Purpose and characteristics of the rectification process. Description of the rectification process. B) chemical treatment of raw alcohol

Alcohols have become part of our lives. And it's not just alcohol. It is obtained by fermentation followed by distillation. And often people mistakenly think that rectification is a second distillation. In fact, this is a multiple run of alcohol-containing liquids in special columns. As a result of the meeting of two streams - liquid and vapor - pure alcohol is obtained. Let's see what rectification is in more detail.

Alcohol and its properties

But first, let's figure out what alcohol is. The word was borrowed from the Latin language and means "spirit". If all normal conditions are met, it will be a colorless, transparent liquid with a sharp taste and characteristic aroma. Pure alcohol will be strong in the range of 95.6 to 100%.

Humanity has known alcoholic beverages for a long time, as with fermented juice of natural berries and fruits. Back then, these were drinks with a low alcohol content. But with the development of chemical knowledge, people received more and more strong drinks. But only at the very end of the 18th century they were able to get 100% rectified alcohol. The author of the invention was the Russian chemist T.E. Lovitz.

What is rectification

The word came to our language from the Latin language and means correction, straightening. This is one of the methods used in industry, laboratories or at home to separate mixed liquids.

The rectification process is based on the difference in the distribution of mixed components between the vapor and liquid fractions. In this process, the flow of steam moves towards the flow of liquid, they contact each other, exchanging heat and mass until equilibrium in the system occurs. All this takes place in a special device called a distillation column apparatus.

In the process of meeting the flows, the ascending steam flow absorbs all volatile components, and the flowing liquid - less volatile. As in the process of distillation - another process for obtaining alcohol - the energy consumption during rectification is the same, but the extraction of the desired component (in our case, alcohol) is much more efficient. That's what rectification is.

In order for liquid and vapor to interact more successfully, the devices use contact elements - plates or nozzles. They increase the efficiency and area of \u200b\u200binteraction of two counter streams. The principle of their operation is as follows: the vapor going up passes through the contact element and the liquid accumulated on it, exchanging mass and heat more intensively. The more elements are installed in the design, the faster the balance between the vapor and the liquid fraction will be achieved.

How rectification differs from the distillation process, we will consider in the following table.

The difference between rectification and distillation

The difference	Distillation	Alcohol rectification
Received strength of the drink	Depends on the number of distillations and the quality of the apparatus, it can vary from 40 to 65 vol.	It can reach 96 vol.
Beverage quality	The aroma and aftertaste of the raw materials used are preserved.	Rectified alcohol, without other impurities.
Obtaining pure fractions	The quality of separation is extremely poor, the substances are mixed, and there is no way to fix this.	If there are substances that boil at different temperatures, then they will be clean at the exit.
Removal of substances harmful to health	For the high-quality removal of fusel oils, at least two distillations are required.	If all technologies are followed, they are removed completely.
Alcohol losses	Even if you follow all the rules, only 80% of the total will be extracted.	Virtually no loss. Only 1 to 3% can be lost.
Explosion and fire hazard	The device is quite simple, but there is still a risk.	The equipment is quite complex, and if a mistake is made, an explosion is possible.

Rectification equipment

Two types of equipment can be used for this process: continuous and batch plant. The first type is used in industry, since automation is used to regulate the work, which is expensive and complex. For laboratories, a second, simpler and cheaper type of equipment is used. It contains elementary means of adjusting the selection - a thermometer and a pressure gauge measuring the pressure change on the string.

The structure of the distillation column

The classic scheme looks like this. A vertical column (also called tsarga) and a dephlegmator with a limit switch are installed on the evaporator cube. Such an installation does not require complex mechanisms, only a tap for selection, a sight glass, a thermometer, and sometimes a power regulator.

It should be remembered that the higher the column height, the more intensive the mass and heat exchange of the two streams will be. And the rectification of alcohol will be better.

The principle of the column

The cube is filled with a maximum of two-thirds of its volume with an alcohol-containing mixture, the tightness of the joints is checked, the selection valve is closed and a cooling element (most often water) is supplied. Heating can be switched on only now.

It is important to know: you should never close two nozzles (rectification extraction and water supply) at the same time, from this the column can simply explode under the influence of the resulting excess pressure!

With a heater, the liquid poured into the cube is brought to a boil, and the resulting vapor rises. Then, having entered the reflux condenser, it condenses and flows down the walls, again in contact with the rising new steam. It hits the heater again, becomes steam, and the process repeats.

After some time, the vapor and liquid come to equilibrium, and a fraction with a low boiling point (methanol) accumulates in the upper part. At the bottom - with high (fusel oils). Now they can be selected.

Equilibrium is determined by maintaining the temperature for 10 minutes. Until this moment, you do not need to touch the device.

Column selection unit

What is a selection node? Most often it is a small side that slows down, preventing it from draining, phlegm (liquid condensed from vapor). If you open the tap of the sampling unit, then the delayed phlegm flows into the refrigerator, turning into rectified alcohol.

The same liquid that did not stay on the side flows down further to repeat the cycle again. IN industrial plants you can set the ratio between rectified and reflux, which is returned (reflux ratio) using a tap. The purity and percentage of alcohol depends on this number. The higher it is, the purer the alcohol.

It happens that such an unpleasant phenomenon occurs as the flooding of the distillation column. The information that this happened is shown by a strong gurgling noise inside the structure itself. There may be several reasons why there was a flood, consider them.

When the column drowns

The maximum speed of steam movement in each design is different. When it is reached, phlegm slows down its movement in the cube, and then it can stop altogether. Its accumulation in the rectification part causes a stop of the heat and mass transfer process. The result is a pressure drop (often very sharp) and the appearance of extraneous noise.

Reasons for "choking":

most often this is heating above the permissible;
overflow of the cube or clogging it with particles of an alcohol-containing composition;
in the highlands, the main reason is low atmospheric pressure;
voltage surge, due to which the power of the heating element increases;
faults and errors in design.

Now you know what rectification is. The alcohol obtained as a result of this process has a hard taste (the so-called technical alcohol). It can be used for technical purposes, but for food Industry it will still need to be finalized - diluted, filtered and infused.

For better purification, the resulting raw material is subjected to a carbonation process (passing through activated carbon). As a result of this procedure, the alcohol will acquire "softness", and (a small amount of them always gets into the alcohol, even if you used the fractional selection process) will be bound by coal. In fact, this is a classic procedure for making famous Russian vodka.

After the dilution and carbonation procedures, the drink should be allowed to rest. Just leave it in a glass container for a few days. The vodka will go smoother, and if you don't pick it up, you won't have a hangover.

IN modern world methods of extracting individual elements from multicomponent mixtures are often used, which can subsequently be used for industrial and many other purposes. The rectification process is one of the most efficient. Many people use it to produce alcoholic beverages at home.

Rectification is a procedure for separating mixtures of a multicomponent type into individual elements. The separation is carried out due to the fact that a countercurrent exchange of mass and heat appears between the vapor and the liquid. The rectification process can be carried out continuously and from time to time.

In order to be able to carry out rectification, you must have specialized equipment. It can be represented by a rectification column. It is an equipment that is very often used in industry and at home in the manufacture of highly purified alcohol for the production of alcoholic beverages or medical supplies... Also, with the help of rectification columns, you can get gasoline and other types of fuel.

The distillation column is a vertical vessel with a constant or variable cross section. Inside this cylinder there are special auxiliary units and devices for exchanging mass and temperature in order to be able to separate multicomponent liquids into separate fractions for their subsequent use. In the column, all separated components have a temperature close to the boiling point.

The most common column for carrying out the rectification process is a cylindrical vessel that has built-in contact devices. They are represented by various plates and attachments. It also has auxiliary units, the activity of which is aimed at driving or distributing liquid and steam inside the vessel in the correct way, so that a high-quality product is obtained at the output.

Rectification is used in the modern world different types multicomponent liquids to separate from them the elements necessary for further use. The rectification of alcohol is quite common.

This process consists of several stages:

First of all, it is necessary to bring to a boil the liquid, which, along with other substances, contains alcohol.
After that, the vapors of the heated liquid rise up the column. They accumulate in the reflux condenser.
After that, the reflux condenser is cooled.
After the cooling process, condensation forms on the surface of the reflux condenser.
Drops of condensate flow down the column.
At the bottom of the column, all condensate droplets accumulate in a special packing.
The nozzle lifts them back up.
As a result, liquid accumulates in the upper part of the column, which is easily boiled. Part of it accumulates in the condenser, and the other part enters the receiving container.

In the modern world, oil rectification is used to obtain certain types of fuel. This process takes a lot of time and energy. As a result, it is possible to get gasoline or gas at the outlet. This process is also carried out in rectification columns, which are larger than household ones. Their principle of operation is similar. The stages of oil rectification are the same as for the rectification of sprite-containing liquids. Only at the outlet are separate fuels formed.

The rectification of benzene is widely used at coke-chemical production plants.

In this case, substances such as are formed:

pure benzene,
tluene,
xylene.

In the modern world, there are three types of rectification.

They are represented by the following types:

Simple rectification.

This procedure is typical for those cases where a plate type distillation column is used. As a result of heat and mass exchange of vapors, a purified liquid of the required type is obtained at the outlet.

Steam-circulating rectification.

This fork is based on the use of rectification columns, which operate on the principle of circulating steam. In this case, impurities are distributed differently between the liquid substrate and vapors.

This type of rectification is suitable for isolating desired item from raw materials that contain a minimum amount of non-aromatic hydrocarbons.

All these methods are aimed at isolating certain substances necessary for further use from liquids consisting of many components. Their choice depends on the type of distillation column and on the raw material, which must be divided into fractions and the desired substance isolated.

Table 1. Boiling point of the water-alcohol mixture.

Boiling point of the mixture (0 C)	Alcohol content in mixture (0)	Alcohol content in pairs (0)
100	0	0
98,75	1	13
97,50	2	28
95	5	42
93,75	7	50
92,50	10	55
91,25	12	61
90	15	66
88,75	18	68
87,50	20	71
85	30	78
83,75	40	82

Table 2. The output of alcohol and vodka from one kilogram of raw materials.

Type of raw material	Alcohol output, liter / kg	Vodka output 40 0 \u200b\u200bliter / kg
Potatoes	0,11-0,18	0,35
Rye	0,4-0,43	0,88
Wheat	0,433	0,92
Rice	0,587	1,247
Oats	0,358	0,895
Peas	0,408	0,767
Millet	0,413	0,876
Starch	0,716	1,521
Acorns	0,25	0,558
Beet	0,082-0,12	0,212
Grapes	0,09-0,14	0,247
Apples	0,064	0,138
Pears	0,052-0,10	0,165
Barley	0,344	0,724
Buckwheat	0,473	1,004
Cherry	0,045-0,06	0,121
Sugar	0,511	1,080
Chestnut	0,258	0,572

The purpose of the article is to analyze the theoretical and some practical aspects of the operation of a home distillation column aimed at producing ethyl alcohol, as well as dispel the most common myths on the Internet and clarify the points that equipment sellers are "silent" about.

Alcohol rectification - separation of a multicomponent alcohol-containing mixture into pure fractions (ethyl and methyl alcohols, water, fusel oils, aldehydes and others) having different boiling points, by repeated evaporation of the liquid and condensation of steam on contact devices (plates or nozzles) in special counter-current tower apparatus.

From a physical point of view, rectification is possible, since initially the concentration of individual components of the mixture in the vapor and liquid phases is different, but the system tends to equilibrium - the same pressure, temperature and concentration of all substances in each phase. Upon contact with a liquid, the vapor is enriched with highly volatile (low-boiling) components, while the liquid is enriched with non-volatile (high-boiling) components. Heat exchange takes place simultaneously with enrichment.

Schematic diagram

The moment of contact (interaction of flows) of vapor and liquid is called the process of heat and mass transfer.

Due to the different directions of movements (the vapor rises up and the liquid flows down), after the system reaches equilibrium in the upper part of the distillation column, it is possible to select separately almost pure components that were part of the mixture. First, substances with a lower boiling point (aldehydes, ethers and alcohols) come out, then - with a high one (fusel oils).

Equilibrium state. Appears at the very boundary of phase separation. This is achieved only when two conditions are met simultaneously:

Equal pressure of each individual component of the mixture.
The temperature and concentration of substances in both phases (vapor and liquid) are the same.

The more often the system comes to equilibrium, the more efficient heat and mass transfer and separation of the mixture into separate components.

Difference between distillation and rectification

As you can see in the graph, from a 10% alcohol solution (mash), you can get 40% moonshine, and the second distillation of this mixture will leave a 60-degree distillate, and the third - 70%. The following intervals are possible: 10-40; 40-60; 60-70; 70-75 and so on up to a maximum of 96%.

Theoretically, to obtain pure alcohol, it takes 9-10 consecutive distillations on a moonshine still. In practice, distilling alcohol-containing liquids with a concentration of more than 20-30% is explosive; moreover, it is economically unprofitable because of the high energy and time consumption.

From this point of view, the rectification of alcohol is a minimum of 9-10 simultaneous, stepwise distillations that take place on different contact elements of the column (packing or trays) along the entire height.

The difference	Distillation	Rectification
Organoleptic properties of the drink	Retains the aroma and taste of the raw materials.	It turns out pure alcohol, odorless and tasteless (the problem has a solution).
Fortress at the exit	Depends on the number of distillations and the design of the apparatus (usually 40-65%).	Up to 96%.
Degree of separation into fractions	Low, substances even with different boiling points are mixed, this cannot be fixed.	High, pure substances can be isolated (only with different boiling points).
Ability to remove harmful substances	Low or medium. To improve the quality, at least two distillations with separation into fractions are required at least for one of them.	High, with the right approach, all harmful substances are cut off.
Loss of alcohol	High. Even with the right approach, you can extract up to 80% of the total quantity, while maintaining an acceptable quality.	Low. In theory, it is possible to extract all the ethyl alcohol without losing quality. In practice, at least 1-3% losses.
The complexity of technology for implementation at home	Low and medium. Even the most primitive coil apparatus is suitable. Equipment improvements are possible. Distillation technology is simple and straightforward. A moonshine still does not usually take up much space in working order.	High. Special equipment is required, which cannot be manufactured without knowledge and experience. The process is more difficult to understand; preliminary at least theoretical training is required. The column takes up more space (especially in height).
Danger (in comparison with each other), both processes are fire and explosive.	Due to the simplicity of the moonshine still, distillation is somewhat safer (the subjective opinion of the author of the article).	because of complex equipment, when working with which there is a risk of making more mistakes, rectification is more dangerous.

Rectification column operation

Distillation column - a device designed to separate a multicomponent liquid mixture into separate fractions by boiling point. It is a cylinder of constant or variable section, inside which there are contact elements - plates or nozzles.

Also, almost every column has auxiliary units for supplying the initial mixture (crude alcohol), controlling the rectification process (thermometers, automation) and withdrawing the distillate - a module in which the vapor of a certain substance extracted from the system is condensed and then taken outside.

One of the most common home designs

Raw alcohol - the product of the distillation of the mash by the method of classical distillation, which can be "poured" into the rectification column. In fact, this is moonshine with a strength of 35-45 degrees.

Reflux - steam condensed in the reflux condenser, flowing down the walls of the column.

Reflux number - the ratio of the amount of reflux to the mass of the selected distillate. There are three streams in the alcohol distillation column: steam, reflux and distillate (end goal). At the beginning of the process, the distillate is not withdrawn so that sufficient reflux appears in the column for heat and mass transfer. Then part of the alcohol vapor is condensed and taken from the column, and the remaining alcohol vapor continues to create a reflux stream, ensuring normal operation.

For the operation of most installations, the reflux ratio must be at least 3, that is, 25% of the distillate is taken, the rest is needed in the column for irrigating the contact elements. General rule: the slower the alcohol is taken, the higher the quality.

Distillation column contact devices (trays and packings)

They are responsible for multiple and simultaneous separation of the mixture into liquid and vapor, followed by condensation of vapor into liquid - reaching equilibrium in the column. All other things being equal, the more contact devices are in the design, the more effective the rectification is in terms of alcohol purification, since the surface of phase interaction increases, which intensifies the entire heat and mass exchange.

Theoretical plate - one cycle of leaving the equilibrium state with its repeated achievement. A minimum of 25-30 theoretical plates are required to obtain quality alcohol.

Physical plate - a really working device. The vapor passes through the liquid layer in the tray in the form of multiple bubbles, creating a large contact surface. In the classical design, the physical plate provides about half the conditions for achieving one equilibrium state. Therefore, for the normal operation of the rectification column, twice as many physical trays are required than the theoretical (calculated) minimum - 50-60 pieces.

Nozzles. Often, plates are only used in industrial installations. In laboratory and home rectification columns, nozzles are used as contact elements - copper (or steel) wire or nets for dishwashing in a special way. In this case, the reflux flows down in a thin stream over the entire surface of the packing, providing the maximum contact area with the steam.

Washcloth nozzles are the most practical

There are a lot of designs. The disadvantage of homemade wire nozzles is possible damage to the material (blackening, rust), factory analogues are devoid of such problems.

Distillation column properties

Material and dimensions. The column cylinder, packing, cube and distillers must be made of food-grade, stainless steel, safe when heated (evenly expands) alloy. In homemade constructions, cans and pressure cookers are most often used as a cube.

The minimum pipe length for a domestic rectification column is 120-150 cm, and its diameter is 30-40 mm.

Heating system. During the rectification process, it is very important to control and quickly adjust the heating power. Therefore, the most successful solution is heating with heating elements mounted in the lower part of the cube. The supply of heat through a gas stove is not recommended, since it does not allow you to quickly change the temperature range (high inertness of the system).

Process control. During rectification, it is important to follow the instructions of the column manufacturer, which must indicate the operating features, heating power, reflux ratio and model performance.

The thermometer allows you to accurately control the process of sampling fractions

It is very difficult to control the rectification process without two simple devices - a thermometer (helps to determine the correct degree of heating) and an alcohol meter (measures the strength of the resulting alcohol).

Performance. It does not depend on the size of the column, because the higher the tsar (pipe), the more physical trays are inside, therefore, the better the cleaning. The performance is influenced by the heating power, which determines the speed of movement of the steam and reflux streams. But with an excess of the supplied power, the column chokes (stops working).

Average productivity values \u200b\u200bof domestic rectification columns - 1 liter per hour with a heating power of 1 kW.

Influence of pressure. The boiling point of liquids depends on pressure. For successful rectification of alcohol, the pressure at the top of the column should be close to atmospheric - 720-780 mm Hg. Otherwise, as the pressure decreases, the vapor density will decrease and the evaporation rate will increase, which can cause flooding of the column. If the pressure is too high, the evaporation rate decreases, making the operation of the device ineffective (there is no separation of the mixture into fractions). To maintain the correct pressure, each alcohol distillation column is equipped with an atmosphere pipe.

On the possibility of self-made assembly. In theory, a distillation column is not a very complex device. The designs are successfully implemented by craftsmen at home.

But in practice, without understanding the physical foundations of the rectification process, correct calculations of equipment parameters, selection of materials and high-quality assembly of units, the use of a homemade rectification column turns into a dangerous occupation. Even one mistake can lead to fire, explosion or burns.

In terms of safety, the tested (have supporting documentation) factory columns are more reliable, moreover, they are supplied with instructions (must be detailed). The risk of a critical situation comes down to only two factors - correct assembly and operation according to the instructions, but this is a problem for almost all household appliances, not just columns or moonshine stills.

The principle of operation of the rectification column

The cube is filled to a maximum of 2/3 of the volume. Before switching on the installation, it is imperative to check the tightness of the connections and assemblies, close the distillate selection unit and supply cooling water. Only then can you start heating the cube.

The optimal strength of the alcohol-containing mixture supplied to the column is 35-45%. That is, in any case, distillation of the mash is required before rectification. The resulting product (crude alcohol) is then processed on the column, getting almost pure alcohol.

This means that a home distillation column is not a complete replacement for the classic moonshine still (distiller) and can be considered only as an additional purification step that better replaces re-distillation (second distillation), but levels out the organoleptic properties of the drink.

For the sake of fairness, I note that most modern models of rectification columns assume operation in the mode of a moonshine still. To switch to distillation, you just need to close the connection to the atmosphere and open the distillate selection unit.

If both connections are shut off at the same time, the heated column may explode due to overpressure! Don't make such mistakes!

In industrial installations of continuous operation, mash is often distilled immediately, but this is possible due to the gigantic dimensions and design features. For example, the standard is a pipe 80 meters high and 6 meters in diameter, in which many times more contact elements are installed than on rectification columns for a house.

Size matters. The possibilities of distilleries in terms of cube cleaning are greater than with home rectification

After switching on, the liquid in the cube is brought to a boil by the heater. The formed steam rises up the column, then enters the reflux condenser, where it condenses (reflux appears) and returns along the pipe walls in liquid form to the lower part of the column, on the way back contacting with the rising steam on trays or packing. Under the action of the heater, the reflux becomes vapor again, and the vapor at the top is again condensed by the reflux condenser. The process becomes cyclical, both streams are in continuous contact with each other.

After stabilization (steam and reflux are sufficient for an equilibrium state), in the upper part of the column, pure (separated) fractions with the lowest boiling point (methyl alcohol, acetaldehyde, ethers, ethanol) accumulate, at the bottom - from the highest (fusel oils). As sampling proceeds, the bottom fractions gradually rise up the column.

In most cases, a column in which the temperature does not change for 10 minutes ( total time warming up - 20-60 minutes). Up to this point, the device works "for itself", creating streams of steam and reflux, which tend to balance. After stabilization, the selection of the head fraction containing harmful substances: ethers, aldehydes and methyl alcohol begins.

The distillation column does not eliminate the need to separate the yield into fractions. As in the case with a conventional moonshine still, you have to collect the "head", "body" and "tail". The only difference is the cleanliness of the output. During rectification, the fractions are not "smeared" - substances with close, but at least a tenth of a degree, different boiling points do not intersect, therefore, when the "body" is selected, almost pure alcohol is obtained. During conventional distillation, it is physically impossible to divide the yield into fractions consisting of only one substance, whatever design is used.

If the column is brought to the optimal mode of operation, then there are no difficulties in selecting the “body”, since the temperature is stable all the time.

The lower fractions (“tails”) during rectification are selected based on temperature or smell, but unlike distillation, these substances do not contain alcohol.

Returning organoleptic properties to alcohol. Often, "tails" are required to return the "soul" to the rectified alcohol - the aroma and taste of the raw material, for example, apples or grapes. After the completion of the process, a certain amount of the collected tail fractions is added to pure alcohol. The concentration is calculated empirically by experimenting with a small amount of product.

The advantage of rectification is the ability to extract almost all alcohol contained in the liquid without losing its quality. This means that the "heads" and "tails" obtained on the moonshine still can be processed on the rectification column and get ethyl alcohol safe for health.

Flooding of the distillation column

Each design has a limiting steam velocity, after which the flow of reflux in the cube first slows down and then stops altogether. The liquid accumulates in the distillation part of the column and "flooding" occurs - the termination of the heat and mass transfer process. Inside there is a sharp drop in pressure, extraneous noise or gurgling appears.

The reasons for the flooding of the distillation column:

exceeding the permissible heating power (occurs most often);
clogging of the bottom of the device and overflow of the cube;
very low atmospheric pressure (typical for high mountains);
the voltage in the network is above 220V - as a result, the power of the heating elements increases;
design errors and malfunctions.

The principle of the rectification process.

The device of rectification columns.

Causes of damage to rectification columns and measures for their prevention.

Fire hazard of rectification plants. Fire prevention measures.

Question 1. The principle of rectification

The processes of distillation of solutions, in other words, separation of solutions into their constituent components, are very common in modern technology.

For example, separation of natural petroleum hydrocarbons in order to obtain motor fuels and lubricating oils.

The separation of individual gases from their mixtures occurs by their preliminary liquefaction, followed by distillation of the liquid mixture.

The vast majority of flammable liquids and combustible liquids (alcohol, acetone, acetic acid, benzene and its homologues, gasoline, kerosene and many other products) are also obtained using distillation processes.

Distinguish between simple distillation and rectification.

Simple distillation is the process of partial evaporation of a solution and complete condensation of the resulting vapor.

Rectification is the process of separating homogeneous liquid mixtures into their constituent components as a result of multiple countercurrent interaction of vapor with liquid.

According to according to Konovalov's first law, steam is enriched with that component, the addition of which to the liquid increases the vapor pressure above it or lowers the boiling point.

Ideal mixtures obey Raoult's law, according to which the partial pressure of a component in a system is equal to the vapor pressure of a pure component multiplied by its mole fraction in solution:P parts \u003d P couple M

And according to Dalton's law, the partial pressure of a component in a gas mixture is equal to the total pressure multiplied by the mole fraction of this component in the mixture:P parts \u003d P total M

The rectification process can be schematically represented as follows.

Where m 1 , m 2 , m 3 - the amount of the mixture received for rectification, the resulting distillate and the remaining residue;

X 1 , X 2 , X 3 - the content (mole fraction) of the volatile component in them, respectively.

Question 2. The device of rectification plants

So, rectification is the process of separating liquid homogeneous mixtures into their constituent components as a result of multiple countercurrent interaction of vapor with liquid.

The rectification processes are carried out in rectification units.

Distillation columns are classified:

by design: disc-shaped; packed.

depending on working pressure: working under vacuum; operating at atmospheric pressure; working under pressure.

The plates distinguish : valve; inkjet; perforated; caps.

Let's take a look at how the rectification process is carried out.

In the middle of the column raw material to be rectified is supplied heated to a certain temperature.

The raw material can be fed to the column as a liquid, vapor, or a mixture of vapor and liquid.

When the raw material enters the column, a flash evaporation process occurs, as a result of which vapors and liquid of a certain composition are formed, which are in equilibrium.

To ensure rectification, it is necessary to the upper part of the column towards the vapors to organize the liquid flow (phlegm, irrigation). For this, heat is taken away from the top of the column in one way or another (heat from the partial condenser). Due to this, part of the vapors rising from the upper tray condenses, forming the necessary downward liquid flow.

Reflux number - the ratio of the amount of distillate (reflux) returned to the column to the amount of distillate removed in the form of a product.

The schematic diagram of the rectification column is shown in the figure.

Receipt of raw materials

Phlegm intake

Vapors outlet to reflux condenser

boiler

Distillation residue output

At the bottom of the column, an upward vapor flow must be provided. For this, heat is supplied to the bottom of the column in one way or another (heat of the boiler). In this case, part of the liquid flowing down from the lower tray evaporates, forming a vapor flow.

In this mode: the lowest temperature will be at the top of the column and the highest at the bottom of the column.

The product taken from the top of the column enriched with a low boiling point component is called rectified (or distillate). And at the bottom of the column, the withdrawn product enriched with a high-boiling component is called residue or bottoms (bottom) product.

The part of the column where raw materials are introduced is called power section or evaporation space. The part of the rectification column located above the feedstock input - upper, concentration or strengthening, and below the input of raw materials - bottom, distant or exhaustive .

The same rectification process takes place in both parts of the column.

This figure shows a diagram of simple (I) and complex (Ii) columns:

and - complete; b- strengthening; in - distillation; r - with the selection of additional products from the top and bottom of the main column; d - with withdrawal of additional product from the top of the stripping column.

Introduction

rectification thermal reflux insulation

Rectification has been known since the beginning of the 20th century as one of the most important technological processes mainly alcohol and oil industries. At present, rectification is increasingly used in various fields of chemical technology, where the isolation of components in a pure form is very important (in the production of organic synthesis, isotopes, polymers, semiconductors, and various other substances of high purity). Thus, the rectification process allows one of the main tasks of chemical technology to be solved - the isolation of target products of the required purity.

Rectification is a process of separation of liquid mixtures, which is reduced to simultaneously occurring and repeatedly repeated processes of partial evaporation and condensation of the mixture to be separated on the phase contact surface. Distillation columns are designed for carrying out mass transfer processes in chemical, petrochemical industry... Columns are manufactured with different types of trays depending on the diameter.

Along with general requirements (high intensity per unit volume of the apparatus, its cost, etc.) a number of requirements can be determined by the specifics of production: a large interval of stable operation with changing loads, the ability of the tray to work in an environment of contaminated liquids, protection against corrosion, etc. Often these qualities become prevailing, determining the suitability of the design for use in each specific process.

The calculation of the rectification column is reduced to determining the basic geometric dimensions of the diameter and height. Both parameters are largely determined by the steam and liquid loads, the type of tray, and the properties of the interacting phases.

Theoretical foundations of the rectification process

Rectification is a mass-exchange process of separation of a homogeneous mixture of volatile components, carried out by countercurrent multiple interaction of vapors formed during distillation with a liquid formed during the condensation of these vapors. The separation of the liquid mixture is based on the different volatility of the substances. During rectification, the initial mixture is divided into two parts: the distillate is a mixture enriched with a low-boiling component (HC), and a bottom residue is a mixture enriched with a high-boiling component (HC). The rectification process is carried out in a rectification unit. The main apparatus is a distillation column, in which vapors of the liquid to be distilled rise from the bottom, and liquid flows towards the vapors, supplied in the form of reflux to the upper part of the apparatus. The rectification process can take place at atmospheric pressure, as well as at pressures above and below atmospheric. Under vacuum, rectification is carried out when high-boiling liquid mixtures are subject to separation. Increased pressure is used to separate mixtures in a gaseous state at a lower pressure. Atmospheric pressure is used when separating a mixture of liquids into its constituent components and the purity of the distillate and still bottoms obtained depends on how developed the contact surface of the phases is, on the amount of reflux supplied to the reflux and the device of the distillation column.

The rectification process is used to separate liquids that differ in boiling points, due to countercurrent multiple contacting of vapors and liquids. To create close contact between vapor and liquid, the rectification columns are equipped with special devices - packing or rectification trays.

Packed columns are used in small-scale production and are used in cases where a low pressure drop is required. To fill packed columns, Raschig rings made of various materials, Paul rings, hired from elements of a saddle-shaped profile (Intallox and Berl saddles).

Tray columns are widely used in refineries. Trays are distinguished according to the method of transferring liquid from a tray to a tray (failure and with special overflow devices), according to the nature of the movement of phases on the tray (bubbling and jet), according to the design of devices for introducing steam into a liquid (contact, cap, valve, etc.). Table 5.7 provides information about the main types of trays used in the chemical and oil refining industries.

Technological calculation of the distillation column consists of the following operations: 1) preparation of the material balance; 2) determination of the pressure in the column; 3) calculation of the temperature regime (temperature of feedstock inlet, top and bottom of the column, selection of side straps in complex columns); 4) determination of the reflux ratio (frequency of irrigation), i.e. the ratio of the amount of reflux supplied to the top of the column to the amount of distillate; 5) drawing up a heat balance; 6) definition of internal material flows; 7) calculation of the number of theoretical plates; 8) determination of the number of real (valid) plates.

For the continuous flow of the rectification process, it is necessary that the mixture entering the separation be in contact with the counter flow of steam with a higher concentration of the non-volatile (high-boiling) component than in the mixture. Therefore, in installations for continuous rectification (Fig. 1), the columns are made of two parts: lower (exhaustive) and upper (strengthening). In the exhaustive part of the column, a highly volatile component is removed from the liquid flowing down, and in the upper part, the rising vapors are enriched with a highly volatile component.

The schematic diagram of the rectification plant is shown in Figure 1:

Figure 1. - Schematic diagram of the rectification plant:

1 - container for the initial mixture; 2.9 - pumps; 3 - heat exchanger-heater; 4 - a boiler; 5 - rectification column; 6 - reflux condenser; 7 - distillate refrigerator; 8 - container for collecting distillate; 10 - cooler of vat liquid; 11 - container for vat liquid.

Initial mixture from the intermediate tank 1 centrifugal pump 2 fed to the heat exchanger 3 where it is heated to boiling point. The heated mixture enters the separation into the distillation column 5, where the composition of the liquid is equal to the composition of the initial mixture.

Flowing down the column, the liquid interacts with the rising vapor formed during the boiling of the bottom liquid in the boiler 4. The initial composition of steam is approximately equal to the composition of the distillation residue X W , those. it has a highly volatile component. As a result of mass transfer with liquid, the vapor is enriched with a highly volatile component. For a more complete enrichment, the upper part of the column is refluxed in accordance with a given reflux ratio with a liquid (reflux) , obtained in a reflux condenser 6 by condensation of steam leaving the column. Part of the condensate is removed from the reflux condenser in the form of a finished product of separation - distillate, which is cooled in heat exchanger 7 and sent to an intermediate tank 8. From the bottom of the column by a pump 9 the bottom liquid is continuously removed - a product enriched with a non-volatile component, which is cooled in a heat exchanger 10 and goes to the container 11.

Thus, a continuous non-equilibrium process of separation of the initial mixture into a distillate (with a high content of a volatile component) and a bottom residue (enriched in a non-volatile component) is carried out in the rectification column.

Rectification has been known since the beginning of the 19th century as one of the most important technological processes, mainly in the alcohol and oil industries. At present, rectification is increasingly used in various fields of chemical technology, where the isolation of components in pure form is very important (in the production of organic synthesis, isotopes, polymers, semiconductors and various other substances of high purity).

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