Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-02-16
2001-06-12
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S650000
Reexamination Certificate
active
06244072
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of and plant for air separation. It is particularly concerned with such a method and plant in which the air is separated into an oxygen product and a nitrogen product, and part of the nitrogen product is supplied at an elevated pressure to a gas turbine.
GB-A-2028 991 relates to such a method and plant. A double rectification column is employed to separate the air. (A double rectification column has a higher pressure rectification column, a lower pressure rectification column and a condenser-reboiler placing an upper, usually a top, region of the higher pressure rectification column, in heat exchange relationship with a region, usually a bottom region, of the lower pressure rectification column.) The air is rectified in the higher pressure rectification column, to form an oxygen-enriched liquid fraction and a first vaporous nitrogen fraction. A stream of the oxygen-enriched liquid fraction is withdrawn from the higher pressure rectification column and is used to form a feed stream to the lower pressure rectification column so as to form an oxygen product fraction and a second vaporous nitrogen fraction. At least one stream of a nitrogen product is taken from the double rectification column. A part of the nitrogen product is raised in pressure and is introduced into a gas turbine comprising an air compressor, a combustion chamber which has a first inlet communicating with the air compressor and a second inlet communicating with a source of fuel, and an expander communicating with the combustion chamber for expanding the hot gaseous products of the combustion of the fuel. The nitrogen is introduced into the combustion chamber or the expander normally for the purpose of reducing emissions of oxides of nitrogen in the exhaust of the expander. The work done by the expander is typically used to generate electrical power.
Because the combustion chamber of the gas turbine normally operates at a high pressure, typically in the range of 10 to 20 bar, GB-A-2028991 discloses that downstream of being warmed to ambient temperature a stream of the second vaporous nitrogen fraction is compressed to the necessary high pressure upstream of its being introduced into the gas turbine. The nitrogen feed to the gas turbine is normally formed exclusively of the second vaporous nitrogen fraction, that is the nitrogen fraction separated in the lower pressure rectification column. In order to reduce the work that has to be done in compressing this nitrogen, GB-A-2 028 991 recommends operating the lower pressure rectification column not at its normal pressure in the range of 1 to 2 bar (absolute), but instead at a higher pressure typically in the range of 3 to 5 bar. Several disadvantages arise. First, the higher pressure rectification column now has to be operated at a pressure in the order of 8 to 12 bars rather than at a conventional pressure in the range of 5-6 bar. Therefore, more work needs to be performed in compressing the incoming air for separation, and more compression equipment is required, than when the higher pressure rectification column is operated at its conventional pressure. Second, increasing the operating pressures of the higher and lower pressure rectification columns reduces the volatility of nitrogen relative to oxygen. An increase in the number of distillation stages required to effect the separation results. Third, the amount of co-produced nitrogen at a pressure above atmospheric is fixed by the oxygen
itrogen ratio in the feed air. It is very often the case that the amount of co-produced elevated pressure nitrogen is in excess of the requirements for control of NO
x
, emissions. There are therefore penalties in terms of thermodynamic efficiency to mixing all the nitrogen with the fuel gas.
It is an aim of the present invention to make it possible to provide a method and plant which make it possible to reduce these disadvantages, but not at the cost of significant additional work of compression of nitrogen compared with when the low pressure column is operated at the optimum pressure disclosed in GB-A-2 028 991 A.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of separating air into an oxygen product and a nitrogen product, wherein a part of the nitrogen product is supplied at an elevated pressure to a gas turbine, including the steps of introducing a first stream of air into the higher pressure rectification column of a double rectification column, rectifying the air therein to form an oxygen-enriched liquid fraction and a first vaporous nitrogen fraction, withdrawing a stream of the oxygen-enriched liquid fraction from the higher pressure rectification column and using the stream of the oxygen-enriched liquid fraction to form a feed stream to the lower pressure rectification column of the double rectification column, rectifying the said feed stream in the lower pressure rectification column so as to form an oxygen product fraction and a second vaporous nitrogen fraction, taking at least one stream of a nitrogen product from the double rectification column, and raising the pressure of the nitrogen product and introducing it into the gas turbine, wherein the stream of the oxygen-enriched liquid fraction is subjected upstream of the lower pressure rectification column to further separation so as to form an oxygen-containing fraction from which the said feed stream is taken and a third vaporous nitrogen fraction, a flow of the third vaporous nitrogen fraction is condensed and is used as reflux in the lower pressure rectification column, at least 60% by volume of that part of the nitrogen product that is introduced into the gas turbine is taken from the first nitrogen vapor fraction, and the lower pressure rectification column is operated at a pressure, at its top, of less than 2 bar absolute.
The invention also provides plant for the separation of air and the generation of power, including a double rectification column including a higher pressure rectification column and a lower pressure rectification column; a gas turbine having an inlet for product nitrogen communicating with the double rectification column for a stream of the oxygen-enriched liquid fraction so as to enable a feed stream to the lower pressure rectification column to be formed therefrom; an inlet to the lower pressure rectification column for the feed stream; a first outlet from the lower pressure rectification column for a first product nitrogen stream of a second vaporous nitrogen fraction separated in the lower pressure rectification column; and a second outlet from the lower pressure rectification column for a stream of an oxygen product fraction separated therein, wherein the plant additionally includes further separation means for forming a third vaporous nitrogen fraction and an oxygen-containing fraction from which the said feed stream is taken in operation of the plant, and a condenser having an inlet for a flow of the third vaporous nitrogen fraction and an outlet for nitrogen condensate communicating with the lower pressure rectification column, whereby in operation of the plant the nitrogen condensate provides reflux for the lower pressure rectification column, in that the communication between the gas turbine and the double rectification column is such that, in operation, at least 60% by volume of the nitrogen product flow from the double rectification column to the gas turbine is taken from the first vaporous nitrogen fraction and in that the lower pressure rectification column is arranged to be operated at a pressure at its top of less than two bar absolute.
Preferably at least 90% by volume of the part of the nitrogen product that is supplied to the gas turbine is taken from the first vaporous nitrogen fraction. More preferably all of that part of the nitrogen product is so taken. Thus, in this case, the communication between the gas turbine and the double rectification column is solely with that region of the higher pressure rectification column where the first vaporous nitrogen fraction is
Capossela Ronald
Cohen Joshua L.
Pace Salvatore P.
The BOC Group plc
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