INTEGRATION PROCESS PRINCIPLES FOR MAXIMIZING THE BOIL OFF RECOVERY ON A H2 LIQUEFIER PLANT

Abstract

A method for recovering boil-off gas from a system including one or more liquefaction trains including transport trucks or loading bays, a gaseous hydrogen feed stream, a lower-temperature cold box, and a low-pressure hydrogen compressor. The method including collecting a boil-off gas stream from the transport trucks or loading bays, determining the pressure of the boil-off gas stream, and depending on the pressure, recycling the boo-off gas stream to predetermined destinations. Wherein the boil-off gas stream has either a low-pressure, having a pressure of less than 2 bara, or a medium-pressure, having a pressure equal to or greater than 2 bara.

Description

BACKGROUND

Hydrogen liquefaction is a process well known in the art. One non-limiting example is shown in FIG. 1. Hydrogen stream 105 is introduced into higher-temperature cryogenic cold box 106. In this context, “higher-temperature” is defined as a cold box that operates at or near a temperature of 80 Kelvins (−193 C). With “at or near” being defined as meaning plus or minus 20 C, preferably plus or minus 15 C, and more preferably plus or minus 10 C. Higher-temperature cold box 106 is refrigerated by a nitrogen refrigeration cycle illustrated simplistically by nitrogen compressor 107, and thus produces cold hydrogen stream 108. Cold hydrogen stream 108 is then introduced into lower-temperature cryogenic cold box 109. In this context, “lower-temperature” is defined as a cold box that operates at or near a temperature of 20 Kelvins (−253 C). Lower-temperature cold box 109 is refrigerated by a hydrogen refrigeration cycle illustrated simplistically by low-pressure hydrogen compressor 110 and high-pressure hydrogen compressor 111, and thus produces liquid hydrogen stream 112. The skilled artisan will recognize that there are numerous other systems for liquefying hydrogen and will also recognize that FIG. 1 is a very simple representation of a much more complex system. For example, many systems utilize one or more refrigeration cycles that utilize mixed refrigerant for precooling purpose or Helium refrigerant for Hydrogen liquefaction.

Hydrogen liquefaction is an expensive process and the processing of boil-off gas (BOG) for the recovery of vaporized hydrogen molecules is extremely important. This is especially true when the gaseous hydrogen is produced at a high cost, for example through electrolyzes. BOG generation generally occurs through the liquefier cold end flash, liquid hydrogen piping, liquid hydrogen storage, during the truck loading process and afterwards during the delivery of the liquid hydrogen.

There are different ways known in the art to recover the BOG from those different areas of the plant (liquefier, storage and loading). FIG. 2 illustrates one non-limiting example of the state-of-the-art method of BOG recovery.

Feed stream 101 is introduced into hydrogen generation unit 102. Hydrogen generation unit 102 may be any system known in the art, such as a steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit 102 generates hydrogen output stream 103. Hydrogen output stream 103 may also come from an industrial complex off gas or from a pipeline. Once free of any components that may freeze in the downstream cold box (not shown), hydrogen output stream 103 passes through hydrogen output stream flow control valve 104, the function of which will be described below. Controlled hydrogen output stream 105 then enters the liquefier.

In FIG. 2, the liquefier is based on a cycle similar to the one described above, with a pre-cooling cycle (typically nitrogen) using higher-temperature cold box 106 and a nitrogen refrigeration cycle with nitrogen compression 107 and a hydrogen liquefaction cycle using lower-temperature cold box 109 and a hydrogen refrigeration cycle which includes low-pressure hydrogen compressor 110 and high-pressure hydrogen compressor 111. Once liquefied, liquid hydrogen stream 112 goes to liquid hydrogen storage 113, which is typically a vacuum insulated storage tank(s) or a sphere depending on the capacity. Downstream liquid hydrogen stream 112 is the loading area which includes multiple loading bays 115 and sometimes transfer pumps. (not shown) Liquid hydrogen stream 114 thus exits liquid hydrogen storage 113 and enters loading bay 115.

FIG. 2 also shows the typical main BOG routings. For example, the BOG 120 generated between lower-temperature cold box 109 and through liquid hydrogen storage 113 is generally recycled back into lower-temperature cold box 109. Downstream liquid hydrogen stream 112, at loading bay 115, the trucks liquid and vapor phases are both connected to the plant (not shown). Hence, the vapor phase from the truck (i.e. BOG) is collected through the BOG network, and sent via cool boil-off gas stream 116. Cool boil-off gas stream 116 is typically warmed up through BOG heater 117 and recycled via warm boil-off gas stream 118 to low-pressure hydrogen compressor 110. The BOG flow is then mixed with the hydrogen cycle flow.

First flow indicator 119 senses the flowrate of boil-off gas flowing through warm boil-off gas stream 118 and re-entering low-pressure hydrogen compressor 110. First flow indicator 119 then sends a signal to hydrogen output stream flow control valve 104 which may then adjust to regulate the total hydrogen flowrate through the liquefier. Note, it is shown that first flow indictor 119 sends a signal directly to hydrogen output stream flow control valve 104, but the skilled artisan will recognize that this communication may be controlled by a local computer, distributed control system (DCS), a programmable logic controller (PLC), or other systems known in the art.

FIG. 3 illustrates another non-limiting example of the state-of-the-art method of BOG recovery. In this example, the hydrogen is produced at a relatively low pressure (for example by using an electrolyzer) and then introduced into a feed compressor upstream of the liquefier. The main difference between this example and the one presented in FIG. 2 is that the BOG from the loading bays and truck loading is recycled into the feed line, instead of the hydrogen refrigeration loop.

This cycle has the advantage of recycling the BOG without impacting the liquefier operation. Another advantage is that the BOG flow does not affect the rated flow of the feed compressor train, since the hydrogen generation unit can be turned down to ensure that the feed compressor experiences a constant flowrate. The feed compressor may be located even further from the LP compressor and the distance between the loading area and the feed compressor may be a design concern since the maximum allowable pressure drop may be high, as the return BOG pressure may be very low (e.g. 1-2 bara).

Feed stream 101 is introduced into hydrogen generation unit 102. Hydrogen generation unit 102 may be any system known in the art, such as a steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit 102 generates hydrogen output stream 103. Hydrogen output stream 103 may also come from an industrial complex off gas or from a pipeline. Once free of any components that may freeze in the downstream cold box (not shown), Hydrogen output stream 103 passes through hydrogen output stream flow control valve 104, the function of which will be described below. Controlled hydrogen output stream 105 then combines warm boil-off gas stream 118 thus forming compressor feed stream 301. Compressor feed stream 301 is then compressed in feed compressor 302, thus forming compressed feed stream 303, which enters the liquefier.

Again, in FIG. 3, the liquefier is based on a cycle similar to the one described above, with a pre-cooling cycle (typically nitrogen) using higher-temperature cold box 106 and a nitrogen refrigeration cycle with nitrogen compression 107 and a hydrogen liquefaction cycle using lower-temperature cold box 109 and a hydrogen refrigeration cycle which includes low-pressure hydrogen compressor 110 and high-pressure hydrogen compressor 111. Once liquefied, liquid hydrogen stream 112 goes to liquid hydrogen storage 113, which is typically a vacuum insulated storage tank(s) or a sphere depending on the capacity. Downstream liquid hydrogen stream 112 is the loading area which includes multiple loading bays 115 and sometimes transfer pumps. (not shown) Liquid hydrogen stream 114 thus exits liquid hydrogen storage 113 and enters loading bay 115.

FIG. 3 shows the typical main BOG routings. For example, the BOG 120 generated between lower-temperature cold box 109 and through liquid hydrogen storage 113 is generally recycled back into lower-temperature cold box 109. Downstream liquid hydrogen stream 112, at loading bay 115, the trucks liquid and vapor phases are both connected to the plant (not shown). Hence, the vapor phase from the truck (i.e. BOG) is collected through the BOG network, and sent via cool boil-off gas stream 116. Cool boil-off gas stream 116 is typically warmed up through BOG heater 117 and recycled via warm boil-off gas stream 118 to compressor feed stream 301. The BOG flow is then mixed with the hydrogen cycle flow.

First flow indicator 119 senses the flowrate of boil-off gas flowing through warm boil-off gas stream 118 and then sends a signal to hydrogen output stream flow control valve 104 which may then adjust to regulate the total hydrogen flowrate through the liquefier. Note, it is shown that first flow indictor 119 sends a signal directly to hydrogen output stream flow control valve 104, but the skilled artisan will recognize that this communication may be controlled by a local computer, distributed control system (DCS), a programmable logic controller (PLC), or other systems known in the art.

SUMMARY

A method for recovering boil-off gas from a system including one or more liquefaction trains, the one or more liquefaction trains including transport trucks or loading bays, a gaseous hydrogen feed stream, a lower-temperature cold box, and a low-pressure hydrogen compressor. The method including collecting a boil-off gas stream from the transport trucks or loading bays, determining the pressure of the boil-off gas stream, and depending on the pressure, recycling the boil-off gas stream to predetermined destinations. Wherein the boil-off gas stream has either a low-pressure, having a pressure of less than 2 bara, or a medium-pressure, having a pressure equal to or greater than 2 bara.

A method for recovering boil-off gas from a system including one or more liquefaction trains, the one or more liquefaction trains comprising transport trucks or loading bays, a gaseous hydrogen feed stream, and a low-pressure hydrogen compressor. The method including collecting a boil-off gas stream from the transport trucks or loading bays, and recycling at least a portion of the boil-off gas stream to either the low-pressure hydrogen compressor or the gaseous hydrogen feed stream, or both.

BRIEF DESCRIPTION OF THE FIGURES

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a schematic representation of a typical hydrogen liquefaction process, as known in the art.

FIG. 2 is a schematic representation of typical BOG routings in a hydrogen production facility, hydrogen liquefaction facility, and liquid hydrogen loading station, as known in the art.

FIG. 3 is another schematic representation of typical BOG routings in a hydrogen production facility, hydrogen liquefaction facility, and liquid hydrogen loading station, as known in the art.

FIG. 4 is a schematic representation of a hydrogen production facility, hydrogen liquefaction facility, and liquid hydrogen loading station, accordance with one embodiment of the present invention.

FIG. 5 is another schematic representation of a hydrogen production facility, hydrogen liquefaction facility, and liquid hydrogen loading station, accordance with one embodiment of the present invention.

FIG. 6 is still another schematic representation of a hydrogen production facility, hydrogen liquefaction facility, and liquid hydrogen loading station, accordance with one embodiment of the present invention.

ELEMENT NUMBERS

101=feed stream

102=hydrogen generation unit

103=hydrogen stream

104=hydrogen stream flow control valve

105=gaseous hydrogen feed stream

106=higher-temperature cold box

107=nitrogen compressor

108=cold hydrogen stream

109=lower-temperature cold box

110=low-pressure hydrogen compressor

111=high-pressure hydrogen compressor

112=liquid hydrogen stream (into storage)

113=liquid hydrogen storage unit

114=liquid hydrogen stream (out of storage)

115=loading bay

116=cool boil off gas (from loading bay)

117=boil off gas heater

118=warm boil off gas

119=first flow indicator

120=boil off gas (from liquid hydrogen storage)

301=compressor feed stream

302=feed compressor

303=compressed feed stream

401=boil off gas temperature indicator

402=boil off gas pressure indicator

403=first portion (of cool boil off gas)

404=second portion (of cool boil off gas)

405=third portion (of cool boil off gas)

406=first LP flow control valve

407=MP flow control valve

408=second LP flow control valve

409=LP boil off gas heater

410=MP boil off gas heater

411=warm LP boil off gas stream

412=warm MP boil off gas stream

413=second flow indicator

414=feed bypass stream

415=feed bypass valve

501=feed stream

502=hydrogen generation unit

503=hydrogen output stream

503A-E=compressor feed streams

504A-E=feed compressors

505=compressed feed stream

505 A-E=compressed feed stream s

506=purification unit

507=purified stream

508=portion of purified stream buffer)

509=buffer compressor

510=compressed buffer stream

511=buffer tank

512=outlet buffer stream

513=buffer valve

514=regulated buffer stream

515=combined purified stream

515A-E=inlet streams

601=portion of warm BOG stream (to low-pressure hydrogen compressor)

602=second portion of warm BOG stream

603=BOG compressor

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below, While the invention is susceptible to various modifications and alternative forms. specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The present schemes maximize the recovery of Liquid Hydrogen BOG by valorizing these hydrogen molecules in one of three different ways:

• • By sending the BOG to the liquefier(s) (depending on the global plant configuration several liquefiers train may be implemented) after being compresses and re-liquefied as necessary,
  • By sending the BOG upstream of the liquefier(s), specifically at compression trains inlet, and
  • By valorizing cold BOG directly in the liquefier to benefit from its inherent cold.

For such a configuration, one important aspect of the present system is separating the BOG depending on their pressure level, and temperature conditions and designing the compression trains capacities in consistency with the truck operations. The process control philosophy of the overall system is based on the adaptability of the hydrogen feedstock load to follow BOG generation and BOG recycle.

The present system allows for a reduction of the capacity of the upstream hydrogen generation system (e.g. electrolyzers or steam methane reformer) by controlling the feed flow rate (at the inlet of the hydrogen liquefaction system) with the BOG flow rate (by turning down the production capacity of hydrogen generation unit (or Feed flow rate) simultaneously with the recovery of BOG). The combination of these different BOG recovery options on the same site also improves the mutual flexibility of the different hydrogen compressors by separating the BOG depending on their pressure level and temperature conditions in order to route them to different tie-in points with different inlet pressure to maximize the flexibility of the integrated liquefaction system and speed up the filling sequence of the trucks or increase the recovery of BOG. Finally, the speeding up of the filling sequence will allow for a reduction of the number of truck loading bays.

This document primarily focuses on the BOG generation and recovery from the loading area, which is located downstream the hydrogen liquefaction unit and downstream the liquid hydrogen storage unit. The BOG generated in the loading area presents many challenges, for example the BOG amount generated when a truck returns to the loading bay depends on the logistics chain, delivery method, distance to customers, number of customers delivered, billing method etc. As used herein, the terms “loading area” or “loading” are understood to apply to truck loading. However, one skilled in the art can recognize that the present system may be equally applicable to the loading of ships, or any other means for transporting hydrogen.

Examples of potential sources of BOG generation may be as diversified as the following operations:

• • Truck depressurization, In the case of a truck arriving on site at high pressure with a residue of liquid H2. The truck depressurization is the step that generates the highest peak flow of BOG
  • Truck filling. This operation will generate gaseous hydrogen return at low pressure, and
  • “natural heat release” around the storage systems.

Valorizing all the types of BOG simply according to FIG. 2, within liquefier by reducing the load of the hydrogen generation system have the following disadvantages:

• • A significant overdesign of the liquefier's compression train(s) is required to be able to handle the peak BOG flow, especially during truck depressurization and which could limit the turndown on the liquefier's machines,
  • A process upset on the liquefier due to management of this additional peak.

Managing the BOG only with solution principle described on FIG. 3, would also be challenging as it would lead to:

• • Depending on the suction pressure conditions of the Feed compressor, it may be impossible to recover the low-pressure BOG generated during the filling stage. Hence, this scheme may only allow for partial BOG recovery, forcing a wide range of ramp up/ramp down on feed gas compression train(s). Note that the addition of a buffer to handle this ramp/up difficulty may be cost prohibitive. In the current state-of-the-art, BOG recovery with such schemes is typically limited to ˜70%. It is important to note that when the BOG is to be recycled to the liquefier, the feed gas flowrate is to be reduced by the same amount. Hence, BOG recovery is even more valuable when the hydrogen generation comes at a high OPEX cost such as electrolysis. The H2 generation process via electrolysis is generally operated at low pressure and a feed gas compressor ir generally required before the hydrogen liquefier. Hence the following description is particularly suited when the Hydrogen is generated via electrolysis.

Turning to FIG. 4, one embodiment of the present invention is presented. This proposed system is typically applicable for liquefaction trains from about 5 to above 100 tpd. The invention consists of creating two different BOG networks and separating the high-pressure network from the low-pressure BOG recovery network. The BOG generated during truck filling (<2 bara) is typically a steady flow at low pressure and can usually simply be warmed up and routed to the warm end of the LP cycle compressor, similarly, to FIG. 2. Note that during the filling stage, the BOG recovery pressure in that network is low and so is the BOG temperature. Hence, it could also be envisaged to recover this LP BOG at the cold end of the liquefier, in a similar fashion as what is done on the storage BOG. The BOG generated during the depressurization is at a higher pressure (˜2 to 10 bara) and may be much warmer, It is warmed up and routed to the feed gas compressor located upstream of the liquefier. Ideally, each low pressure and high-pressure BOG network can be connected to every loading bay.

Hydrogen feed stream 101 is introduced into hydrogen generation unit 102.

Hydrogen generation unit 102 may be any system known in the art, such as a steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit 102 generates hydrogen output stream 103. Hydrogen output stream 103 may also come from an industrial complex off gas or from a pipeline. Once free of any components that may freeze in the downstream cold box (purification system not shown), but typically downstream of the compressors), Hydrogen output stream 103 passes through hydrogen output stream flow control valve 104, the function of which will be described below. Controlled hydrogen output stream 105 then combines warm MP BOG stream 412 thus forming compressor feed stream 301. Compressor feed stream 301 is then compressed in feed compressor 302, thus forming compressed feed stream 303, which enters the liquefier. While, typically, the medium-pressure BOG is at a lower pressure than required at the inlet of the liquefier, compressor feed stream 301 may, if necessary or desired, bypass feed compressor 302 by means of feed bypass stream 414 and feed bypass valve 415.

Again, in FIG. 4, the liquefier is based on a cycle similar to the one described above, with a pre-cooling cycle (typically nitrogen) using higher-temperature cold box 106 and a nitrogen refrigeration cycle with nitrogen compressor 107 and a hydrogen liquefaction cycle using lower-temperature cold box 109 and a hydrogen refrigeration cycle which includes low-pressure hydrogen compressor 110 and high-pressure hydrogen compressor 111. Once liquefied, liquid hydrogen stream 112 goes to liquid hydrogen storage 113. which is typically a vacuum insulated storage tank(s) or a sphere depending on the capacity. Downstream liquid hydrogen stream 112 is the loading area which includes multiple loading bays 115 and sometimes transfer pumps. (not shown) Liquid hydrogen stream 114 thus exits liquid hydrogen storage 113 and enters loading bay 115.

FIG. 4 shows the typical main BOG routings. For example, the BOG 120 generated between lower-temperature cold box 109 and through liquid hydrogen storage 113 is generally recycled back into lower-temperature cold box 109. Downstream liquid hydrogen stream 112, at loading bay 115, the trucks liquid and vapor phases are both connected to the plant (not shown). Hence, the vapor phase from the truck (i.e. BOG) is collected through the BOG network, and sent via cool BOG stream 116.

Cool BOG stream 116 is split into at least two of the three portions presented herein. The first portion of cool BOG stream portion 403, the flowrate of which is controlled by first LP flow control valve 406, is warmed up through LP BOG heater 409 and recycled via warm LP BOG stream 411 to low-pressure hydrogen compressor 110. The second portion of cool BOG stream portion 404, the flowrate of which is controlled by MP flow control valve 407, is warmed up through MP BOG heater 410 and recycled via warm MP BOG stream 412 to compressor feed stream 301. The BOG flow is then mixed with the hydrogen cycle flow. The third portion of cool BOG stream portion 405, the flowrate of which is controlled by second LP flow control valve 408, may be recycled back into lower-temperature cold box 109 as an alternative path to first portion 403.

First flow indicator 119 senses the flowrate of BOG flowing through warm MP BOG stream 412 and then sends a signal to hydrogen output stream flow control valve 104. Second flow indicator 413 senses the flowrate of BOG flowing through warm LP BOG stream 411 and then sends a signal to hydrogen output stream flow control valve 104. Hydrogen output stream flow control valve 104 then adjusts to regulate the total hydrogen flowrate through the liquefier. Note, it is shown that first flow indictor 119 sends a signal directly to hydrogen output stream flow control valve 104, but the skilled artisan will recognize that this communication may be controlled by a local computer, distributed control system (DCS), a programmable logic controller (PLC), or other systems known in the art.

Turning to FIG. 5, another embodiment of the present invention is presented. FIG. 5 is similar to FIG. 4 except that it includes 2 liquefaction trains, One skilled in the art will recognize that while two trains are illustrated for simplicity, this process scheme will be equally applicable to systems with more than 2 trains. The BOG system for both trains are connected. The benefit of the invention when there are multiple liquefaction trains is that the BOG recovery is quasi-independent of whether or not one liquefaction train is shut down.

FIG. 5 illustrates another example of the present system of BOG recovery. In this example, the hydrogen may be produced at a relatively low pressure (for example by using an electrolyzer) and then introduced into a feed compressor upstream of the liquefier. This system may also be used if the hydrogen is produced at higher pressure by bypassing the feed compressor.

This process scheme is typically applicable for liquefaction trains from about 5 to about 100 tpd and consists of producing two different BOG networks and separating the high pressure from the low-pressure truck BOG recovery networks. The BOG generated during the filling of the truck (<2 bara) is routed either to the warm end of the LP cycle compressor, similar to FIG. 2. Since the BOG recovery pressure in that network is limited, the BOG temperature leaving the truck is better managed. Hence, it is much easier to recover the LP BOG at the cold end of the liquefier, in a similar fashion as what is done on the storage BOG. The BOG generated during the depressurization are at a higher pressure (˜2 to 10 bara) and is routed to the feed gas compressor. Each BOG network may be connected to every loading bay.

Feed stream 501 is introduced into hydrogen generation unit 502. Hydrogen generation unit 502 may be any system known in the art, such as a steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit 502 generates hydrogen output stream 503. Hydrogen output stream 503 may also come from an industrial complex off gas or from a pipeline. Once free of any components that may freeze in the downstream cold box (not shown), Hydrogen output stream 503 combines with warm MP BOG stream 412 and is split into multiple compressor feed streams 503A-503E. It should be noted that while 5 separate compressor streams are indicated, one skilled in the art will recognize that this number may be as few as 2 or as many as necessary for the design of the system. Compressor feed streams 503A-503E are then compressed in feed compressors 504A-504E, thus forming compressed feed streams 505A-505E, which combine into compressed feed stream 505 which may then enter purification unit 506.

Purification unit 506 produce purified stream 507, which then enters one or more liquefaction trains A/B. It should be noted that while 2 separate liquefaction trains are indicated, one skilled in the art will recognize that this number may be many as necessary for the design of the system. At least a portion 508 of purified stream 507 may be introduced into buffer compressor 509, thereby producing compressed buffer stream 510, which may then enter buffer tank 511. Buffer tank 511, as needed, may the release outlet buffer stream 512, the flowrate of which may be regulated by buffer valve 513, thus producing regulated buffer stream 514, which may combine with purified stream 507 as needed, thus forming combined purified stream 515, which is split into inlet streams 515A/B, which the then enter multiple liquefaction trains A/B

Again, in FIG. 5, each liquefier is based on a cycle similar to the one described above, with a pre-cooling cycle (typically nitrogen) using higher-temperature cold boxes 106A/B and a nitrogen refrigeration cycles with nitrogen compressors 107A/B and a hydrogen liquefaction cycle using lower-temperature cold boxes 109A/B and a hydrogen refrigeration cycle which includes low-pressure hydrogen compressors 110A/B and high-pressure hydrogen compressors 111A/B. Once liquefied, liquid hydrogen streams 112A/B are combined into liquid hydrogen stream 112 which goes to liquid hydrogen storage 113. Liquid hydrogen storage 113 is typically a vacuum insulated storage tank(s) or a sphere depending on the capacity. Downstream liquid hydrogen stream 112 is the loading area which includes multiple loading bays 115 and sometimes transfer pumps (not shown). Liquid hydrogen stream 114 thus exits liquid hydrogen storage 113 and enters loading bay 115.

FIG. 5 shows the typical main BOG routings. For example, the BOG 120 generated between lower-temperature cold box 109 and through liquid hydrogen storage 113 is split into BOG streams 120A/B recycled back into lower-temperature cold boxes 109A/B respectively. Downstream liquid hydrogen stream 112, at loading bay 115, the trucks liquid and vapor phases are both connected to the plant (not shown), Hence, the vapor phase from the truck (i.e. BOG) is collected through the BOG network, and sent via cool BOG stream 116.

Cool BOG stream 116 is split into at least two portions. The first portion of cool BOG stream portion 403, the flowrate of which is controlled by first LP flow control valve 406, is warmed up through LP BOG heater 409, thus producing warm LP BOG stream 411. Warm LP BOG stream 411 is then split into warm LP BOG streams 411A/B which are recycled to low-pressure hydrogen compressors 110A/B. The second portion of cool BOG stream portion 404, the flowrate of which is controlled by MP flow control valve 407, is warmed up through MP BOG heater 410 and recycled via warm MP BOG stream 412 to compressor feed stream 503.

Turning to FIG. 6, another embodiment of the present invention is presented. FIG. 6 is an alternative solution to FIG. 4 above. In FIG. 6, the LP and HP BOG streams are combined similarly to FIG. 2 or FIG. 3. Since the average BOG flow of a truck during an entire loading sequence is relatively small, and depressurization BOG peaks are more rare, it may make sense that the BOG is preferentially sent to the LP compressor. In the case of peak BOG flow, for example during depressurization of a truck, BOG compressor 603 may be added in order to recycle the extra BOG flow at the hydrogen liquefaction system feeds inlet by increasing the pressure of the extra BOG flow up to the suction pressure of the feed system (typically in the above scheme, up to the suction of the feed compressor).

FIG. 6 illustrates another example of the present system of BOG recovery. In this example, the hydrogen may be produced at a relatively low pressure (for example by using an electrolyzer) and then introduced into a feed compressor upstream of the liquefier. This system may also be used if the hydrogen is produced at higher pressure by bypassing the feed compressor.

This process scheme is typically applicable for liquefaction trains from about 5 to about 100 tpd and consists of producing one single BOG network. BOG generated during filling are at low pressure and preferentially routed to the LP compressor of the liquefier. BOG generated during depressurization are at a higher pressure and also much higher flowrate. These BOG are all collected and letdown to the same low-pressure level and recycled preferentially to the LP compressor. The extra capacity that cannot be handled by the LP compressor (for example during a peak of BOG) is routed to the BOG compressor and/or feed gas compressor. Depending on the suction conditions of feed gas compressor 302, BOG compressor 603 may or may not be required.

Hydrogen feed stream 101 is introduced into hydrogen generation unit 102. Hydrogen generation unit 102 may be any system known in the art, such as a steam methane reformer (SMR) or Electrolysis. Hydrogen generation unit 102 generates hydrogen output stream 103. Hydrogen output stream 103 may also come from an industrial complex off gas or from a pipeline. Hydrogen output stream 103 then combines warm BOG stream 602 thus forming compressor feed stream 301. Compressor feed stream 301 is then compressed in feed compressor 302, thus forming compressed feed stream 303, which enters the liquefier.

Again, in FIG. 6, the liquefier is based on a cycle similar to the one described above, with a pre-cooling cycle (typically nitrogen) using higher-temperature cold box 106 and a nitrogen refrigeration cycle with nitrogen compressor 107 and a hydrogen liquefaction cycle using lower-temperature cold box 109 and a hydrogen refrigeration cycle which includes low-pressure hydrogen compressor 110 and high-pressure hydrogen compressor 111. Once liquefied, liquid hydrogen stream 112 goes to liquid hydrogen storage 113, which is typically a vacuum insulated storage tank(s) or a sphere depending on the capacity. Downstream liquid hydrogen stream 112 is the loading area which includes multiple loading bays 115 and sometimes transfer pumps, (not shown) Liquid hydrogen stream 114 thus exits liquid hydrogen storage 113 and enters loading bay 115.

FIG. 6 shows a typical main BOG routing. For example, the BOG 120 generated between lower-temperature cold box 109 and through liquid hydrogen storage 113 is generally recycled back into lower-temperature cold box 109. Downstream liquid hydrogen stream 112, at loading bay 115, the trucks liquid and vapor phases are both connected to the plant (not shown). Hence, the vapor phase from the truck (i.e. BOG) is collected through the BOG network, and sent via cool BOG stream 116.

Cool BOG stream 116 is warmed up through BOG heater 117. At least a portion 601 of warm BOG stream 118 is recycled to low-pressure hydrogen compressor 110. Another portion 602 of warm BOG stream 118 is then mixed with the hydrogen output stream 103.

ADVANTAGES

Operating range of the LP compressor is much narrower and LP compressor design gets simpler. The LP compressor can easily operate considering 0 BOG returning to the liquefier but also can well perform if multiple trucks are being filled simultaneously. The ramp-up and down between the running cases with or without BOG recovering is therefore reduced and the reactivity of the LP cycle compressor is enhanced

The liquefier production capacity becomes independent from the truck BOG recycle to the LP machine since LP BOG represent a much smaller and much more stable flow

As mentioned above, the Feed gas compressor system design can handle the depressurization BOG by reducing the production of the upstream H2 generation unit The BOG flow is typically much smaller compared to the nominal capacity of the feed compressor; hence the depressurization BOG flow can be increased and the depressurization time decreased. That way, the entire loading process duration might be reduced to the point that it may be possible to consider one less loading bay for the design of the plant. The ramping up and down of the integrated system to recover depressurization BOG is therefore beneficiating from the mutualized flexibility of the hydrogen generation system as well as from the Feed compressor.

BOG recovery is maximized (up to more than 90% recovery) and the sizing of the equipment is very little dependent on the BOG flowrate recycled.

Claims

1. A method for recovering boil-off gas from a system comprising one or more liquefaction trains, the one or more liquefaction trains comprising: transport trucks or loading bays,a gaseous hydrogen feed stream,a lower-temperature cold box, anda low-pressure hydrogen compressor;

2. The method of claim 1, wherein at least a portion of the low-pressure boil-off gas stream is recycled to the lower-temperature cold box.

3. The method of claim 1, wherein at least a portion of the low-pressure boil-off gas stream is recycled to the low-pressure hydrogen compressor.

4. The method of claim 1, wherein at least a portion of the medium-pressure boil-off gas stream is recycled to the gaseous hydrogen feed stream, thereby forming a compressor feed stream.

5. The method of claim 4, further comprising: a hydrogen generation unit configured to produce a gaseous hydrogen stream,a hydrogen stream flow control valve, anda first boil-off gas flow indicator,

6. The method of claim 1, further comprising a liquid hydrogen storage unit, wherein the liquid hydrogen storage unit provides liquid hydrogen to the transport trucks or loading bays and collecting a second boil-off gas stream from the liquid hydrogen storage unit and recycling the second boil-off gas stream to the lower-temperature cold box.

7. The method of claim 1, wherein the boil-off gas is hydrogen.

8. A method for recovering boil-off gas from a system comprising one or more liquefaction trains, the one or more liquefaction trains comprising: transport trucks or loading bays,a gaseous hydrogen feed stream, anda low-pressure hydrogen compressor;

9. The method of claim 8, further comprising a liquid hydrogen storage unit, and a lower-temperature cold boxy, wherein the liquid hydrogen storage unit provides liquid hydrogen to the transport trucks or loading bays and collecting a second boil-off gas stream from the liquid hydrogen storage unit and recycling the second boil-off gas stream to the lower-temperature cold box.

10. The method of claim 8, wherein the boil-off gas is hydrogen.