The basic ideal reactor types

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The basic types are ? you know them, or ?... more?

The characterization of the 3 basic (ideal) reactor types

There are two points of view for the characterization of the basic reactor types:

• the characteristics of macro-mixing, represented by the residence time distribution (RTD),( - see answer to the question: 'which information do you get from the RTD ?' for further details).
  In this case you 'see only' information on the vessel's mixing-behaviour as you get it by labelling with an inert tracer. I want to visualize the 'mere mixing bevaviour' as we could see it with our bare eyes when a colour tracer is applied in a very simple animation. The red colour tracer is added in a step function, - the response is the cumulative RTD. The draft can be seen here (animated gif, about 600 k):
  
  Don't forget: no reaction!!
  You see the moving plug flow front in the ideal TFR (is that realistic ??) and the steadily growing red of the colour in the CSTR . When the cumulative RTD is finished, the system starts again with the tracer concentration switched off simultaneously, here you see the signal course of the 'displacement method', the tracer fluid (colour solution) gets displaced by the pure 'basic fluid' (solvent, e.g. water). Did you know that for the ideal CSTR, and only for this reactor, the displacement curve represents the RTD spectral curve ?!!
  
  
• the (typical) concentration course as you can measure it when carrying out an experiment with a 'running' reaction. (But don't forget that the mixing-behaviour is an information that is immanent in the concentration course)

Therefore: 'More' information on the behaviour under reaction conditions can be received by regarding the concentration curves vs. time and space (length, volume) in the 3 basic types when carrying out a simple reaction in the reactors:*

• the batch stirred tank reactor: concentration is is constant overall in the whole vessel (stationary in space and totally back-mixed in space) but it is dropping down with the time ( not stationary in time, - that is equivalent to 'not back-mixed in time' but this statement may be a bit less understandable), this can also be seen by the different constant concentration levels in space that are dropping down with time (parallel lines!). The reactor is not stationary in (both) space .. and time.
  
  
• the continuous tubular flow reactor: concentration is constant over the time at different length positions along the tube ( parallel lines with the parameter L), it is stationary in time. But the reactor is not stationary in space, as you can see from the concentration dropping down along the reactor length, - the reactor is not stationary in space. The reactor is 'not-back-mixed' in space (concentration gradient along reactor length). The reactor is not stationary in (both) space ..and time.
  
  
• the continuous stirred tank reactor: concentration is stationary in space and time. The reactor is totally back-mixed in space and time. The reactor is stationary in space and time. But you have to realize: that is valid only for the reactor when running stationary, not in the start-up- , shutdown- or disturbed operation phase !!!!

*And once again: the statements above are made for the reactors with a 'running' reaction inside, do not change that with the properties of macro-mixing alone, as characterized by the RTD from an inert tracer signal. But - as already mentioned - the macro-mixing properties are the cause for the whole behaviour. What is the difference ? Take a very simple statement: With a RTD experiment the inert tracer is diluted by the convective flow and disappears on account of that dilution. The 'running' chemical reaction makes together with the convective flow the educts disappear, products appear and disappear, concentrations change, kinetics get influenced. You see the difference between 'only' RTD and 'reaction running' very comprehensive, when you regard a serial combination of an ideal CSTR and an ideal TFR: for the RTD it is not important, which reactor type is the first of the series, - wheres for a running reaction this topic becomes 'decisive' (see general remarks on reactor combinations and especially two graphs showing the two alternatives of a series combination TFR/CSTR for a simple 1st order reaction). By the way: think about the question whether a RTD for a STR makes any sense in practice. First of all: you can not add a step function to a 'batch' (true or not?). But you could perhaps add a Dirac pulse to a STR. What would you see: you could see the mixing quality of the tank, isn't it? In spite of being interesting, this information is not a RTD in the usual sense. And: never take the word 'reaction' into your mouth, when talking about RTD, - unless you are a specialist and talking about 'reactive marking in RTD' ! The 'normal' RTD doesn't characterize reactive behaviour ! But: both aspects (RTD and reactive system) are entirely represented by the material balance of the reactor. Only in the case of the RTD the term of the chemical reaction is omitted in the material balance, - and when you solve the material balance equation for this case, you get the observable function of the RTD. For a visualized example I want to show you a 'virtual snapshot' of an 'open' parallel combination of an ideal CSTR and a TFR. The continuous reactor system is running stationary and a reaction is carried out in it where a red dye is decomposing to a colorless product in a 'definite reaction' started at the reactor inlets ( thermal or light induction or something like that). In the CSTR you get some rest colour corresponding to the reached conversion, whereas in the TFR you see a gradient of the colour along the tube, ending at the white colour of the solvent. I tried to put a black line to the place where the colour is the same as in the CSTR. With this line I wanted to demonstrate that the same conversion can be reached in the TFR within a clearly smaller reactor volume. Realize: reaction takes place !!! The colour gradient along the tube is a bit dense, but I think you realize the intention. Did you realize that we need no animation here, because in the stationary state we would have always the same image (repeatedly) ? ! In a further animation you see (repeatedly) the colour decay in a batch STR, where the 'x-coordinate' for the concentration gradient is the time axis (compare to the graph on top ): Realize: reaction takes place !!! here you need an animation for a good visualization!! back to top

Immediate test of your knowledge:
Do you now understand the following statements?

A CSTR is working stationary in space and time, it is 'totally' back-mixed

A Batch STR is working stationary only in space, but not in time, it is only back-mixed in space, but not in time

A TFR is working stationary in time, but not in space (there is a gradient along the length-axis), it is not-back-mixed in space

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Important 'Add-on' to the classification of basic reactor types

Mainly in context with reactors for heterogeneous systems - but also as a general mode of reactor classification - we can define:

• *integral reactors:
  
  Integral reactors are reactors running in a high conversion mode. They are 'good' reactors for high specific product efficiencies. Fixed bed reactors with considerable heights of the fixed bed are typical integral reactors (integral TFR). For kinetic measurements the integral reactors are problematic, as they exhibit a concentration gradient along the fixed bed and the solution of their material balance demands an integration. For this integration you normally have to know your reaction kinetics, - but that is what you want to measure when carrying out kinetic measurements !!
  
  
• *differential reactors:
  
  Differential reactors are running in a very low conversion mode. They are 'good' reactors for kinetic measurements. When the conversion is infinitesimally small, our conversion represents directly the reaction rate. The only disadvantage of these reactors are the analytical problems caused by the minimal difference between inlet- and outlet-concentrations. Typical differential reactors are fixed bed reactors with very low fixed bed heights. For getting rid of the problem of very small concentration differences we can take the differential loop reactor with high recirculation factors. Here we get an 'enhancement' by multiple passing of the 'differential bed' and as a further advantage that we can use the material balance of a CSTR for the differential loop reactor with high recirculation.
  

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And last not least a more complicated question: what makes the relationship between batch STR and continuous TFR, what is their difference to the CSTR, - and what is the difference between them among themselves ? help?

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