Reactants particle size distributions

The counterparts of dissolving particles are the processes of precipitation and crystallization the description and simulation of which involve several additional aspects however. First of all, the interest in commercial operations often relates to the average particle size and the particle size distribution at the completion of the (batch) operation. In precipitation reactors, particle sizes strongly depend on the (variations in the) local concentrations of the reactants, this dependence being quite complicated because of the nonlinear interactions of fluctuations in velocities, reactant concentrations, and temperature. 

A schematic representation of this reactor model is shown in Figure 22.2. Particles of solid reactant B are in BMF, and fluid reactant A is uniform in composition, regardless of its flow pattern. The solid product, consisting of reacted and/or partially reacted particles of B, leaves in only one exit stream as indicated. That is, we assume that no solid particles leave in the exit fluid stream (no elutriation or entrainment of solid). This assumption, together with the assumption, as in the SCM, that particle size does not change with reaction, has an important implication for any particle-size distribution, represented by P(R). The implication is that P(R) must be the same for both the solid feed and the solid exit stream, since there is no accumulation in the vessel in continuous operation. Furthermore, in BMF, the exit-stream properties are the same as those in the vessel Thus, P(R) is the same throughout the system ... 

Particle size distributions of the reactant as well as of the product powders, were measured by a HORIBA (model CAPA-700) particle size analyzer. This instrument uses a non-contact method based on liquid-phase sedimentation, and has a measuring range between 0.01 and 300 pm. 

Emulsion polymerization reactors are made of stainless steel and are normally equipped with top-entry stirrers and ports for addition of reactants. Control of the reaction exotherm and particle size distribution of the polymer latex is achieved most readily by semibatch (also called semicontinuous) processes, in which some or all of the reactants are fed into the reactor during the course of the polymerization. Examples are given in Chapter 8. In vinyl acetate copolymerizations, a convenient monomer addition rate is such that keeps the vinyl acetate/water azeotrope retluxing. at about 70°C. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

There are certain essential differences between solid state reactions and reactions involving gaseous or liquid phases. In the latter case, the kinetic motion of the reactant molecules ensures that they are available to one another for reaction under conditions which can be defined by statistical laws. Solid state reactions occur between apparently regular crystal lattices, in which the kinetic motion is very restricted and depends on the presence of lattice defects. Interaction can only occur at points of contact between the reacting phases and is therefore dependent on particle size and particle size distribution. The factors which govern the rate of a solid state reaction are (/) the rate of the boundary phase processes which lead to the consumption of the original lattices, and (ii) the rate of particle transfer through the product layer. 

Attempts have been made to allow for the influence of particle-size distributions on kinetic behaviour [76-83], but most usually it is assumed (perhaps implicitly) that all reactant particles behave similarly. Allowances for size effects, which are difficult to quantify, are most often based on numerical integration across an assumed or measured distribution of particle dimensions [82,83]. 

Precipitation is carried out by a controlled mixing of the reactants in order to obtain a supersaturated solution from which nucleation takes place. Amorphous primary particles are formed that later crystallise into desired phases and in parallel agglomerate to larger secondary particles. Precipitation processes need in line pH meters and possibilities for automatic particle size distribution analysis coupled to the ageing vessel. 

Reactant particle size distributions Kinetic characteristics of some reactions of solids depend sensitively on reactant particle sizes (29). Ideally, reactants to be used in kinetic studies should be composed of crystallites of identical (known) sizes and shapes, to which the geometry of interface advance can be related quantitatively. This is not, however, always (or easily) achieved in experimental studies, and most powder samples contain particles of disparate sizes for example, sometimes crystals are mixed with fine powder. The kinetic model giving the best apparent fit to the data then may not accurately represent the reaction. Dependencies of rate on particle size are only rarely investigated. The state of subdivision of a solid reactant is most frequently described in literature reports only by qualitative terms, such as single crystals or crushed powder. 

We have seen how problems of particle size distribution of reactant and solid products can be employed in the design of fluid-bed reactors. The conversion obtained at the reactor exit depends on these distributions plus various other factors. The equations presented so far were based on continuous solids feed. In calculating the conversions, it is easier to divide the solid reactants into discrete ranges (each with an average size) and express the conversion as the sum from all the ranges. Furthermore, size distribution is usually determined by screen analysis, which gives discrete measurements. 

An important technique is that in which it is the precursor of the final colloidal particle that is reduced to a colloidal size. Thus a liquid reactant may be emulsified and then caused to react to form a colloidal dispersion of solid particles whose particle size distribution is related to that of the emulsion precursor. The commonest application of this method is in suspension polymerisation, in which an emulsion of monomer droplets, stabilised by a surfactant, is polymerised by adding an initiator which is soluble in the monomer. Polymerisation occurs within the monomer droplet, leading to the formation of a polymer latex. 

In an agitated reactor, the effect of mass transfer resistance can be reduced to a minimum by adjusting the stirring speed. The mass transfer coefficient is also a function of the size of suspended particles. From the point of view of reactor design, to maintain the uniformity of the desired product from batch to batch the particle size distribution of the solid reactant should be in a rather narrow range to render the mass transfer resistance unimportant. 

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