SYSTEM AND METHOD FOR LIQUEFYING NATURAL GAS EMPLOYING TURBO EXPANDER

Abstract

An improved system and method for liquefying natural gas employing liquid nitrogen is disclosed. The improved system and method lowers the nitrogen consumption rate by using an expander, for example, a radial inflow turbo-expander, on the nitrogen side. This reduction in nitrogen consumption rate substantially reduces system operating costs.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for liquefying natural gas. More specifically, the present invention relates to systems and methods for liquefying natural gas employing a cryogenic liquid such as liquid nitrogen.

2. Description of the Prior Art and Related Information

Currently there are numerous ways to liquefy natural gas (NG). One of the ways is to use a cryogenic liquid such as nitrogen to liquefy natural gas (LNG). The present invention is directed to this type of approach. As an example, Cosmodyne LNG plant model PGL100 plant uses an open nitrogen refrigeration cycle to liquefy natural gas (NG).

FIG. 1 shows a schematic sketch of such a prior art cryogenic natural gas liquefier system 10. Nitrogen liquid from a storage tank 12 is pumped by a pump 14 into a coldbox 16 and is vaporized by the warm natural gas. This heat exchange will result in liquefying the natural gas.

The nitrogen refrigerant is supplied as a cryogenic liquid, liquid nitrogen (LN). It is boiled, superheated and eventually discharged as its refrigeration capacity is exhausted from cooling and liquefying the natural gas. During the discharge process the nitrogen gas may pass through a heater 24 and adsorption beds to form part of the NG pre-treatment system 18. The handling of the nitrogen gas exhaust may vary depending on the intended use of the gas after it is discharged.

The discharged nitrogen gas is provided to an outlet vent 20 where it can be vented to atmosphere or utilized in many different ways. Some uses include: reliquefaction in a nitrogen liquefier to be sent to a LN storage tank; compression to be stored in standard nitrogen cylinders or used as an inert gas supply; or used as a regeneration gas for the NG pre-treatment system.

The coldbox will have a NG inlet, NG liquid outlets, a liquid nitrogen inlet and nitrogen gas outlets. The coldbox 16 typically comprises a steel frame which houses the heat exchangers (two sections; nitrogen superheating region and reboiler/condenser), subcooling coils, piping, and pressure vessels (separators). The coldbox will be filled with insulation material to minimize refrigeration loss to the ambient environment.

The NG is received from a supply 22 such as a pipeline. The NG will be first directed to a pre-treatment system 18 to remove carbon dioxide (CO2), water (H2O), mercury (Hg), sulfur components (such as H2S), and other impurities. Depending on the feed gas composition a pre-treatment system 18 can consist of just adsorption equipment or can include a feed gas inlet filter, separator, amine system, mercury guard bed, and adsorption equipment. The treated NG is then directed to the liquefier module including the coldbox 16. The pre-treatment of the natural gas upstream of the PGL100 system and the receipt of the product LNG from the coldbox at 26 may be conventional and are not described.

The advantage of this system is its simplicity, low capital investment cost, and high reliability. There are very few components, making it relatively cheap and quick to install. In addition, there are limited moving parts within the major equipment which avoids downtime for frequent maintenance or repairs.

The drawback of this system is the high operating cost. The cryogenic liquid, usually liquid nitrogen, is expensive to source and supply. Additionally, the liquefier will require about 1.5 gallons of liquid nitrogen to produce 1 gallon of LNG. The actual ratio will depend on the supply temperature and pressure of the liquid nitrogen.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a natural gas liquefier system comprising of: a natural gas input coupled to a source of natural gas; a liquid nitrogen input coupled to a source of liquid nitrogen; a liquefier module coupled to receive the natural gas and liquid nitrogen and liquefy the natural gas by boiling the liquid nitrogen; a turbo expander module coupled to the liquefier module to receive the boiled gaseous nitrogen, cool the gaseous nitrogen by expansion, and reintroduce the colder gaseous nitrogen into the liquefier module; and a liquefied natural gas output coupled to the liquefier module.

In another aspect the present invention provides a natural gas liquefier system, comprising a natural gas flow path including a natural gas input and liquefied natural gas output, a separate nitrogen flow path including a liquid nitrogen input and a gaseous nitrogen output, a liquefier module coupled to both the natural gas and nitrogen flow paths wherein the liquid nitrogen is brought into thermal contact with the natural gas to liquefy the natural gas and boil the nitrogen, and a turbo expander coupled to the nitrogen flow path in a closed loop to receive boiled gaseous nitrogen, cool the nitrogen gas through expansion and reintroduce the cooler nitrogen gas into the nitrogen flow path.

In another aspect the present invention provides a method of reducing liquid nitrogen usage in a natural gas liquefaction process in a system having a natural gas liquefier module and a cryogenic turbo expander. The method comprises boiling liquid nitrogen in the natural gas liquefier module creating a nitrogen superheating region in the liquefier module and introducing two pressure levels of nitrogen cooling media in the nitrogen superheating region by using expansion of boiled nitrogen in a turbo expander.

Further features and aspects of the invention are set out in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a prior art cryogenic natural gas liquefier system.

FIG. 2 is a schematic drawing of an improved cryogenic natural gas liquefier system in accordance with the present invention.

FIG. 3 is a schematic drawing of a prior art coldbox employed in a system such as that of FIG. 1.

FIG. 4 is a schematic drawing of a turbo expander module and coldbox in accordance with the present invention.

FIG. 5 is a cross section of a suitable turbo expander employed in the system shown in FIG. 4.

FIG. 6 is a process flow diagram as implemented in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved system and method for liquefying natural gas employing a cryogenic liquid such as liquid nitrogen (LN). The preferred implementation of the improved system and method lowers the nitrogen consumption rate by using an expander like a radial inflow turbo expander in the nitrogen side. This reduction in nitrogen consumption rate is very important as it makes the described natural gas liquefier competitive with other designs. As used herein the term natural gas refers to both gas having naturally occurring hydrocarbons such as methane and ethane as well as various impurities and also the treated form of natural gas having some or all of the impurities removed.

FIG. 2 shows a schematic sketch of an improved cryogenic natural gas liquefier system 100 improved in accordance with the present invention as discussed in detail below. Several of the components illustrated may be conventional in nature and will not be described in detail. The overall system has three main equipment systems, a LN pump module 110, a liquefier module or coldbox 120 and the turbo expander module 150. The LN storage 122 and feed to the LN pump module may be conventional and details are not included in the present description. The pre-treatment of the natural gas upstream of the system 100 and the receipt of the product LNG from the coldbox 120 at 124 are also conventional and not described.

The coldbox 120 will have liquid and gas inlets and outlets for both natural gas and nitrogen shown in more detail in FIG. 4. The coldbox preferably comprises a steel frame housing the heat exchanger(s) (two sections; nitrogen superheating and reboiler/condenser), recondensate coils, piping, and pressure vessels (separator), as shown in FIG. 4. The turbo expander module 150 will be comprised of a turbo expander, lubrication, cooling and seal gas system. The coldbox and cold sections of the turbo expander module 150 will be filled with insulation material to minimize refrigeration losses.

The LN pump module 110 and turbo expander module 150 are controlled to optimize nitrogen usage by the monitoring and control system 130 which receives inputs 132 from the coldbox 120 and turbo expander instrumentation as discussed below. Other conventional monitoring and control functions are also provided which need not be described in further detail.

The effluent nitrogen gas from the coldbox can be used for the adsorber bed regeneration by temperature swing adsorption regeneration, shown as path 144 to the NG pre-treatment system 142. The nitrogen gas is then provided to an outlet vent 146 where it can be vented to atmosphere. Alternatively nitrogen gas from the coldbox can be used as feedstock to a nitrogen liquefier system 160 and sent to the LN storage tank 122 forming a close loop nitrogen refrigeration system, thus further reducing operating cost.

On the natural gas (NG) side the NG will be first directed from a conventional supply 140 (such as a pipeline) to a pre-treatment system 142 to remove carbon dioxide (CO2), water (H2O), mercury (Hg), sulfur components (such as H2S), and other impurities. Depending on the feed gas composition the pre-treatment system can employ just adsorption equipment 148 or can include a feed gas inlet filter, separator, amine system, mercury guard bed, and adsorption equipment. The treated NG is fed directly to the liquefier module including coldbox 120.

As noted above, the present invention provides a turbo expander module 150. To better appreciate the advantages over the conventional approach without a turbo expander the conventional approach will first be described in relation to FIG. 3. As shown in FIG. 3 the NG pressure controller 300 will provide a limiting function to a NG feed rate and will be set consistently with the desired tank pressure.

The treated NG (Stream 310) enters the coldbox heat exchanger 301 where it is precooled and withdrawn to a separator tank 302 (Stream 323), where the heavy hydrocarbons are removed in the liquid bottom stream of the separator. These are removed at this point of the process to prevent them from freezing inside the heat exchanger 301.

The gas stream from separator 302 will return to the heat exchanger 301 (Stream 304) where it is cooled and partially condensed. The condensate, comprised predominantly of ethane and heavier hydrocarbons, is removed in separator 303, creating the capability to adjust the Wobbe number of the final LNG product. If the feed gas is rich in ethane, the bulk of the ethane can be removed from the process by the use of valve 305 (Stream 321). For the expected feed gas composition, ethane rejection will not be necessary and the liquids removed in separator 303 will be re-injected into the product stream via LCV (Level Control Valve) 324. In this case valve 305 would remain closed.

The natural gas from separator 303 will flow (Stream 310) to the main reboiler 314/condenser 312, a thermosyphon type heat exchanger, where NG is fully condensed and sub-cooled with a pool of boiling nitrogen. The condensed LNG drained from the reboiler vessel 314 (Stream 315) is blended with the sub-cooled ethane rich separator 303 bottoms (Stream 316) to yield the final product stream (Stream 318). The final product temperature will be in equilibrium with storage pressure.

On the nitrogen side, the nitrogen refrigerant is supplied as a cryogenic liquid (LN) (Stream 320). LN from a storage tank is pressurized by pump modules 322 to provide positive control of the LN boiling temperature in the reboiler 314 (Stream 321). The gaseous nitrogen vapor (GN) created in the boiling process will flow out of reboiler 314 and into heat exchanger 301 (Stream 315) to support NG cooling and liquid extraction in separator 303. In the process the GN vapor will become superheated. The temperature of the saturated GN vapor entering heat exchanger 301 must be controlled to achieve the desired cooling of the NG feed gas and this is accomplished by monitoring the vapor temperature and controlling the LN level in reboiler vessel 314. The superheated gaseous nitrogen (GN) exits heat exchanger 301 for further utilization (Stream 317) which may be controlled by valve 325. To maintain the required temperature differential limits of heat exchanger 301, intermediate vents from heat exchanger 301 (Stream 326) may be provided when necessary. These vents can be routed and merged together with final vent (Stream 317) if desired.

Next, the liquid nitrogen pump module 322 will be described. The purpose of the LN pump module is to deliver pressurized LN to the liquefier. Liquid nitrogen from storage will be pumped to elevated pressure consistent with the condensing natural gas pressure and composition. The pump module has two identical process pumps, as shown. One is used in normal operation and one is installed as a spare. Pressure control is achieved by PCV 325 located at the outlet of the coldbox.

The pump in operation is on an automatic speed control to maintain LN level in the boiler 314. This level is indicated by level indicator 330. Variable frequency drives (VFDs) located in the motor control center (MCC) room provide the speed loop control and the VFDs receive a remote set point signal from the control system based on the LN level in boiler 314. In this manner, the operator can change the liquid level in reboiler 314 by adjusting the pump speed set point. This capability is built into the liquefier to control the heat transfer rate in heat exchanger 312. The pump module is equipped with a local panel where the operator can switch the pumps on/off.

A preferred embodiment of the turbo expander module 150 as coupled to the coldbox 120 in the improved liquefaction system of the present invention is shown in FIG. 4.

The treated NG (Stream 406) enters the coldbox heat exchanger 401 where it is precooled and withdrawn to a separator tank 402 (Stream 423), where the heavy hydrocarbons are removed from the feed. These are removed at this point in the process to prevent them from freezing inside the heat exchanger 401.

The gases from separator 402 return to the heat exchanger 401 (Stream 304) where they are cooled and partially condensed. The condensate, comprised predominantly of ethane and heavier components is removed from the separator 303, creating the capability to adjust the Wobbe number of the final LNG product. If the feed gas is rich in ethane, the bulk of the ethane can be removed from the process by the use of valve 405 (Stream 421). For the expected feed gas composition, ethane rejection will not be necessary and the liquids removed in separator 403 will be re-injected into the product stream via LCV (Level Control Valve) 424. In this case valve 405 would remain closed.

The natural gas from separator 403 will flow (Stream 410) to the main reboiler (414)/condenser (412), a thermosyphon type heat exchanger, where NG is fully condensed and sub-cooled in a pool of boiling nitrogen. The condensed LNG drained from reboiler vessel 414 (Stream 415) is blended with the sub-cooled ethane rich separator 403 bottoms (Stream 416) to yield the final product stream (Stream 418). The final product temperature will be in equilibrium with storage pressure.

On the nitrogen side, the nitrogen refrigerant is supplied as a cryogenic liquid (LN) (Stream 420) from a storage tank 122 (FIG. 2). LN from the storage tank is pressurized by pump modules 422 (Stream 421). The nitrogen vapor created during the boiling process in reboiler 414 exits the reboiler and flows into heat exchanger 401 (Stream 417). After partially heating, the nitrogen vapor is removed from heat exchanger 401 and enters the turbo expander 430 (Stream 432). The radial inflow turbo expander may be conventional, such as a commercially available turbo expander like an ACD Corporation TC series turbo expander. A cross section of a suitable expander is shown in FIG. 5, discussed below. The temperature of the saturated GN vapor entering heat exchanger 401 will be a function of the LN flowrate and pressure to the turbo expander inlet. The optimum operating point shall be a result of the following control signals and inputs: First, the expander inlet pressure sensor (signal 435) will signal and control the expander inlet guide vane (IGV) positioning (signal 434) to maintain expander inlet pressure; second, the liquid level sensor 450 will set the pump VFT controls (signal 132, FIG. 2) to regulate the flow rate of nitrogen. For a given natural gas feed to the liquefier, there will be an optimum operating point controlled as described above by LN pressure and flowrate.

The turbo expander 430 will lower the nitrogen pressure and temperature through an isentropic expansion. The cold stream from the turbo expander exhaust (Stream 436) will flow back into heat exchanger 401 to provide additional refrigeration at the pinch point of the heat exchanger (401). This, in turn, will reduce the rate of liquid nitrogen required to liquefy the NG. The power generated by the turbo expander via isentropic expansion can be utilized by a compressor mounted on the common shaft as the expander to compress gas. As another option, the expander can be connected to a generator instead and produce electrical power when feasible. As another option, the power generated by the expander may be simply dissipated using an oil or air brake.

The superheated GN exits heat exchanger 401 for further utilization or to be vented (Stream 438). The pressure of this exiting nitrogen gas will be controlled by valve 440 and will be set based on its intended future utilization.

The above description is particular to the thermosyphon type nitrogen boiling. It is selected to avoid freezing the methane. However, if the liquid nitrogen storage pressure is above 40 PSIG an alternative design with a single heat exchanger shall be acceptable.

In such an alternate embodiment of the invention, a single heat exchanger (such as heat exchanger 401) may be sufficient for the desired cooling of the natural gas. In such an embodiment the turbo expander would be coupled to the heat exchanger in a closed loop to enhance efficiency, as in the embodiment shown in FIG. 4. In such an embodiment, LN from stream 420 and pumps 422 would be provided directly to heat exchanger 401 (similar to Stream 417 in FIG. 4). The output to the expander and return of cooled GN (Stream 436) back to the exchanger will be coupled at the top section of the exchanger vessel. LNG in turn would be provided from heat exchanger 401 similarly to streams 415, 416 in the illustrated embodiment.

A suitable expander 430 employed in the system shown in FIG. 4 is shown in a sectional view in FIG. 5. The expander has an inlet 500 which receives GN stream 432 (FIG. 4). The inlet flow volume is controlled by inlet vane 502 which is mechanically opened or closed in response to control signal 434 (FIG. 4) and the input nitrogen pressure is monitored to provide feedback along line 435 (FIG. 4) to the control system 130 (FIG. 2). The inlet control vane or other inlet volume control mechanism may be configured as part of an input conduit to the expander instead of part of the expander, and the structure schematically shown is merely illustrative in nature. The expander may operate using a conventional expander wheel 504 to provide low pressure, cooled GN flow (Stream 436) at expander outlet 506 (FIG. 4). As noted above, the power generated by the turbo expander through isentropic expansion can be used to compress gas by a compressor mounted on the common shaft 510 as the expander. The compressor includes gas inlet 512 and outlet 514 and compressor wheel 516 in a conventional configuration. Alternatively, shaft 510 may connect the expander to a generator to produce electrical power when feasible. Another option is to simply dissipate the energy using an oil or air brake. The illustrated turbo expander is one preferred expander option, but other expander designs may be employed to provide the desired GN cooling.

Referring to FIG. 6, a simplified exemplary process flow implemented by control system 130 is illustrated as it pertains to the modified control of the present invention employing an expander in the control loop. It will be noted that a complete LNG plant control system may include many other functions conventional in nature and not described. Initially, a target reboiler LN level is set by the plant operator and is shown as initial process step 600. The LN pump speed is then automatically controlled by the process at step 602 to set and maintain LN level in the reboiler 414 within the desired range. This level is maintained under the control of monitoring and control system 130 (FIG. 2). Next, at step 604 an initial expander inlet guide vane position for turbo expander 430 is set for an estimated desired minimal differential temperature (delta T) in the main heat exchanger. Next, at 606 the temperature of the GN in heat exchanger 401 is monitored and provided to control system 130 which also receives input from the expander IGV values and provides an adjusted signal to control LNG pump speed (602). Adjustment of the refrigeration capacity is performed at step 608. By controlling the expander suction pressure (by adjusting IGV) and LN rates (by adjusting pump speed) in conjunction with monitoring GN temperature in the heat exchanger (606), optimal cooling of the natural gas may be achieved. This makes it possible to minimize the consumption of LN while achieving the desired production rate of liquefied natural gas. As a result, overall usage of LN is reduced, decreasing the operation costs of the system.

Therefore, the present invention provides a number of features and aspects with attendant advantages. These include the following features, aspects and advantages. Enhancement of the natural gas open loop liquefaction process, where refrigeration is provided by boiling nitrogen by utilizing two pressure levels of nitrogen cooling media in the nitrogen superheating region introduced by using expansion in the cryogenic turbo expander. Employing advanced, interactive, process controls to minimize nitrogen usage. Introducing a method for reclaiming waste nitrogen back into the reliquefaction process.

It will be appreciated by those skilled in the art that the foregoing is merely an illustration of the present invention in currently preferred implementations. A wide variety of modifications to the illustrated embodiments are possible while remaining within the scope of the present convention. Therefore, the above description should not be viewed as limiting but merely exemplary in nature.

Claims

1. A natural gas liquefier system, comprising: a natural gas input coupled to a source of natural gas;a liquid nitrogen input coupled to a source of liquid nitrogen;a liquefier module coupled to receive the natural gas and liquid nitrogen and liquefy the natural gas by boiling the liquid nitrogen;a turbo expander module coupled to the liquefier module to receive the boiled gaseous nitrogen, cool the gaseous nitrogen by expansion and reintroduce the colder gaseous nitrogen into the liquefier module; anda liquefied natural gas output coupled to the liquefier module.

2. A natural gas liquefier system as set out in claim 1, wherein the turbo expander module comprises a turbo expander coupled to the liquefier module in a closed loop.

3. A natural gas liquefier system as set out in claim 1, wherein the liquefier module comprises at least one heat exchanger.

4. A natural gas liquefier system as set out in claim 3, wherein the at least one heat exchanger comprises a first heat exchanger and a second heat exchanger, wherein the second heat exchanger is coupled to the liquid nitrogen source and liquefies the natural gas by boiling the liquid nitrogen and outputs liquefied natural gas to the liquefied natural gas output and outputs boiled gaseous nitrogen to the first heat exchanger, and wherein the turbo expander is coupled in a closed loop to the first heat exchanger.

5. A natural gas liquefier system as set out in claim 4, wherein the first heat exchanger has first and second inputs for receiving gaseous nitrogen at a different temperature and different pressure from the second heat exchanger and expander, respectively.

6. A natural gas liquefier system as set out in claim 1, further comprising a liquid nitrogen pump coupled between the liquid nitrogen input and the liquefier module.

7. A natural gas liquefier system as set out in claim 6, further comprising a monitoring and control system coupled to the liquid nitrogen pump, turbo expander module and liquefier module.

8. A natural gas liquefier system as set out in claim 7, wherein the turbo expander module comprises an input volume flow control mechanism.

9. A natural gas liquefier system as set out in claim 8, wherein the monitoring and control system controls the liquid nitrogen pump and turbo expander module input volume flow control mechanism.

10. A natural gas liquefier system as set out in claim 9, wherein the monitoring and control system receives a liquid nitrogen level signal from the liquefier module and input pressure signal from the turbo expander module.

11. A natural gas liquefier system as set out in claim 1, further comprising a nitrogen reclaiming system coupled to receive used nitrogen gas from the liquefier module, liquefy the nitrogen gas and return the liquid nitrogen to the liquid nitrogen source.

12. A natural gas liquefier system, comprising: a natural gas flow path including a natural gas input and liquefied natural gas outputs;a separate nitrogen flow path including a liquid nitrogen input and a gaseous nitrogen outputs;a liquefier module coupled to both the natural gas and nitrogen flow paths wherein the liquid nitrogen is brought into thermal contact with the natural gas to liquefy the natural gas and boil the nitrogen; andan expander coupled to the nitrogen flow path in a closed loop to receive boiled gaseous nitrogen, cool the nitrogen gas through expansion and reintroduce the cooler nitrogen gas into the nitrogen flow path.

13. A natural gas liquefier system as set out in claim 12, wherein the nitrogen flow path includes a nitrogen reliquefaction system coupled to receive nitrogen gas from the liquefier module, liquefy the nitrogen gas and return the liquid nitrogen to the nitrogen supply to the liquefier module.

14. A natural gas liquefier system as set out in claim 12, wherein the liquefier module includes first and second heat exchangers, wherein the first heat exchanger is configured in the nitrogen flow path in a nitrogen superheating region having boiled nitrogen in gaseous form, and wherein the expander is coupled to the first heat exchanger.

15. A natural gas liquefier system as set out in claim 12, wherein the second heat exchanger comprises a reboiler which receives the liquid nitrogen and the natural gas, liquefies the natural gas and boils the nitrogen and provides the boiled nitrogen gas to the first heat exchanger at a first temperature and pressure.

16. A natural gas liquefier system as set out in claim 15, wherein the expander provides cooled nitrogen gas to the first heat exchanger at a second temperature and pressure.

17. A method of reducing liquid nitrogen usage in a natural gas liquefaction process in a system having a natural gas liquefier module and a cryogenic turbo expander, comprising: boiling liquid nitrogen in the natural gas liquefier module creating a nitrogen superheating region in the liquefier module; andintroducing two pressure levels of nitrogen cooling media in the nitrogen superheating region by using expansion of boiled nitrogen in the turbo expander.

18. A method as set out in claim 17, further comprising: monitoring nitrogen pressure input to the turbo expander; andcontrolling the flow of boiled nitrogen into the turbo expander to control the pressure of boiled nitrogen.

19. A method as set out in claim 18, further comprising: monitoring liquid nitrogen level in the liquefier module; andcontrolling the pressure of liquid nitrogen supplied to the liquefier module.

20. A method as set out in claim 17, further comprising reclaiming and liquefying waste nitrogen and reintroducing the reclaimed liquid nitrogen into the liquefaction system.