FIELD OF INVENTION

The present invention relates to the field of natural gas and condensate processing for the extraction of natural gas liquids.

BACKGROUND OF THE INVENTION

Hydrocarbon production streams in the upstream and midstream sectors of the industry frequently comprise a mixture of natural gas, natural gas liquids, condensate and a proportion of non-hydrocarbon components including water. It is typically required to separate such hydrocarbon streams according to boiling point into products and remove impurities such as water for downstream processes and consumers.

Depending on the nature of the reservoir from which the gas is produced, production of hydrocarbon gas frequently results in a mixed phase fluid stream at surface production facilities, comprising hydrocarbon gas, hydrocarbon liquid and free water.

Generally, the first processing step at the surface facilities is gravity separation of the produced well fluid to produce separate gas, liquid hydrocarbon and produced water streams. The water stream is disposed of after further treatment to reduce the level of hydrocarbons and other contaminants in the water to within disposal limits. The gas and liquid hydrocarbon streams both require further processing before they can meet specifications for transport, storage and sale to downstream users.

The gas stream is saturated with both water vapour and heavier hydrocarbons at separation conditions, and partial condensation of both phases will occur in the event of temperature reduction or pressure change. Both the water and hydrocarbon dewpoint must be reduced in order to bring the gas to sales quality. The liquid stream comprises mixed hydrocarbons of varying molecular weights. Any pressure reduction will result in vaporisation of the lighter hydrocarbon species. This presents problems in the handling and storage of the liquid. Furthermore, both the gas and liquid streams contain significant quantities of propane and butane, which have a higher value as separate products, compared to leaving them in the sales gas and stabilised condensate. Recovery of these valuable components from gas is important in the industry using well established processes. Therefore there is a need for a process system that can extract NGLs from water saturated, gas, liquid or mixed phase hydrocarbon feed streams where the process prevents hydrate formation, freezing and blockage, is thermodynamically efficient and minimises equipment requirements, yet still achieves high NGL recovery.

SUMMARY OF THE PRESENT INVENTION

The invention integrates the gas and liquid treatment processes into a single process that is tolerant to the presence of water in some of the feedstreams, whilst also using cryogenic fractionation to achieve high NGL recovery.

One embodiment of the invention is a system for processing gas, liquid or mixed phase hydrocarbon feed streams, using a single fractionation column that comprises; an upper section that operates below hydrate formation and water freezing temperatures, receiving feed streams that have been dehydrated, typically to ppm or lower levels; a lower section that operates at elevated temperatures and receives gas, liquid or mixed phase hydrocarbon feed streams that may contain water and/or water vapour; a reboiler unit for re-boiling the liquid from the bottom of the lower section and returns gas to the lower section; an inter-stage dehydration unit for dehydrating the gas stream from the top stage of the lower section and feeding the dehydrated gas to the bottom stage of the upper section; an inter-stage pump for pumping the liquid from the bottom stage of the upper section to the top stage of the lower section; the entire system forming a single fractionation unit, characterized in that the fractionation unit has a warm section that is tolerant to the presence of water, a single mid-section, an inter-stage dehydration unit, wherein the vapour from the lower stage is dehydrated before passing to the upper stage, and the bottom stage of the upper section and the top stage of the lower section constitute two adjacent stages of the fractionation unit, in such a way that the Natural Gas Liquids content is efficiently separated from the feed streams by operating the fractionation unit at temperatures down to and below the hydrate formation temperature and/or the freezing point of water.

Preferably, the hydrocarbon gas and liquid fractions are processed in a single fractionation unit, in which a dehydration unit is positioned between adjacent intermediate stages of the fractionation unit, such that the system efficiently process and allows low temperature fractionation stages leading to a high Natural Gas Liquids (NGL) recovery. Preferably, the upper section and the lower section are incorporated in a single vessel, such that the bottom stage of the upper section and the top stage of the lower section constitute two adjacent stages of the fractionator, therefore gas evolved from the top stage of the lower section is channelled to the bottom stage of the upper section, whilst liquid from the bottom stage of the upper section channelled to the top stage of the lower section.

Preferably, the inter-stage pump is used for overcoming pressure difference between the lower section and the upper section resulting from the inter-stage dehydration unit.

Preferably, the gas from the top stage of the lower section is channelled to the bottom stage of the upper section, whilst liquid from the bottom stage of the upper section channelled to the top stage of the lower section.

Preferably, the upper section operates with a minimum temperature that is below the water freezing and/or hydrate formation temperature.

Preferably, the hydrocarbon gas which contain water vapour is dehydrated by molecular sieve dehydration.

Preferably, the dehydrated hydrocarbon gas is cooled, partially condensed, separated and processed for purposes of improving product recovery or improving thermodynamic efficiency.

Preferably, at least one gas evolved from the top stage of the lower section passes through the dehydration unit to remove water vapour before being channelled to the bottom of the upper section.

Preferably, the dehydration unit introduces a pressure drop to the gas stream, such that the lower section operates at one to two bar higher pressure than the upper section and the liquid from the bottom stage of the upper section is pumped to the top stage of the lower section. An alternative embodiment of the invention is a system for processing gas, liquid or mixed phase hydrocarbon feed streams, using a single fractionation unit in two separate vessels that comprises; an upper column that operates below hydrate formation and/or water freezing temperatures, receiving feed streams that have been dehydrated, typically to ppm or lower levels; a lower column that operates at elevated temperatures and receives gas, liquid or mixed phase hydrocarbon feed streams that may contain water and/or water vapour; a reboiler unit for re-boiling the liquid from the bottom of the lower column and returns gas to the lower column; an inter-stage dehydration unit for dehydrating the gas stream from top stage of the lower column and feeding the dehydrated gas to the bottom stage of the upper column; an inter stage pump for pumping the liquid from the bottom stage of the upper column to the top stage of the lower column; the entire system forming a single fractionation unit, characterized in that the fractionation unit has a warm section that is tolerant to the presence of water, a single dehydration unit situated between the two columns, wherein the vapour from the lower column is dehydrated before passing to the upper column, and the bottom stage of the upper column and the top stage of the lower column constitute two adjacent stages of the fractionation unit, in such a way that the Natural Gas Liquids content is efficiently separated from the feed streams by operating the fractionation unit at temperatures down to and below the hydrate formation temperature and/or the freezing point of water.

Preferably, the upper section and the lower section are incorporated in two separate vessels, such that the bottom stage of the upper section and the top stage of the lower section constitute two adjacent stages of the fractionator, therefore gas evolved from the top stage of the lower section is channelled to the bottom stage of the upper section, whilst liquid from the bottom stage of the upper section channelled to the top stage of the lower section. Another embodiment of invention is a method of processing gas and liquid fractionation, comprising steps of; setting a upper section having the gas, liquid or mixed phase hydrocarbon feed streams dehydrated typically to ppm or lower levels, to operate below hydrate formation and/or water freezing temperatures; setting a lower section having the gas, liquid or mixed phase hydrocarbon feed streams with a water or water vapour content to an elevated temperatures; re-boiling the gas, liquid or mixed phase hydrocarbon feed streams using a reboiler unit; dehydrating the re-boiled gas, liquid or mixed phase hydrocarbon feed streams coming from top stage of the lower section and feeding to bottom stage of the upper section, using an inter-stage dehydration unit; pumping at least one liquid formed at the bottom stage of the upper section to the top stage of the lower section using an inter-stage pump; and performing fractionation process using a fractionation unit, characterized in that the fractionation unit having a warm section for fractionation in the presence of water, channelling a plurality of dehydrated vapour traffic from the warm section to the upper fractionation section using the inter-stage dehydration unit, in such a way that a Natural Gas Liquids content are separated from the gas, liquid or mixed phase hydrocarbon feed streams by operating the fractionation unit at temperatures below at least one hydrate formation temperature or at a freezing point of water.

Preferably, the hydrocarbon gas and liquid fractions processed in a single fractionation unit, in which the dehydration unit positioned in the mid-section of the fractionation unit, such that all of the gas exiting the fractionation unit has passed through the lowest temperature section of the unit, such that the system efficiently processes and allows low temperature fractionation leading to a high Natural Gas Liquids (NGL) recovery.

Preferably, the upper section and the lower section are incorporated in a single vessel to constitute two adjacent stages of the fractionator unit, such that a gas evolved from the top stage of the lower section channelled to the bottom stage of the upper section, whilst a liquid from the bottom stage of the upper section channelled to the top stage of the lower section. The present invention consists of features and a combination of parts hereinafter fully described and illustrated in the accompanying drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify various aspects of some embodiments of the present invention, a more particular description of the invention will be rendered by references to specific embodiments thereof, which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the accompanying drawings in which:

Figure 1 illustrates a processing system for gas, liquid or mixed phase hydrocarbon feed streams or Split Deethaniser, having an inter-stage dehydration.

Figure 2 illustrates an overall process scheme from well fluids reception through to Natural Gas Liquids (NGL) production that utilises a split deethaniser.

DETAILED DESCRIPTION OF THE INVENTION

The general principles of the present invention address the issue of loss of thermodynamic efficiency associated with the traditional approach of separate, independent processing of the gas and liquid fractions of a mixed phase gas- condensate well fluid stream for the extraction of Natural Gas Liquids (NGL).

The hydrocarbon gas and hydrocarbon liquid phases are processed separately for the recovery of NGL’s: i) The hydrocarbon liquid stream (which contains water) is processed by fractional distillation, stabilised to remove predominantly butane and lighter components, leaving a stable condensate product of predominantly pentane and heavier components, which has a vapour pressure of less than atmospheric pressure at prevailing ambient temperatures. The condensate stabiliser operates at minimum temperatures that are higher than water freezing or hydrate formation temperatures, and can thus tolerate the presence of water in the feed stream. The removed NGLs and lighter hydrocarbons are extracted from the stabiliser together with water vapour as a gas which may then be recycled to the gas stream for further processing (see (ii) below). ii) The water saturated hydrocarbon gas stream is processed to recover NGL’s by absorption, cryogenic fractionation, or a combination of the two. Where the selected NGL recovery process operates at sub-zero temperatures, the gas must first be dehydrated, typically to sub ppm levels, to prevent hydrate formation, freezing and blockage within the process equipment. The water saturated overhead gas stream from condensate stabilisation (described in (i) above) may also be fed to the NGL recovery system, and would also require pre-treatment for water removal. This separate processing of the gas and liquid phases and recycle of the overhead gas stream from the liquid treatment process to the gas treatment process results in; a) thermodynamic inefficiency as the liquid stabilisation process must typically input sufficient energy to heat the entire overhead gas stream to the required operating temperature to exit the fractionation column. This stream must then be cooled for subsequent compression and gas phase processing. b) compression equipment for the recycle of the overhead gas stream from the typically low liquid treatment process stabilisation pressure to the typically higher gas treatment process operating pressure, c) both the liquid processing and the gas processing system equipment to be of a size and capacity that accounts for the overhead gas stream flow rates as the stream components pass through both the liquid processing and the gas processing systems. d) a second set of dehydration equipment for the recycled gas stream in cases where this stream is comingled with the gas phase downstream of the gas phase stream dehydration process,

Improved thermodynamic efficiency could be achieved by processing the hydrocarbon gas and liquid fractions together in a single fractionation step, but the presence of water in the liquid hydrocarbon phase introduces an obstacle to the use of low temperature processes which are essential for high Natural Gas Liquids (NGL) recovery.

The proposed invention allows the hydrocarbon gas and liquid fractions to be processed together in a single fractionation unit, referred to here as the “Split Deethaniser” by the addition of a dehydration stage located in the mid-section of the fractionator. This provides a thermodynamically efficient process and allows low temperature fractionation stages leading to high Natural Gas Liquids (NGL) recovery.

The upper section of the Split Deethaniser operates with a minimum temperature that is below the water freezing and/or hydrate formation temperature.

The water saturated hydrocarbon gas phase is dehydrated, typically by molecular sieve dehydration. It is then cooled, partially condensed, separated and possibly undergoes further processing stages for purposes of improving product recovery and/or improving thermodynamic efficiency before being sent to the upper section of the Split Deethaniser.

Further processing steps may involve turbo expansion, cooling by mechanical refrigeration and/or heat and mass transfer integration by a variety of proprietary and open art methods and are not the subject of the proposed innovation.

Hydrocarbon liquids, containing water, are fed into the top of the Split Deethaniser lower section, together with liquid from the bottom of the Split Deethaniser upper section.

If conditions in the lower section lead to the formation of a free water phase then a water draw must be added however any water that remains dissolved in the liquid hydrocarbon does not need to be otherwise removed.

The upper and lower sections of the Split Deethaniser may be incorporated in a single vessel, or may be accommodated in separate vessels. The bottom stage of the Split Deethaniser upper section and the top stage of the Split Deethaniser lower section constitute two adjacent stages of the fractionator, therefore gas evolved from the top stage of the lower section flows to the bottom stage of the upper section, whilst liquid from the bottom stage of the upper section flows to the top stage of the lower section.

However, the gas evolved from the top of the lower section passes through a dehydration stage (typically molecular sieve dehydration), to remove water vapour before being passed to the bottom of the upper column.

As the inter-stage dehydration introduces a pressure drop to the gas stream, the lower section of the Split Deethaniser must operate at typically one to two bar higher pressure than the upper section. Therefore liquids from the bottom of the upper section must be pumped As illustrated in Figure 1 , the entire Split Deethaniser, comprises the parts listed below:

(i) Deethaniser Upper Section (101) - operates at sub-zero temperatures, with water content of feed streams reduced to sufficiently low level to prevent hydrate formation or freezing in the column ;

(ii) Deethaniser Lower Section (102) - operates at elevated temperatures (higher than water freezing and hydrate formation temperature), contains water / water vapour;

(iii) Deethaniser reboiler (103) - a heat exchanger which heats liquids from the bottom of the lower section of the Deethaniser, partially vapourises the liquids and returns the vapour to the bottom tray of the lower section of the Deethaniser;

(iv) Deethaniser Interstage dehydration (104) - gas from top of lower section is dehydrated and fed to bottom stage of upper section; (v) Deethaniser Interstage Pump. (105)- Liquid from base of upper section is pumped into top of lower section. Pump is required to overcome pressure difference between lower and upper stages resulting from interstage dehydration;

(vi) Deethaniser Upper Section Top Feed stream (106) (typically dry, lean, low temperature);

(vii), (viii) Additional feed streams (107,108) to Deethaniser Upper Section (all dry);

(ix) Deethaniser Lower Section Top Feed stream (109) (typically water wet, rich, ambient or higher temperature); (x) Additional feed streams to Deethaniser Upper Section (110) (rich, may be water wet or dry);

(xi) Deethaniser overhead gas stream (111) (primarily ethane and lighter components) exiting top of deethaniser upper section directed to downstream natural gas consumers; (xii) Deethaniser bottoms liquid stream (112) of recovered NGLs (primarily propane and heavier components) exiting deethaniser reboiler directed to downstream storage and/or NGL consumers. Figure 2 illustrates a typical overall process scheme from well fluids reception through to Natural Gas Liquids (NGL) production that utilises a split deethaniser. The key parts of the scheme are described below

(i) Inlet separator (201) - provides bulk separation of the hydrocarbon gas, hydrocarbon liquid and aqueous phases;

(ii) Condensate coalescer (202) - removes distributed water from the hydrocarbon liquids down to the level required for operation of the deethaniser;

(iii) Produced Water Treatment system (203) - removes hydrocarbons from produced water down to levels required for water disposal;

(iv) Feed Gas Dehydration (204) - removes water vapour from the feed gas down to levels required for satisfactory operation of the cryogenic Natural Gas Liquids (NGL) removal system (likely to ppm levels or lower);

(v) Main Exchanger (205) - to effect heat transfer between the warm feed gas and returning cold streams;

(vi) Low Temperature Separator (206) - removes condensed hydrocarbon liquid from the chilled feed gas;

(vii) Turboexpander (207) - provides work expansion of the chilled feed gas down to deethaniser operating pressure to produce the necessary refrigeration to drive the Natural Gas Liquids (NGL) extraction;

(viii) Subcooler (208) - heat exchanger to chill a portion of chilled feed gas against returning deethaniser overhead gas, to produce a liquid wash for the deethaniser;

(ix) Deethaniser (209) - see description of component parts above.

Best Mode for Carrying Out the Invention

The Split Deethaniser system is best engaged in a scenario where water wet gaseous and liquid hydrocarbon phases are present, recovery of Natural Gas Liquids (NGL) is required and there are no existing process units with available capacities or synergies that would make alternative Natural Gas Liquids (NGL) recovery schemes more economic. This would typically be the case for Natural Gas Liquids (NGL) recovery from well fluids with a high Condensate-Gas Ratio (CGR) in a remote, greenfield location. Industrial Applicability

The proposed invention is applicable to Upstream and Midstream oil and gas production (onshore and offshore). The proposed invention is also applicable but not limited to Gas processing facilities or refineries where multiple, multiphase gas and liquid feed streams are present.

The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore indicated by the appended claims rather than by the foregoing description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.