GB2117368A

GB2117368A - A process and an apparatus for the preparation of sulfuric acid

Info

Publication number: GB2117368A
Application number: GB08308081A
Authority: GB
Inventors: Peter Carl Sehestedt Schoubye
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 1982-03-25
Filing date: 1983-03-24
Publication date: 1983-10-12
Also published as: GB2117368B; SE8301537D0; DE3310779A1; GB8308081D0; CA1205614A; SE8301537L; DK136182A; JPS5918103A; DK155723C; DK155723B; FR2523948A1

Abstract

In the preparation of sulfuric acid, a 240-330 DEG C hot gas stream containing SO3 and H2SO4-vapour in a total amount of up to 10% b.v. and up to 50% b.v. of steam, the ratio steam/SO3 being at least 1:1, is passed upwards through a plurality of externally cooled, vertical, acid resistant tubes (7) in a heat exchange tower for condensing sulfuric acid. The tubes are cooled with air introduced through an inlet opening (12) in the upper part of the tower and discharged through one or more exit openings in the lower half of the tower. By regulating cooling air velocities and levels from the bottom of the tower of discharging the cooling air the cooling is regulated so as to determine the discharge temperature T4 DEG C by the formula T4>125+6 alpha + beta +0.2(T1-Td> (where alpha is the %b.v. content of SO3+H2SO4-vapour in the feed gas, beta its % b.v. content of steam, T1 its temperature in DEG C and Td its dew point for H2SO4-vapour in DEG C). Sulfuric acid is condensed on the inner tube walls as a film of liquid flowing downwards and in contact with the hot feed gas is concentrated to 93-98% H2SO4. <IMAGE>

Description

SPECIFICATION A process and an apparatus for the preparation of sulfuric acid Field of the invention The present invention relates to a process for the preparation of sulfuric acid, comprising the steps of passing, in a sulfuric acid tower constructed as a heat exchanger containing a plurality or bundle of exteriorly cooled, vertical, acid resistant tubes, a gas stream having a temperature of 2403300C and containing sulfur trioxide and sulfuric acid vapour in a total amount of up to 10% by volume and steam in an amount of up to 50% by volume, the lower limit of the steam/S03 ratio being 1 :In in an upward direction through said vertical tubes to condense sulfuric acid as a film of liquid flowing downwards on the inner tube walls.

The invention also relates to apparatus in which the process may be carried out.

Technical background From US Patent Specification No.4,348,373 (GB Patent Specification No. 2,072,642A)- there is known a process for the preparation of sulfuric acid, which includes the steps of passing, in a sulfuric acid tower, a gas stream containing sulfur trioxide and sulfuric acid vapour in a total amount of up to 10% by volume and up to 50% by volume of water vapour (i.e. steam) in countercurrent with formed liquid sulfuric acid through a concentrating zone containing filler bodies trickled with sulfuric acid and subsequently an absorption zone wherein sulfuric acid vapour is absorbed in recycled sulfuric acid on filler bodies trickled therewith.There is obtained a substantial decrease in the amount of sulfuric acid mist formed if there are maintained in the sulfuric acid tower temperature conditions such that the recycie acid is removed from the tower at a temperature T4 C, determined by the formula T4 > 1 40+6a++O.2(Ti-T) where a is the concentration in % by volume of SO3+H2SO4 vapour in the inlet gas entering the tower, p the concentration of water vapour in the same inlet gas, T1 the temperature of the same inlet gas in C and Td the dew point of the sulfuric acid vapour in the same inlet gas.The highly reduced amount of acid mist permits filters for the discharge gas placed in the stack of the tower to be dimensioned smaller than would otherwise be necessary to obtain contents of acid mist of below 5-10 mg/Nm3 in the discharge gas in these filters. Also the filters are only charged to a small degree whereby the pressure drop and hence the energy consumption in the tower are reduced significantly. Moreover, the diameter of the tower may be reduced and the filling for catching drops of acid may be replaced by a simple demister. The filler bodies may be increased in size, which also reduces the pressure drop.

However, the process described hereinabove still involves certain drawbacks.

One of these drawbacks is that the recycle acid ordinarily must be cooled with water whereby the heat generated by the cooling of the gas and the condensation of sulfuric acid in general either is lost in cooling water or can be utilized only for heating water for heating houses.

Another drawback is that the circulation system for sulfuric acid with acid coolers and pumps belonging thereto represents a big investment and may cause difficulties in operation and maintenance.

A third drawback is that the circulating sulfuric acid uppermost in the condensating zone absorbs steam from the gas phase, and in the case of large excesses of steam in the gas this involves considerable heat generation which is transferred to the discharge gas from the tower. As a result the discharge gas attains a temperature which is up to 500 higher than that of the cooler and becomes so high that comparatively expensive construction materials are required for the mist filter.

Brief description of the invention It is an object of the invention to reduce or avoid the abovementioned drawbacks and at the same time maintain a low degree of acid mist formation. This according to the invention is obtained by cooling the tubes in the sulfuric acid tower with air introduced via at least one inlet opening near the top of the tower and discharged via at least one exit opening in the lower half of the tower, the cooling being regulated by means of the flow velocity of the cooling air and the distance from the bottom of the tower at which it is discharged, so as to discharge the cooling air from the tower at a temperature T4 C, determined by the formula T4 > 1 25+6a+Pt-0.l(t,--T,) (wherein a is the concentration in % by volume of SO3+H2SO4-vapour in the feed gas stream fed to the tower, p the concentration in % by volume of steam in the same feed gas, T, the temperature of the same feed gas in C and Td the dew point of the sulfuric acid vapour in the same feed gas in OC), the sulfuric acid being thereby condensed as a film of liquid flowing downwards on the inner wall of the tubes, which liquid by contact with the hot feed gas is concentrated to a concentration of 9398% by weight of H2SO4.

In this way the heat produced is transferred to process air or to process gases at temperatures close to 2000C so that the heat can be utilized in a more valuable way, e.g. for preheating process air or process gas whereby the production of steam is increased correspondingly, or for concentrating diluted sulfuric acid in plants where the process here described is used for regenerating spent sulfuric acid.

The process according to the invention is based on the same principles as to technical background in so far as formation of acid mist is concerned, viz. that acid mist is avoided when the gas containing sulfuric acid vapour nowhere comes into contact with surfaces which are colder than 20500 below the sulfuric acid dew point of the gas.

According to a preferred embodiment of the invention the sulfuric acid condensed in the tubes is concentrated in a part of the tubes not cooled externally. Advantageously this may be achieved by extending the cooling tubes below the lower tube sheet into the lower compartment. As a result it is ensured that the lower part of the tubes is not cooled externally, which improves the process of concentrating the sulfuric acid.

The invention also relates to an apparatus for use in this preferred embodiment of the invention comprising a heat exchange tower provided with top and bottom covers, a plurality of vertical, acid resistant tubes and two horizontal tube sheets serving to define three compartments in the tower which compartments do not communicate directly with each other, the upper compartment, defined by the upper tube sheet and the top cover, communicating with the interior of the tubes and being provided with a vent opening, the middle compartment, defined by the two tube sheets, being provided in its uppermost part with at least one inlet opening for cooling gas and in its lower half with at least one discharge opening for cooling gas, and the lower compartment, being defined by the lower tube sheet and the bottom cover, communicating with the interior of the tubes and being provided with an inlet opening for feed gas, the tubes extending below the lower tube sheet into the lower compartment.

The apparatus naturally is also equipped with valves, pumps and other devices to ensure proper gas and liquid velocities and proper cooling by the cooling air, including means to direct the discharge of the cooling air at desired levels above the bottom of the tower and means to control flow directions.

Brief description of the drawings The process and the apparatus according to the invention will now be described more fully with reference to the accompanying drawings, in which Fig. 1 shows a preferred embodiment of the apparatus according to the invention, Fig. 2 shows part of a tube and the lowermost tube sheet in the apparatus shown in Fig. 1, and Fig. 3 shows a pilot plant where the experiments described in the Example were carried out.

In Fig. 1, an inlet gas containing up to 50% H2O and up to 10% H2SO4 vapour and SO3, and with a lower limit of the steam to SO3 ratio of 1:1 is fed into the tower through an acid resistant tube 1 to a compartment or chamber 2 from where the gas proceeds further upwardly through acid resistant tubes 7.

In the part of the tubes 7 positioned between tube sheets 5 and 10 and referred to as the condensation zone the gas is cooled and the sulfuric acid condenses on the inner wall of the tubes as a downwards flowing film of liquid which, after having passed the condensation zone, is concentrated to 94-98% sulfuric acid by passing a concentrating zone. In the preferred embodiment the concentrating zone is constituted by the lower part of tubes 7 which in that embodiment projects below the lower tube sheet 5 at a length a into chamber 2 wherein no exterior cooling of pipes 7 takes place. Tubes 7 when extending into chamber or compartment 2 preferably extend into it by 0.4-0.8 m.

The extension of pipes 7 into chamber 2 below the lower tube sheet 5 as mentioned is the preferred embodiment; however, it is also possible to arrange tubes 7 to end at the lower tube sheet and in another manner ensure that the lower end thereof is not cooled externally. Likewise it is possible to carry out the concentrating of the liquid film after the passage thereof through the tubes, e.g. over filler bodies placed in chamber 2.

Tubes 7 typically have an inner diameter of 25-35 mm and may in principle be made of any acid proof material which has a thermal conductivity of at least 0.5 kcal/h and which has the needful mechanical properties at the actual conditions. A preferred tube material is glass.

At tube sheet 5 gas and liquid will be in vapour pressure equilibrium with respect to H2O and H2SO4 at the dew point of the acid, which typically is between 230 and 2600 C, depending on the contents of H2O and H2SO4 in the gas.

The vapour phase hydrolysis of SO3 possibly present in the feed gas will rapidly proceed to completion in the lower part of tubes 7 so as to form H2SO4-vapour.

The cooling air is introduced through an inlet conduit or opening 12 (a plurality of such inlets may be present) at the upper part of the middle compartment defined by the two tube sheets, and passes the outer surface of tubes 7 in a manner so as to ensure an efficient cooling thereof. After this the air is discharged through one or more discharge conduits or opening 1 3 and 14 positioned in the lower half of the middle compartment of the tower and preferably situated at different levels, i.e. at different heights above tube sheet 5. An efficient cooling may for instance be ensured by the aid of horizontal guide plates 9 transverse to tubes 7 but not extending over the entire cross section of the tower.

Hereby it is possible according to the invention to have the middle compartment subdivided into sections one above the other so that the cooling air passes from the top sectionwise in a transverse flow past the tubes, and downwards. To enable reducing the cooling effect in the critical lower part of the condensation zone suitable regulation means such as valves (not shown) may be present so as to permit discharge of part of the cooling air via discharge opening 1 4 somewhat above discharge opening 13. Likewise it is possible to discharge the entire amount of cooling air via opening 14 whereby cooling of the lower part of tubes 7 in the condensating zone is avoided. However, according to the invention the major part of the cooling air is discharged via the lower opening.It has been found to be essential for the avoidance of acid mist that the temperature difference between the liquid film on the inner wall of tubes 7 and the upward flowing gas is below a certain limit which is lowest at the bottom of tubes 7 where the gas phase concentration of sulfuric acid is highest. The permissible temperature difference may be considerably higher in the top of the tower, a fact which is illustrated more fully by the experiment described in the Example.

In Fig. 2 there is shown a radial temperature profile, calculated according to known principles, through the lower part of the condensation zone. 70-80% of the total temperature difference between the cooling air and the gas inside the tubes is present over the gas film on the inner wall of the tubes because the gas velocity here nowhere should exceed 5-6 m/sec. For, it has been found that the sulfuric acid liquid film will be carried upwards by the gas stream at higher gas velocities; hereby part of the sulfuric acid will be brought upward with the liquid film and pass out of the pipe in the form of droplets in the gas stream. By experiments it has been found that this critical gas velocity is practically the same for all tube diameters from 20 to 45 mm.

According to the invention the acid tower is preferably dimensioned so as to achieve a maximum linear velocity of 5 m/sec in the part of the tubes where the liquid film of condensed sulfuric acid flows downwards in countercurrent with the gas, the velocity being highest just above the lower tube orifices where gas temperatures of 250-270 may be expected. The calculation of length and number of tubes 7 is made on the basis of known principles for calculating heat transfer numbers with and without condensation on the tube wall.

The experiment described in the Example shows that with tubes of an inner diameter of 35 mm there is obtained a substantially complete concentrating of the acid when the length a of tubes 7 below lower tube plate 5 is about 600 mm or more.

The Table summarizes the results of experiments with the condensation of sulfuric acid vapour according to the abovementioned principles in glass tubes having an inner diameter of 35 or 25 mm, using a feed gas fed to the glass tube tower containing 10% H20 and 1.0, 3.5 or 6% H2SO4 vapour and in one experiment containing 30% H20. The gas velocity in the glass tubes was 4.5-5 m/sec, calculated at 2500C. The inlet temperature of the gas conveyed to the glass tube tower in all of the experiments was 290-3000C. The exit temperature was 100-1 200 C, which is more than is ordinarily, for reasons of energy economy, desired in industrial plants. The high exit temperature is due to the fact that comparatively short glass tubes were chosen for the experiments, viz. tubes of a length of 4.5 m.This, however, is without any consequence for the investigation of the formation of acid mist because the acid mist is formed exclusively in the part of the glass tubes where the acid condenses since there practicaily cannot be condensed more acid when the gas temperature is only about 100- 1 200C or below; likewise, as mentioned hereinabove, the temperature differences may be much bigger without causing the formation of acid mist by condensing when the gas phase is saturated at lower temperatures.

It is seen from the Table that the temperature T4 of the cooling air around the tubes in the lower part of the cooling zone (above the lower tube sheet) determines whether or not substantial amounts of acid mist are present in the discharge gas. During the experiments it could be observed that the acid mist was formed just above the lower tube sheet and that apparently it passed unaffected out through the tube once it was formed. Moreover it was found that one had to operate at a higher and higher temperature T4 the more sulfuric acid vapour and steam there were present in the gas to avoid the formation of acid mist. Qualitatively this is similar to the situation described in the US Patent Specification 4,348,373 according to which the coolant instead of air was circulating sulfuric acid in direct contact with the gas in a trickled tower.The essential difference seems to be, however, that when using air as the coolant one can operate at an exit temperature of the coolant which here is about 15"C lower than when using acid in the trickled tower, which is explained more fully in the following.

On the basis of these experimental results it is possible to set up the following equation for determining the temperature T4 of the air emanating from the sulfuric acid tower, the temperature T4 being that above which one should operate in order to avoid larger amounts of acid mist in the process gas emanating from the tower: T4 > 1 25+6a++0.2(T1-Th)0C where a is the concentration in % by volume of S03+H2S04 vapour in the inlet gas to the tower, P the concentration in % by volume of steam in the same inlet gas, T, the temperature of the same inlet gas in OC and Td the dew point of the sulfuric acid vapour in the same inlet gas in OC.

Part of the explanation of the fact that the critical values ofT4 here are about 1 50C lower than for the trickled tower probably is that the critical temperature in reality is the temperature difference T6-T2 between the liquid film on the inner wall of the glass pipes and the temperature in the condensing gas phase (see Fig. 2). T2 usually is 5-1 00C above Td, calculated for the inlet gas, because a certain evaporation of acid takes place in the concentrating zone, depending on the excess temperature of the inlet gas: the equation pays regard to this by means of the part 0.2(Tl-Td). As regards T6 the equation T6=T4-AT applies; in this AT is the sum of the temperature drop over the gas film on the air side and the temperature drop through the tube wall.AT moreover is approximately the temperature difference which in known processes is calculated for the heat transfer values in the test setup, which furthermore corresponds to the heat transfer values for sulfuric acid towers according to the invention for industrial use. It may be expected that for sulfuric acid towers having better or poorer heat transfer conditions at the cooling side of the tubes one must expect marginal values ofT4 being correspondingly higher or lower, respectively, than calculated by means of the above equation. The heat transfer condition interiorly in the tubes in contradistinction hardly play any important role for the critical value ofT4 for the formation of acid mist.

Example The experiments were carried out while using the sulfuric acid tower shown in Fig. 3. Experiments were carried out with three glass tubes of length 4.5 meters, and an outer diameter of 40 mum and an inner diameter of 35 mm (40/35 tubes), or an outer diameter of 28 mm and an inner diameter of 25 mm (28/25 tubes), respectively. With three 40/35 tubes the feed gas stream was about 29 Nm3/h and with three 28/25 tubes it was about 14 Nm3/h, in- both cases corresponding to a linear velocity in the tubes of about 4.5 m/sec, calculated at 2500C. The feed gas was prepared by mixing SO2 and air, preheating and adding steam upstream of converter 33 in which about 95% of the S02 was converted in known manner to SO3 at 400-4500C over a vanadium catalyst.After cooling to T, in a heat exchanger 34 the gas was passed into the tower at 35, upwards through three glass tubes 36 which by means of an upsetting rest on a tube sheet 37, and out of the tubes through a discharge opening 38.

The temperature T2 inside one of the glass tubes was measured by the aid of a 3 mm thermowell of glass, led out at top 39 of the tower. The cooing air was introduced at the top through inlet opening 40 and moved downwards in countercurrent with the process gas. The cooling air was discharged at exit opening 41, at a temperature T4, it being necessary to discharge some of the air via another exit opening 42, situated at a greater distance above tube sheet 37 than exit opening 41, notably when high values of T4 had to be obtained. The condensed acid was discharged via an overflow tube. In order to obtain reasonably good heat transfer values on the air side the glass tubes had been wrapped with about 2 mm steel wireat one winding per 10 cm. The mantle 44 was of glass and insulated by 1 50 mm mineral wool.The concentration of acid mist was measured by filtering off all droplets of liquid from a gas of known volume by the aid of a glass fiber filter and titration of the sulfuric acid contained in the liquid filtered off. 2-4 measurements were carried out for each set of operation parameters shown in the Table. The measurements were reproducible within an uncertainty of +30%.

It was found that a tube length of 400 mm below the tube plate was insufficient to obtain maximum concentration of H2504 in the acid and that about 600 mm concentrating zone was needed to obtain an acid strength that could not be further increased substantially by a further elongation of the concentrating zone at gas velocities of about 4.8 m/sec.

Table Inner diameter of glass a Td T1 T4 Acid mist tubes % (H2SO J % H20 bC OC OC g H2SO,/Nm3 35 mm 1 10 228 290 130 5 35 mm 1 10 228 290 150 0.2 35 mm 1 10 228 290 160 0.1 35 mm 1 10 228 290 180 0.08 35 mm 1 10 228 290 200 0.08 35 mm 3.5 10 237 290 140 5 35 mm 3.5 10 237 290 160 1 35 mm 3.5 10 237 290 180 0.1 35 mm 3.5 10 237 290 200 0.1 35 mm 3.5 30 245 290 180 0.2 35 mm 6.0 10 245 300 170 5 35 mm 6.0 10 245 300 190 0.1 35 mm 6.0 10 245 300 200 0.1 35 mm 6.0 10 245 300 210 0.1 25 mm 3.5 10 237 290 130 2 25 mm 3.5 10 237 290 150 1 25 mm 3.5 10 237 290 170 0.05 25 mm 3.5 10 237 290 190 0.05

Claims

1 A process for preparing sulfuric acid which includes the steps of passing a gas stream having a temperature of 240-3300C and containing sulfur trioxide and sulfuric acid vapour in a total amount of up to 10% by volume and steam in an amount of up to 50% by volume, the lower limit of the steam to SO3 ratio being 1::1, in a sulfuric acid tower comprising a heat exchanger containing a plurality of externally cooled, vertical, acid resistant tubes, in an upward direction through the said tubes to condense sulfuric acid, the said tubes being cooled with air introduced via at least one inlet opening near the top of the tower and discharged via at least one exit opening in the lower half of the tower, the cooling being regulated by means of the flow velocity of the cooling air and the distance from the bottom of the tower at which it is discharged, so as to discharge the cooling air from the tower at a temperature T4 C, determined by the formula T4 > 1 25+6a++O.2(Ti-T) (wherein a is the concentration in % by volume of SO3+H2SO4-vapour in the feed gas stream fed to the tower, p is the concentration in % by volume of steam in the same feed gas, T, is the temperature of the same feed gas in OC and Td is the dew point of the sulfuric acid vapour in the same feed gas in OC), the sulfuric acid being thereby condensed as a film of liquid flowing downwards on the inner wall of the tubes, which liquid by contact with the hot feed gas is concentrated to a concentration of 93-98% by weight H2SO4.

2. A process as claimed in claim 1 wherein the sulfuric acid condensed in the tubes is concentrated in a part of the tubes not cooled externally.

3. A process as claimed in claim 1 or claim 2 wherein the maximum linear velocity of the gas stream in the part of the tubes where the liquid film of condensed sulfuric acid flow downwards in countercurrent with the gas is about 5 m/sec.

4. A process as claimed in any preceding claim wherein the cooling air is discharged via at least two exit openings at different distances from the bottom of the tower, the major part of the cooling air being discharged via the lowermost of these openings.

5. A process as claimed in any preceding claim wherein the tower is subdivided by horizontal guide plates into a number of sections placed one on top of the other whereby the cooling air is passed from the top sectionwise in a transverse flow past the tubes, downwards from one section to the next lower section until it is discharged.

6. A process as claimed in claim 1 substantially as herein described.

7. A process for preparing sulfuric acid substantially as herein described with reference to any of the accompanying drawings and/or the Example.

8. Sulfuric acid whenever prepared by a process as claimed in any one of claims 1 to 7.

9. An apparatus for preparing sulfuric acid comprising a heat exchange tower provided with top and bottom covers, a plurality of vertical, acid resistant tubes and two horizontal tube sheets serving to define three compartments in the tower which compartments do not communicate directly with each other, the upper compartment, defined by the upper tube sheet and the top cover, communicating with the interior of the tubes and being provided with a vent opening, the middle compartment, defined by the two tube sheets, being provided in its uppermost part with at least one inlet opening for cooling gas and in its lower half with at least one discharge opening for cooling gas, and the lower compartment, being defined by the lower tube sheet and the bottom cover, communicating with the interior of the tubes and being provided with an inlet opening for feed gas, the tubes extending below the lower tube sheet into the lower compartment.

10. An apparatus as claimed in claim 9 wherein the tubes extend into the lower compartment by 0.40.8 m.

11. An apparatus as claimed in claim 9 or claim 10 wherein the tubes are glass tubes.

12. An apparatus as claimed in claim 9 substantially as herein described.

1 3. An apparatus substantially as herein described with reference to any of the accompanying drawings and/or the Example.

14. Each and every novel method, process, product and apparatus herein disclosed.