Field of the invention

The present invention relates to the production and distribution of natural gas derived from low pressure sources .

Background To The Invention

Natural gas (methane) has traditionally been produced from large, high pressure gas wells with individual discrete sources providing flow rates of some 2 to 100kg/s. With these high mass flow rates, the construction of fixed installations and extensive pipeline networks has been economic. More recently, lower flow, lower pressure and more diffuse sources such as coal and biomass derived methane have been utilized. For these cases, the high capital costs of pipeline construction and gas gathering networks have had a negative impact on both the investment decision and the operating profitability of these schemes. In some cases, methane has been combusted in internal combustion engine/generator sets with energy sold on as electricity. This approach still requires the investment in generator sets and electrical transmission lines, and the relative inefficiency of the energy conversion and delivery process in comparison with large scale electricity generation further reduces the potential for profitability. In addition, the visual and noise impact of generating sets and transmission cables may limit the use of this approach in certain areas.

A second potential approach to avoid the significant investment in pipeline construction would be for methane to be converted to Liquefied Natural Gas (LNG) at or close to the source of the gas by mechanical refrigeration and transported to markets by insulated tanker. The capital investment in the refrigeration equipment for this method is still significant, and operating costs are relatively high due to the low efficiency of small scale cryogenic refrigeration equipment which can use up to 25% of the gas mass flow to achieve the liquefaction of the remainder, 75% in this case. Though the methane will be increased in value by its conversion to LNG, a market for LNG within a reasonable distance of the methane source would be necessary, and if this was not in existence the regasification of the LNG would entail the total loss of the energy expended in the original conversion of the gas to LNG.

A third approach would be to use a system such as that suggested by Williams et al (US 3,400,547), with methane liquefied at or close to its source by heat exchange against boiling liquid air or nitrogen, transported to a "market site" as LNG in an insulated tanker and regasified against pressurized air or nitrogen which is liquefied for transport to and subsequent use at the methane liquefaction site.

This method was originally suggested for use with large flow rate, high pressure natural gas sources to be transported over long distances by sea, and has significant difficulties associated with its implementation for small, low pressure sources of methane. The most significant difficulty in utilization of this method is the discrepancy between the mass flows of the LNG and liquid nitrogen streams. A mass flow of liquid nitrogen some double that of the LNG stream is required to liquefy natural gas, and though the liquid nitrogen is around double the density of the LNG, this can create significant transport difficulties with a tractor and trailer unit specified for the mass of liquid nitrogen to be carried having an LNG capacity of only around half of this figure. Thus an insulated semi-trailer and tractor unit optimised for transport of LNG will be unable to transport sufficient liquid nitrogen from the market site to the production site in order to liquefy that methane stream. In addition, the difference in the boiling points of methane and nitrogen makes it necessary to compress the nitrogen to a significant pressure before the boiling point is increased sufficiently for the nitrogen to be liquefied by the LNG stream. A further problem to be addressed is the provision of a nitrogen stream for conversion to liquid nitrogen. Air separation units and other methods for the generation of nitrogen from air have a non-negligible capital and operating cost and have a secondary stream of oxygen or liquid oxygen, a market for which must be found if the air separation unit is to be subsidised to any degree.

It is therefore apparent that to increase the financial viability of both the second and third approach, a low capital cost and efficient means of refrigeration of the methane stream must be provided.

In both cases, the use of some 20 to 25% of the gas in the liquefaction process will significantly reduce the low- carbon nature of the fuel and can result in far higher releases of Carbon Dioxide to the atmosphere than utilization of more traditional routes to market.

A complete natural gas distribution system with which the present invention is concerned is composed of two installations, one at the production site and the other at the market site. The production site installation sets out to convert natural gas efficiently into LNG which is then transported in containers to the market site. At the market site, the LNG is converted efficiently back into natural gas but part of the energy used to liquefy the natural gas is recovered and used to produce liquid nitrogen which can then be transported back to the production site to assist in the refrigeration of the natural gas. The concept of recovering energy in this way already been proposed in US 3400547. Both installations will be described below but the present invention is concerned only with the production site at which it is desired to receive liquid nitrogen in containers, to use it in liquefying natural gas derived from a local supply and to return the containers filled with LNG to the market site. The market site is claimed in a separate patent application which, in common with the present application, claims priority from GB 0907905.4 filed on 8 May 2009.

Object of the invention

The present invention therefore sets out to provide an efficient method and apparatus to produce LNG from natural gas.

Summary of the invention

According to a first aspect of the present invention, there is provided a method of liquefying natural gas, which comprises adding to the natural gas a refrigerant having a higher boiling point than natural gas, compressing the gaseous mixture, cooling the compressed mixture to liquefy the refrigerant, separating the liquid refrigerant from the cooled natural gas, passing the liquid refrigerant through an expansion valve to vaporise the refrigerant, using the refrigerant fluid to cool the natural gas further in a heat exchanger, returning the refrigerant gas to the compressor intake for recycling and subsequently applying further cooling to liquefy the natural gas passing through the heat exchanger .

The first aspect of the invention takes advantage of the fact that the natural gas sources need to be compressed before transport, treatment or liquefaction processes are applied. Use is made of this compression process to introduce a pre-cooling step before liquefaction, by adding a refrigerant gas to the natural gas prior to compression and condensing it after compression and cooling of the compressed gas mix. By separating the condensed refrigerant and passing it through an expansion valve to flash it to around compressor inlet pressure, a sub-ambient temperature refrigerant fluid is produced to cool the compressed natural gas to a sub-ambient temperature before the refrigerant is returned as gas to the inlet side of the compressor. Further cooling to liquefy the natural gas can be performed by a Brayton cycle refrigeration system or by sacrificial liquid nitrogen cooling.

According to a second aspect of the invention, there is provided an apparatus for liquefying natural gas, which comprises a compressor for compressing an intake gas consisting of a mixture of the natural gas and an added refrigerant having a higher boiling point than natural gas, an after-cooler for cooling the compressed mixture to cause the refrigerant to condense, a separator for separating condensed refrigerant from the natural gas, an expansion valve connected to receive the liquid refrigerant from the separator and operative to cool the refrigerant by partial vaporisation, a heat exchanger for using the refrigerant fluid to cool the natural gas further, a line for returning refrigerant gas from the heat exchanger to the compressor intake for recycling and a refrigeration stage for cooling the natural gas passing through the heat exchanger so as to liquefy the natural gas.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a schematic diagram of a production site installation to liquefy natural gas, and Figure 2 is a schematic diagram of a market site installation to gasify LNG and produce liquid nitrogen.

Detailed description of the preferred embodiment

In Figure 1, a mixture of natural gas NG derived from a low pressure source such as coal bed methane, and a refrigerant, such as propane or butane circulating in a refrigerant circuit described below, is compressed in a compressor 10 and cooled in an after-cooler 12. The compressed refrigerant condenses in the after-cooler 12 and the liquid refrigerant is removed from the compressed gas by a separator 14. The liquid refrigerant is then "flashed" in an expansion valve 16 (which is essentially a throttle) which has the effect of cooling the refrigerant by partial evaporation. The vaporised refrigerant and the compressed natural gas are then passed through a heat exchanger to gasify the refrigerant and cool the natural gas. The gasified refrigerant is then returned to the intake of the compressor 10 for remixing with fresh natural gas.

Some refrigerant may remain in the compressed natural gas stream and it will condense within the heat exchanger 18. Once again, the liquid refrigerant is removed from the gas stream by a second separator 24 and that liquid refrigerant can either be recycled through the previously described expansion valve 16 (as shown by the broken line arrow) or it can be passed through a second expansion valve 20 and heat exchanger 22 to reduce the temperature of the natural gas still further before it too is returned to the intake of the compressor 10.

At this point, the natural gas will have been compressed and cooled but it will still remain as a gas until cooled further; for example, the natural gas may be at a pressure of 15 bar and a temperature of -60°C. At this pressure, the temperature of the natural gas would need to be reduced to -115°C before it will condense to form LNG.

The necessary further cooling is carried out in the illustrated embodiment in two stages, passing first through a heat exchanger 50 then through a heat exchanger 40. In each of these two heat exchangers, the natural gas can lose heat to a reverse Brayton cycle refrigeration circuit, as will be described below, or to a stream of liquid nitrogen obtained from an insulated container, as will also be described below. Unless there is a local requirement for the gasified nitrogen, it can be sacrificed by being discharged into the ambient atmosphere.

Figure 1 shows two reverse Brayton cycle refrigeration units that are bootstrapped so that only one motor is required to operate both of them. These units are well known in the art and are often referred to as companders. The companders use air, nitrogen, or any suitable gas, as the refrigerant and this remains in a gaseous state at all stages of the refrigeration cycle.

The compander supplying cold gas to the heat exchanger 40 comprises a compressor 30 driven by a motor 32. The compressed gas passes through an after-cooler 36 before flowing through a recuperator 38, which is itself a counter- current heat exchanger where the compressed gas loses heat to the gas entering the compressor 30. The gas is then used to drive a turbine that is mechanically coupled to the motor 32 to assist in driving the compressor 30. The expansion of the gas in the turbine results in a drop in pressure and temperature so that when it is passed through the heat exchanger 40 it lowers the temperature of the natural gas to the point where it liquefies.

The second compander comprises a compressor 42 driven by a turbine 44 and the circulating gas once again passes through an after-cooler 46 and a recuperator 48. The difference from the first compander is that it does not have a separate motor and is instead powered by the compressed gas generated by the compressor 30 to which it is connected in a bootstrap configuration.

After leaving the heat exchanger 40, the natural gas will have been fully liquefied but, to lower its temperature and pressure to values suitable for transportation of the LNG, the liquefied natural gas is flashed in an expansion valve 52 creating some NG vapour and lowering the temperature of the liquid natural gas. A separator 54 separates the liquid natural gas for storage in insulated transportation containers, while the vaporised natural gas is returned via the two heat exchangers 40 and 50 to the intake of the compressor 10.

To assist in understanding the drawing, heavier lines have been drawn to represent the flow of the natural gas, lighter lines to represent the flow of different refrigerant and a dotted line to represent the flow of nitrogen.

Figure 2 shows an installation at a market site, where the LNG arrives in containers to be gasified and, if necessary, pressurised for distribution through a pipeline. It will be clear from the description given above of the production site installation that considerable energy needs to be expended to liquefy natural gas. When the liquid is gasified at the market site, all the energy consumed in the liquefaction process is ordinarily wasted. The distribution system of the present invention reduces such energy wastage by allowing the "coolth" of the liquefied natural gas to be recycled and returned to the production site in the form of liquid nitrogen, to enable the natural gas to be liquefied at the production site using less energy. The installation in Figure 2 therefore has LNG as its input and at the same time as gasifying the LNG it generates liquid nitrogen to be returned to a production site to be used in the previously described heat exchangers 40 and 50.

As the liquefied gas to be returned to the production site is stored in the same container as the LNG, it is important that it should be substantially devoid of oxygen. Air is of course a suitable source of nitrogen but before it can be used it has to be stripped of its oxygen content.

The market site installation shown in Figure 2 comprises as a source of nitrogen an engine 60 which runs on natural gas, for example obtained from the pipeline to be supplied. The engine may suitably be a reciprocating piston engine but it could equally be a rotary or gas turbine engine.

When the engine burns methane in air, its combustion products will include water and carbon dioxide in addition to the required nitrogen. These must also be removed before the exhaust gases are liquefied because they would otherwise freeze and block the various heat exchangers.

The exhaust gases of the engine 60 are cooled in an after-cooler 62 to cause the steam to condense. A separator 64 removes the liquid water and allows the remainder of the exhaust gas stream to enter the intake of a two-stage compressor 68, 70 driven by the output shaft 66 of the engine 60. An intercooler 72 is arranged in the normal way between the two stages 68, 70 of the compressor. A second after-cooler 74 results in more of the water condensing and this is removed by a second separator 76.

The remaining exhaust gas stream which still contains carbon dioxide is passed through a two-stage counter-current heat exchanger where the other fluid is the liquid natural gas to be gasified. The carbon dioxide condenses in the first stage 78 of the heat exchanger and is removed in liquid form by a separator 80. It should be noted that because the exhaust gases have been pressurised by the compressor 68, 70 they are above the pressure at which carbon dioxide will sublimate. The exhaust gas stream entering the second stage 82 of the heat exchanger will therefore consist predominantly of nitrogen and because its boiling point will have been raised by its high pressure it will condense on losing its heat to the liquid natural gas stream.

The temperature of the liquefied nitrogen is lowered by partial vaporisation in an expansion valve 84. The further cooled liquid nitrogen is then stored in the insulated containers that were used for the liquid natural gas for return to a production site while the nitrogen vapour is returned to an earlier compressor stage for recycling after having first passed through the two-stage counter-current heat exchanger 78, 82. Depending on the pressure of the liquid nitrogen, the recycled nitrogen can either be returned to the second stage 70 of the compressor as shown in solid lines or to the first stage 68 as shown by a dotted line .

Instead of relying exclusively on thermal contact to transfer heat from the nitrogen stream to the liquid natural gas stream in the heat exchangers 78, 82, it would be possible to provide a mechanically powered refrigeration circuit to act effectively as a heat pump, in order to pump heat from the nitrogen stream into the liquid natural gas stream.

The volume of liquid nitrogen produced can be substantially the same as the volume of LNG that is gasified because of the difference in the specific heats of the two gases. However, the weight of the liquid nitrogen will be substantially greater than the LNG and it may not therefore be acceptable to fill the containers to their maximum capacity for their return to the production site. It is therefore possible that a production site will not receive all the liquid nitrogen that is needed to liquefy the natural gas .

This inequality can be allowed for by combining liquid nitrogen cooling with reverse Brayton cycle cooling as described above but this would necessitate expensive heat exchangers with multiple passages.

An alternative approach in a network comprising several production sites is to design some sites to use only liquid nitrogen sacrificial cooling as the last cooling stage in the liquefaction of the natural gas while other sites rely solely on reverse Brayton cycle cooling for the last cooling stage. In this way, cost of individual production sites can be minimised by avoiding the need for dual purpose heat exchangers in all production sites, and the need for companders in the sites that rely on liquid nitrogen cooling.