Dehydrogenation of butane to butenes

Dehydrogenation of butane is carried out in the boiling or moving layer of the chromium and aluminum catalyst. The process is carried out at a temperature in the range of 550 to 575 degrees. Among the features of the reaction, we note the continuity of the technological chain.

Technology features

Dehydrogenation of butane is mainly carried out in contact adiabatic reactors. The reaction is carried out in the presence of water vapor, which significantly reduces the partial pressure of the interacting gaseous substances. Compensation in the surface reaction apparatuses of the endothermic heat effect is accomplished by the supply of flue gases through the surface of the heat.

Simplified version

Dehydrogenation of butane in the simplest way involves impregnating alumina with a solution of chromic anhydride or with potassium chromate.

The resulting catalyst promotes a rapid and qualitative flow of the process. This accelerator of the chemical process is available in the price range.

Production Scheme

Dehydrogenation of butane is a reaction in which no significant consumption of the catalyst is expected. The dehydrogenation products of the starting material enter the extractive rectification unit, where the necessary olefin fraction is recovered. Dehydrogenation of butane to butadiene in a tubular reactor having an external heating option allows for a good yield of the product.

Specificity of the reaction in its relative safety, as well as in the minimal application of complex automatic systems and devices. Among the advantages of this technology can be mentioned the simplicity of the designs, as well as the low consumption of inexpensive catalyst.

Process Features

Dehydrogenation of butane is a reversible process, with an increase in the volume of the mixture. According to the Le Chatelier principle, in order to shift the chemical equilibrium in this process towards the production of reaction products, it is necessary to lower the pressure in the reaction mixture.

Optimum is considered the atmospheric pressure at a temperature of up to 575 degrees, using a mixed chromium-aluminum catalyst. As the accelerator of the chemical process deposits on the surface of carbon-containing substances that are formed during the course of side reactions of the deep degradation of the initial hydrocarbon, its activity decreases. To restore its original activity, the catalyst is regenerated by blowing it with air, which is mixed with flue gases.

Flow conditions

In the dehydrogenation of butane, an unlimited butene is formed in cylindrical reactors. In the reactor there are special gas distribution grids, cyclones are installed, which allow to catch the catalyst dust carried away by the gas flow.

Dehydrogenation of butane into butenes is the basis for the modernization of industrial processes for obtaining unsaturated hydrocarbons. In addition to this interaction, this technology is used to obtain other variants of paraffins. Dehydrogenation of n-butane became the basis for the production of isobutane, n-butylene, ethylbenzene.

There are some differences between the technological processes, for example, when dehydrogenating all hydrocarbons of a number of paraffins, analogous catalysts are used. The analogy between the production of ethylbenzene and olefins is not only in the use of one process accelerator, but also in the use of similar equipment.

Duration of use of the catalyst

What is the characteristic of dehydrogenation of butane? The formula of the catalyst used for this process is chromium oxide (3). It is deposited on amphoteric alumina. To increase the stability and selectivity of the process accelerator, it is simulated with potassium oxide. With proper use, the average duration of a full-time catalyst works is a year.

As it is used, a gradual deposition of a mixture of oxides of solid compounds is observed. They must be timely burned using special chemical processes.

The poisoning of the catalyst is by steam. It is on this catalyst mixture that dehydrogenation of butane occurs. The reaction equation is considered at the school in the course of organic chemistry.

In the case of an increase in temperature, the chemical process accelerates. But at the same time, the selectivity of the process decreases and sedimentation of the coke layer on the catalyst is observed. In addition, in high school, this task is often proposed: write the equation of the reaction of dehydrogenation of butane, burning ethane. These processes are not expected to be of particular complexity.

Write the equation of dehydrogenation reaction, and you will understand that this reaction proceeds in two mutually opposite directions. One liter of the volume of the reaction accelerator accounts for approximately 1000 liters of butane in gaseous form per hour, this is how dehydrogenation of butane occurs. The reaction of the compound of the unsaturated butene with hydrogen is the reverse process to the dehydrogenation of normal butane. The yield of butylene in the direct reaction averages 50 percent. Out of 100 kilograms of the initial alkane, after the dehydrogenation, about 90 kilograms of butylene are formed in the event that the process takes place at atmospheric pressure and a temperature of about 60 degrees.

Raw materials for production

Let us consider in more detail the dehydrogenation of butane. The equation of the process is based on the use of the feedstock (mixture of gases) formed during oil refining. At the initial stage, the butane fraction is thoroughly purified from pentenes and isobutenes, which interfere with the normal course of the dehydrogenation reaction.

How does dehydrogenation of butane occur? The equation of this process assumes several stages. After purification the dehydrogenated butenes are dehydrated to butadiene 1, 3. Butene-1, n-butane and butenes-2 are present in the concentrate containing four carbon atoms, which is obtained in the case of catalytic dehydrogenation of n-butane.

To carry out an ideal separation of a mixture is problematic enough. With the use of extraction and fractional distillation with a solvent, this separation can be effected, and the efficiency of this separation can be increased.

When fractional distillation is carried out on apparatuses having a large separating capacity, it becomes possible to completely separate butane-1 from normal butane as well as butene-2.

From the economic point of view, the process of dehydrogenation of butane to unsaturated hydrocarbons is considered an inexpensive production. This technology allows you to get motor gasoline, as well as a huge variety of chemical products.

This process is mainly carried out only in those areas where unsaturated alkene is needed, and butane has a low cost. Thanks to the cheaper and improved procedure of dehydrogenation of butane, the scope of use of diolefins and mono-olefins has expanded significantly.

The process of dehydrogenation of butane is carried out in one or two stages, the unreacted raw material is returned to the reactor. For the first time in the Soviet Union, butane was dehydrogenated in the catalyst bed.

Chemical properties of butane

In addition to the polymerization process, butane has a combustion reaction. Ethane, propane, other representatives of saturated hydrocarbons is contained in natural gas, so it is the raw material for all transformations, including combustion.

In butane, the carbon atoms are in the sp3-hybrid state, so all the bonds are single, simple. A similar structure (tetrahedral form) determines the chemical properties of butane.

It is incapable of joining the addition reactions, only the processes of isomerization, substitution, dehydrogenation are characteristic for it.

Replacement with diatomic molecules of halogens is carried out by a radical mechanism, and for this chemical interaction, rather stringent conditions (ultraviolet irradiation) are necessary. Practical value of all properties of butane is its combustion, accompanied by the allocation of a sufficient amount of heat. In addition, the process of dehydrogenation of the ultimate hydrocarbon is of special interest for production.

Specificity of dehydrogenation

The butane dehydrogenation procedure is carried out in a tubular reactor having external heating on a fixed catalyst. In this case, the yield of butylene increases, the automation of production is simplified.

Among the main advantages of this process is the minimum consumption of the catalyst. Among the shortcomings noted the significant consumption of alloyed steels, high capital investment. In addition, catalytic dehydration of butane involves the use of a significant number of aggregates, since they have low productivity.

The production has a low productivity, since some of the reactors are oriented to dehydrogenation, and the second part is based on regeneration. In addition, the minus of this technological chain is considered and the number of employees in production. It must be remembered that the reaction is endothermic, so the process proceeds at elevated temperature, in the presence of an inert substance.

But in such a situation there is a risk of accidents. This is possible if seals in the equipment are broken. The air that penetrates the reactor, when mixed with hydrocarbons, forms an explosive mixture. In order to prevent such a situation, the chemical equilibrium is shifted to the right by introducing water vapor into the reaction mixture.

The one-stage process variant

For example, in the course of organic chemistry the following task is proposed: make the equation of the reaction of dehydrogenation of butane. In order to cope with this task, it is enough to recall the main chemical properties of hydrocarbons of the class of limiting hydrocarbons. Let's analyze the features of butadiene production by a one-stage process of dehydrogenation of butane.

The butane dehydrogenation battery includes several individual reactors, their number depends on the cycle of operation, and also on the volume of the sections. In general, the battery includes five to eight reactors.

The process of dehydrogenation and reverse regeneration is 5-9 minutes, the purging stage takes 5 to 20 minutes.

Due to the fact that the dehydrogenation of butane is carried out in a continuously moving bed, the process is stable. This contributes to improved production performance, improves reactor productivity.

A one-stage dehydrogenation of n-butane is carried out at low pressure (up to 0.72 Mpa) at a temperature higher than that used for production carried out on an aluminum chromium catalyst.

Since the technology involves the use of a regenerative-type reactor, the use of water vapor is excluded. In addition to butadiene, butenes are formed in the mixture, they are reintroduced into the reaction mixture.

One stage is calculated through the ratio of the butanes in the contact gas to their number in the reactor charge.

Among the merits of this method of dehydrogenation of butane, we note a simplified technological scheme of production, lowering the consumable amount of raw materials, as well as reducing the cost of electrical energy for the process.

Negative parameters of this technology are represented by short periods of contact of reacting components. To correct this problem requires complex automation. Even with these problems in mind, the one-stage dehydrogenation of butane is more favorable than the two-stage production.

When the butane is dehydrogenated in one step, the feedstock is heated to a temperature of 620 degrees. The mixture is sent to the reactor, its direct contact with the catalyst is carried out.

To create vacuum in the reactors, vacuum compressors are used. The contact gas goes from the reactor to the cooling, then it goes to the separation. After completion of the dehydrogenation cycle, the feedstock is transferred to the following reactors, and from those where the chemical process has already passed, hydrocarbon vapors are removed by purging. The products are evacuated, and the reactors are again used for the dehydrogenation of butane.

Conclusion

The main reaction of dehydrogenation of butane of normal structure is the catalytic preparation of a mixture of hydrogen and butenes. In addition to the main process, it is possible that there are many side processes that significantly hamper the technological chain. The product that is obtained as a result of dehydrogenation is considered a valuable chemical raw material. It is the demand for production that is the main reason for the search for new technological chains for the conversion of hydrocarbons of the terminal series to alkenes.