The invention relates to a process for liquefaction of natural gas and other methane-rich gas streams, and more particularly relates to a process to produce pressurized liquid natural gas (PLNG).
Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called xe2x80x9cLNGxe2x80x9d) for transport to market.
In the design of a LNG plant, one of the most important considerations is the process for converting natural gas feed stream into LNG. The most common liquefaction processes use some form of refrigeration system.
LNG refrigeration systems are expensive because so much refrigeration is needed to liquefy natural gas. A typical natural gas stream enters a LNG plant at pressures from about 4,830 kPa (700 psia) to about 7,600 kPa (1,100 psia) and temperatures from about 20xc2x0 C. (68xc2x0 F.) to about 40xc2x0 C. (104xc2x0 F.). Natural gas, which is predominantly methane, cannot be liquefied by simply increasing the pressure, as is the case with heavier hydrocarbons used for energy purposes. The critical temperature of methane is xe2x88x9282.5xc2x0 C. (xe2x88x92116.5xc2x0 F.). This means that methane can only be liquefied below that temperature regardless of the pressure applied. Since natural gas is a mixture of gases, it liquefies over a range of temperatures. The critical temperature of natural gas is between about xe2x88x9285xc2x0 C. (xe2x88x92121xc2x0 F.) and xe2x88x9262xc2x0 C. (xe2x88x9280xc2x0 F.). Typically, natural gas compositions at atmospheric pressure will liquefy in the temperature range between about xe2x88x92165xc2x0 C. (xe2x88x92265xc2x0 F.) and xe2x88x92155xc2x0 C. (xe2x88x92247xc2x0 F.). Since refrigeration equipment represents such a significant part of the LNG facility cost, considerable effort has been made to reduce the refrigeration costs and to reduce the weight of the liquefaction process for offshore applications. There is an incentive to keep the weight of liquefaction equipment as low as possible to reduce the structural support requirements for liquefaction plants on such structures.
Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) xe2x80x9ccascade cyclexe2x80x9d which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature, (2) xe2x80x9cmulti-component refrigeration cyclexe2x80x9d which uses a multi-component refrigerant in specially designed exchangers, and (3) xe2x80x9cexpander cyclexe2x80x9d which expands gas from a high pressure to a low pressure with a corresponding reduction in temperature. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
The cascade system generally uses two or more refrigeration loops in which the expanded refrigerant from one stage is used to condense the compressed refrigerant in the next stage. Each successive stage uses a lighter, more volatile refrigerant which, when expanded, provides a lower level of refrigeration and is therefore able to cool to a lower temperature. To diminish the power required by the compressors, each refrigeration cycle is typically divided into several pressure stages (three or four stages is common). The pressure stages have the effect of dividing the work of refrigeration into several temperature steps. Propane, ethane, ethylene, and methane are commonly used refrigerants. Since propane can be condensed at a relatively low pressure by air coolers or water coolers, propane is normally the first-stage refrigerant. Ethane or ethylene can be used as the second-stage refrigerant. Condensing the ethane exiting the ethane compressor requires a low-temperature coolant. Propane provides this low-temperature coolant function. Similarly, if methane is used as a final-stage coolant, ethane is used to condense methane exiting the methane compressor. The propane refrigeration system is therefore used to cool the feed gas and to condense the ethane refrigerant and ethane is used to further cool the feed gas and to condense the methane refrigerant.
A mixed refrigerant system involves the circulation of a multi-component refrigeration stream, usually after precooling to about xe2x88x9235xc2x0 C. (xe2x88x9231xc2x0 F.) with propane. A typical multi-component system will comprise methane, ethane, propane, and optionally other light components. Without propane precooling, heavier components such as butanes and pentanes may be included in the multi-component refrigerant. The nature of the mixed refrigerant cycle is such that the heat exchangers in the process must routinely handle the flow of a two-phase refrigerant. This requires the use of large specialized heat exchangers. Mixed refrigerants exhibit the desirable property of condensing over a range of temperatures, which allows the design of heat exchange systems that can be thermodynamically more efficient than pure component refrigerant systems.
The expander system operates on the principle that gas can be compressed to a selected pressure, cooled, typically be external refrigeration, then allowed to expand through an expansion turbine, thereby performing work and reducing the temperature of the gas. It is possible to liquefy a portion of the gas in such an expansion. The low temperature gas is then heat exchanged to effect liquefaction of the feed. The power obtained from the expansion is usually used to supply part of the main compression power used in the refrigeration cycle. The typical expander cycle for making LNG operates at pressures under about 6,895 kPa (1,000 psia). The cooling has been made more efficient by causing the components of the warming stream to undergo a plurality of work expansion steps.
It has been recently proposed to transport natural gas at temperatures above xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. For most natural gas compositions, the pressure of the natural gas at temperatures above xe2x88x92112xc2x0 C. will be between about 1,380 kPa (200 psia) and about 4,480 kPa (650 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.). Processes for making PLNG are disclosed in U.S. Pat. No. 5,950,453 by R. R. Bowen et al., U.S. Pat. No. 5,956,971 by E. T. Cole et al., U.S. Pat. No. 6,023,942 by E. R. Thomas et al., and U.S. Pat. No. 6,016,665 by E. T. Cole et al.
U.S. Pat. No. 6,023,942 by E. R. Thomas et al. discloses a process for making PLNG by expanding feed gas stream rich in methane. The feed gas stream is provided with an initial pressure above about 3,100 kPa (450 psia). The gas is liquefied by a suitable expansion means to produce a liquid product having a temperature above about xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.) and a pressure sufficient for the liquid product to be at or below its bubble point temperature. Prior to the expansion, the gas can be cooled by recycle vapor that passes through the expansion means without being liquefied. A phase separator separates the PLNG product from gases not liquefied by the expansion means. Although the process of U.S. Pat. No. 6,023,942 can effectively produce PLNG, there is a continuing need in the industry for a more efficient process for producing PLNG.
This invention discloses a process for liquefying a pressurized gas stream rich in methane. In a first step, a first fraction of a pressurized feed stream, preferably at a pressure above 11,032 kPa (1,600 psia), is withdrawn and entropically expanded to a lower pressure to cool and at least partially liquefy the withdrawn first fraction. A second fraction of the feed stream is cooled by indirect heat exchange with the expanded first fraction. The second fraction is subsequently expanded to a lower pressure, thereby at least partially liquefying the second fraction of the pressurized gas stream. The liquefied second fraction is withdrawn from the process as a pressurized product stream having a temperature above xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.) and a pressure at or above its bubble point pressure.