METHOD AND APPARATUS FOR THE LIQUEFACTION OF NATURAL GAS

A method for the liquefaction of natural gas, comprising: taking unpurified natural gas from a gas well; pre-treating the gas for removing impurities; performing a first compression stage; performing a first heat exchange stage; performing a second compression stage; performing a second heat exchange stage; performing a third compression stage; performing a third heat exchange stage; performing an additional regeneration heat exchange stage; performing a first main independent heat exchange cycle; performing a second main heat exchange cycle; passing the gas through a Joule-Thomson valve; sending the liquefied gas to storage; injecting the portion of the gas in the gaseous state into the second main heat exchange stage; and the remaining gas in the gaseous state is injected into the first main heat exchange stage.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is referred to a method and apparatus for the liquefaction of natural gas that includes an exclusive high pressure LNG liquefaction cycle for small scale production. This cycle allows the production at very economical costs of up to 20000 Nm3/day of LNG with a very low operating cost and a significantly lower level investment in comparison with any other known facility available in the market.

Making first reference toFIG. 1, the cycle100starts at the inlet101of unpurified natural gas. This inlet may come from a as well an oil well, a natural gas production pipe, or the like. That gas is pretreated at the treatment device102to suit it to liquefaction conditions. This natural gas contains, at this stage, impurities that need to be removed. Some of these impurities may be carbon dioxide, nitrogen, and other gases. These impurities must be removed to make the gas fit for this cycle.

Once the gas is purified at102, it enters into a first high-pressure multi-stage compressor103. After the stage103, the gas enters into a beat exchanger104after which the mixture of liquid and gaseous natural gas enters a cold box105from which the liquefied gas is transferred to a flash liquid/gas separator106for Its storage107and/or transportation108. Between the cold box and the separator an expansion valve is included to reduce the pressure creating a Joule Thompson effect.

An independent propane refrigeration cycle120including an additional high-pressure multi-stage109and a heat exchange stage110helps the general cycle100to perform the last liquefaction process in the cold box105.

The natural gas that still remains in the gaseous state in the cold box105is then transferred to a booster compressor111and a heat exchanger112before it is re-injected into the inlet pipe being mixed with the as incoming from the treatment plant.

FIG. 2shows in more detail the present method100for the liquefaction of natural gas. The high-pressure multi-stage compressor103is shown in this figure as a three-stage process including a first compression stage201for compressing the natural gas coming from the inlet pipe122at around 10/11 bar to around 30/35 bar. During this compression stage, the temperatures rise from around 36° C. to 148° C.; therefore, it is necessary to reduce this temperature drastically for which the gas enters into a first heat exchange stage202to cool down the temperature of the gas from 148° C. to 40° C.

Immediately afterwards, a second compression stage203starts for compressing the natural gas from around 33 bar at the end of the first stage to around 95 bar, and during which the temperatures rise from 40° C. to 149° C. Therefore, a second heat exchange stage204for cooling down the temperature of the gas from 149° C. to 40° C. is performed.

At the outlet of the second stage, a third compression process205is performed for compressing the natural gas from around 95 bar to around 250 bar, and in which the temperatures rise from 40° C. to 136° C. Therefore, in a heat exchanger206, a third heat exchange stage cools down the temperature of the gas from 136° C. to 40° C.

At the end of this process the natural gas is at 250 bar and 40° C. It is then injected into a regeneration cycle207that cools down the temperature of the gas from 40° C. to 7° C. keeping the pressure at 250 bar.

A first main independent heat exchange cycle208cools down the temperature of the gas from 7° C. to −47° C. while pressure is kept at 250 bar. This independent cycle is fed by a propane cycle120that includes a compressor121, a heat exchanger122, a second compressor123and a second heat exchanger124that basically liquefies, expands and vaporizes said propane gas.

At the exit of this first main independent heat exchange cycle208, a second main heat exchange cycle209cools down the temperature of the gas from −47° C. to −77° C. while pressure is kept at 250 bar. This second main heat exchange cycle209uses the energy of the returning gas through line210as will be explained in detail below. At this stage, part of the mass of the natural gas is already in a liquid state. The mix of liquid and gaseous natural gas at −77° C. and 250 bar is expanded in an separator device211including an internal thermal expansion valve that reduces the gas pressure from 250 bar to 2 bar. This significant, drop in the gas pressure also produces a significant drop in the gas temperature because of the Joule-Thomson effect. The Joule-Thomson expansion describes the temperature change of a gas when it is forced through a valve while kept insulated so that no heat is exchanged with the environment. This procedure is called a throttling process or Joule-Thomson process.

Thus, at the exit of this evaporator211, around half of the mass of natural gas has been liquefied as it is at −151° C. and 2 bar. It is then collected for further processing (storage of transportation) through a pipe215. The gaseous portion of the gas at −151° C. and 2 bar is injected through the pipe210into the second main heat exchange cycle209. Because of the temperature difference between the gas returning from pipe210(−151° C.) and the gas entering the second main heat exchange cycle209at −47° C., this returning gas helps the heat exchange process. At the exit of said second main heat exchange cycle209, through pipe212, said returning gas is at −60° C. and 2 bar. It is finally injected, into the first main heat exchange cycle207through pipe213to help in the heat exchange process of this cycle. As in the previous case, the temperature difference between the returning gas (−60° C.) and the entering gas (40° C.) makes this returning gas an important helping role in the beat exchange cycle.

Before this returning gas can be incorporated, in the aspiration pipe122, it must be compressed, as the entrance gas is already at 11 bar and this returning gas is at 2 bar. Thus, this returning gas at 37° C. and 1.9 bar enters a compressor216that raises the gas pressure from 2 bar to 11 bar and the temperature rises from 37° C. to 224° C. Therefore, before injecting it into the inlet pipe122is injected into a heat exchanger217that cools it down from 224° C. to 40° C. Through pipe218, this returning gas is finally reincorporated in the circuit and the process may start again.

FIG. 3shows one example of a transportable and compact plant used to perform the liquefaction method of the present invention. In the module300illustrated inFIG. 3all the necessary elements are included. Therefore once the inlet has pipe and the LNG outlet liquid gas are connected the plant is fully operational. Instead of having all the typical complex and expensive means used in the LNG plants of the prior art, this solution provides a flexible, affordable solution for low volumes application.

In the general perspective view ofFIG. 3some of the parts can be distinguished, including a compressor301, three heat exchangers302, a GNL module303, the GNL outlet304, refrigerating funs305, two vent chimneys306and a display control307.

FIGS. 4-6show an internal unit of the module ofFIG. 3. The unit includes accumulators321, a GNL outlet322, and a propane inlet323. The heat exchangers325of the unit320are protected by an external isolating cover324. The unit also includes a propane outlet326and a natural gas outlet327.