PROCESSES IN THE GAS-SOLID»

Homogeneous processes leak at the micro level, ie. at the level of interaction between molecules; processes are high speed, and for an intensification of processes using temperature change, pressure, the concentration of the reacting substances and catalysts. In practice, If possible, heterogeneous process translate homogeneous. An example of such a reaction is to get the sulphur dioxide: S + O₂= SO₂.

Melt through the nozzle sulphur furnace, exothermic reaction; at a temperature of 444 ⁰With the sulfur turns into vapor State and reaction becomes gas phase. Gas-phase reactions, as a rule, proceed with the formation of by-products, and here is the important factor is the concentration of the reacting substances.

Most often from heterogeneous reactions used reaction, Leaky gas systems-solid» (G–T), as well as gas-liquid» (G–W), "liquid-liquid (W–W), "solid-solid» (T–T), "liquid-solid» (W–T). A special class of heterogeneous catalytic processes are. Heterogeneous process in contrast to the homogeneous flows on the boundary of phases, and in the presence of an inert layer or ash inside the solid phase chemical reaction will be one of the stages of the CHARACTERISTIC TOURISM INDUSTRIES.

Heterogeneous process consists of five stages:

- external diffusion of the reagent through boundary layer of gas;

- internal diffusion of the reagent through the layer of ash or inerta;

- chemical reaction;

- internal diffusion of products through the layer of ash or inerta;

- external diffusion of products through the border gas layer.

A classic example of a heterogeneous process-burning coal (figure 5.1).

Each stage flows with its speed, with the process, flowing at a minimum speed, will limit the total speed of the heterogeneous process.

The speed of the CHARACTERISTIC TOURISM INDUSTRIES is expressed by the equation:

, (5.1)

where is the coefficient of heterogeneous process speed;

A-phase contact surface;

∆ P-the driving force of the process.

1 -charcoal; 2 -Ash; 3 -boundary layer of gas;
_{, ,} -the partial pressure of oxygen;
, , -partial pressure of carbon dioxide
Figure 5.1 -Scheme of coal burning

Most slow stages, each of which can slow down the entire process, are:

- a chemical reaction;

- diffusion;

- at the same time a chemical reaction and diffusion.

In the first case, the speed of diffusion is large compared with the speed of a chemical reaction; then say, What process occurs in the kinetic field. In the second case, the velocity of chemical reaction-diffusion speed much more process flows in the diffusion field (in externally or internally diffusion). In the third case, the speed of individual stages commensurate, then say, that process is in transition (mixed) the field.

After the establishment of a limited process stage steps are being taken, providing increase of speed this stage.

Known, that when you increase the temperature on the 10 ⁰With the speed of the process increases 2-4 times, While the diffusion coefficient increases always only 5 ... 10 %, therefore, determining the speed of heterogeneous CHARACTERISTIC TOURISM INDUSTRIES depending on temperature, You can determine a sunset phase (figure 5.2).

WRA 3’ A 3 4 T 1 -the speed of diffusion; 2 -the speed of a chemical reaction; 3 -overall speed the process of theoretical; 4 -overall speed process, installed experimentally Figure 5.2 -Dependence of the speed of diffusion, chemical reactions and overall speed of process temperature

Sunset stage of heterogeneous process, you can define, changing the temperature and gas flow rate (figure 5.3).

Keane. Toward. The WPPT. The WPPT. Toward. Kinetič.

T

Figure 5.3 -Dependence of the overall speed of the process

the temperature and flow rate

Changing the temperature and speed gas flow, You can determine the impact of the external diffusion and chemical reaction speed to the speed of a heterogeneous process.

Effects of internal diffusion is explored when resizing the solid phase. When the effect of temperature and gas flow rate at the speed of the CHARACTERISTIC TOURISM INDUSTRIES do not significantly, change the radius of the particles (figure 5.4).

Figure 5.4 -Dependence of the overall speed of the process

of the radius of the particles of a solid reagent

If the 1/R<1/R₁, the precipitate is internal diffusion.

Consider a heterogeneous reaction And_(g)+In_(TV)=R_(TV) (spherical particles) for the three cases.

1. The process is limited by the external diffusion:

or (5.2)

where -duration of a heterogeneous process, with;

is the density of the substance in, kg/m³;

R₀ -radius of a solid phase with the kernel and a layer of ash, m;

-massootdači factor in the gas phase, kg/[m²·with·(kg/m³)];

With_And(g) is the concentration of the reactant and in the gas phase, kg/m³;

X_In -the degree of reaction reagent in;

-complete transformation of particles, with.

The dependence degree of reaction reagent in the attitude _n is straightforward (figure 5.5).

1 -the process of limited external diffusion;

2 -the limited internal diffusion;

3 -limited chemical reaction process

Figure 5.5 -Dependence of degree of reaction

from relationship/τ τ_n

For the intensification of this process (to reduce the value of τ) need:

- reduce the size of the solid phase (R₀ worth in the numerator);

- increase the coefficient of massootdači (use mixing and increase flow speed);

- increase the concentration_And(g) in the gas phase.

2. The process of limited internal diffusion:

or , (5.3)

where D is the diffusion coefficient of reactant and through the layer of ash, m²/with;

R is the radius of the nucleus, m.

Relative speed this process above process speed, flowing on external diffusion.

For the intensification of this process must be:

- create conditions, accelerating the diffusion, ie. enter into the system the substance, increase the fenestration inert layer or ash;

- increase the concentration_And(g) in the gas phase.

3. The process is limited by the speed of a chemical reaction:

, or

If a process is limited by the speed of a chemical reaction, the dependence of the transformation curve values of τ / τ_p similar to internal diffusion curve, the difference is small and comparable to experiment errors.

For the intensification of this process must be:

- reduce the size of the particles (R)₀;

- increase the constant chemical reaction by increasing the temperature;

- increase the concentration_And in the gas phase.

Dependencies x_In= f( _n) allow you to define the scope of the. To do this, carry out a number of experiments, build dependencies and compared with the theoretical curves.