Air separation

Abstract: Air is separated into oxygen and nitrogen in rectification columns 28 and 30. A stream of nitrogen is withdrawn from the top of the column 30 through an outlet 54, is warmed to about ambient temperature by passage through heat exchangers 34, 46 and 24, and is then heated at a pressure in the range 2 to 7 atmospheres absolute by heat exchange in heat exchanger 56 with a hot stream of fluid initially at a temperature of less than 600°C without said fluid undergoing a change of phase. The resulting hot nitrogen is then expanded in turbine 58 with the performance of external work, e.g. the generation of electricity.

Description: This invention relates to air separation. It is known to be advantageous in certain circumstances to recover work from nitrogen produced in a cryogenic air separation plant. Most proposals for so doing are dependent upon the presence of a gas turbine employed to drive an alternator to generate electricity. See for example US-A-2 520 862 and US-A-3 371 495 in which compressed nitrogen is employed to control the pressure in the combustion chamber associated with the gas turbine, and energy is then recovered in the expansion of the gas. Accordingly, most if not all of the energy requirements of the air separation process can be met thereby. Frequently, however, a suitable gas turbine is not available on site to enable such processes to be used. In US-A-3 950 957 (corresponding to UK-A-1 455 960) there is described an alternative process for recovering work from the nitrogen product. This method involves a thermodynamic linking of the air separation plant with a steam generator. The nitrogen product is heat exchanged first with compressed air and then with flue gases intended for generation of the steam in the steam generator so as to impart high grade heat to the nitrogen product and thus heat it to a temperature greater than 600°C. The nitrogen is then work expanded to convert most of its required heat energy into the mechanical energy. Steam is generated by the flue gases downstream of their heat exchange with the nitrogen product. Residual, available heat in the work-expanded nitrogen product is used to reheat fluids re-entering the steam generator. The process described in UK patent specification 1 455 960 has a number of drawbacks. First, the use of high-grade heat to raise steam is relatively inefficient. Second, there is a significant cost involved in steam raising. Third, although there is the potential to use work recovered from the air separation process to generate large excess quantities of electricity for export, the process according to UK 1 455 960 does not avail itself of this possibility. Fourth, suitable steam generation plant may frequently not be available on the site of the air separation plant. Fifth, there may not be readily available a suitable source of high grade heat, and if there is, there may be more efficient ways of using it. The present invention relates to a method and apparatus for recovering work from a nitrogen stream, in which the nitrogen is pre-heated by heat exchange with a fluid stream embodying low grade heat (ie at a temperature of 600°C or less) typically generated from a chemical or other process in which the oxygen product of the air separation partakes. According to the present invention there is provided a process for separating air and recovering work from a waste gas embodying low grade heat, comprising the steps of separating air by rectification into oxygen and nitrogen; taking a stream of nitrogen from a rectification column in which the separation is performed; heating the stream of nitrogen at a pressure in the range of 203 to 709 kPa (2 to 7 atmospheres absolute) by heat exchange with a stream of fluid which enters at a temperature of less than 600°C into said heat exchange and which does not undergo a change of phase during said heat exchange, there being no compression of the nitrogen intermediate said rectification column and said heat exchange; and without any intervening step of further heating the heated nitrogen stream expanding the heated nitrogen stream in a turbine with the performance of external work, wherein the said fluid comprises said waste gas or a heat transfer medium that has been heat exchanged without change of phase with a stream of said waste gas. The external work performed in the method according to the invention may be the compression of an air stream entering or product stream leaving the air separation process but is preferably the generation of electricity for another process then the air separation or for export. The stream of fluid is preferably initially (ie before heat exchange) at a temperature in the range 200-400°C, and more preferably in the range 300-400°C. It is not usually possible to recover work efficiently from such streams and therefore the invention is advantageous in providing a unique and relatively efficient way of recovering work. Typically, the stream at a temperature 600°C or less is a waste gas stream from an industrial or chemical process in which said oxygen is used or alternatively heat may be available from an industrial process where there is a requirement to cool a process stream. The heat exchange is preferably performed in a direct gas-to-gas heat exchanger. Another alternative is to use the fluid stream from an industrial or chemical process to raise the temperature of a heat transfer medium (without changing its state) and use the medium to heat the nitrogen by direct heat exchange, without the medium change state. The medium may be a heat transfer oil. The optimum pressure at which the nitrogen is brought into heat exchange relationship with the fluid stream depends on the temperature of the fluid stream. The higher the temperature of the fluid stream, the higher the preferred nitrogen stream pressure, so that at about 400°C the preferred nitrogen pressure is approximately 405 kPa (4 atmospheres). Typically, the nitrogen stream is employed at a pressure in the range 203 to 507 kPa (2 to 5 atmospheres), particularly if the fluid stream is initially at a temperature in the range 200 to 400°C. If the air is separated in a double column of the conventional kind as described in Ruhemann's "Separation of Gases", Oxford University Press, 1945, the lower pressure column may advantageously be operated at a pressure of from 3 to 4 atmospheres absolute, with a resultant increase in efficiency in comparison with conventional operation of such column at a pressure between 1 and 2 atmospheres absolute. Upstream of being heat exchanged with the fluid stream, the nitrogen stream is typically used to regenerate apparatus used to remove water vapour and other relatively non-volatile components from the air for separation, be such apparatus of the reverse in heat exchange kind or of the adsorbent kind. The oxygen separated from the air may typically be used in a chemical, metallurgical or other industrial process from which the waste heat is generated. The process according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic circuit diagram of a combined air separation plant - chemical or metallurgical plant - electrical power generator; and Figure 2 is a schematic circuit diagram of an air separation plant for use in the apparatus shown in Figure 1. Air is separated in an air separation plant 2 to provide oxygen and nitrogen products which need not be pure. The oxygen product is supplied to a plant 4 in which it is used to take part in a chemical or metallurgical reaction. The plant 4 produces amongst other products a waste gas stream 6 at a temperature of 395°C. This gas stream is then brought into countercurrent heat exchange in heat exchanger 8 with a nitrogen product stream from the air separation plant 2. The nitrogen product stream typically enters the heat exchanger 8 at a pressure of four atmospheres absolute (406 kPa). The resulting nitrogen stream is thereby heated to a temperature of about 350°C and then enters an expansion turbine 10 where it is expanded with the performance of external work. Typically the turbine is used to drive an alternator 12 used to generate electrical power, which may be employed in the air separation plant 2 or the chemical/metallurgical plant 4. Alternatively, the shaft may be directly coupled to compressors used in the air separation plant. The gas stream from the plant 4 after heat exchange with the nitrogen may typically be vented to the atmosphere through a stack (not shown). Referring to Figure 2 of the drawings, air is supplied at a chosen pressure from the outlet of an air compressor 20. The air is passed through a purification apparatus 22 effective to remove water vapour and carbon dioxide from the compressed air. The apparatus 22 is of the kind which employs beds of adsorbent to adsorb water vapour and carbon dioxide from the incoming air. The beds may be operated out of sequence with one another such that while one bed is being used to purify air the other is being regenerated, typically by means of a stream of nitrogen. The purified air stream is then divided into major and minor streams. The major stream passes through a heat exchanger 24 in which its temperature is reduced to a level suitable for the separation of the air by cryogenic rectification. Typically therefore the major air stream is cooled to is saturation temperature at the prevailing pressure. The major air stream is then introduced through an inlet 26 into a higher pressure rectification column 28 in which it is separated into oxygen-enriched and nitrogen fractions. The higher pressure rectification column forms part of a double column arrangement. The other column of the double column arrangement is a lower pressure rectification column 30. Both rectification columns 28 and 30 contain liquid vapour contact trays and associated downcomers (or other means) whereby a descending liquid phase is brought into intimate contact with an ascending vapour phase such that mass transfer occurs between the two phases. The descending liquid phase becomes progressively richer in oxygen and the ascending vapour phase progressively richer in nitrogen. Typically, the higher pressure rectification column 28 operates at a pressure substantially the same as that to which the incoming air is compressed. The column 28 is preferably operated so as to give a substantially pure nitrogen fraction at its top but an oxygen fraction at its bottom which still contains a substantial proportion of nitrogen. The columns 28 and 30 are linked together by a condenser-reboiler 32. The condenser-reboiler 32 receives nitrogen vapour from the top of the higher pressure column 28 and condenses it by heat exchange with boiling liquid oxygen in the column 30. The resulting condensate is returned to the higher pressure column 28. Part of the condensate provides reflux for the column 28 while the remainder is collected, sub-cooled in a heat exchanger 34 and passed into the top of the lower pressure column 30 through an expansion valve 36 and thereby provides reflux for the column 30. The lower pressure rectification column 30 operates at a pressure lower than that of the column 28 and receives oxygen-nitrogen mixture for separation from two sources. The first source is the minor air stream formed by dividing the stream of air leaving the purification apparatus 22. The minor air stream upstream of its introduction into the column 30 is first compressed in a compressor 38, is then cooled to a temperature of about 200K in the heat exchanger 24, is withdrawn from the heat exchanger 24 and is expanded in an expansion turbine 40 to the operating pressure of the column 30, thereby providing refrigeration for the process. This air stream is then introduced into the column 30 through inlet 42. If desired, the expansion turbine 40 may be employed to drive the compressor 38, or alternatively the two machines, namely the compressor 38 and the turbine 40, may be independent of one another. The independent arrangement is often preferred since it enables the outlet pressure of both machines to be set independently of one another. The second source of oxygen-nitrogen mixture for separation in the column 30 is a liquid stream of oxygen-enriched fraction taken from the bottom of the higher pressure column 50. This stream is withdrawn through an outlet 44, is sub-cooled in a heat exchanger 46, and is then passed through a Joule-Thomson valve 48 and flows into the column 30 at an intermediate level thereof. The apparatus shown in the drawing produces three product streams. The first is a gaseous oxygen product stream which is withdrawn from the bottom of the lower pressure column 30 through an outlet 48. This stream is then warmed to at or near ambient temperature in the heat exchanger 24 by countercurrent heat exchange with the incoming air. The oxygen may for example be used in a gasification, steel making or partial oxidation plant and may, if desired, be compressed in a compressor (not shown) to raise it to a desired operating pressure. Two nitrogen product streams are additionally taken. The first nitrogen product stream is taken as vapour from the nitrogen-enriched fraction (typically substantially pure nitrogen) collecting at the top of the column 28. This nitrogen stream is withdrawn through an outlet 52 and is warmed to approximately ambient temperature by countercurrent heat exchange with the air stream in the heat exchanger 24. The other nitrogen product stream is taken directly from the top of the lower pressure column 30 through an outlet 54. This nitrogen stream flows through the heat exchanger 34 countercurrently to the liquid nitrogen stream withdrawn from the higher pressure column and effects the sub-cooling of this stream. The nitrogen product stream then flows through the heat exchanger 46 countercurrently to the liquid stream of oxygen-enriched fraction and effects the sub-cooling of this liquid stream. The nitrogen stream taken from the top of the column 30 then flows through the heat exchanger 24 countercurrently to the major air stream and is thus warmed to approximately ambient temperature. This nitrogen stream is at least in part heat exchanged in a heat exchanger 56 with a fluid stream embodying low grade heat. The resultant hot nitrogen stream is then expanded in a turbine 58 which is used to drive an alternator 60. If desired, some of the nitrogen product stream from the lower pressure column may be used to purge the adsorbent beds of water vapour and carbon dioxide in the purification apparatus 22. Such use of nitrogen, which is typically pre-heated (by means not shown) is well known in the art. The resultant impurity-laden nitrogen may if desired be recombined with the nitrogen product stream upstream of the heat exchanger 56. In a typical operation of the apparatus shown in Figure 2, the column 28 may operate at about 1280 kPa (12.8 bar) and the column 30 at about 420 kPa (4.2 bar). Accordingly the compressor 18 compresses the air to about 1300 kPa (13.0 bar) and the compressor 38 has an outlet pressure of about 1820 kPa (18.2 bar). Operation of the plan under these conditions to give 30,000 m³/hr of oxygen at 8 bar and 95% purity and 10,000 m³/hr of nitrogen from the column 28 at 1000 kPa (10 bar) consumes the following power: However, assuming that 10.4 MW of waste heat are available to the heat exchanger 56 from a fluid stream at 350°C, then 6.7 MW may be recovered from the turbine 58, leaving the net power consumption at 8.7 MW. This net power consumption compares favourably with operation of comparable plants to produce the same oxygen and nitrogen products in which: (A) the column 28 is operated at about 600 kPa (6 bar) and the column 30 at about 130 kPa (1.3 bar); or (B) the column 28 is operated at about 600 kPa (6 bar) and the column 30 at about 130 kPa (1.3 bar) and no waste heat is recovered; (C) the column 28 is operated at about 600 kPa (6 bar) and the column 30 at about 130 kPa (1.3 bar) and there is no heating of the nitrogen stream. Instead the waste heat stream is used to raise steam which is then expanded in a steam turbine; (D) the column 28 is operated at about 1280 kPa (12.8 bar) and the column 30 at about 420 kPa (4.2 bar). No waste heat is transferred to the nitrogen stream, which is expanded to atmospheric pressure from ambient temperature; or (E) the plant is operated as in paragraph D above and waste heat is used to raise steam which is expanded in a steam turbine to recover additional work. The comparative net power consumptions are shown in the Table below in which all quantities are MegaWatts (MW). It can thus be appreciated that when work is recovered from nitrogen at an elevated pressure by a process comprising heat exchange if the nitrogen with a fluid stream initially at a temperature of 600°C or less which does not change its state during the heat exchange, followed by turbine expansion of the resultant hot nitrogen stream, there is a net power saving over any alternative comparable process.

Claims: A process for separating air and recovering work from a waste gas embodying low grade heat, comprising the steps of separating air by rectification into oxygen and nitrogen; taking a stream of nitrogen from a rectification column (30) in which the separation is performed; heating the stream of nitrogen at a pressure in the range of 203 to 709 kPa (2 to 7 atmospheres absolute) by heat exchange with a stream of fluid which enters at a temperature of less than 600°C into said heat exchange and which does not undergo a change of phase during said heat exchange, there being no compression of the nitrogen intermediate said rectification column and said heat exchange; and without any intervening step of further heating the heated nitrogen stream expanding the heated nitrogen stream in a turbine (58) with the performance of external work, wherein the said fluid comprises said waste gas or a heat transfer medium that has been heat exchanged without change of phase with a stream of said waste gas. A process as claimed in claim 1, in which the external work is the production of electricity. A process as claimed in claim 1 or 2, in which the said stream of fluid is at a temperature in the range 200 to 400°C on entering into heat exchange with the stream of nitrogen. A process according to claim 3, in which the nitrogen stream is at a pressure from 203 to 507 kPa (2 to 5 atmospheres absolute). A process according to any one of the preceding claims, in which the said waste gas is from a chemical or metallurgical process. A process according to claim 5, in which said oxygen is used in said chemical or metallurgical process. A process according to any one of the preceding claims, in which the said medium is a heat transfer oil. A process according to any one of the preceding claims, in which after said expansion the nitrogen is vented to the atmosphere. A process according to any one of the preceding claims, in which the nitrogen stream is warmed to about ambient temperature intermediate said rectification column and its heat exchange with said fluid stream. A process according to any one of the preceding claims, in which the rectification column is the lower pressure column of a double column arrangement.

Inventors: RATHBONE THOMAS, RATHBONE, THOMAS
Applicants: BOC GROUP PLC, THE BOC GROUP PLC
Application year: 1990
Main classification: F25J3
IPC codes: F01D15, F01K25, F25J3