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A BRIEF REVIEW OF A, PORTION OF THE LITERATUREDEALING WITH SULPHUR BURNING AND THE FORMATION

AND DISSOCIATION OF SULPHUR TRIOXIDER. j.HOGARTH and C. T. JENKIN

, '

In this paper an attempt will be made to sumup the facts and figures contained in a host ofpapers and books already published on the subjectof sulphur burning. For the sake of clarity thetwo sections :--

I. Theory.II. Practice.

have been chosen to enable the reader to be in abetter position to differentiate between theoreticalhypotheses and conjectures and actual observationsrecorded from plant practice. In the theoreticalsection, especially, all calculations have been madewith a view to simulating those conditions pre­vailing in the sugar industry in this country.

It sho.l.1ld be explained that the sequence ofoperations.vwhich may be generally referred to assulpliur burning, has' been dealt with mainly fromthe point of view of standard sulphite pulp andpaper mill practice. This angle of approach waschosen because of the quantity and quality ofliterature that is readily available from this source.However, this should not cloud the issue at allfrom the sugar technologist's point of view as,basically, the problem of the production of sulphurdioxide is the same in both cases.' Like the sugarchemist, the pulp and paper technician is concernedwith the production of a burner gas which is richin SO 2 and free from all but the smallest possibleamounts of SO 3, under the operating conditionsprevailing. Only after the sulphur dioxide isproduced does the application differ for the twoindustries. We shall therefore concern ourselvesonly with those operations that are common toeither industry.

The source of information. has been indicatedwhen a particular work is referred to, and, wherepossible, the more modern methods of plant practiceor theory have been chosen, although in some casespersonal preference or dislikes may have predom­inated and influenced the choice of certain facts andtheories.

PART I. THEORY.

When 'sulphur is heated hi the presence, ofexcessair it will, under favourable conditions, ignite andcontinue burning to form, in the main," sulphurdioxide.

If the reaction:-

S + O 2-+ S02is considered, it will be seen that one moleculeof sulphur and one of oxygen combine to form amolecule of sulphur dioxide. However, it is notnecessary to use pure oxygen for the purposes ofcombustion: air will meet the requirement equallywell, and, furthermore, its use is more economical.Basically, air may be considered to consist of21 per cent. oxygen and 79 per' cent, nitrogen byvolume. Thus, when one volume ~foxygeri.· isused to produce one volume of SO 2' there will be

~~ or 3,76 volumes of nitrogen involved. The

final burner gas would then consist of:-

1 .00 volume SO 2

3.76 volumes N 2

4:. 76 total volume of burner gas.

H is obvious, theri, that the theoretical maximumsulphur dioxide content of the burner gas would be

4.\6 X 100 = 21 per cent. by volume. This

calculation assumes that dry air is used and noS03 is formed during the actual burning of thesulphur. .

Continuing, it is apparent from the above equationthat in order to completely oxidise lib, of sulphur,it is necessary to have 1 lb. of oxygen present when2 lbs. of SO 2 will be formed. This amount ofoxygen is present in 53.4 cubic feet of air at S.T.P.Practical considerations, however, demand that theair supplied to the burneris in excess of the theoreticalminimum in order to ensure that all the sulphuris completely burned off in the space of time allowed.Because of this, the maximum S02 content of aburner gas in practice would be less than thetheoretical maximum. In fact, with the con­ventional rotary burner the maximum SO 2' contentis approximately 18 per cent. .

Quantity' of Burner Gas.

Having made these preliminary calculations,the next point of interest is the variation of thequantity of burner gas produced when the SO 2content is raised from a minimum to a maximum value.

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On top of this it would be of assistance to knowhow moisture in 'the atmosphere would effect thesevalues.

Table I and II, Graphs A and B, supply theanswers here" and all calculations were madeaccording to the methods outlined in Lundberg'sbook."

Where moist air was assumed in the theory inthe calculations, this was taken as air 74 per centsaturated with water vapour at 66. 6°F. Thisrepresents roughly the average humidity andtemperature taken over a number of years at MountEdgecombe during the months of May to December.'

"'Graphs A and B show how the volume of theburrier gas: produced decreases with increasingSO 2 concentration. A comparison of the resultswithdry and moist air indicates that the effect ofmoisture is '. not very marked upon the final gascoming from the burner.

... ~

. Theoretical Flame Temperature.

.: When sulphur is burned in air, the reaction isexothermic and therefore the products are at a highertemperature than that of the reactants.' For everyparticular concentration of SO 2 in a gas, there willbe a corresponding flame temperature to be reachedin the burner.

U~fq~tunately, the deduction' of these flame.temperatures ,from basic principles is a fairly longand' tedious procedure: A method suggested byLundberg" was usedhere and it is reasonably rapidand convenient, although comparison with flametemperatures calculated from basic thermodynamicprinciples indicates that this rapid method may beslightly inaccurate and especially so at the lower­concentrations of, SO2' It must be stressed thatthe, temperatures listed in Tables III and IV are'approximate only! and should be taken more as anindication of the order of magnitude of temperatureand not as accurate flame temperatures.. Anyonewishing to derive theoretical flame temperatures;may.do so by consulting the technical literatured.e~.1irig with this. subject 5 8 _9 10

'.. Once again, results have been caleulated fordry and 'moist air, and in order to show how thetemperature of the flame may vary due to heatlosses by radiation, the two cases:

(a) No Heat Losses

, (b) Radiation of 15 per cent. of the total heat inputhave been considered. ' /

, .

• ~ T~e':Tables and Graphs show how dilution Of theburner.gas with air causes the flametemperature to

-drop.' ;;Moi;;tiire 'again appears to have a negligible·effect.' -;

Su~phur T~ioxide .Formation.

Most of the points of theoretical importance havebeen listed, and all that remains to completejhepicture is a consideration of the factors influencingthe formation of SO 3' It is known in practice thata small amount of this compound is always formedalong with the SO2 in the burner, and thereforea knowledge of 'the conditions which favour SO3

formation should be useful in that it would thenbe possible to arrange an atmosphere and conditionsin the burner which would, keep the amount of SO 3 '

formed at an. absolute minimum.

When SO3 is formed in the presence of excessair, the following reaction occurs: . .

250 2 + 02¢ 2S0 3

This reaction is reversihle and therefore it willproceed from left to right until equilibrium isreached. At this point, as much SO 3 as is beingformed will be dissociating once more into' itscomponents., The equilibrium can only be dis­turbed by removing one of the reactants or productof the. reaction, .or changing the steady state ofconditions prevailing, otherwise, if temperature,concentration, etc. are kept steady, then 'theequilibrium will be stable and constant for thosesets of conditions. .

Fortunately, this reaction has been studied. insome detail by many observers,and it is possibleto calculate the amount of SO 3 that will be formedunder almost any given set of 'conditions: Thesubject of catalysis and the effects of catalysts haveno direct bearing on the theory of SO 3 formationas it is conceded that catalysts do not disturb theequilibrium' point, but merely serve to speed upthe rate Of reaction, thereby ensuring that equili­brium will be reached in a shorter space of time. Theyalso allow lower temperatures to be used when S03is being formed. Catalysts are therefore of im­portance in' the Contact Process for theproductioriof sulphuric acid: " .

The theoretical conversion percentages whichhave been calculated here do not take into accountthe time factor, that is to say; no allowance hasbeen made forthe period that is necessary to ensurethat the reaction reaches equilibrium before thegases pass from the burning apparatus. Oftenit will be found in plant practice that the conversionof SO 2 to SO 3 as measured, is considerably lessthan that predicted in theory under the conditionsspecified. Thus, the conversion figures listed maybe taken as the maximum possible, which mightnever be attained in the plant. .' " ....' . ,

In orderto be in a position to predictthe amountof conversion of SO 2 to SO3 under a given set of

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conditions, it is necessary to know the equilibriumconstant, K, for the reaction:

S02 + i 0 2¢ S03

which occurs at various temperatures. This re­action, and not the previous one having double thequantities, has been chosen purely for convenience,The equilibrium constant, K, differs for the tworeactions, the former being the square of the latter,but the calculated conversions would' be the samein each case.

For the purpose of this paper, the equilibriumconstant has been calculated from the average ofthree authorities, viz.

Fairlie 4-

T 476010glOI<..· = T - 4.474

Lewis and Randall--

493610glOK = T - 4.665

5186.510glOK = ~ + O.611log10T - 6.7497

where K is the equilibrium constant and T is theabsolute temperature in degrees Kelvin.

The above equations showthat K is dependentupon temperature only. Naturally, the equili­brium constant can be calculated from thermo­dynamic first principles along these lines:

K ["S03J· h . th tivit. ["S02J ["OJt' were a IS re ac IVI y co-

efficient for each respective reactant or product,and this equation may be re-written as

K () (PSO 3) h . h . I~ (PSG 2) (PO 2)f were p IS t e partia pressure

of each substance as. indicated. However, thecalculation of K from first principles is somewhatlengthy, but should this expedient be necessaryor desired, reference should be made to textbookson thermodynamics." B 9 10

Knowing K, it is then possible to deduce theexpected conversion of SO 2 to SO'3 tinder variousconditions. However, the conversions calculatedin this paper were carried out on the lines suggestedby Browning andKress.? . The equilibrium constant,

. K, was calculated from an equahonwhere thevariables were in terms of concentrations of thevarious products as follows:

K=_x_ 1l-'Xjb'1..- 2 ax

, 100 -'-. lax

where a = initial concentration of SO 2in %

where b = initial concentration of °2 in %

where x = Fraction (%cOnverSion) of S03100

K is calculated from assumed values of conversionfor a gas of specific SO 2 content, and then theequilibrium temperature for that value of K cal­culated from the Lewis and Randall equation givenpreviously. . ,. ~

Table V and Graph Edetail the variation ofequilibrium constant, K, with temperature, Tables VIVII and Graphs F and G, record the equilibriumtemperature for any assumed degree of conversionwith a gas of known composition. '

With a gas of known strength, the amount ofSO 3likely to be formed can be reduced by increasingthe temperature. Generally, in order to reducethe possibility of trioxide formation, the SO 2content of the gas should be increased, .i.e. theamount of excess air used, in burning should bekept at a minimum. On top of this, high corn­bustion chamber temperatures help to reduce theincidence of SO:I formation. However, theoryindicates that it is literally impossible to preventSO 2 undergoing a certain amount. of oxidation,albeit a very small amount at temperatures over1,000°C. .

The sugar chemist is attempting to produce aburner gas containing S02 only, and none of thehigher oxide, and in the light of the foregoing .theoryit would appear that this could best be accomplishedby the following:-

(a) Maintaining the quantity of excess air usedat an absolute minimum for the plant inquestion. .

(b) Ensuring that the flame temperature. in theburner is kept as high as possible and heatlosses due to radiation are minimised.

(c) Arranging for the hot burner gases to becooled as rapidly as possible in order that theyneed not be held at those temperatures for anylength of time, where maximum conversionof SO 2 to SO:i is likely to occur.

(d) On top of this it would be helpful to drawthe gases through the burner as rapidly. aspossible, with the apparatus in use, to en~

deavour to reach a state 'of affairs where thereaction 250 2 + 0 2 ~' 2'50 3, does riot havetime to reach equilibrium.' However,' thisrate of flow of the gases in the burner willnaturally be dictated largely by the rate atwhich sulphur can be burned without sublim­ation taking place.

This completes the theoretical study of sulphurburning, and all that remains is to observe whatoccurs in practice. The theory is in fair agreementwith what is experienced in actual pant operationandit is hoped that the connections between theoryand practice will be obvious.

PART II. PRACTICE.The most convenient raw material for the pro­

duction of SO 2 is sulphur, and although there aremany alternative sources available, sulphur stillproves the popular choice for a variety of obviousreasons. The sulphur used in the sugar industryin this country is obtained almost solely from theU.S.A., this source being chosen with considerationto price, quality and availability.

In 1949 the production of crude sulphur inAmerica amounted to approximately 41 millionlong tons.v mined by the Frasch-process and it isinteresting to note that the Texas Gulf SulphurCompany was the major producer. During thesame period the Union of South Africa is creditedwith the importation of just over 65,000 long tonsof crude sulphur.P most of which was used in themanufacture of sulphuric acid. It can be seen,therefore, that the sugar industry's requirements ofbetween 4,500 and 5,500 long tons per annum, arequite small when compared with the total importsinto the country.

Sulphur is abundantly distributed in nature,and on the Gulf Coast it is usually associated withsalt dome intrusions. The element is found tooccur in the limestone, gypsum and anhydrite caprock and a few of these domes contain commercialquantities of sulphur.i! The occurrence of thesedomes appears to be limited mainly to the GulfCoast region of the States of Texas and Louisiana.Details of the mining and production of sulphurare contained in a highly informative circular en­titled "Sulphur-General Information" issued bythe United States Bureau of Mines-! and anyoneinterested would be well-advised to procure a copyof this publication.

The element sulphur is non-metallic and existsin a variety of allotropic forms and therefore maybe said to have a series of properties depending onthe form in which it is found. Natural sulphurusually occurs as the more stable rhombic ora-sulphur form. However, it can exist in manyforms," but they all tend to revert to the rhombicform. A study of the various properties andallotropic forms of sulphur is outside the scopeof this paper and a brief account of those propertiesof relative importance to sulphur burning only,will be given. Sulphur, S8' is a brittle, yellow ele­ment which is solid at room temperature, is in­soluble in water, but will dissolve in carbon bisul-

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phide. The rhombic form melts at 112. 8°C. and themelt forms a mobile liquid which becomes moreviscous as the temperature is increased, until at200°C. it becomes so thick it will not flow. However,at 350°C., the melt again becomes mobile. It ignitesat about 248°C., finally boiling at 444.6°C.2 Theabove temperatures should be taken as approximateonly as Mellor" gives specific figures for the knownallotropic forms and any particular allotropicmixture would probably have its own characteristicphysical and chemical constants.

A typical analysis of the sulphur supplied to thesugar industry is roughly as follows:-16

Moisture at 105°C. 0.10%Total Residue on Burning 0.13%Organic residue 0.03%Mineral residue 0.10%Arsenic as As 20 3 ... 2 p.p.m.Selenium as Se NegligibleAcidity as H 2S0 4 <0.1 %Sulphur (by difference) 99.6%

Sulphur as supplied contains very little sulphuricacid, but storing in the presence of air and moisturemay cause sulphuric acid to be formed." It hasbeen found that the oxidation of sulphur is negligibleif moisture is absent." As sulphur contains a smallamount of hydrocarbons which form water oncombustion, this causing mists in the burner, andfurthermore, as a finite quantity of moisture may bepresent originally, it is often considered goodpractice to melt and strain all sulphur going to theburner. This process ensures that moisture andhydrocarbons are removed before combustion takesplace.

From these brief observations, it is fairly safeto conclude that sulphur should be stored in acool, dry place under cover to prevent contaminationby moisture, carbonaceous matter, etc. Whenstored in the open,a certain amount of moisturewill be absorbed from the air or rains, as will becarbon and silica which are present in the particlesof dust in the air. The first component, moisture,helps the oxidation of sulphur to H 2S0 4' whilethe second, carbon, may form a carbonaceous scumon the surface of the molten sulphur in the burner,or, alternatively, ignition of the carbon will occur,the CO2 formed causing dilution of the burner gas.The silica increases the ash content of the sulphurand this in itself is a nuisance in that it wouldinvolve frequent ash removal from the burner.

Obviously, these three constituents, i.e. H 20,C and SiO2' would, if present in sufficient quantity,affect the rate of burning of the sulphur in additionto altering the final composition of the burner gas.It is not suggested here that storing sulphur in theopen would involve the above contaminants beingpresent in quantities sufficient to affect the efficiency

c.

of the burner, but it is possible that in many casesthey may reach a proportion where they causetrouble.

Types of Burners.

(a) Rotary Burner.This burner is proably the most commonly used

and is known as the Glens Falls Rotary Burner.The makers, Glens Falls Machine Works of America,claim that maximum efficiency can be attained withunskilled labour and that installation and operatingcosts are low. Furthermore, they emphasise thatlosses due to sulphur sublimation or formation ofSO 3 can be eliminated almost entirely.'! Withthis type of burner it is possible to operate con­tinuously at SO 2 concentrations ranging between5 per cent. and about 18 per cent. In order toprevent fluctuations in gas strength it is desirableto feed the sulphur by mechnical means and notby hand.

The sulphur in the lower part of the horizontalrotating drum forms a molten pool as well as a thinfilm around the circumference of the drum. Thisfilm, in effect, increases the surface area of thesulphur exposed to the incoming air and a furtherincrease in surface area is brought about by thesulphur that drips down from the top of the drum.These factors help to increase the capacity of theburner and ensure that combustion is complete.On top of this, it is claimed that the sulphur filmprotects the drum metal from deterioration due tohot SO 2 gases as the heat conductivity of sulphuris less than that of cork.

A combustion chamber is fitted at the back ofthe horizontal drum and this serves to completethe combustion of any sublimed sulphur as well asto mix the gases and dilute them to the desiredstrength. This chamber is therefore constructedso as to have air ports and one or more baffles.

Impurities in the sulphur do not readily affectthe operation of these burners because the rotationof the drum agitates the molten sulphur sufficientlyto prevent any blanketing films forming on thesurface. A further refinement that can be addedis a sulphur melter and the heat of radiation fromthe burner may be utilised to keep the sulphurmolten. Steam heating coils in the Melting Tankare only required for starting up the burner aftera long shut-down. In order to further improvethe efficiency of this machine, the molten sulphurshould be strained prior to passing it to the burner.The Teeding of molten sulphur ensures that theburner is handling moisture-free material.

Ideal results are obtained with this type of burnerby using sulphur of a minimum purity of 99.6per cent. .The molten sulphur in the horizontaldrum should be at the highest possible level withoutallowing any overflow to take place.

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The makers suggest the following capacities fora burner 30 in. in diameter and 8 ft. long.

Capacity with l in. water draught is 1351bs. Sulphurper hour.

Capacity with ~--1 in. water draught is 200 lbs.Sulphur per hour.

Capacity with 2 in. water draught is 270 lbs. Sulphurper hour.

A burner of identical diameter, but only 6 ft. longwould have corresponding capacities of about 75per cent. of those given above. As a rough guide,it may be taken that:- .

For a 1 in. draught, the burner consumes approxi­mately 2 lbs. Sulphur/sq. ft.yhr.

For a £-1 in. draught, the burner consumes approxi­mately 3 lbs. Sulphur/sq. ft./hr.

For a 2 in. draught, the burner consumes approxi­mately 4:.2 lbs. Sulphur/sq. ft.yhr.

Another authoritv" considers that a burner 36 in.in diameter and '·8 ft. long is capable ·of handlingnearly 8 lbs. of Sulphur/sq. ft./hour. Fairlie!finds it practical, without resorting to high draught,to run a burner 4: ft. in diameter at a rate of 1 ton­of Sulphur per day per foot of length.

The size of the combustion chamber is variablebetween fairly wide limits and it should be bornein mind that a large chamber permits a higherconcentration of SO 2 in the burner gas withoutdanger of sulphur sublimation. It is suggested thata chamber spa e of 60 cubic feet per ton of sulphurper 24 hours- is adequate, while the makers of thistype of burner indicate that a minimum space of(it cubic feet per pound of sulphur per hour persquare foot of burner area should suffice for burnersup to 30 in. in diameter. These figures should betaken as approximate indications only and no hardand fast rules seem to apply. The size of thecombustion chamber should be dictated by practicalconsiderations and previous experience.

Sutermeister'" states that rotary sulphur burnersproduce a gas of varying composition owing tosudden rushes of cold air through the apparatus,but Fairlie" claims that with a continuous feed, it ispossible to maintain the gas at a uniform con­centration of SO 2 with only minor fluctuations.He stipulates, however, that this is dependent onthe depth of the molten sulphur in the rotating­cylinder being kept uniform. Any change in thisdepth will alter the area of the unsubmerged film­covered surface and this in tum will alter the rateof combustion of sulphur.

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(b) The Acme Burner.

This type is deigned to give a constant rate ofburning for any given concentration of SO 2 in thegas. The makers, Acme Coppersmithing andMachine Company, point out that the rate of com­bustion of sulphur bears a definite relation to theamount of air supplied. The amount of air re­quired for a given concentration of SO 2 is given bythe expression:-

o/SO . C b ti G 90.65/0 2 In om us IOn ases = ---A + 0.026

where A = lbs. of air at 70°F. per lb. of sulphurburned when the burner is operating ata rate of2t lbs. of sulphur per square foot of burning surface.Should the sulphur burning rate be changed, thenthe above formula has to be modified slightly.

It is claimed that a tray or rotary burner does notsatisfy the requirements of a definite constantburning rate because of the following factors:-

(1) Ordinary sulphur contains organic matterand this accumulates as a carbonaceous filmon the surface of the burning sulphur whichmay eventually extinguish the flame.

(2) Additions of fresh charges of sulphur to theburning surface disrupts the burning rate andnormal conditions are only returned to aftersever~l hours have elapsed.

With this burner, the makers have overcome thefirst drawback by operating the burner at a rategreater than 2 lbs.of Sulphur per Square foot ofburning surface per hour, finding that this preventsthe formation of carbonaceous scum by burning itoff with the rest _of the gases. The drawbacks offactor (2) were prevented by arranging a specialfeeding device that fed molten sulphur in a mannerthat did not disturb the burning surface, but, atthe same time, ensured that a constant level ofmolten sulphur was maintained in the burner.

This burner is operated on compressed air anda special sulphur melter is supplied with theapparatus. Sulphur is fed to the bottom of theburner via a main feed pipe, and a side feeder armon this main feed allows for positive control ofsulphur feed, thereby guaranteeing a constantlevel of molten sulphur in the burner.

All the data." given here and the brief descriptionof the principles behind this burner were taken froma pamphlet issued by. the manufacturers, viz.Acme Coppersmithing and Machine Company ofOreland, Pennsylvania. A description of this burnermay be found in the technical literature. 19

(c) Spray Type Burner.

The Research Department of the Texas GulfSulphur Company is accredited with the invention

of this burner. The development of apparatuswas described in a technical paper issued in 193420

and it was therein explained how the burner wasoperated. Briefly, molten sulphur is fed to a spraynozzle which injects a fine spray of this materialinto the burner. At the same time the desiredamount of air is forced into the burner chamberand ignition of the sulphur takes place. Theburner chamber is fitted with baffies to mix theresultant gases and prevent any unburned sulphurpassing through to the absorption apparatus.The burning and combustion chambers are in oneunit. Normally, the burner chamber is lined withfire-brick to reduce heat losses and minimise theformation of SO 3'

Certain advantages are claimed for this burnerand among those listed are:-

(1) Operation is simple and the burner requireslittle attention once the rate of combustionhas been set. The combustion rate can bechanged simply by altering the stroke of thesulphur pump and adjusting the compressedair supply accordingly.

(2) Starting up and shutting down operations arecomparatively easy, and if the burner is startedup when cold, maximum gas concentration canbe reached within 2;} hours. In order to shutdown it is merely necessary to stop the sulphurmetering pump and shut off the compressedair supply.

(3) The burner is extremely flexible insofaras rate of combustion is concerned and there­fore it is possible to vary the capacity of theburner within fairly wide limits.

(4) It will produce a gas with an SO 2 content ofanything up to 20 per cent. when run con­tinuously.

(5) Sublimation of sulphur should not occurprovided proper care is taken, and since thereis no burning-down period, as with conventionalequipment, the hazards of possible sublim­ation are reduced to an absolute minimum.

(6) Provided the gas concentration is kept highand the burner is lined with refractory brick,little SO 3 should be formed at the elevatedtemperatures (2,400° to 2,700°F.) of operation.During tests'" it was found that the SO 3

content of the gas passing to the cooler wasonly 0.14 per cent. of the sulphur burned.

(7) The maintenance costs are not likely to behigh and the initial cost of installation shouldcompare favourably with that of any of themore usual types of burner.

(8) Kress" found that power costs could be re­duced by 75 per cent. over that of conven­tional burners.

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Anyone wishing' to study in detail the design ofa spray type burner is well-advised, to read thepaper by Conroy and johnstone" on this subject.The authors indicate that the internal volume of arotary burner should be 13/14 cubic feet per ton ofSulphur per day and on top of this a combustionchamber space of 60 cubic feet per ton of Sulphurper day is required, whereas the total volume of aspray type burner is 24 cubic feet per ton of Sulphurper day.

It is felt that the three burners listed cover thetypes of particular and possible interest to the sugarindustry. In order to complete our practical con­siderations of sulphur burning, it is only necessaryto study how SO3 formation. occurs and what canbe done to minimise its formation as this informationshould then make it possible to successfully producea gas that is rich in SO2and, at the same time, freeof all but traces of SO3'

The Formation and Dissociation of SO3'

The conditions favouring the formation of SOaare:-

(i) Decomposition of any sulphuric acid thatmay be present in the commercial sulphur.

(ii) Formation of trioxide during the actualburning of the sulphur.

(iii) Oxidation of the SO 2 present in the burnergas.

The first source accounts for only a relativelysmall amount of the SO3 normally found in burnergas. Naturally, sulphur will oxidise. to a certainextent in the presence of moist air, but the amountof sulphuric acid formed is bound to be extremelysmall.

Undoubtedly, a certain amount of trioxide willbe' formed during the burning of the sulphur, butthis is difficult to determine accurately. It willdepend, to a large extent, upon the proportion ofexcess air present and upon the temperature reachedin the burner. It can be seen therefore that theSO ~ content of the gas should be as high as possiblefor the type of burner in use.

The third source, i.e. the oxidation of SO2.accounts for the major portion of trioxide found ina burner gas and consequently this aspect of theproblem will have to be examined in fairly minutedetail. In practice it has been found that oxidationwill take place only until a certain ratio of dioxideto trioxide is reached.. At this point equilibriumis established between the reactants and products

oof the reaction and as fast as SO3 is being formed,it"is again decomposed into SO2 and oxygen.. Thisequilibrium may be distrubed or changed by alteringthe concentration of the reactants on either side ofthe equation:

O2 + 2S0 2~ 2S0 3

A rotary steel or cast-iron burner operatingcontinuously has been found to convert between1 per cent. and 2 per cent. of the S02 into S032,while a similar burner lined with refractory brickwill produce a gas of much lower trioxide content.Obviously, then, the materials of construction havean effect upon the oxidation of SO2and an investig­ation led to the discovery that certain metals ortheir oxides could increase the rate at which trioxidewas formed. These metallic substances are knownas catalysts and while they do not take part in thereaction in a quantitative way, that is to say, theyare not used up or depleted during the course ofoxidation, they most certainly speed up the rate ofreaction. These substances, therefore, warrantstudy as well. .

It will probably be advisable to study the variouseffects of physical and chemical conditions tlpontrioxide formation separately to ensure that thepicture be complete.

Effects of Excess Air,

Comog'" and co-workers found that when sulphurvapour was burned in the presence of 250 per cent.excess air at 4:60°C., approximately 3.4 per cent.to 3.8 per cent of the sulphur appeared as thetrioxide. However, Browning and Kress'" observedless than 0.02 per cent. conversion under similarconditions in the absence of a flame. The obviousinference is that atomic oxygen in a flame has aconsiderable influence upon the reaction. Theresults of their experiinents indicated that trioxideformation could be reduced by decreasing theequilibrium conversion of SO2 to SO 3 by keepingthe quantity of excess air at a minimum in con­junction with high combustion chamber temper­atures. A perusal of the theory will show agreementwith this statement.

Obviously, then, the amount of air supplied tothe burner should be carefully metered, or, failingthat, regular analyses of the gases from the com­bustion chamber should be carried out to ensurethat the SO2 content is being maintained at amaximum.

A rough guide to the correct amount of air re­quired is based on observation of the flame in theburner. If the sulphur burns with a blue flame,conditions are just right. A flame with a browntinge indicates insufficient air and sublimation ofsulphur. However, these observations do notprevent one from operating the burner with' anexcess quantity of air and one is well-advised torely more on chemical tests and mechanical aidsthan on rule-of-thumb methods such as those men­tioned above.

A study of Table VII will show how the quantityof excess air can influence the formation of SO3'

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Should the burner be operated with insufficientair to complete the combustion of sulphur, thensublimation takes place and slight mists of unburnedsulphur may form.

Influence of Temperature.

Theory has already indicated to us that temper­ature exerts a considerable influence upon theamount of trioxide formed. In practice it has beenfound that the optimum temperature range favour­able to the formation of SO3 lies between MO°Cand 980°C22. Frohberg-" found that maximumoxidation occurred at 4000-500°e., while at anapproximate temperature of 900°/1,000°C., dissoci­ation took place. It has been shown that at1,000°C. nearly 50 per cent. of the SO3 formed isdecomposed, this decomposition starting at 700°e.at which temperature the conversion of SO2 toS03 is about 60 per cent." Above 1,000°e. thedissociation of the trioxide is more rapid than itsformation. It may be generally stated that below400°e. the rate of formation of SO3 is too slow tobe regarded as a nuisance, while above 1,000°e.,even though SO3 is formed rapidly, the rate ofdecomposition predominates. However, even at305°e., oxidation of S02 has been found to occur,albeit at a very slow rate." Thus we have two con­flicting states, viz. where the rate of conversion ofSO2 to SO3 increases rapidly with increasingtemperature, but where the dissociation of SO3

overhauls the rate of conversion, and at the highertemperature (above 1,000°e.), therefore, the per­centage conversion, as measured, tends to decrease.

The reaction:

2S0 2 + O 2 ¢ 2S0 3

takes place nearly to completion at 450°C. in thepresence of a catalyst, and. it has been shown that,'while the rate of the forward reaction is onlymoderate at 400°e., it increases to 40 times thisvalue at 500°e.,the decomposition reaction onlybecoming perceptible at 550°e. and upwards.It may therefore be said that up to a temperatureof 450°C., reaction of formation prevails, and onlywell above this temperature does dissociation comeinto play.25

Sulphur trioxide is very stable in the absenceof contact substances. and once formed is difficultto dissociate. Generally, decomposition will nottake place completely at temperatures as high as1,100°/1,200°e. Decomposition of trioxide alreadyformed cannot be relied upon to keep the loss fromthis source at a reasonable value at temperaturesbelow 1,00°e., the amonnt of dissociation at thistemperature only reducing the loss by an amountof less than 0.5 per cent.

It ' is well worth .remembering 'that the finalequilibrium condition of the reaction depends onlyupon the temperature and the composition of thegas mixture, and oxidation of SO2 will take placeuntil a certain ratio of trioxide to dioxide is reached.High temperatures favour a low ratio and lowtemperatures a high ratio of SO 3 to SO 2.7

Presence of Moisture.

The air drawn into a burner is not normallydried, but to prevent the formation of corrosivesulphuric acid mists, the water content of the airused should not be more than 5 mgm. per cubicfoot of air at S.T.P.24 Under conditions prevailingin Natal, this would involve drying the air witheither concentrated sulphuric acid or P 205 or :asimilar efficient drying agent.

If the air is thoroughly dried with P 205' thereis little oxidation of SO2 up to a temperature of450°e. and in general it may be stated that dryair helps to retard oxidation. In the presence ofwater vapour, oxygen does not combine with SO2at 100°e. but oxidation does occur, even at thislow temperature, if particles of liquid water arepresent. 6 Mellor" found that moisture does notappear to affect the oxidation of SO 2' but thepresence of CO2 and nitrogen causes more SO3

to be formed.

Moisture has a poisoning effect on some catalystsand Tolley" has shown this to be true for ironoxide. Upon continued exposure, however, cata­lysis slowly increases until after 40 hours thecatalysis with wet gases is half that of dry gases.He found that water vapour has its greatest in­hibiting action at a temperature of 475°e., puteven at 635°e., the catalytic activity of iron oxideis reduced to ! of that with dry gases.

Browning and Kress? investigated the dew-pointof burner-gas mixtures and showed that the cor­rosion to be expected would be greatest at the dew­point of the particular gas because of the combinedaction of scale and condensed liquid upon the metalbase. Above the dew-point, only gases are presentand corrosion is much reduced, while at temper­atures below the dew-point, the rate of corrosionis considerably less on account of the slowing upof the chemical reactions at these lower temperatures.Their experiments revealed that the dew-pointincreased as the SO 2 content of the gas was raisedand for safe operating practice, using moist air,the temperature of the gas in iron should be kept­above 200°C. to prevent undue corrosion of themetal. When the gas is required to be cooledbelow this temperature, it should be transferred toa lead pipe.

As previously stated,. moisture in the air cancause mists to form in the burner gas and these aremost difficult to, remove. A method which helpsin the removal of mists consists of cooling the mixedgas to below 46°C., when the S03 will condense.The gas should now be passed through a filter-boxpacked with charcoal or iron borings and thenthrough a water-spray .scrubbing tower. Thismethod, however, is not always very successfuland it is easier to prevent mists than, having onceformed, to remove them.

Action of Catalysts.

Catalysts do not affect the equilibrium point ofa gas, mixture, but merely alter the rate at whichequilibrium is approached. The ratio of the originaland final substances present in the gas will not bechanged upon contact with a catalyst. The contactsubstance will only ensure that this equilibriumratio, is reached in less time.

A list of various contact substances normallyencountered follows, and these, with their effectsupon SO 3 formation, have been listed separatelyfor the sake of clarity. Anyone wishing to knowmore about the effects of various catalysts on SO3formation, should read the monumental work ofBrowning and Kress," as this covers the subjectmost comprehensively.

A. Iron Compounds.

Iron or its oxides have long been known to exhibitcatalytic properties as far as the oxidation of SO2is concerned and the Mannheim Process for themanufacture of suphuric acid utilised iron oxideas a catalyst. For reasons of efficiency, this oxidehas been replaced by either platinum or vanadiumin the Contact Process.

Tolley" predicted that the first reaction to occur·when steel was in contact with sulphur dioxideand oxygen at reasonably high temperatures, wouldbe a combination of oxide and sulphide formation,represented as:-

2Fe + S02-+ 2FeO + SFe + S-+ FeS

or alternatively:-

5Fe + 2S0 2-+ 2FeS' + Fe 30 4

He pointed out that as FeO cannot exist below570°C., the first reaction shown above could onlyoccur above this temperature. The experimentsshowed that the catalytic activitiy of mild steelincreased rapidly during the first few hours ofexposure to S02' but after about 10 hours theactivity became reasonably constant. It was feltthat this initial rapid increase in the rate of oxid­ation of SO2 was probably due to the formation ofiron oxide.

95

The catalytic effect of iron compounds depends toa large extent upon their physical and chemicalstate. Experiments have shown that iron oxides,as such, are not good catalysts, but their activitymay be increased after a certain period by theformation of sulphates and other compounds.At elevated temperatures both the dioxide andtrioxide react with iron, and this would explainthe short life of iron under these conditions ofservice.

In a rotary sulphur burner, there will be surgesof SO3 formed when starting up or burning downas more iron will be exposed during these periods.Furthermore, the temperature range at which ­maximum SO3 formation occurs will be passedthrough during these stages."

Reverting to the influence of physical state uponcatalytic activity, it is worth noting that ferricoxides may exhibit contact properties, and thesurface of the material will exert the least influencewhen freshly precipitated oxides, which are not yetdried, are used. The activity increases if the oxidehas been moderately heated or kept for a long timeso as to become dry. Oxides obtained by heatingferric- or ferrosulphate give a much lower contactaction than that obtained with an oxide preparedby igniting a precipitated hydroxide or pyrites­cinders." An oxide containing 21 per cent. Arsenicas As shows considerably more contact reactionthan that of a pure oxide at 700°C., while theaddition of copper oxide to an oxide of iron isfavourable to the formation of S03. 26

Observations have shown that when a burnergas is in contact with an iron pipe, maximumconversion occurs at 750°C., there being a 13 per cent.maximum conversion for a gas with a 10.5 per cent.S02 concentration. This conversion drops to 10per cent. when the SO2 content is raised to 14 percent. and a further decrease in conversion to 3 percent. for a gas of 19.5 per cent. content is observed. 23At 1,000°C., the conversion at all concentrationsIS zero.

Ferric oxide begins to exhibit catalytic activityat 550°C., and this activity increases to a maximumin the temperature range 6000-620°C.6 Increasingthe S02 content of the gas from 2-12 per cent.does not affect the conversion to any appreciableextent, although with higher concentrations, theyield of SO3 is lower. As a contact substance,this oxide only becomes really effective at a temper­ature of 600°C. when a percentage conversion ofSO2 to SO3 of 40 to 50 per. cent.! is attainable.In practice, however, the conversion never exceeds60-66 per cent.!"

Iron oxide formed from pyrites-cinders showslittle activity, and this only begins above 650°C., .increasing with temperature and reachirig a con-

version of 1 percent. at 1,000°C. with a gas con­taining 15 per cent. SO2' However, it should beconsidered worthy of note that if 3 per cent. of SO3

is formed in a burner gas during normal burningoperations, this figure may, under favourableconditions, be raised to three or four times thisvalue if the gases are passed through red-hotpyrites-cinders. 26

Pure iron oxide exhibits maximum contact actionat 650°C., the respective conversion figures being15 and 5t per cent. for gases of 10.5 and 19.5 percent. SO2 content. These conversion figures drop

•to 4i and Ii per cent. respectively when the temper­ature is raised to 1,000°C., the activity in thiscase being only above 500°C.7

Ferrous sulphate has a maximum contact actionat 650°C. when the conversion of a gas containing15 per cent. S02 is 12t per cent., this conversiondecreasing to t per cent. at 1,000°C. Similar con­version figures for ferric sulphate are 8 per cent. at650°C. and t per cent. at 1,000°C.; indicating thatit is less active than the ferrous form.

All the above observations indicate that iron inits many forms exhibits varying degrees of catalyticactivity and if the formation of. SO3 is to be mini­mised then obviously gases of high S02 concen­tration must be produced at the highest possiblecombustion chamber temperature that can beattained in practice. It is of further benefit tonote that in many instances, the amount of SO3

that will be formed according to theory will not bereached in practice as the gas passes through thecombustion apparatus before equilibrium is reached.

Silica and Silicates.

It is generally conceded that vitreous fusedsilica exerts no catalytic effect on the oxidation ofS02.7 The same applies to Dialite brick, andtherefore these materials are considered satisfactoryfor the lining of furnaces, combustion chambers,etc.

The Efficient Operation of a Burner.

In America it is a generally accepted fact thatthe air to the burner should be carefully meteredby mechanical means and furthermore, allowanceis made for fluctuations in the burner. Thesechanges in burning rate are catered for by theinstallation of an automatic recording device whichcontinuously analyses the SO2 content of thegases from the combustion chamber. A diaphragmoperated valve then automatically operates theair inlet and sulphur feed and regulates these,thereby keeping the SO2 content of the burner gasconstant within fairly narrow limits.

96

Furthermore, the sulphur feed should bemechnical to enable a constant rate of feed to bemaintained. With. all these refinements it ispossible to arrange for high combustion temper­atures and a burner gas containing the highestconcentration of SO2 possible with the type ofapparatus in use. Even better results will beobtained if the sulphur. is melted and strainedbefore it is fed to the burner, for reasons alreadygIven.

The combustion chamber should be fitted withone or more baffles to ensure thorough mixture ofthe gases and prevent any possibility of unburnedsulphur passing to the absorption plant. Coolersare normally fitted after the combustion chamber,and it is imperative that cooling of the gas be asrapid as possible to minimise SO 3 formation.'Initially the gases would be air- or. water-cooledand then passed to either a direct or indirect coolerto bring the temperature down to 200°/300°C.At this stage, if further cooling is attempted, thepipes conducting the gases should be lead-lined.The subject of cooling is outside the scope of thispaper, but is well-worth : pursuing. by readingLundberg- and others.

Summary.

The practice of sulphur burning, the productionof SO2 and the formation and dissociation of SO3

has been outlined and the effects of:Excess Air,Temperature, andCatalysts

upon SO 3 formation detailed. Where possible acomparison with the theory has been made and ithas been shown that the following points all helpto increase the efficiency of this type of plant:-

(a) It is preferable to feed the sulphur con­tinuously by mechnical feeder. The feedingof strained molten sulphur is preferable asthis eliminates moisture and deleterious hydro­carbons.

(b) The quantity of excess air should be carefullycontrolled and the SO2 content of the burner­gas kept at a maximum.

(c) The temperature of the burner and com­bustion chamber must be maintained above1,000°C. if at all possible.

(d) Automatic recording and operating apparatus.to maintain an even feed of sulphur andconstant SO2 content of the gas is helpful.

(e) The gas from the combustion chamber shouldbe filtered and cooled as rapidly as possibleto prevent formation of SO3'

Conclusions.

The authors have made everv endeavour andtaken all possible precaution's to ensure that theinformation given is accurate. However, mistakesmay have cropped up and no responsibility can betaken for such errata.

Many of the improvements listed may possiblymake the production of SO 2 by these methods rathercostly and their inclusion should not be taken asa recommendation, but rather as a guide or exampleof how efficiency may be improved. .These refine­ments are in practice in America in many of thelarger paper mills, so obviously they are economicalfor the production of large quantities of SO 2'

It is admitted that it is literally impossible toprevent the formation of SO3 entirely, but if pre­cautions, along the lines of those mentioned in thispaper, are taken, the quantity of SO3 can be reducedto such a low level that it no longer constitutesa nuisance.

The types of burners listed were limited to thosegenerally used in the. Sugar Industry and thosewhich may be of interest. The spray type burneris proving very popular in America and it producesa gas of low SO 3 content and furthermore it is veryflexible in that it may be started or shut down ina very short space of time. Control is easier thanwith the conventional rotary burner and papermills in America have found that installation costsare not excessive. Maintenance costs are lowand power consumption compares most favourablywith other types.

For the sake of those who would like to knowmore about sulphur burning and its applications,a reading list has been added.

Acknowledgments.

Our sincere appreciation and thanks are dulymade to those firms and Institutions in Americawho corresponded with us, supplied technicalarticles and generally went out of their way to behelpful.' In particular we would like to mention:­The Paper Institute, Texas Gulf Sulphur Company,Acme Coppersmithing and Machine Co., and theGlens Falls Machine Works-all American organis­ations that were extremely helpful and co-operative.

REFERENCES.1 Beater: The Distribution of Temperature in the Sugar Belt

of Natal and Zululand-S.A.S.T.A. 1949.2Lundberg: Acid Making in the Sulphite Pulp Industry.3 Chemical Control Plant Data: Booklet issued by Chemical

Construction Company.4 Fairlie: Sulphuric Acid Manufacture.5Lewis and Randall: Thermodynamics and the Free Energy

of Chemical Substances.6 Mellor: A Comprehensive Treatise on Inorganic and

Theoretical Chemistry-Vol. X..

97

7 Browning and Kress: A Study of Some Factors Influencingthe Formation and Dissociation of 50 3 in Burner Gases-PaperTrade Journal, 100 No. 19, 31-43, 1935.

8 Glasstone : Thermodynamics for Chemists.o Dodge: Chemical Engineering Thermodynamics.10 Whitney, Elias and May: Chemical Reaction Equilibria-

TAPPI; 34, No.9, 11.)51. .11 The Glens Galls Rotary Sulphur Burner: Pamphlet from

Glens Falls Machine Works.12 Darrah: The Preparation of S02-Paper Trade Journal,

pp. 132, Nov. 30, 11.)50.13 Sutermeister: Chemistry of Pulp and Paper Making.14 Josephson and Downey: Sulphur and Pyrites-U.S. Bureau

of Mines Yearbook, lU49.15Ridgway: Sulphur-General Information-U.S. Bureau

of Mines I.C. 6329. . .16 Sulphur: Technical Service Note No. 32-African Explosives

and Chemical Industries, Ltd.17 Furniss: Rogers Manual of Industrial Chemistry.18 General Description of Acme Burner-Pamphlet from

Acme Coppersmithlng and Machine Co.10 Cain and Chatelain: New Low-Capacity Sulphur Burner­

Chemical and Metallurgical Engineering, 46, Oct. 1939.20 Kress and Others: Spray Type. Sulphur Burner-The Paper

Mill, Oct. 11.)34.21 Conroy and Johnstone: Combustion of Sulphur in a Venturi

Spray Burner-Industrial and Engineering Chemistry 41, pp.2741, Dec. 194U.

22 Barker: Sulphite Acid Preparation-Paper Trade Journal,pp. 136, Nov. 30, lU50.

23 Newell. Stephenson: Pulp and Paper Manufacture, Vols. III and III.

2< Modern Chemical Processes.25 Riegel: Industrial Chemistry.26 Lunge: Manufacture of Sulphuric Acid and. Alkali, 3rd

Edition.27 Tolley: The Catalytic Oxidation of SO 2 on Metal SurfacesJournal Society for Chemical Industry, 67, pp. 369, Oct. 1948.

READING LIST.

Chemical Engineering-53, 225 (1946).54, 221 (1947).

Chemical Engineering Process-46,614 (1950).Chemical Markets-30, 261 (1932).Chemical and Metallurgical Engineering 42, No. 7,374-7 (1935)Chemical Process Principles-Hougen and Watson.Industrial and Engineering Chemistry-17, 593 (1925).

34, No.9, 1017 (1942).35, 522, 541-5 (1943).41, 2741 (1949).42, No.4, 713-8 (1950).42, 2215-7 (1950)

Industrial Chemistry-Riegel.Journal Am. Chern. Soc.-48, No. 11 (1926).

Chern, Soc.-123, 3203 (1923).Paper Trade Journal-94, No. 15, 39-42 (1932).

" " " 99, No. 17,48-51 (1934).Proc. Roy. Soc. London-Series A, 138, 635 (1932).Pulp and Paper Mag. Canada-31, No. 52, 1390-3 (1931).

33, No.2, 78-80 (1932).Conv. 37, No.3, 140-8 (1938).44, No.4, 320 (1943).52, 108-111 115 (1951).

"" "" Conv. 243-8 (1951).Soc. Chern. Ind. Journal-67, 369-73 401-4 (1948).Textbook of Physical Chemistry-Glasstone.Trans. Am. Inst. Chern. Engs.-27, 264 (1931).U.S. Bureau of Mines-Bulletin 406 (1\)37),.

98,

TABLE V.Variation of Equilibrium Constant, K, with Temperature for the

Reaction SO. + !O. = SOa

APPENDIX OF TABLES.

TABLE I.

Weight and Volume of 'Burner Gas at S.T.P. hased on One Poundof Sulphur-Dry Air. Temperature.

°C.

Equilibrium Constant J{,Fairlie Lewisand Miles

Randall.Average

K.Average.Log: 10K.

Weight and Volume'of Burner Gas at S.T.P. based on One Poundof Sulphur-Air Saturated with Water Vapour.

Compositionof Burner Gas. Quantity of Quantity of Air.% by Volume.. %50, Burner Gas.

Oxygen. SO, Nitrogen. Water Weight CubicFeet, Lbs. CubicFeet. Lbs.

15 6 79 12.48 187.0 16.02 187.0 15. I13 8 79 16.31 140.3 12.26 140.3 1l.3II 10 79 19.98 112.2 10.01 112.2 9.19 12 79 ' 23.50 93.5 8.51 93.5 7.55 16 79 30.1.5 70.1 6.63 70.1 5.73 18 79 33.30 62.3 6.01 62.3 5.01 20 79 36.33 56.1 5.51 56.1 4.5

Variation of Theoretical 'Flame Temperature and MolecularWeight with Burner Gas Composition.

A.-Dry Air-No Radiation Losses.

B.-Dry Air-Assuming 15 per cent. of Total Heat Lost byRadiation.

TABLE VII.Variation of Equilibrium Temperature with Burner Gas Com­position Assuming Degree of Conversion of SO. to SO,,-Dry Air.

500° 48.2 52.5 48.7 49.8 1.697

6000 9.53 9.75 9.74 9.67 0.985

7000 2.62 2.55 2.56 2.58 0.412

8000 0.915 0.859 0.864 0.879 1. 944

,900° 0.384 0.349 0.353 0.362 1.559

1000° 0.1845 0.163 0.167 o. in 1.236

1200° 0.0573 0.0485 0.O511 0.0523 2.719

14000 0.0235 0.0193 0.0209 0.0212 2.326

16000 0.01l7 0.0093 0.0104 0.0105 2.021

1800° 0.0066 0.0052 0.0060 0.0059 3.771

2000° 0.0041 0.0032 0.0038 0.0037 3.568

TABLE VI.V!'riation of Equilibrium Temperature with Burner Gas Com-position Assuming Degree of Conversion of S02 to SOa-Dry Air.

Equilibrium TemperatureDegreesCentigrade.

Gas SO, 6% 8% 10% 12% 16% 18%%Conversion. Composition 0, 15% 13% 11% 9% 5% 3%

99 421° 415° 407° 394°95 499° 492° 482° 467°90 540° 533° 5230 5070

80 5910 583° 573° 557°70 6290 621° 6100 5960

60 662° 654° 644° 630° 526°50 6950 6870 6770 664° 6000

40 731° 7230 713° 700° 649°30 7730 7650 7550 742° 698° 600°25 798° 789° 779° 7670 7250 6680

20 8280 819° 809° 797° 7570 710°10 9230 9130 902° 8890 849°

Quantity of Air.CubicFee!. Lbs.

Quantity of BurnerGas.

CubicFeet. Lbs.

TABLE II.

%05,By

Weight

Composition of Burner Gas. Molecular Theortetical Flame% by Volume. Weightof Temperature-e-X'.

Oxygen. SO, Nitrogen. Burner Gas. A. B.

15 6 79 30.8 593° 508°13 8 79 31.4 766° 659°II 10 79 . 32.0 928° 795°9 12 79 32.7 10950 \J44°5 16 79 34.0 1384° 1224°3 18 79 34.6 1534° 1328°1 20 79 35.3 1667° 1411°

14.8 5.9 77.7 1.6 12.36 190.2 16.18 190.2 15.312.8 7.9 77.7 1. 6. 16.21 142.0 12.34 142.0 1l.410.9 9.8 77.7 1.6 19.71 114.5 10.15 114.5 9.28.9 1l.8 77.7 1. 6, 23.26 95.1 8.60 95.1 7.64.9 15.8 77.7 1.6 29.97 71. 0 6.67 71. 0 5.73.0 17.7 77.7 1.6 32.99 63.4 6.06 63.4 5.11.0 19.7 77.7 1:6 36.05 57.0 5.55 57.0 4.6

TABLE III.

Composition of BurnerGas.

% by Volume.Oxygen. SO, Nit.

TABLE IV.

Equilibrium Temperature-Degrees Centigrade.Gas SO, 6% 8% 10% 12% 16% 18% 20%

Composition Os 150/0 130/0 110/0 9~'i:. 50/0 3°,fo 1%0;0 Conversion.

14.8 5.9 77.7 1.6 30.5 582° 496°12.8 7.9 77.7 1.6 31. 2 757° 646°10.9 9.9 77.7 1.6 31.8 914° 7830

8.9 1l.8 77.7 1.6 32.5 1080° 927°4.9 15.8 77.7 1.6 33.7 1368° 1182°3.0 17.7 77.7 1.6 34.3 1496° 1292°1.0 19.7 77.7 1.6 35.0 1624° 1403°

Variation of Theoretical Flame Temperature and MolecularWeight with Gas Composition.

A.-Air 74 per cent. Saturated with Water Vapour-No RadiationLosses.

B.-Air 74 per cent. Saturated with Water Vapour-I:) per cent.of Total Heat Lost by Radiation.

Composition of Burner Gas.% by Volume. ,

Oxygen. SO, Nitrogen.

MolecularWeight of

Water. Burner Gas.

Theoretical FlameTemperature-e-v'C.

A. B.

5 1025° 1015° 1002° 988° 945° 815°4 1061° 1050° 1037° 1021" 977° 938° 850°3 1l10° 10980 1084° 1068° 1020° 965° 8930

2 1184° 1170° 1155° 1137° 10860 1044° 9540

1.5 1241° 1227° 1210° 1190° 1138° 1091" 998°1.0 1328° 1312° 12950 1276° 1212° 1173° 1064°0.9 1354° 13360 1317° 1295° 1233° 1183° 1082°0.8 13810 1364° 1345° 13210 12570 1207° 1l02°0.7 1414° 1397° 1376° 1352° 12860 1232° 1125~

0.6 1454° 1436° 1414° 1389° 13200 12640 1154°0.5 1503° 1484° 1461° 1435° 13620 1303° 1188°0.4 1568° 1547° 15220 1495° 1417° 1355° 1233°0.3 1659° 1635° 1608° 15780 14930 14250 1294°0.2 18010 1775° 1745° 17090 1612° 1535° 1388°0.1 2103° 2068° 2028° 1982· 1858° 1760° 1577°0.05 25050 2458° 2405° 2343° 2176° 2048° 1814°0.02 3308° 3230° 3142° 30410 2780° 2584° 2237°0.01 4308° 4182° 40410 3881° 3481" 3188° 2691°

19

17

IS

GRAPH "A"

VARIATION OF QUANTITY OF BURNER GAS

WITH SO, CONCENTRATION.

VOLUME OF BURNER GAS-DRY AIR.

WEIGHT OF BURNER GAS DRY AIR

SCALE A: UNITS = LBS.

UNITS x 10 "CUBIC FEET.

~ 13

Z:JCl:III

":"1

oJ "~~ ;III

9

6 10 -e- 6 18 2

17

IS

13

"

9

7

6

% SO, BY VOLUME.

GRAPH "B"

VARIATION OF QUANTITY OF BURNER GAS

WITH SO, CONCENTRATION.

VOLUME OF BURNER GAS MOIST AIR.

~ WEIGHT OF BURNER GAS-MOIST AIR.

SCALE A: UNITS LBS.

UNITS x 10 co CUBIC FEET.

0;.50, BY VOLUME.

20

17

GRAPH ..·C"

FLAME T£MPEAATURES yo. '50, CONC:ENTRATION.

ORY AIR.

100

. GRAPH "0"

FLAME TEMPERATURES \'I. SO CONCENTRATION.,

MOIST AIR.1750

1500

U0III

1250a: 36

~.36:J

~... ~o ,{J"" U 8'Pi ~o

~ ~-4i i' :r <t': ~..~ III

~O~""'" :rIII o ' q,1i ... \ 0 a:.. ~ $\0 ~..~~ r- SI~. , ~",1l 0

J: m 34 r:III ~..I n ~\~

m... ~\tf-~ C ns III

otP\V C.. s:JD J:~

III :JD

m ... -750 32 ~Ci m

:I s:-4 :I

:-430 500 30

6 I~ I~ 6 10 104 18 22

%50, BY VOLUME. %50, BY VOLUME.

GRAPH "E"

Log" K. \'I. TEMPERATURE

FOR REACTION: S0, + ~O, SO,.

+

+1

0

il500 700 900 1100 1,3pO l~ h l.7oo 1900 2100

I

TEMPERATURE °C.

'-1

100

80

Z0 -60iiiII:

'">Z0 --40U

~

20

101

GRAPH "F"

EQUILIBRIUM TEMPERATURE VI. % CONVERSION.

5

4

Z ~0 0ll'

~ -!<~

0ll'

'" 9>z0U 2~

5 1,100

TEMPERATURE C.

GRAPH "G"

VARIATION OF EQUILIBRIUM TEMPERATURE WITM

SO, CONCENTRATION AT LOW CONVERSION PERCENTAGES.

BOO 1000 12 20

TEMPERATURE C.

2

102

Mr. Dymond said that they were indebted to theauthors for this very important paper: .He expressedthe view that the standard of papers at the Congresswas very high. This paper provided data that hadbeen wanted for many years.

Mr. Main agreed and said that the paper was amost valuable addition to the technical knowledge.He referred to Maxwell's book on sulphur burningwhich was now out of print. Since then he had notfound anything as valuable as the paper presented.He hoped that it might be possible to provideillustrations.

Mr. Hogarth said the point had been discussedand photostat copies of a paper dealing moreespecially with one type of sulphur burner could bemade. He hoped that these could be fairly generallydistributed.

Mr. Rohloff said he thought the three types ofsulphur burners could be classed as three separatestations in the development of sulphur burners.Much the same could besaid about the developmentof boilers. He drew a parallel between the differenttypes of sulphur burners and the different types ofboilers. This development seemed to suggest thatthe trend was towards greater flexibility and hethought it was fit to mention this, as it indicateda desire to make sulphur burners for the SugarIndustry more flexible and more applicable to sugarproduction. Mr. Rohloff said that they had askedthe S.M.R.I. about the Acme burner and were toldthat this had been investigated, but the type ofsulphur used in the Industry was not applicable tothis burner.

Mr. Hogarth .said the sulphur supplied to theSugar Industry did not vary in composition froin thesulphur used overseas and presumably used inAcme burners.

Mr. Perk said that speaking from memory, hehad told Mr. Rohloff that the method of storingsulphur in Natal precluded the sulphur being usedin the closed type of burner used in J ava. In orderto operate these burners for months without loss

of capacity or interruption for cleaning, the sulphurhad to be kept free from dust and moisture. Itwas therefore routine at the Java factories to storethe sulphur in a separate compartment, andparticularly not in the store-room where lime waskept.

Mr. Main expressed the view that sulphur storageconditions did not materially affect the sulphurused in burners in Natal.

Mr. Hogarth said he was inclined to agree withMr. Main, as the contaminants in the sulphursupplied to the Sugar Industry were very smallindeed.

Mr. Rault said the problem in the Sugar Industrywas not only to burn the sulphur but also to absorbit. He asked whether there was a more modern,compact' and controllable absorbtion system thanthe "eye sore" and dirty plant called the sulphurtower, commonly used in our factories.

Mr. Hogarth said that the absorption of SO2 wasa most complicated question, but there was equip­ment available which would assist in the absorptionmore efficiently than was done at the moment inthe Industry. He offered to, supply this inform­ation to Mr. Rault.

Mr. Dymond agreed that the problem of hotgases going into the juices required further in­vestigation and he hoped this would be done by theS.M.R.I.

Mr. Barnes referred to the possibility of usingS02 instead of raw sulphur in the Sugar Industry, asincreasing quantities of SO2 were being generatedby industry.. He thought the possibility of usingcold gases should be investigated more thoroughly.He referred to the use of liquid ammonia as afertiliser and said it was not many yeCjlrs since thishad been thought impossible, but today most of thenitrogen used in Louisiana was applied in the formof gas or liquid. He felt the same might apply togaseous S02 in the Sugar Industry.

Mr. Dymond concluded by calling for a heartyvote of thanks to Mr. Hogarth for hisexcellent paper.