Principal types of reactor

The following characteristics are normally used to classify reactor designs  

Reactor geometry flow pattern and manner of contacting the phases 

In a batch process all the reagents are added at the commencement the reaction proceeds, the compositions changing with time, and the reaction is stopped and the product withdrawn when the required conversion has been reached. Batch processes are suitable for small-scale production and for processes where a range of different products, or grades, is to be produced in the same equipment for instance, pigments, dyestuffs and polymers. 

In continuous processes the reactants are fed to the reactor and the products withdrawn continuously the reactor operates under steady-state conditions. Continuous production will normally give lower production costs than batch production, but lacks the flexibility of batch production. Continuous reactors will usually be selected for large-scale production. Processes that do not fit the definition of batch or continuous are often referred to as 

Homogeneous reactions are those in which the reactants, products, and any catalyst used form one continuous phase gaseous or liquid. 

---

In addition to these three principal types of reactor, there is also the semi-batch reactor in which one reactant is added gradually to the others. This is a convenient manner of operation for some highly exothermic reactions since the temperature can be controlled by adjusting the rate of addition. 

The four principal types of reactors used for bench-scale kinetic studies are batch, continuous stirred-tank (CSTR), tubular, and differential reactors. Which of these to choose is essentially a matter of the reaction conditions, available equipment, and the chemist s or engineer s predilections. The discussion here will focus on facets that pertain specifically to quantitative kinetic studies of homogeneous reactions. 

We turn now to consider the principal types of reactors and derive a set of equations for each that will describe the transformation 5 of the state of the feed into the state of the product. The continuous flow stirred tank reactor is one of the simplest in basic design and is widely used in chemical industry. Basically it consists in a vessel of volume V furnished with one or more inlets, an outlet, a means of cooling and a stirrer which keeps its composition and temperature essentially uniform. We shall assume that there is complete mixing on the molecular scale. It would be possible to treat of other cases following the work of Danckwerts (1958) and Zweitering (1959), but the corresponding transformation is much less wieldy. If the reactants flow in and out at a constant rate q, the mean residence time T/g is known as the holding time of the reactor. 

Let us - after these rather extensive introductory remarks -turn to the specific problems encountered in design of ammonia and methanol synthesis reactors. We shall not endeavour to treat all of the above mentioned aspects, but mainly concentrate on the initial steps and on the basis of this illustrate how the various principal types of reactors can be applied in these syntheses. We shall discuss the reaction kinetics for the reactions and the calculation of reactor performance and some of the problems encountered in the calculation of reactor performance. The mathematical procedure used for the computer calculations is discussed by Christiansen and Jarvan W in a separate presentation in this volume. 

As important as kinetic mechanism are the phase changes that occur in polymerization. Only a small fraction of polymerizations are carried out only in one phase thus thermodynamics, heat and mass transfer, and the kinetics of the phase change itself all play a role in determining the properties of the product polymer. Table IV indicates the principal types of kinetic mechanisms and reaction media which arise in polymerization reactors. Each of these classes of systems has its own peculiar problems so that polymerization reactor design can often be much more challenging than the design of reactors for short chain molecules. 

FIG. 7-17 Principal types of laboratory reactors for gas-liquid reactions. [From Fig. 8 in J. C. Charpentier. Mass Transfer Rates in Gas-Liquid Absorbers and Reactors in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981. ]... 

There are many different types of reactors. In the United States, the majority of the reactors are pressurized water reactors with graphite moderators. The Canadians built the CANDU reactor using heavy water as both moderator and coolant. Naval ship reactors are graphite moderated liquid metal cooled reactors. The detailed differences between the reactor types will not be examined, but the operating principal common to all will be discussed. 

Practical activities are used when catalyst comparisons only are required. Measurements are faster and less demanding. The same types of reactor discussed above are used or larger units may be required. The principal function of pilot unit operations is to explore real feeds, process conditions, and deactivation, but practical activities are usually measured as these change. Three types are encountered measurement of (1) conversion at fixed space velocity, (2) space velocity for a fixed conversion, and (3) temperature for a fixed conversion. 

From the end of sixties, the principal studies in the theory of chemical technology were based on mathematical and physical modelling of the total set of superimposed processes. Vigorous development of computers and numerical methods of analysis promoted a fast development of investigations and continuous complication of models, which enable us to percive new details of the processes. At present, the fundamental physical principles and phenomena are understood in principle, mathematical models of processes have been developed in the main types of reactors, the fields of their application have been determined and computational methods for solution and analysis have been defined. Since the mid 1970s, the main attention of the researchers has been attracted to the study of the peculiarities of processes. 

This Safety Guide deals principally with light water reactors. However, some considerations may be of interest for other types of reactor. 

The first section of this chapter is a brief discussion of those aspects of nuclear reactions which are of principal interest to reactor physics. This presentation assumes that the reader is equipped with an introductory course in nuclear physics. The second section is an outline of the basic nuclear components of reactors and of the various types of reactors, and the last section is a summary of the principal problems of reactor physics and the analytical methods of attack. It is intended that the last section be used primarily for purposes of review and to aid the reader in orienting the various topics with regard to the over-all structure and scope of the subject. 

Reactor pressure vessels. Three principal types of stresses should be considered in designing the pressure vessels of one- or two-region reactors ... 

Sasol produces synthetic fuels and chemicals from coal-derived synthesis gas. Two significant variations of this technology have been commercialized, and new process variations are continually under development. Sasol One used both the fixed-bed (Arge) process, operated at about 240°C, as weU as a circulating fluidized-bed (Synthol) system operating at 340°C. Each ET reactor type has a characteristic product distribution that includes coproducts isolated for use in the chemical industry. Paraffin wax is one of the principal coproducts of the low temperature Arge process. Alcohols, ketones, and lower paraffins are among the valuable coproducts obtained from the Synthol process. 

Retrofitting features of the more efficient reactor types have been the principal thmst of older methanol plant modernization (17). Conversion of quench converters to radial flow improves mixing and distribution, while reducing pressure drop. Installing an additional converter on the synthesis loop purge or before the final stage of the synthesis gas compressor has been proposed as a debotdenecking measure. 

The point is that the same population dynamics are applicable to remediation systems. The principal difference is that in a soil/water system, one has essentially the growth rate of the bacteria as a limiting condition. This is also akin to another type of system known as the Sequencing Batch Reactor or SBR. 

The IWA (International Water Association), formerly known as the IWQA, has had several task forces working on model development for various types of processes. I believe that these reactor models have a good potential application for remedial treatment. The subject of the models is extremely complex and too involved for this discussion, as it is a Master s Level course in Environmental Engineering. However, let me indicate that there are several types of models which may have some application to the bioremediation field. The principal models are... 

The principal use of tubular reactors for kinetic studies is as catalytic fixed-bed reactors in heterogeneous catalysis. They are rarely used for quantitative studies of homogeneous reactions because these are difficult to confine sharply to reactors of this type (see farther below). 

---