WO2018115442A1

WO2018115442A1 - Hydrate inhibitor recovery system

Info

Publication number: WO2018115442A1
Application number: PCT/EP2017/084390
Authority: WO
Inventors: Dieter Müller
Original assignee: Nov Process & Flow Technologies As
Priority date: 2016-12-23
Filing date: 2017-12-22
Publication date: 2018-06-28
Also published as: NO342676B1; NO20162051A1

Abstract

A hydrate inhibitor recovery system and a method of operating the system is disclosed. The system comprises - a flash separator (32) with a main stream rich hydrate inhibitor inlet (29), - a recycle heating loop (35, 44, 46, 37); - a salt separator (48) in fluid communication with the recycle heating loop, and - a main stream vapour outlet with an unidirectional fluid connection to a distillation column (34) with a main stream vapour inlet, wherein the distillation column comprises a rich hydrate inhibitor slip stream inlet (53), a reflux inlet (39), a steam outlet (31) and a lean hydrate inhibitor outlet (33).

Description

HYDRATE INHIBITOR RECOVERY SYSTEM

The present invention relates to a system for recovery of hydrate inhibitor, especially an improved system for reclamation and re-concentration of hydrate inhibitor including the use of flash separation. Background

Glycol based hydrate inhibitors such as Mono Ethylene Glycol (MEG), Di-ethylene glycol (DEG), or tri-ethylene glycol (TEG) are used in hydrocarbon gas and/or condensate pipelines e.g. in gas fields, to absorb moisture and prevent formation of hydrate in the pipeline and other equipment. Typically, the MEG is injected into the upstream end of the pipeline and is separated from the hydrocarbon flow at downstream end. The separated MEG (approximately 50 % MEG, 50 % water), denoted as rich MEG, carries the absorbed water. This rich MEG is re-concentrated by a water removal process to produce "lean MEG" (approximately 90 % MEG, 10 water) for re-use. Water removal is normally performed by evaporation of the water. The MEG is also contaminated with other components from the well and the pipeline. Pipeline corrosion products, scale and other contaminants such as hydrocarbons, salts from formation water or production chemicals including other types of hydrate inhibitors are present. These impurities have to be fully or partially removed in the reclamation process otherwise the concentration of these compounds would be increase every time the inhibitor is recycled.

Herein the present invention is exemplified by referring to MEG as the glycol based hydrate inhibitor, however a person skilled in the art will understand that the invention is equally applicable for other types of inhibitors comprising glycol inhibitors and the invention is not limited to the recovery of MEG. Prior art

In WO 2007/073204 Al a process and a plant is described for regeneration of glycol from a mixture comprising glycol, water and salts, the salts comprising carbonate and/or bicarbonate ions. The mixture is flash distilled to obtain a salt-free solution of glycol and water. This solution is condensed and distilled to obtain glycol with reduced water content. The salts are concentrated in the vacuum boiler and removed from a sub- stream taken out of a return circuit to the vacuum boiler.

US 2005/0072663 disclose a method of regenerating a glycol solution containing water, hydrocarbons and salts. The glycol solution is expanded in a drum, then distilled in a column. The concentrated glycol collected at the level of a reboiler is placed under vacuum to vaporize the water and to precipitate the salts. The salts are separated from the glycol in a separation device. The concentrated glycol feed of the salts is stored in capacity. In industry two main types of systems are commonly used for MEG reclamation and re-concentration: the Full Stream system and the Slip Stream system. WO

2007/073204 and US 2005/0072663 referred to above are specific examples of these two main systems respectively. By re-concentration is meant concentrating the rich MEG to lean MEG, and by reclamation is meant removing contaminants as salts and corrosion products. In a Slip Stream system known from prior art, the MEG is only partly reclaimed, meaning only a partial stream undergoes reclamation to control the level of impurities like salts in the lean MEG being reintroduced to the well stream, whereas the rest of the rich MEG only undergoes re-concentration.

Objectives of the invention

The main objective of the present invention is to provide more energy efficient recovery system and method. Wherein the system has a compact design, and the improvement reduces the weight and amount of equipment compared to a tradition slip stream system. Therefore, the invention may reduce both the investment and operation costs.

The present invention provides a hydrate inhibitor recovery system comprising

- a flash separator with a main stream rich hydrate inhibitor inlet,

- a recycle heating loop, - a salt separator in fluid communication with the recycle heating loop, and

- a main stream vapour outlet with an unidirectional fluid connection to a distillation column with a main stream vapour inlet, wherein the distillation column comprises a rich hydrate inhibitor slip stream inlet, a reflux inlet, a steam outlet and a lean hydrate inhibitor outlet. The recycle heating loop on the flash separator will normally comprise a recycle loop outlet in fluid communication with the flash separator, a recycle loop pump with an inlet in fluid communication with the recycle loop outlet and a pump outlet in fluid communication with an inlet to a recycle heater comprising an outlet in fluid communication with a recycle loop inlet arranged on the flash separator. In second aspect of the hydrate inhibitor recovery system the main stream vapour outlet is fluidly connected to the distillation column through a chimney tray. In one aspect, the chimney height of the chimney tray is taller than 400mm- 1000mm or 0.3-0.7 times the inner diameter of the distillation column. In one aspect of the hydrate inhibitor recovery system, the distillation column comprises a further tray between chimney tray and the slip stream inlet. This further tray may be a further chimney tray of a perforated tray.

In a further aspect of the hydrate inhibitor recovery system the rich hydrate inhibitor slip stream inlet is arranged above the chimney tray and a deflector is arranged at the inlet for deflecting the rich hydrate inhibitor slip stream towards the chimney tray or the further tray.

In yet another aspect of the hydrate inhibitor recovery system, the system comprises a heat exchanger connected to the lean hydrate inhibitor outlet and the rich hydrate inhibitor slip stream to transfer heat from the lean stream to the slip stream.

In another aspect of the hydrate inhibitor recovery system, the system further comprises a slip stream valve and a salt content analyser in fluid connection with the rich hydrate inhibitor main stream inlet, wherein the analyser is in signal communication with a control unit and wherein the slip stream valve is controllably connected to the control unit which is adapted to control the valve depending on the salt content. If the salt content increases the slip stream feed rate can be reduced to avoid formation of solids or scaling on the chimney tray. In yet a further aspect the system comprises a reflux valve upstream the reflux inlet controllably connected to the control unit for controlling the reflux and thereby temperature in the distillation column and water content in the lean hydrate inhibitor stream directed out from the column through the lean hydrate inhibitor outlet. If the slip stream feed rate is reduced the reflux feed rate can be increased to adjust the water content in the lean hydrate inhibitor product, since an important feature of the present invention involves that the latent heat of vaporized fluid from the main stream in the flash separator must be able to evaporate the rich hydrate inhibitor slip stream entering the distillation column.

If there are expected gas production periods were the salt levels in the rich hydrate inhibitor are so low that there would be negligible salt accumulation in the flash separator, the hydrate inhibitor recovery system may then comprise a reversibly closable lean return stream conduit fluidly connecting the lean hydrate inhibitor outlet with the flash separator. The lean hydrate inhibitor product may then be obtained directly from the recycle heating loop, hereinafter referred to as a salt free mode. One example of such periods could be when there is nil, or extremely low formation water being produced from the reservoirs, where basically the only water being produced is condensed water from the produced gas. One example of negligible salt levels could be when the ppm level of total dissolved salt ions are below 20 ppm. Another example of negligible salt levels could be when the ppm when the divalent ions are below 1 ppm, but where the monovalent salts ions are above 20 ppm, allowing some accumulation of monovalent salts in the lean hydrate inhibitor injected to pipeline.

When the salt accumulation in the lean hydrate inhibitor injected to the pipeline reaches a significant or pre-set value, then the lean return stream would be closed for a period and the system is at least for a period not ran in the salt free mode.

The present invention further provides a method to operate a hydrate inhibitor recovery system according to the present invention, wherein the rich hydrate inhibitor slip stream feed rate is in the range of 1% to 35% of the feed rate of the rich hydrate inhibitor main stream. Preferably, in the hydrate inhibitor recovery system according to the present invention the slip stream is in the range of 27% to 33% of the feed rate of the rich hydrate inhibitor main stream.

In one aspect of the method, it comprises:

analysing the salt content of the rich hydrate inhibitor main stream;

adjusting the rich hydrate inhibitor slip stream feed rate depending on the salt content; and

adjusting the reflux stream feed rate depending on the slip stream feed rate.

In a further aspect, the method comprises deflecting the slip stream on the inside of the tank, collecting non-flashed liquids from this deflected slip stream collected on a perforated tray below, where at least a portion of this non-flashed liquid spills downwards into the section of the chimney tray through the perforations.

In yet another aspect one or more chimney tray hats deflects the evaporated flow from the mainstream into a flow pattern so it heats the non-flashed liquid that is spilled downwards into the section of the chimney tray.

In a further aspect, the hydrate inhibitor recovery system is operated in batches of in various modes of

a) full stream mode

b) slip stream-full stream hybrid mode.

In yet a further aspect the system is periodically operated in a salt free mode wherein the method comprises adjusting the rich hydrate inhibitor slip stream feed rate to nil and returning the lean hydrate inhibitor stream to the flash separator via an open and closable lean return stream conduit and producing the lean hydrate inhibitor stream from the liquid phase in the flash separator.

The salt content analyser and the control unit may, depending on the stability of the well stream treated with the hydrate inhibitor, be applied only initially to adjust the system to the well. Alternatively, it may be used more regularly if there are significant variations in the salt content. The rich hydrate inhibitor pump may also be connected controllably to the control unit so that this may control the overall feed rate into the recovery system.

The present invention may also be referred to as hybrid reclamation. When the invention is employed on a MEG system the rich MEG is fed to the flash separator while a smaller slip stream of rich MEG is sent directly to the bottom stage of the distillation column below the packing and above the chimney tray. Similar to a full stream design, both the reclamation system (i.e. the flash separator) and re- concentration system (i.e. distillation column) are operating under vacuum. The advantage to the hybrid configuration is that only a partial stream is sent to the reclamation for removing and controlling the salt levels, while the slip stream is sent directly to the distillation column for distillation obtaining the lean MEG product, thereby reducing the total energy usage and optimizing on equipment size and weight. This allows the reclamation (i.e. flash separator, recycle heater and pump) to be smaller. The vapour from the flash separator is providing the re-boiling heat in the column for the distillation of the MEG. The concept of removing a portion of salts is similar to the slip stream design known from prior art whereby the salt levels in the MEG loop are controlled, however, the difference being that the slip stream for reclamation is taken from the front of the process as rich MEG instead of from the lean MEG product according to the present invention. One essential feature for the latent heat of vaporized fluid from the main stream to be able to evaporate the slip stream, is to set up the system to a thermodynamic equilibrium where there still is some remaining water content in the produced lean MEG, typically 10% wt water, i.e. 90% MEG.

According to one aspect of the invention the lean hydrate inhibitor contains above 5 wt.% water, preferably above 7 wt.% water, or in another aspect it contains in the range of 7- 15 wt.% water, specifically around.10 wt.% water.

Brief description of the drawings

The present invention will be described in further detail with reference to the enclosed figures, wherein: Figure 1 is a schematic illustration of an embodiment of the system according to the present invention.

Figure 2 is a schematic illustration of an embodiment of the system according to the present invention including a control unit.

Figure 3 is a schematic view of an embodiment with separate flash separator and distillation column. Figure 4 is a schematic illustration of an embodiment of the system according to the present invention comprising additional heat exchange.

Figure 5a, 5b, 5c, 5d illustrated different configurations of the lower section of the distillation column of the combined flash separator and distillation column containing the chimney tray and the slip stream inlet.

Figure 6 is schematic illustration of a recovery system according to the present invention including injections to control and minimize precipitation, adjustment

Figure 7 is schematic illustration of a recovery system according to the present invention including alternative injections to control and minimize precipitation. adjustment

Figure 8 is schematic illustration of a recovery system according to the present invention including injections to control and minimize precipitation. Adjustment and an additional tray.

Figure 9 illustrates an embodiment especially adapted to a "salt free" rich MEG stream.

Principal description of the invention

In the drawings, equal reference numbers refer to equal equipment. All the reference numbers are listed in a table at the end of the description to provide a full overview. Although the figures illustrate different embodiments of the invention a person skilled in the art will appreciate that the different embodiments can be combined to form alternative embodiments of the invention.

Figure 1 illustrates one embodiment of the present invention, it should be understood that the invention primarily relates to the flash separation and re- concentration system and that this inventive central part can be combined with other known methods of regeneration of hydrate inhibitor and treatment of a well stream.

Illustrated on figure 1 is a formation 1 , with a seafloor 2 and a sea surface 3. A well stream 1 1 is obtained from the formation that contains natural gas, water and salts, but which may also comprise condensate, liquid hydrocarbons and production chemicals. The well stream is mixed with lean hydrate inhibitor from conduit 13 before it is past through the pipeline 10, thereby avoiding and/or minimising formation of hydrates during transport in the pipeline. Via conduit/riser 15 the inhibited well stream is fed to a hydrocarbon separator 12 providing a hydrocarbon gas stream 17 and a rich hydrate inhibitor stream 19. By separator pump 14 the rich hydrate inhibitor is fed to a rich hydrate inhibitor storage tank 16. Via rich hydrate inhibitor conduit 21 and rich hydrate inhibitor pump 18 the rich hydrate inhibitor is fed via pressurised rich hydrate inhibitor conduit 23 to the recovery system. A main stream 25 passes valve 20 and enters the flash separator 32 of a combined flash separator and distillation column 30 via main stream inlet 29. A rich hydrate inhibitor slip stream 27 passes valve 22 and enters the distillation column 34 of the combined flash separator and distillation column 30 via slip stream inlet 53. The slip stream feed rate is between 1 and 30% of the mainstream feed rate. The combined separator and column 30 is operated under vacuum. Separating the flash separator 32 and the distillation column 34 is a chimney tray 38 which allows main stream vapour to pass from the flash separator 32 to the distillation column 34, and which collects the flashed and distilled lean hydrate inhibitor and directs it out trough lean hydrate inhibitor outlet 33. The lean hydrate inhibitor pump 42 transfers the lean hydrate inhibitor via conduit 45 to the lean hydrate inhibitor tank 50 from where it via lean hydrate inhibitor pump 52 can be supplied to conduit 13.

To provide heat to the flash separator 32 and to the distillation column 34 a recycle heating loop is installed. The loop comprises a recycle loop outlet 35 in fluid communication with a bottom section of the flash separator 32, a recycle loop pump 44 with an inlet in fluid communication with the recycle loop outlet 35 and a pump outlet in fluid communication with an inlet to a recycle heater 46 comprising an outlet in fluid communication with a recycle loop inlet 37 arranged on the flash separator 32 in a section above the liquid filled bottom section. To remove salts a salt removal slip stream conduit 47 is connected to the recycle heating loop and the fluid stream is passed into a salt remover 48 where the salts are separated out through a salt outlet 49. The salt remover can be any type of equipment known for separation of solid salt particles from a fluid stream including equipment such as centrifuges and filters.

The distillation column is further equipment with a reflux loop. Steam is removed trough the steam/vapour outlet 31 arranged in the top of the column. It is cooled by passing through cooler 24 and water is condensed in condenser 26. Remaining vapour leaves through vapour outlet 41. The condensed water is past via reflux pump 28, then reflux valve 40 and to recycle inlet 39 into the distillation column. As the purpose of the distillation is to remove water to concentrate the hydrate inhibitor water is removed through water outlet 43.

Figure 2 shows an embodiment similar to figure 1 but further comprising a salt content analyser 54 connected to a control unit 56. The control unit is connected to the rich hydrate inhibitor pump 18, slip stream valve 22 and reflux valve 40, and can control these based on the input from the analyser. The embodiment illustrated on figure 3 contains the same control units as the one illustrated on figure 2 but in this embodiment the flash separator 32 and the distillation column 34 are separate units. Lean hydrate inhibitor is collected in the bottom of the separate distillation column and no chimney tray is therefore required. The embodiment illustrated on figure 4 is equivalent to figure 1 except that a heat exchanger 58 is installed on the slip stream conduit wherein the slip stream is heated by indirect heat exchange with the lean hydrate inhibitor stream coming from the lean hydrate inhibitor outlet 33. Figures 5a, 5b, 5c, 5d illustrate different configurations of the lower section of the distillation column 34 of the combined flash separator and distillation column 30 containing the chimney tray 38 and the slip stream inlet 53.

In figure 5a the rich hydrate inhibitor slip stream inlet 53 is arranged near the bottom of the chimney tray 38 providing a submerge inlet. The vapour will flash when entering the column at vacuum conditions and proceed through the lean hydrate inhibitor liquid collected on the chimney tray 38 and continue upwards through the packing 36 of the distillation column.

In figure 5b the inlet 53 is arranged higher up and a second chimney tray 60 is arranged above the main chimney tray 38 but below the inlet. The slip stream will flash when entering this section and liquid collected by the second tray is allowed to spill down onto the main chimney tray 38.

In figure 5c the slip stream inlet 53 is arranged high above the chimney tray 38 and a deflector 64 is arranged above the inlet deflecting the slip stream downwards to provide for mainly the vapour phase to enter the packing 36 of the distillation column.

In figure 5d the configuration is similar to the configuration in figure 5b but here the second tray 160 is a perforated plate without chimney hats.

In figure 5b and 5d the slip stream is deflected on the inside of the lower section of the distillation column, where non-flashed liquids from this deflected slip stream is collected on a second tray 60, 160 below, where at least a portion of this non-flashed liquid spills downwards into the section of the chimney tray 38 through the perforations. It is believed that when one or more chimney tray hats deflects the evaporated flow from the main stream into a flow pattern, it heats the non-flashed liquid that is spilled downwards from the second tray 60, 160 into the section of the chimney tray 38.

Figure 6 illustrate an embodiment comprising acid injection 61 in slip stream by upstream injection into evaporating tank 62 to suppress precipitation of divalent ions from the slip stream by shifting the solubility product of the divalent salts as a function of the following feed conditions

-salt concentrations

-pH

-alkalinity -temperature

- MEG concentration

Acid injection 61 is supplied to the slip stream 27 in a treated slip stream tank 62 resulting in a pH of 4-6 or lower in the slip stream past though inlet 153. The lean hydrate inhibitor 33 is at least partly neutralized by injecting base through base injection inlet 59, providing the lean hydrate inhibitor 33 with a pH of

approximately 6-7. This reduces or eliminates the scaling and salt precipitation on the chimney tray. In the illustrated embodiment, there is also provided for adding chemicals to the main stream 25 via inlet 57, where such chemicals that minimise scaling in the flash separator 32 and control the salt precipitation could be injected here. The main stream including chemicals are referred to as 129.

Figure 7 illustrates a similar system to figure 6 but here the acid 61 is injected directly into the slip stream 27.

In figure 8 the injection systems are equivalent to figure 7 but here a deflector according to figure 5c and a perforated plate according to figure 5d are installed in the distillation column.

Figure 9 is an illustration of the periods when the system runs in the salt free mode, where in the embodiment of figure 8 a lean return stream conduit 63 is included directing the lean hydrate inhibitor 33 down into the flash separator 32. The valve 22 controlling the slip stream 27 and the valve on the acid injection 61 are closed during this mode. As there are no salts, the recovered lean hydrate inhibitor can be removed from the recycle heating loop as stream 65. This mode allows for pump 42 to be switched of.

Reference numbers

1 Formation 30 Combined flash separator and distillation column

2 Seafloor 31 Distillation column vapour outlet

3 Sea surface 32 Flash separator

10 Pipeline 33 Lean hydrate inhibitor outlet

11 Well stream (gas, water, salts) 34 Distillation column

12 Hydrocarbon separator 35 Recycle loop outlet

13 Lean hydrate inhibitor supply 36 Packing

conduit

14 Separator pump 37 Recycle loop inlet

15 Inhibited well-stream 38 Chimney tray

16 Rich hydrate inhibitor storage 39 Reflux inlet

tank

17 Produced gas conduit 40 Reflux valve

18 Rich hydrate inhibitor pump 41 Vapour outlet

19 Separated rich hydrate inhibitor 42 Produced lean hydrate inhibitor pump

20 Mainstream valve 43 Water outlet

21 Rich hydrate inhibitor conduit 44 Recycle loop pump

22 Slip stream valve 45 Lean hydrate inhibitor conduit

23 Pressurised rich hydrate 46 Recycle heater

inhibitor conduit

24 Cooler 47 Salt removal slip stream

25 Rich hydrate inhibitor main 48 Salt remover

stream conduit

26 Condenser 49 Salt outlet

27 Rich hydrate inhibitor slip 50 Lean hydrate inhibitor tank

stream conduit

28 Reflux pump

29 Rich hydrate inhibitor main 52 Lean hydrate inhibitor pump stream inlet Rich hydrate inhibitor slip stream inlet

Salt content analyser

Main stream vapour outlet

Control unit

Chemical injection

Heat exchanger

Base injection

Second tray, further tray

Acid injection

Treated slip stream tank

Lean return stream conduit

Deflector

Lean hydrate inhibitor directly from flash separator

Mainstream with injected chemicals

Acid slip stream inlet

Perforated tray

Claims

1. Hydrate inhibitor recovery system comprising

- a flash separator with a main stream rich hydrate inhibitor inlet,

- a recycle heating loop,

- a salt separator in fluid communication with the recycle heating loop, and

- a main stream vapour outlet with an unidirectional fluid connection to a distillation column with a main stream vapour inlet,

wherein the distillation column comprises a rich hydrate inhibitor slip stream inlet, a reflux inlet, a steam outlet and a lean hydrate inhibitor outlet.

2. The hydrate inhibitor recovery system according to claim 1 wherein the main stream vapour outlet is fluidly connected to the distillation column through a chimney tray.

3. The hydrate inhibition recovery system according to claim 2, wherein the chimney height is taller than 400mm- 1000mm or 0.3-0.7 times the diameter of the distillation column.

4. The hydrate inhibitor recovery system according to claim 2 wherein the distillation column comprises a further tray between chimney tray and the slip stream inlet.

5. The hydrate inhibitor recovery system according to any one of the claims 2-4, wherein the rich hydrate inhibitor slip stream inlet is arranged above the chimney tray and a deflector is arranged at the inlet for deflecting the rich hydrate inhibitor slip stream towards the chimney tray or the further tray.

6. The hydrate inhibitor recovery system according to any one of the claims 4-5, wherein the further tray has perforations.

7. The hydrate inhibitor recovery system according to any one of the claims 2-6, wherein the system comprises a heat exchanger connected to the lean hydrate inhibitor outlet and the rich hydrate inhibitor slip stream to transfer heat from the lean stream to the slip stream.

8. The hydrate inhibitor recovery system according to any one of the claims 2-7, further comprising a slip stream valve and a salt content analyser in fluid connection with the rich hydrate inhibitor main stream inlet, wherein the analyser is in signal communication with a control unit and wherein the slip stream valve is controllably connected to the control unit which is adapted to control the valve depending on the salt content.

9. The hydrate inhibitor recovery system according to claim 8, wherein the system comprises a reflux valve upstream the reflux inlet controllably connected to the control unit for controlling the reflux and thereby temperature in the distillation column and water content in a lean hydrate inhibitor stream obtained from the lean hydrate inhibitor outlet.

10. The hydrate inhibitor recovery system according to any one of the claims 1-9, further comprising a reversibly closable lean return stream conduit fluidly connecting the lean hydrate inhibitor stream with the flash separator.

11. The method to operate a hydrate inhibitor recovery system according to any one of the claims 1- 10, characterized in that the rich hydrate inhibitor slip stream feed rate is in the range of 1% to 35% of the feed rate of the rich hydrate inhibitor main stream.

12. The method according to claim 11 wherein the method comprises

analysing the salt content of the rich hydrate inhibitor main stream;

adjusting the reflux stream feed rate depending on the slip stream feed rate.

13. The method according to claim 11 or 12, wherein the method comprises deflecting the slip stream on the inside of the tank, collecting non-flashed liquids from this deflected slip stream collected on a perforated tray below, where at least a portion of this non-flashed liquid spills downwards into the section of the chimney tray through the perforations.

14. The method according to claim 13 wherein one or more chimney tray hats deflects the evaporated flow from the mainstream into a flow pattern so it heats the non-flashed liquid that is spilled downwards into the section of the chimney tray.

15. The method according to claim 11, 12, 13 or 14, wherein the hydrate inhibitor recovery system is operated in batches of in various modes of

a) full stream mode

b) slip stream-full stream hybrid mode.

16. The method according to any one of the claims 11- 15, wherein the method further comprises periods of a salt free mode comprising adjusting the slip stream feed rate to nil and returning lean hydrate inhibitor to the flash separator via an open and closable lean return stream conduit and producing the lean hydrate inhibitor from the liquid phase in the flash separator.