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CHEMISTRY AND TECHNOLOGY OF PETROLEUM By Dr. Dang Saebea REFINING CHEMISTRY INTRODUCTION Petroleum refining plays an important role in our lives. Most transportation vehicles are powered by refined products such as gasoline, diesel, aviation turbine kerosene (ATK) and fuel oil. THE REFINING INDUSTRY IN THREE WAYS Ø An increased search for fuel products from non-fossil sources such as biodiesel and alcohols from vegetable sources. Ø The development of better methods to process tar sand, coal gasification and synthesis of fuels by new technology. Ø The initiation of long-term plans to look for renewable energy sources. REFINING MEANS. . . 1.To a pure state, to remove impurities 2.To improve product REFINING IS CARRIED OUT IN THREE MAIN STEPS Step 1 – Separation Step 2 – Conversion Step3 - Purification REFINING IS CARRIED OUT Step 1 - Separation The oil is separated into its constituents by distillation, and some of these components (such as the refinery gas) are further separated with chemical reactions and by using solvents. REFINING IS CARRIED OUT Step 2 - Conversion o The various hydrocarbons produced are then chemically altered to make them more suitable for their intended purpose. o For example, naphthas are "reformed" from paraffins and naphthenes into aromatics. REFINING IS CARRIED OUT Step3 - Purification o The hydrogen sulfide gas which was extracted from the refinery gas in Step 1 is converted to sulfur, which is solid in liquid form to fertiliser manufacturers. INTEGRATION 1. Physical Separation Processes 2. Chemical Catalytic Conversion Processes INTEGRATION 3. Thermal Chemical Conversion Processes 1. Physical Separation Processes INTEGRATION Crude Distillation Crude Distillation q Crude oils are first desalted and then introduced with steam to an atmospheric distillation column. q The atmospheric residue is then introduced to a vacuum distillation tower operating at about 50 mmHg, where heavier products are obtained. Atmospheric distillation Vacuum distillation 1. Physical Separation Processes INTEGRATION Solvent Deasphalting Ø This is the only physical process where carbon is rejected from heavy petroleum fraction such as vacuum residue. Ø Propane in liquid form (at moderate pressure) is usually to dissolve the whole oil, leaving asphaltene to precipitate. Ø The deasphalted oil (DAO) has low sulphur and metal contents since these are removed with asphaltene. This oil is also called ‘‘Bright Stock’’ and is used as feedstock for lube oil plant. Ø The DAO can also be sent to cracking units to increase light oil production. Solvent deasphalting process 1. Physical Separation Processes INTEGRATION Solvent Extraction q In this process, lube oil stock is treated by a solvent, such as phenol and furfural, which can dissolve the aromatic components in one phase (extract) and the rest of the oil in another phase (raffinate). q The solvent is removed from both phases and the raffinate is dewaxed. Solvent Extraction 1. Physical Separation Processes INTEGRATION Solvent Dewaxing q The raffinate is dissolved in a solvent (methyl ethyl ketone, MEK) and the solution is gradually chilled, during which high molecular weight paraffin (wax) is crystallized, and the remaining solution is filtered. q The extracted and dewaxed resulting oil is called ‘‘lube oil’’. q In some modern refineries removal of aromatics and waxes is carried out by catalytic processes in all ‘‘ hydrogenation process” 2. Chemical Catalytic Conversion Processes INTEGRATION Catalytic Reforming q In this process a special catalyst (platinum metal supported on silica or silica base alumina) is used to restructure naphtha fraction (C6–C10) into aromatics and isoparaffins. q The produced naphtha reformate has a much higher octane number than the feed. This reformate is used in gasoline formulation and as a feedstock for aromatic production (benzene–toluene–xylene, BTX). 2. Chemical Catalytic Conversion Processes INTEGRATION Hydrotreating q This is one of the major processes for the cleaning of petroleum fractions from impurities such as sulphur, nitrogen, oxy-compounds, chloro-compounds, aromatics, waxes and metals using hydrogen. q The catalyst is selected to suit the degree of hydrotreating and type of impurity. Catalysts, such as cobalt and molybdenum oxides on alumina matrix, are commonly used. 2. Chemical Catalytic Conversion Processes INTEGRATION Catalytic Hydrocracking q For higher molecular weight fractions such as atmospheric residues (AR) and vacuum gas oils (VGOs), cracking in the presence of hydrogen is required to get light products. q In this case a dual function catalyst is used. It is composed of a zeolite catalyst for the cracking function and rare earth metals supported on alumina for the hydrogenation function. q The main products are kerosene, jet fuel, diesel and fuel oil. 2. Chemical Catalytic Conversion Processes INTEGRATION Catalytic Cracking q Fluid catalytic cracking (FCC) is the main player for the production of gasoline. The catalyst in this case is a zeolite base for the cracking function. q The main feed to FCC is VGO and the product is gasoline, but some gas oil and refinery gases are also produced. 2. Chemical Catalytic Conversion Processes INTEGRATION Alkylation q Alkylation is the process in which isobutane reacts with olefins such as butylene (C4 ) to produce a gasoline range alkylate. q The catalyst in this case is either sulphuric acid or hydrofluoric acid. The hydrocarbons and acid react in liquid phase. q Isobutane and olefins are collected mainly from FCC and delayed coker 2. Chemical Catalytic Conversion Processes INTEGRATION Isomerization q Isomerization of light naphtha is the process in which low octane number hydrocarbons (C4, C5, C6) are transformed to a branched product with the same carbon number. This process produces high octane number products. q One main advantage of this process is to separate hexane (C6) before it enters the reformer, thus preventing the formation of benzene which produces carcinogenic products on combustion with gasoline. q The main catalyst in this case is a Pt-zeolite base. 3. Thermal Chemical Conversion Processes Delayed Coking q This process is based on the thermal cracking of vacuum residue by carbon rejection forming coke and lighter products such as gases, gasoline and gas oils. q The vacuum residue is heated in a furnace and flashed into large drums where coke is deposited on the walls of these drums, and the rest of the products are separated by distillation. Flexicoking q In this thermal process, most of the coke is gasified into fuel gas using steam and air. q The burning of coke by air will provide the heat required for thermal cracking. q The products are gases, gasoline and gas oils with very little coke. 3. Thermal Chemical Conversion Processes Visbreaking q This is a mild thermal cracking process used to break the high viscosity and pour points of vacuum residue to the level which can be used in further downstream processes. q In this case, the residue is either broken in the furnace coil (coil visbreaking) or soaked in a reactor for a few minutes (soaker visbreaker). q The products are gases, gasoline, gas oil and the unconverted residue. The End CRUDE DISTILLATION CRUDE DISTILLATION Crude distillation unit (CDU) is at the front-end of the refinery, also known as topping unit, or atmospheric distillation unit. It receives high flow rates hence its size and operating cost are the largest in the refinery. This involves the removal of undesirable components like sulphur, nitrogen and metal compounds, and limiting the aromatic contents. TYPICAL PRODUCTS FROM THE UNIT ARE: CRUDE OIL DESALTING o The crude oil contains salt in the form of dissolved salt in the tiny droplet of water which forms a water-in oil emulsion. o This water cannot be separated by gravity or through mechanical means. o It is separated through electrostatic water separation. This process is called desalting. CRUDE OIL DESALTING In the electrostatic desalter, the salty water droplets are caused to coalesce and migrate to the aqueous phase by gravity. It involves mixing the crude with dilution water (5–6 vol%) through a mixing valve. POOR DESALTING HAS THE FOLLOWING EFFECTS: 1. Salts deposit inside the tubes of furnaces and on the tube bundles of heat exchangers creating fouling, thus reducing the heat transfer efficiency; 2. Corrosion of overhead equipment. 3. The salts carried with the products act as catalyst poisons in catalytic cracking units. TYPES OF SALTS IN CRUDE OIL Salts in the crude oil are mostly in the form of dissolved salts in fine water droplets emulsified in the crude oil. The salts can also be present in the form of salts crystals suspended in the crude oil. These are mostly magnesium, calcium and sodium chlorides with sodium chloride being the abundant type. TYPES OF SALTS IN CRUDE OIL Ø These chlorides, except for NaCl, hydrolyze at high temperatures to hydrogen chloride: Hydrogen chloride dissolves in the overhead system water, producing hydrochloric acid, an extremely corrosive acid DESALTING PROCESS The process is accomplished through the following steps: 1. Water washing: - Water is mixed with the incoming crude oil through a mixing valve. - The water dissolves salt crystals and the mixing distributes the salts into the water, uniformly producing very tiny droplets. - Demulsifying agents are added at this stage to aide in breaking the emulsion by removing the asphaltenes from the surface of the droplets. DESALTING PROCESS 2. Heating: - The crude oil temperature should be in the range of 49-54 ˚C (120–130 F) since the water–oil separation is affected by the viscosity and density of the oil. DESALTING PROCESS 3. Coalescence: - The water droplets are so fine in diameter in the range of 1–10 mm that they do not settle by gravity. Coalescence produces larger drops that can be settled by gravity. - This is accomplished through an electrostatic electric field between two electrodes. - The electric field ionizes the water droplets and orients them so that they are attracted to each other. - Agitation is also produced and aides in coalescence. DESALTING PROCESS 4. Settling: According to Stock’s law the settling rate of the water droplets after coalescence is given by where is the density is the viscosity, d is the droplet diameter k is a constant. DESCRIPTION OF DESALTER Two electrodes Simplified flow diagram of an electrostatic desalter DESCRIPTION OF DESALTER A primary field of about 600 V/cm This field helps the water droplets settle faster. Simplified flow diagram of an electrostatic desalter DESCRIPTION OF DESALTER A secondary field of about 1000 V/cm The ionization of the water droplets and coalescence takes place here Simplified flow diagram of an electrostatic desalter TWO-STAGE DESALTING - The desalter of this design achieves 90% salt removal. However 99% salt removal is possible with two-stage desalters. -A second stage is also essential since desalter maintenance requires a lengthy amount of time to remove the dirt and sediment which settle at the bottom. -The crude unit can be operated with a one stage desalter while the other is cleaned. DESALTER OPERATING VARIABLES For an efficient desalter operation, the following variables are controlled: Ø Desalting temperature: The settling rate depends on the density and viscosity of the crude T density & viscosity settling rate Desalting temperature can vary between 50 and 150 ˚C ØWashing water ratio: Adding water to the crude oil helps in salt removal. Wash water rate Coalescence rate DESALTER OPERATING VARIABLES ØWater level: - Raising the water level reduces the settling time for the water droplets in the crude oil -However, if the water level gets too high and reaches the lower electrode, it shorts out the desalter. - it is always better to keep the level constant for stable operation. DESALTER OPERATING VARIABLES ØWashing water injection point: -Usually the washing water is injected at the mixing valve. - However, if it is feared that salt deposition may occur in the preheat exchangers, part or all of the washing water is injected right after the crude feed pump. DESALTER OPERATING VARIABLES ØType of washing water: -Process water in addition of fresh water is used for desalting. The water should be relatively soft in order to prevent scaling. - It should be slightly acidic with a pH in the range of 6. It should be free from hydrogen sulphide and ammonia so as to not create more corrosion problems. ATMOSPHERIC DISTILLATION 330 ˚C Process flow diagram of an atmospheric distillation unit COMPONENT OF ATMOSPHERIC DISTILLATION Rectifying section Flash zone Stripping section Description OF ATMOSPHERIC DISTILLATION ØThe vapor from pipestill furnace discharge as a foaming stream into distillation tower. ØThe partially vaporized crude is transferred to the flash zone. ØThe vapour goes up the tower to be fractionated into gas oil that is called the overhead product . Øliquid portion of feed go down to bottom of tower Description OF STRIPPING SECTION Ø Steam reboilers may take the form of a steam coil in the bottom of the tower or a separate vessel. ØThe bottom product from the tower enters the rebolier where part is vaporized by heat from steam coil. ØThe hot vapor is directed back to the bottom of the tower and the nonvolatile leaves the rebolier and passes through a heat exchanger, where its heat is transferred to the feed to the tower. Description OF STRIPPING SECTION Ø Steam is also injected into the column -To strip the atmospheric residue of any light hydrocarbon. - To lower the partial pressure of the hydrocarbon vapours in the flash zone. This has the effect of lowering the boiling point of the hydrocarbons and causing more hydrocarbons to boil and go up the column to be eventually condensed and withdrawn as side streams. Description OF RECTIFYING SECTION ØAs the hot vapours from the flash zone rise through the trays up the column, they are contacted by the colder reflux down the column. ØIn the overhead condenser, the vapours are condensed and part of the light naphtha is returned to the column as reflux. Ø Reflux is provided by several pumparound streams along the column. IMPROVEMENT OF DISTILLATION EFFICIENCY WITH PUMPAROUND Ø Vapours Cold liquid condenses Reflux To compensate for the withdrawal of products from the column. Ø The addition to the heat removal from the condenser. The thermal efficiency of the column is improved and the required furnace duty is reduced. Description OF RECTIFYING SECTION Liquid collects on each tray to a depth, and the depth controlled by a dam or weir. As the liquid spills over the weir into a channel, which carries the liquid to the tray below. IMPORTANT OF STRIPPING AND RECTIFYING SECTION v Stripping section The more volatile component are stripped from the descending liquid v Rectifying section The concentration of the less volatile component in the vapor is reduced Straight-Run Naphtha and Gases 125 ˚C Heavy Naphtha 160 ˚C Kerosene 250 ˚C Gas Oil Crude Oil 300 ˚C The Temperature of tray is progressively cooler from bottom to top Residuum THE EFFICIENT OPERATION OF THE DISTILLATION ØTower requires the rising vapors to mix with liquid on each tray. ØThis is usually achieved by installing a bubble caps. ØThe cap forces the vapor to go below the surface of the liquid and to bubble up through it. LIMITING TEMPERATURE OF ATMOSPHERERIC DISTILLATION It is important not to subject the crude oil to temperatures above 350 °C because the high molecular weight components in the crude oil will undergo thermal cracking and form petroleum coke . Formation of coke would result in plugging the tubes in the furnace and the piping from the furnace to the distillation column as well as in the column itself. The constraint imposed by limiting the column inlet crude oil to a temperature of less than 350 °C yields a residual oil from the bottom of the atmospheric distillation column. VACUUM DISTILLATION To further distill the residual oil from the atmospheric distillation column, the distillation must be performed at absolute pressures as low as 10 to 50 mmHg so as to limit the operating temperature to less than 350°C. Vacuum distillation is the reduced temperature requirement at lower pressures. Vacuum distillation increases the relative volatility of the key components. Vacuum distillation can improve a separation by: Prevention of product degradation or polymer formation because of reduced pressure leading to lower tower bottoms temperatures. Reduction of product degradation or polymer formation because of reduced mean residence time especially in columns using packing rather than trays. Reduction of capital cost because of reduced the height and diameter. FRACTIONS OBTAINED BY VACUUM DISTILLATION used as catalytic cracking stock Gas Oil or, after suitable treatment, light lubricating oil Light Medium Lube oil Heavy used directly as asphalt or Residuum, converted to asphalt Nonvolatile OPERATION OF CRUDE DISTILLATION UNITS The factors affect the design and operation of the unit are explored 1. Fractionation The degree of fractionation in a crude unit is determined by the gap or overlap between two adjacent side stream products. Example: The gap or overlap in the boiling point range between kerosene and LGO. Lighter product: kerosene end boiling point Heavier product: LGO initial boiling point In the ideal case there would be no overlap However, if we compare the ASTM distillation boiling points, and since ASTM distillation does not give perfect fractionation. Since determining the initial and end point on the laboratory test is not always possible or accurate. The fractionation gap is defined as the difference between the ASTM 5% boiling point of the heavier product and the 95% point of the lighter product. a gap indicating good fractionation some of the light product is still in the heavier product 2. Overflash The partially vaporized crude is transferred to the flash zone. The furnace outlet temperature should be enough to vaporize all products withdrawn above the flash zone plus about 3–5 vol% of the bottom product. This overflash has the function of providing liquid wash to the vapours going up the column from the flash zone. The overflash improve fractionation on the trays above the flash zone, thereby improving the quality of the HGO and reducing the overlap with the bottom products below the flash zone. 3. Column Pressure The pressure inside the CDU column is controlled by the back pressure of the overhead reflux drum at about 0.2–0.34 bar gauge (3–5 psig). The top tray pressure is 0.4–0.7 bar gauge (6–10 psig) higher than the reflux drum. The flash zone pressure is usually 0.34–0.54 bar (5–8 psi) higher than the top tray. Pflash zone > PTop tray > Preflux drum 4. Overhead Temperature The overhead temperature must be controlled to be 14–17 ˚C higher than the dew point temperature for the water at the column overhead pressure so that no liquid water is condensed in the column. This is to prevent corrosion due to the hydrogen chloride dissolved in liquid water (hydrochloric acid). EXAMPLE If the overhead stream contains 8.5 mol% water at a pressure of 34.7 psia (2.36 bars), calculate the overhead temperature for safe operation. EXAMPLE If the overhead stream contains 8.5 mol% water at a pressure of 34.7 psia (2.36 bars), calculate the overhead temperature for safe operation. Solution: The saturation temperature of water at the partial pressure of water in the overhead vapour. Water partial pressure = 0.085 x 2.36 =0.2 bars From the steam tables: Saturated steam temperature at 0.2 bars = 61 ˚C Safe overhead operating temperature = 61+17 ˚C The End
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