Control system for reliquefaction system of ship

Abstract

Disclosed is a control system for a ship reliquefaction system. A control system for a ship reliquefaction system according to the present invention is used in a reliquefaction system in which boil-off gas generated from a ship storage tank is compressed in a compressor, and the compressed boil-off gas is cooled by a heat exchanger and thus reliquefied. The control system is characterized by comprising: a reliquefaction line connected from the compressor to the storage tank; a control valve provided downstream of the heat exchanger in the reliquefaction line and controlling a flow rate of the boil-off gas flowing through the reliquefaction line; a flow rate regulator for controlling the opening degree of the control valve; and a load controller transmitting a signal for adjusting a load of the reliquefaction system to the flow rate adjuster. When the load of the reliquefaction system changes, the load controller adjusts the load of the reliquefaction system by delivering a predetermined set point for the boil-off gas flow rate at the corresponding load to the flow rate adjuster.

Description

Technical Field

[0001] The present invention relates to a control system for a reliquefaction system for a ship, which enables boil-off gas (BOG) generated from liquefied gas stored in an on-board storage tank to be cooled and reliquefied and can quickly control the reliquefaction system in response to a change in the load of the reliquefaction system. Background Art

[0002] Natural gas contains methane as a main component and is favored as an eco-friendly fuel that emits little or no environmental pollutants during combustion. Liquefied natural gas (LNG) is obtained by cooling natural gas to about -163°C at normal pressure, and since the volume of LNG is about 1 / 600 of the volume of natural gas in a gaseous state, it is suitable for long-distance maritime transportation. Therefore, natural gas is stored and transported in the form of LNG, which is easy to store and transport.

[0003] Since natural gas is liquefied at a low temperature of -163°C at normal pressure, LNG storage tanks are usually insulated to keep LNG in a liquid state. However, even though they are insulated, the ability of such storage tanks to block external heat is still limited. Therefore, since external heat continuously transfers to the LNG storage tank, the LNG stored in the LNG tank continuously evaporates naturally during transportation, generating boil-off gas (BOG).

[0004] The continuous generation of boil-off gas in the LNG storage tank increases the internal pressure of the LNG storage tank. If the internal pressure of the storage tank exceeds a predetermined safety pressure, this may lead to an emergency situation such as the rupture of the storage tank. Therefore, it is necessary to discharge the boil-off gas from the storage tank using a safety valve. However, boil-off gas is a loss of LNG and is an important issue for the transportation efficiency and fuel efficiency of LNG. Therefore, various methods are adopted to dispose of the boil-off gas generated in the LNG storage tank.

[0005] Recently, a method of using boil-off gas at a fuel demand site (e.g., the engine of a ship), a method of reliquefying boil-off gas and returning the reliquefied boil-off gas to the LNG storage tank, and a method of combining these two methods have been developed and put into use. Summary of the Invention

[0006] Technical Problem

[0007] Methods for re-liquefying evaporation gas include methods using a refrigeration cycle (the refrigeration cycle uses a separate refrigerant and re-liquefies the evaporation gas by heat exchange with the refrigerant), methods using the evaporation gas itself as a refrigerant without using a separate refrigerant to re-liquefy the evaporation gas, and similar methods.

[0008] As a method of using the evaporation gas itself as a refrigerant without using a separate refrigerant to re-liquefy the evaporation gas, a re-liquefaction system has been developed and applied to ships. The re-liquefaction system is configured to cool the compressed evaporation gas by heat exchange with the uncompressed evaporation gas and re-liquefy the cooled evaporation gas by adiabatic expansion.

[0009] Systems using a separate refrigeration cycle include, for example, systems that use nitrogen as a refrigerant in the re-liquefaction process.

[0010] Although this re-liquefaction process using nitrogen refrigerant is relatively less efficient compared to the re-liquefaction cycle using a mixed refrigerant, it is safer due to the inert nature of the nitrogen refrigerant and is more easily applicable to ships because the nitrogen refrigerant does not undergo a phase change.

[0011] In this way, the evaporation gas cooled by heat exchange with a separate refrigerant or the evaporation gas itself is introduced into a separator, where the evaporation gas is separated into a gas phase and a liquid phase, and the resulting re-liquefied gas is returned to the storage tank.

[0012] One aspect of the present invention provides a re-liquefaction system and a method of operating the re-liquefaction system, which enables effective load adjustment according to the amount of evaporation gas generated in the storage tank and to be re-liquefied, thus ensuring efficient operation.

[0013] Technical solution

[0014] According to one aspect of the present invention, there is provided a control system for a re-liquefaction system for a ship, in which evaporation gas generated from liquefied gas in an on-board storage tank is compressed in a compressor, and the compressed evaporation gas is cooled and re-liquefied by a heat exchanger. The control system includes:

[0015] A re-liquefaction pipeline connecting the compressor to the storage tank;

[0016] A control valve provided downstream of the heat exchanger in the re-liquefaction pipeline to adjust the flow rate of the evaporation gas flowing through the re-liquefaction pipeline;

[0017] A flow rate regulator configured to control the opening degree of the control valve; and

[0018] A load controller, configured to send a signal for adjusting the load of the re-liquefaction system to the flow rate regulator,

[0019] wherein, when the load of the re-liquefaction system changes, the load controller sends a pre-determined set point indicating the evaporation gas flow rate at the corresponding load to the flow rate regulator to adjust the load of the re-liquefaction system.

[0020] Preferably, the control system further includes: a temperature regulator, configured to detect the temperature of the compressed evaporation gas downstream of the heat exchanger on the re-liquefaction pipeline and send an adjustment value for finely adjusting the load of the re-liquefaction system; and a calculation unit, configured to receive the pre-determined set point from the load controller and the adjustment value from the temperature regulator and send an adjusted set point for adjusting the opening degree of the control valve to the flow rate regulator.

[0021] Preferably, the control system further includes: a flow meter, disposed upstream of the heat exchanger on the re-liquefaction pipeline to detect the flow rate of the evaporation gas introduced into the heat exchanger and send the detected data to the flow rate regulator; and a temperature detector, configured to detect the temperature of the compressed evaporation gas downstream of the heat exchanger on the re-liquefaction pipeline and send the detected data to the temperature regulator.

[0022] Preferably, when the load of the re-liquefaction system changes, the flow rate regulator and the temperature regulator operate in a master-slave mode to enhance the response to load changes.

[0023] Preferably, the re-liquefaction system further includes a refrigerant circulation system, the refrigerant circulation system includes: a refrigerant circulation pipeline through which the refrigerant circulates, the refrigerant exchanges heat with the compressed evaporation gas in the heat exchanger; a refrigerant expander, configured to expand and cool the refrigerant to be supplied to the heat exchanger; and a refrigerant compressor, configured to compress the refrigerant discharged from the heat exchanger after heat exchange.

[0024] Preferably, the refrigerant circulated through the refrigerant circulation system is nitrogen, and in the case where the load of the re-liquefaction system changes, the mass flow rate of the refrigerant is adjusted by discharging some refrigerant from the refrigerant circulation system or adding more refrigerant to the refrigerant circulation system, so as to adjust the amount of cold heat supplied to the re-liquefaction system.

[0025] Preferably, the evaporated gas cooled by the heat exchanger undergoes gas-liquid separation in a separator, the separated liquefied gas is supplied to the storage tank, and the flash gas separated in the separator merges with the evaporated gas to be introduced into the compressor and is supplied to the compressor after undergoing cold heat recovery in the heat exchanger.

[0026] Advantageous Effects

[0027] The control system according to the present invention enables the evaporated gas to be re-liquefied to be cooled more effectively by utilizing both the cold heat of the evaporated gas itself and the cold heat in the refrigerant circulation, thereby providing an enhanced re-liquefaction rate.

[0028] In the case where the load of the re-liquefaction system changes due to a change in the amount of evaporated gas generated in the storage tank and to be re-liquefied, the control system according to the present invention can quickly adjust the load of the re-liquefaction system by adjusting the flow rate of the evaporated gas introduced into the heat exchanger. Description of the Drawings

[0029] Figure 1 is a schematic diagram of a control system for a re-liquefaction system for a ship according to an embodiment of the present invention.

[0030] Figure 2 is a graph showing the change in the flow rate of the evaporated gas according to the load of the re-liquefaction system under the control of the control system according to the present invention.

[0031] Figure 3 is for Figure 1 a more detailed view of the control system for the re-liquefaction system shown in Detailed Description of the Invention

[0032] To fully understand the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings showing the preferred embodiments of the present invention and their descriptions.

[0033] Hereinafter, exemplary embodiments of the present invention will be described in detail in terms of the features and effects of the present invention with reference to the accompanying drawings. It should be noted that throughout the specification and all the drawings, the same components will be denoted by the same reference numerals.

[0034] As used herein, the term "ship" may refer to any type of ship provided with a liquefied gas storage tank. For example, the ship may include: self-propelled vessels, such as LNG carriers, liquid hydrogen carriers, and LNG regasification vessels (RVs); and non-self-propelled floating offshore structures, such as floating production storage and offloading (FPSO) units and floating storage regasification units (FSRUs).

[0035] In addition, embodiments of the present invention may be applied to any type of liquefied gas re-liquefaction cycle that can be transported in a liquid state by liquefaction at low temperature and can generate evaporation gas during storage. For example, such liquefied gases may include liquefied petrochemical gases, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene, and liquefied propylene gas. In the following embodiments, LPG, a typical liquefied gas, will be used as an example to illustrate the present invention.

[0036] Figure 1 is a schematic diagram of a control system for a re-liquefaction system for a ship according to an embodiment of the present invention.

[0037] Referring to Figure 1 , the control system according to this embodiment is used in a re-liquefaction system for a ship, in which evaporation gas generated from liquefied gas in an on-board storage tank (not shown) is compressed by a compressor, cooled and re-liquefied by a heat exchanger, and returned to the storage tank to quickly adjust the load of the re-liquefaction system in response to a change in the load of the re-liquefaction system.

[0038] The re-liquefaction system includes a compressor (not shown), an evaporation gas supply line (not shown), and a reliquefication line (RL). The compressor is configured to receive and compress evaporation gas. The evaporation gas from the storage tank is supplied to the compressor through the evaporation gas supply line. The reliquefication line connects the compressor to the storage tank and enables the evaporation gas to be re-liquefied and returned to the storage tank along the reliquefication line.

[0039] The vapor supply line extends from the storage tank to the compressor through the heat exchanger (100) such that the uncompressed vapor from the storage tank is supplied to the compressor for compression after supplying cold heat to the heat exchanger.

[0040] A compressor (not shown) can compress the vapor to a specific pressure, for example, to the fuel supply pressure required for the main engine of the ship. For example, the compressor can compress the vapor to a pressure of about 5.5 barg for a DF engine, to a pressure of about 15 barg for an X-DF engine, and to a pressure of about 300 barg for an ME-GI engine. The compressed vapor can be supplied as fuel to the demand site (such as the main engine (not shown) of the ship), and the excess vapor can be re-liquefied along the re-liquefaction line (RL) through the re-liquefaction system.

[0041] The compressed vapor from the compressor is introduced into the heat exchanger (100) along the re-liquefaction line (RL) to be cooled by heat exchange. The cooled vapor undergoes gas-liquid separation in a separator (not shown), and the separated re-liquefied gas is returned to the storage tank.

[0042] In the heat exchanger (100), the vapor is cooled by both the refrigerant circulating through the refrigerant circulation system and the cold heat of the uncompressed vapor to be introduced into the compressor.

[0043] The refrigerant circulation system (not shown) includes a refrigerant circulation line (not shown) through which the refrigerant circulates. The refrigerant circulation line can be provided with a refrigerant expander (not shown), a refrigerant compressor (not shown), and a motor. The refrigerant expander is configured to expand and cool the refrigerant to be supplied to the heat exchanger, the refrigerant compressor is configured to compress the refrigerant discharged from the heat exchanger, and the motor is configured to drive the refrigerant compressor. The refrigerant compressor and the refrigerant expander can be connected to each other through a common shaft to use the expansion energy of the refrigerant to compress the refrigerant in the refrigerant compressor, thereby reducing the power consumption required for treating the refrigerant circulation.

[0044] For example, nitrogen (N2) can be used as the refrigerant circulating through the refrigerant circulation line to supply cold heat to the heat exchanger. The amount of cold heat delivered to the heat exchanger can be adjusted by adding more nitrogen refrigerant from a refrigerant inventory system (not shown) to the circulation line or by discharging some refrigerant from the circulation line.

[0045] The compressed refrigerant from the refrigerant compressor is cooled by the heat exchanger (100) while circulating through the refrigerant circulation pipeline, expanded and cooled by the refrigerant expander, and supplied to the heat exchanger (100) as a refrigerant. Therefore, in the heat exchanger (100), four streams including the evaporated gas stream compressed by the compressor and to be re-liquefied, the uncompressed evaporated gas stream to be introduced into the compressor, the refrigerant stream expanded and cooled by the refrigerant expander, and the refrigerant stream compressed by the refrigerant compressor will undergo heat exchange.

[0046] The cooled evaporated gas from the heat exchanger can undergo gas-liquid separation in a separator (not shown) after passing through a control valve (200) provided downstream of the heat exchanger. Then, the separated re-liquefied gas from the separator (not shown) can be supplied to a storage tank for re-storage, and the resulting flash gas can be combined with the uncompressed evaporated gas stream upstream of the heat exchanger in the evaporated gas supply pipeline, or can be transported to a Gas Combustion Unit (GCU).

[0047] A change in the amount of evaporated gas generated in the storage tank, a change in the amount of evaporated gas consumed as fuel for engines and similar components, or a change in the composition of the evaporated gas causes a change in the amount of cooling and heating required for the re-liquefaction system, so it is necessary to adjust the load of the re-liquefaction system. Figure 1 is a schematic diagram of a control system for a re-liquefaction system for a ship according to an embodiment, and Figure 3 is Figure 1 a more detailed diagram of the control system shown.

[0048] The control system according to this embodiment is used to adjust the load of the re-liquefaction system and includes: a control valve (200) provided downstream of the heat exchanger (100) to adjust the flow rate of the evaporated gas flowing through the re-liquefaction pipeline; a flow rate regulator (FIC) configured to control the opening degree of the control valve; and a load controller (LC) configured to send a signal for adjusting the load of the re-liquefaction system to the flow rate regulator, as Figure 1 and Figure 3 shown in.

[0049] In addition, the control system includes: a temperature regulator (TIC) configured to detect the temperature of the compressed evaporated gas in the re-liquefaction pipeline downstream of the heat exchanger and send an adjustment value for fine-tuning the load of the re-liquefaction system; and a calculation unit (FX) configured to receive a predetermined set point from the load controller and an adjustment value from the temperature regulator and send an adjusted set point for adjusting the opening degree of the control valve to the flow rate regulator.

[0050] The re-liquefaction pipeline (RL) is provided with: a flowmeter (FM), arranged upstream of the heat exchanger (100) to detect the flow rate of the evaporation gas introduced into the heat exchanger (100) and send the detected data to the flow rate regulator; and a temperature detector (T1), arranged downstream of the heat exchanger (100) to detect the temperature of the compressed evaporation gas in the re-liquefaction pipeline downstream of the heat exchanger and send the detected data to the temperature regulator.

[0051] In this embodiment, when the load of the re-liquefaction system changes, the load controller (LC) sends a predetermined setpoint indicating the flow rate of the evaporation gas at the corresponding load to the flow rate regulator (FIC) to adjust the load of the re-liquefaction system.

[0052] In a re-liquefaction system using nitrogen as the refrigerant, the load adjustment of the system is mainly performed by adjusting the amount of cold and heat in the nitrogen (N2) refrigerant cycle, and secondly by controlling the temperature of the evaporation gas to be re-liquefied to achieve the load balance of the system. However, relying solely on temperature control has limitations in responding to load changes, which may lead to energy imbalance, and the energy imbalance may cause the re-liquefaction process to be unstable and even lead to an emergency shutdown.

[0053] To solve this problem, in this embodiment, when the load of the re-liquefaction system changes, the load of the re-liquefaction system is adjusted by regulating the flow rate of the evaporation gas. In addition, the flow rate regulator and the temperature regulator operate in a master-slave mode to ensure improved response to load changes and improved control accuracy.

[0054] That is to say, in the case where the load of the re-liquefaction system changes, the main load adjustment can be performed by adjusting the mass flow rate of the refrigerant in the refrigerant circulation pipeline to adjust the amount of cold and heat supplied to the re-liquefaction system, which can be achieved by discharging some refrigerant from the refrigerant circulation system or by adding more refrigerant to the refrigerant circulation pipeline. Secondly, the load controller (LC) sends a pre-determined set point (PSP) to the flow rate regulator (FIC), and the pre-determined set point (PSP) is calculated using the actual test data of the evaporation gas flow rate under the corresponding load. The flow rate regulator (FIC) then adjusts the opening degree of the control valve (200) based on the pre-determined set point to adjust the flow rate of the evaporation gas introduced into the heat exchanger along the re-liquefaction pipeline. The temperature detector (T1) detects the temperature of the compressed evaporation gas downstream of the heat exchanger, and the temperature regulator (TIC) sends an adjustment value (AJ) for fine load adjustment based on the temperature of the compressed evaporation gas detected downstream of the heat exchanger. The calculation unit (FX) calculates the adjusted set point (ASP) by applying the adjustment value received from the temperature regulator to the pre-determined set point received from the load controller, and sends the calculated adjusted set point to the flow rate regulator. Then, the flow rate regulator adjusts the opening degree of the control valve based on the received adjusted set value to adjust the flow rate of the evaporation gas introduced into the heat exchanger, thereby achieving the load balance of the re-liquefaction system.

[0055] Figure 2 It is a graph showing the change in the flow rate (tons per hour) of the evaporation gas in response to the change in the load (load, %) of the re-liquefaction system under the control system according to this embodiment. In the graph, the top dashed line (UB) represents the upper boundary of the load-dependent flow rate of the evaporation gas, and the bottom dashed line (LB) represents the lower boundary of the load-dependent flow rate of the evaporation gas. The thick straight line (PSP) represents the change in the flow rate of the evaporation gas according to the change in the load of the re-liquefaction system, and the change in the flow rate is calculated using the actual load test data. The flow rate of the evaporation gas under the corresponding load represented by the thick straight line is transmitted from the load controller to the flow rate regulator as the pre-determined set point. The shaded area (AJ) represents the adjustment value transmitted by the temperature regulator based on the temperature of the evaporation gas downstream of the heat exchanger.

[0056] It has been confirmed that, compared with the load adjustment using the temperature control method alone, the load adjustment using the master-slave method in this embodiment can improve the control speed by 35% or more than 35%, indicating a significant improvement in the response.

[0057] In addition, assuming that even when the amount of cold heat supplied to the reliquefaction system remains constant, the amount of reliquefied boil-off gas varies depending on the composition of the boil-off gas, the control system according to this embodiment performs load adjustment by adjusting the flow rate of the boil-off gas and then performs correction by a temperature regulator, thereby ensuring improved control accuracy and thus ensuring stable operation of the reliquefaction system.

[0058] Although some embodiments have been described, it will be apparent to those skilled in the art that these embodiments are given by way of example only, and that various modifications, changes, variations and equivalent embodiments may exist without departing from the spirit and scope of the present invention.

Claims

1. A control system for a re-liquefaction system of a ship, in which boil-off gas generated from liquefied gas in an on-board storage tank in the re-liquefaction system is compressed by a compressor and cooled and re-liquefied by a heat exchanger, the control system comprising: A re-liquefaction pipeline connecting the compressor to the storage tank; A control valve provided downstream of the heat exchanger on the re-liquefaction pipeline to adjust the flow rate of the boil-off gas flowing through the re-liquefaction pipeline; A flow rate regulator configured to control the opening degree of the control valve; And A load controller configured to send a signal for adjusting the load of the re-liquefaction system to the flow rate regulator, wherein in the case where the load of the re-liquefaction system changes, the load controller sends a predetermined set point indicating the flow rate of the boil-off gas at the corresponding load to the flow rate regulator to adjust the load of the re-liquefaction system.

2. The control system according to claim 1, further comprising: A temperature regulator configured to detect the temperature of the compressed boil-off gas on the re-liquefaction pipeline downstream of the heat exchanger and send an adjustment value for finely adjusting the load of the re-liquefaction system; And A calculation unit configured to receive the predetermined set point from the load controller and the adjustment value from the temperature regulator and send an adjusted set point for adjusting the opening degree of the control valve to the flow rate regulator.

3. The control system according to claim 2, further comprising: A flow meter provided upstream of the heat exchanger on the re-liquefaction pipeline to detect the flow rate of the boil-off gas introduced into the heat exchanger and send the detected data to the flow rate regulator; And A temperature detector configured to detect the temperature of the compressed boil-off gas on the re-liquefaction pipeline downstream of the heat exchanger and send the detected data to the temperature regulator.

4. The control system according to claim 3, wherein in the case where the load of the re-liquefaction system changes, the flow rate regulator and the temperature regulator operate in a master-slave mode to enhance the response to load changes.

5. The control system according to claim 3, wherein the reliquefaction system further comprises a refrigerant circulation system, the refrigerant circulation system comprising: A refrigerant circulation pipeline through which refrigerant exchanging heat with the compressed boil-off gas in the heat exchanger circulates; A refrigerant expander configured to expand and cool the refrigerant to be supplied to the heat exchanger; And a refrigerant compressor configured to compress the refrigerant discharged from the heat exchanger after heat exchange.

6. The control system according to claim 5, wherein The refrigerant circulated through the refrigerant circulation system is nitrogen, and When the load of the reliquefaction system changes, the mass flow rate of the refrigerant is adjusted by draining some of the refrigerant from the refrigerant cycle system or adding more refrigerant to the refrigerant cycle system, thereby adjusting the amount of cold and heat supplied to the reliquefaction system.

7. The control system according to any one of claims 1 to 6, wherein the boil-off gas cooled by the heat exchanger undergoes gas-liquid separation in a separator, the separated liquefied gas is supplied to the storage tank, and the flash gas separated in the separator is combined with the boil-off gas to be introduced into the compressor and is supplied to the compressor after undergoing cold and heat recovery in the heat exchanger.

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