Page 347 - Petroleum and Gas Field Processing

Ethane recovery, however, is quite small. This process is being
phased out.

Refrigeration Process
The production of NGL at low temperature is practiced in many gas
processing plants in order to condense NGL from gas streams. As
indicated in Figure 2, using nontoxic and noncorrosive refrigerants to chill

the feed natural gas to a temperature between 0 F and 40 F using a low-
level one-component refrigerant system provides external refrigeration.
When using a high-level cascade refrigerant system, a much lower

temperature in the range of 100 Fto 150 F is reached. Liquids are
separated from the residue gas at multiple temperatures and then
fractionated into final products. Ethane recovery is a strong function of
the operating temperatures as is explained next.
The following operating conditions are important in the separation
of NGL constituents: Two main objectives are usually targeted when the
desired temperature of the gas–liquid stream leaving the chiller is specified:
(1) dew point control (dpc) of the hydrocarbons and (2) liquid recovery.
Now, if the dpc of the hydrocarbon is the primary target, then the

temperature of the gas–liquid stream is set at about 6–10 C below the
desired dew point temperature. On the other hand, if the liquid recovery is
the main objective, then condensing the least amount of nonsalable
components should be achieved. In other words, condensing methane is
not cost-effective.
As far as the operating pressure for the refrigeration system, the
recommended operating pressure for maximum liquid recovery is set
between 400 and 600 psia. Condensation of methane increases with higher
pressure; therefore, optimum pressure must minimize the total cost of the
system. In general, separation is carried out at a pressure corresponding to
the sales gas pressure simply to eliminate the cost of gas recompression. To
summarize the above, for a given selected separation pressure the
corresponding operating temperature is chosen based on the type of
product:
If the liquid product is to be sold as ‘‘crude oil,’’ then the

separation temperature is between 0 C and 5 C.
If the liquid product contains propane as the lightest component,

temperature is about 30 Cto 18 C. In this case, temperature

depends on whether absorption or adsorption is combined with
refrigeration.
If the operating temperature is set below 30 C, a cryogenic range

of ethane recovery is encountered.

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