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Wiring diagram for connecting the boiler kiturami sts 17. Gas heating boilers Kiturami - the art of heating from South Korea. Installation and operation rules

Basic concepts

The absorbed substance, which is still in the volume of the phase, is called adsorbent, absorbed - adsorbate. In a narrower sense, adsorption is often understood as the absorption of an impurity from a gas or liquid by a solid (in the case of gas and liquid) or liquid (in the case of gas) - adsorbent . In this case, as in the general case of adsorption, the impurity is concentrated at the adsorbent-liquid or adsorbent-gas interface. The process that is the reverse of adsorption, that is, the transfer of a substance from the interface to the volume of the phase, is called desorption. If the rates of adsorption and desorption are equal, then one speaks of the establishment adsorption equilibrium. In a state of equilibrium, the number of adsorbed molecules remains constant for an arbitrarily long time, if the external conditions (pressure, temperature, and composition of the system) are unchanged.

adsorption and chemisorption

At the interface between two phases, in addition to adsorption, which is mainly due to physical interactions (mainly this is Van der Waals forces), a chemical reaction can take place. This process is called chemisorption. A clear distinction between adsorption and chemisorption is not always possible. One of the main parameters by which these phenomena differ is the thermal effect: for example, the thermal effect of physical adsorption is usually close to the heat of adsorbate liquefaction, while the thermal effect of chemisorption is much higher. Moreover, unlike adsorption, chemisorption is usually irreversible and localized. An example of intermediate options that combine the features of both adsorption and chemisorption is the interaction of oxygen on metals and hydrogen on nickel: at low temperatures they are adsorbed according to the laws of physical adsorption, but as the temperature rises, chemisorption begins to occur.

Similar phenomena

In the previous section, we discussed the case of a heterogeneous reaction occurring on a surface—chemisorption. However, there are cases of heterogeneous reactions throughout the volume, and not just on the surface; this is the usual heterogeneous reaction. Absorption over the entire volume can also take place under the influence of physical forces - this case is called absorption.

physical adsorption

Physical adsorption models
Monolayer formation energy diagram

Rice. one: a) adsorbent, b) adsorbate, c) adsorbent (gas phase or solution) Rice. 2: a) adsorbent, b) adsorbate, c) gas phase, d - distance, E - energy, E b - adsorption energy, (1) desorption, (2) adsorption
polycondensation Selective adsorption
Rice. 3: a) adsorbent, b) adsorbate, c) condensate, d) adsorbent (gas phase or solution) Rice. 4: a) adsorbent, b) adsorbate, c) adsorbents (gas phase or solution): predominant adsorption of blue particles is shown

Adsorption is caused by non-specific (that is, substances that do not depend on the nature) Van der Waals forces. Adsorption complicated by chemical interaction between adsorbent and adsorbate is a special case. Phenomena of this kind are called chemisorption and chemical adsorption. "Ordinary" adsorption in the case when it is required to emphasize the nature of the interaction forces is called physical adsorption.

Physical adsorption is a reversible process, the equilibrium condition is determined by equal adsorption rates of adsorbate molecules P on vacant sites of the adsorbent surface S* and desorption - release of the adsorbate from the bound state S-P:

;

the equilibrium equation in this case:

, ,

where is the proportion of the surface area of ​​the adsorbent occupied by the adsorbate, is the Langmuir adsorption coefficient, and P is the concentration of the adsorbate.

Since and, respectively, , the adsorption equilibrium equation can be written as follows:

The Langmuir equation is a form of the adsorption isotherm equation. The adsorption isotherm equation (the abbreviated term adsorption isotherm is more often used) is understood as the dependence of the equilibrium adsorption value on the adsorbate concentration a=f(С) at a constant temperature ( T=const). The concentration of the adsorbent for the case of adsorption from a liquid is expressed, as a rule, in mole or mass fractions. Often, especially in the case of adsorption from solutions, the relative value is used: C / C s , where C is the concentration, C s is the limiting concentration (saturation concentration) of the adsorptive at a given temperature. In the case of adsorption from the gas phase, the concentration can be expressed in units of absolute pressure, or, which is especially typical for vapor adsorption, in relative units: P/P s , where P is the vapor pressure, P s is the saturated vapor pressure of this substance. The adsorption value itself can also be expressed in units of concentration (the ratio of the number of adsorbate molecules to the total number of molecules at the interface). For adsorption on solid adsorbents, especially when considering practical problems, the ratio of the mass or amount of absorbed substance to the mass of the adsorbent is used, for example, mg/g or mmol/g.

Adsorption value

Adsorption is a universal and ubiquitous phenomenon that takes place always and everywhere where there is an interface between phases. Adsorption is of the greatest practical importance. surfactants and adsorption of impurities from gas or liquids with special highly effective adsorbents. Various materials with a high specific surface can act as adsorbents: porous carbon(the most common form is Activated carbon), silica gels , zeolites as well as some other groups of natural minerals and synthetic substances.

The adsorption plant is called adsorber.

see also

Notes

Literature

  • Frolov Yu. G. Course of colloid chemistry. Surface phenomena and dispersed systems. - M.: Chemistry, 1989. - 464 p.
  • Keltsev NV Fundamentals of adsorption technology. - M.: Chemistry, 1984. - 592 p.
  • Greg S., Sing K. Adsorption, surface area, porosity. - M.: Mir, 1984. - 310 p. *
  • Adamson A. Physical chemistry of surfaces. – M.: Mir. 1979. - 568 p.
  • Oura K., Lifshits V. G., Saranin A. A. et al. Introduction to surface physics / Ed. V. I. Sergienko. - M.: Nauka, 2006. - 490 p.
  • Karnaukhov A.P. Adsorption. Texture of dispersed and porous materials. - Novosibirsk: Science. 1999. - 470 p.
  • Chemical encyclopedia. T. 1. - M.: Soviet Encyclopedia, 1990. - 623 p.
  • Poltorak O.M. Thermodynamics in physical chemistry. - M.: graduate School, 1991. - 319 p.

Links

Sorption units are used to obtain target products, separate components from gas mixtures, remove impurities from gas and liquid mixtures, dry and in other cases. Sorption - a physical and chemical process, as a result of which the absorption by any body of gases, vapors or solutes from environment. The concept of sorption includes both absorption, and adsorption.


Absorption - absorption of gas in volume, as well as selective absorption of one or more components of the gas mixture by a liquid absorbent (absorbent). Absorption of gas can occur either as a result of its dissolution in the absorbent, or as a result of its chemical interaction with the absorbent. In the first case, the process is called physical absorption, and in the second chemisorption. A combination of both process mechanisms is also possible.


Physical absorption is reversible in most cases. This property of absorption processes is based on the release of absorbed gas from a solution - desorption . The combination of absorption and desorption makes it possible to repeatedly use the absorber and isolate the absorbed component in its pure form.


Absorbents are homogeneous liquids or solutions of the active ingredient in a liquid solvent. In all cases, a number of requirements are imposed on absorbents, among which the most significant are high absorption capacity, selectivity, low vapor pressure, chemical inertness with respect to common structural materials (in case of physical absorption, also to components of gas mixtures), non-toxicity, fire and explosion safety. , availability and low cost.


From a technological point of view, the best absorbent is the one whose consumption for a given process is less, i.e., in which the solubility of the absorbed substance is higher. Therefore, absorbents are chosen mainly according to the solubility of absorbed substances in them.


The process of physical absorption of gas is accompanied by the release of heat and, consequently, an increase in the temperature of the absorbent and the gas mixture in contact with it. With a significant increase in temperature, a sharp decrease in the solubility of the gas is possible, therefore, in order to maintain the required performance of the absorber, in some cases it is necessary to resort to cooling it with internal or external cooling elements.


Two phases are involved in absorption processes - gas and liquid. The gas phase consists of a non-absorbable carrier gas and one or more absorbable components. The liquid phase is a solution of the absorbed (target) component in a liquid absorbent. During physical absorption, the carrier gas and the liquid absorbent (absorbent) are mutually inert and in relation to the passing component.


Equilibrium in absorption processes determines the state that is established with prolonged contact of the phases and depends on the composition of the phases, temperature, pressure and thermodynamic properties of the component and absorbent.


The technique uses the following circuit diagrams absorption processes: direct-flow, counter-flow, single-stage with recirculation and multi-stage with recirculation.


The direct-flow scheme of the interaction of substances in the process of absorption is shown in fig. 1, a. In this case, the gas and absorbent streams move parallel to each other, while a gas with a higher concentration of the distributed substance is brought into contact with a liquid having a lower concentration of the distributed substance, and vice versa. The countercurrent absorption scheme is shown in fig. 1b. According to this scheme, at one end of the apparatus, fresh gas and liquid are brought into contact, having large concentrations of the distributed substance, and at the opposite end, smaller ones. In schemes with recirculation, a multiple return of liquid or gas to the apparatus is provided. The circuit with liquid recirculation is shown in fig. 1, c. The gas passes through the apparatus from the bottom upwards, and the concentration of the distributed substance in it changes from Yn to Yk. The absorbing liquid is supplied to the upper part of the apparatus at a concentration of the distributed substance Yn, then it is mixed with the liquid leaving the apparatus, as a result of which its concentration rises to Xs. The working line is represented on the diagram by a straight line segment: its extreme points have coordinates Yn, Xk and Xk, Xs, respectively. The Xc value is determined from the material balance equation.



Rice. one. : a - straight-through; b - countercurrent; c - with liquid recirculation; g - with gas recirculation; e - multistage with liquid recirculation; e is the proportion of the component used for recycling


The scheme of absorption with gas recirculation is shown in fig. 1, d. The material relations here are similar to the previous ones, and the position of the working line is determined by the points Ac*(Yc, Xk) and B*(Yk, Xn). The Yc ordinate is found from the material balance equation. Single-stage circuits with recirculation can be either cocurrent or countercurrent.


Multi-stage circuits with recirculation can be co-current and counter-current, with gas and liquid recirculation. On fig. 1e shows a multi-stage countercurrent circuit with liquid recirculation in each stage. On the diagram, the working lines are plotted separately for each stage, as in the case of several separate stage apparatus. In the case under consideration, the working line consists of segments A1B1, A2B2 and A3B3.


An analysis of the described processes allows us to conclude that single-stage schemes with absorbent or gas recirculation have the following differences compared to schemes without recirculation: at the same fresh absorbent flow rate, the amount of liquid passing through the apparatus is much larger; the result of such a regime is an increase in the mass transfer coefficient and a decrease in the driving force of the process. At a certain ratio between the diffusion resistances in the liquid and gas phases, such a scheme can help reduce the dimensions of the apparatus. Obviously, liquid recirculation is appropriate if the main resistance to mass transfer is the transition of a substance from the phase interface to the liquid, and gas recirculation is when the main process resistance is the transition of the substance from the gas phase to the phase interface.


Multi-stage recirculation schemes have all the advantages of single-stage schemes and at the same time provide more driving force for the process. Therefore, variants of schemes with multi-stage recirculation are more often chosen.


It should be noted that absorption processes are characterized by the fact that, due to the low relative volatility of the absorbent, the transfer of a substance occurs predominantly in one direction - from the gas phase to the liquid phase. The transition of the absorbed substance from the gas state to the condensed (liquid) state is accompanied by a decrease in energy in it. Thus, as a result of absorption, heat is released, the amount of which is equal to the product of the amount of absorbed substance by the heat of its condensation. The associated increase in the temperature of the interacting phases, which is determined using the heat balance equation, reduces the equilibrium content of the absorbed substance in the liquid phase, i.e., worsens the separation. Therefore, if necessary, it is advisable to remove the heat of absorption.


Structurally, absorption apparatuses are performed similarly to heat exchange, distillation, evaporation and drying apparatuses. According to the principle of operation, absorption devices can be divided into surface, bubbling and spraying.

2. Adsorption. Designs, principle of operation of adsorption apparatus

Adsorption - the process of absorption of gases (vapors) or liquids by the surface of solids (adsorbents). The phenomenon of adsorption is associated with the presence of attractive forces between the molecules of the adsorbent and the absorbed substance. Compared to other mass transfer processes, adsorption is most effective in the case of a low content of extractable components in the initial mixture.


There are two main types of adsorption: physical and chemical(or chemisorption). Physical adsorption is caused by the forces of interaction of the molecules of the absorbed substance with the adsorbent (dispersion or van der Waals). However, molecules, in contact with the surface of the adsorbent, saturate its surface, which worsens the adsorption process. Chemical adsorption is characterized by chemical interaction between the medium and the adsorbent, which can form new chemical compounds on the surface of the adsorbent. Both types of adsorption are exothermic.


The transition of a substance from the gas and liquid phases to the adsorbed state is associated with the loss of one degree of freedom, i.e., it is accompanied by a decrease in the entropy and enthalpy of the system, and, consequently, by the release of heat. In this case, differential and integral heats of adsorption are distinguished; the first expresses the amount of heat released when a very small amount of substance is absorbed (2 g/100 g of the adsorbent), the second - when absorbed until the adsorbent is completely saturated. The temperature increase in each adsorption process depends on the heat of adsorption and the mass velocity of the gas (vapor) flow, on the thermal diffusivity of this flow and the adsorbent, the amount of adsorbed substance and its concentration. Since the adsorption capacity of the adsorbent decreases with increasing temperature, the exothermicity of the process must be taken into account in engineering calculations. At high heat releases, the adsorbent layer is cooled.


Adsorption processes are selective and reversible, allowing one or more components to be absorbed (adsorbed) from gas (vapor) mixtures and solutions, and then, under other conditions, to isolate (desorb) them from the solid phase. In this case, the selectivity depends on the nature of the adsorbent and adsorbed substances, and the limiting specific quantity absorbed substance also depends on its concentration in the initial mixture and temperature, and in the case of gases - also on pressure.


Adsorbents are porous bodies with a highly developed pore surface. The specific surface of pores can reach 1000 m2/g. Adsorbents are used in the form of tablets or balls with a size of 2 to 6 mm, as well as powders with a particle size of 20 to 50 microns. Activated carbon, silica gel, aluminosilicates, zeolites (molecular sieves), etc. are used as adsorbents. An important characteristic of adsorbents is their activity, which is understood as the mass of adsorbed substance per unit mass of the adsorbent under equilibrium conditions. The activity of the adsorbent is equal to:



where M is the mass of absorbed components; G is the mass of the adsorbent.


Adsorbents are also characterized protective action time, which is understood as the time during which the concentration of absorbed substances at the exit from the adsorbent layer does not change. With a longer time of operation of the adsorbent, a breakthrough of the absorbed components occurs, associated with the exhaustion of the activity of the adsorbent. In this case, regeneration or replacement of the adsorbent is necessary.


Due to the variety of adsorbents and adsorbed substances, a unified theory of adsorption has not yet been developed. The patterns of adsorption processes, in which the van der Waals forces of attraction play a decisive role, can be satisfactorily described by the so-called potential theory of adsorption. According to this theory, a polymolecular adsorption layer is formed on the surface of the adsorbent, the energy state of the molecules in which is determined by the value of the adsorption potential, which is a function of the distance from the surface, and does not depend on temperature. The adsorption potential has the greatest knowledge on the surface of the adsorbent. Potential theory is applicable to adsorption processes on adsorbents whose pore sizes are commensurate with the sizes of absorbed molecules. In such cases, not layer-by-layer, but volumetric filling of the pores occurs.


To describe the process of monomolecular adsorption, the Langmuir theory, according to which, due to uncompensated forces at the surface atom or adsorbent molecule, the adsorbed molecule is retained for some time on the surface, has received the greatest use. Adsorption occurs at special points on the surface - adsorption centers. The material flows involved in the processes of adsorption and desorption contain transportable and "inert" components. The first refers to substances that pass from one phase to another, and the second - those that do not participate in such a transfer. In the solid phase, the "inert" component is the adsorbent.


The rate of the adsorption process depends on the conditions of transport of the adsorbed substance to the surface of the adsorbent (external transfer), as well as on the transfer of the adsorbed substance inside the adsorbent grains (internal transfer). The rate of external transfer is determined by the hydrodynamic conditions of the process, while the rate of internal transfer is determined by the structure of the adsorbent and the physicochemical properties of the system.


Adsorption processes are carried out mainly in the following ways:


1) with a fixed adsorbent bed;


2) with a moving adsorbent bed;


3) with a fluidized adsorbent bed.


Principal schemes of adsorption processes are shown in fig. 2.



Rice. 2. : a - with a fixed adsorbent layer; b - with a moving adsorbent layer; c - with a fluidized bed of adsorbent


When using a granular adsorbent, schemes with a fixed (Fig. 2, a) and with a moving (Fig. 2, b) adsorbent are used. In the first case, the process is carried out periodically. First, a gas-vapor mixture G is passed through the adsorbent L and saturated with the absorbed substance; after that, the displacing agent B is passed through or the adsorbent is heated, thus carrying out desorption (regeneration of the adsorbent).


In the second case, the adsorbent L circulates in a closed system: its saturation occurs in the upper - adsorption - zone of the apparatus, and regeneration - in the lower - desorption zone. When using a pulverized adsorbent, a scheme is used (Fig. 8.2, c) with a recirculating fluidized adsorbent.