Sulphur Burning and the Formation of So3
Transcript of Sulphur Burning and the Formation of So3
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8 8
On to p of this it would be of assistance to know
how moisture in
t he
atmosphere would effect these
values.
Table I an d II Graphs A an d B, supply the
answers here an d all calculations were made
according to
th e
methods outl ined in Lundberg 's
book.
Where moist air was assumed in the theory in
th e calculations, this was taken as air 74 per
cent
saturated with water vapour
at
66. 6F. This
represents roughly
th e
average humidity an d
temperature taken over a number of years
at
Mount
Edgecombe during
th e
months of May to December. '
'Graphs
A
an d B show how the volume of the
burrier
gas:
produced decreases with increasing
SO
2
concentration.
A
comparison of the results
withdry an d
moist air indicates
that
the effect of
moisture is
no t
very marked upon the final gas
coming from the burner.
...
.
Theoretical lame Temperature
When sulphur is burned in air, th e reaction is
exothermic an d therefore the products are at a higher
temperature than that of the reactants.'
For
every
particular concentration of SO2 in a gas, there will
be a corresponding flame temperature to be reached
in
th e
burner.
U ~ f q ~ t u n a t e l y
th e
deduction of these flame
.temperatures ,from basic principles is a fairly long
and
tedious procedure: A method suggested by
Lundberg was usedhere an d it is reasonably rapid
an d
convenient, although comparison with flame
temperatures calculated from basic thermodynamic
principles indicates that this rapid method ma y be
slightly inaccurate an d especially so at the ow r-
concentrations of, SO2' It must be stressed that
the
temperatures listed in Tables
II I an d
IV are
'approximate only and should be taken more as an
indication of the order of magnitude of
temperature
an d
not as accurate flame temperatures
Anyone
wishing to derive theoretical flame tempera tures
;may.do so
b y
consulting th e technical literature
d e ~ 1 i r i g with this. subject 5 8 _9 10
'.. Once again, results have been caleulated for
dr y an d 'moist air an d in order to show how the
temperature
of the flame
ma y
vary due to
heat
losses by radiation, th e two cases:
a
No Heat Losses
b Radiation of 15 per cent. of the total heat input
have been considered. ' /
.
T ~ e : T a b l e s an d Graphs show how dilution Of the
burner .gas with ai r causes
th e
flametemperature to
-drop.' ;;Moi;;tiire'again appears to have a negligible
effect.
u ~ p u r ~ i o x i d e
ormation
Most of the points of theoretical importance have
been listed, an d all that remains to completejhe
picture is a consideration of
th e
factors influencing
the formation of SO
3
is known in practice
that
a small
amount
of this compound is always formed
along with th e SO2 in the burner, an d therefore
a knowledge of 'the conditions which favour SO
3
formation should be useful in that it would then
be possible to arrange an atmosphere an d conditions
in the burner which would, keep th e amount of SO 3 '
formed at an. absolute minimum.
When SO 3 is formed in the presence of excess
air, the following reaction occurs: . .
25 0
2
2S 0
3
This reaction is reversihle
an d
therefore it will
proceed from left to right until equilibrium is
reached. At this point, as much SO 3 as is being
formed will be dissociating once more
into
its
components., The equilibrium can only be dis
turbed by
removing one of the reactants or product
of
the.
reaction, .or changing the steady state of
conditions prevailing, otherwise,
if
temperature,
concentration, etc. are kept steady, then
t he
equilibrium will be stable an d constant for those
sets of conditions. .
For tuna te ly, this reaction has been studied. in
some detail by
many
observers and it is possible
to calculate the amount of SO
3 that
will be formed
under almost
an y
given
se t
of 'conditions: The
subject of catalysis
an d
the effects of catalysts have
no direct bearing on the theory of SO 3 formation
as it is conceded
that
catalysts do
no t
disturb the
equilibrium' point, bu t merely serve to speed up
the rate
Of
reaction, thereby ensuring
that
equili
brium will be reached in a shorter space of time. They
also allow lower temperatures to be used when
S0 3
is being formed. Catalysts are therefore of im
portance in' th e Contact Process for theproductiori
of sulphuric acid: .
The theoretical conversion percentages which
have been calculated here do not
take
into account
the time factor, that is to say; no allowance has
been made
forthe
period
that
is necessary to ensure
that
th e reaction reaches equilibrium before the
gases pass from the burning apparatus. Often
it will be found in plant practice
that
the conversion
of SO
2
to SO
3
as measured, is considerably less
than
that
predicted in theory under th e conditions
specified. Thus, the conversion figures listed ma y
be
taken
as the maximum possible, which might
never be attained in the plant.
.
....' . ,
In
orderto be in a position to predictthe amount
of conversion of SO
2
to SO3 under a given set of
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89
conditions, it is necessary to know
the
equilibrium
constant, K, for
the
reaction:
S02
+
i
0
2
S03
which occurs
at
various temperatures. This re
action, and not the previous one having double
the
quantities, has been chosen purely for convenience,
The
equilibrium
constant,
K, differs for
the
two
reactions, the former being
the
square of the lat ter,
but the
calculated conversions
would'
be
the
same
in each case.
For the
purpose of this paper,
the
equilibrium
constant has
been calculated from
the
average of
three
authorities, viz.
Fairlie
4
T 476
10glOI< =
T - 4.474
Lewis and Randall
4936
10glOK
=
T -
4.665
5186.5
10glOK
=
O.611log
1
- 6.7497
where K is the equilibrium constant
and
T is the
absolute temperature
in degrees Kelvin.
The above equations showthat K is dependent
upon temperatu re only. Natu ra lly, the equili
brium constant can be calculated from thermo
dynamic
first principles along these lines:
K [
S03J
h . th tivit
. [
S02J
[ OJt'
were a IS re ac IVI y co-
efficient for each respect ive reactant or produc t,
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, the
calculat ion of K from first principles is somewhat
lengthy,
but
should this expedient be necessary
or desired, reference should be made to
textbooks
on thermodynamics.
B 9 10
Knowing K, it is then possible to deduce the
expected conversion of SO
2
to
SO 3
tinder various
conditions. However, the conversions calculated
in this paper
were carried
out
on
the
lines suggested
by
Browning andKress.? . The equilibrium constant,
.K, was calculated
from
an equahonwhere the
variables were in terms of concentrat ions of the
various products as follows:
K=_x_
l Xjb 1..
2
ax
, 100 - -. lax
where a = initial concentration of SO2 in
where b = initial concentration of
in
where x = Fraction
( cOnverSion)
of S03
100
K is calculated from assumed values of
conversion
for a gas of specific SO2 content, and then the
equilibrium temperature for
that
value of K cal
culated from
the
Lewis and Randall equation given
previously. . ,.
Table
V
and Graph
Edetai l the
var iation of
equilibriumconstant, K, with temperature, Tables VI
VII
and Graphs
F and
G, record
the
equilibrium
temperature for
any
assumed degree of conversion
with a gas of known composition. '
With
a gas of known strength, t he amount of
SO3likely to be formed can be reduced
by
increasing
the
temperature. Generally, in order to reduce
the
possibility of trioxide format ion,
the
SO2
content of the gas should be increased,
.i.e. the
amount of excess air
used,
in burning should be
kept
at
a minimum. On
top
of this, high
corn
bustion chamber tempera tu res help to reduce
the
incidence of SO:I formation. However,
theory
indicates that it is literally impossible to prevent
SO2 undergoing a certain amount.of oxidation,
albeit a very small amount at temperatures over
1,000C. .
The sugar chemist is attempting to produce a
burner
gas containing S02 only,
and
none of
the
higher oxide,
and
in
the
light of
the
foregoing .theory
it would appear
that
this could best be accomplished
by the following:-
a
Maintaining the
quantity
of excess
air
used
at an absolute minimum for the plant in
question. .
b
Ensuring that the flame temperature. in
the
burner
is
kept
as high as possible
and heat
losses due to radiation are minimised.
c Arranging for
the
hot burner gases to be
cooled as rapidly as possible in order that they
need not be held at those temperatures for
any
length of time, where maximum conversion
of SO2 to SO:i is likely to occur.
d On top of this it would be helpful to
draw
the
gases through
the
burner as
rapidly.
as
possible, with the apparatus in use, to
deavour to reach a
state
'of affairs where the
reaction
250
2
0 2 2'50
3
, does riot have
time to reach equilibrium.' However,' this
rate of flow of
the
gases in
the
burner will
naturally be dictated largely by the rate at
which
sulphur can be burned without sublim
ation taking place.
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This completes
the
theoretical study of sulphur
burning, and all that remains is to observe what
occurs in practice. The theory is in fair agreement
with what
is experienced in
actual
pant operation
andit is hoped that
the
connections between theory
and practice will be obvious.
P RT
PR CTICE
The most convenient raw material for
the
pro
duction of SO2 is sulphur, and although there are
many
alternative sources available, sulphur still
proves
the
popular choice for a
variety
of obvious
reasons. The sulphur used in
the
sugar
industry
in this country is obtained almost solely from
the
U.S.A., this source being chosen with consideration
to price, quality
and
availability.
In
1949
the
production of crude sulphur in
America amounted to approximately 41 million
long
tons.v
mined by
the
Frasch-process
and
it is
interesting to note that
the
Texas Gulf
Sulphur
Company was the major producer. During the
same period
the
Union of South Africa is credited
with
the
importation of
just
over 65,000 long tons
of crude
sulphur.P
most of which was used in
the
manufacture of sulphuric acid.
can be seen,
therefore, that
the
sugar industry's requirements of
between 4,500
and
5,500 long tons per annum, are
quite small when compared with
the
total imports
into the country.
Sulphur is
abundantly
distributed in nature,
and
on the Gulf Coast it is usually associated with
salt dome intrusions. The element is found to
occur in
the
limestone, gypsum
and
anhydrite cap
rock
and
a few of these domes contain commercial
quantit ies of sulphur .i The occurrence of these
domes appears to be limited mainly to
the
Gulf
Coast region of
the States
of Texas
and
Louisiana.
Detai ls of the mining and production of sulphur
are contained in a highly informative circular en
titled Sulphur-General Information issued
by
the United
States Bureau of Mines-
and
anyone
interested would be well-advised to procure a copy
of this publication.
The element sulphur is non-metall ic
and
exists
in a var ie ty of allotropic forms
and
therefore
may
be said to have a series of properties depending on
the
form in which it
found.
Natural
sulphur
usually occurs as the
more
stable rhombic or
a-sulphur form. However, it can exist in
many
forms,
bu t
they all tend to revert to the rhombic
form. A study of the various properties
and
allotropic forms of sulphur is outside the scope
of this
paper
and a brief account of those properties
of relative importance to sulphur burning only,
will be given. Sulphur, S 8 ' is a bri tt le, yellow ele
ment which is solid at room temperature, is in
soluble in water, bu t
will dissolve in carbon bisul-
90
phide. The rhombic form melts at 112. 8C. and
the
melt forms a mobile liquid which becomes more
viscous as the temperature is increased, unti l at
2 C it becomes so thick it will not flow. However,
at
350C., the melt again becomes mobile. It ignites
at
about 248C., finally boiling
at
444.6C.2 The
above temperatures should be
taken
as approximate
only as Mellor gives specific figures for
the
known
allotropic forms
and any
particular allotropic
mixture would probably have its own characteristic
physical
and
chemical constants.
A typical analysis of the sulphur supplied to the
sugar industry is roughly as follows:-
16
Moisture
at
105C.
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 Negligible
Acidity as H 2S0
4
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of
the
burner, bu t it is possible that in
many
cases
they
may
reach a proportion where they cause
trouble.
Types of urners
a
Rotary urner
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
with
unskilled labour
and
that installation
and
operating
costs are low. Furthermore, they emphasise that
losses due to sulphur sublimation or formation of
SO
3
can be eliminated almost entirely.' With
this type of burner it is possible to operate con
tinuously at SO 2 concentrations ranging between
5 per cent. and about 18 per cent. In order to
prevent fluctuations in gas strength it is desirable
to
feed
the
sulphur by mechnical means and not
by hand.
The sulphur in the lower part of the horizontal
rotat ing drum forms a molten pool as well as a thin
film around the circumference of
the
drum. This
film, in effect, increases
the
surface area of the
sulphur exposed to the incoming air
and
a further
increase in surface area is brought about by the
sulphur that drips down from the
top
of the drum.
These factors help to increase the capacity of
the
burner and ensure that combustion is complete.
On
top
of this, it is claimed
that the
sulphur film
protects the drum metal from deteriorat ion due to
hot
SO 2 gases as
the
heat conductivity of sulphur
is less
than that
of cork.
A combustion chamber is fitted at
the
back of
the horizontal drum and this serves to complete
the
combustion of any sublimed sulphur as well as
to mix the gases and dilute them to
the
desired
strength. This chamber is therefore constructed
so as to have air ports and one or more baffles.
Impurities in the sulphur do not readily affect
the
operation of these burners because
the
rotation
of
the
drum agitates the molten sulphur sufficiently
to prevent any blanketing films forming on the
surface. A further refinement that can be added
is a sulphur melter and the heat of radiation from
the
burner
may
be utilised to keep the sulphur
molten. Steam heating coils in
the
Melting Tank
are only required for starting up the burner after
a long shut-down. In order to further improve
the
efficiency of this machine, the molten sulphur
should be strained prior to passing it to the burner.
The Teeding of molten sulphur ensures that the
burner is handling moisture-free material.
Ideal results are obtained with this type of burner
by using sulphur of a minimum
purity
of 99.6
per cent. .The molten sulphur in the horizontal
drum should be at the highest possible level without
allowing any overflow to take place.
91
The makers suggest
the
following capacities for
a burner 30 in. in diameter
and
8 ft. long.
Capacity with l in. water draught is 1351bs. Sulphur
per hour.
Capacity with in. water draught is 200 lbs.
Sulphur per hour.
Capacity with 2 in. water draught is 270 lbs. Sulphur
per hour.
A burner of identical diameter,
but
only 6 ft. long
would have corresponding capacities of about 75
per cent. of those given above. As a rough guide,
it may be taken tha t : .
For a
1
in. draught,
the
burner consumes approxi
mately 2 lbs. Sulphur/sq. ft yhr
For
a
in. draught, the burner consumes approxi
mately 3 lbs. Sulphur/sq.
ft /hr
For
a 2 in. draught, the burner consumes approxi
mately 4:.2 lbs. Sulphur/sq.
ft.yhr.
Another authoritv considers that a burner 36 in.
in diameter
and
ft. long is capable of handling
nearly 8 lbs. of Sulphur/sq. ft./hour. Fairlie
finds it practical, without resorting to high draught,
to run a burner
ft. in diameter at a rate of 1 ton
of Sulphur per
day
per foot of length.
The size of the combustion chamber is variable
between fairly wide limits and it should be borne
in mind that a large chamber permits a higher
concentration of SO 2 in
the
burner gas without
danger of sulphur sublimation. is suggested that
a chamber spa e of 60 cubic feet per ton of sulphur
per 24 hours- is adequate, while the makers of this
type of burner indicate that a minimum space of
it cubic feet per pound of sulphur per hour per
square foot of burner area should suffice for burners
up to 30 in. in diameter. These figures should be
taken as approximate indications only
and
no
hard
and fast rules seem to apply. The size of
the
combustion chamber should be dictated by practical
considerations
and
previous experience.
Sutermeister' states that
rotary
sulphur burners
produce a gas of varying composition owing to
sudden rushes of cold air through the apparatus,
but
Fairlie claims that with a continuous feed, it is
possible to maintain
the
gas at a uniform con
centration of SO 2 with only minor fluctuations.
He stipulates, however, that this is dependent on
the
depth of the molten sulphur in
the
rotating
cylinder being kept uniform. Any change in this
depth will alter
the
area of
the
unsubmerged film
covered surface and this in tum will alter
the
rate
of combustion of sulphur.
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b The cme Burner
This
type
is deigned to give a constant rate of
burning for any given concentration of SO 2 in the
gas. The makers, Acme Coppersmithing and
Machine Company, point out that the rate of com
bustion of sulphur bears a definite relation to the
amount
of air supplied. The amount of air re
quired for a given concentrat ion of SO 2 is given by
the
expression:-
o/SO . C b ti G 90.65
2 In om us On ases
A 0.026
where A
lbs. of air
at
70F. per lb. of sulphur
burned when the burner is operating a ta
rate
of
t
lbs. of sulphur per square foot of burning surface.
Should the sulphur burning rate be changed, then
the above formula has to be modified slightly.
is claimed
that
a
tray
or rotary burner does
not
satisfy the requirements of a definite constant
burning
rate
because of the following
factors:-
1
Ordinary sulphur contains organic matter
and this accumulates as a carbonaceous film
on the surface of the burning sulphur which
may eventually extinguish the flame.
(2) Additions of fresh charges of sulphur to the
burning surface disrupts the burning rate and
normal conditions are only returned to afte r
s v r ~
hours have elapsed.
With this burner, the makers have overcome the
first drawback by operating the burner at a rate
greater
than
2
lbs.of
Sulphur per Square foot of
burning surface per hour, finding that this prevents
the formation of carbonaceous scum by burning it
off with the rest _of the gases. The drawbacks of
factor (2) were prevented
by
arranging a special
feeding device
that fed molten sulphur in a manner
that did not dis turb the burning surface, but at
the same time, ensured that a constant level of
molten sulphur was maintained in the burner.
This
burner is opera ted on compressed air
and
a special sulphur melter is supplied with the
apparatus. Sulphur is fed to the
bottom
of the
burner
via a main feed pipe, and a side feeder arm
on this main feed allows for positive control of
sulphur feed, thereby guaranteeing a constant
level of molten sulphur in the burner.
All the data. given here and the brief description
of the principles behind this burner were taken from
a pamphlet issued
by.
the manufacturers, viz.
Acme Coppersmithing and Machine Company of
Oreland, Pennsylvania. A description of this burner
may be found in the technical literature.19
c
pray ype
Burner
The Research Department of the Texas Gulf
Sulphur Company is accredited with
the
invention
of this burner. The development of apparatus
was described in a technical paper issued in 1934
20
and it was therein explained how the burner was
operated. Briefly, molten sulphur is fed to a spray
nozzle which injects a fine spray of this material
into the burner. At the same time the desired
amount of air is forced into the burner chamber
and ignition of
the
sulphur takes place. The
burner chamber is fitted with baffies to mix the
resultant gases and prevent any unburned sulphur
passing through to the absorption apparatus.
The burning
and
combustion chambers are in one
uni t. Normally,
the
burner chamber is lined with
fire-brick to reduce heat losses and minimise the
formation of SO 3
Certain advantages are claimed for this burner
and among those listed are:-
(1) Operation is simple and the burner requires
little
attention
once
the rate
of combustion
has been set. The combustion rate can be
changed simply by altering the stroke of the
sulphur
pump
and adjusting the compressed
air supply accordingly.
(2) Starting up and shutting down operations are
comparatively easy, and if the burner is started
up when cold, maximum gas concentration can
be reached within } hours. In order to shut
down it is merely necessary to stop the sulphur
metering pump and shu t off
the
compressed
air supply.
(3) The burner is extremely flexible insofar
as
rate
of combustion is concerned
and
there
fore it is possible to vary the capacity of the
burner within fairly wide limits.
(4) will produce a gas with an SO
2
content of
anything up to 20 per cent. when run con
tinuously.
(5) Sublimation of sulphur should not occur
provided proper care is taken, and since there
is no burning-down period, aswith conventional
equipment, the hazards of possible sublim
ation are reduced to an absolute minimum.
(6) Provided the gas concentration is kept high
and
the burner is lined with refractory brick,
l itt le SO 3 should be formed
at
the elevated
temperatures (2,400 to 2,700F.) of operation.
During tests it was found that the SO 3
content of the gas passing to the cooler was
only 0.14 per cent. of the sulphur burned.
(7) The maintenance costs are not likely to be
high and the initial cost of installation should
compare favourably with that of any of the
more usual types of burner.
(8)
Kress
found
that
power costs could be re
duced by 75 per cent. over that of conven
tional burners.
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93
Anyone wishing to study in detail the design of
a spray type burner is well-advised, to read the
paper by Conroy
and
johnstone on this subject.
The authors indicate that the internal volume of a
rotary burner should be 13/14 cubic feet per ton of
Sulphur per day and on top of this a combustion
chamber space of 60 cubic feet per ton of Sulphur
per day
is required, whereas the
total
volume of a
spray type burner is 24 cubic feet per ton of Sulphur
per day.
It is felt
that
the three burners listed cover the
types of particular and possible interest to the sugar
industry. In order to complete our practical con
siderations of sulphur burning, it is only necessary
to study how SO3 formation. occurs and what can
be done to minimise its formation as this information
should then make it possible to successfully produce
a gas that is rich in SO2and, at the same time, free
of all but traces of SO3
The Formation and Dissociation
SO
3
The conditions favouring the formation of SOa
are:-
(i) Decomposition of any sulphuric acid
that
may be present in the commercial sulphur.
(ii) Formation of trioxide during the actual
burning of the sulphur.
(iii) Oxidation of the SO 2 present in the burner
gas.
The first source accounts for only a relatively
small amount of the SO3normally found in burner
gas. Naturally, sulphur will oxidise. to a certain
extent in the presence of moist air, but the amount
of sulphuric acid formed is bound to be extremely
small.
Undoubtedly, a certain amount of trioxide will
be formed during the burning of the sulphur, but
this is difficult to determine accurately. t will
depend, to a large extent, upon the proportion of
excess air present and upon the temperature reached
in the burner.
t
can be seen therefore that the
SO content of the gas should be as high as possible
for the type of burner in use.
The third source, i.e. the oxidation of SO
accounts for the major portion of trioxide found in
a burner gas
and
consequently this aspect of the
problem will have to be examined in fairly minute
detail. In practice it has been found that oxidation
will take place only unti l a certain ratio of dioxide
to trioxide is reached At this point equilibrium
is 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.. This
equilibrium may be distrubed or changed by altering
the concentration of the reactants on either side of
the equation:
O
2
2S0
2
2S0
3
A rotary steel or cast-iron burner operating
continuously has been found to convert between
1 per cent. and 2 per cent. of the S02 into S032,
while a similar burner lined with refractory brick
will produce a gas of much lower trioxide content.
Obviously, then, the materials of construction have
an effect upon the oxidation of SO2and an investig
ation led to the discovery
that
certain metals or
their oxides could increase the rate at which trioxide
was formed. These metallic substances are known
as catalysts and while they do not take part in the
reaction in a quantitative way, that is to say,
they
are not used up or depleted during the course of
oxidation, they most certainly speed up the rate of
reaction. These substances, therefore, warrant
study as well. .
t will probably be advisable to study the various
effects of physical and chemical conditions tlpon
trioxide formation separately to ensure that the
picture be complete.
Effects
Excess
ir
Comog and co-workers found that when sulphur
vapour was burned in the presence of 250 per cent.
excess air at 4:60C approximately 3.4 per cent.
to 3.8 per cent of the sulphur appeared as the
trioxide. However, Browning and Kress observed
less than 0.02 per cent. conversion under similar
conditions in the absence of a flame. The obvious
inference is that atomic oxygen in a flame has a
considerable influence upon the reaction. The
results of their experiinents indicated
that
trioxide
formation could be reduced by decreasing the
equilibrium conversion of SO2 to SO 3 by keeping
the quantity of excess air at a minimum in con
junction with high combustion chamber temper
atures. A perusal of the theory will show agreement
with this statement.
Obviously, then, the amount of air supplied to
the burner should be carefully metered, or, failing
that, regular analyses of the gases from the com
bustion chamber should be carried out to ensure
that the SO
2
content is being maintained at a
maximum.
A rough guide to the correct amount of air re
quired is based on observation of the flame in the
burner. the sulphur burns with a blue flame,
conditions are just right. A flame with a brown
tinge indicates insufficient air and sublimation of
sulphur. However, these observations do not
prevent one from operating the burner with an
excess quantity of air and one is well-advised to
rely more on chemical tests and mechanical aids
than on rule-of-thumb methods such as those men
tioned above.
A study of Table
VII
will show how the quantity
of excess air can influence the formation of SO3
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94
Should the burner be operated with insufficient
air to complete the combustion of sulphur, then
sublimation takes place and slight mists of unburned
sulphur may form.
nfluence Temperature
Theory has already indicated to us that temper
ature exerts a considerable influence upon the
amount of trioxide formed. In practice it has been
found that the optimum temperature range favour
able to the formation of SO 3 lies between
MOC
and 980C22 Frohberg- found that maximum
oxidation occurred at 400
0 500e.
while at an
approximate temperature of 900/1,000C., dissoci
ation took place. t has been shown that at
1,000C. nearly 50 per cent. of the SO
3
formed is
decomposed, this decomposition starting at 700e.
at which temperature the conversion of SO2 to
S03
is about 60 per
cent.
Above 1,000e. the
dissociation of the trioxide is more rapid than its
formation. t may be generally stated that below
400e. the rate of formation of SO3 is too slow to
be regarded as a nuisance, while above 1,000e.,
even though SO 3 is formed rapidly, the rate of
decomposition predominates. However, even at
305e., oxidation of S02 has been found to occur,
albeit at a very slow rate. Thus we have two con
flicting states, viz. where the rate of conversion of
SO2 to SO
3
increases rapidly with increasing
temperature,
but
where the dissociation of SO3
overhauls the rate of conversion, and at the higher
temperature (above 1,000e.), therefore, the per
centage conversion, as measured, tends to decrease.
The reaction:
2S0
2
O
2
2S0
3
takes place nearly to completion at 450C in the
presence of a catalyst , and. it has been shown
that
while the
rate of the forward reaction is only
moderate at 400e., it increases to 40 times this
value at 500e. the decomposition reaction only
becoming perceptible at 550e. and upwards.
t may therefore be said that up to a temperature
of 450C., reaction of formation prevails, and only
well above this temperature does dissociation come
into play.25
Sulphur trioxide is very stable in the absence
of contact substances.
and
once formed is difficult
to dissociate. Generally, decomposition will not
take place completely at temperatures as high as
1,100/1,200e. Decomposition of trioxide already
formed cannot be relied upon to keep the loss from
this source at a reasonable value at temperatures
below 1,00e., the amonnt of dissociation at this
temperature only reducing the loss by an amount
of less than 0.5 per cent.
It is well worth .remembering that the final
equilibrium condition of the reaction depends only
upon the temperature
and
the composition of the
gas mixture, and oxidation of SO2 will take place
until a certain ratio of trioxide to dioxide is reached.
High temperatures favour a low ratio and low
temperatures a high ratio of SO
3
to SO
2
7
Presence Moisture
The air drawn into a burner is not normally
dried,
but
to prevent the formation of corrosive
sulphuric acid mists, the water content of the air
used should not be more than 5 mgm. per cubic
foot of air at S.T.P.24 Under conditions prevailing
in Natal, this would involve drying the air with
either concentrated sulphuric acid or P 205 or :a
similar efficient drying agent.
the air is thoroughly dried with P 205 there
is little oxidation of SO2 up to a temperature of
450e. and in general it may be stated that dry
air helps to retard oxidation. In the presence of
water vapour, oxygen does not combine with SO2
at 100e.
but oxidation does occur, even at this
low temperature, if particles of liquid water are
present.6 Mellor found that moisture does not
appear to affect the oxidation of SO 2
but
the
presence of CO2 and nitrogen causes more SO3
to be formed.
Moisture has a poisoning effect on some catalysts
and Tolley has shown this to be true for iron
oxide. Upon continued exposure, however, cata
lysis slowly increases until after 40 hours the
catalysis with wet gases is half that of dry gases.
He found
that
water vapour has its greatest in
hibiting action at a temperature of 475e., put
even at 635e., the catalytic act ivity of iron oxide
is reduced to of
that
with dry gases.
Browning and Kress? investigated the dew-point
of burner-gas mixtures and showed that the cor
rosion to be expected would be greatest at the dew
point of the particular gas because of the combined
action of scale and condensed liquid upon the metal
base. Above the dew-point, only gases are present
and corrosion is much reduced, while at temper
atures below the dew-point, the rate of corrosion
is considerably less on account of the slowing up
of the chemical reactions at these lower temperatures.
Their experiments revealed that the dew-point
increased as the SO 2 content of the gas was raised
and for safe operating practice, using moist air,
the temperature of the gas in iron should be kept
above
200C
to prevent undue corrosion of the
metal. When the gas is required to be cooled
below this temperature, it should be transferred to
a lead pipe.
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As previously stated,. moisture in the air can
cause mists to form in the burner gas
and
these are
most difficult to, remove. A method which helps
in the removal of mists consists of cooling the mixed
gas to below 46C., when the S03 will condense.
The gas should now be passed through a filter-box
packed with charcoal or iron borings
and
then
through a water-spray .scrubbing tower. This
method, however, is not always very successful
and it is easier to prevent mists than, having once
formed, to remove them.
ction Catalysts
Catalysts do not affect the equilibrium point of
a
gas
mixture,
bu t
merely alter the rate at which
equilibrium is approached. The ratio of the original
and
final substances present in the gas will not be
changed upon contact with a catalyst . The contact
substance will only ensure that this equilibrium
ratio, is reached in less time.
A list of various contact substances normally
encountered follows, and these, with their effects
upon SO3 formation, have been listed separately
for the sake of clarity. Anyone wishing to know
more about the effects of various catalysts on SO3
formation, should read the monumental work of
Browning and Kress, as this covers the subject
most comprehensively.
Iron Compounds
Iron or its oxides have long been known to exhibit
cata lytic properties as far as the oxidation of SO2
is concerned and the Mannheim Process for the
manufacture of suphuric acid utilised iron oxide
as a catalyst.
For
reasons of efficiency, this oxide
has been replaced by either plat inum or vanadium
in
the
Contact Process.
Tolley predicted tha t the first reaction to occur
when steel was in contact with sulphur dioxide
and
oxygen at reasonably high temperatures, would
be a combination of oxide and sulphide formation,
represented as
2Fe
S02-
2FeO
S
Fe
S- FeS
or
alternatively:-
5Fe
2S0
2
- 2FeS'
Fe 30 4
He pointed out that as FeO cannot exist below
570C., the first reaction shown above could only
occur above this temperature. The experiments
showed that the catalytic activitiy of mild steel
increased rapidly during the first few hours of
exposure to S02 but after about 10 hours the
activity became reasonably constant. was felt
that this initial rapid increase in the rate of oxid
ation of SO2was probably due to the formation of
iron oxide.
95
The catalytic effect of iron compounds depends to
a large extent upon their physical
and
chemical
state . Experiments have shown
that
iron oxides,
as such, are not good catalysts, bu t their activity
may be increased after a certain period by the
formation of sulphates and other compounds.
At elevated temperatures both the dioxide and
trioxide
react with iron, and this would explain
the short life of iron under these conditions of
service.
In a rotary sulphur burner, there will be surges
of SO3 formed when star ting up or burning down
as more iron will be exposed during these periods.
Furthermore, the temperature range at which
maximum SO3 formation occurs will be passed
through during these stages.
Reverting to the influence of physical state upon
catalytic activity, it is worth noting
that
ferric
oxides may exhibit contact properties,
and the
surface of the material will exert the least influence
when freshly precipitated oxides, which are not yet
dried, are used. The activity increases the oxide
has been moderately heated or kept for a long t ime
so as to become dry. Oxides obtained by heating
ferric- or ferrosulphate give a much lower contact
action than that obtained with an oxide prepared
by igniting a precipitated hydroxide or pyrites
cinders. An oxide containing 21 per cent. Arsenic
as As shows considerably more contac t reaction
than that of a pure oxide at 700C., while
the
addition of copper oxide to an oxide of iron is
favourable to the formation of S03.
26
Observations have shown
that
when a burner
gas is in contact with an iron pipe, maximum
conversion occurs at 750C., there being a 13per cent.
maximum conversion for a gas with a
10.5
per cent.
S02 concentration. This conversion drops to 10
per cent. when the SO2content is raised to 14 per
cent. and a further decrease in conversion to 3 per
cent. for a gas of 19.5 per cent. content is observed.
At 1,000C., the conversion at all concentrations
IS
zero.
Ferric oxide begins to exhibit catalytic act iv ity
at 550C., and this activity increases to a maximum
in the temperature range 600
0 620C
6
Increasing
the
S02
content of the gas from
2-12
per cent.
does not affect the conversion to
any
appreciable
extent, although with higher concentrations,
the
yield of SO3 is lower. As a contact substance,
this oxide only becomes really effective at a temper
ature of
600C
when a percentage conversion of
SO2 to SO3 of 40 to 50 per. cent. is attainable.
In practice, however, the conversion never exceeds
60-66 per cent.
Iron oxide formed from pyrites-cinders shows
lit tle activi ty, and this only begins above 650C., .
increasing with temperature
and
reachirig a con-
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10/16
version of 1
percent.
at 1 000C. with a gas con
taining 15 per cent. SO
However it should be
considered worthy of note that if 3 per cent. of SO
3
is formed in a burner gas during normal burning
operations this figure may under favourable
conditions be raised to three or four times this
value if the gases are passed through red-hot
pyrites-cinders.
Pure iron oxide exhibits maximum contact action
at 650C. the respective conversion figures being
15
and 5t
per cent. for gases of 10.5 and 19.5 per
cent. SO 2 con tent . These conversion figures drop
to 4i and
Ii
per cent. respectively when the temper
ature is raised to 1 000C. the activity in this
case being only above
5 C 7
Ferrous sulphate has a m aximum c onta ct action
at
65 C
when t he conversion of a gas containing
15 per cent. S02 is 12t per cent. this conversion
decreasing to t per cent. at 1 000C. Similar con
version figures for ferric sulphate are 8 per cent. at
65 C and
t
per cent. at 1 000C.; indica ting
that
it is less a ctive
than
the ferrous form.
All the above observa tions indica te that iron in
its many forms exhibits varying degrees of catalytic
a ct iv it y and if the formation of. SO
3
is to be mini
mised then obviously gases of high S02 concen
tratio n must be produced at the highest possible
combustion c ha mber tem pe ra ture that can be
a tt ai ne d in practice.
is of further benefit to
note
that
in many instances t he amount of SO
3
that
will be formed according to t heo ry will not be
reached in practice as the gas passes through the
combustion apparatus before equilibrium is reached.
Silica and Silicates
is generally conceded that vitreous fused
silica exerts no c at al yt ic effect on the oxidation of
S02.7 The same applies to Dialite brick and
therefore these materials are considered satisfactory
for the lining of furnaces c ombustion chambers
etc.
The Efficient Operation a urner
In America it is a generally accepted fact
that
the air to the burner should be carefully metered
by mechanical means and furthermore allowance
is made fo r fluctuations in the burner. These
changes in burning rate are catered for by the
installation of an automatic recording device which
continuously analyses the SO 2 content of the
gases from the combustion chamber. A dia phra gm
operated valve then automatically operates the
air inlet and sulphur feed and regulates these
t he re by keeping t he SO 2 co nt en t of the b ur ner gas
constant within fairly narrow limits.
96
Furthermore the sulphur feed should be
mechnical to enable a constant rate of feed to be
maintained.
With.
all these refinements it is
possible to arrange for high combustion temper
atures and a burner gas containing the highest
concentrati on of SO 2 possible with the type of
apparatus in use. Even
better
results will be
obtained if the sulphur. is melted
and
strained
before it is fed to the burner for reasons already
gIven.
The combustion chamber should be fitted with
one or more baffles to ensure thorou gh m ix tu re of
the gases and p reven t
any
possibility of unburned
sulphur passing to the absorption plant. Coolers
are normally fitte d a fter the combustion chamber
and it is imperative that cooling of the gas be as
rapid as possible to minimise SO
3
formation.
In it iall y the gases would be air- o r. water-cooled
and t hen passed to eit her a direct or indirect cooler
to bring the temperature down to 200/300C.
At this stage if further cooling is a ttem pted the
pipes conducting the gases should be lead-lined.
The subject of cooling is outside the scope of th is
paper but is well-worth : pursuing. by reading
Lundberg- and others.
Summary
The practice of sulphur burning the production
of SO 2 and the formation and dissociation of SO
3
has been outlined and the effects of:
Excess Air
Temperature and
Catalysts
upon SO
3
formation detailed. Where possible a
comparison with t he th eo ry has been made and it
has been shown
that
the following points all help
to increase the efficiency of this type of
plant:
a
is preferable to feed the sulphur con
tinuously by mechnical feeder. The feeding
of strained molten sulphur is preferable as
this eliminates moisture and deleterious hydro
carbons.
b The quantity of excess air should be carefully
controlled and the SO2c ontent of the burne r
gas k ep t at a maximum.
c
The temperature of the burner and com
bustion ch amber m us t be mai nt ai ned above
1 000C. if at all possible.
d Automatic recording
and
operating
apparatus
.to maintain an even feed of sulphur and
constant SO 2 content of the gas is helpful.
e
The gas from the combustion chamber should
be filtered and cooled as rapidly as possible
to pre ve nt formation of SO3
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Conclusions.
The authors have made everv endeavour
and
taken all possible precaution s to ensure that the
information given is accurate. However, mistakes
may
have cropped up
and
no responsibility can be
taken for such errata.
Many of the improvements listed
may
possibly
make the production of SO 2 by these methods rather
costly
and
their inclusion should not be taken as
a recommendation, but rather as a guide or example
of how efficiency may be improved. .These refine
ments are in practice in America in many of the
larger paper mills, so obviously they are economical
for the production of large quantities of SO
is admitted that it is literally impossible to
prevent the formation of SO3 entirely, bu t if pre
cautions, along the lines of those mentioned in this
paper, are taken, the
quantity
of SO3 can be reduced
to such a low level
that
it no longer constitutes
a nuisance.
The types of burners listed were limited to those
generally used in the. Sugar Industry and those
which may be of interest. The spray type burner
is proving very popular in America and it produces
a gas of low SO 3 content and furthermore it is very
flexible in that it may be started or shut down in
a very short space of time. Control is easier than
with the conventional rotary burner and paper
mills in America have found
that
installation costs
are not excessive. Maintenance costs are low
and power consumption compares most favourably
with other types.
For the sake of those who would like to know
more about sulphur burning and its applications,
a reading list has been added.
Acknowledgments.
Our sincere appreciation
and
thanks are duly
made to those firms and Institutions in America
who corresponded with us, supplied technical
articles and generally went out of their way to be
helpful. In particular we would like to mention:-
The Paper Institute, Texas Gulf Sulphur Company,
Acme Coppersmithing
and
Machine Co.,
and
the
Glens Falls Machine Works all
American organis
ations that were extremely helpful
and
co-operative.
REFERENCES.
1Beater:
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.
3Chemical Control
Plant Da ta :
Booklet issued
by
Chemical
Construction Company.
4
Fairlie: Sulphuric Acid Manufacture.
5Lewis
and Randall : Thermodynamics a nd th e Free Energy
of Chemical Substances.
6 Mellor: A Comprehens ive Treatise on Inorganic an d
Theoretical
Chemistry Vol.
X
97
7Browning and Kress : A
Study
of Some Factors Influencing
t he Formation and Dissociation of 50
3
in Burner Gases Paper
Trade Journal 100 No. 19, 31-43, 1935.
8Glasstone: Thermodynamics for Chemists.
oDodge: Chemical Engineering Thermodynamics.
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.
2 Darrah: The Preparation of S02 Paper
Trade Journal
pp. 132, Nov. 30, 11 50
3
Sutermeister: Chemistry of
Pulp
and Paper Making.
14 Josephson an d Downey: Sulphur and Pyrites U.S. Bureau
of Mines Yearbook, lU49.
15Ridgway:
Sulphur General Information U.S.
Bureau
of Mines I.C. 6329. . .
6 Sulphur: Technical Service Note No. 32 African Explosives
an d
Chemical Industries,
Ltd.
7 Furniss: Rogers Manual of
Industrial
Chemistry.
8
General Description of Acme Burner Pamphlet from
Acme Coppersmithlng
and
Machine Co.
Cain
an d
Chatelain: New Low-Capacity
Sulphur
Burner-
Chemical
and
Metallurgical Engineering, 46, Oct. 1939.
2 Kress
and
Others:
Spray
Type. Sulphur Burner The Paper
Mill, Oct.
11 34
2
Conroy
and
Johnstone: Combustion of
Sulphur
in a
Venturi
Spray
Burner Industrial
an d
Engineering Chemistry 41, pp.
2741, Dec. 194U.
22
Barker:
Sulphite Acid Preparation Paper
Trade Journal
pp. 136, Nov. 30, lU50.
23
Newell. Stephenson: P ulp a nd P ap er Manufacture Vols. I
II and III.
2