Runaway limits for adiabatic packed-bed catalytic reactors

Most existing criteria for predicting the critical residence time (or flow rate) at which runaway occurs in an adiabatic catalytic packed-bed reactor do not account for interphase heat-transfer resistance and intraparticle diffusion. When these transport limitations exist, the critical residence time at which runaway occurs is given by tau=RTo/E T-o/Delta T-ad f(phi(o))/k(T-o) -2.72(rho fC(pf)/ha(v)). This simplified equation give the runaway locus as a function of the feed temperature (T-o), adiabatic temperature rise (Delta T-ad), activation energy (E), reaction rate constant at feed temperature [k(T-o)], fluid-solid heat-transfer coefficient (h), the interfacial area per unit bed volume (a(u)), and the normalized thiele modulus (phi(o)). The function of f(phi(o)) accounts for the intraparticle diffusional effects and may be approximated by the two asymptotes, f(phi(o)) = 1 when phi(o) < 1/2 and f(phi(o)) = 2 phi(o) when phi(o) > 1/2. Similar analytical results are presented for other reactor models and are for more complicated reaction networks. A practical example shows the calculation of the runaway boundary for industrial hydrogenation reactions obeying Langmuir-Hinshelwood kinetics.