GB2365441A - Enhanced natural gas liquid (NGL) recovery - Google Patents

Abstract

This invention relates to improving hydrocarbon liquids recovery in oil and gas production onshore and offshore fixed or floating installations. The new invention could use a conventional absorption column or a novel absorption column to re-contact a proportion of the export crude oil, stabilized oil or condensate with the gas to maximise hydrocarbon liquid recovery. The recovered hydrocarbons could be methane or heavier hydrocarbons. The crude oil used in the absorption process could originate from a three-phase separator or a storage tank. Quite often it is necessary to store the produced crude oil in a storage tank for final water separation or for other operational reasons. This invention enables such crude oil to be exported with a higher True Vapour Pressure, TVP. The new method can offer greater hydrocarbon liquids recovery within the vapour pressure limitations of the export oil pipeline, with lower maintenance and operating cost than associated with conventional cooling or refrigeration.

Description

<Desc/Clms Page number 1> Background of the Invention Hydrocarbon recovery by absorption or fractional distillation from various gas mixtures is widely used in the oil and gas production and refineries industry. There are numerous types of liquid that could be used to recover particular hydrocarbon components from a gas mixture. There are also several types of process designs to carry out the absorption or fractionation process.

Philip H. Deming reported in his USA patent No 2,516,507 of 251h jUly 1950 an invention relating to a process for the separation of lower molecular weight hydrocarbons, and more particularly to an improved absorption separation process for the separation of a given hydrocarbon component from gaseous admixture thereof with both more and less volatile hydrocarbons components.

Deming stated that it is conventional to separate a mixture of lower molecular weight hydrocarbons, such as a mixture of Cl-C, carbon-atoms-per-molecule hydrocarbons, including saturated and unsaturated hydrocarbons, into desired fractions by conventional fractional distillation processes by effecting the fractionation in a suitable fractionating column under sufficiently low temperatures and/or under super atmospheric pressures so as to obtain adequate rectification in the fractionator.

Golden A. Moyer reported, in his USA Patent No 2,938,865 of 31't May 1960, a method for controlling the operation of separation equipment. In one aspect, it relates to a method for controlling the operation of the lower or stripping section of a separation column. In another aspect, it relates to a system for controlling the operation of a separation column.

He pointed out that, in the petroleum industry, the separation of hydrocarbons into light and heavy fractions is often accomplished by the use of separation columns, which employ an external reboiler. The purpose of the reboiler is to supply heat to the lower or stripping section of the separation equipment. In any particular separation, with a feed material of uniform quality being supplied to the equipment at a constant rate, it is necessary that a definite amount of reboiler heat be supplied if an efficient separation is to be obtained. It is seen that any column upset, such as a change in the quantity or quality of the feed material, requires an adjustment of the amount of reboiler heat supplied to the separation column. In conventional separation systems, this adjustment is generally made manually, but such an expedient is unsatisfactory because it fails to immediately correct the column upset, with the result that there are periods of operation when a specification product is not being produced.

Moyer stated that it is an objective of his invention, therefore, to provide an improved method for operating the lower or stripping section of a separation column.

He further stated that another objective of the invention is to provide a method of controlling the supply of reboiler heat to a separation column.

<Desc/Clms Page number 2>

An additional objective of Moyer's invention is to provide a system for controlling the rate at which reboiler heat is supplied to the stripping section of a separation column.

David Zudkevitch stated in his European Patent Application 84105721-9 of 18 1h May 1984, that his invention relates to a process for separating mixtures containing light hydrocarbons, carbon dioxide, hydrogen suiphide, mercaptans and carbonyl sulfide. More particularly, his invention relates to an integrated process for recovering a separate stream of fuel-grade natural gas which is mainly methane with some quantities of nitrogen, ethane and smaller quantities of carbon dioxide, a separate stream of propane and heavier hydrocarbons, and a separate stream of carbon dioxide. Further, such a gas having a heating value of between 500 and 1300 Btu per standard cubic foot, known as "sales-gas", is separated from its original mixture with carbon dioxide and other compounds such as ethane, hydrogen sulphide, propane and heavier hydrocarbons. The heavier hydrocarbons are sometimes referred to as "LPG" or "NGL" while the original gas may be referred to in commerce as an "associated gas" or "separator gas". The said sales-gas stream of this process is virtually free of hydrogen sulfide.

Zudkevitch further explained that the process of his invention also encompasses within its scope the recovery of, as a separate stream, most of the carbon dioxide which is present in the original mixture. By putting this process into practice, a stream of carbon dioxide is obtained which is substantially free of hydrogen sulphide. In most instances by controlling the process variable, carbon dioxide streams of sufficient purity and properties suitable for re-injection into petroleum oil reservoirs are recovered.

Yuv Mehra explained, in his International Application Number PCT/US86/01477 of 17" July 1986, his invention which relates to removing and recovering methane and higher boiling hydrocarbons from a natural gas stream which contains large quantities of inert gases, which may contain acidic components such as CO, and H2S, and may vary in moisture content from dry to saturated. It specifically relates to the upgrading of heating values of natural gas streams having heating values below desired specifications. It further relates to adapting the extractive flashing and extractive stripping versions of the Mehra Process for processing of nitrogen-rich natural gas streams.

Again,Yuv Mehra detailed in his USA patent No 4,696,688 of 29t' September 1987, a continuous process for selective counter-current extraction of C2+ hydrocarbons from a gas stream with a lean oil as solvent, which is rejuvenated by substitution of a preferential physical solvent for the lean oil solvent to produce a residue gas stream, meeting specifications for methane content and a hydrocarbon product having a composition which can be adjusted to a selected minimum degree for ethane, as low as 2% ethane, and produce at the maximum propane content that is available from the equipment. - In this way, profitability of the extraction operation can be maximised at all times.

<Desc/Clms Page number 3>

The preferential physical solvent has a minimum relative volatility of methane over ethane of at least 6.0, and a solubility of at least 1.0 standard cubic foot of gaseous hydrocarbons per gallon of the solvent (thereby defining its hydrocarbon loading capacity) or, alternatively, a preferential factor of at least 6.0. Preferred solvents include polyalkylene ethers of dialkylene glycol and alkyl-substituted monocyclic Cs- C,, aromatic hydrocarbons, preferably including mesitylene, reformates, crackates and mixed xylenes.

This Invention.

This invention relates to improving hydrocarbon liquids recovery during oil and gas production onshore and offshore, fixed or floating installations.

Oil and gas production offshore currently uses three phase separators to separate oil, gas and water. The separation efficiency and oil recovery is improved by using multistage separation, usually one, two or three separators at progressively reducing pressure in series. Recovery of valuable hydrocarbon liquids from the gas is carried out by recycling the condensate from the compressor suction vessels to the production separators or by injection direct into the export oil. Additional recovery can be achieved by using refrigeration to cool the gas. Figure 1 illustrates the current practice for a two-stage separation system.

The new invention uses one or more absorption columns to re-contact a proportion of the export crude oil stabilized oil or condensate with the gas to maximise hydrocarbon liquid recovery. The recovered hydrocarbons could be methane or higher hydrocarbons. The crude oil used in the absorption process could originate from a three-phase separator or a storage tank as shown in Figures 2 and 3. Sometimes it is necessary to store the produced crude oil in a storage tank for final water separation or for other operational reasons. This invention enables such crude oil to be exported with a higher True Vapour Pressure, TVP. The new method can offer greater hydrocarbon liquids recovery within the vapour pressure limitations of the export oil pipeline, with lower maintenance and operating cost than associated with conventional cooling or refrigeration.

Figure 2 illustrates one layout version of the new enhanced NGL recovery technology using leaner gas. To achieve lower vapour pressure export oil the absorption column could be supplied with a reboiler.

The quantity of oil and gas stream to be used is the subject of an optimisation study for each individual installation. In this example all the gas from the separators is re- contacted. To minimise the size of the absorber for certain operations, only the gas stream containing the highest amount of heavy hydrocarbons can be processed though the absorber. In this case the 111 stage can be operated at a sufficiently high pressure to minimise hydrocarbon- liquids in the gas, the absorber can then be used to re-contact the gas stream from the 2 m stage separator only.

<Desc/Clms Page number 4>

The gas leaving the compressor could be injected directly into the absorption column without the need for cooling and phase separation. This may be advantageous for some operational conditions.

Other sources of gas could be used in the absorption process without the need to use the pre-absorber compressor. Examples of these gases are, the gas leaving the high-pressure separator, an imported gas stream or any other suitable gas stream.

Salah Baker explained in his USA patent No 5,632,962 of 27 th of May 1997, a novel gas-liquid contactor with improved performance in vertical, tilt and motion operation. Baker's novel column could be compact and suitable for operation under tilt and motion caused by wind and waves particularly on floating and semi-submersible installations. The novel packed column design can be used for this gas absorption process to improve operational and design flexibility and efficiency and reduce the absorber size by improving liquid and gas distribution. (See Figures 4 and 5).

A cooling coil located at the top part of the absorption column can cool the lean gas to recover some of the heavier hydrocarbons. A cooling condenser can be used to cool the outlet gas to separate some of the heavier hydrocarbons that escape with the gas. The collected liquid from the condenser can be injected into the packed bed utilising the helical redistributor of the novel packed column by using a simple injection pipe as shown in Fig 5.

An example can be used to show the advantages of this new NGL recovery process over a conventional system.

Fig 6 shows the flow diagram of a conventional process of oil'and gas production. It basically consists of a three-phase separation with three outlet streams of oil, water and gas. The operating pressure of the three phase-separator is 5 bara.

Oil production is 65000 bpd (barrels per day) and gas production is 28.6 MMscfd (millions standard cubic feet per day).

Fig 7 shows ALTRA's NGL Recovery Process using HiPerColumn TM . The same mixture quantity and quality of feed stream (oil, water and gas) to the three phase- separation was used.

Using ALTRA's NGL recovery process, the quantity of produced oil is 66174 bpd. This is an increase of 1174 bpd over the basic conventional process. The produced gas is much leaner than the gas produced in the conventional process of Fig 6.

Fig 7 is not the only design scenario for the new ALTRA NGL recovery. There are many other uses for the ALTRA NGL recovery process. It can offer improved NGL recovery and/ or selective loading of hydrocarbons to the exported oil.

Amongst the many possible variations to Fig 6 and Fig 7 examples are: # Having more than one production separator.

# Operating production separators at various pressures. # Using more than one absorber in the design.

# Feeding various ratios of more than one gas stream to the absorber.

<Desc/Clms Page number 5>

# Selection of various absorber operation pressures and temperatures to optimise the oil and gas production.

# Varying the quantity of the crude oil used in the absorption process.

# Feeding the compressed gas directly to the absorber without passing it through the gas cooler or the gas scrubber.

# Using more than one type of crude oil in the absorption process. The crude oil could be from the three-phase separation system, stabilised oil crude from the oil storage tanks or other sources of crude oil.

# Other process parameters can be manipulated to optimise the NGL recovery or selective loading of gas into crude oil.

# A combination of HiPerColumn and any other type of absorbers could be used in this new process.

# Conventional absorber(s) could be used alone in this new process. Advantages of the new patent: This new invention offers several advantages over other designs of NGL recovery techniques. Some of these are detailed below: # The use of a novel packed column (HiPerColumn TM) offers a higher coefficient of heat and mass transfer and higher operational range of gas to liquid volumetric ratio.

# The NGL recovery of this invention could be designed to eliminate the effect of tilt and motion on the performance of the system, making it optimal for onshore and offshore application including floating production facilities.

# Reduces weight and foot print compared to alternative designs. # Could offer low capital and operating cost.

# Ability to use stabilised or non-stabilised oil, including stabilized and stored crude oil for the absorption process.

# It offers high flexibility in selecting the gas stream(s) for the absorber to achieve optimum NGL recovery or optimum selective gas loading in the crude oil.

# It reduces the total power consumption.

# It reduces the number of gas coolers and scrubbers downstream of the gas compressor; prior to entering the absorber.

# It can re-load the stabilised oil (from storage tanks and other sources) with selective hydrocarbons up to the TVP limit of the pipeline.

# It eliminates the need for gas dehydration prior to NGL recovery associated with low temperature recovery process # It needs minimal change to convert existing NGL recovery systems into ALTRA's design.

# Environmentally friendly and does not use refrigeration fluid, which may lead to ozone depletion or global warming.

# More energy efficient.

# Reduced capital and running costs.

In this invention the NGL is dispersed in the whole of the crude oil slipstream. This reduces possibility of localised problems or cavitation compared to condensate spiking i.e. less risk of damage/problems to export meter and/or main oil line (MOL) pumps.

<Desc/Clms Page number 6>

Claims (1)

1. We Claim:- 1 - Process for onshore and for offshore applications for absorbing Natural Gas Liquid (NGL) from a natural gas . Iv stream using side stream crude oil as an absorbent fluid and HiPerColumn m or a conventional packed columns as an absorption column as shown in Fig 2 of the patent application. 2- Process according to claiml, wherein the absorbent crude oil used has been stored in storage tanks. 3- Process according to claims 1 and 2 wherein the absorption column is supplied within an internal or external vapour condenser as shown in Fig 4 of the patent application. 4- Process according to claim 1, 2 and 3, wherein the NGL and methane gas is absorbed into the crude oil prior to export into an oil export pipeline to maximise the amount of exported gas within the allowable True Vapour Pressure (TVP) in the export pipeline.