Процессы массопереноса с участием твердой фазы. Учебное пособие

well continuously, released the spent adsorbent on tube 7. In case, if a given degree of purification in one device cannot be achieved, then can be sequentially installed a sequence of devices. In any adsorber takes place only the stage of adsorption, but the stage of regeneration of spent adsorbent is performed in other devices, which are not shown in the scheme.

1 - body;

2 - gas distribution grid;

3 - separator;

4 - socket for withdrawal of cleared gases;

5 - socket for supply (delivery) of initial gas mixture;

6 - pipe for fresh adsorbent supply;

7 - pipe for drainage of waste adsorbent;

8 - fluidized layer of adsorbent.

Fig. 2.9. Adsorber with a fluidized layer of adsorbent:

Continuous Adsorber with a Moving Dense Layer of Adsorbent.

In Fig. 2.10 is shown a device of continuous action with a moving thick layer of granular adsorbent. Adsorber can be described as a column with the built-in refrigerator 1, heater 7 and distributive plates 2. Granular adsorbent, which enter the unit, moves down from the top to the bottom, and the rate of this movement is regulated by the shutter - drainer 8 at the bottom.

Distributive plates of adsorber serve for a uniform distribution of adsorbent along cross-section of the device and prevent the gas phase transition from one zone to another.

Refrigerator 1 is used for cooling of heated regenerated adsorbent, which moves through the tubes, so, a cold water circulates in the annular space.

Warming of desorption zone 7 is used for heating of adsorbent during the desorption. Adsorbent moves through the pipes, and in annular

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space comes in hot water. While driving from top to bottom, the adsorbent firstly cools down to the given temperature in pipes of refrigerator 1, and then through the distributive plate comes into adsorption section 3. Here it interacts with the original gas mixture, which comes from the distributive device 4, and moves as counter-currently to the direction of the adsorbent motion, i.e. up (to the top).

1 - refrigerator;

2 – Diverse (distribution) plates;

3 - adsorption section;

4 - switchgear for the source gas mixture

5 - fitting for the source gas mixture;

6,10 - fittings for input and output of heat-carrier;

7 - heater of desorption section;

8 - adsorbent shutter – drainer;

9 - switchgear for acute water vapor;

11 - fitting for water drainage of vapor with desorption products;

12 - fitting for cleared gases output; 13, 14 - fittings for cooling water input

and output;

15 - fitting for input of acute water vapor;

16 - fitting for input of purified absorbent

17 - fitting for drainage of wet purified adsorbent.

Fig. 2.10. Adsorber with a moving dense layer of adsorbent:

Peeled off absorbing component of gases goes out through fitting 12. But adsorbent via a distributive plate 2 enters the zone of desorption, passes through heater tube 7, where it heats. The replacement substance (acute water vapor), which enters desorption zone through the switchgear 9, moves as a counter flow in respect to the absorbent. Water vapor with the ousted from adsorbent component goes out from desorption zone through the socket 11. Regenerated wet absorbent comes out of the apparatus

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through socket 17 and with the help of pneumatic transport is sent to socket 16. The adsorbent, while moving on pneumatic transport line (in diagram pneumatic transport line is not shown), becomes dry in the flow of heated air.

2.8. Calculation of Adsorbers

Periodic adsorbers. Calculation of periodically acting vertical adsorber with a stationary layer of adsorbent depends on the statement of the task: either the definition of skipping time (time of the stage of adsorption) with the given height of the apparatus, or finding of the height

the value of dummy speed and given gas flow, we can easily obtain crosssectional area of the apparatus and its diameter.

If you want to find the time of adsorption, then the height of the apparatus must be given (for standard vertical adsorbers H is in the range of 0.5 - 1.2 m), possibly, with the subsequent specification. At the initial stage of calculation, for determination of the limit value, you can use the model of ideal equilibrium adsorption and define the maximum skipping time from the relation (2.9). To find the current skipping time, you should use the expressions (2.11) and (2.12), or must solve the system of equations (2.4) - (2.7), preliminarily have calculated the volumetric coefficient of mass transfer v . This system of equations has an analytical solution for

the case of linear isotherm of equilibrium, however, after some certain corrections, it can be used and for non-linear isotherms. Otherwise, the system of equations (2.4) - (2.7 ) can be solved with the help of numerical methods.

If you want to find the height of the device in case of a given time of adsorption, then this problem can be solved in the same way, using a given skipping time and unknown height of the apparatus.

Continuous adsorbers. For calculation of continuous countercurrent adsorber, for example, depicted in Fig. 2.10, you must, at the beginning, determine the minimum flow of adsorbent G from the

equation (2.17), then choose the working consumption of the adsorbent

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G = (1,1 − 1, 5)Ga,min . The ratio G / Ga,min can be used as a parameter of optimization, because the growth of this ratio increases average moving force of the process of mass transfer, what leads to the reduction of the height of the device, but at the same time - to the increase of the apparatus diameter and the cost of the process of adsorbent regeneration.

Speed of a gas must be selected a bit smaller, than the velocity of the fluidizing process beginning. Then can be calculated cross-sectional area of the device S, volumetric ratio of mass transfer v , and height of the adsorption zone according to formula (2.21).

2.9. Ion Exchange

Ion exchange - the exchange between ions, which exist in solution, and ions in solid body (ionite). The difference between ion exchange and adsorption consists in the fact, that between liquid and solid phases occurs the mutual exchange of components, rather than their unilateral absorption, and components represent the charged particles (ions), but not neutral molecules. In addition, substitution of ions by other ions is a heterogeneous chemical reaction, which rate can limit the process of ion exchange.

Ion-exchange resins are usually used as industrial ionites. They have ions, fixed in the matrix, and counter-ions, which are able to exchange with the ions of solution. Ionites, exchanging cations (anions are fixed), are called cationites, and ionites, exchanging anions (cations are fixed), are called anionites. In addition, there are ampholytes, which act as cationites or anionites, depending on conditions.

Reactions of cation (2.22) and anion (2.23) exchange can be represented as follows:

where R is ion, fixed in the matrix, ion+ - cation of ionite (ion), capable of exchange on cation of the solution (sol) sol+ ; ion− - anion of ionite, capable of exchange on anion of the solution sol− .

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Processes of ion exchange can be used for cleaning solutions, extraction of valuable metals from them, softening and demineralization of water (in the latter case, you should skip the salt containing water through a

cationite, which exchange cations of salt on + , and anionite, which exchange anions of salts on − ).

Despite of the apparent differences, processes of ion exchange and adsorption are very similar. Ionites, as a rule, represent a porous granular particles, mass transfer in which can also be characterized by the effective diffusion coefficients. The isotherm of solution-ionite equilibrium is determined, mainly, from the law of mass action and is similar to the shown in figure 2.1. Methods of ion exchange processes and their hardware design is also similar to the process of adsorption. Therefore, for description of ion exchange process and calculation of the apparatus, can be used mathematical models and methods, considered in relation to the adsorption process.

Some difference exist in auxiliary stages of the process of ion exchange: washing of an ionite from initial solution, regeneration of an ionite with the help of a regenerative solution, and washing of an ionite from regenerative solution.

CHAPTER 3. CRYSTALLIZATION AND DISSOLUTION

3.1. General Information

Crystals represent homogeneous solid bodies of various geometrical forms, limited by flat surfaces. Specific crystals feature is a well-defined, recurring in three dimensions, location of ions, atoms or molecules, forming a crystal lattice.

Crystal lattice is determined by the shape of crystals. Depending on the nature of a substance, crystals can be simple (cube, rectangular, etc.) or have more complex form. Crystals of one and the same substance, really obtained in mass crystallization, differ from each other in appearance. However, despite this the angles between facets in all crystals of this substance remain constant.

A number of substances, usually chemically similar, which have such a crystal lattice, with crystallization from solutions are allocated together. They form a combined or mixed crystals of variable composition. Such substances are said to be isomorphic, and the phenomenon -

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isomorphism. Typical representatives of such substances are crystals KCl, KBr; KH2PO4, KH2AsO4 and NH4H2PO4, etc.

It is characteristically for some substances, that, depending on thermodynamic conditions of a process (temperature, pressure), one and the same substance forms different crystals. This phenomenon is called polymorphism, and various forms of crystals are usually denoted by α, β, etc. So ammonium nitrate (NH4NO3) has five polymorphic modifications.

Crystallization is called the process of a solid phase extraction from solutions, melts and gas in the form of crystals. In chemical technology most widely used crystallization from solutions. So we will discuss firstly crystallization from solutions. Production of a large number of crystals simultaneously in industrial scale is called mass crystallization. Emergence of the term «mass crystallization» is associated with those features, which are inherent to a mass simultaneous formation and growth of a large number of crystals. These features are: formation of a large number of centres of crystallization, crystals growth in conditions of competition, collisions, of complex concentration and temperature fields, recrystallization, etc. The purpose of crystallization is to get various substances in pure form or obtain crystals of specified size. Crystallization process is used in production of mineral fertilizers, soda, chemicals, rubbers, photo emulsions, polymers, plastics, vitamins, construction materials, explosives, etc.

Simultaneous emergence and growth of a large number of crystals subject to general laws of phase transformations and, in this sense, mass crystallization does not differ from the growth and emergence of single crystal. Crystallization process consists of the following stages: formation of supersaturated solution, emergence of embryos crystals, crystal growth, rinsing and drying of crystals. The order of priority of individual stages can be different and they can proceed sequentially one after others or simultaneously.

During the mass crystallization can be observed phenomenon of recrystallization. Recrystallization consists of the following. In low supersaturated solution or in a saturated solution takes place the disappearance of small and increase of large crystals size, in addition, there happens some change of crystals forms.

Supersaturation leads to the appearance of moving forces of crystallization process. Supersaturated are called solutions, whose concentration exceeds equilibrium concentration * (solubility) for these conditions. Moving force (saturation) is characterized by the expression:

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