Fundamentals of reaction engineering

We seek to design reaction vessels, i.e. chemical reactors, where a particular chemical reaction (or set of reactions) is carried out. The first decision we take involves the configuration of the reactor and its mode of operation. This means we must decide what reactor type (and reactor shape) to select and whether it would be advantageous to operate in batch or continuous mode. Other design decisions regarding the new reactor will be affected by a multiplicity of factors. To arrive at an appropriate design, we need information on the reaction kinetics and the required daily output. For relatively small daily production rates, we need to choose between batch and continuous operation, while, large throughputs usually require operation in continuous mode. If the reaction is rapid, we would need short residence times in the reactor. This might imply the use of high fluid velocities or small reactor dimensions, or a combination of the two. If we choose continuous operation, we still need to select the type of reactor to design and use. We will primarily focus on tubular reactors and continuous stirred tank reactors (CSTR). We will then need to size the reactor. Together with the throughput requirement and available flow rates, the data will allow us to calculate the residence time and help us decide on the shape of the reactor calculate the required size. The operating temperature and pressure are usually selected on the basis of the kinetics of the reaction and whether the reaction gives off heat (exothermic) or requires a heat input to proceed (endothermic). We will also need to know initial reactant concentrations available to us. Depending on how the problem is presented, some of these operating conditions may have already been specified or we may have to specify these parameters as part of the design process. Criteria for choosing between batchwise and continuous operation: Batch operation For small production rates (like pharmaceuticals, dyestuffs etc) of say a few tons/day, batch operations are generally more flexible and economical. Batch operations tend to require smaller capital expenditure than corresponding continuous processes, especially when the required production is of relatively low tonnage. However, quality control may be a problem, as replicating identical conditions in each batch may prove problematic. It is commonly said that no two vats of dye have exactly the same colour.