Chemical reactor

Encyclopedia : C : CH : CHE : Chemical reactor

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Chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction. Designers ensure that the reaction proceeds with the highest efficiency towards the desired output product, producing the highest yield of product while requiring the least amount of money to purchase and operate. Normal operating expenses include energy input, energy removal, raw material costs, labor, etc. Energy changes can come in the form of heating or cooling, pumping to increase pressure, frictional pressure loss (such as pressure drop across a 90^o elbow or an orifice plate), agitation, etc.

There are two main basic vessel types:

• tank reactor - a tank
• tubular reactor - a pipe or tube

Both types can be used as continuous reactors or batch reactors. Most commonly, reactors are run at steady-state, but can also be operated in a transient state. When a reactor is first brought back into operation (after maintenance or inoperation) it would be considered to be in a transient state, where key process variables change with time. Both types of reactors may also accommodate one or more solids (reagents, catalyst, or inert materials), but the reagents and products are typically liquids and gases.

There are three main basic models used to estimate the most important process variables of different chemical reactors:

• batch reactor model (batch),
• continuous stirred-tank reactor model (CSTR), and
• plug flow reactor model (PFR).

Furthermore, catalytic reactors require separate treatment, whether they are batch, CST or PF reactors, as the many assumptions of the simpler models are not valid.

Key process variables include:

• residence time (τ, lower case Greek tau)
• volume (V)
• temperature (T)
• pressure (P)
• concentrations of chemical species (C₁, C₂, C₃, ... C_n)
• heat transfer coefficients (h, U)

CSTR (Continuous Stirred-Tank Reactor)

In a CSTR, one or more fluid reagents are introduced into a tank reactor equipped with an impeller while the reactor effluent is removed. The impeller stirs the reagents to ensure proper mixing. Simply dividing the volume of the tank by the average volumetric flow rate through the tank gives the residence time, or the average amount of time a discrete quantity of reagent spends inside the tank. Using chemical kinetics, the reaction's expected percent completion can be calculated. Some important aspects of the CSTR:

• At steady-state, the flow rate in must equal the mass flow rate out, otherwise the tank will overflow or go empty (transient state). While the reactor is in a transient state the model equation must be derived from the differential mass and energy balances.
• All calculations performed with CSTRs assume perfect mixing.
• The reaction proceeds at the reaction rate associated with the final (output) concentration.
• Often, it is economically beneficial to operate several CSTRs in series or in parallel. This allows, for example, the first CSTR to operate at a higher reagent concentration and therefore a higher reaction rate. In these cases, the sizes of the reactors may be varied in order to minimize the total capital investment required to implement the process.
• It can be seen that an infinite number of infinitely small CSTRs operating in series would be equivalent to a PFR.

PFR (Plug Flow Reactor)

In a PFR, one or more fluid reagents are pumped through a pipe or tube. The chemical reaction proceeds as the reagents travel through the PFR. In this type of reactor, the reaction rate is a gradient; at the inlet to the PFR the rate is very high, but as the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate slows. Some important aspects of the PFR:

• All calculations performed with PFRs assume no upstream or downstream mixing, as implied by the term "plug flow".
• Reagents may be introduced into the PFR at locations in the reactor other than the inlet. In this way, a higher efficiency may be obtained, or the size and cost of the PFR may be reduced.
• A PFR typically has a higher efficiency than a CSTR of the same volume. That is, given the same space-time, a reaction will proceed to a higher percentage completion in a PFR than in a CSTR.

For most chemical reactions, it is impossible for the reaction to proceed to 100% completion. The rate of reaction decreases as the percent completion increases until the point where the system reachs dynamic equilibrium (no net reaction, or change in chemical species occurs). The equilibrium point for most systems is less than 100% complete. For this reason a separation process, such as distillation, often follows a chemical reactor in order to separate any remaining reagents or byproducts from the desired product. These reagents may sometimes be reused at the beginning of the process, such as in the Haber process.

See also

• chemical engineering
• mass transfer
• mass balance
• heat transfer
• chemical kinetics
• thermodynamics

References

Schmidt, Lanny D., The Engineering of Chemical Reactions. New York: Oxford University Press, 1998. ISBN 0-19-510588-5.

 

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