FE 462 BIOCHEMICAL ENGINEERING

Cell Kinetics and Fermenter Design

INTRODUCTION

Understanding the growth kinetics of microbial, animal, or plant cells is important for the design and operation of fermentation systems employing them. Cell kinetics deals with the rate of cell growth and how it is affected by various chemical and physical conditions. Unlike enzyme kinetics, cell kinetics is the result of numerous complicated networks of biochemical and chemical reactions and transport phenomena, which involves multiple phases and multicomponent systems.
The heterogeneous mixture of young and old cells is continuously changing and adapting itself in the media environment which is also continuously changing in physical and chemical conditions. As a result, accurate mathematical modeling of growth kinetics is impossible to achieve. Even with such a realistic model, this approach is usually useless because the model may contain many parameters which are impossible to determine

GROWTH CYCLE FOR BATCH CULTIVATION

1. Lag phase: The lag phase (or initial stationary, or latent) is an initial period of cultivation during which the change of cell number is zero or negligible. Even though the cell number does not increase, the cells may grow in size during this period.

The length of this lag period depends on many factors such as the type and age of the microorganisms, the size of the inoculum, and culture conditions.

2. Exponential phase: In unicellular organisms, the progressive doubling of cell number results in a continually increasing rate of growth in the population. A bacterial culture undergoing balanced growth mimics a first-order autocatalytic chemical reaction.

The rate of the cell population increase at any particular time is proportional to the number density of bacteria present at that time:

Factors affecting the specific growth rate

Substrate Concentration: One of the most widely employed expressions for the effect of substrate concentration on μ is the Monod equation, which is an empirical expression based on the form of equation normally associated with enzyme kinetics :

The value of Ks is equal to the concentration of nutrient when the specific growth rate is half of its maximum value μmax. According to the Monod equation, further increase in the nutrient concentration after μ reaches μmax does not affect the specific growth rate.

Factors affecting the specific growth rate

Product Concentration: As cells grow they produce metabolic byproducts which can accumulate in the medium. The growth of microorganisms is usually inhibited by these products, whose effect can be added to the Monod equation as follows:

3. Stationary phase: The growth of microbial populations is normally limited either by the exhaustion of available nutrients or by the accumulation of toxic products of metabolism. As a consequence, the rate of growth declines and growth eventually stops. At this point a culture is said to be in the stationary phase.

4. Death phase: The stationary phase is usually followed by a death phase in which the organisms in the population die. Death occurs either because of the depletion of the cellular reserves of energy, or the accumulation of toxic products. Like growth, death is an exponential function. In some cases, the organisms not only die but also disintegrate, a process called lysis.

Modeling of the Bacterial Growth Curve

M. H. ZWIETERING,* I. JONGENBURGER, F. M. ROMBOUTS, AND K. VAN ‘T RIET

APPLIED AND ENVIRONMENTAL MICROBIOLOGY, June 1990, p. 1875-1881

Bioreactors

Stirred Tank Fermenters
A bioreactor is a device within which biochemical transformations are caused by the action of enzymes or living cells. The bioreactor is frequently called a fermenter whether the transformation is carried out by living cells or in vivo cellular components (enzymes).

For a large-scale operation the stirred-tank fermenter (STF) is the most widely used design in industrial fermentation. It can be employed for both aerobic or anaerobic fermentation of a wide range of cells including microbial, animal, and plant cells.

Stirred Tank
Fermenters
Kinetics of Substrate Utilization, Product Formation, and Biomass Production in Cell Cultures

It is difficult to obtain useful kinetic information on microbial populations from reactors that have spatially no uniform conditions. Hence it is desirable to study kinetics in reactors that are well mixed.

Ideal Batch Reactor

Many biochemical processes involve batch growth of cell populations. After seeding a liquid medium with an inoculum of living cells, nothing (except possibly some gas) is added to the culture or removed from it as growth proceeds. Typically in such reactor, the concentrations of nutrients cells, and products vary with time as growth proceeds.
A material balance on moles of component i;

The Ideal Continuous Flow Stirred Tank Reactor (CSTR)
The diagram of this process is shown in fig.2, which is a schematic diagram of completely mixed stirred tank reactor. Such configurations for cultivation of cells are frequently called chemostats.

The Ideal Continuous Flow Stirred Tank Reactor (CSTR)

Productivity of CSTF
MULTIPLE FERMENTERS CONNECTED IN SERIES

A question arises frequently whether it may be more efficient to use multiple fermenters connected in series instead of one large fermenter. Choosing the optimum fermenter system for maximum productivity depends on the shape of the l/rx versus Cx curve and the process requirement, such as the final conversion.

Cell Recycling
For the continuous operation of a PFF or CSTF, cells are discharged with the outlet stream which limits the productivity of fermenters. The productivity can be improved by recycling the cells from the outlet stream to the fermenter.

Alternative Fermenters
Many alternative fermenters have been proposed and tested. These fermenters were designed to improve either the disadvantages of the stirred tank fermenter-high power consumption and shear damage, or to meet a specific requirement of a certain fermentation process, such as better aeration, effective heat removal, cell separation or retention, immobilization of cells, the reduction of equipment and operating costs for inexpensive bulk products, and unusually large designs.

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