US3812683A

US3812683A - Method for storing a subcooled liquid

Info

Publication number: US3812683A
Authority: US
Inventors: R Laverman
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 1972-05-15
Filing date: 1972-05-15
Publication date: 1974-05-28
Anticipated expiration: 1991-05-28
Also published as: CA977669A

Abstract

An enclosed cryogenic storage tank having an insulated bottom, a vertical insulated wall projecting upwardly from the bottom, an insulated roof projecting to the wall and completely covering the space surrounded by the wall, and an insulated deck inside and extending across the tank at a fixed location between the tank bottom and tank roof. A process of removing a subcooled liquefied gas from the space beneath an insulated deck in an insulated cryogenic storage tank, feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, returning the subcooled liquefied gas to the tank below the deck, and feeding the cooled stream of liquefied gas to the tank above the deck. Alternatively, a stream of the subcooled liquefied gas can be mixed with the steam of warm liquefied gas and the combined stream then fed to the tank above the insulated deck.

Description

United States Patent 1 Laverman [111 3,812,683 I451 May 28, 1974 METHOD FOR STORING A SUBCOOLED LIQUID [75] Inventor: Royce Jay Laverman, South Holland, 111.

[22] Filed: May 15, 1972 [2l] Appl. No.: 253,110

Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. Capossela Attorney, Agent, or Firm-Merriam, M arshall, Shapiro & Klose [57] ABSTRACT An enclosed cryogenic storage tank having an insulated bottom, a vertical insulated wall projecting upwardly from the bottom, an insulated roof projecting to the wall and completely covering the space surrounded by the wall, and an insulated deck inside and extending across the tank at a fixed location between the tank bottom and tank roof.

A process of removing a subcooled liquefied gas from the space beneath an insulated deck in an insulated cryogenic storage tank, feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, returning the subcooled liquefied gas to the tank below the deck, and feeding the cooled stream of liquefied gas to the tank above the deck. Alternatively, a stream of the subcooled liquefied gas can be mixed with the steam of warm liquefied gas and the combined stream then fed to the tank above the insulated deck.

8 Clainis, 3brawingFigures 1 METHOD FOR STORING A SUBCOOLED LIQUID This invention relates to processes and apparatus for storing liquids. More particularly, this invention is concerned with improvements in liquid storage tanks and the use of such tanks in processes of cooling warm liquids.

Storage tanks of various sizes are used to store liquids. The tanks have'various shapes including spherical, elliptical and cylindrical. The most common shape of ,tank is one which has a flat bottom, vertical circular cylindrical wall and a conical or domed roof. Such tanks are usedto store materials which are liquid at ambient temperatures and pressures, as well as gases which have been refrigerated to liquid form. To retard heat leak, storage tanks for liquefied gases are fully insulated.

Because of the low boiling point of many liquefied gases, it is advisable to store them in insulated tanks at a temperature which gives a vapor pressure of atmospheric pressure or slightly thereabove. This necessitates cooling the liquefied gases to. very low temperatures. If the low boiling liquefied gases were to be stored at higher pressures, the strength of the tank, and capital cost,- would need to be greatly increased. It is generally less expensive to cool' the liquefied gas to a low temperature suitable for storage at atmospheric pressure thanto build atank adequate to store the product at a higher pressure.

In addition to storing liquefied gases at atmospheric pressure and a boiling point temperature consistent therewith. it is sometimes desirableto subcool a liquefied gas below its boiling point and to store it at such temperature in a tank to provide stored refrigeration. Thus, a subcooled heel of liquefied gas can be provided in the tank to store refrigeration which could later be used to minimize vapor, generation when a warm stream of liquefied gas is fed to the tank for storage. A subcooled liquefied gas, however, will cause a vacuum in'the tank vapor space because of the subcooled temperature and if means are not provided to prevent this, the tank may. be crushed by atmospheric pressure. Increasing the strength of the tank to withstand the vacuum is not a desirable solution because of the increased cost. An alternative means is accordingly needed to provide for suitable storage of a subcooled liquefied gas, and particularly storage of subcooled liquefied gas in the same tank to be used for storage of a larger quantity of the liquefied gas at its boiling point under atmospheric pressure.

According to one aspect of the invention there is pro vided an insulated liquid storage tank enclosing a volume and having an insulated deck inside the tank extending across an area of the tank and positioned and fixed to be below the full level, and abovethe empty level. of the tank. The deck'can be fixed stationary in one position or location,'or it can be arranged to be vertically movable within a certain vertical distance between the full and empty levels of the tank with the said movement being limited by suitable stops, chains or other restraining means. A quantity of subcooled liquefied gas can be stored in the tank in the space below the deck to cool a stream of warm liquefied gas fed to the tank interior space above the deck. The insulated deck prevents a vacuum from developing in the tank above the deck when a subcooled liquefiedjgas is stored below the deck. As a result, the tank need-not be able to withstand a pressure differential caused by a reduction in internal pressure. The deck also serves to separate warmer liquid placed above the deck from colder liquid below it.

An opening is advisably located in the deck to permit flow of liquid and/or vapor from above the deck to below the deck and, in some circumstances, from below the deck to above it. Furthermore, the deck is adapted to be totally submerged in liquefied gas in the tank. It is therefore essential that it be constructed to withstand hydrostatic pressures applied to it. In addi-' tion, the deck advisablyis built so that it has no or very little buoyancy when submerged so as to avoid the application of significant upward forces on the tank wall. The deck however is advisably made slightly buoyant when it is to be displaceable within predetermined limits in the tank. An advantage of having the deck displaceable to a limited extent is that it permits an increase in the volume or space below the deck for storing subcooled liquid.

The shape of the deck is notnarrowly critical. For most purposes however an essentially flat horizontal deck is suitable, and particularly one of substantially uniform thickness. Regardless of the shape of the deck,

it can be supported by the tank wall, or by posts or columns extending from the deck to the tank surface there below, or by suspension members extending from the tank roof to the deck, or any combination of such systems. The deck nevertheless is advisably made rigid although for some purposes it can be flexible.

The tank is generally provided with one or more conduits communicating from the tank space below the deck to outside the tank. It is also advisably provided with a conduit communicating from the tank vapor space above the deck to outside the tank.

According .to a further aspect of the invention, the tank can be used in a process of cooling warm liquefied gas for storage in the tank. Thus, subcooled liquefied gas from the space beneath the insulated deck can be removed and placed in heat exchange with a warm stream of liquefied gas of the same general composition. The warm stream is thereby cooled, after which it can be fed to the space in the tank above the deck for storage at,'or about at, atmospheric pressure. The subcooled liquefied gas can then be returned to the tank space below the deck. In an alternative process, the subcooled liquefied gas removedfrom the tank space below the deck can be mixed with the warm stream of liquefied gas and the combined stream then fed to the tank space above the deck for storage at atmospheric pressure. Such processes are especially useful for cooling a warm stream of a cryogenic liquefied gas, such as natural gas which is being unloaded after transportation by ship orbarge. The subcooled liquefied gas provides ready refrigeration to cool the warm stream.

The invention will be described further in conjunction with the attached drawings in which:

FIG. 1 is a vertical cross section'of a storage tank having an insulated deck;

FIG. 2 is a schematic representation of means to cool a warm stream of liquefied gas after ship transportation using the storage tank of FIG. 1; and I FIG. 3 is a schematic representation of another means to cool a warm stream of liquefied gas after ship transportation using the storage tank of FIG. 1.

So far as is practical the same identifying numbers will be used for the same or similar parts which appear in the various drawings. v

With reference to FIG. 1, tank has an outer shell comprising a flat metal outer bottom 11, a vertical circular cylindrical outer wall 12 and a domed metal roof 13'. An inner shell comprising flat metal inner bottom '14 and vertical cylindrical circular metal inner wall 15 is positioned insideof the outer shell with insulation 16 between outer bottom 11 and inner bottom 14 and insulation 17 between outer wall 12 and inner wall 15. Insulated ceiling 20 has metal sheet 18, suspended by rods2l from roof 13, which supports insulation 19 thereon. Tank 10 is intended to store a liquefied gas at, orslightly above, atmospheric pressure.

Insulated deck is horizontally positioned in tank 10 above inner bottom 14 in fixed position. The deck 25 thus divides the tank interior volume into a space 26 below the deck and a space 27 above the deck.

Columns 28, vertically positioned and spaced apart from one another, support deck 25 above inner bottom 14. The columns can be made of any suitable material but advisably are made of metal; Guy rods' 29 serve to hold the columns vertical.v 1

Insulated deck 25 has a metal bottom 31,.metal top 32 and insulation33 ther'ebetween. The edge of the deck extends to the surface of inner wall 15 and, .if advisable, can be joined thereto. Holeor opening 34 ex-' tends through deck 25 so there can be flow of liquid back and forth between the spaces above and below the deck. v

Conduit 35 communicates with conduit 37 through valve 36, and conduit 37 communicates with space 26 below deck 25 for delivering liquefied gas thereto.

I Conduit 38 also communicates withspace 26 below deck 25 and it can be used for removing liquefied gas from the tank. Conduit 38 communicates with conduit 40 through control valve 39.

- Conduit extends through insulated ceiling 20 and domed roof l3. Vapor generated during filling of the tank, or by heat leak during storage, can be removed by conduit 45 to prevent the internal pressure from rising morev than slightly above atmospheric pressure. Conduit 45 also permits air or some other'gas to flow into the tank during emptying t'o'avoid the creation of a vacuum in the tank high enoughto cause it to collapse fromatmospheric pressure."

Subcooled liquefied gas can be stored in space 26 below insulated deck 25. Because 'of the insulated deck, the subcooled liquid 'will not cause the vapor pressureof the portion'of liquid stored in the space above the deck to be lowered below atmospheric pressure. Tank 10 accordingly need not be constructed as a vacuum vessel with the increased costs this would involve. Tank 10 thus can be constructedto store a liquefied gas at atmospheric pressure.

Representative ways in which the tank of FIG. I can be used are illustrated schematically by FIGS. 2 and 3, it being understood that such conduits as are required are incorporated into tank 10 to make-it suitable for the intended purpose.

With reference to FIG. 2, ship arrives at a destination carrying liquefied natural gas at approximately 254 F. and 16.7 psia. The warm liquefied natural gas is removed from ship 50 by pump15l which delivers it to conduit 52 which feeds it to .pump'53. From pump 53 the liquefied natural gas isfed to conduit 55 which.

feeds it through heat exchanger tubes 56 in heat exchanger 57. The liquefied natural gas is fed, after being cooled toapproximately 258 F. at l5.2 psia from tubes56 to conduit 58 which feeds the liquefied natural gas to space 27 in tank 10 above insulated deck 25.

Subcooled liquefied natural gas having an initial temperature of approximately -285 F. and a vapor pressure of about 3.4 psia is withdrawn by conduit 59 from space 26 below deck 25 by means of pump 60 and is fed to conduit 61 which delivers it to heat exchange tubes 62 inheat exchanger 57. The Subcooled natural gas provides refrigeration, as it passes through the heat exchanger, to cool the warm liquefied natural gas passing through tubes 56. The subcooled liquefied natural gas at approximately -260 F. is fed from tubes 62 to conduit 63 which returns it to space 26 below deck 25.

The system shown schematically in FIG. 3 also uti- I lizes the novel tank of FIG. 1. With reference to FIG. 3, Subcooled natural gas having an initial temperature of approximately -285 F. .and a vapor pressure of 3.4

psia IS removed from space 26 by conduit and pump 71 which delivers the stream of Subcooled liquefied natural gas to conduit 72. The warm stream of liquefied natural gas at approximately 254 F. and l6.7 psia removed from ship 50 (as described with reference to FIG. 2) is fed by pump 53 to conduit 73. The stream of Subcooled liquefied natural gas is fed by conduit 72 to conduit 73 where the two streams are intermixed to form a single stream alt-approximately ,258 F. and 15.2 psia. Conduit 73 feeds the combined single stream to space 27 above deck 25 for storage at atmospheric pressure.

' As liquefiednatural gas is removed by conduit 70 from below deck 25, an equal volume of liquefied natural gas flows from the space 27 above deck 25 through hole 34into the space 26 below the deck.

It is highly advisable in'practicing the invention to always maintain some liquid at a temperature consistent with storage at atmospheric pressure above deck 25 when space 26 contains a subcooled liquefied gas so that creation of a vacuum by reduction of the vapor pressure in space 27 is avoided. Desirably a volume of liquefied gas a few feet thick is maintained on deck 25 at all times so that the vapor pressure in space 27 will always be maintained at some small positive value as contrasted with a vacuum.

The subject invention .has several advantages over the use of an unrestricted floating roof. One advantage is that less insulation is required because the temperature differential between space '27 and space 26 is not great compared to the temperature differential between the temperature of liquefied gas below an insulated floating roof and the temperature of vapor above it. Furthermore, whenan insulated floating roof is used, the liquefied gas to be stored must be fed directly into the mass of subcooled liquefied gas being used as tank of this invention, the subcooled liquefied gas is maintained generally separate from the product being stored and thus at a lower temperature more suitable for providing refrigeration to a warm stream. Another advantage is the lack of moving parts when a fixed roofed is used.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed isi l. The process which comprises:

providing an insulated storage tank having an insulated deck therein with subcooled liquefied gas below the deck,

removing the subcooled liquefied gas from the space beneath the insulated deck in the insulated cryogenic storage tank, s'aid deck being positioned and fixed to be between the full level and empty level of the tank,

feeding the removed subcooled liquefied gas in heat exchangewith a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, 3 1 returning the subcooled liquefiedgas to the tan below the deck, and I feeding the cooled stream of liquefied gas to thetank above the deck. 2. The process of claim 1, in which the liquefied is liquefied natural gas.,

3. The process of claim 1, in which the pressure in the tank above the deck is at an internal pressure of generally atmospheric pressure.

4. The process which comprises: providing an insulated storage tank having an insulated deck therein with subcooled liquefied gas below the deck,

removing a subcooled liquefied gas stream from the space beneath the insulated deck in the insulated cryogenic storage tank, said deck being positioned and fixed to be between the full level and empty level of the tank,

combining the removed subcooled liquefied gas gas stream with a stream of warm liquefied gas of the I 5. The process of claim 4, in which the deck has an opening through which liquefied gas can flow from above to below the deck.

6. The process of claim 4, in which the pressure in the tank is above the deck at an internal pressure of generally atmospheric pressure.

7. The process which comprises: providing an insulated storage tank having an insulated deck therein with subcooled liquified gas below the deck,

removing the subcooled liquefied gas from the space beneath the insulated deck in the insulated cryogenic storage tank, said deck being positioned and fixed to be between the full level and empty level of the tank and permitting liquid flow from the space on either side of the deck to the space on the other side,

feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of tioning an insulated deck within said tank, said deck having an aperture extending from both sides through the deck surrounding the deck with liquefied gas including a subcooled liquefied gas volume beneath said deck, i removing a portion of the subcooled liquefied gas from beneath the deck for heat exchange purposes and causing a flow of liquefied gas from above the deck through the aperture'thereby replacing the removed subcooled volume of liquefied gas below the deck, maintaining the insulated deck surrounded with liquefied g'as, v I feeding the removed'subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, and I returning the volume'of liquefied gas, removed from belowthe deck, and I the cooled stream of liquefied gas to the tank above the deck. I

Claims

1. The process which comprises: providing an insulated storage tank having an insulated deck therein with subcooled liquefied gas below the deck, removing the subcooled liquefied gas from the space beneath the insulated deck in the insulated cryogenic storage tank, said deck being positioned and fixed to be between the full level and empty level of the tank, feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, returning the subcooled liquefied gas to the tank below the deck, and feeding the cooled stream of liquefied gas to the tank above the deck.

2. The process of claim 1, in which the liquefied gas is liquefied natural gas.

4. The process which comprises: providing an insulated storage tank having an insulated deck therein with subcooled liquefied gas below the deck, removing a subcooled liquefied gas stream from the space beneath the insulated deck in the insulated cryogenic storage tank, said deck being positioned and fixed to be between the full level and empty level of the tank, combining the removed subcooled liquefied gas stream with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas, and feeding the combined stream of liquefied gas into the tank above the deck.

5. The process of claim 4, in which the deck has an opening through which liquefied gas can flow from above to below the deck.

7. The process which comprises: providing an insulated storage tank having an insulated deck therein with subcooled liquified gas below the deck, removing the subcooled liquefied gas from the space beneath the insulated deck in the insulated cryogenic storage tank, said deck being positioned and fixed to be between the full level and empty level of the tank and permitting liquid flow from the space on either side of the deck to the space on the other side, feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, returning the subcooled liquefied gas to the tank below the deck, and feeding the cooled stream of liquefied gas to the tank above the deck.

8. The process which comprises: providing an insulated liquid storage tank and positioning an insulated deck within said tank, said deck having an aperture extending from both sides through the deck, surrounding the deck with liquefied gas including a subcooled liquefied gas volume beneath said deck, removing a portion of the subcooled liquefied gas from beneath the deck for heat exchange purposes and causing a flow of liquefied gas from above the deck through the aperture thereby replacing the removed subcooled volume of liquefied gas below the deck, maintaining the insulated deck surrounded with liquefied gas, feeding the removed subcooled liquefied gas in heat exchange with a stream of warm liquefied gas of the same composition as the subcooled liquefied gas to cool the warm stream, and returning the volume of liquefied gas, removed from below the deck, and the cooled stream of liquefied gas to the tank above the deck.