Chemostat reactors

The major advantage of immobilized systems is that the cells are maintained in the reactor vessel and thus higher cell concentrations can be achieved compared to a chemostat reactor. The reactor system can be operated at dilution rates greater than the maximum specific growth rate of the culture. These reactors can... 

We wish to compare the performance of two reactor types plug flow versus CSTR with a substrate concentration of Csf = 60g-m 3 and a biomass yield of Y = 0.1. In a plug flow bioreactor with volume of 1 m3 and volumetric flow rate of 2.5 m -li what would be the recycle ratio for maximum qx compared with corresponding results and rate models proposed for the chemostat ... 

The performance data for plug versus mix reactor were obtained. The data were collected as the inverse of qx vs inverse of substrate concentration. Table E.1.1 shows the data based on obtained kinetic data. From the data plotted in Figure E.1.1, we can minimise the volume of the chemostat. A CSTR works better than a plug flow reactor for the production of biomass. Maximum qx is obtained with a substrate concentration in the leaving stream of 12g m 3. 

A tubular bioreactor design with operational may lead to a CSTR, having sufficient recycle ratio for plug flow that behave like chemostat. The recirculation plug flow reactor is better than a chemostat, with maximum productivity at C, 3 g-m 3. Combination of plug flow with CSTR which behave like chemostat was obtained from the illustration minimised curve with maximum rate at CSf = 3 g-m-3. 

The easiest reactor to analyze is a steady-state CSTR. Biochemists call it a chemostat because the chemistry within a CSTR is maintained in a static condition. Biochemists use the dilution rate to characterize the flow through a CSTR. The dilution rate is the reciprocal of the mean residence time. 

Continuous Stirred Tanks Without Biomass Recycle. The chemostat without biomass recycle is a classic CSTR. The reactor is started in the batch mode. 

Reaction times of fermentation range from a few hours to several days. Batch processes are common, but continuous stirred tanks also are used either singly or in stages. A continuous stirred tank reactor (CSTR) also is called a chemostat. Figure 8.4 is a schematic of a fermentor with representative dimensions from the literature. 

Operate the chemostat initially as a batch reactor with D = 0, and then switch to chemostat operation with D [Pg.541]

Many reviews and several books [61,62] have appeared on the theoretical and experimental aspects of the continuous, stirred tank reactor - the so-called chemostat. Properties of the chemostat are not discussed here. The concentrations of the reagents and products can not be calculated by the algebraic equations obtained for steady-state conditions, when ji = D (the left-hand sides of Eqs. 27-29 are equal to zero), because of the double-substrate-limitation model (Eq. 26) used. These values were obtained from the time course of the concentrations obtained by simulation of the fermentation. It was assumed that the dispersed organic phase remains in the reactor and the dispersed phase holdup does not change during the process. The inlet liquid phase does not contain either organic phase or biomass. 

The CSTR is particularly useful for reaction schemes that require low concentration, such as selectivity between multiple reactions or substrate inhibition in a chemostat (see Section IV). The reactor also has applications for heterogeneous systems where high mixing gives high contact time between phases. Liquid-liquid CSTRs are used for the saponification of fats and for suspension and emulsion polymerizations. Gas-liquid mixers are used for the oxidation of cyclohexane. Gas homogeneous CSTRs are extremely rare. 

The chemostat is a biological CSTR where the substrate concentration in the tank is maintained constant. The tur-bidostat is similar to the chemostat except that the cell mass in the reactor is kept constant. The primary distinction between the two reactors is the control mechanism used to maintain continuous operation. A unique feature of a biological CSTR is the washout point. When the flow rate is increased so that the microbes can no longer reproduce fast enough to maintain a population, the microbes wash out of the tank, and the reaction ceases. This washout point represents the limits of maximum flow rate for operation. 

Continuous fermentation processes are primarily used in the research and development stage. However, more chemostat operations are being used at the production level as the understanding of this reactor increases. Examples include ethanol fermentation for the production of fuel grade ethanol and single-cell protein production from methanol substrates. 

The term fermentation is used to describe the biological transformation of chemicals. In its most generic application, a fermentor may be batch, continuous-stirred tank (chemostat), or continuous plug flow (immobilized cell). Most industrial fermentors are batch. Several configurations exist for these batch reactors to facilitate aeration. These include sparged tanks, horizontal fermentors, and biological towers. 

Several special terms are used to describe traditional reaction engineering concepts. Examples include yield coefficients for the generally fermentation environment-dependent stoichiometric coefficients, metabolic network for reaction network, substrate for feed, metabolite for secreted bioreaction products, biomass for cells, broth for the fermenter medium, aeration rate for the rate of air addition, vvm for volumetric airflow rate per broth volume, OUR for 02 uptake rate per broth volume, and CER for C02 evolution rate per broth volume. For continuous fermentation, dilution rate stands for feed or effluent rate (equal at steady state), washout for a condition where the feed rate exceeds the cell growth rate, resulting in washout of cells from the reactor. Section 7 discusses a simple model of a CSTR reactor (called a chemostat) using empirical kinetics. 

The preferred reactor for kinetics is the chemostat, but semibatch reactors are more often used owing to their simpler operation. 

Rhodobacter sphaeroides RV Fascetti and Todini (1995) lactic acid (100 mM) NH4C1 (1.7 mM) Continuous (2-stages chemostat) 0.5 3 000 lux 2 tungsten lamps 100 W inside each reactor... 

This study demonstrates the principal possibility of hydrogen production in an outdoor photobioreactor (PhBR) incorporating a cyanobacterial mutant of Anabaena variabilis (PK84) under aerobic conditions. A computer-controlled helical tubular PhBR was operated over 4 summer months. A maximum rate of 80 mL H2 per hr per reactor volume (4.35 L) was obtained on a bright day (400 W m 2) from a batch culture. Also the culture was grown in chemostat mode at dilution rate D of 0.02 h 1. The maximum efficiency of conversion of light to chemical energy of H2 in the PhBR was 0.33% and 0.14% on a cloudy and a sunny day, respectively. 

Stirred tank reactor systems can also operate in a continuous mode. In this configuration, fresh medium is continually supplied to the reactor and the desired products are continuously removed in the course of production. A continuous system is referred to as a chemostat when the flow rate is set to a constant value. It is further known as a turbidostat when the flow rate is set to maintain a constant turbidity or cellular concentration.f Continuous reactor systems are commonly abbreviated as CSTR or CSTF and they refer to continuous stirred tank reactor and continuous stirred tank fermenter, respectively. 

Fig. 2 Reactor types used for fermentation processes (A) batch reactor, (B) continuous reactor or chemostat, and (C) fed-batch reactors with either a feed stream or a product withdrawal stream.

In the pharmaceutical industry, the reactors are closed to maintain aseptic conditions, and agitation and aeration are provided as needed. The classic example is the Monod chemostat, often found in laboratories. A representative chemostat, fermentation vessel, or bioreactor, is shown in Fig. 2 with an air sparger, agitator, and feed or exit streams. The continuous flow system is usually replaced by a batch process for production in the pharmaceutical and biotechnology industries. The general relationships for a constant volume system are developed in terms of cell concentration (A), often the component of interest, and substrate concentration (5), such as a sugar or oxygen. 

---