Process and plant for the regeneration of glycol

The present invention relates to a process for the regeneration of glycol and a plant for carrying out the process.

hi the first stage of oil and gas production, a mixture of hydrocarbons, water and dissolved salts is obtained. This mixture is then transported to a further-processing plant for the recovery of the desired oil and gas products. During this transport, the mixture is subjected to other physical conditions than those that prevail in the reservoir. This involves, in particular at low temperatures (< 25 ⁰ C) and high pressures (> 10 bar), a risk of gas hydrate formation. Gas hydrates are solid compounds formed of gas and water which can clog pipelines and other equipment. The addition of a hydrate inhibitor will inhibit the formation of gas hydrates, even in long transport systems in cool surroundings. On completion of transport, the hydrocarbons are separated off, leaving a residue consisting of a mixture of water, hydrate inhibitor, dissolved salts and residual dissolved hydrocarbons. As it is necessary to add substantial amounts of the inhibitor in order to obtain the desired inhibition, this mixture constitutes a substantial volume, which if it were to be disposed of would be regarded as hazardous waste, but it also represents a considerable investment in the form of the hydrate inhibitor. Consequently, it is desirable to be able to recover the inhibitor and reuse it to the greatest extent possible.

Gas from a reservoir normally contains some CO ₂ which in contact with water will form carbonic acid. The amount of CO ₂ varies from reservoir to reservoir. To prevent corrosion of pipelines and other equipment not made of rust-proof, acid-resistant material, a base is added to the mixture, preferably sodium hydroxide or sodium bicarbonate, in order to increase the pH of the system. Base addition usually takes place in the hydrate inhibitor before it is injected into the reservoir stream prior to transport. This method of reducing corrosion is normally referred to as "pH stabilisation", hi addition to oil, condensate and gas, the reservoir stream may contain formation water which can contain NaCl and other ions, typically alkali and earth alkali metals and iron as well as bromide and sulphate.

Today, a number of different types of hydrate inhibitors are known, including different glycol compounds such as monoethylene glycol, diethylene glycol and Methylene glycol or alcohols such as methanol and ethanol. The glycol compounds are characterised by a boiling point that is higher than the boiling point of water. Therefore,

in ordinary distillation, the water will boil off and the glycol-rich phase will contain all the dissolved salts.

US2005/0072663 describes a known process for regenerating a glycol solution containing water, hydrocarbons and dissolved salts. Water and hydrocarbons are distilled off under reduced pressure. The glycol solution is then further concentrated, which results in a concentration of salts such that they precipitate and can be separated off. The document says nothing about what ions are present or what salts are precipitated out. The concentrated glycol solution obtained will be saturated with salts and this could lead to deposits in the equipment used to transport the glycol solution, especially if temperature and pressure conditions change underway.

US6023003 and US6444095 disclose a process and a system for recovering glycol from a glycol and brine mixture. The process comprises three cyclic repetitions of the following steps: Flash distillation of water, removal of water vapour and removal of solid salt particles from the glycol phase. As a result, a reduced water content and a reduced total salt content are obtained once the mixture has undergone such a process three times, but the concentrated glycol solution obtained will be saturated with salt. The documents refer only to the presence of sodium chloride.

US6733636, US2003/0127226 and US2005/0022989 disclose a treatment of produced water from oil production. After the oil has been removed, the pH is lowered to remove carbonates such as CO ₂ gas. The water is then passed to an evaporator for precipitation of salts and silica under pH control. The use and recovery of a hydrate inhibitor is not mentioned in these documents.

Another known method for regenerating glycol consists of passing the salt-containing glycol and water mixture into a vacuum boiler and evaporating water and glycol. This vapour is condensed and distilled until a salt-free glycol solution having suitably low water content is obtained. A certain water content will often be acceptable. The regenerated glycol solution may comprise between 0 and 20% water, preferably about 10% water. During the process, the concentration of the dissolved salts in the liquid phase in the vacuum boiler increases until a saturated solution is obtained and solid salt particles are formed. To remove the salt particles, a sub-stream is led out of the boiler and the solid salt particles are removed therefrom before it is passed back to the vacuum boiler. The salts which are formed during this process from a pH stabilised system will essentially be sodium carbonate. If the system also contains formation water, NaCl and

carbon salts of alkali and earth alkali metals will also be precipitated. However, several of these salts have a tendency to form very small crystals. This means that they can be exceptionally difficult to remove from the liquid phase by known separation methods, hi existing plants, this has made it necessary to close the plant in order to remove the formed salts in thick slush or paste form and flush the plant clean. For reasons of costs, such regeneration plants are often not made of an acid-resistant material and all processes must therefore take place in the neutral to basic pH range.

hi the continuous production of hydrocarbons, it is desirable to have a continuous process for regenerating glycol. Furthermore, operational shut-downs should be avoided as far as possible as regeneration plants often merely have at their disposal a limited storage capacity both for the glycol mixture and the regenerated glycol.

The object of the present invention is to solve these problems of the prior art, and the invention is based on the following observation. On the introduction of the mixture of glycol, water and salts into the vacuum boiler, the pH increases from about 6-9 to about 11-12. This change cannot be due to the accumulation of salts alone, as the precipitation of carbonates and bicarbonates will lower the pH. The cause is, however, that CO ₂ evaporates together with water and glycol. This is surprising as the equilibrium between CO ₂ , HCO ₃ ^" and CO ₃ ^2" at these high pH values suggests a very low concentration and low partial pressure of CO ₂ .

The reason that CO ₂ nevertheless evaporates is that the gas phase, including a low concentration of CO ₂ , is continuously removed from the vacuum boiler and to re- establish equilibrium between the concentration of carbon dioxide gas and dissolved carbon dioxide some of the carbon dioxide will evaporate.

The equilibrium between the concentrations of CO ₂ , HCO ₃ ^" and CO ₃ ^2" is dependent upon the pH value. Thus, CO ₂ dominates in the range from low pH to the first dissociation constant at a pH of about 6.5. At pH values between about 6.5 and about 10.3, bicarbonate dominates and the partial pressure of CO ₂ drops by a power often for each pH unit. At pH values higher than the second dissociation constant of about 10.3, the partial pressure of CO ₂ falls even faster by two powers often per pH unit. Consequently, when the aqueous phase is passed into the vacuum boiler, CO ₂ evaporates quickly until the pH has increased to about 10, then the rate drops until it settles at a stable state where the evaporation of CO ₂ corresponds to a stable portion of the amount of CO ₂ admitted, whilst the rest is precipitated out as Na ₂ CO ₃ . In order that

the electroneutrality balance of the system should be maintained, an equally large amount OfHCO ₃ ^" is converted into CO ₂ as into CO ₃ ^2" , independent of the pressure and temperature of the system. Based on the aforementioned observation, the following new process has been developed for solving the aforementioned problems. By evaporating as much CO ₂ as possible in the vacuum boiler, the amount of carbonates formed will be able to be reduced. By adjusting and controlling the pH of the mixture preferably using hydrochloric acid, it will be possible to avoid high concentrations of carbonate ions and bicarbonate ions in that HCO ₃ ^" is converted into CO ₂ and the salts will precipitate as chloride salts instead of carbonate or bicarbonate salts. The properties of chloride salts are well known and entail a number of advantages.

Accordingly, the present invention provides a process for the regeneration of glycol from a mixture comprising glycol, water and salts, the salts comprising carbonate and/or bicarbonate ions, comprising: - heating the mixture; evaporating water and glycol under reduced pressure: condensing water and glycol vapour: distilling water from the condensed vapour in order to form a salt-free glycol stream with reduced water content; - removing a sub-stream from the heated mixture; removing precipitated salts from the sub-stream, characterised in that it comprises removing CO ₂ from the mixture by evaporation together with glycol and water.

The invention further provides a plant for the regeneration of glycol from a mixture comprising glycol, water and salts, comprising: -a feed pipeline for feeding the mixture to the plant;

- a vacuum boiler, with a return circuit comprising a pipeline from the vacuum boiler to a compression device/pressure-increasing device, a pipeline leading to a pressure- reducing device and a pipeline leading into the vacuum boiler;

- a device for heating the vacuum boiler, its contents or the return stream:

- a pipeline from the circuit after the compression device connected to a salt/solids separation device with an outlet for essentially solid salts, and a pipeline connected to the vacuum boiler; - an outlet from the vacuum boiler for the discharge of vapour containing water, glycol, CO ₂ and volatile hydrocarbon components;

- a condenser for condensing water and glycol; and

- a distillation installation with an outlet for regenerated glycol with reduced water content, characterised in that it further comprises

- a device for adding acid to the mixture before it is passed into the vacuum boiler, and - a device for measuring the pH of the mixture before the mixture is fed upstream of the vacuum boiler or into the vacuum boiler.

Preferred embodiments of the present invention are set forth in the subsidiary claims.

By "glycol" is meant, within the scope of this invention, different glycol compounds such as monoethylene glycol, diethylene glycol and trietheylene glycol and the like and mixtures thereof.

The device and the method used for separation of precipitated and crystallised salts may be any known method such as sedimentation, filtration, centrifugation and the like.

The present invention will be explained in more detail with the aid of the attached Figure 1 which shows an outline of a preferred embodiment of a plant according to the invention.

A mixture comprising glycol, water and dissolved salts is passed into the plant through pipeline 1. The salt comprises carbonate and/or bicarbonate ions and formation water salts. This mixture may further comprise, for example, residual hydrocarbons or additives. One of the main components of the plant is a vacuum boiler 10, also termed a flash separator, in which an evaporation of water and glycol takes place at reduced pressure. Pressure and temperature in the vacuum boiler 10 may vary depending on factors including the glycol type, the glycol concentration and the heat source that is used. Preferably, the pressure is in the range of 10-lOOkPa and the temperature is in the range of 100-140 ⁰ C.

Connected to the vacuum boiler 10 is a return circuit comprising a pipeline 22 connected to the lower part, preferably the bottom, of the vacuum boiler 10. A bottom stream consisting of glycol, water and a concentrated salt content is passed to a compression device 11, consisting, for example, of a pump. The compression device ensures that the pressure in the circuit, also after heating, is sufficient to avoid boiling in the pipelines. It also provides an adequate circulation rate to prevent the salt particles from sedimenting whilst held in a slurry/suspension. The pressurised stream is passed

through pipeline 23 and 24 to a heat exchanger 14 which supplies heat to the system. It is also possible to supply heat by means of, for example, a heating coil in the liquid phase or by heating the vacuum boiler itself. The pressurised heated stream is passed through pipeline 25 and 20 to a pressure-reducing means 15, preferably a valve, before 5 it is passed through pipeline 21 back to the vacuum boiler 10.

The feed stream 1 can, as shown, be fed to the circuit before it is depressurised, but it may also be fed directly to the vacuum boiler 10 or before the heat exchanger in stream 24. A small side stream of the pressurised bottom stream is taken out through pipeline 4 o and passed into a separation device 12 for the separation of solid salt particles.

However, it is also possible to take this sub-stream out of the stream 25 after the heat exchanger 14. In a preferred embodiment, the device 12 comprises a sedimentation tank in which solid salt crystals are sedimented out. The device may also consist of other known devices for separating solid particles from a liquid phase, for example, s filtration devices, centrifugation devices and the like. The solid particles exit the device via pipeline 5, in the preferred embodiment together with a small portion of liquid, sufficient to ensure the mixture is transportable. The substantially particle-free liquid phase is passed through a pipeline 26 back to the vacuum boiler 10. The admission of return stream 21, particle-free liquid stream 26 and optionally a direct feed stream can 0 be carried out at any level in the boiler 10. However, it is advantageous not to admit streams too close to the outlet arranged at the upper end of the boiler as this may result in solid particles and liquid droplets being passed out with the vapour phase. Return stream 21 should preferably also not be introduced into the liquid phase in the boiler 10 because evaporation of water and glycol may then give increased foaming. 5

The vapour phase in the vacuum boiler 10 consists essentially of glycol and water vapour, but also of CO ₂ and volatile hydrocarbon components. It is passed through pipeline 2 to a condenser 13 where the vapour is condensed. The condensate is passed via pipeline 27 to a distillation installation 16 where water is distilled off and removed o via pipeline 28 whilst regenerated glycol with a reduced water content is passed out via pipeline 29.

In a preferred embodiment of the invention, the method according to the invention comprises the addition of acid. In Figure 1 this is shown by pipeline 3 which introduces ₅ acid into the stream 20 before it is depressurised in device 15. The acid addition may also take place elsewhere, for example, in stream 24 or 25. The acid should preferably not be added in the feed stream 1 because the pH in this stream may then be too low,

which could cause corrosion. At the same time, the circuit is provided with a pH meter (not shown) which measures the pH value and a control unit which, on the basis of the measurement, controls the acid addition so that the pH of the mixture is maintained at a desired level. With a view to obtaining fastest possible evaporation of CO ₂ , the pH should be as low as possible, but to prevent corrosion it cannot be less than 7 and often not less than 10. Lowering the pH in the vacuum boiler from 12 to 10 will result in the equilibrium shifting from carbonate to bicarbonate and the partial pressure of CO ₂ increasing by more than two powers often, that is to say that it becomes at least 100 times as great.

The solid salts formed will essentially consist of NaCl. Preferably NaCl will represent more than 80% by weight, more preferably more than 90% by weight and most preferably about 99% by weight of the solid salts. It is therefore not possible to completely avoid the formation of solid carbonate salts such as CaCO ₃ , but the proportion thereof will be small in relation to the amount of the sodium salt of the added acid.