Polyhydroxybutyrate Production from Switchgrass

The process under analysis comprises three major sections: (1) Syngas Formation; (2) Fermentation; and (3) Products Treatment.

Syngas Formation. Switchgrass is fed to a mill grinder and sent to a fluidized bed reactor along with preheated air from the Syngas Conditioning downstream. The biomass is oxidized into a mixture of carbon compounds and hydrogen. The outlet gas stream from the reactor is fed to a set of cyclones to remove solids, that are disposed as waste, while the gas is routed to a Syngas Reformer, which converts hydrocarbons to hydrogen and carbon monoxide. The reformer outlet stream is thermally treated to 30 °C in the Syngas Conditioning area, which is also fed with process water make-up.

Fermentation. The fermentation is carried out under anaerobic conditions in agitated jacketed fermenters. Fresh Syngas is mixed with recycled gas from the Hydrogen Purification and fed to the fermenters. The bacteria “Rhodospirillum rubrum” uses carbon monoxide to produce polyhydroxybutyrate (PHB). Fermentation broth is sent to the PHB Recovery area, while a hydrogen gas stream is sent to Hydrogen Purification.

Products Treatment. The hydrogen-rich stream from the fermenters is to a hybrid membrane/pressure swing adsorption system. The exhaust gas from the membrane is recycled to the fermenters, and a 99.9% purity hydrogen stream is obtained from the PSA. The fermentation broth is fed to centrifuges to separate fermentation media from cells, which are sent to a stirred tank. Surfactant is also fed, breaking cell walls and liberating the PHB granules in the liquid medium, which is mixed with sodium hypochlorite solution and sent to centrifuges. The concentrated PHB is routed to a spray dryer, then to an extruder, and finally is packed in bags and stored.