Note: Descriptions are shown in the official language in which they were submitted.
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EFFICIENT SYSTEM FOR PRODUCING SULFURIC ACID USING A SPRAY TOWER
Technical Field
The present invention pertains to energy efficient systems for producing
sulphuric acid. In particular, it
relates to systems using spray tower absorption.
Background
Sulfuric acid is one of the most produced commodity chemicals in the world and
is widely used in the
chemical industry and commercial products. Generally, production methods
involve converting sulphur
dioxide first to sulphur trioxide which is then later converted to sulphuric
acid. In 1831, P. Phillips
developed the contact process which is used to produce most of today's supply
of sulphuric acid.
The basics of the contact process involve obtaining a supply of sulphur
dioxide (e.g. commonly obtained
by burning sulphur) and then oxidizing the sulphur dioxide with oxygen in the
presence of a catalyst
(typically vanadium oxide) to accelerate the reaction in order to produce
sulphur trioxide. The reaction is
reversible and exothermic and it is important to appropriately control the
temperature of the gases over the
catalyst in order to achieve the desired conversion without damaging the
contact apparatus which
comprises the catalyst.
Then, the produced sulphur trioxide is absorbed into a concentrated sulphuric
acid solution to form oleum,
which is then diluted to produce another concentrated sulphuric acid solution.
This avoids the
consequences of directly dissolving sulphur trioxide into water which is a
highly exothermic reaction.
While the fundamentals of the contact process are relatively simple, it is
desirable to maximize the
conversion of sulfur dioxide into sulphuric acid. Thus, modern plants for
producing sulphuric acid often
involve more than one absorption stage to improve conversion and absorption.
Commonly, a double
absorption process is employed in which process gases are subjected to two
contact and absorption stages
in series, (i.e. a first catalytic conversion and subsequent absorption step
followed by a second catalytic
conversion and absorption step). Details regarding the conventional options
available and preferences for
sulphuric acid production and the contact process are well known and can be
found for instance in
"Handbook of Sulfuric Acid Manufacturing", Douglas Louie, ISBN 0-9738992-0-4,
2005, published by
DKL Engineering. Inc., Ontario, Canada.
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It is also desirable to minimize the energy requirement in the industrial
production of sulphuric acid. In the
numerous processes involved, there are substantial sources and requirements
for heat. Energy efficiency
can desirably be improved with the use of complex heat exchanger arrangements
to maximize energy
recovery.
Many years ago in US4576813, Monsanto disclosed a method and apparatus for
significantly improving
the efficiency of plants that used the double absorption process. A heat
recovery system was disclosed
that raised the temperature at which the absorption of sulphur trioxide takes
place in the intermediate
adsorption stage. By operating at these higher temperatures, the heat of
absorption and dilution can be
used to generate useful steam instead of rejecting the heat to a cooling
tower. Overall efficiency could thus
be substantially improved. In the apparatus, the conventional intermediate
absorption tower is replaced
with a two-stage absorbing tower, a recirculating pump, a heat exchanger, and
a boiler. The two-stage
absorbing tower comprises two packed beds in series within the same absorption
tower. However, at
these higher temperatures, prior art materials of construction were subject to
substantially greater
corrosion rates. In order to enable the method commercially, both greater
sulphuric acid concentrations
and different, specially selected construction alloys had to be employed in
order that the apparatus was
suitably resistant to corrosion at these greater temperatures.
A disadvantage of the above approach however is that the function of the
previous conventional
intermediate absorption tower is integrated into the two-stage absorbing tower
of the energy efficient
system. In the event that a problem occurs with the heat recovery subsystem
during operation, the entire
plant may now need to be shutdown for repair.
Outotec has produced sulphuric acid plants that employ a similar heat recovery
method but different
apparatus to implement it. In the Outotec system, a combined quench venturi
and packed bed absorber is
employed for the intermediate absorption stage instead of a single, two-stage
absorption tower. An
advantage of this approach is that the quench venturi and heat exchanger
subsystem can be bypassed in the
event of a problem during operation. This allows the remaining packed bed
absorber to operate as a
conventional intermediate absorption tower (without improved heat recovery of
course) and thus the
sulphuric acid plant can at least continue to operate in the event of a
problem with the heat recovery
subsystem. A disadvantage of using a quench venturi however is that
significant energy is required to
operate it and provide the pressure drop therein.
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As mentioned, specially selected metal alloys are required to provide
acceptable corrosion resistance in
such improved efficiency, heat recovery subsystems due to the higher
temperatures and different sulphuric
acid concentrations involved. Zeron 100 (UNS S32760) is an alloy promoted for
use in sulphuric acid
manufacturing applications at elevated temperatures up to 200 C.
There remains however a desire for yet further improvements in energy
efficiency in the industrial
production of sulphuric acid. The present invention addresses this desire and
provides other benefits as
disclosed below.
Summary
The present invention provides for an energy efficient system for producing
sulphuric acid using a co-
current, open spray tower preferably with multiple spray levels. The
absorption of SO3 is increased since
each level of spray nozzles provides the same driving force for absorption
(since the absorbing acid
temperature and concentration is essentially the same at each level). This
compares favorably to use of a
packed tower or a quench venturi scrubber where SO3 absorption is diminished
as the gas stream travels
through the apparatus due to increasing absorbing acid temperature and
concentration at the latter stages
of absorption.
Specifically, the invention includes an intermediate absorption subsystem for
a sulfuric acid production
system which comprises a co-current open spray tower, an energy recovery
subsystem, and an
intermediate absorption tower. The co-current open spray tower comprises a top
inlet for a gas stream
comprising SO2 and SO3, a lower outlet for the gas stream after spray
absorption, at least one spray inlet,
at least one spray nozzle within the spray tower and connected to the spray
inlet, an inlet for dilution
water, and a bottom outlet for sulfuric acid liquid. The energy recovery
subsystem comprises a pump and
a heat exchanger arranged in series, in which the bottom outlet of the spray
tower is fluidly connected to
an inlet of the first side of the heat exchanger, the spray inlet of the spray
tower is fluidly connected to an
outlet of the first side of the heat exchanger, a water supply is fluidly
connected to an inlet of the second
side of the heat exchanger, and the outlet of the second side of the heat
exchanger is a steam output. The
energy recovery subsystem typically generates low pressure steam, for instance
at about 7 barg. The
intermediate absorption tower comprises a lower inlet for the gas stream
connected to the spray tower
lower outlet, an upper outlet for the gas stream after packed bed absorption,
a distributor inlet, a liquid
distributor within the intermediate absorption tower, a packed bed within the
intermediate absorption
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tower and below the liquid distributor, a bottom outlet for sulfuric acid
liquid, and a pump fluidly
connected to the bottom outlet of the intermediate absorption tower and to the
distributor inlet.
The spray tower in the intermediate absorption subsystem preferably comprises
a plurality of spray
nozzles on at least two levels, for example three levels. The spray nozzles
desirably produce a spray less
than 300 micron in average size, such that the droplets are of relatively high
surface area for better SO3
absorption.
A disadvantage of employing a spray tower in such systems is the production of
sub-micron mist.
However, this can be removed by high efficiency mist filter eliminators, e.g.
Brownian motion mist filters.
Thus, the intermediate absorption tower in the intermediate absorption
subsystem can comprise a mist
filter eliminator between the packed bed and the upper outlet for the gas
stream after packed bed
absorption.
As with other systems operating at higher temperatures in the intermediate
adsorption stage, special
corrosion resistant metal alloys are required in the heat exchanger in the
energy recovery subsystem.
Suitable such corrosion resistant metal alloys include superduplex alloys such
as alloys with UNS number
S32760 or chromium-based austenitic alloys such as alloys with UNS number
R20033.
The invention also includes complete double absorption systems for oxidizing
sulfur dioxide to produce
sulfuric acid. The first oxidation subsystem in such double absorption systems
comprises contact catalyst,
the aforementioned intermediate absorption subsystem of the invention, a
second oxidation subsystem
comprising contact catalyst, and a final absorption tower.
The present invention offers the advantage that it can operate in a
conventional mode when the energy
recovery system is not operational (by bypassing the spray tower and energy
recovery subsystem or by
simply turning off the energy recovery subsystem pump), and thus the double
absorption system can
continue to produce sulphuric acid. It also offers the advantage of a low
pressure drop requirement in the
spray tower and thus requires less energy (i.e. is more energy efficient) than
a quench venturi apparatus.
Brief Description of the Drawings
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Figure 1 shows a schematic of an intermediate absorption subsystem of the
invention comprising a spray
tower, an energy recovery system, and an intermediate absorption tower for a
sulphuric acid production
system.
Figure 2 shows a schematic of a double absorption system for producing
sulphuric acid in which the
system comprises the intermediate absorption subsystem of Figure 1.
Detailed Description
Unless the context requires otherwise, throughout this specification and
claims, the words "comprise",
"comprising" and the like are to be construed in an open, inclusive sense. The
words "a", "an", and the
like are to be considered as meaning at least one and are not limited to just
one.
In a numerical context, the word "about" is to be construed as meaning plus or
minus 10%.
Figure 1 shows a schematic of an intermediate absorption subsystem of the
invention for use in a sulphuric
acid production system. As shown, intermediate absorption subsystem 1 includes
open spray tower 2,
energy recovery subsystem 9, and intermediate absorption tower 14. A hot (e.g.
- 200 C) gas stream
comprising SO3 and SO2 is directed into top inlet 3 of spray tower 2. Therein,
SO3 is absorbed by acid
spray introduced co-currently at an appropriate temperature (e.g. -120 C) and
concentration (e.g. 98.5 wt
%). As shown, a cooler acid supply for the spray is provided at spray inlet 5
and directed to a plurality of
spray nozzles 6a, 6b, 6c. The nozzles here are located at three different
levels in spray tower 2. Nozzles
at the same level have been denoted with the same identifying numeral (i.e.
either 6a, 6b, or 6c). The spray
nozzles desirably produce a spray less than 300 micron in average size, such
that the droplets are of
relatively high surface area for better SO3 absorption.
Sulfuric acid comprising the absorbed SO3 falls to the bottom of spray tower 2
at a temperature ,.. 170 C
and is diluted appropriately with dilution water provided at inlet 7. This hot
acid (e.g. - 170 C) is then
pumped via pump 11 from bottom outlet 8 to heat exchanger 10 in energy
recovery subsystem 9. Hot acid
is directed to inlet 12a of the first side of heat exchanger 10, whereupon
heat is exchanged with a cold
water supply provided to inlet 13a of the second side of heat exchanger 10.
The cooled acid is now ready
for use as absorption media and is then directed from outlet 12b of the first
side of heat exchanger 10 to
spray inlet 5. The water on the second side has now been heated sufficiently
to become low pressure
steam and thus outlet 13b of the second side of heat exchanger 10 is a steam
output which can be used
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elsewhere as desired. Heat exchanger 10 is made of a special corrosion
resistant metal alloy such as
superduplex alloy UNS number S32760 or chromium-based austenitic alloy UNS
number R20033.
In spray tower 2, the somewhat cooler (e.g. - 150 C) gas stream comprising SO2
and unabsorbed SO3
exits at lower outlet 4 and is directed to lower inlet 15 of intermediate
absorption tower 14 and through
packed bed 19 in which additional SO3 is absorbed. To accomplish this,
absorbing acid is provided at
distributor inlet 17 to liquid distributor 18 which distributes the acid over
packed bed 19. Sulfuric acid
comprising the absorbed SO3 falls to the bottom of intermediate absorption
tower 14 to become the supply
of absorbing acid used over the packed bed. (Dilution water can be added as
necessary or desired, but is
not shown in Figure 1.) This liquid acid is pumped via pump 21 from bottom
outlet 20 to distributor inlet
17. A significant amount of sub-micron mist can be created when using a spray
tower and so intermediate
absorption tower 14 is equipped with high efficiency Brownian motion mist
filter eliminator 22. After
absorption, the gas stream exits intermediate absorption tower 14 at upper
outlet 16.
Figure 2 shows a schematic of an exemplary double absorption system 23 for
producing sulphuric acid.
Therein, system 23 comprises a first oxidation subsystem comprising contact
catalyst, intermediate
absorption subsystem 1 of Figure 1, a second oxidation subsystem comprising
contact catalyst, and a final
absorption tower. In this complex system, the first oxidation stage occurs in
catalyst beds 24-26 which is
followed by intermediate absorption and then the second oxidation stage occurs
in bed 27 which is
followed by final absorption. Hence, the first oxidation subsystem comprises
catalyst beds 24-26 and the
second oxidation subsystem comprises catalyst bed 27.
Double absorption system 23 in Figure 2 comprises four separate contact
catalyst beds 24, 25, 26, and 27,
each containing contact catalyst mass. System 23 also comprises intermediate
absorption subsystem 1
(comprising spray tower 2, energy recovery subsystem 9 and intermediate
absorption tower 14) and final
absorption tower 28, and output stack 30. System 23 also comprises four
separate heat exchangers in
order to obtain a high conversion efficiency in the overall process. These
heat exchangers are denoted as
cold exchanger 32, hot exchanger 33, inter reheat exchanger 34, and cold
reheat exchanger 35. To avoid
clutter in Figure 2, the separating structures within those heat exchangers
shown and their various inlets
and outlets have not been called out. Instead, arrows are provided on all the
interconnecting lines to show
the flow of the gas stream being processed. The inlets and outlets can thus be
readily inferred by the
direction of the arrows.
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In brief then, the double absorption process in Figure 2 proceeds as follows.
A gas stream comprising
sulfur dioxide and oxygen is provided at a supply temperature to cold
exchanger 32 where heat is
exchanged with the hotter gas stream at the fourth catalyst bed outlet
temperature. The gas stream exits
cold exchanger 32 and is directed to hot exchanger 33 where heat is exchanged
with the hotter gas stream
at the first catalyst bed outlet temperature. The gas stream exits hot
exchanger 33 at the first catalyst inlet
temperature and is directed to first catalyst bed 24. A first oxidation of SO2
in the gas stream takes place
over catalyst bed 24 and the exiting gas stream is now at the hotter first
catalyst bed outlet temperature.
After exchanging heat in hot exchanger 33, the gas stream exits at the second
catalyst bed inlet
temperature and is directed to second catalyst bed 25. A second oxidation of
SO2 in the gas stream takes
place over catalyst bed 25 and the exiting gas stream is now at the hotter
second catalyst bed outlet
temperature.
The gas stream at the second catalyst bed outlet temperature is directed to
inter reheat exchanger 34 where
heat is exchanged with the colder gas stream exiting cold reheat exchanger 35.
The gas stream then exits
inter reheat exchanger 34 at the third catalyst bed inlet temperature and is
directed to third catalyst bed 26.
A third oxidation of SO2 in the gas stream takes place over catalyst bed 26
and the exiting gas stream is
now at the hotter third catalyst bed outlet temperature.
The gas stream at the third catalyst bed outlet temperature is directed to
cold reheat exchanger 35 where
heat is exchanged with the colder gas stream coming from intermediate
absorption subsystem 1. The gas
stream then exits cold reheat exchanger 35 at the intermediate absorption
subsystem inlet temperature and
is directed to top inlet 3 of spray tower 2 in intermediate absorption
subsystem 1 in which a first
intermediate absorption of SO3 from the gas stream takes place. Absorption and
energy recovery takes
place as described above in Figure 1. The gas stream exits intermediate
absorption tower 14 at upper
outlet 16 and is then heated in two stages. The first stage involves
exchanging heat with the gas stream at
the third catalyst bed outlet temperature in cold reheat exchanger 35 to
produce a gas stream exiting cold
reheat exchanger 35. Then, the second stage involves directing the gas stream
to inter reheat exchanger 34
where heat is exchanged with the hotter gas stream at the second catalyst bed
outlet temperature, to
produce a gas stream exiting inter reheat exchanger 34 at the fourth catalyst
bed inlet temperature.
The gas stream at the fourth catalyst bed inlet temperature is next directed
to fourth catalyst bed 27. A
fourth oxidation of SO2 in the gas stream takes place over catalyst bed 27 and
the exiting gas stream is
now at the hotter fourth catalyst bed outlet temperature.
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The gas stream at the fourth catalyst bed outlet temperature is directed to
cold exchanger 32 where heat is
exchanged with the colder supplied gas stream. The gas stream then exits cold
exchanger 32 at the final
absorption inlet temperature and is directed to final absorption tower 28 in
which a second and final
absorption of SO2 from the gas stream takes place. The exiting gas stream is
then discharged at stack 30.
The double absorption system in Figure 2 produces sulfuric acid with high
conversion efficiency. And use
of the inventive intermediate absorption subsystem allows for greater energy
efficiency. As is apparent
from Figures 1 and 2, if for some reason the energy recovery system or spray
tower is not functional, they
can be readily bypassed and the sulfuric acid system can operate without them
(i.e. just by using
intermediate absorption tower 14) and thus system 23 can operate in such
circumstances in an historic
conventional manner.
While particular elements, embodiments and applications of the present
invention have been shown and
described, it will be understood, of course, that the invention is not limited
thereto since modifications
may be made by those skilled in the art without departing from the spirit and
scope of the present
disclosure, particularly in light of the foregoing teachings. Such
modifications are to be considered within
the purview and scope of the claims appended hereto.
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