As hydrocarbon resources become scarcer and more difficult to recover, secondary recovery operations are becoming more widespread. Such secondary recovery operations are commonly referred to as enhanced oil recovery (EOR) and enhanced gas recovery (EGR) operations. One such secondary recovery technique involves the injection of a gas which does not support combustion into a reservoir to raise reservoir pressure in order to remove hydrocarbons which cannot be removed from the reservoir by natural reservoir pressure. A commonly used gas for this process is nitrogen because it is relatively abundant and inexpensive and can be produced in large quantities at the reservoir site.
Over time, nitrogen injected at the reservoir will begin to be removed with the hydrocarbons. This requires the removal of the nitrogen from the hydrocarbon stream in order to satisfy minimum heat content requirements or maximum inert content requirements of the product fuel gas.
Conventional processes for removing nitrogen from natural gases, often termed nitrogen rejection processes, separate the reservoir stream into methane, nitrogen, and hydrocarbons having two or more carbon atoms which are often termed natural gas liquids. Conventional processes effect this separation by first separating the reservoir stream into a liquid stream containing primarily natural gas liquids and into a vapor stream containing primarily nitrogen and methane. The liquid stream is recovered and the vapor stream is separated cryogenically in one or more rectification columns into nitrogen and methane. When a single rectification column is used to make the cryogenic rectification, the column is often driven by a recirculating fluid heat pump. The fluid for this heat pump is generally methane since it can match the reboiler and condenser temperatures required with reasonable pressure levels.
One problem with such conventional processes is that some of the natural gas liquids are not separated out in the first separation but instead remain with and are recovered with the methane. This is disadvantageous since the natural gas liquids are more profitably employed in uses other than as fuel to which the methane is put. Thus it would be desirable to recover more natural gas liquids separate from methane than is possible by conventional nitrogen rejection processes.
Another problem with such conventional processes is that the use of methane as the heat pump fluid for the cryogenic nitrogen-methane separation column requires either the column operate at a relatively high pressure or else requires the heat pump circuit to operate at vacuum conditions on its low pressure side. Vacuum conditions in the methane circuit are undesirable first because a vacuum is inherently an unsafe operating condition because of possible air infiltration into the circuit, and second because vacuum conditions cause relatively high power inputs because the pumping energies required are substantial. However operation of the nitrogen-methane cryogenic distillation column at high pressure to avoid heat pump circuit vacuum conditions is disadvantageous because the higher pressure puts an operating penalty on the column in the form of increased separation stages, increased reflux liquid requirements, or some combination of the two. It would therefore be desirable to operate the nitrogen-methane distillation column at a lower pressure without requiring the heat pump circuit to operate under vacuum conditions.
It is therefore an object of this invention to provide an improved process for removing nitrogen from natural gases.
It is another object of this invention to provide an improved process for removing nitrogen from natural gases wherein a greater amount of natural gas liquids are recovered separate from methane than is possible by conventional nitrogen rejection processes.
It is a further object of this invention to provide an improved process for removing nitrogen from natural gas wherein a nitrogen-methane cryogenic separation column is operated at lower than conventional pressures while avoiding the need to operate a heat pump under vacuum conditions.