Patent Application
20190225891
The invention relates to a method for recovering natural gas liquid from a low pressure source.
A natural gas stream often contains light hydrocarbons. Natural gas liquids (NGL) is the general term for liquids extracted from the natural gas stream (ethane and heavier products) and within this liquefied petroleum gas (LPG) is the term used to refer to extracted liquids where the main components are propane, n-butane and iso-butane.
Removal of NGLs from natural gas is desirable for the following reasons:
Depending on the requirement, hydrocarbon dew point control packages or cryogenic plants can be used to extract NGL from gas streams. Hydrocarbon dew point refers to the temperature at any pressure range or the pressure at any temperature range where hydrocarbons begin to condense from the gas mixture. At the same temperature, heavier hydrocarbons' dew point temperature increases as the pressure is reduced
There are various types of Hydrocarbon Dew Point Control (HCDPC) units available in the market to extract NGL (Natural Gas Liquid) from a natural gas stream (associated or non-associated gas). The following is a brief review of the methods used to reduce hydrocarbon dew point in gas streams. As these processes are well known, for the sake of brevity, process descriptions are not included as they are well covered in the literature:
If the raw gas is at high pressure, the removal of hydrocarbons can be accomplished by refrigeration obtained through the expansion of gas by means of a Joule-Thomson (JT) valve. Injection of glycol is required to prevent the formation of hydrates. However, if the raw feed gas pressure is low, this will require booster compression for a JT valve system to be viable.
This process is a variation of the LTS process in which the energy pressure held in the gas is used to move an expander turbine, which in the isoentropic expansion generates refrigeration and exports mechanical work. This work is used to drive a compressor to partially restore the gas pressure. Here again, the raw feed gas pressure has to be relatively high to generate adequate chilling for NGL recovery.
The most common method used for gas dew point control is mechanical refrigeration. This technology is suited especially when pressure is not available to be used to self-refrigerate the gas. Mechanical refrigeration system however are bulky and expensive which includes compression equipment and power consumption.
This method uses adsorbents like silica gel that have the capability to adsorb heavy hydrocarbons. The system is set up in multiple beds cycling in short operating cycles of adsorption, desorption, of approximately 20 minutes. This method was well used in the 60s and early 70s and was gradually abandoned. Recently, new adsorption materials are making this method economically attractive for certain project applications. However, these adsorbent again typically operate effectively with higher feed gas pressures with regeneration and recovery of NGLs being undertaken at lower pressure and higher temperatures.
The Vortex-Tube Device and the Supersonic Tube technology. For these devices also require high pressure gas for the system to generate adequate chilling of the gas stream for NGL extraction.
Silicon rubber membranes, for example, have the ability to permeate heavy hydrocarbons rather than light. This makes them a potential candidate for dew point control. However, these systems require some amount of pre-treatment to protect the membranes and compression of the permeate stream to minimize NGL losses. In addition, to be economically viable, these systems require relatively high inlet gas pressures.
As can been seen from the preceding discussions, whilst there are many NGL recovery systems by means of various types of HCDPC units, these are only really suitable for feed gas streams that operate at relatively high pressures. In addition, the few, like refrigeration systems that can handle low pressure feed gas streams, are very bulky, complex and costly, making them economically not viable for many low pressure applications.
An aim of the invention therefore is to provide a system for recovering NGLs which operates effectively with a low pressure source of natural gas.
In an aspect of the invention, there is provided a system for recovering natural gas liquid from a gas source, comprising:
Advantageously, cooling the gas after separation and injecting liquid thereinto to saturate the gas with evaporant allows the gas/gas heat exchanger to be much more effective at low gas source pressure.
In one embodiment the expansion means is a Joule-Thomson valve. In another embodiment the expansion means is a turbo expander. In yet another embodiment, the expansion means is a Static Expansion Device such as a Vortex-Tube Device or Supersonic Tube technology. Typically the expansion means reduces the pressure of the gas and as a result reduces the temperature thereof.
In one embodiment the liquid used to saturate the gas is water. However it will be appreciated that other liquids e.g. propane may be used as a suitable evaporant depending on the temperature and pressure conditions.
In one embodiment at least part of the water used to saturate the gas is derived from the separator and/or other downstream sources.
Typically the liquid injection means is downstream of the expansion means.
In one embodiment the liquid is injected downstream of the expansion means at a rate to enable the gas at the second outlet to be saturated. This maximizes evaporative cooling duty at the gas-gas heat exchanger.
Typically the liquid injection means is a sprayer for spraying the liquid into the gas as a mist.
In one embodiment a hydrate or ice inhibitor is mixed with the injection liquid to prevent freezing thereof. Typically the hydrate or ice inhibitor is methanol or Mono Ethylene Glycol (MEG).
In one embodiment liquid is injected in excess of the saturation amount to avoid solid deposition at the gas/gas heat exchanger. Advantageously the excess liquid ensures that total dissolved solids (TDS) content of the liquid phase does not exceed its saturation point, thereby helping to prevent precipitation of any of the TDS in the system.
In a conventional system the JT valve or other expansion means is upstream of the separator. When the gas source is at high pressure a large pressure drop can take place at the JT valve resulting in a large reduction in temperature. However, for low pressure gas sources only a small pressure drop can take place, so the reduction in temperature is smaller. Thus in a conventional system the heat exchanger is ineffective for low pressure gas sources.
However, in the present invention the JT valve or other expansion means is downstream of the separator, and liquid evaporant such as water is injected to increase the enthalpy of the cooling fluid, reducing the temperature thereof compared to a conventional system and making the heat exchanger effective even for low pressure gas sources.
In one embodiment the gas from the second outlet may be flared off.
In one embodiment the liquid from the separator comprises water and hydrocarbons (including NGL), which are directed to respective outlets for further treatment.
In one embodiment the system is effective at recovering NGL from gas sources at less than 20 barg (2.1 MPa), such as 10 barg (1.1 MPa) or 3 barg (400 kPA) or less. Typically the NGL recovery is 80% or more at 5 barg (600 kPa) compared to a conventional system at 50 barg (5.1 MPa; a conventional system at 5 barg (600 kPA) may only have NGL recovery of 1% or less compared to its operation at 50 barg (5.1 MPa)).
It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.
Hydrocarbon Dew Point Control (HCDPC) of low pressure gas uses the concept of evaporative cooling, coupled with a gas expansion device which may either be a JT Valve, Static Expansion Devices or a Turbo-Expander, to chill the gas stream to condense and remove the heavier hydrocarbon components (NGLs) from the natural gas stream.
Evaporative cooling is the addition of water vapour into gas that is water dew pointed, which causes a lowering of the temperature of the gas. The energy needed to evaporate the water is taken from the gas in the form of sensible heat, which reduces the temperature of the gas, and converted into latent heat, the energy present in the water vapour component of the gas, whilst the gas remains at a constant enthalpy value. This conversion of sensible heat to latent heat is known as an adiabatic process because it occurs at a constant enthalpy value. Evaporative cooling therefore causes a drop in the temperature of gas proportional to the sensible heat drop and an increase in humidity (or water vapour content) of the gas proportional to the latent heat gain.
A simple example of natural evaporative cooling is perspiration, or sweat, secreted by the body, evaporation of which cools the body. The amount of heat transfer depends on the evaporation rate, however for each kilogram of water vaporized 2257 kJ of energy at 35° C. are transferred. The evaporation rate depends on the temperature and humidity of the air, which is why sweat accumulates more on humid days, as it does not evaporate fast enough.
The evaporative cooling medium as used in this invention is typically fresh (demineralized) water but may be any medium that achieves vaporization in the gas stream to convert sensible heat in the gas to latent heat of vaporization of the medium.
It is also noted that the description of the system as detailed in this document are mainly applicable for low pressure systems, where typically water is used as the evaporative medium, the concept as detailed here may also be used for high operating pressure systems with a suitable alternative evaporative medium.
In the case where water is used as an evaporative medium, this concept is particularly suited for low pressure gas stream which does not have enough upstream pressure to chill the gas on expansion through either a JT Valve, Static Expansion Devices or a Turbo-Expander (or a combination). It is noted that, typically on expansion of low pressure gas, the water dew point of the expanded (lower pressure) gas is significantly lowered. As illustrated in
There are many facilities where natural gas is produced at low pressures of between 3 to 20 barg (400 kPa-2.1 Mpa) and these include:
The installation of HCDPC units for NGL extraction from low pressure natural gas has both the economic and environmental benefits as the main polluting components from the off gas are separated and the value added products like lean natural gas and NGL are produced. The burning of methane rich gas produced from this unit without polluting and soot forming components is beneficial from an environmental point of view.
While it is noted that NGLs constitute a small fraction of natural gas from wells and various other sources, its contribution towards greenhouse gas emissions is significant when the gas is burnt as fuel gas or flared. Typically, CO2 emissions can be reduced by approximately 30% with extraction of NGLs from the gas. It is more significant for low pressure natural gas as the vapour liquid equilibrium favours vaporization of heavy ends into the gas phase resulting in higher content of NGLs in the gas stream. Ironically though, it is the low pressure natural gas streams that are typically disposed as fuel gas or flared as is uneconomical to recover.
With regard to
The conventional system 2 comprises a gas/gas heat exchanger 4, a JT valve 6, and a separator 8. Feed gas 10 at 5 barg (600 kPa) and 68.8° C. is passed through the heat exchanger where it is cooled to 58.8° C. at 4.8 barg (580 kPa). The gas is then passed through the JT valve where it is further cooled to 40.6° C. at 0.5 barg (150 kPa) into the separator where gas is separated from liquid. The liquid NGL and water components are directed to respective outlets 12, 14 for further processing. The gas is directed to a pipe 16 for use as a cooling fluid in the gas/gas heat exchanger 4, before being sent to a flare point 18.
As can be seen, minimum temperatures achievable with the conventional configuration, with feed gas pressure of 5 barg (600 kPa) and temperature of 68° C., is approximately 40.5° C. In addition, the Cold Separator being located downstream of the JT Valve, thus operating close to atmospheric pressure, will not result in significant NGL (condensate) recovery as indicated in the above example where only 0.4 bpd of NGLs is condensed at the Cold Separator.
As such the industry is striving for a new flexible, reliable and a safe process that can cost effectively extract NGLs from low pressure natural gas.
In more detail:
Unlike conventional JT Valve systems, where the JT Valve is located upstream of the cold separator, according to this embodiment of the invention the JT Valve is located downstream of the Cold Separator. This configuration maximizes liquid drop-out from the associated gas stream for low operating pressures of the associated gas as the condensate removal is done at higher pressures for the reasons set out hereinbefore.
Note that if the operating temperatures are dropped to below the hydrate or ice formation temperature, hydrate (or anti-freeze) injection will be required. The advantage of this system is that the HCDPC unit is operated at low pressure, resulting in hydrate formation temperatures being significantly lower, thus mostly avoiding the need for hydrate inhibitor. If, however, operating temperatures are below 0° C., anti-freeze injection will be required.
Downstream of the JT Valve, with pressure letdown, the water content in the gas is lower than saturation. In addition, with the gas being further heated at the Gas/Gas Exchanger, the gas stream is further under-saturated. This is demonstrated in
For the current example, Point B on the above plot corresponds to the water content of gas from the Cold Separator, upstream of the JT Valve (which is at saturated water content conditions). If no water is added to the gas stream downstream of the JT-Valve, the water content of the gas at the outlet of the Gas-Gas-Exchanger will remain as that at Point B. To maximize the chilling effect due to evaporation of water, water is required to be injected to fully water saturate the gas at the outlet of the Gas-Gas Exchanger. This corresponds to Point C in the above plot. Thus, the amount of fresh water needed to be injected corresponds to the differential water content of the gas between Point B and Point C. This is equivalent to a water injection rate of approximately 2750 kg/MMscf of gas. Point A of the above plot corresponds to the water content of the hot feed gas stream at inlet to the Gas-Gas Exchanger. An excess amount of water is dosed into the gas to prevent fouling at the Gas-Gas Exchanger. The amount of excess water dosed is such that at the outlet of the Gas-Gas Exchanger, the TDS in the water phase is below its saturation TDS content. This is to ensure that solids do not precipitate out of the water as it vaporized in the Gas-Gas Exchanger.
Thus, the sensible heat due to heating the gas from Point B to Point C plus the latent heat due to vaporization of the water phase as the fluid transverses the Gas-Gas Exchanger is exchanged with the hot feed gas stream to cool the latter. The latent heat due to vaporization of the water phase provides the additional chilling duty to maximize the chilling duty of the exchanger, thus, further cooling the hot feed gas stream.
Another feature of the system is to utilize condensed water, separated from condensed NGLs, in the Cold Separator as part or all of the water dosing requirements. This will eliminate or minimize the need for fresh water requirements from an external source. For the above example, the amount of condensed water collected in the Cold Separator is equivalent to water content of Point A (1150 kg/MMscf) minus water content of Point B (˜0 kg/MMscf) which corresponds to approximately 1150 kg/MMscf This is the amount of water that is re-injected into the gas stream downstream of the JT-Valve. Thus, the net amount of make-up fresh water required is 1600 kg/MMscf (2750 kg/MMscf minus 1150 kg/MMscf) to fully saturate the gas at cold side outlet of the Gas-Gas Exchanger. Note also that the fresh water may also be sourced from upstream or downstream of the process provided it has low impurities and TDS that will not contribute to fouling of the Gas-Gas Exchanger.
Another feature of this invention is that, whilst for conventional systems, the cold separator is installed downstream of the gas expansion device where the gas supply pressure is typically above its cricondentherm, for low pressure feed gas, the cold separator is upstream of the device as it is at the higher pressure where higher liquid drop-out occurs when gas feed is lower than its cricondentherm pressure, although temperatures are higher than that downstream of the gas expansion device. This point is demonstrated on the phase envelope of the gas in
In the plot, Point A is the operating point of the feed gas stream, Point B is the operating point of the gas upstream of the expansion device and Point C is the operating point of the gas downstream of the expansion device. It is noted that if the gas/liquid separation is performed at Point B, i.e. downstream of the expansion device, as is done conventionally, Point B sits on the quality line 0.015. If however, the gas/liquid separation is undertaken upstream of the expansion device i.e. at Point C, in accordance with the invention, Point C sits on the quality line 0.025. This indicates that with gas/liquid separation being performed upstream of the expansion device when operating pressures are low (approximately less than the cricondentherm), the operating point moves deeper into the phase envelope, thus resulting in higher amounts of NGL recovery from the natural gas stream, although temperatures are higher upstream of the expansion device.
Downstream of the expansion device which is at a lower pressure, the water dew point of gas, in many cases, is expected to be lower than the operating temperature of the gas. An exception may be when a turbo-expander is used as the expansion device where, due to the relatively deep chilling inherent with these devices, the exiting gas temperature may be below its water dew point temperature. In either case, as the expanded gas is heated-up via cross heat exchange with incoming feed gas at the Gas-Gas Exchanger, the gas will be superheated.
Conventionally, the leaner gas after the expansion device (with condensed liquids already removed) has a smaller mass flow (and enthalpy) than the incoming rich feed gas stream. This results in limited heat transfer between the two streams at the Gas-Gas Exchanger. This makes the system in its conventional form very ineffective particularly when feed gas pressures are low.
To overcome the above issues, the strategy adopted, in accordance with the invention, is to inject an evaporative medium; in this case water, into the low pressure gas stream, downstream (or upstream) of the gas expansion device, to provide additional chilling duty to cool the water dew-pointed gas which in turn cools the incoming feed gas stream. To maximize the chilling duty of the Gas-Gas Exchanger, water is injected at a rate such that the gas exiting the Gas-Gas Exchanger is water saturated with a slight excess water to ensure that the Total Dissolved Solid (TDS) content in the water does not exceed its saturation point. This is to avoid precipitation of solids from the water at the Gas-Gas Exchanger.
It is noted that if hydrates or ice formation is envisaged under operating conditions, appropriate hydrate inhibitor, like methanol, MEG, etc. will be required.
With reference to
Crude is spiked into the associated gas stream to shift the vapor faction of the gas stream from single-phase region into two-phase region. Thus, the phase envelope of the stream to the HCDPC unit changes and maximizes condensate recovery on cooling to a specified temperature.
The advantage is that the dewpointing of the gas and the recovery of the C4+ components from the gas stream can be undertaken at relatively high temperatures (of approximately 12° C.) which will avoid operating within the hydrate formation and icing temperature range, thus avoiding the need for hydrate inhibitors. Other than that, crude spiked can act as absorbent for the absorption of some C4+ component from the gas stream thus maximizing oil recovery.
The system 202 is similar to the prior art illustrated in
As with the JT Valve system, whilst the Turbo-Expander is widely used for hydrocarbon dew point control, this system is not suitable for low pressure gas e.g. below 20 barg (2.1 MPa). As can be seen, minimum temperatures achievable with the conventional configuration, with feed gas pressure of 5 barg and temperature of 68.8° C., is approximately 26.4° C. and will not result in significant NGL (condensate) recovery as indicated in the above example where only 112 BPD of NGLs is condensed at the separators.
The following describes the configuration of the system 302:
Unlike conventional Turbo-Expander systems, most of the condensed hydrocarbon liquids are collected in the separator upstream of Turbo-Expander. This configuration maximizes liquid drop-out from the associated gas stream for low operating pressures of the associated gas as the condensate removal is done at higher pressures.
Unlike the case with the JT Valve system, downstream of the Turbo-Expander, due to the deep chilling effect, temperatures are expected to drop to below the gas water dew-point unless the gas is dehydrated upstream of the Turbo-Expander. Notwithstanding this, the gas phase is already saturated with methanol and will prevent freezing of water drop-out. With the gas being further heated at the Gas/Gas Exchanger, the gas stream exiting the Gas-Gas Exchanger will conventionally be under-saturated.
In the similar fashion to that of the JT-Valve embodiment, to maximize the chilling effect due to evaporation of water, water with an appropriate concentration of hydrate or ice inhibitor is injected fully water saturate the gas at the outlet of the Gas-Gas Exchanger. An excess amount of water is dosed into the gas to prevent fouling at the Gas-Gas Exchanger. The amount of excess water dosed is such that at the outlet of the Gas-Gas Exchanger, the TDS in the water phase is below its saturation TDS content. This is to ensure that solids do not precipitate out of the water as it vaporized in the Gas-Gas Exchanger.
Another feature of the system is to utilize condensed water, separated from condensed NGLs, in the Cold Separator and downstream gas scrubber as part or all of the water dosing requirements. This will eliminate or minimize the need for fresh water requirements from an external source. Note also that the fresh water may also be sourced from upstream or downstream of the process provided it has low impurities and TDS that will not contribute to fouling of the Gas-Gas Exchanger.
It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the system which does not affect the overall functioning of the system.
| Number | Date | Country | Kind |
|---|---|---|---|
| PI 2016001286 | Jul 2016 | MY | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/MY2017/050036 | 7/10/2017 | WO | 00 |
