US2964917A

US2964917A - Evaporation of liquefied gases

Info

Publication number: US2964917A
Application number: US682533A
Authority: US
Inventors: Webster Thomas John
Original assignee: British Oxigen Ltd
Current assignee: BOC Group Ltd
Priority date: 1956-09-19
Filing date: 1957-09-06
Publication date: 1960-12-20
Anticipated expiration: 1977-12-20
Also published as: FR1182614A; GB808535A

Description

Dec. 20, 1960 T. J. WEBSTER EVAPORATION OF LIQUEFIED GASES Filed Sept. 6, 1957 FIG. I

FIG.2

luvenfov THorms JOHN wEesrER A x F COMPRESSOR FIG. 3

United States Patent C EVAPORATION F LIQUEFIED GASES Thomas John Webster, Ashford, England, assignor to The British Oxygen Company Limited, a British corn- P y Filed Sept. 6, 1957, Ser. No. 682,533

Claims priority, application Great Britain Sept. 19, 1956 7 Claims. (Cl. 62-51) This invention relates to the evaporation of liquefied gases and more particularly to the evaporation of liquefied gases of boiling point substantially below atmospheric temperature. Examples of such gases are liquid methane, liquid oxygen and liquid nitrogen.

A supply of gas at a moderate pressure, for example, up to 300 p.s.i.g. is frequently required in industry and such supply is often obtained by using free heat derived from a convenient heat source, such as the atmosphere, a river, or a medium in which waste heat is present, to evaporate and warm to ambient temperature liquefied gas which has been pressurised to the desired delivery pressure of the gas. The amount of free heat extracted from the heat source is limited, as it cannot exceed that necessary to evaporate the liquid and raise the gas to the ambient temperature. 7 Moreover, no mechanical energy can be liberated by the process.

It is an object of the present invention to provide a method for increasing the amount of heat extracted from the heat source and to make this additional heat available for the production of mechanical energy.

According to one aspect of the present invention, a method of evaporating a liquefied gas of boiling point substantially below atmospheric temperature with the simultaneous production of mechanical energy comprises compressing the liquefied gas to a pressure substantially in excess of the required gas delivery pressure, evaporating the compressed liquefied gas at this excess pressure and warming the gas so produced by heat exchange with a heat source and thereafter expanding the compressed gas to the required delivery pressure in an expansion machine generating mechanical energ According to another aspect of the present invention, apparatus forevaporating a liquefied gas of boiling point substantially below atmospheric temperature with the simultaneous production of mechanical energy comprises a thermally insulated container for the liquefied gas, means for, compressing the liquefied gas to a pressure substantially in excess of the required gas delivery pressure, a heat exchanger adapted to vaporise the compressed liquefied gas and to warm the gas so produced by heat exchange with a heat source, and an expansion machine adapted to generate mechanical energy on expansion of thejcornp're'ssed gas therein to the gas delivery pressure. The gas is cooled in performing work in the expansion engine and can therefore receive additional heat from the heat source or its surroundings, both during the expansion and, if required, by a further heat exchange to Warm the gas to ambient temperature prior to delivery. In effect, the process of the present invention enables mechanical work to be produced by expanding the over-compressed evaporated gas in such a way as to lower its temperature enable it to extract additional heat from its surroundings. The amount of mechanical work which can be'obtain'ed by the process of the present invention substantially exceeds the additional work required to compress the liquefied gas to the higher pressure. The "proc- 52,964,917 Patented Dec. 20, 1960 lCQ ess therefore leads to a net gain in mechanical energy at the expense of free heat drawn from the heat source.

While the process of the present invention may be applied to any liquefied gas boiling substantially below atmospheric temperature, and required to be delivered at any reasonable pressure, as a particular example, the case of the evaporation of liquid methane to provide gaseous methane at p.s.i.g. will be considered in more detail, in conjunction with the accompanying drawings in which:

Figure 1 shows diagrammatically the conventional method of generating methane gas from liquid methane;

Figure 2 shows diagrammatically one method of generating methane gas from liquid methane according to the present invention; and

Figure 3 shows diagrammatically a development of the method illustrated in Figure 2.

Referring to Figure 1, in the conventional method of generating methane gas at 100 p.s.i.g. from liquid methane stored at substantially atmospheric pressure, the liquid methane is withdrawn from a thermally-insulated storage tank It by means of a pump 11 which discharges the liquid into a heat exchanger 12 at substantially 100 p.s.i.g pressure. The liquid methane is evaporated and warmed to about 15 C. in the heat exchanger 12 by heat drawn from a convenient heat source and gas is delivered from the exchanger 12 at the required pressure of 100 p.s.i.g. If the evaporative heat required in this process is derived from a source of free heat such as a naturally occurring heat source or a medium in which waste heat is available, then the only power required to maintain the process is that necessary to drive the pump ll. In the case of liquid methane pumped to 100 p.s.i.g., the energy 'consumption of the pump 11 (allowing 50% margin 'over the theoretical requirement to cover pump inefiiciencies) would be 0.6K ca'ls/k'g. of methane. In such a process no mechanical energy would be generated and the amount of heat extracted from the heat source cannot exceed that required to evaporate and warm the methane.

Referring now to Figure 2, in operating according to the method of the present invention, liquid methane 'is withdrawn from the storage tank Ill by the pump 11 and discharged into the heat exchanger 12 at a pressure considerably in excess of that at which it is desired to deliver the gas, for example, a pressure of 1000 p.s.i.g. The compressed liquid is their evaporated and warmed in' the heat exchang'erlZ as before by heat drawn from the heat source and is then expanded to the desired delivery pressure of 100 p.s.i.g. in an expansion machine 13. During the expansion the gas will be cooled somewhat and the cold gas leaving the expansion machine 13 is warmed in a further heat exchanger 14 by free heat drawn from the heat source to the delivery temperature of 15 'C.

The amount of work obtained from the expansion machine 13 operating over a given pressure ratio will vary according to whether the expansion takes place isothermally or adiabatically in the machine. Isothermal expansion provides the maximum Work and should be aimed at by allowing the maximum infiux of heat'tothe machine. Difliculties in transferring heat from the surroundings or heat source to'the machine will meanthat in practice the expansion Will tend to be more adiabatic than isothermal. Regardless, however, of the exact nature of the expansion, the power output of the machine will considerably exceed the additional power requiredfto pump the liquid methane to the higher pressure, and the process will involve a net gain in mechanical energy.

Considering the energy gains and consumption'in producing gaseous methane at 100 p.s.i.g. by the process of the present invention, using an intermediate "xcess' p'res sure of 1000 p.s.i.g. i

The theoretical energy output of the expansion machine 13 assuming adiabatic expansion from 1000 to 100 p.s.i.g will be 56K calsjkg. of methane.

Assuming for the sake of illustration that any increase in energy output arising from a deviation from adiabatic towards isothermal expansion in the machine 13 is offset by mechanical inefficiency, then the above theoretical adiabatic value can be taken as the actual energy output of the machine.

The energy required to pump liquid methane to 1000 p.s.i.g. allowing a 50% margin over theoretical requirements will be 6K cals./ kg. of methane.

Therefore, the net energy output in operating according to the present invention will be (56-6) =50K cals./ kg. of methane (approximately).

As the energy consumption in the conventional process is 0.6K cals./kg. of methane, the net gain in energy obtained by operating according to the present invention as compared with the conventional process is slightly in excess of 50K cals./kg. of methane.

Assuming that the methane is being evaporated at the relatively low rate or" 2 tons per hour, the net power output of the process would be 118.5 kw., equivalent to 159 H.P.

In the above example, compression of the methane to only 1000 p.s.i.g. has been assumed. If the methane were compressed to a higher pressure the net power output from the process would be increased, but ultimately the expansion would have to be carried out in two stages to avoid liquefaction in the machine. The rate of increase in power output would, however, decrease with increasing pressure, and there would therefore be no great advantage in operating at pressures far in excess of 1000 p.s.i.g. A reduction in the delivery pressure of the gas -would also lead to an increase in the power output of the process.

The mechanical energy obtained in the process of the present invention may be utilised in various ways. For instance, it may be employed to generate electrical energy by coupling the expansion machine directly to an electrical generator. Alternatively, the arrangement shown in Figure 3 may be adopted. In this arrangement, gas produced by heat inleak to the storage tank 10 is tapped from the top of the tank 10 and passed through a pipe to a gas compressor 16 which is coupled to the expansion machine 13. In the compressor 16, the gas is compressed to the required delivery pressure of 100 p.s.i.g. and is then delivered through pipe 17 to the outlet from the heat exchanger 14. With such an arrangement, the energy liberated by the evaporation of 2 tons per hour of liquid methane (equivalent to 106,000 cu. ft./hour of gas) to 100 p.s.i.g. pressure would, assuming 60% adiabatic compression efiiciency, enable approximately 34,000 cu. ft./ hour of methane gas at ambient temperature or 113,000 cu. ft./hour of methane gas at -161 C. (the boiling point of methane) to be pressurised from atmospheric pressure to the required delivery pressure of 100 p.s.i.g. Depending on the temperature of the gas being compressed, such a process would thus provide 140,000 or 219,000 cu. ft./hour of methane gas at 100 p.s.i.g for no cost in power whatsoever.

It should be noted that the process of the present invention is not limited to the use of any particular type of expansion machine. The machine may be a turbo or reciprocating expander which may be coupled to an electrical generator or directly or indirectly to a reciprocating or turbo compressor. The expansion machine may also be coupled directly or indirectly to a reciprocating or rotary pump used to compress the liquefied gas. Again, the expansion machine may be a free-piston expander/ compressor or a single piston expander/compressor used both to expand the compressed gas and to compress gas evaporated from the liquefied gas by heat inleak, as shown in Figure 3.

Any available means may be used to compress the liquefied gas prior to its evaporation. Whilst a reciprocating pump is preferred for this duty, a rotary pump may also be used.

If desired, the liquefied gas may be fed to the pump from the container by vaporising a part of the liquid in the container in an external heating coil and feeding the gas so formed back to the container to raise the pressure therein.

I claim:

1. The method of evaporating a liquefied gas of boiling point substantially below atmospheric temperature and delivering the evaporated gas to gas utilising apparatus at. a delivery pressure of up to 300 p.s.i.g. and a delivery temperature substantially above its boiling point with the simultaneous production of mechanical energy, which comprises the steps of pumping the liquefied gas to a pressure substantially in excess of said delivery pressure, evaporating the liquefied gas pumped under pressure and warming the compressed evaporated gas so formed to said delivery temperature by heat exchange with a heat source at substantially ambient temperature, expanding the warmed compressed gas to said delivery pressure in an expansion machine generating mechanical energy, replacing heat lost by the expanded gas during its expansion by a further heat exchange with a heat source at substantially ambient temperature, and delivering the warmed expanded gas to the gas utilising apparatus.

2. The method of evaporating a liquefied gas of boiling point substantially below atmospheric temperature and delivering the evaporated gas to gas utilising apparatus at a delivery pressure of up to 300 p.s.i.g. and a delivery temperature substantially above its boiling point with the simultaneous production of mechanical energy, Which comprises the steps of pumping the liquefied gas to a pressure substantially in excess of said delivery pressure, evaporating the liquefied gas pumped under pressure and warming the compressed evaporated gas so formed to said delivery temperature by heat exchange with a heat source at substantially ambient temperature, expanding the warmed compressed gas to said delivery pressure in an expansion machine generating mechanical energy, replacing heat lost by the expanded gas during its expansion by a further heat exchange with a heat source at substantially ambient temperature, and delivering the. warmed expanded gas to the gas utilising apparatus, and using the mechanical energy generated by said expansion machine to compress to said delivery pressure gas produced by evaporation of said liquefied gas during storage thereof.

3. The method of evaporating a liquefied gas of boiling point substantially below atmospheric temperature and delivering the evaporated gas to gas utilizing apparatus at a delivery pressure of up to 300 p.s.i.g. and a delivery temperature substantially above its boiling point with the simultaneous production of mechanical energy, which comprises the steps of pumping the liquefied gas to a pressure substantially in excess of said delivery pressure, evaporating the liquefied gas pumped under pressure and warming the compressed gas so formed to said delivery temperature by heat exchange with a heat source at substantially ambient temperature, expanding the warmed compressed gas to said delivery pressure in an expansion machine generating mechanical energy, replacing heat lost by the expanded gas during its expansion by a further heat exchange with a heat source at substantially ambient temperature, delivering the warmed evaporated gas to the gas utilizing apparatus, and using the mechanical energy generated by said expansion machine to pump the liquefied gas to the pressure substantially in excess of said delivery pressure.

4. The method of evaporating liquid methane and delivering the gaseous methane to gas utilizing apparatus at a delivery pressure of up to 300 p.s.i.g. and a delivery temperature substantially above the boiling point of methane with the simultaneous production of mechanical energy, which comprises the steps of pumping the liquid methane to a pressure substantially in excess of said delivery pressure, evaporating the liquid methane pumped nnder pressure and warming the compressed gaseous methane so formed to said delivery temperature by heat exchange with a heat source at substantially ambient temperature, expanding the warmed compressed gaseous methane to said delivery pressure in an expansion machine generating mechanical energy, replacing heat lost by the expanded gaseous methane during its expansion by a further heat exchange with a heat source at substantially ambient temperature, and delivering the warmed gaseous methane to the gas utilizing apparatus.

5. Apparatus for evaporating a liquefied gas of boiling point substantially below atmospheric temperature and delivering the evaporated gas to gas utilizing apparatus at a delivery pressure of up to 300 p.s.i.g. and a delivery temperature substantially above its boiling point with the simultaneous production of mechanical energy, comprising a thermally insulated container for the liquefied gas, a pump for pumping the liquefied gas to a pressure substantially in excess of said deli ery pressure, a first heat exchanger adapted to vaporize the liquefied gas pumped under pressure and to warm the gas so produced to said delivery temperature by heat exchange with a heat source at substantially ambient temperature, an expansion machine adapted to generate mechanical energy on expansion of the warmed compressed gas to said delivery pressure therein, a second heat exchanger adapted to replace heat lost by said expanded gas during its expansion by heat exchange with a heat source at substantially ambient temperature, and conduit means connecting said second heat exchanger to the gas utilizing apparatus.

6. Apparatus for evaporating a liquefied gas of boiling point substantially below atmospheric temperature and delivering the evaporated gas to gas utilizing apparatus at a delivery pressure of up to 300 p.s.i.g. and a delivery temperature substantially above its boiling point with the simultaneous production of mechanical energy, comprising a thermally insulated container for the liquefied gas, a pump for pumping the liquefied gas to a pressure substantially in excess of said delivery pressure, a first heat exchanger adapted to vaporize the liquefied gas pumped under pressure and to warm the gas so produced to said delivery temperature by heat exchange with a heat source at substantially ambient temperature, an expansion machine adapted to generate mechanical energy on expansion of the warmed compressed gas to said delivery pressure therein, a second heat exchanger adapted to replace heat lost by said expanded gas during its expansion by heat exchange with a heat source at substantially ambient temperature, conduit means connecting said second heat exchanger to said gas utilizing means, a compressor coupled to said expansion machine, and conduit means connecting said compressor to the gas space of said thermally insulated vessel, whereby gas evaporated by heat inleak to said thermally insulated vessel may be fed to said compressor for compression to said delivery pressure therein.

7. Apparatus according to claim 5 wherein said pump is coupled to said expansion machine.

References Cited in the file of this patent UNITED STATES PATENTS 1,886,076 Abendroth et a1 Nov. 1, 1932 2,511,716 Katzow June 13, 1950 2,750,753 Armstrong June 19, 1956 FOREIGN PATENTS 377,729 Great Britain July 22, 1932