Abstract:
A hydrocarbon-containing gas is mixed with an oxygen gas in a gas mixer in the presence of coarse water droplet environment, e.g., a ‘rainy’ or ‘driving rainstorm’ environment in which the water droplets generally have a size greater than 200 microns SMD. The water droplets surround and contact entrained particles in either the oxygen gas stream or the hydrocarbon-containing gas stream. The water acts to suppress, prevent and quench ignition of the hydrocarbon gas in the mixer which would otherwise be caused by energetic collisions between such particles and structures within the gas mixer. In one configuration the gas mixer includes water pipes having coarse water droplet-producing nozzles at the peripheral end thereof concentrically located within oxygen supply pipes. Additionally, nozzles introduce coarse water droplets into a pipe carrying the hydrocarbon gas and forming a mixing chamber for the hydrocarbon and oxygen gases.

Description:
BACKGROUND 
       [0001]    This invention relates generally to gas mixers used in systems for gas-phase partial oxidation of hydrocarbon-containing gases. An example of where this invention has utility is systems for industrial production of ethylene oxide. 
         [0002]    The chemical compound ethylene oxide (chemical formula C 2 H 4 O) is an important industrial chemical used as an intermediate in the production of ethylene glycol (the main component of automotive antifreeze) and other chemicals. Ethylene oxide is also used as a sterilant for foods and medical supplies. It is a colorless flammable gas at room temperature, and can be cooled and stored as a liquid. 
         [0003]    Ethylene oxide first achieved industrial importance during World War I as a precursor to both ethylene glycol and the chemical weapon mustard gas. In 1931, Theodore Lefort, a 
         [0004]    French chemist, discovered a means to prepare ethylene oxide directly from ethylene and oxygen, using silver as a catalyst. Since 1940, almost all ethylene oxide produced industrially has been made using this method. 
         [0005]    In current industrial processes, ethylene oxide is produced when ethylene (CH 2 ═CFl 2 ) and oxygen (O 2 ) react on a silver catalyst at 200-300° C. showing large Ag nanoparticles supported on Alumina. Typically, chemical modifiers such as chlorine are also included. Pressures used are in the region of 1-2 MPa. The chemical equation for this reaction is: 
         [0000]      CH 2 ═CH 2 +½O 2 →C 2 H 4 O 
         [0006]    In ethylene oxide production systems, a gas mixer is used to mix the hydrocarbon-containing gas stream and the oxygen gas stream just upstream of the reaction chamber where the silver catalyst is present. The gas mixer is typically constructed in the form of a vessel or pipe. The vessel includes an inlet manifold for each of the two gases. The vessel is sometimes constructed with a main outer pipe containing the hydrocarbon gas stream and internal concentric tubes or “fingers” which contain the oxygen stream. Mixing occurs at the point where the internal tubes end, where the oxygen gas flowing out of the fingers meets the main stream of hydrocarbon gas flowing in the outer tube. This basic design is described in U.S. Pat. No. 3,706,534. 
         [0007]    The art has long recognized that there is a risk of ignition of a hydrocarbon-containing gas stream (e.g., a stream of gas containing for example ethylene mixed with other hydrocarbon gases) at the point where it is combined with an oxygen gas in a gas mixer. 
         [0008]    Ignition can occur when a particle (e.g. a piece of sand, rust or pipe scale) entrained in the hydrocarbon-containing gas stream or the oxygen gas stream strikes a metallic surface in the mixer, e.g., the wall of the mixer, thereby producing a spark. If the spark occurs in the hydrocarbon-containing gas stream in the highly flammable zone e.g., at, or close to, the point of mixing of the two gas streams, ignition can occur. The ignition may damage the gas mixer and also requires an interrupt of production to suppress the ignition and allow the gas mixer to cool before recommencing production. The flammable region is confined to the mixing zone of the two gases. The hydrocarbon-containing gas as well as the reactor feed blend are below the lower O 2  flammability limit—i.e., too rich to burn. The art has devised a variety of gas mixer designs. Some of the designs are specifically directed to reducing the risk of ignition of hydrocarbon-oxygen gas streams. The known prior art includes the following patent documents, in addition to the above-cited &#39;534 patent: U.S. Pat. No. 4,573,803; U.S. Pat. No. 3,702,619; U.S. Pat. No. 4,256,604; U.S. Pat. No. 4,415,508; U.S. Pat. No. 6,657,079; U.S. 2003/0021182; U.S. Pat. No. 3,518,284; U.S. U.S. Pat. No. 4,390,346; U.S. Pat. No. 3,237,923; U.S. Pat. No. 3,081,818; U.S. Pat. No. 2,614,616 and U.S. Pat. No. 6,840,256. 
         [0009]    Other prior art of interest include British patents GB 705,176 and 2,357,817; U.S. Pat. No. 5,336,791; and U.S. Pat. No. 4,393,817. 
       SUMMARY 
       [0010]    In a first aspect of this disclosure, industrial production systems for gas-phase partial oxidation of a hydrocarbon-containing gas are disclosed which use a method for mixing the hydrocarbon-containing gas with an oxygen containing gas. The method includes providing a gas mixer having a mixing chamber for mixing the oxygen containing gas with the hydrocarbon-containing gas, producing a coarse water droplet environment in the mixing chamber of the gas mixer, and mixing the oxygen containing gas and the hydrocarbon-containing gas in the presence of the coarse water droplet environment. The invention can be applied to hydrocarbon-air mixers and hydrocarbon-enriched air mixers. Hence, the term “oxygen containing gas” is intended to encompass a stream of a gas containing oxygen generally, such as for example a stream of pure or substantially pure oxygen gas, a stream of air, or a stream of air which is enriched with oxygen gas. Similarly, the hydrocarbon-containing gas may include a pure hydrocarbon gas, or a hydrocarbon gas mixed with other gases, e.g., recycle gas in an EO production scenario. 
         [0011]    In another aspect, an improvement to a gas mixer for an industrial production system for gas-phase partial oxidation of a hydrocarbon-containing gas is provided. The improvement is providing a means for producing a coarse water droplet environment in the gas mixer wherein the oxygen containing gas and the hydrocarbon-containing gas are mixed in a mixing chamber of the gas mixer in the presence of the coarse water droplet environment. Several examples of the means for producing the coarse water droplet environment are described, including atomizers (spray nozzles) which inject coarse water droplets into the mixing chamber, wherein the hydrocarbon gas stream flows through the coarse water droplet environment prior to meeting the oxygen gas stream. The design may also feature water pipes concentrically located within oxygen pipes carrying the supply of the oxygen gas to the mixing chamber and having nozzles at the end thereof which produce coarse water droplets wetting the oxygen gas stream prior to mixing with the hydrocarbon-containing gas stream. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic representation of a gas mixer for an industrial production system for catalytic gas-phase oxidation of a hydrocarbon-containing gas, showing a first embodiment of a means for introducing a coarse water droplet into the gas mixer in the form of (1) atomizers (nozzles) which inject coarse water droplets into one or more pipes carrying a hydrocarbon-containing gas stream and forming a mixing chamber for the two gases and (2) water pipes with coarse droplet-producing nozzles at the ends thereof concentrically located within oxygen pipes supplying oxygen to the mixing chamber. 
           [0013]      FIG. 2  is an end view of the hydrocarbon pipes of  FIG. 1  showing the nozzles injecting water into the hydrocarbon pipes and the nozzles placed within the oxygen pipes. 
           [0014]      FIG. 3  is an alternative embodiment showing a different arrangement of nozzles spaced around the circumference of the pipe carrying the hydrocarbon-containing gas. 
           [0015]      FIG. 4  is a more detailed illustration of the hydrocarbon and oxygen gas streams in the embodiment of  FIGS. 1-3 , showing the coarse water droplets created in both gas streams. 
           [0016]      FIG. 5  is an illustration of an alternative configuration of the gas mixer of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In industrial production systems for gas-phase partial oxidation of a hydrocarbon-containing gas, such as production of ethylene oxide, the mixing of hydrocarbon and oxygen gases in a safe, reliable manner is a continuing problem, particularly when the gases to be mixed go through a flammable zone in the mixing process. The features of this disclosure provide improvements to a gas mixer and method of mixing gases which minimizes the probability of ignition. The mixing of the two gases is performed in a coarse water droplet environment. The coarse water droplet environment can be conceptualized as a rainstorm-like environment in the gas mixer. In a high pressure, high capacity application in which substantial amounts of water are needed, a substantial volume of water droplets are introduced into the gas mixer at high velocity, in effect creating a driving rainstorm environment at the point where the two gases are mixed. 
         [0018]    Several different embodiments of a gas mixer featuring apparatus for producing the coarse water droplet environment will be described in some detail below. Applications include ethylene oxide production in a gas mixer featuring a low shear co-axial gas mixing. However, the invention can be practiced in a high shear gas mixer, such as described in the U.S. patent application of the present inventors, Ser. No. 61/007,790, entitled OXYGEN/HYDROCARBON RAPID (HIGH SHEAR) GAS MIXER, the entire content of which is incorporated by reference herein. 
         [0019]    The purpose of the coarse water droplet environment is to reduce the probability of ignition of the flammable gas envelope where the two gases initially mix, or to quench an ignition should one initiate, by introducing a sufficient quantity of coarse water drops (Sauter Mean Diameter (SMD) greater than 200 microns) into the gas streams at the point of the high flammability gas envelope so as to provide enhanced mixing, wetting of the surface of any entrained particles in either the hydrocarbon stream or the oxygen stream, and a heat sink to transfer any heat generated from particle impact or particle fracture while the particle is still present in the flammable region in the mixer. In general, the gas mixer features atomizers (coarse water droplet producing nozzles) which are designed to produce water drops having a size &gt;200 microns SMD. The term Sauter Mean Diameter (SMD) is used here to mean the diameter of a drop having the same volume/surface area ratio as the entire spray of the drops. 
         [0020]    Materials of construction of the gas mixer and the water droplet generating devices may be stainless steel, Monel, Inconel, or other corrosion and ignition resistant metal. Such metals may also be used in the highest velocity zones and the gas-distributing pipes. 
         [0021]    One application of the invention is direct oxidation ethylene oxide process mixers, which mix oxygen at intermediate pressure (˜20 bar) with recycled hydrocarbon-containing gas containing ethylene and other gases. Oxygen pressures are approximately ˜26 bar. The invention can similarly be used for other partial oxidation processes using pure oxygen or enriched air. 
         [0022]    The features of this disclosure redefines the oxygen/hydrocarbon mixing process to reduce the potential for ignition in the flammable gas envelope that exists for some distance downstream of the point of injection of oxygen into the hydrocarbon-rich stream prior to complete mixing of the oxygen-hydrocarbon stream. The invention accomplishes this by mixing the gases in the presence of a coarse water droplet environment, to provide a heat sink to dissipate the impact energy of entrained particles in either the hydrocarbon or oxygen gas streams and to quench an ignition should one occur. The invention is particularly useful for mixing oxygen into the recycle gas containing ethylene in an ethylene oxide process. 
         [0023]    The features of this disclosure provide a number of advantages and satisfy a long-felt need in the art. In particular, it allows for the injection of oxygen into a hydrocarbon-rich gas stream while minimizing the probability of igniting the gas. The advantage is particularly significant for a range of application in which gas mixing occurs at elevated pressures (e.g. 20 bar), which are commonly found in partial oxidation processes such as ethylene oxide production. 
       Example 1 
       [0024]      FIG. 1  is a schematic representation of a gas mixer featuring a coarse water droplet environment where the hydrocarbon and oxygen gases meet. The gas mixer  10  includes a hydrocarbon-containing gas manifold  12  receiving recycled gas containing hydrocarbons such as ethylene from a source along an inlet pipe  14 . One or more pipes  16  are connected to the hydrocarbon-containing gas manifold  12 . Gas mixing occurs in the pipes  16 , therefore the pipes  16  function as a mixing chamber for the gas mixer  10 . Mixed gases are collected in a second manifold  18 . 
         [0025]    The gas mixer  10  features a means for producing a coarse water droplet environment in the pipes  16 . In particular, water supply lines  20  are provided which supply water to atomizers (nozzles)  22 . The atomizers  22  are of a design to produce coarse water droplets having a SMD of at least  200  microns. Valves  24  are placed downstream of the atomizers  22 . Two or more atomizers  22  may be provided per pipe  16  and may be placed around the periphery of the pipe  16  as shown in  FIG. 3 . An alternative is to mount the spray nozzles  22  in the wall of the pipe  16  eliminating the valves  24 . The arrangement of the nozzles can take many forms, with the water droplet sprays being coaxial with the cycle gas flow or at some angle with respect to this flow. In any event, a coarse water droplet environment is produced in the hydrocarbon-containing gas pipes  16 . 
         [0026]    Oxygen is supplied to the gas mixer via an oxygen gas manifold  36 . Oxygen pipes  38 , sometimes referred to in the art as “fingers”, are connected to the manifold  36 . The oxygen pipes  38  are coaxially located within the hydrocarbon pipes  16 . Oxygen flows into the pipes  38  from the manifold  36  and flows out the distal end  35  of the pipes  38 . 
         [0027]    The mixer  10  further includes a water manifold  30  connected to a water source  31  which supplies water to pipes  32 . Each of the hydrocarbon pipes  16  has one or more oxygen pipes  38  placed within it, and each oxygen pipe  38  has a water pipe  32  coaxially within it, as shown in  FIG. 1 . A nozzle  34  is placed at the end of the water pipes  32 . The nozzle  34 , which may be of a variety of configurations, produces a cone or spray of coarse water droplets. The nozzles  34  are also designed to produce water droplets  28  having a size greater than 200 microns. The tip of the nozzle  34  is positioned a substantially distance “D” from the end  35  of the oxygen pipe  38 . This distance D will vary depending on the application but may for example be between 1-5 meters for many applications. The distance D can be expressed in terms of multiples of the diameter of the oxygen pipe  38 , such as between 5 and 500 times the diameter of the oxygen pipe  38 . This design thereby ensures that entrained particles in the oxygen gas stream are wetted prior to entering the mixing zone  40 . Note that the spray of water drops (indicated at  28 ) created by the atomizers  22  is injected into the hydrocarbon-containing gas stream upstream of the mixing point  40 . In particular, the position of the nozzles  22  is such that the it is also located a substantial distance “upstream” of the open ends  35  of the oxygen pipe  38  to thereby ensure that entrained particles in the hydrocarbon stream are wetted prior to entering the mixing zone  40 . The coarse water drops created in the pipe  16  by both the nozzles  34  and  22  create what could be considered to be a “driving rainstorm” environment in the pipe  16  at the mixing zone  40 . 
         [0028]      FIG. 4  is a more detailed illustration of the hydrocarbon and oxygen gas streams in a configuration where the oxygen pipe  38  has a closed end and the oxygen gas flows out through radial holes  42  formed in the walls of the pipe  38 .  FIG. 4  shows the coarse water droplets  28 A and  28 B created in both gas streams. The oxygen pipe  38  has transverse holes  42  through which the oxygen gas and water droplets  28 B flows out of the pipe. The mixing zone where the oxygen gas/water droplet stream mixes with the hydrocarbon-containing gas stream and water droplets  28 A is indicated at  40 . The end  35  of the oxygen pipe  38  is closed and in the form of a cone in this embodiment with the holes  42  providing for egress of oxygen gas and water droplets. 
         [0029]    The pipes  16  and the manifold  18  may have a drain (not shown) for collecting accumulated water and conducting the water from the pipes  16  and the manifold  18 . Additionally, downstream there may be a device for removing water injected into the mixed gas stream. 
         [0030]    In operation, hydrocarbon-containing gas enters manifold  12  where it is divided into one or more independent pipes  16 . An oxygen-containing stream, preferably pure oxygen, enters manifold  36  where the stream is divided into one or more pipes  38 , smaller than and concentric with pipes  16 . Concentric pipes  38  extend some distance down the outer pipe  16  as determined by engineering calculations to be optimal for mixing and separation of the mixing zone  40  where the oxygen-containing gas mixes with the hydrocarbon-rich gas. In addition, a water stream enters manifold  30 . The manifold is connected to the proximal ends of one or more pipes  32 . The water pipes  32  are smaller in diameter and concentrically located within the oxygen pipe  38 , which are concentric in pipes  16 . Each oxygen pipe  38  has one water pipe  32  located within it. At the end of pipes  32  are affixed atomizing nozzles  34  designed for producing a spray of coarse water droplets having a droplet size of at least 200 microns SMD. The nozzle  34  at the end of pipe  32  terminates a substantial distance from the end of pipe  38  so as to cause the oxygen-containing gas to pass through a coarse water droplet environment before it mixes with the hydrocarbon-rich gas in the pipe  16 . 
         [0031]    As noted above, in addition to the coarse water droplet injected into the oxygen stream, water is introduced into the pipe  16  through one or more atomizing nozzles  22  such that a coarse water droplet environment is present in hydrocarbon-containing gas stream in the pipe  16  at the mixing point  40 . Particles traveling with either the hydrocarbon-containing gas stream or the oxygen gas stream are wetted by the coarse water droplets, reducing the impact energy of the particle if it were to strike a surface of either pipe  16  or pipe  38 . The water droplets also enhance heat transfer away from the particle and quench an ignition, if one should occur. The oxygen/hydrocarbon-containing gas mixture is re-gathered in manifold  18  for transfer to an optional downstream water removal processing station for removal of liquid water vapor from the collected gases, prior to entering a reactor located further downstream. 
         [0032]      FIG. 2  is an end view of two of the pipes  16  carrying the hydrocarbon-containing gas shown in  FIG. 1  with coarse water droplets indicated at droplets  28 A present in the hydrocarbon-containing gas stream due to atomizers  22  and also droplets  28 B present in the oxygen gas stream due to nozzles  34 . 
         [0033]    While in  FIG. 2  there is one oxygen pipe  38  per hydrocarbon pipe  16 , this may of course vary, e.g., depending on the size and number of hydrocarbon pipes  16  in the gas mixer. For example, there may be  2 ,  3  or more oxygen pipes per hydrocarbon pipe  16 , each containing a water pipe and nozzle. 
         [0034]      FIG. 3  is an illustration of an alternative configuration of the nozzles  22  which inject a spray of water droplets into the hydrocarbon-containing gas pipe  16 . In  FIG. 3 , the nozzles  22  are distributed around the periphery of the pipe  16 . While three nozzles  22  are shown, the number may vary depending on the size of the pipe  16  and the velocity and distribution of the coarse water droplets produced by the atomizing nozzles  22 . Furthermore, nozzles could be spaced along the length of the pipe  16 . 
         [0035]    The optional downstream water removal processing station may use a pressure vessel column to coalesce water vapor out of the mixed oxygen/hydrocarbon-containing gas stream. 
         [0036]    The recovered water may be processed by a carbonate scrubber to removed dissolved CO 2  from the recovered water and a water wash column to remove particulate matter, salts, and other impurities from the recovered water and recycle the water back into the water supplies of  FIG. 1 . 
         [0037]    Suitable nozzles for use as the atomizers  22  and the nozzles  34  are available from suppliers such as BETE Fog Nozzle Inc., Greenfield Mass., or Spraying Systems Co., Wheaton Ill. A variety of types of devices can use used for creating the coarse water droplets, including single fluid spray nozzles, dual-fluid spray nozzles, ultrasonic devices for creating a spray of drops, or other means known in the art. The preferred nozzles produce water droplets having a size between 200 microns and 3000 microns SMD. 
       Example 2 
       [0038]      FIG. 5  is an illustration of a gas mixer  10  having a coarse water droplet environment. The construction and arrangement of the embodiment of  FIG. 5  differs from  FIG. 1  in several respects, including nozzles producing water drops in a coaxial direction in the hydrocarbon pipes  16 , and a water manifold  30 A which is located within the oxygen gas manifold  36 . In particular, the gas mixer  10  includes a first water manifold  30 A connected to a source of water which is located within the oxygen manifold  36 . Water pipes  22 A are connected to the water manifold  30 A and have a spray nozzle  34  placed at the end thereof. The water pipes  22 A are positioned within the oxygen pipes  38 . A second water manifold  30 B is connected to a source of water. A second set of water pipes  22 B are connected to the manifold  30 B and placed within the hydrocarbon pipe  16  adjacent to the oxygen pipes  38 . 
         [0039]    Coarse water drops  28  are injected into the hydrocarbon-containing gas stream by means of nozzles  34  positioned at the end of the water pipes  22 B. Coarse water drops  28  are injected into the oxygen gas stream by means of nozzles  34  positioned at the ends of the water pipes  22 A. The water drops serve to wet entrained particles in either the hydrocarbon or oxygen-containing gas streams. Thus, mixing of the oxygen-containing gas stream with the hydrocarbon-containing gas stream occurs downstream of the ends of the oxygen pipes  38  in a coarse water drop environment due to the spray of drops produced by the nozzles  34  in the pipe  16  and the nozzles in the water pipes positioned within the oxygen pipe  38 . 
         [0040]    The gas to liquid mass ratio for the gas mixers of this disclosure is in the range of 0.005 to 1 to 3 to 1. 
         [0041]    In both Example 1 and Example 2, the water drops are preferably injected into hydrocarbon-containing gas stream and into the oxygen gas stream upstream of the mixing point where the oxygen and hydrocarbon-containing gases meet a distance “D” which is between 5 and 500 times the diameter of the oxygen pipes  38 . 
         [0042]    The term “coarse water droplet” is intended to encompass water droplets having a size having a size greater than 200 microns SMD, e.g., droplets between 200 microns and 3000 microns SMD. 
         [0043]    In one embodiment, the temperature of the water used to produce the coarse water droplets is at ambient temperature. In an alternative embodiment, the water temperature is heated above ambient. For example, the water is heated to the temperature of the hydrocarbon-containing gas stream. In an EO production scenario, the temperature of the hydrocarbon recycle gas stream is typically between about 35-40 degrees C. and 65-70 degrees C. The water that is supplied to the spray nozzles can be either at ambient temperature, or water which has been heated to a temperature of between 35 and 70 degrees C. 
         [0044]    While presently preferred embodiments have been described with particularity, variation from the specifics of the disclosed embodiments may be made without departure from the scope of the invention. All questions concerning scope of the invention are to be determined by reference to the appended claims.