Reactor Analysis

Reactor Analysis

DOI: 10.4018/978-1-5225-9441-3.ch002
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Reactor is a device or vessel within which chemical processes are carried out for experimental or manufacturing purposes. The most common basic types of reactors are tanks (where the reactants mix in the whole volume) and pipes or tubes (for laminar flow reactors and plug flow reactors). Both types can be used as continuous reactors or batch reactors, and either may accommodate one or more solids (reagents, catalysts, or inert materials), but the reagents and products are typically fluids (liquids or gases). Reactors in continuous processes are typically run at steady-state, whereas reactors in batch processes are necessarily operated in a transient state. When a reactor is brought into operation, either for the first time or after a shutdown, it is in a transient state, and key process variables change with time. The purpose of this chapter is to discuss the types of the reactors used in the wastewater treatment, modeling ideal, non-ideal flows in the reactor and treatment kinetics. Furthermore, the chapter considers both kinetic and hydrodynamic aspect while designing the reactor.
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Types Of Reactors Used In The Wastewater Treatment

  • Batch reactor

  • Continuous stirred-tank reactor (CSTR)

  • Plug flow reactor (PFR)

Batch Reactor

The batch reactor is the generic term for a type of vessel widely used in the process industries. Its name is something of a misnomer since vessels of this type are used for a variety of process operations such as solids dissolution, product mixing, chemical reactions, batch distillation, crystallization, liquid/liquid extraction and polymerization.

Figure 1.

Schematic diagram of batch reactor


Batch reactors are normally used for small-scale operation, testing new processes, the manufacture of expensive products, and processes difficult to convert to continuous. The advantage is that high conversions can be achieved due to leaving the reactants in reactor. The disadvantages are high labour costs, variability of products (batch to batch), and they are difficult to operate/automate for large-scale production.In order to model a batch reactor, we need to make the assumption that at any given time the reactor is well-mixed so that the composition, temperature, and pressure are the same everywhere in the reactor.

When the reaction takes place in a liquid (or sometimes solids), very often the reacting component occurs at a much lower concentration then the principal component of the liquid (i.e. the solvent). In this case, a good approximation is that the volume of the liquid and pressure in the reactor remain constant with time as there is only a small change in the density of the liquid during the reaction. For gas-phase reactions, the entire volume of the reactor is filled by the gas, so that the reaction volume is equal to the reactor volume (whereas for a liquid, the reaction volume is the volume of the liquid which is less than the reactor volume). Thus for a gas-phase reaction, if the reaction either consumes or generates moles, the net effect will be a change to the pressure in the reactor, because the volume remains constant.

If the reaction generates moles, the pressure in the reactor will increase, whereas if moles are consumed, the pressure in the reactor will decrease. Because we assume that the batch reactor is well mixed (remember the mole balance must be made over a volume element which is spatially uniform with respect to composition and temperature) we can apply the mole balance over the entire volume of the reactor (Thomas Rodgers, 2013) . There is no inflow or outflow in a batch Reactor (Krishna, 2013).

Complete Mix Reactor

Figure 2.

Schematic diagram of complete mix reactor


In completely mixed reactors, the content within the control volume remains in completely mixed (homogeneous) state. Such system often needs a stirring device to ensure the mixing in the reactors and are commonly known as Continuous Stirred Tank Reactor (CSTR). This makes the temperature, concentration, and reaction rate independent of position in the reactor (Rosen, 2014). In continuous flow reactors, there is a continuous inflow to and outflow from control volume. Therefore, the rate of mass flux in and out has to be considered in mass balance. CSTRs are simply well ‐mixed tanks which are used to model well‐mixed environmental reservoirs. In CSTRs, the concentration of a substance in outflow remains equal to that in the reactor.

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