Abstract:
The invention relates to a hydrocracking and hydrotreating process for hydrocracking feedstock oils such as vacuum gas oil to produce diesel and lighter distillate products. A first hydrogenation process is carried out in a main reactor with the feedstock and hydrogen flowing cocurrently down through a top section containing a layered system of hydrotreating and hydrocracking catalyst. The feedstock is substantially desulfurized and denitrified, the aromatics are at least partially saturated and cracked products are formed. The vapor and liquid are separated in a disengaging zone below the top section and the liquid flows down through a bottom section also containing a layered catalyst system countercurrent to make-up hydrogen flowing up. The vapor removed from the disengaging zone and the liquid bottoms are then further processed in a post treatment catalytic distillation reactor having an upper catalytic distillation section and a lower stripping section which may also contain a catalyst. Hydrogen for recycle and hydrogen sulfide and ammonia are removed from the post treatment reactor vapors leaving the product distillates.

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 invention includes a first hydrogenation process in a main reactor employing a combination of cocurrent and countercurrent processing. The hydrocracking feedstock oils together with hydrogen are fed down through the top section of the main reactor containing a layered system of hydrotreating and hydrocracking catalysts for the cocurrent processing. Below the top section is a vapor/liquid disengaging zone where vapor and liquid are separated and the vapor containing the majority of the hydrogen sulfide and ammonia formed by hydrotreating and the majority of the diesel oil and lighter materials formed by hydrocracking are withdrawn. The separated liquid then flows down through the lower section also containing catalyst beds for the countercurrent processing. High purity make-up hydrogen is introduced up into the bottom of the reactor which flows up countercurrent to the downflowing liquid for further hydrogenation and for stripping the diesel and lighter materials present or formed in the lower countercurrent section. The bottoms containing primarily the uncracked heavy oil fraction together with some diesel and lighter materials and the separated vapors from the vapor/liquid disengaging zone are then processed in a post-treatment reactor to complete the hydrotreating and hydrocracking for final product quality and to facilitate the nitrogen and sulfur separation. The process using the cocurrent/countercurrent main reaction system with make up hydrogen addition at the bottom in combination with the post-treatment reaction system favors sulfur and nitrogen removal and aromatic saturation as well as final product yields and quality using lower reactor volume, lower catalyst volume, increased catalyst run length and lower hydrogen consumption. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a process flow diagram illustrating the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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 in line  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 recycle compressor  22 . 
     The heated mixture of feed oil and hydrogen is fed in line  24  from the feed heater  18  to the top of the main reactor  26 . The main reactor  26  is a cocurrent/countercurrent reactor containing an upper cocurrent section generally designated  28 , a center vapor/liquid disengaging zone and a lower countercurrent section  32 . The upper cocurrent section is illustrated as including three catalyst beds  28   a,    28   b  and  28   c  although there could be more or fewer catalyst beds for any particular feed material and operating situation. The lower countercurrent section  32  contains the two catalyst beds  32   a  and  32   b  but once again there could be more or fewer. 
     The heated feed  24  containing the feed oil and the hydrogen is passed down through the catalyst beds  28   a,    28   b  and  28   c  with hydrogen-rich recycle vapor from line  20  being injected at the locations  34  between the catalyst beds to maintain a desired hydrogen partial pressure and as a quench to keep the reaction temperature from rising too high as a result of the exotheric reactions. 
     In this top, cocurrent section  28 , anywhere from 5% to 90% of the feed will be converted to diesel and lighter materials with boiling points nominally below about 700° F. depending on the particular feed oil. Also, a large percentage of the sulfur and nitrogen will be removed from the feed oil by hydrodesulfurization and hydrodenitrification to form hydrogen sulfide and ammonia. In the vapor/liquid disengaging zone  30 , the vapor from the upper cocurrent section and the vapor from the lower countercurrent section, as both described in more detail below, are separated and withdrawn from the reactor  26  as the side stream  36 . 
     The liquid separated from the vapor in the vapor/liquid disengaging zone  30  is redistributed over the catalyst beds in the bottom countercurrent section  32  and flows downwardly countercurrent to the hydrogen flowing up. All of the make-up hydrogen  38  from the hydrogen compressor  40  that is required for the reactions is fed to the bottom of the reactor  26 . The make-up hydrogen is the purest hydrogen stream in the process and the introduction from below the bottom catalyst beds  32   a  and  32   b  leads to a higher hydrogen partial pressure in the countercurrent section  32  than would otherwise be possible. This higher hydrogen partial pressure favors aromatic saturation and greatly reduces catalyst deactivation. This results in less catalyst being needed to achieve a specific level of performance. Also, the operating temperatures in the bottom catalyst beds are lower because of the removal of the sulfur and nitrogen. This lower temperature also favors aromatic saturation. As in the upper section, the hydrogen-rich vapor from the recycle stream  20  is introduced at  42  between the catalyst beds for quench, stripping out light end products and maintenance of the hydrogen partial pressure. The catalysts in the main reactor, as well as the post treatment reactor discussed later, may be any conventional 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. 
     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 50-30 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 35 million standard ft 3 /day 
               
               
                 Cocurrent section: 
               
               
                 Weighted Average Bed Tempera- 
                 range 550-800 
               
               
                 ture of Catalyst, ° F.: 
                 typical 730° F. 
               
               
                 Average Pressure, psig 
                 range 1000-3500 
               
               
                   
                 typical 2300 psig 
               
               
                 Counter-current section: 
               
               
                 Weighted Average Bed Tempera- 
                 range 530-800 
               
               
                 ture of Catalyst, ° F.: 
                 typical 690° F. 
               
               
                 Average Pressure, psig 
                 range 1000-3500 
               
               
                   
                 typical 2250 psig 
               
               
                   
               
             
          
         
       
     
     The vapor stream  36  from the vapor/liquid disengaging zone  30  is the combined vapor from the cocurrent section  28  and the countercurrent section  32  and contains most of the diesel and lighter materials, most of the hydrogen sulfide and ammonia and any unreacted hydrogen. This stream  36  is cooled at  14  by the transfer of heat to the incoming feed stream  12  and is then fed to the center feed section  44  of the post treatment reactor  46 . The bottoms  48  from the main reactor  26  contains mainly the uncracked heavy hydrotreated oil having a nominal 700° F.+boiling point. This bottoms stream  48  will also contain some diesel and lighter materials. The bottoms stream  48  is cooled at  16  also by the transfer of heat to the incoming feed stream  12  and is then also fed to the center feed section  44  of the post treatment sector  46 . 
     The post-treatment reactor  46  contains an upper bed  49  above the feed section  46  and a lower bed  50  below the feed section  46 . Hydrogen  52  is fed to the bottom of the post-treatment reactor  46  and flows up through both beds. A small quantity of cold reflux  54  is added to the top of the post-treatment reactor as a cooling quench and to wash down any unconverted oil. The upper bed  49  is a hydrogenation catalyst bed. The vapor fraction of the feed, essentially the diesel and lighter materials and the hydrogen sulfide and ammonia, flows up through the bed  49  in contact with the hydrogen flowing up in a cocurrent manner to complete the hydrogenation of these products. In the bottom bed  50 , the liquid portion of the feed, essentially the unconverted heavy oil 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  50  can be packed with either a highly efficient inert structural packing for stripping or with an active hydrotreating catalyst for reactive stripping. If light cycle oil  56  obtained from the fluid catalytic cracking of vacuum gas is a desired feed component, it is preferably fed to the process after the main reactor  26  and prior to the post-treatment reactor  46  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  46  are as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Average bed temperature 
                 range 500 to 750° F. 
               
               
                   
                   
                 typical 675° F. 
               
               
                   
                 Operating pressure 
                 range 1000 to 3500 psig 
               
               
                   
                   
                 typical 2300 psig 
               
               
                   
                 Hydrogen feed 
                 range 2 to 30 million standard ft 3 /day 
               
               
                   
                   
                 typical 9 million standard ft 3 /day 
               
               
                   
                   
               
             
          
         
       
     
     The vapor effluent  58  from the post-treatment reactor  46  contains the diesel and lighter distillate products along with the hydrogen sulfide and ammonia and unreacted hydrogen. The effluent  58  is partially cooled by heat exchange at  60  against the hydrogen-rich recycle stream  20 . The partially cooled stream  58  is then injected with water at  62  to prevent the deposition of ammonium bisulfide that may form when the reactor effluent is being cooled. The partially cooled effluent stream  58  is then cooled further at  64  to condense the product hydrocarbons, such as the diesel oil, kerosene and naphtha, leaving the hydrogen and some lighter hydrocarbons as vapor. The stream  66  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  68  with the hydrogen-rich gaseous phase  70  forming the recycle to the recycle compressor  22  and with the sour water phase being discharged at  72 . The liquid hydrocarbon phase is discharged at  74 . 
     Returning now to the post-treatment reactor  46 , the bottoms  76  containing primarily unconverted oil is cooled at  78  to recover heat. Then the unconverted oil is flashed to recover light hydrocarbons which are then separated in the hot low-pressure separator  80  as the vapor stream  82 . The bottoms  84  from the hot low-pressure separator  80  form a portion of the combined product stream  86 . The vapor stream  82  from the hot low-pressure separator  80  is cooled at  88  and combined with the hydrocarbon effluent  74  from the cold high-pressure separator  68 . 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  54  to the post-treatment reactor  46 . The remaining hydrocarbon product stream  100  is combined with the unconverted oil stream  84 . The total product stream  86  is then sent for separation by distillation into the various components such as diesel oil, kerosene, naphtha and unconverted oil. 
     In the present invention, the combination of the cocurrent/countercurrent main reactor and the post-treatment reactor reduces the overall catalyst volume, the reactor weight, the hydrogen consumption, the product giveaway and increases the process flexibility. The reactions are 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. The selective addition of hydrogen in the reactors, 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.