Patent Grant
7143606
Natural gas is often unavailable in regions where consumers are located, making it necessary to move the natural gas from remote areas. Currently, there are four (4) methods for moving the natural gas between locations: transport by pipeline, liquefaction of the light hydrocarbon, conversion of natural gas to a liquid or solid product to allow for transport, and conversion of natural gas to electricity for transport by cable. Each of these methods has its limitations.
Transport by pipeline is a highly popular method for transport. However, this may not be feasible due to the extreme distances between natural gas resources and consumers, which increases cost.
Liquefaction of the light hydrocarbon allows for several different installations and transport. Baseload plants can produce liquefaction of the light hydrocarbon, but are not commonly found. Currently, baseload plants are available at about fifteen (15) sites throughout the world. Each site has at least one train, and each train can carry up to five (5) million tons per year. Methane tankers are another option for transport. Methane tankers can transport a cryogenic liquid at temperatures of about −160° C., but only about one hundred tankers have this capability. Another possibility for liquefaction of the light hydrocarbon is the LNG terminal. At a LNG terminal, the liquefied natural gas from the methane tanker is unloaded, then vaporized and sent to pipelines. A final option for liquefaction is peak-shaving plants. These small liquefaction plants near consumer zones liquefy and store the natural gas when demand is low and vaporize the gas when demand is high.
Converting the natural gas to liquid or solid products, which may easily be transported, is another possibility. The conversion can be done through several methods. The first method, requires that the natural gas be converted to heavy synthetic hydrocarbons in two stages. With the first stage, synthesis gas, an oxygen enriched gas is required to produce a mixture of hydrogen and carbon monoxide by partial oxidation or autothermal reforming. The second stage requires a catalytic reaction, such as the Fischer-Tropsch type. With the second method for converting the natural gas into a liquid or solid product, natural gas is converted into a methanol or used to produce ammonia or fertilizer.
Finally, natural gas can be converted into electricity in cogeneration plants. The electricity is then transported by cable. Similar to transport by pipeline, this is not economical over long distances.
Liquefaction or conversion of the natural gas both require significant investment to make the process profitable. The first synergy between the two processes (liquefaction and conversion) is to be found in the upstream and downstream infrastructures. Upstream if the two units are on the same site, they may use the same gas fields and the same pipeline to transport natural gas to the site. The pretreatment of the natural gas before liquefaction or transformation into synthesis gas can also be common to the two units. The downstream port infrastructures can also be common. The same utilities (water, steam, instrument air) can be common to the two units.
It has been proposed in WO00/71951 to use the energy produced by the vaporization of liquid nitrogen, liquid oxygen or liquid argon to liquefy natural gas. U.S. Pat. No. 5,390,499 and French Patent 2,122,307 concern heat transfer between vaporising liquid nitrogen and liquefying natural gas. UK Patent 2,172,388 describes an air separation unit which produces oxygen and liquid nitrogen. The liquid nitrogen removed from the air separation unit is then transported to a remote site and used to liquefy natural gas. The gaseous nitrogen produced is then used for enhanced oil recovery.
Regarding liquefaction cycles for the production of LNG, several solutions are described in various publications (for example, “Developments in natural gas liquefaction” in Hydrocarbon Processing April 1999). The most efficient is the cascade refrigeration cycle: refrigeration is provided by three different refrigerants, typically methane, ethylene and propane, each been vaporised at several pressure levels. The most used is the mixed refrigerant cycle with propane precooling where a multicomponent mixture of hydrocarbons (typically propane, ethane, methane and/or nitrogen) perform the final cooling of natural gas while a separate propane cycle perform the precooling of natural gas and mixed refrigerant. This cycle is described in U.S. Pat. No. 3,763,658. The last cycle which has never been used in a baseload plant due to its relative high power consumption is the expander cycle. U.S. Pat. No. 5,768,912 shows various possible improvements of such a cycle but none is able to attain the efficiency of the propane precooled mixed refrigerant cycle.
It is an object of this invention to provide a process to liquefy natural gas in combination with an air separation unit with isentropic expansion and without having such a high power consumption. The invention consists in using the cold that can be generated by the air separation, unit through isentropic expansion preferably together with liquid vaporisation in order to liquefy natural gas. The basic idea consists in using the cold streams removed from the distillation section under liquid or gaseous form, enriched in nitrogen, oxygen or argon in order to cool the natural gas by indirect heat exchange. As the heat for warming those cold streams is no longer fully available to cool down the air, isentropic expansion is used to cool down directly the air. Another solution consists in performing isentropic expansion on one of the cold streams in order to increase the quantity of cooling provided by the cold streams and therefore be able to cool down natural gas and air. Air expansion will be the preferred solution because recycling can be either avoided or minimised. Generally, recycling increases the duty of an heat exchanger therefore increasing its irreversibility.
As used herein, the term “recycling” means that at least in a given section of the heat exchanger, at least a portion of the fluid after expansion is being warmed. In this same given section there is at least a portion of the fluid prior to the expansion. The term “liquefaction” also includes the pseudo-liquefaction which occurs when natural gas is cooled down at a pressure above supercritical pressure.
A process as per the invention will benefit from the following advantages as compared to the cascade or mixed refrigerant cycle or a combination of the two which have been used in all the baseload plants to date:
According to one embodiment of the invention, there is provided an integrated process for the separation of air by cryogenic distillation and liquefaction of natural gas in which at least part of the refrigeration required to liquefy the natural gas is derived from at least one cryogenic air distillation plant comprising a main heat exchanger and distillation columns, wherein the natural gas liquefies by indirect heat exchange in a heat exchanger with a cold fluid, the cold fluid being sent to the heat exchanger at least partially in liquid form and undergoing at least a partial vaporization in the heat exchanger.
According to further optional embodiments of the invention:
According to a further embodiment of the invention there is provided integrated apparatus for the separation of air by cryogenic distillation and liquefaction of natural gas in which at least part of the refrigeration required to liquefy the natural gas is derived from at least one cryogenic air distillation plant comprising a main heat exchanger and distillation columns, comprising means for sending natural gas and a cold fluid at least partially in liquid form to a heat exchanger, means for removing liquefied natural gas from the heat exchanger and means for removing at least partially vaporised cold fluid from the heat exchanger.
According to further optional embodiments related to the apparatus features of the invention:
Several embodiments of the invention are possible:
Minimal LNG production using the installation of
Air 1 is compressed in a main air compressor 3 to a pressure of 21.5 bar. and is cooled through the use of a mechanical refrigeration unit or an absorption refrigeration unit to a temperature of 12° C. Air 1 is then purified through adsorbers 5 containing typically and molecular sieve and impurities like water and CO2 are removed. A low temperature for the purification unit is preferred for several reasons air will enter the main heat exchanger at a lower temperature allowing an increase in the LNG production, air will content less water and adsorption is more efficient therefore less alumina and molecular sieve will be required. Air 1 (base=1000 Nm3/h) is then introduced in a main heat exchanger 7 typically of the plate-fin brazed aluminium type (alternately a spiral wound exchanger may be used) and is cooled to a temperature of −145° C. and split in two streams 9, 11: first stream 9 (848 Nm3/h) is expanded through an expansion turbine 13 to a pressure of 5.6 bar, a temperature of −173.5° C. and a liquid fraction of more than 10%. It has been assumed that the energy resuffing from this expansion is recovered in a generator. Nevertheless, several other alternates are available such as:
Intermediate liquid production using the installation of
Large liquid production in the installation of
The table below shows the production of LNG and the power consumption for a GTL plant using 20000 t/day of oxygen.
When comparing minimal LNG production to ASU alone, the air separation unit is much simpler: a single air compressor compared to an air compressor and a booster air compressor, a precooling system and a purification unit operating at a higher pressure allowing a significant reduction in size of those equipment thanks to the smaller volume flow and to a better efficiency of adsorption. Therefore, this minimal liquid production is made available for a negative investment.
Alternatively a process as shown in
In this case air 1 is compressed in a main air compressor 3 to a pressure of 21.5 bar and is cooled through the use of a mechanical refrigeration unit or an absorption refrigeration unit to a temperature of 12° C. Air 1 is then purified through adsorbers 5 containing typically alumina and molecular sieve and impurities like water and CO2 are removed. Air 1 (base=1000 Nm3/h) is then introduced in a main heat exchanger 7 typically of the plate-fin brazed aluminium type (alternately a spiral wound exchanger may be used) and is cooled to a temperature of −145° C. and split in two streams 9, 11: first stream 9 (848 Nm3/h) is expanded through an expansion turbine 13 to a pressure of 5.6 bar, a temperature of −173.5° C. and a liquid fraction of more than 10 mol %. Second stream 11 (152 Nm3/h) is further cooled, condensed and subcooled to a temperature of −174.8° C. Both streams are introduced into the medium pressure column 15 of the cryogenic air separation plant, but at different levels. Oxygen enriched and nitrogen enriched liquid streams are removed from the medium pressure column 15 and sent to the low pressure column 17. Nitrogen enriched gaseous stream 27 (flow: 80 Nm3/h) is also removed from this column. From this distillation column 17, a liquid oxygen enriched stream 21 (200 Nm3/h) is removed and pumped by pump 23 to a pressure of 53.5 bar, a gaseous nitrogen enriched streams 19 is also removed from the low pressure column 17 at low pressure 1.25 bar abs. and a temperature of −176° C. (this stream has been used to subcool streams internal to the distillation section; flow: 720 Nm3/h). Those two streams 19, 21 are warmed in the heat exchanger 7.
A pre-treated natural gas stream GN 25 (from which Hg, H2S, H2O and CO2 have been removed) at a pressure of 60 bar abs. and a temperature dose to ambient is introduced into an additional heat exchanger 32 with a flow of 38 Nm3/h. If stream 25 contains heavy hydrocarbons, it can be removed at an intermediate temperature of the additional exchanger 32 to remove those heavy hydrocarbons as shown in U.S. Pat. No. 5,390,499 and then reintroduced in the additional heat exchanger 32 to be further cooled to a temperature of around −165° C. and sent to storage after expansion through a valve or a liquid turbine as flow GNL. In the additional heat exchanger 32, the natural gas exchanges heat with nitrogen enriched gaseous stream 27 and a fluid flowing in a dosed circuit 26. The fluid in this circuit is typically an inert gas such as argon, nitrogen, CF4, HCF3 or any other refrigerant. It is heated in exchanger 32 where it is at least partially vaporised (or pseudo-vaporised if above supercritical pressure) and cooled down in exchanger 7 where it is at least partially condensed (or pseudo-condensed if above supercritical pressure). The liquefied natural gas is removed from the heat exchanger 32.
A 20,000 ton/day (7.3 million tons per year) oxygen air separation unit cannot be built today in a single train essentially due to size limitations for the columns. Typically 3 to 5 trains are required. On the contrary, it is possible to built a single liquefaction train for a size of 14,000 ton/day (5 million tons per year). Therefore, an optimisation of the solution of
Another optimisation results from the fact that an air separation unit where oxygen is vaporised between 30 and 60 bar can provide cold at very low level of temperature (130° C. to −110° C.). Therefore it is possible to condense natural gas (depending on its composition) at low pressures between 10 and 20 bar abs. Two options are then available:
In
In all the Figures, it is possible to produce argon in classical fashion using stream 18. It is also possible to send part of stream 11 to low pressure column. Moreover, liquids extracted from medium pressure column can be cooled down by indirect heat exchange with stream 19 prior to expand them in a valve and introduce them in the low pressure column. It is also possible to replace the expansion valves on stream 11 and on LNG by liquid turbines. If any of the compressor is driven by a gas turbine it is also possible to extract air from this gas turbine to feed at least partially the air separation unit(s).
Air 1 is compressed to a medium pressure in compressor 3 (5.8 bar) and is cooled through the use of a mechanical refrigeration unit or an absorption refrigeration unit to a temperature of 12° C. Air is then purified through adsorbers 5 containing typically alumina and molecular sieves and impurities like water and CO2 are removed. Air (base=1000 Nm3/h) is then divided in 2 streams. First air stream (flow 455 Nm3/h) is further compressed to a pressure of 66 bar abs. in booster 6, cooled and then introduced in an heat exchanger 7 typically of the plate-fin brazed aluminium type (alternately of the spiral wound exchanger type) and is cooled to a temperature of −98° C. and split in two substreams 9, 11: first stream 9 (65 Nm3/h) is expanded through an expansion turbine 13 to a pressure of 5.6 bar abs., a temperature of −173.4° C. and introduced in the medium pressure column 15. Second substream 11 (390 Nm3/h) is further cooled, condensed and subcooled to a temperature of −168.2° C. It is introduced in the medium pressure column 15. Second air stream (flow 545 Nm3/h) is cooled in an heat exchanger 7 and also introduced in medium pressure column. Oxygen enriched and nitrogen enriched streams are removed from the medium pressure column 15 and sent to the low pressure column 17. From this distillation column 17, a liquid oxygen enriched stream 21 (200 Nm3/h) is removed and pumped to a pressure of 53.5 bar, two gaseous nitrogen enriched streams 19, 27 are also removed, one 19 at low pressure 1.25 bar and a temperature of −175.2° C. (this stream has been used to subcool streams internal to the distillation section; flow: 720 Nm3/h), another 27 at medium pressure 5.5 bar and −177.8° C. (flow 80 Nm3/h). Those three streams are warmed in the heat exchanger and oxygen is vaporised.
Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims. In particular, any precooling cycle already described for natural gas liquefaction could be used and any air separation unit cycle with isentropic expansion could be used to provide refrigeration to liquefy natural gas.
This application claims the benefit under 35 U.S.C. § 119(e) to provisional application No. 60/423,039, filed Nov. 1, 2002, the entire contents of which are incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2909906 | Tung et al. | Oct 1959 | A |
| 3339370 | Tutton et al. | Sep 1967 | A |
| 3593535 | Gaumer et al. | Jul 1971 | A |
| 3763658 | Gaumer et al. | Oct 1973 | A |
| 3768271 | Denis | Oct 1973 | A |
| 3886758 | Perrotin et al. | Jun 1975 | A |
| 5137558 | Agrawal | Aug 1992 | A |
| 5220798 | Nagamura et al. | Jun 1993 | A |
| 5390499 | Rhoades et al. | Feb 1995 | A |
| 5615561 | Houshmand et al. | Apr 1997 | A |
| 5768912 | Dubar | Jun 1998 | A |
| 6131407 | Wissolik | Oct 2000 | A |
| Number | Date | Country |
|---|---|---|
| 199 20 312 | Nov 2000 | DE |
| 2 122 307 | Sep 1972 | FR |
| 2 172 388 | Sep 1986 | GB |
| 2172388 | Sep 1986 | GB |
| WO 98 36038 | Aug 1998 | WO |
| WO 00 71951 | Nov 2000 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20040083756 A1 | May 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60423039 | Nov 2002 | US |
