Abstract:
Disclosed are methods and systems for reducing the risk of oxidation of hydrate inhibitors such as monoethylene glycol used to prevent the formation of hydrates in fluid handling systems including flow lines and pipelines. A hydrate inhibitor storage tank in fluid communication with the fluid handling system is used to hold a volume of liquid hydrate inhibitor, a volume of blanket gas and a layer of a liquid, nonflammable at operating conditions, therebetween. The liquid is immiscible with glycol.

Description:
FIELD 
     The present disclosure relates to methods and systems for inhibiting the formation of natural gas hydrates in fluid handling systems that convey natural gas. 
     BACKGROUND 
     Glycols and methanol are commonly used to inhibit the formation of natural gas clathrate hydrates in pipelines and flow lines in fluid handling systems which convey fluids containing natural gas from oil and gas production sites to oil and gas processing facilities. Glycol is also known as monoethylene glycol, MEG, ethylene glycol, ethylene alcohol, ethane-1,2-diol, and 1,2-ethanediol. Such hydrates are formed from hydrocarbon gases such as methane in the presence of free water at high pressures and low temperatures. Hydrates can accumulate to block pipelines and flow lines, thus impeding production. 
     In many circumstances, glycol is preferred as a hydrate inhibitor over methanol as glycol has lower flammability risk and is easy to recover and reuse. However, the recovery of glycol introduces potential for exposure to air and specifically to oxygen. Oxygen contamination in glycol systems increases corrosion risk in pipelines, flow lines and other exposed equipment. These risks increase as glycols oxidize and decompose to form organic acids, particularly at higher temperatures. The presence of oxygen can interfere with corrosion inhibitors, exacerbate corrosion rates of iron and many other metals by methods including carbon dioxide and hydrogen sulfide corrosion mechanisms, and encourage aerobic bacteria to grow which can further promote pitting of equipment. 
     One known method for managing these risks is to use nitrogen or a blanket of dry hydrocarbon gas to exclude oxygen from glycol storage tanks, sumps and other sources of oxygen ingress. The use of nitrogen as a blanket gas has not been effective to eliminate oxygen, as low-grade nitrogen can have an oxygen content of between 3 and 5 vol %. Even high purity nitrogen produced by the cryogenic distillation of air can have an oxygen content of up to 10 ppm. The use of a hydrocarbon blanket gas can be more effective at eliminating oxygen, but introduces flammability and explosion risks. 
     Another known method for managing these risks is to use chemicals such as oxygen scavengers and oxidation inhibitors. However, such methods have been developed based on pure water systems which raise concerns regarding efficacy in the presence of glycol due to reaction kinetics. 
     It would be desirable to maintain an oxygen content in the process stream of less than 20 ppb to avoid problems with corrosion, pitting, cracking and the like. Such a low level has been difficult to achieve, monitor and enforce. 
     SUMMARY 
     In one aspect, a method is provided for reducing the risk of oxidation of glycol in fluid handling systems that use glycol as a hydrate inhibitor. The method includes providing a tank in fluid communication with the fluid handling system for storing a hydrate inhibitor comprising glycol; and providing a layer of a liquid in direct contact with a surface of the hydrate inhibitor in the tank wherein the liquid is immiscible with glycol and is nonflammable at operating conditions. 
     In another aspect, a system is provided which includes a tank in fluid communication with a fluid handling system for storing a volume of hydrate inhibitor comprising glycol; and a layer of liquid in direct contact with the volume of hydrate inhibitor in the tank wherein the liquid is immiscible with glycol and is nonflammable at operating conditions. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become better understood with reference to the following description, appended claims and accompanying drawings where: 
         FIG. 1  is a simplified view of a prior art system. 
         FIG. 2  is a simplified view of a system according to one exemplary embodiment. 
         FIG. 3  is a simplified view of a system according to one exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a simplified view of a prior art system in which glycol 2, also referred to interchangeably as hydrate inhibitor, monoethylene glycol, MEG, aqueous solution or aqueous glycol solution, is stored as a volume of liquid in a tank  4 . The tank  4  operates at atmospheric pressure. The MEG 2 can be fed into the tank  4  from a source of hydrate inhibitor (not shown) through hydrate inhibitor conduit  1 . The MEG 2 can be stored at atmospheric pressure. Above the volume of MEG  2 , an upper space in the tank  4  contains a volume of blanket gas  3 . Blanket gas  3  can be fed into the tank  4  from a source of the blanket gas (not shown) through a blanket gas inlet conduit  5 . The tank  4  allows blanket gas  3  to pass in and out of the tank  4  in order to prevent failure of the tank  4  due to either overpressure or vacuum as the liquid level changes. As MEG 2 enters and leaves the tank  4 , the level of MEG 2 rises and falls. As the MEG level rises, valve  7  opens to allow the blanket gas  3  to leave the tank  4 . Upon leaving the tank  4 , the blanket gas  3  is directed to a vent  8  and a flame arrestor  9 . A blanket gas inlet valve  6  in the blanket gas inlet conduit  5  can be provided to control the introduction or flow of blanket gas  3  into the tank  4  as the volume of hydrate inhibitor in the tank decreases. A blanket gas outlet valve  7  in the blanket gas outlet conduit  13  can control the exit flow of blanket gas  3  from the tank  4  as the volume of hydrate inhibitor  2  in the tank  4  increases. A blanket gas outlet conduit  13  can connect the blanket gas  3  in the tank  4  with a vent  8  and a flame arrestor  9 . The vent  8  routes the blanket gas  3  to a safe location. The flame arrestor  9  is provided to prevent ignition of the blanket gas  3 . 
     In one embodiment, a volume of liquid hydrate inhibitor is stored in a tank in fluid communication with a fluid handling system which conveys fluids containing natural gas from oil and gas production sites to gas processing facilities. The concentration of the glycol solution used depends on particular operating conditions, particularly ambient seabed temperature. Glycol solution at too high a concentration will become very viscous at low temperature and thus too difficult to pump. In one embodiment, the hydrate inhibitor is an aqueous solution containing from 15 to 95 vol % glycol. The MEG used for hydrate inhibition is typically referred to as “lean MEG” and typically contains from 70 to 90 wt % glycol to meet viscosity and freezing point limits. The returning MEG is referred to as “rich MEG” and due to uncertainties in predicting water content, e.g., injection when no or little water is actually present, such as during start-up, the concentration can vary significantly. In one embodiment, the hydrate inhibitor is a returning MEG having been used for hydrate inhibition containing from 15 to 95 vol % glycol, even from 15 to 20 vol % glycol. 
     In one embodiment, between and in direct contact with both the volume of blanket gas  3  and the volume of liquid MEG 2 is disposed a immiscible liquid layer  10  also referred to as a layer of a nonflammable liquid  10 . The immiscible liquid is immiscible with glycol. The immiscible liquid is suitably a nonflammable liquid at operating conditions, e.g., temperatures up to 65° C., even from −10° C. to 65° C. (minus 10° C. to plus 65° C.). The immiscible liquid layer  10  can be used in addition to or as an alternative to the use of nitrogen or dry hydrocarbon blanket gas. The immiscible liquid layer  10  advantageously reduces the risk of oxidation or degradation of the glycol 2 by inhibiting mass transfer of oxygen from the blanket gas  3  to the glycol 2. 
     In one embodiment, the layer of the liquid  10  has an equilibrium solubility for oxygen that is higher than the equilibrium solubility for oxygen of the aqueous glycol solution  2 . As a result, the layer of the liquid  10  preferentially absorbs oxygen from the aqueous glycol solution  2 , thus minimizing the oxygen content within the glycol 2. In this case, oxygen can move from the hydrate inhibitor  2  and/or the blanket gas  3  to the layer of liquid  10  at a sufficiently high mass transfer rate such that oxygen is absorbed by the liquid  10  preferentially over being absorbed by the hydrate inhibitor  2 . 
     In an alternative embodiment, the layer of the nonflammable liquid  10  is nonflammable at operating conditions and has an equilibrium solubility for oxygen that is lower than the equilibrium solubility for oxygen of the aqueous glycol solution  2 . In this case, the liquid  10  inhibits oxygen mass transfer to the aqueous glycol solution  2  because the liquid layer  10  does not absorb oxygen, or oxygen moves from the blanket gas  3  to the layer of liquid  10  at a sufficiently low mass transfer rate such that the liquid inhibits oxygen mass transfer to be glycol 2. Therefore contact between the glycol 2 and any oxygen present above the liquid layer  10  is minimized 
     The immiscible liquid layer  10  can be provided by any suitable method. In one embodiment, as shown, the immiscible liquid is fed from a source of immiscible liquid (not shown) through a conduit  11  also referred to as a liquid conduit  11  to the layer of nonflammable liquid  10 . The immiscible liquid layer  10  forms directly above and in contact with the MEG 2. There are numerous options; temporary tank, feed from the plant. 
     In one embodiment, a thickness detector  17  can be provided in a location such that the thickness detector can measure a thickness of the layer of liquid  10 . The thickness detector  17  can utilize at least one of radar, bubbler and float technologies. 
     In one embodiment, a processor  15  can be provided in communication with the thickness detector  17  for receiving thickness information from the thickness detector  17  and comparing the thickness of the layer of nonflammable liquid  10  measured by the thickness detector  17  with a predetermined target thickness of the layer of liquid  10 . Furthermore, a controller  14  can be provided in communication with the processor  15  for controlling the addition and removal of nonflammable liquid to or from the layer of liquid  10  in order to achieve the predetermined target thickness. 
     In one embodiment, a chemical composition analysis device  18  can be provided at a location such that the chemical composition analysis device  18  can detect the chemical composition of the layer of nonflammable liquid  10 . In one embodiment, a processor  16  can be provided in communication with the chemical composition analysis device  18  for receiving chemical composition information from the chemical composition analysis device  18  and comparing the chemical composition of the layer of nonflammable liquid  10  with a predetermined desired chemical composition of the layer of liquid  10 . 
     In one embodiment, the tank can be located within the structure or legs of an offshore platform associated with offshore oil and gas production. In the system illustrated in  FIG. 3 , in one embodiment, an offshore platform  32  includes a topsides structure  34  and a substructure  40 , also referred to herein as structure or legs  40 , partially submerged beneath sea level  30 . At least a portion of the structure or legs  40  may be used as the MEG tank  36 . The system further includes at least a riser  38 , a flowline  42  for injection of hydrate inhibitor, a wellhead  44  and a reservoir  48  beneath the earth&#39;s surface  46 . 
     It should be noted that only the components relevant to the disclosure are shown in the figures, and that many other components normally part of a glycol unit are not shown for simplicity. 
     Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention. 
     From the above description, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.