Factors Affecting Reactor Performance

Scale-Up Principles. Key factors affecting scale-up of reactor performance are nature of reaction zones, specific reaction rates, and mass- and heat-transport rates to and from reaction sites. Where considerable uncertainties exist or large quantities of products are needed for market evaluations, intermediate-sized demonstration units between pilot and industrial plants are usehil. Matching overall fluid flow characteristics within the reactor might determine the operative criteria. Ideally, the smaller reactor acts as a volume segment of the larger one. Elow distributions are not markedly influenced by... 

Aspects of the various factors affecting performance in the types of reactions and reactors considered in Chapter 9 and in this chapter are summarized in Figure 22.1. 

Figure 22.1 Factors affecting reactor performance for a fluid (A) + solid (B) reaction, A + hB -> products...

Factors affecting the performance of a reactor. In general we may write... 

Internal and external mass transfer resistances are important factors affecting the catalyst performance. These are determined mainly by the properties of the fluids in the reaction system, the gas-liquid contact area, which is very high for monolith reactors, and the diffusion lengths, which are short in monoliths. The monolith reactor is expected to provide apparent reaction rates near those of intrinsic kinetics due to its simplicity and the absence of diffusional limitations. The high mass transfer rates obtained in the monolith reactors result in higher catalyst utilization and possibly improved selectivity. 

It is important to recognize the limitations of the RTD method. Residence time distribution does not discern between a reacting fluid that is mixed on the molecular level (micromixing) and one that flows in segregated blobs. Also, the same RTD is obtained when the reacting fluid is mixed near the entrance or near the exit. Both of these factors affect the chemical reactions and the performance of the reactor. 

Recently, the fluidized bed membrane reactor (FBMR) has also been examined from the scale-up and practical points of view. Key factors affecting the performance of a commercial FBMR were analysed and compared to corresponding factors in the PBMR. Challenges to the commercial viability of the FBMR were identified. A very important design parameter was determined to be the distribution of membrane area between the dense bed and the dilute phase. Key areas for commercial viability were mechanical stability of reactor internals, the durability of the membrane material, and the effect of gas withdrawal on fluidization. Thermal uniformity was identified as an advantageous property of the FBMR. 

Chin S S, LimT M, Chiang K and Fane A G (2007b), Factors affecting the performance of a low-pressure submerged membrane photocatalytic reactor , Chem Eng 7,130,53-63. 

Daniels, T.C. and Al-Jumaily, F.K. (1975). Investigations of the factors affecting the performance of a rotating heat pipe. Int. J. Heat and Mass Transfer, Vol. 18, pp. 961-973. Goebel, P. (1977). Polymerisation reactor with gilled-tube radiator and axial agitator. US Patent 4029143, 14 June. 

Immobilized enzyme reactors are increasingly popular due to their advantages over conventional catalysts. For efficient reactor design and performance prediction, quantitative knowledge of reaction kinetics and the factors affecting them is required. In this chapter, enzyme catalytic mechanisms are described and the kinetic models developed from these mechanisms are discussed. The chapter also discusses the kinetics of immobilized enzymes and their related mass transfer effects. Diffusion restrictions are described with a particular focus on packed bed reactors. The chapter concludes with a brief discussion of immobilized enzyme reactor design and scale-up. 

Many factors affect optimum fluidized bed reactor performance, including hydrodynamics, heat and mass transfer of interparticles and intraparticles, and complexities of reaction kinetics. The design of fluidized bed reactor processes follows the general approach for multiphase reactor processes. Krishna (1994) and Jazayeri (1995) outlined the general procedure for this process development. The design of the processes can be described by considering various factors as illustrated in Fig. 3. 

As noted in Sec. 2, most applications of CFB technology involve chemical processes where the principal reactions require solid particles, either as catalyst or as reactant, and a gas. The major factors affecting the performance of CFB reactors are related to the findings covered above ... 

Although methods to analyse the instability mechanism have recently become available, small variations in the factors that cause power instabilities affect the predictable performance of any given reactor for a given flow. Those factors are primarily, void fraction, fuel time constant, power level, power shape, feedwater temperature and core flow. Additionally, the design of the fuel rod and bundle can affect the formation and propagation of the void density waves. These factors affect the power/flow region at which power oscillations are probable. 

The preferential CO oxidation in H2-rich streams is a reaction of great relevance due to its application in the purification of feeds for hydrogen fuel cells, and because of the scientific interest. This reaction is known to be very sensitive to catalytic surface structures and to the pretreatments. Au catalysts supported on metal oxides with high metal dispersions have been demonstrated as very effective in this PROX reaction [1]. However, the studies carried out over these systems have allowed us to conclude that several factors affect the performance of the catalyst, such as particle sizes, preparation method, supports, etc. Nevertheless, there is still some controversy with respect to the nature of the active site or about the mechanism of reactioa In the present conununication and with the aim to obtain further information to elucidate these unresolved points a study on the role of the support in the CO PROX mechanism, particularly emphasizing in the aspects related to the preparation of Au-supported catalysts is presented. The use of a TAP (Temporal Analysis of Products) reactor is also applied to reveal elementary processes that are taking place on the surface under reaction conditions, since this reactor system allows the detection of reactants and products with a submilhsecond time resolution [2]. 

Based on experimental results and a model describing the kinetics of the system, it has been found that the temperature has the strongest influence on the performance of the system as it affects both the kinetics of esterification and of pervaporation. The rate of reaction increases with temperature according to Arrhenius law, whereas an increased temperature accelerates the pervaporation process also. Consequently, the water content decreases much faster at a higher temperature. The second important parameter is the initial molar ratio of the reactants involved. It has to be noted, however, that a deviation in the initial molar ratio from the stoichiometric value requires a rather expensive separation step to recover the unreacted component afterwards. The third factor is the ratio of membrane area to reaction volume, at least in the case of a batch reactor. For continuous opera-... 

Effect of mixing factors for nonfirst-order reactions. Segregation plays no role in plug flow however, it increasingly affects the reactor performance as the RTD shifts from plug to mixed flow. 

Factors that are affected by the size of the bed, mainly those that are connected to the bubble behavior. The size of the bubbles has an impact on various physical properties such as the bed density, whereas it also influences the gas-solids contact and reactor performance (Matsen, 1996). For example, in small equipment, the bubbles may have dimensions approaching those of the bed, whereas it is not the case in large beds and a scale-up effect will surely exist. 

Based on these preliminary results, a small library of NCN-pincer nickel-containing metallodendrimers was prepared by Van Koten et al. in order to investigate the factors that can affect the catalyst performance and their applicability in nanofiltration membrane reactors [35,36]. The strategy in this... 

The above issues associated with prediction of trickle-bed reactor performance has motivated a number of researchers over the past two decades to perform laboratory-scale studies using a particular model-reaction system. These are listed in Table I. Although a more detailed summary is given elsewhere (29), a general conclusion seems to be that both incomplete catalyst wetting and mass transfer limitations are significant factors which affect trickle-bed reactor performance. 

When combining the separator and the reactor functions into one compact physical unit, factors related to catalysis need to be considered in addition to those related to selective separation discussed in previous chapters. The selection of catalyst material, dispersion and heat treatment and the strategic placement of catalyst in the membrane reactor all can have profound impacts on the reactor performance. The choice of membrane material and its microstructure may also affect the catalytic aspects of the membrane reactor. Furthermore, when imparting catalytic activity to inorganic membranes, it is important to understand any effects the underlying treatments may have on the permeability and permselectivity of the membranes. 

The objective of scaleup is to design industrial-sized reactors on the basis of experimental data obtained from lab-scale reactors. A rehable scaleup requires insight of the phenomena and mechanisms that affect the performance of the reactor operation. Once these factors are identified and quantified, the task is to establish similar conditions in the industrial-size reactor. The difficulty arises from the fact that not all the factors can be maintained similar simultaneously upon scaleup... 

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