Langmuir-Hinshelwood –Hougen-Watson(LHHW) model

The rate equation derived from mechanistic model that simulates the actual surface phenomenon during the process is preferred for reactions involving solid catalysts. The Langmuir-Hinshelwood–Hougen-Watson(LHHW) approach is one of the most commonly used way of deriving rate expressions for fluid solid catalytic reactions. The advantages of this method are that:

(1)  Rate derived by this method takes into account the adsorption/desorption process occurring over the surface along with the surface reaction.

(2)  Rate equation derived can be extrapolated more accurately to concentrations lying beyond the experimentally measured values.

During this method of derivation of rate expression, all the physical transport steps like mass transfer from bulk phase to catalyst surface or diffusion of reactants from pore mouth to interior pore (intraparticle diffusion) are excluded. Thus, it is assumed that the external and internal mass transport processes are very rapid relative to the chemical rate process occurring on or within the catalyst particle. The chemical rate depends on:

(1)  chemisorption steps

(2)  surface reaction steps

(3)  desorption steps

This simple kinetic model assumes isothermal condition about and within catalyst that is temperature gradient is zero.

In LHHW model development, the rate equation is first derived in terms of surface concentration of adsorbed species and vacant sites. Then, these surface concentrations are related to the fluid or bulk concentration that is directly measurable.