chemistry and technology of petroleum

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 • An increased search for fuel products from non-fossil sources such as biodiesel and alcohols from vegetable sources. • The development of better methods to process tar sand, coal gasification and synthesis of fuels by new technology. • 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 • The various hydrocarbons produced are then chemically altered to make them more suitable for their intended purpose. • For example, naphthas are "reformed" from paraffins and naphthenes into aromatics.

Refining is carried out Step3 - Purification • 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.

Refinery-petrochemical integration 1. Physical Separation Processes

2. Chemical Catalytic Conversion Processes Refinery-petrochemical integration

3. Thermal Chemical Conversion Processes

1. Physical Separation Processes Refinery-petrochemical integration Crude Distillation

Crude Distillation • Crude oils are first desalted and then introduced with steam to an atmospheric distillation column. • 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 Refinery-petrochemical integration

Solvent Deasphalting • This is the only physical process where carbon is rejected from heavy petroleum fraction such as vacuum residue. • Propane in liquid form (at moderate pressure) is usually to dissolve the whole oil, leaving asphalteneto precipitate. • 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. • The DAO can also be sent to cracking units to increase light oil production. Solvent deasphalting process

Solvent Extraction • 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). • The solvent is removed from both phases and the raffinate is dewaxed. Solvent Extraction

Solvent Dewaxing • 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. • The extracted and dewaxed resulting oil is called ‘‘lube oil’’. • In some modern refineries removal of aromatics and waxes is carried out by catalytic processes in all ‘‘ hydrogenation process”

Catalytic Reforming • 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. • 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).

Hydrotreating • 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. • 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.

Catalytic Hydrocracking • 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. • 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. • The main products are kerosene, jet fuel, diesel and fuel oil.

Catalytic Cracking • 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. • The main feed to FCC is VGO and the product is gasoline, but some gas oil and refinery gases are also produced.

Alkylation • Alkylation is the process in which isobutane reacts with olefins such as butylene (C4 ) to produce a gasoline range alkylate. • The catalyst in this case is either sulphuric acid or hydrofluoric acid. The hydrocarbons and acid react in liquid phase. • Isobutane and olefins are collected mainly from FCC and delayed coker

Isomerization • 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. • 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. • The main catalyst in this case is a Pt-zeolite base.

Delayed Coking • 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. • 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 • In this thermal process, most of the coke is gasified into fuel gas using steam and air. • The burning of coke by air will provide the heat required for thermal cracking. • The products are gases, gasoline and gas oils with very little coke.

Visbreaking • 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. • 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). • 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 • The crude oil contains salt in the form of dissolved salt in the tiny droplet of water which forms a water-in oil emulsion. • This water cannot be separated by gravity or through mechanical means. • 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 • 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 of49-54 ˚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.

chemistry and technology of petroleum