Selection of reactors

The selection of reactor pressure for vapor-phase reversible reactions depends on whether there is a decrease or increase in the number of moles and whether there is a system of single or multiple reactions. 

The selection of reactor type in the traditionally continuous bulk chemicals industry has always been dominated by considering the number and type of phases present, the relative importance of transport processes (both heat and mass transfer) and reaction kinetics plus the reaction network relating to required and undesired reactions and any aspects of catalyst deactivation. The opportunity for economic... 

The selection of reactor pressure for vapor-phase reversible reactions depends on whether there is a decrease or an increase in the number of moles. The value of AN in Equation 6.25 dictates whether the equilibrium conversion will increase or decrease with increasing pressure. If AN is negative, the equilibrium conversion will increase with increasing pressure. If AN is positive, it will decrease. The choice of pressure must also take account of whether the system involves multiple reactions. 

The complexity of this relation depends on the reaction orders involved. Generally, one finds that it is not easy to arrive at expressions for the time dependence of the various species concentrations. However, it is often possible to obtain relations for the relative extents of reaction that are useful for design purposes. In Chapter 9 we will see the implications of such relations in the selection of reactor type and modes of contacting. 

The selection of microbiological processes for treating soils and groundwater contaminated with organic pollutants requires characterization of the waste, selection of an appropriate microorganism or consortium, and information about degradation pathway and rates. This chapter will focus on ex situ processes and the selection of reactors for the treatment of soils and groundwater. 

Obviously, rpEi can be expressed in terms of reaction temperature and concentration or partial pressure of chemicals and so on. Therefore, the PEI rate-law expression can be helpful to analyze the effect of concentrations and temperature on the PEI transformation rate, rpEi, and to study the influence of back-mixing on process environmental performance. At least, the PEI rate-law expression can reveal the factors that control the transformation rate of PEI, so as to provide guidance for the selection of reactor type and operation conditions, and the inner structure of the reactor, which produce desired products while creating minimum undesired potential environmental impact. 

Besides the appropriate selection of reactor type, the two problems can be highlighted (1) Atomization of Ca(OH)2 suspension and (2) Caking on the walls and cleaning. 

In general, the temperature of the reactor is established in a number of different ways that depend very strongly on the chemistry and the kinetics. For the simple irreversible reaction studied in this section, in which the only issue is to achieve the desired conversion, it would appear that the reactor temperature should be made as high as possible. This would give the largest specific reaction rate and therefore the smallest reactor size, thus minimizing capital investment. However, as we show below, there are dynamic controllability considerations that must be factored in when selecting reactor temperature. For the more complex reactions considered in later sections (such as reversible, consecutive, or simultaneous reactions), in which issues of both conversion and yield are important, the selection of reactor temperature must consider the production of undesirable products as well as reactant conversion. 

We have already given some qualitative consideration of this question for several of the reaction types. For example, with the simple irreversible, exothermic A —> B reaction conducted in a single CSTR, the highest possible temperature-minimized reactor size and hence capital investment. But small reactors have small heat transfer areas, so dynamic control problems may limit the selection of reactor temperature. 

It might be wise to point out that there can be other problems with a cooled reactor that will influence the selection of reactor type. An adiabatic reactor with a bed of catalyst is certainly mechanically straightforward to construct and maintain. Catalyst is easily loaded or discharged. A cooled reactor with multiple parallel tubes is mechanically more complex. Loading and emptying the tubes of catalyst can be difficult. 

Table 2.9 Heuristics Selection of reactor for homogeneous systems.

The selection of reactors dealing with a heterogeneous reaction should take into account three aspects catalyst selection, reactant injection and dispersion, choice of hydrodynamic flow regime, as illustrated by Figure 2.7 [14]. 

UOP and Norsk Hydro have jointly developed and demonstrated a new MTO process utilizing a SAPO-34 containing catalyst that provides up to 80% yield of ethylene and propylene at near-complete methanol conversion. Some of the key aspects of the work have included the selection of reactor design for the MTO process and determination of the effects of process conditions on product yield. Evaluation of the suitability of the MTO light olefin product as an olefin polymerization feedstock and demonstration of the stability of the MTO-lOO catalyst have also been determined during the development of this process. 

In Figure 2.6 A stands for the reactants, P for the desired products and X and Y for the undesired products. In the same figure examples are given of industrial reactions, which in later chapters will be studied in further detail. Depending on the orders of the relevant reactions and the choice of reaction conditions, by careful selection of reactor type and concentration level the selectivity can be controlled favorably. These reactor properties were defined in Section 1.4. 

In this section we discuss various means of minimizing the undesired product, U, through the selection of reactor type and conditions. We also discuss the development of efficient reactor schemes. 

As a third example, consider the HDA process studied extensively by Douglas (1988). The superstructure for this process is shown in Fig. 17, which is based on a preliminary qualitative analysis of alternatives described in Douglas (1988). Given the basic options considered for the selection of reactors and the use of membrane separators, as well as a restricted set of alternatives for the... 

Insufficient information exists currently for complex selective reactions, limited to phenomenological results with few electrocatalysts and reactants. The design of polymetallic clusters and of catalysts with controlled crystallite size, the exploration of redox catalysts, the tailoring of the physical catalyst structure, and the selection of reactors and operating conditions to enhance or suppress multiple reaction paths await further study. The exploitation of unconventional reduction or oxidation potential regimes for specificity control, which has been only occasionally attempted or appreciated, appears to be especially attractive. 

In an early attempt Kokossis and Floudas (1990, 1994) proposed the use of a redundant MINLP superstructure that includes several CSTR s, and recycles. Integer variables are used to manage the selection of reactors and their connections. It is clear... 

For a constant amount of heat transfer, a degraded heat transfer characteristic requires higher fuel temperature, which is not desirable. Therefore, desirable heat transfer properties in the selection of reactor materials, especially those used as core cladding and heat exchanger tubes, are a major consideration. 

Idealized models of mixing and displacement that are determined by hydrodynamic structure of liquid flows are usually used for theoretical calculation and selection of reactor construction accordingly to the kinetics of process. 

The key parameters for selection of reactor type are residence time and reaction zone dimensions that are necessary for achievement of required monomer conversion in isothermal conditions and consequently the optimal time of process carrying out. 

Selection of reactor for sulfation of a-olefmes caused significant complications due to specificity of chemical process proceeding. Reaction proceeds fast enough (characteristic reaction time according with [276] is lower than 30 sec) and is high-exothermal. In particular, heat of sulfation is equal to q 502,MO kJ per 1 m of a-olefines [277]. As a consequence temperature rise in reaction zone is about 190 5°C. We should also note that due to significant difference in values of viscosity (n)... 

More common configurations of PMRs are systems with suspended photocatalyst. In these PMRs various membrane techniques are utilized MF, UF, NF, dialysis, PV, or MD. The photocatalytic reaction in PMRs with suspended photocatalyst might be conducted in (a) a feed tank, (b) a membrane module or (c) an additional reservoir (photoreactor) which is located between the feed tank and the membrane module. Thus, the danger of polymer membrane destruction by UV light could be avoided by a proper selection of reactor configuration. 

The selection of reactor type is perhaps the most important problem in SSITKA studies. There are two types of reactors, that is, plug-flow and continuous stirred tank (CSTR), which differ in mass transfer regimes. The decisive advantage of CSTR is that the reaction takes place in gradientless conditions, which considerably simplifies the kinetic studies. However, the rather high response time of gas mixing in the reactor volume may distort the true dynamics of label transfer from reactants to reaction products. 

This issue will have an impact on the selection of reactor tubes and is in fact a more or less continuous improvement overtime. Better alloys for new tube materials have creep-rupture strength, which is more than double as that of the traditional HK-40 tube material. 

An on-line gas chromatograph is also used to determine the concentrations of the more important components such as H2> CH4, C2H4, and CsHg, These analyses, in conjunction with the product gas volume measurement, permit yields of the major components to be calculated within 30 minutes after completion of a run. More importantly, the on-line instrument permits the effect of reactor conditions on product yields to be screened rapidly, and enables the selection of reactor conditions required to achieve a particular product distribution. 

Such operation of the reactor requires a proper selection of reactor parameters (Thullie and Burghardt, 1995). When the cycling time is properly determined according to mass flow rate of the main stream a special attention should be given to mass flow rate of the injection. When it is too high in comparison to the main stream the reactor may be cooled down and the reaction on the catalyst surface will be stopped. 

In Chapters 10 - 13 a number of examples will be given of chemical reactor development and of the selection of reactor types, for various categories of chemical processes. 

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