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.