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DEFINITION
Absorption the process of absorption of a substance (sorbate) by another substance (sorbent) is called.
Absorption of gases or vapors by liquids is most often used in technology. The absorption process is reversible and selective. No chemical reactions take place during physical absorption. There is also chemisorption.
The absorption process is accompanied by an increase in the mass and volume of the absorbent, as well as its other changes. physical parameters... It is even possible to change the state of aggregation.
Absorption differs from adsorption in that in the first case, absorption occurs in the entire volume of the sorbent.
The reason for the absorption is the mutual attraction of the molecules of the absorbent and the absorbent substances.
Definition
In accordance with Dalton's law, if a mixture of gases dissolves in a liquid, then each component of the mixture dissolves in proportion to its partial pressure, regardless of the rest of the gases. The absorption coefficient is the degree of dissolution of a gas in a liquid. There are several absorption coefficients: Bunsen, Van Slike, Oswald. The Bunsen coefficient is the most commonly used.
DEFINITION
Gas absorption coefficient(according to Busen) in a liquid at a temperature t o is equal to the volume of gas, which is measured under normal conditions (Pa), absorbed by a unit volume of liquid, when the gas pressure above the liquid is 1 atm .. It is often denoted by the letter
The absorption coefficients of gases by water are found using the formula:
where is the gas temperature; ,, Are constant coefficients, for each gas their own and can vary depending on the temperature.
So, the coefficient of absorption according to Bunsen for the absorption: water is equal to oxygen, water is nitrogen; carbon dioxide water
Oswald absorption coefficient (solubility coefficient) at temperature t o С and partial pressure of gas above liquid at equilibrium p atm. is equal to the volume of gas, which is measured without reduction to normal conditions, dissolved in a unit volume of liquid. It is often denoted by a letter.
The process and result of absorption of gases by liquids is dependent on the type of gas and liquid, gas pressure and temperature. There is a law called Henry's Law, according to which the concentration of a gas (c) that is dissolved in a liquid is related to pressure (p) using the formula:
where, if the concentration is expressed in terms of the volume of gas, which is normalized and dissolved in a unit volume of liquid, the pressure is given in atmospheres. That is, the numerical value of the coefficient k depends on the units in which the pressure and concentration are expressed. In other words, the solubility of a given gas in a liquid at a constant temperature is directly proportional to its pressure in the gas phase. Henry's Law is used for gases with low solubility.
Absorption Ratio Units
The absorption coefficient is dimensionless.
Examples of problem solving
EXAMPLE 1
| Exercise | The absorption coefficient according to Oswald for in water at equals. What mass of carbon dioxide will dissolve in 1 liter of water at the same temperature and pressure of 4 atm. |
| Solution | 1.8 carbon dioxide dissolves in 1 liter of water at a pressure of one atmosphere. At a pressure of 4 atmospheres, 1.8 liters will also dissolve, however, they are at a pressure of 1 atm. will occupy a volume equal to:
Under normal conditions, a gramol occupies a volume of 22.4 liters and has a mass of 44 g. Therefore, a volume of carbon dioxide equal to 7.2 liters has a mass:
|
| Answer | m = 14.2 g |
(l)
(G)EXAMPLE 2
| Exercise | There is a mixture of oxygen and carbon dioxide in the tank above the water (Fig. 1). The mixture contains 75% oxygen (by volume). What is the composition of the gas mixture dissolved in water at, if the absorption coefficients for these gases according to Oswald are equal: and? |
Introduction
Per last years the relevance of this topic has grown significantly in the field of the chemical industry, since the absorption process in the chemical industry can act as a purification equipment for removing impurities in gas mixtures, eliminating large losses of valuable material.
The purpose of this work: to study absorption processes and get acquainted with the equipment in which the absorption processes take place.
Task of the work: analysis of equipment to identify parameters that can affect the choice of absorbers, using the material balance of the process.
Absorption process
Absorption is the process of gas absorption by a liquid absorber, in which the gas is soluble to one degree or another. The reverse process - the separation of dissolved gas from solution - is called desorption.
In absorption processes (absorption, desorption), two phases are involved - liquid and gas, and the substance passes 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.
Industrial carrying out absorption may or may not be combined with desorption. If no desorption is performed, the absorber 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 cleaning of gases, a waste solution is obtained, which is drained (after neutralization) into the sewage system.
The combination of absorption with desorption makes it possible to reuse the absorber and isolate the absorbed component in its pure form. For this, the solution after the absorber is sent to desorption, where the component is released, and the regenerated (freed from the component) solution is returned to absorption again. With such a scheme (circular process), the absorber is not consumed, except for some of its losses, and all the time circulates through the absorber - desorber - absorber system.
In some cases (in the presence of a low-value absorber), multiple use of the absorber is abandoned during the desorption process. After that, the absorber regenerated in the stripper is discharged into the sewer, and fresh absorber is fed into the absorber.
Absorbers, in which absorption is accompanied by an irreversible chemical reaction, cannot be regenerated by desorption. The regeneration of such absorbers can be done by a chemical method.
Absorbers
Apparatus in which absorption processes are carried out are called absorbers.
During absorption processes, mass transfer occurs on the contact surface of the phases. Therefore, absorption devices must have a developed contact surface between gas and liquid. Based on this, absorption devices can be divided into the following groups:
a) Surface absorbers, in which the surface of contact between the phases is a liquid mirror (surface absorbers themselves) or the surface of a flowing liquid film (film absorbers). The same group includes packed absorbers, in which liquid flows over the surface of the packing loaded into the absorber from bodies of various shapes (rings, lumpy 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, packing), although in many cases it is not equal to it.
b) Bubble absorbers, in which the contact surface is developed by gas flows. distributed in the liquid in the form of bubbles and streams. Such movement of gas (bubbling) is carried out by passing it through a liquid-filled apparatus (continuous bubbling) or in column-type apparatus with cap, sieve or failure trays. A similar nature of the interaction of gas and liquid is also observed in packed absorbers with flooded packing. This group also includes bubbling absorbers with liquid mixing with mechanical stirrers. In bubbling absorbers, the contact surface is determined by the hydrodynamic regime (gas and liquid flow rates).
Tray columns with drain devices. In these columns, the overflow of liquid from the tray to the tray is carried out using special devices - drain pipes, pockets, etc. The lower ends of the tubes are immersed in the glass on the lower trays and form hydraulic seals that exclude the possibility of gas passing through the drain device.
Rice. 1 - a disc-shaped column with drain devices: 1 - a plate; 2 - drain devices
The principle of operation of columns of this type can be seen from Fig. 1, where an absorber with perforated trays is shown as an example. The liquid enters the upper tray 1, is discharged from the tray to the tray through the overflow devices 2 and is removed from the bottom of the column. Gas enters the bottom of the apparatus, passes sequentially through the holes or caps of each tray. In this case, the gas is distributed in the form of bubbles and jets in the liquid layer on the tray, forming a foam layer on it, which is the main area of mass transfer and heat transfer on the tray. Waste gas is removed from the top of the column.
The overflow tubes are placed on the trays in such a way that the liquid on adjacent trays flows in mutually opposite directions. Recently, drain devices in the form of segments cut in a plate and limited by a threshold - an overflow - have been increasingly used.
c) Spray absorbers, in which the contact surface is formed by spraying liquid in a mass of gas into small droplets. The contact surface is determined by the hydrodynamic regime (fluid flow rate). This group includes absorbers in which the liquid is atomized by nozzles (nozzle or hollow absorbers), in a stream of gas moving at a high speed (high-speed direct-flow atomizing absorbers) or rotating mechanical devices (mechanical atomizing absorbers).
The above classification of absorption devices is conditional, since it reflects not so much the design of the device as the nature of the contact surface. The same type of device, depending on the operating conditions, may appear in different groups. For example, packed absorbers can operate in both film and bubbling modes. In devices with bubbling trays, modes are possible when there is a significant spraying of liquid and the contact surface is formed mainly by drops.
Applications of absorption processes
The fields 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 a liquid. Examples are: SO3 absorption in sulfuric acid production; absorption of HC1 to obtain 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 absorber used should have the greatest possible absorption capacity with respect to the component to be extracted and possibly lower with respect to other constituents 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 cracking or pyrolysis gases of natural gas, the absorption of butadiene from the contact gas after the decomposition of ethyl alcohol, etc.
Gas purification from impurities of harmful components
Such purification is carried out primarily with the aim of removing impurities that are not permissible in further processing of gases (for example, purification of oil and coke oven gases from H2S, purification of a nitrogen-hydrogen mixture for the synthesis of ammonia from CO2 and CO, dehydration of sulfur dioxide in the production of contact sulfuric acid, etc. etc.). In addition, they carry out sanitary cleaning of exhaust gases released into the atmosphere (for example, cleaning of flue gases from SO2; cleaning of C12 offgas after condensation of liquid chlorine; cleaning from fluoride compounds of gases emitted during the production of mineral fertilizers, etc.).
In this case, the recovered component is usually used, therefore it is isolated by desorption or the solution is sent for appropriate processing. Sometimes, if the amount of the recoverable component is very small and the absorber is not valuable, the solution after absorption is discharged into the sewer.
Capturing valuable components from the gas mixture to prevent their losses, as well as for sanitary reasons, for example, the recovery of volatile solvents (alcohols, ketones, ethers, etc.).
It should be noted that, along with absorption, other methods are used to separate gas mixtures, purify gases and capture valuable components: adsorption, deep cooling, etc. The choice of one method or another is determined by technical and economic considerations. Absorption is generally preferred when very complete component recovery is not required.
absorption gas cleaning
Material balance of the absorption process
Material balance and consumption of absorbent. Let us take the flow rates of the phases along the height of the apparatus to be constant and express the content of the absorbed gas in relative molar concentrations.
Let us designate: G - consumption of inert gas, kmol / sec; nor Yk - initial and final concentration of absorbent in the gas mixture, kmol / kmol of inert gas; consumption of absorbent, kmol / sec; its concentrations Xn and Xk, kmol / kmol of absorbent.
Then the material balance equation will be:
(1)
Hence the total consumption of the absorbent (in kmol / sec)
(2)
and its specific consumption (in kmol / kmol of inert gas)
(3)
This equation can be rewritten as follows:
(4)
Equation (4) shows that the change in concentration in the absorption apparatus occurs in a straight line and, therefore, in the Y-X coordinates, the working line of the absorption process is a straight line with an angle of inclination, the tangent of which is equal to ... There is a definite relationship between the specific consumption of the absorbent and the dimensions of the apparatus. Through point B with coordinates Xn and Yk (Figure 2), we draw, according to equation (4), the working lines BA, BA1, BA2, BA3, corresponding to different concentrations of the absorbent or different specific costs. In this case, points A, A1, A2, A3 will lie on one horizontal line in accordance with the given initial concentration Yн of gas in the mixture.
Figure 2 - To the determination of the specific consumption of absorbent
In the case of solutions of low concentration, for any value of X and the selected value, the driving force of the process is expressed by the difference between the ordinates Y-Y *, depicted by vertical segments connecting the corresponding points of the working line and the equilibrium line.
For the entire apparatus, you can take the average value? Yav, values: which, for example, for the working line BA1, is shown in the figure by the segment? Yav1. The value of? Yav will be the greater, the steeper the slope of the working lines and, therefore, the greater the specific consumption of the absorbent. If the working line VA coincides with the vertical, then the driving force of the process has a maximum value, but the specific consumption of the absorbent in this case will be infinitely large (since Xk = Xn). If the line of working concentrations BA3 touches the equilibrium line, then the specific consumption of the absorbent is minimal (l = lmin), and the driving force at the point of contact is zero, since at this point the working concentration is equal to the equilibrium one. In the first case, the dimensions of the absorption apparatus will be the smallest at an infinitely large flow rate of the absorbent, in the second, the flow rate of the absorbent is the smallest at an infinitely large size of the apparatus. Thus, both cases are limiting and practically impracticable.
In a real absorption apparatus, the equilibrium between the phases is not achieved and always Xk< Х*к, где Х*к - концентрация поглощаемого газа в жидкости, находящейся в равновесии с поступающим газом. Отсюда следует, что значение l всегда должно быть больше минимального значения lmin отвечающего предельному положению рабочей линии (линия BA3 на рисунке 2).
The lmin value can be determined by equation (3) when replacing Xk with X * k:
(5)
It should be noted that an increase in the specific consumption l of the absorbent simultaneously with a decrease in the height of the apparatus leads to a certain increase in its diameter. This is due to the fact that with an increase in l, the consumption of the absorber L also increases, and at the same time, as shown below, the permissible gas velocities in the apparatus decrease, according to which its diameter is found. That is why, in cases where the specific consumption of the absorbent is not specified by the technological conditions, i.e., when the final concentration Xk of the absorbent is not specified, such a ratio should be chosen between the dimensions of the absorption apparatus and the specific consumption l of the absorbent, at which the value of l and the dimensions of the apparatus will be optimal ...
The optimal specific consumption of the absorber lopt can be found only with the help of a technical and economic calculation.
Conclusion
The problems that I encountered in carrying out this work are that at present there is still no completely reliable method for determining the mass transfer coefficient by calculation or on the basis of laboratory or model experiments. However, for some types of devices, it is possible to find the mass transfer coefficients with a sufficiently high accuracy using calculation or comparatively simple experiments.
Another not unimportant problem is the choice of the type and dimensions of the absorber (for example, diameter and height), which is determined by calculation based on the given operating conditions (productivity, required degree of component recovery, etc.). The calculation requires information on the statics and kinetics of the process. Statics data are found from look-up tables, calculated using thermodynamic parameters, or determined empirically. Kinetic data largely depend on the type of apparatus and its mode of operation. The most reliable are the results of experiments carried out under the same conditions. In a number of cases, such data are absent, and one has to resort to calculations or experiments.
Conclusion: the absorption process is currently a topical topic for the chemical industry, since the combination of absorption with desorption allows the absorber to be repeatedly used and the absorbed component is isolated in its pure form. With such a scheme (circular process), the absorber is not consumed, except for some of its losses, and all the time circulates through the absorber - desorber - absorber system.
Bibliography
1. E. Ignatovich. Chemical engineering. Processes and apparatus. Part 2. Moscow: Technosphere, 2007.
... "Calculation of plate-type absorption columns" ed. In A. Ivanova, Moscow, 1985.
... "Basic processes and devices of chemical technology", design manual, ed. Yu.I. Dytnersky. M, "Chemistry" 1991
K.F. Pavlov, P.G. Romankov, A.A. Noskov. "Examples and tasks for the course of processes and devices of chemical technology." L., "Chemistry", 1976.
A.A. Lashchinsky, A.R. Tolchinsky. "Fundamentals of the design and calculation of chemical equipment." M., 1968
Industry standard OST 26-808-73.
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Topic 3.3. Absorption 12h, incl. lab. slave. and practical. busy 6h
The student must:
know:
Physical foundations and theory of the absorption process (balance between phases, principles of compiling the material heat balance, working line equation);
- the procedure for calculating the packed and bubble absorber;
- the essence and methods of desorption;
be able to:
- make up the material and heat balance;
- determine the consumption of the absorber;
- build a balanced and working line of the process;
- determine the main overall dimensions of the absorbers, using reference books.
Appointment of absorption. Absorption during separation of homogeneous gas mixtures and gas purification. The choice of absorbent. Physical absorption and absorption accompanied by chemical interaction. Desorption.
Equilibrium between phases during absorption. Influence of temperature and pressure on the solubility of gases in liquids. Material balance of the process and equations of the working line for absorption and desorption. Absorbent consumption. Heat balance of absorption. Heat dissipation during absorption.
Absorption is called the process of selective absorption of components from gas or vapor-gas mixtures by liquid absorbers - absorbents.
The absorption principle 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 gas and liquid phases;
· Temperature and pressure of the process;
When choosing an absorbent, it is necessary to take into account its properties such as selectivity (selectivity) in relation to the absorbed component, toxicity, fire hazard, cost, availability, etc.
Distinguish 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, then the process is called chemisorption.
The absorption process is usually exothermic, i.e. accompanied by the release of heat.
Absorption is widely used in industry for separating hydrocarbon gases at oil refineries, obtaining hydrochloric and sulfuric acids, ammonia water, purifying gas emissions from harmful impurities, separating valuable components from cracking or methane pyrolysis gases, 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 for heterogeneous equilibrium:
C = K - F + 2.
Since the absorption process is carried out in a two-phase (gas - liquid) and three-component (one distributable and two distributing components) system, the number of degrees of freedom is three.
Thus, the equilibrium in the gas (vapor) - liquid system can be characterized by three parameters, for example, temperature, pressure and composition of one of the phases.
Equilibrium in the gas - liquid system is determined by Henry's law of solubility, according to which at a given temperature the molar fraction of gas in solution (solubility) is proportional to the partial pressure of the gas above the solution:
where p is the partial pressure of the gas above the solution; x is the molar concentration of gas in the solution; E - coefficient of proportionality (Henry's coefficient).
Henry's law applies primarily to poorly soluble gases, as well as solutions with low concentrations of highly soluble gases in the absence of a chemical reaction.
Coefficient E has the dimension of pressure, which coincides with the dimension of p, and depends on the nature of the dissolving substance and temperature. It was found that with an increase in temperature, the solubility of a gas in a liquid decreases. When a mixture of gases is in equilibrium with a liquid, each of the components of the mixture can follow Henry's Law separately.
Since the thermal effect accompanying the absorption process adversely 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 from the dependence
where q d is the differential heat of dissolution within the concentration change x 1 - x 2; L is the amount of absorbent.
If absorption is carried out without heat removal, then it can be assumed that all the heat released goes to heating the liquid, and the temperature of the latter rises by the value
where c is the heat capacity of the solution.
To lower the temperature, the initial gas mixture and the absorbent are cooled by 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 molar fraction of this component in the mixture, i.e.
where R- the 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 = E / P is the phase equilibrium constant applicable to the domains of Henry's and Dalton's laws.
Let P ab - the vapor pressure of the pure absorbent in the conditions of absorption; p ab is the partial vapor pressure of the absorbent in the solution; P is the total pressure; x is the molar fraction of the absorbed gas in the solution; y is the mole fraction of the distributed gas in the gas phase; y ab is the molar fraction of the absorbent in the gas phase.
According to Raoult's law, the partial pressure of a component in solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in solution:
According to Dalton's law (10.2), the partial pressure of the absorbent in the gas phase is
In equilibrium
Analysis of the factors affecting the equilibrium in gas (vapor) - liquid systems made it possible to establish that the parameters that improve the conditions of absorption include high pressure and low temperature, and the factors promoting desorption are low pressure, high temperature and introduction into the absorbent. 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 to - the initial and final content of the distributed substance in the gas phase, kmol / kmol of inert gas; X to and X n - the initial and final content of the distributed substance in the absorbent, kmol / kmol of the absorbent; M is the amount of the distributed substance transferred from the G phase to the L phase per unit time, kmol / s.
From the material balance equation (10.9), you can determine the required total consumption of the absorbent
The absorption process is also characterized by the degree of extraction (absorption), which is the ratio of the amount actually absorbed by the component to the amount absorbed during its complete extraction,
Kinetics of the process absorption is characterized by three main stages, which correspond to the scheme shown 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 interface (liquid surface).
The second stage is the diffusion of the molecules of the absorbed component through the surface layer of the liquid (interface).
The third stage is the transition of molecules of the absorbed substance from the interface to the bulk of the liquid.
The kinetic patterns of absorption correspond to the general equation of mass transfer for two-phase systems:
It has been experimentally established that the second stage of the absorption process proceeds at a higher rate and does not affect the overall rate of the process, limited by the rate 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 in terms of other parameters:
In equations (10.5b) and (10.6b) p is the working partial pressure of the distributed gas in the gas mixture; p is equal to the equilibrium gas pressure above the absorbent, corresponding to the working concentration in the liquid; C is the working volumetric molar concentration of the gas being distributed in the liquid; С equal - equilibrium volumetric molar concentration of the distributed gas in the liquid, corresponding to its working partial pressure in the gas mixture.
With this expression of the driving force of the absorption process, the equation of the equilibrium dependence takes the form
where Ψ is the coefficient of proportionality, kmol / (m 3 * Pa).
The 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 contact surface of the phases; β x, β WITH- coefficients of mass transfer from the phase contact surface to the fluid flow.
The coefficients of mass transfer for gas and liquid β y and β x can be determined from criterion equations having the form:
for the gas phase Nu diff у = f* (Re, Pr diff y);
for the liquid phase Nu diff x = f* (Re, Pr diff x).
The value of the coefficient Ψ has a significant effect 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-change 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 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 dependencies (9.6) and (9.7).
The same dependences can be used when expressing the driving force of the absorption process in terms of the partial pressures of the component to be distributed in the gas or the volumetric molar concentrations of the same component in the liquid in equations (10.5b) and (10.6b)
Here Δр max, Δр min - greater and smaller values of the driving force at the beginning and end of the absorption process, expressed through the difference in the partial pressures of the absorbed component; ΔС max, ΔС min - larger and smaller values of the driving force at the beginning and end of the absorption process, expressed in terms of the volumetric molar concentration 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 relationship, 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, a part of the component to be distributed 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 chemisorption rate depends on both the mass transfer rate and the rate of the chemical reaction. In this case, the diffusion and kinetic regions of chemisorption are distinguished. 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 proceed in a mixed, or diffusion-kinetic, region.
When calculating chemisorption, the mass transfer coefficient in the liquid phase, which takes into account the chemical reaction β 'x in it, can be expressed in terms of the coefficient of mass transfer during 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 ′ х = β х * Ф m
The fm factor is determined by graphical dependencies.
