FI69441C - FOERFARANDE FOER FRAMSTAELLNING AV SVAVELTRIOXID - Google Patents

Description

1 69441

Process for the preparation of sulfur trioxide

The invention relates to a process for the production of sulfur trioxide by a contact process using a fluidized-bed technique, in which the SO 2 gases in the single-stage fluidized bed of the contacting substance are converted into sulfur trioxide by oxygen-containing gases and the contact mass is thermostated by heat exchangers.

In the production of sulfur trioxide by catalytic oxidation of sulfur dioxide 10, solid layer contacting agents are technically used. In this case, the SO2-containing gases are brought into contact with the catalyst masses in a 2- or 3-stage manner in a known manner, whereby mainly vanadium oxide-15 contacting agents are used. In order for the reaction to proceed at the required rate, the SO2-containing gases must be preheated to temperatures above 400 ° C. Above all, the use of well-concentrated SO2-containing gases releases large amounts of energy-20 in a fraction of a second, which must be dissipated in a controlled manner in order to avoid overheating of the gases as well as the contact masses and the contact boiler. Temperatures well above 600 ° C must be avoided to prevent damage to the contact material. Since the optically calculated equilibrium temperature, depending on the SO 2 and oxygen content of the gases, is substantially lower than the above-mentioned temperature, the last contact stage has sought to operate at the lowest possible temperatures, i.e. about 420-460 ° C to achieve a high total SO 2 change. To control the temperature, the gases are cooled between the different stages in the usual way by heat exchange.

DE-A-1 929 388 already describes a process for the production of sulfur trioxide by a contact process using a vortex bed technique, in which the reaction space is cooled by blowing τρ 2 69441 miles into gases with a correspondingly lower temperature SC> 2.

The process described in U.S. Patent No. 2,595,254 operates at bed temperatures of 426-538 ° C. In this known method, changes are also achieved in the usual pre-contact area, but it is obligatory to select a grain size of 50 mm for the contact material. However, this either requires low penetrations or leads to a technically barely manageable dust problem at high temperatures and corresponding gas velocities. It has also been proposed to cool the vortex layers with a built-in heat exchanger during SO 2 catalysis.

The invention relates to a process for the production of sulfur trioxide by a contact process and intermediate absorption 15 and using a fluidized bed technique, characterized in that the height of the single-stage fluidized bed in the fluidized state is about 0.1-2 m, the average grain size of the contact mass is about 0.4-2 m, the gas is about 0.1-6 n ^ / s, the velocity after the inlet-20 flow bottom before contact with the contact granules is less than / equal to 9 π ^ / s, with SO-containing gases having a temperature of about 20-700 ° C and a sulfur dioxide content of up to 66 .7% is converted to more than 90% sulfur trioxide, , hot it is then there and recooled in each case in the heat exchanger by indirect evaporative cooling, the water vapor being kept at the same pressure in the heat exchanger by selecting the number of superheating and recooling cycles 35 so that the ratio of enthalpy differences 1.

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In particular, when using gases with a high sulfur dioxide content, the invention offers advantages, since the thermostating according to the invention can achieve an almost isothermal reaction process.

In the process according to the invention, the SO 2 -containing gases are initially largely converted, i. about 90% and more, in a fluidized bed. The sulfur dioxide content of SO2-containing gases can always be 66.7%. When treating gases resulting from the combustion of sulfur in air, the sulfur dioxide content is preferably adjusted to 10-13.5% by volume, more preferably 11.5-12.5% by volume. At concentrations above about 13% by volume, oxygen or oxygen-enriched air is used. In order to achieve more than 90% change according to the invention, the excess oxygen is selected over the stoichiometrically required amount (reaction: sulfur dioxide + oxygen = sulfur trioxide) so that it is about 1-10% by volume. Preferably, the higher the sulfur dioxide content of the gases to be treated, for example with a SO 2 content of 12% by volume to about 3% by volume, with a sulfur dioxide content of up to about 62% by volume with a sulfur dioxide content of more than 100% of the stoichiometric excess.

With sulfur dioxide-containing gases from the combustion of sulfur, the temperature is preferably adjusted to between about 170-550 ° C. If more oxygen or oxygen-enriched air is introduced into these gases, then the gases can also be introduced into the catalytic reactor at temperatures between about 0-170 ° C. The temperatures in the fluidized bed are adjusted to between about 430-530 ° C, preferably 460-500 ° C. The gas velocity in the fluidized bed should be from the minimum fluidization rate of the contact medium to twice the minimum fluidization rate. The gas velocity in the fluidized bed 35 is calculated in an empty reactor at a temperature of 20 ° C of 0.1 to 0.6 m 2 / s, preferably 0.1 to 0.35 n 2 / s. The height of the fluidized bed in the fluidized state is set to 69441 4 0.1-2 m, preferably 0.6-1.3 m. A structure is preferably chosen for the inlet flow bottom such that the velocity of the gas at the point where it contacts the contact substance does not exceed 9 m / s , preferably not more than 3-4 m / s.

The average grain size of the catalyst material used should be between about 0.4-2 mm, preferably 0.7-1.3 mm.

The preparation of an abrasion-resistant catalyst support for fluidized bed reactors suitable for SO 2 catalysis is described in DE-A-1767 754 and DE-A-1 926 564.

Substantially improved catalysts are obtained according to an unpublished proposal when they are prepared on the basis of precipitated silicic acid 15 using only so much clay minerals that the dried catalyst support contains up to 4% Al 2 O 3, preferably up to 2% Al 2 O 3 and calcination at temperatures up to 600 ° C, preferably ° C. This support is then absorbed without acid treatment with a dilute sulfuric acid solution of vanadium-20-yloxysulfate and then dried. Temperatures of 150-250 ° C are sufficient in this drying process. In any case, heat treatments of the support, as well as the finished contact material, which convert the clay minerals to the reactive form of the aluminum in the crystal space must be omitted.

It is possible to combine both said heat treatments in such a way that the carrier is merely dried before impregnation and then curing takes place on the finished contact material at temperatures below 600 ° C, preferably 200-500 ° C.

Catalyst masses are obtained which are characterized by both high abrasion resistance and the same high activity in long-term use.

The carrier mass can be prepared both by the method according to DE-pa-35 1 926 564 and also by other known granulation methods, such as 5 69441, for example using a granulation disc. Doped silica fillers are used which have a specific surface area of 40-80 m / g according to BET and which preferably contain about 5-10% by weight of calcium oxide. The amount of filler-5 is dimensioned so that the carrier, based on the anhydrous granulate, contains about 20-50% by weight, preferably 35-50% by weight of filler. This filler is premixed with an aqueous, stable silica sol having a specific surface area of 150-450 m / g BET, whereby gelling takes place in a known manner by adding small amounts of highly active magnesium oxide. It is preferred to use hydrated magnesium oxide in amounts of about 0.1 to 3% by weight. When suspending the filler in a silicon sol or granulating, about 10-15% by weight of clay minerals are added. Suitable clay minerals are, for example, kaolinite, montmorillonite, attapulgite, bentonite and / or kaolin.

The granules are dried and then either cured at temperatures up to 600 ° C, preferably 200-20,500 ° C, or, after drying, are passed directly to lactation.

The impregnation takes place with a sulfuric acid solution of vanadyloxysulphate, which preferably contains at most 6% by weight of free sulfuric acid, more preferably at most 3% by weight. The impregnation solution contains vanadyloxysulphate with an oxidation state of vanadium of + IV, 3-30% by weight, preferably 12-23% by weight, and potassium bisulphate 15-40, preferably 20-30% by weight, all values based on the total weight of the solution. These solutions can be strongly supersaturated so that they can be placed on the carrier both cold and warm.

The concentrations of vanadium and potassium in the acid solution must be chosen so that the molar ratio of potassium to vanadium in the dried catalyst is 6 69441

3: 1-2: 1 and the vanadium content, calculated as vanadium pentoxide, is 4-9, preferably 4-6% by weight.

Vanadyl salts, preferably sulfates, are used as tetravalent vanadium salts. For a preferred preparation of the impregnation solution according to the invention, the necessary amount of water is taken in a single step, to which the calculated amount of vanadium pentoxy-10 dia is then suspended with the amount of acid required for reduction and bisulphate formation and the solution is then reduced with sulfur dioxide or sulfur dioxide gas until all vanadium pentoxide is dissolved as vanadyl sulfate (VOSO 2). Potassium sulphate is then introduced into the acidic solution at a temperature of up to 40 ° C, preferably 60-80 ° C. After the formation of the bisulfate is complete, only a small excess of sulfuric acid remains. This solution can be used immediately to absorb the carrier.

The impregnation preferably takes place in a fluidized bed. The material is fluidized at a sufficient fluidization rate and then sprayed with an impregnation solution. This is preferably followed by treatment with warm air (150-300 ° C) until the substance no longer releases moisture.

In particular, when the support was cured at temperatures up to 600 ° C, preferably at about 400 ° C, prior to the impregnation process, it can be used immediately in a fluidized bed to oxidize SC 3 to SO 4 without reductive pretreatment. It has a substantially higher activity than other fluidized bed catalysts at normal operating temperatures of about 380-500 ° C and still has high abrasion resistance and unchanged activity even after several years of use.

35 If curing does not take place before impregnation, 7 69441 it can also occur in connection with impregnation.

With the new catalyst material, for example, at an operating temperature of the fluidized bed of about 470 ° C and with a gas containing 12% by volume of sulfur dioxide, more than 93% conversion of sulfur dioxide to sulfur trioxide was achieved in one fluidized bed stage.

The vortex layer tulovirtauspohjän must be constructed so that one part is achieved by uniform distribution of the gas over the entire bottom 10 on one side there is little energy loss due to pressure drop through the bottom. It has been found that for fluidized beds with a diameter of less than 6 m, a sufficient pressure drop of 0.2 bar, for a diameter of 8 m, a pressure drop of 0.3 bar was required. Even distribution of the gas over the fluidized bed base is achieved, for example, by several nozzle bores in the fluidized bed base.

The decisive advantage of using a vortex contact is that the catalytic reaction can take place at a certain, carefully selected temperature and temperature uniformities and overheating due to the heat of reaction released can be avoided with corresponding heat exchanger interiors.

The contact mass is thermostated via a piping system with water vapor and / or air or gases containing SO 2 - or SO 2 --25. In the event that the amount of heat released during the reaction is greater than that required to heat the incoming gases to the desired fluidized bed temperature, heat must be dissipated through the piping system in the fluidized bed. In this case, it is preferable to distribute the pipe system evenly in the fluidized bed and to dissipate the heat through water vapor and / or air or a SO 2 -containing gas. Preferably, the gases flow through the piping system from the bottom upwards, i. in the direction of the reactor gas, so that, in particular at the top of the fluidized bed, no temperature gradients of more than 8 69441 100 ° C, preferably more than 50 ° C, are formed between the piping system and the eddy current contact material. If it is necessary to conduct large amounts of heat out of the fluidized bed, for example in the reaction of gases hotter than the operating temperature of the fluidized bed. When operating with pure oxygen or oxygen-enriched air, an evaporation-pipe system in a fluidized bed is used in the lower part of the bed according to the invention. The heat dissipated from the fluidized bed, e.g. by means of superheated air or preferably superheated water vapor 10, can then achieve such a high temperature level that in the second heat exchanger the heat of overheating is recovered as useful heat. If the fluidized bed plant is operated at highly fluctuating gas concentrations, as is the case with roasting plants, splitting plants and exhaust gas recovery plants, then the fluidized bed must be thermostated by a piping system. In this case, the flow-through pipes of the fluidized bed must not be too long so that the residence time of the hot gas 20 in the pipes until the nearest cooling, e.g. to the evaporation drum, does not affect the temperature constant of the fluidized bed. If the heat of reaction released by gas mixtures with a low SC 2 concentration and arriving at a low inlet temperature is not sufficient to maintain the desired fluidized bed temperature, then the incoming gas must be heated to the fluidized bed temperature. Such thermostating is easily possible with a built-in piping system.

According to another proposal, which has not been published so far, the thermostating can be carried out as follows: is cooled by indirect evaporative cooling, wherein the heat feeder keeps the water vapor at the same pressure so that the superheat and aftercooling sets are selected such that the ratio of the sum of the enthalpy separations of the aftercooling stages to the saturation steam and the introduced feedwater is at least Ental.

The proposed system consists of superheated heating surfaces located in the reaction space or 10 product paths and indirectly loaded constant pressure heat exchangers located outside the reaction space or products, as shown in Figure 3. In this figure: 15 1 reaction gas inlet 6 sensor 7 temperature control valve 8 superheated stages in vortex bed 9 recooling stages in heat exchanger 10 supply water supply to heat exchanger 25 11 level control valve for supply water 12 pressure control valve 13 excess steam removal 14 mixer 15 temperature control valve 30 16 bypass line 18

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The operation is as follows: Impregnation steam is taken from the heat feeder 4 via line 5, controlled by temperature sensor 6 and valve 7, and superheated several times, at least 4 times, preferably 6-12 times, especially 5, preferably 8-10 times, in superheated stages 2 8 and after each such stage is recooled together with one of the recooling stages 9 located in the heat feeder 4.

The balance between the heat taken from the feeder 4 in the form of steam to cool the vortex-10 bed and the heat returned indirectly via the cooling stages 9 to the feeder 4 is when the sum of the enthalpy separations of the recooled steam 9 5 between the steam taken in.

This balance is adjusted according to the invention independently of the given or selected vapor states in the superheat and recooling series.

However, in the superheating and recooling series, it is also possible to adjust the desired other ratio, which is greater than 1. With this operation according to the invention, more heat is always produced in the heat feeder 4 than is taken from it for cooling purposes.

The higher the ratio of the sum of the enthalpy separations of the recooled steam to the enthalpy separation between the steam and the feed water in the recooling stages, the more flexibly the system responds to fluctuating, dissipated amounts of heat.

30 Excess steam that is not needed for cooling is discharged through line 13 under the pressure control of valve 12.

In order to obtain the useful steam removed via line 20 at the desired degree of superheating, a portion 11,694 41 of the steam used to cool the vortex layer is superheated in the last superheat stage 8 by connecting lines 16 and 17 in mixer 18 and .

The vapor states present in the path described above are illustrated in Figure 3b.

In this figure, mean: tw feed water temperature 10 ts saturation steam temperature tii superheat temperature tk vortex bed temperature tD temperature after the last superheat stage tA delivered useful steam temperature 15 i enthalpy at each temperature.

The figure also clearly shows that by selecting the number of superheating and recooling stages, the temperature range of the steam used for cooling can be selected arbitrarily.

In particular, with a small enthalpy separation in the stages and a corresponding increase in the number of stages, far-reaching thermostating of the reaction process is possible.

The method according to the invention is explained in more detail below by means of examples.

Example 1 (cf. Figure 1)

2 3,000 Nm- * of sulfur fuel gas (gas produced by combustion of sulfur in air) with 12.3% by volume of 30 SO2 and a temperature of 330 ° C flowed into the inlet flow chamber of the fluidized bed 1. In the fluidized bed with a diameter of 6.4 m, the calculated empty speed was set to 20 Ncm / s. The height of the fluidized bed in the resting state was 100 cm, in the fluidizing state about 115 cm. The fluidized bed was thermostated to 475 ° C with horizontally placed, evenly spaced tubes through which steam flowed. The steam entered i2 6 94 41 into several pipe coils from below at a temperature of about 200 ° C and exited from above at a temperature of about 420 ° C. The heat of superheating of the steam was transferred to the steam drum by means of a hot steam condenser and thus a useful heat of steam of about 3.2 t in 5 hours at a pressure of 6 bar was obtained. SO 2 was converted to 92% in the fluidized bed. The gas leaving the reactor was cooled in a known manner in the cross-gas heat exchanger 2 and passed to the intermediate absorption 4. After reheating, the remaining 10 SO 2 in the cross-gas heat exchanger 2 was changed in post-contact so that the total change was more than 99.6-99.8%. The plant produced 310 t of 98% sulfuric acid per day.

Example 2 (cf. Figure 2)

In the air liquefaction plant 8, pure 15 oxygen was obtained, which was used in the furnace 9 to burn sulfur.

3

After several additions of sulfur, 3000 Nm of gas containing about 60% by volume of SO2 and about 39% by volume of SO2 were taken from the furnace. This gas was introduced into the inlet flow chamber of the fluidized bed 12 after being cooled to about 200 ° C with ECO 11. Three heat exchangers were superimposed on a 2.5 m diameter Lei layer, water was evaporated at 32 bar in the lower 13, heat was dissipated by superheating the steam in the middle 14 and upper 15 heat exchangers. The height of the fluidized bed 25 was about 150 cm. The temperature was adjusted to about 450 ° C in the upper third, in the range of both lower heat exchangers the temperatures were 480-350 ° C. After cooling the gas, after 90-97% of the SO 2 had been converted, 60-65% of oleum 18, then 29-35% of oleum 19 and finally 98% of sulfuric acid were produced in superheater 16 and 30 in ECO 17 in 3-stage absorption. . Most of the gas leaving the intermediate sorption was again mixed via the booster fan 21 directly to the charge of the sulfur combustion furnace, 35 a smaller partial flow 22 was fed to the air liquefaction plant 8.

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The plant produced 150-160 tons of SO ^ za per day. More energy was obtained than was needed in the air liquefaction plant connected to the plant.

Claims (5)

A process for producing sulfur trioxide by a contacting method using a fluidized bed technique, wherein the SC 2 gases in a single-stage fluidized bed of 5 contact materials are converted to sulfur trioxide by oxygen-containing gases and the contact mass is thermostated with heat exchangers, characterized in that the single-stage fluidized bed -2 m, the average grain size of the contact-10 mass is about 0.4-2 mm, the gas velocity in the empty reactor space is about 0.1-0.6 ni ^ / s, the velocity after the inlet flow bottom before contact with the contact granules is lower / equal than 9 n 2 / s, where more than 90% of SO 2 -containing gases with a temperature of about 20-700 ° C and a sulfur dioxide content of up to 66.7% are converted to sulfur trioxide, the SO 2 -containing reaction gases are fed to the intermediate absorption and the sulfur trioxide-free gases are further converted in single or double-stage contact and for thermostating, water vapor is introduced into the reaction space in a forced circulation 20 in several cycles; equilibrium heat exchanger filled with boiling water separated from the reaction space, superheated there and then recooled in the heat exchanger by indirect evaporative cooling. with respect to the enthalpy separation between the impregnation steam and the imported feed water is at least 1.1. A method according to claim 1, characterized in that the height of the fluidized bed in the fluidized state is about 0.6-1.3 m. Method according to Claim 1 or 2, characterized in that the average grain size of the contact mass used is approximately 0.7 to 1.3 mm. 15 69441 Process according to one of Claims 1 to 3, characterized in that the gas velocity in the empty reactor space is about 0.15 to 0.35 n 2 / s. Method according to one of Claims 1 to 4, characterized in that the velocity of the gas after the inlet flow base before contact with the contact granules is less than or equal to 3 mN / s. !! 69441