WO2011028131A1

WO2011028131A1 - Device and method for recovering meg

Info

Publication number: WO2011028131A1
Application number: PCT/NO2010/000326
Authority: WO
Inventors: Shanta Seereeram; Lucie Addicks
Original assignee: Aker Process Systems AS
Current assignee: Fjords Processing AS
Priority date: 2009-09-02
Filing date: 2010-09-02
Publication date: 2011-03-10
Anticipated expiration: 2012-03-02
Also published as: GB2473213B; AU2010290195B2; GB0915300D0; GB2473213A; AU2010290195A1

Abstract

This invention relates to a device and method for regenerating mono ethylene glycol (MEG) after use as hydrate inhibiting agent in production lines of gas- and oil fields, where the method comprises subjecting the process fluid from the production lines to a separation in a per se known manner to produce a hydrocarbon fraction and a aqueous fraction of rich mono ethylene glycol, precipitating divalent cations from the aqueous fraction of rich mono ethylene glycol by sufficient alkalinity and heating of the aqueous fraction and hold the aqueous fraction at this conditions for a sufficient period of time to obtain sufficient particle grow of divalent mineral salts and corrosion products as solid particles, removing solid particles from the aqueous fraction of rich mono ethylene glycol by purging the liquid with a purging gas and skim off the foam forming on the surface of the aqueous fraction, and separating the aqueous fraction of rich mono ethylene glycol into a water fraction, a fraction of recovered lean mono ethylene glycol and fraction of other contaminants as solids of mono valent salts by regeneration of rich mono ethylene glycol in a per se known manner.

Description

Device and method for recovering MEG

This invention relates to a device and method for regenerating mono ethylene glycol (MEG) after use as hydrate inhibiting agent in production lines of gas- and oil fields. Background

Off-shore extraction of hydrocarbons from a oil and gas reservoir often involves transportation of a mixture of hydrocarbons, water and dissolved salts in production pipelines from the reservoir up to land based or floating top-side facilities for processing the mixture to recover the desired hydrocarbon products. Due to shifting physical conditions during the pipeline transit, there is a problem with formation of hydrates in the fluid mixture of the pipelines threatening to clog the lines.

One much applied solution to the problem of hydrate formation is to add, at subsea level, low water content mono ethylene glycol (lean MEG) into the process fluid which usually is a mixture of hydrocarbons, water and dissolved salts and then extract the MEG as so-called rich MEG from the process fluid at the top-side facilities. From an operational costs and environmental point of view, the rich MEG should be regenerated to lean MEG and then reused as hydrate inhibiting agent in the production lines. Rich MEG usually contains remains of the hydrocarbons, high water levels, corrosion products, production chemicals and a mixture of dissolved mineral salts.

One of the main challenges associated with maintaining the MEG loop is the presence of salts which can pose problems in the MEG loop, such as scaling of injection points, production pipelines and topside processing facilities. Salt management is therefore a key parameter in MEG regeneration systems. Prior art

In present MEG regenerating systems, salt removal is achieved through controlled precipitation and separation of the particles in different parts of the regeneration plant. In some applications salts of divalent cations and corrosions products are removed in a pre-treatment facility upstream of the MEG regeneration.

The removal of salts and corrosion products during the pre-treatment can pose a great challenge as the precipitated particles with sizes typical of 1 - 30 μπι are difficult to remove from the liquid phase. Current methods for removal of these particles are based on either settling in large tanks, filters or high gravity centrifugal forces.

WO 2009/017971 discloses a method for reclamation of mono ethylene glycol, MEG, wherein the seeding of calcium carbonate into a calcium chloride- contaminated MEG stream accelerates the growth of calcium carbonate particles to a size that enhances their removal from the stream by filtration.

US 7,232,505 discloses a method of regenerating a glycol solution containing water, hydrocarbons and salts wherein the glycol solution is expanded in a drum, then distilled in a column. The concentrated glycol is collected at the level of reboiler is placed under vacuum to vaporize the water and precipitate the salts. The salts are separated from the glycol in separation device, e.g. filtration, centrifugation.

However, since these methods are required to treat the full Rich MEG flow, will either be space demanding as seen with the settling tanks, have high operational costs as seen with filters, or energy demanding and costly as seen with the centrifuges. This means that there is a significant drive to develop more cost effective, compact designs.

Objective of the invention

The main objective of this invention is to provide an improved method and system for purification of MEG solutions containing precipitated salts of divalent cations, corrosion products and hydrocarbons.

Brief description of the drawings

Figure 1 shows a schematic outline of the method according to the first aspect of the invention.

Figure 2 is a sketch showing particles adhering to the surface micro bubbles.

Figure 3 is a sketch showing entrapment of a small particle. Description of the invention

The following terms and expression as used herein shall have the meaning below unless otherwise indicated:

The expressions "salts of divalent cations" and "divalent mineral salts" are used interchangeable herein and means salts wherein the cation is divalent, the anion of these salts may be of any valence.

"Aqueous fraction" is the aqueous part of a process fluid when the hydrocarbon fraction is partly separated.

"Liquid phase" is the aqueous fraction and hydrocarbon fraction

"Total alkalinity" is sum of HCO^3" +C0₃ ^2" +OFT ions The term "production line" as used herein means any form of pipeline employed to extract fluids containing hydrocarbons from reservoirs up to facilities for processing the extracted fluid into a hydrocarbon product.

The term "process fluid" as used herein means the hydrocarbon containing fluid which is extracted from the reservoir, and which normally will comprise water, hydrocarbons, particulate corrosion products, dissolved mineral salts and production chemicals.

The term "lean mono ethylene glycol" as used herein means sufficiently pure mono ethylene glycol to be employed as anti-hydrate forming agent in process fluids. The term "rich mono ethylene glycol" as used herein means mono ethylene glycol which has been used as anti-hydrate forming agent in process fluids, and which is loaded with water and other contaminants which need to be removed before being fit for reuse as anti-hydrate forming agent.

The term 'MEG regeneration unit' as used herein means system which regenerates mono ethylene glycol by distillation at atmospheric pressure, or by reclamation and distillation at vacuum pressure or by distillation at atmospheric pressure combined with reclamation at vacuum pressure.

The invention is based on the realisation that an effective particle removal from the fluid phase may be obtained by use of gas purging. Thus, by combining present systems for recovering MEG in oil and gas production with a gas purging step to remove particles by floatation, a cost effective method for removing particulates from rich MEG which may readily be installed on existing facilities is obtained.

Thus in a first aspect, the present invention relates to a method for recovering mono ethylene glycol used as anti-hydrate forming agent in production lines of oil- and/or gas production, wherein the method comprises the following process steps:

- subjecting the process fluid from the production lines to a separation in a per se known manner to produce a hydrocarbon fraction and an aqueous fraction of rich mono ethylene glycol,

- precipitate the divalent cations and corrosion products from the aqueous fraction of rich mono ethylene glycol by having sufficient alkalinity and temperature between 60 - 100 C,

-hold the aqueous fraction at this conditions until particle size of salts of divalent cations and/or corrosion products is between 1 - 10 microns,

- remove solid particles of salts of divalent cations and/or corrosion products from the aqueous fraction of rich mono ethylene glycol by purging the liquid with purging gas and skim off the foam forming on the surface of the aqueous fraction, and

- separating the aqueous fraction of rich mono ethylene glycol into a water fraction, a fraction of recovered lean mono ethylene glycol and fraction of other

contaminants such as solid particles of mono valent salts by MEG regeneration unit in a per se known manner.

In an embodiment of the method at least a fraction of the hydrocarbons being separated out from the process fluid exiting the production line is inserted into the aqueous fraction.

In another embodiment of the method the purging gas is purged in such a way that micro bubbles are generated by a pump or other means.

In a further embodiment of the method according to the invention the purging gas is an inert gas. Examples of suitable inert gases are nitrogen (N₂) and carbon dioxide (C0₂).

The feature of gas purging solves the problem of presently established technologies which are based on precipitation and removal of particles by mean of settling i.e. settling tanks or centrifuges, or barrier methods i.e. filters where the presence of gas is not desired. The gas purging has the effect of reverse settling (flotation) of particles and hydrocarbons in that the gas bubbles tend to drag along or entrain particulates in the liquid. The gas purging will remove any particulates in the liquid including, but not limited to, divalent mineral salts, corrosion products and residual hydrocarbons. The proposed gas purging step is not the obvious choice for particle separation for this process as the common knowledge is that in order to obtain an effective particle removal by gas purging, the surface tension of the particles need to be lower than that of the liquid in order to obtain an effective adherence to the gas bubbles. Mono ethylene glycol has a low surface tension, such that the particles usually have larger surface tensions than the liquid and therefore not favourable for separation by flotation. However if the surface properties of the particles are altered, for example if coated with hydrocarbons, separation by flotation can be achieved.

Furthermore, prior art has separated precipitated salts by means of acceleration forces (either gravity such as for settling tank or centrifugal acceleration). And in this context entrained and released gasses during the MEG regeneration process have been an obstacle for the acceleration means for separation of precipitated particles. Instead the gas has preferably been removed prior the separations means, and not used as an active element in the separation process. To actually use gas actively instead of trying to remove it in this MEG re concentration process far from obvious. In a second aspect, the invention relates to a device for recovering mono ethylene glycol used as anti-hydrate forming agent in production lines of oil- and/or gas production, where the mono ethylene glycol is first inserted into the process fluid of the production lines and then extracted from the process fluid after being

transported through the production line as a rich mono ethylene glycol fraction, wherein the device comprises:

- a separating vessel (1) for collecting and separating the process fluid (4) from the production lines into a hydrocarbon fraction (6) and a aqueous fraction (5) of rich mono ethylene glycol,

- a holding vessel (2) for collecting and holding the aqueous fraction of rich mono ethylene glycol (5) at temperatures 60 - 100 C,

- means of sufficient alkalinity of the aqueous fraction (5) upstream the holding vessel (2) or in the holding vessel (2) by injecting chemicals (12) such K₂C0₃. NaOH or others

- means for controlled heating of the aqueous fraction of rich mono ethylene glycol (5) upstream of or in the holding vessel (2),

-means for holding the fluid at sufficient alkalinity and temperature 60 - 100 C for a period of time conditions until particle size of salts of divalent cations and/or corrosion products is between 1 - 10 microns,

- a floatation vessel (3) with means (10) for purging the aqueous fraction of the vessel with a purging gas and means (9) for skimming off and discharging the foam formed on the surface of the aqueous fraction in vessel,

- means for transporting the aqueous fraction (7) including entrained particulates of the holding vessel (2) to the floatation vessel (3),

- means for transporting the aqueous fraction (8) from the floatation vessel (3) to a MEG regeneration unit (14), and

- means for separating the aqueous fraction to fractions of recovered lean mono ethylene glycol (16), water (15) and other contaminants as solid particles of mono valent mineral salt (11) in the MEG regeneration unit (14).

The chemicals injected for sufficient alkalinity of the aqueous fraction (5) upstream the holding vessel (2) or in the holding vessel (2) can be e.g. K₂C0₃, NaOH or others

In an embodiment of the invention the chemicals are injected to achieve sufficient alkalinity.

One of the preferred embodiments of the invention is to perform forced

precipitation and particle growth of divalent salts in a MEG solution and combine this with flotation. Preferably also where the hydrocarbons are left in the system where they are allowed to coat the divalent salt precipitates and corrosion products. The mixture is then routed to the flotation unit where it is mixed with inert gas bubbles prior to entering the separation chamber.

The method according to the first aspect is schematically drawn in Figure 1. Process fluid 4 from a production line is passed into a separation vessel 1, where the process fluid is separated into a hydrocarbon fraction 6 and an aqueous fraction 5 of rich

MEG. The Rich MEG fraction 5 is passed to a holding vessel 2 where the alkalinity is controlled and the rich MEG is heated and held to obtain forced precipitation of divalent cations in the rich MEG. After precipitation of the divalent salts, the rich MEG with solid particles 7 is transported to a floatation vessel 3 where the rich MEG is purged with a purging gas 10 to remove particulates. The particulates, which comprise hydrocarbon remains, corrosion products, and precipitated salts, are collected in the foam forming on top of the liquid phase of the floatation vessel 3 and skimmed off to form a discharge fraction 9. The particle deprived MEG solution 8 is passed to a MEG regeneration unit 14 for separating the MEG solution into a water fraction 15, a recovered lean MEG fraction 16 and possibly solid fraction of mono valent salts 11. The figure also shows an optional line 13 for passing a fraction of extracted hydrocarbons from the process fluid into the floatation vessel 3 to allow hydrocarbons to aid the floatation process.

As an alternative embodiment of the invention, it is envisioned that the forced precipitation of divalent cations by heating and sufficient alkalinity of the liquid and the subsequent floatation to remove particles may be performed in the same vessel. Thus the invention relates to a device , wherein the holding vessel (2) and the floatation vessel (3) are combined into one vessel which comprises:

- the means for controlled alkalinity and heating of the liquid fraction of rich mono ethylene glycol (5) in combined holding and flotation vessel and hold the fluid at sufficient alkalinity and temperature for a sufficient period of time to obtain precipitation and grow of divalent mineral salts and corrosion products as solid particles,

- the means (10) for purging the liquid phase in combined holding and flotation vessel with a purging gas and means (9) for skimming off and discharging the foam formed on the surface of the liquid phase in the combined holding and flotation vessel,

- the means for transporting the liquid phase (8) from the combined holding and floatation vessel to a MEG regeneration unit (14), and

- means for separating the fractions of recovered lean mono ethylene glycol (16), water (15) and other contaminants as solid particles of mono valent mineral salts (1 1) in the MEG regeneration unit (14).

Without being bound by theory, it is believed that the gas bubbles in the rich MEG solution are used to promote separation of one of two immiscible fluid phases with different densities and particulates of liquid-solid mixtures. Thus the salt particles, corrosion products and free hydrocarbons are believed to adhere (see Figure 2) or be encapsulated (see Figure 3) by the bubbles. As the bubble and contaminated MEG mixture enters the flotation chamber, due to the density difference of the

hydrocarbons and the hydrophobic nature of hydrocarbon coated particles, the particles and hydrocarbons will follow the bubbles rather than the MEG and be floated to the surface. The floated particles and hydrocarbons are skimmed off and routed to further treatment whilst the clarified MEG can be routed to MEG regeneration.

To further promote flotation of the particles and hydrocarbons the possibility of chemical dosing, with for example 'floating agents', or electrostatic fields, or a combination of the two may be applied to the liquid fraction of the floating vessel. The invention may apply floating agents such as ammoniated compounds. By electrostatic treatment it is meant that the liquid fraction is exposed to an alternating high voltage electrical field, in which electrical field effects breakup the surface tension of particles adhered to the MEG, enabling better separation effect of the inert gas. The idea is to expose this electrical field in the feed to the floatation vessel together or after the purge gas is inserted. Alternatively the electrical field is applied in a recycle loop at the floatation vessel.

The separation effect of the gas purging may be enhanced by allowing hydrocarbons to coat the divalent salt precipitates and corrosion products before purging the liquid phase with inert gas. This will reduce the effective density difference of the particles including hydrocarbon coating and the MEG-solution, and thus increase the floating ability of the particles. It is also believed to increase the adherence of the particles to the rising gas bubbles. This effect may be obtained by i.e. inserting at least a fraction of the hydrocarbons being separated out from the process fluid exiting the production line into the liquid phase of the floatation vessel and thus allow the hydrocarbon phase to coat the precipitated salt particles. The same effect can be achieved by treating the rich mono ethylene glycol without hydrocarbon removal.

The separation effect of the gas purging may be enhanced by arranging the gas purging in such a way that micro bubbles are generated, i.e small inlet nozzles of gas purging, gas purging in pulses, mixers, chemical etc. An example of the effect of using micro bubbles in sketched out in figure 2, where particles are surrounded my multiple bubbles which for a large bubble entrapping the particle.

Claims

1. Method for recovering mono ethylene glycol used as anti-hydrate forming agent in production lines of oil- and/or gas production, wherein the method comprises the following stepwise process steps:

- subjecting a process fluid from the production lines to a separation in a per se known manner to produce a hydrocarbon fraction and an aqueous fraction of rich mono ethylene glycol,

- precipitate divalent cations and corrosion products from the aqueous fraction of rich mono ethylene glycol by having sufficient alkalinity and temperature between 60 - 100 C,

- hold the aqueous fraction at this conditions until particle size of salts of divalent cations and/or corrosion products is between 1 - 10 microns,

- remove solid particles from the aqueous fraction of rich mono ethylene glycol by purging the liquid with a purging gas and skim off any foam forming on a surface of the liquid, and

contaminants such as solid particles of mono valent salts by MEG regeneration unit.

2. Method according to claim 1, wherein

at least a fraction of the hydrocarbons being separated out from the process fluid exiting the production line is inserted into the aqueous fraction of a holding vessel.

3. Method according to claim 1 or 2, wherein the purging gas is purged in such a way that micro bubbles are generated.

4. Method according to any one of claims 1 to 3, wherein

the temperature in the step for salt precipitation and growth is 60-100 °C.

5. Method according to any one of claims 1 to 4, wherein

the alkalinity in the step for salt precipitation is sufficient.

6. Method according to any one of claims 1 to 5, wherein

the purging gas is an inert gas.

7. Device for recovering mono ethylene glycol used as anti-hydrate forming agent in production lines of oil- and/or gas production, where the mono ethylene glycol is first inserted into a process fluid of the production lines and then extracted from the process fluid after being transported through the production line as a rich mono ethylene glycol fraction, wherein the device comprises:

- a separating vessel (1) for collecting and separating the process fluid (4) from the production lines into a hydrocarbon fraction (6) and an aqueous fraction (5) of rich mono ethylene glycol,

- a holding vessel (2) for collecting and holding the aqueous fraction of rich mono ethylene glycol (5) at temperature 60 - 100 C,

- means for sufficient alkalinity of the aqueous fraction of rich mono ethylene glycol (5) upstream of the holding vessel (2) or in the holding vessel (2) by injection of chemicals (12),

- means for controlled heating of the aqueous fraction of rich mono ethylene glycol (5) upstream of the holding vessel (2) or in the holding vessel (2),

- means holding the aqueous fraction at this conditions until particle size of salts of divalent cations and/or corrosion products is between 1 - 10 microns,

- a floatation vessel (3) with means (10) for purging the aqueous fraction of the vessel with a purging gas and means (9) for skimming off and discharging any foam formed on the surface of the liquid in the flotation vessel,

- means for transporting the aqueous fraction (7) including entrained particulates from the holding vessel (2) to the floatation vessel (3),

- means for separating the aqueous fractions of recovered lean mono ethylene glycol (16), water (15) and other contaminants as solid particles of mono valent mineral salts (1 1) in the MEG regeneration unit (14) as per known manner.

8. Device according to claim 7, wherein the holding vessel (2) and the floatation vessel (3) are combined into one vessel which comprises:

- the means for sufficient alkalinity and heating of the liquid phase of rich mono ethylene glycol (5) in the combined holding and flotation vessel and hold the liquid phase at sufficient alkalinity and temperature for a sufficient period of time to obtain precipitation and growth of divalent mineral salts and corrosion products as solid particles,

- the means (10) for purging the liquid phase in the combined holding and flotation vessel with a purging gas and means (9) for skimming off and discharging the foam formed on the surface of the liquid phase in combined holding and flotation vessel,

means for separating the fractions of recovered lean mono ethylene glycol (16), water (15) and other contaminants as solid particles of mono valent mineral salts (1 1) in the MEG regeneration unit (14) as per known manner.

9. Device according to claim 7 or 8, wherein the vessel for floatation of the liquid fraction of rich mono ethylene includes means for inserting floating agents or treating the liquid with an electrostatic field, or a combination of both.

10. Device according to any one of claims 7 to 9 wherein the means for purging generates micro bubbles.