1. FIGURE IV:2-6. REFINERY PROCESS CHART.

    FIGURE IV:2-6. REFINERY PROCESS CHART.


TABLE IV:2-3. OVERVIEW OF PETROLEUM REFINING PROCESSES.
Process name
Action
Method
Purpose
Feedstock(s)
Product(s)
FRACTIONATION PROCESSES
Atmospheric distillation Separation Thermal Separate fractions Desalted crude oil Gas, gas oil, distillate, residual
Vacuum distillation Separation Thermal Separate w/o cracking Atmospheric tower residual Gas oil, lube stock, residual
CONVERSION PROCESSED--DECOMPOSITION
Catalytic cracking Alteration Catalytic Upgrade gasoline Gas oil, coke distillate Gasoline, petrochemical feedstock
Coking Polymerize Thermal Convert vacuum residuals Gas oil, coke distillate Gasoline, petrochemical feedstock
Hydro-cracking Hydrogenate Catalytic Convert to lighter HC's Gas oil, cracked oil, residual Lighter, higher-quality products
*Hydrogen steam reforming Decompose Thermal/ catalytic Produce hydrogen Desulfurized gas, O2, steam Hydrogen, CO, CO2
*Steam cracking Decompose Thermal Crack large molecules Atm tower hvy fuel/ distillate Cracked naphtha, coke, residual
Visbreaking Decompose Thermal reduce viscosity Atmospheric tower residual Distillate, tar
CONVERSION PROCESSES--UNIFICATION
Alkylation Combining Catalytic Unite olefins & isoparaffins Tower isobutane/ cracker olefin Iso-octane (alkylate)
Grease compounding Combining Thermal Combine soaps & oils Lube oil, fatty acid, alky metal Lubricating grease
Polymerizing Polymerize Catalytic Unite 2 or more olefins Cracker olefins High-octane naphtha, petrochemical stocks
CONVERSION PROCESSES--ALTERATION OR REARRANGEMENT
Catalytic reforming Alteration/
dehydration		 Catalytic Upgrade low-octane naphtha Coker/ hydro-cracker naphtha High oct. Reformate/ aromatic
Isomerization Rearrange Catalytic Convert straight chain to branch Butane, pentane, hexane Isobutane/ pentane/ hexane
TREATMENT PROCESSES
*Amine treating Treatment Absorption Remove acidic contaminants Sour gas, HCs w/CO2 & H2S Acid free gases & liquid HCs
Desalting Dehydration Absorption Remove contaminants Crude oil Desalted crude oil
Drying & sweetening Treatment Abspt/ therm Remove H2O & sulfur cmpds Liq Hcs, LPG, alky feedstk Sweet & dry hydrocarbons
*Furfural extraction Solvent extr. Absorption Upgrade mid distillate & lubes Cycle oils & lube feed-stocks High quality diesel & lube oil
Hydrodesulfurization Treatment Catalytic Remove sulfur, contaminants High-sulfur residual/ gas oil Desulfurized olefins
Hydrotreating Hydrogenation Catalytic Remove impurities, saturate HC's Residuals, cracked HC's Cracker feed, distillate, lube
*Phenol extraction Solvent extr. Abspt/ therm Improve visc. index, color Lube oil base stocks High quality lube oils
Solvent deasphalting Treatment Absorption Remove asphalt Vac. tower residual, propane Heavy lube oil, asphalt
Solvent dewaxing Treatment Cool/ filter Remove wax from lube stocks Vac. tower lube oils Dewaxed lube basestock
Solvent extraction Solvent extr. Abspt/ precip. Separate unsat. oils Gas oil, reformate, distillate High-octane gasoline
Sweetening Treatment Catalytic Remv H2S, convert mercaptan Untreated distillate/gasoline High-quality distillate/gasoline
* Note: These processes are not depicted in the refinery process flow chart.

  1. DESCRIPTION OF PETROLEUM REFINING PROCESSES AND RELATED HEALTH AND SAFETY CONSIDERATIONS.

    1. CRUDE OIL PRETREATMENT (DESALTING).

      1. Description.

        a.  Crude oil often contains water, inorganic salts, suspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration).

        b.  The two most typical methods of crude-oil desalting, chemical and electrostatic separation, use hot water as the extraction agent. In chemical desalting, water and chemical surfactant (demulsifiers) are added to the crude, heated so that salts and other impurities dissolve into the water or attach to the water, and then held in a tank where they settle out. Electrical desalting is the application of high-voltage electrostatic charges to concentrate suspended water globules in the bottom of the settling tank. Surfactants are added only when the crude has a large amount of suspended solids. Both methods of desalting are continuous. A third and less-common process involves filtering heated crude using diatomaceous earth.

        c.  The feedstock crude oil is heated to between 150° and 350°F to reduce viscosity and surface tension for easier mixing and separation of the water. The temperature is limited by the vapor pressure of the crude-oil feedstock. In both methods other chemicals may be added. Ammonia is often used to reduce corrosion. Caustic or acid may be added to adjust the pH of the water wash. Wastewater and contaminants are discharged from the bottom of the settling tank to the wastewater treatment facility. The desalted crude is continuously drawn from the top of the settling tanks and sent to the crude distillation (fractionating) tower.

        TABLE IV:2-4. DESALTING PROCESS.
        Feedstock From Process Typical products . . . To
        Crude Storage Treating Desalted crude . . .  Atmospheric distillation tower
        Waste water . . . . . Treatment


        FIGURE IV:2-7. ELECTROSTAITC DESALTING.
        FIGURE IV:2-7. ELECTROSTAITC DESALTING.


      2. Health and Safety Considerations.

        a.  Fire Prevention and Protection. The potential exists for a fire due to a leak or release of crude from heaters in the crude desalting unit. Low boiling point components of crude may also be released if a leak occurs.

        b.  Safety. Inadequate desalting can cause fouling of heater tubes and heat exchangers throughout the refinery. Fouling restricts product flow and heat transfer and leads to failures due to increased pressures and temperatures. Corrosion, which occurs due to the presence of hydrogen sulfide, hydrogen chloride, naphthenic (organic) acids, and other contaminants in the crude oil, also causes equipment failure. Neutralized salts (ammonium chlorides and sulfides), when moistened by condensed water, can cause corrosion. Overpressuring the unit is another potential hazard that causes failures.

        c.  Health. Because this is a closed process, there is little potential for exposure to crude oil unless a leak or release occurs. Where elevated operating temperatures are used when desalting sour crudes, hydrogen sulfide will be present. There is the possibility of exposure to ammonia, dry chemical demulsifiers, caustics, and/or acids during this operation. Safe work practices and/or the use of appropriate personal protective equipment may be needed for exposures to chemicals and other hazards such as heat, and during process sampling, inspection, maintenance, and turnaround activities.

        Depending on the crude feedstock and the treatment chemicals used, the wastewater will contain varying amounts of chlorides, sulfides, bicarbonates, ammonia, hydrocarbons, phenol, and suspended solids. If diatomaceous earth is used in filtration, exposures should be minimized or controlled. Diatomaceous earth can contain silica in very fine particle size, making this a potential respiratory hazard.


    2. CRUDE OIL DISTILLATION (FRACTIONATION).

      1. Description. The first step in the refining process is the separation of crude oil into various fractions or straight-run cuts by distillation in atmospheric and vacuum towers. The main fractions or "cuts" obtained have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils, and residuum.

      2. Atmospheric Distillation Tower.

        a.  At the refinery, the desalted crude feedstock is preheated using recovered process heat. The feedstock then flows to a direct-fired crude charge heater where it is fed into the vertical distillation column just above the bottom, at pressures slightly above atmospheric and at temperatures ranging from 650° to 700° F (heating crude oil above these temperatures may cause undesirable thermal cracking). All but the heaviest fractions flash into vapor. As the hot vapor rises in the tower, its temperature is reduced. Heavy fuel oil or asphalt residue is taken from the bottom. At successively higher points on the tower, the various major products including lubricating oil, heating oil, kerosene, gasoline, and uncondensed gases (which condense at lower temperatures) are drawn off.

        b.  The fractionating tower, a steel cylinder about 120 feet high, contains horizontal steel trays for separating and collecting the liquids. At each tray, vapors from below enter perforations and bubble caps. They permit the vapors to bubble through the liquid on the tray, causing some condensation at the temperature of that tray. An overflow pipe drains the condensed liquids from each tray back to the tray below, where the higher temperature causes re-evaporation. The evaporation, condensing, and scrubbing operation is repeated many times until the desired degree of product purity is reached. Then side streams from certain trays are taken off to obtain the desired fractions. Products ranging from uncondensed fixed gases at the top to heavy fuel oils at the bottom can be taken continuously from a fractionating tower. Steam is often used in towers to lower the vapor pressure and create a partial vacuum. The distillation process separates the major constituents of crude oil into so-called straight-run products. Sometimes crude oil is "topped" by distilling off only the lighter fractions, leaving a heavy residue that is often distilled further under high vacuum.