The majority of typical process units perform some kind of chemical reaction. We can include in this class conversion units, like thermal cracking, as a coking or a visbreaking, cracking processes, like fluid catalytic cracking, steam cracking and hydrocracking, catalytic reforming, isomerization, alkylation and polymerization. In this same category, we can include the treatment processes, hydrodesulfuriza-tion and hydrotreating.
To break and recombine molecules, high pressure, temperature and consequently energy are required over relative small amounts of matter. So, generally, these processes are very energy intensive per mass. On the other hand, they usually run over small shares of the crude oil input of the refinery, so that their energy requirements may not be the most significant in the overall refinery energy balance.
Fluid Catalytic Cracking Catalytic cracking is the most commonly used process to convert heavy oils, like light and heavy gas oil from atmospheric or vacuum distillation, coking and deasphalting units into more valuable lighter products, especially gasoline. It requires both heat and catalyst to break large hydrocarbon molecules into smaller, lighter molecules.
Catalytic cracking reactions occur on moving fluidized bed reactors. In this process, oil streams are in contact with a hot zeolite type catalyst in a so- called 'riser reactor', where the cracking process takes place. The catalyst and the reaction products separate mechanically in a cyclone system in the top of this riser. Any oil remaining on the catalyst is removed by steam fed in a stripping section of the vessel that contains the cyclone on top of the reactor. The catalytic cracking reactions produce coke that is deposited over the catalyst surface, reducing its activity and selectivity. To allow the process to continue, catalyst should be continuously regenerated, by burning off the coke from it, at high temperature in the regenerator. This regeneration demands huge amounts of pressurized air, producing a large flow of a high temperature gas. Regeneration burning can be performed in a complete or partial mode, meaning that this flue gas, although mainly composed of nitrogen, may have approximately 10% of either CO or CO2 gas. Oil products are separated by means of a fractionation train. On average, these units are responsible for processing and producing up to 25% of the overall volume output, so that they are among the most demanding energy units in a refinery, requiring great amounts of heat for the feedstock and a large power demand to achieve the appropriate process pressures.
Common measures to improve energy efficiency in these units include addition of a waste heat boiler to recover heat from the catalyst regeneration flue gas and from completed burning of its CO content, depending on the regeneration process. The installation of a power recovery gas expander turbine for the same hot flue gas is also recommended, to produce shaft power. To avoid hot hydrocarbon vapors flowing inside the boiler furnace, for safety reasons, a certain pressure drop is needed. The huge flow with this pressure drop delivers more than enough power to drive the catalyst regeneration air blower. With this equipment, depending on the unit size and feedstock, the catalytic cracker may become nolonger a net energy consumer, but an energy producer for the refinery. For opportunities in the frac-tionation train, refer to the analysis of the distillation units.
Reforming, Hydrocracking, Hydrotreating and Similar Units Catalytic reforming uses catalytic reactions to transform low octane distillate naphtha into high octane aromatics like benzene, toluene and xylene. Feedstock is in contact with a platinum-containing catalyst at elevated temperatures and hydrogen pressures ranging from 3.5 to 35atm. Four kinds of reaction: dehydrogenation of naphthenes to aromatics; dehydrocyclization of paraffins to aromatics; isomerization; and hydro-cracking may occur. The process might be either continuous, using moving bed reactors, or cyclic and semi-regenerative, using fixed bed reactors. Dehydrogena-tion reactions are very endothermic, requiring the hydrocarbon stream to be heated between each catalyst bed. All hydrocracking reactions release hydrogen, which can be used in the hydrotreating or hydrocracking processes. Feedstocks must be hydrotreated first to remove sulfur, nitrogen and metallic contaminants that cloak the catalyst. The permitted amount of benzene in gasoline has been reduced for environmental restrictions, and the use of catalytic reforming as an octane enhancer is decreasing.
Catalytic hydrocracking occurs in a fixed bed catalytic cracking reactor under high pressures ranging from 80 to 140 atm in the presence of hydrogen. Feedstocks are distillate fractions that cannot be cracked effectively in catalytic cracking units, like residual fuel oils. Hydrogen reduces the formation of heavy residuals streams, while increasing the yield of gasoline by reacting it with cracked products. Depending on products and unit size, it might be a single stage or multistage reactor process. The catalyst is similar to that of the catalytic cracking process, being a silica alumina crystal with scattered rare earth metals.
Hydrotreating and hydroprocessing are similar processes used to remove contaminants such as sulfur, nitrogen, oxygen, halides and trace metal impurities. Hydrotreating also converts olefins into paraffins to reduce gum formation in fuels. These units are usually needed to treat the feedstock of processes for which sulfur and nitrogen are poisons to the catalyst, like catalytic reforming and hydro-cracking. It uses a fixed bed reactor catalyst in the presence of high pressure hydrogen and temperature. It produces the treated streams, fuel gases, hydrogen sulfide and ammonia. The treated product and hydrogen-rich gas are cooled after leaving the reactor and before being separated. Hydrogen is recycled to the reactor. Since these processes have high hydrogen consumption they require a hydrogen plant. This unit usually has a steam reformer and produces syngas (synthetic gas), a hydrogen and carbon monoxide mixture.
Isomerization promotes molecular rearrangement to increase the octane number, that means paraffins being converted to isoparaffins. Reactions occur at temperatures between 100 to 200 °C in the presence of a platinum-based catalyst. This catalysis demands a hydrogen atmosphere to minimize coke deposition; but hydrogen consumption is negligible.
Alkylation is the reaction of propylene and butane olefins with isobutane to form higher molecular weight and high octane number isooctane. It is a low temperature reaction conducted in presence of very strong acids, like hydrofluoric acid or nonfuming sulfuric acid. Hydrofluoric acid alkylation produces a residual acid-soluble oil that is burned in a furnace by a special burner. Sulfuric acid alkylation produces acid sludges which are burned for sulfuric acid regeneration.
All the above processes, with the exception of alkylation, run over contact with a catalyst bed usually under high temperature and high pressure, but most of the reactions involved are exothermic, and products leaving reactors have high energy content. Opportunities for energy efficiency lie in improving heat recovery over products leaving the reactor. So achieving better heat integration through preheating the fired heater feed with the products is essential. Because of high pressures, improving compressor efficiency also gives good return. This recommendation applies in particular to olefin gases in alkylation and hydrogen in hydroprocessing.
Although catalytic reforming is becoming old-fashioned for environmental reasons, these same restrictions are pushing the need for hydrotreating and hydrofinishing processes while, because of its production flexibility, hydrocracking is partially substituting pure catalytic cracking units. This indicates that hydrogenation processes will become more significant in refinery energy consumption profile in the future.
Visbreaking, Coking and Similar Units Visbreaking is a thermal cracking process that uses heat and high pressure to break big hydrocarbon molecules into smaller ones. It aims to produce more gasoline and reduce the viscosity of fuel oil. Heavy gas oils and residue from vacuum distillation are the feedstock for this unit. Feedstock is heated in a furnace up to 480 °C and then fed to a reaction chamber at a pressure of about 10 atm. High temperatures and long residence time result in a high cracking severity that boosts gasoline yield and also produces a low viscosity residue that can be added to the fuel oil pool. After the reaction step, the process stream is mixed with a cooler recycled stream, which stops the cracking, pressure is then reduced in another chamber, and lighter products are vaporized and withdrawn. Products are sent to fractionating, and the distillation bottom residue is partially recycled to cool the process stream leaving the reaction chamber; the rest is blended into residual fuel oil. It is difficult to control reaction pace, so excessive cracking happens and generates unstable compounds. Because of that, visbreaking has been substituted in many refineries by catalytic cracking.
Coking is a more severe thermal cracking process used to convert low value residual fuel oils and transform them into lighter products such as gasoline and gas oils. It also produces petroleum coke, solid carbon, with impurities and approximately 5% of hydrocarbons. There are two types of coking process: delayed coking and fluid coking. The flexicoking process is similar to fluid coking, but it gasifies the fluidized coke to produce coke gas. The vapors from the coke vessels are lighter-cracked hydrocarbon products containing hydrogen sulfide and ammonia, which are sent to fractionating.
Thermal cracking processes demand great amounts of heat at high temperature and products leaving reactors have high energy content. Opportunities for energy efficiency again are on improving heat recovery from the products.
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