Patent Description:
Usually, natural gas is transported, at high pressure, along great distances, inside of gas pipelines or methane pipelines (which form the transmission network), then it is distributed by distribution points known as Metering and Regulating Stations, MRS, from which the gas, suitably processed, is delivered to the end-users (households, public facilities, factories, etcetera) through the transmission network. In the MRSs the gas is subjected to a measuring step (by suitable measuring apparatuses) and to a regulation step, in other words its pressure is reduced by a process reducing the gas pressure in the transmission network to a predefined lower pressure, for the distribution network. In the MRS, the gas is further subjected to additional treatments, particularly to a filtration step, a step preheating the gas to a predefined temperature, and to an odorizing step. The preheating step is performed because the pressure drop (typically from <NUM> bar to <NUM> bar) cools down the gas and such cooling, if not prevented, can freeze the pipes and also the metering and regulating apparatuses, which in turn determines a supply disruption. The preheating step is performed by burning a gas fraction in a water heating boiler, which in turn is used for preheating the gas. Therefore, the preheating step is an additional expense and also an energy waste.

A further alternative approach of delivering natural gas consists of delivering LNG (liquefied natural gas). In this case, the extracted natural gas is subjected to a liquefying step by consecutive cooling and condensing steps, then it is transported inside tanks typically by land or sea. The liquefied natural gas can then be subjected to regasification before being introduced into the distribution network, or can be used in the liquid state, for example, in the automotive and/or industrial field.

The known liquefying plants (for example the Linde cycle plant or the likes) generally compress the gas, cool it down, and then they decompress it, by exploiting this further pressure drop for ultimately cooling down the gas in order to liquefy it. However, the compression required to enable the liquefying process, is also one of the main costs with reference to the power consumption. Moreover, in this liquefying plants, there is always a waste gas component which is difficult to manage. The document <CIT> discloses a plant for treating gas.

Therefore, an object of the present invention consists of making available a plant for treating gas, particularly natural gas, supplied by a transmission network, for supplying, on one side, liquefied gas at a lower pressure, destined for example to the distribution network, and from another side, liquefied gas destined to be used in the automotive and/or industrial fields, enabling to reduce the energy wastes associated to the respective processes according to the beforehand prior art.

This and other objects are obtained by a plant according to claim <NUM>.

Dependent claims define possible advantageous embodiments of the invention.

In order to better understand the invention and appreciate the advantages, some exemplifying non-limiting embodiments thereof will be described in the following with reference to the attached figures, wherein:.

Referring to the attached figures, a plant for treating gas, particularly natural gas (typically comprising methane) is generally indicated by reference <NUM>. The plant <NUM> receives at the inlet gas supplied by a transmission network (not shown in the figures) and, to this purpose, it comprises an inlet <NUM> connectable to said transmission network. The natural gas conveyed by the transmission network which enters the plant <NUM> is at a high pressure, typically between <NUM> and <NUM> bar. The plant <NUM> comprises a first plant portion <NUM> for decompressing a first fraction of the gas supplied by the transmission network and a first outlet <NUM> for supplying the decompressed gas at an outlet predefined pressure from the plant <NUM> itself, particularly to a distribution network (not shown in the figures), which the first outlet <NUM> is connectable to. The decompressed gas delivered at the first outlet <NUM> is typically at a pressure of about <NUM> bar for methane gas, therefore at a pressure lower than the pressure of the transmission network. Moreover, preferably, the gas is supplied at the outlet at an outlet predefined temperature, which can be comprised between <NUM> and <NUM>, for example, generally greater than the temperature of the gas in the transmission network.

The plant <NUM> comprises a second plant portion <NUM> for liquefying a second fraction of the gas supplied by the transmission network and a second outlet <NUM> for supplying the liquefied gas, typically LNG, flowing out the plant <NUM> itself, where the liquefied gas can be stored in tanks in order to be transported away. The liquefied gas supplied at the second outlet <NUM> is typically at a temperature of about -<NUM> and at a pressure comprised between about <NUM> and <NUM> bar in case of LNG.

Advantageously, the plant <NUM> comprises a flow divider <NUM> for separating the gas entering through the inlet <NUM> into the first fraction destined to the first plant portion <NUM> and into the second fraction destined to the second plant portion <NUM>. Preferably, the flow divider <NUM> is configured so that all the entering gas is delivered to the first <NUM> and second plant portions <NUM>. Still more preferably, the first gas fraction is greater than the second gas fraction, in order to meet the working flow rate and the requirements of the distribution network.

In the following description and in the attached claims, the positions of the elements in the plant, will be indicated by the terms "upstream" and "downstream", which should be understood with reference to the direction of the gas flow inside the plant, as shown by the arrows drawn in the attached figures.

With reference to <FIG>, the first plant portion <NUM> comprises a throttle valve <NUM> in which the first gas fraction from the flow divider <NUM> is subjected to a temperature and pressure reductions by generally remaining in the gas state.

Further, the first plant portion <NUM> comprises a first heat exchanger <NUM> establishing a thermal exchange relationship between the segment of the first plant portion <NUM> placed downstream the throttle valve <NUM> and the second plant portion <NUM>, wherein the first gas fraction is heated by the second gas fraction flowing in the second plant portion <NUM>, which cools down accordingly.

Moreover, the first plant portion <NUM> comprises a second heat exchanger <NUM> establishing a thermal exchange relationship between the segment of the first portion <NUM> placed downstream the first heat exchanger <NUM> and the segment of the first plant portion <NUM> placed upstream the throttle valve <NUM>. In this way, the gas in the first plant portion <NUM> supplied by the flow divider <NUM> is precooled by the cooler gas coming from the throttle valve <NUM> and by the first heat exchanger <NUM>, which in turn is heated. Such gas from the second heat exchanger <NUM> can be supplied to the first outlet <NUM> and delivered therefrom to the distribution network.

Referring now to the second plant portion <NUM>, it comprises, as beforehand cited, a first heat exchanger <NUM>, in which the second gas fraction from the flow divider <NUM> is cooled down and at least partially liquefied. The second plant portion <NUM>, downstream the first exchanger <NUM>, comprises a throttle valve <NUM> determining a further temperature and pressure reductions of the liquefied gas. The liquefied gas from the throttle valve <NUM> therefore can be delivered to the second outlet <NUM>, where it can be extracted, stored, and transported in a liquefied state.

As a person skilled in the art will clearly understand, the plant according to the invention substantially reduces the energy wastes present in the systems according to the prior art described in the introductory part. Actually, the first plant portion <NUM> does not require heating burners and the second plant portion <NUM> does not need energy for the compression and for other energy-consuming methods used for liquefying. Such effect is obtained by the synergic relationship between the first and second plant portions, which exchange with each other heat by the above described modes.

Referring to <FIG>, some alternative embodiments of the plant according to the invention will be now described. The plant in <FIG> comprises the same elements of the plants of <FIG>, as well as plural additional elements for a still better efficiency of the processes. It is observed that each of such additional elements can be provided alone or combined with one or more of the further additional elements and that the many alternative embodiments stemming from such combinations are not individually shown only for not making obscure the present description.

Referring to the first plant portion <NUM>, it comprises, downstream the flow divider <NUM>, the throttle valve <NUM>, the first heat exchanger <NUM>, and the second heat exchanger <NUM>, according to what was beforehand described.

According to a possible embodiment, the first plant portion <NUM> comprises a chiller <NUM> located upstream the throttle valve <NUM>, preferably downstream the second heat exchanger <NUM>. The chiller <NUM> further pre-cools the first gas fraction before entering the throttle valve <NUM>.

According to an embodiment, the first plant portion <NUM> comprises a third heat exchanger <NUM> placed downstream the second heat exchanger <NUM> establishing a thermal exchange relationship between the segment of the first plant portion <NUM> downstream the second heat exchanger <NUM> and the segment of the second plant portion <NUM> upstream the first heat exchanger <NUM>. The third heat exchanger <NUM> determines a further heating of the first gas fraction downstream the second exchanger <NUM> and a pre-cooling of the second gas fraction upstream the first heat exchanger <NUM>.

According to an embodiment, the second plant portion <NUM> comprises a section <NUM> for recirculating a possible non-liquefied part of the second gas fraction exiting the throttle valve <NUM>. Advantageously, such recirculation section <NUM> comprises a condensate separator <NUM> downstream the throttle valve <NUM>. Such condensate separator <NUM> separates the liquefied part and the non-liquefied part of the second gas fraction exiting the throttle valve <NUM> and conveys the liquefied part to the second outlet <NUM> and the non-liquefied part to the first outlet <NUM>, where this latter can be mixed in a mixer <NUM> with the first gas fraction from the second heat exchanger <NUM>. The non-liquefied part, being effectively a waste of the second plant portion <NUM>, can be advantageously recovered.

The non-liquefied part of the second gas fraction from the condensate separator <NUM>, at a low temperature, can be advantageously used for further pre-cooling the second gas fraction in the segment upstream the throttle valve <NUM>. For this matter, the recirculation section <NUM> can comprise a fourth heat exchanger <NUM> establishing a thermal exchange relationship between the segment conveying the non-liquefied part of the second gas fraction downstream the condensate separator <NUM> and the segment of the second plant portion <NUM> upstream the throttle valve <NUM>, preferably downstream the first heat exchanger <NUM>.

Still more advantageously, the non-liquefied part of the second gas fraction from the condensate separator <NUM> can be also used for further pre-cooling the first gas fraction in the segment upstream the throttle valve <NUM>. For this purpose, the recirculation section <NUM> can comprise a fifth heat exchanger <NUM>, preferably placed downstream the fourth heat exchanger <NUM>, if provided, which establishes a thermal exchange relationship between the segment conveying the non-liquefied part of the second gas fraction downstream the condensate separator <NUM> and the segment of the first plant portion <NUM> upstream the throttle valve <NUM>, preferably downstream the chiller <NUM>, if provided.

Still more advantageously, the non-liquefied part of the second gas fraction from the condensate separator <NUM> can be also used for further pre-cooling the second gas fraction in the segment upstream the throttle valve <NUM>. For this purpose, the recirculation section <NUM> can comprise a sixth heat exchanger <NUM>, for example placed downstream the third heat exchanger <NUM> and upstream the first heat exchanger <NUM>, which establishes a thermal exchange relationship between the segment of the second plant portion <NUM> which conveys the non-liquefied part of the second gas fraction downstream the condensate separator <NUM> and the segment of the second plant portion <NUM> upstream the first heat exchanger <NUM>.

According to an embodiment, the second plant portion <NUM> comprises a second recirculation section <NUM> placed downstream the throttle valve <NUM>, preferably downstream the recirculation section <NUM>, if provided. The second recirculation section <NUM> comprises a second flow divider <NUM> separating the liquefied part of the second gas fraction into two distinct branches <NUM> and <NUM>. The first branch <NUM> leads to the second outlet <NUM>, while the second branch leads to the mixer <NUM> and from there to the first outlet <NUM>. The second branch <NUM> comprises a second throttle valve <NUM>, which subjects the liquid gas to a further throttling action, in which the liquid gas is subjected to further temperature and pressure reductions, and a seventh heat exchanger <NUM> which establishes a thermal exchange relationship between the first branch <NUM> and the segment of the second branch <NUM> downstream the second throttle valve <NUM>, so that the liquefied gas circulating in the first branch <NUM> is further cooled down before flowing to the second outlet <NUM>, while the liquefied gas downstream the second throttle valve <NUM> in the second segment <NUM> switches back to the gas state before flowing to the mixer <NUM> and then being conveyed to the first outlet <NUM>. Thus, also this latter gas fraction, which is effectively a waste of the second plant portion <NUM>, can be advantageously recovered.

According to a further embodiment, the second section <NUM> comprises an eighth heat exchanger <NUM> establishing a thermal exchange relationship between the second segment <NUM> (and preferably the portion of the second segment <NUM> downstream the seventh heat exchanger <NUM>) and the segment of the second plant portion <NUM> upstream the first heat exchanger <NUM>, preferably downstream the sixth heat exchanger <NUM>. This further pre-cools the second gas fraction upstream the throttle valve <NUM>, and also heats the part of the second gas fraction flowing in the second segment <NUM> before it reaches the mixer <NUM> and from there the first outlet <NUM>. According to a possible embodiment, the second segment <NUM> comprises, downstream the seventh heat exchanger <NUM>, preferably downstream the eighth heat exchanger <NUM>, if provided, a first compressor <NUM> suitable to increase the pressure, and also the temperature, of the gas flowing in the second segment <NUM> before reaching the mixer <NUM> and from there the first outlet <NUM>.

According to an embodiment, the second plant portion <NUM> comprises a second compressor <NUM> placed upstream the first heat exchanger <NUM>, and preferably upstream the third heat exchanger <NUM>, if provided.

From the above given description, a person skilled in the art can appreciate that the plant according to the invention reduces the wastes described with reference to the plants according to the prior art due to an energy synergy between the first and the second plant portions. Indeed, the pressure difference of the process which is performed in the first plant portion is what is required by the process performed in the second plant portion, and the heat difference of the process performed in the second plant portion is what is required by the process performed in the first plant portion. Substantially, by combining the two processes, one process compensates the other one by eliminating the negative drawbacks which each process would have if not combined together.

According to the invention, it is not necessary to counter the cooling of the gas in the first plant portion by preheating it by external boilers because the decompressed gas is recirculated, causing the gas to liquefy in the second plant portion, and at the same time the gas is suitably heated in order to be conveyed into the local distribution network.

Claim 1:
Plant (<NUM>) for treating gas, particularly natural gas, supplied by a transmission network, comprising:
- a gas inlet (<NUM>) connectable to said transmission network;
- a first plant portion (<NUM>) configured to decompress to a predefined outlet pressure a first fraction of the gas from the inlet (<NUM>) and to supply the decompressed gas at a first outlet (<NUM>);
- a second plant portion (<NUM>) configured to liquefy a second fraction of the gas from the inlet (<NUM>) and to supply the liquefied gas at a second outlet (<NUM>),
wherein the first plant portion (<NUM>) comprises:
- a valve (<NUM>) for throttling the first gas fraction;
- a first heat exchanger (<NUM>) establishing a thermal exchange relationship between the segment of the first plant portion (<NUM>) placed downstream the throttle valve (<NUM>) and the second plant portion (<NUM>);
- a second heat exchanger (<NUM>) establishing a thermal exchange relationship between the segment of the first plant portion (<NUM>) placed downstream the first heat exchanger (<NUM>) and the segment of the first plant portion (<NUM>) placed upstream the throttle valve (<NUM>),
and wherein the second plant portion (<NUM>) comprises, downstream the first heat exchanger (<NUM>), a valve (<NUM>) for throttling the second gas fraction.