Abstract:
A method and apparatus of pre-heating LNG boil-off gas stream flowing from a reservoir in a reliquefaction system, before compression. The method comprises heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, where the second coolant stream is obtained by selectively splitting a first coolant stream into second and third coolant streams, third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and the low temperature duty from the BOG is substantially preserved within the reliquefaction system, and thermal stresses in the cold box are reduced. Before the compression step, the BOG is pre-heated to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of re-liquefaction of boil-off gases from liquid natural gas (LNG). More specifically, the invention relates to a method and an apparatus for pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, and a method and an apparatus for cooling an LNG boil-off gas (BOG) stream in a reliquefaction plant. 
       Background 
       [0002]    A new generation of LNG vessels was established in association with the introduction of LNG reliquefaction systems (LNG RS). Prior to this, basically all LNG vessels were driven by steam turbines fuelled by boil off gases (BOG) evaporating from the cargo during transportation. In periods when the total amount of BOG was insufficient to cover the entire power demand, additional LNG had to be fed to the boilers through forced vaporizers. 
       Brief Description of the Prior Art 
       [0003]    The new LNG RS opened the possibility to collect, cool down and reliquefy all BOG and hence preserve the total cargo volume throughout the laden and ballast voyages. Conventional slow speed diesel engines, with high efficiencies compared to the steam turbines, could then be used for propulsion. 
         [0004]    Several patents have described various aspects with such reliquefaction plants, and accordingly improvements to these. The prior art (e.g. Norwegian Patent Application No. 20051315 basically focuses on improvements of the nitrogen Brayton cycle and the utilization of cold nitrogen for pre-cooling. There is, however, a further need to improve the system in order to reduce the power demands. 
         [0005]    Most of today&#39;s LNG vessels utilize low-temperature centrifugal BOG compressors to feed their boilers. Much of the reason for choosing low-temperature compression is that this will reduce the compressor size significantly compared to compression at ambient temperatures. The fan laws are applicable for centrifugal compressors, and show that a low suction temperature will ensure a higher pressure ratio per stage. The density of the gas will accordingly increase, the volume flow is reduced to a minimum, and the size and efficiency of the BOG compressors become more favourable. 
         [0006]    Since there is no need to preserve the low temperature duty in the BOG stream—in fact the BOG is normally additionally heated before introduction to the boilers—the heat of compression is deliberately absorbed by the compressed gas without any means of heat rejection downstream the BOG compression. 
         [0007]    The common practice of low.temperature BOG compression has been further applied to new BOG compressor designs, dedicated for operation towards LNG reliquefaction systems. From an energy point-of-view this results in inefficient operation, since the cooling cycle must be sized to remove the heat of compression from BOG compressors, in addition to the heat of evaporation and the superheating adsorbed in the cargo containment system. 
         [0008]    Also, other problems arise when low-temperature BOG compression is applied. Since no aftercoolers (intercoolers) are employed, recycling at low capacities depend on temperature control upstream the BOG compressor. The cooling duty necessary for this purpose can be difficult to predict since it will depend much on the BOG compressor efficiency, which in turn depends on several properties of the processed stream. Using recondensed BOG to provide this cooling, also reduces the performance of the plant, measured in terms of power per unit reliquefied BOG returned to the tanks. 
       SUMMARY OF THE INVENTION 
       [0009]    It is thus provided a method of A method of pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, the method comprising heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, the method being characterized in that the second coolant stream is obtained by selectively splitting a first coolant stream into said second coolant stream and a third coolant stream, said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and heat exchange with low temperature BOG is done by optimising the split of the coolant in the first heat exchanger in order to minimize exergy losses, and thermal stresses in the cold box are reduced. 
         [0010]    It is also provided a method for cooling an LNG boil-off gas (BOG) stream in a reliquefaction plant, the BOG flowing from a reservoir, the method comprising compressing the BOG; heat exchanging the compressed BOG against a coolant in a cold box; flowing substantially re-liquefied BOG from the cold box to the reservoir, characterized by prior to the compression step, pre-heating the BOG to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG. 
         [0011]    In one embodiment, the pressure of the reliquefied BOG between the cold box and the reservoir is controlled independently of the BOG compressor discharge pressure and the reservoir pressure, and the amount of vent gas generated and the vent gas composition thus may be controlled. 
         [0012]    It is also provided an apparatus for cooling an LNG boil-off gas (BOG) in a reliquefaction system, comprising a closed-loop coolant circuit for heat exchange between a coolant and the BOG; a BOG compressor having an inlet side fluidly connected to an LNG reservoir; a cold box having a BOG flowpath with a BOG inlet fluidly connected to the BOG compressor outlet side; said BOG flowpath having outlet for substantially re-liquefied BOG, fluidly connected to the reservoir; said cold box further comprising coolant flowpaths for heat exchange between the BOG and the coolant; characterized by a first heat exchanger in the fluid connection between the reservoir and the BOG compressor inlet side, said first heat exchanger having a coolant path fluidly connected to the closed-loop coolant circuit, at a point downstream of the coolant circuit&#39;s compander aftercooler but upstream of the coolant flow paths in the cold box, whereby the BOG compressor receives BOG with temperatures near or at the system ambient temperatures. 
         [0013]    In one embodiment, the invention provides a separator in fluid connection with the cold box outlet and with the reservoir, a first valve in the cold box outlet line and a second valve in a line connected to the reservoir, said separator also comprising a vent line ( 11 ), whereby the pressure in the separator may be controlled, and the amount of vent gas and the vent gas composition thus may be adjusted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]      FIG. 1  is a simplified process flow diagram, illustrating the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]    The invention will now be described with reference to  FIG. 1 , illustrating the novel features of the LNG RS with ambient temperature BOG compression. The figure shows schematic a cargo tank  74 , holding a volume of LNG  72 . BOG, evaporating from the LNG, enters a line  1  which is connected to a first heat exchanger H 10 . In this heat exchanger, the BOG is heated up to near-ambient temperatures, as will be described later. Following this pre-heating, the BOG enters the first stage BOG compressor C 11  via line  2 . The BOG compressor is shown as a three-stage centrifugal compressor C 11 , C 12 , C 13 , interconnected via lines  3 - 7  via intercoolers H 11 , H 12  and aftercooler H 13  as shown in the figure, but other compressor types may be equally applicable. The pre-heating ensures that the heat generated by the compression may be rejected through cooling water in the intercoolers H 11 , H 12  and the aftercooler H 13 . 
         [0016]    Pressurized BOG is then, via a line  8 , fed into a second heat exchanger (or “cold box”) H 20  where it is heat exchanged against a coolant, as will be described later. The coolant is preferably nitrogen (N 2 ). Following heat exchange, substantially reliquefied BOG exits the cold box H 20  via a lines  9 ,  10  connected to a separator F 10 . The separator is provided with a vent line  11 . A throttling valve V 10  is arranged in the lines  9 ,  10  between the cold box and the separator, for expanding the reliquefied BOG. Following separation, reliquefied BOG is fed into the LNG  72  in the cargo tank  74  via lines  12 ,  13 , as shown in  FIG. 1 . A valve V 11  is arranged in the lines between the separator F 10  and the tank  74 , the purpose of which will be described later. 
         [0017]    The closed N 2 -Brayton cooling cycle is here represented by a 3-stage compressor C 21 , C 22 , C 23  with intercoolers H 21 , H 22 , aftercooler H 23 , interconnected via lines  51 - 55  as shown in the figure, and a single expander stage E 20 . (Other cooling cycle constellations, for instance as discussed in Norwegian Patent Application No. 20051315 can also be utilized in this context.) Pressurized coolant (N 2 ) exits the compressor and the aftercooler H 23  via a line  56  connected to a three-way valve V 12 . The three-way valve V 12  is controllable to selectively split the high-pressure N 2  stream flowing in the line  56  into two different streams in respective lines  57 ,  59 , as further detailed below. A first outlet of the three-way valve V 12  is connected to a coolant inlet in the first heat exchanger H 10  via a line  59 . A line  60  connects the coolant outlet of the first heat exchanger H 10  with the second heat exchanger&#39;s H 20  middle section, via line  61 , as shown in  FIG. 1 . A line  57  connects a second outlet of the three-way valve V 12  to the inlet of a first coolant passage  82  in the second heat exchanger H 20  upper section. The first coolant passage  82  outlet is connected via a line  58  to an entry point on the line  60  described above. A line  61  connects this entry point to a the inlet of a second coolant passage  84  in the cold box, in the vicinity of the cold box&#39; middle section, as illustrated by  FIG. 1 . Coolant flows through the second coolant passage  84  and into an expander E 20  via a line  62 . The expanded coolant enters the second heat exchanger (cold box) H 20  lower section via a line  63  connected to the inlet of a third coolant passage  86  before it exits the heat exchanger and flows back to the compressor C 21 , C 22 , C 23  via the line  50 . The flow split here described as a three-way valve V 12  can equally be performed by other flow control configurations, such as normal single line control valves, orifices, etc. The important aspect is that the flow split can be controlled in order to cope with varying BOG flow conditions. 
         [0018]    Generally, the process involves three new features which differ from previously suggested reliquefaction designs:
   1. A heat exchanger H 10 , to ensure that most of the low-temperature duty which can be extracted from the BOG in the ship&#39;s vapor header line  1 , remains preserved within the reliquefaction system,   2. A BOG compressor C 11 , C 12 , C 13  working under ambient, or near-ambient conditions, with rejection of its heat of compression H 11 , H 12 , H 13  to the ambience;   3. A generally higher pressure for the BOG stream  8  entering the main heat exchanger (cold box) H 20 , compared to the discharge pressure of common BOG compressors, allowing the condensation to take place at a higher temperature level, and at the same time opens the possibilities for controlling the separation pressure in the separator F 10  at a level between the cold box outlet pressure in the line  9  and the storage pressure in the cargo tanks  74 . This pressure control must be seen in association with flow control through the separator vent line  11  (flow control valve not shown in  FIG. 1 ). By adjusting the separation pressure, the vent flow, as well as the composition of the condensate which is returned to tanks  74 , can be controlled according to the operator preferences. Minimizing the vent gas flow results in higher required reliquefaction power input and vice versa. Adjustments of the separator pressure will therefore allow the operator to select the most favourable conditions for economic optimization of the LNG RS operation.   
 
       1. Heat Exchanger Upstream BOG Compressor 
       [0022]    The heat exchanger H 10  upstream the BOG compressor C 11 , C 12 , C 13  is installed to preserve the low-temperature duty in the BOG coming from the tanks  74 , within the system. To extract as much low temperature duty as possible from this BOG stream, the BOG temperature should be allowed to increase up to near-ambient temperatures. To preserve the low temperature duty within the system, the duty must be absorbed by another stream in the reliquefaction system, originating at a higher temperature than the BOG stream. 
         [0023]    This other stream will typically be a fraction of the warm high-pressure N 2 -stream  59  as shown in  FIG. 1 . Other alternatives, such as using the entire N 2 -stream (not only a part of it), or the BOG-stream from downstream the BOG compressor&#39;s aftercooler are also possible. However, the process of  FIG. 1  will probably be the most beneficial, given the limitations and characteristics of commonly employed equipment for such processes. Consequently, only the process of  FIG. 1 , involving a split of the high-pressure N 2 -stream  56  downstream the N 2 -compander&#39;s aftercooler H 23  into two different streams  57 ,  59 , will be discussed next. 
         [0024]    The BOG pre-heater control is based on controlling the coolant flow (N 2 ) on the secondary side. The energy which is transferred between the compressed N 2  and the BOG in the first heat exchanger H 10  (pre-heater) will depend on the BOG flow and temperature, and consequently be a more or less fixed value [kW] as long as the BOG flow is constant. This means that the temperature of the N 2  flow exiting the pre-heater H 10  will vary with the N 2  flow rate. As long as the heat transfer area of the pre-heater is large enough, the three-way valve V 12  (or equivalent flow split constellations) in the N 2  stream upstream the pre-heater H 10  can be used for two different purposes: 
       A: For Thermodynamic Optimization of the Overall Process: 
       [0025]    The freedom represented by the flow split (three-way valve V 12 ) can be used to ensure a very efficient heat exchange (low LMTD [log mean temp difference], and consequently low exergy losses) in the upper parts of the cold box H 20 . The heating and cooling curves can in theory be designed to be parallel with a constant temperature difference between streams at any temperature in the upper (warm) parts of the cold box. 
         [0026]    Since the Brayton cycle is based on the concept that pressurized N 2  has a higher heat capacity than low pressure N 2 , the heating curves can only be made parallel if the high pressure mass flow is smaller than the cold, low pressure flow. The split of the high pressure stream will consequently cause a very efficient heat exchange in the upper parts of the cold box, and since the branch flow also is cooled independently in the BOG pre-heater, the energy penalty which otherwise would have been associated with the mixing of the two high pressure N 2  streams at a lower temperature is reduced to a minimum. 
         [0027]    The flow split will typically be controlled based on the BOG compressor suction temperature. 
       B: For Reducing Thermal Stress in the Cold Box to a Minimum 
       [0028]    Another benefit of the flow split control made possible by the three-way valve V 12  (or alternative flow split constellations), is that the temperature of the high pressure N 2  stream exiting the pre-heater H 10  and flowing in the line  60 , can be monitored and, if necessary, controlled in order to avoid rapid temperature fluctuations in the flow which is reintroduced to the cold box via the line  61 . 
         [0029]    The cold box is normally made in aluminium and is sensitive to thermal stress. By applying a safety control function which changes the flow through the pre-heater based on undesirable conditions, the temperature of all streams entering the cold box can be carefully controlled. This would not have been possible if the pre-heater was a low pressure BOG vs. high pressure BOG heat exchanger, as the high temperature BOG outlet temperature would change synchronously with the fluctuation in the low pressure incoming BOG. 
         [0030]    Normally, the split ratio defining the flows of streams  57  and  59 , will be adjusted in order to extract as much low temperature duty as possible from the low temperature BOG. However, this configuration also opens for controlling the split ratio with respect to the temperature of the nitrogen stream  61  entering the cold box&#39; middle section. Doing so, conditions which may expose the main heat exchanger H 20  to damaging thermal stresses can easily be eliminated. 
         [0031]    To achieve the optimal heat integration from a thermodynamic point-of-view, the heat exchangers H 10  and H 20  can be combined in one single multi-pass heat exchanger. However, since the main heat exchanger (cold box) H 20  typically will be a plate-fin heat exchanger, which to some extent is sensitive to both rapid temperature fluctuations and large local temperature approaches, it can be feasible to extract some of the heat transfer to an external heat exchanger of a more robust type, as shown at the pre-heater H 10  in  FIG. 1 . 
         [0032]    The heat exchanger configuration shown in  FIG. 1  will also dampen the temperature fluctuations of the flow  61  entering the main heat exchanger&#39;s H 20  middle section, since the N 2 -coolant stream will be very large compared to the BOG flow. This will ensure a much safer operation with respect to thermal stresses in the cold box. 
       2. Ambient Temperature BOG Compressor 
       [0033]    The main incentive for employing ambient temperature BOG compression is the possibility this offers for rejecting heat to the ambience. While today&#39;s commonly used BOG compressors preserves the compression heat within the BOG stream, the compression heat can now be delivered to an external source operating at ambient or near ambient temperatures (e.g. cooling water). 
         [0034]    Ambient temperature compression also offers other benefits. Since an aftercooler H 13  as shown in  FIG. 1  typically will be associated with this system, the temperature of the compressed stream  8  entering the cold box is stabilized relative to the heat rejection source&#39;s temperature. After- and intercooling also represent major advantages with respect to operation in recycle and/or anti surge modes, where the external cooling media ensures stable operation, normally without any additional temperature control. 
         [0035]    Ambient temperature BOG compression is especially favourable for LNG vessels where boil-off rates, compositions, temperatures and pressures may vary considerably with the type of voyage (ballast or laden voyages) and cargo. Inter- and aftercooling towards ambient conditions will stabilize the compression conditions and ease capacity control (recycling, etc.) 
         3 . Benefits of Selecting a Higher Pressure Ratio 
       [0036]    A “higher” pressure ratio over the BOG compressors C 11 ,C 12 ,C 13  will in this context relate to a higher cold box inlet pressure in the line  8  than what is strictly necessary to provide a sufficient differential pressure for forcing the LNG back to the cargo tanks. 
         [0037]    This allows the cryogenic separator F 10  to be placed at an intermediate pressure level, typically limited to a zone between two valves V 10 , V 11  as shown in  FIG. 1 . The pressure in this zone can then be controlled independently of the BOG compressor discharge pressure and the cargo tank pressure. Accordingly, some of the overall system&#39;s capacity control can be performed by pressure adjustments in this region. It will consequently enable the operator or the automated control system to adjust both the amount of vent gas generated as well as the vent gas composition in order to operate under the most economically favourable conditions during all LNG price fluctuations. 
         [0038]    A dedicated line can also be placed in order to bypass the separator under conditions where reliquefied BOG is so much subcooled that the separation pressure otherwise will drop below a defined minimum value. 
         [0039]    The pressure differential between the main heat exchanger H 20  and the separator F 10  ensures that the separator can be placed more independent of the main heat exchanger. 
         [0040]    A higher BOG compressor discharge pressure will increase the gain (either in form of a higher adiabatic temperature change or reduced flash gas generation) during the throttling processes down to tank pressure. 
         [0041]    Last, a higher process pressure will increase the heat transfer coefficient in heat the main heat exchanger H 20  and ensure that the condensation here will be performed at higher temperatures in order to reduce exergy losses. 
         [0042]    The person skilled in the art will appreciate that the purpose of the three-way valve V 12  is to selectively control the flow split between (i) the line  59  connected to the first heat exchanger H 10  and (ii) the line  57  connected to the cold box H 20 . To this end, the three-way valve V 12  described above may be replaced by e.g. a controllable choke valve in the line  60 , downstream of the first heat exchanger H 10 , and a fixed-dimension restriction in the line  57 .