Water gas shift reaction

Water gas shift reactions are carried out at conditions of high and low temperature. For the water gas shift reaction, the equilibrium favors the reaction at lower temperature. However, copper based catalyst, active at low temperature, is highly sensitive to residual sulfur or chloride compounds which have passed through the pre-purification steps. Hence, initially the reaction is carried out over catalyst composed of Fe₂O₃ and Cr₂O₃ which is relatively inexpensive, resistant to sintering and tolerant to sulfur and chloride poisoning. However, at low temperature Fe₃O₄ is not sufficiently active hence, the reaction has to be carried out at higher temperature. The high temperature catalyst adsorbs the residual sulfur or chloride compounds thereby, protecting the low temperature copper based shift catalyst from exposure to these compounds. The high temperature process is followed by shift reaction at thermodynamically favorable lower temperature of 200°C over copper based catalyst.

High temperature water gas shift reaction

Gas from the secondary reformer contain 10-13 % CO and is further processed to increase the H₂ concentration. The high temperature water gas shift reaction lowers the CO concentration to about 2-3 % in a fixed bed reactor.

Process is operated at 350-500⁰C and 20 -30 atm. Since equilibrium favors the WGS at lower temperature, the feed gas is cooled to 350-400⁰C before entering the reactor.

Catalysts:

A typical catalyst is composed of 90 % Fe₃O₄ and 10 % Fe₂O₃. Fe₃O₄ acts as the active metal while Fe₂O₃ acts as a stabilizer to minimize sintering of the active iron oxides. The catalysts are prepared by precipitation.

Kinetics

The rate expression from Langmuir Hinshelwood model proposed is given as [4] :

k is the rate constant and K_i's the respective adsorption constants. The ‘i' represents the chemical species.

Activation energy for the water gas shift reaction on iron oxide/chromia is in the range of 122 kJ/mol. Reaction over small pellets (5 x 4 mm) operates with an effectiveness factor of unity below 370⁰C and 31 atm.

Deactivation

The catalyst is deactivated by adsorption of residual S or Cl containing compounds that escape the initial purification steps.

Low temperature water gas shift

The CO concentration of the existing gas from high temperature WGS is 2-3 % and further reduced to below 0.2 % by low temperature water gas shift reaction. The WGS reaction as favored at low temperature, operating temperature is maintained at 200⁰C, the minimum temperature at which steam is not condensed and operating pressure at 10-30 atm.

Catalysts

A typical catalyst is composed of CuO/ ZnO/Fe₂O₃ and prepared by co-precipitation method. The catalyst is highly selective for shift reaction with low activity for methanation reaction. CuO is the catalytically active metal and ZnO minimize sintering of Cu. Cu is highly sensitive to poisoning by S or Cl compounds even at very low level of 1 ppm. ZnO & Fe₂O₃ also scavenge the S or Cl compounds protecting Cu which is highly sensitive to poisoning by these compounds. Catalyst is reduced very carefully by H₂ and the bed temperature is never allowed to rise above 230⁰C to avoid sintering of Cu. The high temperature shift catalyst also protects the low temperature catalyst from these poisons.

Kinetics

Kinetic m odel is based on surface redox mechanism involving dissociation of water to OH radicals which further dissociate to atomic oxygen. The rate expression as proposed by Ovensen et al.[2] is

A= pre-exponential factor; E_a= activation energies; m, n, x, y = respective reaction orders; β defined as approach to equilibrium and K_WGS is equilibrium constant for WGS reaction.

Final CO/CO₂ removal by methanation

After low temperature WGS, the CO content in hydrogen stream is 0.2-0.5 % and must be reduced to 5 ppm level for using the H₂ for ammonia synthesis. The CO is further reacted with hydrogen and converted to methane. The catalyst is typically Ni/Al₂O₃. MgO is sometimes used as a promoter to minimize sintering of Ni. The process is carried out at 300 -350⁰C and at 30 atm pressure. Care must be taken to avoid formation of poisonous Ni(CO)₆ which occurs below 200⁰C at partial pressure of CO greater than ~ 0.2 atm.

 

Book reference:

•  C. H. Bartholomew and R. J. Farrauto, Fundamentals of Industrial catalytic Processes, Wiley, VCH, 2006

 

Publication reference:

1. 1. M.H Halabi M.H.J.M de Croon, J. van der Schaaf, P.D. Cobden, J.C. Schoulen, Applied catalysis A Gen. 389 (2010) 80-91

2. J. Agrell, H. Birgersson, M. Boutonnet, Journal of Power Sources 106 (2002) 249–257

3. M. Zeppieri, P.L. Villa, N. Verdone, M. Scarsella, P. De Filippis, Applied Catalysis A: General 387 (2010) 147–154

4. J.R . Rostrup –Nielsen, Catalysis Science and technology eds. J.R. Anderson and M. Boudart, Springer –Verlag, Newyork,1984

5. T. Numaguchi and K.Kikuchi, Chemical Engineering Science 43(1988) 2295-2301