The biochemical reactors can be broadly classified into submerged and surface reactors [2,6]. In surface reactors the culture adheres to a solid surface and oxygen is supplied from the gas phase to the continuously wetted solid surface. Wastewater treatment employs such designs. In submerged reactors gas-to-liquid mass transfer is achieved by dispersing the gas in the liquid through continuous input of energy. Submerged reactors can be divided into three groups depending upon the nature of the energy input, viz., (1) mechanically stirred systems with agitators (Chapter 11 deals with this type in more detail), (2) forced convection of the liquid using recirculating pumps, and (3) pneumatic operation by pumping compressed air (bubble columns or airlift reactors). Chapter 12 deals in more detail on tower reactors.
The simplest and widely used reactor is a stirred tank with baffles (Fig. 8.1). Marine propeller, hydrofoil, or pitched blade turbine stirrer is used to achieve axial motion, and flat blade turbine, back swept, or Rushton turbine stirrers are used to achieve radial motion. Rushton turbine is good for gas dispersion. Helical or anchor impellers is used for viscous substrates, while low rpm ribbon agitator is good for ultra-high-viscosity and anchor stirrer is good for high viscosity, blending, and heat transfer applications.
Propeller is used for low to medium viscosities, pitched blade turbine is generally used for solid suspension and blending. Air is introduced from the bottom through a sparging arrangement. The sparger could be single hole, multiple hole, perforated plate, or sintered plate. Gas-aspiring or self-sucking agitator has a hollow shaft through which air is sucked from the top space (or head space) and distributed from the liquid bottom. These agitators are common in food technology, especially in the manufacture of vinegar, yeast,
vitamins, and amino acids. The height to diameter ratio of such reactors is less than 3. The reactor is provided with jacket or coils for heating and cooling purposes.
In baffled stirred reactors, complex flow patterns are observed, which can be overcome by providing coaxially arranged cylindrical tube (draught tube). The circulating flow is well defined (Fig. 8.2), and these reactors consume less power than the conventional stirred tank reactors. Also, the power uptake is lower in overflow operation than in the completely filled state.
Stirred cascade reactors are column reactors in which several sections arranged above one another are formed by intermediate plates (Fig. 8.3) Mixing and dispersion of gas takes place in each chamber.
In plunging jet reactors, the gas is dispersed by the free jet from the nozzle impinging perpendicularly on the surface of the liquid. In jet loop reactors the liquid phase is returned from the outlet of the reactor to the inlet. Recycling can take place through a draught tube placed inside the vessel (Fig. 8.4). A circulation pump produces the driving jet, with both the phases in cocurrent. Submerged reactors of these types are used for wastewater treatment.
Perforated plate or sieve plate cascade reactors have countercurrent gas-liquid contact (Fig. 8.5), in which the liquid flows down from one stage to another by overflow pipes. The liquid collected at the bottom is externally recycled back with the help of pump.
In air lift loop reactors the circulation of the liquid is due to the density difference between the mixed phases in the aerated tower and the liquid in the down comer (Fig. 8.6). These reactors have internal draught tube, internal partition or external loop as shown in the figure. The draft tube gives the airlift reactor a number of advantages like preventing bubble coalescence by causing bubbles to move in one direction, distributing the shear stresses uniformly throughout the reactor and thus providing a healthier environment for cell growth. Also circular movement of fluid through the reactor increases the heat transfer rates. As the bubbles in the draft tube rise they also carry the liquid up with them, and when they disengage at the top, the liquid travel down in the down comer section. The heating/cooling jacket is located on the walls of the airlift reactor. A reactor with an external riser will have the advantage of having greater turbulence near the jacket and thus better heat
Figure 8.3 Multicompartment stirred column reactor.
transfer efficiency. Also the amount of foam produced with an external riser is less than the one with an internal riser. Deep shaft reactors are 50-150 m long and are made of concrete. They are buried underground and are used for sewage treatment. The air in this reactor is not introduced at the bottom, but in the middle. These reactors are also suitable for shear sensitive, foaming, and flocculating organisms (Fig. 8.7).
Bubble columns are slender columns with gas distributor at the bottom (Fig. 8.8). Construction of bubble columns is very simple, and higher mass transfer coefficient than loop reactors can be achieved with them. These reactors can be as large as 5000 m3. Since they have broad residence time distribution and good dispersion property, they can be used for aerobic wastewater treatment and production of yeast.
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Figure 8.4 Jet loop reactor with external liquid recycle.
Gas mixed reactors are not provided with impellers to decrease bubble diameter and increase kL, whereas they have a larger height to diameter ratio so as to improve oxygen transfer efficiency. This design helps in increasing the pressure at the base of the reactor which increases the saturation concentration of oxygen at the base, increasing the bubble residence time and gas holdup. A very tall reactor will have oxygen starved conditions at the top of the reactor and also large bubble circulation times. These reactors are provided with a disengagement zone at the top with a larger cross section, which leads to the reduction in the fluid velocity. So bubbles thus rise less rapidly and disengage slowly leading to less damage to the cells. Aerosol formation and evaporation are reduced. Bubbles disengage from the liquid and proceed to the reactor exit instead of returning down.
Figure 8.4 Jet loop reactor with external liquid recycle.
Li quid pump
Li quid pump
Packed bed column reactors (Fig. 8.9) are used in enzyme-catalyzed reactions, sewage treatment or vinegar production. The nutrient or substrate is evenly distributed over the packing through distributor. Air is introduced countercurrent to the liquid flow. In enzyme-catalyzed reactions, supported enzyme is packed in the reactor tube. The pressure drop in this reactor is generally high, but because of the low voidage, the effective enzyme concentration is high, and hence the reactor volume is generally smaller than the stirred tank reactor. The lower degree of back mixing found in this reactor when compared to stirred reactor also results in a lower enzyme requirement. Industrially pressurized packed column reactors are used for the production of l-alanine and l-aspartic from ammonium fumarate and magnesium chloride. The reactors are packed with immobilized E. coli and P. dacunnae.
Conitinuous production of l-maleic acid from fumaric acid using immobilized B. ammoniagenes is achieved in a packed bed reactor. 6-Amino-penicillanic acid is prepared from penicillin G industrially in immobilized enzyme columns.
In fluidized bed reactor the fluid flow is such that the solids are in a suspended or fluidized state leading to good gas-liquid contact and mixing. Pressure drop here is low when compared to packed bed reactor. Particles carry over is an issue in this design. These reactors, unlike packed bed reactor, show less tendency to blockage and problems associated with heat and mass transport.
Trickle bed reactor contains packing, which act as a support for the growing of the biocell. The substrate, nutrient, and air flow slowly over the packing in a trickling flow. These reactors are used in the manufacture of vinegar and in waste water treatment.
Tubular membrane or hollow fiber reactors consists of tube made of membranes that permit radial diffusion of reactants and products in and out of the membrane while retaining the soluble enzyme inside.
Film reactors consist of vertically arranged tubes or channels, upon which nutrient solution flows from above, while the gas is introduced from the bottom. Unlike packed bed reactors, the gas velocity in these reactors can be varied within wide range. Fermentation is also carried out in tray reactors in which the substrate overflows from one tray to another and the cultures
Figure 8.8 Bubble column reactor.
generally can float on the liquid surface. These reactors are ideal for aerobic wastewater treatment and production of acetic acid. The oxygen transfer occurs through the film and reaches the biomass. The rate of oxygen mass transfer here depends on the diffusion coefficient in the medium and the rate at which the fluid near the surface is renewed by the liquid circulation pattern. The oxygen mass transfer coefficient is given by kL = (8.1) V ks where recirculation time t is equal to the ratio of stream depth to average velocity,
Stirred tank and draft tube reactors have very low oxygen transfer rates (of the order of 4 g/l h), while jet loop, airlift loop, bubble column, and sieve plate column reactors have very high oxygen rates (around 10-12 g/l h). Oxygen transfer rates of packed tubular reactors are of the order of 0.5 g/l h. Power requirements of these reactors are the lowest (0.5 kW/m3), followed by
loop and column reactors (3-5 kW/m3), and highest for stirred tank and draft tube reactors (8 kW/m3). Overall heat transfer coefficient for reactors operating with most of the organisms is in the range of 200-1500 W/m2/k.
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