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Figure 3. Production of a-amylase in a chemostat by recombinant Escherichia coli. The authors (11) used a modified Leude-king-Piret model, qP = (aß + ß)(1 + kß)~1, to describe a-amylase kinetics. The term (1 + kß)-1 accounts for an observed increase in plasmid copy number with decreasing growth rate. Model parameters were regressed from data: a = 34.12 units/mL x OD660, ß = 4.2 x 10-10 U/mL x OD660 x h, and k = 8.63 h. Experimental data are depicted as points, model predictions as solid lines, and trend lines as dotted lines.

In the case of secondary metabolites (i.e., a< b), product concentration is inversely proportional to the dilution rate, and the productivity (D X P) is independent of dilution rate. The low dilution rates favorable for secondary metabolite production approach batch operation, which is generally favored over the CSTR in such instances.

other issues can impact the decision between batch and continuous culture. The ability of the CSTR to maintain an ideal environment for product formation may offer a competitive advantage over the batch fermentation, with its time-varying environment and prolonged lag and stationary growth phases. Regulatory and market factors also play an important role in deciding on the operating mode. The CSTR is a dedicated manufacturing system used to produce a single product. Such a system may not be well suited for the production of specialty chemicals and pharmaceuticals because it can neither adapt to variable market demand nor satisfy demand for multiple products.

Energy Balance. Heat transfer is an important consideration in fermentor design, scale-up, operation, and sterilization. The energy balance is used to determine the time-temperature profile of the fermentation broth by accounting for the transfer and accumulation of energy. The heat transfer rate limits the ability to reduce the cycle time for sterilization (12). More importantly, the rate of heat removal from the broth during cell growth can constrain volumetric productivity, an issue in very large reactors with reduced area-to-volume ratios. Because considerable heat generation accompanies rapid cell growth, the high specific growth rates favored in industrial CSTR applications will exacerbate the problem of heat transfer in large reactors. The steady-state energy balance for the fluid in the CSTR is written as

QAGIT + QMET + dCSENS CLOSS CEVAP CEXCH _ 0

in which CAGIT is the mechanical energy imparted to the fluid through impeller agitation (equal to the gassed power input), QmET is the metabolic heat generated by cell growth, DCsens is the net sensible heat added to the system by streams entering and leaving the system, CLOSS is the sum of the heat losses from the system to the surroundings, CEVAP is the latent heat removed by evaporation, and CEXCH is the heat removed from the system by an appropriate heat exchanger system (13). In some cases, the heats of solution and mixing must be accounted for, but in most cases they are negligible. The terms DCSENS, CLOSS, and CEVAP are comparatively small, leading to the simplified energy balance

For fast-growing microorganisms, the heat exchanger duty can be as high as 7.7 to 23.2 kW/m3, of which QmET and CAGIT typically represent about 75% and 25% of the total, respectively (14).

Approximately 40 to 50% of the energy contained by a substrate is converted into useful chemical energy, whereas the balance is released as heat. If this metabolic heat is not removed from the fermentation broth, the temperature will rise and possibly hinder performance. Metabolic heat generation is a function of the growth rate of the organism, the cell concentration, the fluid volume, and the efficiency of cell growth on a particular substrate (i), which can be expressed as the metabolic heat released per gram of cell produced (1/YH) (kcal/g DCW).

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