Producing chemicals and fuels by fermentation of renewable feedstocks can offer improved sustainability, lower costs and greater safety compared to conventional thermal processes. But realizing these benefits depends on a careful assessment of the process economics, and an understanding of the differences between fermentation-based processes and alternatives. This one-page reference offers a brief discussion.
Key considerations for fermentation processes differ from those using petroleum-based feedstock.
Impurities. Chemicals from fossil feedstocks have characteristic impurities that differ from those derived biologically, even if both technologies offer end products at the same purity level. For example, the feedstock for bio-based processes is often carbohydrates. These can lead to product-quality issues, such as color and odor, if not addressed during process design. Engineers for fermentation-based processes must be familiar with carbohydrate, and protein and amino acid chemistries, as well as methods for separating color and odor-causing compounds.
Separations. Separating the desired chemical product from the fermentation broth often requires different techniques and equipment than what might be found for a conventional process. Effective handling and purification of aqueous streams often dictates specialized unit operations. Key concerns include energy-efficient techniques to remove water and the ability to recycle and reuse water.
Feedstock variations. Designing a process that can handle varying inputs is a fundamental task for process engineers that impacts both capital and operating costs. The techniques for managing variations in feedstocks for bio-based processes are different than for conventional processes, and may include feedstock testing (to determine attributes), collaboration with feedstock suppliers to optimize consistency versus cost, rethinking the microorganism to efficiently handle greater variation in feedstock properties, and adjusting fermentation or other operating parameters.
Sterility requirements. Contamination is a concern in any production plant, but the manner in which it is realized for a bio-based process, and the rigor with which it must be maintained, are different. In particular, it is necessary to design, build and operate a bio-based process to exclude viable foreign microbes. This is particularly critical in fermenters and associated systems, and, depending on the product, can extend into downstream processing as well. Preventing contamination of fermentation systems is of paramount importance.
Managing weather. Large-scale fermentations can be sensitive to the effect of outside temperatures on cooling-tower capacity. Insufficient cooling capacity can ruin a fermentation batch due to temperature run-up, with consequences that can extend into downstream processing. This risk can be addressed through operating procedures that adjust process parameters to slow down the fermentation rate to maintain temperature control of the fermentation process. Fermentation plants are often constructed with minimal enclosure and exposed piping. Given their lower operating temperatures and aqueous streams, it may be necessary to account for the possibility of freezing.
Economics of fermentation
In bio-based processes, a single unit operation (fermentation) frequently replaces multiple unit operations for a conventional chemical process (diagram), so the capital cost per ton may be significantly lower (sometimes 20–40%) for bio-based process technologies than for conventional processes using fossil feedstocks — especially for mid-sized plants. Additionally, capital equipment for bio-based processes may be less expensive because they are run at near-ambient temperature and pressure and near-neutral pH, versus the more challenging conditions often required in a conventional process.
In considering capital and operating costs in fermentation processes, the questions of whether it is better to use a smaller number of large fermentation tanks (for example, one 1,000-m3 tank) or a larger number of smaller tanks (100 m3) should be asked. Other questions also must be addressed, including whether the process will use aerobic or anaerobic microorganisms; whether to control temperature with a cooling jacket, internal coil or external loop; and whether the process will be run as a batch or continuous process.
For separation and purification after the fermentation step, considerations include feedstock quality (more impurities at the start likely mean more effort and cost later); handling of solids, both upstream (for example, biomass pretreatment and sucrose handling) and downstream (crystallization and drying); and the properties of the target chemical (such as solubility, volatility, permeability and target purity). The net effect of these factors can be significant, potentially shifting the balance of capital and operating expenses. One process design might be better at larger scale while another is better at smaller scale.
Editor’s note: The content for this column was adapted from the following articles: Weiss, S., Harnessing Biotechnology: A Practical Guide, Chem. Eng., April 2016, pp. 38–43; amd Miley, B., Riley, J. and Zelmanovich, Y. Large-Scale Fermentation Systems: Hygienic Design Principles, Chem. Eng., November 2015, pp. 59–65.
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