Reaction heat transport

Figure 4 Various measures for the harmonization of reaction, heat transport, and pressure drop in chemical reactors benchmarked against a multitubular reactor.

When heat effects are important, one has to account simultaneously for heat transport, mass transport, and reaction. Heat transport through a catalyst pellet can be described by the Fourier law ... 

To proceed with the topic of this section. Refs. 250 and 251 provide oversights of the application of contemporary surface science and bonding theory to catalytic situations. The development of bimetallic catalysts is discussed in Ref. 252. Finally, Weisz [253] discusses windows on reality the acceptable range of rates for a given type of catalyzed reaction is relatively narrow. The reaction becomes impractical if it is too slow, and if it is too fast, mass and heat transport problems become limiting. 

The productivity of DR processes depeads oa chemical kinetics, as weU as mass and heat transport factors that combine to estabhsh the overall rate and extent of reduction of the charged ore. The rates of the reduction reactions are a function of the temperature and pressure ia the reductioa beds, the porosity and size distribution of the ore, the composition of the reduciag gases, and the effectiveness of gas—sohd contact ia the reductioa beds. The reductioa rate geaerahy iacreases with increasing temperature and pressure up to about 507 kPa (5 atm). 

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

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... 

In the sulfamic acid process, electrical energy is needed for removal of reaction heat, filtration, fluid transportation, etc. Consumption is about 300 kWh/1 of sulfamic acid. Consumption of steam, used for the heat exchanger, crystallizer, and drier, is from 1000 to 1500 kg/1 of sulfamic acid. 

Catalysts are generally developed for a particular process, i.e. for a certain reaction in a certain reactor under certain conditions. Mass and heat transport phenomena put their... 

Controlled elimination of mass and heat transport resistances is an important prerequisite for obtaining intrinsic kinetic parameters of the fast exothermic reaction of partial oxidation of methane to synthesis gas. It has been demonstrated that under conditions of strong transport limitations erroneous conclusions concerning the reaction scheme can be derived [7-9]. It was determined in this laboratory that transport limitations are practically absent over a wide range of operating conditions if one portion of the catalyst (< 40 pm) is diluted with -5 portions of an... 

The reaction was of a scouting nature, actually one of the most common investigations done concerning gas-phase reactions in micro reactors. Hence it addresses in a general way the investigation of general micro reactor properties such as mass and heat transport and residence time. 

If the reactant solid is porous, the reactant fluid would diffuse into it while reacting with it on its path diffusion and chemical reaction would occur in parallel over a diffuse zone. The analysis of such a reaction system is normally more complex as compared to reaction systems involving nonporous solids. Here also it is important to assess the relative importance of chemical reaction kinetics and of mass and heat transport. 

An incompletely reacted and sectioned sample of limestone would appear as shown in Figure 3.27. For the reaction to sustain it is highly required that flow of heat occurs from outside the limestone sample particle to the reaction interface, which represents the shrinking core of limestone. This heat transport by conduction in the pores and solid body of the lime can affect the rate of reaction. 

There may be radial temperature gradients in the reactor that arise from the interaction between the energy released by reaction, heat transfer through the walls of the tube, and convective transport of energy. This factor is the greatest potential source of disparities between the predictions of the model and what is observed for real systems. The deviations are most significant in nonisothermal packed bed reactors. 

Figure 3. Temporal and Spatial Evolution of Reaction Rates in the Liquid Phase Reaction Zone. Rates were calculated as a function of time and distance from the bubble surface assuming only conductive heat transport from a sphere with radius 150ym at 5200K, embedded in an infinite matrix at 300K.

Since publication of the first edition, the held of reaction modeling has continued to grow and hnd increasingly broad application. In particular, the description of microbial activity, surface chemistry, and redox chemistry within reaction models has become broader and more rigorous. Reaction models are commonly coupled to numerical models of mass and heat transport, producing a classification now known as reactive transport modeling. These areas are covered in detail in this new edihon. 

For a more detailed analysis of measured transport restrictions and reaction kinetics, a more complex reactor simulation tool developed at Haldor Topsoe was used. The model used for sulphuric acid catalyst assumes plug flow and integrates differential mass and heat balances through the reactor length [16], The bulk effectiveness factor for the catalyst pellets is determined by solution of differential equations for catalytic reaction coupled with mass and heat transport through the porous catalyst pellet and with a film model for external transport restrictions. The model was used both for optimization of particle size and development of intrinsic rate expressions. Even more complex models including radial profiles or dynamic terms may also be used when appropriate. 

Solid-solid (S-S) systems are now being investigated in view of eliminating solvents from chemical reactions. Here the paradigm is the best solvent is no solvent. Just mixing two solids can often lead efficiently and cleanly to a product however, there are limitations that are mainly due to the choice of reagents and to mass and heat transport. ... 

The mass and heat transport model should be able to predict mass and energy fluxes through a gas/vapour-liquid interface in case a chemical reaction occurs in the liquid phase. In this study the film model will be adopted which postulates the existence of a well-mixed bulk and a stagnant transfer zone near the interface (see Fig. 1). The equations describing the mass and heat fluxes play an important role in our model and will be presented subsequently. 

Endothermic ammonia cracking is regarded as the reverse of the synthesis reaction, and since it is limited by heat transport, its efficiency can potentially be improved using microreactors. In industry, ammonia synthesis occurs at approximately 500 °C and 250 atm, and it is often represented by the following reaction ... 

The char combustion is sustained by its own heat release. The heat release and heat transport is thereby coupled with the oxygen transport, which is usually the controlling factor. The heat evolved from reaction is transported by heat conduction and convection out of the particle. [73]... 

The heat and mass transport phenomena of the char gasification is not described in the literature as much as for the char combustion [11,28,78]. There are good reasons to believe that it is quite analogous to the char combustion phenomenology [79]. However, the heterogeneous gasification reactions are overall endothermic which results in some differences with respect to the intraparticle heat transport [79]. 

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