FOOD AND INDUSTRIAL MICROBIOLOGY

Module 7. Bioreactor configuration

Lesson 24
SUBMERGED FERMENTOR SYSTEM AND THEIR TYPES

24.1 Introduction

The fermentor is the heart of any biochemical process in which microbial, mammalian, or plant cell systems are employed for the economic production of fermentation products. A properly designed fermentor should be used to provide an aseptic, controlled environment to facilitate optimal growth and product formation of a particular cell system.

24.2 Stirred Tank Bioreactor (STR)

The most commonly used bioreactor for industrial applications is the conventional stirred tank reactor (STR) (Figure 24.1). The STR offers the advantages of high oxygen transfer rates required for high biomass productivity coupled with low investment and operating costs, which form the basis for any successful aerobic fermentation process. STRs typically have height to diameter ratios of 1:3 to 1:6. The agitator may be top driven or bottom driven depending on the scale of operation and other operational aspects. The choice of impeller depends on the physical and biological characteristics of the fermentation broth. Usually, a ring-type sparger with perforations is used to supply air to the fermentor. Baffles are provided to avoid vortex formation and improve mixing. Most fermentation processes use complex medium ingredients like corn steep liquor, molasses, and soybean flour as inexpensive nutritional sources (for carbon and nitrogen), supplemented with vital growth factors (amino acids, proteins, and vitamins) . The high turbulence imparted by the impellers in an STR can result in foaming due to the presence of proteinaceous substrates. Although chemical antifoaming agents (silicone or polypropylene glycol) can be added to control the foam, these can have detrimental effects on microbial growth and product recovery. In order to overcome this, mechanical methods of foam suppression such as rakes on the stirrer shaft mounted above the critical level have also been adopted. The emphasis on asepsis of the bioreactor, right from the end of the sterilization cycle to the end of the fermentation, has led to the maintenance of a minimum positive pressure in the fermentor to ensure sterility. A most important aspect of sterility is the point of contact between agitator shaft and vessel, which can be effectively sealed with a lubricated double mechanical seal. The sampling devices and injection ports must be contained in steam sterilizable closures.

Fig. 24.1 Conventional stirred tank reactor (www.googleimages.com)

A Few important types are briefly described below

24.2.1 Stirred tank reactors

In these reactors, mechanical stirrers (using impellers) are used to mix the reactor to distribute heat and materials (such as oxygen and substrates)

24.2.2 Bubble column reactors

These are tall reactors which use air alone to mix the contents

24.2.3 Air lift reactors

These reactors are similar to bubble column reactors, but differ by the fact that they contain a draft tube. The draft tube is typically an inner tube which improves circulation and oxygen transfer and equalizes shear forces in the reactor.

24.2.4 Fluidized bed reactors

In fluidized bed reactors, cells are "immobilized" on small particles which move with the fluid. The small particles create a large surface area for cells to stick to and enable a high rate of transfer of oxygen and nutrients to the cells

24.2.5 Packed bed reactors

In packed bed reactors, cells are immobilized on large particles. These particles do not move with the liquid. Packed bed reactors are simple to construct and operate but can suffer from blockages and from poor oxygen transfer.

24.2.6 Flocculated cell reactors

Flocculated cell reactors retain cells by allow them to flocculate.

24.2.7 Air lift bioreactor

For fermentations that have low shear and energy requirements, an air lift reactor can be useful (Figure 24.2). The amount of air required for the fermentation process is usually sufficient to act as the sole source of liquid mixing. In this process, air pumped from the bottom of the reactor creates buoyant, bubbles, which exert a drag on the surrounding fluid. A riser and a “down comer” inside the bioreactor impose a circulating fluid pattern of movement, which provides for oxygenation and mixing of the fermentation broth. The bottlenecks associated with large scale air lift bioreactors are inadequate sterilization, higher capital investment, and aeration requirements. Since mixing in an air lift is solely caused by aeration, the power required for fluid circulation and dispersion can be higher than that needed by an agitator in a stirred tank bioreactor.

Fig. 24.2 Air lift bioreactor (Okofer et al., 2007)

24.2.8 Fluidized bed bioreactor

In the last few decades, there has been a significant increase reported in the application of fluidized bed reactor systems. These have been mainly used for cells that have been immobilized onto particulate matter. This has the advantage that a high density of particles can be used, and that the flow velocity required for the fluidization can be achieved independently of the reactor throughput. The main advantages of a fluidized bioreactor system as observed in ethanol production from S.cerevisiae) are superior mass and heat transfer characteristics, very good mixing between the three phases, relatively low energy requirements, and low shear rates (which makes a fluidized bed reactor suitable also for shear sensitive cells such as mammalian and plant cells). Fluidized bed reactors have been used with cells adsorbed inside the carrier, made either of glass or of ceramics. The upward feed flow rate in a fluidized bed bioreactor is high enough to provide fluidization of carriers, resulting in improved mixing properties and medium distribution; but this can also induce carrier abrasion and damage. In addition, fluidization of glass and ceramic carriers may require high medium flow rates that could result in higher pumping costs and eventually cell leakage. Gas liquid solid fluidized bed bioreactors have been employed for production of ligninolytic enzymes, treatment of wastewater from refineries, and raw wastewater.

In this type of bioreactor, no mechanical agitation is provided, but the medium can be manually agitated in situ or it can be transferred into a kneading machine and reloaded into the basket. The majority of mammalian cells need a solid surface such as a microcarrier or a packed bed upon which to grow. The growth of cells on microcarrier beads depends directly on the surface available for growth up to the point where the microcarrier particles reach sufficient concentration to inhibit the cells and thus reduce cell yield. The toxicity of the support can cause long lag phases, death of the cells in the early stages of development, and limited cell yields. Microcarrier bioreactor systems (Figure 24.3) have been used for cultivation of human fibroblast cells to produce cell mass and in the production of interferon. A great advantage of microcarriers is the high surface area for cell growth provided under low shear conditions, while still allowing conventional fermentor equipment to be used. However, bead to bead and bead to impeller collisions, and hydrodynamic shear forces, may cause reduced viability.

Fig. 24.3 Microcarrier bioreactor

24.2.9 Membrane bioreactor

Membrane bioreactors comprising hollow fiber systems have been developed and tested for the growth of mammalian and plant cells, and for the immobilization of bacteria, yeast and enzymes. Hollow fiber reactors have been used in the enzymatic hydrolysis of cellulose, penicillin, starch, haemoglobin, protein synthesis, and the culture of plant cells and mammalian cells. The advantages of using a hollow fiber reactor for microbial systems include high density of cell growth, using a perfusion system for simultaneous separation of product and biomass, and biocatalyst regeneration. However, a major disadvantage is the difficulty in monitoring and controlling the growth and metabolism of the culture. Other process constraints associated with microbial hollow fiber reactors are low oxygen transfer rates at high cell density and blockage, and rupture of the membranes due to excessive growth. The accumulation of toxic products in the hollow fiber might also inhibit the metabolic activity of the cell system.

24.2.10 Photobioreactor

Microalgae have been used successfully, with high productivity compared to higher plants. The high productivity in these systems is due to the high biomass produced in the bioreactor. Microalgae have been used for preparation of vitamins, pigments, antioxidants, and fatty acids, and as feed for aquaculture. The cultivation techniques employed are open systems and closed or semiclosed outdoor photobioreactors. The common photobioreactors used are tubular-type and plate-type reactors The cyanobacterium Spirulina platensis has been studied in batch and continuous photobioreactors under varying conditions of incident light energy and nutrient limitations.