Abstract:
A residue from petroleum refining is thermally cracked to convert the residue to useful cracked products and to generate fuel gas. The residue is cracked by contact with hot synthesis gas produced by the gasification on the tar/pitch residue remaining after the cracking of the residue feed. Waste heat can be recovered from remaining portions of the synthesis gas from the gasifier in the form of steam which can be used in the gasification process and in the cracking process as needed for coke suppression. The combustible synthesis gas and the combustible gasses form the thermal cracking are separated from the cracked product liquid and used for power generation in a combined cycle plant.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for treating the petroleum residue from a refinery by an integrated process of thermal cracking and partial oxidation to obtain higher thermal cracking at reduced investment cost. 
     The residue from a refinery usually comprises the components boiling above about 500-575° C. These residues may comprise any such streams such as vacuum tower residue, visbreaker residue and deasphalting residue. There is a considerable amount of residue from a refinery to be treated. For example, a typical refinery processing 10 million metric tons annually (MTA) of Arabian Mix Crude will produce about 6,500-7,000 MT per stream day (SD) of vacuum tower residue. This residue can be blended into residual fuel oil (which has a low value), upgraded to high value transportation fuels (which is expensive) or gasified to produce power. Without further processing, gasification of this residue will provide about 1,200 MW of electrical power. This is greatly in excess of the amount of power which can be effectively used in the plant. Several processes are available to reduce the amount of the residue but the degree of conversion of the residue is low and/or the cost is high. Examples are: 
     
       
         
               
               
               
             
           
               
                   
               
               
                   
                   
                 Cost of 
               
               
                   
                 Wt % Unconverted 
                 Upgrading Plant 
               
               
                 Process 
                 Residue or Coke Formed 
                 $ MM 
               
               
                   
               
             
             
               
                 Visbreaking 
                 84.5% 
                 $ 29.7 
               
               
                 Visbreaking &amp; 
                 66.5% 
                 $ 38.6 
               
               
                 Vacuum Flasher 
               
               
                 Deasphalting 
                 43.2% 
                 $ 46.0 
               
               
                 Delayed Coking 
                 32.5% 
                 $144.3 
               
               
                   
               
             
          
         
       
     
     Conversions of more than 50% are desired for efficient and effective plant operation but the cost for obtaining such conversions with these prior art processes is high. With respect to visbreaking, the overall conversion to 500° C. and lighter components is limited to 35% in order to maintain the stability of the residue (500° C.+components) for fuel oil blending. Also, the visbreaking process is limited by the maximum skin temperature of the furnace tubes of about 650° C. Although higher yields are possible with visbreaking, the unstable nature of the fuel product and the coking of the tubes pose significant problems. Although the Eureka Process (steam cracking with superheated steam) has a good conversion (67%), it requires the injection of superheated steam to suppress coking which all has to be condensed in a downstream fractionator and then treated in a sour water stripping unit. This adds cost to the unit. 
     SUMMARY OF THE INVENTION 
     The present invention involves the thermal cracking of a residue from petroleum refining to convert the residue at low cost to useful cracked products at a high conversion yield and to generate fuel gas for power production without the need for supplying outside energy for the thermal cracking. 
     The present invention involves thermal cracking of a residue from petroleum refining by contacting the residue feed with hot synthesis gas produced by gasification of the tar/pitch residue remaining after the cracking of the feed. Only a portion of the hot synthesis gas produced via gasification is needed for thermal cracking. Waste heat is recovered in the form of steam from the remaining synthesis gas from the gasifier and a portion of the steam can be used in the gasification process. The cooled, combustible synthesis gas is combined with the combustible gases produced by the thermal cracking for power generation such as in a combined cycle power plant. The cracked liquid converted from the residue feed is similar to thermal products from delayed coking and visbreaking and is hydrotreated in the same manner as existing thermal products. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block flow diagram of the process of the invention. 
     FIG. 2 is a block flow diagram showing a portion of the process incorporating a modification which incorporates hydrotreating of the thermal liquids within the process by utilizing the hydrogen contained in the synthesis gas. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a residue feed stream  10  from a refinery is fed to a contactor/thermal cracker  12  in which the feed  10  is contacted with a hot synthesis gas from a partial oxidation gasifier to be described later. The feed  10  can be any of the pumpable refinery residues previously mentioned such as a vacuum tower residue. Generally, such residue stream will have a boiling range above about 500° C. The sulfur content and the gravity are unimportant for the present invention. In the contactor/thermal cracker  12 , the feed at about 150° C. is contacted with the synthesis gas  14  which is at about 1,250-1,500° C. The synthesis gas is quenched and the residue feed is heated and cracked to produce thermal distillates which are further processed in the refinery in the same manner as other thermal distillates. The presence of hydrogen and steam in the synthesis gas will suppress the formation of coke. However, high pressure steam  16  may be added to the contactor/thermal cracker  12  as needed to assist in the suppression of coke. The operating conditions in the contactor/thermal cracker  12  are in the range of 35-80 kg/cm 2  total pressure, 10-30 kg/cm 2  hydrogen partial pressure and 10-30 kg/cm 2  steam partial pressure. The conditions in the contactor/thermal cracker assuming a typical feed of vacuum tower residue are 70 kg/cm 2  total pressure, 25 kg/cm 2  hydrogen partial pressure and 10 kg/cm 2  steam partial pressure. 
     The effluent  18  from the contactor/thermal cracker  12  for a typical feed of vacuum tower residue would have, as an example, a composition comprising the bulk of the synthesis gas stream  14  plus the following components from the cracked residue feed: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Typical - Wt. % 
                 Range - Wt. % 
               
               
                   
                   
               
             
             
               
                   
                 H 2 S 
                 1.5 
                 1-2 
               
               
                   
                 C 1  to C 4   
                 6.8 
                 5-8 
               
               
                   
                 C 5  to 165° C. 
                 9.6 
                  8-12 
               
               
                   
                 165 to 343° C. 
                 20.1 
                 16-24 
               
               
                   
                 343 to 500° C. 
                 22.0 
                 18-26 
               
               
                   
                 500° C. + 
                 40.0 
                 52-28 
               
               
                   
                   
               
             
          
         
       
     
     The effluent  18  from the contactor/thermal cracker  12  has a temperature in the range of 500 to 550° C. The preferred temperature is selected to produce an effluent in which 50 to 70%, preferably about 60%, of the cracked residue are vapors at the effluent conditions and the remainder are liquids. This effluent  18  is fed to the hot separator  20  for separation of the hot liquid at  22  and the vapor at  24 . 
     The hot liquid  22  from the separator  20 , which is generally referred to as tar or pitch, is recycled to the gasifier  26  in which the pitch is converted to synthesis gas. The hot separator bottoms include most of the 500° C.+ material plus some of the 343/500° C. vacuum gas oil. In this example about 40% of the feed residue is obtained as hot separator bottoms. 
     Also fed to the gasifier  26  is recycle soot  28  to be described later, high pressure steam  30  and oxygen  32 . The partial oxidation gasifier produces synthesis gas effluent  34  at 40-70 Kg/cm 2  containing hydrogen, carbon monoxide and dioxide, water and small amounts of hydrogen sulfide and other minor components. A typical gas composition from a high sulfur vacuum residue is as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Gas 
                 Mole % 
               
               
                   
                   
               
             
             
               
                   
                 H 2   
                 37.0 
               
               
                   
                 CO 
                 39.0 
               
               
                   
                 CO 2   
                 7.0 
               
               
                   
                 H 2 O 
                 14.0 
               
               
                   
                 H 2 S 
                 1.5 
               
               
                   
                 Other 
                 1.5 
               
               
                   
                   
               
             
          
         
       
     
     The temperature of the effluent  18  from the contactor/thermal cracker  12  and therefore the resulting temperature in the hot separator  20  are selected to produce a vapor-liquid separation in the hot separator to yield the desired amount of liquid  22  to recycle to the gasifier  26  for the production of the synthesis gas. Specific amounts will vary depending on the feed composition and the effluent temperature of the contactor. As an example for 100 metric tons (MT)/hr of residue feed  10 , about 108 MT/hr of synthesis gas  34  is produced. This synthesis gas is then divided into streams  14  and  36  with about 50 MT/hr going at  14  to the contactor/thermal cracker  12 . The synthesis gas rate is set by the amount of unconverted residue stream  22  coming from the hot separator as it must all be gasified. The synthesis gas rate is about 2.7 times the unconverted residue, although it will vary a small amount depending upon the feed residue composition. The amount of synthesis gas going to the contactor/thermal reactor will be about 0.5 times the feed residue. The ratio will depend upon the rate of conversion as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 % Conversion 
                 Syn Gas/Feed Ratio 
               
               
                   
                   
               
             
             
               
                   
                 50 
                 0.46 
               
               
                   
                 60 
                 0.50 
               
               
                   
                 70 
                 0.54 
               
               
                   
                   
               
             
          
         
       
     
     The amount of synthesis gas to the contactor/thermal reactor is what is needed to provide the heat for conversion. Any excess synthesis gas (stream  36 ) is cooled separately prior to gas scrubbing. Cooling can be via direct water quench or in a waste heat boiler as shown in FIG.  1 . In this example, about 50 MT/hr is sent to the contactor/thermal reactor and 58 MT is sent to the waste heat boiler. To produce this amount of synthesis gas, about 40.0 MT/hr of tar/pitch residue  22  is required. The hot separator bottoms liquid  22  will contain most of the 500° C.+ material plus a portion of the 343-500° C. fraction. The hot separator does not provide perfect separation. Most of the 500° C.+ material goes with the bottom product, but some goes out with the vapor. Similarly, most of the 343-500° C. heavy gas oil goes out with the vapor, but some of it will go out with the bottoms product. The typical values and the ranges for the temperatures and flow rates for the relevant streams based on 60% conversion are as follows: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 Stream No. 
                 T ° C. Range 
                 Preferred T ° C. 
                 MT/hr 
                 Range 
               
               
                   
               
             
             
               
                 10 
                 150-250 
                 150 
                 100.0 
                 100.0 
               
               
                 34 
                 1300-1400 
                 1400* 
                 108.0 
                  60-150 
               
               
                 14 
                 1300-1400 
                 1400* 
                 50.0 
                 40-60 
               
               
                 36 
                 1300-1400 
                 1400* 
                 58.0 
                  0-110 
               
               
                 30 
                 250-350 
                 300 
                 24.0 
                 14-22 
               
               
                 22 
                 500-550 
                  500** 
                 40.0 
                 23-55 
               
               
                 32 
                  30-100 
                  65 
                 44.0 
                 25-60 
               
               
                 18 
                 500-550 
                  500** 
                 150.0 
                 140-160 
               
               
                   
               
               
                 *The temperature will be between 1300-1400° C. depending on the feed composition. For lower temperatures, more synthesis gas is needed. This example is for 1400°.  
               
               
                 **The preferred temperature is the temperature that results in the proper conversion. In this example, 500° C. and 40% conversion are used.  
               
             
          
         
       
     
     The divided synthesis gas stream  36  at about 1300-1400° C. passes to the waste heat boiler  38  and feed water heater  39  where the sensible heat is transferred from the synthesis gas to the boiler feedwater  40  to produce high pressure steam  42 . The bulk of this high pressure steam can be added at  30  to the gasifier  26  as a component of the gasification or partial oxidation process. The required amount of steam  30  based on the preferred flow rates previously listed is about 24.0 MT/hr. A portion  16  of the remaining high pressure steam can be fed to the contactor/thermal cracker  12  as required for coke suppression. Any excess steam is fed at  44  for other desired uses. The cooled synthesis gas  46  now at about 180-250° C. is fed to an aqueous scrubber  48  where particulates such as soot are removed. The water and particulates are than separated at  50 . The particulates can be recycled to the gasifier  26 . The cleaned water is recycled at  52  to the scrubber and water which is accumulated is purged at  54 . The remaining cooled and cleaned synthesis gas  56  from the scrubber  48  is combined with another synthesis gas stream preferably for power generation as will be explained later. 
     The hot vapor  24  from the hot separator  20  will contain the H 2 S and the cracked hydrocarbons. These hot vapors  24  are cooled at  58  to condense out the converted liquids  60  which are separated in the cold separator  62 . For the specific example previously discussed, the converted liquids  60  will amount to about 50 MT/hr. Since there is no catalyst, the amount of hydrogen saturation is small. In practice, the cold separator  62  may be a fractionator which separates various fractions such as a naptha fraction, a light gas oil fraction and a heavy gas oil fraction. Depending upon the conversion and heat balances, a portion of the heavy gas oil fraction may be recycled to the partial oxidation unit. The remaining gas  64  is a synthesis-type gas which is combined with the synthesis gas  56  from the scrubber  48 . The combined synthesis gas stream  66  of about 118 MT/hr is preferably fed to an acid gas scrubber to remove H 2 S and then fired in a gas turbine to generate power as shown in FIG. 2 described below. 
     FIG. 2 illustrates in block diagram form a modification of the present invention as well as the use of the product synthesis gas in a gas turbine as previously mentioned. Addressing this latter aspect of the invention first, the combined synthesis gas stream  66  is scrubbed at  68  to remove any sulfur containing acid gases such as H 2 S. The cleaned gases  70  are then burned in the gas turbine  72  which powers the generator  74 . 
     In the FIG. 2 embodiment, the hot vapor  24  from the hot separator  20  is cooled at  76  down to a temperature suitable for a catalytic hydrogenation reaction, about 350-400° C. This cooled vapor  78  may be mixed with any desired portion  80  of the cleaned synthesis gas  56  from the scrubber  48  for the catalytic hydrogenation reaction at  82 . This catalytic reactor  82  can operate in a once-through manner since there is more than sufficient hydrogen in the vapors to hydrotreat the converted materials. 
     The cost of the contactor and the hot and cold separators for the invention would be significantly less than the cost of an equivalent visbreaker since the major cost of the visbreaker is the heater. No heater is required for the invention since the hot gases for the cracking are produced in the gasifier.