Mettler Toledo AG

Sept. 15, 2008
Control of Turbidity Optimizes Crystallization Process

An amine (R-NH2) produced by synthesis in the pharmaceutical industry exists in the form of a hydrochloride salt ([R-NH3]+Cl-) and is precipitated from a clear solution through the addition of caustic soda (NaOH). Samples of the precipitated amine revealed strong and unacceptable fluctuations in product purity.

Clear turbidity signal of crystallization is required

Precipitation conditions, solvent composition, concentration and dosage rate of the caustic soda all have an influence on the quality of the intermediate product. The use of an in-line turbidity measurement system has provided important information contributing to optimization of this reaction step.

A linear increase in the dosing rates was accompanied by an increase in the level of turbidity, followed by signal decay before the concentration of solid particles slowly evened out to a stable value. The decay of the turbidity signal points to conglomeration of the finer crystals, possibly also includes entrapped impurities. A more rapid, optimized dosing of caustic soda leads to spontaneous precipitation of the whole product which, furthermore, also displays a higher degree of purity.

The goal: time saving and avoidance of faulty charges

Detailed understanding of the optimum precipitation conditions therefore results in being able to achieve a product with the required degree of purity. The use of an in-line turbidity sensor in the process provides information on how these conditions are to be maintained and therefore gives early indication of any quality deviation in the precipitation reaction. This successfully prevents time-consuming and costly downstream processing of faulty charges.

The turbidity sensor InPro 8200 has been proven to be a solution for this type of application. The use of scratchproof sapphire windows allows long-term operation even in the presence of abrasive crystals. Signal transmission between sensor and transmitter employing optical fiber cable technology in conjunction with the principle of backscattered light results in a sensor with a compact design of which installation is equally possible in small research reactors and in large production-scale vessels.