Continuous-flow stirred-tank reactor

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In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydraulic agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations. The CSTR is the idealized opposite of the well-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, liquid-, and solid-flow behaviors.

Continuous stirred tank reactors, (a) With agitator and internal heat transfer surface, (b) With pump around mixing and external heat transfer surface, (adopted by ref. 5).
Continuous stirred tank reactors, (a) With agitator and internal heat transfer surface, (b) With pump around mixing and external heat transfer surface, (adopted by ref. 5).

Because the compositions of mixtures leaving a CSTR are those within the reactor, the reaction driving forces, usually the reactant concentrations, are necessarily low. Therefore, except for reaction orders zero- and negative, a CSTR requires the largest volume of the reactor types to obtain desired conversions. However, the low driving force makes possible better control of rapid exothermic and endothermic reactions. When high conversions of reactants are needed, several CSTRs in series can be used. Equally good results can be obtained by dividing a single vessel into compartments while minimizing back-mixing and short-circuiting. The larger the number of CSTR stages, the closer the performance approaches that of a tubular plug-flow reactor.

Continuous-flow stirred-tank reactors in series are simpler and easier to design for isothermal operation than are tubular reactors. Reactions with narrow operating temperature ranges or those requiring close control of reactant concentrations for optimum selectivity benefit from series arrangements. If severe heat-transfer requirements are imposed, heating or cooling zones can be incorporated within or external to the CSTR. For example, impellers or centrally mounted draft tubes circulate liquid upward, then downward through vertical heat-exchanger tubes. In a similar fashion, reactor contents can be recycled through external heat exchangers.

The CSTR configuration is widely used in industrial applications and in wastewater treatment units (i.e. activated sludge reactors).

Tubular reactor or plug flow reactor: A tubular reactor is a vessel through which flow is continuous, usually at steady state, and configured so that conversion of the chemicals and other dependent variables are functions of position within the reactor rather than of time. In the ideal tubular reactor, the fluids flow as if they were solid plugs or pistons, and reaction time is the same for all flowing material at any given tube cross section. Tubular reactors resemble batch reactors in providing initially high driving forces, which diminish as the reactions progress down the tubes. Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases. Turbulent flow is generally preferred to laminar flow, because mixing and heat transfer are improved. For slow reactions and especially in small laboratory and pilot-plant reactors, establishing turbulent flow can result in inconveniently long reactors or may require unacceptably high feed rates.

Multiphase reactors – Reactor configurations for biofilm reactors / immobilized cell reactors Multiphase Reactors: The overwhelming majority of industrial reactors are multiphase reactors. Most reactors contain three phases:

  • Solid phase (biomass aggregates or biomass immobilized on carrier material)
  • Liquid phase (water phase with pollutants / nutrient and products)
  • Gas phase (air or gas feed, gaseous products CO2, N2, CH4). Design and operation of two-phase systems (liquid-solid) is considerably easier than three-phase reactors. Depending on the location of the cell aggregates / immobilized cells and the movement of carrier material three reactor categories can be distinguished:
  • Mixed suspended particles (e.g. fluidized beds)
  • Fixed particles or large surfaces (e.g. packed beds, trickling filters)
  • Moving surfaces (e.g. RBC, moving bed sand filters) Although for each type of immobilized cell system a variety of reactor types can be selected, optimal performance requires a careful matching of immobilization method and reactor configuration. Design of the cell aggregate and selection of conditions in the reactor should also go hand in hand. Immobilization of biomass removes the washout limitation in continuous operation with free cells. Cell recycling is, however, an alternative to cell immobilization that might be considered for operation at high cell densities, both in fed-batch and continuous modes and also for removing the washout limitation. Biomass recycling is intermediate between freely suspended and immobilized cell systems. The separation and recycling of cells can be achieved with the help of a centrifugation, settling or a membrane. The types of reactors presented below are often employed, but are not the only ones used. The presence of more than one phase, (solid and/or liquid and/or gas), whether or not it is flowing, confounds analyses of reactors and increases the multiplicity of reactor configurations. Gases, liquids, and solids flow in characteristic fashions, either dispersed in other phases or separately. Flow patterns in these reactors are complex and phases rarely exhibit idealized plug-flow or well-stirred flow behavior.

Mixed suspended particles reactors: In these reactors, liquid and solid phase (i.e. cell aggregates or immobilized cells) are completely mixed. Typical examples are fluidized bed reactors (FBR) and stirred tanks. Two phase reactors are generally limited to anaerobic processes because oxygen transfer requires the mixing of three phases. Three phase mixed reactors are widely used in fluidized, semi-fluidized or expanded bed reactors in which the particles are suspended and mixed by the upflow of gas and liquid. The flow pattern and consequently the liquid mixing and gas hold-up depends on the particle density and flow rates. Particle density can be altered during the operation of the reactor due to the growth of the biofilm making the reactor design complicated.

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