In the fluid-solid particles reaction system

the rate of reaction in general is influenced by the solid reactant concentration left in the solid particles as well as by the operating conditions such as temperature and pressure. Under these conditions

calculation of extent of reaction of solid particles in a multistage reactor must be based on the residence time distribution (RTD) of solid particles in each stage of the reactor system. By using either Dirac function d(o) or the probability density function for sums of independent random variables

the RTD of the particles in each stage of continuously stirred tank reactor(CSTR)was derived in this paper. The probability density function was also used to derive the general formula for calculation of extent of reaction of solid particles in CSTR. This equation could be as well obtained from the probability density function of the solid reactant concentration distribution. For several special reaction rate equations

general formulas for calculation of the extent of reaction in any number of stages of CSTR were given

when analytical solution is rather difficult to find with complex reaction rate equations

numerical methods for calculation of the probability density function stage by stage were investigated with either the reaction time or the fractional degree of reaction of solid particles used as calculation parameter.