Patent Grant
8544296
The present invention is generally related to mechanical devices and fluid systems. One embodiment of the present invention relates to a temperature-actuated valve for regulating fluid flow. Another embodiment of the present invention relates to a high-efficiency compression-based heater discharge/expansion station.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Electrical or gas-fired heating, direct or using an intermediate heat transfer fluid, are exemplary methods used to heat a fluid by transferring thermal energy. Energy conversion in these methods is electrical to thermal using resistors, or chemical to thermal using combustion.
Adiabatic compression is known as the process through which gases are reduced in volume and as a byproduct, a large amount of energy is converted into heat. Most commonly, this heat is removed by a cooling fluid through heat exchangers. Immediately or eventually, most of this heat is disposed of into the environment. This heat is generally referred to as heat of compression.
Gas expansion has the opposite effect—the gas cools as it expands and most of the heat is absorbed directly or indirectly from the surrounding environment. Most gas pipelines also suffer from cooling as the gas expands and loses pressure through a pipeline, before coming to a booster station or gate station, where gas is expanded even further to reduce it to local transmission line pressures. Compressor booster stations reside along gas pipelines to increase pressure marginally and many times, due to their minimal temperature rise during compression, they are operated without an after-cooler, leaving most of the heat of compression in the pipeline. Expander stations typically use electrical or gas-fired heaters to increase the temperature to practical levels, for example to avoid hydrate formation.
Most compressors are driven using internal combustion engines, and these so-called drivers tend to have low energy conversion efficiencies, in the order of 25%-50%, with the rest of the energy converted to waste heat, which is disposed of into the surrounding environment.
In short, when a gas or vapor at high pressure expands through a valve into a reservoir at lower pressure, the pressure drop is accompanied by a cooling of the gas called the Joule-Thompson effect. If the gas cools too much, it can freeze in the gas line, plugging it. Additionally, if the temperature drops too low, components in the gas can condense forming droplets in the gas flow, and impurities such as water vapor can freeze on instruments and other parts causing damage.
This problem is particularly acute with wet natural gas, which is sometimes defined as natural gas that contains more than 10% C2 hydrocarbons or more than 5% C3 hydrocarbons. Wet natural gas may also contain some water, and sometimes may be saturated with water. When wet natural gas undergoes a pressure drop and expands through a valve, such as when a high pressure tank of gas is downloaded into a pipeline or to an end user, the resultant cooling can cause the high molecular weight components of the natural gas to condense, cause impurities such as water vapor or carbon dioxide to freeze, thus subsequently clogging the line, or cause solid chemical complexes called hydrates to form, also clogging the line.
Currently, the pipe leading from an expansion valve when natural gas is downloaded is heated to prevent condensation, freezing, and the formation of hydrates. During the course of a downloading process, the pressure drop varies, the amount of cooling changes, and hence the amount of heating needed to prevent problems changes. However, the current practice is to provide an excess of heat at all times during a natural gas pressure letdown procedure. This is fine at the beginning of the process when the need for heat is greatest, but is a waste of energy later in the process as more heat is being put into the expanding gas than is needed to prevent condensation, freezing, and hydrate formation. Given the rising cost of energy, this is also a waste of money.
It is against this background that various embodiments of the present invention were developed.
The present invention relates to a high-efficiency compression-based heater discharge/expansion station. The invention also features an apparatus and method for using a temperature actuated valve to automatically heat an expanding substance flowing through a pipe.
Therefore, one embodiment of the present invention is a fluid pressure letdown apparatus, comprising a first valve receiving a fluid via a first pipe with a pressure drop across said valve cooling the fluid; a heat exchanger for heating said cooled fluid received from the first valve via a second pipe; a temperature-measuring device disposed after the heat exchanger for measuring a temperature signal of the heated fluid via a third pipe; and a second valve that is automatically actuated by the temperature signal received from said temperature-measuring device that controls a flow of the fluid through the heat exchanger. When this fluid pressure letdown apparatus is used in the context of a large system, such as the natural gas discharge station described below, it is referred to as a “temperature-actuated valve.”
Another embodiment of the present invention is the system described above, wherein the heat exchanger comprises coolant fluid from an internal combustion engine that provides heat.
Another embodiment of the present invention is the system described above, wherein the heat exchanger is heated by electrical power.
Another embodiment of the present invention is the system described above, wherein the heat exchanger comprises heat that is provided by a hot fluid.
Another embodiment of the present invention is the system described above, wherein the heat exchanger comprises heat that is provided by a heat pump.
Another embodiment of the present invention is the system described above, wherein the heat exchanger comprises heat that is provided by waste heat from an external source.
Another embodiment of the present invention is the system described above, wherein the heat exchanger comprises heat that is provided by waste heat from a steam condensate return.
Another embodiment of the present invention is the system described above, wherein the temperature-measuring device is a thermostat.
Another embodiment of the present invention is the system described above, wherein the temperature-measuring device is a thermistor.
Another embodiment of the present invention is the system described above, wherein the temperature-measuring device is a thermocouple.
Another embodiment of the present invention is the system described above, wherein said second valve is automatically actuated by a signal carried through a wire from the temperature-measuring device.
Another embodiment of the present invention is the system described above, wherein said second valve is automatically actuated by a wireless signal from the temperature-measuring device.
Another embodiment of the present invention is a method for preventing a freezing of substance lines during a pressure drop across a valve and subsequent cooling, the method comprising the steps of measuring a temperature signal at an input of said valve; and actuating the valve to regulate a flow of a substance through a heat exchanger using the temperature signal such that if the temperature is too high said valve will open wider so that said substance spends less time in the heat exchanger reducing its temperature, and if the fluid temperature is too low the valve will tighten so the substance spends more time in the heat exchanger increasing its temperature.
Another embodiment of the present invention is the method described above, wherein the substance is natural gas.
Another embodiment of the present invention is the method described above, wherein the substance is wet natural gas.
Another embodiment of the present invention is the method described above, wherein the substance is a liquid.
Another embodiment of the present invention is the method described above, wherein the substance is a gas.
Another embodiment of the present invention is the method described above, wherein the substance is a powder.
Another embodiment of the present invention is the method described above, wherein the substance is a gel.
Yet another embodiment of the present invention is a natural gas discharge system for discharging high-pressure natural gas into a medium-pressure receiving location (such as interstate lines that typically operate over 1,000 psig), comprising an inlet port for receiving the high-pressure natural gas at a high inlet pressure; an expansion valve for regulating the pressure to a stable intermediate pressure; a cryogenic line disposed after the expansion valve for carrying a two-phase fluid mix comprising natural gas liquids and natural gas; a natural gas liquids recovery unit for recovering a portion of the natural gas liquids having a discharge line into a storage vessel adapted to store the recovered natural gas liquids for later pickup; a main heat exchanger for heating up a remaining fluid mix; a filtration vessel for vaporizing all remaining liquids and for filtering particulate matter resulting in a substantially pure natural gas stream; a compressor for compressing the natural gas stream and heating up the natural gas stream using heat of compression; and a discharge port for discharging the compressed, heated-up natural gas stream into the medium-pressure receiving location.
According to another embodiment of the present invention, the temperature-actuated valve described above is used in place of the compressor in the natural gas discharge system described above when discharging into a low-pressure receiving location.
According to yet another embodiment of the present invention, the temperature-actuated valve described above is used in the natural gas discharge system described above in addition to the compressor as a backup safety valve when discharging into a medium-pressure receiving location, such as interstate lines that typically operate over 1,000 psig.
Other embodiments of the present invention include the methods corresponding to the systems above, the systems constructed from the apparatus described above, and the methods of operation of the systems and apparatus described above. Other features and advantages of the various embodiments of the present invention will be apparent from the following more particular description of embodiments of the invention as illustrated in the accompanying drawings.
The following terms of art shall have the below ascribed meanings throughout this specification.
CNG is an acronym for Compressed Natural Gas, which is natural gas typically compressed to a pressure above approx. 2,000 psig.
Wet gas is natural gas that contains a high proportion of C2+ components (more than 10%); typically anything more than 5% C3+ is also considered wet gas. This is not an absolute definition, but a rule of thumb used in the literature. A dominant majority of wet gas is also often, but not always, saturated with water vapor.
Natural gas liquids (NGL)—C2+ components, including ethane, propane and heavier hydrocarbons.
Saturated gas is natural gas that is saturated with water vapor.
Dry gas is natural gas with <5% of C3+ components, or <10% C2+ components.
LPG is an acronym for Liquefied Propane Gas, which is generally a term for gas mixtures of C3+ components.
High-pressure or medium pressure receiving location is any receiving location that is over approximately 1,000 psig, such as interstate lines.
Low-pressure receiving location is any receiving location that accepts natural gas below approximately 1,000 psig, such as an end-user or industrial facility.
Joule-Thomson (“J-T”) Effect, also known as the Joule-Kelvin effect or the Kelvin-Joule effect, describes the temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment. This procedure is called a throttling process or Joule-Thomson process. At room temperature, all gases except hydrogen, helium and neon cool upon expansion by the Joule-Thomson process.
In order to reduce pressure in a container, such as a gas cylinder, expansion of gas through a valve is performed. This can be done in either of two modes:
(1) Fast discharge—intended to discharge the container as rapidly as possible, quickly cycling or emptying the cylinders. Usually this has to be discharged into an open-ended area that can absorb the discharge, typically a pipeline. When the pipeline operates at high pressure, the discharge will likely be to the inlet of a compressor that can boost the pressure back up to pipeline levels.
(2) Variable discharge—intended to feed the consumption of an industrial operation, a distribution/consumption line, or a vehicle/machine such as a drilling rig generator.
As a byproduct of expansion (for practical purposes expansion through a valve is to be considered isenthalpic), temperature of the gas drops as it expands (the J-T effect). Without adding heat to this now cold gas, it starts causing a host of issues, including but not limited to:
(1) Material failure due to exceeding the low temperature limits of the pipeline or coatings.
(2) Freezing CO2 and other non-hydrocarbon streams in the gas.
(3) Creating hydrates in the pipeline due to the reduced temperatures.
Heat is added after the expansion valve in order to bring it up to a desirable condition, typically in the 60 F-100 F range. Unless a significantly oversized heater and heat addition source is provided, controlling for the variations in heat required during the course of discharge is very challenging. At the start of the discharge, when the pressure differential is highest, the most amount of heat is required (on a per unit of mass basis), while little to no heat is required at the end of the cycle. Current practice is to supply an excess amount of heat at all times during the downloading process, keeping the gas above the freezing and hydrate formation temperature. Later in the downloading process much of this heat is not needed, which is a waste of energy and money.
In order to maintain a stable temperature given a pre-selected heat exchanger (sized for required flow conditions), as well as to maximize the heat added, one feature of one embodiment of the present invention is a temperature-actuated balancing valve used at the outlet. The temperature-actuated valve will only allow gas to flow through if it has attained a sufficiently high temperature. This temperature-based control allows for reduced flows at the onset of the cycle (given that the heat requirements are highest), and very high and full open flows at the end of the cycle (when practically no heat is required).
This temperature control in turn allows the utilization of a variable heat source, such as that found in waste heat streams such as cylinder jacket water from a combustion engine, steam condensate return, among others. The temperature-actuated valve eliminates gas flow through the heat exchanger in case insufficient heat is available, avoiding freezing incidents that could in turn burst the tubes or surfaces of the heat exchanger, causing a serious accident.
Therefore, one embodiment of the present invention is a gas discharge station utilizing a temperature-actuated valve (fluid pressure letdown apparatus). The temperature-actuated valve uses a temperature-measuring device to sense the temperature of the natural gas after it expands through an expansion valve and after it passes through a heat exchanger inside the discharge station. This temperature-measuring device sends signals to a valve that is automatically actuated. If the temperature of the gas is too low, the valve is tightened, increasing the residence time in the heat exchanger and increasing the gas temperature. If the gas temperature is too high, the valve is widened, reducing the residence time in the heat exchanger, and decreasing gas temperature. Using this temperature-actuated valve to control the temperature of a wet gas discharge station is described in greater detail below.
The present invention also allows pretreatment and cooling upstream of the expansion valve, in order to further maximize the J-T effect cooling and integrate cryogenic separation, for example. Pre-conditioning of gas before sending through a cryogenic expander is another possible use of the present invention. Allowing a safe, single-step reduction in pressure, which could in turn be utilized in a pressure letdown station at a city gate from a major pipeline, is another use.
There are many applications of the present invention, including discharge/unloading stations that have an isenthalpic expansion valve, or other pressure reduction device, and due to the related Joule-Thompson cooling effect, require heat to be added in order to avoid phase-separation, freezing, or adverse effects down the line. In particular, the present invention may be used to unload a predetermined amount of gas, stored in high-pressure cylinders, into a pipeline or other industrial/final user of the gas at a lower pressure.
The present invention can also be used in pipeline “city gate” pressure letdown locations, in liquefaction operations, and in natural gas liquids processing and separation plants.
Fluid Pressure Letdown Apparatus (“Temperature-Actuated Valve”)
Accordingly,
In the case of the fluid being wet natural gas, after exiting the valve 312, the expanded natural gas may safely enter a gas line 318, which may have additional wet gas 320 from another source, and safely supplied to an end user 322 without the issues, problems, risks, and safety concerns associated with prior art pressure letdown devices.
In one embodiment of the present invention, the heat exchanger obtains heat from a coolant fluid coming from an internal combustion engine. In another embodiment, the heat exchanger can obtain waste heat from a steam condensate return. In yet another embodiment, the heat exchanger can obtain heat by electrical means, such as a heating coil or heating tape. In yet another embodiment, the heat exchanger can obtain heat from a flow of hot gas, such as from the exhaust of any device that gives off waste heat. In yet another embodiment, the heat exchanger can obtain heat from a heat pump. Any device that gives off heat could be used in the heat exchanger to heat gas flowing through it.
In various embodiments, the temperature-sensing device could be a thermostat, a thermocouple, or a thermistor.
In one embodiment, the automatically-actuated valve can receive its signal from the temperature-sensing device through a wire. In another embodiment, the automatically-actuated valve can receive its signal from the temperature-sensing device wirelessly.
In one embodiment, the fluid flowing through the pressure letdown apparatus is natural gas. However, the present invention could be used to control the flow of any material passing through a pipe, such as any gas, vapor, liquid, powder, gel, or paste. The pressure letdown apparatus is particularly applicable to wet gas applications, since hydrate formation and freezing gas lines are a particular problem in wet gas discharge situations.
In summary, the fluid pressure letdown apparatus allows the flow-rate to be automatically adjusted depending on the heat capacity that is available. Thus, one advantage of the present invention is that the heat source can be swapped or switched when necessary without concern about heat mismatch.
In the prior art systems that do not utilize the fluid pressure letdown apparatus of the present invention, when wet gas is discharged, the pipes end up clogged as a result. For example, this occurs in Nigeria that is a typical place for flare gas recovery. Hydrate formation is an issue in natural gas pipelines, but since stranded associated wet gas (which is normally flared) hadn't been transported at pressure before, this has not been previously recognized.
One of the advantages of the present invention is that the heater can run at a lower temperature than in the prior art but still do its job effectively because of the feedback loop. The present invention also nearly eliminates the possibility of a heat exchanger freeze-up accident. In essence, the present invention allows one to have equivalent safety to an over-sized heat source, without the costs and inefficiency of running an oversized heating system or having to do multiple pressure letdowns in series, as typically done in the prior art.
Several flow control apparatus are described in the prior art that utilize temperature sensing. U.S. Pat. No. 6,125,873 issued to Daniel H. Brown describes a device for preventing water line freeze damage. The device incorporates air temperature sensing means to control a trickle flow in a water system, so that a trickle flow is initiated whenever the ambient air temperature drops below a predetermined point. The trickle flow inhibits freezing in the water system.
U.S. Pat. Nos. 6,626,202; 6,722,386; and 6,918,402 all issued to Bruce Harvey describes a flow control apparatus comprising a thermostat that automatically actuates a valve to enable water to flow through the valve when the temperature of the air or water is at or near the freezing temperature of water. When the temperature of the air or water rises above freezing, the thermostat causes the valve to close, thereby preventing water from flowing through the valve. Therefore, when the apparatus is coupled to an end of a water conduit, such as a water spigot or hose, water is allowed to flow through the conduit when the air or water temperature is at or near freezing to prevent the conduit from bursting due to water freezing and expanding within the conduit.
However, none of the prior art discloses or suggests a fluid pressure letdown apparatus, comprising a first valve receiving a fluid via a first pipe with a pressure drop across said valve cooling the fluid; a heat exchanger for heating said cooled fluid received from the first valve via a second pipe; a temperature-measuring device after the heat exchanger for measuring a temperature signal of the heated fluid via a third pipe; and a second valve that is automatically actuated by the temperature signal received from said temperature measuring device that controls a flow of the fluid through the fluid pressure letdown apparatus.
Natural Gas Discharge Station for Discharging into High-Pressure or Medium-Pressure Receiving Locations
Another embodiment of the present invention is a natural gas discharge station for discharging into high-pressure or medium-pressure receiving locations. One illustrative embodiment of the discharge station includes an expansion valve, followed by a heat exchanger, a gas/liquid separator/scrubber, and a subsequent compressor stage. After the final process, additional heat may be added or withdrawn from the system using an additional heat exchanger. As a heating fluid, waste heat from an internal combustion engine or driver may be used. To increase further the heat content of the heating liquid, cylinder jacket liquid may be circulated through a heat recovery exchanger at the exhaust of the engine, before transferring the thermal energy to the cool expanded gas. Thermostatic valves may be used throughout the process to regulate and stabilize operating temperatures in the auxiliary and main fluid circuits. To enhance the recovery of natural gas liquids (NGLs)—including ethane, propane and heavier hydrocarbons—an additional refrigeration circuit may be added mid-process, consisting of multiple heat exchangers and thermal transfer devices, as well as controls.
Referring now to aspects of the invention in more detail in
The heating circuit consists of a liquid coolant, which may be a mix of water and glycol or others, in any proportion, which flows through a coolant line (120) into a combustion engine (108), which typically serves as the driver for the compressor. Here, heat is extracted from the combustion process from cylinder jackets (109) and the resulting temperature in the hot post cylinder jacket coolant (112) is usually above 180 F. Afterwards, the hot coolant goes through a second heat exchanger (110) for recovering heat from the exhaust gases flowing through an engine combustion exhaust stack (111) in order to gather even more heat into line (113) which flows into the main heat exchanger (104) in order to transfer the thermal energy into the natural gas fluid coming from line (122).
All captured natural gas liquids flow through a discharge line (114) into an insulated or non-insulated capture vessel (115) in order to store the liquids for later pickup by a transport (117). In order for the liquids to be pumped into such transport, they flow through an exit line (116).
According to one embodiment of the present invention, shown in
In one embodiment, the refrigerated condenser (204) may have an external closed-loop refrigeration or heating system, to regulate the temperature of the fluid mix to optimal NGL extraction temperatures. The refrigeration/heating loop consists of a reversible rotary refrigeration compressor (212) running on nitrogen or propane, a condenser/evaporator (214), and an expansion valve (210).
Natural Gas Discharge Station for Discharging into a Low-Pressure Receiving Location Utilizing the Temperature-Actuated Valve
Yet another embodiment of the present invention is a natural gas discharge station for discharging into a low-pressure receiving location utilizing the temperature-actuated valve.
Unlike the embodiment shown in
A variation of this is discussed above in relation to a discharge station which unloads into a high-pressure or medium-pressure receiving location. In that embodiment, a compressor that accepts a fixed amount of mass while pressure is kept constant by the first valve 403 replaces the actuated valve 408. The heat added is variable and will depend at which point in the cycle the system is operating in. The role for this is to have a fixed/pre-determined discharge time for a high-pressure vessel while using the heat of compression as a means to reduce the total heat required. The compressor adds pressure and further depletes the incoming gas containers, which is particularly useful when unloading into high-pressure or medium-pressure receiving locations, such as interstate pipelines that typically operate over 1,000 psig.
In the application of tube trailer discharge stations, heating of the gas has been applied to compensate for the significant cooling effect caused by the large pressure drop from the storage containers, and to elevate the operating temperatures above freezing or the hydrate formation point. At times, tube trailers must discharge into high-pressure pipelines and thus leaving a significant volume of gas in the trailers, or use a booster compressor to continue depleting the tube trailer cylinders. Compressor cylinders are of standard design with a minimum inlet temperature, and to reach this temperature the cold expanded gas must be heated. In practice, a significant amount of energy is spent in heating the expanded gas to acceptable pipeline levels. The present invention alleviates these problems.
Fluid Pressure Letdown Method
Natural Gas Discharge Method for Discharging into a High-Pressure Receiving Location
While the methods disclosed herein have been described and shown with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form equivalent methods without departing from the teachings of the present invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations is not a limitation of the present invention.
Finally, while the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiments, methods, and examples herein. The invention should therefore not be limited by the above described embodiments, methods, and examples, but by all embodiments and methods within the scope of the invention, as defined in the appended claims.
This application is a non-provisional of and claims priority from provisional application U.S. Ser. No. 61/462,459, filed on Feb. 2, 2011, and entitled “High-Efficiency Compression-based Heater Discharge/Expansion Station,” the entirety of which is hereby incorporated by reference herein.
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| Number | Date | Country | |
|---|---|---|---|
| 20120192580 A1 | Aug 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61462459 | Feb 2011 | US |
