分子筛分离乙醇和水

0263–8762/04/$30.00+0.00 # 2004 Institution of Chemical Engineers www.ingentaselect.com=titles=02638762.htm Trans IChemE, Part A, July 2004 Chemical Engineering Research and Design, 82(A7): 855–864 SEPARATION OF ETHANOL–WATER MIXTURES USING MOLECULAR SIEVES AND BIOBASED ADSORBENTS S. AL-ASHEH*,{, F. BANAT and N. AL-LAGTAH Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan T he potential use of new biobased adsorbents and different types of molecular sieve was assessed in the separation of the ethanol–water azeotrope. Molecular sieves of type 3A, type 4A, and type 5A and biobased adsorbents such as natural corncobs, natural and activated palm stone and oak, were used in this study. Each of these adsorbents was packed in a column, which was surrounded by a heating jacket. The jacket temperature was maintained constant in an attempt to maintain the packed-column at constant temperature. The water concentration in the feed solution was varied from 5% to 12% (weight basis). Although the initial concentration of ethanol in the flask was constant at the beginning of the experiment, it varied with time due to boiling. The breakthrough curves of water sorption on these adsorbents at different water contents showed that among the molecular sieves examined, type 3A molecular sieves gave the best separation of the ethanol–water system and among biobased adsorbents examined, natural palm stone was the best. The Guggenheim, Anderson, and De Boer (GAB) model at different water contents represented the isotherms for water sorption on molecular sieves and biobased adsorbents. In addition, the surface area of adsorbents and maximum water uptake were calculated. Type 3A molecular sieves were determined to have the largest surface area and the highest value of water uptake compared with the other two types of molecular sieve. However, for the biobased adsorbents, activated palm stone was found to have the largest surface area and the highest value of water uptake compared to the other biobased adsorbents. It was found that upon activation, the adsorption capacity of biobased adsorbents was not enhanced. On the contrary, the activation gave negative results on some occasions. Keywords: azeotrope; ethanol–water; molecular sieves; biobased sorbents. INTRODUCTION Ethanol is considered as one of the most useful synthetic oxygen-containing organic chemicals. This is due to its unique combination of properties as a solvent, a beverage, antifreeze, a fuel, a depressant, and especially its versatility as a chemical intermediate for other organic chemicals (Othmer, 1981). Usually it is produced in an aqueous form either by synthetic methods or fermentation processes. To obtain pure ethanol, the conventional separation methods of water removal include distillation processes. However, these are inapplicable since at atmospheric pressure ethanol forms an azeotrope with water at an ethanol composition of 95 wt% and a temperature of 78.1�C (Walas, 1985). There- fore, special methods are used to break the azeotrope; these include azeotropic distillation or extractive distillation where ethylene glycol is added as a solvent (Treybal, 1980). In addition, a membrane process known as pervaporation has recently been used to break the ethanol–water azeotrope using polyvinyl alcohol membranes (Cussler, 1997; Hickey and Slater, 1990; Strathmann and Gudernatsch, 1991; Choi et al., 1992; Bruschke, 1990). Pervaporation involves the preferential passage of ethanol through a dense membrane matrix. However, these polymeric membranes were not found to be suitable for applications involving high water concentrations or to de-water ethanol to very low dryness. Zeolite-based membranes have been found efficient in separating the ethanol–water azeotrope (Shah et al., 2000; Nomura et al., 1998). One of the methods used in separating water from ethanol is adsorptive distillation. The adsorbents used are called desiccants. These include molecular sieves, chloride salts, silica gel, and biobased desiccants. Type A molecular sieves are the most used in the application of ethanol–water azeotrope separation. Sowerby and Crittenden (1998) studied the recovery of ethanol from the ethanol–water system at the azeotrope composition by adsorption using 855 *Correspondence to: Dr S. Al-Asheh, Department of Chemical Engineering, Jordan University of Science and Technology, PO Box 3030, Irbid 22110, Jordan. E-mail: alasheh@just.edu.jo {Currectly on sabbatical leave at Qatar University, Department of Chemical Engineering, PO Box 2713, Doha, Qatar E-mail: alasheh@qu.edu.qa fixed beds packed with various pore sizes of type A molecular sieves. They found that type 4A are superior to type 3A molecular sieves, with higher adsorption capacities, while the type 5A molecular sieves were determined not to be suitable since there is a great possibility of ethanol adsorption within their pores. However, it was found that the use of a type 3A molecular sieve for drying the ethanol– water azeotrope instead of a type 4A is more efficient in terms of energy saving since the regeneration energy requirements for type 3A are lower than those of type 4A (Trent, 1993). It has been shown that biobased desiccants, which are mostly composed of cellulose and starch, are also able to remove water from a wide range of organics (Ladisch, 1997). Ladisch and Dyck (1979) first demonstrated the separation of ethanol–water vapour mixtures over cracked corn, starch and carboxymethyl cellulose, with a positive energy balance compared with other adsorbents, such as calcium oxide and sodium hydroxide. The authors found that the total energy consumed for drying of ethanol by calcium oxide is 3669 kJ kg�1 ethanol, while the energy consumed using cellouse is 2873 kJ kg�1 ethanol (Ladisch and Dyck, 1979). It was also found by the same authors that the adsorption process using biobased adsorbents is more energy-efficient than azeotropic distillation. The energy consumption in azeotropic distillation for the ethanol– water azeotrope is about 88 kJ m�3 ethanol, which is higher than the energy consumption needed for the adsorp- tion process using corn grits by 32 kJ m�3 ethanol (Ladisch and Dyck, 1979). This indicates that the adsorption process is less expensive than the azeotropic distillation process (Lee and Ladisch, 1987). Corn grits are currently being used in industry to dry over 750 million gallons of ethanol per year, with 99.8% dryness by weight (Beery and Ladisch, 2001). However, biobased desiccants are biodegradable and are derived from renewable resources (Lee and Ladisch, 1987; Beery and Ladisch, 2001). In this work, new biobased desiccants that are able to separate the azeotropic water–ethanol mixture are devel- oped. Molecular sieves were taken as reference materials for comparison purposes. The water content in the feed solution was varied and the breakthrough behaviour at each water concentration was determined. The isotherms of water sorption are also presented and discussed at different inlet water contents. MATERIALS AND METHODS Desiccants Three different types of molecular sieve, namely type 3A, type 4A, and type 5A, were used in this work. These were purchased from Scharlau Chemie (Spain) and The Asso- ciated Cement companies Ltd. (India). The molecular sieves were dried in an oven at temperatures of 190–210�C for 24 hours. This is to make sure that all humidity within the crystals of molecular sieves is evaporated (Trent, 1993). The dried molecular sieves were kept in bottles, which then were stored in a glass chamber. The glass chamber contained silica gel to make sure that no humidity would be sorbed on the molecular sieves. Palm stones (date pits), oak, and corncobs were used in this work as natural biobased adsorbents. The palm stone and oak were washed with distilled water and then dried in an oven at temperatures of 70–80�C for 24 hours. The dried natural palm stone and oak were crushed and dried again. The dried palm stones and oak were crushed, milled and sieved into different particle sizes. They were then kept in bottles and stored in a glass chamber, which contained silica gel. Studies were focused on the size fraction of 0.125– 0.212 mm. Corncobs were cut into small pieces with a knife and washed with distilled water before being dried in a packed bed using nitrogen at a temperature of 90�C for six hours. This was to ensure that no biological degradation occurred to the polysaccharides within the corncobs (Westgate and Ladisch, 1993). The dried corncobs grits were then kept in bottles that are stored in a glass chamber containing silica gel. Crushed palm stone and oak also underwent a physical activation process using a vertical fluidized backed column oven. The procedure of activation is summarized as follows. The column was filled with the natural material. The temperature of the bed was raised to 700�C, using electrical heaters, in the presence of nitrogen gas at a flow rate of 1.5– 2.0 cm3 min�1. The material was kept at 700�C for two hours. After that, the temperature was raised to 900�C in the presence of nitrogen gas. Pyrolysis gases were formed and these were released to the atmosphere directly. When the temperature reached 900�C, the nitrogen gas flow was replaced by carbon dioxide gas for 30 minutes. At the end, the heaters were switched off and cooling was allowed. The activated materials were then removed from the column. They were dried in the oven at temperatures of 190–210�C for 24 hours to make sure that no humidity would be sorbed on the activated materials during the cooling process. The dried activated biobased adsorbents were kept in bottles that were stored in a glass chamber containing silica gel. Raw palm stones and activated palm stones have BET surface areas of 1.56 and 870 m2 g�1, respectively, and micropore surface areas of 0.19 and 364 m2 g�1, respectively (Guo and Lua, 2001). Bench-Scale Fixed Bed Adsorber A bench-scale fixed bed adsorber apparatus has been designed and is shown schematically in Figure 1. The adsorbent was packed in the fixed bed (14.6 mm ID (internal diameter); 25 cm depth). The column wall temperature was maintained at the initial bed temperature during the experi- ment by circulating hot water through the jacket. The jacket was insulated with heating tape. In a typical experiment, vapour was boiled up from a 500 ml flask surrounded by an electric heating mantle. Heat input was controlled by a variable transformer. The tempera- ture of the mixture in the boiling flask was measured by a thermocouple. The atmospheric pressure was assumed constant at 760 mmHg. The temperature of the jacket was controlled and kept constant during the runs. The exit stream was condensed using water as a cooling medium. The condensate was removed every four minutes; a volume of about 6 ml was collected. Thus, the average flow of the condensate was about 1.5 ml min�1. This was achieved by controlling the heat input by manipulating the power supply. A Karl Fischer titrator measured the water content of the condensate. At the end of experiment, the adsorbent was removed from the bed and dried for further use. Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A7): 855–864 856 S. AL-ASHEH et al. The adsorbents were efficient even after 10 cycles of the adsorption–drying process. The aqueous ethanol mixtures were prepared at four water levels, namely, 5 (the azeotropic concentration), 8, 10, and 12 wt%. They were prepared by weighing the desired amounts of distilled water and ethanol (Frutarom Ltd., UK) to make about 350 ml solution. RESULTS AND DISCUSSION Breakthrough Curves Using Molecular Sieves Three molecular sieves of type A were used to study the breakthrough behaviour, a sudden increase in water concen- tration in the effluent, of water sorption using four different water concentrations of ethanol–water mixtures. The break- through curve of water sorption is plotted for the three different pore sizes of molecular sieve at the different water concentrations. The results are shown in Figure 2 for water contents of 5, 8, 10, and 12 wt%, respectively. It is seen (Figure 2) that the time required to reach a plateau value varies as a function of the inlet water content of ethanol–water mixture. Note that some curves do not show the plateau behaviour since more time was required for these curves to attain the plateau value and these data are not shown. The time needed to reach a plateau value in the case of 5 wt% was about 144 minutes. This is longer than that of other water contents, which is about 120 minutes. It is also noted (Table 1) that the plateau time, time at which the water content in the exit becomes the same as the inlet, becomes shorter as the water concentration increases. This behaviour is attributed to the fact that the 5 wt% water content represents the azeotropic composition of the mixture; i.e., the compositions of ethanol and water in the liquid and vapour phases are the same. The separation of ethanol from water is only achieved by the selective sorption of the molecular sieves used. It is also worth mentioning that the breakthrough behaviour likely resulted in less than complete water removal given that the column was relatively short. To distinguish between the three types of molecular sieve, the average water content in the samples of the effluent stream was considered as well as the calculated breakthrough time for each type of molecular sieve used. The breakthrough time is arbitrarily designated to be the time required to reach 2 wt% of water content in the effluent stream. This value was chosen because it happens that water concentration in the first droplet of condensate was greater than 1 wt% for some of the sorbents. It is important to emphasize that the data are plotted as water content in effluent stream divided by initial water content (C=C0) versus time, therefore, the breakthough time varies depending on the initial water content. A 2 wt% was selected since the sorption of water on type 5A molecular sieves usually gives outlet water content more than 1 wt%. The average content of the effluent and the breakthrough time for the different types of molecular sieve are displayed in Table 1. It can be concluded from the data of Table 1 that at a given water content, type 3A molecular sieves has the best sorption performance, followed by type 4A, and lastly by type 5A. The longer the breakthrough time and the lower the average water content in the outlet, the better is the sorption performance. The diameter of the ethanol molecule, which is 4.46 A˚ (Westgate and Ladisch, 1993), is the main deter- mining factor of the performance of all types of molecular sieve. Since the size of the ethanol molecule is less than the pore diameter of type 5A molecular sieves, ethanol molecules can pass the pores of the molecular sieves and be sorbed there. However, as the diameter of the water molecule is 2.8 A˚ (Trent, 1993), which is less than any of the three types of molecular sieve used in this work, the three types of the molecular sieve can easily sorb water mole- cules. The higher adsorption capacity of type 3A molecular sieves can be attributed to its high surface area compared to that of type 4A and type 5A molecular sieves (Sowerby and Crittenden, 1988). It was expected that when molecular sieves were exposed to more water vapour, the breakthrough time would be shorter and the average outlet water content would be higher. This was verified by the results of Table 1. This can be related to the fact that as the water content is increased in the feed, the amount of vapour that passes through the bed will be higher, thus more water molecules will pass through the pores. For example, for type 3A molecular sieves, the breakthrough time was shortened from 88.9 to 61.2 minutes when the water content was increased from 5 to 12 wt%. Moreover, the average water content in the condensate increased to 2.01 mol l�1 at 12 wt%, which is about three times larger than the average water content of condensate when using 5 wt% inlet water content. Figure 1. Schematic diagram of bench-scale fixed bed adsorber. Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A7): 855–864 SEPARATION OF ETHANOL–WATER MIXTURES USING MOLECULAR SIEVES 857 The main conclusion of these results is that the best type of molecular sieve that can be used for drying of ethanol–water mixtures type 3A. Therefore, the perfor- mance of type 3A was considered as a datum to which the performance of the examined biobased adsorbents was compared. Breakthrough Curves Using Biobased Adsorbents Three types of biobased adsorbents were utilized for the drying of ethanol–water mixtures using four different initial water contents. These biobased adsorbents were: palm stone, oak (with and without activation) and natural corncobs. The breakthrough curves of these biobased adsorbents are illustrated in Figure 3 using water contents of 5, 8, 10, and 12 wt%, respectively. The shape of the breakthrough curves obtained for these biobased adsorbents was similar to those obtained when type A molecular sieves were used (Figure 3). It can be seen that the time needed to reach a plateau value using the biobased adsorbents is shorter than that for the molecular sieves (Figure 3). This can be attributed to the uniform and non-uniform structures of the molecular sieves and the biobased adsorbents, respectively. As such, there is a possibility of ethanol being sorbed on the biobased adsorbents. Lee et al. (1991) studied the sorption pheno- Figure 2. Breakthrough curves for ethanol–water system during water sorption on different types of molecular sieve using (A) 5 wt%; (B) 8 wt%; (C) 10 wt%; and (D) 12 wt% inlet water content. Trans IChemE, Part A, Chemical Engineering Research and Design, 2004, 82(A7): 855–864 858 S. AL-ASHEH et al. mena of ethanol and water on starch, which is the main constituent of the biobased adsorbents, and found that ethanol sorption can be assumed to be negligible compared to water sorption by the starchy material. This assumption can also be applied to the results of this work. The breakthrough time and the outlet average water content are reported in Table 2. The water concentration for each outlet-collected sample was determined from the knowledge of concentration and volume of samples collected at different time intervals. The average concentration of all outlet-collected samples, Cw,avg., was then calculated using the following expression: Cw,avg: ¼ Pn i¼1 ViCw,iPn i¼1 Vi (1) where Cw,i is the water concentration in ith sample (mol l �1), Vi is the volume of the ith sample (ml), and n is the number of outlet-collected samples. The data of Table 2 can be used to compare the performance of the biobased adsorbents to that of the molecular sieves. It is obvious that natural palm stone, among the tested biobased adsorbents, is the best one for ethanol drying; it showed the lowest average exit water content and the longest breakthrough time compared to the other tested materials. This is true for all inlet water contents (Table 2). This would indicate that the surface-active func- tional groups in palm stone, which are believed to be poly- saccharides, are more numerous than those in the other biobased adsorbents. It is also worthy mentioning that acti- vation of palm stone and oak did not improve their sorption performance. On the contrary, sorption performance of these biobased materials deteriorated upon activation (Table 2). Activation was expected to change the structure of palm stone and oak, so as to make them more porous materials, meaning that there would be more possibility of water molecules being sorbed. However, this is not the case here. It might happen that the sorption process of water within the biobased adsorbents is determined by chem