Description:FIELD OF INVENTION:
[0001] The present invention relates to an apparatus and method for recondensing boil-off gas (BOG) received from an LNG storage tank and recondensing the vapor into an output stream for returning to the storage tank.

BACKGROUND OF THE INVENTION:
[0002] The natural gas is one of the most environmentally friendly energy resource. It easily burns without generating any unburnt solid particulates or any other pollutants. It produces 25–30% less carbon dioxide per joule delivered than oil, and 40–45% less than coal. As on today, the natural gas proven reserve has been reported to be adequate for more than 50 years. Besides, huge investments in natural gas exploration and production have been made which are increasing the proven gas reserve year by year generating a less fear of natural gas getting exhausted soon. As Natural gas production locations and consumption locations are wide apart across large oceans ad tough terrains, the pipeline transportation becomes technically infeasible / economically expensive. As a result, LNG cargo transportation, import and regas terminals have become the most preferred energy fort polio. Most of the developed countries including Europe, USA, China and Japan have been operating LNG import and regas terminals for decades. Now many of the South East Asian counties are also planning to build LNG regas terminals and dispensing stations. The LNG Regas terminals are of smaller capacities compared to developed countries because of limited natural consumption. The conventional Regas process scheme is expensive and tecno-economically challenging. The process of the present invention is expected to meet those challenge cost effectively.
[0003] WO2015126119A1 discloses a a system and a method for processing boil-off gas. A system for processing boil-off gas, which is generated in an LNG storage tank in a ship or a marine structure, according to the present invention comprises: a regasification line for supplying natural gas by regasifying LNG stored in the LNG storage tank; and a high-pressure compression line for compressing the boil-off gas generated in the LNG storage tank to a high pressure and supplying to a point down-stream from the regasification line.
[0004] BOG generation is any LNG handling system is inevitable due to heat transfer from the atmospheric surroundings to the cryogenic system, as liquefied natural gas (LNG) is stored at a temperature of about -160°C under ambient pressure.
[0005] It is therefore discerned that reasonable and effective management of BOG, can not only reduces capital expenditure, the waste of energy at LNG terminals, but also reduce atmospheric emission.
[0006] In view of the gaps existing in the prior art, the present invention discloses a process and device for recondensation of BOG, to improve operability and increase economic significance.

SUMMARY OF THE INVENTION:
[0007] According to embodiments of the present invention, it is provided for a BOG Liquefaction Process for LNG export / import terminals and Dispensing Station consists of adding additional impellers to the first send out pump (InTank Pump) installed in the LNG tank to generate the required gas supply pressure eliminating the use of another set of HP LNG pump (second send-out pump).
[0008] The BOG from the LNG Tank is compressed to 3-4 Barg using either high pressure natural gas driven Ejector or single stage Mechanical compressor and the compressed BOG is then condensed in a standard shell and tube heat exchanger using cold LNG from InTank pump and condensed BOG is returned to the storage tank through a pressure control valve with 5-6% flashing. The LNG from shell-tube condenser picks-up heat of condensation of BOG. The LNG from shell-tube condenser is then passed through LNG vaporizer where entire LNG vaporizes into gas is then fed to the gas grid as in the case of current process scheme.

OBJECT OF THE INVENTION:
[0009] It is an object of the present invention to provide for cost effective BOG management system for LNG export / import / regas terminals and LNG dispensing stations with zero emission.
[0010] It is another object of the present invention to reduce Initial Capital Expenditure (Capex) and Operating Expenditure (Opex) significantly.
[0011] It is yet another object of the present invention to operate the Regas terminal with lower minimum send-out capacity than the current direct-gas-liquid contract recondenser.
[0012] It is yet another object of the present invention to provide for a Regas terminal which is easier to operate.
[0013] It is yet another object of the present invention to provide for a Regas terminal which is reliable due to a smaller number of rotating equipment.

BRIEF DESCRIPTION OF THE DRAWINGS:
[0014] The present invention will be described in more detail hereinafter with the aid of the accompanying drawings. The drawings are illustrative of one or more embodiments of the invention and do not in any manner limit the scope.
[0015] Figure 1 illustrates the existing LNG Regas system.
[0016] Figure 2 illustrates the recondensation device and process, according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION:
[0017] The following description illustrates various embodiments of the present invention and ways of implementation. The embodiments described herein are not intended to be limited to the disclosure and that the same is in no way a limitation. The invention may be embodied in different forms without departing from the scope and spirit of the disclosure.
[0018] The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The illustrations are merely representational and may not be drawn to scale. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive.
[0019] It is also to be understood that the specific device and method illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Specific dimensions and/or other physical characteristics relating to the embodiments disclosed herein are therefore not to be considered as limiting, unless the claims expressly state otherwise.
[0020] Further, reference numerals are used only as an aid to explain the invention and they do not in any matter restrict the scope of the invention.
[0021] Generally referring to the existing state of art, the present invention relates to recondensation of boil off gas (BOG) in a liquefied natural gas (LNG) processing system. The conventional process has mechanical BOG compressors and a large heavy gas-liquid direct contact BOG recondenser with minimum Send Out capacity which leads to higher Capex, Opex and minimum send-out capacity.
[0022] The present invention discloses a device and a process for Boil-off gas (BOG) re-condensation at LNG export terminals, dispensing stations and import terminals by using high pressure natural gas driven ejector instead of mechanical compressor and standard shell; and a tube condenser instead of gas-liquid direct BOG recondenser resulting in significantly saving cost and improving operability.
[0023] The present invention comprises an ejector to compress boil-off gas using high pressure regas as ejector’s motive fluid where ejector sucks atmospheric boil-off gas, compresses it, and makes it suitable for recondensation. The compressed boil-off gas is then sent to shell & tube condenser where compressed boil-off gas is condensed using cold LNG from in-tank pumps / HP LNG pumps. The condensed LNG from the condenser is returned to LNG storage tank through flashing pressure control valve.
[0024] Recondensation of Boil-off gas (BOG) is generated in LNG storage terminals using high pressure natural gas driven ejector instead of mechanical compressors to increase the BOG pressure to make it thermodynamically suitable for recondensation and use of standard shell and tube heat exchanger to recondense the BOG instead of direct contact gas liquid re-condenser.
[0025] Use of high pressure driven natural gas ejector reduces initial capital expenditure and operating cost by more than 50% of the existing design. Use of standard shell and tube BOG condenser reduces initial capital expenditure about 40% and completely prevents cavitation problems in HP LNG pumps & sloshing problem in floating terminals. It also reduces minimum send-out capacity of the terminal by about 10%-15% compared to the that of conventional designs.
[0026] The present invention also eliminates use of two set of LNG pumps where the send-out pressure is of medium pressure (40-50 Barg) which results in initial capital expenditure saving by about 50% compared to that of two set of LNG pumps.
[0027] The present invention discloses a process and a device for BOG recondensation at LNG terminals and dispensing stations which reduces the Capex and Opex significantly of the terminal and also minimizes emissions. It comprises of adding additional impellers to in-tank LNG pumps, shell, and tube condenser, natural regas driven ejector to compress BOG (Boil-off Gas) or single stage BOG (Boil-off Gas) compressor.
[0028] Referring to Figure 1, it discloses a convention process, wherein Liquefied Natural Gas (LNG) is unloaded from an LNG carrier/ship to onshore cryogenic storage tanks in which LNG is stored at -160 Degree Celsius and at nearly atmospheric pressure. Atmospheric heat ingress in to the storage tank generates Boil-off-Gas (BOG) continuously. The BOG is compressed and recondensed in a Recondenser. LNG pump (first send-out pump) installed in the storage tank gives LNG flow at 5-10 Barg pressure which is fed to the BOG Recondenser where it condenses BOG fed from BOG compressor discharge. The LNG from BOG Recondenser bottom is fed to HP LNG pump suction and the pump increases the pressure to the required gas grid pressure which then passes through the LNG vaporizer and the regas is sent to the gas grid.
[0029] The present invention discloses a process and a setup/terminal/apparatus which has the following distinguishing features, in comparison with the existing state of art. The present invention uses an LNG Boil-off Gas (BOG) standard shell and tube condenser to manage BOG instead of BOG Recondenser and eliminates complex BOG Recondenser and cavitation problems in HP LNG pumps.
[0030] Firstly, additional impellers are added to the in-tank pumps to avoid another set of HP LNG Pumps, wherein medium pressure is generated for send-out pressure operations, thereby eliminating HP LNG pumps. Secondly high-pressure natural gas driven ejector instead of mechanical compressor is used to compress the BOG, and thirdly, a standard shell and tube condenser is utilised in the place of gas-liquid direct contact recondenser.
[0031] Referring to Figure 2, it identifies a process and a setup according to a preferred embodiment of the present invention. It discloses additional impellers to enhance the efficiency of BOG recondensation systems, especially in liquefied natural gas (LNG) terminals.
[0032] The impellers are connected/added to the in-tank pumps (1) installed in the LNG tank to use it as a send out pump to generate the required gas supply pressure instead of using another set of HP LNG pump (2) (second send-out pump). The conventional setup, as disclosed in Figure 1, uses a second HP LNG pump to act as a send out pump.
[0033] The BOG from the LNG Tank is compressed to a pressure of 3-4 Barg by using a compressor. In an embodiment ejector compressor (3) is utilised for compressing the BIG. In an alternate embodiment, a Mechanical compressor (4) is used. The use of the varying compressor is dependent on process optimization requirements.
[0034] The present invention comprises ejector to compressor boil-off gas using high pressure regas as ejector’s motive fluid where ejector sucks atmospheric boil-off gas, compresses it and makes it suitable for recondensation. The compressed boil-off gas is then sent to shell & tube condenser where compressed boil-off gas is condensed using cold LNG from in-tank pumps / HP LNG pumps. The condensed LNG from the condenser is returned to LNG storage tank through flashing pressure control valve.
[0035] The pressurised BOG emanating from the compressor is then condensed in a standard shell and tube heat exchanger and condenser (5) using cold LNG from InTank pump (1) together with the impellers and is returned to the tank with 5-6% flashing through a pressure control valve.
[0036] During recondensation, the LNG from shell-tube condenser (5) pick-up heat of condensation of BOG. The pre-heated LNG from shell-tube condenser (5) is then passed through LNG vaporizer (6) wherein entire LNG vaporizes into gas. Depending upon the usage, it may be recirculated to be used for the ejector compressor (3). A portion of the gas is diverted to a gas grid to be used for supply for consumption.
[0037] Furthermore, in the present invention, the use of inexpensive gas ejector, standard shell & tube condenser contribute significantly in reducing the Capex and Opex and minimum send-out limitation.
[0038] The process of the present invention is minimizing the BOG venting (by reducing minimum send-out capacity) and reducing power consumption (ejector system consumes lower power compared to mechanical compressor) due to which CO2 emission is drastically reduced compared to the conventional design.
[0039] The following table provides comparative data regarding performance and other categories with respect to the present invention and a conventional process.
TABLE 1
COMPARISON BETWEEN PRESENT INVENTION AND CONVENTIONAL PROCESS
S.No. Description Conventional Process Present Invention
1. BOG condenser BOG gas-liquid directly contact with the Recondenser, in a complex and expensive design. BOG gas is recondensed in a standard shell & tube condenser which reduces Capex by about 50%
2. Minimum send out rate 6-12 times of the BOG generation 3-5 times of the BOG generation
3. BOG compressor / Ejector discharge pressure Two stage, 6-8 Barg Ejector or Single stage 3-4 Barg which reduces both Capex and Opex
4. HP LNG Pumps Required May not be required if send out pressure can be achieved using additional impellers in InTank pumps
5. Capex Much higher due to large and complex Recondenser, HP LNG Pumps, two stage BOG compressor. Much lower due to possible elimination of HP LNG pumps and single stage BOG compressor or ejector
6. Opex High due to requirement of additional set of HP LNG pumps and mechanical BOG compressor Low due to possible elimination of additional set of HP LNG Pumps, and BOG ejector instead of mechanical compressor
7. Operability Operability is difficult due to HP Pump and Recondenser. Operability is much simpler due to shell & tube BOG condenser and no cavitation problem in HP LNG pumps
8. Reliability Relatively lower because of more rotating equipment and complex BOG recondenser Relatively higher because of lower rotating equipment and simpler shell & tube BOG condenser
ADVANTAGES
[0040] The present invention comprises by Adding additional impellers / stages to InTank LNG pumps and eliminating the requirement of HP LNG pump which is expensive
[0041] The present invention discloses a conventional Shell and Tube condenser which is easy to operate instead of a complex re-condenser which is difficult to operate.
[0042] The present invention discloses a Single stage high pressure natural gas motive fluid ejector to compress the BOG compressor the BOG from atmosphere instead of two stage BOG compressor
[0043] The present invention discloses a Single stage conventional mechanical compressor to compress the BOG compressor instead of two stage BOG compressor
[0044] The present invention discloses a Lower Capital Investment for LNG Regas Terminals
[0045] The present invention discloses a Lower Operating Cost due to elimination of capital equipment of the current design
[0046] The present invention discloses a higher reliability of the terminal due to lower rotating equipment.
[0047] The present invention discloses better operability due to elimination of complex BOG condenser
[0048] The present invention discloses minimization of natural gas flaring / venting for want of adequate send-out capacity.
[0049] The present invention minimizes the BOG (natural gas) venting and power consumption due to which CO2 emission is drastically reduced compared to the conventional / current design.
[0050] The invention as described above refers to a preferred embodiment. Naturally, while the principle of the invention remains the same, the details of construction and the embodiments may widely vary with respect to what has been described and illustrated purely by way of the example, without departing from the scope of the present invention. , Claims:WE CLAIM:

1. A Boil-Off Gas (BOG) recondensation device, comprising:
a BOG inlet for receiving boil-off gas from a storage tank;
an LNG In tank pump (1) for supplying LNG for the recondensation,
wherein the in-tank pump comprises an impeller to generate medium pressure for send-out pressure operations;
a compressor for pressurizing the BOG;
a heat exchanger (5) configured to recondense the BOG;
a storage vessel for storing the recondensed BOG.

2. The device as claimed in claim 1, wherein the device further comprises a vaporiser (6) for vaporising pre-heated LNG from the heat exchanger (5).

3. The device as claimed in claim 1, wherein the heat exchanger (5) is a shell and tube condenser.

4. The device as claimed in claim 1, wherein the compressor may be selected from an ejector compressor (3) or a mechanical compressor (4).

5. A method for processing BOG, comprising:
receiving BOG from a storage tank;
pressurizing the recondensed BOG using a compressor;
recondensing the BOG into liquid using a heat exchanger (5),
wherein LNG In tank pump (1) supplies LNG for the recondensation, and
wherein the in-tank pump (1) comprises an impeller to generate medium pressure for send-out pressure operations.

6. A method as claimed in claim 5, wherein a vaporiser (6) vaporises pre-heated LNG from the heat exchanger (5).