Biomedical Engineering Reference
In-Depth Information
biochemistry, process engineering and process economics (Cooney, 1983; Maiorella et al .,
1984). The main goal in bioreactor design is to minimize the overall cost of production of
the desired product. These include costs associated with raw material conversion to product,
cost of product recovery and capital cost.
The performance of a desired bioreactor depends on many parameters such as the ability
to (Aiba et al ., 1973 ):
(1) maintain high cell concentrations to provide high product yields;
(2) sustain cell metabolic activities by controlling process parameters such as pH, dis-
solved oxygen, redox potential, nutrient concentrations and temperature;
(3)
achieve and preserve sterile conditions throughout the process to avoid contamination
of pure cultures;
(4)
provide adequate agitation to achieve homogeneous mixing of substrate and suspended
cells and reduce heat and mass transfer limitations.
7.4.1 Types of reactors
The selection of a bioreactor and operation mode starts with the type of biocatalysts (enzymes
or whole cells) that will be used to produce the desired product, and if the biocatalysts will be
suspended or immobilized with a recycle option. Each of these possibilities should be examined
while taking into consideration the reactions stoichiometry and kinetics, mixing needs, gas
exchange, heat removal and capital and operating costs (Merchuk and Asenjo, 1995).
For example, different fermentation schemes have been developed for the production of
ethanol. Conventional batch, continuous, cell recycle and immobilized cell processes, as
well as membrane, extraction and vacuum processes, which selectively remove ethanol
from the fermentation medium as it is formed, were compared on identical bases using a
consistent model for yeast metabolism (Maiorella et al ., 1984 ). The continuous flow stirred
tank reactor (CSTR) with cell recycle, tower and plug flow reactors all showed similar cost
savings of about 10% compared to batch fermentation. Cell recycle increases cell density
inside the fermentor, which is important in reducing fermentation cost.
Several bioreactors of interest are discussed in the next section, including the stirred tank
reactor (STR), bubble column, hollow fiber and monolithic reactors.
7.4.1.1
Stirred tank reactor
The stirred tank reactor is one of the most commonly used bioreactors for fermentation
(Figure 7.5). The fermentor shape can be rectangular or cylindrical. Currently, cylindrical
fermentor design is most commonly used. The height-to-diameter ratio of the cylindrical
vessel varies between 2:1 and 6:1. Agitation is facilitated with multiple impellers mounted
on a shaft driven by a motor. Agitation is required to facilitate mixing and heat and mass
transfer to obtain homogenous suspension of cells and nutrients, pH and temperature.
Rushton turbine, hollow blade, three blade pitched and marine are some of the types of
impellers that can be used. The number of impellers used depends on the size of the vessel
(Doran, 1995). A gas sparger is placed below the impeller for aeration. A STR can be oper-
ated in batch, fed-batch or continuous modes.
The temperature inside a large bioreactor is maintained by heating and cooling coils
placed in the vessel. However, a heating jacket maintains the operating temperature for
small bioreactors.
Search WWH ::




Custom Search