US2550886A

US2550886A - System for conserving liquefied gases

Info

Publication number: US2550886A
Application number: US728072A
Authority: US
Inventors: Harold E Thompson
Original assignee: Union Carbide and Carbon Corp
Current assignee: Union Carbide Corp
Priority date: 1947-02-12
Filing date: 1947-02-12
Publication date: 1951-05-01
Anticipated expiration: 1968-05-01

Description

May 1, 1951 H. E. THOMPSON Filed Feb. 12, 1947 SYSTEM FOR CONSERVING LIQUEFIED GASES 2 Sheets-Sheet l ggggffgi-fe 42 52 50 5/ T0 ATMOS. 22' v 47 v v v 46 45 V 44 5.4 A g 52 56 3g 6/ 54 v Rsssue/A/c LINE 5/ k DISCHARG/NG LINEgO a 3 3 VAPOR LINE J CHARGING 1.1/35 j 77 1 v fl k lfmlylmlfllwlvlllllnlfllfi 'wmmmmnvrII lg INVENTOR HAROLD E. THOMPSON 44 2 1. BY 2E ATTORNEY 2 Sheets-Sheet 2 H. E. THOMPSON SYSTEM FOR CONSERVING LIQUEFIED GASES N Am MUWQ May 1, 1951 Filed Feb. 12, 1947 MED QOQVA INVENTOR HAROLD E. THOMPSON ATTORNEY Patented May 1, 1951 SYSTEM FOR CONSERVING LIQUEFIED GASES Haroid E. Thompson, Rye, N. Y., assignor, by

mesne assignments, to Union Carbide and Carben Corporation, a corporation of New York Application February 12, 1947, Serial No. "128,072

This invention relates to a system for conserving liquefied gases and more particularly to a system for the economical marine transportation of liquefied hydrocarbons and for conserving such gases during shipment. Large quantities of propane are now either wasted or are unavailable for useful purposes due to the lack of a satisfactory and economical method of large bulk transportation of such gases.

Liquefied hydrocarbon gases such as propane, propylene, butane and the like are commonly stored and transported in closed pressure vessels at atmospheric temperatures. It is necessary in accordance with marine shipping regulations that such pressure vessels shall be designed for working pressures equal to the vapor pressure of the liquefied gas at a maximum summer temperature of 115 F. At such temperatures propane, for example, has a high vapor pressure of 213 p. s. 1. gauge and a relatively low liquid density (3.83 pounds per gallon). It is further required that such containers shall not be com" pletely filled with liquid at the maximum working pressure and at 115 F. Thereiore, the

weight of metal in the vessels is high relative to the amount of liquefied gas contained therein. Shipment by high pressure tank car is expensive and involves high investment costs. The excessive steel requirements have prevented the development of a satisfactory high pressure tank barge for the waterway shipment of liquefied hydrocarbon gases. With propane as a cargo, for example, the design of a tank barge for transportation at atmospheric temperatures would result in excessively thick-walled pressure containers such that for a standard size of barge, 195 feet long by 35 feet wide, the weight of steel required f 1 the pressure containers would be approximately 700 tons and the weight of the empty containers and the barge would approximate 900 tons, such that the useful carrying capacity for the propane cargo would only be in the order of 300 tons.

Liquefied propane could be transported at atmospheric pressure if its temperature were maintained at the boiling point corresponding to atmospheric pressure, which is 44.1'7 F. The maintenance of this temperature, however, would require an excessive weight and thickness of insulation for the container or the evaporation loss would be high. A certain minimum wall thickness for the container would still be necessary, particularly for a large container, merely to hold the weight of liquid cargo and for structural selfsupport. In order to prevent large evaporation losses, the energy required for refrigerating the liquid would be so great as to be economically prohibitive. Great difificulties are also encountered in order to avoid pressure changes which large 5 Claims. (01. 62-1) tanks are not able to withstand unless the Walls are made heavy.

By the present invention these and other difficulties are overcome and large bulk shipment of liquefied hydrocarbon gas is economically carried out by maintaining the cargo at moderate superatmosph-eric pressure and at a depressed temperature which is intermediate between the maximum summer temperature and the boiling point of the liquefied gas at atmospheric pres-'- sure. The liquefied gas cargo is maintained at the depressed temperature by a refrigerating system preferably involving the removal of a portion of the vapor phase of the cargo from the shipping container, compression of such vapor to a pressure sufiicient to permit liquefaction thereof by heat exchange with a suitable cooling medium, for example, the river or canal water during waterway shipment, and finally expansion and readmission of the expanded gas ma terial into the shipping container. The maximum working pressure is chosen such that the wall thickness of the containers need be only slightly greater than would be required for structural strength to hold the liquid at atmospheric pressure. A moderate thickness only of insulation about the container is necessary, and therefore the tare weight is proportionately small compared to the weight of cargo. In addition to the advantage of lighter weight shipping containers, the maintenance of intermediate temperature and moderate pressure provides conditions under which the density or weight per cubic foot of the cargo is considerably greater than when shipping the liquefied gas under high pressure, and thus a greater amount of liquefied gas can be transported in a given size container. At the same time refrigeration is not continuously required and the energy required for refrigeration when needed is relatively small. Large quantities of liquefied hydrocarbon gas can be transported safely for long distances with both a low investment cost and a low transportation cost and with immaterial gas loss.

The principal objects of the invention are, therefore, to provide a method of and apparatus for conserving and shipping liquefied hydrocarbon gases which maintains the cargo of liquefied gas at a moderately reduced temperature and a low superatmospheric pressure, such that the quantity of liquefied gas conserved and shipped is large relative to the weight of its container, which permits large bulk shipments of liquefied gas with safety, with immaterial gas loss, and with economy of investment and transportation costs; to provide such a method and apparatus in which continuous production of refrigeration is not essential and the amout of refrigeration which is required is moderate, in which advantage may be taken of periods when atmospheric temperatures are low to eliminate the need for refrigeration, in which the cargo itself is employed in the refrigeration cycle and the energy for the refrigeration production is furnished by a small portion of the cargo to eliminate the need for an independent refrigeration cycle and a separate source of energy to operate same, and by which a large quantity of liquefied gas can be economically transported for long distances by waterway and the cooling power of the water taken from the waterway employed to reduce the energy required for refrigeration.

The above and other objects and novel features of the invention will become apparent from the following description having reference to the annexed drawings wherein:

Fig. 1 diagrammatically illustrates an exemplary system for conserving a liquid hydrocarbon gas while being transported; and

Figs. 2 and 3 are respectively plan and and elevational views of a barge for waterway transportation of liquefied gas.

The invention will be described particularly for the transportation of propane, but the principles thereof are adapted for the transportation of other combustible gases, for example, ethane, propylene, ethylene, etc. The pressures mentioned herein are to be understood as gauge pressures unless otherwise specified.

Propane has a boiling point at atmospheric pressure of ll.17 F. and a vapor pressure at 115 F. of 213 p. s. i. The characteristic vapor pressure curve for propane is such that at increasing temperatures the change in vapor pressure becomes increasingly greater, particularly at the higher temperatures. It is, therefore, possible to effect a major reduction of vapor pressure by the relatively moderate reduction of temperature. For shipping propane according to the invention an upper pressure limit of p. s. i. is preferred and a normal operating pressure limit is preferably chosen at 75 p. s. i. The boiling point of propane corresponding to 100 p. s. i. is 64 F., and that corresponding to 75 p. s. i. is 43 F., and, therefore, no refrigeration would be required to prevent loss of gas by venting whenever the atmospheric temperature is below 64" F., as often occurs during the colder months.

To maintain the pressure at 75 p. s. i. no refrigeration would be needed whenever the atmospheric temperature is below 48 F. If, however, the large container were designed for holding propane .at atmospheric pressure, the liquid would have to be maintained at a temperature of at least 90 F. lower than an atmospheric temperature of 48 F. and it will be seen that under such conditions the insulation heat leakage would be large and the requirement for refrigeration would be continuous. Thus by designing the container for 100 p. s. i. maximum pressure instead of a maximum pressure of 213 p. s. i., a major reduction of wall thickness and weight of container is effected and only an inappreciable amount of refrigeration is required. There also will be long periods when no refrigeration at all will be needed.

Even though the containers are designed fo 100 p. s. i. maximum pressure a further advantage results from maintaining a normal maximum operating pressure less than 100 p. s. i. and preferably about '75 p. s. i., in that it will take a long, time for heat leak during hot summer conditions to raise the pressure up to 100 p. s. i. if

the refrigeration device were inoperative for any reason, such as for minor repairs.

For providing the moderate amount of refrigeration when necessary, a compressor is arranged to draw vapor from the container or containers and to compress it into a condenser where it is liquefied by heat exchange with cooling water pumped from the waterway. The liquefied vapors are expanded through a valve and passed into the container to cool the contents. The compressor and cooling water pump are preferably driven by a gas engine which burn a portion of the vapor to provide the power. Thus a portion of the cargo supplies the energy required for its refrigeration and the small portion of vapor used by the engine constitutes the only loss of cargo during storage and shipment, which loss is relatively immaterial in amount. For example, with a full barge load of about 700 tons of propane, the fuel consumption may be only 0.2%

of the total cargo during a twenty-three day trip under hot summer conditions. A special fuel for the engine can be provided if it is desired to avoid any loss of cargo.

Referring now to the drawings and particularly to Fig. 1, one of the large containers for holding liquefied hydrocarbon gases as indicated in schematic cross-section at H], the container [0 may have any suitable shape and is preferably cylindrical with hemispherical ellipsoidal or elliptical ends. The upper portion of the container in may be provided with a dome H of customary construction to which various conduits may be connected and, if required, two 01' more sealed manholes it may also be provided. As indicated hereinbefore, the container It is constructed of metal which is thick enough to withstand internal working pressure of p. s. i. The container is covered with layers of heat insulation l3 and i4, and particularly when the container is mounted in a barge a layer of insulation l3 covering the lower half of the container may be thinner than the layer It covering the upper half of the container. The thicker insulation covers the portion of the container above the main deck level of the barge, which is subject to higher atmospheric temperatures and radiation from the sun. For example the layer I3 may be about two inches of cork and the layer it may be about four inches thick.

A preferred assemblage of apparatus for refrigerating a body of liquefied gas l5 held in the container is also diagrammatically illustrated in Fig. l. The liquefied hydrocarbon gas is filled into the container I 6 through a charging line it, controlled by a valve ll and entering through the dome II. For discharging liquefied gas when desired, for example at the destination, a discharge line is preferably leads from a sump H9 at the bottom of the container IE] and upwardly through the dome II. This is controlled by a valve 20. Other auxiliary devices of customary type are also provided such as a pressure relief va1ve 2l, connected through the dome l l and set to discharge gas at pressures above 100 p. s. i., and liquid level indicating devices which may extend into the container at a manhole l2.

For drawing gas from the container a vapor line 22, controlled by a valve 23, connects to the gas space in the dome H and conducts gas from the container to the inlet 2d of a gas compressor 25. The compressed vapor is conducted from the conduit 2?. denser 26 passes from the lower end thereof through a conduit 28 to the inlet of an expansion valve 29 and from the discharge of the expansion valve 29 a conduit 30 conducts the resulting expanded gas material through the dome H to a point near the bottom of the container l8. A shut-off valve 3! is preferably provided at the point where the conduit 38 enters the dome ll. The conduit 30 within the container IE3 is preferably provided with an extension 32 which leads to a point near the bottom of the container It at a substantial distance from the dome II. This is to insure thorough mixture of the expanded and cooled incoming gas material with the body of liquid in the container IE3. The expansion valve 29 is preferably of an automatically operated type, and it is adjusted according to the liquid level in the condenser 25. To this end there is provided a float chamber 33 having a float therein connected to regulate the valve 29. The float chamber 33 is connected by conduits 3d and 35 with the upper and lower portions of the condenser 28, such that the levels of liquid in the float chamber 33 and condenser 25 are at the same height. If it should be desired to by-pass the valve 29, a valved by-pass connection 36 between

conduits

28 and 38 may be provided, and for such purpose shut-off

valves

37 and 38 at the inlet and outlet of the valve 29 are also provided. The condenser 26 is also preferably provided with a pressure gauge 39 and a vent valve 6t, which may be used to release non-condensable gases that may accumulate in the condenser 25 and interfere with the operation thereof.

The compressor 25 is preferably directly coupled to a gas engine 4i. Such gas engine is provided with the customary air and gas mixing device 42. For supplying the gas a branch connection d3, controlled by a valve 44 is preferably provided from the vapor line 22. Interposed in the connection 43 there may be a gas metering device 4-5 and other auxiliaries, such as a first pressure reducing valve 45 discharging into a chamber 4? in which gas is maintained at an intermediate pressure. A second pressure regulator 48 is provided to receive gas from the chamber 4'! and discharge it into a surge chamber 49. The regulator 48 is preferably made responsive to the pressure in the connection 50 between the chamber 49 and the gas mixer 42, by a pressure connection 5|. At the inlet of the mixer 42 there is a control valve 52. The pressure maintained in the chamber 49 is preferably only slightly above atmospheric pressure or it may be slightly below atmospheric pressure because the suction of the engine can draw gas from the chamber 29 as required, and when the engine does not run, or if the engine should stop, no gas would flow toward the mixer 42. It is also preferable to provide a metering device in the vapor line 22 before the branch 43, for example, an orifice 53 to which i connected a meter or indicating device 54. The device 54 provides data to indicate the rate of circulation of gas for refrigerating purpose, while the meter 45 will indicate the consumption of gas by the engine 4! Only the consumption registered by the meter 45 represents a loss of cargo.

A water circulating pump 55 is also driven by the engine 4!. The pump 55 has a suction line 56 that leads from an intake 51 in the wall of a barge at the point below the normal water line and preferably near the bottom to draw in the coolest water. Interposed in the suction line 56 there is preferably provided a strainer 58. A discharge line 59 of the pump 55 connects to the in, let 60 of the cooling water tubes in the condenser 26, and a discharge line 5i conducts the used cooling water from the water discharge outlet 62 of the condenser 26 to a discharge point outside of the barge at a distance from the intake line 5?.

In Figs. 2 and 3 is shown a preferred arrangement of four of the containers it upon a standard barge. The barge structure may include a hull B4 of more or less standard design havin a main deck 65, the tanks I!) being supported in the hull on suitable cradles, not shown, in such manner that the upper portions of the tanks protrude above the level of the main deck 65. At one end of the barge is a deck house 56, within which the compressor 25, condenser 26 and engine 4! may be housed. The domes H of each of the containers ID are positioned so that they are close together, and interconnecting manifolds are provided for the conduits connected to each dome. For example, the vapor line 22 may be connected to each of the domes by four branches 22a, each of which may have a stop valve therein which is not shown in the interest of clearness of the drawing. Such stop valves, however, may be' used so that vapor can be drawn through the vapor line 22 from any one of the four containers of which it may be desired to reduce the pressure, or any two or all of the containers can be opened to the vapor line 22 for reducing the pressure of all four containers simultaneously. The pressuring line 30 may similarly be connected to each of the containers through branches and the charging and discharging lines i5 and 58 are also connected to each of the containers. It will also be preferable to provide a sheet metal cover over the containers Ill, which cover is joined tightly to the main deck 65 to keep rain water and hydrocarbon vapor out of the hull.

In operation the containers are filled at a loading dock through the chargin line and vapors displaced from the containers may be returned to the loading dock container through a connection to the vapor line. A relatively small vapor space is left above the liquid in each container to allow for liquid expansion up to the maximum working pressure of about p. s. i. If during the trip, the pressure of one or more of the containers should rise to about 75 p. s. i., the engine 4! will be started and the

valves

31 and 23 opened in order to draw Vapor from the container or containers having the high pressure. Starting of the engine may be facilitated b providing a supply of compressed air which is conducted to the engine intake by a valve controlled line 58. The vapors will be compressed by the compressor 25, liquefied under pressure in the condenser 26 and with valve 36 closed and

valves

37 and 38 open, the liquid will be expanded through valve 29 and then conducted through conduit 38 and extension 32 into the container ii]. The liquid portion will mix with the liquid in the container l8 and the portion of the liquid which is flashed into vapor by expansion (about one-quarter) will bubble up through the body of liquid [5 and thus create a movement thereof for circulating the body of liquid so that the cooling of it will be uniform. When the body of liquid has been cooled enough to cause the pressure in the container to reduce to a preselected pressure below 75 p. s. i., for example 65 p. s. i., the compressor may be shut down.

By designing the containers for 100 p. s. i. pressure a major reduction in weight of container is effected without incurring a large refrigeration requirement. Thus a barge having four containers designed for 100 p. s. i. and insulated as herein described would, to maintain a pressure of about '75 p. s. i. during hottest summer conditions, require the operation of a '25 horsepower refrigeration compressor for onl approximately of the time. Furthermorethe safety of shipment according to the invention is assured by the long time (approximately seven days at hot summer conditions) for heat leak to increase the pressure from '75 p. s. i. to 100 p. s. i.

When the barge arrives at the unloading dock the liquid discharging line may be connected to the receiving containers on shore and pressure is built up to about 100 p. s. i. on the barge containers by adding superheated vapor through the pressuring line from a shore compressor. I'he propane is discharged from the containers ll) through line 18 by pressure difference preferably to the suction side of pumps at the shore station, which pumps may force the liquid into permanent storage tanks. When the containers are empty of liquid the vapor pressure therein is drawn down to about '75 p. s. land the containers are closed off. A positive vapor pressure is always maintained in the containers.

While one embodiment of the invention has been disclosed and described, it is contemplated that modifications of the method and the apparatus may be made and that some features of the invention may be employed without others without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of conserving a cargo of liquefied hydrocarbon gas, such as liquefied propane, in a large insulated container constructed for a maximum internal pressure of 100 s. i. and sub jected to atmospheric temperatures, which method comprises utilizing a small quantity of said cargo as fuel for generating power, employing a major portion of said power for compressing a refrigerant, cooling said refrigerant by heat exchange with cooling water, utilizing such cooled refrigerant for cooling said cargo suificient to maintain the vapor pressure thereof close to but not exceeding a predetermined working pressure below 100 p. s. i., and employing another portion of said power for pumping cooling water for said heat exchange.

2. A method of transport-ing a cargo of liquefield hydrocarbon gas, such as propane, in a large insulated container adapted to hold such cargo under superatmospheric.pressure on a marine vessel, such as a barge, over a waterway, which method comprises utilizing a small quantity of vapor of said liquefied gas as fuel for generating power, employing a major portion of said power for compressing gas material drawn from the vapor phase of said container, condensing said gas material by heat exchange with cooling water, expanding said condensed gas material and returning same to said container to maintain a pressure therein intermediate be tween the vapor pressure of said cargo at 115 F. and atmospheric pressure, and employing another portion of said power for pumping cooling water from said waterway for said heat exchange. I

3. Apparatus for transporting a cargo of liquefied hydrocarbon gas such as propane, which comprises at least one large horizontally elongated insulated container constructed no heavier than required for internal pressures not exceeding about 100 p. s. i., a gas engine, means for supplying a small quantity of the cargo to said gas engine for use as a fuel therein to generate power, a compressor coupled to be driven by said gas engine, conduit means connecting the suction of said compressor to the vapor space of said container, a condenser connected to receive compressed vapor from said compressor for liquefying the vapor, and means for expanding liquid from said condenser and delivering the expanded material to said container to cool the cargo and maintain the vapor pressure thereof at a desired value below p. s. i., said means for delivering expanded material opening into the liquid space of said container at a distance from the point of connection of said conduit means to the vapor space of said container.

4. Apparatus for transporting a cargo of liquefled hydrocarbon gas, such as propane, which comprises at least one large insulated container constructed to hold such cargo under a pressure not exceeding about 100 p. s. i. a marine vessel, such as a barge, supporting said container; said vessel also having thereon a gas engine, means for supplying a small quantity of said cargo to said gas engine for use as fuel therein to generate power, a compressor coupled to be driven by said gas engine, conduit means connecting the suction of said compressor to the vapor space of said container, a condenser connected to receive compressed vapor from said compressor for liquefying the vapor, means for extending liquid from said condenser and delivering the expanded material to said container to cool the cargo and maintain the vapor pressure thereof at a desired value below 100 p. s. i., and a pump coupled to be driven by said engine and connected to draw cooling water fromthe water supporting said vessel to force said cooling water through said condenser.

5. Apparatus for transporting a cargo of liquefied hydrocarbon gas, such as propane, which comprises at least one large container constructed of metal having a thickness for a maximum internal working pressure not exceeding about 160 p. s. 1.; heat insulation around said container equivalent on parts exposed to atmospheric temperature to about 4 inches of cork; a refrigerating system including a refrigerant compressor and a water-cooled refrigerant condenser and constructed and arranged to cool said cargo when the vapor pressure thereof tends to exceed about '75 p. s. i. under conditions of atmospheric temperature as high as F.; an engine coupled to drive said compressor; and a water pump coupled to be driven by said engine and connected for circulating the cooling water through said condenser.

HAROLD E. il IOMPfSON.

REFERENCES CITED .n. following references are of record in the file of this patent:

UNITED STATES PATENTS

Claims

1. A METHOD OF CONSERVING A CARGO OF LIQUEFIED HYDROCARBON GAS, SUCH AS LIQUEFIED PROPANE, IN A LARGE INSULATED CONTAINER CONSTRUCTED FOR A MAXIMUM INTERNAL PRESSURE OR 100 P. S. I. AND SUBJECTED TO ATMOSPHERIC TEMPERATURES, WHICH METHOD COMPRISES UTILIZING A SMALL QUANTITY OF SAID CARGO AS FUEL FOR GENERATING POWER, EMPLOYING A MAJOR PORTION OF SAID POWER FOR COMPRESSING A REFRIGERANT, COOLING SAID REFRIGERANT BY HEAT EXCHANGE WITH COOLING WATER, UTILIZING SUCH COOLED REFRIGERANT FOR COOLING SAID CARGO SUFFICIENT TO MAINTAIN THE VAPOR PRESSURE THEREOF CLOSE TO BUT NOT EXCEEDING A PREDETERMINED WORKING PRESSURE BELOW 100 P. S. I., AND EMPLOYING ANOTHER PORTION OF SAID POWER FOR PUMPING COOLING WATER FOR SAID HEAT EXCHANGE.