 min. Gas and liquid samples were then taken and analyzed by gas chromatography. Gas samples were taken in gas-tight syringes and liquid samples in chilled, stoppered vials which were immediately frozen in a Dry Ice-acetone cold bath. Equilibrium solubility measurements for dissolved but unreacted isobutylene in 70% trifluoroacetic acid were complicated by chemical reaction. As isobutylene was added to the equilibrium vessel. it dissolved and was consumed by reaction. Initial reaction rates were too fast to obtain gas and liquid samples in physical equilibrium. Depending upon the amount of isobutylene added to the system. between 4 and 27 hr was required for reaction to slow to a degree such that the isobutylene partial pressure decreased by less than 1.0 mm in 10 min. Since a maximum of 3 min was required for equilibration of the l - b u tene-aqueous acid system. it was assumed that physical equilibrium between gaseous isobutylene and dissolved but unreacted isobutvlene existed a t these low reaction rate levels. Solubilities thus measured are only estimates
of those in 70% acid since a t high isobutylene partial pressures the liquid contained as much as 40% ester and alcohol. Specific Absorption Rate llleasurements. Absorption rates of isobutylene and 1-butene in 70% trifluoroacetic acid were determined in a stirred cell absorber similar to that described by Danckwerts and Gillham (1966). The cell consisted of a thick-walled Pyrex resin flask 21.6 cm tall having an inside diameter of 13.01 cm. A stainless steel stirrer shaft connected by flexible cable to a variable speed motor entered the top of the cell through a mercury seal. At the bottom of this centered shaft was affixed a thin stainless blade that extended to within 0.15 cm of the flask wall at each end. Above the lower blade was mounted a second wider blade that stirred the cell's gas volume. Stirring rates could be varied between 2 and 50 rpm. The stirred cell was designed to allow gas absorption into an agitated liquid with the gas-liquid interfacial area known. This area is simply the cross-sectional area of the cell equal to 133.0 cm2. The lower stirrer blade was set such t h a t it just cut the liquid surface with its lower edge, forming a thin liquid meniscus approximately 0.05 cm high on each side of the blade. With this arrangement stirring rates up to 40 rpm could be attained without rippling the liquid surface or forming a vortex. Stirred cell absorption measurements began by charging 1.25 1. of 70% acid to the cell immersed in a temperature bath. Air was purged from the space above the liquid surface with ten volumes of feed gas, either pure isobutylene or 1-butene or mixtures with nitrogen. Following purging the purge and exhaust lines were shut simultaneously. Pure solute gas was then fed to the cell through a 3-ft copper line immersed in the temperature bath. Solute gas volumes added to the cell were measured by calibrated burets connected to leveling bottles containing saturated salt solutions. As solute gas was absorbed in the stirred cell, the leveling bottle was raised continuously to maintain atmospheric pressure. Gas volumes displaced by salt solution in the buret in a measured period of time represented the total absorption rate at any stirring speed. This rate divided by the known interfacial area in the cell yielded the specific rate of absorption. Packed Column Absorption. The experimental column used in the present study consisted of three &in. sections of 1-in. i.d. stainless pipe joined together by stainless couplings. Each column section was packed with 0.25-in. ceramic Berl saddles supported a t the bottom of each section by stainless wire mesh shaped into inverted cones. These mesh cones redistributed liquid to the lower column sections. Below the bottom packed section was fitted a short length of 1-in. i.d. glass pipe which served as a sight gauge for maintaining a liquid seal at the bottom of the column. Gas and liquid sampling valves were located at the top and bottom of the column with the samples collected and analyzed as described for the solubility measurements. Column temperatures were monitored by thermocouples located at trisecting points in each column section. Pressure was measured a t the top and bottom of the column with a mercury manometer. The liquid used in all column experiments was 70% trifluoroacetic acid fed to the column at flow rates of 10-120 ml/min. Gas phases used were mixtures of isobutylene or 1-butene in nitrogen. The solute gas and nitrogen passed separately through calibrated rotameters and were mixed prior to entering the column.
Results and Discussion Solubility Measurements. Solubilities of isobutylene and 1-butene at 25 and 35" in 70% trifluoroacetic acid are Ind. Eng. Chem., Process Des. Develop., Vol. 1 3 , No. 1, 1974
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