Operations and Recovery Improvement Via Heavy Hydrocarbon Extraction
Authors: Iyengar J.N. , Sibal P.W. , Clarke D.S. 
Published: December 2007

The Mary Ann facilities process sour gas from offshore facilities and produce pipeline quality gas. The plant treating facilities consist of Sulfinol-M and MDEA treating systems. Both treating systems experienced foaming problems that were related to the presence of heavy hydrocarbons in the feed gas. In addition, the sulfur recovery units experienced operating problems due to heavy hydrocarbons in the acid gas feed. A silica gel heavy hydrocarbon unit was installed upstream of each treating unit to reduce the C6+ hydrocarbons in the feed. Although the operations of the treating units improved significantly, the treating units continue to experience sporadic foaming problems. A proprietary analytical technique was developed to determine the hydrocarbon contents of the solution. A carbon filtration system was installed to treat the Sulfinol-M solution to remove the dissolved heavy hydrocarbons. The installation of the carbon bed dramatically reduced the foaming incidences of the Sulfinol-M system and permitted operation of the treating units at the design capacity. In addition, the performance of the sulfur recovery units also improved.

Mary Ann Facilities Background

Mobil E&P U.S. (MEPUS) owns and operates two onshore natural gas treating systems in Alabama's Mobile Bay area, the Mary Ann Gas Plant and the 823 Gas Plant. Two separate plants were constructed within the same area to allow economical processing of relatively high hydrogen sulfide (H2S) content natural gas, ranging in concentration from 2.5 to 10 percent H2S, and near pipeline quality gas (a few hundred parts per million H2S) from offshore leases in both state and federal waters. These plants were brought onstream in 1988 and 1991, respectively.

All of MEPUS' offshore production is cooled, separated from its produced/condensed water and condensed heavy hydrocarbons, and dehydrated offshore prior to pipeline transport to the onshore processing facilities. For the high H2S content production, a closed loop of the condensed heavy hydrocarbons coupled with a corrosion inhibiting agent (called Corrosion Inhibitor Oil or C/O) protects the top sides metallurgy upstream of the offshore dehydration units. MEPUS employs both silica gel and triethylene glycol (TEG) offshore dehydration systems.

The Mary Ann Gas Plant receives dehydrated sour gas and CIO/water from the offshore facilities. The sour gas is treated in two parallel Sulfinol-M Units to remove essentially all of the H2S and some of the CO2. The sweetened gas is then dehydrated in two parallel TEG Units and sold to three pipeline transmission systems. Each parallel Sulfinol/TEG train has a design capacity of 80 MMSCFD; the second train was brought onstream in 1994. The acid gas removed from the plant feed is stripped from the circulating sulfinol solution and routed to the Sulfur Recovery Unit. Sulfur is recovered as a molten product in a three-bed modified Claus Unit. A SCOT tail gas unit converts all residual sulfur species back into H2S and recycles them back to the inlet of the Claus Unit. This plant achieves typical sulfur recovery efficiencies of 99.5 percent. The Sulfur Recovery Unit's capacity was increased from 285 long-tons per day (LTPD) to 470 LTPD, via a 1997 plant expansion employing oxygen enrichment technology.

The 823 Plant contains two parallel contactors and a common regenerator to remove the H2S and some of the CO2 from a design plant feed rate of 250 MMSCFD. This amine unit uses a generic MDEA solution. Sweetened gas is dehydrated in a single TEG Unit, combined with the Mary Ann Plant sales gas, and sold to the same pipeline systems. The acid gas stream derived from the overhead of the Primary Amine Unit's regenerator can either be routed through or bypassed around an Acid Gas Enrichment (AGE) Unit that also uses a generic MDEA solution. This AGE Unit allows slippage of CO2 in the acid gas stream directly to the Thermal Oxidizer, facilitating the sulfur recovery process. The enriched or non-enriched acid gas stream is then sent either to a MODOP (Mobil Oil Direct Oxidation Process) sulfur recovery unit within the 823 Plant, or it can be routed to the inlet of the Claus Unit in the Mary Ann Plant.

Figure 1 provides a simplified process flow diagram of the treating units.

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