Ind. Eng. Chem. Res. 1990,29,307-309
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COMMUNICATIONS Sorption of Ethanol and Water by Starchy Materials A simple apparatus for the experimental determination of both the amount and composition of binary vapor sorbed onto solid sorbents is described. Materials composed mostly of starch, such as corn, were found to sorb considerable quantities of both ethanol and water from ethanol/water vapor mixtures a t 90 "C. Selectivity for water is improved by maximizing the ratio of amylopectin to amylose in pure starches. In grits, the selectivity seems to be reduced by grinding. If corn grits are to be used as sorbents for the drying of ethanol, attention must be paid to the recovery of sorbed ethanol. Ethanol has been used for many years as an alternative fuel to petrol. It can be produced from renewable resources such as starch and sugar; however, the product of fermentation is a dilute solution of less than 10% ethanol in water, and for mixing with petrol, the ethanol must be dry. Thus,efficient separation processes to remove water from ethanol are important. The separation problem can be divided into two regions. From 10 to 85 wt % ethanol, the vapor/liquid equilibrium is favorable and distillation is effective. Above 85 w t 70, as the azeotrope at 95.6 wt ?% is approached, distillation becomes costly, requiring high reflux ratios and additional equipment if very dry ethanol is required. Selective adsorption is a promising alternative to distillation for the final drying stage (Serra et al., 1987; Sowerby and Crittenden, 1988). Zeolites (type 3 and 4A) have been used successfully (Garg and Ausikaitis, 1983), they are perfectly selective, but water is very strongly sorbed and high temperatures and/or low pressures are required to regenerate them. Starchy materials have received much attention (Ladisch and Dyck, 1979; Ladisch et al., 1984; Newman et al., 1986) as they are cheap and require only moderate temperatures of 1W120 "C to dry them again after water sorption. Very little data have been published on the simultaneous sorption of ethanol and water, while some workers have reported no ethanol sorption at temperatures of 50-100 OC (Hong et al., 1982; Rebar et al., 1984); other works suggest significant sorption at these temperatures (Hassaballah, 1986). The aim of this work is to experimentally determine the extent of ethanol and water sorption onto a variety of dry starchy materials from vapor mixtures at 90 "C and atmospheric pressure.
Materials and Apparatus Factors that may influence the sorption properties of starch are the relative amounts of amylose and amylopectin (Rebar et al., 1984) and pretreatment such as heat/moisture treatment, mechanical damage, and gelatinization. The sorbents used, see Table I, cover a wide range of these properties. The particle size of the starches was determined by laser scattering (Frock, 1987) using a Malvern Series 2600 droplet and particle sizer and that of the grits was determined by sieve analysis or optical microscopy. The sample of YG16/18 grits used in the sorption experiments was milled and returned to the sorrption tube to form sample "grits M/R". This sample had a wide range of particle sizes but was identical in composition with YG16/18. The sample grits M/S was 0888-5885/90/2629-0307$02.50/0
Table I. Approximate Sorbent Compositiona and Promrtiese amyloapprox mean amylose, pectin, other: particle diam, gelatinsorbent % % % Lcm ized Hylon VI1 70 30 0 12 no starch Amiocastarch 0 100 0 16 no maize starch 27 73 0 19 no cooked corn 19 53 28 240 yes grits YG16/18 19 53 28 850 no grits M/Rc 19 53 28 300 no grits M/Sd 19 53 28 120 no nDry weight basis. Mostly protein, cellulose, fat, and sugars. All the milled sample was returned to the sorption tube. dA large batch of grits was milled and sieved. eSource of composition data: Nation starch and Chemical. Personal communication. Matz S. A. Cereal Science; AVI Publishing: Westport, CT, 1969;p 50. Table 11. Corrected Equilibrium Sorption Results feed vapor, wt % amt sorbed? g/g (&/&)/ sorbent ethanol total water ethanol ( Y J Y . ) maize starch 87.7 0.1005 0.0648 0.0357 12.9 11.4 maize starch 95.9 0.0694 0.0227 0.0467 8.3 Hylon VI1 88.4 0.1021 0.0531 0.0490 7.1 Hylon VI1 95.9 0.0684 0.0159 0.0525 11.4 Amioca 87.8 0.1069 0.0659 0.0413 10.6 Amioca 95.9 0.0714 0.0223 0.0491 9.1 Amioca 99.9 0.0526 0.0005 0.0521 10.3 cooked corn 95.9 0.0711 0.0217 0.0494* 10.9 cooked corn 88.4 0.1042 0.0614 0.0428* 34.2 grits YG16/18 96.3 0.0483 0.0274 0.0209* 31.9 grits YG160/18 88.8 0.0860 0.0657 0.0157* 18.4 grits M/R 88.0 0.0986 0.0705 0.0281* 12.3 grits M/S 96.3 0.0727 0.0233 0.0494* 13.4 grits M/S 88.6 0.1066 0.0677 0.0389 The * means possibly not true equilibrium results.
a fresh sample of YG16/18 milled and sieved to a narrow particle size range. The compositions of the materials reported in Table I1 are typical values and were not determined for the actual samples used. The apparatus used for sorption is shown in Figure 1A. Ethanol/water mixtures were boiled in the flask, producing vapor that was heated to the required temperature in the copper coil before passing up through the small packed bed of approximately 20 g of sorbent. Unsorbed vapor was condensed and returned to the liquid reservoir to maintain an effectively constant ethanol concentration during a run. For desorption, the apparatus was rearranged as shown in Figure 1B. A slow flow of dry helium was heated to the 0 1990 American Chemical Society
308 Ind. Eng. Chem. Res., Vol. 29, No. 2, 1990
conductivity detector (Pirzada and Hills, 1983). A sample of the feed during sorption was taken, for analysis, from below the condenser at the end of the sorption cycle when the sorbent was saturated, and the composition of the vapor did not change across the bed.
Vl
.w Figure 1. Diagram of the apparatus: (A) sorption, (B) desorption. (a) 1-L glass flask; (b) electrothermal mantle; (c) copper coil, 1 m long, 6-mm diameter; (d) glass sorption tube (bed approximately 120 X 25 mm); (e) heated cabinet; (fj water-cooled condenser; (8) cold trap; (h) Dewar filled with liquid nitrogen; (vl and v2) rotaflo taps.
cabinet temperature in the copper coil before passing up through the saturated sorbent. Desorbed vapor was collected in the cold trap which was cooled by liquid nitrogen. Procedure Each sample of sorbent was dried for 4-5 h at 105-110 "C with nitrogen, having a water content of less than 0.1 wt 70,and then conditioned by one sorption/desorption cycle at 90 "C before the first sorption results were recorded for that material. For all sorption runs, the cabinet temperature was maintained at 90 "C, well above the dew point of the wettest feed, and the sorption tube was always allowed to warm up to the cabinet temperature before the vapor flow was started. The time for the sorbent sample to warm up was established before the firt run by monitoring the temperature in the center of the bed after the sorption tube was placed in the hot cabinet. At intervals of several hours during a run, the sorption tube was isolated at valves V1 and V2, removed from the hot cabinet, and allowed to cool to room temperature before it was weighed. Equilibrium was assumed to have been reached when two successive weight readings differed by less than 0.01 g. At the end of a sorption run, the void space in the sorption tube and the lines above and below it were full of vapor. The lines were flushed with dry helium before the start of desorption, but the vapor in the void space of the bed was collected in the cold trap, weighed, and analyzed together with the desorbate. A correction was made for the vapor in the voids of the bed by calculating the amount of ethanol and water in the void volume, assuming the ideal gas equation to apply. The void volume of the bed was calculated by subtracting the volume of sorbent (estimated from its mass and specific volume) from the total volume of the sorption tube. A correction was also made for the difference in density between the air filling the cold trap when it was weighed before a run and the helium filling it at the end of desorption. All ethanol/water samples were analyzed by gas chromatography on a Porapak Q column using a thermal
Results and Discussion The mass balance calculated for each run confirmed that almost all the material desorbed (>98%) was collected in the cold trap. The equilibrium sorption results, summarized in Table 11, have been corrected by subtracting the mass of vapor in the void space from the total weight gain as described above. Under the experimental conditions, no significant capillary condensation occured. This was confirmed by following the usual sorption-desorption procedure with nonporous glass spheres of 250-pm diameter as the packing in the sorption tube. The weight gain during this sorption was negligible compared to the weight gain with the starchy materials. Although the materials tested preferentially sorbed water, all still sorbed significant quantities of ethanol. The total amount sorbed from a particular vapor concentration was similar for all the materials apart from the YG16/18 grits, which sorbed less. To compare the selectivity of the sorbents, it is convenient to define a separation factor:
Xw/Xe Yw/ Ye where X, and Yware, respectively, the mole fractions of water in the sorbed and vapor phases at equilibrium. X e and Ye are the corresponding ethanol mole fraction. The ranking of the sorbents according to their a values is grits YG16/18 >> Grits milled > maize starch > Amioca = cooked corn > Hylon VII. Although the selectivities of all the pure starches were similar, the maize starch and Amica starch (both high in amylopectin) were more selective than the high amylose starch, Hylon VII. This trend was also observed by Rebar et al. (1984) using inverse gas chromatography. The effect of gelatinization is best seen by comparing the cooked corn with the grits M/R which were similar in composition and particle size. The gelatinized corn had a slightly greater total sorption capacity but was much less selective for water. Measurement of the X-ray spectra of the samples of starch and grits confirmed that they were not gelatinized during the experiments as would be expected from the low moisture content and moderate temperatures encountered. The most striking feature of the sorption results is that the selectivity for water of the YG16/18 grits was so much greater than that of the other sorbents. The amount of ethanol sorbed by the grits is a strong function of the particle size, while the amount of water sorbed is not. This is not due to a change in the composition of the grita, which could be caused by sieving the wide range of particle sizes that are produced by milling, as the M/R grits sample is identical with the YG16/18 sample in all but particle size, yet it has a different selectivity. The results can be explained if it is assumed that the water vapor penetrates easily to the center of the particles while the much slower diffusing ethanol vapor is sorbed in a thick shell of uniform ethanol concentration, leaving the core of the large particles not yet reached by the ethanol front at the end of sorption. That the amount of ethanol sorbed by the M/S grits is similar to that sorbed by the very small maize starch grains suggests that the maximum depth of ethanol penetration during the run times used (up to 12 h) is approximately 60 hm, the average particle radius of the cy=-
Ind. Eng. Chem. Res. 1990,29, 309-312
Table 111. Results of NonequilibriumRuns for Amioca Starch Corrected for the Void Vapor feed vapor, wt%
ethanol 87.8 87.7 88.0
desorbate, run time, min 300 30 10
total amt
wt%
sorbed, g/g 0.1069 0.0657 0.0305
ethanol 38.7 44.5 49.3
M/S grits. A simple “shrinking coren model of ethanol sorption like this gives good qualitive agreement with the experimental results. According to this model, the ethanol sorption figures for particles larger than 120-pm diameter are not true equilibrium values but are ddfusion controlled; these values are marked with an asterix in Table 11. In an attempt to determine whether kinetic factors could account for the difference between the quantity of sorbed ethanol observed in this work and the negligible ethanol sorption reported by Hong et al. (1982), for experiments with much shorter run times but similar particle size, a number of runs with Amioca starch were stopped after 30 or 10 min, well before the attainment of equilibrium, which required 3-4 h under these conditions. Table I11 summarizes these results and includes one equilibrium run for comparison. There is a slight tendency for the ethanol concentration in the desorbate to rise as the run time is reduced. This suggests that the sorbents would be even less selective when used in the fixed bed sorption process, which is stopped at breakthrough before all the bed has reached equilibrium with the vapor. A detailed investigation of the kinetics of both ethanol and water sorption from mixed vapor is needed to properly explain the surprising selectivity of the YG16/18grits and also to identify the way in which starchy materials could be best used to dry ethanol. Acknowledgment The starch samples used in this work were kindly supplied by National Starch and Chemical, Manchester, UK. The grits were supplied by Smiths Flour Mills Limited,
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Worksop, U.K. Thanks also to J. M. Blanchard, who carried out the X-ray analysis of the starch samples. Registry No. Starch, 9005-25-8; amylose, 9005-82-7; amylowater, 7732-18-5; ethanol, 64-17-5. pectin, 9037-22-3;
Literature Cited Frock, H. N. Particle Size Determination Using Angular Light Scattering. In Particle Size Distribution Assesment and Characterization; Provder, T., Ed.; American Chemical Society: Washington, DC, 1987;pp 146-166. Garg, D. R.; Ausikaitis, J. P. Molecular Sieve Dehydraton Cycle for High Water Content Streams. Chem. Eng. B o g . 1983, 79 ( 4 ) , 60-65. Hassaballah, A. A. Drying of Alcohol by Adsorption. Ph.D. Dissertation, Nottingham University, Nottingham, U.K., 1986. Hong, J.; Voloch, M.; Ladisch, M. R.; Tsao, G. T. Adsorption of Ethanol-Water Mixtures by Biomass Materials. Biotechnol. Bioeng. 1982,24,725-730. Ladisch, M. R.; Dyck, K. Dehydration of Ethanol: New Approach Gives Positive Energy Balance. Science 1979,205,896900. Ladisch, M. R.; Voloch, M.; Hong, J.; Blenkowski, P.; Tsao, T. Cornmeal Adsorber For Dehydrating Ethanol Vapours. Znd. Eng. Chem. Process Des. Dev. 1984,23,437-443. Newman, R.; Voloch, M.; Blenkowski, P.; Ladisch, M. R. Water Sorption Properties of a Polysaccharide Adsorbent. Znd. Eng. Chem. Fundam. 1986,25,422-425. Pirzada, I. M.; Hills, J. H. Determination of Water and Ethanol in Moist Air. Analyst 1983,108, 1096-1101. Rebar, V.; Fischacch, E. R.; Apostolopoulous, D.; Kokini, J. L. Thermodynamics of Water and Ethanol Adsorption on Four Starches as Model Biomass Separation Systems. Biotechnol. Bioeng. 1984,26,513-517. Serra, A.; Poch, M.; Sola, C. A Survay of Separation Systems for Fermentation Ethanol Recovery. Process Biochem. 1987,Oct, 154-158. Sowerby, B.; Crittenden, B. D. An Experimental Comparison of Type A Molecular Sieves for Drying the Ethanol-Water Azeotrope. Gas. Sep. Purif. 1988,2 (2),77-83.
John P. Crawshaw,* John H. Hills Department of Chemical Engineering University of Nottingham University Park, Nottingham, U.K. NG7 2RD Received for review May 10,1989 Accepted October 17, 1989
Enhanced Protein Diffusion in a Solvent Gradient Apparent diffusion rates of a-chymotrypsin across porous membranes were measured as a function of the methanol content of aqueous solutions. The experiments were performed under two conditions: (1) methanol was present at a uniform concentration throughout the system, and (2) a difference in methanol concentration existed across the membrane. The data show that methanol at uniform concentration retarded the diffusion of the enzyme in a manner quantitatively predicted from methanol’s effect on the viscosity of aqueous solutions. The methanol gradients enhanced or retarded the enzyme’s diffusion rate depending on whether the methanol gradient was in the same direction as or in a counterdirection to the enzyme’s concentration gradient. We speculate that the methanol-gradient effect is based on methanol’s influence on the thermodynamic activity of the enzyme. Experiments by Cussler and Breuer (1972) on the diffusion of low molecular weight solutes (e.g., antibiotics in water) across porous membranes have demonstrated that gradients of other chemical species (e.g., dioxane) can significantly accelerate or retard the diffusion rate of the solute. They explained their results by postulating that the transport of the solute is driven by the gradient of its chemical potential and noting that the solute’s activity coefficient is a function of the local solvent composition. Consequently, the effect of the mixed-solvent gradient (water-dioxane) was to alter the activity of the solute such that the gradient of its chemical potential differed sub0888-5885/90/2629-0309$02.50 /0
stantially from the concentration gradient. In this communication, we report the results of similar experiments in which the apparent diffusion rate of a protein is significantly affected by a gradient of solvent composition. In general, water-soluble proteins are charged and contain hydrophobic residues; therefore, their activity should depend on the polarity of aqueous solutions, which can be altered by adding miscible solvents. Indeed, significant differences in the activity of a protein in different solvents account for why precipitation or two-phase aqueous extraction can be used in purification procedures (Albertsson, 1986). 1990 American Chemical Society
