What Is an Alkylating Agent?

The process of introducing an alkyl group into an organic molecule using an addition or displacement reaction. As an important synthetic method, alkylation reaction is widely used in many chemical production processes.

Alkylation chain reaction mechanism

The main product of various butene-isobutane alkylation reactions is 2,2,4-trimethylpentane. In the alkylation reaction of propylene isobutane, trimethylpentane is in the reaction product. It also holds a considerable amount.

The alkylation reaction based on the orthocarbon ion theory can be summarized as the following chain reaction mechanism. Any chain reaction generally includes 3 steps, that is, chain initiation, chain growth, and chain termination.

(1) Chain initiation

In the alkylation reaction between isobutane and olefin, the olefin gets hydrogen protons to form a carboion as a chain initiation process. The carboion on the tert-butyl group plays a vital role in the alkylation reaction. .

Regarding the initiation of the chain, there are several points that need to be explained:

1. The dissociation of sulfuric acid or hydrofluoric acid generates hydrogen protons, which provides a source of positive ions for carbocations, but when the acid is in a state that cannot be dissociated at all, such as under relatively dry conditions, that is to say without polarity For large water molecules, the acid cannot dissociate and the alkylation reaction cannot occur.

2. Only tert-butyl carboion can serve as a chain-carrying function. If other linear olefins accept hydrogen protons, the situation is more complicated: or the linear olefin itself isomerizes to tert-butyl carboion; or the linear The carbene ion of the olefin extracts the hydride ion of isobutane, and the isobutane becomes a tert-butyl n-carbonate to initiate an alkylation reaction.

3. Macromolecular n-carbon ions, especially acid-soluble hydrocarbons, are highly ionized, and can extract hydride ions of olefins or isobutane to generate new tert-butyl n-carbon ions.

The main source of tert-butyl n-carboxion is isobutylene. The n-carbon n-butane ion formed by protons can also be obtained by hydrogen transfer. However, when people studied the mechanism of alkylation, they found that at least part of the traced n-butene became isobutane, indicating that under acidic conditions, n-butene was isomerized to form isobutene. Isobutylene was transferred to isobutene by receiving hydrogen anions. In addition, the alkylation reaction of isobutene / isobutane does not significantly increase the content of trimethylpentane. When hydrogen transfer plays a major role, it is instructed to accelerate the dimerization and multimerization of isobutene, indicating that before the alkylation reaction N-Butene undergoes an isomerization process. This is also the main reason for the similar distribution of the products obtained by the alkylation of isobutane with different olefins.

(2) Growth of the chain

The tert-butyl carboion ion captures the hydrogen anion to generate a product, and ensures the continued existence of the tert-butyl carboion ion.

(3) Termination of the chain

Increasing carboions usually pick up a hydrogen anion from isobutane and stop growing, which is how most alkylated chains terminate. However, the loss of protons of chain-producing carboions into olefins is rare, because olefins are rarely found in the analysis of alkylation products, and once the olefins are formed, they are immediately protonated under alkylation conditions. And rejoined the reaction. ^[1]

Alkylation reactions can be divided into thermal alkylation and catalytic alkylation. Due to the high temperature of the thermal alkylation reaction, it is easy to produce

Industrially catalyzed alkylation processes can be divided into

Alkylation is

Liquid phase alkylation can be in horizontal or tower

in

Alkylation principle

One of the refinery gas processing processes is to make isobutane and butane in the presence of a catalyst (hydrofluoric or sulfuric acid or solid acid (research direction, which can avoid environmental pollution caused by liquid waste acid or high recovery and treatment costs)). A process in which olefin (or a mixture of propylene, butene, and pentene) undergoes an alkylation reaction to produce a high-octane gasoline component. Using isobutane and butene as raw materials, the research method of the product can reach octane number (see octane number) up to 94; using propylene, butene, pentene as raw materials, the octane number is slightly lower. Alkylated gasoline has good sensitivity, low vapor pressure, and good lead sensitivity (adding a small amount of tetraethyl lead can significantly increase the octane number of gasoline). It is an ideal blending component for the production of aviation gasoline and high-grade automotive gasoline. ^[2]

History of Alkylation

During World War II, petroleum hydrocarbon alkylation technology was developed to meet the demand for aviation gasoline. In 1939, the British and Iraqi Petroleum Company used sulfuric acid as a catalyst, and in 1942 the United States Global Oil Company and Phillips Petroleum Company used hydrofluoric acid as a catalyst to build petroleum hydrocarbon alkylation units to produce high-octane gasoline. In the decades after the war, the demand for high-octane motor gasoline continued to grow. China completed the sulfuric acid alkylation unit in the 1960s, and in recent years it is constructing a hydrofluoric acid alkylation unit. ^[2]

Alkylation process

Depending on the catalyst used, it can be divided into hydrofluoric acid alkylation and sulfuric acid alkylation.

Hydrofluoric acid alkylation process usually consists of raw material pretreatment, reaction, product fractionation and treatment, acid regeneration and three waste treatment. The purpose of pretreatment is to control the moisture content of the raw materials (less than 20ppm) to avoid severe corrosion of the equipment, and at the same time, strictly control the content of impurities such as sulfur, butadiene C2, C6 and oxygenates. Due to the greater solubility of hydrocarbons in hydrofluoric acid, the alkylation reaction speed is very fast, which can be basically completed in only tens of seconds, so a tube reactor can be used. The reaction temperature is 20 to 40 ° C, and the pressure is 0.7 to 1.2 MPa. In order to suppress the side reactions, a large amount of isobutane needs to be recycled back to the reaction feed, so that the isobutane and olefin feed maintain a volume ratio of (8-12): 1. The heat of reaction is taken away by the acid cooler. The purpose of acid regeneration is mainly to remove the water produced in the stack and the raw materials generated in the reaction, and to discharge the acid-soluble oil from the bottom of the regenerator to maintain the hydrofluoric acid concentration at about 90%. The alkylated oil is discharged from the bottom of the main fractionation column, and the circulating isobutane is withdrawn from the side line of the column. If aviation fuel is to be produced, the obtained alkylated oil needs to be re-distilled, and light alkylated oil is separated from the top of the tower as a component of aviation gasoline. The exhaust gas or waste liquid containing hydrofluoric acid discharged from the system needs to be treated, and finally reacted with calcium chloride to make it into inert calcium fluoride. Approximately 0.4 to 0.6 kg of hydrofluoric acid is consumed per ton of alkylated gasoline.

The basic process of sulfuric acid alkylation is similar to that of hydrofluoric acid. The main problem is the high acid consumption. 70 to 80 kg of sulfuric acid is consumed for 1t of alkylated oil, and a large amount of dilute acid is produced as a byproduct. If there is no sulfuric acid plant or acid concentration facility nearby, it will cause serious pollution to the environment. ^[2]