Page 65 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

5.1.4 Nitrogen rejection

Excessive nitrogen is removed by cryogenic distillation and higher
concentrations are removed by absorption with lean oil or another special
solvent if a smaller fraction is detected. (See acid gas removal for both
principles). Cryogenic removal also permits production of helium, if present,
as a valuable by-product.

5.1.5 NGL recovery and treatment
Remaining NGLs are recovered from the gas stream in most modern plants
by a cryogenic turbo expander based process followed by a fractionating
process. This process leads the cooled NGLs though distillation columns
called de-ethanizer, de-propanizer and de-butanizer, to extract ethane,
propane and butane respectively and leave a residual stream of pentane and
higher hydrocarbons.

The final step is to remove mercaptans (smelly organic gases e.g. CH 3SH) if
present, in a sweetening process, based on molecular sieves adsorption or
catalytic oxidization such as Merox – mercaptan oxidization or Sulfrex,
where the main difference is a type of catalyst.

5.1.6 Sales gas specifications

The exact sales gas specification is specified by pipeline operators and
distributors. Typical standard sales gas requirements are for the following
parameters:

3
Volume is measured in standard cubic meters (scm) defined as 1 m at 0ºC
3
and 101.325 kPa or standard cubic feet (scf) as 1 ft at 60 °F (16 °C) and
14.73 PSIA

Calorific value specifies the total amount of energy per unit generated
during combustion of the gas. The value is used to calculate the amount of
energy delivered. Several values are listed:

• Gross calorific value or gross heat of combustion: is the heat
released when a specific quantity of fuel in mixture with air is ignited
and the end products have returned to the initial temperature,
normally 25ºC. EU specifications are typically 38.8 MJ (10.8 kWh)
±5% per scm. In the US 1030 BTU ±5% per scf.

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