Types of Continuous Fermentation Reactors
There are two basic types of continuous fermentation reactors (Figure 9.2):
1. Continuous stirred tank reactor (CSTR)
2. Plug flow reactor (PFR)
In the CSTR, fermentation broth is continuously stirred by one or more agitators, and in an ideally mixed CSTR, the composition in the reactor is homogenous and identical to that for the outgoing flow. Continuous stirred-tank reactors are commonly used in syngas fermentation. In this application CSTR has a continuous flow of gas bubbling through the liquid, which typically consists of a dilute solution of essential nutrients for the microorganism to grow and survive on. The performance of continuous stirred-tank reactors has been evaluated in concurrent ethanol and hydrogen fermentation [11,12], as well as in using flocculating yeast strains [13]. In addition, product removal from the fermentation medium by applying gas stripping has also been reported with CSTR systems [14]. In the plug flow reactor (PFR), the reactants are pumped
Figure 9.2 Two types of continuous fermentation reactors: (A) Stirred Tank Reactor (CSTR) and, (B) Plug Flow Reactor (PFR).
through a pipe or tube with a uniform velocity profile across the radius, and the reaction proceeds as the reagents travel through the PFR, with diffusion assumed to be negligible in the axial direction. Consequently, PFR operations imply that inoculum has to be constantly fed to the reactor for fermentation processes.
Dale and Musgrove have describe the use of the CSTR system in a pilot plant-scale study on cellulosic ethanol production from municipal solid waste (MSW) [15]. Their project consisted of (1) separating paper from MSW using a unique pulverizer-air classifier, (2) taking the collected light-fraction waste paper stream and pulping the entire unsorted fraction using a low-shear mechanical pulper to produce a clean paper pulp fiber stream, and (3) converting the pulp fiber to bioethanol by simultaneous saccharification fermentation (SSF) in a 100 L pilot scaled Continuous Multi Stage Stirred Reactor Separator (CMSRS). The CMSRS system was used for gas stripping of ethanol along with the simultaneous saccharification and fermentation of the cellulose fibers and allowed recycling of the fermentation broth/cellulase enzyme [15].