lecture 2 of refinery 0111.pdf
- 1. Refinery of Petroleum Technique Crude oil Distillation: Electrical desalting of crude oil, desulphurization of crude oil. Lecturer: Zheno Kareem Ahmed 1 November 2014 1
- 2. Content: • Crude oil pre-treatment: • Desalting of Crude oil • Sweetening of Crude oil 2
- 3. Crude oil pre-treatment: Crude oil-gas-water mixture produced from well are generally directed through flow lines and manifold system, to a central processing and treatment facility normally called the gas-oil separation plan (GOSP). The first step in processing of the produces stream is the separation of the phases (oil, gas and water) into separate system. This take place in mechanical devices known as two phase gas-oil separator when the produced stream contains no water or three-phase separators when the produced stream contains water. Gas-oil separation carried out in these separators is recognised as the backbone process in a train of field processing units of oil and gas operations. The separators are used to relive the excess pressure due to the gas associated with the produced crude and, consequently, separating it from the oil (three stages separators and the main separator are gravity type). When water exists in the produced stream, separators are also used to separate the free water from the oil. Once separation is done each stream undergoes the proper processing for further field treatment. 3
- 4. The presence of this salt water presents serious corrosion and scaling problems in transportation and refinery operations. Water remaining in the oil is known as the basic sediment and water (BS & W). A maximum of 1% BS & W and in some cases less than 0.5 % BS & W is acceptable. The limit on the salt content of the remnant water in oil is usually in the range of 10 to 15 PTB (Ib/1000bbl). If these specifications are not met, then further treatment of oil leaving the separator will be needed. Such treatment involves emulsion treatment/dehydration and desalting processes. Some produce crude oils contain hydrogen sulphide and other sulfur products. When it contains more than 400 ppm H2S gas, the oil is classified as sour crude. Sour crude oils present serious safety and corrosion problems. In such cases, another treatment known as the sweetening process is needed to remove hydrogen sulphide or reduce its content to acceptable limits. 4
- 5. 5 Desalting • Desalting required to reduce corrosion, plugging, fouling & catalyst poisoning; using: – chemical desalting – electrical desalting – filtration through diatomaceous earth (clays) • H2O added to feedstock crude & the mixture heated to 150-350 oF– reduces viscosity & increases separation from H2O • pH adjusted – NH3, caustic or acids • Waste H2O & contaminants discharged from bottom of a settling tank • Desalted crude removed from top settling tank
- 6. 6 The removal of salt from crude oil for refinery feedstock has been and still is a mandatory step. Desalting of crude oil will eliminate or minimize problem from the presences of mineral salts in crude oil. These salts often deposit chlorides on the heat transfer equipment of the distillation units and cause fouling effects. In addition, some chlorides will decompose under high temperature, forming corrosive hydrochloric acid. MgCl2 at high temperature +2H2O = Mg(OH)2 + 2HCl The reduction of salt content down to 5 PTB is feasible. Electrical desalting of crude oils: It consist of two steps: Forming an emulsion of crude oil and water. Demulsification process in which the emulsion of crude oil and water formed in the first step is broken by means of an electrical field. Process variables: • Pressure drop through mixing valve (1.5- 4) kgf/cm2 • water injection rate (3-10)vol% • conductivity of fluid flow • voltage for good desalting (12,000-35000 volt/in2) • Desalter temperature (200-300) oF • Desalter pressure (2-3)kgf/cm2 • PH value (6-8) • Solid at water-oil interface must be removing (cause trouble of electrode)
- 7. 7 Figure (1): Desalting process
- 8. Crude oil Sweeting (Desulphurization): Crude oil is considered “sweet” if the dangerous acidic gases are removed from it. On the other hand, it is classified as “sour” if it contains as much as 0.05 ft3 of dissolved H2S in 100 gal of oil. The poor quality of crude oil currently can obviously result in the high sulfur contents of oil products, which can lead to corrosion, catalyst poisoning, environmental pollution and other negative consequences see figure (2). The increasing sulfur content of crude oil also results in an increase in sulfur content in automotive gasoline, diesel fuel, and jet fuel. Sulphur when involved in the combustion process produce sulphur dioxide (SO2) which is a major pollutant involved in acid rain, and important component in the formation of fine particulate matter. Sulphur can reduce the effectiveness of some pollution control technology, such as catalytic converters that reduce other types of pollutants such as nitrogen oxides Nox, carbon monoxide Co and HC from the combustion of gasoline and diesel in our vehicles. 8
- 9. 9 Figure (2): Global trend on crude oil quality
- 10. 10 Most common forms of sulphur in crude oils: Hydrogen sulphide (H2S): 0.1% in air is toxically fatal in 30 min. Mercaptan (H-S-R): when an alkyl group replaces the hydrogen in the sulfur-containing molecule, the order is less obnoxious. Polysulfides (R-S-S-R) these are more complicated sulfur compounds and they may decompose, in some cases depositing elemental sulfur.
- 11. To meet the needs for producing clean fuels, decreasing the sulfur content of crude oil becomes an urgent task. Studying desulfurization technology and raising the efficiency of desulfurization processes are the keys to bringing more profits to the oil refining companies. The process of removing sulphur from crude oil is called desulphurization or sweetening and uses a technique called Hydrotreating and it involves chemical reaction at high pressure and temperature to remove sulphur atoms from oil and replace them with hydrogen ayoms. Hydrodesulphurization is one of the catalytic desulfurization processes, which aims at turning organic sulfur compounds into H2S using H2 as the reactant in the presence of metal catalysts operating at high temperature and pressure. 11
- 12. The resultant hydrogen sulphide is then removed from the system by heating or may be separated by using amine solutions. However, the HDS process features a complicated procedure, high production cost and high materials consumption. With an increasing ratio of heavy crude oil supply to the oil refinery, the sulfur content in crude slate is growing, which leads to a shortened catalyst life at the refinery. Furthermore, the HDS process needs more H2, so the production cost will increase a lot. Sulfur removal occurs via the conversion to H2S of the organic sulfur compounds present in the feedstock. Sulfur is found throughout the boiling range of petroleum fractions in the form of many hundreds of different organic sulfur compounds which, in the naphtha to atmospheric residue range, can all be classified as belonging to one of the following six sulfur types: mercaptans, sulfides, di-sulfides, thiophenes, benzo-thiophenes, and di-benzo-thiophenes. Typical reactions for each kind of sulfur compound are shown below: 12
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