The University of Texas at Austin
College of Engineering

Langmuir-Hinshelwood Kinetics

THE PROBLEM STATEMENT

Consider the following heterogeneous reaction in which A and B associatively adsorb on a catalyst surface and undergo reaction to Product P1 that subsequently desorbs.

At 373 K the equilibrium constants for adsorption are

K1 = 190,000 K2 = 580,000 K3 = 75,000 cm3/gmole

The heats of adsorption are

The rate constant for r3 is

where T is in Kelvin

THE QUESTIONS

  1. Develop a Langmuir-Hinshelwood rate expression (i.e., the rate is expressed in terms of gas phase components) for the surface reaction when you assume the surface reaction is the slow step and the adsorption and desorption processes are at equilibrium.

  2. Determine the magnitude of rate of the surface reaction (r3) at 373 K if the gas contains a 50:50 mixture of A and B at a pressure of 1 atm. What is the value of the rate if a 50:10:40 mixture of A:B:P is present instead? = 0.0008 gmoles/gcat. Justify the different values.

  3. Run the simulation and using the reference case examine the effect of changing the composition of the gas in contact with the catalyst on the rate. This program plots r3 for the case where the temperature is 400 K, the total pressure is varied between 1 and 10 atm, the mole fraction of P is zero, the mole fraction of A is 0.1 and the mole fraction of B is varied between 0 and 0.9. (An inert component that does not adsorb makes up the balance of the gas phase.) Discuss the shapes of the curves. Why is there a maximum in the rate with mole fraction of B? Why does the maximum rate occur at different mole fractions for different total pressures? Why does the maximum value change with total pressure?

  4. Continue running the program to explore the effect of changing the temperature on the rate curves as a function of the mole fraction of B at 5 and 10 atm of total pressure. Vary the temperature between 350 and 450 K. Note that if the temperature is too low the rates are very small and when the temperature is at the upper limit the maxima are gone. Explain/justify the effect temperature has on the rate curves.