GAS INDUSTRY ASSESSES NEW WAYS TO REMOVE SMALL AMOUNTS OF H2S

For natural gas with relatively small amounts of sulfur, typically less than 200 ppmv H2S, the Iron Sponge scavenging process was widely used by the industry for many years. Recent safety and environmental concerns associated with the disposal of the spent wood chips has prompted several new H2S scavenger processes to be introduced and used by the industry.

The three major categories of H,S scavengers are:

1. Caustic/sodium nitrite solutions (e.g., Exxon Corp's Sulfa Check)

2. Nonregenerable amines/triazines (e.g., Petrolite Corp's Sulfa Scrub)

3. Iron based (e.g., SulfaTreat Co.'s SulfaTreat).

As these processes become more widely used, information on their application and performance is needed. In addition, regulations are becoming stricter and heightened awareness related to waste generation and disposal has prompted a keen interest by the gas industry in identifying better and more environmentally acceptable H2S scavenging processes.

GRI is addressing these needs through:

• Technical evaluation of vendor literature and user information

• Laboratory testing

• Field testing at commercial facilities.

TECHNICAL EVALUATION

As with the liquid redox process evaluations, GRI initiated this research with an analysis of available literature and estimates provided by the vendors. An index of selection has been developed by M.W. Kellogg for the large number of chemicals on the market (Table 2).

Based on information available to date, this relative index allows the user to weight each ranking factor depending on site specific or other conditions. The factors used in developing the relative index of selection (IOS) and their weights, in parenthesis, include:

• Total plant investment (1.0)

• Operating cost (1.0)

• Process reliability, (1.0)

• Winterization (0.2)

• Ease of operation (0.5)

• Operators' acceptance (0.4)

• Disposal of spent material (1.0).

This work is ongoing (and largely reflects vendor estimates) and will be updated as information and data are generated through the lab and field studies, as well as site visits. The program will be used to prepare process selection and operating guidelines.

LAB STUDIES

Laboratory testing is evaluating scavengers from each scavenger class to obtain general behavioral characteristics and performance data.

About six scavenging agents are being tested with laboratory-scale equipment (Fig. 3d) at ambient pressure. The test variables include H2S concentration, CO2 concentration, and the presence of liquid hydrocarbon condensate to evaluate the potential for hydrocarbon induced foaming.

The data being collected include qualitative information on reaction rates, H2S capacity, solids formation/characteristics, and tendency, to foam.

From initial results, the capacities of the scavenging solutions measured in the laboratory, have been 0.8 lb sulfur/gal for Sulfa Scrub and 0.9 ppg for Gas Treat 114, respectively. Testing of Sulfa Check is in progress. These capacity data are comparable to estimated values obtained from historical process data collected by the operator at the site where the field evaluations are under way.

Historically, the estimated capacities from these earlier field data have ranged from 0.29 0.75 ppg for Sulfa-Check, to 0.47 0.85 ppg for Sulfa-Scrub, and 0.30 0.89 ppg for Gas Treat 114.

GRI will publish the laboratory test results when available later this year.

FIELD EVALUATIONS

Field evaluations is being conducted to obtain accurate and reliable performance, cost, and environmental data on each class of scavenging agents.

The initial field testing is at a major gas producer's operations in South Texas.

The plant has two parallel towers capable of treating up to 15 MMscfd of gas containing up to 20 ppmv of H2S (Figs. 3e and 6). An inlet separator and coalescing filter remove free water and hydrocarbon liquids from the inlet gas. The gas is then introduced at the bottom of the towers, which are partially filled with liquid scavenging agents.

After disengagement from the liquid, the treated gas flows through mist eliminators and leaves the plant. An outlet separator removes any liquids which may carry over from the towers under upset conditions. Automatic valves regulate the flow of gas through the towers.

The testing will continue for several months.

Three different liquid scavenging agents are being evaluated: Sulfa-Check, Sulfa Scrub, and Gas Treat 114. Each agent will be tested twice, once in each tower.

GRI also plans to evaluate in line injection, a common approach for some scavengers. Some of the agent reaction rates are rapid enough so that tower residence times are not always needed.

Limited information is available in the open literature on the design and application considerations required for in line scavenging. GRI's research is designed to develop guidelines in this area.

GRI is also interested in evaluating the solid SulfaTreat product in the field. An initial discussion with the vendor and a meeting with a producer in East Texas have been held that may lead to a cooperative program in this area.

GRI RESEARCH

Although the basic research component of GRI's gas processing/sulfur program has not been discussed in this article, it is an integral aspect in the creation of process improvement ideas.

The applied laboratory and field testing often have their roots in fundamental research and laboratory studies.11 12 The applied testing often develops information fed back into the basic research studies to give new insights. Current emphasis is on mechanisms of liquid redox processes and development of a regenerable scavenger process.

GRI is a not for profit research organization that exists for the benefit of the gas industry and the natural gas rate payer. GRI disseminates its research results to the gas industry through reports, presentations at conferences, journal articles, software, special seminars and site visits, sharing information with other industry groups, and by holding conferences on specific subjects of interest.

GRI also develops technology and supports its commercial deployment to the industry, through strategic partnerships with manufacturers, vendors, and technology users.

Since 1986, GRI has held six conference on technologies for removal of small amounts of sulfur from natural gas and other streams. The most recent conference was May 15 17 near Austin.

Proceedings will be available later this year.13-17

REFERENCES

1. Hugman, R.H., Vidas, E,H., and Springer, P.S., "Chemical Composition of Discovered and Undiscovered Natural Gas in the Lower-48 United States 1993 Update, Vol. 1," Report No. GRI 93/0456.1 (to be published).

2. Leppin, D., Brunsman, B., and Krist, K., "Gas Research Institute Program in Sulfur Removal and Recovery Research 1992 Update," GRI Liquid Redox Sulfur Recovery, Conference, Austin, Oct. 4 6, 1992.

3. Leppin, D., Gamez, J., and Meyer, H., "Developments for Reducing Natural Gas Treating and Processing Costs," SPE Gas Processing Symposium, Calgary, June 28 30, 1993.

4. Leppin, D., and Dalrymple, D.A., "GRI Program in Sulfur Removal and Recovery from Natural Gas," 73rd GPA Annual Convention, New Orleans, Mar. 7 9, 1994.

5. Quinlan, M.P., and Echterhoff, L,W., "Technical and Economic Comparison of LO CAT II with Other Iron based Liquid Redox Processes," GRI Liquid Redox Sulfur Recovery Conference, Austin, Oct. 4 6, 1992.

6. Quinlan, M.P., "Technical and Economic Analysis of the Iron based Liquid Redox Processes," 91st GPA Annual Convention, Anaheim, Calif., May, 16 18, 1992.

7. Johnson, J. E., et al., "Options narrowed to Claus and Redox processes," OGJ, Nov. 22, 1993, pp. 70 76.

8. Trofe, T.W., McIntush, K.E., and Murff, M.C., "Stretford Process Operations and Chemistry Report," Report No. GRI 93/0121, November 1993.

9. Hardison, L.C., "Early Experience with ARI-LO CAT II for Natural Gas Treatment," AIChE Spring National Meeting, New Orleans, Apr. 2, 1992.

10. "Processes for H2S removal from low volume gas streams tested," OGJ, Sept. 13, 1993.

11. DeBerry, D.W., "Rates and Mechanisms of Reactions of Hydrogen Sulfide with Iron Chelates," Report No. GRI 9310019, April 1993.

12. DeBerry, D.W., "Regeneration of Chelated Iron Liquid Redox Sulfur Recovery Sorbent," draft topical report to GRI, December 1993.

13. Scheffel, F.A., and Edge, V.D., "Proceedings of the 1986 GRI Stretford Users' Conference," Report No. GRI 88/0236, November 1986.

14. Dalrymple, D.A., and Trofe, T.W., "Proceedings of the 1987 GRI Liquid Redox Sulfur Recovery Conference," Report No. GRI-88/0078, April 1988.

15. Dalrymple, D.A., and Wessels, J.K,, "Proceedings of the 1989 GRI Liquid Redox Sulfur Recovery Conference, Report No. GRI-89/0206, August 1989.

16. Dalrymple, D.A., and Wessels, J.K., "Proceedings of the 1991 GRI Liquid Redox Sulfur Recovery Conference, Report No. GRI-91/0188, June 1991.

17. Dalrymple, D.A., and Wessels, J.K., "Proceedings of the 1992 GRI Liquid Redox Sulfur Recovery Conference, Report No. GRI-93/0129, April 1993.

Copyright 1994 Oil & Gas Journal. All Rights Reserved.