Comparison of Batch Elution and Continuous Simulated Moving Bed Chromatography

Abstract

The production of chirality with maximum economy is one of the most challenging tasks of today's pharmaceutical industry. Apart from the use of inherent chirality (starting material from the chiral pool, e.g., amino acid derivatives, carbohydrates), the creation of chiral centers via biocatalysis or asymmetric synthesis is commonly used. Another way to obtain pure enantiomers is the separation of racemates via kinetic resolution through preferred crystallization or preparative chromatography on chiral stationary phases. This paper emphasizes this last method, explains the possibilities of this technique, especially in its application form as simulated moving bed (SMB) chromatography, and shows its benefits and limitations. Therefore, comparisons to classical batch elution chromatographic processes as well as other unit operations (such as crystallization, etc.) must take cost calculations into account. In this paper, a theoretical comparison of optimized SMB and batch elution processes by simulation studies based on rigorous process models is presented for the separation of two different binary mixtures. These examples are chosen to demonstrate the different effects which dominate the applications in large-scale isomer separations and production-scale enantiomer separation. The first example is a fructose/glucose separation with linear isotherms. The model parameters are measured by Nicoud. The second characteristic example is an enantioseparation. The corresponding isotherms are of the modified Langmuir type. The performance of each separation process is quantified by three characteristic objective functions: productivity, dilution, and solvent requirement. Last, the specific separation costs or the total costs of separation are calculated as an objective function to lay emphasis on the economy of the separation, including product recovery and solvent recycling. The comparison of these objective functions, which are determined for batch and SMB processes, leads finally to certain rules of consideration to decide what kind of process (either batch elution or SMB) is preferable as a function of the physical properties of the given binary mixture and the separation task.