Reactors for Reactive Solids

Another type of stability problem arises in reactors containing reactive solid or catalyst particles. During chemical reaction the particles themselves pass through various states of thermal equilibrium, and regions of instability will exist along the reactor bed. Consider, for example, a first-order catalytic reaction in an adiabatic tubular reactor and further suppose that the reactor operates in a region where there is no diffusion limitation within the particles. The steady state condition for reaction in the particle may then be expressed by equating the rate of chemical reaction to the rate of mass transfer. The rate of chemical reaction per unit reactor volume will be (1 - e)kCAi since the effectiveness factor rj is considered to be unity. From equation 3.66 the rate of mass transfer per unit volume is (1 - e) (Sx/Vp)hD(CAG CAl) so the steady state condition is ... 

In an ideal fixed-bed reactor, plug flow of gas is assumed. This is, however, not a good assumption for reactive solids, because the bed properties vary with position, mainly due to changing pellet properties (and dimensions in most cases), and hence the use of nonideal models is often necessary. The dispersion model, with all its limitations, is still the most practical one. The equations involved are cumbersome, but their asymptotic solutions are simple, particularly for systems... 

Slurry reactors are popular in industry where the solids either take part in the reaction or act as catalyst.Many aspects of these reactors,particularly for catalytic systems,have been discussed at length in 1iterature(1,2).Catalytic slurry reactors are also reviewed in this proceedings by Hofmann(3).However,there are still aspects which have not been treated in the literature in sufficient detail.Firstly,until recently little attention has been paid to slurry reactors involving reactive solids.Secondly, it is often assumed that steps of diffusion of the dissolved gas from the gas-liquid interface to the bulk liquid phase bulk liquid phase to the solid catalyst surface and surface reaction are steps in series.This leads of course to a specific gas absorption rate which is always smaller than k.A. While this is a representative picture in a majority of cases of industrial relevance,we can conceive situations,where the catalyst particle size may be smaller than the diffusion film(liquid film next to gas-liquid interface) thickness.We may then have steps of the transport of the dissolved gas from the gas-liquid interface and reaction on the catalyst particle in parallel,that is,while the dissolved gas diffuses it reacts on the catalyst surface.This is then in many ways analogous to normal gas-liquid reactions and may lead to the enhancement of specific gas absorption rate so that it exceeds k.A. This point is also relevant to reactive solid systems indeed in an earlier paper,Ramachandran and Sharma(4) had shown that the specific rate of absorption accompanied by an instantaneous reaction in a slurry containing sparingly soluble fine particle size was considerably smaller than the film thickness. Finally,there is substantial information in the literature on the combined effect of solid particles on k a.However,the information... 

An examination of gas absorption mechanisms of three phase slurry reactors indicates that three major points,which are not normally considered in reviews concerning this type reactors(see,for in-stance(l) - (3))deserve further attention.These are the slurry reactors involving reactive solids,the possible gas absorption... 

Roumanie et al. [14] developed a silicon microstructured reactor for gas-solid heterogeneous catalysis, in particular for the dehydrogenation of methylcyclo-hexane. A cathodic Pt film sputtered on silicon pillars of 5 pm x 100 pm was applied. The activity of the sputtered Pt catalyst was an order of magnitude lower as compared to that of a similar catalyst prepared by impregnation of a Pt precursor onto an Y-alumina layer. The sputtering method is thus not advisable for the preparation of supported catalysts in microstructured reactors as lowered reactivity and leaching of the metals during reactions remain serious problems. 

Clays are a family of crystalline aluminosilicate solids that interact with a variety of organic compounds (Theng, 1974). Acid treatment develops acidic sites by removing aluminum from the structure and often enhances the reactivity of the clay with specific families of organic compounds. The acid sites also catalyze the formation of coke, and Houdry developed a moving bed process that continuously removed the coked beads from the reactor for regeneration by oxidation with air (McEvoy, 1996). 

The solid product of hydrolysis is loaded out of the nutsch filter, weighed and sent into reactor 14 through a hatch the reactor also receives solid sodium hydroxide. After that, backflow condenser 13 is filled with water, the agitator is switched on and the reactive mixture is gradually filled with ethyl alcohol (or hydrosite) from batch box 72 at such speed that the temperature in the reactor does not exceed 80 °C. After the whole mixture has been introduced, the jacket is filled with vapour and agitated for 1-2 hours at 75-80 °C. The mixture is cooled to 20 °C and poured into... 

In the middle of the reactor is the fluidized sand bed. The sand serves two major purposes in the experiment. The violent agitation of the sand provides extremely rapid heat transfer to the carbonaceous fuel particles which undergo abrasion and grinding that constantly exposes fresh reactive solid surfaces as well as reduces the tendency for the reacting fuel particles to agglomerate. Also, the sand bed acts as a... 

MASS BALANCES FOR REACTORS CONTAINING A SOLID REACTIVE PHASE... 

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