Description

TECHNICAL FIELD

[0001] The present disclosure concerns a liquid natural gas production unit. Embodiments disclosed herein specifically concern a modularized liquefied natural gas production unit comprising a cold box wherein heat exchange between natural gas to be liquefied and a refrigerant fluid undergoing cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization is performed.

BACKGROUND ART

[0002] For practical and commercially viable transport of natural gas, its volume has to be greatly reduced. To do this, the gas must be liquefied by refrigeration to less than -161°C (the boiling point of methane at atmospheric pressure). Each liquid natural gas production plant consists of one or more liquefaction and purification facilities to convert natural gas into liquefied natural gas.

[0003] The liquefaction process involves removal of certain components, such as dust, acid gases, water, mercury and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is then condensed into a liquid with a vapor pressure close to atmospheric pressure by cooling it to approximately -162°C; maximum transport pressure is set at around 25 kPa (4 psi).

[0004] In order to reduce the temperature of natural gas, the heat of the natural gas is transferred to a refrigerant fluid in controlled conditions through the use of heat exchangers. After having absorbed heat from the natural gas, in order to be reused the refrigerant fluid is conveniently cooled in a closed thermodynamic refrigeration cycle, wherein a cooling effect is produced through cyclic thermodynamic transformations, including compression, cooling, condensation, expansion and vaporization.

[0005] In order to obtain the liquefaction of natural gas through heat exchange with a refrigerant fluid, efficiency of heat exchange is a key issue in order to save costs. Efficiency is optimized on one side by promoting heat transfer between the natural gas and the refrigeration fluid and on the other side by reducing undesired heat exchange with the surrounding environment. The first aim is obtained by increasing the ratio between heat exchange surfaces and heat exchanger volume and by selecting materials with the lowest resistance to heat transfer for the making of heat exchange surfaces while the second aim is obtained by insulating the thermal exchanger from the surrounding environment.

[0006] From the above description, it is evident that the technology of production of liquefied natural gas involves a significant amount of apparatuses, including compressors and thermal exchangers. As a consequence, the success of industrial plants for the production of liquefied natural gas is highly dependent on installation costs. In this regard, modularized liquefaction systems have been proposed to offer a “plug and play” approach that enables faster installation and lower construction and operational costs. The modules are manufactured, assembled and tested in the premises of the producer of these apparatuses and subsequently transported to the installation site, where the different modules only need to be connected and where instruments and electrical connections are provided to complete installation of the plant.

[0007] In a modularized liquefaction system, an important solution to increase the efficiency of heat exchange is the use of the so-called cold boxes. A cold box is a complete package of heat exchangers contained in a casing with structural support, thermal insulation containment, and protection for the internal equipment. The thermal insulation of heat exchangers and piping can be obtained in a single casing, making use of a common insulation, for example by using insulating materials inside the casing.

[0008] Cold boxes allow very compact layout and offer a highly efficient thermal insulation, without maintenance needed, to the heat exchange between natural gas and refrigerant fluid. Additionally, on-site installation work is very limited and access to connection piping is simple due to an optimized design, making construction a very quick and simple step and reducing pre-commissioning.

[0009] At present, the production of modularized liquefaction systems provides for the handling of cold boxes as a separate equipment with respect to other modules of a plant. This means that a cold box is transported separately to the site where the natural gas liquefaction plant is installed, has its own foundations and requires that the piping connections are custom made on site, in order to compensate deviations in the position of different modules and the cold box. Additionally, all instrumentation and other electrical consumers can only be connected after the erection on site is finished.

[0010] This approach negatively affects the installation costs of the system and may have an adverse impact on overall safety of the plant. In fact, even in small and midscale liquefied natural gas facilities the cold box can feature more than a hundred temperature instruments, which could dramatically increase time needed to complete the installation but also improve the risk of erroneously connected instruments.

[0011] Accordingly, an optimized arrangement of a liquefied natural gas production unit to address the issues of complexity of the systems of the current art would be beneficial and would be welcomed in the technology. More in general, it would be desirable to provide an optimized arrangement of a liquefied natural gas production unit adapted to more efficiently address problems entailed by the need of connecting the pipes of the cold box and the other modules and the relative instrumentation on the plant installation site.

SUMMARY

[0012] In one aspect, the subject matter disclosed herein is directed to a modularized liquefied natural gas production unit, wherein the cold box is put directly on the refrigerant fluid closed thermodynamic refrigeration cycle module, to form an integrated refrigeration module. This arrangement allows for saving a lot of time on site as the process, instrumentation and electrical connections are completed and tested in the premises of the producer.

[0013] In another aspect, the subject matter disclosed herein concerns a modularized liquefied natural gas production unit, wherein the integrated refrigeration module further comprises all the instrumentation and related electrical connections.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig.1 illustrates a schematic of an arrangement of a liquefied natural gas production unit according to a first embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0015] According to one aspect, the present subject matter is directed to a modularized liquefied natural gas production unit comprising a plurality of modules, and in particular: a first module, called main exchanger module or cold box module, containing a natural gas main heat exchanger, also named cold box, a refrigerant make-up system and an electrical room, the electrical room being configured as a collector of all the electrical connections, and a second module, called the refrigerant fluid closed thermodynamic refrigeration cycle module.

[0016] In particular, the main exchanger module comprising the main electrical room, with the motor control center, is installed in close proximity to a refrigerant compressor skid.

[0017] More in particular, the refrigerant fluid closed thermodynamic refrigeration cycle module comprises all refrigerant related equipment despite cold-box and refrigerant compressor, that is refrigerant compressor suction and discharge vessels, refrigerant compressor compression stage cooler and condenser and main emergency relief installations, like a flare drum.

[0018] Furthermore, according to one aspect, the refrigerant fluid closed thermodynamic refrigeration cycle module also contains all the interface connections with the outside installation, like feed and product lines or utilities.

[0019] According to one aspect, the present subject matter is directed to a modularized liquefied natural gas production unit comprising a cold box including at least a main heat exchanger, transferring heat from natural gas and warm refrigerant to the cold refrigerant, and related vessels needed to separate cold refrigerant vapor and liquid required to allow even distribution of the refrigerant within the main heat exchanger. The cold box is arranged on a module, called main exchanger module, also containing the mam electrical room. The mam exchanger module is arranged next to the refrigerant fluid closed thermodynamic refrigeration cycle module to form an integrated refrigeration module.

[0020] According to a more general aspect, the subject matter disclosed herein is directed to an optimized arrangement of a liquefied natural gas production unit wherein the Cold-Box module also comprises the electrical room and the refrigerant make-up system.

[0021] Reference now will be made in detail to embodiments of the disclosure, one examples of which is illustrated in the drawing. Such example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0022] When introducing elements of various embodiments, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0023] Referring now to the drawings, Fig.1 shows a schematic of an exemplary arrangement of a modularized liquefied natural gas production unit comprising a plurality of modules, and in particular: a main exchanger module 10, a refrigerant compressor skid 20 and a refrigerant fluid closed thermodynamic refrigeration cycle module 30.

[0024] According to one embodiment, the main exchanger module 10 and the refrigerant fluid closed thermodynamic refrigeration cycle module 30 are arranged together to form an integrated refrigeration module 40.

[0025] According to the present disclosure, the refrigerant fluid closed thermodynamic refrigeration cycle module 30 contains all the interface connections with the outside installation, namely a natural gas feed line 31 and product lines, i.e. a liquefied natural gas line 32 and a liquefied petroleum gas line 33. Additionally, the refrigerant fluid closed thermodynamic refrigeration cycle module 30 also contains utilities such as a debutanizer 34 and related heat utility 341 and a flare vessel 35 and flare line 351, connecting to an outside flare (not shown).

[0026] According to an embodiment, the refrigerant fluid closed thermodynamic refrigeration cycle module 30 comprises a refrigerant compressor suction vessel 36 and a refrigerant compressor discharge vessel 37, a refrigerant compressor intermediate compression stage cooler 38 and separator 381, a refrigerant compressor final compression stage condenser 39 and main emergency relief installations, like connection lines 352 to the flare vessel 35.

[0027] According to an exemplary embodiment, the main exchanger module 10 comprises a cold box including a main heat exchanger 11, a cold gas separator 12 to separate cold refrigerant vapor 121 and liquid 122, a main electrical room 13 and a refrigerant make-up system 14 including a refrigerant make-up feed line 141 and refrigerant make-up lines 142. 143, 144, connected to cooled refrigerant lines.

[0028] According to an exemplary embodiment, the refrigerant fluid from the compressor suction vessel 36 is directed to the compressor skid 20 through a suction line 361. The refrigerant fluid is then compressed in a first compressor stage of the compressor and subsequently directed, though a line 380 to a refrigerant compressor intermediate compression stage cooler 38, wherein the heaviest fracti ons of the refrigerant condense. The cooled refrigerant stream is then routed to the separator 381, wherein it is separated into a liquid stream 382 and a vapor 383. The liquid stream 382 is directed to the main heat exchanger 11 of the cold box and subsequently directed via a collector line 360 to the compressor suction vessel 36.

[0029] The vapor stream 383 from the separator 381 is sent to a second compression stage of the compressor skid 20 and subsequently directed through a line 390 to the condenser 39 wherein it is cooled and wherein other fractions of the refrigerant condense. The cooled refrigerant stream is then routed to refrigerant compressor discharge vessel 37, wherein it is separated into a liquid stream and a vapor stream, the vapor stream being composed of the lightest fractions of the refrigerant. The liqui d stream is directed via a liquid stream line 371 to the main heat exchanger 11 of the cold box, and subsequently to the refrigerant compressor suction vessel 36.

[0030] The vapor stream from the refrigerant compressor discharge vessel 37 is directed via a vapor stream line 372 to the cold end of the main heat exchanger 11 of the cold box, and subsequently to the refrigerant compressor suction vessel 36.

[0031] The refrigerant cycle allows to exchange heat with the natural gas in a plurality of heat exchangers at different temperatures, taking advantage of the vaporization temperature difference between the different generated refrigerant streams to optimize the natural gas liquefaction by approaching the cooling curve of the natural gas from ambient to cryogenic temperatures, minimizing energy requirements and heat exchangers size.

[0032] On the natural gas side of the liquefied natural gas production unit, the natural gas feed is routed via a natural gas feed line 31 to the main heat exchanger 11 of the cold box, to be pre-cooled in order to condense heavier than methane hydrocarbons. The pre-cooled natural gas stream is then routed via a line 120 to the separator 12, wherein it is separated into a liquid stream and a vapor stream, the liquid stream comprising heavier than methane hydrocarbons, together with a certain amount of methane. From the top of the separator 12, the vapor stream is routed via a vapor stream line 121 to the main heat exchanger 11 of the cold box, to be cooled at a temperature causing the condensation of the vapor.

[0033] The liquid stream comprising heavier than methane hydrocarbons is routed via a liquid stream line 122 to the debutanizer 34, to separate methane still present in the liquid stream, from heavier than methane hydrocarbons, in particular from butane. The debutanizer 34, being composed of a pressurized column with a boiler at its bottom, provides heat 341 to the liquid stream, vaporizing the lighter components of the liquid stream, mainly methane with a little amount of propane and some butane, which run through the column wherein a vapor-liquid equilibrium is established between components with different boiling points. A liquid stream from the boiler of the debutanizer 34, comprised mainly of butane, but also compri sing propane and heavi er than butane components, is obtained and is collected via a liquid stream line 33. A vaporized stream from the top of the debutanizer 34, mainly comprising methane, is sent via a vaporized stream line 342 to the main heat exchanger 11 of the cold box, wherein it is condensed to form, together with the condensed vapor stream routed via the vapor stream line 121, a liquefied natural gas +stream, sent via a condensed vapor stream line 110 to a liquefied natural gas stream collection unit 111, before being collected through the line 32.

[0034] From the above description of an exemplary embodiment it is evident that most of the process, instrumentation and electrical connections are shared between the main exchanger module 10, the refrigerant compressor skid 20 and the refrigerant fluid closed thermodynamic refrigeration cycle module 30 and can consequently be completed in the premises of the producer rather than on the plant installation site, allowing for saving a lot of time on site. Depending on the refrigerant system used the number and type of equipment may vary. Key element of the invention is the location of Cold- Box, main electrical room and compressor skid in relation to the residual installation. Depending on the use of other highly instrumented systems, like turboexpanders, allocation of equipment on either the main exchanger module 10 or the refrigerant fluid closed thermodynamic refrigeration cycle module 30 might be adopted.

[0035] While aspects of the invention have been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirt and scope of the claims.