Energy : The Missing Millennium

Abstract

Pseudomonas oleovorans and recombinant strains containing the alkane oxidation genes can pro-duce alkane oxidation products in two-liquid phase bio-reactor systems. In these bioprocesses the cells, which grow in the aqueous phase, oxidize apolar, non-water soluble substrates. The apolar products typically accu-mulate in the emulsified apolar phase. We have studied both the bioconversion systems and several down-stream processing systems to separate and purify al-kanols from these two-liquid phase media. Based on the information generated in these studies, we have now designed bioconversion and downstream processing systems for the production of 1-alkanols from n-alkanes on a 10 kiloton/yr scale, taking the conversion of n-octane to 1-octanol as a model system. Here, we describe overall designs of fed-batch and continuous-fermentation processes for the oxidation of octane to 1-octanol by Pseudomonas oleovorans, and we discuss the economics of these processes. In both systems the two-liquid phase system consists of an apolar phase with hexadecene as the apolar carrier solvent into which n-octane is dissolved, while the cells are present in the aqueous phase. In one system, multiple-batch fermenta-tions are followed by continuous processing of the prod-uct from the separated apolar phase. The second system is based on alkane oxidation by continuously growing cultures, again followed by continuous processing of the product. Fewer fermentors were required and a higher space-time-yield was possible for production of 1-octa-nol in a continuous process. The overall performance of each of these two systems has been modeled with Aspen software. Investment and operating costs were estimated with input from equip-ment manufacturers and bulk-material suppliers. Based on this study, the production cost of 1-octanol is about 7 US$kg −1 when produced in the fed-batch process, and 8 US$kg −1 when produced continuously. The comparison of upstream and downstream capital costs and produc-tion costs showed significantly higher upstream costs for the fed-batch process and slightly higher upstream costs for continuous fermentation. The largest cost contribu-tion was due to variable production costs, mainly result-ing from media costs. The organisms used in these systems are P. putida alk+ recombinants which oxidize alkanes, but cannot oxidize the resulting alkanols further. Hence, such cells need a second carbon source, which in these systems is glu-cose. Although the continuous process is about 10% more expensive than the fed-batch process, improve-ments to reduce overall cost can be achieved more easily for continuous than for fed-batch fermentation by de-creasing the dilution rate while maintaining near con-stant productivity. Improvements relevant to both pro-cesses can be achieved by increasing the biocatalyst per-formance, which results in improved overall efficiency, decreased capital investment, and hence, decreased pro-duction cost.