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SEPARATIONS Adsorption of Water from Liquid-Phase Ethanol-Water Mixtures at Room Temperature Using Starch-Based Adsorbents Kyle E. Beery†,‡ and Michael R. Ladisch*,†,§ Laboratory of Renewable Resources Engineering, Department of Agricultural and Biological Engineering, and Department of Biomedical Engineering, Purdue University, West Lafayette, Indiana 47907-1295
The kinetically controlled, selective removal of water from ethanol vapors by desiccants is well documented. However, studies on the removal of water by liquid-phase contacting of waterethanol mixtures with starch-based material are limited. This research presents a screening study that shows that starch-based adsorbents remove liquid-phase water between 1 and 20 wt % from ethanol without the adsorbent being dissolved. The mass of water adsorbed per gram of dry adsorbent increases with increasing water content. Side by side comparisons of these starchbased adsorbents to silica gel and molecular sieves show that, in a kinetically controlled adsorption scheme below 10 wt % water, the inorganic desiccants have a greater operational (nonequilibrium) adsorption capacity per gram. At water concentrations at or above 10% water, however, the operational adsorptive capacity per gram of the starch-based adsorbents is roughly equivalent to the inorganic adsorbents, when using the same regeneration and adsorption conditions. The starch-based adsorbents adsorb water by forming hydrogen bonds between the hydroxyl groups on the surface of the adsorbent and the water molecules. Introduction Two areas of current research in the biobased fuel alcohol industry that are of concern in this paper are the sorption of ethanol from the fermentation broth and the adsorption of water from the process stream after distillation. Some materials that have been used to research the liquid-phase adsorption of ethanol include divinylbenzene cross-linked polystyrene resins,1,2 hydrophobic molecular sieves,1 and silicalite.3 The materials that have been researched for vapor-phase adsorption of water from ethanol-water mixtures include corn grits,4-8 phillipsite-rich volcanic tuffs,9 saponified starchg-poly(acrylonitrile),10 molecular sieves,11 and activated alumina.12 Starch, starch-based materials, cellulose, and hemicellulose have an affinity for water.4,13-15 Combinations of these materials also have been found to be adsorptive16-18 and are able to be regenerated at temperatures of 80 °C and lower.21 In this study, several combinations of these materials are tested for liquid-phase adsorption of water. Three potential methods of adsorbing water from high ethanol concentration feeds in the liquid phase are described by Robertson et al.,5 Fanta et al.,10 and Rao * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (765) 494-7023. Phone: (765) 4947022. † Laboratory of Renewable Resources Engineering, Department of Agricultural and Biological Engineering. ‡ Former graduate research assistant. Current address: Archer Daniels Midland Co., 1001 N Brush College Road, Decatur, IL 62521. E-mail:
[email protected]. § Department of Biomedical Engineering.
and Sircar.12 Robertson et al.5 proposed the use of a room temperature inert gas to pass through a vessel containing a liquid ethanol-water solution. The gas would become saturated with ethanol and water and would then pass through a grain bed where the water would be removed from the air; the gas would again pass through the vessel and pick up more water vapor. Fanta et al.10 suggested that the liquid-phase ethanol feed could pass through saponified starch-g-poly(acrylonitrile), which would selectively absorb the water in a sponge fashion. The material will absorb the water and will need to be air-dried before being used again. Finally, Rao and Sircar12 stated that the liquid-phase ethanol feed stream could be passed over a bed of activated alumina, which does not have as high of a regeneration temperature as that of molecular sieves. The regeneration temperature of molecular sieves is ∼250 °C, silica gel is ∼200 °C, and alumina is ∼175 °C. The recovery of fuel alcohol from the fermentation broth begins with sending the 5-10 wt % alcohol broth to a distillation column. If simple binary atmospheric distillation is used, then the maximum concentration of alcohol obtainable is 95.6 wt %, the azeotropic concentration. A method of separating the remaining water from the ethanol is to pass the vapors from the distillation column over a fixed bed of corn grits, after which the water concentration is negligible. The ethanol-water mixture needs to be kept in vapor form throughout this entire process. Currently, the production of over 2.8 billion L/year of fermentation ethanol utilizes this method of water vapor adsorption.6,19 This research was focused on the use of biobased adsorbents for the adsorption of water from liquid-phase water-
10.1021/ie0009381 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/06/2001
Ind. Eng. Chem. Res., Vol. 40, No. 9, 2001 2113 Table 1. Operational Adsorption Capacity of Several Starch-Based Adsorbents density of the adsorbent (g/mL) standard unmodified corn grits R-amylase-modified corn grits starch-based synthesized adsorbent 0.04 g of glucose, 4 g of starch, 4 mL of water 2 g of Avicel, 4 mL of water, 4 g of starch new batch of starch-based synthesized adsorbent 0.04 g of glucose, 4 mL of 0.04 M NaOH, 4 g of starch 2 g of Avicel, 4 g of starch, 0.04 g of glucose, 4 mL of 0.04 M NaOH 0.4 g of glucose, 4 mL of 0.04 M NaOH, 2 g of cobs, 4 mL of starch partially gelled starch-based synthesized adsorbent 3.2 g of galactose, 4 mL of 0.04 M NaOH, 4 g of starch, 2 g of cobs 0.8 g of maltose, 4 mL of 0.04 M NaOH, 2 g of cobs, 4 g of starch 0.4 g of sucrose, 4 g of starch, 2 g of cobs standard white corn grits gelatinized corn starch unmodified (native) corn grits R-amylase-modified corn grits starch-based synthesized adsorbent 0.04 g of glucose, 4 g of starch, 2 g of cobs, 4 mL of 0.04 M NaOH second batch of starch-based synthesized adsorbent 2 g of Avicel, 4 g of starch, 0.04 g of glucose, 4 mL of water partially gelled starch-based synthesized adsorbent
ethanol mixtures and compared operational (nonequilibrium) adsorption capacities. The biobased materials used in this study were white corn grits, R-amylase-modified yellow corn grits,20-22 polysaccharide-based synthesized adsorbent,23 and slightly gelled polysaccharide-based synthesized adsorbent. Materials and Methods The procedures for the manufacture of the modified yellow corn grits and polysaccharide-based synthesized adsorbent are found in their respective references.22,23 The partially gelled starch-based adsorbent was produced by heating 100 g of starch and 100 mL of a 0.04 M NaOH solution together on a hot plate until a white starch slurry was obtained. A sample of the starch was removed and examined under a light microscope and found to be partially gelatinized. A total of 50 g of corncob flour (Grit-o’cobs 60, The Andersons’ Corncob Products, Inc.) was then mixed with the slurry, and the material was spread into a thin layer and placed into an oven, where it was heated at 160 °C for 15 min. The material was then ground and sieved, and only the product larger than 0.355 mm (40 mesh) was utilized as adsorbent. The desiccants were screened to select the four best candidates for further tests. In preparation for the first test, 1 mL samples of 16 different adsorbents were dried in an oven for 40 min at 80 °C. Each adsorbent was then immersed in 5 mL of an approximately 1% (by weight) water in ethanol solution in a test tube covered with Parafilm for 10 min. The amount of time of 10 min was chosen as a realistic example of an industrial process. The adsorption in this range will be kinetically controlled instead of equilibrium controlled. A sample of the resulting solution was drawn and injected into a Shimadzu (Kyoto, Japan) GC-14A gas chromatograph containing a Supelco (Bellefonte, PA) HayeSep D column and a TCD detector to analyze the water content of the sample. The starting concentration of the waterethanol solution had been determined by injection into
0.735 0.670 0.447 0.438 0.668 0.451 0.461 0.680 0.561 0.483 0.533 0.485 0.467 0.856 0.608 0.735 0.670 0.447 0.438 0.451 0.663 0.483
% water in the ethanol mixture 1.48 1.42 1.39 1.40 1.45 1.43 1.41 1.44 1.46 1.47 1.42 1.46 1.45 1.46 1.53 1.40 1.44 1.43 1.31 1.45 1.46 1.37 1.44 1.41
water adsorbed from 99% ethanol (mg of water/g of adsorbent) 3.2 5.4 6.8 2.8 2.7 6.5 3.1 1.4 0.83 5.3 1.9 2.9 1.9 6.1 5.8 5.5 13 6.7 6.3 14 5.0 9.7
the gas chromatograph. The 16 adsorbents consisted of white or yellow corn grits or combinations of starch; corn cobs, R-cellulose, or Avicel; glucose, maltose, galactose, or sucrose; and either water or 0.04 M sodium hydroxide. A listing of the preliminary results that were used to select the four best adsorbents for further testing is found in Table 1. Table 1 was actually the same test conducted at two different times; that is why the “standard” in Table 1 is 1.48 and 1.53 wt % water. The 1.48% water in ethanol mixture was prepared the first day of testing, and the 1.53% water in ethanol mixture was prepared the second day of testing. The four best adsorbents were determined to be white corn grits, R-amylase-modified corn grits, starch-based synthesized adsorbent, and partially gelatinized starch-based synthesized adsorbent. These adsorbents along with two sizes of 3 Å molecular sieves (0.710 and ∼2 mm) and silica gel (∼1.70 mm) were used for the first test to determine operational adsorptive capacities at different weight percent water-ethanol mixtures. Two tests were conducted on the ability of starchbased adsorbents to adsorb water from liquid mixtures. The first test determined the operational adsorptive capacity of the adsorbents when using liquid-phase ethanol containing 1% water. This gave a first indication of adsorbent formulations that might be capable of taking up water. The second test was designed to confirm that the water adsorption by the starch-based adsorbents over a range of aqueous ethanol concentrations was scientifically verifiable. The method for the first test consisted of drying five 1 mL samples of each of the seven adsorbents overnight at 80 °C and then placing the samples in test tubes, each with a 2 mL portion of one of five different standards of water-ethanol solutions. The five standard waterethanol mixtures were made at 1, 2.5, 4.5, 10, and 20 wt % water (the 4.5 wt % mixture of water in ethanol was made to test if the adsorbents could dry the ethanol-water mixture past the azeotropic concentration at 95.6 wt % ethanol). The compound n-propyl alcohol was added at 1 wt % to the ethanol-water
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Table 2. Physical Properties of Various Adsorbents composition white corn grits R-amylase-modified yellow corn grits
starch
protein
oil
ash
other
surface area per gram (m2/g)
packing density (g/mL)
88.10% 88.10%
8.47% 8.47%
0.79% 0.79%
0.34% 0.34%
2.30% 2.30%
0.225 (BET) 0.289 (BET)
0.856 0.670
corn starch
corn cobs
0.04 MNaOH
4g 4g crystalline aluminosilicate amorphous silica
2g 2g
4 mL 4 mL
starch-based adsorbent partially gelled starch-based adsorbent molecular sieves blue-indicating silica gel
solutions as an internal standard because the ethanol peak is off-scale. The n-propyl alcohol has a much lower rate of adsorption on starch than water, and the amount of n-propyl alcohol adsorbed onto the starch during the course of the experiment is insignificant. The n-propyl alcohol was added at the time of preparation of the ethanol-water solutions. The test tube was then covered with Parafilm, mixed using a vortex mixer, and set aside for 10 min. After 10 min the test tube was mixed again and a 25 µL syringe was used to draw a 3.5 µL sample (after drawing and expelling the capacity of the syringe three times). This sample was then injected into the gas chromatograph to analyze the water concentration of the remaining solution. The water removed from the solution was considered to be adsorbed by the desiccant. In the second test, 1, 2, and 3 mL samples of the white corn grits, R-amylase-modified corn grits, starch-based synthesized adsorbent, and partially gelled starch-based synthesized adsorbent, which were measured in a 15 mL graduated cylinder, were dried in Petri dishes in a gravity convection oven for 40 min at 80 °C. Each sample was then immediately added to 5 mL of a 1 wt % water-ethanol standard solution and mixed using a vortex mixer. The solution was set aside for 10 min and then mixed again. A 3.5 µL sample of the solution was then injected into the gas chromatograph to determine the amount of water adsorbed. For both of the tests, each run was repeated three times and the results were averaged and compared against the n-propyl alcohol internal standard reading from the respective ethanol-water standard. The peak areas of the n-propyl alcohol and the water from the adsorption run were scaled so that the n-propyl alcohol peak area was equal to the n-propyl alcohol peak area from the water-ethanol standard. The water peak area from the run was then used to find the final water concentration. The water content was calculated, and the mass of water adsorbed per unit mass adsorbent was calculated. The physical properties of the adsorbents used in these tests are shown in Table 2. Further testing was completed to determine the strength of the physical structure of the starch-based synthesized adsorbent after an extended time in various water-ethanol solutions. A total of 11 water-ethanol standards was made, ranging from 0 to 100 vol % water in increments of 10. The 1 cm sphere-shaped starchbased adsorbent particles were prepared by making the standard starch-based adsorbent mixture and then rolling small spheres by hand. These spheres were baked in a Petri dish in a gravity convection oven at 160 °C for 40 min or until dry. One sphere was added into 20 mL of each of the 11 solutions. The particles were checked after 24 h and then after a week to
surface Area (m2/g) ) 300-1000
0.447 0.483 0.763 0.774
Figure 1. Comparison of operational (nonequilibrium) adsorption capacities for different desiccants: mass of water adsorbed per gram of dry adsorbent as the water content of the ethanol-water solution concentration increases.
determine if the spheres had begun to disaggregate into small particles when prodded with a small spatula. Results and Discussion The liquid-phase adsorption of water from organic streams would require that the biobased adsorbent be insoluble at the process conditions and that it be able to be regenerated while maintaining its sorption capacity. Also, for economic viability as a liquid-phase adsorbent in any application, the biobased adsorbents would need to have capacities that are at least similar to commercially used adsorbents, such as molecular sieves or silica gels. The regeneration temperature is also lower for biobased adsorbents than for commercially used adsorbents and could lead to energy savings. The mass of water adsorbed per unit mass of dry adsorbent increases for the biobased adsorbents and silica gel as the water content in the ethanol increases from 1 to 20%. The mass of water adsorbed for molecular sieves increases up to 4.5 wt % water and then decreases (Figure 1). The results seen for the biobased adsorbents and silica gel are expected, in that the water molecules would increasingly find hydroxyl group adsorption sites as the number of water molecules per unit volume increases, thus leading to a higher driving force for adsorption. However, the molecular sieve entraps water in pore spaces having dimensions that match the steric characteristics of the water. Therefore, the decrease in the amount of water adsorbed by the molecular sieves as the water content rises above 4.5 wt % could be due to the temporary associations of water molecules within the molecular sieve pores that cause steric hindrances and temporarily prevent access to the interior adsorption sites for other water molecules.
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cultural Research Programs. We thank Craig Keim and Nathan Mosier for their helpful comments and suggestions. Literature Cited
Figure 2. Comparison of operational (nonequilibrium) adsorption capacities: water adsorbed by organic adsorbents in 5 mL of a 1% water-ethanol mixture with adsorbent added in 1, 2, and 3 mL quantities; absorbent volume measured by a 15 mL graduated cylinder.
When the amount of biobased adsorbent is increased from 1 to 2 to 3 mL in the 5 mL of 1 wt % water-ethanol standard solution, the amount of water adsorbed per gram of dry adsorbent steadily decreases as shown in Figure 2. However, because the amounts of water adsorbed per gram are close for all of the masses of adsorbent tested, it is evident that liquid-phase adsorption is occurring. The decrease may be due to inhibited diffusion of the water in the ethanol-water solution as the layer of adsorbent in the bottom of the test tube is doubled and tripled (even with the tube being mixed at the beginning and end of the test). If equilibrium was reached, the expected results from this test would be that the amount of adsorption would be the same on a per gram of adsorbent basis. The results of the water resistance test, in which the starch-based synthesized adsorbent particles were immersed in ethanol-water solutions levels from 0 to 100%, showed that after a week the starch-based adsorbent particle was unaffected at up to 10 vol % water, began to degrade at 20%, easily broke apart at 70%, and fermented at 100% water. Also, it appears possible that the mechanism occurring below 20% water is still adsorption and not absorption, because the particle shows no swelling or dissolution. Conclusions Biobased adsorbents, specifically starch-based synthesized adsorbents and R-amylase-modified corn grits, adsorb water selectively from ethanol-water mixtures under kinetically controlled conditions. These adsorbents have capacities similar to commercial adsorbents under operational (nonequilibrium) conditions. The biobased adsorbents also have the advantage of being able to be regenerated at temperatures of 80 °C and lower, whereas the commercial adsorbents need to reach at least 175 °C to be completely regenerated. However, the current starch-based adsorbents are limited to a 10 vol % water feed if the contact time is continuous because the adsorbent begins to disaggregate at concentrations above 10% water. If the contact time between the feed and the adsorbent is not continuous, then the concentration of water in the feed could be higher. The 10% limit is not a hindrance, however, because generally distillation columns produce ethanol concentrations of 90% or higher. Acknowledgment The material in the work was supported by USDA Contract 96-35500-3191 and Purdue University Agri-
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Received for review November 1, 2000 Revised manuscript received February 12, 2001 Accepted February 17, 2001 IE0009381
