Chemical reactor

Chemical Reactors deal with several aspects of chemical engineering. It is the job of the chemical engineer to ensure that the reaction proceeds with the highest efficiency, producing the purest product while requiring the least possible amount of input energy. There are two main types of chemical reactors: the CSTR (Continuously-Stirred Tank Reactor) and the PFR (Plug Flow Reactor). Both types can be used as steady-state or batch reactors. Both types may also accommodate one or more solid reagents, but the reagent(s) and product(s) are typically liquids.

CSTR (Continuously-Stirred Tank Reactor)

In a CSTR, one or more liquid reagents are introduced into a storage tank equipped with an impeller. The impeller continuously stirs the reagents to ensure proper mixing. At the same time, product is drawn off from the tank. Simply dividing the volume of the tank by the flow rate through the tank gives the space-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:

• The flow rate in must equal the flow rate out, otherwise the tank will overflow or go empty.
• All calculations performed with CSTRs assume perfect mixing .
• The reaction proceeds at the reaction rate associated with the final (output) concentration. In most chemical reactions, this is also the SLOWEST rate at which the reaction will proceed.
• 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 liquid 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 an 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, because the rate of reaction decreases as the percent completion increases. For this reason, a separation process such as distillation often follows a chemical reactor in order to separate any remaining reagents from the desired product. These reagents may sometimes be reused at the beginning of the process.

See Also

• chemical engineering
• mass transfer
• heat transfer
• chemical kinetics