Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for preparing sulfur
dioxide from liquid sulfur by combustion of the sulfur using technically pure
oxygen.
It is known to prepare sulfur dioxide on an industrial scale by
total or partial condensation or by hydraulic washing (Grillo process~ of
sulfur combustion gases containing S02 or of gases from roasting; Air being
used as the oxidant for the combustion.
It is also known to burn fine grained powdered sulfur ~cf. German
Reichs patent No. 191,956) or vapours sulfur ~cf. German Reichs patent No.
437,910) with oxygen. According to this process an excess of sulfur causes
a reduction of the reaction temperature. Sulfur may also be burnt by intro-
ducing bubbles of heated oxygen into liquid sulfur ~cf. German Reichs patent
No. 539,640), especially by means of an immersion heater ~cf. German
Reichs patent No. 711,537).
German Offenlegungsschrift No. 2,159,790 discloses a process
for partially burning sulfur with oxygen by admixing S02, wherein the maximal
combustion temperatures are at 2000C. German Offenlegungsschrift No.
2,223,131 discloses a process for burning sulfur with oxygen or with oxygen
enriched air while simultaneously recycling cold S02 to the sulfur combustion
chamber.
The processes already carried out on an industrial scale require a
high energy expenditure (total condensation of S02 in the presence of large
quantities of inert gas or in combination with an oxidation or absorption
plant in the case of the partial condensation of sulfur dioxide). The cited
processes also demand high investment costs and are only suitable for plants
having a daily maximum output of 1~ to 20 tonnes of sulfur per combustion
unit. It is important for processes operating in the presence of air to main-
tain the temperature in a range where the formation of nitric oxides is aslow as
possible. In the process of German Offenlegungsschrift No. 2,159,790 operating
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with oxygen several steps are used in order to assure that theoretical com-
bustion temperatures of about 3000C are not produced; processes providing a
combustion in several steps, however, demand high investment costs.
Accordingly, it would be advantageous to prepare pure sulfur dioxide
on a large scale in a manner as cheap as possible by oxidizing sulfur with
technically pure oxygen. The present invention is based on the problem of
avoiding the inconveniences of the known technical processes. It should be
possible to obtain a combustionof more than 99.9% in one step with a
stoichiometric proportion within the accuracy of measurement of the products
used as starting components.
Accordingly, the present invention provides a process for preparing
sulfur dioxide from liquid sulfur and oxygen which comprises combining and
burning technically pure oxygen containing at least 70% by volume 2 and
finely dispersed liquid sulfur using a two component nozzle burner or a
rotation burner without dilution by sulfur dioxide, the molar proportion of
02/S bèing about 1.0 to 1.55, collecting the hot gases in a combustion chamber
having a heat resistant wall, the outer side of said wall being cooled and
the inner side of said wall having a temperature of at least 1200C and
subsequently cooling the hot gases. Under technically pure oxygen there is
especially to be understood oxygen having an 2 content, of more than 90% by
volume. The higher the 2 content is, the less is the ballast gases which
must be fully separated from the S02 for~ed. Pure S02 may be condensed from
the sulfur combustion gas by cooling. The combustion gas may also be further
worked up without separating liquid SO2 by condensation. In this case, it is
sufficient to cool the combustion gas to the temperature required for the further
working up (for example to a temperature of 450C for preparing SO3).
Simultaneously, the heat of reaction may also be utilized for the economic -
production of steam. The process may be carried out a~ elevated pressure, but
this does not considerably reduce the costs of the process, especially with
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respect to the heat e~changer~radiation part of the boiler~ where the released
reaction heat is utilized. The process is preferably operated under a pres-
sure of 1 to 7 bars. The process of the present invention disperses with
cooling by using recycled cold sulfur dioxide.
A molar proportion of 02/S of about 1.55 is advantageous used espe-
cially in the case where SO3 is to the final product produced from the sulfur.
It is quite surprising considering German Offenlegungsschriften 2,159,790 and
2,223,131 that the flame temperatures do not exceed 2000C in the combustion
of stoichiometrical mixtures of S/02. An equilibrium obviously is produced at
high temperatures preventing a complete reaction. The rest of the reaction
would be effected synchronously with the cooling of the flama gases.
According to the process of the present invention only small
quantities of sulfur trioxide are observed in the sulfur dioxide gas.
However, it is nevertheless recommended for the production of
technically pure S02 to connect a washing colunnafter the steam boiler filled
with 99% sulfuric acid to absorb the residual contents of SO3 and to
further cool the gas to a temperature of from 40 to 80C. The small quantities
of any excess oxygen are recovered in the condensation of the sulfur dioxide
and admixed with fresh oxygen.
Since only small gas volumes are used in the process of the
present invention, only a small apparatus is needed; the output however being
high. Accordingly, the investment costs for plants operating according to
the process of the present invention are relatively low. The quantity of
sulfuric acid necessarily obtained can be minimized (about 1 kg/100 kg of
SO2) owing to the small portion of SO3 in S02. The output of inert gas
introduced in small amounts with the technically pure oxygen can also be
rendered insignificant. It is, moreover, possible to free these small
quantities from SO2 by washing them, for example, with sodium hydroxide so-
lution so that noxious emissions do not occur.
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The flame may be produced by used burners which enable the sim-
ultaneous introduction of liquid sulfur and oxygen such as two components
nozzle burners as well as rotation burners. The proportion of liquid sulfur
and oxygen is not cirtical. The process may be operated with stoichio-
metrical quantities, with a slight excess of 2 (for example of up to 7%) or,
if the sulfur dioxide formed is to be further oxidized to give SO3, with a
higher excess of 2 of about 55%. In all cases the measured maximal temperaturesof the flame are at about 2000C.
The invention will now be illustrated by way of example using the
accompanying drawing which depicts a flow scheme. The hot gases produced in
the burner (1) are collected in a combustion chamber (2) having heat resistant
walls. Chambers lined with highly fire-proof stones have proved particularly
successful. The walls are advantageously cooled on the outside. Uncoated
steel tubes which are cooled may also be used; the good heat tranfer, however,
results in a rapid temperature decrease of the reaction gases and, consequently,in a possible residual content of unburnt sulfur. The thickness of the wall
lining must be such that the inner wall temperature is at least 1200C, pre-
ferably from 1400 to 1500C. In this case the temperature of the zone of in-
candenscence formed at the wall is still so high that inspite of the outside
cooling a sufficient quantity of sulfur is still burnt. Only in the case of
small sulfur outputs i.e. in the case of sulfur outputs substantially smaller
; than those just obtainable in a given appratus the process may be operated
from time to time without outside cooling owing to the backflow of the cooled
flame gases. The combustion gas only contains a small quantity of SO3 (of
about 0.1 to 1% by volume). A heat exchanger preferably consisting of a
radiator and a convection part is connected after the reactor (3) in order
to utilize the reaction heat. In order to control the temperature of the
waste gas in the case of varying charges the cooling surfaces may be ad-
vantageously subdivided. Cooling of the combustion gases to about 280 to
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100C may be effected, for example, in an economizer (not illustrated in thedrawing). Thereby the feeding water of a steam producing boiler is heated at
the pressure side of the pump. The combustion gas is then further cooled to
about 40C in the following sulfuric acid washer (4). The small S03 content
is simultaneously removed thereby. Pure S02 is precipitated in a liquid form
in the condenser (5) by brine cooling. Any excess of oxygen is either recycled
to the burner or released to the open air after the removal of small portions
of sulfur dioxide in the washer (6).
The following example illustrates the invention.
EXAMPLE
In a two component nozzle burner having a circular nozzle of a
diameter of 2.5 mm for sulfur and a concentric ring nozzle for oxygen (inner
diameter 7.5 mm, outer diameter 70 mm) 90 kg/h of liquid sulfur and 92 to 96
kg/h of oxygen were introduced (excess of oxygen varying from 3 to 7%). The
burner was fixed at one end of a cylindrical steel tube (diameter 50 cm,
length 3m) the tube being provided on the outside with a spiral condenser welded
thereon for a water cooling and at the inside with a 3 mm asbestos coating
(R Cerafelt).
The ignition of the flame was effected by means of a natural
gas burner, the length of the flame being about 50 cm, the maximal flame
temperature 1950C and the temperature of the wall of the combustion chamber
~inner side) 1400C. The end of the steel tube was connected to a second
cooling segment ~length 3 m) being water-cooled as well. Thereafter the gas
still had a temperature of 280C.
The gas contained from 0.13 to 1.87% by volume of S03, depending
on the excess of oxygen.
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