Petroleum refining processes stabilize and upgrade petroleum products by separating them from less desirable products and by removing objectionable elements. Undesirable elements such as sulfur, nitrogen and oxygen are removed by hydrodesulfurization, water treatment, chemical sweetening and removal of acid gases. Refining processes, mainly used to separate petroleum products, include processes such as deasphalting. Desalination is used to remove salt, minerals, sand and water from raw crude oil before refining. Asphalt blowing is used to polymerize and stabilize asphalt in order to improve its weathering characteristics. The most important oil refining processes are:
- Hydro-desulphurization
- Hydro-treating
- Chemical sweetening
- De-asphalting
Hydro-desulphurization
Although reactions related to the catalytic hydrogenation of organic materials were known before 1897, the property of finely divided nickel to catalyze the fixation of hydrogen on hydrocarbon double bonds. Hydrodesulfurization (HDS) is a catalytic chemical process widely used to remove sulfur compounds from refined petroleum products such as:
- Gasoline or Petrol
- Jet fuel
- Diesel fuel
- Fuel oils
One of the goals of sulfur removal is to reduce sulfur dioxide emissions from the use of those fuels in automobiles, airplanes, railroad locomotives, ships or oil-burning power plants, residential and industrial furnaces, and other forms of fuel combustion.
Another important reason for removing sulfur from intermediate product naphtha streams in an oil refinery is that sulfur, even at very low concentrations, poisons the platinum and rhenium noble metal catalysts in the catalytic reformer units that are subsequently used to upgrade the naphtha stream. will be Hydrogenation of sulfur compounds leads to the formation of hydrogen sulfide, an undesirable and toxic gas. Industrial hydrogen desulfurization processes include facilities for absorbing and removing hydrogen sulfide gas. In oil refineries, hydrogen sulfide gas is subsequently converted to byproduct elemental sulfur. In fact, the vast majority of the 68,000,000 metric tons of sulfur produced worldwide in 2010 was by-product sulfur from oil refining and natural gas processing plants.
Desulfurization or desulfurization with the help of hydrogen is a catalytic process that is carried out in refineries to remove sulfur compounds from fossil fuels. Most metals catalyze the hydrodesulfurization process, but intermediate metals are more active. Ruthenium disulfide is the most active of the catalysts, but a relatively high binary combination of cobalt and molybdenum is usually used as the main catalyst in the process.
The reaction takes place in a trickle bed reactor, where the direction of movement of the liquid phase is always from top to bottom, but the direction of movement of the gas phase can be from top to bottom or from bottom to top. This reactor is usually used in hydrogenation reactions. The process temperature and operating pressure are 330°C and 4000 kPa respectively.
In this reactor, sulfur compounds are converted into hydrogen sulfide in the presence of hydrogen. All reactions are exothermic and the reaction rate decreases as the reactor length increases. Some of the most important reactions that occur in this process are listed below:
Reactions A and B for mercaptans and reactions C and D are related to dimethyl disulfide and dimethyl disulfide, which are in the flow of disulfide oils with hydrogen, respectively. Also, reaction E is for benzothiophene.
- CH3SH+H2 CH4+H2S
- C2H6S2+H2 2CH4+2H2S
- C3H9S2+7/2H2 3CH4+2H2S
- C4H10S2+3H2 2C2H6+2H2S
- C3H6S+3H2 C8H10+H2S
Hydro-treating
The term hydrogen refining is used to describe the process of removing sulfur, nitrogen and metal impurities in the raw material by hydrogen in the presence of a catalyst. Hydrocracking is the process of catalytic cracking of raw materials into lower boiling point products by reacting them with hydrogen. Hydrogenation is used when aromatics are saturated by hydrogen to the corresponding naphthenes. The use of hydraulic conversion technique depends on the type of raw material and desired products, which are shown in the table below.
The Hydrotreating process achieves the following objectives:
- Removing impurities, such as sulphur, nitrogen and oxygen for the control of a final product specification or for the preparation of feed for further processing (naphtha reformer feed and FCC feed);
- Removal of metals, usually in a separate guard catalytic reactor when the organo-metallic compounds are hydrogenated and decomposed, resulting in metal deposition on the catalyst pores (e.g. atmospheric residue desulphurization (ARDS) guard reactor);
- Saturation of olefins and their unstable compounds.
Chemical sweetening
Sweetening of distillates is accomplished by the conversion of mercaptans to alkyl disulfides in the presence of a catalyst. Conversion may be followed by an extraction step for removal of the alkyl disulfides. In the conversion process, sulfur is added to the sour distillate with a small amount of caustic and air. The mixture is then passed upward through a fixed-bed catalyst, counter to a flow of caustic entering at the top of the vessel. In the conversion and extraction process, the sour distillate is washed with caustic and then is contacted in the extractor with a solution of catalyst and caustic. The extracted distillate is then contacted with air to convert mercaptans to disulfides. After oxidation, the distillate is settled, inhibitors are added, and the distillate is sent to storage. Regeneration is accomplished by mixing caustic from the bottom of the extractor with air and then separating the disulfides and excess air.
Amine gas treating, also known as gas sweetening and acid gas (AG) removal, refers to a group of processes that use aqueous solutions of various amines to remove H2S and CO from gases. Sweetening processes involve the removal of H2S and mercaptans from refinery streams.
Amines have a functional group that contains nitrogen. Primary amines arise when one of the three hydrogen atoms in ammonia is replaced by an organic substituent. Secondary amines have two organic substituents bound to N together with one H. The most commonly used amines in gas treating are:
- Primary monoethanolamine (MEA)
- Secondary diethanolamine (DEA)
- Tertiary methyldiethanolamine (MDEA)
Propane Deasphalting
Solvent Deasphalting (SDA) is a unique separation process in which the residue is separated by molecular weight (density) instead of boiling point, producing a low pollutant deasphalting oil (DAO) that is rich in paraffin. . This has the advantage of being a relatively low-cost process that has the flexibility to accommodate a wide range of DAO qualities. As with vacuum distillation, there are limitations to how far the SDA unit can upgrade the residue or how much DAO it can produce. These restrictions usually include:
- The DAO quality specifications required by downstream conversion units
- The final high-sulphur residual fuel oil stability and quality
The well-proven SDA process normally separates vacuum residue feedstock into relatively low metal/carbon DAO and a heavy pitch stream containing most of the contaminants. A solvent (typically C3–C7) is used and recovered from both product streams by supercritical recovery methods, thereby minimizing utilities consumption.
One of the well-known solvent deasphalting process is the ROSE process. The ROSE process is an energy efficient and cost-effective solvent deasphalting technology. The following Figure is a simplified process flow diagram of the Rose process.
In the Rose process the feedstock is mixed with a portion of the solvent and fed to an asphaltene separator where additional solvent is contacted with the feed in a countercurrent mode at an elevated temperature and pressure. The heavy asphaltene fraction drops out of the solution and is withdrawn from the bottom. The solvent dissolved in the asphaltenes is separated, recovered and recycled.