Dehydrogenation of butane is carried out in a fluidized or moving bed of chromium and aluminum catalyst. The process is carried out at a temperature in the range from 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 apparatus of the endothermic thermal effect is carried out by supplying 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 potassium chromate.
The resulting catalyst contributes to the rapid and high-quality course of the process. This chemical process accelerator is affordable in the price range.
Production scheme
Butane dehydrogenation is a reaction in which no significant consumption of catalyst is expected. The dehydrogenation products of the starting material fall into the extractive distillation unit, where the necessary olefin fraction is isolated. Dehydrogenation of butane to butadiene in a tubular reactor having an external heating option allows a good yield of the product.
The specificity of the reaction is in its relative safety, as well as in the minimal use of complex automatic systems and devices. Among the advantages of this technology, we can mention the simplicity of the structures, as well as the low consumption of an inexpensive catalyst.
Process features
Butane dehydrogenation is a reversible process, with an increase in mixture volume. According to the Le Chatelier principle, in order to shift the chemical equilibrium in a given process towards obtaining the reaction products, it is necessary to lower the pressure in the reaction mixture.
The atmospheric pressure is considered optimal at temperatures 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, which are formed during the occurrence of side reactions of deep destruction of the initial hydrocarbon, its activity decreases. To return to its original activity, the catalyst is regenerated by blowing it with air, which is mixed with flue gases.
Flow conditions
When butane is dehydrogenated, unsaturated butene is formed in cylindrical reactors. The reactor has special gas distribution grilles, cyclones are installed that allow you to trap catalyst dust carried away by the gas stream.
Dehydrogenation of butane to butenes is the basis for the modernization of industrial processes for the production of unsaturated hydrocarbons. In addition to this interaction, a similar technology is used to obtain other options for paraffins. Dehydrogenation of n-butane became the basis for the production of isobutane, n-butylene, ethylbenzene.
There are some differences between technological processes, for example, when dehydrogenating all hydrocarbons of a number of paraffins, similar catalysts are used. The analogy between the production of ethylbenzene and olefins is not only in the use of a single process accelerator, but also in the use of similar equipment.
Catalyst Duration
What is characterized by butane dehydrogenation? The formula of the catalyst used for this process is chromium oxide (3). It is precipitated on amphoteric alumina. To increase the stability and selectivity of the process accelerator, it is imitated with potassium oxide. When used correctly, the average duration of the full catalyst operation is one year.
As it is being used, gradual deposition of solid compounds oxides on the mixture is observed. They must be burned in a timely manner using special chemical processes.
Catalyst poisoning occurs with water vapor. It is on this mixture of catalysts that butane is dehydrogenated. The reaction equation is considered at school in a course in organic chemistry.
In the case of an increase in temperature, an acceleration of the chemical process is observed. But at the same time, the selectivity of the process also decreases, and a coke layer settles on the catalyst. In addition, the following task is often offered in high school: write the equation for the reaction of butane dehydrogenation and ethane combustion. These processes do not imply any particular difficulties.
Write the equation of the dehydrogenation reaction, and you will understand that this reaction proceeds in two opposite directions. Approximately 1000 liters of butane in gaseous form per hour are produced per liter of reaction accelerator volume, butane dehydrogenation occurs. The reaction of the unsaturated butene compound with hydrogen is the inverse process of the dehydrogenation of normal butane. The yield of butylene in the direct reaction is an average of 50 percent. About 100 kilograms of butylene are formed from 100 kilograms of the initial alkane after dehydrogenation if the process is carried out 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 process equation is based on the use of feedstock (gas mixture) generated 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 butane dehydrogenate? The equation of this process involves several steps. After purification, dehydrogenation of the purified butenes to butadiene 1, 3 takes place. In the concentrate containing four carbon atoms obtained in the case of catalytic dehydrogenation of n-butane, butene-1, n-butane, as well as butenes-2 are present.
Perfect separation of the mixture is quite problematic. When using extraction and fractional distillation with a solvent, it is possible to carry out a similar separation, to increase the efficiency of this separation.
When carrying out fractional distillation on apparatuses with a large separation ability, it becomes possible to fully separate normal butane from butene-1, as well as butene-2.
From an 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.
Basically, this process is carried out only in areas where unsaturated alkene is needed, and butane has a low cost. Due to the reduction in cost and improvement of the butane dehydrogenation procedure, the scope of use of diolefins and monolefins has significantly expanded.
The butane dehydrogenation procedure is carried out in one or two stages, the return of unreacted raw materials to the reactor is observed. For the first time in the Soviet Union, butane was dehydrogenated in a catalyst bed.
Chemical properties of butane
In addition to the polymerization process, butane has a combustion reaction. Ethane, propane, and other representatives of saturated hydrocarbons are sufficiently contained in natural gas, therefore it is it that is the raw material for all transformations, including combustion.
In butane, carbon atoms are in the sp3 hybrid state, so all bonds are single, simple. A similar structure (tetrahedral form) determines the chemical properties of butane.
It is not able to enter into the reaction of accession, it is characterized only by the processes of isomerization, substitution, dehydrogenation.
Substitution with diatomic halogen molecules is carried out by a radical mechanism, and for this chemical interaction to take place, rather stringent conditions (ultraviolet irradiation) are necessary. Of the practical properties of butane is its burning, accompanied by the release of a sufficient amount of heat. In addition, the process of dehydrogenation of a saturated hydrocarbon is of particular interest for production.
Specificity of dehydrogenation
The butane dehydrogenation procedure is carried out in a tubular reactor with external heating on a fixed catalyst. In this case, the butylene yield is increased, and the automation of production is simplified.
Among the main advantages of this process, the minimum consumption of catalyst can be distinguished. Among the shortcomings note the significant consumption of alloy steels, high investment. In addition, the catalytic dehydration of butane involves the use of a significant number of aggregates, since they have a low productivity.
Production has low productivity, as part of the reactors is focused on dehydrogenation, and the second part is based on regeneration. In addition, the disadvantage of this technological chain is also the number of employees in the workplace. 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 the seals in the equipment are broken. The air that enters the reactor, when mixed with hydrocarbons, forms an explosive mixture. In order to prevent this situation, the chemical equilibrium is shifted to the right by introducing water vapor into the reaction mixture.
One-Step Process Option
For example, in a course in organic chemistry, the following task is proposed: write the equation for the butane dehydrogenation reaction. In order to cope with this task, it is enough to recall the basic chemical properties of hydrocarbons of the class of saturated hydrocarbons. Let us analyze the features of producing butadiene by a one-stage process of butane dehydrogenation.
The butane dehydrogenation battery includes several separate reactors; their number depends on the operation cycle, as well as on the volume of the sections. Basically, five to eight reactors are included in the battery.
The process of dehydrogenation and reverse regeneration is 5–9 minutes; it takes from 5 to 20 minutes to the stage of purging with steam.
Due to the fact that the dehydrogenation of butane is carried out in a continuously moving layer, the process is stable. This helps to improve operational performance of the production, increases the productivity of the reactor.
The process of single-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 conducted 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 form in the mixture and 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 advantages of this method of dehydrogenation of butane, we note a simplified technological scheme of production, lower consumption of raw materials, as well as a decrease in the cost of electric energy for carrying out the process.
The negative parameters of this technology are represented by short contact periods of the reacting components. Correcting this problem requires sophisticated automation. Even taking into account such problems, one-stage dehydrogenation of butane is a more favorable process than two-stage production.
When butane is dehydrogenated in one stage, the feedstock is heated to a temperature of 620 degrees. The mixture is sent to the reactor, it is in direct contact with the catalyst.
To create vacuum in the reactor, vacuum compressors are used. The contact gas comes from the reactor for cooling, then it is sent to the separation. After the dehydrogenation cycle is completed, the feed is transferred to the following reactors, and from those where the chemical process has already passed, hydrocarbon vapors are removed by blowing. Products are evacuated, and reactors are again used to dehydrogenate butane.
Conclusion
The main dehydrogenation reaction of normal butane is the catalytic preparation of a mixture of hydrogen and butenes. In addition to the main process, there may be many side effects that significantly complicate the process chain. The product resulting from 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 limiting series to alkenes.