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Obtaining nitrogen and oxygen by fractional distillation of air. Fractional distillation. Fractional distillation of liquid air

25.03.2024

Lesson #6

Subject: Oil distillation. Petroleum products and their use. Fractional distillation of liquid air.

Target: become familiar with petroleum products and their applications; consider the process of oil distillation and liquid air distillation; develop cognitive interest and intellectual abilities; cultivate an attitude towards chemistry as one of the fundamental sciences.

Equipment: collection “oil and petroleum products”; film "Petroleum Products"; presentation on the topic; multimedia projector and screen.

During the classes.

IClass organization.

IIMessage of the topic, lesson goals, motivation for learning activities.

The most important natural raw material for obtaining materials for the manufacture of various things important to us is oil. Today we will look at what oil is, what materials are obtained from it, and where they are used. Let us also consider how oil is divided into its constituent fractions and how air is divided into the individual gases that make up the air.

IIIUpdating basic knowledge.

(Front conversation)

    What methods of separating mixtures do you know?

    What is settling? What mixtures can be separated using this method?

    What is filtering? What mixtures can be separated using this method?

    What are evaporation and crystallization? What is the purpose of this method?

    What is distillation? What is the purpose of this method?

    What is flotation? What mixtures can be separated using this method?

    What is magnetization? What mixtures can be separated using this method?

IVLearning new material.

    What materials are obtained from petroleum products? What items are made from these materials? How are they used by humans? What are they made from? (Statement of the problem. Questions are written on the board). To answer these questions, let's look at the first slide. (Slide 1) What do you see? (Slide 2) What do you see on the second slide? So what is obtained from oil and where is it used? (We listen to the children’s answers, then show

    What is oil? What is she like? (We listen to the children's answers).

    So oil is a mixture. In order to obtain the necessary materials from oil, it is necessary to divide the oil into fractions. This is the primary processing of oil. Oil contains liquid substances with different boiling points. You and I know that such mixtures can be separated using distillation. Let's look at how oil is distilled. (Slide 5). (Teacher's explanation).

    What gases does air consist of? (Children know from biology and natural history courses that air contains oxygen and carbon dioxide. The teacher adds). (Slide 6). Is it possible to isolate the gases that make up it from the air? The release of nitrogen and oxygen is of great importance. The air is first cooled into a liquid state and then distilled. (Slide 7)

VGeneralization and systematization of knowledge.

So let's summarize what we learned today.

    What is oil? Why is it mined? (Students’ answer, show the first line of slide 8.)

    What is oil? (Students’ answer, show the second line of slide 8.)

    What method is used for primary oil refining?

    What fractions is the mixture divided into? (Students’ answer, show the third line of slide 8.)

    By what method are oxygen and nitrogen produced in industry? (Students’ answer, show the fourth line of slide 8.)

VILesson summary.

We have looked at how oil and liquid air are separated into fractions by distillation. You worked very actively in class. Well done! Your reward will be grades: .....

IIHomework message.

You need to learn the supporting notes for this lesson.

The choice of installation (device) for performing work is determined, first of all, by the task facing the experimenter, the conditions of the work, as well as the properties of the initial and final products.

Assembly of the installation must be carried out with great care and precision, as this is an indispensable condition for successful and safe operation.

The following rules for assembling devices and installations can be noted.

    The individual parts of the installation must be connected to each other carefully, selecting plugs, tubes and other parts before securing the device to the tripod.

    If the devices are assembled on ground sections, they should be pre-lubricated.

    The dishes are selected in such a size that the reaction mass occupies no more than 2/3 of the volume.

    If the reaction mixture will be heated, be sure to use a round-bottomed flask of the appropriate size.

    After the individual parts of the installation are assembled, they are secured in the legs of the tripod.

    The installation is always assembled starting from the main block or from its intended “top”.

    For example, when assembling an installation for simple distillation, you should first attach a Wurtz flask to a stand, then attach a downward condenser to it, then an allonge, and finally place a receiver under it.

    The entire installation must be assembled in one plane or along one line (with the exception of some cases), without distortions or stress on the glass parts of the device. This is especially important when working with standard sections, when they must be attached to each other without much effort on the part of the experimenter.

    It is necessary to ensure that when connecting individual parts of the device, tightness conditions are met.

    If the glass parts of the installation are quite heavy (for example, a flask with a reflux condenser, a stirrer, a dropping funnel, a thermometer, etc.), then they should be secured to the tripod with several legs. In this case, reflux condensers, stirrers, and reflux condensers are mounted strictly vertically, and downward condensers are mounted obliquely so that the liquid flows into the receiver without getting into the plugs.

    If the installation is intended to operate under atmospheric pressure, then it is necessary that it communicates freely with the atmosphere in order to avoid an increase in pressure in the system.

If it is necessary to protect reacting substances from air moisture, calcium chloride tubes are used.

12.When starting work, you should carefully inspect the device again and

make sure it is assembled correctly.

4. Isolation and purification methods

organic matter

Substances obtained during synthesis, as a rule, contain a certain amount of impurities (original substances that have not entered into the reaction, by-products, solvents, etc.). To get rid of them, various methods of purification and separation of organic substances are used. These methods are quite diverse and depend mainly on the state of aggregation of the compound.

4.1. Purification of liquid substances The main types of purification of liquid substances

are

Simple distillation

Fractional distillation,

Distillation in vacuum

Steam distillation,

Extraction.

4.1.1. Simple distillation In cases where the substance being distilled is sufficiently resistant to heat and practically does not decompose at boiling point, they are used for purification. .

Typically, this distillation method is advisable to use for liquids with a boiling point up to 180 o C, since above 180 o C many substances decompose noticeably. Often during distillation, the temperature of the boiling liquid due to overheating is slightly higher than the temperature of the steam. Overheating, which occurs in the absence of boiling centers in the distilled liquid, leads to strong shocks, as a result of which the substance, along with impurities and contaminants, can be thrown into the receiver. There are various ways to prevent or reduce boil shock. Most often, so-called “boilers” are added to the flask with the liquid being distilled, the role of which is played by various inert, porous materials (Fig. 57).

Figure 57. - Preparation of the mixture for distillation.

Round-bottomed flasks are usually used as a working vessel (Fig. 58). To distill low-boiling liquids, take a flask with a high-soldered outlet tube, for high-boiling liquids - with a low-soldered one. The boiling point is usually controlled by a thermometer, the mercury ball of which must be completely washed by the vapors of the boiling substance, i.e. The upper edge of the ball should be installed approximately 0.5 cm below the opening of the flask outlet tube.

The size of the distillation flask is selected depending on the amount of liquid being distilled and its boiling point. The liquid should occupy no more than 2/3 of the volume of the flask. The flask should not be too large, especially when distilling high-boiling liquids, since a large amount of distilled substance remains in it. The flask is secured in a stand, clamping it with a paw above the outlet tube. To avoid contamination of the substance, the distillate should come into contact with the stoppers as little as possible, so the outlet tube of the distillation flask is connected to the refrigerator so that its end protrudes from the stopper into the refrigerator by at least 4-5 cm and reaches the part of the refrigerator that is cooled by water. The size of the refrigerator (cooling area) is selected depending on the boiling point of the distilled liquid.

Vapors of substances that easily crystallize at room temperature should not be cooled in a refrigerator to the solidification temperature. To do this, the refrigerator can be periodically disconnected from running water. Liquids boiling within 200-300 o C are distilled without a refrigerator, the function of which in this case can be performed by the outlet tube of the distillation flask. The refrigerator is connected to the receiver via an allonge. Conical or flat-bottomed flasks that can be placed on a surface are usually used as a receiver. When using round-bottomed flasks as receivers, they must be additionally secured. For more complete condensation of vapors of low-boiling liquids, the receiver is placed in a vessel with a cooling mixture.

The installation diagram for simple distillation is shown in Fig. 58, 59. It consists of a distillation flask 1 (or Wurtz flask), a thermometer 3, a Liebig downward condenser 4, a longue 5, a receiver 6, a heating element 7. Parts of the device are mounted on stands 8 using couplings 10 and legs 9. The assembly procedure for the installation is shown in Fig. . 61. Before assembly, it is necessary to check the flask for cracks (Fig. 60)

When the entire device is assembled, it is carefully checked and only then they begin to heat it. Depending on the boiling point, heating is carried out using various types of heating baths (Fig. 59). The distillation speed is usually chosen so that no more than 1-2 drops of distillate flow per second.

Simple distillation is often used to purify absolute solvents, but in this case a calcium chloride tube must be attached to the allonge.

Figure 58. - Installation diagram for simple distillation without a bath.

Figure 59. Installation diagram for simple distillation in a bath.

Figure 60 Flask with a crack (star)


Figure 61. Installation procedure for simple distillation

Fractional distillation has a number of important applications, for example, the production of oxygen, nitrogen and noble gases from liquid air, oil refining, the production of alcoholic beverages (see introductory text to this chapter), etc.

In Fig. Figure 6.16 shows a schematic diagram of a typical laboratory fractional distillation setup. The vertical column is filled with glass beads or randomly oriented short lengths of glass tubes. A bubble column may be used instead. Such a column allows sublimating vapors to come into contact with the liquid flowing down.

Let's see what happens when fractional distillation of a two-component mixture of composition xA(C) (Fig. 6.17). When this mixture is heated, its temperature rises to point C. Then the liquid begins to boil. The resulting vapor is richer than the liquid in the more volatile component A. At the boiling point, this vapor and liquid are in equilibrium. This equilibrium corresponds to the connecting line CD in the phase diagram. The vapor rising through the fractionation column gradually cools and eventually condenses into a liquid. This decrease in temperature is represented on the phase diagram by the vertical line DD. At point D, a new equilibrium is established between the condensate, which has the composition xA(D), and its steam, which has the composition xA(E). Liquid condensate flows down the column, and steam rises through it. Thus, at each level of the column, the flowing liquid and rising vapor are in equilibrium. These equilibria are represented by connecting lines. As the vapor rises through the column, passing through each successive equilibrium, it becomes increasingly enriched in the more volatile component. Eventually the steam exits through an opening at the top of the column, condenses, and the resulting liquid flows into the receiver. Meanwhile, the liquid in the flask becomes increasingly enriched in the less volatile component, and as a result its boiling point gradually increases.

Due to the removal of steam through the hole at the top of the column, the equilibrium in it is continuously shifted. Good separation is only achieved if the flask is heated slowly enough to allow time for equilibria to settle. In practice, fractional distillation is usually used to separate multicomponent liquid mixtures.


In Uganda, the production of the alcoholic drink “inguli” is widespread, which is obtained by fractional distillation of beer in homemade distillation apparatuses. In Uganda, holders of licenses for the production of inguli sell their products to industrial distilleries, where an alcoholic drink called “waragi” is obtained from it. Homemade inguli and similar homemade alcoholic beverages produced in East African countries are dangerous to drink because the second fraction often contains toxic impurities from the first and third fractions. For this reason, in most East African countries there is a ban on the production and consumption of such alcoholic beverages.

Inguli. Fermenting the wort from molasses and banana juice produces African beer "Inguli", from which three fractions are collected by distillation.

The first fraction contains toxic low-boiling aldehydes, ketones and alcohols. For example, propanal (bp. 48 °C, toxic), propanone (bp. 56 °C, toxic) and methanol (bp. 64 °C, very toxic, causes loss of vision). This fraction is destroyed.

The second distillation fraction represents the target product inguli. O. contains water and ethanol. Ethanol (ethyl alcohol) has a boiling point of 78 0C. when consumed in small quantities it is not harmful (see, however, the introductory text at the beginning of this chapter).

The third fraction contains alcohols with boiling points ranging from 12 to 130°C. This faction is also destroyed.

O.S.GABRIELYAN,
I.G. OSTROUMOV,
A.K.AKHLEBININ

START IN CHEMISTRY

7th grade

Continuation. For the beginning, see No. 1, 2, 3, 4, 5, 6, 7, 8, 9/2006

Chapter 3.
Phenomena occurring with substances

(ending)

§ 17. Distillation or distillation

Obtaining distilled water

Tap water is clean, transparent, odorless... But is this substance pure from the point of view of a chemist? Look into the kettle: scale and brownish deposits that appear on the spiral and walls of the kettle as a result of repeated boiling of water in it are easily detected.
(Fig. 71). What about limescale on taps? Both natural and tap water are homogeneous mixtures, solutions of solid and gaseous substances. Of course, their content in water is very small, but these impurities can lead not only to scale formation, but also to more serious consequences. It is no coincidence that injection medications are prepared only using specially purified water, called.

distilled Where did this name come from? Water and other liquids are purified of impurities through a process called distillation, or distillation

.

The essence of distillation is that the mixture is heated to a boil, the resulting vapors of the pure substance are removed, cooled and again converted into a liquid that no longer contains contaminants.
A laboratory installation for the distillation of liquids is assembled on the teacher’s desk (Fig. 72).
What temperature does the thermometer show? What outlet do you think cold water is supplied to the refrigerator through, and through which it is drained?

Distilled water is used not only to prepare medicines, but also to obtain solutions used in chemical laboratories. Even motorists use distilled water, adding it to batteries to maintain electrolyte levels.

And if it is necessary to obtain a solid substance from a homogeneous solution, then use evaporation, or crystallization

Crystallization

One way to isolate and purify solids is crystallization. It is known that when heated, the solubility of a substance in water increases. This means that when the solution is cooled, a certain amount of the substance precipitates in the form of crystals. Let's check this experimentally.

Demonstration experiment. Remember the beautiful orange crystals of potassium dichromate that the teacher used to “color” the water for distillation? Let's take about 30 g of this salt and “contaminate” it with several crystals of potassium permanganate. How to clean the main substance from the introduced impurity? The mixture is dissolved in 50 ml of boiling water. When the solution is cooled, the solubility of the dichromate decreases sharply, and the substance is released in the form of crystals, which can be separated by filtration and then washed on a filter with several milliliters of ice water.

If you dissolve the purified substance in water, then by the color of the solution you can determine that it does not contain potassium permanganate. Potassium permanganate remained in the original solution.

Crystallization of a solid from a solution can be achieved by evaporating the solvent. This is what the evaporation cups you encountered while learning about chemical glassware are designed for.

If the evaporation of liquid from a solution occurs naturally, then for this purpose special thick-walled glass vessels are used, which are called crystallizers. You also became acquainted with them in practical work No. 1.

In nature, salt lakes are unique pools for crystallization. Due to the evaporation of water on the shores of such lakes, a gigantic amount of salt crystallizes, which, after purification, ends up on our table.

Distillation is used not only to purify substances from impurities, but also to separate mixtures into separate portions - fractions that differ in boiling point. For example, oil is a natural mixture of a very complex composition. During the fractional distillation of oil, liquid petroleum products are obtained: gasoline, kerosene, diesel fuel, fuel oil and others. This process is carried out in special devices - distillation columns (Fig. 73). If your city has an oil refinery, you might have seen these chemical machines that continuously separate oil into products that are important and necessary in the life of modern society (Fig. 74).

Gasoline is the main fuel for passenger cars. Tractors and trucks use another petroleum product as such - diesel fuel (diesel fuel).
The fuel for modern aircraft is mainly kerosene. With this small example, you can understand how important a process such as oil distillation is in modern life.
Rice. 74.

Oil and petroleum products

Fractional distillation of liquid air

You already know that any gases are mixed in any ratio. Is it possible to isolate individual components from a mixture of gases? The task is not easy. But chemists have proposed a very effective solution. The mixture of gases can be turned into a liquid solution and subjected to distillation. For example, air is liquefied by strong cooling and compression, and then individual components (fractions) are allowed to boil away one after another, since they have different boiling points. Nitrogen is the first to evaporate from liquid air (Fig. 75); it has the lowest boiling point (–196 °C).

Argon (–186 °C) can then be removed from the liquid mixture of oxygen and argon. What remains is almost pure oxygen, which is quite suitable for technical purposes: gas welding, chemical production. But for medical purposes it needs to be further purified.

1. Nitrogen obtained in this way is used to produce ammonia, which in turn is used to produce nitrogen fertilizers, medicinal and explosives, nitric acid, etc.

2. The noble gas argon is used in a special type of welding, which is called argon.

3. What is distillation or distillation? What is it based on?

4. What kind of water is called distilled?

5. How is evaporation (crystallization) different from distillation (distillation)? What are both methods of separating liquid mixtures based on?

6. What is the difference between the processes of evaporation and crystallization? What are both methods of isolating a solid from a solution based on?

7. Give examples from everyday life in which evaporation and distillation are used.

8. What mass of salt can be obtained by evaporating 250 g of a 5% solution? What volume of water can be obtained from this solution by distillation?

PRACTICAL WORK No. 4.
Growing salt crystals
(home experiment)

Before starting work, carefully read its description to the end.

First of all, select the appropriate salt for the experiment. Any salt that is highly soluble in water (copper or iron sulfate, alum, etc.) is suitable for growing crystals.

Table salt - sodium chloride - will also work.

Equipment you will need:

A liter jar or small saucepan, in which you will prepare the salt solution;

Wooden spoon or stirring stick;

Funnel with cotton wool for filtering the solution;

A thermos with a wide neck with a capacity of 1 liter (it is needed so that the solution cools slowly, then large crystals will grow).

If you don’t have a funnel or the right thermos, you can make them yourself.

To make a funnel, take a plastic drink bottle and carefully cut off the neck with scissors, as shown in Fig. 76.

Instead of a thermos, an ordinary glass liter jar will do. Place it in a cardboard or foam box. There is no need to take a large box, the main thing is that it completely fits the jar. Seal the gaps between the box and the jar tightly with pieces of rag or cotton wool. To seal the jar tightly, you will need a plastic lid.

Prepare a hot saturated salt solution.

Usually several crystals grow on the thread. It is necessary to periodically remove the excess ones so that one large crystal grows.

It is important to record the conditions of the experiment and its result; in our case, these are the characteristics of the resulting crystal. If several crystals are obtained, then a description of the largest is given.

Examine the resulting crystal and answer the questions.

How many days did you grow the crystal?

What is its shape?

What color is the crystal?

Is it transparent or not?

Crystal dimensions: height, width, thickness.

Crystal mass.

Sketch or photograph the resulting crystal.

PRACTICAL WORK No. 5.
Cleaning table salt

The purpose of this work is to purify table salt contaminated with river sand.

The contaminated table salt offered to you is a heterogeneous mixture of sodium chloride crystals and sand. To separate it, it is necessary to take advantage of the difference in the properties of the components of the mixture, for example, different solubility in water. As you know, table salt dissolves well in water, while sand is practically insoluble in it.

Place the contaminated salt provided by the teacher into a beaker and add 50–70 ml of distilled water. Stir the contents with a glass rod until the salt is completely dissolved in the water.

The salt solution can be separated from the sand by filtration. To do this, assemble the installation as shown in Fig. 77. Using a glass rod, carefully pour the contents of the glass onto the filter. The transparent filtrate will flow into a clean glass, while the insoluble components of the original mixture will remain on the filter.

The liquid in the glass is an aqueous solution of table salt. Pure salt can be isolated from it by evaporation. To do this, pour 5–7 ml of the filtrate into a porcelain cup, place the cup in the ring of the tripod and carefully heat it over the flame of an alcohol lamp, constantly stirring the contents with a glass rod.

Compare the salt crystals obtained after evaporation of the solution with the original contaminated salt. List the techniques and operations you used to clean contaminated salt.

6. HYDROCARBON GAS PROCESSING

6.1 HYDROCARBON GAS SEPARATION

Oil is a complex natural mixture of organic substances (hydrocarbons) and is the main source of modern types of liquid fuels - gasoline, kerosene, diesel and boiler fuel, as well as gas fractions. Hydrocarbon gases are produced during the primary distillation of oil, as well as in the processes of catalytic and thermal processing of oil fractions and residues. They mainly consist of C1-C4 hydrocarbons and some heavier components. Depending on the type of process for processing oil fractions, gases may contain mainly saturated hydrocarbons (distillation processes of oil and oil fractions, hydrogenation processes, reforming, isomerization, etc.) or unsaturated hydrocarbons (catalytic cracking, thermal destruction processes.

Saturated hydrocarbon gases are, as a rule, subjected to gas fractionation in HFC units, and unsaturated gases are separated in AGFU (absorption-gas fractionation units).

These plants purify raw materials from the hydrogen sulfide they contain, followed by deep distillation, the product of which is gasoline and narrow gas fractions.

The purification of raw materials from hydrogen sulfide is carried out with an aqueous solution of monoethanolamine (MEA), which interacts with hydrogen sulfide according to the following reaction:

(CH2 CH2 OH) NH2 + H2 S ® (CH2 CH2 OH NH3 ) HS

2(CH2 CH2 OH) NH2 + H2 S ® (CH2 CH2 OH NH3 )2 S

The desulfurization process occurs at temperatures up to 40° C; at higher temperatures, the quality of desulfurization deteriorates, because a reverse reaction process is possible. Regeneration of MEA saturated with hydrogen sulfide is carried out by heating it to a temperature of 105-120 ° C, at which a reverse reaction occurs.

Fractionation of liquefied gases.

The process of separating a multicomponent mixture into fractions based on the difference in boiling point of the components is called rectification. In HFC and AGFU installations, the rectification process is carried out in distillation columns - vertical apparatuses equipped with complex internal devices - plates and packings of various types.

During the rectification process in HFC units, liquefied hydrocarbon gases, subject to separation into fractions, are heated, and some of the components they contain pass into the gas phase. The heated gas-liquid mixture is fed into the middle (or lower) part of the distillation columns. The liquid phase flows down the plates, while low-boiling substances continue to evaporate from it under the influence of vapors rising from the bottom of the column

components, the vapor phase rises. On each plate, gases come into contact with the liquid phase flowing from the overlying plates. As a result, the heaviest components with a higher boiling point condense and, mixing with the liquid flowing from the plate, fall down. The remaining gaseous components rise to the overlying plate, where the described process is repeated.

The flow of liquid flowing down the plates to the bottom of the column is called reflux. It begins with part of the product coming out in the vapor phase from the top of the column, condensed in refrigerator-condensers and returned to the upper plate of the column as acute reflux. Flowing down the plates, the phlegm is enriched with the heaviest components condensing in it from the flow of gases rising upward. By condensing, the components of the gas flow give off heat to the reflux flow, due to which the lightest components, boiling at a lower temperature, evaporate from it. Thus, on the trays of the distillation column, the processes of heat exchange (heat transfer from a stream of hot gases to a flow of colder reflux) and mass transfer (transition of low-boiling components from a liquid stream to a gas stream, and heavy ones from a gas stream to a liquid stream) simultaneously occur. As a result of these processes, under steady-state conditions, a certain temperature and the corresponding equilibrium composition of the liquid and gaseous phases are established on each tray of the column.

Fractionation of liquefied gases in HFC plants consists of the following processes.

Deethanization of hydrocarbon feedstocks. Consists in the separation of coal

hydrogen raw materials of light hydrocarbons C1 -C2 (methane, ethane). Occurs in the deethanizer - column K-1 (Fig. 5.1). Light hydrocarbons are discharged into the plant's fuel network.

Obtaining propane fraction. The process takes place in a K-2 propane column. The raw material is the deethanized fraction obtained in the K-1 column. As a result of rectification, two fractions are isolated: a propane fraction from the top -K2 and a butane-pentane fraction from the bottom of the column. The propane fraction is removed from the installation as a component of domestic liquefied gas, the fraction of butanes and above is the feedstock of the K-3 column.

Debutanization When obtaining the butane fraction, the process occurs in K-3. The raw material is the butane-pentane fraction obtained during depropanization in the K-2 column. As a result of rectification, two fractions are isolated: butane-

the first fraction from the top of K-3 and fraction C from the bottom of the column. Butane fraction

can be partially discharged from the installation into the fuel network, and the second part into the liquefied gas park as a component of household liquefied gas.

Rice. 6.1 Schematic diagram of HFC

It should also be noted that a number of HFC schemes provide for the separation of the butane fraction into isobutane and n-butane.

Obtaining isopentane or sum of butanes. HFC can operate in one

nom of two options. In the first case, the rectification products are the isopentane fraction and the fraction C5 and higher, in the second the sum of butanes and the fraction sum C5 and higher.

In addition to the components listed above, the HFC installation may include a mercaptan removal unit – “Merox”.

When separating unsaturated hydrocarbon gases, AGFU units are used. Their distinctive feature is the use of the technology of absorption of hydrocarbons C3 and higher by a heavier hydrocarbon component (fractions C5 and higher) to isolate dry gas (C1 - C2) in column K-1 (Fig. 6.2). The use of this technology makes it possible to reduce temperatures in the columns and thereby reduce the likelihood of polymerization of unsaturated hydrocarbons.

In AGFU units, unsaturated hydrocarbon gases, after being compressed by a compressor, are heated and enter the K-1 absorber, into the upper part of which

Fraction C and higher is used as an absorbent. Heavy hydrocarbons

They absorb well components that are similar in structure and molar mass and

do not absorb light C-C gases well. As a result, they are removed from above the co-

columns, and hydrocarbons C3 and higher are carried away by the absorbent and from the bottom of the K-1 column are sent to the K-2 desorber. In it, by means of rectification, a section is carried out -

dividing a mixture of hydrocarbons into two fractions C-C and C and higher. The first of them is

After purification from mercaptans (Merox process), it enters the K-3 column for separation into the propane-propylene fraction (C3) and the butane-butylene fraction (C4).

The propane-propylene fraction is most often used to produce polypropylene, propylene di- and trimers, diisopropyl ether, isopropyl alcohol, and polymer gasoline.

The butane-butylene fraction can serve as a raw material for the production of methyl tert-butyl ether or alkylate. At some refineries, isobutylene is extracted from it, which is used in the production of polyisobutylene.

Fraction C5 and higher is included in the composition of commercial gasoline.

Rice. 6.2 Schematic diagram of AGFU

6.2 Alkylation of isobutane with olefins

In the production of motor gasoline, there is a constant tendency to increase their octane number, since the use of high-octane gasoline makes it possible to increase the power of carburetor engines without increasing dimensions while simultaneously reducing specific fuel consumption. The main types of motor gasoline must have an octane number of about 9395. Along with this, for environmental reasons, the production of leaded gasoline is sharply reduced or the content of tetraalkyl lead in them is significantly reduced, which is caused not only by the release of toxic compounds of carbon, sulfur and nitrogen into the atmosphere in the exhaust gases, but also the poisonous effect of tetraalkyl lead decomposition products on afterburning catalysts of engine exhaust gases. In this regard, it is especially advisable to increase the content of high-octane isoparaffin components in motor gasoline, which, having a high research octane number (RON), have low sensitivity.

The combustion products of isoparaffins contain small amounts of toxic substances. Octane numbers (according to the research method) of the main isoparaffins C5 -C8 formed in alkylation and isomerization reactions are presented in table. 6.1.

The processes for the production of high-octane isoparaffins are based on the isomerization reactions of n-paraffins and the alkylation of paraffinic hydrocarbons with C5 -C8 olefinic hydrocarbons. According to the mechanism, alkylation reactions belong to two main groups:

acid-catalytic alkylation reactions,

· thermal alkylation reactions.

Acid catalytic alkylation

The processes of alkylation of isoparaffins with olefinic hydrocarbons catalyzed by acid catalysts are based on reactions occurring through the carbocation mechanism. Carbocations, depending on the type of acid used, can be formed in several ways:

The lifespan of carbonium ions varies over a wide time interval depending on their solvation, structure and inductive effects.

The formation of carbonium ions follows certain rules. Thus, when a proton interacts with acyclic olefins of normal structure, a secondary carbocation is more likely to be obtained than a primary one:

i.e., the addition of a proton occurs in accordance with Markovnikov's rule. Protonation of acyclic iso-olefins with a double bond in the b-position produces a tertiary carbocation more easily than a secondary one:

This is confirmed by the values ​​of the heats of formation (∆Nobr) of some unsolvated carbocations:

IN In order of decreasing stability, carbocations are located in the series: tertiary > secondary > primary.

IN During the catalytic alkylation of paraffin hydrocarbons, carbocations undergo a number of reactions:

proton abstraction

hydride ion migration

methyl group migration

addition to olefin

cracking (b-cleavage)

elimination or transfer of hydride ion

In accordance with the presented reactions of carbonium ions, the interaction of isoparaffins with acyclic olefin hydrocarbons, for example isobutane with butenes, is carried out according to the following scheme:

C4 H8 + H+ → C4 H9 + (reverse reaction 1)

iso-C4 H10 + C4 H9 + → C4 H10 + iso-C4 H9 + (reaction 6)

iso-C4 H9 + + C4 H8 → iso-C8 H17 + (reaction 4)

iso-C8 H17 + + iso-C4 H10 → iso-C8 H18 + iso-C4 H9 + (reaction 6)

This last step generates the tert-butyl cation, which continues the alkylation chain reaction. In this case, depending on the structure of the butene taken, various octyl carbocations can be formed (by reaction 4):

For the acid-catalytic alkylation of isoparaffins with acyclic olefins, there are a number of common factors that determine the yield and quality of alkylates:

1) although alkylation n-butane and isobutane are thermodynamically equally probable; only isoparaffins with a tertiary carbon atom enter into acid-catalytic alkylation reactions;

2) only strong acids provide transition hydride ion, and the rate of such transitions decreases with decreasing acid concentration;

3) olefin hydrocarbons dissolve well and quickly in acids, which contributes to the occurrence of side reactions that deteriorate the quality of alkylates, therefore the initial content of olefins in the reaction medium should be minimal;

4) poor solubility of paraffins in acids requires a high degree of dispersion of the reaction mass in order to create the largest possible interface between the acid and hydrocarbon phases, at which transition reactions occur hydride ions, which limit the rate of formation of target alkylation products;

5) The selectivity of isoparaffin alkylation reactions is higher, the lower the temperature of the reaction mixture.

Reactions of alkylation of isoparaffins with olefins proceed with the release of a significant amount of heat, the need for removal of which should be taken into account when designing reactor devices. The experimentally determined values ​​of the thermal effects of isobutane alkylation reactions with various olefin feedstocks are presented in Table. 6.2.

Table 6.2. Experimental values ​​of thermal effects (∆H) of isobutane alkylation reactions with olefins

Molar ratio

Thermal efficiency

wearing iso-

reaction effect,

tan/olefin

Isobutylene

Diisobutylene

Triisobutylene

Butane-butylene frac-

fractional 56% wt.

*98% isobutane concentrate was used. **67% isobutane concentrate was used.

The equilibrium constants for the alkylation reactions of isobutane with ethylene, propylene, isobutylene and 2-methyl-2-butene in the temperature range 298-700 K are presented in table. 6.3.