Team:Calgary/PHB Fermentation

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PHB Fermentation

Overview

In this stage of the process, VFA-rich stream from the previous step is fermented by engineered E. coli to produce PHB. This continuous fermentation process occurs in a 5L stirred-tank bioreactor at 37°C 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 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, while the implementation of the self-cleaning filter reduces fouling. An example of a stirred tank bioreactor can be found following this link,certain specifications - like power, mass and volume of the following model were used in the ESM parameters estimation. An Example of smaller-scale self-cleaning filter can be found on the Eaton website. The filter operates by passing the 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 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