Team:Calgary/PHB Fermentation

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PHB Fermentation

Overview

In this stage of the process, the VFA-rich stream from the previous step is fermented by engineered E. coli to produce PHB. This continuous fermentation process occurs in a 5-L stirred-tank bioreactor at 37°C under anaerobic conditions. A continuous flow of the VFA-rich stream is generated by a pump. To achieve continuous fermentation, a mechanical self-cleaning filter with 0.2-micron pores is used to separate and recycle bacteria back to the bioreactor. The resulting bacteria-free harvest stream containing PHB is then passed on to PHB extraction and water recovery stages. The proposed process is shown in Figure 1.

Figure 1: Diagram of the proposed PHB fermentation process.

Design options considered

Considered design options included:

• External membrane bioreactor (EMB)
• Immersed membrane bioreactor (IMB), specifically the hollow fiber and the flat sheet designs
• Stirred-tank bioreactor with self-cleaning filter separation

The major disadvantage of membrane bioreactors is fouling. Because of fouling, frequent filter cleaning and filter replacement might be required, resulting in higher crew time requirements and transportation costs to supply replacement membranes.

The stirred-tank bioreactor, on the other hand, provides continuous agitation and easy temperature control, and the implementation of the self-cleaning filter reduces fouling. An example of a stirred tank bioreactor can be found here. Certain specifications, such as power, mass, and volume of the following model, were used in the ESM parameters estimation. An example of a smaller-scale self-cleaning filter can be found on the Eaton website. This filter operates by passing liquid through a cylindrical drum and using a mechanical "arm" to continuously scrub the fouling particles from the filter surface. With this in mind, the stirred-tank bioreactor combined with the self-cleaning filter was chosen as the optimal design for the process. The ESM analysis for the final PHB fermentation stage is summarized in Table 1.

Table 1: ESM analysis for the proposed PHB fermentation system.

	Self-cleaning filter	Stirred-tank bioreactor
Power (kW)	2	0.38
Weight (kg)	16	15
Volume (m^3)	0.028	0.1153
Spares & Consumables (kg/day)	0	0
Spares & Consumables (m^3)	0	0
ESM Estimation	196	73