DE1251286B - Process for the production of sulfur dioxide - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE Int. Cl .:

COIb

German KL: 12 i- 17/54

Number: 1251286

File number: A 42642IV a / 12 i

Filing date: March 19, 1963

Open date: October 5, 1967

The invention relates to a process for the production of sulfur dioxide by burning sulfur or sulfur-containing compounds with air and condensation of the sulfur dioxide from the Roasting gas, which is characterized in that part 5 of the sulfur dioxide is separated from the roasting gas and along with part of that for total combustion required air is fed back to the burner.

From the German patent specification 458 757 a process for the production of a sulfur dioxide is rich Gas, but not the production of liquid sulfur dioxide. In this procedure pure oxygen is used for the combustion of sulfur.

The German patent 564 058 discloses a method for the recovery of sulfur dioxide from diluted gases containing sulfur dioxide, whereby gaseous sulfur dioxide is obtained. This method shows a very complicated sequence of process steps, and even if the skilled person is aware of the The task would be to carry out this process with a process for the production of sulfur dioxide Combining the combustion of sulfur, no teaching can be found in this German patent specification, the much simpler and far more economical process according to the present Invention leads. The knowledge according to the invention that the used for the expulsion of the sulfur dioxide Air that can be reused as combustion air is supported by the prior art in in no way suggested.

Two common methods that can be used to separate the desired sulfur dioxide are the freezing process for direct condensation of the gas to the liquid or the process optional absorption in a suitable solvent in a column, followed by the solution in a second column is stripped. If the freezing process is used, one essentially requires Complete separation of the sulfur dioxide means that considerable energy is used for freezing or for compressing, or for both. For processes in which solvents are used, the solvent must generally be heated to release the sulfur dioxide, which then by condensation as a liquid (possibly after compression) by means of a condenser Freezing or water cooling is obtained. Sulfur dioxide is mostly sold in liquid form.

According to the method according to the invention, substantial savings in energy are achieved, the required method for the production of sulfur dioxide

Applicant:

Associated Chemical Companies Limited,

Stockton-on-Tees, Durham (UK)

Representative:

Dr.-Ing. Dipl.-Ing. R. Poschenrieder

and Dr.-Ing. Dipl.-Ing. E. Boettner,

Patent Attorneys, Munich 8, Lucile-Grahn-Str. 38

Named as inventor:

Clifford Alfred Vessey,

Harrogate, Yorkshire (UK)

Claimed priority:
Great Britain of March 20, 1962 (10 652),
dated May 11, 1962 (18 223)

It is necessary to obtain the sulfur dioxide from the combustion gas by increasing the amount of sulfur dioxide in the gas being condensed is increased to a value greater than that in the burner gas generated amount, namely by the incineration in connection with the recycling of sulfur dioxide, that is mixed with air. The main advantage of this process is that the total energy, that is required for freezing and compressing is less than that required for one Substantially complete recovery of the sulfur dioxide by known freezing and pressure processes to obtain. The recycling makes it possible to obtain the desired oxide more easily through condensation remove at a rate equivalent to the rate at which the sulfur burns is.

The main devices of a factory plant for carrying out the method according to the invention are a burner, a condenser, an absorption column, a stripping column and means for the Return of the sulfur dioxide.

The invention can be made according to several different methods, all of which work on the same principle, be performed.

709 650/368

According to one embodiment or invention, the method is carried out by adding some of the sulfur dioxide the non-condensed part of the sulfur dioxide is obtained from the roasting gas by condensation is absorbed in a liquid and then expelled from the liquid with air and together is fed back to the burner with this air.

The return of the non-liquefied sulfur dioxide to increase the sulfur dioxide content in the gas to be condensed is carried out as follows:

Essentially all of the uncondensed sulfur dioxide is absorbed in a solvent and thus separated from the undesired gases. The sulfur dioxide in the resulting solution is released from this by dry air. The air that carries the dioxide that comes out of the solvent Stripped is used to make all the air for the combustion of the sulfur or one To deliver part of this air. Because the sulfur dioxide, which was not condensed, with air for combustion is fed back to the burner, the burner gas contains a higher proportion of sulfur dioxide than with procedures as previously carried out.

The plant in which this process can be carried out is arranged so that the flow of the sulfur dioxide goes from the burner to the condenser, from there to an absorption column and a Stripping column and finally back to the burner through suitable devices. Will the procedure carried out at constant pressure throughout the system, they are usually substantial There are differences between the mean temperature of the absorption column and the stripping column, in that the former is cooled and the latter relatively warm. Although this temperature difference does not constitute a serious disadvantage, it may be appropriate to reduce it or to substantially reduce it to cancel. If the stripping column is operated at a lower pressure than the absorption column, the stripping can be carried out at a relatively lower temperature. So can the Absorption column can be operated at a relatively high pressure, and the pressure can be reduced when the sulfur dioxide solution is passed into the stripping column. Another advantage is that the amount of air required for stripping is also reduced. Around to obtain from the stripping column a gas mixture in which the amount of sulfur dioxide is as high as is possible, it is therefore preferable to carry out the stripping procedure at a pressure which is lower than that of the absorption process. In this case, there is an additional advantage that it it is not necessary to add heat during the stripping process.

The solvent used in absorbing and stripping can be water. However, it is preferable Tributyl phosphate or optionally chlorosulfonic acid, as described in British Patent 796 755, to use. Ethylene sulfite can also be used.

Chlorosulfonic acid is a good solvent, but technically it is difficult to use as it is easily hydrolyzed to hydrochloric acid and sulfuric acid, so that water is very strictly excluded must be and precautions must be taken to ensure that in any apparatus or there are no leaks in the system, as the hydrolysis products are highly corrosive.

Glycol sulfite (Äthylensulfit) also has good dissolution properties, but its use is quite limited because its freezing point is -17 ⁰ C while under 0 C ^c its viscosity considerably increases, so that an increased force is required for pumping.

Tri-n-butyl phosphate is a particularly good solvent for SO ₂ . It is impervious to water, freezes at -8O ⁰ C, boils at 290 ° C and has a very low volatility (its vapor pressure is at 140 ° C only lmm). These properties enable this solvent to be used over a very wide range of process conditions and, in addition, this solvent is readily available commercially.

The following table compares the solubility of SO ₂ in the solvents mentioned above.

water

Chlorosulfonic acid ...

Ethylene sulfite

Tri-n-butyl phosphate

Temperature, ° C

10 20 I 100

58.5

13.3

20.9
41.7
36.5

9.7

11.1

30th

29.5

almost insoluble

1.9
3.5
6.3

Solubilities in grams of SO ₂ per 100 g of solution at atmospheric pressure of SO _2. )

When sulfur is burned, a small amount of sulfur trioxide is produced, equivalent to up to 1% of the sulfur burned. This sulfur trioxide, which attacks solvents such as tributyl phosphate, can be removed. Essentially all of the sulfur trioxide condenses in the first stage. If only about 10% SO _{2 is} condensed at this stage, this fraction can easily be re-evaporated by some of the air used for combustion and returned to the burner as additional recycle. When this method is used, the concentration of sulfur trioxide in the burner gas reaches an equilibrium concentration so that further conversion of the sulfur to sulfur trioxide is avoided. The use of air for combustion to bring about this further cycle enables an average recovery of sulfur as liquid sulfur dioxide of more than 99%

The principles that determine the choice of conditions under which the invention according to this Procedures performed are described below.

If a burner with an efficiency of 85%, calculated on the use of oxygen, is used for the combustion of sulfur with air, the proportion of SO ₂ in the burner gases is approximately 18 percent by weight. Use of a freezing temperature of -30 ° C (a practical temperature obtainable using a conventional freezing agent, e.g. anhydrous ammonia) is _{ineffective for condensing SO 2} when operating at atmospheric pressure. if

working with a pressure of 10.5 atmospheres at absolutely the same freezing temperature, 83% SO ₂ would be condensed. However, if the air used for combustion _{is enriched with solvent stripped SO 2} so that the burner gas _{contains approximately 26 volume percent SO 2} , at the same pressure of 10.5 atmospheres, all of the SO ₂ produced in the combustion will be at one Temperature of -5 ⁰ C condenses. In order to condense a certain amount of sulfur dioxide from a burner gas with a certain composition, the condensation temperature must be adjusted in relation to the pressure. The process can be carried out over a wide range of temperatures and pressures. Since both freezing and absorbing are assisted by the use of increased pressure, it is preferable to work at superatmospheric pressures, for example in the range of 5 to 10.5 atmospheres abs. The known values of the ao saturated vapor pressure for pure liquid sulfur dioxide in equilibrium with the gas make it possible to relate the condensation temperature to the pressure when the combustion is carried out under different conditions of oxygen efficiency and the process according to the invention is used. As the temperature of condensation of the product is increased, there is a corresponding increase in the amount of sulfur dioxide that advances to the absorption and stripping system. If the system z. B. with a pressure of 10.5 atmospheres abs. is operated and 85% of the available oxygen is consumed (whereby "true" burner gas strength of approximately 18 volume percent SO _{2 is} obtained), the amount of sulfur dioxide used for the enrichment is as a fraction of the condensate removed as the end product at a temperature of, for example, 0 ° Calculate C as follows:

The number of moles of undesirable ingredients per mole of sulfur dioxide produced is

Table 1

Return ratios when using a pressure of 6.5 atmospheres abs.

(82)
(18)

= 4.56.

If the amount of uncondensed sulfur dioxide is R , then is

(R) x 10.5

45

4.56 + r

the partial pressure of sulfur dioxide in atmospheres in the gases leaving the condenser.

This partial pressure must be equal to the saturated vapor pressure of the sulfur dioxide at the condensation temperature of ⁰ ° C., ie 1.51 atmospheres. Therefore, R is 0.77 for this set of conditions, so that for every mole of sulfur dioxide formed in the burner and removed by condensation, an additional 0.77 moles will be absorbed by the condenser, stripped off, and returned to the burner. The volume concentration of SO ₂ in the gas coming out of the burner is increased from 18 to approximately 28%. Similar calculations make it possible to derive the value of R, called the recycle ratio, for any set of conditions, and values for certain pressures are tabulated in Tables 1 and 2 below.

True strength of the burner gas, 19 ° / o 18% 17% 16% Condensation Volume percentage of SO 4.21 4.49 4.81 5.18 temperature, ⁰ C 2.94 3.14 3.36 3.62 2.22 2.37 2.55 2.74 20th 1.70 1.81 1.94 2.09 15th 1.33 1.42 1.52 1.63 10 0.99 1.06 1.14 1.22 5 ¹ 0.75 0.80 0.86 0.92 0 0.59 0.63 0.67 0.72 -5 0.45 0.48 0.52 0.56 -10 -15 -20

Table 2

Return ratios when using a pressure of 10.5 atmospheres abs.

True strength of the burner gas, 19% 18% 17% 16% Condensation Volume percentage of SO 1.88 2.01 2.16 2.32 temperature, ⁰ C 1.46 1.56 1.68 1.80 1.15 1.22 1.31 1.41 20th 0.91 0.97 1.05 1.12 15th 0.72 0.77 0.82 0.88 10 0.57 0.61 0.65 0.70 5 0.44 0.47 0.50 0.54 0 0.35 0.37 0.40 0.43 -5 0.27 0.29 0.31 0.39 -10 -15 -20

The drawings (F i g. 1 to 4) show schematically different constructions of a system, which suitable for carrying out the method according to the invention.

In the following an example is given for a method using the plant shown in FIG will.

Sulfur S is burned in a burner 1, using dry air under such conditions that an oxygen efficiency of 85% when working at a pressure of 10.5 atmospheres abs. is obtained. After heat is recovered in a steam boiler 2 from waste heat, followed by further cooling and heat exchange in order to reduce the temperature of the burner gas to its dew point in relation to the sulfur dioxide it contains, the gases are passed to a freezing condenser 3 which is treated with anhydrous ammonia as Coolant has cooled to a temperature of -5 ° C. Liquid sulfur dioxide condenses as a recovered product in substantially the amount equivalent to the burned sulfur and is discharged at 13, the non-condensed gases, which contain approximately 11.7 percent by volume of sulfur dioxide, being passed into a cooled absorption column 4, namely in countercurrent to a stream of chlorosulfonic acid, which was ^{cooled to a temperature of -20 to -3O 0} C in the freezer unit 5. The end gas, which leaves the column 4 at the top and is essentially free of sulfur dioxide, is first in heat exchange

with the burner gas going to the freezing condenser and then, if desired, can be expanded by a turbine after heating to recover some of the energy required to compress the combustion air before it is at 12 in lets the atmosphere escape. The solution which exits from the bottom of absorption column 4 and 20 to 25 weight percent sulfur dioxide contains in chlorosulfonic acid is then brought to the upper portion of the heated stripping column 7, which has been preheated in a heater 6 to a temperature of 5O ⁰ C. Between 30 and 45% of the total air of the combustion process, which was first compressed to 10.5 atmospheres by a compressor 8 and dried, are after heating to 50 ° C at the bottom of the stripping column? added and passed up through the stripping column in countercurrent to the hot solution. This air, together with released sulfur dioxide, which forms 20 to 30 percent by volume of the mixture, is passed through a washing tower 9 in which traces of the chlorosulfonic acid in the mixture are removed _{by contact with liquid SO 2.} Then the gases are brought together with the main air flow at 10 and led to the sulfur burner 1. Introducing this recycled sulfur dioxide into the air supply increases the concentration of sulfur dioxide in the burner gas from 18 percent by volume to approximately 26 percent by volume. The chlorosulfonic acid, from which the sulfur dioxide was stripped, is returned to the absorption column 4 by a pump 11.

The in F i g. 1 can, as in F i g. 2 can be modified to eliminate the need to cool the absorption column and the Heating of the stripping column is avoided (the same reference numerals are used in FIG. 2, if corresponding parts of the system are designated). The solution coming from the bottom of the absorption column 22 emerges and contains 20 to 25 percent by weight of sulfur dioxide in chlorosulfonic acid, is in this case passed through an expansion valve 20 so that the pressure is reduced to substantially 1.0 atmospheres Section. is decreased. This solution is then passed to the upper part of the stripping column 23, no prior heating is necessary. Between 30 and 45% of the total air for the combustion process, which was dried first is added to the bottom of the stripping column 23 and guided in countercurrent with the warm solution. This air is then freed along with that Sulfur dioxide, which represents 20 to 30 percent by volume of the mixture, by means of compressor 21 compressed, combined with the main air flow and fed to the sulfur burner. Introducing this returned sulfur dioxide into the air supply increases the concentration of the sulfur dioxide in the burner gas from 18 to about 26 volume percent.

In these two examples, sulfur dioxide is condensed per unit in the freezer 0.62 units in the system in the circuit.

In a system that is suitable for working with the method described above, the main components are a burner, a condenser, an absorption column and a stripping column (in this sequence) and a device for conveying the gas from the stripping column to one Set point in front of the burner. In a second method of practicing the invention is the plant arranged differently so that the burner gas flows from the burner to the absorption column and the gas mixture, containing the sulfur dioxide released in the stripping column, flows directly to the condenser. It is necessary to strip the sulfur dioxide from the solvent in such a way that a gas mixture is obtained, which is richer in sulfur dioxide than the gas leaving the burner. It can be seen that with an in This plant carried out the process the sulfur dioxide is substantially completely absorbed in a solvent in the combustion product and so separated from the undesired gases, the sulfur dioxide being obtained in the Solution from this is made free by a gas in such an amount that a gas mixture is generated which is richer in sulfur dioxide than the burner gas, and this gas mixture becomes condensation subject, e.g. B. at a temperature that can be achieved by cooling water, or by direct cooling with anhydrous ammonia.

According to a further embodiment of the invention, all of the sulfur dioxide is derived from the roasting gas absorbed in a liquid, expelled again with air and then partially recovered by condensation, a non-condensed part of the sulfur dioxide being sent to the burner together with the air is fed back.

The amount of sulfur dioxide in the gas flowing from the condenser in relation to the Sulfur dioxide flowing into this is of the condensation temperature and pressure of the gas dependent in the capacitor. The amount of sulfur dioxide in the condenser flowing gases depends on the amount of air used for stripping. The uncondensed Dioxide, together with the air it carries, is possibly circulated through an expansion valve led to the burner to take all or part of the air necessary for the combustion to deliver this air.

This second method of practicing the invention makes it possible to give the capacitor a To supply a gas mixture which is richer in sulfur dioxide than is possible, λνεηη the first mentioned above Method for a similar recycle ratio is used. (The feedback ratio is again the number of moles of uncondensed gaseous sulfur dioxide per mole of liquid gaseous sulfur dioxide, which is removed as a product from the condenser.) Another benefit of carrying out the condensation after stripping is that the pressure under which the condensation is carried out, independently of the pressure at which the combustion and Absorption process works. The most economical pressures chosen for absorption can now be lower than those used in the first procedure.

Of course, the same solvent can be used for the absorption of the sulfur dioxide as in the first procedure, and in fact the same principles apply with regard to removal of the sulfur trioxide and the relative temperatures and pressures of the absorption and stripping column.

ίο

Tables 1 and 2 give values for the recirculation ratio when the entire system is operated at pressures of 6.5 and 10.5 atmospheres abs. works, whereby the true burner gas strength _{varies between 16 and 19 volume percent SO 2} and the product is condensed _{as liquid SO 2} at temperatures between 20 and -20 ° C. Under such conditions, the amount of air used for stripping does not affect the recycle ratio, provided, of course, that it does not exceed the amount required for the combustion process. When operating according to the second embodiment of the process, the recycle ratio is determined by the amount of air used for the stripping. In practice, the amount of air used for stripping must exceed the theoretical minimum amount so that the stripping column can be constructed economically. The theoretical minimum amount of stripping air is determined by the partial pressure of the sulfur dioxide above the solution at the temperature at which it enters the stripping column and the total pressure in the stripping column.

If the partial pressure is ρ and the total pressure is P , then the theoretical amount of stripping air which is required per mole of sulfur dioxide is the same

(P ~ p)

Solution at the temperature at which the solution enters the stripping column, possible the total pressure to be exceeded in the stripping column. In such cases, flash evaporation occurs Dioxydes, and such a condition is created that the partial pressure of the sulfur dioxide is above the solution (neglecting the partial pressure due to the solvent) is the total pressure in the stripping column. It is now only necessary to supply air for the to free remaining in the solution after flash evaporation.

Provided that a fraction λ 'of the dioxide is flash evaporated, that is per mole of dioxide The amount of air required for wiping, again assuming a driving force of 0.67 atmospheres, is the same

-X) ' 0.67

ρ - 0.67

The recycle ratio for various conditions of the condensation pressure, the condensation temperature and the absorption pressure was calculated and is given in Table 3 under the following assumptions:

For the purposes of practical construction, a driving force, ie a pressure difference between ρ and the actual partial pressure of the dioxide in the gas leaving the stripping column, must be taken into account. For example, if one takes a driving force of 0.67 atmospheres, the number of moles of stripping air per mole of dioxide is the same

P- (P- 0.67)

ρ - 0.67

If the pressure change between the absorption column and the stripping column is large, so is it for the partial pressure of sulfur dioxide above that

40

1. Working the stripping column at an atmosphere of absolute pressure.

2. True burner gas strength 18 volume percent SO ₂ .

3. The solvent is chlorosulfonic acid.

4. Constant driving pressure of 0.67 atmospheres at the top of the stripping column.

5. Constant driving pressure of 0.13 atmospheres at the bottom of the absorption column.

6. Condensation rate of 1.1 moles of SO ₂ per 1.0 mole of SO ₂ used product, the 0.1 mole excess being used to scrub solvent vapors from the gases exiting the top of the stripping column; this amount of SO ₂ is then fed back to the base part of the absorption column.

Tabel

r \ DöOrpHUnS " Condensation Return
relationship Volume percentage SO ₂ Volume percentage SO ₂ Condensation pressure pressure
in atmospheres temperature the in the the in the in atmospheres
Section. EDS. ⁰ C 0.69 capacitor
entering absorber
entering 4th -20 0.24 Gases Gases 4th 6.5 -20 0.62 32.5 28.2 6.5 6.5 5 0.18 37.3 22.7 6.5 8th -20 0.40 39.5 27.4 8th 8th 5 1.32 37.6 21.9 8th 8th 18th 0.10 40.7 24.8 10 11 -20 0.21 44.0 34.8 11 11 -5 1.36 42.1 20.9 11 6th 18th 0.69 44.3 22.4 11 8th 18th 0.45 40.4 35.0 14th 12th 18th 0.31 41.4 28.2 15th 12th 18th 46.3 25.4 20th 44.5 23.7

Let us now give an example of the production of SO ₂ according to this second embodiment of the method according to the invention using the plant which is shown in FIG. 3 is illustrated. Sulfur is burned in a burner 30 using dry air which is compressed by a compressor 31 under controlled conditions to provide oxygen efficiency

709 650/368

of 85% results, with a pressure of 8 atmospheres abs. is working. After heat has been recovered in a waste heat boiler 32 and subsequent further cooling and heat exchange with exhaust gas to about 20 ^° C., the gas is passed to an absorption column 33. In the absorption column 33, the gas runs in countercurrent to a chlorosulfonic acid stream, which enters at a temperature of -25 ° C. and has been cooled in a freezing unit 34, the speed being sufficient to cause the solution leaving the column 33 to form an SO ₂ have vapor pressure which is less than 0.13 atmospheres of SO ₂ partial pressure of the incoming gas mixture containing 20.1 weight percent of SO ₂ at a temperature of 30 ⁰ C. The exhaust gas leaving the upper part of the column 33 at 35 is then essentially free of SO ₂ . The solution emerging at the bottom of the absorption column 33 runs through a pressure reducing valve 36 to a stripping column 37, which is at a pressure of 1 atmosphere abs. is held. The SO ₂ -TeU pressure above the solution of SO ₂ in chlorosulfonic acid is about 1270 mm and is therefore greater than 1 atmosphere, so that flash evaporation takes place, the evaporated fraction making up about 26 percent by weight, the strength of the solution at the same time to 15.7 Percent by weight SO ₂ falls and the temperature of the solution drops to 18 ⁰ C. If the driving force for the stripping at the top of the stripping column is defined as 0.67 atmospheres, the amount of stripping gas required which enters the stripping column is the same

0.67
0.33 '

0.74 moles per mole of SO ₂ .

The mixture of air and SO ₂ , which the stripping column emits, is reduced by a compressor 38 to a pressure of 8 atmospheres abs. compressed and runs to a condenser 39, which ^{operates at a temperature of -5 0} C and uses anhydrous ammonia as a freezing agent. The SO ₂ concentration of the gases entering the condenser is 40.7 percent by volume, and they leave the condenser at 15.7 percent by volume. The gases discharged from the condenser are then returned to the gas system and enter the burner at 40 while liquid SO ₂ is removed at 44. The recycle ratio for the conditions defined in this example is 0.40, and the introduction of this recirculated gas into the burner increases the SO ₂ concentration in the burner gas to about 24.8 percent by volume and increases its partial pressure to about 1510 mm.

Chlorosulfonic acid is transferred from the stripping column 37 to the absorption column 33 by means of a pump 43 pumped.

Since the SO ₂ -air mixture leaving the top of the column 37 contains some chlorosulfonic acid, it is desirable to remove this before the mixture reaches the condenser 39. This is effected in a scrubber 41, the scrubbing liquid of which is a small fraction of the liquid SO ₂ product from the condenser 41. The liquid SO ₂ , which contains the chlorosulfonic acid removed in the scrubber, then enters the gas mixture which is fed to the absorption column 33 at 42.

According to a further embodiment of the invention, all of the sulfur dioxide is obtained from the roasting gas absorbed under high pressure in a liquid, the absorption solution in a relaxation chamber relaxed and the so released sulfur dioxide condenses and recovered, while from the relaxed Absorption liquid with air expelled the remaining sulfur dioxide and together with this Air is fed back to the burner. Through this

ίο method is the sulfur dioxide content in the gas, which is subjected to condensation increased to essentially 100%, while the im Sulfur dioxide remaining in solution is released from it by air and recycled will.

According to this method, the system is arranged so that burner gas from the burner to a Absorption column flows; Means are provided for releasing the pressure above the solution, which leaves the absorption column; a condenser is provided for this by flash evaporation to condense generated gaseous sulfur dioxide; there is also a stripping column for clearing the remaining sulfur dioxide from the solution; there are also funds for recycling the liberated sulfur dioxide to the burner.

To achieve the pressure reduction which is necessary so that the flash evaporation of the solution of SO ₃ in the solvent (again preferably chlorosulphonic acid) takes place, one can expediently use either a gas compressor which effectively _{draws SO 2} out of the solution and compresses it to such a pressure that condensation occurs at cooling water temperatures, or one freezes, for example by means of anhydrous ammonia, so that the sulfur dioxide is condensed directly under a pressure which is equal to the partial vapor pressure of the dioxide. The reduced pressure obtained in this way is suitably 1 atmosphere or less.

The most economical pressures for this third method can be much lower than what one would get in either of the first two methods above. The amount of dioxide returning to the burner is independent of the air flow rate, provided that the amount of air used does not exceed the amount required for combustion. If the relaxation evaporation amckp atmospheres abs. and the stripping column pressure P atmospheres abs. is, the theoretical minimum amount per mole of dioxide stripping gas required is the same

p ~ p

In practice, however, when all of the combustion air goes through the stripping column, the amount of stripping gas is in excess of this theoretical minimum amount.

If, for example, a burner is used with an efficiency of 85% in terms of the use of oxygen, the ratio of SO ₂ in the burner gases is about 18 percent by volume.

If the _{amount of SO 2 is} increased by providing a return of R volume of SO ₂ per volume of SO ₂ produced in the burner, then the burner has it

leaving fraction has an SO ₂ volume content of R ± 1

iöö '

R +

In this way, for any chosen absorption _{pressure, the SO 2} partial pressure in the burner gas which is fed to the absorption column is known. In the absorption column, the burner gas can flow in countercurrent to a chlorosulfonic acid stream, which enters at a temperature of, for example -25 ° C at a rate to cause the solution leaving the column to have a partial vapor pressure of SO ₂ of 0.13 atmospheres less than the SO ₂ partial pressure of the inflowing gas mixture. The relationship to the solubility data for SO ₂ in chlorosulfonic acid shows the actual temperature and composition of the solution leaving the base of the column. If this solution passes through an expansion valve so that the total pressure in the system is less than the partial vapor pressure of the SO ₂ above the solution, flash evaporation occurs and a fraction

1
1 + R

of the SO ₂ is evaporated while the rest

1 H- R

is made free from the solution.

The recycle ratio for various conditions of absorption column pressure and flash evaporation pressure is shown in Table 4 under the following assumptions:

1. Operation of the stripping column at one atmosphere of absolute pressure.

2. True burner gas strength of 18 volume percent SO ₂ .

3. The solvent is chlorosulfonic acid.

4. Constant driving pressure of 0.13 atmospheres at the bottom of the absorption column.

Table 4

Change of the recycle ratio with the absorption pressure and flash evaporation pressure. Using dry air under controlled conditions that result in an oxygen efficiency of 85%, sulfur is burned in a burner 51, whereby at a pressure of 4.08 atmospheres abs. is working. After the recovery of heat in a waste heat boiler 52, after which a cooling and heat exchanging with the exhaust gas to a temperature of about 20 ⁰ C follows, the burner gas flows to an absorption column 53. In the absorption column

ίο the gas runs in countercurrent to a chlorosulfonic acid stream, which, ^{cooled by means of a freezing unit 65 to a temperature of -25 0} C, enters at a speed which is sufficient _{to absorb substantially all of the SO 2} from the burner gas and to add a solution result, which contains _{15.5 percent by weight SO 2} at a temperature of 17.5 ° C. Exhaust gas 56 exits from the top of column 53. The solution emerging from the bottom of the absorption column 53 runs through a pressure reducing valve 54 to an expansion evaporation vessel 55, which at a pressure of 0.25 atmospheres abs. is working. Since the partial vapor pressure (770 mm) of the SO ₂ above the solution which enters the flash evaporation vessel 55 is greater than the pressure in the flash evaporation vessel, SO _{2 is} evaporated until the SO ₂ vapor pressure equals 0.25 atmospheres abs. will. The SO ₂ vapors are removed and by a compressor 57 to a pressure of 3 atmospheres abs. before being subjected to condensation by water cooling in a condenser 58. The liquid SO ₂ product is then collected at 67.

The remaining solution, which consists of 9.3 percent by weight SO ₂ in chlorosulfonic acid at a temperature of 0.5 ^° C., is then pumped by a pump 59 to the upper part of a stripping column 69, which at a pressure of 1 atmosphere abs. is working. Here it is directed in countercurrent to a stream of dry air 61, and it is in the

"Do essentially all of the remaining SO ₂ freed from the solution. The chlorosulfonic acid is then recycled through a pump 66.

The composition of the gas mixture which leaves the top of the stripping column 60 is 0.78 moles of SO ₂ and 5.55 moles of air, which corresponds to a partial pressure of _{SO 2}

0.78
0.78 + "5755

= 0.123 atmospheres abs.

Absorption pressure
Atmospheres
Section. Relaxation
evaporation pressure
Atmospheres
Section. Return
relationship 2.38 0.10 0.49 2.38 0.25 1.22 2.38 0.50 2.60 4.08 0.25 0.78 4.08 0.50 1.59 4.08 1.00 3.50 6.80 0.25 0.61 6.80 0.50 1.19 6.80 1.00 2.30

An example of this method, which is shown in a Fig. 4 is carried out, is given below:

results. The partial vapor pressure of SO ₂ above the solution entering the stripping column 60 is about 0.25 atmospheres abs. (neglecting the vapor pressure of the solvent), and hence the stripping can be easily effected.

The stripped gas mixture exiting the stripping column is then passed through a compressor 62 to a pressure of 4.08 atmospheres abs. compressed and runs over a scrubber 63 for chlorosulfonic acid in the gas mixture to the burner.

Washing is effected by means of a fraction of liquid SO ₂ product from condenser 58, the resulting solution of chlorosulfonic acid and liquid SO _{2 being} combined with the solution exiting absorption column 53 at 64.

The SO ₂ concentration in the gases which go from the stripping column 60 to the burner 51 is 12.3 percent by volume, and when leaving the

Brenner's concentration is 28.1 percent by volume. The recycle ratio for the conditions defined in this example is 0.78. The introduction of this recirculated gas into the burner gas increases the SO ₂ concentration from 18 to 28.1 percent by volume and increases its partial pressure from 872 mm.

Claims (4)

Patent claims: 1. Process for the production of sulfur dioxide by burning sulfur or Sulfur-containing compounds with air and condensation of sulfur dioxide from the roasting gas, characterized in that a Part of the sulfur dioxide separated from the roasting gas and used together with part of the Total combustion required air is fed back to the burner. 2. The method according to claim 1, characterized in that part of the sulfur dioxide the non-condensed part of the sulfur dioxide is obtained from the roasting gas by condensation is absorbed in a liquid and then expelled from the liquid with air and fed back to the burner together with this air will. 3. The method according to claim 1, characterized in that all of the sulfur dioxide the roasting gas is absorbed in a liquid, expelled again with air and then partially through Condensation is obtained, with a non-condensed part of the sulfur dioxide together with the air is fed back to the burner. 4. The method according to claim 1, characterized in that that all the sulfur dioxide from the roasting gas under high pressure in a liquid is absorbed, the absorption solution is relaxed in a relaxation chamber and that way released sulfur dioxide is condensed and recovered while from the relaxed Absorption liquid with air expelled the remaining sulfur dioxide and together with this air is fed back to the burner. Considered publications:
German patent specifications No. 458 757, 564 058. 1 sheet of drawings 709 650/368 9. 67 © Bundesdruckerei Berlin