Abstract:
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.

Description:
CROSS-REFERENCE To RELATED APPLICATIONS 
       [0001]    This application is a divisional application which claims priority to U.S. patent application Ser. No. 14/051,138 filed on Oct. 10, 2013, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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). 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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. 
         [0010]    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 
       [0011]    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. 
         [0012]    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 acetate-rich overhead stream from the reaction vessel. The acidification of the cooled and diluted MEG stream occurs under a vacuum. 
         [0013]    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. 
         [0014]    Preferably, the cooling and diluting means results in the incoming MEG stream to be 50wt % 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. 
         [0015]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1A  is a preferred embodiment of a system and process practiced according to this invention. A single reaction vessel is used for the removal of calcium and carboxylates from a mono ethylene (MEG) water stream. The reaction vessel swings or cycles between those two removal modes. The acetate-rich overhead stream is routed directly to a knock-out drum. 
           [0017]      FIG. 1B  is another preferred embodiment of a system and process practiced according to this invention. A single reaction vessel is used for the removal of calcium and carboxylates from a mono ethylene (MEG) water stream. Similar to  FIG. 1A , the vessel cycles between those two removal modes. The acetate-rich overhead stream is routed directly to a distillation column where it is mixed with a MEG-water overhead stream from a reclaimer. 
           [0018]      FIG. 2  is yet another preferred embodiment of a system and process practiced according to this invention. One reaction vessel is used for the removal of calcium from the MEG-water stream and another reaction vessel is used for the removal of carboxylates. The high pH stream generated in the calcium removal vessel is filtered to remove solids and then treated with hydrochloric acid in the acetate removal vessel operated at sub-atmospheric pressure. 
           [0019]      FIGS. 3 to 7  show simulated results of a system and process made according to  FIGS. 1A, 1B and 2  for acetate removal from a model solution where solution pH was reduced to 4.5 prior to raising temperature and reducing pressure. 
           [0020]      FIG. 3  is a graph of acetate removal as a function of temperature and pressure. 
           [0021]      FIG. 4  is a graph of MEG losses in the reaction vessel overheads as a function of temperature and pressure. 
           [0022]      FIG. 5  is a graph of acetate removal against MEG loss for three temperatures (80, 90 and 100° C.). 
           [0023]      FIG. 6  is a graph of calculated MEG loss per kg of acetate rejected as a function of operating pressure. 
           [0024]      FIG. 7  is a graph of calculated acetate removed from the reaction vessel as a function of initial pH (3.5, 4.5, 5.5), temperature (60-100° C.), and operating pressure (0.1-0.3 bara). 
           [0025]      FIG. 8  is schematic of an apparatus used to test preferred embodiments of the system and process of this invention. 
           [0026]    Elements and Element Numbering Used in the Drawing Figures 
           [0027]      10  System 
           [0028]      11  Reaction vessel 
           [0029]      13  Incoming carboxylates-rich MEG stream 
           [0030]      15  Water stream 
           [0031]      17  Cooled and diluted MEG water stream 
           [0032]      19  Precipitating means 
           [0033]      21  Acidifying means 
           [0034]      23  Acetate-rich overhead stream 
           [0035]      25  Condensed acetate-rich overhead stream 
           [0036]      27  Calcium carbonates bottoms stream 
           [0037]      29  Filtered calcium carbonates bottoms stream 
           [0038]      101  Reactor vessel 
           [0039]      102  pH probe 
           [0040]      103  Dissolved oxygen probe 
           [0041]      104  Redox probe 
           [0042]      105  Electrical conductivity probe 
           [0043]      106  Stirrer 
           [0044]      107  Hot oil circulator bath and heater 
           [0045]      108  Condenser 
           [0046]      109  Distillate collection vessel 
           [0047]      110  Vacuum pump 
           [0048]      111  Pressure control valve 
           [0049]      112  Pressure transducer 
           [0050]      113  Oxygen-free nitrogen supply 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]    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. 
         [0052]    Referring to the drawings and first to  FIGS. 1A  and B, a system  10  for removing organic carboxylates like acetates from a MEG water stream includes a reaction vessel  11  as 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 vessel  11  is closely coupled to the recycle loop of the package. 
         [0053]    The incoming carboxylate-rich MEG stream  13  is typically at 80-90 wt % MEG and high ph (&gt;9.5). The incoming MEG stream  13  is cooled and diluted with water  15  to yield a cooled and diluted MEG water stream  17  which enters and resides within reaction vessel  11  as a MEG water mixture. The cooled and diluted MEG stream  17  is preferably at a temperature of about 80-100° C. and 50 wt % MEG. 
         [0054]    The reaction vessel  11  can 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. 
         [0055]    When reaction vessel  11  is in a calcium removal mode or cycle, the vessel  11  operates at atmospheric pressure and removes calcium and other divalent cations from the incoming MEG water stream by elevating pH. Precipitating means  19  such as sodium or potassium carbonate or sodium or potassium hydroxide are introduced to the reaction vessel  11 . 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 stream  27 . 
         [0056]    When reaction vessel  11  is in an acetate removal mode, the vessel  11  operates 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 stream  17  is acidified within the reaction vessel  11  using acidifying means  21  such 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 vessel  11  is 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 acetate-rich overhead stream  23  from the vessel  11 . 
         [0057]    The composition of the overhead stream  23  from the reaction vessel  11  is primarily a function of temperature and pressure (see  FIGS. 3 &amp; 4 ). Ideally, the overhead stream  23  contains the maximum quantity of acetic acid and a minimum quantity of MEG. The results reported below in  FIGS. 3 &amp; 4  can 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 stream  23 . 
         [0058]    In the system of  FIG. 1A , the overhead stream  23  is condensed and the condensed stream  25  (low pH&lt;3) may be routed to water treatment equipment or neutralized in a knock-out drum. 
         [0059]    In the system of  FIG. 1B , the overhead stream  23  is condensed and the condense stream  25  is 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. 
         [0060]    Referring now to  FIG. 2 , separate vessels  31 ,  33  are 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 vessel  31  at approximately 80° C. and 1.0 bar by raising the pH using precipitating means  19  and filtering the resulting calcium carbonates stream  27 . The centrate/filtrate steam  29  is acidified using acidifying means  21  at 80-100° C. and at sub-atmospheric pressure (about 0.1 to 0.3 bar) in vessel  33  to remove the organic acids and water as an overhead stream  23 . The calcium-free and organic acid-free MEG can be returned to the production process. 
         [0061]    However, the same calcium removal process as that described for vessel  11  (see  FIGS. 1A and 1B ) is employed within vessel  31 , as is the same acetate removal process for vessel  33 . 
       Simulated Results 
       [0062]    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. 
         [0063]      FIG. 3  shows acetate removed as a function of temperature and pressure. The highest levels of acetate are removed at elevated temperature and low pressure.  FIG. 4  shows 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. 
         [0064]      FIG. 5  plots 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. 6  plots 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). 
         [0065]      FIG. 7  plots 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. 
         [0066]    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 
       [0067]    The apparatus used in the test is shown in  FIG. 8 . A double skinned 5L glass reactor vessel  101 —fitted with a pH probe  102  (a Hamilton Polilyte Plus ARC 425), a dissolved oxygen probe  103  (Hamilton Oxygold G ARC 425), a redox probe  104  (Hamilton Polilyte Plus ORP ARC 425), a electrical conductivity probe  105  (Hamilton Conducell 4USF ARC PG425), and a stirrer  106 —was heated by means of a hot oil circulator bath and heater  107 . (Although probes  103 ,  104  and  105  were fitted to the reactor vessel  101 , those probes were not used to collect data during the experiments.) The reactor vessel  101  was connected to a condenser  108  and a distillate collection vessel  109 . The reactor vessel  101 , condenser  108 , and distillate collection vessel  109  were evacuated using a vacuum pump  110 , a pressure control valve  111  and a pressure transducer  112 . An oxygen-free nitrogen supply  113  was connected to the reactor vessel  101  to provide gas blanketing function. 
         [0068]    A MEG-water-acetic acid solution was prepared in the reactor vessel  101  by 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 %. 
         [0069]    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. 
         [0070]    In conventional MEG loops the organic acids in the formation water and condensed water are usually present as the sodium salt: 
         [0000]      2CH 3 CO 2 H (aq)+Na 2 CO 3 (aq)→2CH 3 CO 2 Na+CO 2 (aq)+H 2 O
 
         [0000]      CH 3 CO 2 H(aq)+NaOH(aq)→CH 3 CO 2 Na+H 2 O
 
         [0071]    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 vessel  101  was reduced to 0.15 barA and the temperature was raised to 80° C. The reactor vessel  101  was held at 0.15 barA/80° C. for approximately 3.3 hours. 
         [0072]    The reactor vessel  101  was allowed to cool and the residue in the vessel  101  and the distillate collected in the MEG-water collection vessel  109  were weighed and analyzed. The results are shown in Table 1 below. 
         [0073]    Table 1 shows that the MEG level in the reactor vessel  101  rises 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 vessel  101  is 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). 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Analysis of Reactor and Distillate Vessel Contents 
               
             
          
           
               
                   
                   
                 MEG 
                 Estimated 
                 Acetate 
                 Acetate 
               
               
                   
                 Inven- 
                 by GC 
                 Density 
                 by IC 
                 Calculated 
               
               
                   
                 tory 
                 [Note 1] 
                 [Note 2] 
                 [Note 3] 
                 [Note 4] 
               
               
                   
                 (g) 
                 (wt %) 
                 (g/L) 
                 (mg/L) 
                 (g) 
               
               
                   
                   
               
             
          
           
               
                 Reactor @ 
                 3,781 
                 53.6 
                 1,065 
                 20,964 
                 75.49 
               
               
                 t = 0 
               
               
                 Reactor @ 
                 2,079 
                 93.7 
                 1,109 
                 17,326 
                 32.75 
               
               
                 t = 3.33 hrs 
               
               
                 Distillate @ 
                 1,580 
                 6.7 
                 1,006 
                 29,093 
                 45.69 
               
               
                 t = 3.33 hrs 
               
               
                   
               
               
                 [Note 1]: 
               
               
                 MEG content measured by Gas Chromatography 
               
               
                 [Note 2]: 
               
               
                 Density estimated as a function of MEG: water: dissolved salt at 20° C. 
               
               
                 [Note 3]: 
               
               
                 Acetate content measured using Ion Chromatography 
               
               
                 [Note 4]: 
               
               
                 Acetate(g) = Acetate (mg/L) × Inventory (g)/Density (g/L) 
               
             
          
         
       
     
         [0074]    Based on a total final acetate measurement of 78.44 g (32.75 g remaining in the reactor vessel  101  plus 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. 
         [0075]    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. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Experimental acetate removal compared with simulated results 
               
             
          
           
               
                   
                 pH 
                 TEMP 
                 PRESSURE 
                 ACETATE 
               
               
                   
                 — 
                 deg C. 
                 barA 
                 REMOVED % 
               
               
                   
                   
               
             
          
           
               
                 OLI Prediction 
                 3.50 
                 80 
                 0.15 
                 70.0 
               
               
                 Experimental 
                 3.34-3.78 
                 66-80 
                 0.14-0.17 
                 58.3 
               
               
                   
               
             
          
         
       
     
       SUMMARY 
       [0076]    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.