| Glutamic acid (GA) is not only one of proteinogenic amino acids, but also an important free amino acid which is widely used in the fields of food industry, medicine industry, chemical industry for daily supplies and feed industry. Total output of the GA was 1.6 million tons in China in 2009, accounting for 75 per cent of global output. GA has now become one of the domestic staple fermentation products with strong competitive advantage in the global market. In our country, isoelectric crystallization with ion exchange (IEIE) is now the most common GA extracting process. Although the yield coefficient is high in IEIE, large amount of raw material is consumed and environmental pollution is serious. Instead, a concentrated continuous isoelectric process (CCIC) with low level of raw material consumption is applied to extract molasses-based GA. However the yield coefficient is not high enough in CCIC. Therefore modification of the current GA extraction technology to achieve the target of "energy-saving, raw material consumption and emission reduction" is essential for the promotion of the sustainable development and international competitiveness of Chinese GA industry. In this paper, key technologies of GA evaporative crystallization in the high viscosity solution, continuous isoelectric crystallization with eliminating fine crystal, and bacterium removal by combining thermal denaturation with flocculation were studied using Biotin-limited GA fermentation broth as the research object. A novel and cleaner GA extraction technique, called two-stage crystallization technology (TSC), was proposed and tested in pilot scale (76 m3). The main research work is summarized as follows:(1) pH-shift model was adopted to investigate the effect of supersaturation on the GA crystallization using monosodium glutamate (MSG) solution instead of fermentation broth. Results showed that the supersaturation influenced both the crystal habit and size distribution. The main crystal habit wasα-form and the crystal size showed a logarithmic symmetric distribution when the supersaturation S≤7.15 at 25℃. With the increase of the supersaturation, the percentage ofβ-form crystal increased whereas the median diameter decreased with a wider size distribution. Almost all the crystal habit wereβ-form when the S≥14.17. According to the theoretical calculation and measurement of the induce time of nucleation, the increase of nucleation rate with the supersaturation is the primary reason for the widened crystal size distribution range. A GA C-pH Phase diagram model was proposed based on the results. This model explained how the supersaturation affected the GA crystal habit theoretically. The supersaturation could be efficiently reduced, and GA crystal habit as well as the crystal size distribution range could be improved by controlling the rate of acidity adjustment or by adding crystal seed.(2) The pH-shift model was used to study how the viscosity of solution influenced the GA crystallization by adding sucrose to change the viscosity of solution. Results indicated that the viscosity did not affect the GA habit but reduce the nucleation rate in the range of 1.00-10.26 mP·s. The increase of viscosity led to smaller crystal size. The rate of crystallization was controlled by the solute diffusion rate in the high viscosity solution. Nucleation rate and crystal growth rate decreased with the increased viscosity which caused the smaller crystal size. The viscosity could be reduced by enhancing the temperature to increase the nucleation rate and crystal growth rate. Larger crystal size and better crystal habit could be obtained by adding crystal seed properly.(3) The effects of temperature and condensed fold on the viscosity of condensed mother liquor and GA solubility were investigated using the isoelectric mother liquor in which the bacterium was removed. Results suggested that the viscosity increased as the folds of concentration enhanced. The viscosity could be reduced by rising the temperature. Besides, the viscosity under different concentration was similar when the temperature was over 60℃. On the other hand, high temperature could also promote the coking rate of GA. So the temperature should be controlled under 70℃to avoid the loss of GA in the evaporation crystallization process. To control the GA habit and size distribution, experiments were carried out to study the effects of crystallization temperature, strategy of seed addition, evaporation rate, folds of concentration and other factors during the evaporation crystallization process. The optimum operation conditions were: the optimum crystallization temperature at 60℃, seed crystal size of 120 mesh, the amount of seed addition of 30% of the theoretical GA quantity, evaporation rate of 207 L/m~2·h and 6 folds. Under these conditions, granularα-form crystal with median diameter of 122.2μm was obtained, and the yield coefficient of extraction reached to more than 72.8%.(4) The maximum cycle rate of fine crystal elimination (R) was 1.3. Further research was done to investigate the effects of the technological parameters such as R, crystallization retention time and other parameters on the GA crystallization. As the increase of R, the cutting particle size of fine crystal was increased from 36.44μm to 51.35μm, and the crystal size was also increased. At the same time size distribution tended to be centralized. This phenomenon was due to the decrease of supersaturation and the nucleation rate, in contrast the increase of crystal growth rate with the increase of R value. Crystallization retention time could also influence the supersaturation and then crystal size distribution. This system was proved to efficiently eliminate the fine crystal and improve the crystal quality by comparing the size distribution of the three crystals in the processes of IEIE, evaporation isoelectric crystallization and continuous isoelectric crystallization respectively.(5) In order to determine the optimum condition for the removal of bacterium, COD and soluble protein, the effects of flocculants, dosage, pH, temperature and time on flocculation were carried out using Response Surface Analysis. Highest bacterium, COD and soluble protein removal capacities (99%, 47% and 65%, respectively) were obtained when the PAAS was used under the condition of pH 3.1, dosage 60 mg/L, temperature 80℃and 30min. And the filtering rate was 280 L/m2·h when filter cloth was used. The mechanism of thermal denaturation with flocculation technology was studied. Results indicated that higher temperature could reduce the zeta potential on the surface of bacterium, compress the thickness of diffusion layer around the surface of particle, and enhance the bridge effect. The median diameter of the floccules reached to more than 148μm. In this condition, high filtering rate could be obtained using common filter cloth.(6) Based on the above results, a novel and cleaner GA extraction technique, called two-stage crystallization technology (TSC), was proposed and tested in pilot scale (76 m3). Crystal separation and crystal quality were optimized in the pilot scale test to form a more perfect two-stage crystallization technology. Average yield coefficient of extraction was 93.4% which is higher than that of evaporation isoelectric crystallization process by 5.4% and lower than that of IEIE process by only 1.6%. Purity, SO42- concentration and transmittance were superior to those of the other two processes. Compared with IEIE process, sulfuric acid and liquid ammonia consumptions were reduced by 53% and 100%, respectively. Technical analysis shows that production cost of TSC process would be lower than those of IEIE process and evaporation isoelectric crystallization process by 15.8% and 14.6%, respectively, provided that the fermentation level, the GA quality and effect on environment of the three processes were same.
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