Water adsorption in the zeolite bed ? study on thermal effects
Jan RAKOCZY, Krzysztof KUPIEC, Monika GWADERA ? Faculty of Chemical Engineering, Tadeusz Kościuszko Cracow University of Technology, Poland

Please cite as: CHEMIK 2013, 67, 8, 711?718

This paper presents the results of studies on adsorptive dehydration of fuel ethanol on zeolites. During the process, a huge amount of heat is generated which causes a significant increase of the bed temperature. The changes of the temperature of the adsorbent bed were determined in time and at different levels. It was stated that the content of water in the resource has the highest impact on the thermal effects of the process.

Keywords: fuel bioethanol, dehydration, adsorption, zeolites

Introduction

There is a tendency to replace the conventional fuels with the alternative ones. This reflects the depletion of fossil fuels resources, unstable situation on the worldwide fuel market, increased emission of CO2 to the atmosphere and the implementation of more strict standards of pollution emission. Thus, the worldwide production of fuel ethanol is constantly increasing. The ethanol produced by fermentation (bioethanol) is a renewable fuel, and therefore, its production as an additive to fuels has a proecological character.

Because ethanol?s toxicity is relatively little, it is well soluble in water and biodegradable, the consequences of its emergency spills are less dangerous for the environment than in the case of paraffin products. Because nowadays, many regions of the world suffer from food shortage, the production of biofuels from fully-fledges resources containing starch or sugar (biofuels of the first generation) seems of little help. As a result of using food resources for the production of biofuels, their market price has increased. Thus, biofuels of the second generation are becoming more popular. They are produced from lignocellulosic biomass which is useless in the food industry. Using the biomass or waste material containing starch enables a drastic reduction of the resources costs. However, the process of ethanol production from starch is still not common because of high durability of plant tissues, a complex technological process and high cost of obtaining the enzymes [1]. In a near future, it is expected that biofuels of the second generation will be more common than biofuels of the first generation, mainly because of their higher energy efficiency and resources diversity (no competition with the food production). For their production, the following resources will be used: waste wood or wood of low energetic value, straw, oilcakes and other agricultural waste, organic waste e.g. beet molasses, corn stems, grasses, lucernes and fast growing plants (giant grass, woodland sunflowers). As much as 73% of the worldwide production of ethanol is used as an additive to fuels. The United States and Brazil are the two world?s largest producers of fuel ethanol. These counties produce 87% of the world?s fuel ethanol, with the corn as the main resource in the United States and the sugar cane in Brazil. Table 1 presents data on the world production of fuel ethanol on each continent and in few countries which are its largest producers.

The traditional biological processes of ethanol production are well known. In the fermentation industry, in the last few years only slight improvements were implemented. The main resources are as follows: sucrose (from sugar cane, sugar beets) and starch (from corn, wheat, barley). Starting from the fermentation phase of candied mash, all technologies of ethanol production are almost the same regardless the used resource. In spite of high efficiency, the process of conversion of starch resources to ethanol requires much more energy than a method based on resources containing simple sugars. The production of ethanol as an additive to fuels consists of two phases. The first production phase (ethanol fermentation) is conducted in alcohol factories. This is a typical biotechnological process carried on in an aqueous environment, under conditions similar to the typical ones and with low concentrations of substrates. The obtained dilute solution of ethanol is then condensed by distillation to the content of ethanol above 90 %vol. (distilling spirit). The second phase relates to the further dehydration of the obtained solution and is described in the second chapter.

The adsorptive ethanol-water separation has its specific character. There are two causes for it: the heat of water adsorption is high and water content in the dehydrated resource is significant, and usually exceeds 10% of mass. Thus, the adsorptive dehydration leads to generation of huge amounts of heat and a significant increase of the bed temperature. If the maximum temperature of the adsorbent grains is high, there can be problems with the practical realisation of the adsorption process. When zeolite adsorbents are used, a high increase of the bed temperature may damage their structure. This problem emerges in the commissioning phase of the installation containing a fresh load of the adsorbent. This work presents the results of studies on the adsorptive dehydration of ethanol solutions on zeolites. During the process, changes of the bed temperature at different levels were defined. The aim of these studies was to determine the impact of process parameters on the thermal effects, and in particular on the temperature increase.

Methods for dehydration of ethanol solutions

The fuel ethanol should contain at most 0.32% vol. of water, however, the raw spirit produced in alcohol factories contains only about 90?92% of ethanol. With the multi-stage distillation it is possible to obtain the maximum concentration of ethanol of 95.6%, because water and ethanol create an azeotrope of this concentration. The further dehydration is conducted with other methods, among others:
? extraction methods; with the use of liquid carbon dioxide which replaces water in a solution with ethanol; the extract is in the form of a mixture of ethanol and carbon dioxide which is easy to separate
? membrane methods; a mixture of ethanol and water is directed into three sequential membrane modules separated by heat exchangers; elastomers are used as membranes; the obtained ethanol has a concentration above 99.9 %vol.; the main advantage of this method is low energy consumption of the process, but because of very high investment costs (equipment) this method is not very popular
? adsorptive methods; zeolites or bioadsorbents are used for water absorption (e.g. corn starch); the main advantage of bioadsorbents is that they can be further used in the form of fodder for cattle and are relatively inexpensive; the disadvantage is much higher concentration of water in the dehydrated ethanol than in the case of zeolites.

The development of adsorption processes led to introduction of the adsorption technique with regeneration by the pressure reduction (pressure swing adsorption ? PSA). The total pressure reduction in combination with purging the bed with an inertial factor (rinsing) is particularly effective. This process leads to an effective desorption of the previously adsorbed ingredient which is necessary for the next cycle. The process conducted in a gas phase is characterised by high energy efficiency. For the first time, the PSA was used for ethanol dehydration as a fuel in the 80s of XX century. The synthetic zeolites 3A are most often used as water adsorbents in ethanol dehydration because of a selective water adsorption. The ethanol dehydration is mostly conducted in a gas phase. In many papers, the studies were conducted with the use of plant adsorbents; in the paper [3], the results of studies on using corn grains as the water absorbing adsorbent from the steam mixtures with ethanol were described. Air or nitrogen at the temperature of 80?120°C was used for thermal regeneration of the bed. The corn based bioadsorbent was also used in studies described in [4÷6]. In the paper [5], the adsorption equilibrium of water and ethanol with the use of corn starch was investigated. The studies were conducted at the temperature of 82?100°C for different humidity contents in the steam mixtures of ethanol and water. The studies on the adsorption equilibrium during the removal of water from the steam mixtures with ethanol are described in [6]; different adsorbents were used, both synthetic and natural: zeolites 3A, 4A and 5A, palm seeds, corn cobs and others. Based on the analysis of the breakthrough curves it was stated, that in a case of synthetic adsorbents the best results can be achieved using zeolites 3A and in the case of biological adsorbents ? the palm grains.

The paper [7] deals with the separation of an ethanol-water mixture by the pressure swing adsorption. Based on the experimental results, the empirical model of the process was developed and its optimisation conducted. In the paper [8], the process of ethanol dehydration by the PSA method in a one column installation is described. The researchers were dehydrating a solution containing 93.5% mass of ethanol. The pressure of the feed mixture was changed from 0.2 to 0.5 MPa, whereas, the process temperature from 120 to 150°C. 99.9% ethanol was obtained at the pressure of 0.35 MPa. Papers [9÷12] describe the adsorption process modelling of the ethanol dehydration and the studies on a laboratory scale.

In the paper [9], the adsorption of water vapour from an ethanol-water mixture on zeolites 3A was analysed. A mathematical model of the adsorptiondesorption cycle was developed, a digital simulation of the process conducted and the model was verified by comparing experimental and model results. In the papers [10, 11], the analysis of adsorptiondesorption cycles in the presence of the ethanol dehydration was described. The calculations results from a developed model were compared with the results of measurements conducted in the laboratory installation. Paper [12] considers the impact of pressure loss in the gas phase at the stage of vacuum rinsing on the process course. In the paper [13], the adsorption of water and ethanol on zeolites 3A and 5A with the use of a flow microcalorimeter was examined. Both adsorbents had similar characteristics of the water adsorption; the differences were apparent regarding the adsorption of ethanol which was just slightly absorbed by the zeolite 3A in comparison with the zeolite 5A. The difference was explained by the steric effect. The heat of water adsorption on zeolites 3A at different water contents in zeolite grains was determined.

Sowerby and Crittenden [14] were investigating the ethanol dehydration in a fixed bed. They were using zeolites 3A, 4A, 5A and 10A. They stated that zeolites 5A and 10A are not adequate because of their reactivity and cause undesirable products of the ethanol conversion. Simo, Brown and Hlavacek [15, 16] developed a model of ethanol dehydration using the method of pressure swing adsorption. The modelled cycle consisted of five stages, including the pressure swing stages. The non-izothermal model with dispersive flow, variable flow rate and the momentum balance equation was considered. The numerical examinations demonstrated the existence of hot spots in the bed at the beginning of the process. These hot spots were disappearing with the movement toward the bed exit. The cyclic steady state was reached after about 350 cycles and in all examined cases the single steady state was reached. The numeric values of the temperature front were used to monitor the concentration front, because both waves were transmitted together. The impact of the feed stream temperature, duration of a rinsing stage, water concentration in the feed stream and pressures ratio on the dehydration process were examined. It was stated that an increase of the resource temperature can improve the product quality. Also duration of the rinsing stage has a positive impact on the purity of the resultant product, thus, a lower bed can be used. Pruksathorn and Vitidsand [17] presented the results of studies on the separation of a ethanol-water mixture using the PSA method. The aim of studies conducted under the atmospheric pressure in the adsorber with a stationary bed of 1.59 cm in diameter was to determine the optimal adsorption conditions.

Equilibrium of water adsorption on zeolites

The equilibrium of water adsorption on zeolites 3A is presented in the Figure 1. Based on the equilibrium data, it is possible to determine the isosteric heat of adsorption. The adsorption isosters are the parallel lines in the coordinate system ?element partial pressure – temperature? for a determined content of the adsorbed element in grains qm. Figure 2 presents the adsorption isosters for different qm values. They were determined based on the data from Figure 1. In the coordinate system in which the absolute inverse temperature (1/T) is on the abscissa and the logarithm of the partial pressure is on the ordinate, the adsorption isosters are similar to straight lines. A dotted line relates to the water condensation. From the equation of Clausius-Clapeyron it comes out that:

This dependence was used to determine the isosteric adsorption heat Qst (Fig. 3). As it can be seen, the adsorption heat is highly dependent on the content of water in grains and much less on the temperature. Extrapolating the values from the graph for the maximum values of qm, which correspond to the pressure of water vapour saturation, enables one to obtain the values of heat of water vapour condensation; for example, for a temperature of 100°C the heat of condensation amounts to 40,600 J/mole. From the Figure 3, it is possible to estimate a range of values of heat of water adsorption on zeolites: for an average content of water in grains qm = 0.1 kg/kg Qst values are in the range of 56,000?60,000 J/mole.

Experimental part

Studies on the ethanol dehydration were conducted in the installation presented in the Figure 4 [19, 20]. The main element of the installation was the adsorptive column of 13.6 mm in diameter filled with grains of zeolite adsorbents up to the height of 330 mm. From bellow, 3 thermocouples, which were located in the bed axis and were 50, 167 and 287 mm away from the inlet of the resource into the column, were inserted into the column. The water vapour under the atmospheric pressure was supplied to the heating jacket. The aqueous solutions of ethanol in different concentrations were used as a resource. The resource was fed from the top with the use of a syringe pump (type 610-2). The resource was completely evaporated in an evaporator 3 before its introduction to the column. During the flow of ethanol and water vapours through the zeolite bed, only water was adsorbed. The ethanol vapours leaving the column were condensed, and the dehydrated ethanol collected in the receiver. During the process course, the temperature changes on three bed levels were determined. The measurements results were registered automatically.

Three series of studies were conducted. Different contents of water in the resource ymol0 and different flow rates of the resource m0 were used for each study. The values of temporal dependence of bed temperature at points situated at different levels of the bed achieved in individual measurement series are presented in the Figures 5÷7 in
the form of graphic symbols.

All relationships between the bed temperature and the process time (Fig. 5÷7) have a similar shape. Initially, the bed temperature is rising, reaching its maximum value and then decreasing. The time of reaching the maximum temperature depends on the location coordinate in the bed. Longer distance to the bed inlet causes a longer time of reaching the maximum value. The maximum temperature increases depend on the water content in the resource: more water in the resource allows reaching higher temperatures. In the Figures 5÷7, solid lines represent temperature courses determined by the numerical solving of the mathematical model equations for the process of adsorption in a column, as presented in the paper [20]. It can be seen (Figures 5÷7) that the conformity between experimental and computational courses is quite satisfactory, but there are a few differences in some courses. They result from the errors in measurements and the model. The second ones are mainly a consequence of the simplifying assumptions. Moreover, some parameters of the model are erroneous, what also has an impact on the computational results. However, attention should be paid to the fact that the differences between computational and experimental temperatures are not significant and the model quality is fully adequate.

Conclusion

The process of ethanol dehydration on zeolite absorbents is strongly exothermic. Under conditions of the measurements, an increase of the bed temperature was in the range of 50?80 K. For a gas inlet temperature of 100°C, the maximum temperatures are in the range of 150?180°C. The difference between maximum temperatures from the measurements and calculations was slight. The content of water in the resource has the highest impact on the increase of bed temperature. Thus, in case of concerns about reaching too high bed temperature, it is recommended to use a resource containing a small amount of water in the period of installation commissioning (e.g. mixed with a dehydrated product).

Publications
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Jan RAKOCZY, Ph. D. Eng., graduated from the Faculty of Chemistry at the Cracow University of Technology in 1971. In 1997, he obtained the degree of doctor habilitated of technical sciences at the Faculty of Chemistry at the Silesian University of Technology. Since 2006, he has been a professor of the Cracow University of Technology. Specialisation ? industrial catalysis, chemical technology. e-mail:

Krzysztof KUPIEC, Ph. D. Eng., graduated from the Faculty of Chemistry at the Cracow University of Technology in 1972. He obtained the degree of doctor habilitated at the Faculty of Chemical and Process Engineering at the Warsaw University of Technology. Currently, he is working as an associate professor in the Institute of Chemical and Process Engineering of the Cracow University of Technology. Specialisation ? chemical and process engineering.

Monika GWADERA, M. Sc. Eng., graduated from the Faculty of Chemical Engineering and Technology at the Cracow University of Technology in 2009. She is an assistant lecturer in the Institute of Chemical and Process Engineering of this university. Specialisation ? engineering of removable energy sources.