Most natural-gas production contains, to varying degrees, small (two to eight carbons) hydrocarbon; such as Methane (CH4), Benzene (C6H6), and Butane (C4H10). The subsequent high level of contamination of the spent molecular sieve became evident on the first change out with concentrations of in excess of 3000ppm. With this waste stream being the largest single stream generated, a suitable treatment solution was required to address this significant waste stream that in turn would lead to a recyclable non-hazardous waste stream.

In order to reduce the Benzene concentration on the spent molecular sieve, the thermal desorption process was chosen because it is environmentally clean and can be applied to varying levels of contamination. The technology has come a long way since the early 1990's, WRT safety and environmental controls. However, some of the variables must be controlled in order to be process and cost effective.

This paper presents an analysis of the selection and use of the thermal desorption process to reduce the various levels of hydrocarbon on the spent molecular sieve. This project was investigated, evaluated and run at the RasGas facility operated within the Ras Laffan Industrial City in the State of Qatar.

A short description of the type of unit selected, the environmental controls required in order to meet the Industrial City and State emissions requirements. Including a summary of the operation, geographic information and a description of the various waste stream compositions. How the operating conditions impacted on the unit's overall performance.

Key parameters in the process efficiency were considered, comparing actual results obtained in the field as well as theoretical concepts regarding the treatment process prior to implementation.

Three groups of variables were analyzed:

  • Related to waste composition

    1. Water fraction

    2. Total liquid fraction

    3. Particle size

  • Related to Weather Conditions

    1. Environment humidity

    2. External temperature

  • Related to Process Operation

    1. Burners; and fuel feed rate

    2. Direct vs indirect contact

    3. Feed rate and burn duration (retention time)

    4. Recovered elements

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