Short contact high temperature MEG recovery

Abstract

Monoethylene glycol (MEG) can be recovered by a process comprising contacting a MEG-water-salt stream with a heat transfer fluid in a flash separator vessel and then flash separating the MEG and water, wherein the pressure is above 0.3 bar absolute (0.03 MPa), the temperature is in the range of above 120 °C to about 250 °C, and the residence time of the MEG and water ranges from about 1 second to about 10 minutes, and then removing the MEG and water in the head of the flash separator vessel and removing the salt from the flash separator vessel. In some embodiments, it is contemplated that the temperature of the process can range from above 165 °C to about 250 °C, and / or the pressure can be atmospheric pressure.

Description

BACKGROUND

[0001] The present invention relates generally to a process for recovering monoethylene glycol (MEG) from a water-miscible liquid, and more particularly to a process for recovering MEG with a heat transfer fluid at high temperature and at pressures higher than conventionally employed.

[0002] In conventional MEG recovery processes involving removal of salts from MEG-water solutions, a MEG-water-salt stream is contacted with a recycle stream of salt-saturated MEG operating at a temperature above the dew point of the incoming feed, such that the volatile components of the feed are sufficiently vaporized, and the dissolved salt components of the feed are precipitated and removed from the heat transfer fluid. The recycle MEG stream thus serves as the heat transfer fluid. A hydrocarbon stream (or other non-volatile fluid) can also be used as the recycle stream.

[0003] Conventional MEG recovery processes include, but are not necessarily limited to, the processes described in U.S. Patent No. 6,685,802 ("the '802 patent"), U.S. Patent No. 5,993,608 ("the '608 patent"), U.S. Patent No. 6,340,373 ("the '373 patent").

[0004] As noted, these processes involve removal of salts from glycols used for natural gas dehydration and for preventing hydrate formation in oil and gas production facilities. And as mentioned, demineralization is typically accomplished by a flash process, in which a heated recycle liquid provides heat to vaporize the aqueous stream of glycol, while collecting precipitated salts and other solid materials in a liquid residue, which can then be removed from the process. The processes described in the '802, '608 and '373 patents each include a flash process similar to that described above, and such flash processes have been or are being applied in the oil and gas industry for removing unwanted salts from glycols.

[0005] It is widely accepted that MEG degrades significantly above a temperature of about 165°C. Therefore, in order to achieve complete vaporization of MEG and water components at temperatures well below the accepted 165°C degradation temperature, the flash process is carried out at sub-atmospheric pressures (0.1-0.3 bar absolute (bar A); 0.01-0.03 MPa). The recycle heater outlet temperature in conventional flash separators is typically limited to an upper value of 150°C due to concerns about MEG degradation.

[0006] Conventional MEG recovery processes use a large circulating inventory of concentrated MEG, which is used to provide the heating duty, as exemplified in U.S. Patent No. 8,728,321. This circulating MEG has a very long residence time in the MEG recovery system (on the order of months to years at the high temperatures required for complete vaporization.

[0007] The long residence time of the recirculated MEG stream (commonly referred to as the recycle MEG) that is saturated with the circulating brine makes the degradation process problematic. Because of this long residence time, it is commonly believed that operation under reduced pressure is necessary in order to prevent significant MEG degradation, which in turn lowers the pH in the system through MEG degradation into formic acid, acetic acid, glycolic acid, and other carboxylic acids. The lowering of the pH potentially leads to an increase in the corrosion rate. In such a case, in order to maintain the system operable, it is necessary to periodically blow down and dispose of the degraded MEG from the system, resulting in significant MEG loss and potential environmental impact from the disposal of this waste product. The cost of disposal and replacement of the MEG can be significant.

[0008] It would therefore be desirable to find an improved process for recovering MEG that minimizes or avoids one or more of these problems. SUMMARY

[0009] A process for recovering monoethylene glycol (MEG) is provided in one non-limiting embodiment, wherein the process includes contacting a stream comprising MEG, water, and at least one salt with a heat transfer fluid, optionally in a flash separator vessel, flashing the MEG and water from the stream in the flash separator vessel, wherein the pressure is greater than 0.3 bar absolute (0.03 MPa), the temperature is in the range of 110 °C to about 250 °C; or 150 °C or higher to about 220 °C, and the residence time of the MEG and water is in the range of about 1 second to about 10 minutes. The process further includes removing the MEG and water in the head of the flash separator vessel and removing the at least one salt from the flash separator vessel. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 A flash separator vessel used in a process for removing solid material from a lower portion of a vessel containing process liquid and unwanted solid material and for removing MEG and water in the head is schematically illustrated.

[0011] It is to be understood that the drawings are schematic and not to scale, and thus some of the important components of the application can be exaggerated in the drawings for the sake of illustration. DETAILED DESCRIPTION

[0012] The process involves a process in which a hydrocarbon stream (or other non-volatile fluid) is used to provide the heating duty to vaporize the MEG and water components of the feed. It has been found that MEG degradation does not occur instantaneously, and it has further been found that the flash separator vessel can be operated at higher pressures without observing any significant increase in MEG degradation products, provided that the residence time of the MEG in the flash separator vessel is kept to a minimum. Experiments have been conducted at atmospheric pressure, and higher pressure operation should in principle also be achievable.

[0013] This discovery allows the flash separator vessel to be operated at pressures above the currently accepted range of 0.1 to 0.3 bar absolute (0.01 to 0.03 MPa); or at pressures at or above atmospheric pressure; and in various non-limiting embodiments at pressures of about atmospheric pressure plus 5 psi (1.34 bar absolute), and thus at temperatures significantly higher than the previously accepted 165°C limit. The benefits of operating in the range of 0.1 to 0.3 bar absolute (0.01 to 0.03 MPa) include, but are not necessarily limited to:

[0014] 1) Equipment size is reduced due to lower vapor phase velocities at higher pressures. The flash separator vapor can be vented to the site flare system without the need for a vacuum system.

[0015] 2) It is possible to eliminate the vacuum system, which not only saves cost, but also eliminates the complexity of the equipment.

[0016] 3) The potential for oxygen ingress is reduced, which is another cause of MEG degradation.

[0017] More specifically, the method involves the MEG / water inlet stream entering the flash separator vessel, where the MEG and water rapidly and completely vaporize at temperatures above 150°C and pressures above 0.3 bar absolute (0.03 MPa); or above 0.5 bar absolute (0.05 MPa), with no MEG / water inventory remaining in the equipment that can be exposed to high temperatures for long periods of time. In one non-limiting embodiment, the flash separator temperature ranges from about 110°C or 110°C or above, or about 120°C or 120°C or above, independently to about 250°C; in another non-limiting embodiment, 150°C or above, independently to about 220°C; or 165°C or above, independently to about 220°C. When the term "independently" is used herein in reference to a range, it means that any lower threshold can be combined with any upper threshold to give a suitable alternative range. The optimum temperature range depends on the composition of the feed input and the operating pressure. Higher MEG content requires higher temperature. In one non-limiting embodiment, if the sump equilibrium temperature is maintained at about 220°C, but there is a temperature differential across the recirculation heat exchanger, and if high temperature is required because MEG residence time is kept to a minimum, then the recirculation heater outlet temperature, and thus the recirculation heater temperature differential, can be increased. This can provide the further advantage of reducing the recirculation flow rate, allowing the use of smaller pumps and smaller diameter piping.

[0018] The residence times for MEG and water range from about 1 second independently to about 10 minutes, or from about 10 seconds independently to about 5 minutes, and in various non-limiting embodiments from about 20 seconds independently to about 1 minute. Generally, shorter residence times are preferred, but a limited amount of time is required to heat the feed to its bubble point and then evaporate it.

[0019] Suitable heat transfer fluids include, but are not limited to, aromatic hydrocarbons, alcohols, glycols, amines, silicone-based liquids, and mixtures thereof (where these heat transfer fluids are immiscible with MEG). In a non-limiting example, a suitable mixture of synthesized aromatic hydrocarbons is available from Solutia Inc. ADX-10 heat transfer fluid. Such as and A mixture of paraffinic hydrocarbons at 600°C can also be used as a heat transfer fluid. Suitable silicone-based liquids include, but are not limited to, DURATHERM S, etc.

[0020] Salts that may contaminate the feed stream include, but are not limited to, sodium chloride (NaCl), calcium chloride (CaCl2), and other chlorides, oxides, sulfates, acetates, nitrates, phosphates, bicarbonates, and carbonates of sodium, potassium, calcium, magnesium, iron, copper, lead, barium, strontium, etc., as well as combinations thereof. In addition, the feed stream may contain optional flow protection chemicals such as scale inhibitors, corrosion inhibitors, wax inhibitors, and oxygen scavengers.

[0021] like Figure 1 As shown, the lower part of the flash separator vessel 10 contains a mixture comprising a process liquid 12 (i.e., a heat transfer fluid) that is substantially immiscible with water and undissolved solid matter or salt in particulate form. Undissolved solids can be removed from the process using various techniques, including, but not limited to, conveying devices such as downcomer 14, which connects to a solids collection tank 16 near the bottom of vessel 10. Removal of solids from the process can be achieved using one of several commercially viable solid-liquid separation methods, such as the use of settling tanks (e.g.,...). Figure 1 (as shown in the diagram) or centrifuge. The feed stream 18 is a free-flowing mixture comprising two or more miscible liquids (e.g., MEG and water) and dissolved solids (e.g., sodium chloride, magnesium chloride, calcium chloride from gas wells, and flow assurance chemicals added to the MEG-water solution to minimize pipe scaling and corrosion). Examples of such mixtures include glycol / water and amine / water mixtures containing dissolved salts, corrosion products, and / or other unwanted solid contaminants.

[0022] The method can be referred to as a "Flash-on-Oil" method (FoA). Indeed for FoO as well as for the conventional MEG method, one or more of the liquid components boils at a temperature significantly higher than the other liquid components, but the operation of the present FoO method is such that both (or all) components in the feed are completely vaporized regardless of the boiling point differences. This is not the case for the conventional system where there is a balance between the incoming MEG-water and the recirculated MEG such that a significant inventory of MEG remains in the vessel. If the FoO method is properly optimized, the amount of MEG (water) in the liquid phase present in the vessel will be minimal.

[0023] The feed stream 18 enters the flash separator vessel 10 and mixes with the larger and hotter stream of recirculated liquid 20 that has also entered the separation vessel 10. In one non-limiting embodiment, the recirculated liquid 20 immediately heats the feed stream 18 and thereby causes the volatile components in the feed stream 18 to rapidly boil or flash.

[0024] Alternatively, the feed stream 18 and the recirculated liquid 20 can be mixed upstream (not shown) of the separator vessel 10 and the combined stream injected into the separation vessel 10.

[0025] The vapor 22 produced by the flash of the feed stream exits the separation vessel through an outlet passage 24. Unless there is significant entrainment of small particles or droplets into the vapor, this vapor is essentially free of solids. In one non-limiting embodiment, the vapor is MEG and water.

[0026] The solids and unvaporized liquids collect in a liquid pool 12 in the lower half of the separation vessel 10. The flash that has occurred ensures that the liquid pool is primarily composed of higher boiling liquids (i.e., heat transfer fluid) and solids. The recirculated liquid 20 is withdrawn from the liquid pool and enters a recirculation loop 26 where it is pumped by a recirculation pump 28, heated by a recirculation heater 30, and mixed with the feed stream 18 as described above. It is an objective of the method herein to minimize or prevent the accumulation of any MEG in the loop. This can be achieved by operating at suitable temperature-pressure-residence time.

[0027] The method employs a short residence time for the MEG molecules in the flash zone in order to minimize thermal degradation and oxidation-thermal degradation of the MEG molecules to organic acids and other species. Employing a short residence time allows for the use of operating pressures at or near ambient (atmospheric pressure). The temperature-pressure regime employed in the method is such that the MEG and water components of the incoming feed stream are brought to complete vaporization.

[0028] Operating at near atmospheric pressure instead of the 0.1-0.3 bar absolute units (0.01-0.03 MPa) pressure conventionally employed in flash evaporation processes has significant and substantial benefits. These are illustrated as follows:

[0029] 1) MEG degradation and corrosion: In conventional MEG recovery processes operated at partial vacuum, the potential for air to enter the flash separator vessel through leaks at flanges and fittings is increased. The oxygen present in the incoming air can cause increased degradation of MEG to organic acids, which in turn can cause increased corrosion of the piping. Operating at atmospheric pressure or above will significantly reduce the potential for air ingress. In one non-limiting embodiment, the flash separator vessel will be operated at a pressure slightly above atmospheric pressure. In another non-limiting form, the actual operating pressure will be determined by the equipment and piping downstream of the vessel, but the operating pressure will most likely be around 5 psiG (20 psiA, 1.35 bar absolute).

[0030] 2) Complex vacuum system: Operating at reduced pressure in conventional MEG recovery processes requires a vacuum system. Vacuum systems can be complex and add additional weight, space requirements, and utility (electric and cooling medium) requirements to the MEG package. Space and weight constraints are important, particularly for offshore applications where space and weight are at a premium. Operating the flash separator at atmospheric pressure as in the present process eliminates the need for a vacuum package.

[0031] 3) Large diameter vessels and piping: Operating at low pressure (0.1-0.3 bar absolute units (0.01-0.03 MPa)) results in large volumes of low density vapor (MEG-water) being produced by the flash process. This large volume flow requires large flash separator vessels, large diameter piping, and larger equipment downstream of the flash separator vessel (typically condensers, knockout drums, and distillation columns). Operating at atmospheric pressure in the present process will reduce the size of the flash separator vessel, the piping, and the downstream equipment to handle the overhead vapor. Operating at atmospheric pressure will also reduce the size (surface area) of the condenser equipment in the recovery unit section.

[0032] In the foregoing specification, the disclosure has been described with reference to specific embodiments thereof, and is intended to effectively provide a method and apparatus for improving the recovery of MEG by allowing the flash separator to operate at pressures above the currently accepted range, even at atmospheric pressure and thus at temperatures significantly above the conventionally accepted 165°C limit. It will, however, be evident that various modifications and changes can be made thereto without departing from the broader scope of the disclosure as set forth in the appended claims. The specification is therefore to be regarded in an illustrative rather than a restrictive sense. For example, the MEG-water-salt compositions, heat transfer fluids, pressures, temperatures, residence times, and / or flow rates can be varied or optimized in accordance with what is illustrated and described, and even though certain additional features can not have been specifically identified or attempted in the particular systems, methods or apparatus described herein, it is contemplated that they will be within the scope of the disclosure. For instance, it is contemplated that the parameters, compositions and processing of any of the components and equipment described will be of utility and covered by the appended claims.

[0033] The word "comprising" as used in the claims should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus be understood that it is meant to be inclusive rather than restrictive in that it specifies the presence of stated features or steps and does not preclude the presence or addition of one or more other features, steps, aspects, or steps.

[0034] The disclosure can suitably include, consist of, or consist essentially of, the disclosed elements, and can be implemented with or without the disclosed elements lacking. For example, in one non-limiting embodiment, a method for recovering monoethylene glycol (MEG) can be provided that essentially consists of, or consists of, contacting a stream comprising MEG, water, and at least one salt with a heat transfer fluid, optionally in a flash separator vessel; flash separating MEG and water from the stream in the flash separator vessel, wherein: the pressure is greater than 0.3 bar absolute (0.03 MPa); the temperature is in the range of about 110°C to about 250°C; and the residence time of MEG and water ranges from about 1 second to about 10 minutes. The method can further essentially consist of, or consist of, removing MEG and water in the head of the flash separator vessel, and removing at least one salt from the flash separator vessel.

Claims

1. A method of recovering mono-ethylene glycol (MEG), the method comprising: contacting a stream comprising MEG, water, and at least one salt with a heat transfer fluid in a flash separator, wherein the heat transfer fluid is immiscible with MEG; flash separating the MEG and water from the stream by thermal contact with the heat transfer fluid in a flash separator vessel without the use of vacuum, wherein: the pressure is atmospheric pressure and above; the temperature is in the range of 110 °C to 250 °C; and the residence time of the MEG and water ranges from 1 second to 10 minutes; and removing the MEG and water in the top of the flash separator vessel; and removing the at least one salt from the flash separator vessel; wherein the temperature is 165 °C and above to 220 °C.

2. The method of claim 1, wherein the heat transfer fluid is selected from the group consisting of aromatic hydrocarbons, paraffinic hydrocarbons, DURATHERM S, and mixtures thereof.

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