Moisture must be removed from natural gas to reduce corrosion problems
and to prevent hydrate formation.
Hydrates are solid white compounds
formed from a physical-chemical reaction between hydrocarbons
and water under the high pressures and low temperatures used to transport
natural gas via pipeline. Hydrates reduce pipeline efficiency.
To prevent hydrate formation, natural gas may be treated with glycols,
which dissolve water efficiently. Ethylene glycol (EG), diethylene glycol
(DEG), and triethylene glycol (TEG) are typical solvents for water
removal. Triethylene glycol is preferable in vapor phase processes
because of its low vapor pressure, which results in less glycol loss. The
TEG absorber normally contains 6 to 12 bubble-cap trays to accomplish
the water absorption. However, more contact stages may be required to
reach dew points below –40°F. Calculations to determine the number of
trays or feet of packing, the required glycol concentration, or the glycol
circulation rate require vapor-liquid equilibrium data. Predicting the interaction
between TEG and water vapor in natural gas over a broad range
allows the designs for ultra-low dew point applications to be made.6
A computer program was developed by Grandhidsan et al., to estimate
the number of trays and the circulation rate of lean TEG needed to dry natual
gas. It was found that more accurate predictions of the rate could be
achieved using this program than using hand calculation.7
Figure 1-4 shows the Dehydrate process where EG, DEG, or TEG
could be used as an absorbent.8 One alternative to using bubble-cap trays
is structural packing, which improves control of mass transfer. Flow passages
direct the gas and liquid flows countercurrent to each other. The use
of structural packing in TEG operations has been reviewed by Kean et al.9
Another way to dehydrate natural gas is by injecting methanol into gas
lines to lower the hydrate-formation temperature below ambient.10 Water
can also be reduced or removed from natural gas by using solid adsorbents
such as molecular sieves or silica gel.
Condensable Hydrocarbon Recovery
Hydrocarbons heavier than methane that are present in natural gases
are valuable raw materials and important fuels. They can be recovered by
lean oil extraction. The first step in this scheme is to cool the treated gas
by exchange with liquid propane. The cooled gas is then washed with a
cold hydrocarbon liquid, which dissolves most of the condensable hydrocarbons.
The uncondensed gas is dry natural gas and is composed mainly
of methane with small amounts of ethane and heavier hydrocarbons. The
condensed hydrocarbons or natural gas liquids (NGL) are stripped from
the rich solvent, which is recycled. Table 1-2 compares the analysis of
natural gas before and after treatment.11 Dry natural gas may then be
used either as a fuel or as a chemical feedstock.
Another way to recover NGL is through cryogenic cooling to very low
temperatures (–150 to –180°F), which are achieved primarily through
adiabatic expansion of the inlet gas. The inlet gas is first treated to
remove water and acid gases, then cooled via heat exchange and refrigeration.
Further cooling of the gas is accomplished through turbo
expanders, and the gas is sent to a demethanizer to separate methane
from NGL. Improved NGL recovery could be achieved through better
control strategies and use of on-line gas chromatographic analysis.12
and to prevent hydrate formation.
Hydrates are solid white compounds
formed from a physical-chemical reaction between hydrocarbons
and water under the high pressures and low temperatures used to transport
natural gas via pipeline. Hydrates reduce pipeline efficiency.
To prevent hydrate formation, natural gas may be treated with glycols,
which dissolve water efficiently. Ethylene glycol (EG), diethylene glycol
(DEG), and triethylene glycol (TEG) are typical solvents for water
removal. Triethylene glycol is preferable in vapor phase processes
because of its low vapor pressure, which results in less glycol loss. The
TEG absorber normally contains 6 to 12 bubble-cap trays to accomplish
the water absorption. However, more contact stages may be required to
reach dew points below –40°F. Calculations to determine the number of
trays or feet of packing, the required glycol concentration, or the glycol
circulation rate require vapor-liquid equilibrium data. Predicting the interaction
between TEG and water vapor in natural gas over a broad range
allows the designs for ultra-low dew point applications to be made.6
A computer program was developed by Grandhidsan et al., to estimate
the number of trays and the circulation rate of lean TEG needed to dry natual
gas. It was found that more accurate predictions of the rate could be
achieved using this program than using hand calculation.7
Figure 1-4 shows the Dehydrate process where EG, DEG, or TEG
could be used as an absorbent.8 One alternative to using bubble-cap trays
is structural packing, which improves control of mass transfer. Flow passages
direct the gas and liquid flows countercurrent to each other. The use
of structural packing in TEG operations has been reviewed by Kean et al.9
Another way to dehydrate natural gas is by injecting methanol into gas
lines to lower the hydrate-formation temperature below ambient.10 Water
can also be reduced or removed from natural gas by using solid adsorbents
such as molecular sieves or silica gel.
Condensable Hydrocarbon Recovery
Hydrocarbons heavier than methane that are present in natural gases
are valuable raw materials and important fuels. They can be recovered by
lean oil extraction. The first step in this scheme is to cool the treated gas
by exchange with liquid propane. The cooled gas is then washed with a
cold hydrocarbon liquid, which dissolves most of the condensable hydrocarbons.
The uncondensed gas is dry natural gas and is composed mainly
of methane with small amounts of ethane and heavier hydrocarbons. The
condensed hydrocarbons or natural gas liquids (NGL) are stripped from
the rich solvent, which is recycled. Table 1-2 compares the analysis of
natural gas before and after treatment.11 Dry natural gas may then be
used either as a fuel or as a chemical feedstock.
Another way to recover NGL is through cryogenic cooling to very low
temperatures (–150 to –180°F), which are achieved primarily through
adiabatic expansion of the inlet gas. The inlet gas is first treated to
remove water and acid gases, then cooled via heat exchange and refrigeration.
Further cooling of the gas is accomplished through turbo
expanders, and the gas is sent to a demethanizer to separate methane
from NGL. Improved NGL recovery could be achieved through better
control strategies and use of on-line gas chromatographic analysis.12
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