COS and CS2 Formation in the Claus Unit Acid Gas Burner

Lili Lyddon

March 1, 2007

In a Claus Sulfur Recovery Unit, COS and CS2 are formed in the acid gas burner and are partially destroyed in the first Claus bed (provided the appropriate catalyst is present and the temperature is sufficiently high). Equilibrium models do not adequately predict the formation of COS and/or CS2 in the burner as observed concentrations can be significantly higher than equilibrium predictions. A number of semi-empirical correlations have been developed to more accurately predict COS and CS2 formation in the burner:

  • Fischer 1974 - Fischer, H., "Burner/Fire Box Design Improves Sulfur Recovery," Hydrocarbon Processing, October, 1974, pp. 125-129.
  • Luinstra d'Haene 1989 - This correlation predicts the concentration of CS2 out of the burner and requires a residence time specification. If no data is available, use a residence time of 2 seconds. Reference: Luinstra, E. A., and P. E. d'Haene, "Catalyst Added to Claus Furnace Reduces Sulfur Losses," Hydrocarbon Processing, July, 1989, pp. 53-57.
  • NSERC 1993 - Monnery, W. D., W. Y. Svrcek, and L. A. Behie, “Modelling the Modified Claus Process Reaction Furnace and the Implications on Plant Design and Recovery,” Proceedings of the 43rd Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, 1993, pp. 261-309.
  • NSERC 2002 - Pollock, A. E., P. D. Clark, N. I . Dowling, M. Huang, W. D. Monnery, and W. Y. Svrcek, “Finally – A Kinetic Model of the Modified Claus Process Reaction Furnace,” Proceedings of the 52nd Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, 2002, pp. 43-62. Pollock, A. E., "Kinetic Modelling of the Modified Claus Reaction Furnace," Ph.D. Dissertation, The University of Calgary, Calgary, Alberta, Canada, 2001.

ProMax offers all of these correlations to predict COS and CS2 formation in the Acid Gas Burner. Correlations are selected in the Constraints Grouping of the Reactor Process Data Tab. NSERC 1993 is the default correlation for both COS and CS2 formation in the Acid Gas Burner. Depending on the conditions (e.g. sour water stripper gas present, oxygen enhanced air feed, fuel gas added, etc.), one correlation may be recommended over the others or over equilibrium calculations. The table below presents recommended correlations for various acid gas burner conditions.

Component With No Sour Gas (No NH3) With Sour Gas (NH3 Present)
Air Only Fuel Gas Added Oxygen Enhanced Air Only Fuel Gas Added Oxygen Enhanced
COS NSERC 1993 NSERC 1993 NSERC 1993 Fischer 1974 Fischer 1974 NSERC 1993
CS2 NSERC 1993 NSERC 1993 Equilibrium Fischer 1974 Fischer 1974 Equilibrium

To calculate compositions using Equilibrium, be sure no constraints for CS2 formation are set in the burner. You can delete the Constraint or clear the Active checkbox for the Constraint. If the acid gas feed stream has greater than 2 mole % hydrocarbons or high concentrations of heavy hydrocarbons with carbon number greater than 3, it is recommended to use the Fischer 1974 correlation for CS2 since the NSERC 1993 correlation appears to predict excessively high amounts of CS2 in the burner effluent. Although not listed in the table, the Luinstra d'Haene 1989 correlation is also available for CS2 formation prediction in the acid gas burner. If no data is available for the required residence time parameter, a value of 2 seconds is recommended. If no constraints are set, or if the checkboxes associated with the constraints are not selected, the COS and CS2 formation in the burner will be based on equilibrium calculations.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)