Chapter 8
      Hydroprocessing and Resid Processing
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       The term resid refers to the bottom of the barrel and is usually the atmospheric tower 
        bottoms (atmospheric reduced crude, or ARC) with an initial boiling point of 343°C or 
        vacuum tower bottoms (vacuum reduced crude, or VRC) with an initial boiling point of 
        566°C.
       These streams contain higher concentrations of sulfur, nitrogen, and metals than does 
        the  crude  oil  from  which  they  were  obtained,  and  hydrogen/carbon  ratios  in  the 
        molecules are much lower. These concentrations are much higher in the case of the VRC.
       In the past this resid has been sold as asphalt (if the qualities of the crude permit) or as 
        heavy fuel oil (No. 6 or bunker fuel oil).
       Today,  more  of  these  must  be  converted  in  the  refinery  to  feed  stocks  for  refining 
        processes that will convert them to transportation fuel blending stocks.
       High carbon forming potentials of resids, caused by the low hydrogen/
       carbon ratios in the cause rapid catalyst deactivation and high catalyst costs, and the 
        nickel and vanadium in the resids act as catalysts for reactions creating carbon and light 
        gaseous hydrocarbons.
       Catalytic processes for converting resids usually use ARC for their feedstocks, and VRC 
        feedstocks are usually processed by noncatalytic processes.
        The processes most commonly used for processing ARC are reduced crude catalytic 
        cracking units and hydroprocessing units.
        Thermal cracking processes such as delayed coking and Flexicoking or solvent extraction 
        processes for VRC feedstocks.
       The term hydroprocessing is used to denote those processes used to reduce the boiling 
        range of the feedstock as well as to remove substantial amounts of impurities such as 
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        metals, sulfur, nitrogen, and high carbon forming compounds.
      COMPOSITION OF VACUUM TOWER 
      BOTTOMS
      Vacuum tower bottoms are complex mixtures of high molecular weight and 
        high boiling point compounds containing thousands of hydrocarbon and 
        organic compounds.
      All of the bad processing features of refinery feedstocks are present in the 
        bottoms streams in greater concentrations than in any of the distillate 
        feedstocks.
      Because they are so complex it has been difficult to express the compositions 
        in ways meaningful to processing operations.
      Liquid propane is used to extract the oil fraction from vacuum tower bottoms, 
        and liquid n-pentane, n-hexane, or n-heptane is then used to extract the resin 
        fraction from the residue from the propane extraction. The material insoluble 
        in either the propane or the higher hydrocarbons is termed the asphaltene 
        fraction.
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      Asphaltene fraction
       The asphaltene fraction has a very low hydrogen-to-carbon ration and consists of highly 
        condensed ring compounds with predominating molecular weights in the 5000 to 10,000 
        range. 
       The  molecule  is  built  up  of  sheets  of  these  highly  condensed  ring  structures  held 
        together by valence bonds between hetro atoms such as sulfur, oxygen, and metals. 
       An asphaltene  molecule  contains  three  to  five  unit  sheets  consisting  of  condensed 
        aromatic  and  naphthenic  rings  with  paraffinic  side  chains.  These  sheets  are  held 
        together  by  hetro  atoms  such  as  sulfur  or  nitrogen  and/or  polymethylene  bridges, 
        thioether bonds, and vanadium and nickel complexes.
        Separation into unit sheets is accompanied by sulfur and vanadium removal.
        A significant feature of the asphaltene fraction is that 80 to 90% of the metals in the 
        crude (nickel and vanadium) are contained in this material
         Apparently  25  to  35%  of  these  metals  are  held  in  porphryin  structures  and  the 
        remainder in some undetermined type of organic structure.
       The asphaltene fraction contains a higher content of sulfur and nitrogen than does the 
        vacuum resid and also contains higher concentrations of carbon forming.
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 Hypothetical asphaltene 
 molecule structure
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