Insulated vessel for storing cold fluids and insulation method

Abstract

An insulated vessel for storing cryogenic liquids and other cold fluids features an inner tank and an outer jacket with spherical aerogel particles positioned in the insulation space formed there between. The insulation material is installed by fluidizing the spherical aerogel particles with dry gaseous nitrogen and subjecting them to a slight positive pressure. A vacuum is simultaneously pulled on the insulation space of the vessel through a filter by a vacuum pump so that the spherical aerogel particles are transferred into the insulation space. The insulation space is then evacuated by the vacuum pump.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from currently pending U.S. Provisional Patent Application Serial No. 60/308,629, filed Jul. 30, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to containers for storing cryogenic or other very cold fluids and, more particularly, to an improved insulated vessel for storing cryogenic or other very cold fluids and a method for insulating such vessels.

[0003] Cryogenic gases such as nitrogen, hydrogen, ethylene, argon and oxygen find use in a variety of industrial and medical applications. These gases are frequently liquefied to reduce the amount of space required for storage or transport. Vessels for storing cryogenic liquids (which boil at −50° F. or less at atmospheric pressure), and other very cold liquids, are well known in the art. Such vessels may be mobile, as used by over-the-road trailers and rail cars, or stationary. The vessels must be very well insulated to prevent ambient heat from reaching the cryogenic liquids and vaporizing them. Vaporized cryogen causes an undesirable pressure increase within the vessel. In addition, if this pressure increase becomes great enough, venting of the cryogenic vapor may be necessary to reduce the pressure within the tank to an acceptable level. Such venting results in wasted cryogen which is also undesirable. This may cause air pollution problems as well.

[0004] Cryogenic vessels typically include three main elements. These elements include an inner tank, an outer tank or jacket and insulation. The inner tank contains the cryogenic liquid and is surrounded by the outer jacket. The insulation is positioned within the annular space between inner tank and the outer jacket. As a result, heat transfer between the outer jacket, which is warmed by ambient heat, and the inner tank is reduced due to the insulation. Evacuation of gas from the insulation space increases the insulating capability of the vessel as heat transfer by convection and conduction through the gases that otherwise would be present is eliminated. Examples of such vessels are presented in U.S. Pat. Nos. 3,166,511 to Matsch et al., 3,298,185 to Loudon and 4,718,239 to Nowobilski et al.

[0005] Materials that are used as insulation in such tanks must be compatible with vacuum conditions and extremely cold temperatures. Perlite is commonly used as insulation in cryogenic vessels. Perlite is a white particulate substance manufactured from a volcanic ore that occurs in widely distributed deposits. The rock is ground into a fine powder and then “popped” by heating it to temperatures above 800° C. Microscopic voids formed during the popping account for low density and, consequentially, low conductivity. Perlite has been found to be most effective as an evacuated insulation when used at a bulk density of 145 kg/m3.

[0006] There are a number of disadvantages associated with using perlite as an insulation for cryogenic vessels. Perlite tends to absorb moisture from the air. This increases the time it takes to evacuate the insulation space of a cryogenic storage vessel to the desired vacuum level. This variability in moisture content and resulting variability in pump down time causes unpredictability in the production schedule which is also problematic and undesirable. In addition, perlite tends to settle over time causing increases in thermal conductivity to occur as spaces between the particles are eliminated. In addition, perlite is easily powdered when its container is subjected to vibrations, such as those that are present with mobile tanks during transport. Such powdering, which is due to the rough surfaces of the perlite particles, also increases thermal conductivity. Perlite is also moderately heavy as compared to other insulations and may be difficult to fluidize due to wide variation in particle sizes. As a result of such shortcomings, insulation materials other than perlite have been proposed for use in cryogenic vessels.

[0007] Silica aerogels are a silica composite, that is, a composite formed from silicon and oxygen, with a very porous structure. They consist of a silica matrix that is approximately 96% air. They can be fabricated in slabs, pellets, powders or any other desired shape. They offer low thermal conductivity at relatively higher pressures, when compared to other insulations, and are extremely lightweight. These characteristics make them excellent for vacuum insulation purposes. Silica aerogels are also nonflammable, nontoxic and thermally stable to about 650° C.

[0008] Aerogels in fine powder form have been suggested as insulation in the annular space between the inner tank and the outer jacket of cryogenic vessels. Examples of such broad applications are described in U.S. Pat. Nos. 3,481,504 to Nelson and 3,583,592 to Kerfman. The use of aerogel in amorphous fine powder form presents a number of disadvantages, however. As with powdered perlite, the powdered aerogel eliminates spaces between the insulation particles so that thermal conductivity is increased. In addition, it is difficult to evacuate the annular space between the inner tank and outer jacket of the cryogenic vessels when the space is filled with the fine aerogel powder as the powder will clog the evacuation filter. The aerogel powder also settles during transport of the vessel which increases thermal conductivity even further.

[0009] U.S. Pat. No. 5,386,706 to Bergsten et al. discloses the use of aerogels in coherent form. Such panels, however, must be preformed to conform to the cryogenic vessel shape. This makes manufacture and assembly of the cryogenic vessel difficult and expensive. In addition, the Bersten et al. '706 patent discloses the use of the coherent aerogel as part of a composite. Such composites do not evacuate well and, as a result, the insulation does not have optimum insulating capability.

[0010] Accordingly, it is an object of the present invention is to provide an improved insulation for cryogenic vessels and the like.

[0011] It is another object of the present invention to provide an insulation for cryogenic vessels and the like that is quick and easy to install.

[0012] It is another object of the present invention to provide an insulation for cryogenic vessels and the like that permits the annular space within which it is installed to be easily evacuated of gases.

[0013] It is another object of the present invention to provide an insulation for cryogenic vessels and the like that offers low thermal conductivity in a vacuum or at low pressures.

[0014] It is another object of the present invention to provide an insulation for cryogenic vessels and the like that offers high insulating capability over a relatively broad range of pressures.

[0015] It is another object of the present invention to provide an insulation for cryogenic vessels and the like that resists settling during transport.

[0016] It is still another object of the present invention to provide an insulation for cryogenic vessels that resists powdering during transport.

[0017] It is still another object of the present invention to provide an insulation for cryogenic vessels and the like that may be easily fluidized and transported.

[0018] It is still another object of the present invention to provide an insulation for cryogenic vessels and the like that resists dusting during fluidization.

SUMMARY OF THE INVENTION

[0019] The present invention is directed to an insulated vessel for storing cryogenic liquid and other cold fluids and a system for and method of insulating such vessels. The insulated vessel includes an inner tank adapted to contain the fluid, an outer jacket surrounding the inner tank so that an insulation space is formed there between and spherical aerogel particles positioned within the insulation space. The insulation space is evacuated to a pressure below atmospheric pressure. The outer jacket of the vessel features evacuation and insulation fill ports.

[0020] A system and method for installing the spherical aerogel particles as insulation in the vessel includes a storage silo containing a supply of the spherical aerogel particles that is in communication with the insulation fill port of the vessel. A tank containing a supply of pressurized nitrogen gas is also in communication with the silo so that the spherical aerogel particles are fluidized and the storage silo is slightly pressurized. A sintered metal filter is positioned between the evacuation port and the insulation space of the vessel and also communicates with a vacuum pump. The vacuum pump evacuates gas from the insulation space of the vessel through the filter so that the spherical aerogel particles are transferred into the insulation space. The insulation space is evacuated after it is filled with the spherical aerogel particles. The spherical aerogel particles may be pre-processed by evacuating them in a storage container prior to fluidization.

[0021] The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a vertical sectional view of a cryogenic vessel constructed in accordance with the present invention;

[0023] FIG. 2 is a chart showing effective thermal conductivity for aerogel as a function of pressure (vacuum);

[0024] FIG. 3 is a schematic of a system for installing the insulation of the present invention using the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] With reference to FIG. 1, a vessel for storing cryogenic fluids, or other very cold fluids, is indicated in general at 10. The term vessel, as used herein, may be used interchangeably with the terms tank or container. In addition, while the present invention is described below in terms of a vessel for containing a cryogenic liquid, it is to be understood that the invention may be used with vessels for holding other types of cold fluids.

[0026] The vessel 10 includes an inner tank 12, an outer jacket 14 and an annular insulation space there between 16. The inner tank 12 contains a supply of cryogenic liquid 18. Examples of cryogenic liquids include liquid nitrogen, liquid oxygen, liquid argon, liquid ethylene, and liquid hydrogen as well as other similar substances. A vapor or head space 22 is located above the liquid 18. A fill port 24 (shown diagramatically) permits liquid to be added to the vessel, while a liquid withdrawal port 26 permits the liquid to be directed to a use device or the like when valve 28 is open. As is known in the art, the vessel may also include a vapor withdrawal port (not shown) in communication with the head space 22 whereby cryogenic vapor in the head space 22 may be used or vented.

[0027] The tank 10 also includes an evacuation port 32 and an insulation fill port 34, both of which communicate with the annular space 16 surrounding the inner tank 12. When not in use, the ports are covered in a sealed fashion by caps 33 and 35, respectively. While only one evacuation port and one fill port are illustrated, the vessel may instead feature a number of each type of port. Using the method and system described below with respect to FIG. 3, the annular space 16 of the vessel is filled with spherical particulate aerogel, represented by hashed line 40, and gas is evacuated therefrom. The spherical aerogel particles preferably have a diameter of approximately 1 mm and the particles preferably are generally uniform in size. On source of such material is available under the trade name NANOGEL from the Cabot Corporation of Boston, Mass.

[0028] Such an arrangement presents several advantages. More specifically, stationary cryogenic vessels are typically constructed and then transported to their use points, such as an industrial plant or hospital. The vessel is often placed horizontally on its side and transported by rail or truck. When the vessel is on its side, the insulation naturally has a tendency to gather in the lowest portion of the insulation space 16 (FIG. 1). When the vessel arrives at its use point, it is raised to a vertical position. Prior art insulation, such as perlite, has a tendency to stay clumped on the side of the vessel that rested on the rail car or truck bed. Due to the spherical nature of the aerogel, however, the insulation material of the present invention redistributes itself around the inner tank (12 in FIG. 1). As a result, the vessel 10 has superior insulating performance when compared to prior art vessels.

[0029] Another advantage is that, due to their spherical configuration and mechanical stability, the aerogel particles of the insulation of the present invention resist settling and dusting when the vessel 10 of FIG. 1 is subjected to vibrations. As described above, settling and dusting reduces or eliminates the spaces between insulation particles so that thermal conductivity increases and insulating capability decreases. While such considerations are important during transport of a stationary vessel, they are even more important for truck or train vessels.

[0030] As illustrated in FIG. 2, the vacuum that is present in the insulation space 16 of the vessel of FIG. 1 actually improves the insulation performance of the spherical aerogel particles of the present invention. The insulation also performs quite well even in the presence of a poor vacuum within the insulation space. As a result, in some applications, it may not be desirable or necessary to completely evacuate the insulation space of the vessel so that time may be saved in the manufacturing cycle for the vessel.

[0031] Indeed, as evidenced by the rather flat profile of the curve of FIG. 2, the spherical aerogel of the insulation of the present invention offers high insulation efficiency over a wide range of vacuum levels (1-1000 microns). Such a characteristic is particularly advantageous for cryogenic vessels and the like as they may experience vacuum deterioration over time after installation in the field. As FIG. 2 illustrates, the insulating performance of the insulation would not be adversely effected, at least to a significant degree, by this deterioration. Prior art insulations, such as perlite, usually exhibit high efficiency over a much narrow range of vacuum levels.

[0032] Due to the high efficiency of the aerogel insulation of FIG. 1, as compared to prior art insulations such as perlite, it is possible to use a lesser thickness of the aerogel insulation. This reduces the overall size of the vessel 10 of FIG. 1 for a given inner tank capacity. As a result, the vessel is easier and cheaper to ship, particularly to distant destinations.

[0033] The spherical particle aerogel, when installed using the method and system described below, has a density in the range of 30-145 kg/m3. This compares to an optimum density of 145 kg/m3 for prior art insulations such as perlite. The resulting lighter weight of the vessel of FIG. 1 is important for use in mobile equipment where gross vehicle weight limits apply. With regard to stationary vessels, the reduced weight could result in lower shipping costs. Under these circumstances it could be possible to transport an increased volume of liquid for the same gross weight of the vessel.

[0034] As stated previously, aerogel is a precipitated silica composite with a very porous structure. The aerogel spheres of the insulation of the present invention contain approximately 1% water or less (as a product specification) which is resident in the porous structure. The material may be purchased in bulk form, transported in trailers of the type that are used to transport plastic pellets and dry bulk items, typically with a volume of approximately 60 m3, and stored in silos at the point of use, that is, the vessel manufacturing facility. The silos preferably have a volume of 300 m3 or more.

[0035] At each handling step after the spherical particle aerogel is produced, the material must be prevented from absorbing moisture because moisture in the insulation space of the vessel 10 of FIG. 1 will prevent or retard attainment of a suitable vacuum in the final product in a reasonable amount of time. As such, the truck and trailer transports are purged with dry nitrogen gas prior to receiving the aerogel from the aerogel manufacturer and, after filling, a slight positive internal pressure of dry nitrogen gas is maintained in the transport to preclude in-leakage of air containing moisture from contacting the aerogel. Liquid nitrogen having less than 1 ppm of water may be stored by the vessel manufacturer at its manufacturing site and the dry nitrogen gas produced using ambient vaporizers. Such an arrangement is efficient as dry nitrogen gas is needed for a multitude of other manufacturing purposes as well.

[0036] During the assembly of the vessel of FIG. 1, the liquid storage (pressure) vessel is inserted into the vacuum vessel and held in place with low-conductivity supports, such as those illustrated at 42 and 44 in FIG. 1. Such low-conductivity supports are well known in the art. In addition, the tank inlet conduit 46 and outlet conduit 48 are constructed of materials that resist heat transfer, such as thin-walled, insulated stainless steel. After testing the assembly (inner tank, supports and outer jacket), the aerogel insulation is installed in the vessel space 16 of FIG .1 between the inner tank and the outer jacket.

[0037] FIG. 3 illustrates a system that may be used to install the spherical particle aerogel insulation in the insulation space 16 of vessel 10 using the method of the present invention. As described above, the aerogel spherical particles 50 are stored in a storage silo 52. A supply of dry nitrogen gas 54 communicates with the silo via line 56. As a result, as indicated by arrow 58, nitrogen is transferred to the silo 52 so that a slight positive pressure, for example around 1.4 bars, is maintained in silo 52 and the aerogel 50 therein is fluidized, as illustrated at 59. The storage silo 52 is connected to the insulation fill port(s) 34 of tank 10 by a hose 60.

[0038] One or more vacuum pumps 62 are connected to the evacuation port(s) 32 of the tank 10 by line 63. The pump 62 is preferably of the mechanical two-stage variety. An example of a suitable vacuum pump is the two-stage rotary piston pump model DK 200, available from Leybold Vacuum Products of Export, Pennsylvania. In addition, a filter 64 is connected to each evacuation port and positioned within the insulating space 16 of vessel 10. As the aerogel in silo 52 is being fluidized, vacuum pump 62 is activated so that gas is withdrawn from space 16, as illustrated by arrow 66. As a result, a pressure drop within space 16 occurs, preferably to around 0.5 bars. Due to the resulting pressure difference between the silo 52 and the space 16, the fluidized spherical aerogel particles travel to the space 16 so that it is filled with the insulation material. As such, the aerogel is in effect sucked into the insulation space 16.

[0039] The spherical configuration of the aerogel particles results in them having smooth surfaces. This eases fluidization of the particles. In addition, the smooth surfaces of the particles minimize breakage and dusting during fluidization and, after installation, when vibrations occur as the spherical particles don't abrade one another.

[0040] The filter(s) 64 may be constructed of bronze and can be of the sintered metal variety, although other suitable filter types and materials or construction may be used. The pores of the filter 64 are sized to prevent the aerogel particles from being sucked out of the insulation space and into the pump system. Accordingly, if 1 mm diameter spherical aerogel particles are being used, the pores of the filter should be slightly smaller than 1 mm diameter each. Suitable filters include the grade F60 which may be obtained from the Capstan California company of Gardena, Calif.

[0041] While aerogel spherical particle sizes of around 1 mm diameter are described above, it appears that the range of particle diameters for optimal performance can be from 0.5 mm to 3.0 mm. The larger the particle sizes, the larger the pores of the filter may be. Larger filter pores results in quicker evacuation times.

[0042] After the transfer of the aerogel insulation into the final vessel assembly, the space containing the aerogel is evacuated using the vacuum pump. The aerogel is evacuated to approximately 50 microns and held at the vacuum level for enough time to be sure that all of the undesirable components (components that will cause a vacuum to prematurely degrade) have been removed. The spherical particle aerogel evacuates quickly. Laboratory studies indicate that this is because of its structure and lack of affinity to absorb water.

[0043] If desired, for high throughput assembly situations, the aerogel can be “pre-processed” by evacuating the material in a separate storage vessel (not shown) and keeping it under evacuated conditions until immediately prior to the fluidized transfer. This extra “pre-processing” step reduces the final in-line evacuation time by removing moisture and other products before the insulation is put in the final assembly. While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.

Claims

1. An insulated vessel for storing a cold fluid comprising:

a. an inner tank adapted to contain the fluid;

b. an outer jacket surrounding the inner tank so that an insulation space is formed there between; and

c. insulation disposed within the insulation space, said insulation including a plurality of spherical aerogel particles.

2. The insulated vessel of claim 1 wherein the insulation space is evacuated to a pressure below atmospheric pressure.

3. The insulated vessel of claim 2 wherein the insulated space is a vacuum.

4. The insulated vessel of claim 1 wherein said plurality of spherical aerogel particles are generally uniform in diameter.

5. The insulated vessel of claim 1 wherein diameters of said plurality of spherical aerogel particles are larger than 0.5 mm.

6. The insulated vessel of claim 1 wherein diameters of said plurality of spherical aerogel particles range from 0.5 mm to 3.0 mm.

7. The vessel of claim 1 further comprising an evacuation port formed in the outer jacket and in communication with the insulation space.

8. The vessel of claim 1 further comprising an insulation fill port formed in the outer jacket and in communication with the insulation space.

9. The vessel of claim 1 wherein the inner tank is supported within the outer jacket by non-conductive supports.

10. A method for insulating a vessel having an inner tank, an outer jacket and an insulation space formed there between comprising the steps of:

a. providing a supply of spherical aerogel particles;

b. fluidizing the spherical aerogel particles; and

c. evacuating gas from the insulation space of the vessel so that the spherical aerogel particles are transferred thereto.

11. The method of claim 10 further comprising the step of evacuating the insulation space after it is filled with the spherical aerogel particles.

12. The method of claim 10 further comprising the step of pressurizing the supply of spherical aerogel particles.

13. The method of claim 12 further comprising the step of providing a supply of pressurized nitrogen gas and using the pressurized nitrogen gas to pressurize the supply of spherical aerogel particles.

14. The method of claim 10 further comprising the step of providing a supply of nitrogen gas and using the nitrogen gas in step b to fluidize the spherical aerogel particles.

15. The method of claim 10 further comprising the step of pre-processing the spherical aerogel particles by evacuating them in a storage container prior to step b.

16. The method of claim 10 wherein diameters of said supply of spherical aerogel particles are larger than 0.5 mm.

17. The method of claim 10 wherein diameters of said supply of spherical aerogel particles range from 0.5 mm to 3.0 mm.

18. A system for installing insulation in a vessel for storing cold fluids where the vessel includes an inner tank, an outer jacket with an evacuation port and an insulation fill port formed therein and an insulation space disposed between the inner tank and the outer jacket comprising:

a. a storage silo containing a supply of spherical aerogel particles and adapted to communicate with the insulation fill port of said vessel;

b. a tank containing a supply of a pressurized gas, said tank in communication with said silo so that said spherical aerogel particles are fluidized and said storage silo is pressurized;

c. a filter adapted to be positioned between the evacuation port and the insulation space of the vessel; and

d. a vacuum pump adapted to communicate with the evacuation port of the vessel and evacuate gas from the insulation space of the vessel through said filter so that the spherical aerogel particles are transferred into the insulation space.

19. The system of claim 18 wherein the pressurized gas is nitrogen.

20. The system of claim 18 wherein the filter is a sintered metal filter.