Abstract:
The invention relates to a hydrotreating and hydrocracking process for various oils nominally boiling between 600 and 1500° F. to produce diesel oil and lighter materials. The process includes a first hydrogenation reaction in the presence of multiple hydrogenation catalyst beds which is limited to the hydrogenation level needed for the removal of sulfur and nitrogen and for aromatic saturation and to produce an effluent of both hydrocracked oil and uncracked heavy oil. The effluent is then flashed to produce hydrocracked oil vapors and liquid uncracked heavy oil. The hydrocracked oil fraction is further hydrotreated by catalytic distillation in a post-treatment reactor to give the final product quality while the liquid uncracked heavy oil bypasses the post-treatment reactor. The process significantly reduces hydrogen consumption and reduces the overall reactor and catalyst volumes for a given level of performance.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the hydrocracking of vacuum gas oil or various other typical hydrocracking feedstock oils or mixtures thereof. 
     In hydrocracking technology, reactor operating conditions are dictated either by product quality requirements or by catalyst life. It is impossible to optimize processing conditions in a single reactor because operating conditions in the reactor are set by the most difficult components of the feed. For example, the conditions in the reactor could be set by the amount of nitrogen in the feed. Typically, in the first reactor treating raw feed, conditions are severe (high-temperature) and not conducive to aromatic saturation. Moreover, once products are formed from hydrocracking reactions, they compete with the heaviest fractions of the feed (nominally 700° F.+ material) to gain access to the active catalyst sites. Occlusion of the products (700° F.− material) from the active sites by the heavy products is very likely. 
     Consequently, for a given conversion level, single reactor systems operating at the same pressure levels as multi-reactor systems produce inferior quality products. In order to compensate for this shortfall in product quality, units are run at higher pressures and with lower space velocities. In most cases, there is considerable giveaway in product quality for at least one major product especially at start-of-run conditions, as operators select an operating pressure level to guarantee the quality of all products and extend the catalyst run length. For example, the hydrocracked Jet/Kerosene Smoke Point is often 30 mm at start-of-run when the specification requires 20 mm. Similarly, the hydrocracked Diesel Cetane Index is often around 60 when the required value is 50. This product quality giveaway translates to a waste of hydrogen. In most refineries, hydrogen is an expensive commodity. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a hydrocracking and hydrotreating process which minimizes hydrogen consumption and reduces the overall reactor and catalyst volumes for a given level of performance for the production of diesel oil and lighter materials including kerosene and naphtha. The process provides a first hydrogenation reaction which is limited to the hydrogenation level needed for hydrotreating the feed for the reduction of sulfur and nitrogen and for aromatic saturation and for the hydrocracking to form the diesel and lighter materials. The uncracked heavy fraction that does not require hydrogenation beyond the sulfur and nitrogen removal and aromatic saturation is separated and bypassed around a second, post-treatment hydrogenation in which only the diesel and lighter materials are further hydrogenated thereby reducing the hydrogen consumption. The objects of the invention are accomplished through the use of a main catalytic reactor operating at conditions which produce an effluent of hydrocracked oil and uncracked heavy oil followed by an intermediate vapor/liquid separator and a post-treatment reactor involving reactive distillation for final hydrocracking and hydrotreating. The primary reaction achieves a partial level of conversion without meeting final product quality with the post-treatment reaction operating to hydrogenate only the separated distillates to meet final product specifications. The invention also allows for advantageous feed locations for certain specific feed materials. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a process flow diagram illustrating the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention relates to the hydrocracking and hydrotreating of various oils from distillation or from solvent extraction nominally boiling between 600 or 700° F. up to about 1500° F. In particular, the invention relates to the hydrocracking and hydrotreating of vacuum gas oil or various other known feedstock oils typically processed by hydrocracking such as light cycle oil, coker gas oil, visbreaker gas oil and deasphalted oil. Typically, vacuum gas oil forms the bulk of the feed usually with some quantity of one or more of the other oils. By way of explanation, vacuum gas oil is that fraction of the crude oil that typically boils between about 600° F. and 1200° F. and is derived by the vacuum distillation of residue from the atmospheric distillation column in a petroleum refinery. Depending on the crude source and the boiling range, the composition of paraffins, naphthenes and aromatics and the level of contaminants like sulfur, nitrogen, metals, asphaltenes, etc. can vary widely. Vacuum gas oil is the primary component of feedstock to conversion units such as hydrocracking. A typical vacuum gas oil has the following properties: 
     Specific Gravity: 0.85 to 0.98 
     Total Nitrogen, ppm: 100 to 5000 
     Total Sulfur, ppm: 0.1 to 4.0 
     Metals (Ni+V), ppm: 0.1 to 2 
     Distillation range: 600° F. to 1200° F. 
     Light cycle oil is the light distillate obtained from fluid catalytic cracking of vacuum gas oil in a petroleum refinery. The typical boiling range is 400° F. to 800° F. Light cycle oil is a highly aromatic compound (40-90 wt. % aromatics) and is also high in sulfur. Visbreaker gas oil is the distillate obtained after the fractionation of products obtained from thermally cracking vacuum residue in a visbreaking process. It is high in olefins, nitrogen and sulfur. The typical boiling range is 600° F. to 1000° F. Deasphalted oil is obtained after solvent extraction of the vacuum residue fraction of crude oil in a solvent deasphalting unit. The solvent is typically propane, butane or pentane, and the deasphalted oil is high in metals, nitrogen and sulfur. The typical boiling range is 900° F. to 1500° F. 
     Referring to the flow diagram of the drawing, a preheated feed  12  of vacuum gas oil and/or other typical hydrocracking feedstock oils, such as coker gas oil and visbreaker gas oil, is fed through and further heated in the heat exchangers  14  and  16  and then fed at  17  to the feed heater  18  in admixture with the hydrogen-rich gas from line  20 . The hydrogen-rich gas in line  20  is the hydrogen-rich recycle from the compressor  22  and the make-up hydrogen  23  from the compressor  24 . 
     The mixture of feed oil and hydrogen is fed from the feed heater  18  to the top of the main reactor  26 . The main reactor  26  is a cocurrent, downflow reactor containing a plurality of catalyst beds  28   a ,  28   b  and  28   c . Although three beds have been illustrated, there could be more or less for any particular operating situation. The catalyst may be any hydrogenation catalyst such as those from the following list: 
     Nickel-molybdenum on alumina 
     Nickel-molybdenum on silica-alumina with zeolites 
     Paladium/alumina/zeolite 
     Nickel/tungsten/titanium silica-alumina with zeolites 
     Nickel/tungsten on zeolite 
     Cobalt-molybdenum on alumina 
     Cobalt-molybdenum on zeolite 
     The catalyst metals may be impregnated, co-gelled or co-mulled on the base. 
     In the main reactor  26 , the feed is hydrogenated in the presence of the catalyst to hydrotreat for the removal of sulfur and nitrogen compounds and for the saturation of aromatics and to upgrade the feed oils by hydrocracking to produce the lighter products. Although the bulk of the hydrotreating and hydrocracking reactions occur in the main reactor, conditions are maintained including a reduced hydrogen partial pressure and/or a high space velocity whereby fairly high conversions are still achieved but without expending the large quantities of hydrogen which would otherwise be required to fully hydrogenate the heavy oils and to meet the final quality required for the diesel and lighter distillate products. The space velocity may be as much as 15% higher or the hydrogen partial pressure as much as 20% lower or some combination of these changes as compared to a conventional hydrogenation process. 
     In the main reactor  26 , the heated hydrogen/feed mixture  27  flows down through each of the beds  28   a ,  28   b  and  28   c  in series with additional hydrogen  30 , preferably from the recycle compressor  22  as shown, being added between the beds to quench and maintain the hydrogen partial pressure. For a typical light vacuum gas oil feed with a feed rate of 35,000 barrels per day and containing 800 ppm nitrogen and 2.3 weight percent sulfur, a typical example of the operating conditions within the main reactor  26  are as follows: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 H 2 -rich gas with feed 
                 range 50-300 million standard ft 3 /day 
               
               
                   
                 typical 175 million standard ft 3 /day 
               
               
                 Recycle quench gas 
                 range 0-200 million standard ft 3 /day 
               
               
                   
                 typical 105 million standard ft 3 /day 
               
               
                 Make-up hydrogen 
                 range 10-70 million standard ft 3 /day 
               
               
                   
                 typical 30 million standard ft 3 /day 
               
               
                 Weighted average bed temperature 
                 range 550 to 800° F. 
               
               
                   
                 typical 730° F. 
               
               
                 Operating pressure 
                 range 1000 to 3500 psig 
               
               
                   
                 typical 1900 psig 
               
               
                   
               
             
          
         
       
     
     Exiting the bottom of the main reactor  26  is the partially hydrogenated intermediate product stream  32  which now contains hydrogen sulfide, ammonia, some excess hydrogen, uncracked heavy hydrotreated oil having a nominal 700° F.+ boiling point, and the hydrocracked product diesel and lighter materials having a nominal 700° F.− boiling point. This product stream  32  passes through the heat exchanger  16  to transfer heat to the incoming feed stream  17 . The partially hydrogenated intermediate product stream  32  is flashed in the hot, high-pressure separator  34  to vaporize and recover the majority of the distillates (diesel fuel, kerosene, naphtha) as overhead  36 . In the example, the hot separator operates at a temperature of about 600 to 800° F. and a pressure in the range of 1,000-3,500 psig. The temperature in the hot separator  34  is regulated to minimize the vaporization of unconverted oil in the overhead. The heavy product oil effluent  38  from the bottom of the hot separator  34  is the unconverted portion of the feed oil. Although this is basically an unconverted oil, it has undergone hydrodenitrification and hydrodesulfurization and also a substantial amount of aromatic saturation. One of the features of the invention is that the amount of hydrogen used by the heavy oil product is minimized. This is done by bypassing the heavy product oil effluent  38  around the portion of the active catalyst in the post-treatment reactor  40 . The heavy product oil effluent  38  may later be combined with the overall product as will be described or it may be separately processed. 
     The overhead  36  from the hot separator  34  is fed as stream  44  to the post-treatment reactor  40 . The post-treatment reactor  40  contains an upper bed  42  above the feed  44  and a lower bed  46  below the feed. The feed is primarily a mixture of vapor with some condensate. Hydrogen  48  is fed to the bottom of the post-treatment reactor and flows up through both beds. A small quantity of cold reflux  50  is added to the top of the post-treatment reactor as a cooling quench and to wash down any unconverted oil. The upper bed  42  is a hydrogenation catalyst bed. The vapor fraction of the feed  44 , essentially the diesel and lighter materials, flows up through the bed  42  in contact with the hydrogen flowing up in a cocurrent manner to complete the hydrogenation of these products. In the bottom bed  46 ., the liquid portion of the feed, essentially entrained unconverted oil from the hot separator with some diesel and perhaps lighter material, is stripped of the diesel and lighter material by the hydrogen moving up through the bed counter-current to the liquid flowing down. Depending on the degree of post treatment required for any particular situation, the bottom bed  46  can be packed with either a highly efficient inert structural packing for stripping or with an active hydrotreating catalyst for reactive stripping. If it is required to meet the post treatment reactor requirements, the vapor  44  from the hot separator  34  can be cooled by heat exchange at  14  against the main reactor feed  12 . As a further alternative, if light cycle oil  52  obtained from the fluid catalytic cracking of vacuum gas is a desired feed component, it is preferably fed to the process after the hot separator  34  and prior to the post-treatment reactor  40  because it can cause rapid catalyst deactivation. However, it can also be fed to the main reactor  26  along with the other oils. Following up on the specific operating conditions previously recited, a specific example of the operating conditions in the post-treatment reactor  40  are as follows: 
     Average bed temperature 
     range 500 to 750° F. 
     typical 675° F. 
     Operating pressure 
     range 1000 to 3500 psig 
     typical 1900 psig 
     Hydrogen feed (48) 
     range 2 to 30 million standard ft 3 /day 
     typical 9 million standard ft 3 /day 
     The vapor effluent  54  from the post-treatment reactor  40  contains the diesel and lighter distillate products along with the remaining hydrogen and the hydrogen sulfide and ammonia from the sulfur and nitrogen removed from the feed. The effluent  54  is partially cooled by heat exchange at  56  against the hydrogen feed  20 . The partially cooled stream  54  is then injected with water at  58  to prevent the deposition of ammonium bisulfide that may form when the reactor effluent is being cooled. The partially cooled effluent stream  54  is then cooled further at  60  to condense the product hydrocarbons, such as the diesel oil, kerosene and naphtha, leaving the hydrogen and some lighter hydrocarbons as vapor. The stream  62  is now a three-phase mixture of gases, liquid hydrocarbon and an aqueous phase. These three phases are separated in the cold high-pressure separator  64  with the hydrogen-rich gaseous phase  66  forming the recycle to the recycle compressor  22  and with the sour water phase being discharged at  68 . The liquid hydrocarbon phase is discharged at  70 . 
     Returning now to the post-treatment reactor  40 , the bottoms  72  containing primarily unconverted oil is combined with the unconverted oil  38  from the bottom of the hot separator  34 . This combined stream  74  is cooled at  76  to recover heat by heating other process streams in this unit. Then the unconverted oil is flashed in the hot low-pressure separator  78  to recover light gases and hydrogen. The bottoms  82  from the hot low-pressure separator  78  form a portion of the combined product stream  84 . The vapor stream  80  from the hot low-pressure separator  78  is partially cooled at  86  and then further cooled at  88  and combined with the hydrocarbon effluent  70  from the cold high-pressure separator  64 . This forms the stream  90  which again is a three-phase stream which is separated at  92  to form the vapor stream  94 , the sour water stream  96  and the hydrocarbon product stream  98 . The vapor stream  94  containing some hydrogen is sent for recovery of that hydrogen and any other desired constituents. 
     A portion of the hydrocarbon product stream  98  is withdrawn to form the reflux  50  to the post-treatment reactor  40 . The remaining hydrocarbon product stream  100  passes through the heat exchanger  86  and is combined with the unconverted oil stream  82 . The total product stream  84  is then sent for separation, such as in the generally designated distillation system  102 , into the various components such as diesel oil, kerosene, naphtha and unconverted oil. 
     In the present invention, two distinct reactor stages, the main reactor and the post-treatment reactor, are combined with an intermediate vapor/liquid separation to reduce the overall catalyst volume, the reactor weight, the hydrogen consumption, the product quality giveaway and to increase the process flexibility. The first or main reactor stage is operated at conditions including the hydrogen level and space velocity whereby the unconverted oil is only treated to the level necessary to meet the quality requirements such as saturation of aromatics and hydrodesulfurization and hydrodenitrification. Essentially all of the hydrotreating and most of the hydrocracking takes place in this first reactor. The unconverted oil then bypasses the post-treatment reactor in which the hydrocracking of the distillates is completed to the extent required to meet the final product specifications. This selective addition of hydrogen, as opposed to the addition of all of the hydrogen in a single reactor under non-optimum conditions, leads to a significant reduction in hydrogen consumption, perhaps by 5-30%. Further, the operating pressures can be lowered for the same catalyst volume, perhaps by about 5-30%, or the catalyst volume can be lowered by about 5-30% at the same operating pressure. 
     In the invention, the heaviest portion of the feed that contains the bulk of the sulfur and nitrogen, is hydrotreated only to the extent necessary and is then separated so that it does not come into contact with the portion of the catalyst in the post-treatment reactor which would otherwise be deactivated at a higher rate.