TECHNOLOGY Texas plant treats natural gas for use as alternative fuel

Amine Plant Flow Diagram [77119 bytes]

Pushed by the Clean Air Act Amendments of 1990, the Metropolitan Transit Authority of Harris County (known as Houston Metro) is using clean-burning liquefied methane to fuel some of Houston's city buses.

Houston Metro's primary supply of liquefied methane is a 12 MMscfd amine-treating unit in Willis, Tex. The Willis plant uses Dow Chemical Co.'s GAS/SPEC process to treat natural gas.

Regulatory incentive

The U.S. Environmental Protection Agency (EPA) monitors the country's ambient air quality and sets forth regulations governing emissions from mobile and stationary sources.

Under the Clean Air Act Amendments, companies operating 10 or more trucks in metropolitan areas EPA says have high concentrations of air pollutants must run half of their fleets on alternative fuels. Although Houston Metro and other public agencies are not affected by this rule, Metro uses liquefied methane as an alternative fuel to meet other governmental requirements.

Process description

The GAS/SPEC process uses a specialty amine solvent to remove carbon dioxide (CO2) and hydrogen sulfide from natural gas.

To convert natural gas to a liquid, processers cool the gas to cryogenic temperatures and compress it. If the gas contains carbon dioxide, the CO2 will freeze, subsequently blocking transmission lines and damaging equipment. Processors therefore must remove CO2 from natural gas to prevent these problems.

The natural gas processed at the Willis amine treater contains 1.5-2.5% CO2. During treatment, the CO2 concentration is reduced to 50 ppm (flow diagram).

The solvent used in the Willis plant is called GAS/SPEC CS-Plus. Dow makes seven other solvents for use with the process.

Morton Aubrey, marketing manager for Dow's GAS/SPEC process, says it is not necessary to use corrosion inhibitors with the solvents, as it is with traditional amines. In addition, according to Aubrey, the specialty solvents used in the process do not degrade over time.

Traditional amines must be disposed of and replaced, increasing processing costs. But the GAS/SPEC specialty solvents can be recycled and reused because they do not degrade.

Aubrey says the solvent can be recycled as long as it does not contain any contaminant that would cause it to be classified as a hazardous material.

When the solvent in an operating unit has degraded to a degree that warrants replacement, a new solvent charge is delivered to the plant. The used solvent is transported to a reclamation facility, where the contaminants are removed via thermal distillation.

"Typically," says Aubrey, "the recycled solvent is sold as a replacement charge to the next customer with a reclamation need."

The specialty amines for this process are used in concentrations as high as 50 wt %. Concentrations of more-corrosive amines, like mono ethanolamine and di ethanolamine, are limited to, respectively, 15 and 30 wt %.

The higher concentration of the specialty amines allows operators to remove more acid gas per gallon of solution. As a result, the capacity of existing plants can be increased, says Dow, often without adding equipment. And because the process is more efficient, it requires smaller pumps and uses less energy.

According to Aubrey, Dow converted more than 20 plants from generic amines to the specialty amines during 1995. That is greater capacity without any additional capital expense, he said.