System and Process For Removal Of Organic Carboxylates From Mono Ethylene Glycol (MEG) Water Streams By Acidification and Vaporization Under Vacuum

A system and method for removing organic carboxylates from a mono ethylene glycol (“MEG”) stream includes a reaction vessel; means for cooling and diluting the MEG stream being routed to the reaction vessel; means for acidifying the cooled and diluted MEG stream during its residence time within the reaction vessel; and means for removing an acetic-rich overhead stream from the reaction vessel. The acidification of the cooled and diluted MEG stream occurs under a vacuum. The reaction vessel may be located downstream of a calcium removal vessel and receive a filtered bottom stream from that vessel, or it may be a single reaction vessel that cycles between a calcium removal mode and an acetate removal mode, with the pressure of the single vessel being greater during the calcium removal mode than during the acetate removal mode.

BACKGROUND OF THE INVENTION

This invention relates to systems and methods designed to treat mono ethylene glycol (“MEG”) streams used in the oil and gas industry to control hydrates formation in the production pipeline. More particularly, the invention relates to systems and processes which allow for removal of carboxylates from the MEG water stream of a MEG reclamation and regeneration package to reduce MEG losses.

In the oil and gas industry, dry (lean) MEG is injected into the production pipeline to control the formation of hydrates within the produced stream. The MEG injection is part of a MEG loop of a gas production facility. The loop typically includes a reclamation and regeneration package to treat the wet (rich) MEG and reclaim as much MEG as possible for reinjection into the pipeline.

The formation waters and condensed waters, which arrive at the gas production facility along with the raw hydrocarbon products, contain organic acids. Because these organic acids are highly soluble in MEG-water mixtures, they tend to follow the MEG-water stream. Additionally, in order to protect production pipelines against corrosion, and to remove dissolved divalent cations from the MEG stream, the pH of the MEG-water in the pipeline is elevated by the addition of bases such as sodium hydroxide. At elevated pH, the organic acids are present as a carboxylate salt (e.g., sodium acetate).

The low volatility of the carboxylate salts results in their accumulation in MEG process streams within the MEG loop. This accumulation, in turn, results in increased viscosity and density, making the MEG streams more difficult to pump.

To control the carboxylate levels in the MEG loop, the carboxylate-rich MEG is periodically discharged. However, this results in loss of MEG from the system and requires replacement to ensure the MEG inventory of the loop is maintained. Therefore, a need exists for systems and processes which control the carboxylate levels and reduce or eliminate MEG loss in the loop.

Carboxylate accumulation (either as the organic acid or as the corresponding salt) is an issue for MEG reclamation and regeneration packages due to the high solubility of these species in the water-MEG aqueous phase.

In order to minimize corrosion issues, the MEG Loop is operated at relatively high pH whereby the carboxylic acids are present predominantly as the carboxylate salts which have low volatility and, thus, are not removed in the overheads (produced water) stream from the regenerators or reclaimers of the MEG Recovery Package. Their high solubility in alkaline MEG solutions means that they do not precipitate when the pH is raised to remove the calcium, magnesium and other divalent cations.

Accumulation of acetates can lead to elevated density and viscosity in MEG streams which, in turn, lead to operational difficulties. Therefore, a need exists for a system and process to remove organic carboxylates from the MEG water stream.

SUMMARY OF THE INVENTION

By employing a system and process made and practiced according to this invention, the problems discussed in the above background section are minimized because the acetate levels are controlled to a manageable level while MEG losses associated with the acetate removal process are kept to a minimum when compared to the alternative.

A system and process for removing organic carboxylates from a mono ethylene glycol (“MEG”) stream includes a reaction vessel; means for cooling and diluting the MEG stream being routed to the reaction vessel; means for acidifying the cooled and diluted MEG stream during its residence time within the reaction vessel; and means for removing an acetic-rich overhead stream from the reaction vessel. The acidification of the cooled and diluted MEG stream occurs under a vacuum.

The reaction vessel may be located downstream of a calcium removal vessel and receive a filtered bottom stream from that vessel, or it may be a single reaction vessel that cycles between a calcium removal mode and an acetate removal mode, with the pressure of the single vessel being greater during the calcium removal mode than during the acetate removal mode.

Preferably, the cooling and diluting means results in the incoming MEG stream to be 50 wt % MEG at a temperature in a range of 80° to 100° C. In the acetate removal mode, the pressure is sub-atmospheric, preferably in a range of 0.1 to 0.3 bar. The acidifying means, which may be hydrochloric acid, results in the cooled and diluted MEG stream during its residence time within the reaction vessel to have a pH in a range of 3.5 to 5.5.

Objects of the invention include providing a system and process which: (1) can be retrofitted into existing MEG loops; (2) controls and reduces the amount of acetates in the MEG water stream; (3) extends the length of the production run; (4) reduces MEG loss; and (5) increases MEG recovery for re-use.

ELEMENTS AND ELEMENT NUMBERING USED IN THE DRAWING FIGURES

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system and process made and practiced according to this invention allows a target salt (in this case acetate) to accumulate in a reaction vessel and then removes the acetate from a concentrated liquor within the reaction vessel, thereby keeping vessel site and inventory to a minimum.

Referring to the drawings and first toFIGS. 1Aand B, a system10for removing organic carboxylates like acetates from a MEG water stream includes a reaction vessel11as part of the MEG loop of a MEG reclamation and regeneration package. The MEG reclamation unit (not shown) is typically operated at a temperature in a range of about 120°-140° C. and at sub-atmospheric pressure (about 0.1-0.3 bara). The recycle loop within the reclamation unit is operated at elevated pressure (about 4 bar). The reaction vessel11is closely coupled to the recycle loop of the package.

The incoming carboxylate-rich MEG stream13is typically at 80-90 wt % MEG and high ph (>9.5). The incoming MEG stream13is cooled and diluted with water15to yield a cooled and diluted MEG water stream17which enters and resides within reaction vessel11as a MEG water mixture. The cooled and diluted MEG stream17is preferably at a temperature of about 80-100° C. and 50 wt % MEG.

The reaction vessel11can be switched between a calcium removal mode (high pH, atmospheric pressure) and an acetate removal mode (low pH, sub-ambient pressure). The frequency of calcium removal cycles and acetate removal cycles can be varied to control the levels of calcium and organic acids in the MEG loop depending on the composition of the MEG feed entering the MEG regeneration package.

When reaction vessel11is in a calcium removal mode or cycle, the vessel11operates at atmospheric pressure and removes calcium and other divalent cations from the incoming MEG water stream by elevating pH. Precipitating means19such as sodium or potassium carbonate or sodium or potassium hydroxide are introduced to the reaction vessel11. Salts residing in the MEG water mixture—such as calcium chloride and, commonly, lesser amounts of other divalent salts like magnesium, barium and strontium chlorides—react with the precipitant agent and precipitate out of the MEG water mixture as solid crystals. The solid crystals are removed as a bottom stream27.

When reaction vessel11is in an acetate removal mode, the vessel11operates under a vacuum (preferably in a range of about 0.1-0.3 bar) and removes carboxylates by lowering pH (preferably in a range of about 3.5-5.5). The stream17is acidified within the reaction vessel11using acidifying means21such as hydrochloric acid (e.g., 30 w % HCl in water) to achieve a pH in a range of about 3.5-5.5. The pressure in reaction vessel11is then reduced to 0.1-0.3 bar and acetates are evolved along with water, some carbon dioxide and some MEG. Means well known in the art are employed to remove the acetic-rich overhead stream23from the vessel11.

The composition of the overhead stream23from the reaction vessel11is primarily a function of temperature and pressure (seeFIGS. 3 & 4). Ideally, the overhead stream23contains the maximum quantity of acetic acid and a minimum quantity of MEG. The results reported below inFIGS. 3 & 4can be applied to determine an optimum initial pH, temperature and pressure regime whereby the maximum yield of acetic acid is combined with a reduced yield of MEG in the stream23.

In the system ofFIG. 1A, the overhead stream23is condensed and the condensed stream25(low pH<3) may be routed to water treatment equipment or neutralized in a knock-out drum. In the system ofFIG. 1B, the overhead stream23is condensed and the condense stream25is routed directly to the distillation column where it is mixed with a MEG-water overhead stream from the reclaimer. The acetates partition between the aqueous phase (produced water) and the lean MEG phase. Routine experimentation can be used to determine the partitioning of acetic acid between water and lean MEG to determine the preferred routing of acetic acid-MEG-water stream from the acetate removal process.

Referring now toFIG. 2, separate vessels31,33are used for calcium removal and carboxylates removal, respectively. During simultaneous production of calcium and carboxylic acids the calcium (and other divalent cations) are precipitated in vessel31at approximately 80° C. and 1.0 bar by raising the pH using precipitating means19and filtering the resulting calcium carbonates stream27. The centrate/filtrate steam29is acidified using acidifying means21at 80-100° C. and at sub-atmospheric pressure (about 0.1 to 0.3 bar) in vessel33to remove the organic acids and water as an overhead stream23. The calcium-free and organic acid-free MEG can be returned to the production process.

However, the same calcium removal process as that described for vessel11(seeFIGS. 1A and 1B) is employed within vessel31, as is the same acetate removal process for vessel33.

Simulated Results

Simulated results were obtained employing OLI STREAM ANALYZER™ software (OLI Systems, Inc., Cedar Knolls, N.J. A model feed representing a high pH, 50% MEG solution with 3 wt % dissolved sodium acetate and excess sodium hydroxide and sodium bicarbonate was reacted with hydrochloric acid (as HCl) to reduce the pH to 3.5-5.5. The temperature of the solution and the reaction pressure were adjusted and the composition of the predicted overhead stream was calculated. The acetate content of the reaction mixture was fixed at 30 kg sodium acetate.

FIG. 3shows acetate removed as a function of temperature and pressure. The highest levels of acetate are removed at elevated temperature and low pressure.FIG. 4shows that MEG losses are highest at elevated temperature and reduced pressure. Therefore, an optimum temperature/pressure condition is required to maximize acetate removal while reducing or minimizing MEG losses to acceptable levels.

FIG. 5plots acetate removal against MEG loss for three temperatures (80, 90 and 100° C.). Lower temperatures and low pressure are preferred to maximize acetate removal.FIG. 6plots MEG loss as a function of acetate rejected from the MEG Loop. Using simple blowdown, 950 kg of MEG will be ejected with every 30 kg of sodium acetate (44 kg MEG per kg of acetate).

FIG. 7plots acetate removal as a function of temperature and pressure for three initial pH conditions: 3.5, 4.5, 5.5). Acetate removal efficiency is increased as the starting solution pH is reduced. For a starting pH of 3.5, 89% of the acetate in the reaction vessel is removed in the overhead stream at 100° C. and 0.1 bara compared to 48.3% for an initial solution pH of 4.5 and 8.8% for an initial solution pH of 5.5.

Using the acidification/vaporization scheme practiced according to this invention can significantly reduce this MEG loss. At 80° C. and 0.15 bar, the simulation software predicts 73.8 kg of MEG in the overhead stream along with 8.75 kg of acetic acid (8.58 kg MEG per kg acetate). At 80° C. and 0.15 bar only 40% of the acetate is removed per batch with the acetate remaining in the liquid phase being routed back to the reclaimer for re-processing.

Experimental Results

The apparatus used in the test is shown inFIG. 8. A double skinned5L glass reactor vessel101—fitted with a pH probe102(a Hamilton Polilyte Plus ARC 425), a dissolved oxygen probe103(Hamilton Oxygold G ARC 425), a RedOx probe104(Hamilton Polilyte Plus ORP ARC 425), a electrical conductivity probe105(Hamilton Conducell 4USF ARC PG425), and a stirrer106—was heated by means of a hot oil circulator bath and heater107. The reactor vessel101was connected to a condenser108and a distillate collection vessel109. The reactor vessel101, condenser108, and distillate collection vessel109were evacuated using a vacuum pump110, a pressure control valve111and a pressure transducer112. An oxygen-free nitrogen supply113was connected to the reactor vessel101to provide gas blanketing function.

A MEG-water-acetic acid solution was prepared in the reactor vessel101by adding 93 g of acetic acid (99-100% ex Sigma-Aldrich) to a mixture of monoethyleneglycol (1,737 g, Uninhibited CoolFlow MEG ex Hydra Technologies Limited, Fforestfach, Swansea SA5 4AJ, UK) and de-ionised water (1,710 g). The pH of this solution was measured as 2.61 at 23° C. The acetate content of the solution was calculated as 2.58 wt %.

The pH of the test mixture was elevated to 10.2 by addition of 100 g of anhydrous sodium carbonate (ex Sigma-Aldrich) and 1 g of sodium hydroxide pellets (ex Sigma-Aldrich). At this high pH, the acetic acid is converted to sodium acetate which is non-volatile and which would not be removed from the reaction vessel by elevating the temperature and reducing the operating pressure.

In conventional MEG loops the organic acids in the formation water and condensed water are usually present as the sodium salt:

In order to effectively remove the dissolved acetate the pH of the solution was reduced from 10.2 to 3.5 by addition of 70 g of 37 wt % hydrochloric acid solution (Sigma-Aldrich). The pressure in the reactor vessel101was reduced to 0.15 barA and the temperature was raised to 80° C. The reactor vessel101was held at 0.15 barA/80° C. for approximately 3.3 hours.

The reactor vessel101was allowed to cool and the residue in the vessel101and the distillate collected in the MEG-water collection vessel109were weighed and analyzed. The results are shown in Table 1 below.

Table 1 shows that the MEG level in the reactor vessel101rises from 53.6 wt % to 93.7 wt % as the water component is removed in preference to the less volatile MEG at low pressure and elevated temperatures. Table 1 also shows that the acetate component in the reactor vessel101is also removed in preference to the MEG component and that the acetate content of the distillate (predominantly water) is higher (29,093 mg/L) than in the original reactor mixture (measured at 20,994 mg/L, calculated from starting composition at 25,396 mg/L).

Based on a total final acetate measurement of 78.44 g (32.75 g remaining in the reactor vessel101plus 45.69 g collected in the distillate), it is calculated that 58.3% in the acetate present as sodium acetate in the reactor vessel at pH=10.2 was removed as acetic acid by reducing pH to 3.5 then reducing the pressure to 0.15-0.17 barA and raising the temperature to 80° C.

Experimental results and predicted values from the OLI STREAM ANALYZER™ are shown in Table 2, below. The OLI model predicts 70% removal of acetate at 80oC/1.5 barA.

Summary

The above preferred embodiments of a system and method made and practice according to this invention are not all possible embodiments. The claims listed below define the scope of the invention, including equivalents to the elements listed.