Team:Calgary/PHB Fermentation

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PHB Fermentation

Overview

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

Figure 1: Diagram of the proposed PHB fermentation process.

Design options considered

Other considered design options include:

• External membrane bioreactor (EMB)
• Immersed membrane bioreactor (IMB): specifically the hollow fiber and the flat sheet designs
• Stirred-tank bioreactor with acoustic cell separation
• 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.

After literature search on acoustic cell separation, we concluded that this option is not ideal for the scale of our process.

The stirred-tank bioreactor, on the other hand, provides continuous agitation and easy temperature control, while the implementation of the self-cleaning filter reduces fouling. 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 system 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