20.3 Petroleum Refining

  • Desalting: Crude oils delivered to the refinery frequently contain substantial quantities of water, silts, sand, salts, etc. “Desalting” involves removal of most of these impurities before further processing.

    Two methods:

    • Chemical desalting is accomplished by addition of about 10% water to the oil to be treated, sulfuric acid or sodium hydroxide as necessary for crude pH adjustment. Demulsification takes place during this process.

    • Electrostatic desalting is accomplished by addition of water and applying high potential electric field across the electrodes. This is a comparatively faster process.

  • Sweetening

    • Sweetening is the removal of sulfur and its compounds like sulfides, mercaptants, etc, from petroleum products. Presence of these compounds makes the oil sour which has a bad odour. Sweetening makes foul smelling petroleum products into sweet smelling or odorless products.

    • Several processes are available for the removal of objectionable sulfur thus improving its odour. The Doctor treatment was the original method of “doctoring” of oil to reduce the odour and utilized sodium plumbite (\(\ce{Na2PbO2}\)) to convert mercaptants to less objectionable disulfides.

  • Modern crude distillation towers can process about 200,000 barrels (~30,000 m\(^3\)) of oil per day.

  • The atmospheric fractionator normally contains 30 to 50 fractionation trays, typically about 50 m height with valve trays.

  • Topping Refineries—produces LPG, gasoline blending stocks, jet fuel, diesel fuel.

  • Hydroskimming Refineries—make extensive use of hydrogen treating processes for cleaning up naphthas and distillate streams.

  • A rule of thumb used by some refiners is that it takes 1 barrel of oil-equivalent energy to process 10 barrels of crude oil.

  • Gross Refinery Margin (GRM): It is a parameter used to assess the performance of a refinery.

    \[\text{GRM} =\frac{\text{Value of petroleum products (output)} -\text{Price of crude or raw material (input)}}{\text{Amount of crude (in barrel)}}\] International unit of GRM is US dollars per barrel ($/bbl). GRM can be negative also.

  • Engines can run with crude oil itself. Crude oil being a mixture of lighter and heavier components leads to mixture of properties reducing the ease of handling.