EP3803188B1

EP3803188B1 - Method and system for storage and transport of liquefied petroleum gases

Info

Publication number: EP3803188B1
Application number: EP19732545.9A
Authority: EP
Inventors: Lars Grønnæss; Leidulf Dyrland; Øyvind Skjerven
Original assignee: Waertsilae Gas Solutions Norway AS
Current assignee: Waertsilae Gas Solutions Norway AS
Priority date: 2018-06-04
Filing date: 2019-06-04
Publication date: 2022-01-26
Anticipated expiration: 2039-06-04
Also published as: KR20210016584A; CN112243479A; CN112243479B; NO344169B1; WO2019234025A1; EP3803188A1

Description

Technical Field

The present invention relates to a method and system for storage and transport of liquefied petroleum gases, normally known as LPG, on a tanker vessel, hereinafter referred to as LPG carriers.
The present method and system are equally applicable for the use on floating production storage and offloading vessels (FPSO) for liquefied petroleum gases (LPG), and also equally applicable for the use on vessels carrying liquefied ethane and liquefied ethylene gas.

Background Art

WO 2011/002299 describes a typical known reliquefaction unit with a multi stage compressor and one intercooler arrangement. Similar arrangements can be found also in WO 2016/027098 A1 and WO 2017/171171 A1 .
WO 2012/143699 extends the principles shown in WO 2011/002299 by adding an additional feature for cooling non-condensable gases in a dedicated heat exchanger. WO 2012/143699 is applicable for multistage compressors with two or more stages of compression. Non-condensed boil off gas components are separated from the condensed part of the boil off gas in a receiver and the non-condensed boil off gas components are further cooled in a heat exchanger where said components are re-liquefied and returned to the cargo tank. A heat exchanger embedded into a vent gas condenser is utilised, where the heat exchanger is a coil inside a vessel; i.e., the coil is submerged into liquid, known as liquid pool cooling. Intercooler/economiser of similar design is also used. When starting a reliquefaction unit based on the principles in prior art arrangements with intercooler liquid pool cooling, it may take up to two hours before the intercooler liquid pool is in equilibrium with the vapour phase. Similar will also be for an additional vent gas condenser. Further, in prior art arrangements, the energy losses might be higher than what is desirable.
When using intercooler(s) based on the principle of liquid pool cooling, there might be a temperature difference at equilibrium of up to 20°C for some LPG types. Obviously, this temperature difference will also be present in a vent gas condenser and will limit the rate of condensation in the heat exchanger.
Due to an increasing focus on energy efficiency and green house gas emissions it is a need for a more efficient recovery of non-condensed boil off gas components as well as a more efficient sub-cooling and to further improve the coefficient of performance (COP) of the reliquefaction system used on LPG carriers.
Thus, the object of the present invention is to provide a more efficient method and system for reliquefaction of boil off gas overcoming the drawbacks of prior art solutions.

Brief description of the Invention

The inventors surprisingly found that by utilising glide refrigeration, improved condensation rate of the non-condensed boil off gas components was achieved and additional sub-cooling not feasible with liquid pool cooling was provided, all with both simplifications in the piping arrangement and amount of equipment.
Thus, in one aspect, the present invention provides a method for storage and transport of liquefied petroleum gases (LPG) on LPG carriers according to independent claim 1. In another aspect, the present invention provides a system for storage and transport of liquefied petroleum gases (LPG) on LPG carriers, according to independent claim 12.
Further embodiments of the present invention are as set out in the dependent claims.

Brief Description of the Drawings

Embodiments of the invention will now be described in further detail with reference to the following figures in order to exemplify its principles, operation and advantages.
Equivalent parts in the figures have been given the same reference numerals.

Fig. 1 shows a principle sketch of a ship with four cargo tanks and the flow lines of vapour (BOG) to the reliquefaction units and condensate lines returning to the cargo tanks with a prior art reliquefaction unit.
Fig. 2 illustrates the principles of an intercooler based on the principle of liquid pool cooling.
Fig. 3 shows schematically a prior art reliquefaction unit with one intercooler arrangement.
Fig. 4 illustrates schematically a prior art reliquefaction unit with one intercooler arrangement and one vent gas condenser.
Fig. 5 shows a schematic diagram of an example, not covered by the appended claims, of a
reliquefaction unit with a combined glide condenser and intercooler between compression stage 1 and 2, any controls are omitted from the figure.
Fig 6 illustrates schematically the controls around a combined condensate sub-cooler and condenser for non-condensed boil off gas components.
Fig 7 illustrates schematically the controls around each stage of compression and a receiver.
Fig. 8 illustrates schematically an example, not covered by the appended claims,
of a reliquefaction unit with one combined condensate sub-cooler and condenser for non-condensed boil off gas components with controls shown.
Fig. 9 shows schematically an embodiment of the present invention of a reliquefaction unit having a liquid drum.
Fig. 10 shows schematically an embodiment according to the present invention.
Fig. 11 shows a mixer for mixing of gas and liquid at a receiver outlet.
Fig. 12 shows the controls associated with a first condensate sub-cooler for a three stage compressor.
Fig 13 illustrates schematically the controls around a second combined condensate sub-cooler and condenser for non-condensed boil off gas components.
Fig. 14 shows the controls associated around three stages of compression and a receiver.
Fig. 15 shows schematically an embodiment of the invention with controls and three stage compression.
Fig. 16 shows a pressure-enthalpy diagram for the condensate sub-cooler process.
Fig. 17 shows refrigeration capacity vs. ethane content in prior art solutions and the present invention.
Fig 18 shows cooling curves for a flooded intercooler.
Fig 19 shows cooling curves for a heat exchanger utilizing glide refrigeration according to the invention.

Detailed description of the Invention

The present invention shall be understood as applicable for liquefied gases having a boiling point of -110°C (163 Kelvin) and above at 1 atmosphere and for the sake of convenience the term LPG covers the range of gases with boiling point from -110°C and above.
LPG is to be understood as a range of different grades or products of petroleum gases stored and transported as liquid. Of the various petroleum gases, propane and butane are the principal examples in which propane typically includes a concentration of ethane from 0 mole % up to 10 mole % and may also contain small fractions of other petroleum constituents as e.g. butane. Butane can be any mixture of normal and iso-butane. In addition to the above-mentioned hydrocarbons, LPG should as a minimum also include the following list of liquefied products:

ammonia;
butadiene;
butane - propane mixture (any mixture);
butylenes;
diethyl ether;
propylene;
vinyl chloride;
ethane (typically comprising fractions of methane, and may also contain small constituents of heavier compounds);
ethylene.

Hydrocarbon products such as ethane, propane and butane are normally distillate products from natural gas, gas condensate or light petroleum fractions and consequently also holds smaller fractions of other compounds as well. These additional compounds may be less or more volatile.
LPGs are transported in liquid form either at pressures greater than atmospheric or at temperatures below ambient, or a combination of both. The present invention relates to:

1. LPG carriers transporting liquefied cargoes, LPG, at temperatures below ambient and pressures close to atmospheric, known as fully refrigerated, and
2. LPG carriers transporting liquefied cargoes, LPG, at pressures greater than atmospheric and temperatures below ambient. The latter is known as semirefrigerated / semi-pressurised.

LPG stored and transported at temperatures below ambient releases continuously a certain amount of vapour. The normal manner of maintaining the pressure in the cargo tanks is to extract the released vapour, then liquefy it, and return it back to the cargo tanks as condensate.
In the present specification, condensate is to be understood as liquefied vapour whereas vapour is meant to be the product of vapours comprising vapours generated by heat input to the LPG and any vapour generated when the condensate is returned. Vapour also includes displaced gas during loading the cargo tanks and also cargo handling operations involving using the reliquefaction plant to reduce the tank pressure. Preferably, the condensate should be further sub-cooled to provide the needed refrigeration duty to handle the heat ingress into the cargo tanks where sub-cooling is provided by using a cooled portion of the warm condensate stream as a vaporising refrigerant and return this portion back to suitable locations in the refrigeration unit. The term sub-cooling refers to a liquid existing at a temperature below its normal boiling point.
A multi stage compressor shall be understood as a compressor with two stages of compression or more. Typically, all stages of compression are constituted within a single body machine but may also be split into more than one body as e.g. one compressor body per compression stage.
In the present specification, a reliquefaction unit is meant to be a refrigeration unit which duty is to liquefy vapour and the prefix "re" points to liquefaction of vapour from liquefied gases. A cargo tank is one or more liquid tight containers intended to hold LPG.
A cargo type is any of the LPG grades and/or products mentioned above. As an example; a cargo type can be commercial propane with 5 mole % ethane.
The different cargoes carried by a liquefied gas carrier may require different demands for compressor intercooling. E.g., cargoes as ammonia and vinyl chloride generate high interstage temperature and require a larger degree of cooling between first and second stage of compression compared to e.g. cargoes as propane. This is normally solved by having a first filled intercooler where the compressed vapour from the first stage is ejected through a liquid bath and hence the suction gas to the second stage compressor inlet is close to its dew point temperature.
Thus for ships that shall be able to carry a wide range of cargoes (wide cargo list) the first stage intercooler may have a different piping arrangement than for ships with a more narrow cargo list.
A narrow cargo list shall be understood as a list with reduced number of possible liquefied products that a specific ship can carry, e.g. a list without ammonia and possible other liquefied products. Common for both the wide and narrow cargo list is that they normally both contain the standard hydrocarbons as propane and butane. Occasionally, the ship loads a cargo with a higher than normal amount of volatile components and thus the boil off gas will also have a high concentration of these volatile components. At some concentration level the reliquefaction plant cannot fully liquefy the boil off gas in the cargo condenser and venting the non-condensed portions is needed.
For illustration, Figure 1 shows a principle sketch of a ship with four cargo tanks and the flow lines of vapour (boil off gas - BOG) to the reliquefaction units and condensate lines returning to the cargo tanks where the reliquefaction unit is a prior art unit as disclosed for example in WO 2011/002299 . A ship can have one or more reliquefaction units, typically from two to four units. The number of cargo tanks shown in Figure 1 is for illustration only, and can be any number.
Figure 2 shows the principles of an intercooler based on the principle of liquid pool cooling:

8: Condensate flow to be sub-cooled
9: Expanded portion of total condensate flow
12: Sub-cooled condensate flow
13: Vapour flow exiting the intercooler
15: Total condensate flow
22: Portion of total condensate flow
30: Liquid pool
31: Vapour emitting from liquid pool
32: Liquid portion of stream 9
33: Vapour portion of stream 9
34: Total vapour exiting the intercooler
40: Heat exchanger

At equilibrium, and for condensates constituting two or more chemical compounds as e.g. propane with ethane the temperature of liquid 32 is not the same as the temperature of liquid 30. The temperature of liquid 30 approaches the dew point temperature of the liquid 32 which is higher than the actual temperature of liquid 32. It is not uncommon to see a temperature difference of 20°C for some LPG types.
Hence, the temperature in sub-cooled condensate flow 12 can never be lower than the dew point temperature of liquid 32. Vapour sensible heat and mixing enthalpies have limited effect and are thus neglected here.
Figure 3 shows a schematic diagram of a prior art reliquefaction unit with one intercooler between first and second stage of compression. For the cargo vapours that will generate high interstage temperatures, the flow is routed via line 2a, via the liquid pool intercooler 170, then to the second stage of compression. For cargo vapour as e.g. propane, there is no flow in line 2a.

The high pressure condensate stream leaves condenser 130 as stream 7 where it enters a liquid receiver 140 where any non-condensable gases are separated and leaves via line 20. Valve 220 will typically open on a predefined set point releasing these non-condensable gases. Non-condensable gases can be inert gases as e.g. nitrogen or even volatile hydrocarbon components. High pressure is related to the needed discharge pressure of

compressor

100, 120 in stream 6 to liquefy the boil off gas in cargo condenser 130 against the heat exchange medium. The below table 1 gives typical pressure values in stream 6 as a function of condensing temperature where condensing temperature is 4 °C above seawater temperature.

Table 1

Temperature °C	Pure propane	Propane with 8 mole % ethane	Propane with 5 mole % ethane	Propane with 2.5 mole % ethane
24	9.36 bara	19.71 bara	16.38 bara	13.12 bara
36	12.61 bara	25.06 bara	21.04 bara	17.12 bara
40	13.86 bara	27.04 bara	22.77 bara	18.63 bara

The non-condensable gases can be released back to the cargo tank to be absorbed into the cargo liquid, routed back to a suitable location in the reliquefaction plant, to a dedicated heat exchanger for recovery of cargo vapour following the inert gases or to other suitable location as a combustion unit or lastly to the vent mast.
Figure 4 shows schematically a prior art reliquefaction unit with one intercooler and one vent gas condenser. The source for the coolant to the vent gas condenser is taken as a separate portion of the uncooled condensate exiting a first heat exchanger. Non-condensed boil off gas components are separated from the condensed part of the boil off gas in a receiver 140 and via a line 8 the non-condensed boil off gas components are sent to further cooling in a heat echanger 330 where said components are re-liquefied and returned to the cargo tank. An additional heat exchanger 330 are embedded into a vent gas condenser 300 which enables a higher refrigeration capacity for high content of volatile component as ethane.
The present invention shows that further improvements in both liquefaction capacity and COP compared to the system shown in figure 4 is achieved.
Normally, a cargo compressor is electrically driven and when on a ship the electricity is produced in large generators consuming primarily a hydrocarbon based fuel. Thus in order to reduce the green house gas emissions from a ship carrying liquefied petroleum gases all energy consuming systems should be as efficient as possible. Thus, increasing the COP reduces the required energy consumption to the reliquefaction unit (higher refrigeration capacity gives a reduction in total running time for a given heat ingress). Hence, for environmental reasons it is desirable to have a reliquefaction system with better COP than present technology may offer.
Glide, also referred to as non-azeotropic, refrigerants are mixtures of two or more refrigerants where the components have different saturation temperatures at the same pressure level. As a glide refrigerant enters an evaporator, the most volatile component will vaporise first. When the concentration of the most volatile refrigerant decreases, the temperature of the remaining refrigerant mixture will also increase, approaching the saturation temperature of the second least volatile refrigerant, and so on. The evaporating temperature will be lower at the entry point of the evaporator than at the exit point, even when the evaporation pressure remains constant. The opposite then applies when the refrigerant is condensing, i.e as the glide refrigerant enters a condenser, the least volatile component will condense first. When the concentration of the least volatile refrigerant decreases, the temperature of the remaining refrigerant mixture wil also decrease, approaching the saturation temperature of the second least volatile refrigerant, and so on. The condensing temperature will be higher at the entry point of the condenser than at the exit point, even when the condensing pressure remains constant.
In an evaporator operating with glide refrigerants, there are three temperatures of special interest: the dew point (the highest evaporation temperature which is achieved when all refrigerant has been transferred to vapour), the mean evaporation temperature and the bubble point (lowest evaporation temperature), which is achieved just before the refrigerant starts to evaporate.
The principle of using glide refrigerants is referred to as glide refrigeration.
The inventors surprisingly found that by utilising glide refrigerants, a more efficient reliquefaction of boil off gas could be achieved. By applying an advanced calculation method, it was found that by developing a novel control strategy it was possible to utilise glide refrigeration providing additional condensation and sub-cooling compared to liquid pool cooling. With the present invention, there is a temperature glide where the vapour / liquid composition changes along the heat transfer process.
In order to minimise energy losses, heat exchange should be by a gliding profile where a cooled and expanded portion of the BOG condensate stream could function as a glide refrigerant. A glide refrigerant is best utilised in a counter-current heat exchange regime. The more volatile components will boil first during heat exchange with the warmer fluid in the counter current passages of the heat exchanger and thus provide a lower resulting temperature of the fluid to be cooled compared to liquid pool cooling according to the prior art.
To utilise glide refrigerants most optimally it is essential that most of, or preferably all of the volatile components remain in the liquid phase of the coolant/refrigerant after expansion so that more of the latent heat of vaporisation of the volatile components is made available for heat transfer and not lost during the expansion. To achieve this, the portion of the warm condensate stream used to sub-cool the portion of the warm condensate stream to be returned to the cargo tanks must also be sub-cooled prior to it being expanded. The inventors found that the portion of the warm condensate used for sub-cooling should be sub-cooled to the same degree as the condensate being returned back to the cargo tanks.
Further, the inventors found that by not separating out the non-condensed portions from the condensed boil off gas but instead combining the principle of glide condensation and glide evaporation, an improved condensation and sub-cooling was achieved.
Figure 5 shows a schematic diagram of a system, not covered by the appended claims, with a condensate sub-cooler between compression stage 1 and stage 2 and a droplet separator to protect the second stage suction from any droplets that might follow the vapour flow. The figure does not show a line 2a with liquid sub-cooling but it can equally well be arranged with a line 2a in the same manner as shown in Figure 3.
In the system shown in figure 5, boil off gas (BOG) emitted from LPG cargo, the LPG having a boiling temperature of -110°C or higher at 1 atmosphere flows by pressure via stream 1 to a cargo compressor 100, 120 with minimum two stages of compression where it is firstly compressed in cargo compressor 100 in stage 1 to a first intermediate pressure stream 2 where the first intermediate stream 2 is cooled by physically mixing with a stream 14 at a lower temperature.
The sum of streams 2 and 14 forms stream 5 and enters the second cargo compressor 120 in a second stage of compression where it is compressed to a final pressure stream 6 and then flows by pressure to a cargo condenser 130 where the final compressed vapour is cooled and condensed.
The cooling medium used in the cargo condenser 130 can be seawater, glycol / water mixture or a suitable refrigerant as e.g. propylene. Even multi-component refrigerant may be used if a lower temperature than what is achievable with e.g. propylene is needed.
The final discharge pressure of cargo compressor 120 is such that without the presence of inert gases or large contents of volatile cargo components in the boil off gas, the condensate is at its bubble point pressure in the receiver 140, this is a self adjusting process where the discharge pressure automatically adjusts towards equilibrium conditions in the receiver 140.
For this condition whitout inert gases or large contents of volatile components, examples of typical pressure values in stream 6 as a function of condensing temperature where condensing temperature is 4 °C above seawater temperature are the same as indicated in Table 1 above.
When the cargo vapour canot be fully condensed in the cargo condenser 130 the cargo compressor will typicaly be operated close to its allowable limits. This depends obviously on type of compressor, but typical for LPG reliquefaction this will range between 20 - 35 barg. This range should however not be a limiting factor for the invention.
When the condition in the receiver 140 is at its bubble point, the warm and saturated condensate stream 15 leaves the receiver 140 and enters a condensate sub-cooler 150 where it is heat exchanged against an intermediate expanded portion 19 of the further cooled condensate stream 8.
By the term warm it shall be understood a temperature close to the coolant / refrigerant temperature used in cargo condenser 130, typically this temperature is about 4 - 6 °C above the coolant / refrigerant temperature. The coolant / refrigerant temperature depends on the heat exchange medium used in the cargo condenser 130. For seawater as heat exchange medium, the temperature will be in the range from 0 °C to about 40 °C. When there is applied a refrigerant as heat exchange medium the temperature might be as low as -50 °C for single component refrigerants.
Intermediate pressure shall be understood as an intermediate pressure between two stages of compression, e.g. between first and second stage of compression or between a second highest compression stage and final compression stage; i.e. for a three stage compressor, intermediate pressure is the pressure between the second and third stage of compression. For a four stage compressor, intermediate pressure may also be the pressure between third and fourth stage of compression.
The warm condensate stream 15 leaves the condensate sub-cooler 150 as a further cooled condensate stream 8 where it is split in two portions to conduct internal cooling duties; stream 12 and stream 18. Stream 18 being the portion of the further cooled condensate stream 8 used for internal cooling and stream 12 is the final condensate returned back to the ship's cargo tanks. All of or part of stream 12 might also be led to at least one fuel tank; eg., for the main propulsion machinery (not shown in figure 5). The at least one fuel tank may for example be a deck tank serving as a fuel tank, e.g., for the main propulsion machinery. Whether all of or part of stream 12 is returned to the cargo tank(s) and/or transported to the fuel tank(s) might vary during different operating conditions and during time. Condensate return to the cargo tanks shall thus be understood as return to or transport to any liquid tight tank. Valve 200 is typically a level control valve controlling the level in receiver 140.
The condensate sub-cooler 150 can for example be a compact heat exchanger suitable for glide refrigeration. Typical alternatives are plate & plate exchangers, shell & plate may also be used. The invention shall not be limited by the type of heat transfer equipment.
Stream 18 is the second portion of the further cooled condensate stream 8 and is expanded in valve 190 into stream 19 being an intermediate expanded additional cooled portion of the further cooled condensate stream 8. Stream 19 enters the condensate sub-cooler 150 where it is heat exchanged against the warm condensate stream 15. Stream 19 exits the condensate sub-cooler 150 fully vaporised as stream 11 and enters a droplet separator 160 for the removal of any droplets, if present. Normally, there will not be any droplets present and the droplet separator 160 is only a protective feature for the cargo compressor 120 in compression stage 2. Any liquid can be drained back to one or more of the cargo tanks via line 17 by opening valve 210.
Stream 14 leaves the droplet separator 160 and mixes with the compressed stream 2 into a cooled compressed stream 5.
Figure 6 shows schematically the controls associated with the condensate sub-cooler 150 and droplet separator 160.
With reference to Figure 6, the following guide is given:

1:: The point where line 14 connects to line 2 and forms line 5 before flowing to the cargo compressor 120;
2:: The point where line 15 connects to the receiver 140;
3:: The point where line 8 connects to the splitting device (not shown) splitting stream 8 into streams 18 and 12;
5:: The point where line 18 connects to the splitting device;
L3:: Level transmitter measuring liquid level in droplet separator 160;
VC1:: Valve controller for valve 190;
TD1:: Temperature difference between streams 8 and 19;
TD2:: Temperature difference between streams 15 and 11.

A splitting device is normally a pipe segment of Tee type.
The best possible COP is normally achieved when operated with maximum possible recycle flow (stream 18) through the condensate sub-cooler 150. This means operating the outlet stream 11 at dew point. Recycle flow is understood as the portion of the warm condensate stream 15 being returned to a lower pressure level via the condensate sub-cooler 150 and thereby conducts a refrigeration duty in the sub-cooler 150. A common and known method to assure that the stream 11 is operated at dew point is to introduce a fixed liquid level in the droplet separator 160.
However, controlling the liquid level in the droplet separator 160 is difficult as the liquid level will increase quickly when stream 11 is entering as two-phase flow and decrease slowly when stream 11 enter as superheated gas.
It was therefore a target to find a control strategy other than controlling the liquid level directly.
For the case where it is desired to control stream 11 of the condensate sub-cooler 150 at dew point and achieve a liquid level in the droplet separator 160, it is found that controlling the temperature of stream 11 gives a stable liquid level and will remain fixed if the temperature of stream 11 is always kept at the dew point temperature.
The temperature of stream 11 shall thus be controlled to its dew point temperature at the given pressure in droplet separator 160. Since the pressure in droplet separator 160 is governed by the performance of cargo compressor 100, 120 where interstage pressures dynamically fluctuate, e.g. due to sea states or ambient temperatures, it will be necessary to dynamically modify the set point for the temperature controller of stream 11.
Figure 16 shows a pressure - enthalpy diagram for the condensate sub-cooler process where:

T19 is the temperature in stream 19;
T15 is the temperature in stream 15;
T11 is the temperature in stream 11;
T8 is the temperature in stream 8.

It can be seen from Figure 16 that stream 11 will be more superheated when reducing the temperature difference between T11 and T15. This temperature difference is denoted TD2, while the temperature difference between T8 and T19 is denoted TD1. On the other hand, stream 11 will be less superheated when increasing TD2 and will at a certain TD2 be at its dew point. If TD2 is further increased this will lead to a liquid increase in the droplet separator.
The challenge involved is to operate the TD2 at exactly the correct value to achieve a temperature equal to the dew point temperature of stream 11. The required TD2 will change when intermediate pressure is changing and is thus an adjustable input parameter to the control logic.
It was found that by introducing a factor X which is the ratio between TD2 and TD1 and controlling this ratio at a desired value increases the stability of the liquid level in droplet separator 160. The factor X thus indirectly controls the flow ratio between stream 11 and condensate stream 15 and it was found that a value of X equal 1 gave least energy losses, i.e. optimal conditions. The value of X is multiplied by a correction factor based on the variations in liquid level measured by level transmitter L3, when the level increases above operating point the correction factor is decreased from unity (one). For reducing levels the correction factor is increased towards unity (one).
If more than one condensate sub-cooler is used (not shown in Figure 16), T24 is the temperature in stream 24, T10 the temperature in stream 10 and T9 the temperature in stream 9. TD3 is the temperature difference between streams 10 and 9 and TD4 is the temperature difference between streams 8 and 24, and a L4 is a level transmitter measuring the liquid level in droplet separator 260. The factor X will here be the ratio between TD4 and TD2.
Figure 7 shows schematically the controls for the cargo compressor 100, 120 and receiver 140.
P1, P2, P5, P6 and P7 are pressure readings. P2 and P5 shall be understood as separate or combined pressure reading instrument. With combined pressure reading instrument it shall be understood as one instrument, typically this is sufficient since the pressure losses in the piping between the stages are marginal and e.g. P2 and P5 will read the same pressure.
PC1 and PC 3 are pressure controllers ensuring that the pressure ratio across each stage of compression is kept within acceptable levels.
PC6 is a compressor discharge pressure controller monitoring and controlling the discharge pressure of cargo compressor 120. Figure 7 shows P6 feeding the pressure signal to PC6, it can also be a separate instrument, i.e. not common with the PC3 function.
PC7 is a receiver pressure controller.
LC1 is a level controller.
PC1, PC3, PC6, PC7 and LC1 all send their values to a select block selecting the signal requesting the highest opening value of valve 200 and control the opening of valve 200 accordingly and thus also the feed conditions to condensate sub-cooler 150. The select block can be denoted a high select block or high select function block when it is the highest value that shall be selected.
A compressor has normally a mechanical limitation on allowable pressure ratio across each stage of compression and if PC1 or PC3 reaches maximum allowable value, the high select function will control valve 200 so that the pressure ratio does not exceed maximum allowable value. Maximum allowable pressure ratio is a set of values given by the compressor manufacturer and these values are programmed into the high select block as constraints to control against.
If neither of PC1, PC2 or PC3 reaches maximum allowable value the high select block will select the compressor discharge pressure controller PC6 to control the valve 200. PC6 controls on a predefined maximum allowable operating discharge pressure.
The compressor can then be operated at or close to its maximum permissible / allowable limits in order to condense as much as possible of the vapour in the cargo condenser 130.
Additional to PC6, pressures can be read from stream 7, stream 20, stream 15, or in the receiver via PC7 for the control of valve 200.
Figure 8 shows an example, not covered by the claims, with figures 5, 6 and 7 combined in one figure. The following description applies when the high pressure and cooled stream 7 leaving the cargo condenser 130 is not fully condensed, i.e. inside the phase envelope.
The inventors found that the principles of glide refrigeration are an efficient means of handling cargo boil off vapours that cannot be fully condensed at the pressure achieveable by the selected compressor at the temperature available for cooling in the cargo condenser 130. This could be the case for propane with high ethane content where sea water is applied for cooling in the cargo condenser or it could be the case for ethane with high methane content where a refrigerated cooling medium is applied in cargo condenser 130 or optionally in series with cargo condenser 130, not shown. For the latter, a possible refrigeration medium can be propylene. For such conditions, it is possible to utilise the condensate sub-cooler 150 as a partial condenser and sub-cooler. In Figure 8, boil off gas emitted from LPG cargo having a temperature of -110°C or higher flows by pressure via stream 1 to a cargo compressor 100, 120 with minimum two stages of compression where it is firstly compressed in cargo compressor 100 in stage 1 to a first intermediate pressure stream 2 where the first intermediate stream 2 is cooled by physically mixing with a stream 14 at a lower temperature.
The sum of streams 2 and 14 forms stream 5 and enters the second cargo compressor 120 in a second stage of compression where it is compressed to a final pressure stream 6.
The cooling medium used in the cargo condenser 130 can be seawater, glycol / water mixture or a suitable refrigerant as e.g. propylene. Even multi component refrigerant may be used if a lower temperature than what is achieveable with e.g. propylene is needed.
The high pressure stream leaves condenser 130 as a cooled and partly condensed stream 7 where it enters a liquid receiver 140 that ensures proper mixing of gas and liquid for stable flow regime out via stream 15.
A warm and mixed two-phase stream 15 leaves the receiver 140 and enters a condensate sub-cooler 150 which now functions as a combined final condenser and condensate sub-cooler. The mixed phase flow 15 entering the condensate sub-cooler150 is heat exchanged against an intermediate expanded portion 19 of the further cooled condensate stream 8.
By warm it shall be understood a temperature close to the coolant / refrigerant temperature used in cargo condenser 130, typically this temperature is about 4 - 6 °C above the coolant / refrigerant temperature. The coolant / refrigerant temperature depends on the heat exchange medium used in the cargo condenser 130. For seawater as heat exchange medium, the temperature will be in the range from 0 °C to about 40 °C. When there is applied a refrigerant as heat exchange medium the temperature might be as low as -50 °C for single component refrigerants.
The warm mixed phase stream 15 leaves the condensate sub-cooler 150 as a fully condensed and further cooled condensate stream 8 where it is being split into two portions; stream 18 and stream 12. The warm and mixed phase 15 is thus subject to complete condensation and further cooling before being split in portions to conduct internal cooling duty. Stream 12 is the first portion of the fully condensed and further cooled condensate stream 8 and is the final condensate returned back to the ship's cargo tanks (not shown in figure 8). Valve 200 is typically a level control valve controlling the level in receiver 140.
The portion of the fully condensed and further cooled condensate stream 8 flowing through valve 190 becomes an expanded portion of the condensate stream 8 and is typically a stream with mixed phases. This stream is denoted stream 19 in Figure 8.
Heat exchange in the condensate sub-cooler 150 is by utilising the principle of glide refrigeration where the gas / liquid is not separated but kept as a mixed two-phase flow throughout the heat exchange and stream 19 flows out of the condensate sub-cooler 150 as stream 11 normally fully vaporised and enters a droplet separator 160 for the removal of any droplets if present. Normally, there will not be any droplets present and the droplet separator 160 is only a protective feature for the cargo compressor 120 in compression stage 2. Any liquid can be drained back to one or more of the cargo tanks, not shown in figure 8.
Stream 14 leaves the droplet separator 160 and mixes with the compressed stream 2 and forms a cooled compressed stream 5.
Figure 8 thus shows the same process and control scheme for handling boil off gas that can either be fully condensed or partially condensed in the cargo condenser 130.
Situations with rough weather may also create excessive boil off of more volatile components hindering full liquefaction. Hence it is not only the quality of the loaded cargo that could result in partial condensation in cargo condenser 130.
When the cargo boil off gas is fully condensable at the temperature available in the cargo condenser 130, the pressure from the compressor 120 in the final compression stage will drop, and level will build up in the receiver 140 at a pressure P7 below the set point of the pressure controller acting on valve 200. With the utilisation of a high select function block in the control system, the valve 200 will automatically switch to controlling level for such cargo vapours.
If the loaded cargo has higher fractions of lighter components than normal, e.g., more than 5 mole % ethane in propane (the amount of ethane depends on the number of compressor stages - typical values are 5 mole % for a two-stage compressor and 8 mole % for a three-stage compressor), the cargo may not be possible to fully condense with the maximum operating compressor discharge pressure at the temperature available in the cargo condenser 130. The pressure will increase to the set point of the pressure controller, which normally is set close to the maximum permissible / allowable operating pressure of the compressor. The pressure controller will open the valve 200 and the level will eventually drop until two-phase flow is released through the bottom outlet of receiver 140. The vapour fraction will after a short period stabilise at a fraction that can be condensed at the given pressure as controlled by the pressure controller and the available temperature that can be provided by the cargo condenser 130. The pressure controller is selected between PC1, PC3 and PC6.
Refrigeration capacity is calculated as the massflow entering the reliquefaction plant in stream 1 multiplied with the enthalpy difference between stream 12 and the saturated vapour enthalpy in the cargo tanks. Prior art and the present invention give moderately different interstage temperatures and it was found that the massflow through a defined reciprocating compressor will in all practical senses not change for identical suction and delivery conditions, i.e., same suction pressure and temperature pluss same delivery pressure when used in either prior art or this invention. Thus, to achive a higher refrigeration capacity and better COP more enthalpy must be removed from the condensate stream 12. In order to achieve this, the temperature of recycle flow 18 should be as low as possible and the sub-cooling should be controlled such that the recycle stream exits the sub-cooler 150 at correct conditions. It may e.g. be benefical that stream 11 is superheated thus utilising the sensible heat in addition to the latent heat in the condensate sub-cooler 150, this allows for a lower sub-cooling temperature in condensate sub-cooler 150. By controlling the temperature difference on each side of the sub-cooler and the ratio between the temperature differences the sub-cooler 150 can be operated at the most optimal operating point.
It may also be desirable to have stable operations even when all vapours cannot be liquefied in the cargo condenser, this is achieved by routing the two-phase flow exiting the cargo condenser via the bottom outlet of receiver 140 to the condensate sub-cooler 150 where the two-phase flow undergoes further condensation.
Figure 9 shows an embodiment of the invention where a liquid drum 400 has been included and connected to stream 8. The liquid drum 400 functions as a separation vessel if the inert gas concentrations are of such a high concentration that full condensation in the condensate sub-cooler 150 is not achieveable.
Normally this drum will be liquid filled but will generate a level when vapour is present in stream 8.
When L2 measures a certain loss of liquid level, it sends a signal to LC2 (level controller) which will then open valve 500 until the liquid level has been restored. L2 is an instrument for measuring the liquid level in liquid drum 400.
The functionality of line 2a as shown in Figure 3 utilised in prior art reliquefaction units is applicable for vapours that produce high interstage temperatures. This interstage cooling can also be utilised in the present invention as shown in the schematic diagram in Figure 10.
In Figure 10, the vapour exiting the cargo compressor 100 flows via line 2a when the valve 250 is closed and the warm vapour mixes with stream 11. By adding line 2a it is possible to cool down the flow 2 from cargo compressor 100 in the first compression stage more than without utilising stream 2a. If the droplet separator 160 is operated with a liquid level in combination with vapour from cargo compressor 100 entering droplet separator via stream 2a, the vapour entering cargo compressor 120 in the second compressor stage will be close to its dew point. This gives the maximum interstage cooling possible. The optimal interstage cooling effect can be controlled with valve 190. The optimal cooling will depend on the cargo medium. Line 2a can also be connected directly to the droplet separator 160 achieving the same functionality.
For the functionality of the receiver 140 to handle conditions with full condensation in the cargo condenser 130 as well as partial condensation in the cargo condenser 130, it is important that liquid can be drained uninterrupted as well as that two-phase flow is properly mixed at the bottom outlet of the receiver 140, i.e., where stream 15 leaves the receiver 140.
Figure 11 shows a mixer, comprising a cylinder with distributed holes that will provide even mixture of the gas and liquid phases at said receiver outlet. As the liquid level surrounding the cylinder increases, the liquid flow through the outlet will increase as more holes are available for liquid flow, and higher hydrostatic pressure increases the liquid flow through the holes located near the bottom of the cylinder. A similar functionality can be provided by vertical slots in the cylinder.
With reference to Figure 11, the following guide is given:

1:: The flow entering the receiver 140, this is equivalent to stream 7 in Figure 10;
2:: The liquid portion of the flow entering the receiver 140;
3:: The vapour portion of the flow entering the receiver 140;
4:: Liquid collecting at the receiver 140 bottom;
5:: The pipe opening inside the receiver 140 where the vapour will flow through;
6:: The liquid draining through the distributed holes;
7:: The distributed holes;
8:: The stabilised flow leaving the bottom of the receiver 140;
9:: The vapour outlet of the receiver, normally closed off.

Figure 12 shows the controls across a first condensate sub-cooler in an arrangement with more than two stages of compression.
With reference to Figure 12, the following guide is given:

1:: The point where line 14 connects to line 4 and forms line 5 before flowing to the cargo compressor 120;
2:: The point where line 15 connects to the receiver 140;
3:: Connects to a next sub-cooler;
4:: The coolant flow to the next sub-cooler;
5:: The point where stream 16 connects to the splitting device;
L3:: Level transmitter measuring liquid level in droplet separator 160;
VC1:: Valve controller for valve 190;
TD1:: Temperature difference between streams 8 and 19;
TD2:: Temperature difference between streams 15 and 11.

Figure 13 shows the controls across a second sub-cooler in an arrangement with more than two stages of compression.
With reference to Figure 13, the following guide is given:

7:: The point where line 13 connects to line 2 and forms line 3 before flowing to the cargo compressor 110;
3:: The point where line 8 connects to the condensate sub-cooler 150;
4:: The point where line 22 connects to the splitting device;
6:: The point where line 10 connects to the liquid drum 400;
L4:: Level transmitter measuring liquid level in droplet separator 260;
VC2:: Valve controller for valve 180;
TD3:: Temperature difference between streams 10 and 9;
TD4:: Temperature difference between streams 8 and 24.

Figure 14 shows the compressor control for a compressor with at least three stages of compression and with sub-coolers 150, 240 between each stage of compression.
P1, P2, P3, P4, P5, P6 and P7 are pressure readings. P2 and P3 shall be understood as separate or combined pressure reading instrument. P4 and P5 shall also be understood as separate or combined pressure reading instruments, with combined pressure reading instrument it shall be understood as one instrument, typically this is sufficient since the pressure losses in the piping between the stages are marginal and e.g. P2 and P3 will read the same pressure.
PC1, PC2 and PC3 are pressure controllers ensuring that the pressure ratio across each stage of compression is kept with acceptable levels.
PC6 is a compressor discharge pressure controller monitoring and controlling the discharge pressure of cargo compressor 120 in stage 3. Figure 14 shows P6 feeding the pressure signal to PC6, it can also be a separate instrument, i.e. not common with the PC3 function.
PC7 is a receiver pressure controller.
LC1 is a level controller.
PC1, PC2, PC3, PC6, PC7 and LC1 all send their values to a select block selecting the highest requested opening value of valve 200 and thus controlling the feed conditions to condensate sub-cooler 150. Feed conditions are to be understood as pressure and quality where quality is the vapour fraction.
A compressor has normally a mechanical limitation on allowable pressure ratio across each stage of compression and if any of PC1, PC2 or PC3 reaches maximum allowable value, the high select function will control valve 200 so that the pressure ratio does not exceed maximum allowable value. Maximum allowable pressure ratio is a set of values given by the compressor manufacturer and these values are programmed into the high select block as constraints to control against.
If neither of PC1, PC2 or PC3 reaches maximum allowable value the high select block will select the compressor discharge pressure controller PC6 to control the valve 200. PC6 controls on a predefined maximum allowable operating discharge pressure.
The compressor can then be operated at or close to its maximum permissible / allowable limits in order to condense as much as possible of the vapour in the cargo condenser 130.
Additional to PC6, pressures can be read from streams 7, stream 20, stream 1, or in the receiver via PC7 for the control of valve 200.
Figure 15 shows how the method of glide refrigeration can be utilised in combination with a three stage compressor. The inventors found that in the same manner as described above for a two stage compressor the principles of glide refrigeration are an efficient means of handling cargo boil off vapours that cannot be fully condensed at the pressure achieveable by the selected compressor at the temperature available for cooling in the cargo condenser 130. This could be the case for propane with high ethane content where sea water is applied for cooling in the cargo condenser or it could be the case for ethane with high methane content where a refrigerated cooling medium is applied in cargo condenser 130 or optionally in series with cargo condenser 130, not shown. For the latter, a possible refrigeration medium can be propylene. For such conditions, it is possible to utilise the condensate sub-cooler 150 as a partial condenser and sub-cooler.
Figure 15 includes the controls shown in figure 14 together with the process schematics for an embodiment of the invention with at least three stages of compression. The following description applies when the high pressure and cooled stream 7 leaving the cargo condenser 130 is not fully condensed, i.e. inside the phase envelope.
In Figure 15, boil off gas emitted from LPG cargo having a temperature of -110°C or higher flows by pressure via stream 1 to a cargo compressor 100, 110, 120 with minimum three stages of compression where it is firstly compressed in cargo compressor 100 in stage 1 to a first intermediate pressure stream 2 where the first intermediate stream 2 is cooled by physically mixing with a stream 13 at a lower temperature.
The sum of streams 2 and 13 forms stream 3 and enters the second cargo compressor 110 in a second stage of compression where it is compressed to a second intermediate pressure stream 4. The second intermediate pressure stream 4 is cooled by physically mixing with stream 14.
The sum of stream 4 and 14 forms stream 5 and enters the third cargo compressor 120 in a third stage of compression where it is compressed to a final pressure stream 6 and then flows by pressure to a cargo condenser 130 where the final compressed vapour is cooled and partly condensed.
The cooling medium used in the cargo condenser 130 can be seawater, glycol / water mixture or a suitable refrigerant as e.g. propylene. Even multi component refrigerant may be used if a lower temperature than what is achieveable with e.g. propylene is needed.
The final discharge pressure of cargo compressor 120 is such that without the presence of inert gases or large contents of volatile cargo components in the boil off gas, the condensate is at its bubble point pressure in the receiver 140, this is a self adjusting process where the discharge pressure automatically adjusts towards equilibrium conditions in the receiver 140.
The high pressure condensate stream leaves condenser 130 as stream 7 where it enters a liquid receiver 140 where on prior art solutions any non-condensable gases are separated and leaves via line 20. For this invention valve 220 will be closed and normally only operated during situations with known large amount of inert gases as e.g. after docking where cargo tanks, piping and reliquefaction system has been inerted (gaseous atmosphere has been replaced with nitrogen).
The discharge pressure of cargo compressor 120 in the third cargo compressor stage is such that without the presence of inert gases in receiver 140 the condensate is at its bubble point pressure, this is a self adjusting process where the discharge pressure automatically adjusts towards equilibrium conditions in the receiver 140.
When full condensation occurs in the condenser 130, the receiver 140 will be operated with a liquid level and warm and saturated condensate stream 15 (i.e. at its bubble point) leaves the receiver 140 and flows towards the condensate sub-cooler 150 where it is heat exchanged against an intermediate expanded portion 19 of the further cooled warm and saturated condensate stream 15.
When full condensation is not achieveable in the condenser 130, the high pressure stream leaves condenser 130 as a cooled and partly condensed stream 7 where it enters the liquid receiver 140 that ensures proper mixing of gas and liquid for stable flow regime out via stream 15.
By warm it shall be understood a temperature close to the coolant / refrigerant temperature used in cargo condenser 130, typically this temperature is about 4 - 6 °C above the coolant / refrigerant temperature. The coolant / refrigerant temperature depends on the heat exchange medium used in the cargo condenser 130. For seawater as heat exchange medium, the temperature will be in the range from 0 °C to about 40 °C. When there is applied a refrigerant as heat exchange medium the temperature might be as low as -50 °C for single component refrigerants.
Intermediate pressure shall be understood as an intermediate pressure between second compression stage and final compression stage; i.e. for a three stage compressor, intermediate pressure is the pressure between the second and third stage of compression. For a four stage compressor, intermediate pressure may also be the pressure between third and fourth stage of compression.
The warm mixed phase stream 15 leaves the condensate sub-cooler 150 as a fully condensed and further cooled condensate stream 8 where it flows to a second condensate sub-cooler 240 for additional cooling and becomes an additional cooled warm condensate stream 10. The warm and mixed phase stream 15 is thus subject to two stages of cooling before being split in portions to conduct internal cooling duties. It leaves the condensate sub-cooler 240 as stream 10, enters an optional additional liquid drum 400 for the disposal of non-condensables, downstream the optional liquid drum 400 it is subject to its first splitting. Here it is divided into two portions, stream 12 and stream 16. Stream 12 being the first portion of the additional cooled warm condensate stream and is the final condensate returned back to the ship's cargo tanks. Stream 12 or part of stream 12 might also be led to at least one fuel tank; eg., for the main propulsion machinery (not shown in figure 15). The at least one fuel tank may for example be a deck tank serving as a fuel tank, e.g., for the main propulsion machinery. Whether all of or part of stream 12 is returned to the cargo tank(s) and/or transported to the fuel tank(s) might vary during different operating conditions and during time. Condensate return to the cargo tanks shall thus be understood as return to or transport to any liquid tight tank. Valve 200 is typically a level control valve controlling the level in receiver 140. Stream 16 may be omitted such that stream 10 is split directly into three portions streams 12, 18 and 22.
The two condensate sub-coolers 150, 240 are typically compact heat exchangers such as e.g. plate & plate, shell & plate or any other compact heat exchangers. The invention shall not be limited by the type of heat transfer equipment.
The second portion of the additional cooled warm condensate stream 15 denoted stream 16 is further split into additional two portions, stream 18 and stream 22.
Stream 22 is subject to a final expansion via valve 180 and becomes an additional cooled final expanded mixed phase stream 9. Final shall be understood as the intermediate pressure between first and second compressions stage of a multi stage compressor.
The additional cooled final expanded mixed phase stream 9 is the third portion of the warm mixed phase stream 15 and enters the condensate sub-cooler 240 where the entire stream content is heat exchanged against the further cooled condensate stream 8. Heat exchange is by utilising the principle of glide where the gas / liquid is not separated but kept as a mixed two-phase flow throughout the heat exchange and the resulting normally fully vaporised stream 9 flows out of the condensate sub-cooler 240 as stream 24 and enters a droplet separator 260 for the removal of any droplets if present. Normally, there will not be any droplets present and the droplet separator 260 is only a protective feature for the cargo compressor 110 in compression stage 2. Any liquid can be drained back to one or more cargo tanks (not shown).
Stream 13 leaves the droplet separator 260 and mixes with the compressed stream 2 and forms a cooled compressed stream 3.
Stream 18 is the second portion of the warm condensate stream 15 and is expanded in valve 190 into stream 19 being an intermediate expanded additional cooled portion of the warm mixed phase stream 15. Stream 19 enters the condensate sub-cooler 150 where it is heat exchanged against the warm mixed phase stream 15. Stream 19 exits the condensate sub-cooler 150 normally fully vaporised as stream 11 and enters a droplet separator 160 for the removal of any droplets if present. Normally, there will not be any droplets present and the droplet separator 160 is only a protective feature for the cargo compressor 120 in compression stage 3. Any liquid can be drained back to the one or more cargo tanks (not shown).
Stream 14 leaves the droplet separator 160 and mixes with the compressed stream 4 and forms a second cooled compressed stream 5.
The functionality of line 2a as shown in Figure 3 utilised in prior art reliquefaction units is applicable for vapours that produce high interstage temperatures. This interstage cooling can also be utilised in the present invention as shown in the schematic diagram in Figure 10. A line 4a (not shown) running from outlet side of the second compressions stage 110 to the inlet side of droplet separator 160 may also be possible.
For the conditions when full condensation is possible in condenser 130, the stream 15 will be a warm condensate stream.

Example 1

Taken basis on the capacity of a three stage reciprocating compressor of type 3K160-3L by Burckhardt Compression AG a set of curves was established to show the superiority of the present invention compared to prior art solutions a series of detailed analysis to calculate the effect that increasing ethane content in propane has to the refrigeration capacity.
Figure 17 shows the shows refrigeration capacity vs. ethane content in prior art solutions and the present invention.
From Figure 17 it is seen that a prior art solution without ventgas cooling shows a steep drop in performance for ethane contents of 6 mole%, this is when full reliquefaction is no longer possible and non-condensed gases must be disposed off.
Further, it is shown in Figure 17 that a prior art solution with vent gas cooling has a less steep drop in performance loss but in this solution with vent gas cooling an an additional heat exchanger is necessary to provide this performance improvement.
The present invention does not need an additional heat exchanger and from Figure 17 it can be seen that the present invention still shows a significant performance improvement compared to prior art solutions both when full liquefaction is possible and during partial liquefaction. An increase in liquefaction capacity of 18% compared to prior art solutions was shown.

Example 2

Taken basis on the capacity of another three stage reciprocating compressor of type 3K160-3K from Burckhardt Compression AG the following curves have been developed presenting the performance improvements in condensate sub-cooling using glide refrigeration:

Figure 18 shows the cooling curves for a flooded intercooler
Figure 19 shows the cooling curves for a compact heat exchanger suitable for glide refrigeration

For a typical commercial propane cargo with 5 mole % ethane contained in the liquid phase it will not be uncommon to see above 26 mole % ethane in the boil off gas. In a reliquefaction plant with flooded intercooler, the ethane content in the liquid pool stabilises at a significantly lower level than what is contained in the inlet stream 1. Actual content is dependent on seawater temperature and intercooler pressure but a change in ethane content from 26 mole % down to 7 mole % should be expected.
The following tables list the performance of above mentioned compressor.
Table 2 shows the calculated suction pressure and discharge pressure at the three stages of compression and the condensate temperature for glide refrigeration according to the invention compared with flooded intercooler. Table 2

Case Suction P kPa 1^st Stage Out kPa 2^nd Stage Out kPa 3^rd Stage Out kPa Condensate °C

Glide (invention) 100 391 1 078 2 287 -27.2

Flooded 100 386 1 033 2 287 -11.3
Table 3 shows the calculated power consumption, liquefaction capacity and coefficient of performance (COP) for glide refrigeration according to the invention and flooded intercooler. Table 3

Case Power Consumed kW Liquefaction Capacity kW COP

Glide (invention) 447 563 1.259

Flooded 442 509 1.152
The results given in the above tables clearly show that glide refrigeration gives a significant performance increase compared to flooded intercooler.
The given examples describe various options using reciprocating compressors, however, the invention shall not be limited by such compressors. For instance, the invention works perfectly well with centrifugal and other rotary compressors as e.g. rotary screw compressors. For rotary screw compressors, the pressure at the economiser port can be regarded as an intermediate pressure between suction and final discharge pressure. Hence a rotary screw compressor with one set of rotors, male and female, with one economiser port can in this invention context be understood as a two stage compressor. Also screw compressors with more than one set of rotors may be used. The function and design of a rotary screw compressor is well known by the industry and will not be described any further.
Although the above description refers specifically to LPG, it should be noted that the invention is equally applicable to the regasification of other liquefied gases such as ethane, propane, N₂, and CO₂. As an alternative, it is understood that the present plant also may be installed onshore and the present method may be used in an onshore plant.

Claims

A method for storage and transport of liquefied petroleum gases (LPG) on LPG carriers, comprising compressing boil off gas emitted from one or more LPG cargo tanks, said LPG having a boiling point temperature of -110°C or higher at 1 atmosphere in a cargo compressor with minimum two stages of compression (100, 110, 120), providing at least one intermediate pressure stream (2, 4), at least one further cooled, compressed stream (3, 5), and a final compressed stream (6); cooling and condensing said final compressed stream (6) in a cargo condenser (130) to provide a high pressure condensate stream (7) which enters a liquid receiver (140) for separation of any non-condensable gases and mixing of gas and liquid; characterized in comprising the steps of:
- cooling a warm and mixed two-phase stream (15) leaving the receiver (140) in at least one condensate sub-cooler (150, 240), in which the warm and mixed two-phase stream (15) is heat exchanged with at least one glide refrigerant (9, 19) thereby producing at least one further cooled condensate stream (8, 10);

- separating any gas from the at least one further cooled condensate stream (8, 10) in a liquid drum (400) separator;

- dividing the further cooled condensate stream (8, 10) into a stream (12), which is sent to one or more liquid tight tanks, and at least one stream (18, 22), which is expanded to at least one mixed phase refrigerant stream (9, 19), which is introduced into the at least one condensate sub-cooler (150, 240) countercurrently heat exchanged with the warm and mixed two-phase stream (15);

- passing the at least one mixed phase refrigerant stream (9, 19) leaving the condensate sub-cooler (150, 240) as at least one stream (11, 24) to at least one droplet separator (160, 260) for removal of any droplets, if present;

- passing and mixing at least one stream (13, 14) leaving the at least one droplet separator (160, 260) with the at least one intermediate pressure stream (2, 4) from the cargo compressor with at least two stages of compression (100,110, 120) thus forming the at least one further cooled, compressed stream (3, 5).
A method according to claim 1, controlling the ratio of the temperature difference between streams (11, 24) and (15, 8) (TD2, TD4) on one side of the condensate sub-cooler (150, 240) and the temperature difference between streams (8, 10) and (19, 9) (TD1, TD3) on the other side of the condensate sub-cooler (150, 240).
A method according to any of the preceding claims, controlling the temperature (T11, T24) of stream (11, 24) to its dew point temperature at a given pressure in droplet separator (160, 260) by controlling a temperature difference (TD2, TD4) between the temperature (T11, T24) in stream (11, 24) and the temperature (T15, T8) in stream (15, 8).
A method according to any of the preceding claims, controlling the temperature (T11, T24) of stream (11, 24) to its dew point temperature at a given pressure in droplet separator (160, 260) by controlling the ratio of the temperature difference between streams (11, 24) and (15, 8) (TD2, TD4) on one side of the condensate sub-cooler (150, 240) and the temperature difference between streams (8, 10) and (19, 9) (TD1, TD3) on the other side of the condensate sub-cooler (150, 240).
A method according to any of the preceeding claims, correcting the ratio of the temperature difference between streams (11, 24) and (15, 8) (TD2, TD4) on one side of the condensate sub-cooler (150, 240) and the temperature difference between streams (8, 10) and (19, 9) (TD1, TD3)) on the other side of the condensate sub-cooler (150, 240) based on the liquid level in droplet separator (160, 260).
A method according to any of the preceding claims, controlling the pressures in the cargo compressor with at least two stages of compression (100, 110, 120) and receiver (140) by reading pressures by pressure reading instruments (P1, P2, P5, P6, P7), controlling pressure ratio across each compression stage by pressure controllers (PC1, PC2, PC3), controlling discharge pressure from the compressor by a compressor discharge pressure controller (PC6), controlling pressure from receiver (140) by a receiver pressure controller (PC7) or controlling the level in receiver (140) by a level controller (LC1).
A method according to any of the preceding claims, by mixing the two-phase gas liquid stream (15) at a bottom outlet of receiver (140).
A method according to any of the preceding claims, wherein vapour exiting the first compression stage in the cargo compressor (100) flows via a line (2a) when a valve (250) is closed and said vapour mixes with stream (11).
A method according to any of the preceding claims, compressing the LPG in three stages of compression (100, 110, 120).
A method according to any of the preceding claims, comprising heat exchanging in two condensate sub-coolers (150, 240).
A method according to any of the preceding claims, returning stream (12) to the one or more LPG cargo tanks and/or flowing stream (12) to one or more fuel tanks.
A system for storage and transport of liquefied petroleum gases (LPG) on LPG carriers, comprising at least one cargo tank for LPG, said LPG having a boiling point temperature of -110°C or higher at 1 atmosphere, a cargo compressor with minimum two stages of compression (100, 110, 120) for compressing boil off gas from the LPG, to provide at least one intermediate pressure stream (2, 4), at least one further cooled, compressed stream (3,5) and a final compressed stream (6), a cargo condenser (130) for condensing said final compressed stream (6) to provide a high pressure condensate stream (7) and a liquid receiver (140) for separation of any non-condensable gases and mixing of gas and liquid; characterized in that said system further comprises
- at least one condensate sub-cooler (150, 240) configured for heat exchanging by glide refrigeration a warm and mixed two-phase condensate stream (15) from the liquid receiver (140) against at least one glide refrigerant, the at least one glide refrigerant being an intermediate expanded portion (9, 19) of the further cooled warm and mixed two-phase stream condensate stream (15) configured to be introduced into the at least one condensate sub-cooler (150, 240) countercurrent to the warm and mixed two-phase condensate stream (15);

- a liquid drum (400) connected to an at least one further cooled condensate stream (8, 10) for separation of gas and liquid; and

- at least one droplet separator (160, 260) configured for removal of any droplets, if present, from at least one glide refrigerant stream (11, 24) leaving the at least one condensate sub-cooler (150, 240), and further configured to pass and mix at least one stream (13, 14) leaving the at least one droplet separator (160, 260) with the at least one intermediate pressure stream (2, 4), thus forming 2. the further cooled, compressed stream (3, 5).
A system according to claim 12, wherein said system comprises a temperature control system for controlling the temperature of streams (11, 24), comprising controllers for controlling temperature differences (TD1, TD2, TD3, TD4) on each side of the condensate sub-cooler (150, 240) and level transmitter (L3, L4) for measuring liquid level in the droplet separator (160, 260).
A system according to any of claims 12 - 13, wherein said system comprises a pressure control system for controlling the pressures in the cargo compressor with minimum two stages of compression (100, 110, 120) and receiver (140), comprising pressure reading instruments (PI, P2, P3, P4, P5, P6, P7), pressure controllers (PC1, PC2, PC3), a compressor discharge pressure controller (PC6), a receiver pressure controller (PC7) and a level controller (LC1).
A system according to any of claims 12 - 14, wherein the at least one condensate sub-cooler (150, 240) is a compact heat exchanger such as plate & plate exchanger, shell & plate heat exchanger.
A system according to any of claims 12 - 15, wherein said system comprises a line arranged to connect stream (2) from cargo compressor (100) with stream (11).
A system according to any of claims 12 - 16, wherein said system comprises a cargo condenser with three stages of compression (100, 110, 120).
A system according to any of claims 12 - 17, wherein said system comprises two condensate sub-coolers (150, 240) configured for heat exchanging by glide refrigeration.