Abstract:
A hydrocarbon-containing gas is mixed with an oxygen-containing gas in a gas mixer in the presence of a water mist. The water mist surrounds and contacts entrained particles in either the oxygen-containing gas stream or the hydrocarbon-containing gas stream. The water acts to suppress and prevent ignition of the hydrocarbon gas in the mixer by serving as a sink for heat created by energetic collisions between such particles and structures within the gas mixer. The water mist also acts to quench ignition caused by such collisions. The water mist can be introduced into the gas mixer in a number of different configurations, including via nozzles injecting a mist into a hydrocarbon gas manifold or an oxygen gas manifold, nozzles placed within the gas mixer adjacent to ends of the oxygen supply pipes, and nozzles placed coaxially within the oxygen supply pipes in the gas mixer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase of International Application No. PCT/US2008/012586, filed Nov. 7, 2008, which claims priority to U.S. Provisional Application No. 61/007,734, filed Dec. 14, 2007, all of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     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. 
     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. 
     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 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. 
     In current industrial processes, ethylene oxide is produced when ethylene (CH 2 ═CH 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:
 
CH 2 ═CH 2+ ½O 2 →C 2 H 4 O
 
     In ethylene oxide production systems, a gas mixer is used to mix the hydrocarbon and oxygen gas streams 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. 
     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. Ignition can occur when a particle (e.g. a piece of sand, rust or pipe scale) entrained in the hydrocarbon or 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 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 damages 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 and oxygen gas stream. The known prior art includes the following patent documents, in addition to the above-cited ′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. 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. 
     Other prior art of interest include British patents GB 705,176 and 2,357,318; U.S. Pat. No. 5,336,791; and U.S. Pat. No. 4,393,817. 
     SUMMARY 
     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 for mixing the oxygen-containing gas with the hydrocarbon-containing gas, introducing a water mist into the gas mixer, and mixing the oxygen gas and the hydrocarbon-containing gas in the presence of the water mist. 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. 
     In another aspect, an improvement to a gas mixer for an industrial production system for gas-phase oxidation of a hydrocarbon-containing gas is provided. The improvement is providing a means for producing a water mist in the gas mixer wherein the oxygen-containing gas and the hydrocarbon-containing gas are mixed in the gas mixer in the presence of the water mist. Several examples of the means for producing the water mist are described, including atomizers (nozzles) which inject a water mist into a hydrocarbon gas manifold, nozzles which inject a water mist into an oxygen gas manifold, and water pipes with mist-producing nozzles at the ends thereof either (1) concentrically located within oxygen pipes supplying oxygen in the gas mixer, (2) positioned along side the oxygen pipes, or both (1) and (2). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a gas mixer for an industrial production system for gas-phase oxidation of a hydrocarbon-containing gas, showing a first embodiment of a means for introducing a water mist into the gas mixer in the form of (1) atomizers (nozzles) which inject a water mist into a hydrocarbon gas manifold and (2) water pipes with mist-producing nozzles at the ends thereof concentrically located within oxygen pipes supplying oxygen to the gas mixer. 
         FIG. 2  is an end view of the one of the pipes carrying the hydrocarbon-containing gas shown in  FIG. 1 , showing the water pipes within the oxygen pipes and the mist provided by the atomizers and the nozzles at the end of the water pipes. 
         FIG. 3  is an illustration of a second embodiment of a gas mixer which features water pipes carrying water and nozzles at the end thereof which produce a mist at the mixing point where oxygen and hydrocarbon-containing gases are mixed. 
         FIG. 4  is an end view of the gas mixer of  FIG. 3  showing the water pipes placed both adjacent to the oxygen pipes and concentrically within the oxygen pipes. 
         FIG. 5  is an illustration of a third embodiment of a gas mixer which features atomizing nozzles injecting a water mist into an oxygen gas manifold. 
         FIG. 6  is a detailed cross-section of the ends of the oxygen pipes of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     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 may 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 water mist environment. 
     Several different embodiments of a gas mixer featuring apparatus for producing the water mist environment will be described in some detail below. These embodiments illustrate applications suitable for ethylene oxide production in a gas mixer featuring low shear co-axial gas mixing. However, variation from the disclosed embodiments is of course possible and the invention can be practiced in a high shear gas mixer, such as described in WO 2009/078899, entitled Oxygen/Hydrocarbon Rapid (High Shear) Gas Mixer, Particularly For The Production Of Ethylene Oxide, the entire content of which is incorporated by reference herein. In the low shear embodiments, a water mist is injected coaxially in one gas stream, particularly a high-purity oxygen feed, and/or alternatively surrounding one gas stream at the point of entry into the second gas stream, particularly the mixing of a high-purity oxygen stream into a hydrocarbon-containing gas stream. 
     The purpose of the water mist 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 small water droplets into the gas stream 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 of the flammable gas stream. In general, the gas mixer features atomizers (water mist producing nozzles) which are designed to produce the water droplet having a size approximately ≦200 microns SMD (Sauter Mean Diameter). This term is defined as the drop diameter that has the same surface area to volume ratio as the entire spray. However, the droplets could be considerably smaller, such as for example as small as 1 micron. At elevated pressures, such as found in the gas mixers of the type of this disclosure, the water mist may act like a dense gas which travels with the oxygen gas into the high flammability zone and act as an ignition suppressant. Materials for construction of the gas mixer and the mist/fog 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. 
     One application of the invention is direct oxidation ethylene oxide process mixers, which mix oxygen at intermediate pressure (˜20 bar) with recycled hydrocarbon gas containing ethylene and other gases. Oxygen pressures run around ˜26 bar. The invention can similarly be used for other partial oxidation processes using pure oxygen or enriched air. 
     The features of this disclosure redefines the oxygen/hydrocarbon mixing process to reduce the potential for ignition in the high 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 fine water mist, and most preferably in the presence of a droplet size of 1-15 micron SMD, which behaves as a dense gas, to provide a heat sink to dissipate the impact energy of entrained particles in either the hydrocarbon or oxygen gas streams or 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. 
     The methods and gas mixer of this disclosure differs from prior technology in that it introduces a fine water mist directly and concurrently into either or both of the oxygen and hydrocarbon streams. Ideally, the mist is formed from water droplets at or less than 200 microns in size to wet the surface of any particles traveling with the gas stream(s) to reduce the energy of impact of the wet particle on mixing device wall and/or act as an ignition suppressant. Water injection for the purpose of flame suppression is commonly employed in a variety of applications like turbine engines and oil well fires, however, no applications of the type described have been found that specifically both minimize the occurrence of ignition sources and suppress the growth of an incipient flame. 
     The features of this disclosure satisfy a long-felt need in the art in that it allows for the injection of oxygen into a hydrocarbon-rich gas stream while minimizing the probability of igniting the mixture. 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 
       FIG. 1  is a schematic representation of a gas mixer featuring a water mist environment where the hydrocarbon-containing gas and oxygen-containing gases meet. The gas mixer  10  includes a hydrocarbon gas manifold  12  receiving recycled gas containing hydrocarbons such as ethylene from a source along an inlet pipe  14 . Although ethylene is the hydrocarbon gas of commercial interest in Example 1, it is typically controlled to about 20-35% by volume within a ballast gas such as methane or nitrogen. One or more pipes  16  are connected to the hydrocarbon gas manifold  12 . Gas mixing occurs in the pipes  16 . The pipes function as mixing chambers for the gas mixer  10 . Mixed gases are collected in a second manifold  18 . 
     The gas mixer  10  features a means for producing a water mist in the pipes  16 . In particular, water supply lines  20  are provided which supply water to atomizers  22 . The atomizers  22  produce a fine water mist. In one embodiment, the atomizers are designed to produce water droplets of a size of about 200 microns or less. Valves  24  are placed downstream of the atomizers  22 . Tubes  26  carry mist produced by the atomizers  22  are mounted in the hydrocarbon gas manifold  12 . Thus, a fine mist or fog is produced in the hydrocarbon gas manifold  12 , droplets being indicated at  28 . The mist created in the manifold  12  is mixed and carried into the pipes  16  and thus is present in the hydrocarbon-containing gas stream in the pipes  16 . 
     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 located within the hydrocarbon pipes  16 . Oxygen flows into the pipes  38  from the manifold  36  and flows out the distal open end of the pipes  38 . 
     The mixer  10  further includes a water manifold  30  connected to a water source which supplies water to the proximal end of water mist pipes  32 . Each of the hydrocarbon pipes  16  has one or more oxygen pipes  38  placed within it, and each oxygen pipe has a water mist pipe  32  coaxially within it, as shown in  FIG. 1 . A nozzle  34  is placed at the distal end of the pipes  32 . The nozzle  34  is preferably a pig-tail type nozzle which produces a cone of fine water droplets. Alternatively, the nozzle is a hollow cone pressure swirl nozzle. The tip of the nozzle  34  is positioned either adjacent to, or slightly inward from the distal end of the oxygen pipe  38 , as shown in  FIG. 1 . 
     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 where the oxygen-containing gas mixes with the hydrocarbon-rich gas. In addition, a water stream enters manifold  30 . The manifold  30  could be positioned inside the oxygen manifold  36 . The manifold  30  is connected to the proximal ends of one or more water 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 fine water mist having a droplet size of approximately 200 microns or less. The nozzle  34  at the end of pipe  32  terminates approximately coincident with the end of pipe  38  and before the end of pipe  16  so as to cause the oxygen-containing gas to pass through a fine water mist as it mixes with the hydrocarbon-rich gas in the pipe  16 . 
     As noted above, in addition to the water mist injected into the oxygen stream, water is introduced into the hydrocarbon manifold  12  through one or more atomizing nozzles  22  such that a fine water mist mixes with the hydrocarbon stream. Particles traveling with either gas stream are wetted by the mist, reducing the impact energy of the particle if it were to strike a surface of either pipe  16  or pipe  38 . The mist also enhances heat transfer away from the particle and quenches an ignition, if one should occur. The oxygen/water/hydrocarbon-containing gas mixture is re-gathered in manifold  18  for transfer to downstream water removal processing station, prior to entering a reactor located further downstream. 
       FIG. 2  is an end view of the one of the pipes  16  carrying the hydrocarbon-containing gas shown in  FIG. 1  with mist (indicated by droplets  28 A) present in the hydrocarbon-containing gas stream. The Figure also shows the water pipes  32  coaxially located within the oxygen pipes  38  and the mist  28 B provided by the nozzles  34  located at the end of the water pipes  32 . While in  FIG. 2  there are three oxygen pipes 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. 
     The downstream water removal processing station may use a pressure vessel column to coalesce water out of the mixed gas stream. For example, this vessel could be an integral part of the CO 2  removal column, which is nearby in a typical processing scheme. The recovered water may be filtered to remove particulate matter and recycled back into the water supplies of  FIG. 1 . 
     The nozzles  34 , like the atomizers  22  of  FIG. 1 , are also designed to produce a droplet size of approximately 200 microns or less SMD. In one possible embodiment, the nozzles  34  and/or  22  are designed to produce water droplets of a size of 1-20 microns SMD, whereby a micron-sized droplet mist is produced. 
     In a variation to the embodiment of  FIG. 1 , the water pipes  32  are positioned in the pipes  16  but not within the oxygen pipes. Rather, the water pipes are positioned adjacent to the oxygen pipes  38  such that the nozzles  34  at the end of the water pipes  32  direct a water mist into the mixing zone where oxygen gas is mixed with the hydrocarbon-containing gas in the pipes  16 . 
     EXAMPLE 2 
       FIG. 3  is a schematic illustration of a second example of a gas mixer incorporating the water mist features of this invention. 
     In this embodiment the hydrocarbon-rich gas is supplied from a manifold to a single vessel or pipe  16  which functions as a mixing chamber for the gas mixer  10 . In this embodiment, the hydrocarbon-containing gas is not separated into multiple parallel pipes such as pipes  16  in the previous embodiment, but rather flows around one or more oxygen-carrying pipes  38  positioned within the vessel  16 . As in Example 1, oxygen-containing gas, preferably high purity oxygen, enters manifold  36  where the stream of gas is divided into one or more pipes  38 . The oxygen pipes have a distal open end through which oxygen gas flows out of the oxygen pipes  38 . 
     The gas mixer  10  features a means for producing a water mist in the pipe or vessel  16 . In particular, water enters manifold  30 A where it is divided into pipes  32 A. The pipes  32 A have a nozzle  34  at the end thereof for producing a water mist having a droplet size smaller than about 200 microns SMD. Water also enters a water manifold  30 B where it is divided into multiple water mist pipes  32 B, which are smaller than and concentrically located in the oxygen pipes  38 . At the end of pipes  32 B are affixed atomizing nozzles  34  capable of producing a fine water mist having droplet sizes smaller than about 200 microns. The nozzles  34  at the end of pipes  32 B terminate coincident with the end of pipe  38  so as to cause the oxygen-containing gas to pass through a fine water mist as it mixes with the hydrocarbon-rich gas. The water pipes  32 A are closely adjacent and parallel to oxygen pipes  38  and interspersed between pipes  38  in a pattern such as shown in  FIG. 4 . One water mist generating pipe  32 A is provided for every three or four oxygen injection pipes  38 . 
     Particles traveling with either gas stream are wetted by a mist, reducing the impact energy of the particle if it were to strike a surface of either pipe  32 ,  38  or vessel  16 . The oxygen/water/hydrocarbon-containing gas mixture is transferred to downstream water removal equipment, prior to entering a reactor located further downstream. The use of multiple water atomizers in this embodiment improves operating reliability of the system. 
     EXAMPLE 3 
     A third embodiment of this invention is shown in  FIG. 5 . In this embodiment, oxygen gas is supplied to an oxygen gas manifold  36 . Hydrocarbon-containing gas is supplied via an inlet  14  to a pipe  16  which functions as a mixing chamber in the gas mixer. 
     The gas mixer  10  features a means for producing a water mist in the pipe  16 . In this embodiment a fine water mist, preferably with a droplet size at or below about 200 microns, is generated in the oxygen manifold  36  by a series of two or more atomizing nozzles  22  connected to a water supply  20 . The nozzles  22  are arranged around the circumference of the manifold  36  and inject a water mist into the manifold  36 . Valves  24  and pipe segments  26  connect the nozzles  22  to the manifold  36 . Oxygen enters the misty environment of manifold  36  where the wet oxygen-water mist is divided into one or more parallel oxygen pipes  38 . 
     Hydrocarbon-rich gas enters the gas mixer  10  from the side inlet  14  and flows into the vessel  16  in a direction parallel to pipes  38 . Flow straighteners  62  may be provided to provide axial flow of the hydrocarbon-containing gas. Properly designed, these serve to equalize the flow distribution of the cycle gas across the cross-section of the pipe  16 . Supports  60  are provided to support the oxygen pipes  38  and prevent vibration of the pipes  38 . The wet oxygen gas exits the distal open end of pipes  38  and mixes with the hydrocarbon-rich gas. Particles traveling with either gas stream are wetted by the mist, reducing the impact energy of the particle if it were to strike a surface of either the outer containment pipe  16  or oxygen pipes  38 . The mist also enhances heat transfer away from the particle and quenches an ignition, if one should occur. The length of pipes  38  is determined to minimize residence time and reduce flow eddies. The distal end  64  of one of the oxygen pipes  38  is shown isolated and in cross-section in  FIG. 6 . The pipe  38  ejects an oxygen/water mist in a jet mixing zone  66  where the oxygen gas is mixed with hydrocarbon-containing gas flowing over the outer peripheral surface of the pipe  38 . 
     Nozzles  22  are preferably designed to produce droplets having a size at or below approximately 200 microns SMD. 
     Suitable nozzles for the design of Examples 1-3 are pig-tail type nozzles, commercially available from BETE Fog Nozzle, Inc., Greenfield, Mass., or Spraying Systems Co., Wheaton Ill. Other nozzles may be used, including spiral pintle, hollow cone pressure swirl, and ultrasonic atomizing nozzles. 
     In one embodiment, the temperature of the water used to produce the water mist is at ambient temperature. In an alternative embodiment, the water 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. 
     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.