Separation of a Fluid Mixture Using Self-Cooling of the Mixture

Abstract

The use of an inexpensive heat exchanger for self-cooling of a fluid mixture is disclosed. The heat exchanger is made up of a plurality of plates and each plate has at least two channels defined in the plate. A fluid mixture is cooled and expanded, and separated, generating at least two process streams. One or more of the process streams are passed back to the heat exchanger to cool the fluid mixture.

Description

FIELD OF THE INVENTION

The present invention relates to the cooling of fluids through the self-cooling from the fluid. More particularly this invention goes to the cooling of a fluid to self-cool the fluid and to cool and separate the self-cooled fluid.

BACKGROUND OF THE INVENTION

The demands for natural gas have increased in recent years. The transport of natural gas is through pipelines or through the transportation on ships. Many areas where natural gas is located are remote in the sense that there are no convenient pipelines to readily transfer the natural gas to. Therefore natural gas is frequently transported by ship. The transport of natural gas on ships requires a means to reduce the volume and one method of reducing the volume is to liquefy the natural gas. The process of liquefaction requires cooling the gas to very low temperatures. There are several known methods of liquefying natural gas as can be found in U.S. Pat. No. 6,367,286; U.S. Pat. No. 6,564,578; U.S. Pat. No. 6,742,358; U.S. Pat. No. 6,763,680; and U.S. Pat. No. 6,886,362.

One of the methods is a cascade method using a series of shell and tube heat exchangers. These shell and tube heat exchangers are very large and very expensive, and present problems of economics and feasibility for remote and smaller natural gas fields. It would be desirable to have a device for liquefying natural gas that is compact and relatively inexpensive to ship and use in remote locations, especially for natural gas fields found under the ocean floor, where collection and liquefaction of the natural gas can be performed on board a floating platform using a compact unit.

There is also an increasing demand for methods of cooling gases to condense them for transport or for separation purposes. The self cooling provides an opportunity to separate fluids that will generate liquids during the cooling process. Improvements over the current commercial design can include lower cost, lower weight, and provide a more compact structure as well as provide improved heat transfer characteristics, for providing for on site separation of hydrocarbons.

BRIEF SUMMARY OF THE INVENTION

The use of a compact self-cooling heat exchanger for separating a fluid mixture is disclosed. The heat exchanger is made up of a plurality of plates and each plate has at least two channels defined in the plate. A fluid mixture is passed through a heat exchanger and cooled. The fluid mixture is expanded through a controlled expansion to provide the desired cooling load, and conditions for passing the expanded fluid mixture to a separation unit. The fluid mixture is expanded to create a two phase mixture of liquid and vapor. The two phase mixture is passed to a separation unit, where a first process stream enriched in at least one component is produced, and a second process stream enriched in at least a second component is produced. The first process stream and the second process stream are passed to the heat exchanger and provide cooling to the feed mixture passed into the heat exchanger. Alternatively, if the separation is preferentially accomplished at elevated pressure, the fluid mixture, being cooled by passing through the heat exchanger, can be separated in a separation unit, upon which both streams can be expanded separately and generate the cooling load. Both expanded streams are then passed back to the heat exchanger, to provide the cooling to the feed mixture. The location of the diversion of the fluid mixture to the separation unit and its potential re-introduction into the heat exchange device, can be chosen such that the separation proceeds under conditions of optimum energy efficiency.

Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the separation through self cooling process;

FIG. 2 is a diagram of the separation process with liquids separated at an intermediate stage of cooling; and

FIG. 3 is a diagram of the process using a self-cooling refrigerant.

DETAILED DESCRIPTION OF THE INVENTION

The use of liquefied natural gas (LNG) is increasing, as fuel and a means of transporting natural gas from remote sites having natural gas, without a nearby gas pipeline, to more distant areas where the natural gas is consumed. Natural gas is typically recovered from gas wells that have been drilled and is in the gas phase at high pressure. The high pressure gas is then treated and passed to a pipeline for transport. However, there are an increasing number of natural gas fields that are in remote locations relative to natural gas pipelines. The present invention is directed to a heat exchanger for cooling the natural gas at the gas wells. By providing an inexpensive heat exchanger for cooling and liquefying natural gas in remote locations, natural gas can be recovered on site and transported as LNG, rather than requiring a natural gas pipeline, or transporting the gas at very high pressures. In addition, improved methods for liquefying natural gas can also lead to improved methods for separation of some of the components of natural gas during the process of liquefaction.

The present invention provides for the separation of components of a mixture using self-cooling of the mixture to facilitate the separation process. The heat exchanger of the present invention is fabricated by plates that are bonded together to form an integral unit. The plates have channels etched, milled, pressed, stamped, inflated, or by other methods known in the art, into them for the transport fluid or fluids. When the plates are bonded together, the channels are covered and form conduits through which fluids can flow. The bonding method will depend on the materials of construction, such as with aluminum plates, bonding involves brazing the aluminum plates together. With steel, diffusion bonding or welding can be performed to bond the steel plates together. Other means of bonding plates are known to those skilled in the art. Fluid access to the plurality of plates can be provided through one or more manifolds, wherein each manifold includes at least one channel that is in fluid communication with all corresponding channel inlets or channel outlets of the plates.

The present invention takes advantage of the use of a compact self-cooling heat exchanger to separate the components of a mixture. A simplified form of the process is shown in FIG. 1. One plate 10 of the self-cooling heat exchanger is shown, where a mixture enters a first channel 12. The mixture passes through an expansion device 20, where the fluid is expanded substantially adiabatically and cools the fluid. The fluid can become an intermediate two phase fluid comprising a liquid and vapor phase. The expanded fluid, or intermediate stream, is passed through part of the heat exchanger to provide a portion of the self-cooling of the mixture. The intermediate stream is passed to a separation unit 30 wherein the intermediate stream is separated into a first stream 32 and a second stream 34. The first stream 32 is passed back to the self-cooling heat exchanger through a second channel 22 to contribute to the cooling of the fluid mixture in the first channel 12. The second stream 34 can also be passed through the heat exchanger in a third channel 24 to further contribute to the cooling of the mixture in the first channel 12. The separation can comprise separating a liquid and vapor, such that the first stream 32 is a vapor stream and the second stream 34 is a liquid stream.

While the channels 12, 22, 24 are substantially parallel to provide good heat transfer characteristics, the flow of the first stream 32 in the second channel 22 is in a counter current direction relative to the flow of the mixture in the first channel 12. The flow of the second stream 34 in the third channel 24 is also in a counter current direction relative to the flow of the mixture in the first channel 12. It is preferred that both streams 32 and 34 are passed back through the heat exchanger. The efficiency of the heat exchanger is partially dependent on the form of the hot and cold composite curves in the temperature-enthalpy diagram (T, h). Maintaining the slopes of the two temperature-enthalpy curves to be substantially parallel and in close proximity to each other, substantially sets the efficiency of the cooling process. The slopes of the curves are related to the total enthalpy, the mass flows and the heat capacity of the various streams, through the formula dT/dh−1/(m*Cp). The re-introduction of all mass of the separated streams back into the exchanger impacts the slope of the composite curves and allows them to be substantially more parallel to each other, which, in turn, allows the process to run more efficiently.

The expansion device, such as an orifice or restriction, is for controlling the expansion of the fluid mixture. One expansion device 20 can be a Joule-Thomson valve, comprising a conic shaped needle valve that matches a conic shaped seat. For multiple plates, the Joule-Thomson valves in each plate can be commonly connected through a shaft that extends through each valve.

In another embodiment, the expansion device 20 can be a mechanical device for extracting work. One example for extracting work is a micro-turbine, such that during the expansion, the micro-turbine is connected through a drive shaft to an external device for performing work, such as a generator. The amount of energy recovered, or conversely, the level of fluid expansion attained, can be controlled by means of variable resistance to the drive shaft of the turbine, or turbines. This allows operation according to the cooling demand required.

The intermediate stream passed to the separation unit 30 can be expanded to cool the intermediate stream and produce a two phase stream. The two phase stream can be separated in a separation unit 30 to produce a liquid phase 34 bottoms stream and a vapor phase 32 overhead stream. The separation unit 30 can be a simple vapor-liquid separator, or can comprise a fractionation column for producing two streams having increased purity over a simple vapor-liquid separator. The vapor phase 32 will be richer in at least one component of the mixture, and the liquid phase 34 will be richer in at least one other component of the mixture. When the mixture is natural gas, as extracted from the ground, the mixture is at a high pressure and at a relatively warm temperature. The mixture contains natural gas liquids (NGL), which can be recovered upon cooling of the natural gas mixture. Natural gas liquids comprise C3 and higher hydrocarbons, and some ethane that can be found in natural gas.

A second design for the process is shown in FIG. 2. The process comprises passing a mixture to a heat exchanger. In each plate 10 of the heat exchanger, the mixture is partially cooled in a first channel 12. The mixture can comprise a two phase stream of vapor and liquid, and with partial cooling can increase the liquid content of the stream. The mixture is passed to a separation unit 30, where the mixture is separated into a vapor stream 32 and a liquid stream 34. The vapor stream 32 is passed to the heat exchanger to a second channel 22. The vapor stream 32 is expanded through and expansion device 20 to produce a cooled stream. The cooled stream can comprise a two phase stream of liquid and vapor, or can remain a vapor stream. The cooled vapor stream in the second channel 22 flows in a generally counter current direction to flow of the mixture in the first channel 12, providing cooling for the mixture. The liquid stream 34 is passed to the heat exchanger to a third channel 24 and is cooled by the cooled vapor stream. When the mixture is a natural gas feed, the partial cooling and separation can remove natural gas liquids from the natural gas feed, and, in doing so, make sure the natural gas has the right energy content prior to liquefaction.

A third design for the process is presented in FIG. 3. The figure presents one plate 10 that is part of a larger heat exchanger comprising a plurality of plates. The process is for separating and cooling a mixture by creating a stream comprising a vapor and a liquid. The mixture is passed to a channel 12 in a heat exchanger and partially cooled. The partially cooled mixture is passed to a separation unit 30 where a vapor stream 32 and a liquid stream 34 are created. The vapor stream 32 is passed to a second channel 22 for cooling, and the liquid stream 34 is passed to a third channel 24 for cooling. A refrigerant is used to self-cool the refrigerant and to cool the vapor stream 22, the liquid stream 24 and the mixture 12. The refrigerant is passed to a fourth channel 32 where the refrigerant is cooled. The refrigerant is expanded through an expansion device 20, to produce an expanded and cooled refrigerant. The cooled refrigerant is passed back through fifth channel 34 providing cooling to the refrigerant in the fourth channel 32. The expansion is controlled to generate the desired cooling load for the refrigerant and for the mixture, and for the liquid and vapor streams.

The process of the present invention provides for the separation of hydrocarbon mixtures by using the cooling effect of the hydrocarbon mixture when expanded under substantially adiabatic conditions. One hydrocarbon mixture of interest is natural gas. When natural gas is recovered, it needs to be processed before transport. This is particularly true for natural gas recovered in remote locations where access to a pipeline is not available. The natural gas can be fed to a heat exchanger to be expanded and cooled to cool the natural gas, and to separate out the natural gas liquids, and further using the separated natural gas and natural gas liquids to contribute to the cooling of the natural gas feed. Other components in the natural gas feed can also be removed by controlling the amount of expansion of the natural gas feed.

The process is also applicable to other hydrocarbon mixtures. Mixtures of paraffins and olefins can be separated by cooling and performing cryogenic separation. One example is propane and propylene, where propane has a boiling point of −42° C. and propylene has a boiling point of −47.6° C. at atmospheric pressure. Cooling a propane/propylene mixture to form a two phase system at a low temperature, and then passing the cooled mixture to a fractionation column permits separation. The separated streams are then passed back to the heat exchanger to provide cooling of the feed stream mixture. Other separation processes are known to those skilled in the art, and are applicable for many of the hydrocarbon separations, including adsorption separation processes.

While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims

1. A process for separating the components of a mixture, by creating a stream comprising a liquid and a vapor, the process comprising:

passing the mixture through a self-cooling heat exchanger;

controlling the expansion of the mixture within the heat exchanger;

passing the liquid and vapor though a separation unit, thereby creating a first stream and a second stream; and

passing the first stream or the second stream through the self-cooling heat exchanger

2. The process of claim 1 further comprising passing the other of the first or second streams through the self-cooling heat exchanger.

3. The process of claim 1 wherein the first stream comprises a vapor stream.

4. The process of claim 2 wherein the second stream comprises a liquid stream.

5. The process of claim 1 wherein the separation unit comprises a fractionation column, thereby generating an overhead first stream, and a bottoms second stream.

6. The process of claim 1 wherein the separation unit comprises a vapor/liquid separator.

7. The process of claim 1 wherein the mixture comprises a hydrocarbon mixture.

8. The process of claim 7 wherein the mixture comprises natural gas containing natural gas liquids.

9. The process of claim 7 wherein the mixture comprises olefins and paraffins

10. The process of claim 1 wherein the control of the expansion is through a Joule-Thomson valve.

11. The process of claim 1 wherein the control of the expansion is through a mechanical device for extracting work.

12. The process of claim 11 wherein the mechanical device is a micro-turbine.

13. The process of claim 11 further comprising recovering power from the micro-turbine.

14. The process of claim 11 further comprising controlling the expansion with the micro-turbine to control the amount of cooling.

15. The process of claim 1 wherein the first or second stream flows in a substantially counter-current direction to the flow of the mixture in the heat exchanger.

16. The process of claim 2 wherein the second or first stream flows in a substantially counter-current direction to the flow of the mixture in the heat exchanger.

17. A process for separating the components of a mixture, by self-cooling the mixture and creating a stream comprising a liquid and a vapor, the process comprising:

partially cooling the mixture through a heat exchanger;

passing the partially cooled mixture to a separation unit, thereby creating a liquid stream and a vapor stream;

passing the liquid stream through the heat exchanger;

passing the vapor stream through the heat exchanger;

controlling the expansion of the vapor stream in the heat exchanger to generate the desired cooling, thereby creating a cooled stream; and

passing the cooled stream through the heat exchanger to cool the mixture, liquid, and vapor streams.

18. The process of claim 17 wherein the mixture comprises a natural gas stream and the liquid stream comprises natural gas liquids.

19. A process for cooling and separating a mixture by creating a stream comprising a liquid and a vapor, the process comprising;

partially cooling the mixture through a heat exchanger;

passing the partially cooled mixture to a separation unit thereby creating a liquid stream and a vapor stream;

passing the liquid stream through the heat exchanger;

passing the vapor stream through the heat exchanger;

passing a refrigerant through the heat exchanger;

controlling the expansion of the refrigerant to generate the desired cooling and to generate a cooled refrigerant; and

passing the cooled refrigerant through the heat exchanger to self-cool the refrigerant and to cool the mixture, liquid and vapor streams.