Reactions of Nonporous Particles

In a situation where the solid participating in the reaction is nonporous to start with, the reaction occurs at a sharp interphase, either fluid-solid or solid-solid, depending as whether a solid product forms or not and if it does then whether it is porous or nonporous. 

In some fluid-solid reactions, nucleation of the product constitutes an important step. With increasing size of the solid or with increasing reaction temperature, the time within which nucleation is important becomes small compared to the total reaction time, thereby making nucleation relatively less important. 

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. 

---

The changing particle size, however, will alter the relative importance of the rates of chemical reaction and the transport of heat and mass. It is conceivable then that there may occur a switch from transport-controlled regime to kinetic-controlled regime during the reaction of nonporous particles even when conditions external to the particle remain constant throughout the reaction period. 

---