Tubular reactor or plug flow reactor

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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 generally is 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: 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) are considerably easier than for 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.
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