where Ci is the concentration of Ri and kf and kb are the rate constants for the forward and backward reaction processes, respectively. This reaction is regarded as irreversible in that it occurs toward equilibrium. In Figure , chemical substances R1 to R6 and R13 flow into an ORN as source substances, whereas all or some of the reaction products are discharged from the ORN. The rate for the inflow or outflow of Ri can be approximately expressed, on the basis of Fick's law of diffusion, as