Physico-chemical basis of the sulfur combustion process.

The combustion of S occurs with the release of a large amount of heat: 0.5S 2g + O 2g = SO 2g, ΔH = -362.43 kJ

Combustion is a complex of chemical and physical phenomena. In a combustion device one has to deal with complex fields of velocities, concentrations and temperatures that are difficult to describe mathematically.

The combustion of molten S depends on the conditions of interaction and combustion of individual droplets. The efficiency of the combustion process is determined by the time of complete combustion of each particle of sulfur. The combustion of sulfur, which occurs only in the gas phase, is preceded by the evaporation of S, mixing of its vapors with air and heating of the mixture to t, which ensures the required reaction rate. Since more intense evaporation from the surface of a drop begins only at a certain t, each drop of liquid sulfur must be heated to this t. The higher t, the more time it will take to warm up the drop. When a flammable mixture of vapor S and air of maximum concentration and t is formed above the surface of the drop, ignition occurs. The combustion process of a drop of S depends on the combustion conditions: t and the relative speed of the gas flow, and the physical and chemical properties of liquid S (for example, the presence of solid ash impurities in S), and consists of stages: 1-mixing drops of liquid S with air; 2-heating of these drops and evaporation; 3-thermal splitting of S vapors; 4-formation of the gas phase and its ignition; 5-combustion of the gas phase.

These stages occur almost simultaneously.

As a result of heating, a drop of liquid S begins to evaporate, S vapors diffuse to the combustion zone, where at high t they begin to actively react with O 2 in the air, and the process of diffusion combustion of S occurs with the formation of SO 2.

At high t, the rate of the oxidation reaction S is greater than the rate of physical processes, therefore the overall rate of the combustion process is determined by the processes of mass and heat transfer.

Molecular diffusion determines a calm, relatively slow combustion process, while turbulent diffusion accelerates it. As the droplet size decreases, the evaporation time decreases. Fine atomization of sulfur particles and their uniform distribution in the air flow increases the contact surface, facilitating heating and evaporation of particles. When burning each single drop S in the torch composition, 3 periods should be distinguished: I-incubation; II- intense combustion; III- the period of afterburning.

When a drop burns, flames emit from its surface, reminiscent of solar flares. In contrast to ordinary diffusion combustion with the emission of flames from the surface of a burning drop, it is called “explosive combustion”.

Combustion of a droplet S in the diffusion mode occurs through the evaporation of molecules from the surface of the droplet. The rate of evaporation depends on the physical properties of the liquid and t of the environment, and is determined by the characteristic of the evaporation rate. In differential mode, S lights up in periods I and III. Explosive combustion of a drop is observed only during the period of intense combustion in period II. The duration of the period of intense combustion is proportional to the cube of the initial diameter of the drop. This is due to the fact that explosive combustion is a consequence of processes occurring in the volume of the drop. Characteristics of burning rate calc. by f-le: TO= /τ сг;

d n – initial diameter of the drop, mm; τ – time of complete combustion of the drop, s.

The characteristic of the droplet burning rate is equal to the sum of the characteristics of diffusion and explosive combustion: TO= K in + K diff; Kvz= 0.78∙exp(-(1.59∙р) 2.58); K diff= 1.21∙r +0.23; K T2= K T1 ∙exp(E a /R∙(1/T 1 – 1/T 2)); K T1 – combustion rate constant at t 1 = 1073 K. K T2 – constant. heating rate at t different from t 1. E a – activation energy (7850 kJ/mol).

THAT. The main conditions for effective combustion of liquid S are: supply of the entire required amount of air to the mouth of the torch, fine and uniform spraying of liquid S, turbulence of the flow and high t.

The general dependence of the intensity of evaporation of liquid S on gas velocity and t: K 1= a∙V/(b+V); a, b are constants depending on t. V – speed gas, m/s. At higher t, the dependence of the evaporation intensity S on the gas velocity is: K 1= K o ∙ V n ;

With an increase in t from 120 to 180 o C, the evaporation intensity S increases by 5-10 times, and from 180 to 440 o C by 300-500 times.

The evaporation rate at a gas speed of 0.104 m/s is determined: = 8.745 – 2600/T (at 120-140 o C); = 7.346 –2025/T (at 140-200 o C); = 10.415 – 3480/T (at 200-440 o C).

To determine the evaporation rate S at any t from 140 to 440 o C and gas speed in the range of 0.026-0.26 m/s, it is first found for a gas speed of 0.104 m/s and recalculated to another speed: lg = lg + n ∙ lgV `` /V ` ; A comparison of the intensity of evaporation of liquid sulfur and the combustion rate suggests that the intensity of combustion cannot exceed the intensity of evaporation at the boiling point of sulfur. This confirms the correctness of the combustion mechanism, according to which sulfur burns only in the vapor state. The rate constant for the oxidation of sulfur vapor (the reaction proceeds according to a second-order equation) is determined by the kinetic equation: -dС S /d = К∙С S ∙С О2 ; С S – vapor concentration S; C O2 – concentration of O 2 vapor; K is the reaction rate constant. The total concentration of S and O 2 vapors is: With S= a(1-x); With O2= b – 2ax; a is the initial vapor concentration S; b – initial concentration of O 2 vapor; x is the oxidation state of vapor S. Then:

K∙τ= (2.3 /(b – 2a)) ∙ (log(b – ax/b(1 - x)));

Rate constant for the oxidation of S to SO 2: lgK= B – A/T;

Sulfur drops d< 100мкм сгорают в диффузионном режиме; d>100 µm in the explosion, in the area of ​​100-160 µm the burning time of the droplets does not increase.

That. To intensify the combustion process, it is advisable to spray sulfur into droplets d = 130-200 μm, which requires additional energy. When burning the same quantity, S is obtained. SO 2 is more concentrated, the smaller the volume of furnace gas and the higher its t.

1 – C O2; 2 – С SO2

The figure shows the approximate relationship between t and the concentration of SO 2 in the furnace gas formed during the adiabatic combustion of sulfur in air. In practice, highly concentrated SO 2 is obtained, limited by the fact that at t > 1300 the lining of the furnace and gas ducts quickly collapses. In addition, under these conditions, side reactions can occur between O 2 and N 2 of the air with the formation of nitrogen oxides, which is an undesirable impurity in SO 2, therefore t = 1000-1200 is usually maintained in sulfur furnaces. And furnace gases contain 12-14 vol% SO 2. From one volume of O 2 one volume of SO 2 is formed, therefore the maximum theoretical content of SO 2 in the calcining gas when burning S in air is 21%. When burning S in air, it burns. O 2 SO 2 content in a gas mixture can increase depending on the O 2 concentration. The theoretical content of SO 2 when burning S in pure O 2 can reach 100%. The possible composition of the roasting gas obtained by burning S in air and in various oxygen-nitrogen mixtures is shown in the figure:

Furnaces for burning sulfur.

The combustion of S in sulfuric acid production is carried out in furnaces in atomized or solid state. For burning molten S, nozzle, cyclone and vibration furnaces are used. The most widely used are cyclone and nozzle. These furnaces are classified according to the following criteria:- by the type of installed nozzles (mechanical, pneumatic, hydraulic) and their location in the furnace (radial, tangential); - the presence of screens inside the combustion chambers; - according to execution (horizontal, vertical); - according to the location of the inlet holes for air supply; - on devices for mixing air flows with vapors S; - on equipment for using combustion heat S; - by the number of cameras.

Nozzle furnace (rice)

1 - steel cylinder, 2 - lining. 3 - asbestos, 4 - partitions. 5 - nozzle for spraying fuel, 6 - nozzle for spraying sulfur,

7 - box for supplying air to the furnace.

It has a fairly simple design, easy to maintain, it produces gas with a constant concentration of SO 2. To serious deficiencies include: gradual destruction of partitions due to high t; low heat stress of the combustion chamber; difficulty in obtaining high concentration gas, because use up a large excess of air; dependence of the percentage of combustion on the quality of atomization S; means fuel consumption when starting and warming up the furnace; comparatively large dimensions and weight, and as a result, significant capital investment, derived areas, operating costs and large heat losses to the environment.

More perfect cyclone ovens.

1 - prechamber, 2 - air box, 3, 5 - afterburning chambers, 4. 6 - pinch rings, 7, 9 - nozzles for air supply, 8, 10 - nozzles for sulfur supply.

Access: tangential air and S input; ensures uniform combustion of S in the furnace due to better turbulization of flows; possibility of obtaining concentrated process gas up to 18 vol% SO 2; high thermal voltage of the combustion space (4.6 10 6 W/m 3); the volume of the apparatus will be reduced by 30-40 times compared to the volume of a nozzle furnace of the same productivity; constant concentration of SO 2; simple regulation of combustion percentage S and its automation; low consumption of time and combustible material for heating and starting the furnace after a long stop; lower content of nitrogen oxides after the furnace. Main weeks associated with high t in the combustion percentage; cracking of the lining and welds is possible; unsatisfactory atomization of S leads to the breakthrough of its vapors into the exchange equipment after the furnace, and consequently to corrosion of the equipment and instability of t at the entrance to the exchange equipment.

Molten S can enter the furnace through nozzles with a tangential or axial arrangement. With the axial arrangement of the nozzles, the combustion zone is closer to the periphery. With tangen - closer to the center, due to which the effect of high t on the lining is reduced. (fig) The gas flow speed is 100-120 m/s - this creates favorable conditions for mass and heat transfer, and increases the combustion rate S.

Vibrating oven (rice).

1 – burner furnace head; 2 – return valves; 3 – vibration channel.

During vibration combustion, all parameters of the process periodically change (pressure in the chamber, speed and composition of the gas mixture, t). Device for vibration combustion S is called a burner stove. Before the furnace, S and air are mixed, and they flow through check valves (2) into the head of the furnace-burner, where the mixture is burned. The supply of raw materials is carried out in portions (cyclic). In this version of the furnace, the heat stress and combustion rate will increase significantly, but before igniting the mixture, a good mixing of the sprayed S with air is necessary so that the process occurs instantly. In this case, the combustion products are well mixed, the SO 2 gas film surrounding the S particles is destroyed and facilitates the access of new portions of O 2 in the combustion zone. In such a furnace, the SO 2 formed does not remove unburned particles; its concentration is high.

A cyclone furnace, compared to a nozzle furnace, is characterized by 40-65 times greater thermal stress, the possibility of obtaining more concentrated gas and greater steam production.

The most important equipment for combustion furnaces is liquid S nozzles, which must ensure a fine and uniform spraying of liquid S, good mixing of it with air in the nozzle itself and behind it, quick adjustment of the flow rate of liquid S while maintaining the necessary its relationship with the air, the stability of a certain shape, the length of the torch, and also have a durable design, reliable and easy to use. For smooth operation of the injectors, it is important that S is well cleaned of ash and bitumen. Nozzles can be mechanical (liquid under its own pressure) or pneumatic (air also participates in the spraying).

Utilization of the heat of combustion of sulfur.

The reaction is highly exothermic, as a result, a large amount of heat is released and the gas temperature at the outlet of the furnaces is 1100-1300 0 C. For contact oxidation of SO 2, the gas temperature at the entrance to the 1st layer of the furnace should not exceed 420 - 450 0 C. Therefore, before the SO 2 oxidation stage, it is necessary to cool the gas flow and utilize excess heat. In sulfuric acid systems operating on sulfur for heat recovery, water-tube waste heat boilers with natural heat circulation are most widely used. SETA – C (25 - 24); RKS 95/4.0 – 440.

The energy-technological boiler RKS 95/4.0 – 440 is a water-tube, natural circulation, gas-tight boiler, designed to operate with pressurization. The boiler consists of evaporation devices of the 1st and 2nd stages, remote economizers of the 1st and 2nd stages, remote superheaters of the 1st and 2nd stages, a drum, and furnaces for burning sulfur. The firebox is designed to burn up to 650 tons of liquid. Sulfur per day. The furnace consists of two cyclones connected relative to each other at an angle of 110 0 and a transition chamber.

The inner casing has a diameter of 2.6 m and rests freely on supports. The outer casing has a diameter of 3 m. Air is introduced into the annular space formed by the inner and outer casings, which then enters the combustion chamber through nozzles. Sulfur is supplied to the furnace using 8 sulfur nozzles, 4 on each cyclone. Sulfur combustion occurs in a swirling gas-air flow. Flow swirl is achieved by tangentially introducing air into the combustion cyclone through air nozzles, 3 in each cyclone. The amount of air is regulated by electrically driven flaps on each air nozzle. The transition chamber is designed to direct the gas flow from horizontal cyclones into the vertical gas duct of the evaporation device. The internal surface of the firebox is lined with mulite-corundum brick, grade MKS-72, 250 mm thick.

1 – cyclones

2 - transition chamber

3 – evaporation devices

When producing roasting gas by burning sulfur, there is no need to purify it from impurities. The preparation stage will only include gas drying and acid disposal. When sulfur is burned, an irreversible exothermic reaction occurs:

S + O 2 = SO 2 (1)

with the release of a very large amount of heat: change H = -362.4 kJ/mol, or in terms of unit mass 362.4/32 = 11.325 kJ/t = 11325 kJ/kg S.

Molten liquid sulfur supplied for combustion evaporates (boils) at a temperature of 444.6 * C; the heat of evaporation is 288 kJ/kg. As can be seen from the data presented, the heat of the sulfur combustion reaction is quite sufficient to evaporate the feedstock, therefore the interaction of sulfur and oxygen occurs in the gas phase (homogeneous reaction).

Sulfur combustion in industry is carried out as follows. The sulfur is preliminarily melted (for this you can use water vapor obtained by recycling the heat of the main combustion reaction of sulfur). Since the melting point of sulfur is relatively low, by settling and subsequent filtration from sulfur it is easy to separate mechanical impurities that have not passed into the liquid phase and obtain feedstock of a sufficient degree of purity. Two types of furnaces are used to burn molten sulfur - nozzle and cyclone. They must provide for the spraying of liquid sulfur to quickly evaporate it and ensure reliable contact with air in all parts of the apparatus.

From the furnace, the roasting gas enters the waste heat boiler and then into subsequent devices.

The concentration of sulfur dioxide in the calcining gas depends on the ratio of sulfur and air supplied to combustion. If air is taken in stoichiometric quantity, i.e. for every mole of sulfur there is 1 mole of oxygen, then with complete combustion of sulfur the concentration will be equal to the volume fraction of oxygen in the air C so 2. max = 21%. However, air is usually taken in excess, since otherwise the temperature in the oven will be too high.

During adiabatic combustion of sulfur, the firing temperature for a reaction mixture of stoichiometric composition will be ~ 1500*C. In practical conditions, the possibilities of increasing the temperature in the furnace are limited by the fact that above 1300 * C the lining of the furnace and gas ducts quickly collapses. Typically, when sulfur is burned, a calcining gas containing 13–14% SO 2 is obtained.

2. Contact oxidation of so2 to so3

Contact oxidation of sulfur dioxide is a typical example of heterogeneous oxidative exothermic catalysis.

This is one of the most studied catalytic syntheses. In the USSR, the most thorough work on the study of the oxidation of SO 2 to SO 3 and the development of catalysts was carried out by G.K. Boreskov. Sulfur dioxide oxidation reaction

SO 2 + 0,5 O 2 = SO 3 (2)

characterized by a very high activation energy and therefore its practical implementation is possible only in the presence of a catalyst.

In industry, the main catalyst for SO 2 oxidation is a catalyst based on vanadium oxide V 2 O 5 (vanadium contact mass). Other compounds, primarily platinum, also exhibit catalytic activity in this reaction. However, platinum catalysts are extremely sensitive to even traces of arsenic, selenium, chlorine and other impurities and therefore were gradually replaced by the vanadium catalyst.

The reaction rate increases with increasing oxygen concentration, so the process in industry is carried out in excess.

Since the SO2 oxidation reaction is exothermic, the temperature regime for its implementation should approach the optimal temperature line. The choice of temperature regime is additionally subject to two restrictions related to the properties of the catalyst. The lower temperature limit is the ignition temperature of vanadium catalysts, which, depending on the specific type of catalyst and gas composition, is 400 - 440 * C. the upper temperature limit is 600 – 650*C and is determined by the fact that above these temperatures the structure of the catalyst undergoes a restructuring and it loses its activity.

In the range of 400 - 600*C, they strive to carry out the process so that as the degree of conversion increases, the temperature decreases.

Most often in industry, shelf contact devices with external heat exchange are used. The heat exchange scheme involves maximum use of the heat of reaction to heat the source gas and simultaneous cooling of the gas between the shelves.

One of the most important tasks facing the sulfuric acid industry is to increase the degree of conversion of sulfur dioxide and reduce its emissions into the atmosphere. This problem can be solved by several methods.

One of the most rational methods for solving this problem, widely used in the sulfuric acid industry, is the double contact and double absorption (DCDA) method. To shift the equilibrium to the right and increase the yield of the process, as well as to increase the speed of the process, the process is carried out using this method. Its essence lies in the fact that the reaction mixture, in which the degree of conversion of SO 2 is 90 - 95%, is cooled and sent to an intermediate absorber to separate SO 3. In the remaining reaction gas, the O 2:SO 2 ratio increases significantly, which leads to a shift in the reaction equilibrium to the right. The newly heated reaction gas is again fed into the contact apparatus, where 95% of the degree of conversion of the remaining SO 2 is achieved on one or two layers of catalyst. The total degree of conversion of SO 2 in this process is 99.5% - 99.8%.