Classification of Bioreactors.docx

Classification Classificat ion of Bioreactors Accord to Current bioreactors are grouped into either culture vessels and reactors using biocatalysts (e.g., immobilized enzymes/microorganisms) or plant and animal tissues. The latter is sometimes used to mean the bioreactor. Table 1 shows a number of aerobic fermentation systems which are schematically classified into i. ii. iii.

internal mechanical agitation reactors external circulation reactors bubble column and air-lift loop reactors.

This classification is based on both agitation and aeration as it relates to oxygen supply. In this table, reactor 1 is often used at the industrial level and reactors (a)2, (b)2, (c)2, and (c)3, can be fitted with draught tubes to improve both mixing and oxygen supply efficiencies. Culture techniques can be classified into batch, fed-batch, and continuous operation (Table 2). In batch processes, all the nutrients required for cell growth and product formation are present in the medium prior to cultivation. Oxygen is supplied by aeration. The cessation of growth reflects the exhaustion of the limiting substrate in the medium. For fed-batch processes, the usual fed-batch and the repeated fed-batch operations are listed in Table 2. A fed-batch operation is that operation in which one or more nutrients are added continuously or intermittently to the initial medium after the start of cultivation or from the halfway point through the batch process. Details of fed-batch operation are summarized in Table 3. In the table the fedbatch operation is divided into two basic models, one without feedback control and the other with feedback control. Fed-batch processes have been utilized to avoid substrate inhibition, glucose effect, and catabolite repression, as well as for auxotrophic mutants. Table 1. Classification of Aerobic Fermentation Systems 

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Internal mechanical agitation reactors Turbine-stirring installation o Stirred vessel with draft tube o Stirred vessel with suction tube o External circulation reactors Water jet aerator o Forced water jet aerator o Recycling aerator with fritted disc o Bubble column and air-loop reactors Bubble column with fritted disc o Bubble column with a draft tube for g yration flow o Air lift reactor o Pressure cycle reactor o Sieve plate cascade system o

Table 2. Classification of Fermentation Processes

1. 2. 3. 1. 4.

Batch process Fed-batch process (semi-batch process) Repeated fed-batch process (cyclic fed-batch process) 4 Repeated fed-batch process (semi-continuous process or cyclic batch process) Continuous process

Table 3. Classification of Fed-Batch Processes in Fermentation

1. Without feedback control a. Intermittent fed-batch b. Constant rate fed-batch c. Exponentially fed-batch d. Optimized fed-batch 2. With feedback control a. Indirect control b. Direct control i. Setpoint control (constant value control) ii. Program Control iii. Optimal control

The continuous operations of Table 2 are elaborated in Table 3 as three types of operations. In a chemostat without feedback control, the feed medium containing all the nutrients is continuously fed at a constant rate (dilution rate) and the cultured broth is simultaneously removed from the fermenter at the same rate. A typical chemostat is shown in Fig. 1. The chemostat is quite useful in the optimization of media formulation and to investigate the physiological state of the microorganism.  A turbidostat with feedback control is a continuous process to maintain the cell concentration at a constant level by controlling the medium feeding rate.  A nutristut with feedback control is a cultivation technique to maintain a nutrient concentration at a constant level. A  phuuxostut is an extended nutristat which maintains the pH value of the medium in the fermenter at a preset value. Table 4. Classification of continuous fermentation processes

1. Without feedback control a. Chemostat 2. With feedback control a. Turbidostat b. Nutristat c. Phauxostat