Ind. Eng. Chem. Res. 1992, 31,887-892
887
Experimental Determination of Binary Sorption and Desorption Kinetics for the System Ethanol, Water, and Maize at 90 "C John P. Crawshaw* and John H. Hills Department of Chemical Engineering, University of Nottingham, University Park, Nottingham, U.K. NG7 2RD
Methods are described to measure the simultaneous rates of sorption for both components of a binary vapor mixture of ethanol and water, and the subsequent desorption into an air stream. The shape of the ethanol vapor sorption curve at 90 "C and atmospheric pressure was very unusual. For vapor feeds of less than 90% ethanol a rapid initial uptake was followed by temporary desorption and f d y a slow resorption to equilibrium uptake. At higher ethanol vapor concentrations the temporary desorption was less apparent. The water sorption-time curves were inflected, particularly in experiments with smaller sorbent particles, indicating that water sorption cannot be controlled by diffusion with a constant coefficient. The sorption behavior can be interpreted in terms of the physical properties of the starch polymer matrix, in which the transition from a rigid, glassy state to a more flexible, rubbery state occurs as water is sorbed.
Introduction Ethanol may be used as a fuel extender and octane enhancer when mixed with gasoline. To avoid problems with phase separation after the gasoline and ethanol are mixed, the ethanol must be dry, whereas the three major processes for the production of ethanol (synthetic production from ethylene, from synthesis gas, and by fermentation of sugars) all produce a dilute solution of ethanol in water. The separation of ethanol from water must be considered as part of an integrated fuel ethanol production process. A recent comparative study (Serra et al., 1987) has confirmed that selective sorption of water is a promising process for the drying of ethanol containing less than 20% water, exactly the region in which distillation is least efficient. Many solid sorbents have been proposed in this context, but materials composed mostly of starch, such as maize grits, possess an attractive combination of low cost, selectivity, and ease of regeneration (Ladish and Dyck, 1979; Lee and Ladisch, 1987). Initially starch was thought to be almost perfectly selective for water from the vapor phase at temperatures of 50-100 "C (Hong et al., 1982; Rebar et al., 1984))but recent work (Hassaballah, 1986; Crawahaw and Hills, 1990) has demonstrated considerable ethanol sorption under these conditions. Sorption rates of pure ethanol have been shown to be very low (Lee et al., 19911,but these workers only touch very briefly on simultaneous sorption of ethanol and water and they did not measure rates of uptake of the individual components. The aim of this work is to produce some basic data on the rate of the simultaneous sorption of ethanol and water by starch and maize from the vapor phase at 90 "C in order that an informed decision may be made on the design of processes which minimize the sorption of ethanol and, in necessary, recover what ethanol is sorbed. Choice of Experimental Methods The simplest system to study would be the sorption of a single component by a single particle from a fluid of constant concentration. However, this would give no information as to the influence of ethanol on water sorption or that of water on ethanol sorption, either of which could be important in the exploitation of the process. So it was necessary to measure the simultaneous sorption of both ethanol and water. The most convenient experimental arrangement for this is the packed bed. For the bed to
model a single particle in a well-mixed fluid, the flow rate of vapor should be maximized both to reduce the effect of any mass-transfer resistance in the fluid film and to eliminate concentration gradients along the length of the bed. Two simultaneous, independent measurements are needed to evaluate the quantities of sorbed ethanol and water. Ideally we would like to measure the mass and concentration of the sorbed material, but determining the concentration of sorbate within the solid is difficult, so the concentration of the sorbed phase must be inferred from changes in the fluid around the bed. One method is to measure the concentration and flow differences across the bed, but these cannot be measured with any accuracy if, at the same time, concentration gradients along the bed are to be minimized. An alternative is to circulate sorbate around a closed loop including the sorbent tube. A rapid circulation rate can eliminate the concentration gradient along the bed, and a small total quantity of sorbate in the loop will give a measurable concentration change in the fluid ouside the bed over the time of the experiment. The two independent variables chosen were, therefore, the concentration of ethanol and the total mass of sorbate remaining in the fluid phase. For the concentration change to be measured with the greatest accuracy the change should be large, but for easy interpretation of the experimental results the fluid-phase concentration should remain as constant as possible. A compromise between these two conflicting demands was made-a concentration change of between 5 and 10% during a sorption run was aimed for. Desorption is conveniently carried out at atmospheric pressure with a hot purge gas. To measure the desorption kinetics, the flow of the purge gas and the concentration of ethanol and water in the desorbate are required as a function of time. The analysis of the desorbate must be carried out in the vapor phase, as it would be impractical to condense the vapor and measure the rate and composition of the desorbate. The interpretation of the data is simplified if the flow rate of the purge gas is very high, allowing the assumption of constant (zero) concentration of the sorbate in the fluid phase. However, this would result in a very dilute purge gas which is difficult to analyze, so again a compromise had to be reached giving sufficient desorbate concentration for an accurate response from the chromatographic analysis used.
0 1992 American Chemical Society Q888-5885/92/2631-0S~~~03.QQ~Q
888 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 Reference
water
q*
Carrier
Purge Figure 2. Diagram of the desorption apparatus: (a) guard bed of silica gel; (b) orifice plate flowmeter; (c) copper coil; (d) sorbent tube, (e) gas sampling valve; (f) cold trap; (GC)gas chromatograph.
Figure 1. Diagram of the sorption apparatus: (a) liquid reservoir; (b) balance; (c) Minipuls peristaltic pump; (d) evaporator; (e) copper coil; (0 sorbent tube, (g) bypass; (h) condenser; (i) polythene bag; (i) liquid return line; (k) flexible tubing; (V) Rotaflo taps.
Experimental Section Sorption. The equipment used to measure the sorption kinetics is shown in Figure 1. The sorbent sample was kept in a glass tube of 25-mm internal diameter, supported on a sintered glass disk. Entrainment of the sorbent particles was prevented by a glass wool plug at the top of the tube. The sample tube was maintained at 90 "C during a sorption run by heating the air in a lagged cabinet. The sorption tube and the hot cabinet were as described earlier (Crawshaw and Hills, 1990) for equilibrium experiments. The liquid reservoir, from which the sorbate was circulated, had a capacity of approximately 30 mL and was sealed at the top with a rubber disk through which liquid samples could be withdrawn by syringe. The reservoir was connected to the liquid return line and to the peristaltic pump by plastic tubing of 1-mm internal diameter. The tubing was very flexible and coiled to allow free movement of the reservoir over the 12 mm required by the balance mechanism. The Sartorious balance used had a sensitivity of 0.001 g. Liquid from the reservoir was pumped to the evaporator, which consisted of a 500-mm length of 4-mm-diameter copper tubing packed with copper wool to enhance heat transfer. The heating tape wrapped around the tubing was connected to a variable power supply (not shown), used to control the temperature in the evaporator at 100 OC. The evaporator temperature was measured by a thermocouple placed between the heating tape and the copper
tube and indicated on the Cormark thermometer used to control the air temperature in the cabinet. Vapor from the evaporator passed into a second copper coil inside the hot cabinet which ensured that the vapor was at 90 "C before it came into contact with the sorbent. The three Rotaflo taps could be operated remotely, without opening the cabinet, and were used to direct the vapor flow through either the bypass line or the sorbent tube. Unsorbed vapor passed out of the hot cabinet to the condenser, and the condensate was returned to the reservoir through a glass capillary of 1-mm internal diameter. The small diameter of all the tubing which carried liquid minimized the holdup of sorbate outside the liquid reservoir. The condenser was sealed at the top by a polythene bag, which allowed some change in the vapor volume without a change in pressure or escape of vapor. Desorption. The apparatus used for measuring desorption kinetics is shown in Figure 2. The purge gas, nitrogen, was dried in a guard bed of silica gel, and ita flow rate was measured using a specially made orifice plate flowmeter connected to a water manometer. The nitrogen was heated to the cabinet temperature of 100 O C in the copper coil. The mixture of purge gas and desorbate leaving the packed bed passed through the Valco 10 port gas sampling valve and then out of the hot cabinet to a cold trap. The flow path through the sample valve was chosen to allow a sample of the desorbate vapor to be injected into the chromatograph each time the valve was switched. The sample size was 10 mL. The chromatograph was a Varian Vista with a Porapak Q column and thermal conductivity detector (TCD). The column and detector were maintained at 175 and 200 "C, respectively. The output of the TCD was to a chart recorder, so peak heights were used to determine the concentration of ethanol and water in the samples. This has previously proved reliable (Pinada and Hills, 1983).
Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 889
Procedure Sorption. The sorption tube, filled with previously dried or regenerated sorbent, was weighed and fitted into the rig. The air heater was then switched on and the rig allowed to warm up to its operating temperature. The reservoir was charged with an ethanol/water mixture of known mass and concentration which was then circulated around the system, bypassing the sorbent, until a steady reading was obtained. To start the sorption, the taps V1 and V2 were opened and V3 was closed. During the run, the weight of the liquid reservoir was recorded frequently. Liquid samples, of approximately 1.0 pL each, were withdrawn less frequently for analysis by chromatography, and interpolation was used to estimate the liquid concentration at the time each weight recording was made. The total number of liquid samples taken during a run did not exceed 20, corresponding to a total weight of less than 0.016 g. This was considered small enough to neglect. A crude estimate was made of the circulation rate of the ethanol/water mixture by counting the number of drops of liquid returning to the reservoir in a known time. The maas flow rate calculated from thiswas 0.06 g/s. Awuming that the vapor was an ideal gas, with a pressure in the sorbent tube of 100 kPa and a temperature of 363 K, the volumetric flow rate was estimated to be 4 X 10” m3/s. The residence time of the vapor in the bed was therefore around 1 s. Knowing the mass of unsorbed vapor, Ut,at time t, and the corresponding ethanol concentration, C,, a simple mass balance results in the mass fractions of ethanol and water on the sorbent, QE,t and QW,t, respectively: (1) Q E , ~= (UoCo - UtCt)/S (2) = (U&1 - C,) - Ut(1 - Ct))/S where U,and C, are the initial values and S is the mass of sorbent. Just before the start of a run the void space of the sealed sorption tube was full of nitrogen and the rest of the hot tubing was filled with vapor of the starting concentration. When the taps sealing the sorption tube were opened and the bypass was closed, the nitrogen was swept out of the sorption tube by the vapor. The nitrogen collected in the top of the condenser, unable to reenter the apparatus because of the flow of vapor at the top and the liquid sealing the tubing at the bottom. The quantity of vapor in the apparatus immediately after the start of sorption was, therefore, increased by the volume of the void space in the sorption tube. As the sorption progressed, the concentration of the vapor inside the apparatus changed and this also caused a small change in the mass of ethanol and water in the liquid reservoir which was not due to sorption. Corrections were made for these two effects before calculating QE,t and QW,, from eqs 1 and 2. Desorption. A desorption run was usually started the day after a sorption run, and the sample was stored overnight a t room temperature, sealed in the sorbent tube. Both the heated cabinet (with the sealed tube installed) and the chromatograph were allowed to warm up for at least 2 h to obtain a steady baseline before the Rotaflo tap were opened and the dry nitrogen flow was set to the desired rate to start the run. The top tap (V2) was always opened first to avoid the slightly pressurized vapor in the sorbent tube blowing back through the orifice plate meter and overloading the manometer. The firat vapor sample was taken at a time of 1 min from the start of desorption, and for the next 20 min samples Qw,t
Table I. Exwrimental Conditions ~~
ethanol run 1 26
3 4 5
6 7 8 9 10 11
sorbent type YG16/18 YG16j18 YG16/18 YG16/18 YG16/18 YG16/18 YG16/18 YG16/18 YGM/S YGM/S YGM/S
sorbent mass,’g 25.17 25.17 25.17 25.17 25.17 25.17 25.17 25.17 27.84 27.84 27.84
sorbate mass,g 22.22 16.01 19.12 18.66 15.61 18.04 18.73 19.79 19.77 19.27 18.51
‘Oncn?
start
80.1 80.4 80.8 90.4 94.5 77.7 77.8 90.0 90.2 90.2 90.2
%
E
end 86.6 87.4 85.7 93.8 97.1 86.0 85.9 93.3 93.4 93.8 93.8
a When dry. bGC failed during desorption: full results are not available.
were taken every 4 min, which was as often as the chromatograph allowed. The sampling rate was then reduced as an experiment progressed and the rate of change of the desorbate concentrations slowed down. A desorption run was ended when both the ethanol and the water peaks disappeared. The pressure drop through the apparatus was measured using a mercury manometer. At the highest gas flow rate the total pressure drop was 30 mmHg, almost all of which was caused by the gas sample valve. The pressure drop across the packed bed was negligible. The presssure drop was added to the atmospheric pressure on the day of an experiment when the mass flow rate of the dry nitrogen was calculated. The rates of ethanol and water desorption a t any of the sample instants was found by the product of the nitrogen flow rate and the appropriate mass ratio for that sample. The cumulative amounts of ethanol and water desorbed at any time were then found by numerical intergration using the trapezoid rule. This procedure, assuming straight lines between the sample points, slightly overestimates the area under curves which are concave upward. The first sample was taken after the vapor in the void space had been swept through the gas sample valve (the residence time of the purge gas in the sorbent tube was approximately 30 s for all runs) and there was no need, therefore, to make a correction for this. However, as desorption was most rapid at time t = 0, a signifcant amount of material was desorbed before the first sample was taken. The rate of desorption during the time before the first sample was estimated by using the Newton-Gregory forward difference equation to extrapolate the first five values calculated for the water and ethanol desorption back to t = 0. The summation above could be used to calculate the total amount desorbed from the start of the experiment onward.
Results and Discussion The experimental conditions for all the runs are summarized in Table I. Two sorbents were used: the Y G16/ 18 grits, which had proved to be the most selective in previous experimenb (Crawshaw and Hills, 1990),and the milled and sieved grits YGM/S. The latter were chosen for the kinetics experiments because of their narrow particle size distribution. If the rate of sorption is determined by diffusion within the particles, then the results of an experiment with a wide range of particle sizes would be more difficult to interpret. Even for sorption with constant diffusion coefficient the rate at which sorbent enters the particle depends on the group D / a 2where D is the diffusion coefficient and a is the particle radius.
890 Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 Table 11. Results: Sorption and Desorption at the End of Each RunD sorption desorption run 1
3 4 5 6 7 8 9 10 11
total m a w g/g weiehinn calc 0.0927 0.0958 0.0605 0.0596 0.0630 0.0612 0.0504 0.0479 0.0814 0.0815 0.0977 0.0976 0.0692 0.0675 0.0866 0.0857 0.0714 0.0735 0.0727 0.0633
totalm=s,g/g weighing calc 0.0806 0.0820 0.0606 0.0602 0.0542 0.0498 0.0473 0.0442 0.0928 0.0940 0.0962 0.0929 0.0588 0.0552 0.0604 0.0599 0.0669 0.0649 0.0663 0.0635
%
ethanol 26.4 23.4 53.5 63.5 13.2 23.7 55.3 66.5 59.7 56.5
0' 0 o o o o 0 0 0 0
0.030
0
O
%
0
ethanol 26.3 13.0 46.2 63.4 24.3 18.4 47.0 48.1 52.2 56.0
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x
m 0.025
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All results have been corrected as explained in the text.
t
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x X X X
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,
,
,
0
,
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,
100
,
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TIME Figure 4. Run 4 sorption profile.
300
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/ min
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w
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, 100
TIME Figure 3. Run 1 sorption profile.
O
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I 150
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0
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Mass Balance. From initial exploratory experiments, it soon became apparent that the weight gained by the sorption tube during a sorption run was less than the weight loss from the liquid reservoir. The leak of sorbent from the system was persistent and resisted all attempts to seal the apparatus completely. The most likely cause of the leak was the tubing connecting the liquid reservoir to the pump and to the liquid return line. This tubing may have been permeable, allowing sorbate to diffuse slowly through it. The rate of loss was low, approximately 4.22 X g/min, and was constant and uniform for all runs. As the ethanol concentration did not change detectably during a 3-h blank run with the sorption tube empty, the liquid lost was presumed to have the same concentration as the liquid in the reservoir. It was therefore decided to subtract the estimated loss from the weight change of the reservoir before applying eqs 1and 2 to calculate the quantities of ethanol and water sorbed. The total amounts sorbed and desorbed (about correction) at the end of each experiment are given in Table 11, and the total weight changes calculated from the liquid reservoir weight and the GC results agree well with the value obtained directly by weighing the sorbent tube. In the first run with a fresh sorbent sample not all the sorbed ethanol could be removed under the mild desorption conditions used here. The sorption of water was reversible as has been observed in many studies of single-
0 . 0 0 0 7 , 0
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Water Ethonol
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1
I
100
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,
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300
/ min
Figure 5. Run 4 desorption profile.
component water sorption (Van den Berg, 1981) where hysteresis may occur in the middle of the isotherm, but there is no residual weight gain at zero humidity. Studies of single-componentethanol aorption at 35 "C (Gupta and Bhatia, 1969)and 27 "C (Bushuk and Winkler, 1957) found that not all the sorbed ethanol could be removed even after drastic evacuation for 8-10 hours; the residual ethanol could be removed, however, by first exposing the sorbent sample to water vapor) and then continuing the desorption to equilibrium. This is duplicated in this work when a run with low water concentration during sorption is followed with a run at higher water concentration, for example, runs 5 and 6. In this case during the second desorption more ethanol was removed than was added during the second sorption. Shape of the Sorption- and Desorption-Time Curves. Figures 3-6 show the quantities of ethanol and water sorbed and desorbed per gram of dry sorbent plotted against time for a representative sample of the runs. In many cases the ethanol sorption profiles show a characteristic peak at between 10 and 20 min from the start of
Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 891
X
x X
4
m
x
\
0.04
X
\
*
X
x
n
9%
w
0.03
ln
a m
0.02
.x
*
0.01
W
g
0.02
ln Q
x
0.01
i *
x .*
* 0
100
200
Run 10
Run 9 Run 10 Run 11 300
i
400
TIME / m i n Figure 6. Runs 9, 10, and 11 sorption profiles: (A) ethanol; (B) water.
sorption, followed by a desorption to a minimum and finally a slow resorption to the final uptake. This is shown in Figure 3 for run 1and a similar result was found for runs 3 and 6-11 inclusive. However, for runs 4 and 5 the peak was less apparent (Figure 4) or nonexistent. These runs were characterized by high ethanol concentrations in the feed, and the greater uptake of ethanol in the second stage of the sorption seems to mask the temporary desorption. This simpler shape of the sorption time curves was also seen when YG16/18 grits were exposed to pure ethanol vapor on the pan of a balance (Crawshaw, 1990). The plots of the sorption of water against time are of a more simple shape with the sorption increasing monotonically. In run 1 (Figure 3) there is a point of inflection in the water sorption-time curve at around 20 min as the rate of water sorption reaches a maximum. This is not readily apparent in the other runs with the YG16/18 grits, but can clearly be seen in the three runs with the YGM/S grits (Figure 6), where there is a significant delay before the most rapid water sorption takes place. The inflection in the water sorption curves for the YG16/18 grits may be more difficult to observe due to the "blurring" effect of the wide range of particle sizes. The water sorption curve for run 9, the first sorption for the YGM/S grits, shows an overshoot in the uptake before
a lower equilibrium value was reached. No other water sorption curve shows this feature, suggesting that the overshoot is caused by an irreversible change in the solid structure during the first vapor sorption. Effect of Sample History. The three runs 9,10, and 11had almost identical experimental conditions, and they have been plotted on the same axes in Figure 6 to draw attention to the slow approach to a repeatable sorptiondesorption cycle. The first run with the YGM/S grits, run 9, shows the expected large ethanol sorption which was not all removed during the subsequent desorption. Conversely the amounts of water sorbed at the end of all three runs were very similar and were matched quite closely by the water desorption. The ethanol sorption had not reached equilibrium during the first run, and consequently the second run also showed some irreversible ethanol sorption, although the effect was much smaller. By the third sorption-desorption cycle the quantity of ethanol sorbed was matched by the desorption and equilibrium appeared to have been reached during the time of the experiment. Rates of Sorption and Desorption. In run 4, where the initial peak in the ethanol concentration is not important, a crude comparison of the rates of sorption and desorption can be made (see Figures 4 and 5). It can be seen that the sorption of water is initially much faster than that of ethanol although the final equilibrium concentrations are similar, whereas the situation is reversed for desorption. A Physical Interpretation of the Kinetics Results. It is apparent that a description of this complex sorption process ae simple diffusion is inadequate, not only for the ethanol sorption curves but also for the inflected water sorption curves. Crank and Park (1951) noted that a point of inflection cannot occur in the sorption-time curves of a system which is controlled by diffusion where the diffusion coefficient is either constant or a function of the (constant) sorbate concentration at the solid surface. The shape of the ethanol sorption profile is very unusual. Complex behavior is often seen in multicomponent sorption in large packed beds where temperature effects are large and may cause desorption and then resorption at a later stage when the heat of sorption has been carried out of the bed,but it is more Micult to explain in experiments which are approximately isothermal and which model the behavior of a single sorbent particle. A rapid sorption of one component from a two-component mixture, followed by ita partial desorption, is seen when the two sorbates compete for the same sites within a rigid sorbent and the more rapidly diffusing component is also the more weakly sorbed (Farooq and Ruthven, 1991). However, this "roll-up" phenomenon could not account for a third phase of slow resorption. Sorption curves similar to those described in this work have been observed in the sorption of vinyl chloride (VCM) by poly(viny1 chloride) (PVC) (Berens, 1978; Berens and Hopfenberg, 1978). For small vapor concentration steps the uptake of vinyl chloride showed a maximum and a minimum followed by a slow climb to the equilibrium sorption. This behavior was attributed to an initial stage of rapid sorption controlled by Fickian diffusion, followed by a collapse in the structure of the amorphous, glassy PVC after the start of the sorption of VCM, and this resulted in the temporary desorption. The collapse occurs when the glass transition temperature becomes lower than the experiment temperature and the polymer chains making up the solid sorbent suddenly become more mobile. This can happen even in an isothermal experiment, as sorption
892 Ind. Eng. Chem. Res., Vol. 31,No. 3, 1992
of a plasticizing sorbate (in this case PVC) lowers the glass transition temperature. This fiial, slow climb to equilibrium was assumed to be controlled by a further rearrangement of the polymer structure. Both the collapse and slow rearrangement were characterized by relaxation rates much slower than the initial diffusive uptake. The above mechanism could account for the shape of the sorption curves measured in this work as water is a plasticizer for the polymers of which starch is composed. The glass transition temperature, which is around 100 "C for dry starch, is reduced to around room temperature (Van den Berg, 1981) by water sorption to a final moisture content of around 0.2 g of water/g of dry starch. It is possible, therefore, that the starch can change from the glassy to the rubbery state during the sorption experiments, although more experimental work is necessary to prove that the glass transition temperature is passed during the sorption. That the pure ethanol sorption curves show no minimum is further evidence for the proposed mechanism; if no water is present then the starch will remain in the glassy state and the solid structure cannot collapse and expel sorbed ethanol. While it has not been proved that a change in the polymer structure underlies the effects observed, the fact that such a change is known to take place with water and starch makes it a very likely hypothesis. On this basis, the picture of the physical processes underlying the sorption and desorption of ethanol and water which is emerging from the experimental results is as follows: For sorption the initial rapid uptake of ethanol by the sorbents is due to the existence of voids in the rigid, glassy structure of the main component, starch. The rate of this initial sorption is limited only by diffusion within the solid sorbent. As water is also sorbed, the glass transition temperature becomes lower until it is below the temperature at which the sorption is taking place; at this time the starch polymers become rubbery and some of the voids collapse as stresses within the polymer chains are released. This causes the temporary desorption as ethanol is squeezed out. Over a longer period the additional sorption is possible as the now rubbery polymer chains are rearranged allowing the ethanol into previously inaccessible regions. Ethanol desorption is rapid from the already expanded polymer network, limited initially by diffusion only. Later as the matrix begins to shrink and trap ethanol further desorption becomes slow, governed by the relaxation of the starch polymer. When most of the water has been desorbed, the glass transition temperature again exceeds the temperature of the experiment. At this time the polymer structure is effectively frozen and additional desorption leaves the voids into which the ethanol initially diffuses.
Conclusions The simultaneous sorption of ethanol and water by maize grita is effectively measured by monitoring the mass
and concentration changes of the unsorbed sorbate in a small liquid reservoir. The sorption of ethanol, which is undesirable for this separation, is complicated by the simultaneous sorption of water which brings about the glass-rubber transition of the starch sorbent. The temporary desorption observed with feeds of less than 90% ethanol could be exploited in a countercurrent sorption process in which the time for which the solid is exposed to the vapor is controlled to minimize ethanol sorption. Desorption of ethanol is initially rapid, which indicates that ethanol recovery from the purge gas need only be used as the start of regeneration. Registry No. EtOH, 64-17-5; H20, 7732-18-5.
Literature Cited Berens, A. R. Analysis of Transport Behaviour in Polymer Powders. J. Membr. Sci. 1978, 3, 247-264. Berens, A. R.; Hopfenberg, H. B. Diffusion and Relaxation in Polymer Powders:2 Separation of the Diffusion and Relaxation Parameters. Polymer 1978,19,489-496. Bushuk, W.; Winkler, C. A. Sorption of Organic Vapours on Wheat Flour at 27 OC. Cereal Chem. 1957,34,87-93. Crank, J.; Park, G. S. Diffusion in High Polymers: Some Anomalies 1951,47,1072-1084. and Their Significance. Trans. Faraday SOC. Crawshaw, J. P. Simultaneous Sorption of Ethanol and Water by Starch and Corn. Ph.D. Dissertation, Nottingham University, UK, 1990. Crawshaw, J. P.; Hills, J. H. H. Sorption of Ethanol and Water by Starchy Materials. Ind. Eng. Chem. Res. 1990,29,307-309. Farooq, S.; Ruthven, D. M. Dynamics of Kinetically Controlled Binary Adsorption in a Fixed Bed. NChE J. 1991,37 (2), 299-301. Gupta, S. L; Ghatia, R. K. S. Sorption of Water and Organic Vapoura on Starch at 35 'C. Indian J. Chem. 1969, 17 (2), 1231-1233. Hassaballah, A. A. Drying of Alcohol by Adsorption. Ph.D. Dissertation, Nottingham University, Nottingham, UK, 1986. Hong, J.; Voloch, M.; Ladisch, M. R.; Tsao, C. 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, 898-900. Lee, J. Y.; Ladisch, M. R. Polysaccharides as Adsorbents. An Update on Fundamental Properties and Commercial Prospecta. Ann. N.Y. Acad. Sci. 1987,506,492-497. Lee, J. Y.; Westgate, P. J.; Ladisch, M. R. Water and Ethanol Sorption Phenomena on Starch. AIChE J. 1991, 37 (8), 1187-1195.
Pirzada, I. M.; Hills, J. H. 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 Survey of Separation Systems for Fermentation Recovery. Process Biochem. 1987, Oct. 154-158. Van den Berg, C. Vapour Sorption Equilibria and Other Water Starch Interactions. A Physic0 Chemical Approach. Ph.D. Dissertation Agricultural University, Wageningen, 1981.
Received for review July 16, 1991 Accepted November 26, 1991
