Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-07-10
2003-10-28
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S927000
Reexamination Certificate
active
06637239
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of and apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product.
BACKGROUND OF THE INVENTION
It is known to extract natural gas from underground reservoirs. The natural gas often contains nitrogen. The nitrogen may be in part or totally derived from nitrogen which has been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. A feature of such operations is that the concentration of nitrogen in the natural gas tends to increase with the passage of time from about 5% by volume to about 60% by volume.
U.S. Pat. No. 4,415,345 discloses a process for rejecting the nitrogen from the methane in a double rectification column operating at cryogenic temperatures. A double rectification column comprises a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the top of the higher pressure rectification column in indirect heat exchange relationship with a region, usually the bottom, of the lower pressure rectification column. In the process according to U.S. Pat. No. 4,415,345 a stream of a mixture of nitrogen and methane is cooled at elevated pressure to a temperature suitable for its separation by rectification. A part of the feed gas is liquefied. The resulting gas mixture is separated by rectification in a lower pressure rectification column having a condenser-reboiler operatively associated with a bottom region thereof. The reboiling passages of the condenser-reboiler are heated by pressurised nitrogen. Typically, a part of the nitrogen is separated in a higher pressure rectification column and another part flows in a heat pump circuit to and from the top of the lower pressure rectification column. At low nitrogen levels in the feed gas, the higher pressure rectification column is by-passed and all the nitrogen for the condenser-reboiler flows in the heat pump circuit, which is separate from the main heat exchanger used to cool the feed gas stream. As the nitrogen content in the feed gas gradually increases so the higher pressure column is used to provide some of the nitrogen. Eventually, the heat pump circuit is closed and the higher pressure column produces all the nitrogen.
The higher pressure rectification column used in the process according to U.S. Pat. No. 4,415,345 is operated with a second condenser-reboiler at its bottom. As a consequence, there is a high vapour flow therethrough and it needs to be of a relatively large diameter even though it is operated at maximum capacity only at high nitrogen concentrations.
It is an aim of the present invention to provide a method and apparatus that keeps down the vapour traffic through the higher pressure rectification column and thereby makes it possible to keep down the diameter of this column.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, the mole fraction of nitrogen in the feed gas increasing over a period of time. The method comprises cooling the feed gas stream in a main heat exchanger, and rectifying the cooled feed gas stream in a first double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, expanding at least part of the feed gas stream into the higher pressure rectification column, partially liquefying the feed gas stream upstream of the first double rectification column, and operating a second double rectification column in parallel with the first double rectification column when the nitrogen mole fraction in the feed gas stream has increased to at least a predetermined value.
The invention also provides apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, the apparatus comprising a main heat exchanger for cooling the feed gas stream, a first double rectification column for rectifying the feed gas stream comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, and an expansion device downstream of the main heat exchanger communicating with the higher pressure rectification column, the expansion device being arranged so as, in use, to introduce a part of the feed gas stream into the higher pressure rectification column in liquid state, characterised in that there is a second double rectification column in parallel with the first double rectification column.
As the mole fraction of nitrogen in the feed gas mixture becomes greater with the passage of time, so the flow of product methane becomes less, and so less of the feed gas mixture is liquefied against the reduced flow of the product stream. In consequence, there is a tendency for an ever increasing proportion of the feed gas mixture to enter the higher pressure rectification column in vapour state with the passage of time. Thus, in its normal operating lifetime, which may well exceed ten years or more, and last until the reservoir which is the source of the feed gas mixture is effectively exhausted, the first double rectification column may have to cope with a very wide range of vapour loadings, posing considerable design problems.
The obvious solution to these problems would be to arrange for all the feed gas mixture to enter the higher pressure rectification column in vapour state. Therefore, changes in the composition of the feed gas mixture would not substantially affect the vapour loading of the higher pressure rectification column. However, one disadvantage of such a procedure, is that the vapour loading of the higher pressure rectification column would always be at a maximum.
The method and apparatus according to the invention make it possible, however, to reduce the effective range of vapour loadings that both rectification columns of the first double rectification column have to face during their operating lifetime by providing a second double rectification column in parallel with the first double rectification column, which second double rectification column can be brought into operation at a suitable nitrogen mole fraction. Accordingly, if the vapour loading on either column of the first double rectification column approaches a level at which its performance would become unsatisfactory, the second double rectification column may be used to share the load. Typically, the apparatus according to the present invention may be installed as a complete unit. Alternatively the second double rectification column may be retrofitted to the first double rectification column whenever the latter is approaching an operational stage at which either rectification column is overloaded with vapour. The latter alternative offers the advantage that the initial size, complexity and cost of the apparatus according to the invention can be kept down.
Preferably, the operating pressure of the higher pressure column of the first double rectification column is periodically increased. To this end, the lower pressure column of the first double rectification column preferably has a back pressure regulating valve associated with it which is operable to increase the pressure therein. Increasing the operating pressure of the lower pressure rectification column brings about a concomitant increase in the operating pressure of the higher pressure rectification column. Increasing the operating pressure of the higher pressure rectification column enables it to receive more vapour per unit time for operation at a given constant percentage of flood. In addition, increasing the operating pressure of the higher pressure rectification column tends to increase the propor
Doerrler William C.
Neida Philip H. Von
The BOC Group plc
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