Abstract:
A light cracked naphtha is treated to convert mercaptans to sulfides and saturate dienes and then subjected to destructive hydrodesulfurization (HDS) to convert the organic sulfur compounds to hydrogen sulfide. The recombinant mercaptans formed by reaction of hydrogen sulfide and olefins at the outlet of the HDS are generally heavier than the light cracked naphtha is fractionated in admixture with a heavy cracked naphtha. A low sulfur content light cracked naphtha is produced as an overheads and the major portion of the mercaptans leave with heavy cracked naphtha as bottoms. It also advantageous to pass the heavy cracked naphtha through the HDS in admixture with the light cracked naphtha, since the recombinant mercaptans formed with the heavy cracked naphtha olefins (which displace some of the lower mercaptans which would form the light cracked naphtha olefins) will be even higher boiling and easier to separate by fractionation.

Description:
The present application claims the benefit of U.S. Provisional Application 60/356,474, filed Feb. 13, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a process for removal of sulfur compounds to low levels while minimizing loss of octane. More particularly the invention relates to a process for the removal of mercaptans and thiophenes from a light fluid catalytic cracked naphtha stream. 
     2. Related Information 
     Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, for downstream processing or in order to comply with product specifications or to ensure compliance with environmental regulations. 
     The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
         (1) RSH+H 2 →RH+H 2 S   (2) RCl+H 2 →RH+HCl   (3) 2RN+4H 2 →RH+NH 3      (4) ROOH+2H 2 →RH+H 2 O       

     Typical operating conditions for the HDS reactions are: 
                                                 Temperature, ° F.   600–780           Pressure, psig    600–3000           H 2  recycle rate, SCF/bbl   1500–3000           Fresh H 2  makeup, SCF/bbl    700–1000                        
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha.
 
     In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins. 
     Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C 5  to about 250° F. for light cracked naphtha (LCN) and a heavy boiling range naphtha which boils in the range of from about 250–475° F. or heavy cracked naphtha (HCN). 
     The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. Fractionation alone of the LCN will not remove the mercaptans. Often, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides. 
     In another process the mercaptans in the light cracked naphtha are reacted directly with the dienes contained within the naphtha to form the disulfides. The disulfides may then be subjected to the standard hydrodesulfurization process. However, in the hydrodesulfurization of naphtha which still contains olefins the H 2 S can recombine with olefins at the reactor outlet to produce mercaptans. 
     It is an advantage of the present invention that the sulfur may be removed from an LCN stream without any substantial loss of olefins. 
     SUMMARY OF THE INVENTION 
     Briefly the present invention in its broader view is a process for the removal of organic sulfur compounds comprising mercaptans, preferably recombinant mercaptans, from LCN, which comprises feeding LCN containing, a first amount of organic sulfur compounds comprising mercaptans to a fractionation zone in admixture with a petroleum fraction having a boiling range higher than the boiling range of LCN, such as HCN, and fractionating said admixture under conditions of temperature and pressure to remove a bottoms fraction comprising said petroleum fraction and a portion of the organic sulfur compounds from the LCN and an overheads comprising LCN and a lesser amount of the organic sulfur compounds than that fed to the fractionation zone. 
     In a more specific embodiment the process comprises: hydrotreating an LCN having a first organic sulfur compound content to convert a portion of said organic sulfur compounds to H 2 S and the corresponding olefins and alkanes, removing the H 2 S, recovering said LCN having second organic sulfur compound content, fractionating said LCN in admixture with an HCN under conditions of temperature and pressure to provide an overheads comprising LCN having a third organic compound sulfur content lower than said second organic compound content and a bottoms comprising HCN. 
     In a preferred embodiment the process comprises mixing a light cracked naphtha feed having an organic sulfur content with heavy cracked naphtha having a sulfur content to form a mixture contacting the mixture with an HDS catalyst to convert a portion of the sulfur compounds to H 2 S, removing the H 2 S from the mixture and fractionating the mixture under conditions of temperature and pressure to provide a bottoms comprising HCN and preferably returning a portion of said bottoms to mix with said LCN feed and an overheads comprising LCN having lower organic sulfur content than said LCN feed fractionated in the absence of said HCN. In theory in this embodiment the HCN is recycling within loop of the HDS and the fractionation and to remove recombinant mercaptans in the bottoms. In practice the HCN is purged to prevent a buildup of the heavy organic sulfur compounds and other heavy byproducts and makeup HCN is added. The HCN purge may be hydrotreated to reduce sulfur content then returned as makeup. 
     The LCN feed may previously have been subjected to thioetherification of mercaptans with diolefins to form sulfides and selective hydrogenation of diolefins. Thus, the LCN feed may comprise mercaptans and the sulfides, both of which will react with hydrogen to form H 2 S and the corresponding olefins or alkanes. In this step there is also a recombination of olefins (new and original) with the H 2 S to produce mercaptans (recombinant mercaptans) which frequently end up in the LCN in prior processes, thus not providing the low sulfur content now required in gasolines. 
     The HCN provides a higher boiling material that remains two-phase under hydrodesulfurization conditions so that the reactor does not run dry. The presence of HCN also reduces the AT across the reactor. This helps minimize fouling and extends catalyst life. In the distillation column, the recombinant mercaptans, which are often higher boiling than the initial mercaptans, also distill into the HCN, which contributes to the low sulfur content in the LCN product 
     The olefin content of the LCN is not greatly diminished although larger equipment would be required for the same level of LCN throughput without the HCN recycle, which is small cost to pay for cleaner gasoline. 
     In one embodiment the diolefins in the light cracked naphtha are selectively hydrogenated in a first reactor and then the effluent from the first reactor is combined with heavy cracked naphtha and subjected to destructive hydrodesulfurization in a second reactor to react most of the remaining organic sulfur compound along with the sulfides formed in the first reactor with hydrogen to form H 2 S, which may be stripped out. The effluent from the second reactor is distilled by fractional distillation in a rerun column where a heavy stream is taken as bottoms, comprising the HCN, which may be returned to the second reactor. The recombinant mercaptans formed at the outlet of the second reactor are generally higher boiling than the light naphtha product and are therefore removed and recycled with the heavy bottoms. 
     The HCN or petroleum fraction is preferably present with the LCN in a volume ratio of HCN:LCN generally of 4:1 to 1:4, preferably 3:1 to 1:3, and more preferably 1.5:1 to 1:1.5. Equal amounts of LCN and HCN have been found very effective at achieving low sulfur overheads in the fractionation. Thus, in operating the present process, these and other competing factors, are apparent from the description and examples must be considered in finding the optimum set of conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified flow diagram of one embodiment of the invention. 
         FIG. 2  is a simplified flow diagram of a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The feeds to the process comprise sulfur-containing petroleum fractions which boils in the gasoline boiling range. Feeds of this type include light naphthas having a boiling range of about C 5  to 250° F. and heavy range naphthas having a boiling range of 250 to 475° F. Generally the process is useful on the light naphtha boiling range material from catalytic cracker products because they contain the desired olefins and unwanted sulfur compounds. Straight run naphthas have very little olefinic material, and unless the crude source is “sour”, very little sulfur. 
     The sulfur content of the catalytically cracked fractions will depend upon the sulfur content of the feed to the cracker as well as the boiling range of the selected fraction used as feed to the process. Lighter fractions will have lower sulfur contents than higher boiling fractions. The front end of the naphtha contains most of the high octane olefins but relatively little of the sulfur. The sulfur components in the front end are mainly mercaptans and typical of those compounds are: methyl mercaptan (b.p. 43° F.), ethyl mercaptan (b.p. 99° F.), n-propyl mercaptan (b.p. 154° F.), iso-propyl mercaptan (b.p. 135–140° F.), iso-butyl mercaptan (b.p. 190° F.), tert-butyl mercaptan (b.p. 147° F.), n-butyl mercaptan (b.p. 208° F.), sec-butyl mercaptan (b.p. 203° F.), iso-amyl mercaptan (b.p. 250° F.), n-amyl mercaptan (b.p. 259° F.), a-methylbutyl mercaptan (b.p. 234° F.), α-ethylpropyl mercaptan (b.p. 293° F.), n-hexyl mercaptan (b.p. 304° F.), 2-mercapto hexane (b.p. 284° F.), and 3-mercapto hexane (b.p. 135° F.). Typical sulfur compounds found in the heavier boiling fraction include the heavier mercaptans, thiophenes, sulfides and disulfides. 
     The reaction of mercaptans and diolefins to form sulfides is commonly called thioetherification. A catalyst useful for the mercaptan-diolefin reaction is 58 wt. % Ni on 8 to 14 mesh alumina spheres, supplied by Calcicat, designated as E-475-SR. Typical physical and chemical properties of the catalyst as provided by the manufacturer are as follows: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Designation 
                 E-475-SR 
               
               
                   
                 Form 
                 Spheres 
               
               
                   
                 Nominal size 
                 8 × 14 Mesh 
               
               
                   
                 Ni wt. % 
                 54 
               
               
                   
                 Support 
                 Alumina 
               
               
                   
                   
               
             
          
         
       
     
     The reaction of organic sulfur compounds in a refinery stream with hydrogen over a catalyst to form H 2 S is typically called hydrodesulfurization (HDS). Hydrotreating is a broader term which includes saturation of olefins and aromatics and the reaction of organic nitrogen compounds to form ammonia. However hydrodesulfurization is included and is sometimes simply referred to as hydrotreating. 
     Catalysts which are useful for the hydrodesulfurization reaction contain components from Group V, VIB, VIII metals of the Periodic Table or mixtures thereof. The Group VIII metal provides increased overall average activity. Catalysts containing a Group VIB metal such as molybdenum and a Group VIII such as cobalt or nickel are preferred. Catalysts suitable for the hydrodesulfurization reaction include cobalt-molybdenum, nickel-molybdenum and nickel-tungsten. The metals are generally present as oxides supported on a neutral base such as alumina, silica-alumina or the like. The metals are converted to the sulfide either in use or prior to use by exposure to sulfur compound containing streams. The catalyst may also catalyze the hydrogenation of the olefins contained within the light cracked naphtha and to a lesser degree the isomerization of some of the mono-olefins. The hydrogenation, especially of the mono-olefins in the lighter fraction may not be desirable. 
     The properties of a typical hydrodesulfurization catalyst are shown in Table II below. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
             
             
               
                   
                 Manufacture 
                 Criterion Catalyst Co. 
               
               
                   
                 Designation 
                 C-448 
               
               
                   
                 Form 
                 Tri-lobe Extrudate 
               
               
                   
                 Nominal size 
                 1.2 mm diameter 
               
               
                   
                 Metal, Wt. % 
               
               
                   
                 Cobalt 
                 2–5% 
               
               
                   
                 Molybdenum 
                 5–20% 
               
               
                   
                 Support 
                 Alumina 
               
               
                   
                   
               
             
          
         
       
     
     The catalyst typically is in the form of extrudates having a diameter of ⅛, 1/16 or 1/32 inches and an L/D of 1.5 to 10. The catalyst also may be in the form of spheres having the same diameters. They may be directly loaded into the single pass fixed bed reactor which includes supports and reactant distribution structures. 
     Reaction conditions for sulfur removal only in a standard single pass fixed bed reactor are in the range of 500–700° F. at pressures of between 400–1000 psig. Residence times expressed as liquid hourly space velocity are generally typically between 1.0 and 10. The naphtha in the single pass fixed bed reaction may be in the liquid phase or gaseous phase depending on the temperature and pressure, with total pressure and hydrogen gas rate adjusted to attain hydrogen partial pressures in the 100–700 psia range. The operation of the single pass fixed bed hydrodesulfurization is otherwise well known in the art. These reactions are very effective and may be operated to convert substantially all of the organic sulfur compounds to H 2 S and the corresponding olefins (some corresponding alkanes are also produced). However, the recovered streams, will still contain some mercaptans, regardless of the severity of the HDS conditions (note more sever conditions will result in the hydrogenation of olefins and the corresponding loss of octane), which have been found to result from the reversible reaction of H 2 S with available olefins. 
     As described above, by mixing the HCN with the LCN for the HDS, results in a greater variety of olefins being available for the recombination, particularly higher boiling olefins, which produce mercaptans of higher boiling point, i.e., well above the end point of the LCN fraction. Hence, the benefit of the HCN in the process can be at least twofold depending on its entry in the present process. Even if the HCN is added only to the fractionation, the presence of the substantial amount of heavier components improves and facilitates the removal of the higher boiling mercaptans from olefins of the LCN, for example: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Species 
                 Normal Boiling Point 
               
               
                   
                   
               
             
             
               
                   
                 1-hexene 
                 336.6 Kelvin 
               
               
                   
                 1-hexanethiol 
                 424.0 Kelvin 
               
               
                   
                 2-hexanethiol 
                 415.0 Kelvin 
               
               
                   
                 1-heptene 
                 366.8 Kelvin 
               
               
                   
                 1-heptanethiol 
                 450.0 Kelvin 
               
               
                   
                 2-heptanethiol 
                 409.8 Kelvin 
               
               
                   
                   
               
             
          
         
       
     
     Referring now to  FIG. 1  one embodiment of the invention is shown. The light cracked naphtha in flow line  101  is combined with hydrogen from flow line  103  and fed to a hydrogenation reactor  10  containing beds  12   a  and  12   b  of selective hydrogenation catalyst where the mercaptans in the light cracked naphtha are converted to H 2 S (and the corresponding olefins) and the diolefins are saturated. The effluent from the hydrogenation and the HCN from flow line  102  in flow line  106 / 107  is combined with hydrogen from flow line  105  and fed to hydrodesulfurization reactor  20  containing beds  22   a  and  22   b  of hydrodesulfurization catalysts where the thiophenes and other sulfur species are reacted with hydrogen to form hydrogen sulfide. At the same time a portion of the hydrogen sulfide reacts with olefins in the light cracked naphtha to form recombinant mercaptans which generally are higher boiling than the light cracked naphtha. 
     The effluent from the hydrodesulfurization reactor is fed via flow line  108  to a high pressure separator  30  where the hydrogen and most of the hydrogen sulfide are flashed overheads with the liquid taken via flow line  112 . The overheads are then cooled and sent to secondary separator  40  where the hydrogen and hydrogen sulfide are removed. The vapors in flow line  110  may be scrubbed to remove hydrogen sulfide and the hydrogen recycled. The liquids from the separators in flow lines  111  and  112  are fed to stabilizer column  50  where C 5 &#39;s and lighter material are taken as overheads via flow line  113 . The stabilized product is taken as bottoms via flow line  114  and fed to distillation column  60  where the heavier mercaptans and the HCN are separated from the hydrotreated light naphtha as bottoms in line  116 . The hydrotreated light naphtha product is taken as overheads via flow line  118 . 
     The conditions in the hydrodesulfurization reactor may be such that the entire feed is vaporized or is maintained to provide a liquid phase in the hydrodesulfurization. The fixed bed, straight pass liquid phase is preferably operated as a trickle bed. 
     In  FIG. 2  as in the  FIG. 1  the light cracked naphtha in flow line  101  is combined with hydrogen from flow line  103  and fed to etherification reactor  10  containing beds  12   a  and  12   b  of thioetherification catalyst where the mercaptans in the light cracked naphtha are reacted with diolefins in the light cracked naphtha to form sulfides. 
     The effluent from the thioetherification reactor in flow line  106  is combined with hydrogen from flow line  105  and the hydrotreated heavy naphtha in flow line  102  and fed to hydrodesulfurization reactor  20  containing beds  22   a  and  22   b  of hydrodesulfurization catalysts where the thiophenes and sulfides are reacted with hydrogen to form hydrogen sulfide. Makeup HCN is added vial line  102   a . At the same time a portion of the hydrogen sulfide reacts with olefins in the light cracked naphtha to form recombinant mercaptans which generally are higher boiling than the light cracked naphtha. 
     The effluent from the hydrodesulfurization reactor is fed via flow line  108  to a high pressure separator  30  where the hydrogen and most of the hydrogen sulfide are flashed overheads with the liquid taken via flow line  112 . The overheads are then cooled and sent to secondary separator  40  where the hydrogen and hydrogen sulfide are removed. The vapors in flow line  110  may be scrubbed to remove hydrogen sulfide and the hydrogen recycled. The liquids from the separators in flow lines  111  and  112  are fed to stabilizer column  50  where C 5 &#39;s and lighter material are taken as overheads via flow line  113 . The stabilized product is taken as bottoms via flow line  114  and fed to distillation column  60  where the heavier mercaptans and the heavy naphtha are separated from the hydrotreated light naphtha as bottoms in line  116 . The hydrotreated light naphtha product is taken as overheads via flow line  118 . A portion of the bottoms may be purged via flow line  117  or combined with the light naphtha product (depending on the limitations on sulfur content for the intended market of the product). The remainder of the bottoms is recycled via flow line  102  to the hydrodesulfurization reactor. 
     EXAMPLE 1 
     Light cracked naphtha (LCN) and a heavy cracked naphtha (HCN) having the characteristics shown in TABLE III was used as feed to the HDS process (13 lb/hr each). The LCN used had been previously subjected to selective hydrogenation of the dienes in a reactor containing a 20% Ni catalyst at the following conditions: Inlet Temp of 220° F.; Inlet pressure of 231 psig and a 2 WHSV to yield a 90+% diene saturation rate. 
     The mixture was passed through a fixed bed cocurrent reactor containing a standard hydrodesulfurization catalyst at the following conditions:
         Inlet temperature 479° F.;   Inlet pressure 219 psig   WHSV of 6.5.       

     Following the reaction the H 2 S was stripped and removed. The LCN was recovered by distillation and the HCN recycled. The final LCN product had the properties shown in TABLE III. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                   
                 HCN 
                   
               
               
                   
                 LCN 
                 ASTM D3710, ° F. 
                 LCN PRODUCT 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 IBP 
                 137 
                 231 
                 136 
               
               
                  5% 
                 141 
                 259 
                 140 
               
               
                 10% 
                 143 
                 281 
                 141 
               
               
                 20% 
                 150 
                 300 
                 148 
               
               
                 30% 
                 161 
                 326 
                 158 
               
               
                 40% 
                 175 
                 337 
                 173 
               
               
                 50% 
                 186 
                 343 
                 184 
               
               
                 60% 
                 196 
                 364 
                 193 
               
               
                 70% 
                 205 
                 376 
                 201 
               
               
                 80% 
                 220 
                 398 
                 215 
               
               
                 90% 
                 235 
                 413 
                 229 
               
               
                 95% 
                 240 
                 431 
                 231 
               
               
                 EP 
                 359 
                 455 
                 248 
               
               
                 Total sulfur 
                 289 wppm 
                 12.2 wppm 
                 1.2 wppm 
               
               
                 Bromine # 
                 60 g/100 g 
                 2.9 g/100 g 
                 43 g/100 g 
               
               
                 R Octane 
                  88.8 
                 N/A 
                  85 
               
               
                 M Octane 
                  79.4 
                 N/A 
                  78.1 
               
               
                   
               
             
          
         
       
     
     As shown by above the distillation curve, the LCN was recovered according expectations from the LCN/HCN mixture. There were a few heavy components in the tail of the LCN which were dropped into the HCN fraction, but there is good agreement between the feed and product boiling points from the initial to the 95% points. The data shows a 99.6 sulfur reduction with only a 28.3% Br# loss, and only an R octane loss of 3.9 and M octane loss of 1.3. 
     EXAMPLE 2 
     A second LCN and HCN having the characteristics shown in TABLE IV were used as feed to the process (13 lb/hr each). The LCN had been previously subjected to selective hydrogenation (thioetherification and saturation) of the dienes in a reactor containing a 20% Ni catalyst at the following conditions: Inlet Temp of 235° F.; Inlet pressure of 260 psig and a 2 WHSV to yield a 90+% diene saturation rate. 
     The mixture was passed through a fixed bed containing a standard hydrodesulfurization catalyst at the following conditions: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Inlet temperature, ° F. 
                 472 
               
               
                   
                 Outlet temperature, ° F. 
                 534 
               
               
                   
                 Operating pressure, psig 
                 250 
               
               
                   
                 H 2  rate, scf/bbl 
                 389 
               
               
                   
                 WHSV 
                 6.5. 
               
               
                   
                   
               
             
          
         
       
     
     Following the reaction the H 2 S was stripped and removed. Most of the sulfur was observed to be in the form of mercaptans. The LCN was recovered by distillation and the HCN recycled. The final LCN product had the properties shown in TABLE IV 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
               
               
             
           
               
                   
                 TABLE IV 
               
             
             
               
                   
                   
               
               
                   
                 FEED 
                 FINAL MXD. 
                 After Distillation 
               
             
          
           
               
                   
                 LCN 
                 HCN 
                 PRODUCT 
                 LCN 
                 HCN 
               
               
                   
                   
               
             
          
           
               
                 Total S (mg S/L) 
                 191 
                 17.79 
                 10.81 
                 6.39 
                 18.43 
               
               
                 Bromine # 
                 62.86 
                 5.01 
                 27.77 
                 53.11 
                 5.03 
               
               
                 Density (g/cc) 
                 0.7197 
                 0.8322 
                 0.7728 
                 0.7206 
                 0.832 
               
               
                 Mercaptan (ppm) 
                 2.4 
                 13.3 
                 11.8 
                 1.5 
                 14.5 
               
             
          
           
               
                 Boiling Range D3710 
               
             
          
           
               
                 ibp ° F. 
                 135 
                 237 
                 139 
                 136 
                 237 
               
               
                  5% pt ° F. 
                 139 
                 259 
                 147 
                 140 
                 259 
               
               
                 10% pt ° F. 
                 141 
                 278 
                 158 
                 146 
                 278 
               
               
                 20% pt ° F. 
                 148 
                 285 
                 178 
                 156 
                 285 
               
               
                 30% pt ° F. 
                 159 
                 319 
                 198 
                 168 
                 319 
               
               
                 40% pt ° F. 
                 172 
                 329 
                 218 
                 182 
                 329 
               
               
                 50% pt ° F. 
                 183 
                 341 
                 242 
                 190 
                 341 
               
               
                 60% pt ° F. 
                 193 
                 355 
                 282 
                 195 
                 355 
               
               
                 70% pt ° F. 
                 204 
                 374 
                 326 
                 206 
                 374 
               
               
                 80% pt ° F. 
                 218 
                 396 
                 352 
                 219 
                 396 
               
               
                 90% pt ° F. 
                 229 
                 413 
                 394 
                 229 
                 413 
               
               
                 95% pt ° F. 
                 235 
                 432 
                 411 
                 230 
                 432 
               
               
                 fbp ° F. 
                 357 
                 458 
                 449 
                 247 
                 459 
               
             
          
           
               
                 Amount of LCN (lb/h) 
                 13 
               
               
                 Amount of HCN (lb/h) 
                 13 
               
               
                   
               
             
          
         
       
     
     In the distillation, the mercaptans tend to distill downward, and out of the final product, leaving very low residual S in the finished LCN. The mercaptans are then recycled back to the HDS reactor where they are reconverted to H 2 S.