Ind. Eng. Chem. Res. 1993,32, 1676-1680
1676
SEPARATIONS Sorption of Organics and Water on Starch Paul J. Westgate and Michael R. Ladisch’ Laboratory of Renewable Resources Engineering, Purdue University, 1295 Potter Center, West Lafayette, Indiana 47907-1295
Starch is a well-established adsorption agent for drying ethanol. This work examines its potential for other gas-phase drying applications. Results from gas chromatography studies confirm that starch separates water from organic acids, alcohols, ketones, ethers, and aromatics, many of which form azeotropes with water. Trends in organics with respect to size and functional group show that the efficiency of this separation is related to both transport properties and strength of interaction between the organic components and starch. Small, polar molecules such as methanol and formic acid that have rapid mass-transfer characteristics and relatively strong interactions with starch are retained to a greater degree and are more difficult to separate from water than either compounds of higher molecular weight or decreased polarity. The large number of possible separations indicates that starch is a versatile material for use in sorbents for vapor-phase separations.
Introduction The separation of ethanol-water mixtures by passing the vapors over cellulose or starch adsorbents was first demonstrated by Ladisch and Dyck (1979). Since then, numerous studies that show this approach can be used to dehydrate ethanol have been carried out with various biomass materials such as corn (Robertson et al., 1983; Crawshaw and Hills, 1990),corn meal (Hong et al., 1982; Ladisch et al., 1984;Hills and Pirzada, 1989; Hassaballah and Hills, 1990), corn grits (Bienkowski et al., 1986; Neuman et al., 1986;CrawshawandHills,1990),cellulosics (Hong et al., 1982; Walsh et al., 1983),and starch (Hong et al., 1982; Crawshaw and Hills, 1990). The advantages of dehydrating ethanol using starch-containing sorbents include efficiency,relatively low adsorbent cost, and ready disposal of spent sorbent by fermentation or animal feeding. Starch and starch-containing sorbents are also environmentally attractive for other drying applications since they are nontoxic, biodegradable materials derived from renewable sources. Although ethanol is the only material dried by biomass adsorbents on a commercial scale, it has been shown by Ladisch et al. (1984) and Bienkowski et al. (1986) that dehydration of other alcohol-water mixtures is possible. We have also recently shown that corn grits can dehumidify air in a pressure swing adsorber (Westgate and Ladisch, 1993). Until recently, it appearedthat water had a much greater affinity for starch than the various nonaqueous components. A number of studies reported that alcohols were not appreciably adsorbed (Hong et al., 1982;Rebar et al., 1984; Bienkowski et al., 1986; Hills and Pirzada, 1989). Under some conditions,however,noticeablealcoholuptake has been observed (Robertson et al., 1983; Hassaballah and Hills, 1990). Previous work by Bushuk and Winkler (1957) and Gupta and Bhatia (1969) showed that substantial adsorption of organic compounds occurs at close to ambient temperatures on wheat flour and starch. Work by Lee et al. (1991) found that ethanol adsorption at high
* To whom correspondence should be addressed (ladischa stab1e.ecn.purdue.edu).
temperatures is similar to that of water if sufficient contact time is given for equilibrium to be achieved. At a given temperature, the rate of adsorption was 100-1OOO times slower for ethanol than for water. It was concluded that the separation of water from alcohol in a fixed bed adsorber is in part related to these differencesin mass-transfer rates. These conclusions agree with the recent results of Crawshaw and Hills (1992) for aqueous mixtures containing greater than 90% ethanol in which the rate of water sorption was much greater than that of ethanol during continuous contact of adsorbent with ethanol-water vapor. The relationship between rates of adsorption and separation led to the hypothesis that other organic molecules which have a slower rate of diffusion than water should also be readily separated from water. This paper demonstrates that organic acids, alcohols, ethers, and aldehydes, many of which form aqueous azeotropes (Table I), separate from water over an adsorbent formed from Pregel20 starch.
Materials and Methods Retention behavior of aqueous and pure component forms of the organic compounds was evaluated by inverse gas chromatography (Gray and Guillet, 1972,1973;Conder, 1974). This method has been used to examine the thermodynamics of adsorption of water and ethanol on starch (Rebar et al., 1984; Ostroff et al., 1988). Two 10-cmby 4-mm nominal internal diameter columns (total volume 1.6 mL) were gravity packed with 0.98 and 0.85 g of Pregel20 starch particles using the standard tap and fill method. Pregel20is manufacturedby A. E. Staley Company using a pregelatinization process in which the starch is f i t cooked in water and then dried. This material was compressed into particles by the Monsanto EnviroChem Company. The average size of the particles, determined by weighing fractions passing through standard size sieves, was 1.15 mm. After packing, the columns were placed in a Varian Model 3700 gas chromatograph with a thermal conductivity detector connected to a Linear 1200 pen recorder. The particles were pretreated for 12 h at a temperature of 85 OC and a helium carrier gas flow rate of 19 mL/min.
0888-588519312632-1676$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1677 Table I. Organic Compounds Studied That Form Azeotropes with Water
~
comDound methanol ethanol n-propyl alcohol n-butyl alcohol isopropyl alcohol isobutyl alcohol tert-butyl alcohol sec-butyl alcohol allyl alcohol formic acid acetic acid propionic acid butyric acid isobutyric acid ethyl ether MtBE EtBE acetonitrile pyridine methyl ethyl ketone propionaldehyde 2-picoline 4-picoline methyl acetate
boiling points ("C) azeotrope com~osn0 azeotrope" pure oraanicb 64.8 96.0 78.2 78.5 71.7 87.7 97.4 62.0 92.4 117.9 87.4 80.3 82.4 67.0 89.8 108.0 88.2 79.9 82.6 73.2 87.0 99.7 72.3 88.9 97.0 77.4 107.2c 100.8 118.1 17.8 99.1 141.0 18.5 99.4 163.4 21.0 98.8 154.9 98.7 34.1 34.7 55.2 73.1 83.5 81.6 81.8 57.0 94.0 115.5 88.7 73.4 79.8 98.0 47.5 48.0 52.0 93.5 128 37.2 97.4 145 96.5 56.6 57.1
Azeotrope composition is weight percent of the organic. Both composition and boiling point are a t 1atm. Data are from Horsely (1973). Boiling points of the pure substances are from Reid et al. (1977)with the exception of picoline,methyl tert-butyl ether (MtBE), and ethyl tert-butyl ether (EtBE) (Merck Index, 1989). Formic acid forms a negative azeotrope. 0
The porosities of the two columns, evaluated based on injections of air, were 0.50 and 0.59. Unless otherwisenoted, retention behavior of dry organic compounds was evaluated by injecting 0.2 pL of liquid at an oven temperature of 145 "C, injector and detector temperatures of 150 OC, a filament temperature of 170 "C, and a helium gas flow rate of 19 mL/min (25 OC, 1atm). This flow rate was regulated using a Matheson Model 8240 mass flow controller calibrated for helium. Flow settings were twice the desired value with flow evenly divided between the two cells of the thermal conductivity detector. All injections of the dry organics were made using a 10-pL Hamilton syringe.
Criteria Used To Select Operating Conditions The operating temperature was chosen to obtain reasonable water retention times in the range of 80-150 "C. At 145 O C , injections of 0.015 pL of water onto 10-cm columns resulted in broad elution profiles with retention times in excess of 5 min. Longer columns or lower temperatures increased the retention and peak width making the peak indistinguishable from the baseline. Larger injections of water that increased the peak height also gave disproportionate increases in peak tailing. Injection of 0.2 pL of a water-free organic component resulted in most of the organic component eluting within a few seconds in the column void volume. Detectablelevels of some of the organic eluted for several minutes as a peak tail indicating that a small portion of the organic pulse did adsorb. In a dehydration process the organic that adsorbs must be removed with the water when the bed is regenerated. Hence, partial sorption of the component decreases the efficiency of the separation. Measurement of the tailing effect is illustrated in Figure 1. The distance between the front of the peak and the point when the tail was 2 mm above the baseline (essentiallycomplete elution) is defined as d , and corresponded to a tailing time t,.
l-4 Figure 1. Measurement of tailing on the strip chart. Table 11. Reproducibility of Injections of 0.2 fiL of Acetonitrile at 145 "C
10-WL-syringe injection 1
2 3 4 t-
tf (rnin)
1.175 1.300 1.275 1.275
10-pL-syringe injection 5 6 7 8
tf (rnin)
1.325 1.300 1.250 1.225
= 1.266; u = 0.048.
A 10-pL syringe was chosen to inject samples since it gave a short, consistent pulse. The reproducibility of the injections was confirmed by making eight injections of 0.2 pL of acetonitrile at different times over the course of 2 days (Table 11). The tailing distances and times were consistent, exhibiting a standard deviation of only 0.048 min (2.9 8). The elution profile for water alone was compared to that of the aqueous organic mixtures. In this fashion it was possible to evaluate the effect of the organic on water retention and to determine whether a water-free organic peak could be cleanly separated from the water.
Results and Discussion The bulk of each of the injected organic components eluted close to the column void volume (retention time 0.05-0.1 min) while water eluted as a broad peak at approximately 4.8 min. This is explained by the interactions between the compounds and the adsorbent. First the compounds form hydrogen bonds with hydroxyl groups on the starch surface. Compounds that can disrupt intrastarch hydrogen bonds then apparently penetrate the adsorbent matrix. Diffusion of these compounds out of the adsorbent would then result in slow desorptionkinetics and extensive tailing (Gray and Guillet, 1973). This appears to explainwhy organicacids,which can form strong hydrogen bonds, exhibited a high degree of tailing while ethers exhibited little (Table 111). Tailing also decreased as the molecular weight of the injected probe increased. These results are consistentwith those of Bushukand Winkler (1957)and Gupta and Bhatia (1969) on wheat flour and corn starch. The rates of adsorption in both cases were reported to decrease with increasing molecular weight for the drying of methanol and ethanol. While Bienkowski et al. (1986) also reported that the operational capacity of a fixed bed of corn grits increasedwith increasingmolecularweight when methanol, ethanol, isopropyl alcohol, and tert-butyl alcohol were dried in a nonisothermal adsorber, this effect followed increases in molar heat capacities. In this study differences between probes of a given functional group are most likely related to mass-transfer effects. As a pulse of a compound passes through the column, interaction with the solid phase is mediated by
1678 Ind. Eng. Chem. Res., Val. 32, No. 8,1993
Table 111. Tailing Characteristics of Selected Organics. comnound D A . ~(em%) . diamb (A) t. (minl methanolc ethanolc n-propyl alcohola n-butyl alcoholc isopropyl alcohol* isobutyl alcohol' see-butyl alcoholc tert-butyl alcohol< allyl alcohol formic acid acetic acid propionic acid butyric acid isobutyric acid ethyl ether EtBE MtBE acetonitrile' pyridine< methyl ethyl ketone' propionaldehydec
0.93 0.77 0.67 0.60 0.67 0.60 0.60 0.60 0.68 0.87 0.73 0.63 0.57 0.57
3.82 4.46 4.96 5.40 4.99 5.40 5.36 5.40 4.68 3.91 4.59 5.10 5.55 5.56 5.47 6.16 5.84 3.44 5.14 4.75 4.57
0.60
0.50 0.55 0.96
0.63 0.68 0.73
3.98 0.70 0.40 0.27 0.28 0.20 0.23 0.18 0.63 15.05 1.60 0.83 0.60 0.43 0.25 0.20 0.23 1.25 0.58 0.35 0.33
a In all cases, 0.2 rrL of the compounds was injected on the Pregel particles at 145 "C and a helium flow rate of 19 mL/min (25 "C, 1 atm). Water elutes as a broad peak at 4.8 min; all other compounds elute at 6-10 8. Calculated as described in Reid et al. (1977). Compounds inject4 as mixtures with water to c o n f m separation (2-picoline. 4-picoline, and methyl acetate were ala0 injeeted with water).
I
,Desorption
O
10
20
30
40
50
I
60
Water Sorbed (g/i 00 g) Figure 2. Conceptual representation of the swelling of wary corn starch at 25 "C (data from Hellman et al. (1952)). transfer toanduptake bytheadsorbent particles. Transfer to the particles can be evaluated in terms of an average film coefficient which, at the flow rates (Re < 1)used in this study, is proportional to the diffusivity of the compound through the carrier gas. (Diffusivitiescan be determined by the method of Brokaw described in Reid et al. (1977)). Uptake by and transport within a granular adsorbent such as the pregel starch particles, on the other hand, is extremely complex (Vagenas and Karathanos, 1991). A complete model would need to consider molecular transport in the macropores between starch particles and through the starch matrix itself. The low loadings in this study simplify the analysis since most of the adsorption occurs on an outer shell beyond which intraparticle transport can be ignored. Little is known,however, about the extent of loading on the surface or penetration into the starch for the various organics. Equilibriumand rates of adsorption have only been studied for water and a few alcohols. Penetration is particularly complex since the starch matrix is swollen during loading resulting in apparent diffusivities that vary with concentration (Fish, 1957;Hansonet al., 1971;Karathanoset al., 1991). During desorption the swelling is reversed and could potentially trapsome of the adsorbateas illustrated in Figure 2, which is based on the data of Hellman et al. (1952). This
phenomenonmayhavecontributedtothe ethanoltrapping described by Crawshaw and Hills (1992). The factors affecting each stage of adsorption and desorption can be qualitatively evaluated although it is not currently possible to model this entire process. For a constant pulse width, the mass transfer to the surface will be dictated by the driving force and the film transfer coefficient which, as previously stated, is proportional to DA-H~, thediffwivityin thecarrier gas. The concentration driving force (the difference between the concentration in the bulkphaseandthatatthe particlesurface) is primarily affected by equilibrium with the solid phase. Compounds that form strong hydrogen bonds will have lower vaporphase concentrations at the particle surface. The rate of penetration through the starch is also most likely related toasmallmoleculardiameter and the abilitytoform strong hydrogen bonds. Desorption is influenced by these same factors. Large diffusivitiesand smallmoleculardiameters would be expected to contribute to fast desorption. The ability to form strong hydrogen bonds, on the other hand, decreases the drivingforcefor transfer from the adsorbent to the bulk phase. The greatest differencesbetween rates of adsorption and desorption would appear to be related to the postulated trapping mechanism. Adsorbate that penetrated deep into the swollen adsorbent matrix during adsorption must pass through the unswollen matrix during desorption. Hence, adsorbates that rapidly reach and penetrate the surface during adsorption are expected to take the longest to completely desorb. Since the collision diameter is inversely proportional to diffusivity and should strongly affect penetration, it was expected that it should stronglycorrelate with tailing. The data show that when injections were made with the 10-pL syringe, acids (Figure 3a), alcohols (Figure 3b), and other types of organics (Figure 3c) exhibit tailing in a manner which is inversely related to collision diameter. It should be noted that other properties such as molecular weights, molar volume, and diffusivity are correlatedto thecollision diameter and hence give similar relationships with tailing. As mentioned earlier, compounds of similar size and diffusivity tended to exhibit tailing proportional to the abilityto form hydrogen bondsas determined by functional group. It would appear that smallmolecules that exhibit strong binding with the adsorbent, such as methanol, formic acid, and acetic acid, would be the most difficult to separate from water. The qualitative analysis indicates that changes in the operating conditions may decrease the amount of organic that would be retained with water since both mass transfer and equilibrium adsorption properties are affected. In other words, it may be possible to enhance water retention while decreasing holdup of the organic component. This phenomenon was observed when the operating temperature was decreased from 145 to 85 OC. The decrease in mass-transferrates reduced ethanol tailing to t , = 0.23 min. The opposite was true for water which, due to an increased affinity for starch, appeared to be indefinitely retained at the lower temperature. The results of the tailing studies indicated that any of these organics should be readily separated from water if an aqueous mixture were injected. Only select watermiscible compounds were studied. The mixtures were made up such that they consisted of 80% organic and 20% water on amolar basis. Injectionvolumes werevaried (using a 1-pL syringe) resulting in a constant molar quantity of injection (3.2 pmol of organic, 0.8 pmol of water). The helium flow rate was decreased to a value of 14.3 mL/min (25 "C, 1atm) which corresponds to 20 mL/
"1
4
Ind. Eng. Chem. Res., Vol. 32, No. 8,1993 1679
9 15.05
A - 145OC
A
Formic Add
Methanol
h
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3-
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Acetic
2-
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5 0 B Collision Diameter (A)
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