WO2014048845A1

WO2014048845A1 - Cooling circuit for the liquefaction of natural gas

Info

Publication number: WO2014048845A1
Application number: PCT/EP2013/069553
Authority: WO
Inventors: Tiziano DE PAOLIS; Marco GUARNONE; Davide BARBATTI; Francesco Rossi
Original assignee: Eni SpA
Current assignee: Eni SpA
Priority date: 2012-09-28
Filing date: 2013-09-20
Publication date: 2014-04-03
Anticipated expiration: 2015-03-28
Also published as: AU2018202194A1; AU2013322818A1; JP2016500800A; AP2015008323A0; JP6329154B2; ITMI20121625A1

Description

COOLING CIRCUIT FOR THE LIQUEFACTION OF NATURAL GAS

The present invention relates to technologies for the liquefaction of natural gas and in particular a cooling circuit to be used in the liquefaction of natural gas .

The liquefaction of natural gas is normally used for making the transportation and storage of the same more convenient.

The present invention relates in particular to the liquefaction technology of natural gas known with the name of C3-MR, i.e. propane-mixed refrigerant.

The C3-MR process has been known since the 70ties' and dominates the relative market.

Most plants for the liquefaction of natural gas operate on the basis of this technology.

Its functioning is based on two cooling cycles: a pre-cooling cycle with a pure cooling fluid, i.e. propane (C3) and a liquefaction and sub-cooling cycle with a mixture of cooling fluids (MR), i.e. a mixture of nitrogen, methane, ethane, and propane.

The propane cycle develops on three or four pressure levels and has the function of pre-cooling the natural gas to temperatures ranging from -30°C to -40°C.

This cycle has the additional function of cooling and partially liquefying the mixed refrigerant, hereinafter MR, used in the second cycle.

The heat exchangers used for this cycle are of the tube-bundle kettle type, in which the propane on evaporating on the shell side cools the hot fluid which is flowing in the pipes.

In the MR cycle, on the contrary, the heat exchange between the coolant and natural gas takes place in a spiral exchanger called "main cryogenic heat exchanger" (MCHE) which represents the core of the liquefaction process .

The compressors of the cooling cycles are operated by large gas turbines or, less frequently, by vapour turbines or electric motors.

In particular, optimizing the process and configuration of the machines used in a C3-MR liquefaction plant has played one of the most important roles in the evolution of the liquefaction capacity of a LNG (Liquefied Natural Gas) train.

As operators prefer to use gas turbines similar to each other for optimizing maintenance processes, one of the most evolute and diffused scheme is the so-called Split-MR™, conceived by the company APCI (Air Products & Chemicals Int.), wherein a portion of the power required for the compression of the mixed refrigerant is provided by the same turbine used for the compression of propane.

This configuration allows full use of the power of gas turbines and minimizes the number of turbines present in the cooling circuit.

The Split-MR™ configuration enables a production capacity of about 5 MTPA per year, to the detriment, however, of the operative flexibility.

In this configuration, in fact, the compressors rotate at a constant rpm and lie on two shafts entrained by respective gas turbines of the heavy-duty type .

These shafts are supported by auxiliary motors and, together with the turbines, provide the C3 and MR compressors with the necessary power, which is equal to about 200 MW.

As the various stages of the MR compressor are installed on the shafts of the two turbines, a malfunctioning of a compressor not only causes the stoppage of the turbine on which the damaged compressor is installed, but also the other turbine, generating the stoppage of the whole plant.

The Split-MR™ configuration therefore allows the exploitation of the power of gas turbines to be optimized, to the detriment, however, of the operative flexibility of the circuit.

Furthermore, this known configuration uses large- sized turbines which are more difficult to purchase as the number of producers is extremely limited.

The lack of competition on the market of large- sized turbines causes an increase in the selling price and prolonged delivery times.

In addition, heavy-duty gas turbines have thermodynamic efficiency characteristics which have now been outdated by new types of gas turbines of the "aeroderivative" type which claim specific fuel consumptions at least 20% lower.

An objective of the present invention is to overcome the drawbacks of the known art and in particular to provide an alternative configuration of the machines present in a natural gas cooling plant.

In particular, an objective of the present invention is to provide a cooling circuit for a natural gas liquefaction plant which allows an increase in the annual production, mitigating the bottleneck generated during hot periods .

A further objective of the present invention is to provide a cooling circuit for a liquefaction plant of natural gas which allows an increase in the annual production .

Another objective of the present invention is to provide a cooling circuit for a liquefaction plant of natural gas which allows a reduction in the consumption of natural gas required by the plant.

An additional objective of the present invention is to provide a cooling circuit for a liquefaction plant of natural gas which allows the plant flexibility to be increased on the basis of the operability of the cooling compressors installed in parallel.

A further objective of the present invention is to provide a cooling circuit for a liquefaction plant of natural gas which allows to have a further cooling capacity to be used for the extraction of heavy components of natural gas (LPG/gasoline) , increasing the flexibility of the plant with respect to the quality and composition of the natural gas at the inlet.

These and additional objectives according to the present invention are achieved by providing a cooling circuit for a liquefaction plant of natural gas, comprising :

- a pre-cooling system of natural gas and mixed refrigerant, comprising two parallel compression lines of propane, each having a first compressor activated by a first gas turbine;

a liquefaction system of natural gas comprising three parallel compression lines of the mixed refrigerant, each having a second compressor activated by a second gas turbine.

Further characteristics of the cooling circuit according to the present invention are object of the dependent claims.

In particular, said first and second gas turbines can be of the aeroderivative type and the same as each other .

Said mixed refrigerant can be a mixture of propane, ethane, methane and nitrogen.

The air at the inlet of said second gas turbines of said liquefaction system of natural gas can be cooled by a portion of cooling power provided by said pre- cooling system.

A portion of cooling power provided by said pre- cooling system can be used for increasing the extraction of heavy components of natural gas, such as LPG and gasoline.

Said second compressor can comprise a third compressor for effecting a first compression step of the mixed refrigerant at low/medium pressure, and a fourth compressor for effecting a second compression step of the mixed refrigerant at high pressure, situated in series with respect to each other.

Said first compressor can divide the propane compression into three or four compression steps at different pressure levels.

The characteristics and advantages of a cooling circuit for a natural gas liquefaction plant according to the present invention will appear more evident from the following illustrative and non-limiting description, referring to the enclosed schematic drawings, in which:

figure 1 is a schematic view of a cooling circuit for a liquefaction plant of natural gas having two compression lines of propane and three compression lines of mixed refrigerant;

- figure 2 is a schematic view of a cooling circuit of the Split-MR™ type, for a liquefaction plant of natural gas;

figure 3 is a schematic view of a preferred embodiment of two compression lines of propane and three compression lines of mixed refrigerant;

figure 4 is a graph relating to the yields of the liquefaction plant of natural gas, during various months of the year.

With reference to figure 1, this shows a cooling circuit 100 for a liquefaction plant of natural gas.

In particular, said liquefaction plant of natural gas is schematized in its two main sub-systems: the pre-cooling system 105, which cools the natural gas introduced into the plant to a temperature ranging from -30°C to -40°C, and the liquefaction system 106, which liquefies and sub-cools the pre-cooled gas to a temperature ranging from -145°C to -160°C.

The natural gas is first pre-cooled in a pre- cooling system comprising two parallel compression lines of propane, each having a first compressor 102 activated by a first gas turbine 101' . Said system is also configurated to pre-cool the mixed refrigerant used in the liquefaction system 106.

The cooled natural gas is subsequently liquefied and sub-cooled in a liquefaction system of natural gas comprising three parallel lines for the compression of the mixed refrigerant, each having a second compressor 103 activated by a second gas turbine 101".

The pre-cooling system 105, fed by the two compression lines of propane, allows the cooling of natural gas to a temperature ranging from -30°C to -40°C.

The liquefaction system 106, fed by the three compression lines of the mixed refrigerant, allows a further removal of heat from the natural gas, transforming said natural gas into liquid natural gas (LNG) .

Said first gas turbines 101' are preferably of the medium-sized aeroderivative type, for example with a power ranging from 30 MW to 60 MW.

Said second gas turbines 101" are preferably also of the medium-sized aeroderivative type, for example with a power ranging from 30 MW to 60 MW.

Said compression lines of propane or mixed refrigerant are in parallel with respect to each other and provide an autonomous contribution to the heat removal from the natural gas.

In particular, each of said lines comprises at least one compressor 102 or 103, operated by a gas turbine 101' or 101", for compressing a refrigerant suitable for expanding in the pre-cooling system 105 or in the liquefaction system 106.

In particular, in the liquefaction process of natural gas known as C3-MR, the power absorbed for the compression of propane is equal to about 35% of the total power required, whereas the power absorbed for the compression of the mixed refrigerant is equal to about 65% of the total power required.

Five aeroderivative gas turbines 101, 101" positioned in parallel and the same as each other, are used for providing the power necessary for activating said compressors 102 and 103, of which two (101') are destined for activating said first compressors 102 of propane and three (101") for activating said second compressors 103 of mixed refrigerant.

This configuration of the compression lines allows the first two gas turbines 101' to make two fifths of the total power installed available, i.e. 40%, against a power requirement on the part of said compressors 102 equal to about 35% of the total power required. This relationship between the power required by said first and second compressors 102, 103 and the total power available and supplied by said first and second gas turbines 101', 101", allows the generation of an additional cooling power in the pre-cooling system 105. This additional cooling power can be used for cooling the air at the inlet to said second gas turbines 101".

In particular, using five gas turbines 101', 101" of the aeroderivative type, having an ISO nominal power of 43.9 MW, it is possible to supply an effective power at 25°C of about 34 MW for each gas turbine 101' and 101".

Said first and second gas turbines 101', 101" of the aeroderivative type allow the number of revolutions of the shaft to be modulated, thus regulating the power supplied in relation to the effective absorption requirements . As illustrated in figure 3, in order to compress the mixed refrigerant in the liquefaction system of natural gas, said second centrifugal compressor 103 preferably comprises a third compressor 103' , for effecting a first compression step of the mixed refrigerant at low/medium pressure, and a fourth compressor 103" for effecting a second compression step of the mixed refrigerant at high pressure, situated in series with respect to each other. Said low/medium pressure level can range from 20 to 35 bar. Said high pressure level can range from 55 to 65 bar.

In this embodiment, a first heat exchanger 109' is installed between said third compressor 103' and said fourth compressor 103", suitable for absorbing heat from the mixed refrigerant after the first compression step, and a second heat exchanger 109" for absorbing additional heat from the mixed refrigerant after the second compression step.

In particular, said third and fourth compressors 103' , 103" are preferably selected so as to make full use of the power that can be supplied by said second gas turbine 101".

Analogously, said first compressor 102 divides the propane compression into three or four compression steps at different pressure levels.

Preferably, in order to compress the propane in the pre-cooling system of natural gas, two first compressors 102 of the centrifugal type may be installed, each activated by a first gas turbine 101'. The power absorbed by said first compressors 102 follows that absorbed by said third and fourth compressors 103' , 103" associated with the lines of the mixed refrigerant, and the process parameters relating to the natural gas to be liquefied. The power absorbed by said first compressors 102 normally ranges from 80 % to 100% of the total power that can be supplied by said first gas turbines 101', depending on the environmental conditions.

In the cooling cycle of the Split-MR™ type represented in figure 2, the power necessary for activating a fifth compressor 112 of propane and a sixth compressor 113 at high pressure of mixed refrigerant, is provided by a first traditional gas turbine 116 of the heavy-duty type, and the power necessary for activating a seventh compressor 114 at low pressure and an eighth compressor 115 at medium pressure, is provided by a second traditional gas turbine 117 of the heavy-duty type.

Said traditional gas turbines 116, 117 have an ISO nominal power of 86.2 MW equal to a power supplied at 25°C of about 72 MW, and a constant number of revolutions of the shaft.

In particular, according to said Split-MR™ configuration, said compressor 112 of propane provides the cooling power necessary for pre-cooling the natural gas in said pre-cooling system 105, whereas said compressors 113, 114, 115, arranged in series with respect to each other, provide the cooling power necessary for liquefying and sub-cooling the natural gas in said liquefaction system 106.

Said traditional gas turbines 116, 117 of the heavy-duty type, are of the single-shaft type and preferably require large-sized auxiliary motors 111', 111" for the start-up. These motors are necessary in the start-up phase for activating the gas turbines and bringing them to a rev regime which allows them to be self-sustained.

Said auxiliary motors 111', 111" are also used for producing additional power to that supplied by the gas turbines, so as to allow a higher potentiality of the cooling circuits.

In particular, said auxiliary motors 111', 111" are installed on respective common turbine shafts and compressors, and have a power of about 20 MW each.

Viceversa, according to the present invention, the compression lines of both propane and mixed refrigerant, are in parallel in order to prevent the failure of a gas turbine or compressor from leading to the stoppage of the whole liquefaction plant.

The use of gas turbines of the aeroderivative type, moreover, allows the number of revs of the turbine to be regulated and consequently the power supplied in relation to the load and functioning conditions of the remaining components of the circuit. The possibility of regulating the speed of these aeroderivative gas turbines and their arrangement on parallel compression lines, allows maintenance interventions to be effected without stopping the cooling circuit. In this way, the availability of the plant is maximized.

Said gas turbines of the aeroderivative type are also more compact with respect to the common industrial gas turbines of the heavy-duty type, thus reducing the overall footprint of the cooling circuit.

A further advantage in the use of gas turbines of the aeroderivative type lies in the fact that this type of turbine has a lower gas consumption with respect to common alternative solutions.

An additional advantage in the use of gas turbines of the aeroderivative type lies in the fact that this type of turbine does not need great power for the start-up, thus avoiding the use of auxiliary motors.

By arranging said compression lines in parallel, a wide operative flexibility can be obtained, as these first and second gas turbines of the aeroderivative type can be started without depressurizing said first and second compressors.

In addition, the cooling circuit in accordance with the present invention allows some of the equipment installed in said plant according to the Split-MR™ configuration, to be eliminated. In particular, said auxiliary motors 111', 111" of the Split-MR™ scheme are not necessary in the scheme proposed according to the present invention.

As the power required by said first compressors 102 ranges from 80% to 100% of the total power suppliable by the gas turbines 101', depending on the environmental conditions, an excess cooling power of up to about 20% of the power suppliable by said first gas turbines 101', is available.

Said portion of cooling power in excess provided by the pre-cooling system can be used for cooling the air at the inlet of said second gas turbines 101".

As is known, the power a turbine can supply is inversely proportional to the temperature of the air at the inlet of the turbine, and, on increasing the temperature of the air at the inlet, its volume increases and the yield of the turbine decreases.

In particular, auxiliary exchangers 104 pre-cool the air using the cooling power of the propane circuit. The heat exchange preferably, but not necessarily, takes place through the use of an intermediate coolant.

By creating an inlet air chilling, i.e. by cooling the air at the inlet to said second gas turbines 101', the production of the same can be improved, as illustrated in the illustrative graph of figure 4.

In particular, figure 4 shows a first curve 401 relating to the yields of a plant configured according to the logic of aeroderivative turbines in parallel without pre-cooling the air at the inlet, and a second curve 402 relating to the yields of the same plant configured according to the logic of aeroderivative turbines in parallel and comprising said auxiliary exchangers 104 for pre-cooling the air at the inlet of said second gas turbines 101".

In the example of figure 4, it can be noted that, by cooling the air at the inlet to said second gas turbines 101", a higher annual production of the liquefaction plant is obtained, ranging from about 10% to about 20%.

This benefit can be mainly obtained in places where the temperature exceeds 20°C for many months of the year .

In very hot places, in fact, the yields of gas turbines decrease with an increase in the temperature of the air, consequently a re-use of the excess cooling power for cooling the air at the inlet to said second gas turbines 101", avoids obtaining lower yields which would otherwise be verified.

According to the present invention, the number of compressors and the number of turbines increases, with the advantage of having a longer average availability of the plant.

By installing five aeroderivative gas turbines model LM6000 PF of General Electric, for example, an availability of a natural gas liquefaction plant can be obtained of about 5 ÷ 10 days/year higher than that obtainable with a plant equipped with two heavy-duty turbines model Frame 7 installed according to the Split-MR™ logic.

Said mixed refrigerant can be a mixture of methane, nitrogen, ethane, ethylene, propane, propylene, butane and pentanes.

Finally, a cooling circuit for a natural gas liquefaction plant thus conceived can evidently undergo numerous modifications and variants, all included in the invention; furthermore, all the details can be substituted by technically equivalent elements. In practice, the materials used, as also the dimensions, can vary according to technical requirements.

Claims

1) A cooling circuit (100) for a liquefaction plant of natural gas comprising:

- a pre-cooling system (105) of natural gas and mixed refrigerant, comprising two parallel compression lines of propane, each having a first compressor (102) activated by a first gas turbine (101');

- a liquefaction system (106) of natural gas comprising three parallel compression lines of the mixed refrigerant, each having a second compressor (103) activated by a second gas turbine (101'') .

2) The cooling circuit (100) according to claim 1, wherein said first and second gas turbines (101', 101'') are of the aeroderivative type.

3) The cooling circuit (100) according to claim 1 or 2, wherein said first and second gas turbines (101', 101'') are the same as each other.

4) The cooling circuit (100) according to any of the previous claims, wherein said mixed refrigerant is a mixture of propane, ethane, methane and nitrogen.

5) The cooling circuit (100) according to any of the previous claims, wherein the air at the inlet of said second gas turbines (101'') of said liquefaction system (106) of natural gas is cooled by a portion in excess of cooling power supplied by said pre-cooling system (105), to increase the power supplied by said second gas turbines (101' ' ) .

6) The cooling circuit (100) according to any of the previous claims, wherein a portion of cooling power supplied by said pre-cooling system (105) is used for increasing the extraction of heavy components of natural gas of the LPG and Gasoline type.

7) The cooling circuit (100) according to any of the previous claims, wherein said second compressor (103) comprises a third compressor (103') for effecting a first compression step of the mixed refrigerant at low/medium pressure, and a fourth compressor (103'') for effecting a second compression step of the mixed refrigerant at high pressure, situated in series with respect to each other.

8) The cooling circuit (100) according to any of the previous claims, wherein said first compressor (102) divides the compression of propane into three or four compression steps at different pressure levels.