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Chemical reactor selection and design

Rajesh Kumar Bhagat B.E.(CHE) 4th year

Introduction Almost all chemical engineering process contains three operations. Unit Raw operation material (cleaning )

Chemical reactor

Unit operation (separatio n)

What does chemical reactor design means ?

Product

Reactor System

Homogenous Heterogeneous

Types of reactors 1.Batch- uniform composition everywhere in reactor but changes with time 2. Semi batch- in semi-batch one reactant will be added when reaction will proceed 3. Continuous reactor a. Mixed flow- this is uniformly mixed , same composition everywhere, within the reactor and at exit b. Plug flow- flow of fluid through reactor with order so that only lateral mixing is possible.

Reactor design parameter Reactor design basically means which type and size of reactor and method of operation we should employ for a given conversation Parameters • • • • • •

Volume of reactor Flow rate Concentration of feed Reaction kinetic Temperature pressure

Isothermal reactor design algorithm

Plug flow and mixed flow reactor design Mixed flow reactor design Applying mass balance performance equation for mixed flow reactor

Plug flow reactor design Performance equation for plug flow reactor

Performance equation

Plug flow vs CSTR •

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For any particular duty and for all positive reaction order the volume of mixed flow reactor will always be grater then plug flow Area under curve in figure is very small for plug flow as compared to mixed flow so volume is small for plug flow. When conversion is small, the reactor performance is only slightly affected by flow type. the perforation ratio very rapidly at high conversion. Density variation during reaction affects design, however it is normally of secondary importance compared to the difference in flow type.

Multiple reactor system • Number of plug flow reactor in series are theoretically same as equivalent volume of a single plug flow reactor. • Number of mixed flow reactor of equal size in series may be used when we need high conversion and can’t perform in a single reactor. • From the given graph, for first order reaction, conversion for series of equal size reactor can be find

Mixed flow reactor of different size in series •

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From the fig it is clear that for plug flow reactor volume can be find by dashed area and for mixed flow whole area. When we are have to use mixed flow reactor, then we can use different size mixed flow reactor so, that over all volume would be small To optimized or to find how different size of mixed flow reactor should used we have to maximized lower dashed rectangle. This optimization gives the slope of diagonal of the rectangle should be equal to slope of curve at intersection of these two reactor. Levenspiel , has proved that after overall economic consideration equal size reactors in series are economical.

Autocatalytic reactor • •

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When a product will act like a catalyst then it is called auto catalytic reaction. In mixed flow reactor at fixed product concentration for high yield, efficiency of reactor will be very low. For no recycle for low product concentration mixed flow reactor will be preferred and for high conversion plug flow . For optimum efficiency we can use a recycle or back mixing plug flow reactors. For a particular exit concentration a particular optimum recycle ratio should be used. Optimum recycle ratio introduced to the reactor feed’s 1/(-r) value should be equal to average 1/(-r) value for whole reactor.

Fig-1 Plug flow reactor with recycle

Fig-2

Fig-3

Fig-4

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Design for parallel reaction

When a reactant gives two product (desired, and undesired)simultaneously with different rate constant then this is called a parallel reaction. To keep maximum amount of desired product we can take following steps. Ifa1>a2 or the desired reaction is of higher order then keep reactant concentration high for high product concentration. If a1