Selection of the Process Design

The process is generally designed on the basis of a bench-scale development study, here executed on working with a representative spread of original fermentation broths. The main task of the performed bench-scale tests is to decide for the process technology, that is, between crop system and recrystallization, but at the same time to collect all the needed chemophysical properties needed for the design, for example, thermal decomposition rate, the solubility system, boiling point elevations, desupersaturation rate as a function of suspension density, densities of solution and suspensions, composition of the solutions, viscosities of solution and suspensions, and so on. 

The crop system, the first to be investigated because of its superior economy, can prove here its ability to recover the product A at the desired yield and quality. Therefore, the recrystallization process alternative need not be applied. 

The feed stream, for example, the preflltered, decolored, and preconcentrated fermentation broth, enters the storage tank on the right-hand side by level control. This tank also receives the second crop filter cake from the centrifuge that mostly is a filter decanter or a filter. The second crop crystals are redissolved in the feed liquor, which is withdrawn slightly undersaturated from the preconcentration step for this purpose, before being sent to the first crop stage for crystallization of the product A. The feed flow follows the liquor level in the crystallizer. If required for other applications, this solution could be further treated for purification purposes (e.g., sent through an activated carbon station for intense decolorization). 

The first crop concentrates the mixed feed to such a degree following the above mass balance that the product can safely reach the purity demand. Therefore, the concentration factor must be monitored and controlled by keeping constant one certain and selective physical property that changes with concentration. Those properties usually are viscosity, boiling point elevation, density, and so on. The control action is pointed out below. 

During the concentration, the crystallization of the product A takes place. The suspension produced is withdrawn continuously, keeping constant the suspension density within this crystallizer at a sufficient value, for example, at 20 mass%. The suspension density is monitored, for example, by radiometric measurement. The suspension is separated on the centrifuge that preferably is of pusher-type to allow best washing efficiencies. 

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