Abstract:
A MEG stream having a first pH level is contacted with a CO 2 -rich gas stream to yield a MEG product having a second different and lower pH level. The system and method can be readily incorporated into a slipstream MEG recovery package, with a source of the MEG stream being a MEG regeneration section of the package. The CO 2 -rich gas could be a vented CO 2  stream from the MEG reclamation section of the package. Unlike hydrochloric and acetic acid overdosing, CO 2  overdosing of the lean MEG stream does not lead to rapid acidification of the MEG product to be stored or injected.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application which claims priority to U.S. patent application Ser. No. 14/973,443, filed Dec. 17, 2015, which was a divisional application that claimed priority to U.S. patent application Ser. No. 14/500,295 filed on Sep. 29, 2014, U.S. Pat. No. 9,216,934, all of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Slipstream MEG recovery packages use a regeneration section to remove water from an incoming rich MEG feed stream and produce a lean MEG stream. A portion of this lean MEG stream is routed to a reclamation unit or section where the salt component is removed to yield a salt-free, pH neutral, lean MEG stream. This salt-free lean MEG stream is then blended with the remaining lean MEG stream to produce a lean MEG product having up to 3 wt % dissolved salts and available for re-injection into the gas production line as hydrate inhibitor. 
         [0003]    For gas fields where significant quantities of calcium and other divalent cations are present in the formation water, a calcium removal unit or section is located upstream of the regeneration section. The calcium is removed from the rich MEG stream by elevating the pH through the addition of sodium or potassium carbonates, hydroxides, or some combination thereof. The lean MEG exits the calcium removal section with an elevated pH, typically above 9.5. 
         [0004]    Because carbonate and hydroxide are often added in excess of the required stoichiometric quantity, un-reacted carbonate and hydroxide is carried through the regeneration system and into the lean MEG product. Removal of water from the rich MEG in the regeneration section further elevates the pH of the lean MEG product sent for reinjection. Mixing this high pH lean MEG with the calcium-rich formation water in the gas production pipeline can lead to increased scaling of the pipeline by precipitation of, for example, calcium carbonate. 
         [0005]    Therefore, a need exists to reduce the pH of the lean MEG product prior to injection and, in turn, mitigate pipeline scaling. Acidification of the lean MEG with hydrochloric acid (HCl) is an option but overdosing with hydrochloric acid can lead to rapid reduction in pH to levels at which corrosion of carbon steel pipework and vessels may occur. 
       SUMMARY 
       [0006]    A lean MEG stream having a first pH level (e.g., pH&gt;9.5) is contacted with a CO 2 -rich gas stream to yield a lean MEG product having a second different pH level, which may be in a range of 6.5 to 7.0. The CO 2 -rich gas could be a vented CO 2  stream from a MEG reclamation unit. 
         [0007]    Carbon dioxide has advantages to hydrochloric acid (HCl) and acetic acid (CH 3 CO 2 H) for pH control because overdosing with CO 2 —i.e., adding it in excess of the required stoichiometric quantity—does not lead to the reduction in pH observed with hydrochloric acid or the accumulation of acetates observed with acetic acid. 
         [0008]    Embodiments of this disclosure may reduce the pH of lean MEG product prior to injection, mitigate the potential for pipeline scaling, reduce or eliminate use of dosing with organic or inorganic acids to control the pH of the lean MEG product, may be less sensitive to overdosing conditions, and does not cause rapid reduction in pH levels when an overdose condition occurs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the above recited features can be understood in detail, a more particular description may be had by reference to embodiments, some of which are illustrated in the appended drawings, wherein like reference numerals denote like elements. It is to be noted, however, that the appended drawings illustrate various embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments. 
           [0010]      FIG. 1  is a schematic of an embodiment of a system and method of this disclosure. A vessel located downstream of a MEG regeneration section receives a high pH lean MEG stream and allows the steam to come into contact with a CO 2 -rich gas. 
           [0011]      FIG. 2  is a graph illustrating a lean MEG stream with alkalinity present as sodium carbonate as the stream is treated with CO 2 , acetic acid, and hydrochloric acid. 
           [0012]      FIG. 3  is a graph illustrating a lean MEG stream with alkalinity present as sodium hydroxide as the stream is treated with CO 2 , acetic acid, and hydrochloric acid. 
       
    
    
     ELEMENTS AND NUMBERING USED IN THE DRAWINGS 
       [0013]      10  Vessel 
         [0014]      15  Rich MEG steam (untreated stream) 
         [0015]      20  Lean MEG stream (treated stream) 
         [0016]      21  Portion of lean MEG stream  20   
         [0017]      30  MEG regeneration unit or section 
         [0018]    CO 2 -rich gas  40   
         [0019]    Lean MEG product exiting  10   
         [0020]      60  MEG reclamation unit or section 
         [0021]      61  Salt-free lean MEG stream 
         [0022]      70  Calcium removal unit or section 
       DETAILED DESCRIPTION 
       [0023]    In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0024]    In the specification and appended claims: the terms “connect”, “connection”, “connected”, “in connection with”, and “connecting” are used to mean “in direct connection with” or “in connection with via one or more elements”; and the term “set” is used to mean “one element” or “more than one element”. Further, the terms “couple”, “coupling”, “coupled”, “coupled together”, and “coupled with” are used to mean “directly coupled together” or “coupled together via one or more elements”. As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and downwardly”, “upstream” and “downstream”; “above” and “below”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the disclosure. 
         [0025]    Referring to  FIG. 1 , an embodiment of a system and method for adjusting a pH level of a lean MEG (treated) steam includes a vessel  10  which receives a lean MEG stream  20  from a lean MEG source such as a regeneration unit or section  30  of a slipstream MEG recovery package. Typically, stream  20  has a pH level above 9.5, as does rich MEG (untreated) stream  15  upstream of the regeneration section  30 . Within vessel  10 , this high pH lean MEG stream  20  comes into contact with a CO 2 -rich gas  40  (i.e., greater than 50% CO 2  content). Vessel  10  can be a contactor vessel of a kind known in the art. 
         [0026]    The CO 2  in gas  40  forms acidic solutions when dissolved in the MEG-water mixture of stream  20 , thereby reducing the pH. A lean MEG product  50  having a second lower pH exits the vessel  10 . In embodiments, product  50  may have a pH level of 6.5 to 7. No inorganic acids such as HCl or organic acids such as acetic or citric acid are used for reducing the pH to this level. 
         [0027]    The CO 2 -rich gas  40  can be from any source but, in some embodiments, is a vent stream from a reclamation unit or section  60  of the slipstream MEG recovery package. Similar to MEG regeneration section  30 , MEG reclamation section  60  is of a kind well-known in the art. 
         [0028]    A salt-free lean MEG stream  61  which exits the reclamation section  60  can be mixed with the lean MEG stream  20  prior to stream  20  entering vessel  10 . Additionally, a portion  21  of the lean MEG stream  20  which exits the regeneration section  30  can be routed to the reclamation unit  60 . 
         [0029]    In slipstream MEG recovery packages that make use of a calcium removal unit or section  70  upstream of the regeneration unit  30 , excess carbonate that finds its way into the reclamation section  60  degrades to form CO 2  (and hydroxide) under the elevated temperature, low pressure regime of a flash separator (not shown). 
         [0030]    Referring to  FIGS. 2 and 3 , unlike hydrochloric and acetic acid overdosing, CO 2  overdosing within vessel  10  does not lead to rapid acidification of the lean MEG product  50 . In a CO 2  overdosing condition, the pH level remains above 6 whereas in an acetic acid and hydrochloric acid overdosing condition the pH level falls below 4 and 2 respectively. Therefore, the system and method of this disclosure is less sensitive to overdosing conditions than prior art methods. 
         [0031]    As mentioned above, acidification with CO 2  removes the risk which occurs with inorganic acids (HCl) and the absence of carboxylates (acetate), namely, overdosing to the point of potentially damaging pH levels. In addition, carboxylates are highly soluble in MEG and are difficult to remove once added to the MEG system. The accumulation of carboxylates can lead to operational problems as the density and viscosity of the MEG increases with increasing carboxylate content. Hydrochloric acid converts readily to salt plus water; carbon dioxide converts to bicarbonate which is much more easily managed in the MEG system than carboxylates. Although the CO 2  reduces the pH, the ‘alkalinity’ (OH— plus HCO 3 — plus CO 2 ) is not reduced. 
         [0032]    To examine the “scaling” potential for the system and method, the following software simulation was run employing OLI Analyzer v 9.1.5 (OLI Systems, Inc., Cedar Knolls, N.J.). 
         [0033]    Starting solution: 90 wt % MEG (on salt-free basis) at 40° C. containing 30,000 mg/kg solvent  sodium chloride, 250 mg/kg solvent  of sodium carbonate and 25 mg/kg solvent  of sodium hydroxide. The pH of this mixture was 10.053 or about 10 (see Table 1, col. A, below). 
         [0034]    Acidification: The MEG solution was neutralized to pH=7.0 and to pH=6.5 using HCl acetic acid and CO 2 . Quantities of HCl, CH 3 CO 2 H and CO 2  added are shown in Table 1, rows 12-14, below. 
         [0035]    Scaling Test: Scaling potential of the acidified solutions was determined by adding in separate simulations MgCl 2 , CaCl 2 , FeCl 2 , SrCl 2  and BaCl 2  to the lean MEG solutions at the quantities shown in Table 1, rows 19-23. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Software Simulation of Scaling Potential. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                   
                   
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 3 
                 TEMP 
                   
                 40 
                 40 
                 40 
                 40 
                 40 
                 40 
                 40 
               
               
                 4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 5 
                 H2O 
                 g 
                 100,000 
                 100,000 
                 100,000 
                 100,000 
                 100,000 
                 100,000 
                 100,000 
               
               
                 6 
                 MEG 
                 g 
                 900,000 
                 900,000 
                 900,000 
                 900,000 
                 900,000 
                 900,000 
                 900,000 
               
               
                 7 
                 NaCl 
                 g 
                 30,000 
                 30,000 
                 30,000 
                 30,000 
                 30,000 
                 30,000 
                 30,000 
               
               
                 8 
                 Na2CO3 
                 g 
                 250 
                 250 
                 250 
                 250 
                 250 
                 250 
                 250 
               
               
                 9 
                 NaOH 
                 g 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
                 25 
               
               
                 10 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 11 
                 ACIDIFICATION 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 12 
                 HCl 
                 g 
                 0 
                 136 
                 — 
                 — 
                 160 
                 — 
                 — 
               
               
                 13 
                 CHCO2H 
                 g 
                 0 
                 — 
                 228 
                 — 
                 — 
                 279 
                 — 
               
               
                 14 
                 CO 2   
                 g 
                 0 
                 — 
                 — 
                 239 
                 — 
                 — 
                 478 
               
               
                 15 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 16 
                 pH 
                 — 
                 10.05 
                 7.01 
                 7.01 
                 7.01 
                 6.50 
                 6.50 
                 6.50 
               
               
                 17 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 18 
                 SCALING TEST POST  
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 ACIDIFICATION 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 19 
                 MgCl2 for Mg precipitation  
                 g 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 as Mg(OH)2 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 20 
                 CaCl2 for Ca precipitation  
                 g 
                 1.4 
                 840 
                 880 
                 250 
                 4300 
                 5300 
                 800 
               
               
                   
                 as CaCO3 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 21 
                 FeCl2 for Fe precipitation  
                 g 
                 0.1 
                 1.5 
                 1.7 
                 0.8 
                 6.7 
                 7.7 
                 2.4 
               
               
                   
                 as FeCO3 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 22 
                 SrCl2 for Sr precipitation  
                 g 
                 0.5 
                 650 
                 660 
                 200 
                 3900 
                 3950 
                 620 
               
               
                   
                 as SrCO3 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 23 
                 BaCl2 for Ba precipitation  
                 g 
                 0.2 
                 7 
                 13.7 
                 2.1 
                 35 
                 80 
                 6.3 
               
               
                   
                 as BaCO3 
               
               
                   
               
             
          
         
       
     
         [0036]    Results: For the starting solution (col. A, pH=10.0) precipitation of divalent cations as carbonate occurs on addition of 1.4 g of CaCl 2 . After acidification to pH 7.0 with HCl, the quantity of calcium chloride added before precipitation of CaCO 3  increases to 840 g from 1.4 g. The effect with acetic acid is similar with precipitation starting at 880 g of CaCl 2 . The equivalent scaling point with carbon dioxide occurs at 250 g, less than that for HCl or acetic acid but a considerable improvement on the 1.4 g for the untreated sample. 
         [0037]    Similar trends are observed for the other divalent cations (Fe, Sr, Ba) although some are more insoluble than others. Iron, in particular, tends to precipitate out readily. At pH=6.5 (col. E-G) the trends agree with those shown at pH=7.0 (col. B-D), i.e. precipitation of divalent cations (Ca, Fe, Sr, and Ba) from the lean MEG is inhibited by addition of CO 2  to the alkaline lean MEG mixture. 
         [0038]    Although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims.