Method for the production of a densified liquid oxygen product

Abstract

A method for the liquefaction and densification of oxygen for use as a fuel in space vehicle launch applications that subcools and densifies a liquid oxygen stream to a temperature at or below 60 K using a liquid refrigerant that has an oxygen concentration in a range of 20% to 77% by mole fraction. The liquid refrigerant preferably comprises a mixture of liquid oxygen with liquid air or a mixture of liquid oxygen with liquid nitrogen with the liquid oxygen, liquid air, and / or liquid nitrogen sourced from a nearby air separation unit.

Description

METHOD FOR THE PRODUCTION OF A DENSIFIED LIQUID OXYGEN PRODUCTTechnical Field

[0001] The present invention relates to a method for the production of a densified liquid oxygen product for use in space vehicle applications, and more particularly, to a method for production of a densified, liquid oxygen stream using an oxygen-nitrogen mixture sourced from a nearby air separation unit as the refrigerant to subcool and densify the liquid oxygen.Background

[0002] Space vehicle launch applications require large quantities of liquid fuels and liquid oxygen to create the propulsion necessary to get the vehicle into space. The quantities of liquid oxygen are very large and often exceed 1000 mtpd and the liquid oxygen must be densified by deep sub-cooling to maximize the oxidant payload of the launch. Subcooling the liquid oxygen below its’ boiling point creates a denser oxidant which then requires smaller tanks to hold the same volume. The limit to the level of densification is solidification at the triple point of the material. Minimizing the volume and weight of the fuel / oxygen tanks is critically important to the viability of a rocket design and it is desired to fit all the necessary fuel / oxygen into as small a volume as possible.

[0003] The use of liquid nitrogen for densification of liquid oxygen in space vehicle launch applications is well established. Liquid nitrogen is typically employed to subcool and densify the liquid oxygen as the liquid oxygen is loaded onto a launch vehicle. However, since the freezing point of liquid nitrogen is roughly 63.1 K at atmospheric pressure, the subcooling and densification of liquid oxygen using liquid nitrogen as the refrigerant is limited by this characteristic value.

[0004] Moreover, the current approach to the design of a tonnage liquid co-product air separation unit or plant for dedicated supply of liquid oxygen and liquid nitrogen to space vehicle launch facilities involves liquefying essentially all of the oxygen molecules available in addition to appreciable fractions of liquid nitrogen. The liquid nitrogen to liquid oxygen product ratio (LIN / LOX) in excess of 0.40 are anticipated for such air separation unit. The liquid nitrogen demands from the air separation unit in addition to the liquid oxygen requirements presents a design challenge to produce an integrated ASU-Liquefier design.Production of liquid nitrogen product in appreciable quantities would impose a significant burden on conventional air separation units since liquid nitrogen is the primary reflux stream in most air separation cycles.

[0005] Challenges facing the space industry related to oxygen densification include reducing the operational costs, including the power costs as well as reducing the capital costs of oxygen densification. Depending on the size of the space vehicle, supply of the liquid oxygen to a launch platform at a launch facility typically requires in excess of 40 trailers of liquid oxygen to be trucked into the launch facility where the liquid oxygen is then densified. The use of so many liquid trailers for the delivery of the liquid oxygen is problematic from both a logistical and safety prospective. Most current oxidant densification processes conducted at space vehicle launch facilities also typically require more trailers of liquid nitrogen required to densify the liquid oxygen to be trucked in from external sources than trailers of liquid oxygen. This further increases the overall logistics burden of trucking in numerous truckloads of liquid nitrogen or other cryogenic liquids to the launch facility and presents significant challenges to production of both liquid oxygen and liquid nitrogen from nearby air separation units.

[0006] What is needed is improved methods of liquid oxygen densification that are commercially viable and can be supported by conventional air separation units.Summary

[0007] The present invention may be characterized as a method for production of a densified, liquid oxygen stream comprising the steps of: (a) producing a stream of liquid oxygen at a temperature between 91 K and 77 K and having an oxygen concentration of at least 95% by mole fraction; (b) producing a liquid refrigerant comprising a mixture of oxygen and nitrogen and wherein the liquid refrigerant has an oxygen concentration in a range of 20% to 77% by mole fraction, and more preferably an oxygen concentration in a range of 50% to 65% by mole fraction; and (c) densifying the stream of liquid oxygen to a temperature at or below 60 K via indirect heat exchange with the liquid refrigerant to yield the densified liquid oxygen stream and at least a cold, partially vaporized refrigerant stream. The liquid refrigerant preferably comprises a mixture of liquid oxygen with liquid air or a mixture of liquid oxygen with liquid nitrogen with the liquid oxygen, liquid air, and / or liquid nitrogen sourced from a nearby air separation unit. More specifically, the liquid oxygenand / or liquid nitrogen are produced via fractional distillation of a purified air stream in a distillation column system of the air separation unit.

[0008] In various embodiments of the present method, the liquid oxygen stream may be pumped to a pressure equal to or greater than about 1 bar(a) while the liquid refrigerant mixture may be expanded in an expansion valve to yield a liquid refrigerant having a pressure less than or equal to about 1 bar(a). After subcooling and densifying the liquid oxygen stream, the cold, partially vaporized refrigerant stream is then vacuum pumped to a pressure equal to or greater than about 1 bar(a) and vented or otherwise recycled to other uses at the launch facility or within the air separation unit plant.Brief Description of the Drawings

[0009] It is believed that the claimed method will be better understood when taken in connection with the accompanying drawings in which:

[0010] Fig. 1 depicts a typical oxygen-nitrogen phase diagram depicting liquid and solid phases of an oxygen and nitrogen mixture;

[0011] Fig. 2 shows a schematic illustration of the present arrangement for the production of a densified, liquid oxygen stream for use in a space vehicle launch from a launch facility using a mixture of liquid oxygen and liquid air sourced from a nearby air separation unit as the refrigerant needed to subcool and densify the liquid oxygen at the launch facility; and

[0012] Fig. 3 shows a schematic of an alternate embodiment of an arrangement for the production of a densified, liquid oxygen stream for use in a space vehicle launch from a launch facility using a mixture of liquid oxygen and liquid nitrogen sourced from a nearby air separation unit as the refrigerant to subcool and densify the liquid oxygen at the launch facility.Detailed Description

[0013] The present method for production of a densified, liquid oxygen stream for space vehicle applications subcools the liquid oxygen product stream with a refrigerant comprised of a mixture of oxygen and nitrogen which enables the subcooling (and densification) of the liquid oxygen product to a temperatures close to its freezing point.

[0014] A consideration of the oxygen-nitrogen phase diagram shown in Fig. 1 indicates an appreciably lower freezing point of when using the eutectic blend of oxygen and nitrogen (e.g. 0.23 / 0.77 N2 / O2 mole fraction) compared to use of either liquid nitrogen or liquid air. As a consequence, a liquid mixture of oxygen and nitrogen having an oxygen concentration higher than liquid air is capable of subcooling liquid oxygen far closer to its freezing point. Thus, the use of an oxygen-nitrogen mixture will enable a further densification of the liquid oxygen which will drive down the cost of any given space vehicle launch mission. It is anticipated that the density of liquid oxygen can be increased by an additional 3% to 4% when using an oxygen-nitrogen mixture that has an oxygen concentration higher than liquid air and that has a freezing point below the freezing point of nitrogen.

[0015] By subcooling and densifying liquid oxygen with a customized oxygen-nitrogen mixture, the separation load upon the integrated air separation unit (ASU)-liquefaction plant used to separate air and produce the liquid oxygen and the liquid refrigerant is substantially reduced. This in turn, could save significant power and will facilitate higher recovery of oxygen and argon by the air separation unit.

[0016] The use of a customized oxygen-nitrogen mixture from an integrated ASU- liquefaction plant for use in the subcooling and densification of liquid oxygen for space vehicle launch applications is believed to be novel. Designing of an integrated ASU- liquefaction plant that produces liquid air and / or for customized oxygen-nitrogen mixtures can be tailored or scaled for the actual volumes of products needed for the space vehicle launch application which has the potential to save appreciable air separation liquefaction power and associated plant costs. For example, the power consumption of the air separation unit can be reduced by simply unloading the liquid nitrogen production.

[0017] Turning now to Fig. 2, there is shown a highly simplified depiction of the general processing scheme for the production of a densified, liquid oxidant to a launch facility for use in a space vehicle launch. As seen therein, a liquid oxygen 60 is produced by an air separation unit 10 located in proximity to a space vehicle launch facility 100. The liquid oxygen 60 is delivered or transported from one or more storage tanks 62 located at or near the air separation unit 10 to the space vehicle launch facility 100 via a carrier or transport means 63 such as cryogenic tanker vehicles, rail cars, or pipelines. The liquid oxygen 60 is preferably stored on-site at the space launch facility 100 in one or more storage tanks 64.

[0018] In addition, a liquid air product stream 40 or synthetic air stream, comprised of a mixture of oxygen and nitrogen from the air separation unit 10, may also be directed or transported from one or more storage tanks 42 located at or near the air separation unit 10 to the space vehicle launch facility 100 via a transport means 43 such as tanker vehicles, rail cars, or pipelines. The liquid air product stream 40 is also preferably stored on-site at the space launch facility 100 in one or more storage tanks 44.

[0019] When needed for a space vehicle launch, a portion of the liquid oxygen 65 is pumped via pump 66 to a desired pressure, preferably over 1 bar(a). The pumped liquid oxygen stream 65 is then subcooled or densified in a second heat exchanger 75 to a temperature at or below 60 K against an oxygen-rich liquid refrigerant 68. The oxygen-rich liquid refrigerant 68 preferably comprises a mixture of another portion of the liquid oxygen 67 from storage tank 64 with a liquid air stream 45 from storage tank 44. The oxygen-rich liquid refrigerant 68 preferably has an oxygen concentration in a range of 20% to 77% by mole fraction, and more preferably has an oxygen concentration in a range of 50% to 65% by mole fraction, making it an oxygen-rich liquid refrigerant.

[0020] The oxygen-rich liquid refrigerant 68 is expanded in expansion valve 69 to yield the liquid refrigerant 70 preferably having a pressure less than or equal to about 1 bar(a). The liquid refrigerant 70 is used to subcool and densify the liquid oxygen stream 65 in the second heat exchange device 75. The resulting densified liquid oxygen 80 is loaded onto the space vehicle for use as a fuel oxidant while the resulting cold vaporized refrigerant stream 76 is pulled through the vacuum pump 77 and repressurized to a pressure of about 1.2 bar(a). The repressurized cold vaporized refrigerant stream 78 may be vented or recycled back to other uses of clean dry air within the launch facility or the air separation unit plant.

[0021] The air separation unit 10 shown in Fig. 2 is equipped with the conventional warm-end processing equipment and cold-end processing equipment. The warm-end processing equipment within the air separation unit 10 typically comprises a main feed air compression train 15 with aftercoolers 16 and air pre-purification units (not shown) configured to substantially compress and pre-purify an incoming feed air stream 12 and yield a compressed and pre-purified air stream 17.

[0022] The cold-end processing equipment within the air separation unit 10 typically comprises a main or primary heat exchanger 23 and a distillation column system 30 that aretypically housed in one or more insulated cold boxes. The cold-end processing equipment within the air separation unit 10 is configured to cool a portion of the substantially compressed and purified air stream 17 and then fractionally distill or separate the cooled air stream 26 in the distillation column system to yield a plurality of products, including liquid oxygen and liquid nitrogen as well as other products including argon products.

[0023] The air separation unit 10 shown in Fig. 2 is also equipped with one or more turbine air circuits configured to provide refrigeration to the air separation cycle process. The supplemental or excess refrigeration is needed for the air separation cycle process as significant quantities of liquid air and liquid oxygen are taken as products and eventually transported to the launch facility.

[0024] In the illustrated embodiment, a diverted portion of the compressed and prepurified air stream 17, referred to as a turbine air stream 24, is partially cooled in the main heat exchanger 23 of the air separation unit 10. The partially cooled turbine air stream 19 is then expanded in a turbine-expander 22 to yield an exhaust stream 27. A first portion 28 of the exhaust stream is directed to the distillation column system 30 of the air separation unit 10 where it is separated into oxygen, nitrogen and argon. A second portion 29 of the exhaust stream may be diverted or returned to the cold end of the main heat exchanger 23 and used to help cool the various portions of the compressed and pre -purified air stream 17. The warmed second portion of the exhaust stream exits the warm end of the main heat exchanger 23 as warmed stream 33 and is then further expanded in turbine-expander 35 to yield a second exhaust stream 36. The second exhaust stream 36 is returned to an intermediate location of main heat exchanger 23 and also provides supplemental refrigeration to the air separation cycle by also cooling various portions of the compressed and pre-purified air stream 17.

[0025] The remainder portion of the compressed and pre-purified air stream 17 is the primary air stream 18 and is fully cooled in the main heat exchanger 23 of the air separation unit 10 to a point where it is substantially liquefied to yield a liquid air stream 25. A first portion of the liquid air stream 25 is taken as liquid air product stream 40, reduced in pressure and stored in one or more storage tanks 42 located at or near air separation unit 10. The liquid air product stream 40 is subsequently transported to the space vehicle launch facility 100 via a transport means 43 such as tanker vehicles or pipeline and preferably storedon-site at the space launch facility 100 in one or more additional storage tanks 44. Another portion of the liquid air stream 25 is maintained at higher pressures and this higher pressure liquid air stream 26 is directed to the distillation column system 30 of the air separation unit 10 where it is separated into other products, including liquid oxygen 60 (to be stored / transported to launch facility), liquid nitrogen 50 (to be stored in storage tank 52) and argon (not shown) as well as a gaseous nitrogen stream 32. In the illustrated embodiment, gaseous nitrogen stream is preferably warmed in main heat exchanger 23 with the resulting warmed nitrogen stream 34 discarded as waste nitrogen or available to be used, as needed.

[0026] The schematic and process flow diagram depicted in Fig. 3 is very similar to the schematic and process flow diagram of Fig. 2 described above and, for sake of brevity, much of the descriptions of the detailed arrangements will not be repeated. Rather, the following discussion will focus on the differences and additions depicted in the process flow diagram of Fig. 3 when compared to the process flow diagram depicted in Fig. 2.

[0027] The key differences between the embodiment of Fig. 3 and the embodiment of Fig. 2 is that in the embodiment of Fig. 3 sufficient quantities of liquid nitrogen are stored in tank 52 and transported via a transport means 53 such as trailers, rail cars, or pipeline to the launch facility 100 in lieu storage and transport of liquid air to the launch facility. At the launch facility 100, a stream of liquid nitrogen 55 originating from the air separation unit and stored in tank 54 at the launch facility together with a diverted stream of liquid oxygen 67 are combined to form the oxygen-rich liquid refrigerant 68. As with the previous embodiments, the oxygen-rich liquid refrigerant 68 preferably has an oxygen concentration in a range of 20% to 77% by mole fraction, and more preferably an oxygen concentration in a range of 50% to 65% by mole fraction. The oxygen-rich liquid refrigerant 68 is then expanded in expansion valve 69 to yield liquid refrigerant 70 at a pressure less than or equal to 1 bar(a).

[0028] Similar to the embodiment of Fig. 2, the liquid refrigerant 70 having a pressure less than or equal to about 1 bar(a) is used to subcool and densify the liquid oxygen stream 65 in the second heat exchange device 75. The resulting densified liquid oxygen 80 is loaded onto the space vehicle for use as a fuel oxidant while the resulting cold vaporized refrigerant stream 76 is pulled through the vacuum pump 77 and repressurized to a pressure of about 1.2 bar(a). The repressurized cold vaporized refrigerant stream 78 may be vented or recycled back to other uses within the launch facility or the air separation unit plant.Industrial Applicability

[0029] The most obvious technical advantages realized by the present method for production of a densified, liquid oxygen stream is with respect to the reduced separation load upon the air separation units that supply the liquid products to the space vehicle launch facility as well as achieving a colder and denser liquid oxygen product stream for purposes of maximizing the oxygen that can be loaded onto the space vehicle. Subcooling the liquid oxygen product stream with a refrigerant comprised of a mixture of oxygen and nitrogen, and more particularly with a mixture composed of 55% to 60% oxygen, with balance substantially nitrogen should enable the subcooling (and densification) of the liquid oxygen product to a temperature very close to its freezing point of 54.4 K. Such an oxygen-nitrogen mixture can be easily synthesized from the distillation columns and associated condenserreboilers of the nearby air separation unit or can be produced by mixing sources of liquid oxygen and liquid nitrogen from storage tanks located at or near the air separation plant or the space vehicle launch facility. It should be noted that smaller amounts of argon and rare gases can be present in the mixtures.

[0030] Alternatively, the use of liquid air as a starting point in the production of the customized oxygen-nitrogen mixture is a convenient means to achieve the desired oxygenrich liquid refrigerant. Simply adding a portion of the liquid oxygen produced by the distillation column system of the nearby air separation unit to the liquid air can yield the oxygen-enriched liquid refrigerant. Moreover, the use of liquid air as the starting fluid may be actually preferred given the fact that no additional separation load is imposed upon the distillation column system of the air separation unit.

[0031] While the present method for the production of a densified liquid oxygen product has been described with reference to one or more preferred embodiments, it is understood that numerous variations, additions, changes, and omissions can be made without departing from the spirit and scope of the present methods as set forth in the appended claims.

Claims

ClaimsWhat is claimed is:

1. A method for production of a densified liquid oxygen stream, the method comprising the steps of:(a) producing a stream of liquid oxygen at a temperature between 91 K and 77 K and having an oxygen concentration of at least 95% by mole fraction;(b) producing a liquid refrigerant comprising a mixture of oxygen and nitrogen and wherein the liquid refrigerant has an oxygen concentration in a range of 20% to 77% by mole fraction; and(c) densifying the stream of liquid oxygen to a temperature at or below 60 K via indirect heat exchange with the liquid refrigerant to yield the densified liquid oxygen stream and at least a cold, partially vaporized refrigerant stream.

2. The method for production of the densified liquid oxygen stream of claim 1, wherein the liquid refrigerant has the oxygen concentration in a range of 50% to 65% by mole fraction.

3. The method for production of the densified liquid oxygen stream of claim 1, wherein the step of producing the stream of liquid oxygen further comprises fractional distillation of a purified air stream in a distillation column system of an air separation unit.

4. The method for production of the densified liquid oxygen stream of claim 1, further comprising the step of pumping the liquid oxygen stream to a pressure equal to or greater than about 1 bar(a).

5. The method for production of the densified liquid oxygen stream of claim 3, wherein the step of producing a liquid refrigerant further comprises the steps of:(bl) producing liquid air from the air separation unit; and(b2) mixing a portion of the liquid air produced from the air separation unit with a portion of the liquid oxygen produced from the air separation unit to yield the liquid refrigerant.

6. The method for production of the densified liquid oxygen stream of claim 3, wherein the step of producing a liquid refrigerant further comprises the steps of:(bl) producing liquid nitrogen from the air separation unit; and(b2) mixing a portion of the liquid nitrogen produced from the air separation unit with a portion of the liquid oxygen produced from the air separation unit to yield the liquid refrigerant.

7. The method for production of the densified liquid oxygen stream of claim 1, wherein the liquid refrigerant is at a pressure less than or equal to about 1 bar(a) and the method further comprises the step of: (d) vacuum pumping the cold, partially vaporized refrigerant stream to a pressure equal to or greater than about 1 bar(a).

8. The method for production of the densified liquid oxygen stream of claim 5, wherein the step of producing a liquid refrigerant further comprises the steps of: (b3) expanding the liquid refrigerant to yield the liquid refrigerant having a pressure less than or equal to about 1 bar(a).

9. The method for production of the densified liquid oxygen stream of claim 6, wherein the step of producing a liquid refrigerant further comprises the steps of: (b3) expanding the liquid refrigerant to yield the liquid refrigerant having a pressure less than or equal to about 1 bar(a).

10. The method for production of the densified, liquid oxygen stream of claim 3, wherein a diverted portion of the purified air stream is partially cooled in a main heat exchanger of the air separation unit and expanded in a turbine to yield an exhaust stream that is directed to the distillation column system of the air separation unit.

11. The method for production of the densified, liquid oxygen stream of claim 10, wherein a diverted portion of the exhaust stream is warmed in the main heat exchanger of the air separation unit and then further expanded to yield a second exhaust stream that is directed to the main heat exchanger of the air separation unit to provide supplemental refrigeration.

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