Petrochemical & Refining
Isomerization is the process of converting normal paraffins into isoparaffins, which offer a higher value than their normal counterparts. Normal paraffins have a linear structure, which can be rearranged to create branches, thus forming isomers that have the same number of carbons, but have a different geometric structure.
Normal butane (n-butane) is converted into isobutane (i-butane), which will then be used in alkylation units. Before n-butane can be isomerized, first it is dehydrated and sulfur is removed to create a cleaner butane stream. Hydrogen is added to the purified stream to suppress dehydrogenation and the formation of coke. The stream is then processed in the isomerization reactor where normal paraffin chains are converted into isoparaffins chains. The butane mixture that is produced will then enter the fractionator, also known as a deisobutanizer, to separate normal butane from the desired isobutane stream.
Pentane and Hexane Isomerization
Normal pentane (n-C5) and normal hexane (n-C6) are converted into isopentane (i-C5) and isohexane (i-C6) for use in gasoline blending because the isomerization process increases the octane rating of these compounds. To process n-C5 and n-C6 into isomers, the feedstock is dehydrated and desulfurized, mixed with organic chloride and hydrogen, then heated. The addition of hydrogen to the stream will suppress dehydrogenation and coking. The stream is then introduced to catalysts that cause benzene and olefins to hydrogenate. Next, the stream enters the isomerization reactor where the conversion of normal paraffins to isoparaffins occurs in the presence of catalysts. After isomerization, the stream is cooled and separated into recycled hydrogen gas and a liquid product stream, known as isomerate. To finish processing the isomerate, caustic and water is used to wash the stream, acid is used to strip the stream, and then the stream is stabilized before being stored or used in the gasoline blending pool.
Hydrogen holds a high value for refineries and is used in processes such as hydrotreating and hydrocracking. Hydrogen can be collected from various processes in the refinery such as off-gas streams, hydrocracker and hydrotreater purge gas, and most critically, from catalytic reforming, where hydrogen is formed as a byproduct of the reformation process. The hydrogen collected from these processes has to be purified in pressure swing adsorption (PSA) units to produce hydrogen. Hengye’s specialized adsorbents, used in PSA units, are able to remove water, CO, CO2, nitrogen, and methane to create an end product that is about 99.9% pure and suitable for use in refineries.
Catalysts are commonly used in petroleum refineries to convert straight run naphtha, which has a typically low octane rating, into reformates. Reformates are used in gasoline blending because of their high octane rating. The catalysts will facilitate the reformation, or rearrangement, of the molecular structure of low value hydrocarbons in naphtha feedstocks and creates a more complex hydrocarbon with a higher value.
Upgrading these low octane hydrocarbons creates a higher value product to increase profits for the refinery. While reforming naphtha, some byproducts are formed such as C1-C4 hydrocarbons. Catalytic reforming is a critical source of hydrogen, another byproduct from reforming, which is of high value in other refining processes.
In catalytic reforming processes, the catalysts are treated with organic chloride to enhance isomerization activity and promote surface acidity, which improves the qualities of the desired reformate. However, these organic chlorides can cause issues in catalytic reforming separator and in downstream equipment including poisoning of catalysts, corrosion, specification issues, and can form into ammonium chloride (NH4Cl) and hydrogen chloride (HCl). Hydrogen chloride can rapidly deactivate palladium, nickel, and copper based catalysts, and will react with the ammonia (NH3) formed in the catalytic reformer feed from nitrogen compounds. When operation temperatures fall below 100˚C, HCl and ammonia will form into ammonium chloride, which can deposit, corrode, and poison downstream catalysts.
To prevent these issues, refineries use chloride guards, filled with catalysts to remove chloride from process streams. Using regular activated alumina is possible, but not the best option because hydrogen chloride, as a polar molecule, will react with the hydroxyl groups of the activated alumina as a means of physical adsorption (physisorption). While activated alumina can be used in gas phase processes, the resulting physisorption is undesirable and promoted alumina products are often chosen instead.
Promoted alumina is commonly used in liquid phase processes, providing chemisorption capabilities that can function at higher temperatures compared to nonpromoted alumina. The promoter reacts with HCl to create a chemical bond, which removes the chloride from the process stream; this product however, cannot be regenerated.
Sometimes referred to as steam methane reforming (SMR), this process involves the heating of methane (CH4) with steam, in the presence of a catalyst, to create hydrogen (H2) and carbon oxide byproducts. After the initial reforming reaction, the steam undergoes a water-gas shift reaction, which converts carbon monoxide (CO) into carbon dioxide (CO2) and more pure hydrogen. Finally, pressure swing adsorption is used to isolate the carbon oxides and leave a nearly pure stream of hydrogen. Since hydrogen does not naturally occur in an isolated form, steam reforming is a crucial process for creating hydrogen. Visit our steam methane reforming (SMR) Page to learn more about the process.
Before being used in steam reforming, the hydrocarbon feedstock stream undergoes purification processes to remove chloride (Cl), sulfur (S), and other impurities. After the stream has been reformed, the steam is put through a methanation step, which removes residual carbon oxides that were not removed in the water gas shift reactions. In place of the methanation step, some newer design SMR plants use a Pressure Swing Adsorption unit that utilizes molecular sieve to create a hydrogen stream with high purity.
Steam reformers can be used in an array of applications including petrochemical refining, ammonia synthesis, chemical production, electronics manufacturing, and more. In petrochemical refining, hydrogen is used to refine crude oil to create high value fuels, such as gasoline and diesel, and can be used to remove sulfur from these hydrocarbon streams. Some small-scale SMR units are used, but these units are typically large, centralized, industrial sized plants. High purity hydrogen is commonly used in fuel cells, which converts chemical energy into electrical energy. Being an energy carrier, hydrogen is combined with oxygen in the fuel cell, which produces heat, electricity, and a byproduct consisting of water vapor.
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