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Gas treating

ExxonMobil has developed and commercialized the FLEXSORB™ suite of gas treating technologies and absorbents.

The FLEXSORB SE technology is designed for the selective removal of H2S in the presence of C02 and utilizes proprietary severely sterically hindered amines. This allows FLEXSORB SE solvent to achieve high H2S cleanup selectively at low solvent circulation rates.

Table 1 provides an overview of the available FLEXSORB solvents and their applications. The technology and absorbents have been widely applied in petroleum refining, natural gas production and petrochemical operations. The FLEXSORB solvents have been applied successfully at offshore and onshore production sites around the world.

Table 1: Solvent type and application
Photo — Table 1: Solvent Type and Application

The FLEXSORB SE process has been shown to be the most selective and cost-effective amine solvent process. It’s reliable, robust and simple to operate. Operating experience has shown low corrosion and lower foaming than with conventional amines. Corrosion is low even at high rich loadings or high levels of heat-stable salts. Conventional equipment used for other amine solvents, such as countercurrent towers, is used for the FLEXSORB SE process as well.

Tail Gas Treating

In sulfur plant Tail Gas Treating Unit (TGTU) applications, FLEXSORB SE solvents can use about half of the circulation rate and regeneration energy typically required by MDEA-based solvents. CO2 rejection in TGTU applications is very high, typically above 90 percent. FLEXSORB SE provides a reduced vapor and liquid load to the regenerator tower, resulting in a smaller tower diameter compared with competing technologies.

Al Khaleej Phase 2 Qatar

A simplified process flow diagram is shown in Figure 1. The feed gas is contacted countercurrently with lean FLEXSORB SE solution in the absorber tower. With low-pressure applications, it is recommended that the top of this tower include a water wash zone to minimize the loss of FLEXSORB SE amine with the treated gas. The water wash purge is combined with the rich amine to recover the FLEXSORB SE amine.

Figure 1: Simplified Flow Diagram
Photo — Figure 1: Simplified Flow Diagram — FLEXSORB® SE Tail Gas Treating Unit (TGTU)

Natural gas treating

Selective H2S removal from natural gas can be advantageous whenever a portion of the CO2 can remain in the treated gas. One such application treats a pipeline gas stored in a depleted oil reservoir. The pipeline gas is stored during the summer months and withdrawn during the heating season to supplement pipeline gas flow during peak demands. The unit was originally designed to treat the gas with a conventional hybrid solvent. The need to process at least 25 percent more gas, drove the licensee to change solvents. A comparison of operating data for the two solvents is given in Table 2. FLEXSORB SE solvent allowed an increase in gas rate from 400 to 510 MSCFD without changing the existing hardware. This was done at a lower solvent circulation, 300 gpm versus 460 gpm, and a higher CO2 slip. FLEXSORB SE also improved the operability of the unit by reducing the co-absorption of heavy hydrocarbons, which previously caused numerous plant upsets.

Table 2: Onshore treating facility
Photo — Table 2: Onshore Treating Facility

Acid gas enrichment

Many natural gas fields contain more CO2 than H2S. When these acid gases are removed from the methane sales gas, the resulting acid gas stream is likely to contain low H2S concentrations (< 20 mol%), making it unsuitable for sulfur recovery using the conventional Claus process. Acid Gas Enrichment (AGE) has become an enabling technology to economically produce these natural gas reserves. As the name implies, an AGE unit enriches the H2S content of the acid gas stream, making it practical to recover sulfur in a Claus unit. Alternatively, AGE technology can also be used to minimize the volume of acid gas for re-injection or to debottleneck existing facilities if new, sourer fields are developed. A limited number of these AGE units are in operation (some report as few as 25 worldwide). FLEXSORB SE or SE PLUS solvents have been specified for 20 AGE units, including the Qatargas II LNG Project (Trains 4 and 5) gas treating facilities that were successfully started in 2009. As the name implies, Acid Gas Enrichment concentrates the H2S from the AGR system by further gas treatment in a second amine unit utilizing a selective amine solvent. Except for the use of the selective amine solvent, an AGE unit is similar to other traditional amine treating units.

To achieve the twin goals of low H2S in the treated gas and low CO2 in the enriched acid gas, the AGE amine solvent must maximize the selectivity for absorbing H2S. AGE process simulations by Weiland (2008) have demonstrated that the CO2 and H2S partial pressure driving forces in the AGE absorber work against achieving these goals simultaneously. As the gas moves up the absorber tower, the H2S partial pressure is decreasing, reducing the mass transfer driving force. At the same time, the CO2 partial pressure is increasing, making CO2 pickup more difficult to avoid.

The level of H2S enrichment depends primarily on the H2S concentration in the feed gas and, of course, the solvent selection. The H2S specification in the treated gas, process operating temperatures and the design of the AGE absorber tower internals can also be important considerations. Higher processing temperatures or lower specification for H2S in the treated gas stream may require increased lean amine circulation rate and/or increased mass transfer area to ensure that the H2S specification in the treated gas is met. Increasing the amine rat or mass transfer area will directionally reduce H2S selectivity.

To successfully achieve the AGE treating goals of a low H2S content in the treated gas and maximum CO2 slip requires a solid base of supporting data and careful engineering.

Figure 2 below illustrates the range of H2S enrichment that can be achieved using FLEXSORB SE or SE PLUS solvent in a single enrichment step.

Figure 2: Acid gas enrichment capabilities
Photo — Figure 2: Acid Gas Enrichment Capabilities

Table 3 provides a concise summary of the range of experience for FLEXSORB SE and SE PLUS units. 

Table 3: FLEXSORB AGE experience bands
Photo — Table 3: FLEXSORB® AGE Experience Bands

ExxonMobil’s FLEXSORB SE or SE PLUS solvents are in use in a wide variety of Acid Gas Enrichment unit designs around the world. These units include both ExxonMobil affiliates and licensees. These plants range in size from three tons to more than 1,000 tons per day of sulfur capacity. CO2 slip ranges from 73 to 94 percent.

100 Commercial apps

More than 120 commercial applications have repeatedly demonstrated the advantages of FLEXSORB SE and SE PLUS over competing solvents since the first commercial unit was started up in 1983. Commercial applications include ExxonMobil affiliates as well as numerous licensee applications in locations around the world, shown graphically in Figure 4. FLEXSORB SE and SE PLUS solvents are used in a variety of gas treating applications, including Acid Gas Removal (AGR), Acid Gas Enrichment (AGE), and Tail Gas Cleanup Units (TGCU). FLEXSORB technology easily fits into natural gas processing (including onshore and offshore) and refining and petrochemical operations using standard gas treating equipment.

Figure 4: Graphical representation of commercial applications
Photo — Figure 4: Graphical representation of commercial applications which include ExxonMobil affiliates as well as numerous licensee applications in locations around the world

Harmony of solutions for sour gas treating problems

  • Absorbs the H2S, rejects the CO2
  • Cost-effective for grassroots and retrofits
  • Removes H2S to less than 10 ppm
  • Vast commercial experience (100 + units)
  • Enables simple, low-cost retrofits

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