that allow the acid
gases, but not the hydrocarbons, to diffuse through the membrane. This
procedure can be performed alone or in combination with absorption liquid.
Sulfur Unit. The H2S-rich stripped gas stream is then fed to a Claus
process – a multistage process with two main sections: A thermal section
fires H2S with air or oxygen to produce SO2 and elemental sulfur, which is
released when cooled. A catalytic section allows more H2S to react with SO2
with alumina or titanium dioxide (TiO2) to produce water and elemental sulfur
(the Claus reaction: 2H2S + SO2 → 3S + 2H2O). The Claus process can
recover 95-97% of the sulfur in the feed gases.
A tail gas treatment unit serves to reduce the sulfur content to below 250
ppm, corresponding to a total sulfur recovery of 99.9%. More complex
solutions can reduce total sulfur down to 10 ppm. Some important processes
include SCOT (Shell Claus offgas treatment) which removes SO2 by
combustion with hydrogen over catalysts to produce H2S and water. H2S is
recycled to the Claus unit. Other solutions are the Beavon sulfur removal
process (BSR), based on amine solvent and catalysts.
5.3.2 Dehydration
Dehydration is either glycol-based scrubbers as described in chapter 4.3.2 or
based on pressure swing adsorption (PSA).
Newer processes also use
membranes.
5.3.3 Mercury removal
Mercury removal is generally based on molecular sieves. A molecular sieve
is a substance containing a material with tiny pores to achieve a large
surface area, such as activated carbon. The surface of the material allows
certain molecules to bind by surface tension. The molecules can later be
extracted and the sieve material regenerated by heating, pressure and/or
purging with a carrier gas.
A molecular sieve is commonly cyclic with one active unit and one (or more)
units in regeneration.
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5.3.4 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 byproduct.
5.3.5 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., CH3SH)
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 the type of catalyst.
5.3.6 Sales gas specifications
The exact sales gas specification is specified by pipeline operators and
distributors. Typical standard sales gas requirements use the following
parameters:
Volume is measured in standard cubic meters (scm) defined as 1 m3
at 0 ºC
and 101.325 kPa or standard cubic feet (scf) as 1 ft3
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 25 ºC. EU specifications are typically 38.8 MJ (10.8 kWh)
±5% per scm. In the US 1030 BTU ±5% per scf.
• Net calorific value or net heat of combustion is the net heat
generated when the water vapor in the gas does not condense
(water forms during combustion) and can be 10% lower.
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Wobbe index measures the heating effect that a burner is exposed to during
combustion. A higher value means a greater thermal load on the burner.
Different gases with the same Wobbe index will impose the same load on
the burner. An excessively high value is a safety hazard, as it can lead to
burner overheating and to excess production of carbon monoxide during
combustion.
Calorific value and Wobbe index can be adjusted by blending gas from
different sources as well as by addition or removal of nitrogen (N2).
Methane number is a value similar to octane value for gasoline, and is
important when the gas is used for internal combustion engines (as CNG).
Hydrogen sulfide and overall sulfur content: Both hydrogen sulfide (H2S)
and total sulfur must be reduced. H2S is toxic as well as corrosive for the
pipeline, as it forms sulfuric acid (H2SO4) and should be kept as low as
possible. Typical maximum values are 5 mg per scm of H2S and total sulfur
at 10 mg per scm.
Mercury should be kept below 0.001 ppb (parts-per-billion) which is its
detectable limit. The goal is to limit emissions and to prevent damage to
equipment and pipelines by mercury amalgamation, which makes aluminum
and other metals brittle.
Dew point is a temperature below which some of the hydrocarbons in the
gas can condense at pipeline pressure, forming liquid slugs that can damage
the pipeline. The gas must also be clear of all water vapor to prevent the
formation of methane hydrates within the gas processing plant or within the
sales gas transmission pipeline.
Particles and other substances must be free of particulate solids and all
liquids to prevent erosion, corrosion or other damage to the pipeline and
satisfy limits on carbon dioxide, nitrogen, mercaptans, etc.
Additives: When the natural gas is intended for domestic use,
tetrahydrothiophene (THT) is added so that the otherwise odorless natural
gas can be detected in the event of a gas leak. The sulfurous-smelling
substance added is equal to a sulfur content of 4-7 mg per scm.
