Reactors are at the core of most processes in the chemical process industries (CPI) and are broken down most broadly by whether their operating mode is non-flow (batch) or continuous. Each reactor type has advantages and disadvantages depending on the phases and properties of the reactants, the thermodynamics and kinetics of the reactions and the type of products being generated. This one-page reference provides a refresher on the most common types of reactors used in chemical manufacturing.
Batch reactors are closed vessels where reactants are added sequentially. Stirred tanks, with an agitator to mix the reactants thoroughly, are the most common batch reactor. Stirring mixes the ingredients initially, maintains homogeneity during the reaction and enhances heat transfer at internal surfaces and jacket walls. Batch processing is used generally when the reaction times are long or the required production volumes are small.
Semi-batch reactors are modified versions where reactants are periodically added or products are periodically removed. Semi-batch reactors may offer greater control over yield or selectivity of the products. This reactor type is useful for carrying out exothermic reactions because the flow of added reactants can be varied to better control the reaction. Scaling up semi-batch processes generally has higher capital costs than those for continuous process reactors.
Continuous stirred-tank reactor
In a continuous stirred-tank reactor (CSTR), reactants are continuously fed into the reactor vessel, where an agitator mixes them to produce desired products, which are removed continuously from the reactor.
The agitator maintains a constant concentration throughout the reactor. Mixing times (length of time needed to achieve homogeneity of mixture of inputs) depends on the geometry of the vessel and the speed and power of the agitator. The average residence time for fluid inside the tank at steady-state flow is the ratio of the total reactor volume to the volumetric steady-state flowrate of fluid exiting the reactor.
One major advantage of employing a CSTR is generating a massive product volume. These are continuous reactors that can be run for extended periods. CSTRs are unsuitable for reactions with extremely slow kinetics.
In many cases, CSTR reaction proceses are carried out in a series of reactors, called CSTR cascades, to provide higher conversions.
Plug flow reactors
In plug flow reactors, also known as tubular reactors, reactants and products flow through a cylindrical pipe with openings at each end. “Plugs” of reactants are continuously fed into the reactor, and as the plug flows along the length of the reactor, the reaction takes place. This results in an axial concentration gradient. Products, along with unreacted reactants, continuously exit the reactor.
Plug flow reactors are mechanically simple and easy to maintain, with high conversion rates for a given reactor volume generally being observed. Because reactor temperature is difficult to control, these reactors can be suboptimal for exothermic reactions.
Often deployed for catalytic processes, fixed-bed reactors are versatile, used in applications such as absorption, distillation, stripping, separation processes and catalytic reactions. Although the physical dimensions of the beds can vary greatly, typical fixed-bed reactors consist of a cylindrical chamber containing catalyst particles or pellets (Figure 1). Fluid flows through the catalyst bed, allowing the desired reaction to occur. Fixed beds contain catalyst particles that usually fall into the range of 2–5 mm dia. Catalysts can be loaded in several ways, including a single large bed, several horizontal beds, several packed tubes in a single shell, single bed with embedded tubes and beds in separate shells.
In fluidized-bed reactors, a heterogeneous catalyst is fluidized by an upward-flowing gas or liquid This allows for extensive mixing in all directions. A result of the mixing is excellent temperature stability and increased mass transfer and reaction rates.
Fluidized-bed reactors must be designed so that the fluid flowrate is sufficient to suspend the catalyst particles. The particles typically range in size from 10–300 μm.
When designing a fluidized-bed reactor, the catalyst life must also be taken into account. Most fluidized-bed reactors have a separate system to regenerate the catalyst.
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