Increasing interest in sustainable industry, renewable feedstock utilization and increase in oil prices has led to renewed attention for Acetone Butanol and Ethanol (ABE) fermentation from industry and academia, in particular for 1-butanol.

Our aim is to develop an efficient butanol production process taking into account Clostridial fermentation as well as downstream processing.

The biological production of butanol by ABE fermentation has been shown to provide a total solvent concentration in the fermentation broth of around 20 g/L. The product concentration is limited mostly by product inhibition of butanol. Product removal during fermentation will increase the productivity and help the overall economy of the production process. An efficient product removal operation will be able to minimize the production costs, including the fermenter costs, by maintaining a non-inhibiting butanol concentration in the fermenter. Therefore the key downstream step in the process is the separation of cell-free fermentation broth into a small butanol-rich and a large butanol-poor fraction.

A large array of options for butanol recovery from aqueous solutions exists. It is difficult to compare these without having to design all the potential integrated process options on the same basis in full detail. Therefore a simpler method to rank the possible recovery options has been developed. This method focuses on the selectivity between butanol and water of the recovery, the resulting mass flows, and the corresponding energy consumption. Energy consumption is a key operational cost factor with respect to the recovery.

According to this analysis, one of the best options for recovery is adsorption of butanol by high silica zeolites. Potentially this can directly lead to relatively pure butanol streams upon desorption. Scaling-up an adsorption/desorption operation leads to a number of complications that will be illustrated.

The design of an overall process including fermentation and adsorption shows that the process can be economically feasible, depending on the prices taken for the raw materials and the product. Important design issues that will be discussed are:

• Continuous fermentation

• Cell recycle

• Minimizing acetone and ethanol by metabolic engineering

• The role of H2 in the process

This study is financially supported by the Netherlands Ministry of Economic Affairs and the B-Basic partner organizations (www.b-basic.nl) through B-Basic, a public private NWO-ACTS programme (ACTS: Advanced Chemical Technologies for Sustainability).