Reactors, selection

The selection of a reactor, from those described above as an alternative to the STR, depends on the heat reaction, the nature of the phases involved and the rate of production. The safety, reliability, energy consumption and cost have to be considered. The volumetric heat- and mass-transfer coefficients can be viewed as measures of process intensification. The attainable coefficients in the reactors dictate the yield, selectivity and size reduction. 

Unfortunately, the available data are sparse at present. From the heat transfer point of view, the suitability of the reactors may be put in deceasing order as SSTR, stator-rotor SDR, HEX reactors, AFR, SDR, ACR/ATR, OBR and HIGEE. If the interphase transfer dictates the reaction rate, the volumetric mass-transfer coefficient plays a dominant role. The micro-mixing and residence time also are of importance in the case of fast reactions and thermally unstable products. The above ordering (but for the SDR) could be used for gas-liquid or liquid-liquid phases. If solid-liquid or solid-liquid-gas are involved, the decreasing order could be OBR, ACR/ATR, AFR, HIGEE, stator-rotor SDR, and SSTR. Studies are underway on the characteristics of the reactors. These would help to ascertain their relative merits quantitatively in the future. 

The selection of a chemical reactor should ensure safe operation, high productivity and yield, low capital and operating costs, environmental acceptability, and acceptable flexibility with respect to production rate and raw materials composition. 

The primary reason for choosing a particular reactor type is the influence of mixing on the reaction rates. Since the rates affect conversion, yield, and selectivity we can select a reactor that optimizes the steady-state economics of the process. For example, the plug-flow reactor has a smaller volume than the CSTR for the same production rate under isothermal conditions and kinetics dominated by the reactant concentrations. The opposite may be true for adiabatic operation or autocata-lytic reactions. For those situations, the CSTR would have the smaller volume since it could operate at the exit conditions of a plug-flow reactor and thus achieve a higher overall rate of reaction. 

While reactor size may be important for the economics of the process, other factors such as yield and selectivity typically play a greater role. A classic situation is when the main products suffer degradation by consecutive reactions. In the scheme 

As is often the case, a completely different set of factors influences the choice of reactor from a control standpoint. Our main focus for control is stability and responsiveness to changes in the manipulated variables. We look at the det ails of these issues in the following sections but make some broad brush generalizations at this point. 

Improving the performance of a thermal incinerator basically involves the optimization of the combustion process. Ideally, no more combustion air should be used than is required for complete combustion of the fiunes and the auxiUary fuel. The auxiUary fuel should be used only in amounts requited to maintain the design furnace temperature. 

An incinerator operating efficiently should normally have only 1-2% O2 and 0-1% combustibles in the outlet gases. Monitors are available that can indicate these parameters to the operator as well as provide automatic control of the incinerator when required. 

The following questions often require answers. If odors are involved, has their concentration been reduced or eliminated Has there been adequate reduction in the emissions of all types of reactive hydrocarbons Has there been a reduction in any possible explosion or pubUc health hazard that might exist in the manufacturing process If so, the air pollution control system should be well designed, and operated and maintained properly. 

There ate a number of factors to be considered prior to selecting a reactor. In general, they can be grouped into three categories engineering, economic, and environmental. 

Contaminant characteristics (i.e., physical and chemical properties, concentration, particulate shape and size distribution—in the case of particulates, chemical reactivity, corrosivity, abrasiveness, toxicity, etc.) 

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In this case, because there are no raw materials losses in the separation and recycle system, the only yield loss is in the reactor, and the process yield equals the reactor selectivity. 

Most reactors have evolved from concentrated efforts focused on one type of reactor. Some processes have emerged from parallel developments using markedly different reactor types. In most cases, the reactor selected for laboratory study has become the reactor type used industrially because further development usually favors extending this technology. Descriptions of some industrially important petrochemical processes and their reactors are available (74—76). Following are illustrative examples of reactor usage, classified according to reactor type. 

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

Packed-bed selectivity. Membrane reactor selectivity. Packed bed yield. 

R. Krishna, A Systems Approach to Multiphase Reactor Selection... 

Gezdhmte Chemie im Mikroreaktor, VDI Nachrichten, June 2000 Micro-reactor enterprises shape and material variety of micro reactors selectivity gains and new project regimes direct fluorination faster process development BASF investigations safety increase speed-up of catalyst development production for fine chemistry and pharmacy numbering-up first industrial examples for micro-reactor production [215]. 

All three comparisons reveal that the micro reactor catalysts have higher conversion and yield, owing to enhanced mass transfer (20 vol.-% ethylene, 80 vol.-% oxygen 0.3 MPa 3.17 1 h 230/250 °C) [44]. The selectivity is slightly better in the fixed beds, but, only by about 1-5%. The best fixed-bed and micro reactor selectivity is 65 and 63%, respectively. The best fixed-bed and micro reactor conversion is 37 and 66%, respectively. 

In fine chemistry, mathematical models are scarce yet. However, even gross kinetics provides a lot of information on the influence of the mode of operation on seleetivity. In general, semi-quantitative criteria are used in preliminary reactor selection. They are mainly based mainly on operational characteristics, experience, and a rough economic estimation. Factors affecting the choice of the reactor and mode of operation are listed in Table 5.4-42. 

Many fine chemistry proces.ses can be lumped into a system of two parallel or two con.secutive reactions. Selectivity can roughly be assessed using the gross kinetics for such lumped schemes, and this can be used to derive approximate criteria for reactor selection. 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

Reactor Selectivity (%) Yield (%) Flowrate of chlorine (kmols-1) Volume (m3)... 

S Entropy (kJ-K-1, kJkg-1-K-1, kJkmol-1-K-1), or number of streams in a heat exchanger network (-), or reactor selectivity (-), or reboil ratio for distillation (-), or selectivity of a reaction (-), or slack variable in optimization (units depend on application), or solvent flowrate (kg s-1, kmol-s-1), or stripping factor in absorption (-)... 

In the sonochemical reactors, selection of suitable operating parameters such as the intensity and the frequency of ultrasound and the vapor pressure of the cavitating media is an essential factor as the bubble behavior and hence the yields of sonochemical transformation are significantly altered due to these parameters. It is necessary that both the frequency and intensity of irradiation should not be increased beyond an optimum value, which is also a function of the type of the application and the equipment under consideration. The liquid phase physicochemical properties should be adjusted in such a way that generation of cavitation events is eased and also large number of smaller size cavities are formed in the system. 

Probability in Receptor Binding and Signaling Rakesh K. Jain, Transport Phenomena in Tumors R. Krishna, A Systems Approach to Multiphase Reactor Selection... 

Because of the ubiquitous nature of polymers and plastics (synthetic rubbers, nylon, polyesters, polyethylene, etc.) in everyday life, we should consider the kinetics of their formation (the focus here is on kinetics the significance of some features of kinetics in relation to polymer characteristics for reactor selection is treated in Chapter 18). 

Autocatalysis is a special type of molecular catalysis in which one of the products of reaction acts as a catalyst for the reaction. As a consequence, the concentration of this product appears in the observed rate law with a positive exponent if a catalyst in the usual sense, or with a negative exponent if an inhibitor. A characteristic of an autocat-alytic reaction is that the rate increases initially as the concentration of catalytic product increases, but eventually goes through a maximum and decreases as reactant is used up. The initial behavior may be described as abnormal kinetics, and has important consequences for reactor selection for such reactions. 

Fox, JM., Degen, B.D., Slurry Reactor Design Studies, Topical Report, Reactor Selection Criteria, US Department of Commerce, National Technical Information Service, USA (1990)... 

As well as the operating conditions inside the reactor, the design features have a powerful influence on reactor performance. The type of reactor selected has an influence on efficiencies, on corrosion endurance, solids-operation feasibility, or even reactor reliability. The most important SCWO reactor configurations are listed in the following. 

There are two criteria which can be used to compare the performances of different types of reactor. The first, which is a measure of reactor productivity, is the output of product in relation to reactor size. The second, which relates to reactor selectivity, is the extent to which formation of unwanted byproducts can be suppressed. When comparing reactions on the basis of output as in the present section, only one reaction need be considered, but when in the next section the question of byproduct formation is taken up, more complex schemes of two or more reactions must necessarily be introduced. 

Krishna R, Sie ST. Strategies for multiphase reactor selection. Chem. Eng. Sci. 1994 ... 

J. J. Linderman, P. A. Mahama, K. E. Forsten, and D. A. Lauffenburger, Diffusion and Probability in Receptor Binding and Signaling Rakesh K. Jain, Transport Phenomena in Tumors R. Krishna, A Systems Approach to Multiphase Reactor Selection... 

Reactor Selection Ideal CSTR and PFR models are extreme cases of complete axial dispersion (De = oo) and no axial dispersion (De = 0), respectively. As discussed earlier, staged ideal CSTRs may be used to represent intermediate axial dispersion. Alternatively, within the context of a PFR, the dispersion (or a PFR with recycle) model may be used to represent increased dispersion. Real reactors inevitably have a level of dispersion in between that for a PFR or an ideal CSTR. The level of dispersion may depend on fluid properties (e.g., is the fluid newtonian),... 

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