PROCESS AND APPARATUS FOR SUPPLYING A BACKUP GAS UNDER PRESSURE

Abstract

The present invention relates to a process and a system for supplying a backup gas at a higher pressure from a source gas at a lower pressure. The backup gas at the lower pressure is at least partially condensed against a backup liquid at a higher pressure in a reprocessing heat exchanger and as a result, the backup liquid is at least partially vaporized. The backup liquid at the higher pressure is formed from boosting liquefied backup gas at the lower pressure. A backup vaporizer is disposed downstream of the reprocessing heat exchanger to completely vaporize the backup liquid at a higher pressure before it was delivered to the customer. The present invention eliminates the use of costly gas compressor and mitigates associated safety risks, in particular when the backup gas is oxygen.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a § 371 of International PCT Application PCT/CN2019/073114, filed Jan. 25, 2019, which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a process and a system for supplying a backup gas, in particular a backup gas at an elevated pressure.

BACKGROUND OF THE INVENTION

In industries such as petrochemical industry, steel industry, glass industry or coal gasification, large quantities of gases, including oxygen, nitrogen, hydrogen, argon and so forth are often requested. These gases maybe supplied through on-site gas production facilities, such as an air separation unit. Inevitably, normal production of gaseous production is disrupted upon occasion by events such as purity upset, scheduled or non-scheduled shutdowns, or other reasons. Thus, the delivery of gaseous products to the customer needs to be maintained by a backup system that may include one or more liquid storage tanks, pumps, or a backup vaporizer of various types. In some cases, backup gases maybe supplemented from a network of gas-transporting pipelines.

Various industrial applications require gases at different pressure. For instance, the pressure of gaseous products from an ASU facility or in a pipeline network is commonly below 20˜30 bara, while partial oxidation reactors could require gases at above 70 bara.

To comply with such constraints, a backup system needs to provide gases at elevated pressure, thus a costly gas turbine has to be employed to raise the pressure of the gaseous product to a desire level.

U.S. Pat. No. 7,409,835B2 discloses a method and a system for reducing pressure fluctuation in the supply of pressurized gaseous products to consumers during a switch from a normal operation mode to a standby mode of an air separation unit. In addition to a normally operating heat exchanger, a backup heat exchanger is maintained at a cold standby by diverting a small portion of liquefied gas stream through it. As a result, switching between the two modes can be effected with low energy consumption and a fast response time.

US2008/0184736A1 describes a method for the emergency backup supply of a gas under pressure by vaporization of a pressurized liquid, this gas normally being supplied by vaporization of liquid in a first exchanger of a pumped air separation unit, during the step of operating a second exchanger in order to produce the make-up gas, pressurized liquid and high-pressure air continue to be sent to the first exchanger.

SUMMARY OF THE INVENTION

During disruption of normal operation in industrial gas production facilities, a backup system and process to ensure delivery of pressurized gases within a small pressure fluctuation is necessary. In addition to liquid storage tanks, pumps or backup vaporizers, backup gases may also be provided from nearby air separation unit or pipeline networks.

When the pressure of gases from the above sources is lower than that required at the customer's site, they cannot be supplied directly to the customer; instead, their pressure needs to be raised by a gas compressor or a booster. Additional gas compressors increase capital and operational cost of the backup system, and may also carry certain safety concerns, especially when compressing oxygen.

The objective of certain embodiments of the present invention is to eliminate the use of a gas compressor or a booster in a backup system, even when the output pressure of the gases needs to be higher, or even significantly higher than the pressure of the source gases.

In one aspect, the present invention discloses a process for supplying a backup gas at an elevated pressure, comprising the following steps: providing a source of backup gas at a first pressure; providing at least a reprocessing heat exchanger, a backup vaporizer and a liquid pump; then heat-exchange between the backup gas at the first pressure and a backup liquid at a second pressure in the reprocessing heat exchanger to produce the at least partially liquefied backup gas at the first pressure and the at least partially vaporized backup liquid at the second pressure; followed up by warming up the at least partially vaporized backup liquid at the second pressure in the backup vaporizer to produce the backup gas at an elevated pressure. In this process, the second pressure is higher than the first pressure, and the backup liquid at the second pressure is obtained by elevating the liquefied backup gas to the second pressure with the liquid pump.

The above process may also comprise a step of expanding the at least partially liquefied backup gas at the first pressure through an expansion valve. With the provision of a liquid storage tank, the process can also include a step of storing the expanded liquefied backup gas in the liquid storage tank before transferring it to the liquid pump.

In another aspect, the process comprises by-passing part of the backup liquid at the second pressure from the reprocessing heat exchanger through a by-pass circuit.

Additionally, certain embodiments of the present invention describe a system for supplying a backup gas at an elevated pressure, comprising a reprocessing heat exchanger, a backup vaporizer and a liquid pump, a first conduit for delivering a backup gas at a first pressure into a warm end of the reprocessing heat exchanger and a second conduit for transporting an at least partially liquefied backup gas at a first pressure from the cold end of the reprocessing heat exchanger into the liquid pump. It further comprises a third conduit for delivering a backup liquid at a second pressure from the outlet of the liquid pump into a cold end of the reprocessing heat exchanger, a fourth conduit for transporting an at least partially vaporized backup liquid at the second pressure from a warm end of the reprocessing heat exchanger to the backup vaporizer, and a fifth conduit for supplying the backup gas at an elevated pressure from the backup vaporizer; wherein the second pressure is higher than the first pressure.

In one aspect, the system further comprises an expansion valve and a liquid storage tank.

In another aspect, the reprocessing heat exchanger of the system has separate flow channels for the backup gas at the first pressure and the backup liquid at the second pressure; and there exists a by-pass circuit, with one end connecting to the flow channel for the backup liquid at the second pressure inside the reprocessing heat exchanger and one end connecting to the fourth conduit. The by-pass circuit may also have a flow-control valve disposed on it.

In both the process and the system for supplying backup gas at an elevated pressure, the reprocessing heat exchanger may comprise aluminum plate fin exchanger or printed plate exchanger. The pressure ratio of the second pressure to the first pressure is in the range of 3˜1, preferably in the range of 2.5˜1.2. The backup gas or the backup liquid comprises oxygen.

Certain embodiments of the present invention eliminate the use of a gas compressor (or a booster) when a backup gas stream is at a pressure lower than required by the customer; thus mitigates associated cost and safety concerns. They may also provide an energy-efficient solution for simultaneously vaporizing a backup liquid and condensing a backup gas through heat exchange in a heat exchanger. Since the backup liquid is already partially vaporized before entering the backup vaporizer, the energy consumed therein is also reduced. In addition, if the backup gas were only supplied from a liquid storage, the capacity of the liquid storage needs to be substantial in order to sustain a long backup period. With certain embodiments of the present invention, the liquid storage can be supplemented with gases from other sources regardless of their pressure; as a result, a large capacity of liquid storage is not necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

For further understanding the nature and objects of the present invention, references should be made to the following detailed description, taken in conjunction with the accompanying drawings.

FIG. 1 provides an illustration of a backup system for producing gaseous product in accord with the present invention.

FIG. 2 is a theoretical representation of the relationship between the pressure of the backup gas and its condensable flow as a function of the total liquid flow to be vaporized, using liquid oxygen (LOX) at 80 bara as an example.

1—low pressure pipeline network; 2—a first conduit (a backup gas at a first pressure); 3—a second conduit (partially liquefied backup gas at a first pressure); 4—a third conduit (a backup liquid at a second pressure); 5—by-pass circuit; 6—a fourth conduit (partially vaporized backup liquid at a second pressure); 7—a fifth conduit (backup gas at an elevated pressure); 8—customer's facility; 10—a liquid storage tank; 11—a liquid pump; 12—a reprocessing heat exchanger; 13—a backup vaporizer; 14—an expansion valve; 15—a flow control valve.

DETAILED DESCRIPTION

Referring to FIG. 1, gases from a low pressure pipeline network 1 can be utilized as a supplemental source of backup gas. Here, “a low pressure pipeline network” means the gas transported in these pipelines is at a lower pressure compared to the pressure of the final backup gas delivered to the customer, and the actual value may range from atmospheric pressure to 70 bara. The transfer line of gaseous product from a nearby air separation unit (ASU) can also be considered as “a low pressure pipeline network”.

Gases from the low pressure pipeline network 1 are then transferred via a first conduit 2 into the warm end of a reprocessing heat exchanger 12. The backup gas inside the first conduit 2 is at a first pressure. The reprocessing heat exchanger 12 comprises any type that allows indirect heat exchange between two fluid steams and can sustain pressure over 50 bara, preferably over 80 bara.

Inside the reprocessing heat exchanger 12, the backup gas at a first pressure is cooled and at least partially condensed through heat exchange with a backup liquid at a second pressure. The backup gas and backup liquid have the same composition, but the second pressure is higher than the first pressure. In addition, the backup gas at a first pressure is usually at room temperature, while the backup liquid at a second pressure is usually at a cryogenic temperature, for instance, below −165° C.

Depending on certain parameters, such as temperature, pressure and flow differences between the two exchanging streams, the backup gas at a first pressure may be entirely or partially condensed or liquefied leaving the cold end of the reprocessing heat exchanger 12. The partially liquefied backup gas at a first pressure is then transferred via a second conduit 3 to a liquid pump 11. Along the second conduit 3, the partially liquefied backup gas first passes through an expansion valve 14 to be further cooled and its pressure is reduced. The expansion valve 14 also controls the flow of the backup gas through the reprocessing heat exchanger 12 to ensure it is cooled to a temperature low enough to avoid flash during the expansion. Optionally on the second conduit 3, a liquid storage tank 10 could also be disposed. The liquid storage tank 10 may hold initial stock of backup liquid to be pumped and vaporized at the start of the backup process. With the continuous supplement of liquefied backup gas at the first pressure, the liquid storage tank does not need to have a capacity to hold the entire quantity of backup liquid needed for sustaining a long period of backup operation. In view of the previous description, the backup liquid at the second pressure may be obtained from elevating the liquefied backup gas to the second pressure with the liquid pump, including directly elevating the liquefied backup gas at the first pressure by a liquid pump to the second pressure; or expanding and storing the backup gas at around atmospheric pressure in the liquid storage tank, then withdrawing from the liquid storage tank a stream to the liquid pump to be boosted to the second pressure.

The liquid pump 11 is used to raise the pressure of backup liquid from the liquid storage tank 10 to a second pressure that is required at the customer's facility 8. The output backup liquid at a second pressure is then transferred through a third conduit 4 into the cold end of the reprocessing heat exchanger 12. After heat exchange with the backup gas at the first pressure, the exit stream from the warm end of the heat exchanger contains at least partially vaporized backup liquid at a second pressure. Via a fourth conduit 6, this stream is delivered into a backup vaporizer 13 and fully vaporized therein. The backup vaporizer has the function to vaporize the liquid under pressure and on leaving this equipment, the gas maintains its pressure and is in general close to the ambient temperature. This backup gas at an elevated pressure is then transferred through a fifth conduit 7 to the customer's facility 8. The word “elevated” means higher than atmospheric pressure, and preferably higher than the first pressure of the backup gas, but not necessarily higher or equal to the second pressure. Depending on the energy sources available on the site and their costs, this vaporizer may use as heat source to vaporize the liquid under pressure, for example air, stream, hot water or combustion flue gas. When the backup gas at the first pressure is not available at some time during the backup operation, the entire flow of backup liquid may be vaporized in the backup vaporizer.

For the reprocessing heat exchanger 12, persons skilled in the art understand that common choices include brazed aluminum plate-fin heat exchanger or printed plate heat exchanger. The brazed aluminum plate-fin heat exchanger provides excellent heat conductivity but cannot withstand great temperature differences between the cold end and the warm end. Thus, for thermal balancing purposes, a by-pass line 5 is placed at an intermediate location of the aluminum plate-fin heat exchanger for the extraction of part of the cold backup liquid at a second pressure. The extracted cold backup liquid passes through a flow control valve 15 before being recombined into the fourth conduit 6. The exit temperature of the at least partially vaporized backup liquid from the warm end can be adjusted by controlling the flow via valve 15.

For other types of heat exchangers, such as printed plate heat exchanger, since the equipment is robust toward temperature differences, the above by-pass line is not necessary.

The present invention may include additional valves, tanks, pumps, flowlines, variations in connections, locations, arrangement, and/or other equipment and interrelated components.

For a given type of gas, the flow that can be condensed within the reprocessing heat exchanger of the present invention depends on a couple factors, such as the initial pressure ratio between the backup gas at a first pressure and the backup liquid at a second pressure and the flow of the backup liquid to be vaporized. FIG. 2 illustrates such a relationship for liquid oxygen (LOX) to be vaporized at 80 bara in a reprocessing heat exchanger. In this figure, the y axis P/p represents a ratio between the higher pressure P of the backup liquid to be vaporized and the lower pressure p of the backup gas to be condensed; and the x axis q/Q represents the flow ratio between the backup gas to be condensed and the backup liquid to be vaporized. For instance, according to this graph, a P/p value of 2 corresponds to a q/Q value of 0.42. Since the P of this graph is set at 80 bara, that means if the backup gas to be condensed is at a pressure of 40 bara (thus P/p=80/40=2), then the current invention has the capability to condense as much as 1000 Nm³/h backup gas at 40 bara by vaporing every 2400 Nm³/h of backup liquid at 80 bara. (q/Q=1000/2400=0.42). Throughout the invention, the flow of both gas and liquid is converted to normal cubic meter per hour at 1 atm and 0° C.

Table 1 and table 2 present simulations of two embodiments according to the current invention. The initial pressure of the backup gas is at 40 bara and 60 bara, respectively.

The required pressure of the product gas is 80 bara. The flow, temperature and pressure of each stream throughout the process are listed in the following:

TABLE 1
Embodiment 1
		Temperature	Flow	Pressure
Flow	Phase	(° C.)	(Nm3/h)	(Bara)
2-backup gas	gas	20	480	40
3-liquified	liquid	−177.8	480	39.9
backup gas
Expanded backup	Gas/liquid	−182.6	480	1.05
gas
4-backup liquid	liquid	−179.8	1000	80
after pumping
5-backup liquid in	liquid	−122	710	79.95
the bypass line
6-vaporized	gas	13	290	79.9
backup liquid
7-backup gas	gas	20	1000	79.8
after backup
vaporizer

TABLE 2
Embodiment 2
		Temperature	Flow	Pressure
Flow	Phase	(° C.)	(Nm3/h)	(Bara)
2-backup gas	gas	20	710	60
3-liquified	liquid	−177.8	710	59.9
backup gas
Expanded backup	Gas/liquid	−182.6	710	1.05
gas
4-backup liquid	liquid	−179.8	1000	80
after pumping
5-backup liquid in	liquid	−110	450	79.95
the bypass line
6-vaporized	gas	11	550	79.9
backup liquid
7-backup gas	gas	20	1000	79.8
after backup
vaporizer

In the column named “Flow”, the numerical number represents the conduit containing the flow, followed by a description of the nature of the flow. The above two examples illustrate the operation of the present invention, but they should not be construed as in any way limiting the scope of the invention.

Although this invention has been described in detail with reference to certain embodiments, those skilled in the art will recognize that variations and modifications of the described embodiments may be used. Accordingly, these variations and modifications are also within the spirit and scope of the invention as defined by the appended claims and their equivalents.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

Claims

1-14. (canceled)

15. A process for supplying a backup gas at an elevated pressure, comprising:

a) providing a source of backup gas at a first pressure;

b) providing at least a reprocessing heat exchanger, a backup vaporizer and a liquid pump;

c) heat-exchange between the backup gas at the first pressure and a backup liquid at a second pressure in the reprocessing heat exchanger to produce the at least partially liquefied backup gas at the first pressure and the at least partially vaporized backup liquid at the second pressure;

d) warming up the at least partially vaporized backup liquid at the second pressure in the backup vaporizer to produce the backup gas at an elevated pressure; and wherein

the second pressure is higher than the first pressure, and the backup liquid at the second pressure is obtained from elevating the pressure of the liquefied backup gas to the second pressure with the liquid pump.

16. The process as claimed in claim 15, further comprising an expansion valve and a liquid storage tank, wherein the at least partially liquefied back-up gas at the first pressure is expanded through the expansion valve before entering the liquid storage tank.

17. The process as claimed in claim 15, further comprising:by-passing part of the back-up liquid at the second pressure from the reprocessing heat exchanger through a by-pass circuit.

18. The process as claimed in claim 15, wherein the pressure ratio of the second pressure to the first pressure is in the range of 1 to 3 bar(a).

19. The process as claimed in claim 18, wherein the pressure ratio of the second pressure to the first pressure is in the range of 1.2 to 2.5 bar(a).

20. The process as claimed in claim 15, wherein the back-up gas and the back-up liquid comprise oxygen.

21. A system for supplying a back-up gas at an elevated pressure, comprising:

a) a reprocessing heat exchanger, a back-up vaporizer and a liquid pump;

b) a first conduit for delivering a back-up gas at a first pressure into a warm end of the reprocessing heat exchanger and a second conduit for transporting an at least partially liquefied back-up gas at a first pressure from the cold end of the reprocessing heat exchanger into the liquid pump;

c) a third conduit for delivering a back-up liquid at a second pressure from the outlet of the liquid pump into a cold end of the reprocessing heat exchanger;

d) a fourth conduit for transporting an at least partially vaporized back-up liquid at a second pressure from a warm end of the reprocessing heat exchanger to the back-up vaporizer;

e) a fifth conduit for supplying the back-up gas at an elevated pressure from the back-up vaporizer, and wherein the second pressure is higher than the first pressure.

22. The system as claimed in claim 21, wherein an expansion valve and a liquid storage tank are in fluid communication with the second conduit.

23. The system as claimed in claim 21, the reprocessing heat exchanger having separate flow channels for the back-up gas at the first pressure and the back-up liquid at the second pressure, further comprising a by-pass circuit, with one end connecting to the flow channel for the back-up liquid at the second pressure inside the reprocessing heat exchanger and one end connecting to the fourth conduit.

24. The system as claimed in claim 23, further comprising a flow-control valve disposed on the by-pass circuit.

25. The system as claimed in claim 23, the reprocessing heat exchanger comprises aluminum plate fin exchanger or printed plate exchanger.

26. The system as claimed in claim 21, wherein a pressure ratio of the second pressure to the first pressure is in the range of 1 to 3 bar(a).

27. The system as claimed in claim 26, wherein a pressure ratio of the second pressure to the first pressure is in the range of 1.2 to 2.5 bar(a).

28. The system as claimed in claim 21, wherein the back-up gas and the back-up liquid comprise oxygen.