Ammonium Bisulfide (ABS) Corrosion

Introduction

Ammonium bisulfide corrosion of reactor effluent air coolers (REAC) and stripper overhead is a reliability issue for the refining industry. This type of corrosion has already resulted in major equipment failures, fires and explosions.

Organic sulfur and nitrogen compounds are converted to hydrogen sulfide and ammonia in hydrotreater reactors. When the effluent is cooled below 250 F, the gases combine to form ammonium bisulfide (ABS) salt. To prevent accumulation of the salt, water is injected before the reactor effluent cools to the ammonium bisulfide deposition temperature. Downstream from the point of injection, the equipment is then subjected to a sour alkaline solution with high concentration of hydrogen sulfide, ammonium bisulfide, and contaminants such as chloride, cyanide, and dissolved oxygen. Change in feedstock, upset conditions and/or wash water injection problems result in plugging of some exchanger tubes causing under-deposit corrosion. The non-plugged tubes are then subjected to high effluent flow and experience severe erosion-corrosion. ABS corrosion has already resulted in major equipment failures, fires and explosions.

Characteristics of ABS Corrosion

ABS corrosion is characterized by severe corrosion of carbon steel at high velocity and point of turbulence such as inlet end of tubes and the U-bends. Heavy deposits of soft iron sulfide were found in the tubes, but the corroded areas were free of corrosion products. Kansite, a non-protective type of iron sulfide, was believed to form at the effluent pH. To prevent erosion-corrosion carbon steel tubes were replaced by 403SS. However, this alloy was found to pit heavily under ABS deposits and was replaced by Alloy 800.

Control of ABS Corrosion

The critical factor in controlling this type of corrosion is the design of a balanced (symmetrical) distribution piping of the air coolers and of an appropriate wash water injection. The other solution is to control the velocity and ammonium bisulfide content of the effluent. Unfortunately, the design rules currently used by the refining industry were derived from empirical fit of plant data obtained since the 1960’s. These rules do not cover the complete range of the environmental conditions currently found in REAC. Thus, operational limits of common refinery materials of construction subject to ammonium bisulfide corrosion are still very insufficient.

References and Further Reading

Piehl, R.L., “Corrosion by Sulfide-Containing Condensate in Hydrocracker Effluent Coolers”, Proceeding, API Division of Refining, May 1968.
Alvarez, A.M and C.A. Robertson, “Materials and Design Considerations in High Pressure HDS Effluent Coolers”, Materials Performance, May 1973.
Sardisco, J.B. and R.E. Pitts, “Corrosion of Iron in an H2S-CO2- H2O System: Composition and Protectiveness of the Sulfide Film as a Function of pH”, Corrosion, November 1965.
Ehmke, E.F., “Corrosion Correlations with Ammonia and Hydrogen Sulfide in Air Coolers”, Materials Performance, July 1975.
Piehl, R.L., “Survey of Corrosion in Hydrocracker Effluent Air Coolers, Materials Performance, January 1976.
“Survey of Construction Materials and Corrosion in Sour Water Strippers-1978”, API Publication 950, April 1983.
Wilson, G.M. et al, “Sour Water Equilibria: Ammonium Volatility Down to ppm Levels, Effect of Electrolytes on Ammonia Volatility, pH vs. Composition”, Research Report RR-34, Gas Process Association, November 1978.
Damin D.G. and McCoy J.D, “Prevention of Corrosion in Hydrodesulfurizer Air Coolers and Condensers, Materials Performance, December, 1978.
Sherrer C. and Durrieu M., ” Distillate and Resid Hydroprocessing: Coping with Corrosion with High Concentrations of Ammonium Bisulfide in the Process Water”, Materials Performance, November 1980.
Newman, S.A, “Sour Water Design by Charts: Part 1”, Hydrocarbon Processing, September 1991.
Newman, S.A, “Sour Water Design by Charts: Part 2”, Hydrocarbon Processing, October 1991.
Newman, S.A, “Sour Water Design by Charts: Part 3”, Hydrocarbon Processing, November 1991.
Fouroulis, Z.A., “Mechanism of Refinery Corrosion by Aqueous Sour Water Condensates”, Middle East Corrosion Conference, Kuwait, 1996.
Shargay, C.A., A.J. Bagdasarian, J.W. Coombs and W.K. Jenkins, “Corrosion in Hydroprocessing Units”, in Corrosion in the Oil Refining Industry, Ed. L. Kaley, J.E. Feather, N. Coble, R. Strong, NACE, Houston, Texas, 1996.
Singh, A., Harvey C. and Piehl R.L., “Corrosion of Reactor Effluent Air Coolers”, CORROSION/97 conference, Paper # 490, NACE.
Turner, J., “Control Corrosion in Washwater Systems”, Hydrocarbon Processing, June 1997.
Turner, J., ” Design of Hydroprocessing Effluent Water Wash Systems”, CORROSION/98 conference, Paper # 593, NACE, 1998.
Shargay, C.A., G.E. Jacobs and M.D. Price, “Ammonium Salt Corrosion in Hydrotreating Unit Stripper Column Overhead Systems”, CORROSION/99, Paper # 392, NACE, 1999.

Capabilities

SET Laboratories, Inc. has capabilities to simulate pitting resulting from under-deposit corrosion and erosion-corrosion resulting from the high velocity effluent. We can generate practical data that will allow the refining operator to:

Improve the operational limits of the existing unit.
Predict the extent of corrosion, in case of upset conditions such as change to a feedstock higher in nitrogen content, sporadic interruption of water wash and/or plugging.

Decide when an inspection of the equipment is justified.
Select the most economical materials when upgrading or planning a new unit.
Call SET laboratories, Inc. at 281-403-0300 or email set@setlab.com for more information about laboratory simulation of ammonium bisulfide corrosion.