LNG reforming system and method of controlling the same

A liquid natural gas (LNG) reforming system of the present invention may include a reformer provided to receive LNG from an LNG tank; a CO2 PSA unit connected to the reformer and configured to extract carbon dioxide from off-gas generated from the reformer; a cooler connected to the CO2 PSA unit and configured to cool and liquefy the carbon dioxide extracted by the CO2 PSA unit using the LNG supplied from the LNG tank to the reformer; a storage tank connected to the cooler and provided to store liquid carbon dioxide of the cooler therein; and a circulation pump provided to pump the liquid carbon dioxide from the cooler into the storage tank and circulate a part of the liquid carbon oxide into the cooler.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2021-0045183, filed Apr. 7, 2021, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a system for producing hydrogen by reforming liquid natural gas (LNG).

Description of Related Art

In a method of producing hydrogen by reforming LNG, there is a problem in that a large amount of carbon oxide (CO2) is generated. Therefore, there is a demand for a technology that properly recovers the carbon dioxide generated in the reforming process such that as little carbon dioxide is released into the atmosphere as possible, and the recovery of carbon dioxide needs to be performed very efficiently.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing an LNG reforming system and a method of controlling the same, which make it possible to effectively separate carbon dioxide generated in the LNG reforming process and to store the separated dioxide in a liquid state and makes it possible to efficiently and stably maintain the pressure in the space storing liquid carbon dioxide so that carbon dioxide may be easily stored for a long time and the space occupied by the entire system may be reduced.

According to various aspects of the present invention, an LNG reforming system includes: a reformer provided to receive LNG from an LNG tank; a CO2 PSA unit connected to the reformer and configured to extract carbon dioxide from off-gas generated from the reformer; a cooler connected to the CO2 PSA unit and configured to cool and liquefy the carbon dioxide extracted by the CO2PSA unit using the LNG supplied from the LNG tank to the reformer; a storage tank connected to the cooler and provided to store liquid carbon dioxide of the cooler therein; and a circulation pump provided to pump the liquid carbon dioxide from the cooler into the storage tank and circulate a part of the liquid carbon oxide into the cooler.

The cooler may be provided with a cooling coil through which the LNG supplied from the LNG tank to the reformer passes, and the cooler may be configured to spray the liquid carbon dioxide circulated by the circulation pump onto the cooling coil.

A gaseous phase supply pipe may be provided between the storage tank and the cooler so that gaseous carbon dioxide in an upper portion of the storage tank is cooled and liquefied in the cooler.

The gaseous phase supply pipe may be provided with a gaseous phase pipe valve configured to open or close the gaseous phase supply pipe.

A liquid phase supply pipe may be provided between the storage tank and the cooler so that the liquid carbon dioxide from the storage tank may be sprayed to the cooling coil through which the LNG in the cooler passes, and a supply pump configured to pump the liquid carbon dioxide to the cooler through the liquid phase supply pipe may be provided.

The liquid phase supply pipe may be provided with a liquid phase pipe valve configured to open or close the liquid phase supply pipe.

A level control valve mounted between the circulation pump and the storage tank and configured to control an amount of the liquid carbon dioxide pumped by the circulation pump and transferred to the storage tank may be provided to maintain the level of the liquid carbon dioxide in the cooler within a predetermined reference range.

The LNG reforming system may further include a heat exchanger provided to allow heat exchange to be performed between the carbon dioxide supplied from the CO2PSA unit to the cooler and the LNG supplied from the cooler to the reformer.

In another aspect of the present invention, a method of controlling an LNG reforming system of the present invention includes: measuring a level of liquid carbon dioxide in a cooler when LNG starts to be supplied to a reformer connected to the cooler; stopping a circulation pump and driving a supply pump to supply liquid carbon dioxide in the storage tank to the cooler when the level of the liquid carbon dioxide in the cooler is less than a predetermined minimum reference level; and stopping the supply pump and driving the circulation pump to circulate the liquid carbon dioxide to the cooler when the controller determines that the level of the liquid carbon dioxide in the cooler is equal to or greater than the predetermined minimum reference level and lower than a predetermined maximum reference level.

The method may further include opening the level control valve so that the liquid carbon dioxide in the cooler is transferred to the storage tank by the circulation pump when the level of the liquid carbon dioxide in the cooler exceeds the maximum reference level.

The method may further include closing the level control valve when the level of the liquid carbon dioxide in the cooler falls within a predetermined closing range determined between the predetermined minimum reference level and the predetermined maximum reference level.

With the present invention it is possible to effectively separate carbon dioxide generated in the LNG reforming process and to store the separated dioxide in a liquid state, and it is possible to efficiently and stably maintain the pressure in the space storing liquid carbon dioxide so that carbon dioxide may be easily stored for a long time and the space occupied by the entire system may be reduced.

DETAILED DESCRIPTION

A specific structural or functional description of embodiments of the present invention disclosed in the specification or application is provided merely for the purpose of describing the exemplary embodiment according to various exemplary embodiments of the present invention. Therefore, the exemplary embodiments according to various exemplary embodiments of the present invention may be implemented in various forms, and the present invention should not be construed as being limited to the exemplary embodiments described in the specification or application.

Various changes and modifications may be made to the exemplary embodiments according to various exemplary embodiments of the present invention, and therefore various exemplary embodiments will be illustrated in the drawings and described in the specification or application. However, it should be understood that embodiments according to the concept of the present invention are not limited to the disclosed exemplary embodiments of the present invention, but the present invention includes all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention.

Such terms as “a first” and/or “a second” may be used to described various elements, but the elements should not be limited by these terms. These terms are intended merely to distinguish one element from other elements. For example, a first element may be named a second element and similarly a second element may be named a second element without departing from the scope of protection of the present invention.

In the case where an element is referred to as being “connected” or “accessed” to other elements, it should be understood that not only the element is directly connected or accessed to the other elements, but also another element may exist between them. Contrarily, in the case where a component is referred to as being “directly connected” or “directly accessed” to any other component, it should be understood that there is no component therebetween. The other expressions of describing a relation between structural elements, i.e. “between” and “merely between” or “neighboring” and “directly neighboring”, should be interpreted similarly to the above description.

The terms used in various exemplary embodiments of the present invention are merely used to describe specific embodiments, and are not intended to limit the present invention. A singular expression may include a plural expression unless they are definitely different in a context. As used herein, the expression “include” or “have” are intended to specify the existence of mentioned features, numbers, steps, operations, elements, components, or combinations thereof, and should be construed as not precluding the possible existence or addition of one or more other features, numbers, steps, operations, elements, components, or combinations thereof.

Unless defined otherwise, all terms used herein, including technical and scientific terms, have the same meaning as those commonly understood by a person skilled in the art to which various exemplary embodiments of the present invention pertains. Such terms as those defined in a generally used dictionary may be interpreted to have the meanings equal to the contextual meanings in the relevant field of art, and are not to be interpreted to have ideal or excessively formal meanings unless clearly defined in various exemplary embodiments of the present invention.

Hereinafter, various exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The same or like reference signs presented in the drawings designate the same or like elements.

Referring toFIG.1andFIG.2, an LNG reforming system according to various exemplary embodiments of the present invention includes: a reformer3provided to receive LNG from an LNG tank1; a CO2pressure swing adsorption (PSA) unit5configured to extract carbon dioxide from off-gas generated from the reformer3; a cooler7configured to cool and liquefy the carbon dioxide extracted by the CO2PSA unit5using the LNG supplied from the LNG tank1to the reformer3; a storage tank9provided to store liquid carbon dioxide of the cooler7; and a circulation pump11provided to pump the liquid carbon dioxide from the cooler7into the storage tank9and circulate the liquid carbon oxide into the cooler7.

Furthermore, an LNG pump13is provided to pump the LNG from the LNG tank1toward the reformer3, a vaporizer15is provided to vaporize the LNG that has passed through the cooler7and supply it to the reformer3, and an off-gas compressor17is provided to compress the off-gas generated from the reformer3and to supply the off-gas to the CO2PSA unit5.

For reference, the off-gas other than carbon dioxide extracted from the CO2PSA unit5is supplied to a burner19of the reformer3to be burned.

The cooler7is provided with a cooling coil21through which the LNG supplied from the LNG tank1to the reformer3passes, and is configured to spray the liquid carbon dioxide circulated by the circulation pump11onto the cooling coil21.

That is, liquid carbon dioxide is stored under the cooler7and the cooling coil21is located above the cooler7so that, when the liquid carbon dioxide is sprayed toward the cooling coil21as described above, the sprayed liquid carbon dioxide is cooled and dropped through contact with the cooling coil21, effectively reducing the temperature in the cooler7and thus efficiently cooling and condensing the gaseous carbon dioxide supplied from the CO2PSA unit5or the storage tank9, converting the gaseous carbon dioxide into a liquid phase.

To the present end, a gaseous phase supply pipe23is provided between the storage tank9and the cooler7so that gaseous carbon dioxide above the storage tank9may be cooled and liquefied in the cooler7.

The gaseous phase supply pipe23may be provided with a gaseous phase pipe valve25configured for opening and closing the pipe. The gaseous phase pipe valve25may be configured to be opened or closed through separate control, but may be configured as a simple check valve so that the gaseous carbon dioxide from the storage tank9is configured for flowing only in a direction toward the cooler7.

The flow of the gaseous carbon dioxide in the gaseous phase supply pipe23is basically made by a pressure difference between the pressure in the cooler7and the pressure in the storage tank9.

That is, when condensation of the gaseous carbon dioxide by cooling occurs in the cooler7, the pressure is lowered so that gaseous carbon dioxide naturally generated in the storage tank9is automatically introduced into the cooler7due to the pressure difference. Therefore, the storage tank9is configured for storing carbon dioxide for a long time period without having a separate gaseous carbon dioxide liquefaction facility to maintain the internal pressure, and such an action is configured for being spontaneously performed without driving a separate pump or the like.

Meanwhile, to ensure that the liquid carbon dioxide in the storage tank9may be sprayed to the cooling coil21through which the LNG in the cooler7passes, a liquid phase supply pipe27may be provided between the storage tank9and the cooler7, and a supply pump29is provided to pump liquid carbon dioxide to the cooler7through the liquid phase supply pipe27.

That is, in the state in which the level of the liquid carbon dioxide in the cooler7is too low, the supply pump29is driven to supply the liquid carbon dioxide from the storage tank9to the cooler7and spray the liquid carbon oxide to the cooling coil21.

Of course, the supply pump29may be provided on the liquid phase supply pipe27or may be provided in the storage tank9.

The liquid phase supply pipe27may be provided with a liquid phase pipe valve31configured for opening and closing the pipe, and the liquid phase pipe valve31may be configured to be opened or closed through separate control, but may be configured as a simple check valve so that the liquid carbon dioxide from the storage tank9is configured for flowing only in the direction toward the cooler7and the liquid carbon dioxide pumped by the circulation pump11is not configured for flowing back to the storage tank9.

As described above, the cooler7is configured for effectively cooling and condensing the gaseous carbon dioxide extracted from the CO2PSA unit5and the gaseous carbon dioxide generated from the storage tank9using LNG supplied to the reformer3, and the continuous cooling function of the cooler7is configured for being maintained using the liquid carbon dioxide and the liquid carbon dioxide is configured for being stored in the storage tank9for a long time period, whereby it is possible to contribute to reducing the space required for facilities of the LNG reforming system.

Meanwhile, to maintain the level of the liquid carbon dioxide in the cooler7within a predetermined reference range, a level control valve33is provided to control the amount of liquid carbon dioxide pumped by the circulation pump11and transferred to the storage tank9.

InFIG.2, the level control valve33is configured to be controlled by a level controller37which is configured to receive a current level value PV, which is a signal of the level sensor35that measures the level of the liquid carbon dioxide in the cooler7, and to control the level control valve33while comparing the current level value PV with a set value SP. The level controller37may be configured as a simple on-off controller rather than a proportional derivation (PD) controller or a proportional integral derivation (PID) controller.

Here, to exclude a situation in which the level of the liquid carbon dioxide in the cooler7is too low to circulate the liquid carbon dioxide by the circulation pump11and thus to smoothly perform cooling, and to exclude a situation in which the level of the liquid carbon dioxide in the cooler7is too high so that the cooling coil21is immersed in liquid carbon dioxide, which makes it difficult to smoothly cool and condense gaseous carbon dioxide, the reference range may be determined by design through a number of tests and analysis, and may be set as a range between a minimum reference level and a maximum reference level to be described later.

FIG.3illustrates another exemplary embodiment of the LNG reforming system according the present invention. The configuration of the present exemplary embodiment of the present invention is the same as that of the exemplary embodiment ofFIG.1, except that the former further includes a heat exchanger39provided to allow heat exchange to be performed between the carbon dioxide supplied from the CO2PSA unit5to the cooler7and the LNG supplied from the cooler7to the reformer3.

When the heat exchanger39is further provided as described above, the carbon dioxide supplied from the CO2PSA unit5to the cooler7is first cooled by the LNG directed from the cooler7to the reformer3, and then is to be supplied to the cooler7. Thus, it is possible to reduce the capacity of the cooler7and to reduce the amount of heat used to drive a carburetor15configured to vaporize the LNG, which makes it possible to operate a more energy efficient LNG reforming system.

Referring toFIG.4, a method of controlling an LNG reforming system according to various exemplary embodiments of the present invention may include: measuring a level of liquid carbon dioxide in a cooler7when LNG starts to be supplied to a reformer3(S10); stopping a circulation pump11and driving a supply pump29to supply liquid carbon dioxide in the storage tank9to the cooler7when the level of the liquid carbon dioxide in the cooler7is less than a predetermined minimum reference level (S20); and stopping the supply pump29and driving the circulation pump11to circulate the liquid carbon dioxide to the cooler7when the level of the liquid carbon dioxide in the cooler7is equal to or greater than the predetermined minimum reference level and lower than a predetermined maximum reference level (S30).

That is, it is determined whether to drive only the circulation pump11or the supply pump29according to the level of liquid carbon dioxide in the cooler7.

The predetermined minimum reference level may be set through a number of tests and analysis in consideration of the minimum level of liquid carbon dioxide at which the state in which the liquid carbon dioxide in the cooler7is sprayed to the cooling coil from the upper side of the cooler7is maintained smoothly and stably through the driving of the circulation pump11, and the cooling action achieved thereby. For example, the predetermined minimum reference level may be set to, for example, 20% of the maximum level in which the cooling coil21is not immersed in the liquid carbon dioxide.

Accordingly, when the cooler7is in the state in which the level of the liquid carbon dioxide is equal to or greater than the predetermined minimum reference level and lower than the maximum reference level, only the circulation pump11is driven to continuously maintain the state in which the liquid carbon dioxide in the cooler7is pumped to the upper side of the cooler7and sprayed.

In various exemplary embodiments of the present invention, when the level of the liquid carbon dioxide in the cooler7exceeds the maximum reference level, it is possible to allow the liquid carbon dioxide in the cooler7to be transferred to the storage tank9by the circulation pump11by opening the level control valve33(S40).

The maximum reference level may be determined by design through a number of tests and analysis such that the cooling coil21of the cooler7may be prevented from being immersed in liquid carbon dioxide and an appropriate level of space for receiving and cooling gaseous carbon dioxide flowing into the cooler7may be secured. For example, the maximum reference level may be set to 70% to 90% of the maximum level or the like.

Meanwhile, the method may further include closing the level control valve33when the level of the liquid carbon dioxide in the cooler7falls within a predetermined closing range determined between the predetermined minimum reference level and the maximum reference level (S50).

That is, when the level of the liquid carbon dioxide in the cooler7exceeds the maximum reference level and the level control valve33is opened so that the level of the liquid carbon dioxide in the cooler7falls within the above-mentioned closing range after the opening, the level control valve33is closed to stop the liquid carbon dioxide from moving from the cooler7to the storage tank9so that the level of the liquid carbon dioxide in the cooler7is at an appropriate level.

Therefore, it may be desirable to set the closing range to a range close to the predetermined minimum reference level in a range equal to or greater than the predetermined minimum reference level to ensure that while preventing frequent opening and closing of the level control valve33, the level of the liquid carbon dioxide in the cooler7may be stably maintained by driving only the circulation pump11, and the closing range may also be determined by design through a number of tests and analysis.

Of course, the closing range may be applied when the control of the level control valve33is implemented by a simple on-off controller.

The control device may be at least one microprocessor operated by a predetermined program which may include a series of commands for carrying out the method disclosed in the aforementioned various exemplary embodiments of the present invention.

The aforementioned invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which may be thereafter read by a computer system. Examples of the computer readable recording medium include hard disk drive (HDD), solid state disk (SSD), silicon disk drive (SDD), read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy discs, optical data storage devices, etc and implementation as carrier waves (e.g., transmission over the Internet).

In various exemplary embodiments of the present invention, each operation described above may be performed by a control device, and the control device may be configured by multiple control devices, or an integrated single control device.

In various exemplary embodiments of the present invention, the control device may be implemented in a form of hardware or software, or may be implemented in a combination of hardware and software.