Outline

Introduction Definitions

Strategy for CSTR Analysis Reaction Kinetics Cell Growth Enzymes Mass Transfer Introduction Variations on the Single CSTR Single CSTR with Recycle CSTRs in Series Nomenclature Bibliography

INTRODUCTION Definitions

A continuous stirred-tank reactor (CSTR) is defined as an agitated vessel with continuous addition and removal of material and energy. The CSTR is one of the basic continuous reactor types widely used in the chemical process industries because of its amenability to process control and scale-up, although in biotechnology applications the CSTR is used more often as a research tool than as a production technology. An idealized, well-mixed CSTR can be modeled as having no spatial variations in temperature, concentration, fluid properties, or reaction rates. Thus, the properties of the exit stream may be considered the same as those throughout the vessel. Although such ideal mixing is never observed, the vessel is designed to provide good mixing through selection of operating conditions and vessel, baffle, and impeller geometries. The stirred tank used in continuous bioprocesses is similar to that used in batch biopro-cesses, with the exception that the CSTR likely has an overflow or other level control device. oxygen can be introduced into the vessel by sparging through inlets at the base of the vessel, where impellers then disperse the bubbles. Vessel jacketing or internal cooling coils provide a means for heat transfer. Continuous systems that are not agitated vessels often are modeled as CSTRs when their behavior approximates that of the ideal CSTR. CSTRs are also known as backmix reactors, continuous-flow stirred-tank reactors (CFSTRs), or chemostats, when used for cell growth.

Strategy for CSTR Analysis

The performance and analysis of a CSTR is based on the material and energy conservation balances and the underlying processes governing the reaction kinetics. Because of the large variety of CSTR applications and limited space for discussion in this article, a brief outline of the steps in systems analysis will be beneficial in understanding any CSTR-based process.

The strategy for analyzing CSTR performance first requires defining the problem statement and goals. The second step is system identification, which includes defining the system boundaries and the interactions between the system and its environment across the system boundaries. The system could be a cell, the fluid in the reactor, or an entire bioprocessing plant. The third step is to identify the state variables that characterize the system. During the course of the analysis, new state variables may be identified and added to the original list. The fourth step is to characterize the state of the system using material and energy balances that account for the accumulation of mass and energy. In a general balance for a particular quantity, the rate of accumulation of that quantity in the system is equal to the net influx of the quantity across the system boundaries plus the rate at which the quantity is generated. Separate material balances are written for the re-actants, products, and the catalyst (e.g., cells or enzymes). The final step is to calculate performance metrics and revisit the assumptions to determine the conditions under which they are valid.

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