Abstract:
A method of safely vaporizing liquid chlorine containing high concentrations of nitrogen trichloride contaminant from a chloralkali plant. In a vertical plug-flow vaporizer having an upward flow direction, a stream of liquid chlorine containing nitrogen trichloride is received. A gas such as air, nitrogen or chlorine gas is introduced into the liquid stream upstream of the boiling zone of the vaporizer to induce a flow regime, for example annular flow or mist flow, that prevents a mass accumulation of nitrogen trichloride in the vaporizer. The liquid chlorine containing nitrogen trichloride is vaporized. The resulting gas stream may be processed to destroy the nitrogen trichloride and recycled to the chlorine production train.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention pertains to the processing of a stream of liquid chlorine containing nitrogen trichloride from a chlorine production process, for example a chloralkali production process. 
         [0002]    In the industrial production of chlorine, a small amount of by-product nitrogen trichloride (NCl 3 ) is produced. In a chloralkali production process the amount formed is proportional to the amount of ammonia present in the salt fed to the process. Nitrogen trichloride follows the product chlorine leaving the chloralkali cell house. 
         [0003]    Nitrogen trichloride is an unstable compound that detonates when it reaches a critical concentration, reported to be about 13 wt %, though it is believed that nitrogen trichloride decomposes to create dangerous conditions at concentrations as low as 3 wt % . However, a critical mass of nitrogen trichloride is also required before it is considered capable of damaging equipment. According to a report of the Chlorine Institute Inc. a typical chlorine vessel with a wall thickness of one-half inch can be fractured with as little as 1.5 gm/cm 2  liquid film of pure nitrogen trichloride. As a compound with a significantly lower vapor pressure than chlorine, it can concentrate if liquid chlorine, containing nitrogen trichloride, is allowed to evaporate. Nitrogen trichloride is thought to be the cause of explosions and fatalities in chloralkali production facilities. 
         [0004]    Chlorine product is typically supplied as a liquid, but the end-user normally evaporates the liquid chlorine prior to use. Depending on how the chlorine is evaporated, this can lead to an increase in nitrogen trichloride concentration. A critical part of the chlorine production process is therefore to keep the nitrogen trichloride concentration low in the final chlorine product, typically only a few parts per million, to allow the end-user to safely evaporate the liquid chlorine. In the chloralkali process, nitrogen trichloride is often removed from the product chlorine through an absorption step, for example in a chlorine scrubber, prior to chlorine compression and liquefaction. In the scrubbing step, nitrogen trichloride is absorbed into fresh, clean product chlorine and pushed down the scrubber and into a holding tank, referred to as the nitrogen trichloride decomposer, containing carbon tetrachloride or sometimes chloroform. In the decomposer, the solvent is maintained at a temperature above the boiling point of the chlorine. When the liquid chlorine contacts the warm solvent, it flashes back into the chlorine scrubber while nitrogen trichloride is absorbed by the solvent. Conditions in the decomposer are selected so that nitrogen trichloride slowly and safely decomposes. In time, tars and other impurities build up in the solvent, and the solvent must be periodically replaced, generating a waste stream that must be disposed of. 
         [0005]    For reasons of both regulatory constraints and product quality (i.e. to reduce organic content in the final product chlorine), it is desirable to avoid the use of solvents such as carbon tetrachloride and chloroform in the chlorine production train. 
         [0006]    A method of disposing of nitrogen trichloride without using carbon tetrachloride or chloroform solvents is described in U.S. Pat. No. 3,568,409, Ferguson et al., in which gas chlorine from the drying tower is contacted with hydrochloric acid upstream of the compression and liquefaction steps. However, the process produces an acidic waste stream that must be disposed of or used elsewhere. 
         [0007]    In the method described in U.S. Pat. No. 3,568,409, chlorine is vaporized and recycled back to process but this is done only after removing or destroying the nitrogen trichloride content. In the currently-practiced industrial process, chlorine is also vaporized and recycled back to process but only after nitrogen trichloride is absorbed into carbon tetrachloride or chloroform or other appropriate organic solvents. The vaporization of chlorine is an important step of most methods proposed for the removal of nitrogen trichloride from the final chlorine product of a chloralkali production facility. 
         [0008]    Although it would be desirable to be able to vaporize the liquid chlorine stream containing a high concentration of nitrogen trichloride from the chlorine scrubber to avoid the use of a decomposer and organic solvents or the use of other chemicals that generate waste streams that must be handled, the chlorine vaporizers currently used in industry have shortcomings in vaporizing such streams. Industrial chlorine vaporizers are generally non-horizontal units, such as vertical bayonet style units, or horizontal vaporizer units, such as kettle reboiler style units. For convenience in the following discussion, non-horizontal units with a positive slope are referred to as “vertical” units, meaning units with an angle from the horizontal from 0.1 to 90 degrees. These horizontal and vertical chlorine vaporizers can be of two types, namely: pool boiling vaporizers and plug-flow vaporizers. In a pool boiling vaporizer, such as a vertical bayonet or kettle reboiler style vaporizer, liquid chlorine is evaporated out of a main body of liquid chlorine. Compounds with lower vapor pressure than chlorine concentrate in the main body of liquid chlorine as chlorine is evaporated. In an upward flow vaporizer, such as a Hooker-style vaporizer or tube vaporizer, liquid chlorine is evaporated as it moves through the vaporizer. In an upward flow vaporizer, depending on the developed flow regime, compounds with lower vapor pressure than chlorine can locally concentrate within the unit, as described below. Neither type of vaporizer is normally used to vaporize liquid chlorine containing a high concentration of nitrogen trichloride, because of the danger of concentrating it. Euro Chlor recommends that the concentration of nitrogen trichloride in liquid chlorine in “reboilers” be maintained below 1000 ppm to avoid excessive concentration. There is a balance between the tendency to concentrate and the tendency for nitrogen trichloride to decompose which is complex and not completely understood. 
         [0009]    In a conventional vertical upward flow vaporizer, chlorine flows the length of the unit through three distinct regime zones. In the first zone, liquid chlorine is heated to its boiling point. In the second zone chlorine is evaporated and in the third zone the resulting chlorine gas is superheated. It is within the second zone, i.e. the boiling zone, of a vertical upward flow vaporizer that nitrogen trichloride can concentrate dangerously. However, reflux of liquid from the boiling zone to the preheat zone can also cause concentration. 
         [0010]    In the boiling zone of a conventional vertical upward flow vaporizer, three flow regimes are commonly encountered. The first flow regime occurs at the beginning of boiling when little vapor has yet formed and a “churn” flow regime develops. In the churn flow regime, vapor and liquid randomly mix and back-mix. Once enough vapor is generated, an “annular” flow regime develops, where vapor flows through the center of the heat transfer chamber (e.g., tube) pushing the liquid forward and against the heat transfer wall. Finally, sufficient vapor is generated to take the system into a “mist” flow regime, where the liquid is broken down into small droplets that randomly mix with the vapor while it moves forward through the boiling zone. It is in the boiling zone where a churn flow regime is present that nitrogen trichloride can concentrate and mass build-up. In this zone, liquid chlorine can back-mix and local “pool” boiling can develop. 
         [0011]    Once the annular flow regime develops within the boiling zone, the liquid is forced forward by the vapor generated, and back-mixing, and therefore nitrogen trichloride accumulation, is minimal. Back-mixing is also minimal in the mist flow regime. As liquid flows forward through the annular and mist flow regimes, liquid chlorine is evaporated faster than the dissolved nitrogen trichloride. However, the mass of nitrogen trichloride in the liquid is decreasing and not accumulating. 
         [0012]    In a conventional vertical upward flow chlorine vaporizer it is difficult to ensure that nitrogen trichloride concentrations do not increase substantially in the zone where the liquid and churn flow regimes are present, and before the annular flow regime is achieved. As a result, this type of vaporizer is not typically used to handle liquid chlorine streams rich in nitrogen trichloride. 
         [0013]    In a horizontal or downward slope plug-flow vaporizer, the opportunity of evaporating the liquid chlorine without back-mixing (i.e., without locally concentrating nitrogen trichloride) is substantially increased relative to a vertical upward flow unit. In these units gravity forces the liquid chlorine to flow forward with more and more of the volume of the unit taken by the generated vapor. If properly designed, the boiling churn zone can be avoided altogether. However, the concern with these types of units is that they can be accidentally flooded, for example, by exceeding their vaporization capacity (i.e., feeding too much liquid chlorine). Once flooded, evaporating the liquid chlorine leads to pool boiling rather than plug-flow vaporization. This causes nitrogen trichloride to concentrate, which can lead to an unsafe condition. On the other hand, a vertical upward flow vaporizer flooded with liquid chlorine is quickly drained back to the feeding tank as soon as enough vapor chlorine is generated again, avoiding pool boiling conditions. 
         [0014]    In summary, vertical upward, horizontal, and downward flow vaporizers all have shortcomings in dealing with liquid chlorine streams rich in nitrogen trichloride. There is a need in the industry for an improved method of vaporizing chlorine streams containing a high concentration of nitrogen trichloride. 
         [0015]    It would also be desirable to provide a process for disposing of the nitrogen trichloride removed from the chlorine stream without using organic solvents such as carbon tetrachloride or chloroform, or other liquid chemicals. 
       SUMMARY OF THE INVENTION 
       [0016]    The invention provides a method of vaporizing liquid chlorine containing nitrogen trichloride in a plug-flow vaporizer oriented non-horizontally and having an upward flow direction. The method comprises receiving a stream comprising liquid chlorine containing nitrogen trichloride, introducing a gas into the liquid stream upstream of a boiling zone of the vaporizer to induce a flow regime in the liquid stream that prevents or minimizes any mass accumulation of nitrogen trichloride in the boiling zone, and vaporizing the liquid stream to produce a stream comprising chlorine gas and nitrogen trichloride gas. 
         [0017]    According to some embodiments, the method includes processing the vaporized stream by destroying the nitrogen trichloride therein, to produce a stream comprising chlorine gas with nitrogen gas formed by decomposition of the nitrogen trichloride. The stream comprising chlorine gas with nitrogen gas may be recycled to the chlorine train of a chlorine production process. 
         [0018]    The invention also provides an apparatus for carrying out the methods of the invention. The apparatus comprises a plug-flow chlorine vaporizer oriented non-horizontally and having an upward flow direction. The apparatus has an inlet for receiving a stream of liquid chlorine containing nitrogen trichloride into the vaporizer. There is a boiling zone in the vaporizer downstream of the liquid inlet. The apparatus has a gas inlet upstream of the boiling zone for introducing a gas into the liquid stream, the gas being, for example, air, nitrogen or chlorine gas. 
         [0019]    The invention accordingly provides a method and apparatus to safely evaporate a liquid chlorine stream rich in nitrogen trichloride. The chlorine vaporization process presents an effective mean of avoiding a churn flow regime within the boiling zone of an upward plug-flow chlorine vaporizer, and therefore providing the means to safely vaporize liquid chlorine containing a high concentration of nitrogen trichloride, e.g. greater than 50 ppm. Because of the vertical position of the vaporizer, accidental flooding of the vaporizer does not lead to pool boiling, avoiding the potential for nitrogen trichloride concentration. The process includes the introduction of gas (vapor), at any point upstream of the boiling zone of a chlorine vaporizer, in sufficient quantity to force the feed liquid chlorine into any flow regime not leading to significant back-mixing or pool boiling within the boiling zone of the vaporizer. The plug-flow vaporizer can be oriented at any angle from 0.1° to 90° from the horizontal position. The gas introduced into the liquid stream can be any suitable gas or vapor, for example but not limited to air, nitrogen, chlorine gas, hydrogen, helium and oxygen, and mixtures thereof. The liquid chlorine feed to the vaporizer is forced into an appropriate flow regime before it reaches the boiling zone of the vaporizer, hence allowing the safe vaporization of liquid chlorine. 
         [0020]    These and other features of the invention will be apparent from the following description and drawings of particular embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a first embodiment of the process of the invention. 
           [0022]      FIG. 2  is a schematic diagram of the chlorine vaporizer. 
           [0023]      FIG. 3  is a schematic diagram of a second embodiment of the process, in which nitrogen trichloride is destroyed using a superheater. 
           [0024]      FIG. 4  is a schematic diagram of a third embodiment of the process, in which nitrogen trichloride is destroyed using a catalytic bed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    In the following description and in the drawings, corresponding and like elements are referred to by the same reference characters. 
         [0026]    In a first embodiment of the process of the invention, illustrated in  FIG. 1 , a vertical upward plug-flow vaporizer  20  receives a stream of liquid chlorine containing nitrogen trichloride (stream  22 ) from the chlorine production train  24  of a chloralkali plant. The production train  24  includes a chloralkali cell house  10  in which chlorine gas is produced by the electrolysis of brine. A chlorine scrubber  11  receives a stream  12  of gas chlorine from the cell house and receives a liquid chlorine stream  13 . Other unit operations usually present between the chloralkali cell house  10  and the chlorine scrubber  11  are not shown in the drawings. A gas chlorine stream  14  from the scrubber is fed to a compressor  15  and is thereafter liquified. From the bottom of the chlorine scrubber  11 , liquid chlorine, rich in nitrogen trichloride (stream  16 ), is fed to a holding tank  17 , from which a stream  22  is routed to the vaporizer  20 . Alternatively, the liquid chlorine, rich in nitrogen trichloride, may be fed directly from the scrubber  11  to the vaporizer  20  (stream  16 A) without using any holding tank. The stream  22  typically has 50 ppm or more of nitrogen trichloride. 
         [0027]    The vaporizer  20  is illustrated in  FIG. 2 . It is oriented substantially vertically, but it may be oriented at any angle from the horizontal in the range of 0.1 to 90°; that is, the vaporizer is non-horizontal, sloping upward and has an upward flow direction. The vaporizer  20  has a body  26  and is heated by a stream  28  of heating medium flowing through a heating jacket  30 . The vaporizer  20  has a heating zone  32  at the inlet end  34 , in which the liquid stream is heated, and a boiling zone  36  downstream of the heating zone, in which the liquid chlorine is evaporated. At the outlet end  38  of the vaporizer, a stream  40  of chlorine gas and nitrogen trichloride gas exits the vaporizer. 
         [0028]    A gas such as air, nitrogen or chlorine gas, or mixtures thereof, from a gas source  42  (stream  44 ) is introduced into the stream  22  of liquid chlorine and nitrogen trichloride upstream of the boiling zone  36  of the vaporizer. The gas stream  44  may be introduced into the liquid stream  22  before entry into the vaporizer, or it may be introduced directly into the heating zone  32  of the vaporizer, as indicated by optional streams  44 A and  44 B shown in  FIG. 2 . 
         [0029]    The gas stream  44  is fed at a flow rate sufficient to force the feed liquid chlorine into a flow regime within the vaporizer  20  that does not permit significant back-mixing or pool boiling within the boiling zone  36  of the vaporizer. Examples of such flow regimes are annular and mist flow regimes. The flow rate of the gas stream  44  may be in the range of 0.01 to 10 kg of gas per kg of liquid chlorine, alternatively 0.01 to 1 kg, alternatively 0.02 to 0.15 kg of gas per kg of liquid chlorine. The effect is to keep the nitrogen trichloride from accumulating within the pool and boiling zone of the vaporizer as the chlorine and nitrogen trichloride evaporate. Although the concentration of nitrogen trichloride increases through the boiling zone, due to the higher vapor pressure (lower boiling point) of chlorine, the induced flow regime limits the concentration increase and the mass accumulation of nitrogen trichloride within the vaporizer to levels that are safe to handle. 
         [0030]    At the outlet end  38  of the vaporizer, a stream  40  comprising chlorine gas with nitrogen trichloride gas and the gas fed into the liquid stream is sent for further processing at step  46 . For example, the stream  40  may be routed to a hydrochloric acid plant, in which chlorine is reacted with hydrogen to make hydrochloric acid. Alternatively, the gas stream  40  may be absorbed in a hypochlorite system. Another option is to destroy the nitrogen trichloride and recycle the stream  40  to the chlorine production train, as explained below. 
         [0031]    In a second embodiment of the process of the invention, the gas mixture produced in the vaporizer  20  is routed to one or more unit operations for the destruction of nitrogen trichloride. The gas leaving the nitrogen trichloride destruction step, i.e. chlorine gas and nitrogen gas, is recycled back to the chlorine train of the chloralkali process. The invention thus avoids the generation of a waste stream or the addition of other chemicals or solvents to deal with the nitrogen trichloride. The step of destroying the nitrogen trichloride can be carried out in various ways. For example, the gas mixture evaporated in the vaporizer may be introduced into a superheater, which may be part of the vaporizer unit. This is illustrated in  FIG. 3 , in which the vaporizer  20  includes a superheater zone  37  downstream of the boiling zone  36 . The operating conditions in the superheater are selected so as to achieve substantially complete destruction of nitrogen trichloride. The average operating temperature of the superheater may be in the range of 30° to 300° C., the operating pressure in the range of 0.5 to 100 bar, and the residence time in the range of 0.5 seconds to 5 minutes. Alternatively, the average operating temperature may be in the range of 35° to 250° C., the operating pressure in the range of atmospheric pressure to 90 bar, and the residence time in the range of 1 second to 3 minutes. 
         [0032]    The gas stream  52  leaving the superheater, comprising chlorine gas and nitrogen gas, is recycled back to the chlorine production train  24  of the chloralkali process. 
         [0033]    As an alternative to using a superheater, and as illustrated in  FIG. 4 , the gas evaporated in the vaporizer  20  can be routed to a catalytic bed  54  in which the nitrogen trichloride is destroyed. The catalytic bed may contain, for example, Monel (trademark) as a catalyst to destroy nitrogen trichloride. The catalytic bed may be operated at temperatures in the range of minus 40 to 300° C., pressures in the range of 0.5 to 100 bar, and a residence time in the range of 0.1 seconds to 5 minutes. The gas stream  52  leaving the catalytic bed, comprising chlorine gas and nitrogen gas, is recycled back to the chlorine production train  24  or can be routed to other unit operations. 
         [0034]    Optionally, the process may use both a superheater and a catalytic bed to destroy the nitrogen trichloride. The catalytic bed may be within a superheater zone of the vaporizer, rather than being a separate unit. 
         [0035]    Optionally, the gas leaving the nitrogen trichloride destruction step, e.g. the superheater or catalytic bed, may be routed to a temperature conditioning step  56  before being recycled back to the chlorine train (stream  60 ), as shown in  FIGS. 3 and 4 . This reduces the temperature of the gas stream leaving the nitrogen trichloride destruction step, which may be at a temperature of about 80° to 120° C., to a lower temperature for introduction into the chlorine train, which may be at a temperature of about minus 35° C. 
         [0036]    Although the invention has been described in terms of various embodiments, it is not intended that the invention be limited to these embodiments. Various modifications within the scope of the invention will be apparent to those skilled in the art. The scope of the invention is defined by the claims that follow.