WO2017052377A1

WO2017052377A1 - Method for removing sodium chloride

Info

Publication number: WO2017052377A1
Application number: PCT/NO2015/050173
Authority: WO
Inventors: Monika FRANKE
Original assignee: Statoil Petroleum As
Priority date: 2015-09-25
Filing date: 2015-09-25
Publication date: 2017-03-30

Abstract

The present invention relates to method for removing sodium chloride from a composition comprising a hydrate inhibitor, said method comprising the steps of: (i) contacting said composition with a solution of sodium chloride to form a mixture; and (ii) withdrawing a vapour comprising water and/or hydrate inhibitor from said mixture, such that (a) a vapour phase comprising water and/or hydrate inhibitor and (b) a liquid containing sodium chloride is formed; and (iii) separating sodium chloride crystals from said liquid.

Description

Method for removing sodium chloride

The present invention relates to the enhancement of salt particle size (and/or shape) in hydrate inhibitor (especially ethylene glycol) reclamation processes.

Hydrate inhibitors such as glycols, e.g. monoethylene glycol (MEG), diethylene glycol (DEG) and triethylene glycol (TEG), are used in hydrocarbon transport pipelines to prevent the formation of gas hydrates, especially methane hydrates, at low

temperatures and high pressures. During transportation of hydrocarbons, the inhibitor becomes "contaminated" by salts from formation water, residual amounts of the drilling fluids, as well as various chemicals that may be added to the inhibitor to keep the loop scaling and corrosion controlled, e.g. scale inhibitors and corrosion inhibitors. As the required inhibitor injection rate may be several hundred cubic metres per day, for economic, logistic and environmental reasons, it is necessary to recover and recycle the hydrate inhibitor. In order to do this, contaminants such as salts have to be removed in order to re-use/re-inject the hydrate inhibitor again to the hydrate inhibitor loop.

A process known as "reclamation" allows removal of salts from the inhibitor feed that enters the reclaiming process. Reclamation typically involves removal of hydrate inhibitor in the gas phase, causing salt concentration in the reclaimer to increase until the salts precipitate out and can be removed. Two types of reclamation processes are commonly used: full reclaiming or slip stream reclaiming. In the full reclamation process, the total rich inhibitor stream is directed and treated in the reclaimer (flash separator), while in the slip stream process a part (e.g. 0-99%) of a full (100%) rich inhibitor stream can be directed for treatment. Regeneration is the process of removing water from a water-rich hydrate inhibitor mixture (sometimes termed a "rich glycol stream") to obtain an inhibitor stream suitable for reuse. Hydrate inhibitor reclamation can be located onshore or offshore and can be downstream of regeneration or upstream, although upstream is more common.

The reclamation process thus serves the purpose of removing the solids from the (usually liquid-rich) inhibitor stream (e.g. MEG, DEG and/or TEG) and further separating them. Solids that are removed are salts that come mostly from production water, back-produced water, and other sources. These salts are monovalent or divalent, but the majority of precipitants are monovalent salts such as sodium chloride. A gentle balance exists between sodium chloride and sodium carbonate as they compete for sodium as a common building block for the crystals that are formed. Sodium chloride levels in rich hydrate inhibitor streams increase when formation water is present, which typically takes place in the later stages of hydrocarbon production in the field.

In an inefficient reclamation process, increasing solid salt content in the reclaimer or other locations causes the density of the inhibitor (e.g. MEG) mixture to increase, which in turn causes the mixture to become more viscous and more difficult to handle by pumps and other rotating equipment. There can also be problems occurring during operation, such as a need for higher heat necessary to run the process, uncontrolled foaming and, as a consequence, carry-over, combined with fluctuating process pressure. It is therefore particularly important that the salt crystals are removed efficiently and this requires them to be of a size sufficient to facilitate their downstream separation. Small particles are problematic to separate with a filter press or centrifuge and thus efficiency of both the reclamation process and separation process is adversely affected by the presence of small salt particles. The separation process therefore becomes ineffective due to inability to separate the tiny particles from the liquid inhibitor phase.

Typically, salt crystals with a particle size distribution such that the majority of the particles have a diameter of at least 20 to 50μηι are required for efficient separation. The growth and shape of crystals such as those of sodium chloride and sodium carbonate can easily be disturbed by usage of chemicals, such as corrosion inhibitors, scale inhibitors and/or the presence of hydrocarbons or organic and inorganic acids. This means that instead of growing bigger, the crystals will nucleate, forming a larger number of small particles.

In general, solids may be crystallised from the liquid phase in a variety of ways, e.g. by cooling, by evaporation, and/or by seeding to promote crystallisation. WO

2009/017971 discloses modifications to a glycol reclamation method in order to tackle calcium chloride contamination of the glycol stream. Firstly, sodium carbonate is used to convert calcium chloride into calcium carbonate. This step produces small crystals of calcium carbonate which are so difficult to separate that they then need to be grown via a separate process in order to enable their removal and thus make the process viable. Seeding, i.e. addition of larger particles/crystals of calcium carbonate is thus then used to grow larger particles of calcium carbonate. This growth mechanism requires a separate "seeding vessel" where the crystals can be grown undisturbed and separate from the flash separator of the reclaiming process.

Seeding is effected by recycling some seeded effluent from the seeding vessel back into the seeding vessel. As the growth of calcium carbonate crystals by seeding is a relatively slow process, this prior art process thus requires a separate vessel in a slip stream relative to the main process, which increases the complexity and costs of the overall reclamation process. Further complexity is added due to the process involving two steps, i.e. the contaminant (CaCI₂) first needs to be converted into CaC0₃ which then needs to be grown. In addition, the slow crystal growth means there is a significant delay in removing the calcium carbonate from the glycol stream, slowing down the entire reclamation process. Furthermore, the process is only applied to divalent calcium salts and is not applicable to sodium chloride, which, as noted above, forms large amounts of very small crystals under typical conditions.

Current methods for dealing with removal of small crystals of sodium chloride are therefore restricted to shutting down the vessel in which the salt has accumulated in order to clean out the salt deposits and/or draining the sediment from the bottom of the vessel. Cleaning a vessel requires either a second vessel that may be used while the first is being cleaned, or complete shut-down of the reclamation process, e.g. in the event that there is no replacement vessel. Draining sediment requires a slipstream via which the small particles may be removed from the vessel, which adds further complexity to the apparatus and a further process step. Clearly, these current solutions to the problem of small sodium chloride particles involve significant time, effort and cost and impact negatively on the efficiency of the overall hydrate inhibitor reclamation process.

There thus exists a need for an improved reclamation process which enables more efficient removal of sodium chloride from hydrate inhibitor streams. It has been surprisingly found that the addition of a sodium chloride solution to the reclamation process (e.g. in the reclamation vessel or upstream of it) enables more efficient removal of the salt from the inhibitor-containing stream. The invention is therefore counter-intuitive in that it adds sodium chloride to a stream in order to enhance efficiency of its removal. The presence of the sodium chloride solution has been surprisingly found to enhance the size of the particles of sodium chloride in the reclamation process, making them easier to separate. Optionally, in addition to the addition of the sodium chloride solution, seeds of sodium chloride can also be added (either via the solution or separately). Thus, viewed from a first aspect, the present invention provides a method for removing sodium chloride from a composition comprising a hydrate inhibitor, said method comprising the steps of:

(i) contacting said composition with a solution of sodium chloride to form a mixture; and

(ii) withdrawing a vapour comprising water and/or hydrate inhibitor from said mixture, such that (a) a vapour phase comprising water and/or hydrate inhibitor and (b) a liquid containing sodium chloride is formed; and

(iii) separating sodium chloride particle, e.g. crystals, from said liquid.

Preferably, the liquid containing sodium chloride contains large sodium chloride crystals. These are easier to separate than small particles.

An optional further step comprises separating water from hydrate inhibitor, e.g. by removing water vapour from the composition comprising hydrate inhibitor, from the mixture of (i) or from the vapour of (ii) (a process known in the art as regeneration). This water removal step may take place prior to, during, or after, the step of contacting the composition (i.e. the composition comprising hydrate inhibitor) with the sodium chloride solution. Moreover, this water removal step may take place prior to, during, or after, the step of withdrawing a vapour comprising water and/or hydrate inhibitor from said mixture.

In the event that the vapour withdrawn in step (ii) comprises both water and hydrate inhibitor (a process known in the art as reclamation), sodium chloride crystals may be removed from liquid (b) without any further vapour removal steps being necessary. Water may be separated from the hydrate inhibitor via a further step comprising removing water vapour from the hydrate inhibitor, e.g. by condensing the vapour phase (a) comprising water and hydrate inhibitor and removing water vapour therefrom as described above.

In the event that hydrate inhibitor is present in the liquid (b) which contains sodium chloride, i.e. the liquid that remains after vapour (a) is removed, the method preferably comprises the step of removing hydrate inhibitor from said liquid, e.g. as a vapour, prior to separation of the sodium chloride crystals from the liquid. Preferably, the sodium chloride solution is contacted with the hydrate inhibitor composition prior to, or during, the removal of a vapour phase comprising hydrate inhibitor.

The stream, from which it is desired to remove sodium chloride, comprises sodium chloride and at least one hydrate inhibitor. This stream is referred to herein as a "hydrate inhibitor composition" or a "composition comprising hydrate inhibitor". Water is preferably also present. Typically, the hydrate inhibitor content of the composition (with respect to the composition as a whole, prior to contact with the sodium chloride solution) is from 20 to 90 wt% hydrate inhibitor, more especially 35 to 75 wt%, e.g. 35 to 50 wt%, 40 to 50 wt%, 50 to 70 wt%, 40 to 75 wt%.

The hydrate inhibitor is typically a glycol, especially an ethylene glycol, e.g.

monoethylene glycol (MEG), diethylene glycol (DEG) or triethylene glycol (TEG). Monoethylene glycol is particularly preferred. Mixtures of different hydrate inhibitors may be present in the composition.

The salt, which the invention aims to remove from the composition which contains hydrate inhibitor, is sodium chloride. Salts other than sodium chloride may also be present in the hydrate inhibitor composition, as well as other components.

As the hydrate inhibitor composition originates from a hydrocarbon pipeline, it can be a complex mixture with many other components present (although some of these may be removed by treatment steps between the pipeline and addition of the sodium chloride solution). There may also be dissolved hydrocarbons and components from the gas/condensate. As the solubility of most hydrocarbons is low, the main dissolved components are carbon dioxide and light hydrocarbons. The content of some hydrocarbons, especially polar, aromatic and cyclic hydrocarbons, can be quite high due to the presence of the inhibitor. If the gas contains hydrogen sulphide, some of this will partition into the aqueous phase. Dissolved carbon dioxide and hydrogen sulphide can result in the presence of bicarbonate, carbonate, and bisulphide ions. Due to corrosion, the aqueous phase will also contain some corrosion products, mainly iron ions and solids such as iron carbonate, iron oxides, etc. The corrosion process will generally also release traces of other components from the metal alloy of the pipeline, for example chromium, manganese, nickel, etc. Normally, the hydrocarbon flow within the pipeline will also contain some water from the subterranean hydrocarbon reservoir, normally referred to as formation water. This formation water contains various dissolved ions, in particular sodium, chloride, potassium, magnesium, calcium, barium, strontium, iron, sulphate, etc. It is frequently the case that the water phase also contains dissolved organic acids, mainly acetic acid; however the short chain alkanoic acids such as formic, propanoic and butanoic acids are commonly also present. The aqueous phase within the pipeline can also contain various chemicals used in the production and transportation process, such as corrosion and scale inhibitors, pH stabilizers, drilling fluids, and pipeline conservation fluids.

Thus, the hydrate inhibitor composition according to the present invention will typically be a rich inhibitor stream, i.e. a composition containing a glycol hydrate inhibitor, preferably MEG, DEG and/or TEG, especially preferably comprising MEG. Other components of the hydrate inhibitor composition may be independently selected from: salts (e.g. those that come from formation water, so this will differ very much from field to field); pH stabilisers; non-glycol-based hydrate inhibitors; corrosion inhibitors and/or scale inhibitors.

The types of additives present in the hydrate inhibitor composition will vary depending on the strategy chosen by the field operatives. The additive strategy is usually chosen after extensive laboratory testing. Typical combinations of additives present in the hydrate inhibitor composition are:

(i) hydrate inhibitor and pH stabiliser;

(ii) hydrate inhibitor, pH stabiliser and corrosion inhibitor

(iii) hydrate inhibitor, pH stabiliser and scale inhibitor

(iv) hydrate inhibitor, corrosion inhibitor and scale inhibitor.

(v) hydrate inhibitor, pH stabiliser, corrosion inhibitor and scale inhibitor.

Preferred pH stabilisers are NaOH or KOH. These are typically added in the whole processing loop and, due to the presence of C0₂, may form Na₂C0₃ and K₂C0₃ respectively, in the reclaiming unit.

One or more of the above-mentioned additives or components may be removed from the hydrate inhibitor composition at any stage, e.g. prior to, during, or after, the method of the invention. A solution of sodium chloride is contacted with the hydrate inhibitor composition in order to enhance the growth of sodium chloride crystals, thus making their subsequent removal easier and more efficient. The non-NaCI component of the solution may comprise water, hydrate inhibitor (as herein defined, e.g. a glycol) or a mixture of water and hydrate inhibitor, i.e. the solution comprises water and/or hydrate inhibitor in addition to sodium chloride. Mixtures of water and hydrate inhibitor (in addition to the sodium chloride) are preferred.

The size of sodium chloride crystals precipitating in a reclamation system without scale and corrosion inhibition typically varies from 50 to 100 μηι, however, in a system with scale and corrosion inhibition, the particles can be as small as 0.2 μηι and thus almost impossible to remove by separation from the hydrate inhibitor stream. In order for the particles to be separated with centrifuge, the particles need to be of an optimal size, depending on the centrifuge, but most common would be 10 to 500 μηι, preferably 50 to 500 μηι, more especially 50 to 300 μηι. The smaller the size of the crystals produced without using the invention, the larger the

concentrations of the salt in the sodium chloride solution should typically be.

Moreover, in hydrate inhibitor processing, the reclaimer is a "desalting" unit that is sensitive in operation under changing conditions. The composition of the sodium chloride solution is therefore crucial.

The optimal or desired composition of the sodium chloride solution may be determined by carrying out tests with rich glycol feeds in the ranges that would be specific to the field of interest. Analytical measurements of chloride, sodium, acids and alkalinity are, in standard reclamation processes, taken from the reclaimer for monitoring the process. Rich glycol and lean glycol streams can be subject to similar monitoring. Additionally, the gas rates, formation water rates and glycol injection rates may also be monitored. On the basis of this operational data, the optimal composition of the sodium chloride solution may be determined on a case by case basis.

Typical values for the amount of hydrate inhibitor in the sodium chloride solution (in relation to the total amount of hydrate inhibitor plus water) are 0 to 100 wt%, e.g. 0 to 45 or 90 to 100, preferably 45 to 90 wt% (e.g. 45 to 62 wt% or 75 to 90 wt%), especially 62 to 75 wt%. Where the sodium chloride solution comprises hydrate inhibitor (optionally in combination with water), the hydrate inhibitor (and water if appropriate) can be sourced from elsewhere in the inhibitor processing system, e.g. from the

regeneration process. Thus, other components, particularly those described herein in relation to the hydrate inhibitor composition, may also be present in the sodium chloride solution. Further components that may optionally be present in solution include salts with a lower solubility than sodium chloride. Preferably however, the sodium chloride solution consists essentially of, or consists of, sodium chloride in combination with water and/or hydrate inhibitor.

The sodium chloride content of the solution is preferably high. Exact concentrations will depend on the other components of the solution and the conditions, e.g.

temperature and pressure. Typical values for sodium chloride, expressed with regard to the solution as a whole, are 5 to 26 wt%, preferably 10 to 26 wt%, especially 20 to 26 wt%; for example at atmospheric pressure and 25 °C, at atmospheric pressure and 30°C, at atmospheric pressure and 40°C, at atmospheric pressure and 45°C, or at atmospheric pressure and 50°C. Especially preferably, the sodium chloride solution is a saturated or supersaturated sodium chloride solution.

Conveniently, the salt content of the solution, or the solution itself, may be obtained from sea water and/or salt produced from reclamation.

In a further aspect of the present invention, the sodium chloride solution may also comprise sodium chloride crystals. Alternatively, sodium chloride crystals may be added, during the method of the invention, but separately from the sodium chloride solution. These crystals may act as a seeding agent to further enhance crystals growth. Said crystals typically have a minimum diameter of 10 to 400 μηι, preferably 50 to 400 μηι, especially 100 to 400 μηι. The seed crystals are typically present in an amount up to 5 wt% of the sodium chloride solution, e.g. 1 to 4 wt%, preferably around 3 wt%. Contact between the seed crystals and the hydrate inhibitor composition may be effected in any of the ways described herein with regard to the sodium chloride solution.

The method of the invention may conveniently form part of a conventional hydrate inhibitor reclamation process. Processes for reclaiming hydrate inhibitor from pipeline mixtures for reuse involve removal of gas and condensates from the pipeline's outlet stream which contain hydrate inhibitor. This will typically take place prior to the method of the invention being carried out. The resulting "rich" hydrate inhibitor stream, e.g. a hydrate inhibitor composition as described herein, may be stored prior to separation of water, inhibitor and/or salts therefrom. Said separation involves processes known in the art as reclamation and/or regeneration.

"Reclamation" is a process for separating liquid hydrate inhibitor from dissolved impurities as described herein, such as salts (e.g. NaCI) and other compounds with high boiling points. Reclamation comprises withdrawal of liquid hydrate inhibitor (and optionally also water) in the gas phase from the hydrate inhibitor composition, for example by vacuum distillation. This typically takes place in a reclamation vessel, e.g. a distillation vessel operating at reduced pressure, for example a flash separator. Reclamation vessels normally operate at a pressure of 0.15 - 0.3 bara (15-30 kPa) and at this pressure MEG can be boiled off at 120-135°C.

Removal of gaseous inhibitor results in the salt concentration in the retentate liquid (i.e. that which is not removed in the gas phase and thus remains in the reclamation vessel) increasing such that salts precipitate out. Precipitated salts are generally removed by taking liquid from the reclamation vessel and removing solids. The liquid is preferably then returned to the reclaimer. The solids removal may be performed, for example, by filtering, settling or centrifuging.

The hydrate inhibitor reclamation vessel can be located onshore or offshore and can be downstream of regeneration or upstream, although reclamation upstream of regeneration is preferred. Thus, the feed to the reclamation vessel may be (directly or indirectly) an output stream from a regeneration vessel, or vice versa. Where both reclamation and regeneration are used, the steps need not be performed in separate vessels, i.e. both steps may take place in the same vessel.

"Regeneration" is primarily concerned with the separation of water from liquid hydrate inhibitor. Regeneration comprises withdrawal of water in the gas phase from the hydrate inhibitor composition, for example by vacuum distillation. This typically takes place in a regeneration vessel, e.g. a distillation vessel operating at reduced pressure, for example a flash separator. This can be carried out for MEG for example at about 140-150°C and 1.1-1.3 bara (110-130 kPa). Water is drawn off as vapour and the "lean" (i.e. with reduced water content) hydrate inhibitor remains in the regeneration vessel as a liquid. The correct hydrate inhibitor concentration within the withdrawn liquid stream may be obtained by adjusting the temperature or pressure within the regeneration vessel. Under normal operation, with this simple distillation of rich MEG, the MEG concentration in the top product, i.e. the water, is generally well below 500ppm, sometimes as low as 50-200ppm.

The vapour withdrawal steps described herein (e.g. step (ii) or other vapour removal steps described herein) are preferably carried out as part of a standard reclamation or regeneration process. The conditions for reclamation and regeneration described herein therefore also apply to the vapour withdrawal step(s) of the invention.

The sodium chloride solution may be contacted with the composition comprising a hydrate inhibitor at any suitable point in the overall hydrate inhibitor reclamation process, i.e. before, during or after reclamation and/or regeneration. For example, step (i) may take place before, during or after step (ii). Preferably step (i) takes place before or during step (ii), i.e. step (ii) is subsequent to, or simultaneous with, step (i). Moreover, regeneration may take place before, during or after contact with the sodium chloride solution. It is especially preferred, however, that the sodium chloride solution has been contacted with the hydrate inhibitor composition at the time of (i.e. before or during) removal of a vapour comprising hydrate inhibitor, e.g. the hydrate inhibitor composition has preferably been contacted with the sodium chloride solution when regeneration takes place.

The invention therefore provides a method comprising adding a sodium chloride solution to a hydrate inhibitor composition before, during or after reclamation and/or before, during or after regeneration. Preferably, the method comprises adding a sodium chloride solution to a hydrate inhibitor composition before or during reclamation.

In one aspect, the method comprises regeneration, followed by addition of the sodium chloride solution, followed by reclamation, e.g. separating water from the hydrate inhibitor composition (e.g. by removing water vapour), prior to contacting the composition with the sodium chloride solution. The vapour removed in step (ii) will then preferably comprise hydrate inhibitor. In another aspect, the method comprises addition of the sodium chloride solution, followed by regeneration, followed by reclamation, e.g. separating water from the hydrate inhibitor composition (e.g. by removing water vapour), after contacting the composition with the sodium chloride solution.

Most preferably, the sodium chloride solution is contacted with the composition prior to or during reclamation, which is then optionally followed by regeneration. Thus, in a particularly preferred aspect, the invention provides a method for removing sodium chloride from a composition comprising a hydrate inhibitor, said method comprising the steps of:

(ii) withdrawing a vapour comprising hydrate inhibitor (and optionally also water) from said mixture, such that (a) a vapour phase comprising hydrate inhibitor (and optionally also water) and (b) a liquid containing sodium chloride is formed; and

(iii) separating sodium chloride crystals from said liquid; and optionally further comprising

(iv) separating water from the phase comprising hydrate inhibitor.

The step of separating water from the phase comprising hydrate inhibitor is preferably carried out by first condensing the vapour produced in step (ii) and then withdrawing water vapour from the condensate.

In all aspects of the invention, the step of contacting the sodium chloride solution with the hydrate inhibitor composition can be effected in a continuous or batch mode. In a preferred aspect, the sodium chloride solution is added until sodium chloride begins to precipitate, at which point, contact between the sodium chloride solution and the hydrate inhibitor composition is stopped. The step of contacting the sodium chloride solution with the hydrate inhibitor composition is thus preferably performed batch- wise.

The dose of the sodium chloride solution (expressed as the vol% of the hydrate inhibitor composition) is typically 0.05 to 20, especially, 0.1 to 16, for example 0.1 to 3, 0.1 to 6, 0.1 to 6.8, 0.1 to 9.5 or 0.1 to 15.2 vol%.

The place at which the sodium chloride solution is contacted with the hydrate inhibitor composition, may or may not be that from where the vapour is removed, i.e. the steps of the method described herein may not necessarily all be performed in the same location. For example, the sodium chloride solution may be contacted with the hydrate inhibitor composition in a storage tank, in a reclamation vessel, in a regeneration vessel and/or in a conduit in contact with any of said vessels. The vapour removal step (ii) may take place in a reclamation vessel and/or in a regeneration vessel, preferably in a reclamation vessel. Preferably, however, all steps take place in a reclamation vessel.

The timeframes for dosing depend on the size of the vessel in which the sodium chloride solution is contacted with the hydrate inhibitor composition. The larger the vessel size, the longer the dosing period must be. This is due to the fact that more time is required to upconcentrate the volume (i.e. to add enough sodium chloride solution to obtain a salt concentration sufficient to enable crystal growth) for a larger vessel. The dosing time will also be dependent on the size of the feed pipelines to and from vessels in the installation and thus may vary between installations. As the salt crystals grow quickly and do not require a settling vessel and time in which to grow, the dosing timeframe refers only to the amount of time that it takes to increase the salt concentration in the vessel to one adequate to enable crystal growth.

Crystal growth of the salt is typically aided by removal of liquid from the hydrate inhibitor composition, preferably as a vapour, e.g. by the evaporation step that is an inherent part of reclamation and/or regeneration. This evaporation increases the concentration of the salt in the remaining liquid (i.e. liquid (b), the retentate) in the vessel, e.g. in the reclamation vessel, until it precipitates as particles/crystals.

Presence of the sodium chloride solution (i.e. contact between the hydrate inhibitor composition and the sodium chloride solution) during this process enhances the growth of the salt crystals according to the invention. Typically, crystal growth will be monitored using standard techniques, such that addition of the sodium chloride solution may be stopped when precipitation of sodium chloride crystals begins.

Preferably, the salt crystals are formed (i.e. the majority of them, preferably substantially all of them) in a single location, especially preferably in the reclamation vessel. As reclamation is intended for the removal of hydrate inhibitor and water in the vapour phase, its aim is to leave salts and the like in the reclaimer vessel.

Reclaimer vessels are therefore designed for the removal of solids, hence are the preferred place for precipitation of the sodium chloride. Thus, the removal of vapour comprising water and/or hydrate inhibitor (preferably comprising hydrate inhibitor) according to the invention aids precipitation of sodium chloride and thus crystal growth. As noted above, vapour may be withdrawn from the hydrate inhibitor composition prior to contact with the sodium chloride solution, but it is particularly preferred that contact is made between the hydrate inhibitor composition and the sodium chloride solution prior to or during, especially prior to, vapour withdrawal, especially withdrawal of a vapour comprising hydrate inhibitor.

As noted above, withdrawal of vapour comprising water and/or hydrate inhibitor may be effected in a manner typical to standard reclamation/regeneration processes. The withdrawal of vapour to aid crystallisation preferably takes place in the reclamation vessel, thus the vapour withdrawal step (ii) of the invention is typically that of a standard reclamation process.

In the case that vapour comprising hydrate inhibitor is removed from the

compositions, it may be reused, e.g. as a hydrate inhibitor in hydrocarbon production, but is more preferably first further processed, e.g. to remove excess water. Removal of excess water from a hydrate inhibitor stream is commonly termed "regeneration" and typically involves a distillation. Thus, the method of the invention may optionally further comprise subjecting the hydrate inhibitor composition (optionally further comprising the sodium chloride solution), or the vapour produced in step (ii), to a second vapour removal step, e.g. to remove water as vapour. Typically, the vapour from which water is to be removed is first condensed and later up-concentrated, e.g. to 90 wt% inhibitor. This upconcentration typically takes place in a further distillation vessel, e.g. in a regeneration vessel.

The vessel, e.g. reclamation or regeneration vessel, is typically a distillation, e.g. flash separator, vessel from which vapour is withdrawn in step (ii) order to facilitate salt crystal growth. The distillation vessels, e.g. flash separators, of the invention may be operated at different pressures and temperatures (thermodynamically, in accordance with the phase diagram for the relevant hydrate inhibitor/water mixtures versus pressure and temperature).

Preferred pressures (expressed in absolute pressure) for the removal of hydrate inhibitor (or hydrate inhibitor and water) according to the invention are those typically used for reclamation, e.g. 0 to 30 kPa, especially 0 to 15 kPa, although up to 150 kPa may be used. Preferred temperatures for the removal of hydrate inhibitor (or hydrate inhibitor and water) according to the invention are 80 to 250 °C, preferably 100 to 200 °C, especially 1 10 to 180 °C, particularly preferably 120 to 135 °C.

In order to remove water vapour, preferred pressures are those typically used for regeneration, e.g. 100 to 200 kPa, preferably 100 to 150 kPa, especially 1 10 to 130 kPa. Preferred temperatures for the removal of water vapour are 100 to 200 °C, preferably 120 to 180 °C, especially preferably 140 to 150 °C.

It should, however, be noted that the method of the invention is not sensitive to pressure or temperature and therefore could be applied to any pressures mentioned herein, e.g. 0-150 kPa.

Sodium chloride crystals are formed in the liquid (b) which remains following a vapour removal step of the invention. The method of the invention promotes the formation of crystals of a size sufficient to enable their removal. The sodium chloride crystals are generally removed from the process by removing liquid from "liquid (b)" and separating the sodium chloride and other solids from the liquid. The separation of the sodium chloride and other solids may be effected, for example, by filtering, settling or centrifuging. The liquid which has been separated from the solids may contain some hydrate inhibitor and thus is preferably returned to the process, e.g. to a distillation vessel.

As reclamation vessels are designed to create the solid deposits, the salt

crystallisation of the present invention preferably takes place in a reclamation vessel, i.e. substantially all of the salt crystals are formed in a reclamation vessel.

Crystallisation of salts elsewhere, e.g. in the regeneration vessel or storage tank should preferably be avoided. In order to maximise the amount of sodium chloride which precipitates in the reclamation vessel (and thus minimise the amount that precipitates in other places) the invention preferably involves contacting the sodium chloride solution with the hydrate inhibitor composition immediately prior to, or during, removal of a vapour comprising hydrate inhibitor.

Rather than using a solution, prior art methods are based on seeding with crystals, which normally target certain polymorphs of the salt. This is the case in respect to CaC0₃, where each polymorph has different crystal lattice and physical properties. The use of a sodium chloride solution to enhance the growth of sodium chloride crystals in a composition comprising a hydrate inhibitor is thus new. Thus, viewed from a further aspect, the present invention provides the use of a sodium chloride solution as herein described to enhance the growth of sodium chloride crystals in a composition comprising a hydrate inhibitor. The sodium chloride solution and inhibitor composition are as described herein in relation to the method of the invention.

The chemistry behind crystallisation can be complicated and can vary depending on the targeted salt/precipitant and the liquid phase from which the crystals shall be obtained. Changing any of the components changes the chemistry of the system. The present invention targets growth of particles that already exist in the system (NaCI) by adding the sodium chloride solution which contains sufficient NaCI to enhance the growth of particles.

By using a sodium chloride solution according to the invention, it is not necessary for the NaCI to reside in a separate vessel, nor for long residence times to be used in order to achieve the desired crystal growth. This means that the crystal growth can be, according to preferred aspect, enhanced directly in the reclaimer, avoiding the need for a separate vessel or other structural components to the overall process.

The invention is particularly applicable to hydrate inhibitor compositions containing various scale and/or corrosion inhibitors (static and/or kinetic inhibitors). Use of these inhibitors is becoming increasingly common and they can affect the reclamation process by decreasing the size of precipitated particles, e.g. sodium chloride and sodium carbonate crystals. As previously mentioned, if these crystals are too small to be removed by separation the process can become inefficient to the degree that the entire hydrocarbon production process may be stopped for the reclamation units to be cleaned. Clearly there is a great need for this to be avoided.

The size to which the sodium chloride crystals can grow depends to some extent on the amounts of formation water produced in the field (i.e. the water content of the hydrate inhibitor composition) and whether any scale or corrosion inhibitors are present. Preferably the majority of particles (e.g. crystals) separated from liquid according to the invention have a minimum diameter of at least 10 μηι, e.g. 10 to 400 μηι, especially at least 15 μηι, preferably 50 to 400 μηι, e.g. 20 to 50μηι, especially 100 to 400 μηι. Typical values are 15 to 1 10 μηι. By producing larger particles (salt crystals), the invention increases the efficiency of hydrate inhibitor reclamation because larger particles are easier to separate from hydrate inhibitor, and additionally form a good filter cake to enhance any secondary filtration.

The amount of salt (NaCI) that is removed from the hydrate inhibitor composition, i.e. the amount of salt that is discharged from the vessel in which it crystallises or is separated from the liquid containing NaCI in step (iii) of the present method, depends on the conditions in the hydrocarbon production field. The composition of a typical rich MEG feed stream is given in Table 1 below. The ranges are dependent on gas rates, formation water rates, glycol concentration etc.

Table 1 : composition of a typical rich MEG feed stream

By facilitating salt removal, via particle size growth and the associated improved salt separation, the method of the present invention removes sodium chloride from the hydrate inhibitor composition. Thus, a hydrate inhibitor composition is produced (e.g. for recycling/reuse) which has a lower concentration of sodium chloride than the original hydrate inhibitor composition to which the method was applied. In one embodiment, the amount of sodium chloride recovered is greater than that recovered without using the method of the invention. Typically, 10 to 100% of the sodium chloride present in the original hydrate inhibitor composition is removed, especially at least 20%, more preferably at least 50%, particularly preferably at least 75% or at least 90%. In a particularly preferred aspect, all, or substantially all, of the sodium chloride that was present in the hydrate inhibitor composition is removed according to the invention.

As mentioned above, accumulation of particles can cause fouling of equipment and eventual breakdown of the inhibitor processing process, e.g. due to blocking of conduits. The invention therefore solves operational problems in a reclamation process (e.g. foaming and carry over, but more especially density and viscosity increase) by more efficiently precipitating the salt particles from the liquid rich hydrate inhibitor feed. This keeps the liquid density and viscosity of the hydrate inhibitor in the reclaimer loop at a fairly constant level which then increases/stabilizes efficiency of the reclaiming process and handling of the inhibitor in the loop by pumps and other rotating equipment. There is thus a reduced need for higher heat requirements and reduction of uncontrolled foaming or fluctuating pressure during operation. This results in lower maintenance of parts such as rotating equipment and avoids the time and costs of unforeseen process optimisations.

A further advantage is that the invention can be readily incorporated into standard reclaiming processes, which already include means for adding extra streams, e.g. conventional sodium bicarbonate addition systems, or sodium hydroxide addition systems. These parts could be used for adding the sodium chloride solution of the present invention to a reclamation process. This ability to adapt existing equipment means that the method of the invention can be implemented at a fairly low cost.

Even if a new batch mixer or tank was required, this would only involve fairly small modification costs.

Moreover, as mentioned above, sodium chloride, hydrate inhibitor and water are all readily available in reclamation processes, e.g. from sea water, and reclamation products. This means that the required components are present at the installations.

The invention also provides apparatus adapted for use in carrying out any of the methods or uses herein described. Thus, viewed from a further aspect, the present invention provides an apparatus for removing sodium chloride from a composition comprising a hydrate inhibitor, said apparatus comprising a first distillation vessel comprising:

(i) a port for the addition of said composition comprising a hydrate inhibitor to said first distillation vessel;

(ii) a conduit for withdrawing a vapour comprising water and/or hydrate inhibitor from said first distillation vessel; and

(iii) a port for the removal of a liquid comprising sodium chloride from said first distillation vessel,

wherein said apparatus (preferably said first distillation vessel) comprises means for contacting a sodium chloride solution with said composition comprising a hydrate inhibitor.

The sodium chloride solution, hydrate inhibitor composition and all other features of the method which are applicable to the apparatus are as herein described with reference to the method and use of the invention and vice versa.

Preferably, the means for contacting the sodium chloride solution with the

composition comprising a hydrate inhibitor is upstream of said first distillation vessel, or in said distillation vessel. The means for contacting is preferably a port or conduit through which the sodium chloride solution may be added to the hydrate inhibitor composition. The means for contacting the sodium chloride solution with the composition comprising a hydrate inhibitor is preferably linked to a source of sodium chloride or sodium chloride solution as herein described. Thus, the apparatus preferably comprises a conduit linked to a source of source of sodium chloride or sodium chloride solution as herein described.

Preferably the apparatus comprises means for separating sodium chloride from the liquid comprising sodium chloride, e.g. a solids separation unit. Said means are preferably a centrifuge, settling tank, decanter, filtration device etc. The port for the removal of a liquid comprising sodium chloride from said first distillation vessel is therefore preferably connected to a solids separation unit.

As noted above, vapour withdrawal steps may take place in addition to that described above in the first distillation vessel. The apparatus may therefore comprise further distillation vessel(s), e.g. a second distillation vessel, either upstream or downstream of the first. The first distillation vessel may be a reclamation vessel as herein described or a regeneration vessel as herein described. Preferably the first distillation vessel is a reclamation vessel.

The second distillation vessel may be a reclamation vessel as herein described or a regeneration vessel as herein described. Preferably the second distillation vessel is a regeneration vessel.

Preferably the first distillation vessel is upstream of said second distillation vessel.

The sodium chloride solution may be contacted, e.g. first contacted, with the hydrate inhibitor composition in a vessel other than the aforementioned distillation vessels. However, convenient and preferred locations for contacting the hydrate inhibitor composition with the sodium chloride solution, are: in a hydrate inhibitor composition storage tank (i.e. a vessel in which hydrate inhibitor composition is stored prior to entering the reclamation or regeneration vessels (e.g. position A in Figure 1), the rich hydrate inhibitor feed conduit (i.e. the conduit through which the hydrate inhibitor composition enters a distillation vessel, preferably the first (e.g. position B in Figure 1) and/or directly into a distillation vessel, preferably the first (e.g. position C in Figure 1). Especially preferably, the sodium chloride solution is added to the rich hydrate inhibitor feed prior to entry into the first distillation vessel (i.e. into the feed to the reclaimer or regenerator vessel, e.g. position B in Figure 1) and/or directly into the reclaimer or regenerator vessel (e.g. position C in Figure 1). Thus, the sodium chloride solution may be contacted with the feed either prior to, or in, the reclaimer or regenerator vessel, preferably either prior to, or in, the reclamation vessel.

The apparatus therefore preferably comprises means for contacting the sodium chloride solution with the hydrate inhibitor composition, said means being located in a hydrate inhibitor composition storage tank, a conduit, or in a distillation vessel (preferably the first distillation vessel), especially preferably located in a conduit located upstream of said distillation vessel, or in a conduit which feeds fluid into a distillation vessel, preferably the first distillation vessel. Said means are preferably connected, e.g. via a conduit, to a source of sodium chloride or sodium chloride solution. In the case of feeding the sodium chloride solution directly to a distillation vessel, preferably the first distillation vessel and/or a reclamation vessel, the solution may be added at any convenient place, but typically at one or more of the following positions:

(1) : at the top of the vessel (i.e. in the gas phase, e.g. position 1 in Figure 1),

(2) at the liquid level in the vessel (e.g. position 2 in Figure 2), and/or

(3) into the liquid in the reclaimer (e.g. position 3 in Figure 1).

The means for contacting the sodium chloride solution and hydrate inhibitor composition in the apparatus of the invention are therefore preferably conduits and/or ports effecting entry of the sodium chloride composition at one or more of the following points:

(1) : at the top of a distillation vessel, preferably the first distillation vessel;

(2) at the liquid level in a distillation vessel, preferably the first distillation vessel;

(3) into the liquid a distillation vessel, preferably the first distillation vessel.

The invention may involve full reclaiming or slip stream reclaiming, preferably full reclaiming.

Preferably, the apparatus comprises a second distillation vessel, connected in series by a conduit to the first distillation vessel, which is upstream or downstream, preferably downstream, of said first distillation vessel. This second vessel typically acts as a regeneration device, and thus vapour comprising water and/or hydrate inhibitor (preferably comprising hydrate inhibitor) preferably passes from the first distillation vessel to the second. Water vapour can then be removed from the second distillation vessel such that a hydrate inhibitor composition which is reduced in, preferably substantially free from, water and sodium chloride is produced.

Preferably, the apparatus comprises a hydrate inhibitor composition storage vessel upstream of said first distillation vessel.

Figure 1 shows an overview of reclamation and regeneration process with the scope of the present invention, where A-C (1-3) are sodium chloride solution addition locations: A- Addition to rich MEG storage tank, B- addition prior to reclaimer, C- Addition directly to reclaimer (preferred) 1- into gas phase, 2- on the liquid surface, 3- in the liquid. As exemplified in Figure 1 , in the method, use or apparatus of the invention, the sodium chloride solution may be added prior to reclamation (or regeneration), e.g. (A) to the hydrate inhibitor composition storage vessel; or (B) to the hydrate inhibitor composition prior to entry to the first distillation vessel (or prior to entry to the second distillation vessel, provided that is upstream of said first distillation vessel) and/or into a distillation vessel, e.g. (C) directly to the first distillation vessel (or directly to the second distillation vessel, provided it is upstream of the first). Where the sodium chloride solution is added directly to a distillation vessel, it may be added (1) in the gas phase, (2) on the liquid surface and/or (3) in the liquid. Thus, the apparatus preferably comprises a port for addition of the sodium chloride solution to the hydrate inhibitor composition storage vessel, a port for addition of the sodium chloride solution to the hydrate inhibitor composition prior to entry to the first distillation vessel (or prior to entry to the second distillation vessel, provided it is upstream of said first distillation vessel) and/or a port for addition of the sodium chloride solution directly to the first distillation vessel (or directly to the second distillation vessel, provided it is upstream of the first). Preferably the ports for addition of the sodium chloride solution are connected via a conduit to a source of sodium chloride or sodium chloride solution. Where the sodium chloride solution is added directly to a distillation vessel, the apparatus comprises a port or ports for the addition of the sodium chloride solution to the gas phase, on the liquid surface and/or in the liquid within said distillation vessel.

Removal of vapour comprising hydrate inhibitor and/or water from the first distillation vessel/reclaimer, e.g. as part of a reclamation process, causes the salt to precipitate in the first distillation vessel. Precipitated salt is generally removed by taking liquid from the first distillation vessel (or optionally the second if it is downstream of the first) and removing solids (including the salt). The liquid is then optionally returned to one of the distillation vessels, preferably the first. The solids removal may be effected, for example, by filtering, settling or centrifuging. Thus, a preferred aspect of the apparatus of the invention is a solids removal unit connected to the port for the removal of a liquid comprising sodium chloride. Said solids removal unit may be a filtration device, a centrifugation device or a settling tank.

The method and apparatus of the invention will now be described further with reference to the accompanying drawing in which Figure 1 shows a typical full reclamation process, incorporating the modifications of the invention. Conventional reclamation processes involve removal of gas and condensates from the pipeline's hydrate inhibitor outlet stream. The resulting "rich" hydrate inhibitor composition may be stored prior to reclamation. To clean the liquid hydrate inhibitor of dissolved impurities like salts and other compounds with high boiling points, rich hydrate inhibitor composition is passed to a reclaimer, where the liquid hydrate inhibitor is withdrawn in the gas phase. This is normally done using a distillation vessel operating at reduced pressure, for example a flash separator or reclaimer. Flash separators normally operate at a pressure of 0.15 to 0.3 bara (15-30kPa) and at this pressure MEG can be boiled off at 120-135°C.

This vacuum distillation of the hydrate inhibitor is referred to as a reclamation process. When the hydrate inhibitor feed into the reclaimer contains salts, the salt concentration in the liquid in the reclaimer will increase and at some point the salts will begin to precipitate out. Precipitated salts are generally removed by taking liquid from the reclaimer, removing solids and returning the liquid to the reclaimer. The solids removal may be, for example, by filtering, settling or centrifuging.

The figure shows regeneration in a distillation column downstream of reclamation (which is preferred), although reclamation may be downstream of regeneration. The regeneration process is primarily concerned with the removal of water from the rich liquid hydrate inhibitor. This can be carried out for MEG for example at about 140- 150°C and 1.1-1.3 bara (110-130 kPa). Water vapour is drawn off and the "lean" (i.e. with reduced water content) hydrate inhibitor is drawn off as a liquid. The correct hydrate inhibitor concentration within the withdrawn liquid stream may be obtained by adjusting the temperature or pressure within the reboiler. Under normal operation, with this simple distillation of rich MEG, the MEG concentration in the top product, i.e. the water, is generally well below 500ppm, sometimes as low as 50-200ppm.

According to certain aspects of the invention, the sodium chloride solution can be added at one or more of the positions shown as A, B and C in Figure 1. Location "A" is addition into the inhibitor (hydrate inhibitor composition) storage tank. B shows addition to the glycol stream prior to entry to the reclamation process (or the regeneration process if regeneration takes place first). C shows the sodium chloride solution being added to the reclaimer directly (although it could additionally or alternatively be added to the regenerator directly) in the gas phase (1), i.e. the composition is added above the liquid level, on the liquid surface (2) and/or in the liquid (3). All references herein to "comprising" should be understood to encompass "including" and "containing" as well as "consisting of" and "consisting essentially of".

The present invention will now be further described with reference to the following non-limiting examples.

Example 1

Process parameters

Laboratory scale experiments were designed to simulate the behaviour of the foaming process that takes place in a flash separator. Table 2 below shows the relationship between the laboratory pieces of apparatus and their industrial scale equivalents.

Table 2:

A known amount of Liquid A (i.e. a rich MEG composition as in table 3 below, i.e. a composition comprising a hydrate inhibitor as herein described) was added to a suction flask, which was placed on a magnetic stirrer with heating plate. A separating funnel was mounted on the suction flask with the feeding cork valve closed. A known amount of Liquid B (i.e. an "enhancement liquid" as in table 3, i.e. a solution of sodium chloride as herein described) was added to the separating funnel. The heating plate was set to the temperature required to boil Liquid A. Liquid B was fed to a suction flask and the effects of this were observed over time. Various sodium chloride compositions were tested in the suction flask and in the separation funnel. Various sizes of separation funnels and suction flasks, as well as dosing speeds were tested. The results are presented in Table 3, which sets out suitable process parameters, i.e. temperature and pressure ranges, typical compositions for a rich- MEG feed composition (i.e. the composition from which the salt needs to be removed via reclamation), composition of the sodium chloride solution in terms of the mother liquor and salt content, dosing speed (i.e. the vol% of the inlet MEG feed to flash

separator which is the amount of sodium chloride solution added to the MEG feed, expressed as vol% of the MEG feed). The resulting sizes of precipitated sodium

chloride particles [μηι] are shown.

Table 3: Experimental parameters and results

Process Rich MEG Enhancement liquid

parameters [wt% EG:wt%_H Resulting

P T 2o] Mother Sodium Dosing sizes of

[bara] [°C] liquor chloride in speed (vol% precipitated a. Wt% EG mother of the inlet sodium b. Wt%H20 liquor [wt%] MEG feed to chloride flash particles separator) [Mm]

O to 1 110 From 35:65 a.100-90 Equivalently 0.1-3

to To 50:50 b.0-10 saturated 18-52

180

O to 1 110 From 40:60 a.89.99-75 Equivalently 0.1-6 21-60

to To 50:50 b.10.01-25 saturated

180

O to 1 110 From 50:50 a. 74.99- Equivalently 0.1-6.8 20-70

to To 70:30 62 saturated

180 b.25.01-38

O to 1 110 From 50:50 a. 61.99- Equivalently 0.1-9.5 22-75

to To 70:30 45 saturated

180 b. 38.01-

49.99

O to 1 110 From 40:50 a. 44.9- 0 Equivalently 0.1-15.2 36-103 to To 75:25 b.45-100 saturated

180

Claims

Claims:

1. A method for removing sodium chloride from a composition comprising a hydrate inhibitor, said method comprising the steps of:

(iii) separating sodium chloride crystals from said liquid.

2. The method as claimed in claim 1 wherein said vapour withdrawn in step (ii) comprises hydrate inhibitor, such that (a) a vapour phase comprising hydrate inhibitor and (b) a liquid containing sodium chloride is formed.

3. The method as claimed in claim 1 or claim 2 further comprising separating water from a composition comprising hydrate inhibitor and water.

4. The method as claimed in any one of the preceding claims wherein the sodium chloride solution comprises sodium chloride, water and hydrate inhibitor.

5. The method as claimed in any one of the preceding claims wherein the amount of hydrate inhibitor in the sodium chloride solution (in relation to the total amount of hydrate inhibitor plus water) is 45 to 90 wt%.

6. The method as claimed in any one of the preceding claims wherein the amount of sodium chloride in the sodium chloride solution, expressed with regard to the solution as a whole, is 5 to 26 wt% at atmospheric pressure and 25 °C.

7. The method as claimed in any one of claims 1 to 5 wherein the sodium chloride solution is a saturated or supersaturated sodium chloride solution

8. The method as claimed in any one of the preceding claims wherein the hydrate inhibitor is monoethylene glycol.

9. Use of a sodium chloride solution to enhance the growth of sodium chloride crystals in a composition comprising a hydrate inhibitor.

10. The use as claimed in claim 9 wherein the sodium chloride solution is as described in any one of claims 4 to 7.

1 1. The use as claimed in claim 9 or claim 10 wherein the hydrate inhibitor is monoethylene glycol.

12. An apparatus for removing sodium chloride from a composition comprising a hydrate inhibitor, said apparatus comprising a first distillation vessel comprising:

wherein said apparatus comprises means for contacting a sodium chloride solution with said composition comprising a hydrate inhibitor.

13. The apparatus as claimed in claim 12, further comprising a conduit linked to a source of sodium chloride solution.