The impact of various components in the alkylation feedstock on the smooth operation of alkylation

A previous topic was published on the effect of various component impurities on the alkylation reaction. You can refer to it

C3–C5 olefins can all be used with isobutane as a raw material for alkylation, but the reaction efficiency varies among different olefins. Using propylene and pentene as raw materials for alkylation, the octane number of the resulting alkylated oils is lower than that of butene-alkylated oils ； Especially for alkyl sulfate production, the acid consumption of propylene and pentene feedstocks is greater than that of butylene. Therefore, isobutane and butylene are used as raw materials for alkylation in industry. For sulfate alkylation, good raw materials are 1-butene and 2-butene. For alkylation using hydrofluoric acid, 2-butene is a better starting material. The butylene produced as a by-product of catalytic cracking units also contains other components and impurities, mainly including butadiene, sulfides, and water; if there is an MTBE unit upstream, the feedstock also contains methanol and dimethyl ether. The presence of ethylene in the raw materials has a significant impact on the operation of sulfate-based alkylation units. The impact of the aforementioned impurities on alkylation is mainly reflected in their effect on acid consumption. 1. Ethylene: In the sulfuric acid-based alkylation process, when ethylene is present in the feedstock, it does not undergo an alkylation reaction with isobutane; instead, it reacts with sulfuric acid to form ethyl hydrogen sulfate, which dissolves in the acid and acts as a diluent for the catalyst, sulfuric acid. In severe cases, this prevents the alkylation reaction from taking place, and polymerization reactions occur instead. Ethylene can also lead to an increase in acid consumption, with 20.9 tons of sulfuric acid being used per ton of ethylene. The way to control ethylene in the raw material is to control the amount of C3 introduced into it. 2. Butadiene: Raw materials typically contain 0.5% to 1% butadiene. Under alkylation conditions, it reacts with sulfuric acid or hydrofluoric acid to form acid-soluble esters or heavy acid-soluble oils (ASO). ASO is a viscous, heavy oil with a high molecular weight, which causes the dry point of the alkylated oil to rise, as well as a decrease in octane number and gasoline yield. Separating ASO also results in acid loss. For sulfate alkylation, 13.4 tons of sulfuric acid are consumed per 1 ton of butadiene ； For alkylation using hydrofluoric acid, 1 ton of butadiene produces 0.7 to 1 ton of ASO, while 1 ton of ASO requires 0.5 to 20 tons of hydrofluoric acid. Therefore, sulfate alkylation requires a butadiene content of less than 0.5%, while hydrofluoric acid alkylation requires a butadiene content of less than 0.2%. Butadiene polymerizes on the surface of solid acid catalysts to form gums, which gradually lead to the formation of coke. Selective hydrogenation is commonly used to remove butadiene. Selective hydrogenation is usually carried out in a fixed-bed reactor. The hydrogenation active component of the catalyst is the precious metal Pd, with a content of 0.2%–0.3%. To improve the selectivity of the catalyst, co-activating components such as Au, Cr, and Ag are usually added. The most widely used carrier is Al2O3. Modifying the Al2O3 carrier with the alkali metal K can reduce surface acidity and improve the stability of the catalyst. Catalysts prepared using composite carriers such as TiO2-Al2O3 exhibit higher activity, selectivity, and resistance to sulfur and arsenic poisoning. Since H2S can cause permanent poisoning of the Pd catalyst, the H2S content in the feedstock is required to be less than 1 μg/g. The selective hydrogenation technology for alkylation feedstocks developed by the Qilu Petrochemical Research Institute of Sinopec enables the hydrogenation of C4 compounds under conditions of a temperature of 60–80°C, a pressure of 1.5–2.0 MPa, an space velocity of ≤5 h-1, and a H2 to C4 molar ratio of 2.0–4.0; as a result, the content of dienes in the C4 compounds is less than 100 μg/g, while the selectivity for monoenes exceeds 100%. After the adoption of selective hydrogenation technology, the dry point of alkylated sulfuric acid gasoline decreased by 5°C, the acid consumption dropped from 88.11 kg/t to 54.5 kg/t, and the research octane number increased from 96.5 to 97.6. The increase in octane number is due to the fact that the precious metal Pd catalyst, in addition to its high hydrogenation activity and selectivity, also possesses high double-bond isomerization activity, which enables the conversion of 1-butene in the feedstock into 2-butene. This effect is even more significant when using the hydrofluoric acid method for alkylation in order to raise the octane number of the alkylated oil. 3. Sulfides: Sulfides have a very significant diluting effect on acids; they promote the occurrence of polymerization reactions while inhibiting alkylation reactions, resulting in an increase in ASO. Meanwhile, 15–60 tons of sulfuric acid can be consumed per ton of sulfide (based on sulfur content). When the sulfur content in the raw material is 20 μg/g, the acid consumption per ton of alkylated hydrofluoric acid oil is 0.608 kg ； When the sulfur content exceeds 50 μg/g, acid consumption increases sharply ； When the sulfur content in the raw material is 100 μg/g, the acid consumption per ton of alkylated hydrofluoric acid oil is 4.05 kg. Therefore, sulfate-based alkylation requires a sulfur content of less than 100 μg/g, while hydrofluoric acid-based alkylation requires a sulfur content of less than 20 μg/g. The existing LPG desulfurization and hydrogen sulfide removal processes can be used to meet the sulfur content requirements. 4. Water: The raw material usually contains about 500 μg/g of saturated water; in particular, when the raw material contains free water, it has a significant impact on alkylation. Water present in the raw materials can lead to acid dilution and increased equipment corrosion. A coalescer can be used to remove free water. In the drying step of the alkylation unit, 3A or 4A molecular sieves or activated alumina are used as desiccants, which can reduce the water content in the raw material to 10–20 μg/g. 5. Methanol and dimethyl ether: The C4 fraction from MTBE plants typically contains 500–2000 μg/g of dimethyl ether and 50–100 μg/g of methanol. Methanol generates dimethyl ether and water in the alkylation unit, while dimethyl ether produces light acid-soluble substances that cannot be separated from the acid, resulting in a decrease in the quality of the circulating acid. This in turn leads to increased consumption of alkylation acid as well as a decline in the yield and octane number of the alkylated oil. Generally, it is required that methanol in the raw materials be ≤50μg/g, and dimethyl ether be ≤100μg/g. Distillation can be used to remove all methanol from the raw material, resulting in a dimethyl ether content of less than 35 μg/g. After implementing measures to remove dimethyl ether and methanol, the acid consumption of 1 ton of alkylated oil can be reduced by 4.38 kg. From http://bbs.hcbbs.com/thread-1351281-1-1.html. Feel free to add more content to help everyone learn*

Take a look at Geng Yingjie’s “Alkylation Production Processes and Technologies”

OP, why do you need to control the amount of C3 added when regulating the ethylene content?

You’re too kind, bro; it’s about exchanging ideas.