Abstract:
A caustic recovery system comprising an oxygen source adapted to provide a gas stream comprising at least 30% oxygen. A method of regenerating caustic comprising intermingling spent caustic with a gas stream comprising at least 30% oxygen.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of the invention is caustic solution regeneration.  
         BACKGROUND OF THE INVENTION  
         [0002]    Current caustic solution regeneration systems often use air as a supply of oxygen with oxygen and water being used to convert mercaptides to disulfide oil and regenerated caustic. Referring to prior art FIG. 1, air B 1  from air source  20  (typically just the atmosphere around the plant in which regeneration is being done) is compressed by compressor  22  and regulated by valve  23 , and then fed as input stream B into oxidizer tower  40  which is typically a co-current flow tower (A plant may utilize a second source of air  21  to provide air B 2  to replace or supplement air B 1 ). Spent caustic A 1  from caustic source  10  is also fed via heat exchanger  15  as input stream A into oxidizer tower  40 , generally with catalyst A 2  from catalyst source  11  mixed in while caustic A 1  passes through mixing device  13 . In oxidizer tower  40 , the air B and caustic A are contacted for oxidation, and regenerated caustic C is transported via pump  24  for reuse in the treating process unit  25 . The regeneration reaction only uses the oxygen portion of the air supplied to the tower. As such, the unused gasses contained in the air are a waste product of the reaction and are vented from the tower as a stream D of mixed gas and hydrocarbons, with oxygen and nitrogen being large components of the vented stream D. In some instances, natural gas from gas source  30  is also fed into oxidizer tower  40  or into downstream piping in order to minimize the risk of explosion posed by the vented stream D.  
           [0003]    After being vented from oxidizer tower  40 , the vent stream D of gas and hydrocarbons will often be scrubbed in a caustic scrubbing system to minimize entrained oils prior to disposal. In FIG. 1, the stream is fed into off gas absorber (“OGA”)  50 . In OGA  50 , the vented waste D from tower  40  is contacted with fresh caustic I from source  12 . From OGA  50 , caustic A 3  is pumped by pump  14  back into input stream A, and the “scrubbed” nitrogen, oxygen, and other gasses E are fed into KO drum  60  prior to disposal to the atmosphere  90  via a downstream combustion device  70  Liquids collected in the KO drum  60 , are returned to the caustic system.  
           [0004]    The air for the oxidizer is generally supplied by an air compressor  22  or from a plant air system. Designs of this type use greater than the stochiometric oxygen requirements thereby increasing air supply and purge requirements.  
           [0005]    Depending on the concentrations of the spent caustic material, variations to FIG. 1 include methods to cool the recycle stream A 3  via heat exchange with the feed stream A 1  and cooling water to moderate the overall temperature in the oxidizer  40  as the caustic regeneration is an exothermic reaction.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to the use of pure oxygen in place of air during caustic recovery. The use of pure oxygen instead of air eliminates the need for treatment and disposal of waste gasses as is required in current caustic recover systems. More particularly, disclosed is a caustic recovery system comprising an oxygen source adapted to provide a gas stream comprising at least 50% oxygen, as is a method of regenerating caustic comprising intermingling spent caustic with a gas stream comprising at least 50% oxygen.  
           [0007]    It is contemplated that elimination of the need of utility consumption (electricity, steam or diesel) for air compression and the elimination of capital equipment for air compression and the disposal of waste gasses will adequately compensate for the increased expense associated with providing pure oxygen. It is also contemplated that the benefit of using pure oxygen will be sufficient to overcome the reluctance of using pure oxygen in a volatile environment considering the hazards and environmental concerns for processing the waste gasses in the current art.  
           [0008]    Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a schematic view of a prior art caustic recovery system.  
         [0010]    [0010]FIG. 2 is a schematic view of a first caustic recovery system embodying the invention.  
         [0011]    [0011]FIG. 3 is a schematic view of a second caustic recovery system embodying the invention.  
         [0012]    [0012]FIG. 4 is a schematic view of a third caustic recovery system embodying the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    In FIG. 2, a caustic recovery system comprises an oxygen source  26 , a caustic solution source  10 , and a catalyst source  11 . Caustic A (mixed with catalyst A 2 ) and oxygen B are fed into oxidizer tower  40  In oxidizer tower  40 , the oxygen B and caustic A are contacted for oxidation, and regenerated caustic C is transported via pump  24  for reuse in the treating process unit  24 . Any unused oxygen D is compressed and fed back into tower  40  as oxygen input J.  
         [0014]    The basic reaction for caustic recovery takes place in the presence of catalyst A 2  introduced into the system. The formula which describes the basic reaction is:  
         4RSNa+O 2 +4H2O=2RSSR+4NaOH+2H2O.  
         [0015]    In addition, when H2S has been present in the hydrocarbon process stream that has been contacted with the caustic solution, sodium sulfide is formed and can be partially regenerated in the oxidizer via the following reaction:  
         2Na2S+4H2O+O2=Na2S2O3+2NaOH+3H2O.  
         [0016]    The primary difference between the method used by the system of FIG. 2 and that of FIG. 1 is that pure (at least relatively) oxygen is used in place of air. Since the oxygen source is free from the unnecessary gasses present in air, the need to handle unused gasses is eliminated.  
         [0017]    One benefit of eliminating substantially all of the waste gasses is that there is no need to mix natural gas in the streams passing through tower  40 . A second benefit is that there are no waste gasses to be disposed of. Any oxygen that has not yet been used to regenerate caustic can simply be compressed and put back into the tower to regenerate additional caustic. Alternative methods for oxygen recycle include caustic recirculation in combination with an eductor as shown in FIG. 3.  
         [0018]    Although it is preferred that pure oxygen be supplied, it is contemplated that supplying a mixture comprising at least 30% oxygen would still be advantageous.  
         [0019]    If less than pure oxygen is used, other gasses may build up in the system and eventually have to be vented and treated in a manner similar to the venting and treatment of waste gasses now. However, the waste stream would be a factor of 10 to 1000 times smaller, would not require a caustic wash, and would be easier to dispose of. Commercially available oxygen typically comprises 88 to 99.5% oxygen, 12 to 0.5% nitrogen and trace components as an impurities. FIG. 4 illustrates the regeneration process when a small quantity of waste gas H is purged from the system. Purge gasses H are fed into KO drum  61  prior to disposal to the atmosphere  91 . If required for environmental purposes, gasses L form KO drum  61  are passed through a downstream combustion device  71  prior to being vented to the atmosphere. Liquids K collected in the KO drum  61 , are returned to the caustic system.  
         [0020]    When using high purity oxygen, depending on regeneration system operating conditions, the nitrogen and trace impurities will not need to be purged from the vapor as the small quantities of these gasses will dissolve into the caustic solution and be purged from the system when recontacted with the process stream the regenerated caustic is used to treat.  
         [0021]    Thus, specific embodiments and applications of caustic recovery systems and methods have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.