Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
8~125
_ackground of the Invention
The present invention relates to the removal of objectionable
components from a moving gas stream, and the principal embodiment
is expected to be the removal of sulphur dioxide, SO2, nitrogen
oxides, NOx, from -~lue gases from industrial and utllity sources.
Other embodiments include the removal of toxic metallic vapors
such as mercury, and other oxidizable gas components as well
as halogens, and acidic and basic components from industrial
effluents. The invention may also be used for purification
and sterilization of air for applications wherever clean air
is required such as hospitals or clean rooms, and also for
deodorization purposes.
Sulphur dioxides and nitrogen oxides are produced in very large
quantities daily from the burning of fossil fuels, and after
oxidation in the atmosphere, are returned to the ground in
what is cal.led "acid rain". The higher levels of acidity in
the rainfall downwind of large 52 and NOx sources in the
form of large industrial areas has a large environmental impact
in many parts of the world. The fish of many lakes and streams
in these areas have vanished, either as a direct kill, or
from a break in their food chain. Agricultural production in
some areas has decreased due to a leaching out of nutrients
from the soil. Many other long term effects are still under
investigation and all effects are causing much concern.
The major sources of SO2 and NOx produced by man derive from
the burning of coal and petroleum products. All fossil fuels
containg sulphur, and the most popular grades of fuel are the
low sulphur varieties, since 1n the burning of sulphur
containing fuels the sulphur present forms SO2 with the
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associated environmental problems. Nitrogen oxides are produced
in the burning process principally from the nitrogen in the
air, and may be controlled to an extent by manipulation of
the burning conditions. Some NOx is nonetheless produced even
in the best practical situations,
Due to the present uncertainties affecting the world petroleu~
supplies, and the limited supply in any case, other energy
sources are being sought to fuel the worlds energy needs,
which are continually increasing. One readily available energy
that is not being fully utilized at present is coal, and
much of this coal that is not being exploited is unused due
to environmental concerns as the coal has an unacceptably
high sulphur content. It has been estimated that 95% of the
sulphur in coal forms SO2 on burning. ~
Over the last few decades , much interest has been shown
in the removal os SO2 from flue gases and many systems for
S2 removal from flue gases have been developed. Many large
power plants have been fitted with some form of SO2 removal
system to date.
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~8~:)25
The efficiency of these desulphurization attempts is in general
insufficient to prevent large scale environmental damage. In
a report to the Air Pollution Control Association in 1978,
William H. Megonnel of the National Association of Electric
Companies in an article entitled "Efficiency and Reliability
of Sulphur Dioxide Scrubbers" examines the systems in operation
ln utilities at that time. Of the thirty-two utilities classed
as operational at that time, virtually all used some variation
on a carbonate or alkaline scrubbing process, involving a
collection of S02 as calcium sulfite;ùltimately. In the
remaining example, the Wellman-Lord,Allied Chemical system
was used involvingS02 recovery and reduction to sulphur at
90% efficiency. The primary deficiency of these system is the
unacceptable removal efficiency obtained in operation, an
average of around 75~ in the examples quoted. As North America
is forced , due to economic and political reasons, to switch
more and more to coal, including high sulphur coal, as an
energy source, this removal efficiency demonstrated is insufficient
to prevent large scale environmental damage.
In the large majority of S02 removal systems in use, the acidic
nature o a water solution of S02 is used to trap the S02 in
the form of a sulfite or bisulfite by reaction with a basic
material such as calcium oxide or hydroxide, or by reaction
with a carbonate such as limestone or dolomite. In some cases,
the sulfite produced is oxidized with atmospheric oxygen
to an lnsoluble sulfate. Although in laboratory scale and
small pilot scale operations these systems show great promise,
it appears that on a large scale, in which non-optimum conditions
are constantly occurring, the efficiencies seen on a small,
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1~80Z~
wellconrolled scale do not seem easily obtainable. One primary
cause of this difficulty seems to be the reversibility of the
reaction used to fix the S02
M2S3 ~~ S2 ~ H20 ~--~ 2MHS03 (1)
The reversibility of the reaction, which forces the use of
a countercurrent scrubber arrangement, leads to a situation
where very careful control of gas flow rate vs S02 concentration
is needed or else the S02 absorption efficiency becomes poor.
Other major problems encountered in the operation of these
cleanup processes include large problems with scaling and
erosion. Much of the work done to date on this kind of system
involves only improvements of various kinds to aleviate the
considerable difficulties involved in the operation of these
systems.
Other research has been carried out on various dry processes,
catalytic processes, and processes involving additions to the
fuel, but to date little practical use ahs been made of these
other processes, although the disadvantages of the carbonate
or hydroxide scrubbing processes described earlier are causing
great interest in alternate cleanup processes.
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Nitrogen oxides have been removed with varying success by a
very large ~umber of methods on a laboratory scale, often in
very uneconomical ways using expensive reagents. Methods used
include dry catalytic reduction or oxidationl wet scrubbing
with baslc solutions or amines, and aqueous scrubbing with
oxidizing solutions. The methods used for nitrogen oxides
removal are very diverse, and it appears that relatively few
processes have been put into practical use, with the emphasis
having been on the removal of S02 , which is generally present
in larger quantities and is much more easily removed by
conventional methods.
:
Generally the wet scrubbing methods, especially the methods
using aqueous oxidizing solutions , are the only systems
readily useable for the simultaneous removal of NOx and S02 .
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SU~ARY OF TflE INVENTION
. _
The inven-tion is directed to providing a method
Eor the removal of oxidizable components from a gas stream
such as, ~ut not limited to, flue gas, into a scrubbing
solution. This is done by using the scrubber effluent
solutions to regenerate the oxidizing agent for reuse in
the process. An economy is obtained by removal of the
o~idlzing agent from the effluent solutions beEore disposal.
Acidic and basic components are removed from the
lQ gas stream due to the chemical composition and pH of the
scrubbing solutions used to accomplish the first object.
Similarly, halogens are removed from the gas stream.
The process may be built on any scale from that
needed for cleaning -the flue gas from an industrial size
utility to a very small unit capable of handling a gas
stream line only a few inches in diameter, in a commercially
reasonable installation.
The process also removes objec-tionable levels
of particulates from -the gas stream if desired.
;8~25
The process for gas stream cleanup is designed to oxidize
objectionable components of the gas and so render them non-
volatile or more readily absorbed. This oxidation will occur
either in the gas phase or in solution in the scrubbers. The
process can be used to remove or reduce the amount presentof
the following: S02, NOx, H2S, ammonia, and mercury and other
metallic vapours, although it will be evident to anyone skilled
in the art that this is by no means a complete list of readily
oxidizable compounds which may be removed from gas streams
by this invention. Because of the pH or chemical compositions
of the scrubber solutions used in the process, the process
will remove almost any acidic or basic compounds as well as
halogens from the gas stream~ The principal embodiment of the
invention is the cleanup of industrial effluents such as flue
gases, and other embodiments include more economical deod-
orization systems, and also sterilization of air.
The process in a brief summary consists of the following:
a). injection of chlorine into the hot gas, which is then left
to react for a time determined by the dimensions of the pipe
used and the gas flow rate.
b). a water quench to cool the gas.
c). gas absorption in a packed scrubber.
d). chlorine absorption in a counter-current packed scrubber,
and return of the effluent solution containing chlorine to
earlier stages of the system.
e). final removal of chorine in a counter-current packed
scrubber with a solution of a carbonate, bicarbonate, or
hydroxide, or packed with a solid carbonate, or using a lime
or limestone slurry or similar. This effluent solution is
mixed with an acid solution for the regeneration of chlorine,
~^ - 5 -
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which is returned to earlier stages of the process.
f)~ demisting of the gas stream , followed by disposal of the
gas.
When particulate removal is desired, either a dry electro-
static precipitator can be used before the chlorine injection,
or else a high energy venturi or similar scrubber may be used
between the quench and first packed scrubberO Chlorine is
removed from the effluent solutions before disposal by bubbling
uslng some of the gas stream.
This summary is only indicative of the many different config-
urations of the system, which can be altered somewhat at almost
every stage in order to fit the system into a certain situation.
Stages a) and b) for example are omitted when the gas stream
temperature is less than 100 degrees C, and chlorine is added
.into stage c). above.
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Brief Description of the Drawings
. _ .
Figure 1 illus-trates an embodiment of the process for use as
a flue gas cleanup system as would be used by an electrical
utility or similar large scale user.
Figure 2 illustrates an embodiment of the process for use in
smaller scale applications.
These two embodiments are only indicative of the many different
forms the process may take.
DETAILED DESCRIPTION OF THE INVENTION
. . .
The process for gas stream cleanup is designed to oxidize
objectionable components of the gas and in doing so to render
them non-volatile or much more readily absorbed. This oxidation
will occur either in the gas phase or in solution in the
scrubbers. The process can be used to remove or reduce the
amount present of the following: S02, NOx, H2S, ammonia,
mercury and other metallic vapors, although it will be evident
to anyone skilled in the art that this is by no means a complete
list of readily oxidizable compounds which may be removed from
gas streams. Because of the pH or chemical compositions of
the scrubber solutions used in the process, the process will
remove almost any acidic or basic compounds as well as halogens
from the gas stream.
The process for gas stream cleanup consists of one option
from each o~ eight sections, and as such gives many hundreds
oE reasonable systems derived from the process.
The first section of the process consists of either a dry
electrostatic precipitator of standard design operated at the
gas stream temperature, or else no precipitator. The function
of the precipitator , if used, is the removal of particulate
matter from the gas stream, as would be found in the use of
this process as a flue gas cleanup system. A high energy wet
scrubber can be used later in the process as an alternative
particulate removal device , if desired~ Particle collection
may be omitted as desired.
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The second stage of the process conslsts of a section of
chemically inert pipe or similar, with or without baffles,
into which chlorine is added in one or more of the following
ways:
1) gaseous chlorine
2) liquid chlorine
3) a mixture of chlorine gas and air, inert gas or
flue gas, which may also contain hydrochloric and/or
nitric acid.
4) a water solution of chlorine.
5) recycled process solution containing chlorine,
and which may also contain hydrochloric and/or
nitric acid.
The chlorine added into the gas stream is for the gaseous
oxidation of objectionable components to form compounds more
readily absorbed in the following scrubbers. The temperature
and/or the water vapour concentration may be adjusted at this
time by the addition of one or more of:
1) liquid water
2) a water solution of chorine.
3) recycled process solution containing chlorine,
and which amy also contain hydrochloric and/or nitric
acid.
although the addition of the water solutions of 2) or 3) may
not be necessary for the control of water vapour concentration
and/or temperature since these solutions may be used rather
for the chlorine addition with no concern
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for other parameters.
The chlorine being added into this section may come from
one of three sources. The chlorine added as a gas , liquid
or as a water solution is from the chlorine cylinder used as
the process chlorine source, and one or more of these is
preferred. the chlorine mi~ed with air, inert gas or flue gas
derives from using gas or warm gas to recover chlorine from
the process effluent solutions by bubbling. The recycled
process solution containing chlorlne is the ef1uent solution
of the sixth stage of the process. The chlorine sources other
than the cylinder are intended for the return of chlorine to
the earliest stage of the process for reuse of unreacted chlorine.
It is anticipated that this stage of the process will be used
only if the gas stream is 100 C or higher in temperature. If
the objectionable components in the gas stream are sufficiently
well absorbed in the following stages of the process without
prior oxidation, this stage of the process may be omitted.
zs
In this stage the gas stream - chlorine mixture is left to
react for a time not less than that needed to result in a 10%
increase in absorption of an objectionable component over that
which would be absorbed with this section omitted, or less
than 10% if this is economically useful. In the case of flue
gas cleanup the component of interest is nitric oxide, NO,
principally, which due to its low solubility is difficu~t to
remove from gases in wet scrubbers. Sulphur dioxide is sufficiently
soluble that this stage should prove unnecessary for adequate
absorption.
The third section of the process is an optional water quench
for cooling the flue gas before entering the wet scrubber stages.
Water or recycled process solution is added as a spray, with
the recycled process solution again coming from the chlorine
reclaiming scrubber as in stage 2. Gaseous chlorine may be
added in this stage.
The fourth stage of the processis a high energy particle
scrubber of standard design and is used when particulate
removal is desired and no electrostatic precipitator is used
in stage 1. Thls scrubber is ideally a high pressure venturi
or venturi with applied high intensity ionisation. This scrubber
will also collect some of the gas stream components, especially
some SO2 in the presence of chlorine.
The scrubber water solution is ideally recirculated in order
to obtain as high a concentration of acid as possible in the
solution. The feedwater used in this scrubber may come from
the water mains or may be recycled process solutions from
later stages of the process. Dissolved chlorine in the effluent
sGlution may by bubbling be blown out using air, an inert gas
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~l6~:s25
or flue gas and added to the second stage of the process. This
givesa greater economy in the use of chlorine without increasing
the complexity of the system unduly., especially if hot flue
gas is used. Chlorine may be added into this scrubber from
the cylinder, either as a gas or into the scrubbing solution.
The fifth stage of the process involves a gas absorbing
scrubber of standard design, ideally a packed scrubber such
as a crossflow scrubber, but almost any scrubber of standard
design may be used, with the suitability being principally
determined by the gas contact time with the scrubbing solution.
Chlorine may be added to this scrubber either in gaseous
form or by dissolving in the scrubber feedwater. If it was
chosen to add no chlorine earlier in the system, chlorine
will be added to this stage in the process from the chlorine
cylinder~ Addition of chlorine as agas may be either into the
gas stream going into the scrubber, or into the interior
of the scrubber. The scrubbing solution in this stage derives
from the same sources, is reciculated similarly to and may
be dechlorinated in the same fashion as that in the fourth
stage of the system, and if desirable, may be mixed with the
solution of the fourth stage.
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The primary function of this stage of the process is the
absorption of the objectionable components into solution,
followed by the rapid, irreversible oxidation of these components
to form either non-vol.atile compounds , such as sulphuric acid
or else highly soluble volatile compounds such as nitric acid.
The size and type of this type of scrubber is determined by the
demands of each situation, and and installation having to deal
only with S02 , as an example , will need only a relatively
simple scrubber due to the relatively high solubility of S02,
whereas an installation handling a large amount of NOx will
need longer residence time scrubber with high liquid contact
due to the lower solubility of nitrogen oxide.
The sixth stage of the process , which may be omitted or
abbreviated if economy in the us~ hlorine is not a major
consideration, is a gas absorbing scrubber of standard design,
ideally a packed scrubber such as a crossflow absorber or a
coutercurrent packed scrubber.Recirculation of the scrubbing
liquid in this absorber is kept to a minimum and is recirc-
ulated to an earlier section of this absorber in the case of
a cross flow absorber. The purpose of this absorber is to
recover chlorine and acid vapours from the gas exiting the /.
previous ahsorber, for return o the chlorine to earlier
sections of the overall process. The effluent solution of this
scrubber may be used as some or all of the feedwater in st~ges
2 through 5.
The size and type of scrubber used in this stage is determined
by the chlorine collection efficiency desired in this scrubber.
The reci.rculation of the scrubbing solution may be accompanied
by dechlorinating the effluent solution as in stage ~ and 5 ,
otherwise the recirculation of the solution is llmited by the
dissolved chlorine in the effluent solution from the scrubber.
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~168~5
The seventh section of the process is ideally a calcium carbonate
or dolomite packed countercurrent scrubber for the removal of
chlorine from the gas stream and for the return of the chlorine
to previous stages of the system. A packed scrubber may also
be used with a recirculating solution of a soluble carbonate,
or bicarbonate, or else a soluble hydroxide such as sodium
hydroxide or calcium hydroxide from slaked lime. A scrubbing
slurry of an alkaline earth carbonate may also be used. In the
case of using solutions or slurries of carbonates, bicarbonates
or hydroxides, however, some caution must be exercised since
the scrubber effluent solution is mixed with acid effluents
from previous stages for the regeneration of chlorine and the
residual carbonate, bicarbonate or hydroxide must be taken into
consideration in the mixing.
The chlorine reacts in the seventh section with the carbonate
bicarbonate or hydroxide to produce either a hypochlorite or
hypochlorous acid, HOC1. Upon mixing with the effluent solutions
from previous scrubbers, which are high in acidity, especially
with hypchloric acid and sulphuric acid, any hypochlorite is
converted to HOC1 , and iithe presence of HCl , the chlorine
dissolution and disproportionation equi;ibrium results in the
regeneration of free chlorine.
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Both HOCl and chlorine are the active oxidizing agents in the
previous sections. The solution from this section may be added
directly to the recirculating solutions in the earlier
scrubbers, or may be added to some of the solution from these
sections separately under controlled conditions.
If the solution Erom this section of the process contains calcium
ions, from a CaC03 , dolomite, slaked lime or slaked calcined
dolomite scrubber, steps must be taken to filter out the CaS04
precipitated during the acidification with the HCl and H2S04
mixture. A controlled amount of excess ~I2S04 is used ideally,in
order ^to minimize the total dissolved calcium in the solution
to prevent scaling in the scrubbers from occurring. The minimum
dissolved calcium is achieved when the volume ratio of the
calcium and sulfate containing solutions are as below, where
the concentrations are the calcium and sulfate concentrations
in the two solutions before mixing:
1 volume calcium TO 2 Calcium + 1 volume of sulfate
containing solution concentration containing sol.
_ , .,
Sulfate
concentration
This minimum total dissolved calcium is desireable to avoid
a high calcium concentration in the recycled solutions which
could precipitate calcium sulfate inside the scrubbers and at
the points of highest sulfate concentration.
If the solution from this seventh section of the process
contains any appreciable amount of dissolved hydroxide,
carbonate or bicarbonate, the mixing with previous stage
solutions must be carried out very carefully, due to the
release of heat or C02, which poses no serious problems if
taken into consideration.
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The evolved C02, if any , is vented into the process no later
than the input of stage 7 and preferrably earlier, since the
evolution of C02 will tend to carry acid vapours and dissolved
chlorine out of the mixture.
The acidified effluent of this stage of the process may be
dechlorinated as in stage 4 or 5 , and the mixture of chlorine
and air, inert gas or flue gas added to the second stage of
the process.
The eiyhth and final stage of the process consists of a demisting
stage of standard design. A packed bed demister is preferred,
but any demister of reasonable efficiency may be used. This
stage may be combined with the previous by making a packed
scrubber and introducing the scrubbing liquid at a point below
the top surface of the packing, and using the packing above
the liquid introduction level as a demlster.
The effluent gas from the system is routed to a stack for
disposal.
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Process Chemical Reactions
~ he hot gas phase reactions with chlorine in the
process have been investiga-ted in the literature, and are
yenerally well known, but only as reactions of the pure
gases. The reaction rates in the gas stream are expected
to be accelerated by the presence of the other gases,
moisture and flue dust surfaces. The equilibrium
C12 + H2O = 2HCl ~ 2 2
may increase the oxidizing power of the gas mix-ture, as
this equilibrium orms a hiyhly oxidizing medium, possibly
due to the production of atomic oxygen or hydroxyl radicals
at high tenlperatures. It appears that this unusual
characteristic of this mixture has never been investigated
since the report in a Russian journallin ].939. No investi-
~ations into the mechanis.ms of this reaction have been
discovered, al~houyh the thexmodynamic constants are known
as the revers~ reaction is the basi.s for the Deacon process
for the manu:Eacture of chlorille from HCl and oxygen.
~1o~her method for increasiny the free radical
concentration in the gas is the rapid vaporiæation of a
solution of chlorine in water, to produce gaseous HOCl, which
decomposes to give hydroxy and chlorine radicals, as in
reaction la. This vaporization
HOCl (gas) - - ~ HO + Cl (la~
is done very rapidly using a fine spray into the hot gas
; in order to evaporate the solution before the equilibrium
of equation 13 can shift to give chlorine gas instead of
HOCl gas.
1. A.P. Kreshkov, J. Chem. Ind. (U.S.S.R.) 16, No. 4-5, 38-41
(1939) Abstracted in Chemical Abstracts 33, no. 77293
.
' ~6~25
The principal reactions expected as forming some
contribu-tion to the gas phase reaction are
SO + O ` SO (2)
NO + O ~ N02 (3)
and similar radical reactions
S2 ~~ C12 = SO2C1
2 12 H20 ~ SO3 ~ 2HCl (5)
2NO + C12 = 2NOCl (6)
2NO2 + C12 = 2NO2Cl (7)
N02Cl + NO ~ N02 ~ NOCl (8)
NOC1 -~ 1/2 2 > NO2 + 1/2 C12 (9)
Hg (gas) ~ C12 ~--~HgC12 very fast (10)
Metal vapor - chlorine reactions in general are expected
to be practically instantaneous and comple-te. Mercury out~
put in the gas stream from this process is expected to be
well below acceptable levels.
1'he water phase reactions in this process are
very rapid and complete. The ~ollowing reactions are
expected.
1~6~ 5
SO3 + H2O ---~ H2SO4 (11)
S2 + H2O ~- H2S 3 (12)
C12 + H2O ~--~ EICl + HOCl (13)
2N2 + H2O ~ HNO2 + HNO3 slow (14)
2NO + H2O -~ HNO3 ---3 3HNO2 (15)
NO2 + 2HCl ---, NOC1 + H2O + ~CL2 (16)
NO + NO2 + H2O ~~ 7 2~N2 (17)
2MO -~ 3HCl + HNO3 ---~ 3NOCl + 2H20 (18)
NOC1 + H2O ---7 HNO + HCl (l9)
NOC1 + C12 + ~2 ~~ ~ HNO3 (20)
NO2Cl + H2O ---~ HNO + HCl (21)
S2C12 + 2H2 ~ H2 4 (22)
H2SO3 ~ HOCl ---~ H SO + HCl (23)
HSO32 ~ HOCl ---~ HSO + HCl (24)
SO3 + HOC1 ---~ SO + HCl (25)
HNO2 ~ HOCl ---~ HNO3 + HCl (26)
NO2 ~ HOCl ~ NO3 + HCl (27)
NH3 -~ H+ ---~ NH4+Xida-t---onNo- (28)
In th~ chlorine removal scrubber, the chlorine and acid vapors
are removed from the gas stream. The preferred method is the
use of a packed tower of calcium carbonate, dolomite or a
similar solid, insoluble and reactive carbonate. This has the
advantage of simplicity of design and a relatively low cost.
Other scrubber sy,stems which may be used are countercurrent
packed scrubbers or simila-r using a scrubbing solution
containing a soluble carbonate, bicarbonate or a h~droxide or
a scrubber using a slurry of an insoluble carbonate or
slaked lime or calcined dolomite or similar.
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The effluent of these scrubbers, upon acidification, can be
used as feedwater in other sections of the process, after
filtering if necessary. The oxidizing power of the chlorine
is not lost in this chlorine removal scrubber, but remains in
the form of hypochlorlte or hypochlorous acid and upon acidif-
ication, chlorine is regenerated as in the equilibrium in
equation 13 , and may be returned to the earlier stages
of the process.
e - lSa -
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The reactions in the chlorine removal scrubber are as follows:
C12 + H20 = HCl + HOCl
HOCl or HCl + M2C03 ~ MOCl or MCl + H20 + CO2
HOCl or HCl + 2MOH ~ MOC1 or MCL + 2H20
HOCl or HC1 + 2MHCO3 ~ MOC1 or MC1 + 2H20 + 2C02
The presence of hypochlorite or hypochlorous acid in the re-
circulated scrubbing liquid results in an oxidising condition
in this scrubber. This results in a contribution to the removal
of objectionable components in the gas stream by reactions
similar to those of the water phase reactions mentioned earlier.
The e~fluent from the chlorine removal scrubber is mixed with
effluent from earlier in the process, containing hydrochloric,
sulfuric and nitric acids. The effluent highest in sulphuric
acid concentration ls preferred. This acidification regenerates
chlorine and precipitates calcium sulfate as in the following
e~uations~
CaClOCl + H2S04 ~ Ca~S04(s) -~ HCl + HOCl
HCl + HOCl ~ C12 + H20
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` Legend of Large Flue Gas System
Illustrated in Figure 1
1. Firebox
2. Stack
3. Addition of Chlorine, etc. followed by the hot section.
4. Quench
5. Venturi and associated cyclone and fan
6. Crossflow gas absorher
7. Crossflow Chlorine absorber
8. CaCO3 to~er
; 9. Cooling Tower
10. Stack reh2~t
11. Flue dust settling and filtration
12. ~ludge washer
13. Collection and silt settllng tank
14. Acid mixing
15. CaSO4 settling and filtration.
16. Chlorine gas removal
17. Chlorine gas removal
18. Demisting
19. CaCo3 addition chute
20 Acid holding tank
A. Water Mains
B. Chlorine reclaim scrubber effluent
C. Venturi cyclone effluent
D. Filtered venturi effluent ~.Dechlorinated D and F
E Sludge wash solution H. Dechlorinated B
F. Chlorine scrubber effluent J. Acidified and filtered CaCO3
column effluent.
K. Acid byproduct output.
s
Description of Large Flue Gas System
The embodiment of figure 1 is an example of a large scale
system for the removal of S02 and NOx from flue gas. For
handling very large flow rates, parallel units are used at
each stage where appropriate, which also gives flexibility
in handling varying flow rates, as well as allow,ing the removal
of a particular unit from use for servicing without forcing
a shutdown of the entire system.
After the firebox 1, the flue gas first enters the hot
reaction section 3 where gaseous chlorine is added into the
gas. Immediately prior to the chlorine addition, some of the
flue gas is diverted to the chlorine sparge sections 16 and
17 where dissolved chlorine is removed from various system
solutions by bubbling the hot flue gas through the solutions.
The flue gas now containing chlorine is reunited with the flue
gas stream at the point of chlori.ne addition 3. Also at this
point effluent solution B from the chlorine reclaim scrubber
7 is added. The flue gas and chlorine mixture passes through
the hot reaction section for a time determined by the dimensions
of the piping used. The gas then enters a quench 4 of solution
B fromthe chlorine reclaim scrubber 7 and recycled quench and
venturi solution D for cooling before entering the venturi
scrubber 5 for particle removal. The effluent solutions from
the quench 4 and venturi scrubber 5 are combined and filtered
11 and the separated flue dust is washed with water from the
mains A, mixed 12 with a small amount of limestone or dolomite
for residual acid removal, and filtered before disposal.
- 18 -
~68~32S
The wash solution E is used as feedwater for the venturi 5 and
first crossflow scrubber6. The filtered effluent solution D
from the venturi scrubber 5 i5 reci~culated to the venturi. A
portion of the filtered solution D is diverted to the flue gas
bubbling section 17 for removal of dissolved chlorine
followed by disposal to a holding tank 20.
After the venturi scrubber the flue gas enters the first
crossflow scrubber 6, which is operated with all of the sections
using the same scrubbing liquid source. The effluent solution
F from this scrubber is treated in the same fashion as the
solution from the venturi scrubber , except with no filtering.
The dechlorinated effluent solution G that is diverted for
disposal is placed in holding tank 20.
The flue gas now enters the second crossflow scrubber 7, the
chlorine reclaim scrubber, which is operated in a counter-
current manner for removal of the majority of chlorine from
the gas stream. The effluent solution B from this scrubber is
used as feedwater ~or the hot gas section 3, the quench 4, the
venturi scrubber 5, and the first crossflow scrubber 6. Any
excess solution to these needs is dechlorinated ln the flue
gas bubbling section 17 and returned to the chlorine reclaim
scrubber 7 for reuse. The feedwater for this scrubber is taken
from the water mains A.
The flue gas then enters a scrubber 8 packed with crushed
limestone for chlorine removal from the gas stream. The
feedwater for this scrubber is taken from the water mains.
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s
The effluent solution from this scrubber is filtered 13 to
remove unreactive solids such as silica, and rec~cled to the
top of the scrubber 8. Some of this solution diverted and mixed
14 with a controlled amount of effluent solution D from the
venturi scrubber ro regenerate chlorine. This mixture is
filtered 15 to remove calcium sulfate, which is washed in a
similar fashion to the treatment of the flue
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dust in 12, and then disposed of. The filtered solution
is dechlorinated in the flue gas bubbling section 16
and then is used as feedwater J ln the ven-turi 5 and first
crossflow scrubber 6.
A cooling -tower 9 is shown in the figure
and is used to cool the effluents from the quench 4 and
the venturi 5 if necessary. A water cooled heat exchanger
is immersed in the tank holding the effluent and the water
is circulated to the tower 9 or similar for cooling.
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Z5
Description of Small Flue Gas System
Figure 2 is a diagram of an apparatus for use in a small scale
flue gas cleanup system. Flue gas leaving the firebox 1 passes
upward through 2 which is a heat exchange section for heating
the cleaned effluent gases inside pipe 21 prior to release.
The hot flue gas continues through 3, which contains an air
radiating cooling section 4 and is drawn by a blower 5, which
forces the gas through the entire system.
The gas then passes through a water cooled condenser 6 which
condenses moisture out of the flue gas and brings the gas
temperature below 100 C. The moisture is collected in 7 and
stored in tank 8, which contains a water cooled heat exchanger
for cooling the temperatures to near ambient temperatures.
The condenser and associated components may be omitted in a
flue gas system for a low water content flue gas. The condensed
water is used for process water and is suplemented with mains
water where necessary.
I'he flue gas then enters 9, and is sprayed with the circulating
chlorine and acid solution for both temperature reduction
and the introduction of chlorine to the incoming flue gas.
The flue gas, at a temperature of 50 to 60 c, then enters the
counterflow packed scrubber 10 for reaction of objectionable
components with gaseous and dissolved chlorine. Chlorine gas
from the cylinders enters the system at 22, within the body
of the scrubber. The temperature of the gas entering the
scrubber and the gas flow rate are used in the bottom half
of the scrubber to partially dechlorinate the effluent
solution and to push the chlorine back up into the scrubber.
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8~ZS
The partially dechlorinated effluent solution from both 9 and
10 are collected in tank 11 containing a water cooled heat
exchanger for cooling the effluent. A small amount of hot flue
gas is bubbled through the solution to remove more free chlorine
from the effluent solution. The effluent is taken off
continually with a surface siphon to storage tank 12, from
which the acid can be shipped for use.
The flue gas, after addition of chlorine at 22, passes up
column 10. The^flow of solution down the column gradually
removes chlorine from the gas stream. The gas then enters
column 13, another packed column, which continues the chlorine
removal and also simultaneously continues the oxidation of
gas components using available chlorine. The circulating
solution in 13 is supplied from either the water mains or
from tank 8 and is recirculated from tank 11. After passing
through the column 13, the solution is collected in tank
14 and from there is circulated to the top of column 10.
The chlorine in the system is thus concentrated in column 10.
In the top of column 13 is a small demisting section 23 for
the removal of acid droplets from the gas stream.
The flue gas is then fed to the bottom of column 15, which
is a counter flow column packed with chunks of calcium
carbonate, for the removal of acid and chlorine from the gas.
The ciculating solution in column 15 is collected in tank 17
and recirculated to the top of the column. Additional water
is provided from the water mains or from tank 8. Insoluble
silt is separated at the bottom of 17 and the excess
solution is siphoned off to tank 18 for acidification with a
controlled amount of solution from tank 11. The calcium
sulphate formed is separated by filtration or centrifuging
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~6~3~1Z~
in 19 , and is disposed of after subsequent washing and mixing
with a small amount of ground calcium carbonate.
The solution leaving 19 contains regenerated chlorine and is
added to the solution ~sed in sections 9 and 10.
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Z5
The flue gas then passes through a demisting section 16,
and is conducted into the heated stack line 21. The updraft
due to the heating of the gas helps drive the gas through the
system and eases the blower requirements in the system.
A cooling tower of standard design 20 is shown in the
figure and is used to cool the circulating water in the heat
exchangers in tanks 8 and 11 and in the condenser 6. The
heat radiated from from the system at 4 and 20 may be used for
heating or for powering the system fans if desired.
All sections of the sy.stem are of standard design. The only
special requirement is corrosion resistance, which means that
the packed columns, tanks, pumps, valves and flue lines are
to be constructed of suitable palstics. Hot flue lines
prior to chlorine addition may be made of conventional
materials.
External to the system is a washing apparatus for the purpose
of cleaning the s.ilt from 17 and the calcium sulfate from 19;`.
The s~ids are washed with clean water and mixed with a small
amount of ground calcium carbonate to remove residual acid
before disposal. If the washing and mixing process is performed
on site, the acidic solution effluent may be used as system
feedwater.
~'
- 21 -
.
,
- ~3 6~
The appatatus of figure 2 differs from that of figure 1 in
a number of ways, re~lecting the versitility of the process.
This apparatus uses no hot chlorine treatment and no attempt
is made to collect particulates , as this particular embodiment
is intended primarily for use in oil well and refinery flares
which have no particulate problems but which have a problem
principally with S02. As these flares have a high temperature,
high water vapour flue gas, both heat and water vapour recovery
are practical, reducing or eliminating need for building heat-
ing in winter and for water forsystem operation from an outside
source. This small system uses simple , vertical scrubbers
made from plastic pipe for economy rather than more complex
scrubbers. Unlike the system of figure l the effluent solids
from this system are not cleaned as a part of the treatment
but are cleaned externally.
As will be apparent to those skilled in the art in the light
of the foregoing disclosure, many alterations and modificati-ns
are possible in the practice of this lnvention without
departing from the spirit or scope thereof. Accordingly,
the scope of the invention is to be construed in accordance
with the substance defined by the following claims.
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