EP3877712A2

EP3877712A2 - Process for the ri-liquefaction and simultaneous reduction of nitrogen content in the bog for self-frigerated absorption

Info

Publication number: EP3877712A2
Application number: EP19831875.0A
Authority: EP
Inventors: Matteo BERRA; Anton Marco FANTOLINI; Fabrizio MELONI
Original assignee: Saipem SpA
Current assignee: Saipem SpA
Priority date: 2018-11-08
Filing date: 2019-11-07
Publication date: 2021-09-15
Also published as: WO2020095246A2; IT201800010171A1; WO2020095246A3

Abstract

The present invention relates to a process for the reliquefaction of the Boil Off Gas and the simultaneous reduction of the nitrogen contained therein, with consequent enrichment in hydrocarbons.

Description

DESCRIPTION

"PROCESS FOR THE RI-LIQUEFACTION AND SIMULTANEOUS REDUCTION OF NITROGEN CONTENT IN THE BOG FOR SELF FRIGERATED ABSORPTION"

Technical field of the invention

The present invention relates to the field of liquefied natural gas (LNG) storage, and in particular for the management of the Boil off Gas (BOG) .

Background art

Boil Off Gas (BOG) is produced in LNG storages, similarly as other low-boiling fluids, because of the inevitable evaporation thereof due to the high difference between the storage temperature (between - 145°C and -161°C) and the external ambient temperature, is typically richer in nitrogen than LNG, due to the greater volatility of this component of LNG compared to methane, which is the main component thereof.

The increase in the nitrogen content alters, however, the calorific value of the BOG, making it often unsuitable for sale and/or for use as a fuel.

An amount of BOG is also generated during the operations of loading and unloading the LNG and in that of recirculating the LNG in loading lines, carried out during the periods of non-use of these lines to keep the temperature constant in the cryogenic circuits to which they belong. The generation of BOGs is a very important problem and in order to manage it, several solutions have been devised ranging from its total or partial re liquefaction to the correction of its calorific value, the latter obtained by separating hydrocarbons from nitrogen (as much as possible) in order to obtain a BOG which is suitable for sale and/or use as natural gas .

Among the different systems for the management of the BOG product, of particular interest are those which allow the re-liquefaction, and in particular the re liquefaction of the hydrocarbon component thereof, thus recovering a product of high economic value, rather than having to dispose of the gas as a low value by product .

The re-liquefaction of BOG, also due to the nitrogen-rich composition, requires refrigeration units supplied at very low temperatures, obtained through particularly energy-intensive refrigeration cycles; the low pressure at which the BOG is generated also requires additional energy for its compression and re-liquefaction .

The known processes, developed in the wake of re liquefaction, always tend to maximize the efficiency of re-liquefaction processes, in order to save energy. Over time, several refrigeration cycles have been proposed, which are able to completely or only partially re-condense the BOG, each with unique features in terms of system simplicity or complexity, energy efficiency (understood as the ratio between energy recovered in the form of LNG and energy spent) , use of particular refrigerants or self-refrigeration.

The Gasconsult Limited patent (GB2522421), the scheme of which is shown in figure 1, utilizes the BOG itself, or rather its incondensable fraction, as a cooling fluid.

The BOG produced in storage is compressed and combined with the recirculation current of the refrigerating cycle to obtain a gas richer in nitrogen than the BOG, which is then pre-cooled and divided into two currents, one main and one service.

The service current is sent to the Refrigeration Cycle Expander, from which it comes out cooler to then supply refrigeration to the main current, which cools down before undergoing an expansion and further cooling in an End Flash Expander.

The main current, thus treated, gives rise to the formation of a mixed phase, in which the liquid fraction is richer in hydrocarbons and is recovered and sent to storage, while the non-condensed fraction, richer in nitrogen (but with a high content of hydrocarbons) forms the recirculation current.

Refrigeration units are recovered from the recirculation current before the necessary recompression; part of the recirculation must be purged from the system, which works cyclically, by release into the atmosphere or by torch.

The systems of the prior art for re-liquefaction have two technical problems which derive from two opposing requirements:

- the total condensation of the BOG requires a lot of energy, due to the presence of nitrogen (without however energy value) , which translates into higher operating costs and higher emissions for the production of this energy.

Alternatively, removal by purging reduces the energy required, but results in an important loss of hydrocarbon recovery, and consequent release of high greenhouse gases into the atmosphere (about 23 times higher for methane than for CO2) ;

- the total condensation of the BOG, in addition to consuming a lot of energy, has the defect of recovering the nitrogen from storage, preventing the aging of the stored LNG; by aging we mean the phenomenon of a progressive decrease in LNG volatility and an increase in its calorific value. It should be noted that, as often occurs, LNG is paid on the basis of its calorific value, therefore the loss of nitrogen is absolutely not a loss of value of the LNG.

The Chinese document CN206721149, the scheme of which is shown in figure 2, uses a low pressure distillation column (3 bar) to separate the BOG into two products: a liquid richer in hydrocarbons, then returned to the gas phase in order to recover the refrigeration units thereof, and a nitrogen-rich gas.

The BOG (11), compressed at a pressure of about 9 bar a, is sent to a heat exchanger for the recovery of the cold currents present in the system and comes out at a temperature between -30°C and -65°C; then (21), it undergoes a further cooling in the reboiler of the distillation column, from where it comes out at a temperature between -80°C and -50°C, to then return to another passage in the first exchanger and reach a final temperature of -90°C (12) .

By means of a valve JT (6) , the BOG thus pre cooled is laminated to a pressure of 3 bar thus becoming a mixed liquid/vapour phase, which is sent to a distillation column (3) provided with both a bottom reboiler and with a head condenser and, from this column, two products come out: a nitrogen-rich gas which, cooled by the refrigeration units produced by evaporation of the bottom liquid, the pressure of which is lowered by the valve JT (7) from 3 bar to a certain pressure (not specified) , it provides the head reflux, which returns to the column.

From the Reflux drum (5), a cold gas (14) comes out, the refrigeration units of which are transferred to the BOG in the pre-cooling step.

The column bottom liquid (32) also returns refrigeration units: first to the column head exchanger (42), where it produces the condensate required to supply a reflux current, then to the BOG in the pre cooling step (13), and in doing so it passes from liquid, at the bottom of the column, to mixed phase, after the depressurization in the valve JT, to gas phase after having transferred the cold both in the column head condenser and in the BOG pre-cooling exchanger .

The above-described system for the re liquefaction and simultaneous decrease of the nitrogen contained in the BOG, with consequent enrichment in hydrocarbons, obtains a low pressure gaseous product, of low value with respect to the LNG and does not solve the problem, much felt especially in bunkering sites, to reduce or zero the loss of product stored in the tanks; moreover, although it is possible to adjust the calorific value, there is no guarantee that it is possible to sell the BOG enriched in hydrocarbons as it is and, if it is possible, the selling price could be so low as to make the enrichment system and that for the necessary re-compression of the BOG up to the network pressure anti-economic, with high energy expenditure .

Summary of the invention

The inventors of the present patent application have surprisingly found that it is possible to simultaneously recover the hydrocarbons present in the Boil Off Gas and separate the nitrogen contained therein into a process of re-liquefaction of the Boil Off Gas in a self-refrigeration cycle.

Object of the invention

Therefore, a first object of the invention is represented by a process for the re-liquefaction of the BOG and the simultaneous decrease of the nitrogen content thereof.

In a second object, a system for carrying out the process of the invention is described.

In a third object, a process for treating an excess amount of BOG and recovering the BOG in storage plants is described. Brief description of the drawings

Figure 1 shows the scheme of the process of Gasconsult Limited (GB 2522421);

figure 2 shows the scheme of the process according to the prior art CN 206721149;

figure 3 shows the scheme of a first embodiment of the invention;

figure 4 shows an example of how a rectification column can be applied to the first embodiment of the invention;

figure 5 shows the scheme of a variant of the first embodiment of the invention with a different configuration of the COLD BOX;

figure 6 shows a particular example of the first embodiment of the invention;

figure 7 shows the thermodynamic passages of the flow of the fluid from the delivery of the compressor up to its separation into reflux liquid (flow 10-11- 12) according to the embodiment in figure 6;

figure 8 shows the scheme of a second embodiment of the invention;

figure 9 shows the scheme of a variant of the second embodiment of the invention with a different configuration of the COLD BOX; figure 10 shows another variant of the configuration of the COLD BOX according to the second embodiment of the invention;

figure 11 shows an example of how a rectification column can be applied to the second embodiment of the invention;

figure 12 shows a particular example of the second embodiment of the invention;

figure 13 shows the thermodynamic passages of the flow of the fluid from the delivery of the compressor up to its separation into reflux liquid (flow 10-11- 12) according to the embodiment in figure 12;

figure 14 shows an example of application of the process of the invention for managing the BOG in excess with respect to the re-liquefaction capacity.

It should be noted that, in general, according to the depiction of the COLD BOX in the figures, the flows heat up from left to right and cool down from right to left .

Detailed description of the invention

The present invention is described in particular in relation to the re-liquefaction of BOG from liquefied natural gas (LNG) , but it is equally applicable to the re-liquefaction of other BOGs from other liquefied fluids stored at low temperatures (below about 0°C) or at cryogenic temperatures (below -45°C) .

In general, the present invention will find application in all those cases in which there is the evaporation of a mixture consisting of two or more components, the most volatiles of which have little or no value, to the point that it is convenient to save the energy required for their recondensation despite having to lose them in the form of a purge current.

In one aspect of the present invention, there are no particular qualitative and quantitative limitations to the BOG which can be used in the described process; in fact, this can contain at least 1% of methane and at least 0.01% of nitrogen, whereas the other components are represented by hydrocarbons with a number of carbon atoms >2.

In the present description, by "liquefied natural gas" (LNG) , hereinafter also referred to as "liquefied gas", it is meant a liquid obtained from natural gas, after appropriate purification treatments from undesired components and dehydration through successive cooling and condensation steps .

More in general, in the present description, by "liquefied gas" it is meant a fluid of mainly liquid component . For the purposes of the present invention, the term "flow" and amount, where used, are to be understood as synonyms.

Moreover, in the present description, the term "cooling fluid" refers to, for example: ambient air, sea water.

A cooling fluid generally operates at temperatures lower than 60°C, preferably lower than 50 °C .

For the purposes of the present invention, the term "atmospheric storage" of LNG means a storage of LNG which is characterized by the atmospheric pressure to which it is stored.

For the purposes of the present invention, by the term "pressure storage" of LNG (for example, in "bullet") it is meant a storage of LNG which is characterized by the pressure at which it is stored, comprised between about 1 and 15 bar g.

According to a first object, the present invention describes a process for the re-liquefaction of the BOG and the simultaneous reduction of the nitrogen content thereof (with consequent enrichment in hydrocarbons) .

In particular, the process of the present invention (for example shown in figure 3) for the re- liquefaction and simultaneous removal of nitrogen from a flow of Boil Off Gas 10 comprises subjecting said flow 10 to the steps of:

a) cooling, thus obtaining a cooled flow of BOG 11, b) subjecting said cooled flow of BOG 11 to an absorption step, thus obtaining a hydrocarbon-enriched flow of BOG 100 and a nitrogen-rich vapour flow 20, c) subjecting said nitrogen-rich vapour flow 20 to a heating step, thus obtaining a heated, nitrogen-rich flow 21,

d) subjecting said flow 21 to a compression step, this obtaining a nitrogen-rich vapour flow and 22, e) subjecting said compressed, nitrogen-rich vapour flow 22 to a cooling step, thus obtaining a compressed and cooled nitrogen-rich flow 31,

f) subjecting said compressed and cooled nitrogen- rich flow 31 to an expansion step, thus obtaining an expanded and further cooled nitrogen-rich flow 32, g) subjecting said expanded and further cooled nitrogen-rich flow 32 to a separation step obtaining a vapour flow 50 which is very rich in nitrogen and a first liquid service flow 40.

According to the present invention, the cooling step a) may be carried out by heat exchange with the vapour flow 50 which is very rich in nitrogen, thus obtaining a heated vapour flow 51 which is very rich in nitrogen.

It should be noted that step a) may also involve the nitrogen-rich flow 20, if required.

In an alternative aspect of the present invention, the cooling step a) may not be carried out; in this case, the initial flow 10 is sent directly to the absorption step (step b) ) , representing the input thereof (in-flow) .

As regards step b) , more particularly it is an absorption step.

In a preferred aspect, this step b) is carried out in an absorption column (BOG Absorber 304 in the figures) .

In particular, in the absorption step b) , said first liquid service flow 40 encounters in counter- current said cooled flow of BOG 11 or BOG 10 if step a) is not carried out.

Inside the absorption column, absorption may take place by stages (plate column) in a variable number according to the conditions, or continuously (filling column) as those skilled in the art may select and optimize .

More specifically, step b) is carried out at a pressure of about 2-20 bar g, preferably at a pressure of 5-15 bar g and even more preferably of about 10 bar g.

According to an aspect of the present invention, a portion of the heated vapour flow 51 which is very rich in nitrogen is sent to step d) , optionally after having been combined with the heated nitrogen-rich vapour flow 21.

According to another aspect of the invention, a portion 52 of the heated vapour flow which is very rich in nitrogen is withdrawn from the current 51 and released, for example into the atmosphere.

For the present purposes, the cooling step e) (which is a second cooling step in the process of the invention, after step a) ) comprises two steps, of which :

- a cooling step el) which is carried out using a cooling fluid 23 as defined above and which allows to obtain a compressed and partially cooled nitrogen-rich flow 30, and

- a step e2) of further cooling which is carried out by heat exchange with the nitrogen-rich vapour flow 20 and/or the vapour flow 50 which is very rich in nitrogen .

For the purposes of the present invention, therefore, step el) is carried out with a cooling fluid which is external to the process circuit and to the exchanger (COLD BOX); step e2), on the other hand, is carried out by heat exchange with one of the flows obtained by the process of the invention and inside the exchanger of the invention (COLD BOX) .

According to a preferred aspect of the invention, the nitrogen-rich flow, compressed and partially cooled, 30 obtained from step el) is used in the heat exchange step c) for heating said nitrogen-rich vapour flow 20.

For the purposes of the present patent application, before the cooling step a) , if carried out, or before the absorption step b) , a compression step aO) of a flow of Boil Off Gas 1 is carried out so as to obtain a compressed flow of the BOG 10.

This step aO) may not be carried out, for example in the case of pressurized storage, while it is necessarily carried out in the case of storage at atmospheric pressure.

According to an aspect of the present invention, for example shown in figure 4, the described process further comprises a rectification step, in which a flow of liquid nitrogen 70 and a portion separated from the vapour flow 60 which is very rich in nitrogen (also referred to as a first service vapour flow) separated from the flow 50 is used, in order to obtain a separate, almost pure, nitrogen flow 80 and a second liquid service flow 90.

In one aspect, in the rectification step, a portion (52') drawn from the heated vapour flow which is very rich in nitrogen may be used, in addition to or alternatively to the flow 60.

As regards the second liquid service flow (90) separated in the rectification step, in one aspect of the invention it may be sent to the absorption step b) , possibly after having been combined with the first liquid service flow (40) forming a third service flow (41) .

According to an embodiment of the present invention (for example shown in figure 8), during step e) , and in particular after step el), a portion of the nitrogen-rich vapour flow compressed and partially cooled 24 is separated and subjected to the steps of: h) cooling, obtaining a refrigerated flow 25,

i) expansion, obtaining a refrigerated and expanded flow 26,

1) heating, obtaining a refrigerated expanded and heated flow 27.

For the purposes of the present invention, such a refrigerated expanded and heated flow 27 is sent to step d) , possibly after being combined with the heated nitrogen-rich flow 21.

In particular, in step h) described above, the heat exchange takes place with the expanded refrigerated flow 26 of step i) and/or with the nitrogen-rich vapour flow 20 of step c) .

According to an alternative aspect of the present invention (for example shown in figure 10), a portion of the nitrogen-rich vapour flow, compressed and partially cooled obtained after steps c) , d) and el) 30' is only partially subjected to the cooling step e2), obtaining a partially refrigerated flow 25'.

In particular, after the partial cooling of step e2), the flow 25' is subjected to the further steps of :

i') expansion and further cooling, obtaining a partially refrigerated and expanded flow 26',

1' ) heating, obtaining a partially refrigerated expanded and heated flow 21 .

In particular, such a partially refrigerated expanded and heated flow 21 is sent to step d) , possibly after being combined with the nitrogen-rich vapour flow and heated 21 (as described above, possibly together with flow 51) .

In particular, in this case, step e2) may be carried out on the portion 30' by heat exchange with the partially refrigerated and expanded flow 26' of step i') and/or with the nitrogen-rich vapour flow 20 of step c) .

As regards the flow of BOG enriched in hydrocarbons 100 obtained with the process of the invention, this is then subjected to an expansion step m) (with a valve VI) obtaining an almost completely liquid BOG flow 110.

This flow is then sent to the LNG tank (301 in the figures) .

According to an aspect of the present invention, from the step m) a gaseous BOG flow is also obtained, which may be combined with the BOG generated in the storage to form the flow 1, which may be sent to the compression step aO) as described.

It should be noted that for the purposes of the present invention, the steps f) and i) described above may be carried out by using valves or machines, even in combination with one another.

For example, step f) may be carried out by using a valve (V2 in the figures) .

For the purposes of the present invention, the thermal exchanges of steps a), c) , e2) h) 1) and 1' ) are preferably carried out inside a first heat exchanger 303 (to which reference will also be made as "COLD BOX"), while step el) is preferably carried out in a second heat exchanger 306.

For the purposes of the present invention, the first heat exchanger 303 ("COLD BOX") is a single exchanger .

It should be noted that, for the purposes of the present invention, depending on the incidental (technical and economic) needs, those skilled in the art may utilize one or more of the other flows (or portions thereof) of the process as a source of calories or refrigeration units for the thermal exchanges required for the process; therefore, the embodiments exemplified in the present description represent some of the possible configurations of the so-called "COLD BOX" (or first heat exchanger 303) without excluding others.

For the purposes of the present invention, the (molar %) content of nitrogen in the described flows increases as follows:

1,10,11 < 20,21,30 < 50,60 < 80.

This means that the flow 80 has a (molar %) content of nitrogen greater than the flow 20 (or 21 or 30), which, in turn, have a higher (% molar) nitrogen content than flow 1 (or 10 or 11) . In fact, the steps b) of absorption, g) of separation, and possibly of rectification, produce flows with a greater (molar %) nitrogen content with respect to the input flow in such a step.

The increase in the (molar %) of nitrogen content obtained with each step depends on the nitrogen content in the initial (input) flow, the efficiency of each step and the operating conditions.

According to a general aspect, reference may be made to the following definitions of the terms used:

The nitrogen-rich flow, such as, for example the flow 20, 21 and 30, comprises nitrogen in a (molar) amount of about 20-98%;

A flow which is very rich in nitrogen, such as the flow 50 (and the flow 60, also referred to as a first service vapour flow, separated therefrom) , comprises nitrogen in a (molar) amount of about 70- 99.5%;

Almost pure nitrogen flow, such as the flow 80, comprises nitrogen in a (molar) amount of about 98-99.9%;

flow of BOG enriched in hydrocarbons (i.e. with a decreased (% molar) nitrogen content with respect to the initial flow) , such as the flow 100 and 110, comprises nitrogen in a (molar) amount of about 0.2-20%;

the first service flow in liquid form, such as for example the flow 40, comprises a (molar) amount of nitrogen of about 20-95%;

the second service flow in liquid form, such as for example the flow 90, comprises a (molar) amount of nitrogen of about 70-95%.

A second object of the invention describes a plant for the re-liquefaction and simultaneous reduction of the nitrogen content of the BOG.

In particular, such a plant comprises:

an LNG tank 301,

a BOG compressor 302 in fluid connection with said LNG tank 301; said BOG compressor 302 being possibly present,

- a first heat exchanger 303 ("COLD BOX") in fluid connection with said BOG compressor 302 and with a BOG Absorber 304,

a BOG Absorber 304 in fluid connection with said first heat exchanger 303,

- a Refrigeration Cycle Compressor 305 in fluid connection with said first heat exchanger 303,

a second heat exchanger 306 in fluid connection with said Refrigeration Cycle Compressor 305 and with said first heat exchanger 303, a Reflux Flash Drum 307 in fluid connection with said heat exchanger 303 and with said BOG Absorber 304,

an expansion valve (VI) in fluid connection with said BOG Absorber 304 and with said LNG tank 301, a pressure decreasing valve (V2) in fluid connection with said Reflux Flash Drum 307.

According to a first embodiment of the invention, the plant 300 may further comprise:

a Refrigeration Cycle Expander 308 in fluid connection with a Reflux Flash Drum 307 and with said first heat exchanger 303 or only with said first heat exchanger 303.

According to another embodiment of the invention, the plant 300 may further comprise:

a Nitrogen Rectification Column 309 in fluid connection with the Reflux Flash Drum 307 and possibly also with the BOG Absorber 304.

In a third object of the present invention, the BOG flow which is subjected to the process according to the above description is obtained by combining the BOG generated in the LNG storage facilities with an additional BOG flow which is an excess flow with respect to the re-liquefaction capacity of the process according to the present invention. In particular, as shown in figure 14, the present invention describes a process which comprises the steps of :

I) compression in a BOG compressor 401 of a flow of BOG 200 obtaining a flow 201 of BOG at high pressure,

II) recondensation of said flow 201 in a BOG recondenser 402 by heat exchange with a flow of LNG 206 obtaining a flow of BOG in liquid form 202,

III) storage of said flow 202 in a storage bullet

403,

IV) reduction of the pressure of a flow 203 taken from the storage of step III) through an expansion valve V3, obtaining an LNG flow 204 and a BOG flow 1, where said BOG flow 1 is then sent to step aO) according to the process described above.

In a preferred aspect of this third object, the compression of step I) is up to a pressure of about 2- 25 bar g, preferably of about 5-15 bar g and even more preferably 10 bar g.

In another aspect of the invention, the condensation step II) is carried out with the LNG flow 206 obtained by pumping an LNG flow 205 taken from the LNG tank 404 by means of a pump 405.

In one aspect of the invention, the storage of step III) is maintained at a pressure of about 2-30 bar and preferably of 5-15 bar g.

The production of BOG in the type of plants in which the present invention may be applied includes: all the LNG storage sites, present in bunkering sites or in LNG production or LNG regasification plants, which produce BOG in a variable manner depending on environmental conditions, daytime and seasonal temperature differences, and especially plant operations, such as the loading or unloading of LNG tanks .

According to an aspect of the present invention, the process described above is applied for treating an excess amount of BOG with respect to the re liquefaction capacities of the process object of the present invention.

According to an aspect of the present invention, it may be applied for the retrofit of LNG production plants

, where usually the BOG is recompressed and used as fuel gas or, in the best cases, recycled at the input of the liquefaction section.

In these cases, the application in retrofit configuration allows to increase the total production of LNG.

As a further application, the process of the invention finds use in bunkering sites, where the LNG is stored, in atmospheric tanks or in bullets under pressure (for subsequent loading on ships, trucks or isocontainers); the generated BOG is sent to torch or fuel gas or compressed to the main pressure possibly available .

For the retrofit application or in the implementation of new LNG regasification terminals, on land or at sea (FSRU), which are exposed to the problem of managing the BOG, especially during storage filling and in case of reduced regasification capacity (minimum send-out or zero send-out) .

With reference to figure 12 and to the graph in figure 13, relating to a second embodiment of the present invention, the following is reported.

Figure 13 shows the thermodynamic transformation undergone by the refrigerating fluid starting from the compressor delivery (current 10* in figure 12), to its cooling by heat exchange (current 11*) and lamination (current 12*) . The current 12* is in mixed phase and is sent to the Reflux Flash Drum where its separation takes place in reflux liquid for the absorber head (current 13*) and in the recirculation gas (current 14*) . The thermodynamic transformation consists of the succession of lines referred to as isoP and isoH, which represent the isobaric refrigeration of the refrigerating fluid obtained from the BOG and its subsequent iso-enthalpy lamination by means of a valve, respectively .

The thermodynamic transformation depicted has the purpose of "bypassing" the equilibrium curve of the refrigerating fluid, allowing it to cool the fluid at super-critical pressure, to avoid phase changes in the cooling itself, and therefore allow the thermal exchange curves in the heat exchanger to be brought closer together, thus increasing the efficiency of the entire process.

This allows, with greater cooling, to reach a point where the subsequent lamination produces a greater amount of liquid; this is in order to reduce the flow processed by the Refrigeration Cycle Compressor .

As for the recirculation gases, these are the current which is the richest in nitrogen of the plants, allowing to minimize the losses of hydrocarbons in the event of purging from this current.

The BOG (currents 1*, 2*, 3*) taken at atmospheric pressure from storage is compressed in the BOG Compressor and refrigerated by heat exchange with the other cold currents generated in the process; it is then fed to the BOG Absorber absorption column where the hydrocarbons are separated from the nitrogen thus producing, at the bottom of the column, the current 4* which, laminated by means of valve VI, produces both liquid, which will fall into storage, and vapour, which will be combined with the BOG and will be recycled to the BOG Compressor.

From the head of the BOG Absorber column, the cold, nitrogen-rich current 5* transfers its refrigeration units to the Cold Box and is fed to the manifold of the Refrigeration Cycle Compressor, which is the engine of the refrigeration cycle. All the compressed gases from said compressor (current 7*) are cooled in an exchanger by means of a cold fluid 23 available in the plant, for example water or air, and then divided into two currents, a service current (current 16*), used for producing the refrigeration units which cannot be recovered from other cold currents, and a process current (current 10*) which undergoes the thermodynamic transformations referred to as IsoP and IsoH, already described with reference to figure 13.

The reflux of the BOG Absorber (current 13) consists of the liquid (reflux liquid) obtained by the lamination in a valve of the current 11*. The separated vapour after the lamination by means of the valve in the Reflux Flash Drum (current 14*) yields its own refrigeration units in favor of the other hot fluids in the Cold Box and is fed to the Refrigeration Cycle Compressor manifold.

The provision of a special tapping line, upstream or preferably downstream of the Cold Box, regulated through a valve, allows the partial removal of nitrogen from the BOG.

As far as the service current (current 16*) is concerned, it undergoes a cycle in which it is cooled in the Cold Box by means of the refrigeration units recovered from the circulating cold currents and sent to the Refrigeration Cycle Expander (current 17*) and, once transferred the refrigeration units in the Cold Box, is combined with the other hot currents in the Refrigeration Cycle Compressor manifold.

If a higher purity, in nitrogen, of the purge current is desired, it is possible to wash it in a further absorption column (the Nitrogen Rectification Column" described above) , with liquid nitrogen possibly imported into the plant (see figure 11, where the liquid deriving from the washing with nitrogen is recovered, sending it to the head of the BOG Absorber) .

With reference to figure 6 to the graph in figure 7, relating to a first embodiment of the present invention, the following is reported.

Figure 7 shows the thermodynamic transformation undergone by the working fluid starting from the compressor delivery up to its separation in reflux liquid for the absorber head and in the recirculation gas. It consists of the succession of lines referred to as isoP and isoH, which represent the isobaric refrigeration of the working fluid (refrigerant obtained from the BOG) and the subsequent iso-enthalpy expansion by means of an expander.

Contrary to the case in figure 13, where the thermodynamic transformation had the purpose of obtaining the greatest possible amount of liquid, now the liquid must be produced in an amount which is compatible with the machine (expander) .

Figure 6 shows a plant working according to the transformation in figure 7.

The BOG (currents 1*, 2*) taken at atmospheric pressure from storage is compressed in the BOG Compressor and refrigerated in the Cold Box by heat exchange with the other cold currents generated in the process; it is then fed to the absorption column where the hydrocarbons are separated from the nitrogen thus producing, at the bottom of the column, the current 14* which, laminated by means of a valve, produces both liquid, which will fall into storage, and vapour, which will be combined with the BOG and will be recycled to the BOG Compressor.

From the head of the BOG Absorber column, the cold nitrogen-rich current 4* transfers its refrigeration units to the Cold Box and is fed to the manifold of the Refrigeration Cycle Compressor, which is the engine of the refrigeration cycle.

All the compressed gases from said compressor

(current 7*) are cooled by means of cold fluid available in the plant, for example water or air, and then undergo the thermodynamic transformations referred to as IsoP and IsoS, described with reference to figure 7; in this case, the entire flow is treated in the Refrigeration Cycle Expander.

The reflux of the BOG Absorber (current 13*) consists of the liquid separated in the Reflux Flash Drum, while the vapour (current 11*) yields its own refrigeration units in the Cold Box in favor of the other hot fluids and, after withdrawing a purge current 15* (from which the removal of nitrogen from the BOG), is fed to the manifold of the Refrigeration Cycle

Compressor . If a higher purity, in nitrogen, of the purge current is desired, it is possible to wash it in a further absorption column, the Nitrogen Rectification Column, with liquid nitrogen possibly imported into the plant (see figure 5, where the liquid deriving from the washing with nitrogen is recovered and sent to the head of the BOG Absorber) .

With reference to figure 14, an excess flow of BOG (200) is compressed at a pressure between 2 and 25 bar g, preferably between 5 and 15 bar g, or more suitably at 10 bar g, to be then recondensed by contact with LNG (206) taken from storage (LNG TANK) and pumped at the same pressure.

The condensate thus obtained is stored under pressure in one or more tanks (bullets) to be then re sent, in the times and at the rate deemed appropriate, to storage, after lamination by means of a valve.

This latter operation causes the separation of LNG, which falls within the storage, and the controlled generation of additional BOG (1), which can be subjected to the process described in the present patent application.

The effectiveness of the solutions proposed by the present invention is evaluated on the basis of two parameters : i) Energy recovery index I:

I = energy recovered in the form of a recondensed BOG/energy spent, where

Energy recovered = (Flow of the liquid hydrocarbons recovered to storage) X (lower calorific value - LHV) and

Energy expenditure = Total mechanical energy spent in the operation; and

ii) Nitrogen purity of the purge current which, even when used for purposes such as compressor seals or torch purge, is discharged into the atmosphere.

The configuration of the plant may include or not the flushing with nitrogen of the purge current depending on the possibility of recovering the nitrogen for plant use or any environmental constraints; in the examples below it is used, if it is convenient.

EXAMPLE 1A

Atmospheric storage of very-nitrogen rich LNG Stored LNG composition

Treated BOG composition The BOG is considered overheated by 12 °C at a temperature of -152°C and the treatment of 1000 kg/h of BOG is considered.

The scheme adopted is that in figure 11.

Two operating possibilities are given, which differ in the purge flow:

1. Production of a purge current with 98.3% nitrogen, based on a flow of 345 kg/h and at a pressure of 10 bar g, 655 kg/h of liquid are recovered with 99.1% of methane .

Recovery I = 11.5 kJthermal/kJmechanical .

Due to the high purity in nitrogen of the purge current produced, no further washing is needed.

2. Production of a purge current with 82.4% nitrogen, based on a flow of 400 kg/h and at a pressure of 10 bar g, 600 kg/h of liquid are recovered with 99.5% of methane .

Recovery I = 12.24 kJthermal/kJmechanical .

Given the significant amount of nitrogen-rich purge current, the washing is anti-economic, therefore the application of the diagram in figure 12 is not considered .

In a further operational possibility, purging nitrogen can be avoided, with a reduction in energy recovery; in this case, with such a high nitrogen content in the BOG, this reduction would be very significant.

EXAMPLE IB

Atmospheric storage of very-nitrogen poor LNG Stored LNG composition

Treated BOG composition

The BOG is considered overheated by about 10°C at a temperature of -152°C and the treatment of 818 kg/h of BOG is considered, corresponding to the evaporation of the BOG considered subject to the same thermal load as in Example la, a plausible hypothesis as it is the same storage park.

The scheme adopted is that in figure 11 under the first operating condition, and in figure 12 under the second operating condition.

The results are described for two operating possibilities :

1. Production of a purge current with 98.2% nitrogen, based on a flow of 65 kg/h and at a pressure of 10 bar g, 753 kg/h of liquid are recovered with 99.7% of methane . Recovery I = 12.64 kJthermal/kJmechanical .

Due to the purity in nitrogen of the purge current produced, no further washing is needed.

2. By washing with 20 kg/h of liquid nitrogen, the production of a purge current with 93.4% nitrogen is obtained, based on a flow of 82 kg/h and at a pressure of 10 bar g, 756 kg/h of liquid are recovered with 99.9% of methane.

Recovery I = 14.03 kJthermal/kJmechanical .

In a further operational possibility, purging nitrogen can be avoided, with a reduction in energy recovery; in this case, with such a modest content in the BOG, this reduction would be modest.

EXAMPLE 2A

Bullet storage of nitrogen-rich LNG Stored LNG composition

Treated BOG composition

The BOG is considered overheated by about 10°C at a temperature of -125.7°C and the treatment of 1000 kg/h of BOG is considered.

The scheme adopted is that in figure 8.

The results are described for two operating possibilities :

1. Production of a purge current with 94.2% nitrogen, based on a flow of 140 kg/h and at a pressure of 10 bar g, 860 kg/h of liquid are recovered with 98.9% of methane .

Recovery I = 21.63 kJthermal/kJmechanical .

2. Production of a purge current with 78.8% nitrogen, based on a flow of 180 kg/h and at a pressure of 10 bar g, 820 kg/h of liquid are recovered with 99.1% of methane .

Recovery I = 25.26 kJthermal/kJmechanical .

Given the significant amount of nitrogen-rich current, the washing is anti-economic.

EXAMPLE 2B

Bullet storage of nitrogen-poor LNG Stored LNG composition

Treated BOG composition

The BOG is considered overheated by about 10°C at a temperature of -125°C and the treatment of 990.5 kg/h of BOG is considered, corresponding to the evaporation of the BOG considered subject to the same thermal load as in Example 2a, a plausible hypothesis as it is the same storage park.

The scheme adopted is that in figure 11.

The best operational possibility is washing with 20 kg/h of liquid nitrogen of the purge current to obtain the production of a current with 99.7% nitrogen, based on a flow of 39 kg/h and at a pressure of 10 bar g, 971.5 kg/h of liquid are recovered with 99.8% of methane .

Recovery I = 26.22 kJthermal/kJmechanical .

In the four cases described above, those skilled in the art select the most suitable plant solutions to meet the contingent needs.

From the above description of the present invention, the advantages provided by the present invention are immediately apparent to those skilled in the art .

With the same energy expenditure, compared to the already known processes of the prior art, the amount of recovered hydrocarbons increases, contributing to a greater efficiency of the process.

From an environmental point of view, the gas eventually released into the atmosphere is mainly nitrogen, with a purity between 70% and 99.6%, depending on the configuration of the process, the composition of the BOG and the operating methods; in particular, the configurations including the nitrogen rectification step ensure a high purity (over 93%) and a constant purge current, with considerable advantages in reducing the environmental impact.

The purge gas, which is characterized by a high nitrogen content, can find practical uses in the plant, also due to the fact it is compressed, for example for machine seals, purge gases for the torch plant, etc.

The stored LNG is allowed to decrease its nitrogen content, reducing its volatility and increasing the specific calorific value and, therefore, the economic value.

Furthermore, the loss of nitrogen leads to a progressive BOG enrichment in methane and this results in a greater efficiency of the process of the invention due to a greater ease in condensing hydrocarbons with the same mechanical energy.

When the stored LNG reaches a certain value of the Wobbe Index it is possible to limit/eliminate the loss of nitrogen by suitably regulating the flow of the purge current .

The flexibility required of the machines is minimal, since when the percentage of nitrogen contained in the BOG changes, the molecular weight of the gas treated by the machines remains almost constant, with the same quality of the purge current.

Particularly important is the aspect relating to the non-use of imported and stored refrigerants in the plant .

Advantageously, the number of machines installed is reduced: a BOG compressor (still necessary and used in sites with LNG storage) , a refrigerant compressor and the related expander; the lower complexity of the plant increases the reliability of the plant compared to other schemes with a higher number of machines. The particular application of the process of the invention for the storage of excess BOG allows the "Peak Shaving", i.e. it allows to reduce remarkably the variations in the flow inputted into the plant of the invention.

The plant described by the present patent application is well suited to the retrofits of existing plants, being able to use different equipment normally available in these sites: BOG Compressor, LNG pumps and BOG Recondenser for the BOG storage plant, possible storage of liquid nitrogen, possible production of liquid nitrogen.

As described above, the present invention has several applications, which include the retrofit of existing systems for the production of LNG and at bunkering sites for the management of BOG production peaks .

k k k

Claims

1. A process for the re-liquefaction and simultaneous reduction of the nitrogen content in the Boil Off Gas (BOG), wherein a flow of Boil Off Gas (10) is subjected to the steps of:

a) cooling, thus obtaining a cooled flow of Boil Off Gas (11), such a step a) being optional,

b) subjecting said flow of Boil Off Gas (10) or said cooled flow of Boil Off Gas (11) to an absorption step, thus obtaining a flow of Boil Off Gas enriched in hydrocarbons (100) and a nitrogen-rich vapour flow (20) ,

c) subjecting said nitrogen-rich vapour flow (20) to a heating step, thus obtaining a heated nitrogen- rich flow (21),

d) subjecting said heated nitrogen-rich flow (21) to a compression step, thus obtaining a nitrogen-rich vapour flow (22),

e) subjecting said nitrogen-rich vapour flow and compressed (22) to a cooling step, thus obtaining a compressed and cooled nitrogen-rich flow (31),

f) subjecting said compressed and cooled nitrogen- rich flow (31) to an expansion step, thus obtaining an expanded and further cooled nitrogen-rich flow (32), g) subjecting said nitrogen-rich flow, expanded and further cooled, (32) to a separation step, thus obtaining a vapour flow (50) which is very rich in nitrogen and a first liquid service flow (40),

wherein in said absorption step b) said first liquid service flow (40) is used.

2. A process according to the preceding claim, wherein step b) is carried out in an absorption column.

3. A process according to any one of the preceding claims, wherein the cooling step a) is carried out by heat exchange with the vapour flow (50) which is very rich in nitrogen, obtaining a heat vapour flow (51) which is very rich in nitrogen.

4. A process according to any one of the preceding claims, wherein said cooling step e) comprises

- a cooling step el) using a cooling fluid obtaining a compressed and partially cooled nitrogen-rich flow ( 30 ) , and

- a cooling step e2) by heat exchange with one or more of the flows selected from the nitrogen-rich vapour flow (20) and the vapour flow (50) which is very rich in nitrogen.

5. A process according to any one of the preceding claims, wherein said step c) of heating said nitrogen- rich vapour flow (20) takes place by heat exchange with said nitrogen-rich vapour flow, compressed and partially cooled (30) .

6. A process according to one of the preceding claims 3 to 5, wherein a portion of the heated vapour flow (51) which is very rich in nitrogen is combined with the heated nitrogen-rich vapour flow (21) and sent to step d) .

7. A process according to any one of the preceding claims, wherein a portion of said heated vapour flow (52) which is very rich in nitrogen is withdrawn.

8. A process according to any one of the preceding claims 1 to 7, further comprising a rectification step, wherein a portion (60) of said vapour flow which is very rich in nitrogen and a flow of pure liquid nitrogen (70) are used, obtaining a separated, almost pure nitrogen flow (80) and a second liquid service flow (90) .

9. A process according to the preceding claim, wherein in the rectification step a portion (52') of the vapour flow which is very rich in nitrogen separated from the heated vapour flow (50) which is very rich in nitrogen may be further used.

10. A process according to claim 8 or 9, wherein said second liquid service flow (90) is sent to step b) .

11. A process according to any one of the preceding claims, wherein in step d) a portion of the heated and compressed nitrogen-rich vapour flow (24) is separated and subjected to the steps:

h) cooling, obtaining a refrigerated flow (25) , i) expansion, obtaining a refrigerated and expanded flow (26),

1) heating, obtaining an expanded and heated refrigerated flow (27), which is sent to step d) , possibly after being combined with said heated nitrogen-rich flow (21),

wherein said step h) is carried out by heat exchange with the expanded refrigerated flow (26) of step i) or with the nitrogen-rich vapour flow (20) of step c) .

12 . A process according to claim 5, wherein a portion (30') of the nitrogen-rich vapour flow, compressed and partially cooled obtained after steps c) and d) and el) is partially subjected to the cooling step e2) obtaining a partially refrigerated flow (25') which is further subjected to the steps of:

i') expansion and further cooling, obtaining a partially refrigerated and expanded flow (26'),

1' ) heating, obtaining a partially refrigerated expanded and heated flow {21’), which is sent to step d) , possibly after being combined with the nitrogen- rich vapour flow and heated (21), wherein said step e2) is carried out by heat exchange with one or more of the partially refrigerated and expanded flow (26') of step ί' ) and with the nitrogen- rich vapour flow (20) of step c) .

13. A process according to any one of the preceding claims, wherein said flow of Boil Off Gas enriched in hydrocarbons (100) is subjected to an expansion step m) producing an almost completely liquid flow of Boil Off Gas (110) which is then sent to a liquefied natural gas (LNG) tank (301) .

14. A process according to any one of the preceding claims, wherein before said step a) , if carried out, or before said step b) , a flow of Boil Off Gas (1) is subjected to a compression step aO) providing said flow of Boil Off Gas (10) which can then be subjected to the process according to any one of the preceding claims.

15. A process according to claim 13 or 14, wherein from step m) a gaseous flow of Boil Off Gas (1) is also obtained, which is subjected to the compression step aO) .

16. A plant for the re-liquefaction and simultaneous reduction of the nitrogen content of Boil Off Gas comprising:

a liquefied natural gas tank (301), a Boil Off Gas compressor (302) in fluid connection with said liquefied natural gas tank (301); said Boil Off Gas compressor (302) being possibly present,

a first heat exchanger (303) in fluid connection with said Boil Off Gas compressor (302) and with a Boil Off Gas Absorber (304),

the Boil Off Gas Absorber (304) in fluid connection with said first heat exchanger (303),

a Refrigeration Cycle Compressor (305) in fluid connection with said first heat exchanger (303), a second heat exchanger (306) in fluid connection with said Refrigeration Cycle Compressor (305) and with said first heat exchanger (303),

a Reflux Flash Drum (307) in fluid connection with said first heat exchanger (303) and with said Boil Off Gas Absorber (304),

an expansion valve (VI) in fluid connection with said Boil Off Gas Absorber (304) and with said LNG tank (301) ,

a pressure decreasing valve (V2) in fluid connection with said Reflux Flash Drum (307); and possibly also comprising:

a Refrigeration Cycle Expander (308) in fluid connection with a Reflux Flash Drum (307) and with said first heat exchanger (303), and/or

a Nitrogen Rectification Column (309) in fluid connection with said Reflux Flash Drum (307) and possibly also with said Boil Off Gas Absorber (304) .

17 . A process according to any one of the preceding claims 1 to 15, wherein said gaseous flow of Boil Off Gas (10) is obtained by a process which comprises the steps of:

I) compression of a Boil Off Gas flow (200) in a Boil Off Gas compressor (401), obtaining a flow (201) of Boil Off Gas at high pressure,

II) recondensation of said flow (201) in a Boil Off Gas recondenser (402) by heat exchange with a flow of liquefied natural gas (206) obtaining a flow of Boil Off Gas in liquid form (202),

III) storage of said Boil Off Gas flow in liquid form (202) thus obtained at the pressure of about 2-30 bar g and preferably of about 5-15 bar g in a storage bullet (403),

IV) reduction of the pressure of a flow (203) withdrawn from the storage of step III) through an expansion valve (V3), thus obtaining a liquefied natural gas (LNG) flow (204) and a flow of Boil Off Gas (1), wherein said flow of Boil Off Gas (1) is then sent to step aO) according to the process of any one of the preceding claims 1 to 15.

18 . A process according to the preceding claim, wherein said initial flow of the Boil Off Gas (200) is an excess amount with respect to the re-liquefaction capacities of the process according to any one of claims 1 to 15.