The present invention relates to a system for recovery of hydrate inhibitor, especially a system with improved handling of by-products such as produced water and salts.

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. This is well known within the industry and especially relevant were the pipeline is passed through water, in relation to subsea wells or wells in the colder regions of the earth. 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 (typically 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” (typically 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. If the formation comprises heavy metals or salts thereof, such as mercury, then such compounds will also have to be separated out, and possibly handled as hazardous waste. All these impurities must 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 but equally applicable to other types of glycol inhibitors such as TEG or DEG, or combinations of the different glycol based inhibitors, including combinations with one or more kinetic inhibitors such at PVCap

Prior art

In WO 2007/073204 A1 a process and a plant are 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.

Within the industry two main types of systems are commonly used for MEG reclamation and re-concentration: the Full Stream system and the Slip Stream system.

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, 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.

When separating out salts from the hydrate inhibitor some of the hydrate inhibitor may be separated out there with which would result in a loss of hydrate inhibitor, any improvements in the system that reduces the loss of hydrate inhibitor will provide both an economical and environmental improvement. Further, the concentrated salts, although primarily comprising NaCl cannot be released into the environment, such as the surrounding sea, due to hydrocarbons and other

contaminants as well as the hydrate inhibitor included therewith.

The rich hydrate inhibitor comprises in addition to the hydrate inhibitor, water, hydrocarbons and other contaminants. During the reconcentration or reclamation some of the hydrocarbon will follow the separated water stream. Such a produced water stream containing hydrocarbons may in most cases, due to the environmental impact of the hydrocarbons not be released into the environment.

Objectives of the invention

The main objective of the present invention is to provide an improved hydrate inhibitor recovery system, especially wherein the separation of salts, and disposal of water and salts from the hydrate inhibitor recovery system is improved, to be more compact, more cost efficient and more environmental friendly.

Wherein the system has a compact design, and the improvement reduces the weight and amount of equipment compared to existing hydrate inhibitor recovery systems. Therefore, the invention may reduce both the investment and operation costs.

A further objection is to provide a system that can extended to effectively and cost effectively separate and handle heavy metal contaminates, especially mercury.

The present invention provides a hydrate inhibitor recovery system comprising:

- a flash evaporator with a rich hydrate inhibitor feed inlet, vapour outlet and a slurry outlet, - a heater in fluid communication with the flash evaporator,

- a condenser in fluid communication with the vapour outlet of the flash evaporator, the condenser further comprising a condensed liquid outlet,

- a hydrocarbon removal unit (HRU) in fluid communication with the condensed liquid outlet of the condenser, wherein the HRU comprises a hydrocarbon deprived condensed water outlet,

- a salt removal and disposal system (SRDS) in fluid communication with the slurry outlet of the flash evaporator, and in fluid communication with the hydrocarbon deprived condensed water outlet, wherein the SRDS comprises a salt deprived liquid outlet in fluid communication with the flash evaporator, and salt enriched hydrocarbon deprived condensed water outlet.

The system is adapted to provide salts dissolved in hydrocarbon deprived water, this stream is environmental friendly as it does not contain hydrocarbons.

The heater can according to common procedure be arranged in recycle heating loop on the flash evaporator. This will comprise a recycle loop outlet in fluid

communication with the flash evaporator, a recycle loop pump with an inlet in fluid communication with a 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 evaporator. The slurry outlet from the flash evaporator may be arranged in the recycle heating loop downstream the recycle loop pump so that no separate pump on the slurry outlet is required, but the slurry outlet may also alternatively be arranged to have a dedicated conduit off take from the flash evaporator, being separately from the recycle heating loop.

The system will according to common procedure preferably comprise a reflux conduit returning a part of the condensed vapour to the top of the evaporator.

In one aspect of the hydrate inhibitor recovery system the HRU comprises a coalescer, more preferably a compact coalescer, and even more preferably a filter/cartridge coalescer. This limits the space required by the HRU.

In a further aspect the HRU comprises a vessel with a lower water outlet and an upper hydrocarbon collection volume comprising a hydrocarbon removal outlet, optionally the vessel is equipped with a hydrocarbon-water interphase measurement and control unit. Accordingly, the HRU is adapted for gravitational separation of the coalesced hydrocarbons.

In yet another aspect the HRU is adapted to coalesce both hydrocarbons and mercury and comprises a mercury trap pot and at least one weir plate. In a further aspect of the hydrate inhibitor recovery system it comprises a mercury removal unit arranged in fluid communication with the hydrocarbon deprived condensed water outlet of the HRU. This system is design to secure that mercury is not introduced in the SRDS via the hydrocarbon deprived condensed water.

In yet a further aspect of the system it comprises a hydrocarbon removal system (HRS), wherein said HRU and said condenser forms part of the HRS, and wherein the condenser comprises a cooler upstream a condensed liquid collector tank, and wherein the condensed liquid collector tank comprises a hydrocarbon overflow weir with an outlet in fluid communication with a hydrocarbon conduit. The hydrocarbon content of the feed stream may variate, and this aspect provides additional flexibility to handle an increase content of hydrocarbons in that such an excess of hydrocarbons can be separated out in the condensed liquid collector tank.

In one aspect the system further comprises a vacuum pump in fluid communication with a vapour outlet of the condenser. By providing the possibility to perform the evaporation and possible other steps of the process at reduced pressure improves the efficiency and provides an additional parameter for controlling the process.

In another aspect the condenser comprises a cooler upstream a condensed liquid collector tank and said condensed liquid outlet is arranged in the lower part of the condensed liquid collector tank and the vapour outlet is arranged in the upper part of the condensed liquid collector tank. Additionally, the condensed liquid collector tank may comprise internals such as baffle plate(s) and weirs and at least one purge gas inlet, to improve the separation efficiency.

In a further aspect the vacuum pump is a liquid ring vacuum pump, and the system comprises a vapour liquid separator in fluid communication with a vacuum pump outlet, and wherein the vapour liquid separator comprises a mercury trap pot. This allows for the system to be used in a method wherein the mercury is maintained in the vapour phase until it has passed through the vacuum pump and the pressure is increased.

In another aspect the system comprises a second cooler in fluid communication with a vapour outlet of the condenser, a second vapor liquid separator in fluid

communication with the second cooler and arranged upstream the vacuum pump, wherein the second vapor liquid separator comprises a mercury trap pot. Here the system is designed to run a process where the mercury is primarily condensed in the second condenser under vacuum conditions.

In these aspects the system is designed to primarily remove the mercury upstream the HRU. In another aspect of the hydrate inhibitor recovery system the SRDS comprises a solid salt separator with a solid salt outlet in fluid communication with a salt dissolution unit which also comprises an inlet in fluid communication with the hydrocarbon deprived condensed water outlet, said solid salt separator further comprises the salt deprived liquid outlet in fluid communication with the flash evaporator.

In a further aspect the solid salt separator is in fluid communication with the hydrocarbon deprived condensed water outlet. This configuration allows for the hydrocarbon deprived condensed water to be used for washing hydrate inhibitor of the separated solid salts. The washing liquid can be returned to the flash evaporator through a hydrate inhibitor return conduit providing the fluid communication with the flash evaporator.

In yet a further aspect the SRDS further comprises a divalent salt separator comprising

- an inlet in fluid communication with the salt deprived liquid outlet of the solid salt separator,

- an outlet for divalent salts deprived hydrate inhibitor in fluid communication with the flash evaporator, and

- a solid divalent salt outlet.

This divalent salt separator will separate out fines including divalent cation salts from the remaining slurry stream before it is returned to the evaporator. Preferably, the divalent salt separator is a cross flow filter, more preferably a dynamic crossflow filter.

In another aspect the SRDS further comprises a reactor with an inlet for a precipitation chemical, an inlet in fluid communication with the salt deprived liquid outlet of the solid salt separator, and an outlet in fluid communication with the inlet of the divalent salt separator. The reactor provides the possibility to precipitate divalent cation salts upstream the divalent salt separator.

In a further aspect of the hydrate inhibitor recovery system the divalent salt separator is in fluid communication with the hydrocarbon deprived condensed water outlet. This configuration allows for the hydrocarbon deprived condensed water to be used for washing hydrate inhibitor of the separated divalent salts. The washing liquid can be returned to the flash evaporator through a hydrate inhibitor return conduit providing the fluid communication with the flash evaporator. Further, this provides for removing the separated divalent salts and other fines out of the separator by forming a slurry with the hydrocarbon deprived condensed water. The system is especially applicable for recovering MEG.

The term“in fluid communication” as used herein refers to a fluid communication in the intended flow direction. Accordingly, the expression“B is in fluid communication with A” should be understood to refer to B being arranged downstream from A with a fluid communication therebetween. Other equipment may be arranged between A and B.

Further the present invention provides a method for recovering hydrate inhibitor comprising

- feeding a rich hydrate inhibitor stream comprising hydrate inhibitor, water, hydrocarbons, salts to a flash evaporator

- obtaining a vapour stream comprising water and hydrocarbons, and slurry of hydrate inhibitor and solid salts,

- obtaining lean hydrate inhibitor from the slurry,

- condensing water and hydrocarbons in the vapour stream,

- separating the condensed vapour in a hydrocarbon deprived water stream and a hydrocarbon stream,

- separating the solid salts from the slurry,

- dissolving the separated salts in the hydrocarbon deprived water stream, and

- disposing the hydrocarbon deprived water stream with dissolved separated salts.

In one aspect the method comprises separation of mercury comprised in the rich hydrate inhibitor stream, wherein the method comprises

- evaporation of the mercury as part of the vapour stream comprising water and hydrocarbons at below atmospheric pressure,

- operating the condensing of water and hydrocarbons in the vapour stream at below atmospheric pressure at conditions at which mercury is maintained in the vapour phase;

- separating mercury by condensation at increased pressure.

In another aspect the method comprises separation of mercury comprised in the rich hydrate inhibitor stream, wherein the method comprises

- evaporation of the mercury as part of the vapour stream comprising water and hydrocarbons at below atmospheric pressure, - operating the condensing of water and hydrocarbons in the vapour stream at below atmospheric pressure at conditions at which mercury is maintained in the vapour phase;

- separating mercury by condensation at lower temperature at below atmospheric pressure.

The temperature in the different steps is regulated by the heaters and coolers. The pressure below atmospheric pressure is primarily provided by a vacuum pump downstream the top vapour outlet of the flash evaporator.

The method can accordingly be extended in several ways to separate mercury from the rich feed securing that the mercury is not included in the water, dissolved salts or vapour streams that are produced by the method.

In a further aspect the method comprises

-washing the separated solid salts with a part of the hydrocarbon deprived water stream to recover hydrate inhibitor therefrom before dissolving the solid salts and returning the obtain washing solution to the evaporation step. This increases the recovery rate of the hydrate inhibitor, and is therefore both economically and environmental attractive.

The salts removed in the solid salt remover is primarily salts with monovalent cations such as NaCl, and KC1, but also other solid particles present in the slurry will be removed.

The divalent cations present in the slurry from the flash evaporator are primarily calcium ions but can also comprise other divalent ions such as magnesium, barium, strontium and iron.

In the divalent cation reactor, a chemical is added to the liquid phase obtained after the solid salts and other solid particles have been removed in the solid salt remover. As the slurry is taken from the lower part of the flash separator it is water deprived. The liquid entering the reactor is thus also water deprived. An objective of the added chemical is to precipitate divalent cation salts such as CaCC> ₃ . Subsequently, then precipitated divalent cation salts are separated in a divalent salt separator, preferably a cross flow filter, or even more preferably a dynamic crossflow filter.

As the divalent salt separator is arranged downstream the solid salt remover, the flux through the divalent salt separator can be adjusted freely and a high flux is possible because the solid salts (monovalent salts) have been removed upstream, and the concentration of particles has been reduced.

The chemical comprises preferably a compound that regulates pH and alkalinity, more preferably the chemical comprises a base such as NaOH, Na ₂ CC> ₃ , NaHCCb, that will result in the precipitation of calcium salts and salts of other present divalent cations.

Furthermore, the hydrate inhibitor recovery system may comprise a fluid connection from the reactor to a sampling point or measurement instrument, to measure the pH or alkalinity of the fluid in the reactor.

Furthermore, the hydrate inhibitor recovery system may also comprise a fluid connection from the flash evaporator to a sampling point or measurement instrument, to measure the pH or alkalinity of the fluid in the flash evaporator. Further the system may comprise a conduit for adding a pH adjustment chemical to the flash evaporator. Preferably, the pH adjustment chemical is an acid, more preferably HC1. The addition of the pH adjustment chemical can be regulated based on the measured pH.

Preferably said measurements of the pH or alkalinity are done regularly, or even more preferably as an inline or real-time measurement, where the results of the measurements are used to calculate the dosing of chemicals to minimize the resulting concentration of carbonate ions [C0 ₃ ² ] inside the flash evaporator.

In one embodiment of the invention, the amount of chemical added to the reactor is adjusted to increases the pH value by at least one, thereby significantly reducing solubility of divalent salts, whilst at the same time the calcium ions still are kept in excess by a factor of larger than 1.1, preferable 1.1 - 10, preferably 1.1-5, more preferably 1.1 to 2, even more preferably 1.1-1.5 compared to the concentration of carbonate ions [C0 ₃ ² ]. The excess of calcium ions secures that significantly all carbonate ions are reacted to form solids that are separated out in the divalent salt separator. Thereby the amount of carbonate ions returned to the flash evaporate is minimized. In the flash evaporate the reaction between carbonate ions and calcium ions could otherwise result in scaling.

The hydrocarbon removal unit (HRU) preferably comprises a coalescer, more preferably a compact coalescer, and even more preferably a filter/cartridge coalescer, preferably utilizing on a polymeric medium to coalesce, such as AquaSep XS Coalescer available from Pall Corporation.

In one aspect the system according to the invention is particularly applicable to remove hydrocarbons from an oxygen free hydrate inhibition stream (comprising glycol, salts, water and hydrocarbon). After the processing step of separating water and hydrocarbon through evaporation and subsequently condensation, the obtained hydrocarbon deprived and oxygen free water is used to dissolve separated salts separated from the accumulated slurry at the bottom of the evaporator. The fact that this water is oxygen free means that this water can dissolve salts without becoming noteworthy corrosive to equipment exposed to the water with comprising dissolved salts. The fact the water is hydrocarbon deprived means it can be discharged to sea.

Oxygen free means substantially oxygen free. However, should there be oxygen ingress somewhere in the system the oxygen will be removed in a full stream system via the vacuum system as that vacuum system then is also functioning as a deaerator.

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

a) full stream mode

b) slip stream-full stream hybrid mode.

Hydrocarbon deprived water means that hydrocarbons has been removed from the water. The hydrocarbon content of the hydrocarbon deprived water is preferably below 20 ppm, more preferably below 10 ppm, and even more preferably below 5 ppm.

Salt deprived liquid means that salts has been removed from the liquid.

Although the present invention is disclosed including the HRS a person skilled in the art will understand that the salt removal system as well as the system features adapted to remove mercury can respectively by applied alone or in combination without including the hydrocarbon removal. Accordingly, the salt removal system may be used alone or combined with a different salt disposal system. The disclosed solutions for mercury removal may be employed alone without hydrocarbon removal and without or with a different system for removal of salts.

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 in a slipstream mode.

Figure 3 is a schematic illustration of an embodiment of the system according to the present invention comprising additional salt removal units.

Figure 4 is a schematic illustration of the details of the hydrocarbon removal unit (HRU). Figure 5 is a schematic illustration of an embodiment of the system according to the present invention comprising the HRU of figure 4.

Figure 6 is schematic illustration of a recovery system according to the present invention including optional equipment for mercury removal.

Figure 7 is schematic illustration of a recovery system according to the present invention including additional optional units for adapting the system to include mercury removal.

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. For elements with similar but not necessarily identical function a digit on the hundred space is added to differentiate. These numbers are not included in the list but discussed in connection with the figure(s) in which they are included.

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 a recovery system comprising both a salt removal and disposal system (SRDS) and a hydrocarbon removal system (HRS) that are interconnected.

The rich hydrate inhibitor feed stream enters the system through feed inlet 1 to the flash evaporator 14. In this embodiment the flash evaporator is a section of a combined compact flash separator further comprising a distillation column 22 and a chimney tray 21 arranged therebetween for collecting the liquid hydrate inhibitor. Both water, hydrate inhibitor and low boiling hydrocarbons will be evaporated in the flash evaporator 14. The vapour passes up through the chimney tray 21 and into the distillation column where the water and low boiling hydrocarbons are separated from the hydrate inhibitor. Hydrocarbons with a boiling point close to the hydrate inhibitor may be included in the lean hydrate inhibitor stream exiting though outlet 4, however as the lean hydrate inhibitor is to be reinjected into the well stream, this is unproblematic. The lean hydrate inhibitor passes via valve 34 and proceeds further back to the system requiring the use of a hydrate inhibitor. Preferably the rich hydrate inhibitor feed stream is substantially oxygen free.

Through the vapour outlet 2 the vapour enters the hydrocarbon removal system (HRS) which also generates the reflux stream that is past via conduit 3 valve 30 and reflux inlet 39 back into the top of the distillation column. The vapour is cooled in cooler 23 and transferred to the condensed liquid collector tank 24. Any remaining volatile compounds are vented through vapour outlet 40. From the condensed liquid collector tank 24 the liquid is transferred via conduit 9 either into the reflux conduit 3 or via valve 31 and conduit 10 into the hydrocarbon removal unit (HRU) 26. The HRU 26 provides for the separation of the hydrocarbon from the produced water. The HRU 26 may include a coalescer or other applicable equipment to

assist/improve the separation of the hydrocarbons from the water, such as a coalescer tank with internals, overflow weir, hydrocyclones, hydrocarbon accumulator with level control, corrugated plate pack etc. The hydrocarbon deprived water leaves the HRU trough conduit 15 whereas the separated

hydrocarbons leave the HRU trough conduit 12 through valve 42 and pass into hydrocarbon conduit 13 which may be combined with other hydrocarbon stream which is not the focus of the present invention.

The HRS in figure 1 includes an additional hydrocarbon removal conduit 11 from an overflow weir 25 within the tank 24 and via valve 41 into conduit 13. These additional features provide the system with increased flexibility to handle an increase in hydrocarbon content in the feed. In this case the tank 24 also functions as a gravity separator, and the excess hydrocarbons are separated in the overflow 25. The valves 41 and 42 will be controlled to allow appropriate flow through the two alternative routes and the control of the valves can be performed based on the measurements/sensors providing information on the hydrocarbon content in the feed and/or the tank 24.

The flash evaporator 14 is provided with a recycle/reboiler loop to add heat to the evaporator 14 the loop comprises a recycle loop outlet 6, a recycle pump 44, a heater 29 and a recycle loop inlet 5. Via valve 45 and slurry outlet 8 a slurry stream is past into the solid salt separator 27. In the disclosed embodiment the slurry outlet is connected to the recycle loop, but the slurry outlet may also be arranged directly on the bottom section of the flash evaporator, which would however require a separate pump. In the separator solid salts are removed from the slurry. The solid salt remover 27 is a conventional solid liquid separator such as a centrifuge, decanter, basket, filter etc. The solid salt remover 27 may depending on the type of equipment selected remove different amounts of the solids present. The main monovalent salt is NaCl, which generally forms relatively large particles/crystals, and which therefore is easily removed to a large extend in a decanter. Other salts, especially salts forming relatively small crystals will only to a limited extend be removed in a decanter. The solid salt deprived liquid phase is via hydrate inhibitor return conduit 7 returned to the flash evaporator.

In the embodiment on figure 1, the hydrocarbon deprived water is via conduit 15 past into the salt removal and disposal system (SRDS) and via conduit 16 and valve 32 passed into solid salt separator 27. This water stream is in the solid salt separator used to wash the separated solids, to remove and wash recovered hydrate inhibitor therefrom. In one preferred embodiment the hydrocarbon deprived water is via said valve 32 passed directly into a spinning salt layer cake in the centrifuge of a decanter centrifuge.

The washing stream including the recovered hydrate inhibitor and used wash water is transferred back to the flash evaporator 14 via conduit 7. Depending on the configuration/structure of separator 27 this salt washing can be performed batch wise or continuously. However, as wash water will dissolve some of the of separate salts, the process involves sending used wash water back to the flash separator will preferably be optimised to limit the amount of washing water used.

The washed solid salts are transferred via conduit 38 to the salt dissolution unit 28. Hydrocarbon deprived water which is also substantially oxygen free is via conduit 15 via valve 33 and conduit 18 added to the dissolution unit 28. The hydrocarbon deprived water is salt free and the dissolution of the separated salts therein is adjusted to provide a salt water applicable for release to the environment via diluted salt outlet 19 valve 37 and conduit 20 which is preferably connected to the sea or other applicable disposal arrangements. The salt dissolution unit 28 may comprise a salt concentration sensor and a control unit regulating the valves on the inlet and outlet of unit 28. In case of excess water compared to the amount of salts being produced the hydrocarbon deprived water can be released directly to the sea, or other applicable disposal arrangements, via conduit 17 valve 35 and conduit 20.

Figure 2 illustrates the present invention in connection with a slip stream system. The HRS and SRDS systems are identical to the systems disclosed on figure 1 and 100 has been added to the reference numbers just to visualise that equipment size will be adapted to the slipstream configuration. In the slip stream system, the rich hydrate inhibitor feed 101 enters a flash evaporator 1 14/distillation vessel 122, wherein water and hydrocarbons are evaporated but the evaporation of hydrate inhibitor is limited. The upper section 122 is different from the distillation column 22 of figure 1 in that it is arranged to avoid droplets of hydrate inhibitor been removed as part of the vapour 102. The vapour 102 is condensed and used partly as reflux and partly after hydrocarbon removal as solvent for the salts in the same way as described in connection with figure 1. The evaporator has a recycle/reboiler loop with and outlet conduit 106 and heater 129 and recycle loop inlet 105, the recycle loop pump has been omitted from the figure to improve the overview.

A slipstream is removed via conduit 59 and valve 201 to slip stream flash evaporator 214 to undergo full reclamation. A part of the partial lean hydrate inhibitor taken from the recycle outlet 106 is via valve 50 past directly into the conduit 204 for returning lean hydrate inhibitor to the directly or indirectly to the well streams. The term partial lean hydrate inhibitor is used here to refer to the situation that water and hydrocarbons have been removed whereas salts are only removed from the slip stream 59.

In slip stream flash evaporator 214 the hydrate inhibitor is evaporated, the vapour in vapour outlet 202 is salt free hydrate inhibitor which is cooled and condensed in cooler 223 and this lean hydrate inhibitor is via conduit 104 added to the outlet 204 of hydrate inhibitor recovered for reuse.

The slip stream flash evaporator 214 has a recycle/reboiler loop with and outlet conduit 206 and heater 229 and recycle loop inlet 205, the recycle loop pump has been omitted from the figure to improve the overview. The salt slurry is feed through slurry outlet 108 into the SRDS, especially the solid salt separator 127.

Figure 3 illustrates a system comprising all the elements of figure 1 explained in detail above. An additional process of precipitating and removing salts with divalent cations has been added. The salt removal system is the equipment arranged within 90. In this embodiment the solid salt deprived liquid phase is primarily past via valve 62 to a divalent cation reactor 65. Conduit 7 back to the flash evaporator 14 may be used when separated salt is washed with hydrocarbon deprived water to remove hydrate inhibitor that is sticking the separated solid salts. The valve 61 is added to control when the return conduit should be open. In the reactor 65 the solid salt deprived hydrate inhibitor is reacted with precipitation chemicals 60 added to the reactor. The precipitation chemicals lower the solvability of divalent cation salts and may be a base. The precipitating chemicals may also comprise carbonates which react with the divalent calcium ions and precipitate as calcium carbonate. After reaction the precipitated solids and the hydrate inhibitor is passed via outlet 72 and valve 66 to the divalent salt separator 70, which is a solid liquid separator, such as a filter, gravitational separator or similar, preferably a cross flow filter, and even more preferably a dynamic cross flow filter. In addition to precipitated divalent cation salts separator 70 will also separate out other fine particles/fines, so although named divalent cation salts separator, it could also be referred to as a fines separator.

The illustrated embodiment comprises two filter cartridges with separate outlets, but the number of filter elements can be freely selected and adapted to the selected filter system and the requirements to the system. The divalent cation salts and other fines deprived hydrate inhibitor is via conduit 71 returned to the flash evaporator 14. An adjustment of the pH of the return stream may be included (not shown) to adjust the pH to the pH of the liquid in the evaporator 14. Also, the salts collected in the divalent salt separator 70 can be washed with hydrocarbon deprived water provided via conduit 73 and valve 67. To recover the hydrate inhibitor the washing liquid is transferred back to the flash evaporator 14 via conduit 71. After washing the valve 68 is opened and the hydrocarbon deprived water is used to flush the divalent cation salts through conduit 69 to be released through conduit 20. The solvability of the divalent cation salts in water is low, but the formed salt particles such as CaCCb are generally environmental friendly in the limited concentration obtained through suspension in the hydrocarbon deprived water.

In an alternative embodiment (not shown), it will be possible to include a bypass conduit bypassing the solid salt separator 27 and optionally the reactor 65 allowing the slurry outlet 8 to be in direct fluid connection with either reactor 65 or separator 70. This bypass could be applied to remove fines.

An alternative embodiment of the system of figure 3, comprises allowing the concentration of divalent ions to become large enough to precipitate in the flash evaporator, the use of divalent cation reactor 65 and precipitation chemical 60 can be omitted, e.g. the solid salt deprived liquid phase 107 is passed directly to the divalent salt separator 70. The divalent cations tend to form very small crystals and therefore these will not or only to a limited extend be removed together with the other solid salts in the solid salt remover 27, when this is a decanter or similar.

Figure 4 is a detailed representation of the HRU 26. Through valve 31 and conduit 10 the condensed liquid enters the HRU 26. The HRU is preferably a compact coalescer, more preferably a filter cartridge coalescer, with coalescer elements 85.

In the HRU the hydrocarbon droplets grow. The droplets with increased size gravity separate easier. The hydrocarbon deprived water outlet 15 is arranged in the lower part of the HRU. The hydrocarbon outlet 12 is arranged in the upper part of the HRU. The hydrocarbons will preferably be concentrated in the hydrocarbon collection volume 36 in the upper part of the HRU. The volume 36 is provided with a hydrocarbon/water interphase measurement and control unit 43. In a batch mode, when the unit 43 detects that the hydrocarbon-water interphase is at lower position, such that the volume 36 is filed with hydrocarbons it opens the valve 42 and the hydrocarbons leaves the HRU trough conduits 12 and 13. In a continuous mode the unit 43 regulates the position of the interphase to be with in a pre-decided level range by adjusting the opening of the valve 42.

Figure 5 illustrates a system similar to the system shown on figure 3 but including the details of the HRU of figure 4 and additional showing equipment for running the system in a vacuum mode. A vacuum pump 47 is arranged in communication with the vapour outlet 40. The vacuum pump is preferably a liquid ring (LR) vacuum pump and therefore a vapour liquid separator 48 is arranged upstream the pump with a liquid return conduit 49 leading back to the pump 47, the vapour is vented through outlet 240. The vapour liquid separator 48 may also be referred to as a vacuum pump scrubber. By applying a vacuum, the temperature in the flash evaporator may be reduced. Further, the vacuum has the effect of removing oxygen from the produced water. This has the effect of significantly reducing the

corrosiveness of the diluted salt solution obtained in the in the salt dissolution unit 28.

Figure 6 illustrates an embodiment similar to figure 5 but further adapted to handle mercury being present in the rich hydrate inhibitor feed 1. Within the flash evaporator 14 elemental mercury will evaporate and follow the vapour stream through the distillation column 22 and be part of the vapour coming out through vapour outlet 402. In the cooler 423 the temperature is reduced to condense water, hydrocarbons but to limited extend the mercury. The tank 424 is preferably equipped with internals to improve the separation of the water, hydrocarbons and mercury. The plate 80 is a liquid weir plate inside condensed liquid tank separating area of the tank 424 comprising the condensed liquid outlet 409. The plate 81 is a vapour top mounted baffle plate inside condensed liquid tank guiding the inlet stream coming from the cooler 423. Hydrocarbons and water are removed via conduit 409 and proceed via pump 446 to the HRU 26. As an extra security a mercury removal unit 58 may optionally be arranged on the water outlet 15 from the HRU 26. The mercury removal unit can be a mercury filter or preferably an absorption/adsorption based mercury removal unit. The conduit comprising the valve 76 is a liquid level adjustment conduit, allow the control of the liquid level within 424. At the bottom of the tank 424 a first mercury trap pot 56 is arranged. Preferably the pressure and temperature within the tank 424 is adjusted to secure condensation of water and hydrocarbons but maintain mercury primarily in the vapour phase. Typically, the conditions in the tank 424 when the hydrate inhibitor is MEG is a temperature of 50-70 °C and a pressure of 0.1 -0.3 bara. In this case the first mercury trap pot 56 is a backup. The main collection of mercury will take place in the second mercury trap pot 52 in the vapour liquid separator 448 downstream the vacuum pump 447. Downstream the vacuum pump 447 the conditions when MEG is the hydrate inhibitor are typically temperature of 30-50 °C and a pressure of 1.1 -1.3 bara. To maintain these conditions a liquid return cooler 75 may be installed in the liquid return loop. The mercury vapour is transported to the pump through conduit 340.

Figure 6 further illustrates an optional purging system to further improve the separation system. Via purge gas conduit 77 gas is added to the lower part of tank 424. Part of the cleaned gas phase 55 obtained from the vapour liquid separator 448 is reused a returned to conduit 77 via conduit 53. Excess vapour is vented through conduit 440. Makeup urge gas, preferably nitrogen, can be added via conduit 54.

In the separator 448 in the illustrated embodiment the liquid outlet nozzle 57 is preferably slightly elevated above the bottom of the separator to avoid mercury being sucked out with the remaining liquid. A similar arrangement of the outlet nozzles may also be included in the tank 424. Figure 7 illustrates an alternative embodiment of the system on figure 6 including arrangements for mercury removal. In this embodiment an additional separator 548 upstream the vacuum pump 447. The vapour phase from tank 424 is cooled in cooler 523 before entering the separator 548. This separator is run at vacuum conditions. The separator comprises a third mercury trap pot 79. The remaining liquid is pumped out by pump 546 and together with any excess liquid from conduit 574 return to the flash evaporator 14 via conduit 78.

In this embodiment temperature in the tank 424 can be more freely adapted to secure that all the mercury remains in the vapour phase and does not enter the HRU. Securing that the hydrocarbon deprived condensed water is mercury free and can be discharged together with the salts. The unit 58 is optional. If traces of mercury are included in the liquid separated from 548 and 448, this mercury is via conduit 78 returned to the flash evaporator and will not be discharged to sea.

The outlet nozzles 57 are preferably to be elevated inside the separator vessel in the gas liquid separator 448, the same is the case in the additional separator 548.

Figure 7 also illustrates a further embodiment of the detailed representation of the HRU 26, where the coalescer filter element 585 in addition to coalescing

hydrocarbons it also coalesces remaining mercury, wherein the bulk portion of the coalesced remaining mercury is accumulated in a HRU mercury trap pot 84, whereas the water with coalesced hydrocarbon is passed over a HRU weir plate 83 downstream, and then process in same manner as described above for figure 4. The mercury loading on the unit 58 will be accordingly reduced by what is removed in the HRU mercury trap pot 84.

Reference numbers