Technical Field

The present invention relates to a plant for the liquefaction of gas, particularly network gas.

Background Art

Various types of plants are known for the liquefaction of gas, in particular network gas.

Usually, by network gas is meant a gaseous mixture of highly volatile hydrocarbons, composed largely of methane, which is present in a percentage between 90% and 99%, while the remaining part consists of ethane, propane and butane, with possible traces of carbon dioxide, nitrogen, noble gases and hydrogen sulfide.

This network gas is extracted from oil or natural gas fields or, alternatively, is produced by anaerobic fermentation processes, and is intended for both domestic and automotive transportation use.

The distribution of gas is carried out, where possible, through a distribution network consisting of pipelines, which transport the gas at high pressure.

Alternatively, the gas can be stored and transported by suitably prepared tankers and then treated, at the user's premises, in regasification stations where it is returned to its aeriform state, and fed into the local gas pipeline network.

However, in some cases, it is useful to transport the network gas in liquid state in cylinders and the like with limited volumes compared to tankers, which is particularly useful when the gas is intended for automotive transportation use. The liquefaction of a gas is a complex process which requires the use of special equipment and the consumption of a large amount of energy.

Typically, the plants for the liquefaction of gas comprise a compressor, generally of the type of a turbocharger, which increases the gas pressure to significantly high values.

Subsequently, the compressed gas is conveyed to a heat exchanger, which significantly reduces its temperature. At this point, the gas is subjected to expansion, so as to bring it to a pressure and a temperature such as to cause the change of its state from gas to liquid. Generally, it is not possible to liquefy the entire flow rate of treated gas, but a gas-liquid mixture is obtained.

The two phases are separated by a gas-liquid separator, which allows obtaining a liquefied gas flow rate, while the fraction in the gaseous phase is mixed with the flow rate of gas to be liquefied entering the liquefaction plant.

If the flow rate of gas to be liquefied is high, the expansion can be carried out by means of a turbo-expander, which permits making a considerable pressure jump, but also producing electricity, which is used to supply the liquefaction plant itself.

In this particular case, the liquefied gas is stored on the LNG carriers, which transport it from the place of extraction to the place of consumption.

This particular type of plant is particularly advantageous inasmuch as it allows the recovery of energy through the turbo-expander, but its use is only useful in the case of sufficiently large flow rates.

If, on the other hand, the flow rate of gas to be liquefied is limited, as in the case of gas to be used for automotive purposes, the liquefaction plant provides for the expansion thereof by means of rolling valves, which, however, does not allow the recovery of energy.

Furthermore, in the case of small flow rates, the liquefied gas is stored in cryogenic cylinders and transported, so as to be available for uses of various kinds, including automotive transportation.

These plants for the liquefaction of gas do have, however, a number of drawbacks.

In fact, the plants for the liquefaction of network gas to be used for automotive transportation involve high operating costs related to the compression of the gas and its subsequent expansion.

Moreover, given the reduced gas flow rates, the expansion is performed by rolling, so there is no possibility of recovery of useful work in the form of electricity. Description of the Invention

The main aim of the present invention is to provide a plant for the liquefaction of gas which permits limiting the operating costs of the process as a whole. Within the illustrated aim, one object of the present invention is to allow the recovery of useful work within the plant.

Another object of the present invention is to provide a plant for the liquefaction of gas which permits maximizing the fraction of liquefied gas obtained, thus minimizing the need for recovery of the fraction in the gas phase.

A further object of the present invention is to provide a plant for the liquefaction of gas which allows overcoming the aforementioned drawbacks of the prior art within the scope of a simple, rational, easy, efficient to use and cost-effective solution.

The aforementioned objects are achieved by the present plant for the liquefaction of gas according to claim 1.

Brief Description of the Drawings

Other characteristics and advantages of the present invention will become more evident from the description of a preferred, but not exclusive embodiment of a plant for the liquefaction of gas, illustrated by way of an indicative, but non- limiting example, in the attached drawings in which:

Figure 1 is a schematic view of the plant according to the invention;

Figure 2 is a schematic sectional view of a detail of the plant of Figure 1 with the fluid- operated distributor in the intake configuration;

Figure 3 is a schematic sectional view of the detail of Figure 2 with the fluid- operated distributor in an expansion configuration;

Figure 4 is a schematic sectional view of the detail of Figure 2 with the fluid- operated distributor in the exhaust configuration.

Embodiments of the Invention

With particular reference to these illustrations, reference numeral 1 globally indicates a plant for the liquefaction of gas.

The plant 1 for the liquefaction of gas, particularly network gas, comprises: at least one compressor 2 adapted to increase the pressure of at least one gas in order to obtain compressed gas;

at least one cooling device 3 connected in a fluid- operated manner to the compressor 2 and adapted to cool the compressed gas;

at least one expansion assembly 6, 10, 20 which is connected in a fluid- operated manner to the cooling device 3 and adapted to reduce the pressure of the compressed gas in order to obtain liquefied gas; and

at least one filling station 4 which is connected in a fluid- operated manner to the expansion assembly 6, 10, 20 and adapted to fill at least one cryogenic reservoir 5 with the liquefied gas.

In particular, the plant 1 is intended to be used for the liquefaction of cryogenic gases, i.e. gases with very low boiling temperatures at atmospheric pressure (generally below -100°C).

It must be pointed out that the plant 1 can be used for the liquefaction of any cryogenic gas by varying the pressure and temperature operating conditions depending on the treated gas.

In the particular case of network gas, this is transported in pipelines at a pressure higher than atmospheric pressure and at a temperature equal to that of the atmosphere.

However, in such temperature and pressure conditions, the network gas, as well as other cryogenic gases, is in supercritical fluid condition, i.e., a gas that cannot be liquefied by simple compression.

The plant 1 therefore exploits the combined effect of compression, cooling and subsequent expansion of the gas in order to liquefy it.

In particular, the gas taken from the network is sent to the compressor 2, which increases the pressure, bringing it to rather high pressure values.

In the particular case of network gas, this is compressed from a few bars up to high pressure values, even 200 bar, so that the compressor 2 makes it possible to obtain high compression ratios.

The compressed gas which is obtained out of such compressor 2 is then cooled by means of the cooling device 3, the purpose of which is to cool the gas down to temperatures well below 0°C. It is, therefore, a cryogenic heat exchanger, in which the coolant liquid, usually liquid nitrogen, is used to cool the compressed gas, bringing it down to a very low temperature, which in the particular case of network gas is about -60°C. Preferably, the cooling device 3 is a cryogenic heat exchanger of the "plate" type, although the possibility of providing a different type of cooling device 3 cannot be ruled out.

In the particular case of using a plate heat exchanger, this allows obtaining high heat exchange outputs, which involve the use of reduced flow rates of coolant liquid, as well as the need for a smaller exchange surface area compared to other types of heat exchangers, which means a small size cooling device 3.

Such advantages have a positive effect on the operating costs of the plant 1 inasmuch as the use is required of a reduced flow rate of liquid nitrogen and, consequently, the cost associated with both the coolant liquid itself and its handling is reduced.

Usefully, since it has to work in contact with fluids at extremely low temperatures, the cooling device 3 is made of special materials, e.g., special steels with the addition of chrome and nickel, which have a very low tendency to brittle fracture.

At this point, the compressed and adequately cooled gas is sent to the expansion assembly 6, 10, 20, in which the actual liquefaction of the gas takes place.

In fact, the pressure reduction of the gas made by the expansion assembly 6, 10, 20 involves a further reduction in temperature, so that the gas is in conditions of temperature and pressure such as to obtain the change of state from gas to liquid, obtaining the liquefied gas.

In the particular case of network gas, the conditions of the liquefied gas out of the expansion assembly 6, 10, 20 are about 3 bar and -145°C.

In actual fact, a liquefied gas is not obtained out of the plant 1 consisting of a flow rate entirely in the liquid phase, but a mist is obtained, i.e., a gas-liquid mixture in the form of aerosol, wherein therefore the liquid fraction is dispersed inside the gaseous fraction in the form of small drops.

Nevertheless, the plant 1 is made to maximize the obtainable fraction of liquefied gas, so as to increase as much as possible the output of the plant itself and reduce overall production costs.

According to the invention, the expansion assembly 6, 10, 20 comprises at least one piston expander 6 comprising at least one expansion chamber 7 which extends along a first axial direction A and is provided with at least one intake and exhaust mouth 8 and at least one piston 9 housed in the expansion chamber 7 and moveable sliding inside the expansion chamber itself along the first axial direction A.

The possibility cannot however be ruled out of providing the piston expander 6 provided with a plurality of expansion chambers 7 in each of which a relevant piston 9 is mobile sliding.

Advantageously, the expansion assembly 6, 10, 20 is able to operate with very high inlet gas pressures, even higher than 150 bar.

Conveniently, the compressed gas is introduced into the expansion chamber 7 and the expansion is performed by varying the volume of the expansion chamber itself by means of the sliding of the piston 9.

In fact, the piston 9 is mobile sliding along the first axial direction A between a first position wherein the piston 9 is placed in the proximity of the mouth 8 and the volume of the expansion chamber 7 is the least possible and a second position wherein the piston itself is separated from the mouth 8 and the volume of the expansion chamber 7 is the largest possible.

In other words, the compressed gas is introduced into the piston expander 6 when the piston 9 is in the first position.

Afterwards, the mouth 8 is closed and the piston 9 is moved as far as the second position, thus increasing the volume of the expansion chamber 7 and, consequently, reducing the pressure of the compressed gas and obtaining liquefied gas.

At this point, the mouth 8 is opened again and the piston 9 is again moved to the first position, so as to empty the expansion chamber 7 of the liquefied gas.

In particular, the piston expander 6 is of the type of a cryogenic piston expander, meaning that it is in contact with a fluid at a temperature even lower than -100°C, so that in this case as well, it is necessary to use steels which are particularly resistant to brittle fracture.

Advantageously, the piston expander 6 comprises at least one fluid- operated distributor 10 associated with the mouth 8 and adapted to control the flow direction of the gas.

In other words, the fluid- operated distributor 10 allows managing the opening and closing of the mouth 8 of the piston expander 6 according to precise timescales, so as to ensure the exhaust of the liquefied gas only once complete expansion has occurred, thus optimizing the operation of the plant 1.

Conveniently, the fluid- operated distributor 10 comprises:

at least one valve body 11 comprising at least one sliding chamber 12a, 12b having a substantially elongated shape which extends along at least a second axial direction B and provided with at least one inlet opening 13 for the inlet of the compressed gas, at least one exhaust opening 14 for the exhaust of the liquefied gas and at least one mouth opening 15 associated with the mouth 8 for the connection of the fluid- operated distributor 10 to the expansion chamber 7 of the piston expander 6; and

at least one slider 16a, 16b having a substantially elongated shape, housed in the sliding chamber 12a, 12b, moveable sliding along the second axial direction B and comprising at least one internal duct 17a, 17b for the passage of the compressed gas and expanded gas and partly liquefied and locatable in fluidic connection with at least two of the inlet opening 13, the mouth opening 15 and the exhaust opening 14.

Conveniently, the fact of providing the internal duct 17a, 17b with an elongated shape, or rather with a width significantly reduced compared to the length, allows obtaining a greater resistance of the fluid- operated distributor 10 to the high pressure of the fluid which passes through it, in particular of the compressed gas entering towards the piston expander 6.

In the particular embodiment shown in the figures, the first axial direction A and the second axial direction B are substantially parallel to one another, so that the piston expander 6 and the fluid- operated distributor 10 are arranged in a manner substantially aligned with one another, obtaining a configuration which allows minimizing the overall dimensions of the expansion assembly 6, 10, 20. The possibility cannot however be ruled out of providing the first axial direction A and the second axial direction B arranged, the one to the other, in a different manner with respect to that shown in the illustrations.

Advantageously, the fluid- operated distributor 10 comprises a plurality of sliding chambers 12a, 12b and a plurality of sliders 16a, 16b, each housed in a respective sliding chamber 12a, 12b and moveable sliding in a substantially staggered manner to one another along the second axial direction B.

In the particular embodiment shown in the illustrations, the fluid- operated distributor 10 comprises:

at least a first sliding chamber 12a in which is housed at least a first slider 16a provided with at least a first internal duct 17a; and

at least a second sliding chamber 12b in which is housed at least a second slider 16b provided with at least a second internal duct 17b.

In particular, the first slider 16a and the second slider 16b are moveable sliding in a substantially alternated manner to one another along the second axial direction B among:

an intake configuration in which the first internal duct 17a is placed in communication with the inlet opening 13 and the mouth opening 15;

an expansion configuration in which the first internal duct 17a and the second internal duct 17b are isolated with respect to the expansion chamber 7; and

an exhaust configuration in which the second internal duct 17b is placed in communication with the mouth opening 15 and with the exhaust opening 14.

Usefully, the movement of the first slider 16a and of the second slider 16b is performed in a manner substantially synchronized with the movement of the piston 9 of the piston expander 6.

In fact, when the compressed gas is made to enter the expansion chamber 7 with the piston 9 in the first position, the fluid- operated distributor 10 is in the intake configuration.

Once the expansion chamber 7 is filled, the fluid- operated distributor 10 is placed in the expansion configuration, so as to occlude the mouth 8, while the piston 9 is moved towards the second position.

Finally, once the expansion of the gas has been completed, the liquefied gas is exhausted, so that the fluid- operated distributor 10 is placed in the exhaust configuration, while the piston 9 is moved again towards the first position.

Such synchronization of movements thus allows optimizing the expansion of the compressed gas, obtaining the largest possible fraction in liquid phase.

In order to ensure the seal and prevent leaks of compressed gas, the fluid- operated distributor 10 is provided with a plurality of seals 21 which are adapted to ensure its fluid- operated seal.

Advantageously, the fluid- operated distributor 10 comprises at least one motorized linear actuator 18 adapted to move at least one of the first slider 16a and the second slider 16b along the second axial direction B.

Preferably, the fluid- operated distributor 10 comprises at least two motorized linear actuators 18, adapted to move the first slider 16a and the second slider 16b, respectively, along the second axial direction B.

This way, therefore, it is possible to move the first slider 16a and the second slider 16b automatically, adjusting times and movements with the utmost precision, in order to obtain a perfect adjustment of the expansion assembly 6, 10, 20.

Advantageously, the plant 1 comprises at least one electric current generator 19 associated with the piston expander 6 and adapted to produce electricity, shown schematically in the illustrations.

Furthermore, the electric current generator 19 also acts as a motor, regulating the speed, start and stop of the piston expander 6.

This way, therefore, it is possible to exploit the movement of the piston 9 so as to produce electricity, which is used to supply at least part of the plant 1.

The fact of preparing the piston expander 6 associated with the electric current generator 19 allows recovering the useful work done by the piston expander itself in order to produce electricity, with a reduction in the energy supplied to the plant through the network and, consequently, a reduction in costs related to production.

Conveniently, the expansion assembly 6, 10, 20 can also comprise at least one rolling valve 20 interposed between the piston expander 6 and the filling station 4 and is adapted to reduce the pressure of the gas out of the piston expander 6. In particular, the rolling valve 20 makes possible a further reduction in the pressure of the liquefied gas out of the piston expander 6, so as to maximize the effect of temperature drop and, consequently, maximize the fraction in liquid phase obtained out of the plant 1.

The possibility of making the expansion assembly 6, 10, 20 without the rolling valve 20 cannot however be ruled out.

The liquefied gas obtained out of the expansion assembly 6, 10, 20 is conveyed to the filling station 4, by means of which it is stored in a cryogenic reservoir 5 to be transported to the end user.

Such cryogenic reservoir 5 is adapted to maintain the liquefied gas under the temperature and pressure conditions obtained out of the expansion assembly 6, 10, 20, so it must be made in such a way as to ensure adequate thermal insulation with respect to the environment.

In fact, the cryogenic reservoir 5 is provided with an inner wall and an outer wall, between which is defined a cavity containing highly insulating materials, so as to thermally insulate the liquefied gas.

The cryogenic reservoir 5 can also comprise a cooling system which exploits the flow of a coolant fluid inside pipes housed inside the cryogenic reservoir itself, so as to keep the liquefied gas under the set temperature conditions, avoiding the heating and subsequent evaporation thereof.

From the construction point of view, the cryogenic reservoir 5 must be resistant to overpressure and to cryogenic temperatures, so it is made using steels resistant to brittle fracture and the wall thicknesses are assessed in accordance with the working pressure.

Advantageously, the fluid- operated connection inside the plant 1 is made by means of a system of pipes made of materials resistant to both high pressures and to cryogenic temperatures.

It has in practice been found that the described invention achieves the intended objects.

In this regard, the fact is underlined that the particular solution of providing an expansion assembly comprising a piston expander makes it possible to limit the operating costs of the plant.

In particular, the fact that the piston expander is associated with an electric current generator makes it possible to recover the useful work done by the expander itself and produce electric current, which is used to supply the plant itself, in order to reduce the amount of electricity supplied by the network.

Furthermore, the particular solution of providing a fluid- operated distributor allows controlling the direction of the flow of gas into and out of the piston expander, optimizing the operation thereof and maximizing the fraction of liquefied gas.