A typical process for the production of a bioproduct like by microbial fermentation consists of seed cultivation,fermentation, product recovery, concentration and purification.Considering that the downstream purification cost in the fermentation based process normally accounts for more than 60% of the total production cost, it is crucial to develop an economical purification process of succinic acid from fermentation broth. In the case of succinic acid purification, separation of byproducts including acetic,formic, lactic and pyruvic acids is the most crucial.
Several methods for the purification of succinic acid, including electrodialysis, acidification and extraction have been developed.
Electrodialysis is a widely used separation process, in which ionized compounds are separated from non-ionized compounds by ion exchange membrane, in wastewater treatment pharmaceutical and food processing industries. Succinic acid normally exists in the form of ionized-succinate salt in a fermentation broth, while others including carbohydrates, proteins and amino acids are mostly non-ionized. Most specialty and commodity applications of succinic acid require the free acid form rather than the salt form. Therefore, the succinic acid purification process composed of conventional electrodialysis followed by water-splitting electrodialysis membrane stacks,which removes most of the salt cation and produces highly pure acid stream, was developed. In order to remove the residual cationic, anionic and amino acids, cation and anion exchange resins were integrated into the above process as the final purification step. Although this process increased the concentration of succinic acid from 51.5% to 79.6% (w/w) and completely removed proteins and salts, the concentration of acetic acid increased from 13.2% to 19.9%.Another purification process employing precipitation of succinate salts was developed. Succinate in the fermentation broth is precipitated as calcium succinate by adding calcium dihydroxide, which can neutralize the fermentation broth at the same time. Calcium succinate is recovered by filtration, and converted to succinic acid by adding sulfuric acid. Succinic acid is recovered by filtration, and further purified by acidic and basic ion exchangers. This process dramatically improved the purity of succinic acid from 44.5% in the fermentation broth to 94.2% (w/w) after the purification. Unlike the succinic acid recovery process based on the electrodialysis, it could not completely remove proteins mainly due to the saturation of the ion exchange sites with the succinate anion. The above two processes are rather complex, which results in relatively high purification costs.
The reactive extraction of succinic acid with amine-based extractant, employing hydrophobic tertiary amines, has been considered as an effective and economical purification method in recent years because the process is operated at normal temperature and pressure. This process is based on reversible reaction between the extractant and the extracted carboxylic acid. The selective separation of specific acid from fermentation broth containing mixed acids can be achieved based on the pKa values of the acids and operating pH. The use of tri-n-octylamine as an extractant resulted in the recovery of succinic acid from the binary mixture of succinic acid and acetic acid with high selectivity and high extraction efficiency. More recently, the integrated succinic acid recovery process composed of reactive extraction, vacuum distillation and crystallization was developed. It allowed purification of succinic acid with the purity of 99.76% (w/w) and the yield of 73.09% (w/w) from the actual fermentation broth of M. succiniciproducens.
Furthermore, no acetic acid was detected after vacuum distillation. This process is much simpler and more cost-effective than those mentioned above.