Study On Cleaner Production Of Organic Acid Based On Precipitation Replacement Reaction

A large amount of acidic calcium sulphate as by-product is produced during the traditional extraction of organic acids from fermentation broth, which is detrimental to the environment. However, calcium alkali is a mild neutralizer for lactic acid bacteria and yields a high lactic acid concentration when added to the broth, and the precipitation of the calcium salt of organic acid plays an important role in promotion of product purity. In this thesis, the precipitation replacement reaction between the calcium salt of organic acid and the soluble carbonate was investigated to solve the problem of calcium sulfate disposal. The problems occurred in precipitation replacement reaction, ion exchange and acid/base regeneration, as well as the corresponding solutions were studied intensively. Based on the results, a new and universal technology for cleaner production of organic acids was established.Firstly, the feasibility of precipitation replacement reaction, and the effects of reaction conditions on the replacement efficiency and the particle size of calcium carbonate were examined. The experimental results showed that the removal rate of calcium was more than 99% when the ratio of ammonium carbonate to calcium lactate was 1.06:1, and such reaction could be completed in 15 min. When the ratio of ammonium carbonate to calcium citrate was 1:1, the recovery rate of citric acid could reach more than 95%, while this reaction happened slowly and took above 2.5 h. Natural sedimentation followed by plate and frame filter press could be used for the solid-liquid separation of the suspension solution obtained in the precipitation replacement reaction.The process parameters in lactic acid separation from supernatant by ion-exchange method were optimized. The results showed that the high pH of supernatant and the residual carbonate ion were detrimental to the connection between precipitation replacement reaction and ion exchange. In order to solve these problems, adjusting pH value to above 7.0 using lactic acid, followed by the heating-decompression (60℃) was used as a pre-treatment step to remove residual carbonate by ion exchange (resin D319).According to the properties of calcium citrate supernatant, the "salt-base" two-cell-BMED was used to regenerate acid/base from supernatant. Since the concentration of residual calcium in supernatant was too high (above 8.0 g/L), the resulting calcium citrate precipitates during the acid/base regeneration. Thus, oxalic acid precipitation was selected and the removal rate of calcium was about 60%. However, the calcium oxalic could also pollute the membrane stack. We found that during the process of regeneration of acid/base, there was a certain time lag between turbidity occurrence and calcium oxalate deposition in the membrane stack. Based on this phenomenon, batch-type-precipitation-out-stack method was established. The experimental results showed that this method could effectively avoid the deposition of calcium oxalate salt solid in membrane stack, and the average current efficiency, flux of NH4+, and energy consumption were 95%,12.5 mol/(m2·h),60 kWh/kmol, respectively.The integration of precipitation displacer production and precipitation replacement reaction could greatly reduce the concentration of displacer in system, thus increasing the apparent formation rate of ammonium carbonate. Meanwhile, the integration of acid/base regeneration and elution step for ion exchange during lactic acid production could also reduce the concentration of H+ and Cl- in solution of acid cell. Moreover, the performance of acid/base regeneration was significantly improved:the flux of Cl-through anion exchange membrane increased by 3.5%, the average current efficiency increased by 56.25%, and energy consumption reduced by 29.61%.The recycling of calcium carbonate during the lactic acid cleaner production was explored. Because of its unique granule structure and constitute, the recycled calcium carbonate could increase the concentration of lactic acid by 32.41%, enhance the productivity of lactic acid by 33.17%, and elevate the cell dry weight by19.34%.The experimental results at laboratory scale showed that the recovery rate of lactic acid could reach above 90%, and the estimated cost of lactic acid by this cleaner production technology was 6782.5 Yuan/ton. The obtained lactic acid could meet the demands of raw material in some polylactic acid plant. The analysis results of citric acid showed that the impurities, such as oxalic acid, ammonium and calcium ion, were not detected.