Naphtha desulfurization process

Abstract

This invention relates to a process for selectively desulfurizing naphtha. More particularly, a low sulfur naphtha feed containing less than 500 wppm sulfur is hydrodesulfurized using a hydrodesulfurization catalyst and a hydrogen treat gas containing at least about 50 vppm hydrogen sulfide followed by mercaptan removal or conversion.

1 . A process for hydrodesulfurizing a low sulfur naphtha feedstock, which process comprises: 
 a) contacting the feedstock containing less than about 500 wppm sulfur, based on feedstock, and greater than about 20 wt. % olefins, based on feedstock, in a first reaction stage under hydrodesulfurization conditions including a hydrogen treat gas provided that the hydrogen treat gas contains at least about 50 vppm of hydrogen sulfide, based on hydrogen, and provided that the hydrogen sulfide may be in the form of a precursor spiking agent in at least one of the feedstock or hydrogen treat gas, with a catalyst comprising at least one Group VIB metal and at least one Group VIII metal on an inorganic refractory support material to yield a first stage reaction product having less than about 50 wppm non-mercaptan sulfur, based on reaction product, and a mercaptan sulfur to non-mercaptan sulfur ratio of greater than 1:1; and    b) passing the first stage product to a second stage wherein mercaptan sulfur is at least partially removed or converted from the first stage product to obtain a second stage product having a reduced amount of mercaptan sulfur.    
   
   
       2 . The process of  claim 1  wherein the hydrogen treat gas contains at least about 100 vppm hydrogen sulfide.  
   
   
       3 . The process of  claim 2  wherein the hydrogen treat gas contains at least about 200 vppm hydrogen sulfide.  
   
   
       4 . The process of  claim 1  wherein the spiking agent is at least one of carbon disulfide, thiophene, mercaptan, organic sulfide, dialkyl disulfide, diaryl disulfide and organic polysulfide.  
   
   
       5 . The process of  claim 4  wherein the spiking agent is dimethyl sulfide or dimethyl disulfide.  
   
   
       6 . The process of  claim 1  wherein the catalyst comprises: (a) about 1 to about 10 wt. % MoO 3 ; (b) about 0.1 to about 5 wt. % CoO; (c) a Co/Mo atomic ratio of about 0.1 to about 1.0; (d) a median pore diameter of about 75 Å to about 175 Å; (e) a MoO 3  surface concentration in g MoO 3 /m 2  of about 0.5×10 −4  to about 3×10 −4 ; (f) an average particle size diameter of less than about 2.0 mm; (g) a metal sulfide edge plane area of from about 760 to about 2800 μmol oxygen/g MoO 3  as measured by oxygen chemisorption; and (h) an inorganic refractory support material.  
   
   
       7 . The process of  claim 1  wherein the olefins content is at least about 30 wt. %, based on feedstock.  
   
   
       8 . The process of  claim 1  wherein the hydrodesulfurization conditions include temperatures of from about 232 to about 371° C., pressures (total) of from about 1480 to about 2514 kPa, liquid hourly space velocities of from about 0.1 to about 15, and hydrogen treat gas rates of from about 36 to about 1780 m 3 /m 3 .  
   
   
       9 . The process of  claim 1  wherein mercaptan sulfur is removed by caustic extraction.  
   
   
       10 . The process of  claim 9  wherein caustic extraction uses an iron-based catalyst that is soluble in caustic, or in the alternative supported on a support, to oxidize mercaptans.  
   
   
       11 . The process of  claim 1  wherein the mercaptan sulfur is extracted to an aqueous treatment solution and converted to mercaptides.  
   
   
       12 . The process of  claim 11  wherein the aqueous treatment solution combines alkali metal hydroxide, alkylphenols, sulfonated cobalt pthalocyanine, and water.  
   
   
       13 . The process of  claim 12  wherein the aqueous treatment solution forms two substantially immiscible phases which are a less dense, homogeneous, top phase of dissolved alkali metal hydroxide, alkali metal alkylphenylate, and water, and a more dense, homogeneous, bottom phase of dissolved alkali metal hydroxide and water.  
   
   
       14 . The process of  claim 13  wherein the two immiscible phase are combined with first stage product and allowed to settle.  
   
   
       15 . The process of  claim 1  wherein mercaptan sulfur is removed by adsorption.  
   
   
       16 . The process of  claim 1  wherein the feedstock has a boiling range from about 18° C. to about 221° C.  
   
   
       17 . The process of  claim 1  wherein the first reaction stage for hydrodesulfurization is preceded by a diolefin reactor.