Reaction solid product layer

It should be noted that in the case of the reaction of a fluid with a nonporous solid, the chemical reaction step and the mass transport step are connected in series. This makes the analysis much simpler as compared to the case of a porous solid. In reactions of nonporous particles there can essentially be two cases one which shows absence of a solid product layer, and the other which shows its presence. 

A reaction in which a solid product layer is formed can be represented as... 

If the product layer is nearly free of pores, then the anodic dissolution of metal will practically cease. The metal is then said to be passivated . The thickness of the compact product layer will reach a stationary value. For oxide products which are essentially electronic conductors, this stationary thickness will be determined by the very low ionic conductivity in the oxide on the one hand, and by the rate of dissolution of the oxide in the electrolyte on the other. However, in many cases the oxide layers are porous, so that the electrolyte can continue to attack the metal, independently of the transport of ions and electrons in the oxide. From the above discussion it can be seen that corrosion reactions in aqueous ionic solutions in which a solid product layer is formed on a metal are among the most complicated of all heterogeneous solid state reactions. The reasons for this are the electrochemical nature of these reactions, the great number of possible elementary steps which can occur at the various phase boundaries, and electrical space charge phenomena which occur in the reaction product. 

Advanced learners may also study the equations needed to describe quantitatively the progress of a gas-sohd reaction in many different cases, for example, for a shrinking non-porous umeacted core with a solid product layer or gaseous products (Sections 4.6.2.2 and 4.6.2.3), or models to evaluate the reaction of a gas with a porous solid for a different strength of the mass transfer resistance (Sections 4.6.3.2-4.6.3.5). 

A solid product or an inert residue is formed and the reaction rate is slow (no influence of diffusion through boundary gas and solid product layer) (model 1 in Figure 4.6.1). 

Figure 4.6.3 clearly shows that by measurement of the conversion of the solid with time (dimensionless time we can hardly decide whether the rate is controlled by the chemical reaction or by diffusion through the solid product layer/film diffusion. Therefore, we need more experimental data and further calculations. For example, the variation of the particle size is helpful, as the final time for conversion is proportional to the initial particle diameter for control by the chemical reaction, Eq. (4.6.18), whereas tfin r if the rate is controlled by diffusion through the solid product layer, Eq. (4.6.22). 

In CaO-SOj reaction, diffusion resistance through the solid product layer is a major rate limiting step. Limitations of chemical reaction rate is important only at the initial stages of the reaction. It is reported that after the formation of about 5 A CaS04 layer, reaction is controlled by the diffusion of SO2 through the product layer.The values of diffosion coefficients reported in the literature are summarized in Section 2.3. 

Under conditions where Vooxide surface will become covered with a metal layer. Importantly the formation of a solid product layer on the oxide surface has the irrrmediate effect of cutting off direct contact between the reactive gas and the underlying oxide phase, resulting in a step change in rate limiting reaction mechanism. 

Diffusion through a porous solid matrix or simply pore diffusion may play an important role in gas-solid reactions. When the reactant solid is porous, diffusion through the pore space is necessary for the reactant gas to gain access to the solid surface in a similar manner, the removal of the gaseous products will also involve this process. However, pore diffusion may also be an important component in the reaction of nonporous solids, when the solid product layer formed is itself porous, because then the supply of gaseous reactant and the removal of gaseous products have to be accomplished by diffusion through this porous product layer. 

The simplest system in fluid-solid reactions is that of a shrinking nonporous particle forming no solid product layer ... 

As we can see in Fig. 3.2, the overall process consists of chemical reaction at the interface, and the diffusion of gaseous reactants and products through the solid-product layer and through the boundary layer at the external surface of the solid. The overall rate may be controlled by the rate of chemical reaction or by the rate of diffusion. In other cases these two steps may present comparable resistances to the progress of reaction and may both influence the overall process. Most models proposed previously assume either of the first two extremes. This is not justified for many systems, nor is it necessary, because they are merely two asymptotes of the general case of both chemical reaction and diffusion determining the overall rate. The solution for the general case encompasses the extreme cases, as we shall see later. 

The main characteristic of attack by halogens at elevated temperatures is that most reaction products are volatile compared with the solid products that form in all cases considered hitherto in this chapter. Thus, in cases where metals are exposed to pure halogen gases large mass losses are usually reported with very little external scale formation. Li and Rapp " showed that internal chloridation occurred when nickel-chromium alloys were exposed to Ni + NiClj powders at 700-900°C. However, where oxide scales can also form, as in combustion gases, the oxide layer was usually highly... 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

Measurements of electrical conductivity permit the identification of the charge-carrying species in the solid phase and also the detection of ionic melts [111,417]. Bradley and Greene [418], for example, could determine the kinetics of reactions between Agl, KI and Rbl because the product (K, Rb)Ag4Is had a considerably higher conductivity than the reactants. The conductivity of the reactant mixture was proportional to the thickness of the product layer. 

Product layer diffusion Many fluid-solid reactions generate ash or oxide layers that impede further reaction. 

Reactions in which a gas or liquid reacts with the surface of a solid are rather common processes in inorganic chemistry. The product that forms as a layer on the surface of the solid may impede the other reactant from contacting the solid. There are several types of behavior that depend on how the product layer affects the mobility of reactants, but in this instance, we will assume that the rate is inversely proportional to the thickness of the product layer. When the rate law is written in terms of the thickness of the product layer, x, the result is... 

Figure 9.2(a) or (b) shows the essence of the SCM, as discussed in outline in Section 9.1.2.1, for a partially reacted particle. There is a sharp boundary (the reaction surface) between the nonporous unreacted core of solid B and the porous outer shell of solid product (sometimes referred to as the ash layer, even though the ash is desired product). Outside the particle, there is a gas film reflecting the resistance to mass transfer of A from the bulk gas to the exterior surface of the particle. As time increases, the reaction surface moves progressively toward the center of the particle that is, the unreacted core of B shrinks (hence the name). The SCM is an idealized model, since the boundary between reacted and unreacted zones would tend to be blurred, which could be revealed by slicing the particle and examining the cross-section. If this... 

When the molar volume of product, i.e. solid (AB) is less than that of solid (A), the product layer will be porous and the rate determining step is the chemical process occurring at the interface of solid (A). Such reactions are also known as topochemical reactions. The rate of reaction may be determined by the available surface area of A. For example, if reaction involves spherical particle, the rate of reaction (i.e. - dV/dt, where V is the volume of particle at time t) is given as... 

When a solid-solid reaction starts, a layer of product is formed between the two reactant phases and a single phase boundary converts into two different phase boundaries as... 

The organic layer is separated and washed twice with a total of 200 mL of water. The aqueous reaction layer is extracted five times with a total of 600 mL of chloroform. The organic extracts are washed twice with a total amount of 250 mL of water. The combined organic layers are dried with magnesium sulfate and evaporated to dryness. The residual solid product is carefully dried under reduced pressure to yield 48-50 g (2 100%) (Note 8). 

If the transport process is rate-determining, the rate is controlled by the diffusion coefficient of the migrating species. There are several models that describe diffusion-controlled processes. A useful model has been proposed for a reaction occurring at the interface between two solid phases A and B [290]. This model can work for both solids and compressed liquids because it doesn t take into account the crystalline environment but only the diffusion coefficient. This model was initially developed for planar interface reactions, and then it was applied by lander [291] to powdered compacts. The starting point is the so-called parabolic law, describing the bulk-diffusion-controlled growth of a product layer in a unidirectional process, occurring on a planar interface where the reaction surface remains constant ... 

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