Note: Descriptions are shown in the official language in which they were submitted.
~ 155634
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PR-4915
PRODUCTION OF LI~UID SULFUR T~IOXIDE
Back~round and Prior Art
~his invention relates to a process for the
productioll of sulfur trioxide in liquid form and more
particularly to its recovery in liquid form by conden-
sation from gases. It is particularly useful for the
recovery or sulfur trioxide in liquid form rom gases
relatively dilute in sulfur trioxide, particularly
such gases containing less than 15% sulfur trioxide
by volume. This process provides a method by which
relatively pure sulfur trioxide can be recovered in
liquid form from gases containing sulfur trioxide and
other gaseous components. Liquid sulfur trioxide is a
valuable product, being useful, for instance, as a
reagent fox production of alkylbenzene sulfonates and
in tertiary oil recovery.
In general, in present pract~ce, liquid sul-
fur trioxide is obtained by absorption of sulfur trioxide
from a gas containing the same in sulfuric acid to form
oleum, followed by boiling the oleum in a separate
operation and condensing the sulfur trioxide evolved.
Such a condensing operation can be troublesome because
1 155634
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of freezing of sulfur trloxide on the condenser tubes.
The freezing point of sulfur trioxide is about 85F.
~29.5C.). In general, in condensing sulfur trioxide
from a gas containing the same, difficulties are
caused by the fact that the temperatures required for
appreciable recovery of sulfur trioxide by condensa-
tion are below this freezing point, resulting in the
formation of solid sulfur trioxide, which can result
in plugging of the equipment, requiring steps to clear
the equipment and remove the sulfur trioxide. Solid
sulfur trioxide may exist in three separate forms,
generally referred to as the alpha-, beta- and gamma-
forms, see for instance, Brasted, "Comprehensive
Inorganic Chemistry", Vol. 8, pp. 134-138 (1961). The
gamma-form exists in ice-like cryst~ls and is the
most volatile and easiest to melt. The beta- and alph3-
forms have asbestos-llke crystals and are more difficult
to melt, with the alpha-form being the most difficult.
In general, when sulfur trioxide is condensed, a liquid
is first formed. This them becomes converted to the
gamma-solid form, which can become further converted to
the beta- and alpha-forms.
In U.S. patent 2,510,684, there is provided
a substantial amount of information concerning the
conditions under which sulfur trioxide can be condensed
from gases containing it. However, this patent requires
that, to effect the condensation of the sulfur trioxide,
the gaseous mixture contain much more than 15% sulfur
dioxide. Condensation under these conditions is affected
by the use of pressure and relatively high concentrations
of sulfur dioxide.
Sulfur trioxide gas is c~nventionally pro-
duced by combustion of sulfur ~r ~sulfur-containing
wastes, such as sludges containing ~ulfuric acid, to
produce sulfur dioxide, followed b~ catalytic oxidation
of the sulfur dioxide to sulfur tri~xide in the presence
1 ~55~;~4
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of a metallic catalyst. When the co~bustion is con-
ducted with air, the gases leaving thie sulfur dioxide
converter generally contain from abouit 5 to about 13%
sulfur trioxide by volume. If oxygen is utiliæed for
combustion, the gas exiting the sulfur dioxide con-
verter will contain a much greater co~centration of sul-
fur trioxide, generally from 50 to 8ff~ by volume, and
a correspondingly lesser amount of in/ert gases (nitro-
gen, carbon dioxide, etc.) However, în order to
condense this sulfur trioxide accord~ng to the process
of U.S. patent 2,510,684, it would be necessary to
conduct an incomplete conversion of the sulfur dioxide,
in order that the gases leaving the converter contain
at leas~ 15% S02. This would require recycle of the
unconverted sulfur dioxide to the converter, which pro- -
vides a disadvantage as opposed to once-through processes
such as absorption of sulfur trioxide in sulfuric acid.
It is particularly difficult to recoYer sulfur trioxide
~y condensation from gases containing lt in relatively
dilute concentrations, for example 15 vol.% or less.
Summary of the Invention
In brief, this invention comprises a process
for recovering sulfur trioxide in liquid form from a
gas containing sulfur trioxide comprising adjusting the
temperature of the gas to a temperature from about
80F. to about 90F. (about 26.7C. to about 32C.),
continuously introducing the gas hav~ng such tempera-
ture into a condensing apparatus maintained at a
temperature of from about 53F. to a~out 0F. (about
11C. to about -18C.), whereby sul~ur trioxide is
caused to condense from the gas; ~ontinuously removing
condensed sulfur trioxide from the con~enser suraces,
continuously conveying the condense~ sulfur trioxide
out of the condensing apparatus, andl subsequently
melting the condensed sulfur trioxide to produce liquid
sulfur trioxide.
.
11~5~
The process as thus defined permits the
recovery of sul~ur trioxide in liquid form from gases
containing it without requiring absorption into
sulfuric acid and boiling and distill~tion of the
oleum thus produced. The process as herein defined
also provides a satisfactory method of recovering
sulfur trioxide in liquid form from gases containing
relatively low concentrations of sulfur trioxide~ by
direct condensation.
Description of the Draw_ g
The drawing represents a sLmplified flow sheet
for the conduct of the process according to one embodi
ment of this invention.
Detailed Description of the Invention
For purposes of convenience, this invention
will be described with reference to the drawing, which
relates to a process for recovery of sulfur trloxide
in liquid form from gases containing relatively low
concentrations of sulfur trioxide, generally from 5%
to less than 15% by volume. However, it should be
understood that the principle of this invention is
applicable to the recovery of sulfur trioxide from
gases con~aining substantially greater concentrations
thereof, for example, from gases produced by combustion
of sulfur or sulfur-containing materials with oxygen
or oxygen-enriched air (as-opposed to air). It should,
furthermore, be understood that the principle of the
present invention is equally applicable to the recovery
of sulfur trioxide from gases containing the same
resulting from any other source or method or production.
In general, it is not now economically practical to
recover sulfur trioxide from gases containing concentra-
tions thereof of less than about 5% by volume, although
the process described herein is capable o~ recovering
.. . . .
11556~4
sulur trioY.ide from such gases containing it in low
concentrations.
Referring now to the clrawing, a gas contain-
ing up to about 15% sul~ur triaxide emanating from a
S source 1 and generally at an elevated temperature, is
passed through line ~ into a heat exchanger 3, which
may be cooled by introduction of water into the shell
side thereof in line 26 and removal of heated water
or steam in line 27. The gas thus cooled is then
passed through line 4 into mist eliminator 5 in which
acid mis~ is removed. The gas is then passed through
line 6 into a heat exchanger 7 in which it is cooled
to a temperature of from about 80F. to about 90F.
(about 26.7C. to about 32C.). As shown in the drawing~
the cooling is preferably accomplished by indirect
heat exchange with gases from which sulfur trioxide has
been removed, introduced into heat exchanger 7 through
line 14 and removed through line 15.
The thus cooled g~s containing sulfur trioxide
is passed through line ~ into a condensing apparatus
generally indicated as 9, and maintained at a tempera-
ture of from about 53F. to abaut 0F. (about 11C. to
about -18C.~, preferably about 53F. to about 10F.
(about 11C. to about -12C.), and most preferably
about 40F. to a~ ut 25F. (about 5C. to about -4C.).
This apparatus contains an interiorly located condensing
section 10 surrounded by a cooling jacket 11. ~lithin
the condensing section 10 are contained helical scraper
blades 12, driven by a motor 13, the blades being
arranged so as to scrape the inner surface of the con-
densing chamber 10. The drawing depicts the condenser
as being horizontally disposed. However, vertical or
incllned condensers may be utilized.
The use of such a scraped surface condenser
with helically arranged scraper blades enables the con-
tinuous removal of product sulfur trioxide from the
_fi_
interior sur~ace of the condenser as it is condensed,
and continuous conveying of the condensed sulfur tri-
oxlde through the condenser, preventing freezing ~nd
plugging of equipment. Preferably, the helical blades
S are mounted on a drum 12a closed at both ends, the
axis or rotation of which is coincidental with the
long axis of the condenser chamber. This continuous
removal and conveying are enhanced by warming the in-
terior of the drum 12a, for example by electrical
heating or by passing a warmer fluid (liquid or gas)
through it, as is conventional in such e~uipment, to
prevent sticking of the solid sulfur trioxide to the
surfaces. Split flights and vanes may also be provided
in the condenser, to break up lumps of solid. Pre-
ferably, the entering gas flow is directed through a
relatively narrow annulus existing between the cold
condenser surface and the rotating drum 12a which bears
the scraper blades 12. The exact clearance between
the condenser and the rotating drum is a function of
the desired gas velocity and the amount of sulfur
trioxide expected to be condensed and removed, and is
readily determinable by those skilled in the art. The
amount of sulfur trioxide expected to be condensed
under these circumstances depends on the concentration
of the sulfur trioxide in the incoming gas and the
temperature and the pressure of the environment, and
is readily determinable by methods known to those
skilled in the art.
The uncondensed gases are removed from the
condenser 9 through line 14 and may be advantageously
utilized to cool the incoming gas in heat exchanger 7.
They are removed via line 15 and are either passed to
treatment to remo~e pollutants and other undesirable
material before being vented to the atmosphere, or
passed to further processing, as discussed below
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115~6~4
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The sulfur trioxide which condenses on the
interior surfaces of the condensing chamber 10 is
removed by the blades 12 and carried thereby along
the length of the chamber 10. The condensed sulfur
trioxide may be in the form of predominantly gamma-
crystals, or may be a slurry or "slush" of such
crystals with liquid sulfur trioxide. The sulfur
trioxide is removed from the condensing chamber via
outlet 16 in which ~s located a star valve 17 which
forms a rotating seal and also serves to extract
condensed sulfur trioxide at a regular rate from the
condenser. Outlet 16 and star valve 17 may be iacketed
and heated in a similar manner to dru~ 12a, to pre-
vent sticking of sulfur trioxide. The sulfur trioxide
is conveyed by the star seal 17 into a melting tank 18
equipped with melting coils 19 through which may be
introduced steam or hot water, and an agitator 26.
Sulfux trioxide is melted in melting tank 18 and liquid
sulfur trioxide is removed via line 20. Preferably
the melting and removai are performed continuously.
The condenser 9 is maintained at the appro-
priate temperature by the use of a liquid or gaseous
refrigerant in a typical refrigerant cycle When
using a gas as a refrigerant, it is introduced into
the shell 11 of the condenser 9 via line 21 and removed
via line 22, passed through a compressor 23, cooled in
heat exchanger 24, passed through expansion valve 25
and returned to the shell 11. As the refrigerant there
may be used any gas which will provide the satisfactory
cooling effect for the condenser, such as sulfur dioxide.
Preferably the jacket temperature is maintained at about
25F. ~4C.). A liquid refrigerant may be similarly
used, without the employment of an expansion valve;
it will be vaporized in shell 11 by the condensing of
- 35 the sulfur trioxide and re-liquefied in compressor 23
--8--
The pressure of the condenser and other
equipment utilized to process sulfur trioxide gas is
maintained substantially at atmospheric pressure.
However, pressures may be as high as 8-10 psig (0.56-
0.7 kg/cm gauge) or, in a high pressure plant, 75
psig ~5.25 kg/cm2 gauge).
Sulfur trioxide thus recovered is substantially
pure sulfur trioxide, although small amounts of sulfur
dioxide may also be condensed and contained in the
product; thus, the process according to the invention is
capable of producing a relatively pure liquid sulur
trioxide product from gases containing relatively
small amounts of this substance. In general, as
known in the art, the condenser will only serve to
condense a portion of the sulfur trioxide contained in
the gases treated. However, the refrigerant gas
utiliæed to cool the condenser 9 can be varied in
order to recover more sulfur trioxide. For e~ample,
if additional recovery is desired, a refrigerant
capable o~ producing lower gas temperatures in the
condenser, such as dichlorodifluoromethane, could be
utilized. Alternatively, the pressure at the inlet
of compressor 22 could be lowered. These techni~ues
for producis~g desired gas temperatures in a condenser
by adjustment of a compression-exparlsion cycle are well
known to those skilled in the art.
In general, recovery o~ additional sulfur
trioxide from the uncondensed gases in line 15 is not
advantageous since it would be necessary to compress
such gases to a higher pressure. However, if such
were desired, the gases would be re-introduced into the
system via line 8. Preferably, the sulfur trioxide
values in these gases would be utilized by psssing the
gases through line 15 into a sulfuric acid drying
tower or an absorption tower in an oleum plaslt. Al-
ternatively, the gases could be passed through
~ ~55~
g
purification equipment to remove sulfur trioxide and/or
other pollutants, and then vented.
Example
The following represents an example of a
process according to the invention, with reference to
the drawing.
A gas containlng 8.16% by volume sulfur tri-
oxide and having a flow rate of 100 lb. mol. per hour
(45.4 kg. mol. per hour) and at a temperature of 350
to 500F. (177 to 260C.) is introduced into a water-
cooled heat exchanger 3 in which it is cooled to a
temperature of about 130F. (54.5C.). The gas is
then passed into a mist eliminator 5 and then further
cooled to a temperature of 80 to 90F, (26.7 to 32C.)
in heat exchanger 7 by heat exchange with exiting
gases in line 14. The gas is then conve~ed via line 8
into a condenser 9 equipped with helically arranged
blades 12 mounted on a rotating drum 12a driven by a
motor 13. The condenser is equipped with a refri~erat~
ing jacket lI supplied with pure sulfur dioxide re-
frigerant such that the jacket temperature is 25F. and
the pressure is 4.38 psig. (0.31 kg./sq. cm.). In
the condenser, the sulfur trioxide con~aining gas is
cooled to 30F., whereupon approximately 30% of the
sulfur trioxide in the gas is condensed, removed from
the condenser through outlet, 16 melted in tank 18
and recovered as liquid product.
