In engineering and chemical technology, absorption (absorption, dissolution) of gases by liquids is most often encountered. But the processes of absorption of gases and liquids by crystalline and amorphous bodies are also known (for example, the absorption of hydrogen by metals, the absorption of low molecular weight liquids and gases by zeolites, the absorption of petroleum products by rubber products, etc.).
Often during the absorption process there is not only an increase in the mass of the absorbent material, but also a significant increase in its volume (swelling), as well as a change in its physical characteristics - up to the state of aggregation.
In practice, absorption is most often used to separate mixtures consisting of substances that have different abilities to be absorbed by suitable absorbents. In this case, the target products can be both absorbed and non-absorbed components of the mixtures.
Typically, in the case of physical absorption, the absorbed substances can be re-extracted from the absorbent by heating it, diluting it with a non-absorbent liquid, or other suitable means. Regeneration of chemically absorbed substances is also sometimes possible. It may be based on chemical or thermal decomposition of the products of chemical absorption, releasing all or some of the absorbed substances. But in many cases, the regeneration of chemically absorbed substances and chemical absorbents is impossible or technologically/economically infeasible.
Absorption phenomena are widespread not only in industry, but also in nature (for example, swelling of seeds), as well as in everyday life. At the same time, they can bring both benefit and harm (for example, physical absorption of atmospheric moisture leads to swelling and subsequent delamination of wooden products, chemical absorption of oxygen by rubber leads to loss of elasticity and cracking).
It is necessary to distinguish absorption (absorption in volume) from adsorption (absorption in the surface layer). Due to the similarity of spelling and pronunciation, as well as the similarity of the concepts designated, these terms are often confused.
Types of absorption
A distinction is made between physical absorption and chemisorption.
During physical absorption, the absorption process is not accompanied by a chemical reaction.
During chemisorption, the absorbed component enters into a chemical reaction with the absorbent substance.
Absorption of gases
Any dense body condenses quite significantly the particles of the gaseous substance surrounding it directly adjacent to its surface. If such a body is porous, such as charcoal or spongy platinum, then this condensation of gases takes place over the entire inner surface of its pores, and thus, consequently, to a much higher degree. Here is a clear example of this: if we take a piece of freshly calcined charcoal, throw it into a bottle containing carbon dioxide or other gas, and immediately closing it with our finger, lower it with the hole down into a mercury bath, we will soon see what rises and enters the bottle; this directly proves that the coal has absorbed carbon dioxide or else compaction and gas absorption have occurred.
Any compaction generates heat; therefore, if coal is ground into powder, which, for example, is practiced in the manufacture of gunpowder, and left to lie in a heap, then due to the absorption of air that occurs here, the mass heats up so much that self-ignition can occur. The device of the Döbereiner platinum burner is based on this absorption-dependent heating. A piece of spongy platinum located there compresses the oxygen of the air and the stream of hydrogen directed at it so strongly that it gradually begins to glow and finally ignites the hydrogen. Substances that absorb - absorb water vapor from the air, condense it in themselves, forming water, and from this they become moist, such as impure table salt, potash, calcium chloride, etc. Such bodies are called hygroscopic.
The absorption of gases by porous bodies was first noticed and studied almost simultaneously by Fontan and Scheele in 1777, and was then studied by many physicists, especially Saussure in 1813. The latter, as the most greedy absorbers, points to beech charcoal and pumice (meerschaum). One volume of such coal at an atmospheric pressure of 724 mil. absorbed 90 volumes of ammonia, 85 - hydrogen chloride, 25 - carbon dioxide, 9.42 - oxygen; Pumice, with the same comparison, had slightly less absorption capacity, but in any case it is also one of the best absorbents.
The more easily a gas condenses into a liquid, the more it is absorbed. At low external pressure and when heated, the amount of absorbed gas decreases. The smaller the pores of the absorber, that is, the denser it is, the greater, in general, its absorption capacity; However, too small pores, such as graphite, are not conducive to absorption. Organic coal absorbs not only gases, but also small solid and liquid bodies, and therefore is used to decolorize sugar, purify alcohol, etc. Due to absorption, every dense body is surrounded by a layer of compacted vapors and gases. This reason, according to Weidel, can serve to explain the curious phenomenon of the so-called sweat patterns discovered by Moser in 1842, that is, those obtained by breathing on glass. Namely, if you apply a cliche or some kind of relief design to a polished glass plane, then, taking it away, breathe on this place, then you get a fairly accurate picture of the design on the glass. This is due to the fact that when the cliche lies on the glass, the gases near the surface of the glass are distributed unevenly, depending on the relief pattern applied to the cliche, and therefore water vapor, when breathing on this place, is also distributed in this order, and having cooled and settled, and reproduce this drawing. But if you pre-heat glass or a cliche, and thus disperse the layer of gases compacted near them, then such sweat patterns cannot be obtained.
According to Dalton's law, from a mixture of gases, each gas dissolves in a liquid in proportion to its partial pressure, regardless of the presence of other gases. The degree of dissolution of gases in a liquid is determined by a coefficient showing how many volumes of gas are absorbed in one volume of liquid at a gas temperature of 0° and a pressure of 760 mm. Absorption coefficients for gases and water are calculated using the formula α = A + IN t+ C t², where α is the required coefficient, t is the gas temperature, A , IN And WITH - constant coefficients determined for each individual gas. According to Bunsen's research, the coefficients of the most important gases are as follows:
In addition to solids, liquids can also be absorbed, especially if they are mixed together in a container. 1 volume of water can at 15 °C and 744 mil. pressure to dissolve in itself, absorb 1/50 of the volume of atmospheric air, 1 volume of carbon dioxide, 43 volumes of sulfur dioxide and 727 volumes of ammonia. The volume of gas that at 0 °C and 760 mil. barometric pressure absorbed by a unit volume of liquid is called the gas absorption coefficient for this liquid. This coefficient is different for different gases and different liquids. The higher the external pressure and the lower the temperature, the more gas dissolves in the liquid, the greater the absorption coefficient. Solids and liquids absorb different amounts of gases at a given time, and therefore it is possible to calculate the amount of gas absorbed for each individual liquid. The study of the absorption of gases by liquids was begun by Henri () and then moved further by Saussure () and W. Bunsen (“Gasometrische Methoden”, Braunschweig, 2nd ed.). - The reason for absorption is the mutual attraction of the molecules of the absorbing and absorbed bodies.
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Excerpt describing Absorption
Pierre did not have that practical tenacity that would give him the opportunity to directly get down to business, and therefore he did not like him and only tried to pretend to the manager that he was busy with business. The manager tried to pretend to the count that he considered these activities very useful for the owner and shy for himself.There were acquaintances in the big city; strangers hastened to get acquainted and cordially welcomed the newly arrived rich man, the largest owner of the province. The temptations regarding Pierre's main weakness, the one that he admitted during his reception to the lodge, were also so strong that Pierre could not refrain from them. Again, whole days, weeks, months of Pierre’s life passed just as anxiously and busyly between evenings, dinners, breakfasts, balls, not giving him time to come to his senses, as in St. Petersburg. Instead of the new life that Pierre hoped to lead, he lived the same old life, only in a different environment.
Of the three purposes of Freemasonry, Pierre was aware that he did not fulfill the one that prescribed every Freemason to be a model of moral life, and of the seven virtues, he completely lacked two in himself: good morals and love of death. He consoled himself with the fact that he was fulfilling another purpose - the correction of the human race and had other virtues, love for one's neighbor and especially generosity.
In the spring of 1807, Pierre decided to go back to St. Petersburg. On the way back, he intended to go around all his estates and personally verify what was done from what was prescribed to them and in what situation the people were now, which God had entrusted to him, and which he sought to benefit.
The chief manager, who considered all the ideas of the young count almost madness, a disadvantage for himself, for him, for the peasants, made concessions. Continuing to make the task of liberation seem impossible, he ordered the construction of large school buildings, hospitals and shelters on all estates; For the master's arrival, he prepared meetings everywhere, not pompously solemn ones, which, he knew, Pierre would not like, but just the kind of religious thanksgiving, with images and bread and salt, just the kind that, as he understood the master, should have an effect on the count and deceive him .
The southern spring, the calm, quick journey in the Viennese carriage and the solitude of the road had a joyful effect on Pierre. There were estates that he had not yet visited - one more picturesque than the other; The people everywhere seemed prosperous and touchingly grateful for the benefits done to them. Everywhere there were meetings that, although they embarrassed Pierre, deep down in his soul evoked a joyful feeling. In one place, the peasants offered him bread and salt and an image of Peter and Paul, and asked permission in honor of his angel Peter and Paul, as a sign of love and gratitude for the good deeds he had done, to erect a new chapel in the church at their own expense. Elsewhere, women with infants met him, thanking him for saving him from hard work. At the third estate he was met by a priest with a cross, surrounded by children, whom, by the grace of the count, he taught literacy and religion. In all the estates, Pierre saw with his own eyes, according to the same plan, the stone buildings of hospitals, schools, and almshouses that were to be opened soon. Everywhere Pierre saw reports from managers about corvée work, reduced compared to the previous one, and heard touching thanks for this from deputations of peasants in blue caftans.
Pierre just didn’t know that where they brought him bread and salt and built the chapel of Peter and Paul, there was a trading village and a fair on Peter’s Day, that the chapel had already been built a long time ago by the rich peasants of the village, those who came to him, and that nine-tenths The peasants of this village were in the greatest ruin. He did not know that due to the fact that, on his orders, they stopped sending children of women with infants to corvee labor, these same children carried out the most difficult work in their half. He did not know that the priest who met him with the cross was burdening the peasants with his extortions, and that the disciples gathered to him with tears were given to him, and were bought off by their parents for a lot of money. He did not know that the stone buildings, according to the plan, were erected by their own workers and increased the corvee of the peasants, reduced only on paper. He did not know that where the manager indicated to him in the book that the quitrent was reduced by one third at his will, the corvée duty was added by half. And therefore Pierre was delighted with his journey through the estates, and completely returned to the philanthropic mood in which he left St. Petersburg, and wrote enthusiastic letters to his mentor brother, as he called the great master.
“How easy, how little effort is needed to do so much good, thought Pierre, and how little we care about it!”
He was happy with the gratitude shown to him, but was ashamed to accept it. This gratitude reminded him how much more he could have done for these simple, kind people.
The chief manager, a very stupid and cunning man, completely understanding the smart and naive count, and playing with him like a toy, seeing the effect produced on Pierre by the prepared techniques, more decisively turned to him with arguments about the impossibility and, most importantly, the unnecessaryness of the liberation of the peasants, who, even without They were completely happy.
Pierre secretly agreed with the manager that it was difficult to imagine happier people, and that God knows what awaited them in the wild; but Pierre, although reluctantly, insisted on what he considered fair. The manager promised to use all his strength to carry out the will of the count, clearly understanding that the count would never be able to trust him not only as to whether all measures had been taken to sell forests and estates, to redeem from the Council, but would also probably never ask or learns how the built buildings stand empty and the peasants continue to give with work and money everything that they give from others, that is, everything that they can give.
In the happiest state of mind, returning from his southern trip, Pierre fulfilled his long-standing intention to call on his friend Bolkonsky, whom he had not seen for two years.
Bogucharovo lay in an ugly, flat area, covered with fields and felled and uncut fir and birch forests. The manor's yard was located at the end of a straight line, along the main road of the village, behind a newly dug, full-filled pond, with the banks not yet overgrown with grass, in the middle of a young forest, between which stood several large pines.
The manor's courtyard consisted of a threshing floor, outbuildings, stables, a bathhouse, an outbuilding and a large stone house with a semicircular pediment, which was still under construction. A young garden was planted around the house. The fences and gates were strong and new; under the canopy stood two fire pipes and a barrel painted green; the roads were straight, the bridges were strong with railings. Everything bore the imprint of neatness and thrift. The servants who met, when asked where the prince lived, pointed to a small, new outbuilding standing at the very edge of the pond. Prince Andrei's old uncle, Anton, dropped Pierre out of the carriage, said that the prince was at home, and led him into a clean, small hallway.
Pierre was struck by the modesty of the small, albeit clean, house after the brilliant conditions in which he last saw his friend in St. Petersburg. He hurriedly entered the still pine-smelling, unplastered, small hall and wanted to move on, but Anton tiptoed forward and knocked on the door.
- Well, what's there? – a sharp, unpleasant voice was heard.
“Guest,” answered Anton.
“Ask me to wait,” and I heard a chair being pushed back. Pierre walked quickly to the door and came face to face with Prince Andrei, who was coming out to him, frowning and aged. Pierre hugged him and, raising his glasses, kissed him on the cheeks and looked at him closely.
“I didn’t expect it, I’m very glad,” said Prince Andrei. Pierre said nothing; He looked at his friend in surprise, without taking his eyes off. He was struck by the change that had taken place in Prince Andrei. The words were affectionate, a smile was on Prince Andrei’s lips and face, but his gaze was dull, dead, to which, despite his apparent desire, Prince Andrei could not give a joyful and cheerful shine. It’s not that his friend has lost weight, turned pale, and matured; but this look and the wrinkle on his forehead, expressing long concentration on one thing, amazed and alienated Pierre until he got used to them.
When meeting after a long separation, as always happens, the conversation could not stop for a long time; they asked and answered briefly about things that they themselves knew should have been discussed at length. Finally, the conversation gradually began to dwell on what had previously been said fragmentarily, on questions about his past life, about plans for the future, about Pierre’s travels, about his activities, about the war, etc. That concentration and depression that Pierre noticed in the look of Prince Andrei now was expressed even more strongly in the smile with which he listened to Pierre, especially when Pierre spoke with animated joy about the past or the future. It was as if Prince Andrei wanted, but could not, take part in what he said. Pierre began to feel that enthusiasm, dreams, hopes for happiness and goodness in front of Prince Andrei were not proper. He was ashamed to express all his new, Masonic thoughts, especially those renewed and excited in him by his last journey. He restrained himself, was afraid to be naive; at the same time, he irresistibly wanted to quickly show his friend that he was now a completely different, better Pierre than the one who was in St. Petersburg.
Topic 3.3. Absorption 12 hours, incl. lab. slave. and practical busy 6 hours
The student must:
know:
Physical foundations and theory of the absorption process (equilibrium between phases, principles of compiling a material heat balance, operating line equation);
- procedure for calculating a packed and bubbling absorber;
- essence and methods of desorption;
be able to:
- draw up material and heat balance;
- determine the absorber consumption;
- build an equilibrium and working process line;
- determine the main overall dimensions of absorbers using reference books.
Purpose of absorption. Absorption in the separation of homogeneous gas mixtures and gas purification. Selecting an absorbent. Physical absorption and absorption accompanied by chemical interaction. Desorption.
Equilibrium between phases during absorption. The influence of temperature and pressure on the solubility of gases in liquids. Material balance of the process and operating line equations for absorption and desorption. Absorbent consumption. Heat balance of absorption. Heat removal during absorption.
Absorption called the process of selective absorption of components from gas or vapor-gas mixtures by liquid absorbers - absorbents.
The principle of absorption is based on the different solubility of the components of gas and vapor-gas mixtures in liquids under the same conditions. Therefore, the choice of absorbents is carried out depending on the solubility of the absorbed components in them, which is determined by:
· physical and chemical properties of gas and liquid phases;
· temperature and pressure of the process;
When choosing an absorbent, it is necessary to take into account such properties as selectivity with respect to the absorbed component, toxicity, fire hazard, cost, availability, etc.
A distinction is made between physical absorption and chemical absorption (chemisorption). During physical absorption, the absorbed component forms only physical bonds with the absorbent. This process is in most cases reversible. The separation of the absorbed component from the solution - desorption - is based on this property. If the absorbed component reacts with the absorbent and forms a chemical compound, the process is called chemisorption.
The absorption process is usually exothermic, that is, it is accompanied by the release of heat.
Absorption is widely used in industry for the separation of hydrocarbon gases at oil refineries, the production of hydrochloric and sulfuric acids, ammonia water, the purification of gas emissions from harmful impurities, the separation of valuable components from cracking gases or methane pyrolysis, from coke oven gases, etc.
Equilibrium in absorption processes is determined by the Gibbs phase rule (B.4), which is a generalization of the conditions of heterogeneous equilibrium:
C = K - F + 2.
Since the absorption process is carried out in a two-phase (gas - liquid) and three-component (one distributed and two distributing components) system, the number of degrees of freedom is three.
Thus, equilibrium in the gas (vapor) - liquid system can be characterized by three parameters, for example, temperature, pressure and the composition of one of the phases.
Equilibrium in the gas-liquid system is determined by Henry’s solubility law, according to which, at a given temperature, the mole fraction of gas in a solution (solubility) is proportional to the partial pressure of the gas above the solution:
where p is the partial pressure of gas above the solution; x – molar concentration of gas in solution; E – proportionality coefficient (Henry coefficient).
Henry's law applies primarily to slightly soluble gases, as well as to solutions with low concentrations of highly soluble gases in the absence of a chemical reaction.
Coefficient E has a pressure dimension coinciding with the p dimension and depends on the nature of the dissolving substance and temperature. It has been established that with increasing temperature the solubility of gas in liquid decreases. When a mixture of gases is in equilibrium with a liquid, Henry's law can be followed by each of the components of the mixture separately.
Since the thermal effect accompanying the absorption process negatively affects the position of the equilibrium line, it must be taken into account in the calculations. The amount of heat released during absorption can be determined by the dependence
where q d is the differential heat of dissolution within the range of changes in concentration x 1 – x 2; L – amount of absorbent.
If absorption is carried out without heat removal, then we can assume that all the heat released goes to heating the liquid, and the temperature of the latter increases by
where c is the heat capacity of the solution.
To lower the temperature, the initial gas mixture and the absorbent are cooled, removing the heat released during the absorption process using built-in (internal) or external heat exchangers.
The partial pressure of the dissolved gas in the gas phase corresponding to equilibrium can be determined by Dalton's law, according to which 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, i.e.
Where R– total pressure of the gas mixture; y is the molar concentration of the gas distributed in the mixture.
Comparing equations (10.2) and (10.1), we find
where A equals = E/P – phase equilibrium constant, applicable for the areas of action of Henry’s and Dalton’s laws.
Let R ab be the vapor pressure of a pure absorbent under absorption conditions; p ab – partial pressure of absorbent vapor in solution; P – total pressure; x – mole fraction of absorbed gas in solution; y is the mole fraction of the distributed gas in the gas phase; yab is the mole fraction of the absorbent in the gas phase.
According to Raoult's law, the partial pressure of a component in a solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution:
According to Dalton's law (10.2), the partial pressure of the absorbent in the gas phase is equal to
At equilibrium
An analysis of the factors influencing the equilibrium in gas (vapor) - liquid systems made it possible to establish that the parameters that improve the conditions of absorption include increased pressure and low temperature, and the factors promoting desorption include low pressure, high temperature and the introduction of additives that reduce the solubility of gases in liquids.
Material balance the absorption process is expressed by the differential equation
where G is the flow of the gas mixture (inert gas), kmol/s; L – absorbent flow, kmol/s; Y n and Y k – initial and final content of the distributed substance in the gas phase, kmol/kmol of inert gas; X k and X n – initial and final content of the distributed substance in the absorbent, kmol/kmol of absorbent; M is the amount of distributed substance transferred from phase G to phase L per unit time, kmol/s.
From the material balance equation (10.9) you can determine the required total consumption of absorbent
The absorption process is also characterized by the degree of extraction (absorption), which represents the ratio of the amount of the actually absorbed component to the amount absorbed when it is completely extracted,
Kinetics of the process absorption is characterized by three main stages, which correspond to the scheme presented in Fig. 9.4.
The first stage is the transfer of molecules of the absorbed component from the core of the gas (vapor) flow to the phase interface (liquid surface).
The second stage is the diffusion of molecules of the absorbed component through the surface layer of the liquid (phase interface).
The third stage is the transition of molecules of the absorbed substance from the phase interface into the bulk of the liquid.
The kinetic patterns of absorption correspond to the general mass transfer equation for two-phase systems:
It has been experimentally established that the second stage of the absorption process occurs at a higher speed and does not affect the overall speed of the process, which is limited by the speed of the slowest stage (first or third).
The driving force of the absorption process for stages I and III in equations (10.5a) and (10.6a) can be expressed through other parameters:
In equations (10.5b) and (10.6b), p is the working partial pressure of the distributed gas in the gas mixture; p equal – equilibrium gas pressure above the absorbent, corresponding to the working concentration in the liquid; C is the working volumetric molar concentration of the distributed gas in the liquid; C equal is the equilibrium volumetric molar concentration of the distributed gas in the liquid, corresponding to its operating partial pressure in the gas mixture.
With this expression of the driving force of the absorption process, the equilibrium equation takes the form
where Ψ is the proportionality coefficient, kmol/(m 3 *Pa).
Mass transfer coefficients are expressed for equations (10.5a) and (10.6a) in the form
for equations (10.5b) and (10.6b)
In equations (10.7) and (10.8), β y, β p are the coefficients of mass transfer from the gas flow to the phase contact surface; β x, β WITH- mass transfer coefficients from the phase contact surface to the liquid flow.
The mass transfer coefficients for gas and liquid β y and β x can be determined from criterion equations having the form:
for the gas phase Nu diff y = f*(Re, Pr diff);
for the liquid phase Nu diff x = f*(Re, Pr diff x).
The value of the coefficient Ψ has a significant impact on the kinetics of the absorption process. If Ψ has high values (high solubility of the component - diffusion resistance is concentrated in the gas phase), then 1/(β c *Ψ)< 1/β р или К Р ≈ β р. Если Ψ мало (извлекаемый компонент трудно растворим – диффузионное сопротивление сосредоточено в жидкой фазе), то Ψ/β р << 1/β с и можно считать К с ≈ β с
Just as for mass exchange processes at L/G = const, the working lines of the absorption process are straight and are described in the case of counterflow by equation (9.4), and in the case of forward flow by equation (9.5).
The average driving force in equations (10.5a) and (10.6a) is determined in the case of a rectilinear equilibrium dependence through the relative molar concentrations of the components according to dependences (9.6) and (9.7).
These same dependencies can also be used to express the driving force of the absorption process through the partial pressures of the distributed component in a gas or the volumetric molar concentrations of the same component in a liquid in equations (10.5b) and (10.6b)
Here Δр max, Δр min are the greater and lesser values of the driving force at the beginning and end of the absorption process, expressed through the difference in partial pressures of the absorbed component; ΔС max, ΔС min – greater and lesser values of the driving force at the beginning and end of the absorption process, expressed in terms of volumetric molar concentrations of the absorbed component in the liquid.
In the case of Δp max /Δp min ≤ 2, ΔC max /ΔC min ≤ 2, while maintaining the linearity of the equilibrium dependence, the average driving force of the absorption process can be equal to the arithmetic mean of these values.
When carrying out the absorption process, accompanied by a chemical reaction (chemisorption) occurring in the liquid phase, part of the distributed component passes into a chemically bound state. As a result, the concentration of the dissolved (physically bound) distributed component in the liquid decreases, which leads to an increase in the driving force of the process compared to purely physical absorption.
The rate of chemisorption depends on both the rate of mass transfer and the rate of chemical reaction. In this case, a distinction is made between the diffusion and kinetic regions of chemisorption. In the diffusion region, the rate of the process is determined by the rate of mass transfer, in the kinetic region - by the rate of the chemical reaction. In cases where the rates of mass transfer and reaction are comparable, chemisorption processes occur in the mixed, or diffusion-kinetic, region.
When calculating chemisorption, the mass transfer coefficient in the liquid phase, taking into account the chemical reaction β′ x occurring in it, can be expressed through the mass transfer coefficient for physical absorption β x, taking into account mass transfer acceleration factor F m, showing how many times the absorption rate will increase due to the occurrence of a chemical reaction:
β′ x = β x * F m
Factor Fm is determined by graphical dependencies.
absorption) - (in physiology) absorption, absorption of liquid or other substances by the tissues of the human body. Digested food is absorbed by the digestive tract and then enters the blood and lymph. Most nutrients are absorbed in the small intestine - in its constituent jejunum and ileum, but alcohol can also be easily absorbed from the stomach. The small intestine is lined from the inside with tiny finger-like protrusions (see Villi), which significantly increase its surface area, as a result of which the absorption of digestive products is significantly accelerated. See also Assimilation, Digestion.
Absorption
Word formation. Comes from Lat. absorptio - absorption.
Specificity. The individual's susceptibility to special states of consciousness (hypnosis, drugs, meditation). In ordinary situations, it manifests itself in an increase in the level of fantasy. It has been shown that absorption is associated with other personal characteristics (positively - with diversity of motives, social adaptability, imaginative thinking, communication, anxiety, as well as with the weakness and dynamism of the nervous system; negatively - with self-control, social status in a small group, level of aspirations, and also with the mobility of the nervous system).
Literature. Grimak L.P. Modeling human states in hypnosis. M.: Nauka, 1978;
Pekala R.J., Wenger C.F., Levine P. Individual differences in phenomenological experience: states of consciousness as a function of absorption // J. Pers. and Soc. Psychol. 1985, 48, N 1, p. 125-132
ABSORPTION
1. When studying sensory processes, the absorption of a chemical, electromagnetic or other physical stimulus by the receptor. For example, see spectral absorption. 2. Busy, absorbed in some activity. The connotation of meaning can be positive when the subject’s attention is focused on performing some task, or negative when absorption of attention is considered as an escape from reality.
Absorption is the process of absorption of gas by a liquid absorber in which the gas is soluble to one degree or another. The reverse process - the release of dissolved gas from solution - is called desorption.
In absorption processes (absorption, desorption) two phases are involved - liquid and gas, and a substance transitions from the gas phase to the liquid phase (during absorption) or, conversely, from the liquid phase to the gas phase (during desorption). Thus, absorption processes are one of the types of mass transfer processes.
In practice, absorption is mostly carried out not by individual gases, but by gas mixtures, the components of which (one or more) can be absorbed by a given absorber in noticeable quantities. These components are called absorbable components or simply components, and non-absorbable components are called inert gas.
The liquid phase consists of an absorber and an absorbed component. In many cases, the absorbent is a solution of the active component that reacts chemically with the absorbed component; in this case, the substance in which the active component is dissolved will be called a solvent.
The inert gas and the absorber are carriers of the component in the gas and liquid phases, respectively. During physical absorption (see below), the inert gas and the absorber are not consumed and do not participate in the processes of transition of the component from one phase to another. During chemisorption (see below), the absorbent can chemically interact with the component.
The course of absorption processes is characterized by their statics and kinetics.
The statics of absorption, i.e., the equilibrium between the liquid and gas phases, determines the state that is established during very long contact of the phases. The equilibrium between the phases is determined by the thermodynamic properties of the component and absorber and depends on the composition of one of the phases, temperature and pressure.
The kinetics of absorption, i.e. the rate of the mass transfer process, is determined by the driving force of the process (i.e. the degree of deviation of the system from the equilibrium state), the properties of the absorber, component and inert gas, as well as the method of contact of the phases (the design of the absorption apparatus and the hydrodynamic mode of its operation ). In absorption apparatuses, the driving force, as a rule, varies along their length and depends on the nature of the mutual movement of the phases (countercurrent, forward flow, cross current, etc.). In this case, continuous or stepped contact is possible. In absorbers with continuous contact, the nature of the phase movement does not change along the length of the apparatus and the change in the driving force occurs continuously. Absorbers with stepped contact consist of several stages connected in series across gas and liquid, and when moving from stage to stage, an abrupt change in force movements occurs.
A distinction is made between chemical absorption and chemisorption. During physical absorption, the dissolution of the gas is not accompanied by a chemical reaction (or at least this reaction does not have a noticeable effect on the process). In this case, there is a more or less significant equilibrium pressure of the component above the solution, and absorption of the latter occurs only as long as its partial pressure in the gas phase is higher than the equilibrium pressure above the solution. In this case, complete extraction of the component from the gas is possible only with countercurrent flow and supply of a clean absorber that does not contain the component into the absorber.
During chemisorption (absorption accompanied by a chemical reaction), the absorbed component is bound in the liquid phase in the form of a chemical compound. In an irreversible reaction, the equilibrium pressure of the component above the solution is negligible and its complete absorption is possible. During a reversible reaction, there is a noticeable pressure of the component above the solution, although less than during physical absorption.
Industrial absorption may or may not be combined with desorption. If desorption is not performed, the absorbent is used once. In this case, as a result of absorption, a finished product, an intermediate product, or, if absorption is carried out for the purpose of sanitary purification of gases, a waste solution is obtained, which is drained (after neutralization) into the sewer.
The combination of absorption and desorption allows the absorbent to be reused and the absorbed component to be isolated in its pure form. To do this, the solution after the absorber is sent for desorption, where the component is separated, and the regenerated (freed from the component) solution is returned to absorption. With this scheme (circular process), the absorber is not consumed, except for some of its losses, and circulates all the time through the absorber-desorber-absorber system.
In some cases (in the presence of a low-value absorber), repeated use of the absorber is abandoned during the desorption process. In this case, the absorber regenerated in the desorber is discharged into the sewer system, and fresh absorber is supplied to the absorber.
Conditions favorable for desorption are opposite to those favorable for absorption. To carry out desorption, there must be a noticeable pressure of the component above the solution so that it can be released into the gas phase. Absorbers in which absorption is accompanied by an irreversible chemical reaction cannot be regenerated by desorption. Regeneration of such absorbers can be done chemically.
The areas of application of absorption processes in the chemical and related industries are very extensive. Some of these areas are listed below:
Obtaining a finished product by absorbing gas into liquid. Examples include: absorption of SO 3 in the production of sulfuric acid; absorption of HCl to produce hydrochloric acid; absorption of nitrogen oxides by water (production of nitric acid) or alkaline solutions (production of nitrates), etc. In this case, absorption is carried out without subsequent desorption.
Separation of gas mixtures to isolate one or more valuable components of the mixture. In this case, the absorbent used must have the greatest possible absorption capacity in relation to the extracted component and the least possible in relation to other components of the gas mixture (selective, or selective, absorption). In this case, absorption is usually combined with desorption in a circular process. Examples include the absorption of benzene from coke oven gas, the absorption of acetylene from gases from cracking or pyrolysis of natural gas, the absorption of butadiene from contact gas after the decomposition of ethyl alcohol, etc.
Purification of gas from impurities of harmful components. Such purification is carried out primarily to remove impurities that are not permissible during further gas processing (for example, purification of oil and coke gases from H 2 S, nitrogen-hydrogen mixture for the synthesis of ammonia from CO 2 and CO, drying of sulfur dioxide in the production of contact sulfuric acid etc.). In addition, sanitary cleaning of exhaust gases released into the atmosphere is carried out (for example, cleaning flue gases from SO 2; cleaning exhaust gases from Cl 2 after condensation of liquid chlorine; cleaning gases released during the production of mineral fertilizers from fluoride compounds, etc.).
In this case, the extracted component is usually used, so it is isolated by desorption or the solution is sent for appropriate processing. Sometimes, if the amount of the extracted component is very small and the absorbent is not valuable, the solution after absorption is discharged into the sewer.
Collecting valuable components from a gas mixture to prevent their losses, as well as for sanitary reasons, for example, recovery of volatile solvents (alcohols, ketones, ethers, etc.).
It should be noted that to separate gas mixtures, purify gases and capture valuable components, along with absorption, other methods are used: adsorption, deep cooling, etc. The choice of one method or another is determined by technical and economic considerations. Absorption is generally preferred in cases where very complete extraction of the component is not required.
During absorption processes, mass transfer occurs at the contact surface of the phases. Therefore, absorption devices must have a developed contact surface between gas and liquid. Based on the method of creating this surface, absorption devices can be divided into the following groups:
a) Surface absorbers, in which the contact surface between the phases is a liquid mirror (surface absorbers themselves) or the surface of a flowing liquid film (film absorbers). This group also includes packed absorbers, in which liquid flows over the surface of a packing loaded into the absorber from bodies of various shapes (rings, lump material, etc.), and mechanical film absorbers. For surface absorbers, the contact surface is to a certain extent determined by the geometric surface of the absorber elements (for example, a nozzle), although in many cases it is not equal to it.
b) Bubbler absorbers, in which the contact surface develops with gas flows distributed in the liquid in the form of bubbles and streams. This movement of gas (bubbling) is carried out by passing it through a liquid-filled apparatus (solid bubbling) or in column-type apparatuses with various types of plates. A similar nature of interaction between gas and liquid is also observed in packed absorbers with a flooded packing.
This group also includes bubbling absorbers with liquid mixing using mechanical stirrers. In bubbling absorbers, the contact surface is determined by the hydrodynamic regime (gas and liquid flow rates).
c) Spray absorbers, in which the contact surface is formed by spraying a liquid in a mass of gas into small droplets. The contact surface is determined by the hydrodynamic regime (fluid flow). This group includes absorbers in which liquid is atomized by nozzles (nozzle, or hollow, absorbers), in a stream of gas moving at high speed (high-speed direct-flow atomizing absorbers) or by rotating mechanical devices (mechanical atomizing absorbers).
Absorption is the process of separating gas mixtures using liquid absorbers - absorbents. If the absorbed gas (absorbent) does not chemically interact with the absorbent, then absorption is called physical (the non-absorbed component of the gas mixture is called inert, or inert gas). If the absorbent forms a chemical compound with the absorbent, then the process is called chemisorption. In technology, a combination of both types of absorption is often found.
Physical absorption (or simply absorption) is usually reversible. The release of absorbed gas from solution - desorption - is based on this property of absorption processes.
The combination of absorption and desorption allows the absorbent to be used repeatedly and the absorbed gas to be released in pure form. Often desorption is not necessary, since the solution obtained as a result of absorption is the final product suitable for further use.
In industry, absorption is used to solve the following main problems:
1) to obtain the finished product (for example, absorption of SO 3 in the production of sulfuric acid); in this case, absorption is carried out without desorption;
2) to isolate valuable components from gas mixtures (for example, absorption of benzene from coke oven gas); in this case, absorption is carried out in combination with desorption;
3) to purify gas emissions from harmful impurities (for example, purification of flue gases from SO 2). In these cases, components extracted from gas mixtures are usually used, so they are isolated by desorption;
4) for drying gases.
The devices in which absorption processes are carried out are called absorbers.
Equilibrium in the absorption process
Henry's law is valid for ideal gases:
Henry's Law: the partial pressure of a component of a gas mixture above a solution is proportional to the mole fraction of that component in the solution when equilibrium is reached. Henry constant ( E) increases with increasing temperature.
According to Dalton's law, the partial pressure of a component of a gas mixture is proportional to its mole fraction in the gas mixture:
,
Where P– total pressure.
By combining Henry's and Dalton's laws, it is possible to establish the influence of conditions on the solubility of a gas in a liquid: 
.
Thus, with increasing pressure in the absorber and decreasing temperature, solubility increases.
The worse the gas dissolves, the more the pressure increases.
When dissolving highly soluble gases, there is no need for a large increase in pressure, but it is necessary to remove heat, which in this case is released in large quantities.
Absorber designs are selected taking into account the solubility of gases. For example, for highly soluble substances (ammonia-water), heat exchanger absorbers can be used. For poorly soluble substances, a developed phase contact surface is required, so packed or plate absorbers are used.
