Closed-loop Hydrocarbon Recovery System Meets Waste Water Regulations

Environmental regulations such as National Emissions Standards for Hazardous Air Pollutants (Neshaps) are forcing refiners to adopt stringent approaches to recovering hydrocarbons from waste water treatment operations.

As a result, many refiners are considering closed-loop treatment methods, according to Amcec Adsorption Systems, Schaumburg, Ill.

According to an unpublished report by Amcec's Jim Graham and Tom Cartwright, "Government agencies are adopting an increasingly multimedia approach in their regulation of emissions from refinery process waste water treatment, with the result that operators are replacing single treatment techniques with integrated systems designed to bring out the best in each method."

The report detailed Amcec's commercially proven recovery process. The process, developed by Texaco and licensed by Amcec in 1992, combines nitrogen stripping, humidity modification, and activated carbon to remove 98% of BTEX from waste water treatment emissions. The process has been installed at 10 U.S. refineries.

Conventional treatment

Waste water from API separators contains hydrocarbons, hydrogen sulfide (H2S), and volatile organic compounds (VOCs). As a result of regulations such as the U.S. Environmental Protection Agency's Neshaps and Hazardous Organic Neshaps (HON), refiners are using a variety of measures to remove these contaminants.

Treatment technologies include:

• Direct treatment with activated carbon
• Air stripping to remove VOCs, followed by recovery onto granulated, activated carbon (GAC)
• Enhanced biodegradation.

The process

The Amcec process integrates several treatment methods in a closed-loop stripping system. A flow diagram of the process is shown in Fig. 1 [33,767 bytes].

The process uses nitrogen to strip benzene, toluene, ethylbenzene, and xylenes (BTEX) from the waste water. The nitrogen in the stripper system is continuously cleaned and recycled.

The stripped BTEX is adsorbed onto activated carbon, which is regenerated using steam. The water-saturated gas stream from the counter-current stripper undergoes relative humidity (RH) modification. This, say Graham and Cartwright, is critical to efficient operation of the GAC adsorber.

RH reduction is accomplished by cooling the gas stream to condense out the water vapor, then reheating it by 20-30° F. The process produces a stream with an RH of less than 50%.

The hydrocarbon-rich gas stream passes through one of two parallel, steam-regenerable carbon beds, where the VOCs and BTEX are adsorbed onto the carbon. While one adsorber is on line, the other is being regenerated.

The effluent gas is compressed in a blower and recirculated to the stripper inlet. The VOC-rich condensate is recycled to the refinery.

The inert nitrogen atmosphere in the closed-loop system inhibits biological growth in the packed towers. It also hinders precipitation of fouling compounds such as iron onto the tower packing. In addition, the low oxygen concentrations prohibit the formation of VOC vapor concentrations that approach the lower explosion limit, thus enhancing the system's safety.

Results

Table 1 [5,271 bytes] shows the properties of a waste water stream that is processed in one of the commercial units. This unit uses 316 stainless steel adsorbers and lined, carbon steel strippers.

Table 2 [10,477 bytes] shows the design parameters and economics of this unit, which has brought into compliance waste water with the following characteristics:

• Flow rates of 150-750 gpm
• BTEX concentrations of 20-150 ppm(wt)
• Benzene levels of 5-100 ppm(wt)
• H2S concentrations of 1-250 ppm(wt)
• Oil and grease concentrations of 20-400 ppm(wt).

A common operating problem has been the presence of fouling agents such as high-molecular-weight hydrocarbons. In addition, hydrogen sulfide has caused sulfur deposition on the GAC.

If the influent contains relatively high concentrations of these two contaminants, which are not readily steam desorbed, they can build up on the GAC beds after several steaming cycles, and fill or block access to the carbon micropores. This buildup reduces the adsorption capacity of the carbon and can reduce cycle length.

The carbon can be fully restored by removing it and reactivating it off site. Alternatively, it can be replaced with fresh charge.

To protect the main carbon adsorbers, refiners can install guard beds containing GAC impregnated with potassium hydroxide. When H2S concentrations are high, a packed-tower caustic scrubber can be installed upstream of the guard beds. According to the report, a system processing 300 gpm of waste water containing 250 ppm(wt) of H2S will use about 75 lb/hr of caustic in the scrubber.

Although the capital cost of a closed-loop system is more than that of a direct carbon-adsorption system, lower operating costs make it a viable approach, says Amcec. In addition, according to Graham and Cartwright, "Permitting a closed-loop system is easier because government agencies view it as a hydrocarbon recovery/recycle unit, not a waste water treatment system."

Mechanics of Hydraulic Fracturing, by Ching H. Yew. Published by Gulf Publishing Co. Book Division, P.O. Box 2608, Houston, Tex. 77001. 183 pp., $95.

The propagation of an hydraulic fracture in a reservoir at great depth is a complicated phenomenon, and the propagation is almost always different from a fracture created in the lab. Without great expense, says the author, you cannot verify the characteristics and geometry of an hydraulic fracture at great depth. So the reliability of your fracture simulator is dependent on the soundness of its underlying mechanics.

This book is a guide for the engineer in accurately determining the vital mechanics information needed for success.

Strategic Geography and the Changing Middle East, by Geoffrey Kemp and Robert E. Harkavy. Published by Carnegie Endowment for International Peace and Brookings Institution Press, Washington D.C. 491 pp., paperback $22.95, hardback $52.95.

With the breakup of the Soviet Union and growing links between the traditional Middle East and the Caucasus as well as Central and South Asia, a new strategic map of the region is emerging that has far-reaching implications for the U.S. and other major powers with interests in the region.

The authors discuss and analyze the impact of new configurations of international relations on the Middle East, the historical background of the regions' strategic geography, emerging linkages between oil and water resources, and demographic trends and current patterns of conflict.

The Russian Far East: A Business Reference Guide, edited by Elisa Miller and Soula Stepanopoulos. Published by Russian Far East Update, P.O. Box 22126, Seattle, Wash. 98122. 282 pp., $59.

The book contains facts about the Russian Far East (the area of Russia east of Siberia), which is now emerging as a new force in Pacific Rim commerce.

There are 18 maps and 55 charts divided into 15 chapters. There is a survey that profiles each of the area's 10 territories, followed by in-depth chapters on major industries, demographics, transportation, foreign trade, privatization, banking and finance, and resource materials.

Produced Water 2: Environmental Issues and Mitigation Technologies, edited by Mark Reed and Stale Johnsen. 536 pp., $139.50.

This book contains the proceedings of the Produced Water Seminar held in Trondheim, Norway, in September 1995. Produced water remains the largest volume waste stream from oil and gas production offshore. In the North and Norwegian Seas, produced water volumes are projected to increase significantly over the coming decades.

The purpose of this seminar was to provide a forum for scientists, legislators, and industrial and environmental representatives to share recent information and research results, and to encourage cooperative pursuit of solutions.

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