CA1154933A - Method of producing sulphur trioxide - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of preparing sulphur trioxide residing in oxidation of sulphur dioxide, contained in a reaction mixture, in a stationary catalyst bed when the direction of the move-ment of the reaction mixture flow along the catalyst bed is reversed periodically (every 10-200 minutes) or when the tem-perature of this reaction mixture is changed in front of the catalyst bed from 20-200°C to 350-600°C during 10-200 minutes.
The proposed invention can be used in sulphur is acid production.

Description

~:~L5~33 Mh~'~HOD OF PRODUCIN~ SULP~nJR ~RIOXIDE
'rhe presen-t invention relates to processing sulphurouæ
gases and more particularly to a method of produci~g sulphur trioxide. '~he proposod inven-tio~ may f'ind applicatio~ in the manu~acture o~ sulphuric acid.
gnown in the ar-t are me-thods of producing sulphur triox-ide by oxidation of sulphur dioxide with subs~que~t produc-tion of sulph~ric acid. ~hese methods ar~ performed in co~-tact apparatus with several (most ~requently with 4 or 5) adiaba-tic beds of a catalyst and with ~uilt-in or separate hsat-excka~gers bstween the beds. ~hs ini-tial gas enters the co~tact apparatus at a te~peratura o~ 20-100C and is heated in the heat-~xcha~gers b~ the reacted gas up to the tempera-ture of the beginning of the reaction o~ the ~irst catalyst bed equal to 390-440. The most intensiva oxidation t~kes place on the ~irst bed where the d~gree of conversion reaches 0.7-0.8 and the gas leavi~g the catalyst has a tsmperature of about 600C. ~he tempsrature conditions i~ the apparatus o~
all catalyst beds is kept constantO The known methods axe used for oxidizi~g gasss with a sulphur dio~ide content of 7.5-12 vol.%, theæe gases being obtained either by burning elemental sulphur or by roasting sulphur-corL-tai~in~ or~s. (I'Spra~roch~lk sernokislotchika" publi~hed in 1971, "Khimiya", Mo~cow~ p.481).
Implementation of such know~ methods re~uires complex conta¢t apparatus. ~he known methods also make possible th~
production of sulphur trioxide ~rom waste ga~es, ~or insta~ce, in no~-ferrous metallurgy, with th~ sulphur dioxide co~-tent in -them o~ 3~5-700 vol~%9 the sur~ac~ area of hea-~-exchanger~

- 2 -`' ~

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in con-tact apparatus being considerably increased.
For oxidation of gases with tha sulphur dioxide con-te~t less than 3.5 vol~% constant heat supply is required, ~or whlch purpose mazut or natural gas is used as a -f'uel.
Gases with the sulphur dioxide content less than 2.0-2.5 vol~% are mostly not processed, since the process become~ u~-economical and the gases are discharged in-to the atmosphere thus co~tami~ating a~d poisoning the en~ironment.
Gases containing more -than 12 vol.% o~ sulphur dioxide are not proc~ssed on conventional catalysts because of high and prolonged o~erheating o~ the catalyst in the reaction zo~e (above 650C).
It is practically impossiblo to process by the known method~q, without rscourse to sp~cial m~asures, waste gases with a variable content of sulphur dioxide in non-~errous ma-tallurgy when -the content varies during short period~ of time.
It follo~s from the above that the known methods are tochnolo ~ically complicated a~d do ~ot allow the processing o~ gase~
with low~ variable, and h~igh conten~ of sulphur dioxid~ wi~hin a wide ra~g~ of concentrations bo~h constant a~d variable wi~h time.
It is an obj~ct of the in~en-tion to provid~ a m~thod of producing sulphur trioxide which will have high tec~nical and economic characteristics a~d make it possibla to process gases of having a composition either consta~t or varying wi~h time with a sulphur dioxide con~e~t of 0~6 to 15.0 vol.%.
~ his objec-t is accomplis~ed in a method of producing sul-phur trioxide by 02idatio~ of sulphur dioxido in a sta-tionary ~ 3 _ ~ 5~93;3~
ca-talyst bed; in said mo-thod, accordi~$ to the inventio~, the process is per~ormed with a periodic (every 10-200 mi~utes) reversal o~ t~e directio~ o~ the flow of the reaction mi~ture, containin~ sulphur dioxide and movi~ alon$ the catalyst bed, wi-th or a change o~ the reac-tion mixture temperature just be-f'ore e~tering the catalyst ~ed from 20-200C to 350~600C dur~
ing 10-200 minutes.
Implementatio~ o~ the process under the above-cited con-dltions allow~ f`or usi~g simple and cheap contact apparatus o~
enhanced reliability and decreasing ~heir metal capacity 10-20 times as compared with the known ones. In addition~ i-t becomes possible to process gases with a sulphur dioxide content in a wide ra~ge of co~centrations both consta~t and varying with time~ ~hus, for ins-tance, gases containing 0.6-2.5 vol.% o~
sulphur dioxide, usually dischar~ed i~-to the atmosphere, can be processe~ without additional hea-t suppl~. I-t is e~pedient to carry out such method of o~idatio~ at aDy low initial tempera-tures whe~ the sulphur dioxide contant in the .reaction mi~ture vaxies with ti~e ~rom 0.6 to 5% or ~rom 2 to 9 vol~%.
~ ere a~d herea~ter in the least the term "i~itial reac-tion mixture" is used to denote the gas at the input o~ -th~
contact apparatus and at the input to the reactio~ zo~e, incl-uding tho unreact~d sulphur dioxide; the term "reaction mixtu-re'l is used to denote gases containing both sulphur dioxide and sulphur trioxide~ residing in the regions of -the begi~ni~g and end of the chemical reac-tion; the term "uL~eact0d reaction mixture'7 is used to denote gases at the outlet from the reac-tion zone and contact apparatus and containing~ predominantly, sulphur trio:~:ide .

~ 3~
r~he method is preferably accomplished in the following way. ~n initial reaction ~ixture with tha sulphur dioxide co~tent of 1.0 or 10 vol.% is fed at 20C onto the catalyst heated up to 500C~ ~he temperature o~ the mixture rises be-oause of direc-t con-tact with the catalys-t and at 390C the reaction of oxidation begins. The part of the catalyst locat-ed at the input o~ the initial reaction mixture is cooled down to the temperature o~ the mix-ture~ thus playi~g ~he role o~ a regenerative h~a-t-exchanger. A~ a result o~ such mechanism of heat transfer alo~g the ca-talyst bed, a moving heat ~ront (reaction zona) arises whose tempera-ture proYile is determined by the ini-tial concentration of sulphur dioxide and other para-meters.
In order to keep the moving heat front o~ the reaction within the catalyst bed~ it is necessary to reverse periodical-ly the direction o~ the moveme~t o~ the initial reac-tion mixtu-re during, say 60 minutes. The cooling o~ that part of the ca-talyst bogins which is adjacent to the irlput o~ the initial raaction mixture and the hea-t fron-t reverses its directiorl.
Appropriate choice o~ a ~umber o~ technological parame-ters (linear velocity, switching-o~er time, e-tc~) elimi~ates prol-o~gad overheating of -the catalyst when gases wi-th a hi~h con-tent o~ sulphur dio~ide (up to 15 vol.%) are processed and en-sures an intonsive course o~ -the chemical reaction when pro-cessing gases with the sulphur dio~ide conterlt as low as 0~6 to 2.5 rol.~0.
The proposed method rules out -the use oY heat-exchangars or additional heat sources for heating the irli-tial reaction mixture sirlce the catalyst bed itscl~ acts as regs~erative $3~
heat-exchangers~ heating the ini.tial reaction mix~ure and coolinæ the r~acted mixture~ '~he temperature re~ime close -to a theor~tical one is established.-thus ensuring a high degree of sulphur dioxide con~ersion into trioxide in o~e catalyst bed.
r~he second varian-t of realizin~ the method is proposed which can be performed in a catalyst bed when the tam~erature of the initial reaction mixture periodically changes ~rom 20-200C to 390-600C. If the initial reac-tion mixture e~teri~g the ca-talyst bed has a high -temperature (for ins-tance~ 420C), the temperature pro~ile is ~ormed at the i~iti~l part of the catal~st ~ed where the chemical tra~sformation -takes place.
A decrease of the tempera~ture o~ the initial mixture to mini-mum values ~or instance~ 20C) coo~s down the catalyst adjac-ent to the i~puto A hea-t front is formsd in the bed which mo~-e~ to the output a~d chemical trans~ormation takes place in this frontg At -time instant when the heat ~ro~t is closs to the eutput of the reaction ~ixture from the catalyst bed, thc temperature o* the i~itial reactio~ mixture is raised agai~
up to maximum and the region of high ~emperatuxes is *ormed on the initial part of -the ca-talyst bed where chemical -trans-formation tak0s placeO r~hen the initial temperature is lowered again and in the ca-talyst bed a heat fron-t re-appears i~stcad of that which has ju~t left the bed~ Such a method of' oxidiz-i~g sulphur dioxide in the catalyst bed ~avours tho f ormation o~ the tcmperature profile clo~o, on the average, to theoreti-cal thus ~suring the degree of con~ersion of sulphur dioxide into trioxide up to 0098 in one catalys-t bed~

Principle scheme~ o~ realizing the proposed method, pre-sented on Figs 1, 2 and 3, are ~iven hereinbelow by way of il-lus-tratio~.
~ `ig. 1 shows -the scheme of oxidatio~ of the initial reaction mixture in one catalyst bed. '~he initial reaction mixture with a sulphur dioxide co~-te~t 3.5 vol.~0 is fed at 20C on the ca-talyst A pre-heated up to 500C; tha direction of the delivery is shown by solid arrows. In -this case ga-tes 2 are opened a~d valves 3 are closed Direct con~act with the catalyst ri~es -the tempsratura of -the initial reaction mixturo up to 390C~ which corresponds to -the temperature of the beg-i~ni~g o~ the reactio~. ~s a result, a heat front al ari~eæ
along the catalyst bed; in 60 mi~utss said fron-t occupios -the position a2. Ahead of the hsat ~rontS which moves in the direc-tion o-~ filtra-tion, a zone of tha catalyst appeares cooled to a temperature of the initial reaction mixture (20C) I~ 60 mi-nutes valves 2 and ~ are switch~d over simultaneously a~d the initial reaction mi~ture reverses the directio~ o~ i-ts move-me~t (dashed arrows). ~he reaction front a2 moves in an oppo-site direction a~d in 60 minutos occupies the posi-tion al.
Af-ter this a complete cycle equal to 120 minutes repeats which provides a continuous out~ou-t of the reacted reactio~ mixture ~rom the catalys-t bed.
It is seen from Fig. 1 that, upon moving the heat front "a" between positions al and a2, there are catal~st zones ahe-ad and behind the ~ront which do no-t particlpate i~ the chemi-cal roaction but play the role o~ regenerative h0at-exchangers heating the initial reaction mixture with a temperature 20C
to the temperature correspo~ding to the beginning of the reaction a~d cooli~g the r~ac-ted reaction mixture at the expense of' trans~er o~ -the reaction heat to -the catalyst cooled to 20C.
~ ig. 2 illustrates the oxidation scheme which is r~alized when the temperature o~ the reac-tion mi~ture chan~es Yrom 20-200C to 390-600C for 10-200 ~inutes. r~'he initial reaction mixture is fed into the catalyst bed i~ one direction. In the catalyst divid~d into two equal parts ~1 and A2 the heat front o~ the reaction periodically moves ~rom position al ~-o a2 and - g e sc eme a2 al a2 al a2 al etc. ~he move-ment of the heat ~ront is per~ormed by alternating switch over of -the valves 1-60 The xeacted reaction mixture is removed ~rom the catalyst bed in the direction point~d by the arrows.
For instance, the initial reaction mixture with a sulphur diox-ide content 9 vol.,0 and a temperature of 70C is delivered to ths catalyst Al pre-h~ated to sooa in the direction indicated by solid arrows. Ths heat ~ront -thus arising begins to move ~rom the position al to a2. ~he gates 1, 3 a~d 5 are ope~ed whereas 2, 4 and 6 are closed. In 100 minutes (duxation o~`
semi-cycle) th~ reaction mixtuxc with a temperature 500C pass-es into a no~-heated part A2 ~ the bad. At this instant th~
1~ 3 gates become closed, -the ga-ts 2 becomes opan~d and -the initial reaction mixture with a temperature of 70C is fed in-to the part ~2 ~ tha cataly~-t bedO When the gates 1~ 3 are completely closed and gate 2 is completely opaned~ the gates 4?
5 and 6 begin to operate simultaneously. The gate 5 bocomes closed and ga-tes 4, 6 become opened; the reacted reaction mix~
ture from -the paxt ~2 of the bed is fed in-to part Al and remov-ed from the catalyst (dashed arrows). When the hcat front passes 4~
~rom -the posi-tion al -to the position a2 a~d vice versa both in upper and low 30ne5 of the parts Al and A2, -the tempera-tu-xe in the zoncs rises -~rom 70C to m~ximum 600C and lowers from 600C -to 70C. ~he mechanism o~ the mo~emen-t o~ the heat front alo~g the catalyst bed with an alternating formation o~
hot and cold zones of the catalys-t, which play -the role of heat-exchangers, is similax to that show~ in ~'i~. 1. llhe suc-cessive switching over of the gates 1-6 ensures a continuous movement of the hea-t front accordin~ to the scheme al-a2~al-a2 and so on in o~e direction and, consequently~ a continuous removal of the reacted reaction mixture from the catalyst bed.
~ ~ig. ~ illustra-tes one more scheme of oxidation ~hich is also realized upon changing -the temperature of the mixturs from 20-200C to 390-6Q0C duri~g 10-200 minu~es~ The initial reaction mixture is fed into a catalyst bed, divided into ~wo unequal parts Al and h2, periodically in two direc-tio~s. ~he part A2 of the bed ser~es as a "primer" o~ the main bed Al;
t~e temperature of the mixture en-tering the part A2 is perio-dically changed. For instance~ an initial reactio~ mixture with a sulphur dioxide conte.nt 0.6 vol.% at a temperature 200C is ~ed on the catalyst Al and ~2 pre-heated up -to 500C
in the direction shown by the arrows ~ig. ~a)~ In each part ~1 and A2 two heat fro~ts arise (al and bl) which begi~ to move in opposite directions. The ga-te 1 is closed and the ga~e

3 is opened; the gate 2 is opened incompletely thus controll-ing a slow mov~ment of the heat ~ront bl with resp~ct to the movement ~ a1~ In 60 mi~utes the heat fronts occupy positions a2 and b2 (~ig.. 3b) after which the gates 2 and 3 become clos-ed and the gate 1 becomes opened and the initial reaction _ g _ -:
.

mix-ture is delivered i~ -the direction shown by axrows. Sulphur dio~ide contained in the initial reaction mixture oxidizes in the reaction zone b2. Since no sulphur dioxîde enters zone a2, the reaction in ~aid zone does not take place and -the heat fro~t a2 removes fxom t~e bed. I~ 10 minutes the heat ~ro~-t b2 o~c~upies the position shown in Fi~. 3a and splits into two heat ~ro~ts al and~ 1- Xn this position the gates 1, 2 and are switched over and the cycle, whose duxation is 70 mi~utes~
repeats. In front o-~ the part Al o~ the ca-talyst bed, where the mixture e~ters -the catalyst~ the temperature o~ the ca-ta-lyst is p~riodically (each 70 minutes) cha~ged ~rom 200C to 390-600C~ Successi~e switching over o~ the gates 1, 2 and 3 provides a pulse arising and dampi~g o~ the heat front al which ensures a continuous removal o~ the reacted reaction mixtuxe from the catalyst bed. The part ~2 of the catalyst periodical-ly ac-ts as a t'primer't.
xam~e 1 An initial reactio~ mixture ~ormed duri~ combustio~ of sulphur and consistin~ o~ 10~5 VO1D% 0~ sulphur dioxide, 10~5 vol.% o~ oxy~en, and 79 vol.% o~ nitro~en is fed into a contact apparatus with one adiaba~ic catalyst bed (~i~. 1).
~he composition of the cat~lyst is as ~ollows (wt~o):
v~o5 6-9 Support SiO2 -the balance~
The tempera-tur0 o~ -the i~itial reactio~ mixture is 20C; the conditional contac~ -time about 6 s~c. l'rior -to introduction o~ the initial reaction mixtl~e~ -the bed is heat~d to 500C.

In this example the direc-tion of movsment of the reactio~
mixtur~ ~low (Figo 1~ is reversed by interchanging the input and ou~put of the mixture in 4Q minutes.. I~ next ~0 minuto~
the direction of movement o~ the initial reaction mixture is re~ersed again, and ~o on. Duration o~ one cycle is 80 mi~u-tes.
An average degree o~ sulphur dioxide conversion into trioxide is 98.3% per cycle which is equivalent to operation in statio-nar~ regime of a contact apparatus wi-th ~ive adiaba-tic cata lyst beds with intermediate heat-exchangersO
xample 2 A~ initial reaction mixture formed during calcination o~
p~rite and consisting of 7.5 volO% o~ ~ulphur dioxids, 10.5 vol.% of ox~gen, a~d 82 ~Ola% of nitrogan is delivered i~to a co~tact apparatus with o~e adiabatic catalyst bsd (~ig. 1)~
~he composition o~ the catalyst is similar to tha-t described in Example 1. ~he temperature of the initial reaction mi~ture is 150G; the conditional con~act time is 5 sec. Prior to in-troduction of tlle initial reaction mi~ture~ -the bed is heated up -to 550C. ~he direction o~ movement of the reaction mixture iæ reversed each 55 minutes; a cycle duratio~ is 110 minute~.
The degree of sulphur dioxide conversion in-to trioxide i~ 98.5%
per cycle~
Example An initial reaction mixture ~ormed during combustion o~
sulphur and consisting o~ 12 vol~0 of sulphux dioxide3 11 vol.%
of oxygen, and 78 volc% o-~ nitrogen is delivered into a co~t-act apparatus ~ith one adiabatic ca-talyst bod (~ig. 1)~ ~he composi-tion o~ the catalyst is similar to that described in ~xample 1. '~he temperatuxe o~ the initial reaction mix-ture is 20C; the conditional contac-t ti.me about 6 sec~ Prior to intr oduction of -the initial reac-tion mixture, thQ catalyst bed is hea-ted ~p to 500Co Each 30 minutes the directio~ o~ the move-ment o~ the reaction mix-ture i~ reversed; cycle duration is 60 minuteq. An average degree o~ sulphur dioxide con~ersion into -trioxide is 98~1~o per cycle.

~4 The procedure is similar to that described in E~ample lo Conditional con~act time is 7 seci cycle duration 200 minutes.
A~ average conversion dagree is 98~0% per cycle.

The procedure is similar to that described in Example 1.
The co~ditional contact time is 5 sec; cycle duration 10 minu-t~so ~n average co~versio~ degree is 98.5% per cycle.
~xample 6 ~ he~fp~ocedure is similar to that described in Exampl0 1 I'he co~position o~ the initial reaction miæture is 0.6 vol.%
of sulphur dioxide~ 5 vol.% o~ oxygen, and 94.4 vol.% o~ ~irgo-ge~, said mi~ture being ~ed into a contact apparatus with one catalyst bed. Cycle duration is 60 minutes; an average co~ver~
sion degree is 99.3% per cycle.
xam;Ple ?
~ he i~itial reaction mi~ture and the catalyst are similar to those described in Example 1. '~he ca-talyst bed is di~ided into two equal parts; the mode of the catal~st action is pre-sented in description of ~i~. 2~ In this example ~h~ tempoxatu-re of the reaction mixture is changed in front of the catal~st , :

' bed and its parts hl and A2 (Fig. 2). The to-tal amoun-t o~ -the catalys-t in both parts corresponds to a conditional contact -time 8 secO The par-t ~1 ~ -the bed is pre-heated up to 500C.
'~he temperature o~ the initial reaction mix-ture at the input Q~ part Al is 20C. The reactiorl mix-tur2, after passing -thro-ugh parts Al a~d A2~ leaves the latter a~d the apparat~s at a temperature which changes smoothly from 20 to 350C during 55 minutes (semi-cycle); t~en -the gates are switched over i~
the succession ~iven in description of Fi~/ 2J I~ 3S minute~
af~er the beg~in~ing o~ the cycle the tempexature o~ the reac~
tion mixture at the input -to the upper part ~2 ~ the bad at--tains 250C, in the next 10 minu-tes it attains 280C, and in another 10 minutes - ~20C; -towards the end o~ semi-cycle ~55 minutes) the temperature attains 350C. After ~witchin~
over the gates in 55 minutes the initial reactio~ mix~ure is ~ed to the part A2 ~ t~e bed i~ the direction shown b~ dashed arrows. In next 55 minutes the heat ~ron-t ~rom the positio~ a2 passes completely into the position al and the cycle with dura-tio~ 110 minutes is repeated. An average con~ersion degree i~
98~% p~r cyclfl which is e~uivalent to the operatio~ o~ a con-tact apparatus with five successively located catalyst beds and intermediate hea-t-exchangersO

~ he i~itial data are similar to -those given in ~`xamples 7 and 1~ ~he di~:Eerence consists in that switching over o~ the ga-tes in 55 minu-tes and re-distribution o~ -the entering i~i-ti-al reaction mixture between the catalyst parts Al and A2 (see Fig. 2) are per~ormed when heat ~ronts al a~d a2 are successi-v81y placed in the middle of the parts A1 and ~2. ~he succes-sion o~ switching ov~r the ga-tes is the same as in descrip--tion o~ . 2. A semi-cycle (55 minutes) begins whe~ tha heat ~ront al occupies the position in -the middle of ~1 and the i~itial r~action mixture at a temperature o~ 20C is ~d in the direction shown by solid arrows (Fig. 2). ~he heat front passes through the upper part A2 oX the bed with the same temperaturas as those described in Example 7 and L~ 55 minu-tes it occupies the position a~ in the middle of A2. '~he gates are switched over a~d the initial raaction mixture at 20C is delivered into the upper part A2 of the bed in the direction shown by dashed arrows (Fi~. 2). In ne~t 55 minu~e~
i.eO in 110 minutes of the complste cycle9 th~ heat ~ront a2 occupies the po~ition a~ the middle o~ Al) and the cycle is repeated. During each semi-cycle the reacted reaction mix--ture is removed successively from the parts Al and A~ o~ the bed and ~rom the apparatus~ the temperature o~ said mixture bei~g smoothly ohanged from 20 to 350a.
Example,~
~ he Example is similar to Example 7. The initial reactio~
mixture consisti~g of 0.6 volO~O o~ sulphur dioxide, 15 vol~%
o~ oxygen a~d 84 vol~70 o~ nitrogen is successivel~ delivered to the parts ~1 and A2 of the bed (Fig. 2) at a temperature of 200C. Cycle duration is 75 mi~utes. From cycle to cycle ~he temperature changes smoothly ~rom 200C to 600C and lower~
down -to 200c at -the i~puts of -the mixture into the parts ~1 and A2. An average conversion degree is 99.4% per cycle~

~ he ~xample is similar to ~xample 7. The reaction mixture with a sulphur dioxide co~-te~-t var~ing i~ time ~rom 0.6 to ~L3LS~
7 vol.% and o~ygen co~tent 9-10 vol.~0 is successiv01y deliv-ered to ~he parts ~ and A2 ~ the ca-talyst bed (~ig. 2) at 100C~ Cycle duration is 80 minutes. ~n average conversion degree is 981~7% per cycle~

~ n initial reaction mixtur~ f'ormed duri~g combu~tion of ~ulPhux and co~sisting of 12 volO~O o~ sulphur dioxide, 9 volO%
of o~ygen, and 79 vol~0 o~ nitrogen is ~ed into a conbact ap-paratus with a "primer" as is shown in Fig. 3. The catalyst is pre-hea-ted to 500C ~ conditional contact time 9 sec 4 ~he temperature o~ the initial reaction mL~ture i5 ~0C. ~he ini-tial rea~-tion mixture is delivered both to the part Al ~nd Q2 (Fig. 3a~ ~or 170 mi~utes a~ter which the heat ~ronts al and bl occupy the positio~s a2 a~d b2. During nsxt 30 minute~ the initial reaction mixture is ~ed into the bed Al through a 1'primer" A2 as is shown in Fig. 3b. During this period o~
time the heat fron-t b2 occupies th0 posi-tion al and b1 where-as the heat front a2 is removed ~rom the part Al. 'F~en the heat front b2 is split, in the upper part of the bed Al and i~ the lower part o~ the bed ~2 the temperature at the mome~t of switchi~ over os the gate is 20C9 i~ next 15 mi~utes -180C 9 in 5 minutes - 350C, and i~ 30 minutes - ~00C. At this moment the gates are swi-tched over again in the same succession which is given in description o~ ~ig. 3. The ini-tial reaction mixture is ~ed a-t 20C into a layer between the parts Al and A2 ~ the ca-talyst bed~ ~he system occupies the position shown in Fi~. 3a ~hich corresponds to the cycle dura--tion 200 minutes~ A~ average conversion degree is 9BD1% per ~' '; ' ,.

~ ~ , ~lS~33 cycle which is eguival~nt to the opera-tion of a co~tact appa-ratus with five adiabatic catalyst beds and with intermediate heat exchang~rs.
xample l_ ~ he E~ample is similar to Example 11. ~he initial reac-tion mixture is delivered on the catalyst at 200C. Cycle duration is 150 mi~utes. A~ average conversion degree is 98~2~o~

~ he Example is similar to Example 11. ~he initial reac-tion mixture containing 1.5 vol.yO o~ ~ulphur dioxide, 15 vol.%
o~ oxygen, and 8~.5 vol.% o~ ~itrogen are processessed at a cycle dura~ion o~ 130 minutes~ ~n average co~versio~ degree is 99.3% per cycle.

An initial reaction mixture ~ormed upon combustion of sulphur and c~ntaining 15 vol.% of sulphur dioxide9 11 vol.~0 o~ oxygan, and 74~0 o~ nitrogen is ~ed into a co~tact apparatus with one catalyst bed. Any scheme shown in ~igs 1, 2 and 3 can be real~zed. ~he temperature o~ the initial reaction miæture at ~irst stage is 50C; conditional contact time - 5 sec~
Prior to introduction o~ the initial reaction miæture, the catalyst ~ed is pre-heated to 500C~ ~n average d~gree oX
sulphur dioxide c o~version into trioxide at this stage is 94%, cycle duration 110 minutes. Such ,a conversion degree is rela-tively low and additional oxidation o~' the ~on-oxidizod sulph-ur dioxide is required~ For this purpose, the reaction mixture obtained is delivered to the ~ir5t absorption sta~e after which the reactio~ ~ixture co~taining 0~75 vol.% o~ sulphur dioxide, 3.6 ~ol.% o~ oxygen, and ni-trogen - the balance is ~ed into the seco~d catalyst bed at 60C and conditio~al con-tact time 5 sec. Said second catalyst bed is the seco~d stage of contactingO ~'here the reactien mixture is oxidized up to a high co~version degree (99.8%) 9 the duration of the cycle being 80 minu-tes. '~he scheme of the contact apparatus may ba similar to that chosen ~or the ~irst stage or be any one of those shown in Figs 1, 2 or 3. '~he catalyst ~or the ~irst and seco~d stage is similar to that used in E~ample 1.

~ n initial reaction mixture formed upon burning ~ulphur in oxygen saturat0d air and consisting o~ 15 vo~0 of sulphur dioxide, 12 volO~O of oxygen~ and 7~ vol.% o~ nitrogen is fed to the first st~ge of con-tac-ting. '~he rest conditions are si-milar to those described in Example 14. ~fter first stage -the conversio~ degree is 94~2% and -the duration of the cycle 160 minutes. '~he reaction mix-ture a~ter the ~irst absorption sta goes to the second s~age of contacting with a sulph~r dioxide conte~t 0.886 vol~%, 4.07 vol.% of ox~gen, and ni-trogen ~ the balance. Cycl~ duration is 80 minutes; the total co~versio~
degr~e 99~87%.

The Example is similar to ~xample 1~ '~he di~ferencc con-sists in -that a sulphur dioxide content in the r~action mixtu-re changes with time at random from 0.6% to 5%, whereas an oxygen co~te~t remains almost constant and e~uals 9~0. Cycle duration is 80 minu-tes, an average conversion degree 98.6%4 "~.

' ~i4~ 3 '~he Example is similar to :Exa~ple 7. ~he di~'ference con-sists in that a sulphur diox.ide content in the i~itial reac-tion mixture changes with -time from 2 to 9 vol~O whereas an oxygen conten-t remains almost ~onstant and equals 12 vol~%~
Cycle duration is 90 minutes; an average conversion degre~
98.2%.

The Example is similar -to Example 14. The difPerence res-ides in that sulphud dioxide conten-t is 11~5%, oxygen conte~t is 9~5~0~ and nitro~en is -the balance~ ~he co~version degree of sulphur dioxide at firs-t stage of contacting is 94.6%; after second stage the conversion degree is 99.9%.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of preparing sulphur trioxide residing in oxidation of sulphur dioxide contained in the reaction mixture in a stationary catalyst bed when the direction of the movement of the reaction mixture flow along the catalyst bed is periodic-ally (each 10-200 minutes) reversed or when the temperature of said reaction mixture is changed in front of the catalyst bed from 20-200°C to 350-600°C during 10-200 minutes.

2. A method as claimed in Claim 1, wherein the process is performed with the sulphur dioxide content in the reaction mixture varying with time from 0.6 to 5 vol.%.

3. A method as claimed in Claim 1, wherein the process is performed with the content of sulphur dioxide in the reaction mixture varying with time from 2 to 9 vol.%.