Abstract:
A method for removing organic sulfur compounds from a vent gas is described. Also described is an apparatus for absorbing organic sulfur compounds from vent gas.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to processes for removing organic sulfur compounds from a vent gas. More specifically, the present invention provides several techniques for the extraction of organic sulfur compounds such as sulfides, including disulfide oils, from a vent gas using a liquid hydrocarbon stream.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many hydrocarbon streams have sulfur-containing compounds as components. These sulfur-containing compounds may make the hydrocarbon stream “sour,” often viewed as undesirable in the industry. One class of the most common form of sulfur-containing compounds present in the hydrocarbon streams is mercaptans, designated R—S—H. R is commonly a light hydrocarbon radical such as a methyl or ethyl group. Mercaptans generally concentrate in hydrocarbon liquid streams during separation in a processing facility.  
           [0003]    Various processes have been used to remove sulfur-containing compounds such as mercaptans. Two of the most common processes are the UOP Merox™ extraction process and the Merichem Thiolex SM /Regens SM  process. In the Merox™ extraction process, the mercaptans are removed in a multistage extraction contactor using high efficiency trays. A caustic regeneration section then converts the extracted mercaptans to disulfide oils, via an air/catalyst oxidation reaction, which are then separated and removed in a disulfide separator vessel. The Thiolex SM /Regen SM  process is similar, except that in the place of the multistage contactor, the Thiolex SM /Regens SM  process uses a fiber bundle to facilitate contact between the caustic stream and the hydrocarbon stream.  
           [0004]    Air is often used for oxidizing the mercaptans to disulfide oils. The unreacted dissolved components of the air stream, i.e., primarily nitrogen and other inert gases and oxygen, are normally separated from the caustic and disulfide oils. The separator vessel allows the unreacted air components to exit in a vent gas stream. This vent gas stream contains primarily air, and small amounts of water, hydrocarbons, and disulfide oils. Because of the presence of disulfide oils, this vent gas is often treated as a waste, which often triggers various state and national regulations for treatment and handling. When vented to the atmosphere directly or incinerated, the amount of sulfur-containing compounds emitted from these processes are normally enough to require permitting and/or various other state and national agency oversight actions. Some operators of these plants have attempted to avoid this result by routing this vent gas through an activated carbon bed. This activated carbon must be replaced or regenerated after having absorbed its limit of sulfur-containing compounds. It is then often necessary to dispose of the spent activated carbon, typically as a hazardous waste. In addition, to avoid unit shutdown while these carbon beds are either regenerated or replaced, an operator will most often have a backup carbon bed to remove the sulfur containing compounds when the primary carbon bed is regenerated or replaced.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, in one embodiment of the present invention, a method of removing organic sulfur compounds from a vent gas stream is described. The vent gas stream is contacted with a liquid hydrocarbon stream, and a portion of the organic sulfur compounds are absorbed from the vent gas stream into the liquid hydrocarbon stream to form an exiting vent gas stream.  
           [0006]    In another embodiment of the present invention, a method for removing organic sulfur compounds from a vent gas stream having organic sulfur compounds is described. In this embodiment, a scrubber with a shell is provided. The scrubber shell has an interior cavity, a diameter, a vent gas entry port, a vent gas exit port, and a hydrocarbon entry port. A hydrocarbon stream is introduced into the scrubber through the hydrocarbon entry port. The vent gas stream is then introduced into the scrubber through the vent gas entry port. A portion of the organic sulfur compounds is absorbed from the vent gas stream into the hydrocarbon stream to form an exiting vent gas stream. Finally, the exiting vent gas stream is removed from the scrubber through the vent gas exit port.  
           [0007]    In still another embodiment of the present invention, a method for removing disulfide oils is described. In this embodiment, a scrubber with a shell is provided. The scrubber shell has an interior cavity, a diameter, a vent gas entry port, a vent gas exit port, and a hydrocarbon entry port. The scrubber shell also has gas/liquid contact material that is located within the interior cavity of the scrubber. A hydrocarbon stream is introduced into the scrubber through the hydrocarbon entry port. The hydrocarbon stream has a least one liquid hydrocarbon that boils between about 180° F. and about 430° F. The vent gas stream is then introduced into the scrubber through the vent gas entry port. A portion of the disulfide oils are absorbed from the vent gas stream into the hydrocarbon stream to form an exiting vent gas stream. Finally, the exiting vent gas stream is removed from the scrubber through the vent gas exit port.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic of a vent gas scrubber for removal of organic sulfur compounds from vent gas according to one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    In one embodiment of the present invention, a portion of the organic sulfur compounds present in an incoming vent gas stream are removed through the use of a scrubber. This scrubber removes sulfides, such as disulfide oils, from the vent gas by absorbtion into a liquid hydrocarbon stream. The sulfides-depleted vent gas may then be routed to the atmosphere, a heater, or an incinerator or other control device, and the liquid hydrocarbon stream may then be treated to remove the sulfur, typically, but not manditorily, in a treater such as a hydrotreater, wherein the sulfur is recovered.  
         [0010]    According to one embodiment of the present invention, as shown in FIG. 1, scrubber  10  is a vertically-oriented, gas-liquid contactor where vent gas and a liquid hydrocarbon stream are commingled. Other orientations, including horizontal, are possible. Scrubber  10  is most often roughly cylindrical, with an interior cavity. The liquid hydrocarbon stream consists of one or more liquid hydrocarbons. This stream should have a high affinity for absorbtion of the particular organic sulfur compounds present in the vent gas stream. Where more than one liquid hydrocarbon is used in the liquid hydrocarbon stream, the overall mixture should be one capable of absorbing the organic sulfur compound present within the vent gas stream. Typically this hydrocarbon stream will be primarily composed of a gasoline boiling range (180° F. to 430° F.) and should contain less than 50 ppm total sulfur and more preferably below 10 ppm total sulfur. Examples of such compounds include benzene, xylene, toluene, hexane, heptane, octane, nonane, and mixtures of hydrocarbons, including hydrogenated naphtha. Any hydrocarbon capable of efficiently absorbing the particular sulfur compounds, either by itself or in combination with other hydrocarbon components may be used.  
         [0011]    In the embodiment depicted in FIG. 1, scrubber  10  consists primarily of shell  100 , which may be constructed out of typical materials used in the manufacture of industrial equipment, often carbon or stainless steel, although certain ceramics and exotic metals such as Inconel alloys, including Inonel alloy 625, able to withstand the conditions associated with scrubber, are appropriate as well. Where scrubber  10  is exposed to high levels of caustic or caustic salts, it is generally not desirable to use carbon steel on process-wetted parts, unless the carbon steel is lined with a suitable noncorrosive material such as rubber, glass, or ceramic. Shell  100  is most commonly a section of piping. Vent gas enters scrubber  10  through vent gas entry point  20 . The vent gas may be composed of the following components, although scrubber  10  is capable of handling any vent gas stream where the sulfides are a minor component:  
                                                   Component   Range (all ranges in mole %)                           Nitrogen   70-90           Oxygen    0-21           Water   0-8           Hydrocarbons    0-10           Disulfide Oils   0-2           Mercaptans   trace           Sodium Thiosulfate   trace                      
 
         [0012]    The vent gas then rises through scrubber  10  where it is contacted by a falling liquid hydrocarbon stream. The exiting vent gas stream leaves scrubber  10  through vent gas exit port  30 . While vent gas entry point  20  and vent gas exit port  30  are shown at the bottom and top of scrubber  10 , this is not limiting, and either or both could be located along the circumference of scrubber  10 .  
         [0013]    The liquid hydrocarbon stream is introduced to scrubber  10  through hydrocarbon entry point  40 . In order to facilitate even dispersal through scrubber  10 , scrubber  10  is generally outfitted with a distributor in functional proximity to hydrocarbon entry port  40 , often within spool piece  105 , located below hydrocarbon entry point  40 . The liquid hydrocarbon stream falls through the interior of scrubber  10  where it contacts the rising vent gas stream, absorbing a portion of the sulfur compounds within the vent gas stream. The absorbed sulfur compounds and the liquid hydrocarbon stream may then either drain into separator  200 , which may be a disulfide separator, or may be removed through hydrocarbon drain  50 . When the liquid hydrocarbon stream and absorbed sulfur compounds are removed from scrubber  10  through hydrocarbon drain  50 , they are typically collected in a drum or other vessel for later treatment such as in a hydrotreater.  
         [0014]    Most of the absorbtion of sulfur compounds by the liquid hydrocarbon stream occurs within contact zone  60  of scrubber  10 . Contact zone  60  is designed to maximize the contact between the falling hydrocarbon stream and the rising vent gas. Various gas/liquid contact materials may be used to facilitate this contact; examples include packing, fiber/film contactors, and tray assemblies. The most common means is through packing, which is depicted in FIG. 1. The packing within contact zone  60  may be of any type designed for liquid/gaseous contact, but should be of a type that is resistant to plugging, as some sulfides such as disulfide oils may congeal and plug holes in the packing and other components within the vent gas stream or hydrocarbon stream may present a similar plugging issue. This plugging will result in column inefficiencies and reduce the overall effectiveness of scrubber  10 . One of the common types of packing used in liquid/gas services is ring packing, such as nutter or raschig rings, which are suitable for use in this application as long as the diameter of the packing is properly chosen. Other types of packing are acceptable, including structured packing. Further, other means of developing contact between the hydrocarbon and rising vent stream are acceptable, including fiber/film contactors and trayed systems, as long as the proper contact area is obtained. Determining the proper contact area for the system is within the capability of one of ordinary skill in the art using traditional methods.  
         [0015]    When ring packing is used, typical sizes of packing range from ½″ to {fraction (2)}″, depending on operator need. Smaller rings are more prone to plugging, but offer greater surface contact per foot of packing and therefore a smaller overall scrubber  10 . One of ordinary skill in the art is capable of evaluating this tradeoff. An operator may change the amount of contact area available for vent gas/hydrocarbon stream contact by altering the height of the packing, packing size, and/or scrubber  10  diameter depending on his need. Scrubber  10  diameter may also be increased in order to avoid flooding at the expected highest flow rates, a condition that will result in decreased efficiency of scrubber  10 . Typically, scrubber  10  diameter will be between 6″ and 24″, although smaller or larger sizes may be used, depending on throughput and the efficiency desired. The material of construction of the packing may be of any material chemically compatible with the vent gas stream and the liquid hydrocarbon stream, as long as it is also able to handle the mechanical stresses within scrubber  10 . The packing is most commonly composed of carbon steel, stainless steel, carbon, or ceramic. The packing is generally supported, such as by packing support  65  as shown in FIG. 1.  
         [0016]    With proper choice of the packing and contact area available for absorption, an operator may obtain removal efficiencies of sulfur from the vent stream of greater than 99%, most often greater than 99.5%. This efficiency may degrade with service of scrubber  10 , as sulfides and other contaminants such as salts from a caustic system will inevitably plug the contact area of the packing. It is often advantageous to design scrubber  10  with a means for cleaning. As shown in FIG. 1, scrubber  10  may be designed with cleaning port  70 . When the efficiency of scrubber  10  drops below a certain predetermined level, it may be isolated, such as by closing isolation valve  80 , and filling scrubber  10  with a solution designed to remove the congealed disulfide oils and salts from the packing within contact area  60 . The solution may be removed through hydrocarbon drain  50 . It is generally preferred to clean the packing with a heated solution, such as for instance condensate. After cleaning, scrubber  10  may be returned to service by opening isolation valve  60 . During cleaning, the vent gas stream may be routed around scrubber  10  through bypass  90 .  
         [0017]    [0017]FIG. 1 depicts the mounting of scrubber  10  on the top of separator  200 . This mounting facilitates the removal of the vent gas from separator  200 . However, depending on physical constraints or other operator needs, scrubber  10  may be mounted adjacent to or remotely from separator  200 . Further, scrubber  10  is versatile enough that it may receive its vent gas stream from other equipment or processes capable of delivering a vent stream containing disulfide oils or sulfides.  
         [0018]    Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.