Abstract:
The seeding of calcium carbonate into a calcium chloride-contaminated monoethylene glycol (MEG) stream accelerates the growth of calcium carbonate particles to a size that enhances their removal from the stream by filtration. A seeding vessel allows the calcium carbonate particles a time period to grow. Sodium carbonate may be added to the contaminated stream to facilitate calcium carbonate particle growth. A recycle seeding conduit may recycle seeds from a filtration unit to the seeding vessel. A base such as sodium hydroxide may be added to accelerate the precipitation process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to methods and apparatus for reclaiming or recovering glycols, and more particularly relates, in one non-limiting embodiment, to methods and apparatus for reclaiming or recovering monoethylene glycol by removing salts therefrom. 
         [0002]    For several decades monoethylene glycol (MEG) and methanol have been the primary chemicals injected offshore to inhibit the formation of hydrates in oil and gas production pipelines and related facilities. At fields that require persistent inhibition, the cost of replacing hydrate inhibitor chemical that is lost to the gas and hydrocarbon liquid product streams is a determining factor in selecting the inhibitor. Methanol solubility in gas and liquid hydrocarbon product streams may be two or more orders of magnitude higher than MEG solubility. This creates a strong economic incentive to use MEG despite the greater quantity of MEG needed per degree of hydrate temperature suppression. Further, methanol is an unwanted contaminant in hydrocarbon sales products. 
         [0003]    Nevertheless, the adoption of MEG over methanol has taken some time to occur, due in part to familiarity with methanol and perceptions in operating difficulties in recovering and recycling MEG. There have been concerns about detrimental effects that saline formation water had on conventional MEG reconcentrator units. Salts are non-volatile, and they will remain in the lean MEG during regeneration as the water is boiled off. If salt enters the MEG system with production fluids, either continuously or in periodic slugs, its concentration in the system will increase during each regeneration cycle until the solubility limit is finally reached and precipitation occurs. Unless MEG is reclaimed by removing the salt, serious fouling and plugging of equipment and flowlines may result. Depending on the magnitude of the contamination problem, MEG reclamation may be accomplished by either continuous or intermittent (i.e. batch) removal from either the total circulation stream or else a partial stream. 
         [0004]    Many MEG reclamation projects involve the removal or reduction of alkali metal salts such as calcium chloride (CaCl 2 ) from the contaminated MEG stream. In some reclamation efforts, the salts are converted to carbonates which may be removed as precipitated particulates. Current or conventional removal methods used include centrifuges, diatomaceous earth (DE) filtration and separation tanks with relatively long retention times (e.g. on the order of 10 hours). 
         [0005]    It would be desirable if methods and apparatus were devised that could easily remove salts and/or carbonates from MEG streams with greater efficiency than at present. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    There is provided, in one non-restrictive form, an exemplary MEG reclamation apparatus that includes a seeding vessel with a vessel inlet and a vessel outlet, where the vessel inlet is adapted to receive a mixture from a mixture conduit. The apparatus further includes a filtration unit having a feed inlet, a solids outlet, a recycle outlet and a permeate outlet. Also included in the apparatus is a seeded effluent conduit from the vessel outlet to the feed inlet on the filtration unit, as well as a recycle conduit from the recycle outlet of the filtration unit to the mixture conduit. 
         [0007]    In another non-restrictive example, there is provided a method for reclaiming MEG that includes contacting an alkali metal salt-contaminated MEG stream with an alkali metal carbonate and a base (accelerator) to form a mixture. The mixture may be contacted with recycled alkali metal carbonate seeds to form a feed. The feed is routed to a seeding vessel for growing alkali metal carbonate particles. A seeded effluent of relatively large particles is transmitted from the seeding vessel to a filtration unit. Alkali metal carbonate solids are discharged from the filtration unit. A relatively reclaimed MEG stream is produced from the filtration unit. Alkali metal carbonate seeds are recycled from the filtration unit to the feed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]      FIG. 1  is a schematic illustration of a MEG reclamation system showing one non-limiting embodiment of the recycle seeding apparatus herein. 
       
    
    
       [0009]    It will be appreciated that the FIGURE is a schematic illustration that is not to scale or proportion, and, as such, some of the important parts of the invention may be exaggerated for illustration. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    In a MEG reclamation systems, sodium carbonate (Na 2 CO 3 ) is injected into a MEG stream containing an alkali metal salt such as calcium chloride (CaCl 2 ) to precipitate calcium carbonate (CaCO 3 ) from the MEG solution to reduce or eliminate CaCO 3  deposition on the heating element of a downstream MEG feed preheater prior to the MEG flash separator of a typical MEG reclamation system. However, the CaCO 3  particles formed are very small in size, on the order of 2-5 microns in average particle size and thus they are difficult to filter out. It has been discovered that by seeding the CaCO 3  in a MEG solution, the CaCO 3  will form crystals or larger particles which are easier to separate and filter out. 
         [0011]    Non-limiting exemplary methods and apparatus described herein enhance the removal of a salt from a MEG stream intermixed therewith by means of salting the contaminated stream with carbonate seeds, and then letting the carbonate particles grow in size in a seeding vessel. The relatively larger size of the carbonate particles makes it easier to subsequently separate them from the MEG stream to reclaim and recover a relatively purified, reclaimed MEG stream. 
         [0012]    The method and apparatus herein are expected to be useful in reclaiming any MEG stream contaminated with a salt, although the chemistry of creating the particles may be different depending on the salts involved. 
         [0013]    The MEG reclamation apparatus described herein may be part of a larger MEG reclamation system that may be used to recondition, reclaim or recover a relatively pure MEG stream. Such MEG reclamation systems may typically include a wet MEG surge drum, one or more wet MEG coalescing filters, a MEG feed preheater, a MEG flash separator, a salt handling system and at least one MEG distillation column, serially connected by lines or conduits transmitting a MEG stream in various stages of recovery. The MEG reclamation apparatus herein would replace the wet MEG coalescing filter(s). The stream egressing from the wet MEG surge drum would feed into the MEG reclamation apparatus as contaminated glycol stream  12  described herein, which in turn would feed the relatively pure MEG stream  40  (permeate) into the MEG preheater, where the heated stream would be fed to the MEG flash separator, where the flashed light components would go to the MEG distillation column. It should also be appreciated that it is not necessary for all of the contaminating salt(s) to be removed from the MEG stream for the apparatus and method described herein to be considered a success, although complete removal would certainly be a worthwhile goal. In one non-limiting embodiment, an objective herein is to remove CaCO 3  particles from the MEG stream to a concentration of 10 ppm or less. 
         [0014]    The overall MEG reclamation apparatus herein is generally referred to in  FIG. 1  as  10 . Contaminated MEG stream enters via line  12 , which in one non-limiting embodiment herein is MEG containing an undesirable level of calcium chloride (CaCl 2 ). To contaminated MEG stream  12  may be added sodium carbonate (Na 2 CO 3 ) via line  14  and a base, in one non-restrictive version an alkali metal hydroxide such as sodium hydroxide (NaOH) via line  16 . The base acts as an accelerator to enhance the precipitation process. These components may be optionally mixed in mixer  18  egressing to mixture conduit  20 . 
         [0015]    To mixture stream  20  is added recycle seeding stream from conduit  22 , which as will be explained is an effluent from the seeding vessel  24  or a seeded effluent of CaCO 3  particles to provide seeds for building relatively larger particles or crystals in seeding vessel  24 . Seeding vessel  24  has vessel inlet  26  and vessel outlet  28 , where the inlet  26  receives the mixture from mixture conduit  20 . Seeding vessel  24  may be preloaded with MEG containing formed CaCO 3  crystals. This preloading may occur before the start-up of system operations. The mixture remains in seeding vessel  24  to grow CaCO 3  particles to a relatively larger size. Conventionally, CaCO 3  particles are in the range of 2-5 microns in average particle size, whereas in the apparatus and method herein, the particles or crystals may grow to at least about 10 microns or larger, and in an alternate non-limiting embodiment may grow to at least about 15 microns, in another non-restrictive version to at least about 20 microns or larger, possibly 30 microns or larger, or even about 40 microns or larger. A typical flow rate through the seeding vessel  24  ranges from about 40 gallons per minute (gpm) to about 120 gpm (about 150 to about 455 liters per minute). In another non-restrictive version the residence time in seeding vessel  24  may range from about 5 to about 10 minutes, alternatively from about 20 independently to about 30 minutes. Seeding vessel  24  may be provided with solids drain line  25 . 
         [0016]    Seeded effluent conduit  30  containing effluent from seeding vessel  24  which effluent contains relatively larger particles, extends from vessel outlet  28  to filtration or separation unit  32  at feed inlet  34  via pump  29 . Solids (CaCO 3 ) are rejected at solids outlet  36 , such as during a flushing or cleaning operation described below, permeate outlet  38  egresses relatively pure MEG (permeate) stream  40 , where recycle seeding conduit  22  exits recycle outlet  42  provides a recycled seeded effluent of CaCO 3  seeds back to mixture in conduit  20  as previously described. In one non-restrictive embodiment, the size of the particles in the seeded effluent exiting outlet  42  is about 20 microns or higher. Solids outlet line  36  may also be used to back flush the filtration unit  32  when high delta pressure in the filtration unit  32  reaches 15 psig (103 kPa) or higher, in one non-limiting embodiment. The seeds or particles in this stream may also be about 20 microns or higher in average particle size. Filtration unit  32  may be a microfiltration unit or any other suitable filtration or separation unit including, but not necessarily limited to, any kind of cross-flow filter and the like. The filter elements for filtration unit  32  may be rated from about 5 to about 10 microns, in one non-limiting embodiment, alternatively from about 10 independently to about 15 in another non-restrictive version. 
         [0017]    There is optionally provided a large particle-containing seeded effluent recycle line  44  from the large particle-containing seeded effluent conduit  30  to the recycle seeding conduit  22 . Particle seeded effluent recycle line  44  is generally used only during preloading of the CaCO 3  particles into seeding vessel  24  when recirculation may be required. 
         [0018]    In the particular embodiment illustrated, the reaction may be schematically outlined as: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0019]    One non-limiting embodiment of the process for reclaiming MEG involves the following operations. 
       CaCO 3  Crystal Preparation Before Start Up 
       [0020]    The seeding vessel  24  will be loaded with predetermined concentration of MEG and CaCO 3  crystal. This mixture will be pumped through line  28 , pump  29 , valve  31  (which is normally closed during regular or normal operation) line  44 ,  20  and vessel inlet  26  into the seeding vessel  24 . The pumping will last approximately 10 to 20 minutes, in one non-limiting version. It should be noted that seeding vessel  24  may be sized for approximately 5 to 10 minutes retention time, in one non-limiting embodiment. 
       Normal Operation 
       [0021]    The MEG and CaCl 2  mixture will flow in line  12 . Na 2 CO 3  will be added through line  14 . NaOH (or other base) may be added in line  16  to accelerate the precipitation of CaCO 3 , if required. The mixture will be mixed by mixer  18  and flow through line  20  and vessel inlet  26 . 
         [0022]    The mixture of MEG and freshly precipitated particles (approximately 2-5 micron size) will flow into the seeding vessel  24 . The mixture flow will flow in the direction of the large arrow over the baffle  27  (more than one baffle may be present), the flow containing mainly MEG and some larger particles due to the seeding effect of the crystals. This MEG and the larger particles will be pumped by pump  29  through lines  30  and  34  to the filtration unit  32  (e.g. a cross-flow filter). In one non-limiting embodiment, the vessel will contain many filter elements. The mixture will flow inside the element(s) and the permeate (clean MEG) will flow through the wall(s) of the element(s). The more concentrated particles will be recycled back to the seeding vessel  24 . 
         [0023]    The permeate (MEG free from particles) will flow out line  40 . A portion of the mixture will be recycled and flow through lines  42 ,  22  and vessel inlet  26 . It should be noted that the recycled flow will contain more concentrated particles. When the pressure difference of the filtration unit  32  exceeds a predetermined pressure, in one non-restrictive embodiment about 15 psig (103 kPa) due to some of the solids adhering the filter elements, a purging or cleaning operation may be performed. 
       Purging Operation 
       [0024]    When pressure difference exceeded about 15 psig (103 kPa) e.g., pump  29  will stop. The filtration unit  32  will be drained through line  36  and open (normally closed) filtration unit drain valve  37 . During draining, all lines will be closed with the exception of line  36  and brine feed purge lines  48  and  50  and purge will be open via normally-closed valve  46  and normally closed drain valve  37 . A clean brine under pressure will flow through purge lines  48 ,  50  and inside the filter element(s) scrubbing the solids from the wall(s) of the filter element(s). The mixture of solids and brine will be directed to drain  36 . 
         [0025]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and is expected to be effective in providing methods and apparatus for reclaiming MEG streams more efficiently. However, it will be evident that various modifications and changes can be made thereto without departing from the broader scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, the seeding vessel may be changed or optimized from that illustrated and described, and even though they were not specifically identified or tried in a particular system, would be anticipated to be within the scope of this invention. For instance, the use of a different separation unit other than a microfiltration apparatus would be expected to find utility and be encompassed by the appended claims. Different contaminated MEG streams other than those described herein may nevertheless be treated and handled in other non-restrictive embodiments of the invention adapted by one having ordinary skill in the art for those streams. 
         [0026]    The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. 
         [0027]    The words “comprising” and “comprises” as used throughout the claims is to interpreted “including but not limited to”.