RU2352877C2 - Method of liquefying natural gas - Google Patents

Abstract

FIELD: heating.

SUBSTANCE: in the method of liquefying a stream reached in hydrocarbons, liquefaction is carried out in a cascade of loops with a coolant, consisting of three refrigeration circuits. The first of the three refrigeration circuits serves to provide preliminary cooling, the second refrigeration circuit is to provide factual liquefaction and the third refrigeration circuit is for guaranteeing supercooling of the liquefied flow, enriched by hydrocarbons. Each refrigerator circuit includes at least one single-step or multi-step compressor. At least part of the flow of the coolant from the second refrigeration circuit is used for cooling the specified flow, enriched by hydrocarbons, to ensure the preliminary cooling of the flow to a temperature, under which the liquefaction of C₃-components begins, such that each compressor has an equal share of the load.

EFFECT: increase in the productivity of the installation for liquefaction of gas.

22 cl, 6 dwg, 3 tbl

Description

The present invention relates to a method for liquefying a hydrocarbon-rich stream.

Natural gas can be obtained from the earth to produce natural gas, which must be processed before being used in industry. Typically, the gas is first pretreated to remove or reduce the content of impurities such as carbon dioxide, water, hydrogen sulfide, mercury, and the like.

Gas is often liquefied before being transported to the place of use to provide liquefied natural gas (LNG; LNG). This makes it possible to reduce the volume of gas to about 600 times, which significantly reduces the cost of transportation. Since natural gas is a mixture of gases, it liquefies over the entire temperature range. At atmospheric pressure, the usual temperature range in which complete liquefaction takes place is from -165 ° C to -155 ° C. However, since the critical temperature of natural gas is about -80 ° C to -90 ° C, the gas cannot be completely liquefied by compression. Therefore, it is necessary to use cooling processes.

Natural gas liquefaction plants are designed either as those known as LNG main load units, i.e. installations for liquefying natural gas with the supply of natural gas as the main energy, or as those known as installations for smoothing peaks, i.e. natural gas liquefaction plants to cover peak requirements.

It is known to cool natural gas by using heat exchangers that use a refrigerant or coolant. One well-known method comprises a series of coolant cycles or cascade-type refrigeration cycles.

Units with the main LNG load usually operate with coolant circuits consisting of a mixture of hydrocarbons. These mixed refrigerant circuits are more energy efficient than expander circuits and make it possible to achieve relatively low energy expenditures at high productivity for liquefying main load plants.

Conventional liquefaction processes using only two refrigerant cycles are limited to approximately 5 million tons per year (MTVG) of LNG unless the use of parallel rows in a single unit of successive elements is considered.

A cascaded mixed-fluid process is known, for example, from U.S. Pat. No. 6,253,574, which uses three independent refrigeration cycles, and it extends the notion of one real unit of serial elements with proven compressor drives to more than 8 MTVG LNG.

This method is also known from German published application 19716415.

With liquefaction methods of this type, in principle, the first circuit with a coolant serves to provide pre-cooling, the second circuit with a coolant serves to provide liquefaction, and the third circuit with a coolant serves to provide subcooling of a hydrocarbon-rich stream or natural gas, respectively.

Between pre-cooling and liquefaction, if necessary, there is a separation of hydrocarbons with higher boiling points. These are at least those components of the hydrocarbon-rich stream or natural gas that will be frozen during the next cooling stage, i.e. C ₅₊ hydrocarbons and aromatic hydrocarbons. Often, in addition, those hydrocarbons meaning in this situation, especially propane and butane, which will cause an undesirable increase in the calorific value of liquefied natural gas, are also separated before the liquefaction stage.

This separation of hydrocarbons with higher boiling points usually occurs by ensuring that the column, known as the TUV (Heavy Hydrocarbon) column, serves to separate heavy hydrocarbons as well as benzene from the hydrocarbon-rich stream to be liquefied. A process step of this type is also described in German published application 19716415 mentioned above.

Due to the fact that this compartment is provided, hereinafter referred to as C ₃₊ compartment, at a given pressure of the source gas, the temperature of separation of these components is set within relatively narrow limits.

If the first refrigerant circuit is now used exclusively for pre-cooling the hydrocarbon-rich stream that is to be liquefied before this C ₃₊ separation, then part of the entire compression effect, from about 40 to 50%, will be definitely spent on it, while the remaining the compression effect of 60 to 50% will be divided between the second and third coolant circuits.

However, in the sense of economical operation of the available compressor and drive units, the inventors have found that it is desirable to maintain approximately the same drive power for the compressors (circuit) of the three loops, i.e. in each case, approximately a third of the total drive power. This is applicable, in particular, to large liquefaction plants with a liquefaction capacity greater than 5 MTVG, since the number of compressors and drive units available for such orders of magnitude is strictly limited. By standardizing the drive units and compressors of these three coolant circuits, it is possible to obtain the maximum available liquefaction capacity during the liquefaction process using proven drive units and compressors, respectively, which are trustworthy.

Thus, in accordance with one aspect of the invention, there is provided a LNG liquefaction process having first and second refrigeration circuits, in which a second refrigeration circuit is used, at least in part, to pre-cool the hydrocarbon-rich stream to be liquefied. Thus, the installed capacity of gas turbines and starters, at least during the normal liquefaction operation, can be fully exploited.

A portion of the refrigerant of the liquefaction cycle (LC; CA) can be vaporized at elevated pressure in the pre-cooling section of the process and fed to the compressor CS as a side stream. Thus, a substantially balanced load between all refrigeration cycles can be achieved.

Therefore, in accordance with one aspect of the present invention, there is provided a method of liquefying a hydrocarbon-rich stream, in particular a natural gas stream, by which the liquefaction of a hydrocarbon-rich stream is carried out in a cascade of refrigeration circuits consisting of three refrigeration circuits, the first of three refrigeration circuits serving providing pre-cooling, the second refrigeration circuit serves to provide the actual liquefaction, and the third refrigeration circuit serves to supercool Ia liquefied hydrocarbon-rich flow, each refrigeration circuit comprises at least one single-stage or multistage compressor, characterized in that at least one partial flow of the refrigerant of the second refrigeration circuit is used for precooling the hydrocarbon-rich flow.

In another aspect, the invention provides a method of liquefying a hydrocarbon-rich gas in which gas flows through a cascade of three cooling stages, each stage comprising a refrigerant circuit and a compressor, wherein at least a portion of the secondary refrigerant stream is used to pre-cool the gas, enriched with hydrocarbons, in the first stage of cooling.

Preferably, the partial refrigerant stream of the second refrigerant (or cooling) circuit used to pre-cool the hydrocarbon-rich stream is vaporized at a pressure higher than the vapor pressure of the remaining portion of the refrigerant stream of the second refrigerant circuit and is passed to the compressor of the second refrigerant circuit at an intermediate pressure value.

Preferably, the separation of the heavier components and / or components of the hydrocarbon-rich stream that are frozen during the liquefaction of the hydrocarbon-rich stream takes place before the actual liquefaction of the hydrocarbon-rich stream.

Preferably, the volumes and / or vapor pressures of the two partial streams of the second cooling circuit are variable.

Preferably, at least one partial stream of one of the two partial flows of the second cooling circuit is used to provide cooling in the heavy hydrocarbon separation unit.

Thus, the invention provides a balanced load LNG liquefaction process in which each compressor can have a substantially equal fraction of the total load, and preferably an equal fraction. This concept can be applied more broadly, and therefore, in another aspect of the present invention, there is provided a liquefaction method comprising a number of cooling circuits arranged in a cascade, each circuit containing a compressor, each compressor having a substantially equal fraction of the total load .

The benefits of a balanced load of refrigeration circuits are not limited to any particular type of refrigerant used. However, since the mixed refrigerant stages mentioned above provide an efficient system, and therefore, in one preferred embodiment, the refrigerant circuits are mixed refrigerant circuits.

Thus, in another aspect of the present invention, there is provided a method of liquefying a hydrocarbon-rich stream, in particular a natural gas stream, by which the liquefaction of a hydrocarbon-rich stream is carried out in a cascade of circuits with a mixed refrigerant consisting of three refrigerant circuits, the first of three refrigerant circuits circuits serves to provide pre-cooling, a second refrigeration circuit serves to provide actual liquefaction, and a third refrigeration circuit serves to supercooling a liquefied hydrocarbon-rich stream, wherein each refrigerant circuit comprises at least one single-stage or multi-stage compressor, characterized in that at least one partial refrigerant stream of the second refrigerant circuit is used to pre-cool the hydrocarbon-rich stream.

It should be noted that the use of hydrocarbons as refrigerants raises the issue of safe release, which is especially important for the environment on the high seas, where it is very undesirable to have large reserves of liquid hydrocarbons, where space is inevitably limited.

Afloat, LNG storage and shipping equipment (LNG FPSOs) is now seen as a viable alternative to remote offshore gas fields that cannot be economically exploited in traditional onshore technologies. The concept of afloat production may soon become the preferred solution for the selection of gas reserves that are deep under water.

Therefore, the need to improve the safety of such installations is of great importance.

One possibility is to use a nitrogen-based process, but it has the significant drawback that thermal efficiency is much lower than hydrocarbon-based plants. In addition, since nitrogen has a low heat transfer coefficient, a large heat transfer surface is required to dissipate the waste heat from the process into the cooling medium. Therefore, despite a safety risk, hydrocarbon-based refrigeration cycles continue to be used.

Another option for a non-flammable and inert refrigerant is carbon dioxide, which can be used in the steam compression cycle, creating sufficient efficiency. Carbon dioxide has a freezing point of -56.6 ° C, which limits the minimum possible evaporation temperature due to the risk of dry ice formation. Therefore, carbon dioxide is an option only for the pre-cooling process. Since most hydrocarbon refrigerant reserves are in the pre-cooling cycle, replacing it with CO ₂ can still significantly increase the safety of the liquefaction process.

Besides the fact that carbon dioxide is non-flammable and has a high triple point, it also differs from conventional hydrocarbon refrigerants for pre-cooling natural gas by a rather low critical temperature (31.1 ° С), which is comparable to the critical temperature of ethane (32.3 ° С )

WO 01/69149 describes the possibility of providing a pre-cooling circuit with carbon dioxide in a cascade device with a main cooling circuit.

The low critical temperature of CO ₂ is a drawback, since losses during throttling and losses during heat removal in the refrigeration cycle will be greater than for C ₃ and C ₃ / C ₂ mixtures _. In addition, losses in heat transfer will be greater than with mixed refrigerant due to evaporation at a constant temperature.

It has been found that replacing the conventional pre-cooling process with C ₃ / C ₂ , for example, which is described in US Pat. No. 6,253,574, with an equivalent process with CO ₂ increases the total energy consumption for liquefaction by about 10%, which is considered unacceptable. This increase in consumption is associated with a decrease in cycle efficiency due to the low critical temperature of carbon dioxide. In addition, the evaporation temperature in the first stage of the pre-cooling cycle with CO _{2 is} only a few degrees higher than the triple point of CO ₂ . This leads to problems in operation and the risk of dry ice formation.

Thus, an efficient liquefaction process should be developed, including a pre-cooling circuit with CO ₂ .

The inventors of the present invention have found that a carbon dioxide pre-cooling circuit can be combined with the balanced load liquefaction process described above to overcome the aforementioned problems using carbon dioxide.

Therefore, in a preferred embodiment of the present invention, the first refrigeration circuit comprises carbon dioxide.

This concept can be considered an invention, and thus, in another aspect of the present invention, there is provided a cascaded process with a mixed refrigerant with a substantially balanced load, including a carbon dioxide pre-cooling circuit.

Since the liquefaction compressor accepts part of the load of the pre-cooling cycle, the carbon dioxide circuit can operate to provide a higher minimum evaporation temperature, and thus the risk of dry ice formation is reduced. In addition, when the load of the carbon dioxide cycle decreases, the effect of lower thermodynamic efficiency of CO ₂ as compared to C ₂ / C _{3 is} facilitated. In a process with a balanced load, where each compressor contributes as a third of the total energy consumption, the increase in energy consumption caused by the use of CO ₂ can be reduced by only a few percent more than when using hydrocarbons.

In order to achieve maximum efficiency from the carbon dioxide circuit, it is preferable that the carbon dioxide is cooled after condensation to a temperature of 20 ° C or less, more preferably to 15 ° C or less. This can be achieved using air cooling, although cold cooling water is preferably used. Since the invention is particularly suitable for use on the high seas, the water is preferably seawater, preferably recovered from a depth suitable to give the desired low temperature.

Thus, preferably, the carbon dioxide pre-cooling cycle includes a subcooling heat exchanger installed downstream of the condenser.

When using this method, the reduction in total energy consumption is large enough to make the use of a pre-cooling circuit with CO _{2 an} feasible choice of equipment for LNG production both onshore and on the high seas.

Preferably, the carbon dioxide cooling circuit contains three pressures to improve the thermodynamic efficiency of the process.

In order to reduce the internal heat load of the pre-cooling circuit, it is preferable that carbon dioxide is not supercooled in the pre-cooling circuit, in contrast to the second and third cycles with refrigerants, the complete supercooling of which increases the process efficiency.

The higher operating pressure required when using CO ₂ means that it may be preferable to use a high pressure housing with a carbon dioxide compressor. More preferably, the compressor can be divided into two housings, and a cylindrical housing is used for the high pressure stage.

In accordance with another aspect of the present invention, there is provided a LNG liquefaction process comprising three cascade cycles, each of which is driven by a compressor, in which the compressors are loaded substantially the same, and one of the cascade cycles is a carbon dioxide cycle.

In accordance with a further aspect of the present invention, there is provided a carbon dioxide pre-cooling circuit for liquefying LNG, in which carbon dioxide has a minimum evaporation temperature of not less than -50 ° C, preferably not less than -40 ° C, and most preferably not less than than -35 ° C.

Preferred embodiments of the present invention will now be described only by way of example with reference to the accompanying drawings, in which:

1 shows a load balanced liquefaction process in accordance with a preferred embodiment of the invention;

figure 2 shows an alternative embodiment of the process with a balanced load;

figure 3 shows a graph of the total energy requirements as a function of reference temperature;

4 shows a load balanced liquefaction process comprising a carbon dioxide pre-cooling circuit;

figure 5 shows the composite curves hot / cold for the processes shown in figure 2 and 4; and

Figure 6 shows a comparison of the stocks of the refrigerant in the processes shown in figures 2 and 4.

In Fig. 1, cooling and liquefaction of a hydrocarbon-rich stream that is passed through line 1 is carried out in a cascade of circuits with a mixed refrigerant, consisting of three circuits with a mixed refrigerant. Here, as a rule, there are various compositions, such as described, for example, in the aforementioned German published application 19716415.

The hydrocarbon-rich stream to be liquefied is cooled in the heat exchanger E1 by two evaporative mixed refrigerant streams 4b to 4d of the first mixture circuit 4a-4e, then cooled by a 3d flow of the evaporated mixed refrigerant and then passed through line 1a to the heavy separation unit S hydrocarbons represented by a rectangle.

In this separation unit S, the C ₃₊ separation described above takes place, whereby the components separated from the hydrocarbon-rich stream are discharged from the heavy hydrocarbon separation unit S through conduit 1b.

In accordance with one advantageous embodiment of the method in accordance with the invention, not shown, at least one partial stream of one of the two partial streams 3b and 3d of the second circuit 3a-3e with a mixed refrigerant, which will be described in more detail below, is used to provide cooling in the compartment unit S. In such a situation, the choice of which of the two partial streams 3b and / or 3d is allocated for this cooling is determined by the temperature value (s) required in the heavy hydrocarbon separation unit S.

The hydrocarbon-rich stream to be liquefied is then passed through conduit 1c to the second heat exchanger E2 and liquefied therein via the vaporized mixed refrigerant stream 3b of the second cooling circuit 3a-3e.

When liquefaction takes place, the hydrocarbon-rich stream is passed through conduit 1d to the third heat exchanger E3 and is cooled here by the mixed refrigerant stream 2b of the third cooling circuit 2a-2c. The supercooled liquid product is then passed through line 1e for its further use.

As can be seen from the drawing, each of the three cooling circuits 2a-2c, 3a-3e and 4a-4e has a compressor V2, V3 and V4, respectively. The corresponding drives for these compressors V2, V3 and V4 are not shown in the drawing. In addition, chillers or heat exchangers that are located downstream than compressors V2, V3 and V4, respectively, are not shown in the drawings, in which the mixture of refrigerants is cooled by means of a cooling medium such as water.

The mixture of primary refrigerants with a refrigerant compressed in compressor V4 is passed through line 4a to the heat exchanger E1 and is divided here into two partial flows 4b and 4d after cooling. The mixture of refrigerants in these partial flows 4b and 4d after throttling in valves d and e, or in throttling devices, evaporates to various pressures in the heat exchanger E1 and then passes through line 4c or 4e to compressor V4 before the first stage (partial flow 4c) or in an intermediate pressure value (partial flow 4e).

The mixture of refrigerants of the second cooling circuit 3a-3e, compressed in compressor V3, is passed through pipe 3a, through heat exchangers E1 and E2 and cooled in them. Then, a partial stream 3b of this refrigerant mixture stream, which is passed through the heat exchanger E2, after expansion in the valve b, is evaporated in the heat exchanger E2 by means of cooling process flows and then passed through the pipe 3c to the inlet stage of the compressor V3.

In accordance with the invention, a partial 3d stream of a mixture of refrigerants of the second circuit 3a-3e with a mixture of refrigerants is discharged after the heat exchanger E1, expands in the valve c and then evaporates in the heat exchanger E1 through flows of the cooling process, before passing through the pipe 3e, at an intermediate pressure into the compressor V3 circuit. Accordingly, a partial 3d stream of a mixture of refrigerants according to the invention contributes to the pre-cooling of the hydrocarbon-rich stream in the heat exchanger E1.

In order to achieve this, a partial 3d stream of a mixture of refrigerants of the second circuit 3a-3e with a mixed refrigerant used to pre-cool the hydrocarbon-rich stream must evaporate at a pressure higher than the vaporization pressure of the partial stream 3b of a mixed refrigerant of the second circuit 3a-3e with a mixed refrigerant.

By selecting the intermediate pressure at which the partial mixed refrigerant stream 3e is vaporized and passed to the compressor V3, and by controlling the volume distribution of the two partial mixed refrigerant streams 3b and 3d, the distribution of the refrigeration capacity of the second circuit with the refrigerant through the heat exchangers E1 and E2 and, therefore, by pre-cooling and liquefying the hydrocarbon-rich stream to be liquefied, it can be controlled mainly arbitrarily.

If, for example, 40% of the total drive power is required for pre-cooling and 60% for liquefaction and subcooling of a hydrocarbon-rich stream, then, in accordance with the concept and method of the invention, one compressor is used in each case with a third of the total drive power in the first and third circuits with a mixture of refrigerants, i.e. for pre-cooling, as well as for sub-cooling of a hydrocarbon-rich stream to be liquefied. The secondary compressor with a mixture of refrigerants works in accordance with the invention in such a way that it uses 20% of its capacity, and therefore 6.66% of the total capacity, for pre-cooling, while the remaining 80%, i.e. . 26.66% of the total capacity used for liquefaction.

Thus, the method according to the invention makes it possible to economically operate the available compressors and drive units, since the (compressor) compressors of the three refrigeration circuits receive approximately the same drive power, i.e. a third of the total capacity in each case. Accordingly, large liquefaction plants, in particular those with a liquefaction capacity of more than 5 million tons of LNG per year, can operate substantially more economically because by standardizing the drives and compressors of the three refrigeration circuits, the achieved liquefaction performance of the liquefaction process can be brought to the maximum using reliable drive units and compressors.

Figure 2 shows an alternative version of the process with a balanced load. As in FIG. 1, the pre-cooling cycle C10 comprises a first circuit driven by the first compressor V10 and one part 22 of the refrigerant stream 21 from the second cycle C20. Three gas turbines General Electric MS 7121 EA (Frame 7) are used to drive the compressors V10, V20, V30. If the highest reliability is essential, three refrigeration cycles can be designed with push-pull 50% gas turbine / compressor series components. In this case, six gas turbines GE MS 6581 B (Frame 6) will be replaced by three Frame 7.

All LNG plants require the extraction of at least those hydrocarbons that will freeze in LNG under storage conditions (e.g. aromatic and C ₅₊ ). In an LNG plant, pre-cooling is generally regarded as the first cooling stage between ambient temperature and recovery of the hydrocarbons mentioned.

It should be emphasized that the method in accordance with the invention can be combined with all known separation methods, which are considered prior art for relatively high boiling hydrocarbons.

A portion of the pre-cooling of the total required power of all compressors for cooling the two gases shown in Table 1 is shown in FIG. 3 as a function of reference temperature. It represents the temperature at which all the main process flows (natural gas, refrigerant fluids) enter the cryogenic heat exchangers.

Table 1 molar% Lean gas Enriched gas N ₂ 5.00 5.00 CH ₄ 88.93 84.07 C ₂ H ₆ 3.96 5.58 C ₃ H ₈ 1.37 2.73 C ₄ H ₁₀ 0.48 1.34 C ₅ H ₁₂ 0.17 0.65 C ₆ H ₁₄ 0.06 0.32 C ₇ H ₁₆ 0.02 0.16 C ₈ H ₁₈ 0.01 0.08 Benzene 0.01 0.08 100.00 100.00

The lower the reference temperature and the richer the gas, the lower the required compressor power for pre-cooling. This situation can be considered by the designers of dual-flow liquefaction processes if the power mismatch between pre-cooling and liquefaction, plus supercooling, is compensated by means of auxiliary mechanisms for gas turbines.

The three-refrigeration cycle process offers a much wider area for even load distribution between cycles. If a portion of the refrigerant of the liquefaction cycle C20 is vaporized at elevated pressure in the pre-cooling section C10 and supplied to the compressor V20 as side stream 22, a perfectly balanced load between all three refrigeration cycles can be achieved. This feature is the main aspect of a project with an effective cost for high productivity. If all three (3) cycles have symmetrical drives, this device is referred to as MFC * s3.

In contrast to the embodiment of FIG. 1, the last compressor V30 of FIG. 2 is divided into two housings V31, V32. The second V32 housing is designed to operate at high pressures, at which a multi-stage compressor operates.

In order to obtain specific values for the design of a real process, a large LNG plant from sequential elements was studied. Based on the composition of lean gas at a pressure of 62 bar and a temperature of 35 ° C at the inlet to the preliminary cooling, a conceptual design of the process was developed. Compressors for cooling are driven by device 7 ′ with an additional 20 MW on each shaft, which were triggered by starters / auxiliary mechanisms. The resulting amount of LNG produced is up to 8.5 MTVG with 333 days of continuous operation, which is supplemented by an additional amount of 0.4 MTVG of natural gas condensate (C ₃₊ hydrocarbons). The specific energy consumption of compressors for cooling is 259 kWh / t _LNG .

In FIG. 4, the pre-cooling circuit C10 of FIG. 2 has been replaced by a pre-cooling circuit C100, which comprises a carbon dioxide stream 101. After compression and condensation / subcooling, stream 101 is divided into three separate streams 102, 103, 104, which then expand to different pressures. This compensates for constant temperature evaporation of CO ₂ . Unlike hydrocarbon streams 201, 301, only a portion of the carbon dioxide stream 101 is subcooled by means of a heat exchanger E100 for pre-cooling before expansion in order to reduce the internal heat load of this heat exchanger.

Due to the higher operating pressure, the V100 compressor for pre-cooling with CO _{2 is} divided into two casings, V110, V120, with a cylindrical housing V120 for the high pressure stage. After compression, carbon dioxide is cooled by means of a water-cooled condenser C20 and an additional subcooling heat exchanger C22 using seawater to sub-cool the liquid refrigerant after the condenser C20 in order to improve process efficiency. In addition, a desuperheater can also be installed downstream of the compressor, as in many standard systems.

As in previous embodiments, a “balanced load” is achieved by the fact that the liquefaction compressor V200 is able to receive a certain amount of load from the pre-cooling cycle, which leads to a “symmetrical” process.

Modeling the process of the above embodiments, as shown in FIG. 4 and FIG. 2, allows to obtain data on the required energy, as shown in table 2, using calculation data, as shown in table 3. As a result of the process with balanced load, the input power in the case pre-cooling of CO ₂ was only 4.4% higher than the baseline. For a given maximum of available power, as determined in the case of a hydrocarbon process, this will correspond to 95.6% LNG capacity with pre-cooling with CO ₂ until a larger power drive is installed.

table 2 Pre-cooling with CO ₂ Pre-cooling with C ₂ / C ₃ MW Total shaft power 162.7 (104%) 155.8 (100%) MW Pre-cooling compressor 49.6 47.6 MW Liquefaction compressor 50,5 47.7 MW Subcooling compressor 50,5 48.5 MW Other power consumers 12.1 12.0 MW

Table 3 LNG performance 5.8 MTVG LNG total calorific value 40 MJ / Sm ³ Supply gas pressure (liquefaction inlet) 69 bar Sea cooling water temperature 5 ° C

The temperature profiles in the form of composite hot / cold curves for these two cases are shown in FIG. Three values of the pre-cooling temperature with CO _{2 are} easily observed in the left diagram. The highest pressure value for the compressor for liquefaction is also considered part of the pre-cooling. Changes in the process of subcooling between the two cases are minimal.

The location, size and weight of the LNG liquefaction module in the open sea with preliminary cooling with CO ₂ were compared with the baseline in the case of hydrocarbons (as shown in FIGS. 1 and 2). Among the factors that contribute to the reduction in the area occupied by the equipment and give smaller dimensions when using CO ₂ , there were reduced sizes of the compressor receiving cylinder for pre-cooling and smaller sizes of the pipe for pre-cooling. However, the additional equipment required due to the third pressure / pre-cooling cylinder and the installation of a refrigerant subcooler make the net reduction in equipment footprint marginal. Ribbed plate heat exchangers were reduced in size due to the higher LMTD (Mean Logarithmic Temperature Difference) and lower internal load. While finned plate heat exchangers were used in this example, it is of course also possible to use other types of heat exchangers, which can also be reduced in size. The dimensions of some of the main pipes in the liquefaction and subcooling circuits did not change significantly, and it is precisely these pipes that determine the height of the deck to a large extent, so that no changes will be considered regarding the elevations of the deck. In general, it was concluded that the size of the liquefaction module would not be larger when using a pre-cooling cycle with CO ₂ , and in fact a reduction of several square meters is possible. In addition, the weight of the module is reduced by 100 tons.

The main threat to the safety of LNG processes with pre-cooling by hydrocarbons, especially when used on the high seas, is the possible formation of a flammable and explosive hydrocarbon / air mixture in the event of a major leak in one of the refrigeration cycles. Thus, minimizing the supply of hydrocarbon refrigerant is very important from a safety point of view.

As can be seen from Fig.6, the supply of HC refrigerant decreases by about 70% during pre-cooling with CO ₂ . Reduced hydrocarbon loading is positive with respect to preventing leaks and achieving the three main safety functions of the LNG barge, which are: (i) the main structural strength, (ii) the main directions of the leaks and (iii) the means of evacuation.

If the molecular weight of the hydrocarbon refrigerant is greater than that of air, a flammable cloud can accumulate inside the modules or between them and on deck surfaces. Thus, in addition to minimizing the total hydrocarbon reserves, the exclusion of heavier components, especially propane (52% heavier than air) and ethane (4% heavier than air), is of particular importance. By replacing hydrocarbons for pre-cooling with

CO ₂ all propane is excluded from the liquefaction module, and even though ethane is present in refrigerants for liquefaction and supercooling, both of these mixtures have a lower molar mass than that of air.

From the above results, it was found that the inclusion of pre-cooling with CO ₂ in a process with a balanced load of MFC * s3 does not significantly increase the requirement for specific power or size / weight / cost of equipment, while process safety can be improved.

Claims (22)

1. A method of liquefying a stream enriched in hydrocarbons, where the liquefaction of a stream enriched in hydrocarbons is carried out in a cascade of circuits with a refrigerant consisting of three refrigeration circuits, the first of the three refrigeration circuits serving to provide pre-cooling, the second refrigeration circuit is used to provide actual liquefaction and the third refrigeration circuit serves to provide subcooling of the liquefied hydrocarbon-rich stream, wherein each refrigeration circuit includes at least m Herein, one single-stage or multi-stage compressor, characterized in that at least a portion of the refrigerant stream from the second refrigerant circuit is used to cool said hydrocarbon-rich stream to facilitate said preliminary cooling of the stream to a temperature at which C ₃ - liquefaction begins components, so that each compressor has a substantially equal share of the load. 2. The method according to claim 1, wherein the partial refrigerant stream from the second refrigerant circuit used to pre-cool the hydrocarbon-rich stream is evaporated at a pressure higher than the vaporization pressure of the remaining partial refrigerant stream from the second cooling circuit, and fed to compressor of the second cooling circuit at an intermediate pressure value. 3. The method according to claim 1 or 2, in which the undesirable components of the hydrocarbon-rich stream are separated after pre-cooling, before the actual liquefaction of the hydrocarbon-rich stream. 4. The method according to claim 3, in which the components of the C ₃ + hydrocarbon-rich stream is separated after pre-cooling, before the actual liquefaction of the hydrocarbon-rich stream. 5. The method according to claim 3, in which at least one partial stream of one of the two partial flows of the second cooling circuit is used to provide cooling in the separation unit. 6. The method according to claims 1, 2 and 5, in which the volumes and / or evaporation pressures of the two partial flows of the second refrigeration circuit are variable. 7. The method according to claims 1, 2 and 5, wherein the hydrocarbon-rich stream is a natural gas stream. 8. The method according to claims 1, 2 and 5, in which all refrigeration circuits include mixed refrigerants. 9. The method according to claims 1, 2 and 5, in which the first refrigeration circuit includes carbon dioxide. 10. The method according to claim 9, in which the carbon dioxide is cooled after condensation to a temperature of 20 ° C or less. 11. The method according to claim 10, in which carbon dioxide is cooled to a temperature of 15 ° C or less. 12. The method according to claim 10 or 11, in which cold cooling water is used to cool carbon dioxide. 13. The method according to item 12, in which the cold cooling water is sea water. 14. The method according to claim 9, in which the pre-cooling circuit with carbon dioxide includes a subcooling heat exchanger installed after the condenser. 15. The method according to PP.10, 11 and 13, in which the pre-cooling circuit with carbon dioxide includes a heat exchanger for subcooling, installed after the condenser. 16. The method according to claim 9, in which the cooling circuit with carbon dioxide includes three pressure values. 17. The method according to PP.10, 11 and 13, in which the cooling circuit with carbon dioxide includes three pressure values. 18. The method according to claim 9, in which carbon dioxide is not supercooled in the pre-cooling circuit. 19. The method according to PP.10, 11 and 13, in which carbon dioxide is not supercooled in the pre-cooling circuit. 20. The method according to claim 9, in which the housing for high pressure is used with a compressor for carbon dioxide. 21. The method according to PP.10, 11 and 13, in which the housing for high pressure is used with a compressor for carbon dioxide. 22. The method according to 17, in which the compressor is divided into two casings, and a cylindrical housing is used for the high pressure stage.

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