XX-th ARS SEPARATORIA Szklarska Poręba, Poland 2005

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1 SEPARATION OF WATER FROM ALIPHATIC ALCOHOLS USING ADSORPTION ON ZEOLITE 4A Elżbieta GABRUŚ ) and Daniela SZANIAWSKA ) ) Chemical Engineering and Environmental Protection Institute, Technical University of Szczecin, Al. Piastow 42, Szczecin, Poland ) Division of Environmental Science, Department of Food Science and Fisheries, Agricultural Academy of Szczecin, ul. K.Kazimierza 4B, Szczecin, Poland ABSTRACT The equilibrium of water adsorption on zeolite 4A sieves from ethanol and propanol solutions with the water concentration in the range of 0 60 g/dm 3 has been analyzed in terms of model combining two adsorption models. The applied model is consisted of Langmuir Freundlich isotherm (LP) and excess surface work model (ESW). The Langmuir Freundlich isotherm describes adsorption equilibrium in the range of lower water concentration with three parameters, affinity and heterogeneity constants and maximum adsorbate concentration. The excess surface work model is based on thermodynamics and describes adsorption equilibrium in the range of higher water concentration with two parameters, monolayer capacity and chemical potential. The parameters of both models for adsorption of water from ethanol and propanol solutions, were evaluated and discussed. INTRODUCTION In the frame of sustainable development concept, there is a great need for developing environmental friendly technologies and techniques for recycling of materials and minimization of waste. To cope with these challenges, the scientists try to improve conventional technologies to be as green as possible. In the field of alcohols production, the attention of researchers is focused on bioprocesses, because of two main reasons. First, the bioprocesses are running in mild conditions, more environmental friendly than chemical synthesis. Second, the renewable materials as well as waste sugar sources can be used as a substrates for fermentation process. The hybrid process of bio-alcohol production is usually consisted of fermentation step and appropriate separation techniques for removal of alcohol solution from fermentation broth, preconcentration of dilute alcohol solutions and final dehydration of azeothropic mixture. The removal of water from azeothropic alcohol mixture using adsorption on molecular sieves seems to be promising, from 10

2 environmental point of view, separation method in comparison to conventional ones, such as extraction distillation or liquid-liquid extraction. In the paper, the results of investigation of equilibrium of water adsorption from ethanol and propanol solutions on zeolite 4A sieves using adsorption process are presented and discussed. Experimentally it was found that adsorption isotherm of water on the zeolite 4A belonged to type II according to the Brunauer classification [1]. The mathematical modelling of such complicated shape of isotherm in wide range of adsorbate concentration using one isotherm model, is impossible. In this study, the combined form of two isotherm models derived for adsorption of gases was adopted and used for handling the experimental results obtained for liquid system. THEORY Type II isotherms do not exhibit a saturation limit [1,2]. Near to the first point of inflection of such isotherms a monolayer is completed following which adsorption occurs in successive layers. For mathematical modeling type II curves should be divided on two parts. Modelling of first part of type II isotherm, in the range of lower concentration of adsorbate can be done with good fitting to the experimental data, using (1), called Langmuir - Freundlich isotherm in the form [2,3] : 1 ( b C ) k e + ( b C ) k nads ns 1 1 e = (1) where n s, b and k are parameters of LF and C e is equilibrium concentration of an adsorbate. The parameter b is called the affinity constant, and it is a measure of interaction between adsorbate molecule and adsorbent surface. The parameter n s is the maximum adsorbed concentration corresponding to the complete monolayer coverage. The parameter k could be regarded as the parameter characterizing the system heterogeneity. If parameter k is unity, Eq.(1) becomes the Langmuir applicable for ideal surface. The parameter k is usually greater than unity, and therefore the larger this parameter the more heterogeneous the system is. For modelling of the second part of type II isotherms, there was applied thermodynamic model, which was developed for gas systems by Adolphs and Setzer [4]. The essential term of this model is the excess surface work, Φ of the adsorbed layer defined for an open isothermal adsorption system with finite changes by Eq.(2): Φ = n Δμ (2) ads where nads and Δ μ are the amount of adsorbed adsorbate and the change of chemical potential during the adsorption process, respectively. 11

3 The ESW model describes adsorption isotherms with a two-parameter function (Eq.3): n = n ln Δμ ads mono (3) Δμ 0 n mono and Δ μ 0, which is necessary for adsorption. The physical meanings of these two parameters are: monolayer capacity, change of chemical potential, The parameters nmono and Δ μ0 can be easily evaluated using the linear plot of ln Δμ against n ads from the reciprocal slope and ordinate intercept, respectively. RESULTS AND DISCUSSION The adsorption equilibrium data experimentally determined for ethanol and propanol water systems were handled with the use of both Langmuir Freundlich isotherm in the range of lower water concentration and ESW model adapted to liquid system in the range of higher water concentration. The change in the chemical potential, Δ μ at constant temperature was expressed for liquid system by Eq.(4): ( 1+ Ce Δ μ = RT ln 1 ) (4) where C e is equilibrium concentration of adsorbate, [g/l]. Taking into account the Eq.(4) the ESW model expressed by Eq.(2) can be rearranged into form: n ( ln RT ln ( C ) Δ μ ) = n ln ads mono e 0 (5) The equilibrium isotherms of water for ethanol water and propanol water systems are presented in Fig.1. 12

4 a) b) n [mol/g] Langmuir- Freundlich Langmuir- Freundlich ESW n [mol/g] ESW C e [g/l] C e [g/l] Fig.1. Equilibrium isotherm of water on zeolite 4A for (a) water-ethanol and (b) water-propanol systems at the temperature 303 K (symbols-data, line-lf and ESW s) a) ϑ b) ϑ φ φ Fig.2. Plot of the normalized excess surface work Φ against the degree of coverage ϑ for ethanol-water system (a) and propanol-water system (b) 13

5 Fig.2 shows plot of the normalized excess surface work, Φ = Φ nmono = ϑ Δμ versus the degree of coverage ϑ = n ads nmono, for ethanolwater system (a) and propanol-water system (b). Using this figure values of Φ was evaluated (Table 1).The parameters evaluated using Langmuir Freundlich and ESW s and Fig.2 are summarized in Table 1. Table 1. Parameters evaluated using (Eq.(1) and Eq.(5) and Fig.2 for ethanol-water and propanol-water systems Equation Parameters Ethanol Propanol n mono (mol/g) 0, ,00335 ESW Δ μ 0 (kj/mol) -38,33-9,55 Φ kj/mol R (-) 0,9982 0,9971 n s (mol/g) 0, ,01559 Langmuir- b (g/l) 1, Freundlich k (-) 1,8707 1,1009 R (-) 0,9991 0,9868 Fig.1 shows the adsorption data (symbols) and the fitting curves for ethanol (a) and propanol (b) solutions. For both the cases all the fitting results in the LF isotherm and ESW model matched well the adsorption data. The values of correlation coefficients R summarized in Table 1 confirm this statement. CONCLUSIONS Modelling of adsorption equilibrium results obtained experimentally on zeolite 4A for ethanol water and propanol water systems using combined form of two adsorption models gives the possibility to predict: - affinity constant; parameter b is higher for propanol-water system than for ethanol-water system; it is consistent with the larger interaction between adsorbate and adsorbent in this system and it indicates that an adsorption layer is hardier to be desorbed - initial change in chemical potential where adsorption starts, Δ μ0, which may be compared to the lowest energy level needed for adsorption occurring - normalized excess surface work, which enable classification of analyzed Φ evaluated for ethanol and system from energetic point of view; values propanol indicate physical adsorption in both system what was already concluded in our earlier paper [5]. 14

6 REFERENCES 1. B. Crittenden, W.J.Thomas, Adsorption Technology and Design, B-H D.D. Do, Adsorption Analysis: Equilibria and Kinetics, ICP H. Ahn, Ch.H.Lee, Chemical Engineering Science, 59 (2004) J.Adolphs, M.J.Setzer, J.Colloid Interface Sci., 184 (1996) E. Gabrus, M. Aksman, D. Szaniawska, Proc. of 31th International Conference of SSCHE, May (2004) Tatranske Maltiare, Slovakia. 15