Patent Application
20170211881
The present invention relates to a method and system for cryogenic air separation involving production of liquid products by using an integrated refrigeration system comprising a primary refrigeration circuit and an auxiliary refrigeration circuit. More particularly, the present invention relates to an auxiliary refrigeration circuit that can be easily tied-in to an existing cryogenic air separation plant and its existing refrigeration system.
Oxygen, nitrogen and argon are separated from air through cryogenic rectification in an air separation plant. Typically, gaseous and/or liquid products are produced for on-site customers or pipeline customers, with any excess products often converted to merchant liquid products for nearby customers. For some cryogenic air separation plants, the on-site or pipeline customer demand for gaseous products, such as gaseous oxygen or gaseous nitrogen, may decrease over time either on a long-term basis or perhaps on a more temporary or mid-term basis. To satisfy the lower gaseous product requirements, the cryogenic air separation plant may be operated so as to vent some of the unneeded gaseous product which is economically inefficient as such venting ultimately wastes the power/energy costs used to produce the vented gaseous products. Alternatively, the air separation plant may be operated in a turn-down mode which produces less gaseous product but at less than full plant capacity and separation efficiency. A third option is to adjust the product slate of the cryogenic air separation plant to produce more liquid products in lieu of the lowered gaseous product requirement.
There have been numerous prior art cryogenic air separation processes designed to address this third option of making additional liquid products to offset decreased requirements of gaseous products. See for example, U.S. Pat. Nos. 6,125,656; 6,666,048; 6,945,076; and 8,397,535; as well as United States Patent Application Publication Nos. 2010-0058805; 2013-0192301; 2007-0101763; and European Patent Publication EP1544559 A1. As seen in these prior art references, refrigeration must be supplied to offset ambient heat leakage, warm end heat exchange losses and to allow the extraction or production of the liquid products, including liquid oxygen, liquid nitrogen, or liquid argon from one or more air separation units. The conventional or main source of refrigeration for a cryogenic rectification plant is typically supplied by a turbine-based refrigeration system capable of expanding part of the feed air stream or a waste stream to generate a refrigeration stream that is then introduced into the main heat exchanger or the distillation column system of the cryogenic air separation plant. Supplemental refrigeration required to produce additional liquid products may be supplied with an additional turbine-based refrigeration source. Such additional turbine-based refrigeration systems involve additional capital costs and are often not optimized or fully integrated with the main source of refrigeration for a cryogenic air separation plant.
What is needed, is an improvement to these prior art supplemental liquid make solutions that allows the additional liquid make system to be configured as an add-on feature to the air separation plant that can be easily added to the cryogenic air separation plant/unit after initial plant construction. Such add-on supplemental liquid-make feature should be integrated with the main source of refrigeration for the cryogenic air separation plant and must also be both efficient and operationally flexible. In other words, the supplemental or auxiliary refrigeration system should be capable of and allow the plant to switch easily between a high liquid make cycle and the original high gaseous product make cycle. Finally, the add-on supplemental or auxiliary refrigeration system should be portable, and preferably skid-mounted.
The present invention may be characterized as a method of separating air in an air separation unit. The air separation unit preferably comprises a main heat exchanger configured to cool a compressed and purified feed air stream to a temperature suitable for the rectification and a distillation column system configured to rectify the compressed, purified and cooled air stream to produce at least one liquid product stream. In such air separation unit, the present method comprises the steps of: (a) compressing and purifying a feed air stream to produce the compressed and purified feed air stream; (b) diverting a first portion of the compressed and purified feed air stream to a first refrigeration circuit configured to produce a first cooled refrigeration stream; (c) diverting a second portion of the compressed and purified feed air stream to the main heat exchanger to cool the second portion of the compressed and purified feed air stream and wherein the cooled second portion of the compressed and purified feed air stream is subsequently directed to the higher pressure column of the distillation column system; (d) diverting a third portion of the compressed and purified feed air stream to a booster air compression circuit configured to produce a further compressed feed air stream and wherein part of the further compressed feed air stream is directed to the main heat exchanger where the further compressed feed air stream is cooled to produce a liquid air stream that is directed to the distillation column system; (e) diverting a fraction of the further compressed feed air stream from the booster air compression circuit to an auxiliary refrigeration circuit configured to produce a second refrigeration stream, the auxiliary refrigeration circuit comprising a second turbo-expander and an auxiliary heat exchanger; (f) diverting a fourth portion of the compressed and purified feed air stream to the auxiliary heat exchanger; (g) diverting part of the first refrigeration stream from the first refrigeration circuit to the auxiliary heat exchanger and warming the diverted portion of the first refrigeration stream in the auxiliary heat exchanger via indirect heat exchange with diverted fourth portion of the compressed and purified feed air stream; (h) directing the fourth portion of the compressed and purified feed air stream exiting auxiliary heat exchanger to distillation column system; (i) directing a remaining portion of the first refrigeration stream to a lower pressure column of the distillation column system to impart a first portion of the refrigeration required by the distillation column system; and (j) directing the cooled second refrigeration stream to the higher pressure column of the distillation column system to impart a second portion of the refrigeration required by the distillation column system.
The present invention may also be characterized as an air separation unit configured to produce at least one liquid product stream. Characterized as such, the air separation unit comprises: (i) an incoming air compression and purification train configured to produce a compressed and purified feed air stream; (ii) a primary refrigeration circuit having a first turbo-expander, the primary refrigeration circuit operatively coupled to the incoming air compression and purification train and configured to receive a first portion of the compressed and purified feed air stream and expand the first portion of the compressed and purified feed air stream in the first turbo-expander to produce a first cooled refrigeration stream; (iii) a main heat exchanger operatively coupled to the incoming air compression and purification train and configured to receive a second portion of the compressed and purified feed air stream and to cool the second portion of the compressed and purified feed stream to a temperature suitable for the rectification of the compressed and purified feed air stream; (iv) a booster air compression circuit operatively coupled to the incoming air compression and purification train and the main heat exchanger, the booster air compression circuit configured to receive a third portion of the compressed and purified feed air stream, further compress the third portion and direct the further compressed third portion to the main heat exchanger to produce a liquid air stream; (v) a second turbo-expander configured to receive a fraction of the further compressed third portion and expand the fraction of the further compressed third portion to produce a second refrigeration stream; (vi) an auxiliary heat exchanger operatively coupled to the incoming air compression and purification train, the booster air compression circuit and the primary refrigeration circuit, the auxiliary heat exchanger configured to receive a fourth portion of the compressed and purified feed air stream and cool the fourth portion of the compressed and purified feed air stream via indirect heat exchange with the second refrigeration stream and a diverted portion of the first refrigeration stream; and (vii) a distillation column system operatively coupled to the primary refrigeration circuit, the booster air compression circuit and the auxiliary heat exchanger, the distillation column system configured to rectifying some or all of the first refrigeration stream, the second refrigeration stream, the liquid air stream, and the cooled second portion of the compressed and purified feed air stream by a cryogenic rectification process to produce the at least one liquid product stream.
In some embodiments, the first refrigeration circuit may include a compressor for further compressing the first portion of the compressed and purified feed air stream; a cooling means such as an aftercooler and/or main heat exchanger configured to cool the further compressed first portion of the compressed and purified feed air stream; and a first turbo-expander disposed within the first refrigeration circuit and configured to expand the further compressed first portion of the compressed and purified feed air stream to produce the first refrigeration stream. Similarly, the auxiliary refrigeration circuit may also include an auxiliary compressor and cooling means.
Other embodiments contemplate diverting a partially cooled portion of the second refrigeration stream from the auxiliary refrigeration circuit to the first refrigeration circuit and combining the diverted portion with the first portion of the compressed and purified feed air stream in the first refrigeration circuit.
Finally, in some embodiments that employ a multi-stage compression system within the booster air compression circuit, the diversion of the fraction of the further compressed feed air stream to the auxiliary refrigeration circuit preferably includes further includes diverting one or more fractions of the third portion of the compressed and purified feed air stream from one or more interstage locations of the plurality of compression stages to the auxiliary refrigeration circuit. One or more flow control valves are disposed between the booster air compression circuit and the second turbo-expander in the auxiliary refrigeration circuit to control the flow of the diverted one or more fractions and the inlet pressure to the second turbo-expander in the auxiliary refrigeration circuit.
While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:
In reference to
In the incoming air purification and compression train or circuit, the incoming feed air is compressed in a multi-stage, intercooled, main air compressor arrangement to a pressure that can be between about 5 bar(a) and about 15 bar(a). This main air compressor arrangement may be an integrally geared compressor or a direct drive compressor arrangement. The compressed air feed is then purified in a pre-purification unit to remove high boiling contaminants from the incoming feed air. A pre-purification unit, as is well known in the art, typically contains beds of alumina and/or molecular sieve operating in accordance with a temperature and/or pressure swing adsorption cycle in which moisture and other impurities, such as carbon dioxide, water vapor and hydrocarbons, are adsorbed.
As described in more detail below, the compressed and purified feed air stream 12 is divided into a plurality of portions which are further compressed and/or cooled. The different portions of the compressed and purified air stream are then separated into oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality of distillation columns that comprise the distillation column system 50. Preferably, the distillation column system 50 may include thermally linked higher pressure column 54 and lower pressure column 56, as well as an optional argon rectification column 58.
Prior to such distillation however, portions of the compressed and purified feed air stream 12 may be further compressed in a booster air compression train or circuit 30 and/or cooled to temperatures suitable for rectification within a primary or main heat exchanger 40. The cooling is typically achieved using refrigeration from the various oxygen, nitrogen and/or argon streams produced by the air separation unit 10 as well as refrigeration generated by one or more refrigeration circuits often as a result of turbo-expansion of various air streams in an upper column turbine (UCT) arrangement, a lower column turbine (LCT) arrangement, and/or a warm recycle turbine (WRT) arrangement as known to persons skilled in the art.
Air Separation Unit with Primary and Auxiliary Refrigeration Circuits
Turning now to
A second portion 15 of the compressed and purified feed air stream is directed or diverted to the main heat exchanger 40 to cool this portion of the compressed and purified feed air stream. The resulting cooled second portion 42 of the compressed and purified feed air stream is then directed to the higher pressure column 54 of the distillation column system 50 as generally known in the art and practiced in many cryogenic air separation units.
In addition, a third portion 17 of the compressed and purified feed air stream is diverted to a booster air compression circuit 30 configured to produce a further compressed, high pressure feed air stream 32. As illustrated, the booster air compression circuit 30 employs a booster air compressor arrangement 33 having a plurality of compression stages with intercoolers and aftercoolers 31 and forms a high pressure air stream 32 that is fed to the main heat exchanger 40. The high pressure air stream forms a liquid phase or a dense fluid if its pressure exceeds the critical pressure after cooling in the main heat exchanger. This liquid air stream 34 is then split into two portions 35, 36, with a first portion 35 being directed through an expansion valve 37 and into the higher pressure column 54 of the distillation column system 50 and a second portion 36 is expanded through another expansion valve 38 and introduced into the lower pressure column 56 of distillation column system 50.
As seen in
The present embodiment also shows a fourth portion 19 of the compressed and purified feed air stream that may also be diverted from the incoming air purification and compression circuit (not shown) as a carrier fluid to the auxiliary heat exchanger 65 where it is cooled and subsequently directed to the higher pressure column 54 of the distillation column system 50 so as to capture the auxiliary refrigeration. As illustrated, this cooled fourth portion 69 of the compressed and purified feed air stream may be combined with the warmed second refrigeration stream 66 and/or the cooled second portion 42 of the compressed and purified feed air stream exiting the main heat exchanger 40 with the resulting combined stream 68 then directed to the higher pressure column 54.
In a preferred embodiment, the first portion of the compressed and purified feed air stream directed to the primary refrigeration circuit represents roughly 8% to 20% of the incoming feed air stream. Of this first portion, 0% to 12% of the incoming feed air stream is diverted as the second portion to the auxiliary heat exchanger to balance the temperatures in the auxiliary heat exchanger. Varying the amount of diverted air from the first refrigeration circuit to the auxiliary refrigeration circuit enables the air separation unit to readily switch between a high gaseous product make cycle and a high liquid product make cycle.
The third portion of the compressed and purified feed air stream represents roughly 25% to 32% of the incoming feed air stream with roughly 5% to 10% of the incoming feed air stream being diverted to the auxiliary refrigeration circuit.
The second portion and fourth portion of the compressed and purified feed air stream combined represents the remainder roughly of the incoming feed air stream 48% to 67% of the incoming feed air stream. The exact split between the second portion and fourth portion of the compressed and purified feed air stream depends on the heat exchange duties in the main heat exchanger and auxiliary heat exchanger.
The main heat exchanger 40 and auxiliary heat exchanger 65 are preferably a brazed aluminum plate-fin type heat exchanger. Such heat exchangers are advantageous due to their compact design, high heat transfer rates and their ability to process multiple streams. They are manufactured as fully brazed and welded pressure vessels. The brazing operation involves stacking corrugated fins, parting sheets and end bars to form a core matrix. The matrix is placed in a vacuum brazing oven where it is heated and held at brazing temperature in a clean vacuum environment. For small plants, a heat exchanger comprising a single core may be sufficient. For higher flows, a heat exchanger may be constructed from several cores which may be connected in parallel or series.
The turbo-expanders 26 and 64 are preferably linked with booster air compressors 24 and 63 respectively, either directly or by appropriate gearing. Although not shown, the turbo-expanders may also to be connected or operatively coupled to a generator. Such generator loaded turbo-expander arrangement allows the speed of the turbo-expander to be maintained constant even at very high or low loads. This arrangement is desirable in some applications because the speed of the turbo-expander would remain generally constant at the ideal efficiency across the entire operating envelope. In such arrangements, the generator load may be connected to the turbo-expander by means of a high speed generator. Alternatively, the generator load may be connected to the turbo-expander by means of a high speed coupling connected to an internal or external gearbox and with a low speed coupling from the gearbox to the generator.
The distillation column system 50 preferably includes a thermally linked higher pressure column 54 and lower pressure column 56 as well as an optional argon rectification column 58. Within the columns, vapor and liquid are counter-currently contacted in order to affect a gas/liquid mass-transfer based separation of the respective feed streams. Such columns will preferably employ structured packing or trays or combinations thereof. The higher pressure column 54 typically operates in the range from between about 20 bar(a) to about 60 bar(a) whereas the lower pressure column 56 typically operates at pressures between about 1.1 bar(a) to about 1.5 bar(a).
As indicated above, the higher pressure column 54 and the lower pressure column 56 are linked in a heat transfer relationship such that a nitrogen-rich vapor column overhead, extracted from the top of higher pressure column as a stream 71, is condensed within a main condenser-reboiler 55 located in the base of lower pressure column 56 against boiling an oxygen-rich liquid column bottoms 72. The boiling of oxygen-rich liquid column bottoms 72 initiates the formation of an ascending vapor phase within lower pressure column 56. The condensation produces a liquid nitrogen containing stream 73 that is divided into streams 74 and 75 that reflux the higher pressure column 54 and the lower pressure column 56, respectively to initiate the formation of descending liquid phases in such columns. If liquid nitrogen product is required, stream 76 may also be recovered.
Streams 34, 66, and 69 are introduced into the higher pressure column 54 along with the expanded liquid air stream 39 for rectification by contacting an ascending vapor phase of such mixture within a plurality of mass transfer contacting elements with a descending liquid phase that is initiated by reflux stream 74. This produces a crude liquid oxygen column bottoms 77, also known as kettle liquid and the nitrogen-rich column overhead 78. A stream 79 representing a portion of the nitrogen-rich column overhead 78 may be directed to the main heat exchanger 40 to provide refrigeration to the feed air streams. In addition, a stream 101 of the crude liquid oxygen column bottoms 77 may be directed to the argon column 58 to as a reflux to aid in the recovery of argon product 93. Alternatively, although not shown, a stream of the crude liquid oxygen column bottoms may be expanded in an expansion valve to the pressure at or near that of the lower pressure column and introduced into the lower pressure column for further rectification.
Lower pressure column 56 is also provided with a plurality of mass transfer contacting elements that can be trays or structured packing or random packing or other known elements in the art of cryogenic air separation. As stated previously, the separation produces an oxygen-rich liquid 80 and a nitrogen-rich vapor column overhead 82 that is extracted as a nitrogen product stream 84. Additionally, a waste stream 85 is also extracted to control the purity of nitrogen product stream 84. Both nitrogen product stream 84 and waste stream 85 are passed through a subcooling unit 90 designed to subcool the reflux stream 75. A portion of the reflux stream may optionally be taken as a liquid product stream 76 and the remaining portion (shown as stream 75B) may be introduced into lower pressure column 56 after passing through expansion valve 99.
After passage through subcooling unit 90, nitrogen vapor product stream 84 and waste stream 85 are fully warmed within main heat exchanger 40 to produce a warmed nitrogen product stream 94 and a warmed waste stream 95. Although not shown, the warmed waste stream 95 may be used to regenerate the adsorbents within pre-purification unit. In addition, an oxygen-rich liquid stream 80 is extracted from the oxygen-rich liquid column bottoms 72 near the bottom of the lower pressure column 56. Oxygen-rich liquid stream 80 can be pumped by a pump 83 to form a pumped product stream as illustrated by pumped liquid oxygen stream 86. Part of the pumped liquid oxygen stream 86 can optionally be taken directly as a liquid oxygen product stream 88, with the remainder, namely stream 87, being directed to the main heat exchanger 40 where it is warmed and vaporized to produce a pressurized oxygen product stream 97. Although only one such stream is shown, there could be a plurality of such streams that are fed into the main heat exchanger 40. Pumped liquid oxygen stream 86 can be pressurized to above or below the critical pressure so that oxygen product stream 97 when discharged from main heat exchanger 40 will be a supercritical fluid. Alternatively, the pressurization of pumped liquid oxygen stream 86 could be lower to produce an oxygen product stream 97 in a vapor form.
Turning now to the embodiment illustrated in
Another difference between the embodiment shown in
Integrating the Auxiliary Refrigeration Circuit with the Air Separation Unit
As indicated above, air separation unit 10 is capable of producing liquid products, namely, nitrogen-rich liquid stream 76 and liquid oxygen product stream 88. In order to increase the production of such liquid products, additional refrigeration is supplied by an add-on or auxiliary refrigeration circuit. In the presently disclosed air separation unit or air separation plant, the add-on refrigeration circuit is the auxiliary refrigeration circuit 60 that is preferably configured to be added to or bolted on the cryogenic air separation unit 10 after initial plant construction. Thus, the design of the auxiliary refrigeration circuit 60 is tailored for such late add-on or retrofit application and the tie-in points to the cryogenic air separation unit 10 are minimized.
In the illustrated embodiments, there are four or five key tie-in points between the cryogenic air separation unit 1 and auxiliary or second refrigeration circuit 60. The first tie-in point 110 preferably occurs downstream of the main air compression train or circuit where the fourth portion 19 of the compressed and purified feed air stream 12 is diverted to the auxiliary or second refrigeration circuit, and more particularly, to the auxiliary heat exchanger 65. This first tie in point 110 is configured to provide the carrier fluid (i.e. compressed and purified air) to which the auxiliary refrigeration from the auxiliary refrigeration circuit 60 is provided.
The second tie-in point 120 is within the booster air compression circuit 30 and is configured to divert a fraction of the further compressed third portion of the compressed and purified stream as compressed stream s 62A, 62B to the auxiliary refrigeration circuit 60. This second tie in point 110 provides a working fluid (i.e. boosted compressed air) that is to be expanded to provide a portion of the auxiliary refrigeration from the auxiliary refrigeration circuit 60.
The third tie-in point 130 is located within the distillation column system 50 and is configured to return the cooled carrier fluid 69 (i.e. compressed and purified air) as well as the warmed working fluid 66 (i.e. fully warmed, expanded working fluid) to the higher pressure column 54.
The fourth tie-in point 140 is located within the first refrigeration circuit 20 and is configured to divert a portion 28 of the first refrigeration stream 22 to the auxiliary refrigeration circuit 60 where it provides further cooling or refrigeration to the carrier stream 19 via indirect heat exchange in the auxiliary heat exchanger 65.
A fifth tie in point 150 is also required in the embodiment shown in
Preferably, the supplemental or auxiliary refrigeration system is configured and constructed as a portable, skid-mounted refrigeration system that can be easily added to the cryogenic air separation plant/unit after initial plant construction in a manner that minimizes cold-box entry. The preferred skid-mounted supplemental or auxiliary refrigeration system would include: (i) one or more auxiliary compressors 63; (ii) the warm second turbo-expander 64; (iii) the auxiliary heat exchanger 65; (iv) associated piping to facilitate the above-identified four or five tie-in points; and (v) one or more control valves 67A, 67B, 67C, and 67D configured to control the air stream flows to the one or more auxiliary compressors 63, second turbo-expander 64, and auxiliary heat exchanger 65 as described above with reference to
By controlling the flow to the supplemental or auxiliary refrigeration circuit via the one or more flow control valves, the presently disclosed system can easily switch between a high gaseous product cycle—when the flow control valves are closed and a high liquid make cycle where the flow control valves are operated to produce an increased amount of refrigeration and associated liquid product make.
An advantage of the present system and method for providing auxiliary refrigeration to a cryogenic air separation plant is the ability to increase the amount of refrigeration and associated liquid product make in a cost-effective manner. The amount of refrigeration produced and amount of liquid make is adjusted by varying the warm turbine inlet pressure and flow in the supplemental or auxiliary refrigeration circuit. Adjustments to the warm turbine inlet pressure and flow are effected by selectively opening and/or closing the one or more flow control valves 67A, 67B, 67C, and 67D. The discharge flow from the warm second turbo-expander is passed through the auxiliary heat exchanger and then directed to the higher pressure column along with the main air (i.e. cooled second portion of the of the compressed and purified feed air stream) and the fourth portion of the of the compressed and purified feed air stream exiting the auxiliary heat exchanger.
An additional advantage presented by the present system and method is that by diverting a portion of the first refrigeration stream from the primary refrigeration circuit to the auxiliary refrigeration circuit and thus bypassing the lower pressure column separation, the gaseous oxygen product produced by the distillation column system is reduced but the argon recovery within the distillation column system can be maintained or possibly enhanced.
Also, diverting a portion of the first refrigeration stream to the auxiliary refrigeration circuit is preferably controlled to balance the temperatures in auxiliary heat exchanger and preserve recovery in the auxiliary booster-turbine arrangement. The flow and pressure ratio within the primary refrigeration circuit is maximized. In this fashion, the upper column turbine arrangement is used more as a heat pump to improve liquid making capability of the cryogenic air separation plant.
Although the present invention has been discussed with reference to preferred embodiments, as would occur to those skilled in the art that numerous changes and omissions can be made without departing from the spirit and scope of the present inventions as set forth in the appended claims.
