SEPARATION OF CITRIC AND LACTIC ACID FROM FERMENTATION LIQUID BY THE CHROMATOGRAPHIC METHOD MODELING AND PARAMETERS ESTIMATION Paweł GLUSZCZ and Jerzy PETERA Faculty of Process and Environmental Engineering, Technical University of Łódź, ul. Wólczańska 213/215, 93-005 Łódź, Poland e-mail: pgluszcz@ck-sg.p.lodz.pl INTRODUCTION In biotechnological production of organic acids the purification process is highly expensive. At present, only two methods of citric acid separation from fermentation broth are used in industry: the precipitation method and extraction in the liquid-liquid system. Both methods consist of many laborious and/or energy-consuming steps, requires a large amount of water and chemicals, and as a result a significant amount of waste is produced. Moreover, both methods do not offer high purity of the product. The most promising method in downstream processing of biotechnology products seems to be Simulated Moving Bed Chromatography (SMBC). The application of an ion-exchange process enables production of high purity material by means of a single sorption/desorption operation, omitting many stages which occur in the technologies applied so far. This method is very efficient and enables selective separation of components with similar properties. However, a correct design of the process based on the SMB chromatography requires the knowledge of many parameters of an ionexchange bed, sorption equilibrium and of bed operation in the ion-exchange column. To identify and to optimise the operation parameters of the column it is most convenient to use a mathematical model of the process. The aim of this work was to determine properties of selected ionexchange resins for citric and lactic acids separation, to define sorption isotherms for citric and lactic acid at different temperatures and to determine diffusion coefficients inside sorbent particles. A mathematical model of the ion-exchange process in the chromatographic column is also presented in this study. This model may be used to identify the bed operation parameters in the column for the needs of the SMBC method. MATERIALS AND METHODS The aim of the preliminary investigations was the evaluation of the applicability of different ion-exchange resins in citric acid and lactic acid
purification. The experiments covered 18 types of anionic resins available on the market: Amberlite types IR-120, IRA-67, IRA-92, IRA-93, IRA 400, IRA 420, IRA-458, IRA-904, XAD-4 and XAD-2, Purolite A-510 and A- 850, Merck Ionenaustauscher I, III and IV, Dowex 200, Dowex WBA and Wofatit SBW. There resins were tested taking into account their selectivity, bed capacity and optimum ion-exchange conditions. The tests were carried out in glass columns of 20 cm 3 bed capacity. The sorption was performed at different temperatures from the range 20-60 C, using the raw fluid of different concentrations of citric acid. The acid was eluted at sorption temperature initially, and then at 25 C for the best resins. The eluents used included distilled water and 0.1, 0.3 and 0.5 M solutions of H 2 SO 4. The concentration of the investigated acid in the outlet stream was analyzed by a spectrophotometer (Unicam UV-300, Great Britain) at the wave length 210 nm. For the needs of mathematical modeling of the bed operation, the sorption isoterms of the citric and lactic acids were investigated for the selected resins in batch, stirred tank experiments, at different temperatures from the range of 20 0 C 60 0 C. The regenerated weighed resin particles were contacted in the glass flasks with the known amount of the acid solution of different initial concentrations from the range of 0.01 0.5 kmol/m 3 and well mixed. The concentration of the acid was measured every day, untill the equilibrium was achieved. The experimental equilibrium data were correlated by the Langmuir equation and then saturation capacity of the resin, q m, and equilibrium constant, b, were calculated. On the basis of transient-state batch sorption experiments the diffusion coefficients of citric acid inside the sorbent particle at different temperatures were also obtained. The experiments were performed in similar conditions as for the equilibria measurements, but after contacting sorbent particles with the solution the citric acid concentration in the liquid phase was measured every two minutes. The diffusion coefficients in the sorbent particle pores, D p, were determined then by numerical fitting of the experimental concentration curve, using the mass balance equation within a spherical particle of radius r and assuming no mass transfer resistance in the liquid film outside the particle. The mass transfer parameters in an ion-exchange column were determined by means of the mathematical model of the process. The model includes mass transfer in the liquid film at the surface of the sorbent particle, diffusion in the liquid phase within the pores of the particle and sorption onto the solid surface. It was assumed, that the process is isotermal, the diffusion coefficient inside the pores is concentration independent, the mass transfer in the liquid film is represented by the linear gradient relationship, the sorption equilibrium is represented by the Langmuir isoterm, the sorbent particles are spherical and uniform in density and radius, the solution flux along the bed may be characterised by the longitudinal dispersion coefficient, D L. The equations for the mass balance of the solute in the liquid
phase flowing along the bed height was formulated assuming that there is no radial flux and no radial concentration gradient in the cross-section of the column. This model was solved by the finite element method in the nonsteady state regime. The calculations verifying the model on the basis of experimetal breakthrough curves were made and a very good agreement of experimental and calculated data was achieved. The presented model was also applied to identify the bed operation parameters in the column, i.e. the mass transfer coefficient in the liquid film, the axial dispersion coefficient in the column, the diffusion coefficient inside sorbent particles and the real liquid velocity in the free cross-section of the column, on the basis of experimentally obtained breakthrough curves. As an optimisation method the Powell s conjugated gradients method was used. RESULTS AND DISCUSSION From the preliminary experiments performed in a small column it was found that for separation of citric acid from the fermentation broth the most suitable is the resin of commercial trademark Amberlite IRA-67; it is a weakly basic gel-type polyacrylic resin with amine functional group. The optimum process conditions for this sorbent were as follows: citric acid concentration in the raw fluid 20%, concentration of H 2 SO 4 used as an eluent 0.1 M, sorption temperature 60 C, elution temperature 20 C. The effective bed sorption capacity of the sorbent in such conditions was 2.71 mval/g, and the efficiency of citric acid recovery from liquid was about 90%. For all investigated resins the effective capacity of the bed increased with the increasing temperature. Tab. 1. Parameters of the Langmuir isoterm and diffusion coefficients for the Amberlite IRA-67 resin Temperature Citric acid Lactic acid [ 0 C] q m b D p *10 10 q m B [g /dm 3 ] [dm 3 /g] [m 2 /s] [g /dm 3 ] [dm 3 /g] 20 316.1 0.49 3.38 226.1 3.54 40 278.9 0.69 4.87 224.0 1.57 60 207.9 1.39 7.19 214.4 0.52 At the same time analysis of the results obtained in sorption equilibrium experiments leads to the conclusion, that the saturation capacity of the resin decreases with the increasing temperature (table 1); this effect is more significant for citric acid than for lactic acid. These apparently contradictory results can be explained, if we take into account the increase of diffusion coefficients in the resin particle at higher temperature. In the transient-state
conditions in a column, when the sorption system is far from equilibrium, the sorption rate, hence the diffusion coefficient, has greater influence on the breakthrough time than the saturation (i.e. equilibrium) capacity of the resin. This conclusion may be justified by the comparison of breakthrough curves obtained in an experimental column at different temperatures: at higher temperature the breakthrough occurs later. For the lactic acid recovery the Amberlite IRA-92 was the most suitable, however IRA-93 and IRA-67 resins have the similar saturation capacity. The corresponding results are presented in table 2. From all experimental data it can be concluded that for recovery of the investigated organic acids weakly basic resins are more suitable than strongly basic, as e.g. IRA-400 or IRA- 458. Table 2. Parameters of the Langmuir isoterm for the lactic acid (temperature 40 0 C) Type of the resin IRA-67 IRA-92 IRA-93 Dowex WBA IRA-400 IRA-458 q m [kg/m 3 ] 224.1 247.5 225.7 200.8 161.7 149.9 b [m 3 /kg] 3.53 0.77 0.28 1.93 0.68 1.54 To determine the bed operation parameters in the experimental column the presented mathematical model was applied. The axial dispersion coefficient, D L, intraparticle diffusion coefficient, D p, film mass transfer coefficent, k f, and the real linear liquid velocity in the free cross-section of the column, ν, were determined by the optimal fitting of the predicted breakthrough profile to the experimental curve. The algorithm of Powell s conjugated gradients routine was used as an optimisation method. The performance index for all calculations was in the range 0.5 3%. In table 3 the model parameters are presented. It may be seen that k f and D L depend on the flow rate; lower values of these parameters correspond to break-through curves with smaller slopes and delayed break-through points. In such conditions the column performance could be increased significatively. Table 3. Calculated values of the experimental column model parameters U * 10 2 [dm 3 /s] ν 10 4 [m/s] D L * 10 5 [m 2 /s] k f * 10 8 [m/s] Performance index [%] 0,7 1,7 0,12 0,91 2,7 2,0 5,4 0,38 1,07 2,5 4,2 8,3 2,73 2,60 0,7 6,0 19,1 2,61 2,86 0,7 6,7 19,9 2,74 3,21 0,9
CONCLUSIONS Analysis of the experimental data leads to the following conclusions: - for recovery of the organic acids from fermentation liquid weakly basic resins are more suitable than strongly basic; - the most suitable for separation of citric acid from the fermentation broth was the resin Amberlite IRA-67, and for lactic acid Amberlite IRA-92; - the saturation capacity of the investigated resins decreased with the increasing temperature and this effect was more significant for citric acid than for lactic acid; - the effective capacity of the bed in the ion-exchange column in transient state increased with the increasing temperature; this effect may be correlated with the increasing diffusion coefficient within the sorbent particle at higher temperature; - the presented mathematical model of the column described the experimental data well; it may be used to determine the bed operation parameters in a single ion-exchange column and/or to simulate operation of the columns in the SMBC system; - the film mass transfer coefficient and axial dispersion coefficient in the liquid phase depended on the flow rate and had an influence on the shape of breakthrough curves. ACKNOWLEDGEMENTS This research was supported by the grant 3 T09C 040 16 of the Polish State Committee for Scientific Research.