Effective Flow Diagram of a Gas Treatment Unit Proposed Due to Increased Requirements to Product Quality

By Alexei Vakulin, April 22, 2013

During preparation of the preliminary design solution for bringing a number of fields into development, an operating company with Russian participation (the project is in a non-CIS country) faced the problem of selecting an effective flowsheet of hydrocarbon mixture treatment (hereinafter – HCM).

The operating company turned to the Ingenix Group experts for help in solving this problem. The peculiarity of the problem is that the tough requirements of state agencies in the host country on the physical properties of the products differ significantly from the Gazprom standards (Table 1).

Crude hydrocarbon gas is characterized by the absence of hydrogen sulfide and mercaptans, low content of СО2, and the content of С5+ does not exceed 1 mole percent.

Hydrocarbon dew-point temperature (HDPT), as a rule, is of decisive significance for the gas treatment flow diagram: it determines the stripping temperature. The water dew-point temperature should be accepted at a minimum of 5 C below that required for low-temperature processing. In this case, there is a rather tough requirement for the gross calorific value (GCV) of the tank gas (GCV should be minimum 9,100 kcal/Cm³ and maximum 9,500 kcal/Cm³). Crude field gas is characterized by the maximum HDPT equal to 3 С at a pressure ≈ 4.8 MPa and the GCV equal to 10,340 kcal/Cm³.

Assurance of the gas gross calorific value not exceeding 9,500 kcal/Cm³ will require high capital and operating costs. Usually the lower calorific value is used for engineering and thermotechnical calculations. At the same time, we can suppose that limitation of the gross calorific value can be related to the idea of limiting the content of С3+ hydrocarbons when it is used as a fuel. As natural gas in Europe (major consumer of gas from this region) is used as fuel, this supposition seems to be most realistic.

The contractor developed the process flow diagram (Fig. 1). To ensure the requirement for GCV, it is necessary to cool the dry gas to a temperature not exceeding minus 53 С. To reach this low temperature, it is possible to apply the proposed flow diagram or use an ethane (ethylene) cooling cycle with a cooling isotherm not exceeding minus 55 С.

To receive gas with a GCV not higher than 9,500 kcal/Cm³, it is necessary to limit the pressure in the stripping assembly to a value not exceeding 4.5 MPa, as higher pressure and a stripping temperature below minus 50 С will facilitate methane condensation, which will result in reduction of tank gas yield and an increase of fuel gas yield.

To ensure proper operation of the stripping assembly, it is necessary to use an assembly of crude gas adsorption dehydration, as the water dew-point temperature should be 5 C lower than the minimum temperature in the low-temperature processing. As the temperature of minus 53 С is achieved in the III stage low-temperature separator, the water dew-point temperature should be not higher than minus 58 С, which could be reliably ensured by application of synthetic zeolites. The semi-continuous process of