Note: Descriptions are shown in the official language in which they were submitted.
101i7680
¦ B~CKGROUND OF THE INVENTION
¦ This invention relates to a relatively simple efficient
and economical process for removing particulates and gases such
as sulfur oxides, hydrogen sulfide and organic sulfur compounds
from an industrial gas stream. Mixed emissions of this type
are commonly found, for e..ample, in Kraft and sulfite recovery
! processes in the pulp and paper industries. Prior art processes
teach various methods of removing these types of emissions
i individually, however, none of the prior art teaches an economical
coordinated process for the removal of all of these components.
Furthermore, in some cases, a prior art process for the removal
of one component interferes with or reduces the efficiency of
subsequent removal steps for other components.
For example, emissions from Kraft recovery boilers
typically consist of hydrogen sulfide and organic sulfur compounds ~-
tdesignated "TRS" for total reduced sulfur), S02 and particulates.
The organic sulfur compounds typically consist of mercaptans such
as methyl mercaptan ~CH3SH), mercapto ethers such as dimethyl
sulfide (CH3SCH3), and disulfides such as dimethyl disulfide
(CH3S-SCH3). Some references indicate the presence of carbonyl
sulfide (COS). The quantity and composition of emissions are a
function of boiler feed and loading, boiler operation, and
process sulfidity.
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1067f~
Emissions from boilcrs are generally in the broad
¦ range of:
TRS: 10-2500 PPM (parts per million)
Particulates: 1-7 gr/sdcf (grains per standard
l dry cubic foot)
¦ so2 10-200 PPM (parts per million)
The permissible emissions from recovery boilers are,
¦ increasingly, being restricted by government authorities.
¦ Although the level of restriction varies with the specific
¦ authority, the emerging standards for 1977 appear to be TRS
l less than 5 PPM and particulates less than 0.08 gr/sdcf.
In some new boiler designs, TRS emissions can be
controlled to 3-10 PPM when operating at 80-100% of design
capacity, but only with close combustion control and decreased
l thermal efficiency. Also, particulate emissions present more
¦ of a problem with this type of design. Black liquor oxidation
processes in combination with existing furnaces can, with close
control, maintain TRS emissions at 4-30 PPM when operating at
l 80-100% of design capacity, but the particulate emissions problem
¦ still exists. Electrostatic precipitators in existing recovery
¦ boilers, after an extended period of operation such as 3-5
years, are reducing particulate emissions to levels of 0.10-
0.25 gr/sdcf at 80-100% of desiqn capacity. When the boilers
are operated at 120~ of design capacity, however, the parti-
l culate emissions level in many cases increases to more than
l 1 gr/sdcf. None of these systems can readily accommodate
fluctuating boiler load levels. Furthermore, electrostatic
l precipitators in themselves do not control TRS emissions.
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Therefore, it app~ars tha~ neith~r ~lectrostatic precipitators
alon~, black liquor oxidation alone, nor a combination of these
two well-evaluated systsms, are consistently capable of meeting
the overall environmental regulations.
Recently, experimental work has been conducted on the
absorption of sulfur oxides and other sulfur compounds in alkaline
slurries of activated carbon, In particular, U.S. Patent Nos.
3,701,824; 2,823,766; 3,486,852; and 3,824,163 teach that water
slurries of activated carbon can be used to scrub sulfur dioxide,
hydrogen sulfide, and organic sulfur compounds such as mercaptans
and alkyl sulfides from a gas stream. These patents appear to
depend on a combination of sorption and oxidation processes. In
general, these patents teach a carbon slurry concentration of
about 0.1-10~ by weight or higher for the cocurrent or counter-
current scrubbing of sulfurous gases having hydrogen sulfide or
organic sulfur compound concentrations on the order ot 100-5000 PPM.
These patents do not discuss the problem of the removal of particu-
lates.
Other prior art patents disclosing alkaline scrubbing
reactions are U.S. Patent Nos. 3,852,408; 3,852,409 and 3,755,990.
U.S. Patent No. 3,324,630 teaches a process for removal
of particulates from a gas stream which utilizes a crossflow
scrubbing technique. The process disclosed is capable of removing
very small particulates on the order of 0.1-10 microns in size.
One feature of the present invention is an improvement
in the particle-removal process of U.S. Patent No. 3,324,630 wherein
the particulate-laden gas stream is first treated under substantially
adiabatic conditions to increase its turbulence and to increase its
~o~idity subs~antially to saturation at a temperature
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1 10~80 Q~JO 5
above about 150F to initiate nucleation of small particulates
by condensat on and/or a~cJlomeration. Thereafter the gas is
contacted with a scrubbing liquor which can be recirculated
through a packed enclosure, usually at a substantially constant
temperature. This improvement normally eliminates the need for
cooling the recirculating liquor at a saving in material and
energy costs.
013JECTS OF TIIE INVENTION
Accordingly, it is a primary object of the present
invention to provide a courdinated and economic process for the
removal of particulates and acid gases from a hot effluent
gas stream.
It is specifically an object of this invention to
provide a process for scrubbing particulates, sulfur dioxide,
hydrogen sulfide and organic sulfur compounds from a gas stream
with an aqueous alkaline carbon slurry in a process which requires
a lesser concentration of carbon than has heretofore been possible .
It is also an object of this invention to provide
a wet scrubbing process for the removal of particulates which
does not normally require the cooling of recycled scrubbing
liquor.
It is further an object of this invention to provide
a process for scrubbing particulates and sulfurous gases from
a gas stream as described wherein efficient removal is obtained
at a minimum caustic and carbon consumption, with reduced re-
quirements or heat and power and with a lower initial cost of
equipment.
These and other objects of the invention will become
apparent from the following description.
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~ DLSCRI~''I'ION OF TIIE DR~WINGS
¦ FIG. 1 is a flow sheet illustrating thc evaporation
and recovery boiler portions of a typical pulping process
l producing an effluent flue gas containing particulates and
¦ sulfur-containing gases;
FIC. 2 is a schematic view of one embodiment of gas
treatment apparatus of this invention;
FIG. 3 is a partially-cutaway perspective view which
illustrates the structure of one form of the apparatus shown
schematically in FIG. 2;
FIG. 4 is a graph comparing the efficiency of TRS .
removal by laminar contact scrubbing with that by a turbulent
¦contactor; and
¦ FIG. 5 is a graph comparing the efficiency of parti-
¦culate removal in the process of this invention at different gas
¦stream dew point temperatures.
¦ FURTHER DESCRIPTION OF THE DRAWINGS
¦ FIG. 1 schematically illustrates one type of recovery
l boiler operation as employed in pulp manufacture. The liquid
¦ containing sulfur compounds and cellulose-lignin organic materials
called "black liquor", from a digester (not shown) is fed into
a black liquor oxidation chamber 10 where it is exposed to
oxygen. The oxidized black liquor is then fed to a steam-heated
evaporator 12 and a direct contact evaporator 14 where water is
evaporated to concentrate organic material to combusti~le lcvels.
The concentrated black liquor is then sprayed into a recovery
boiler 16 where the organic material is burned to recovcr heat
and chemicals. The hot effluent exhaust gases, treatment of
which is one object of the present invention, is then fed back
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¦ pagc 7
~ ~067680
to cvaporator 14 to recover heat, and thence through an electro-
I static precipator 18 to remove particulates. The gas exiting
the electrostatic precipitator (not always ~mployed) contains
l particulates, SO2 and TRS, principally hydrogen sulfide but also
5 ¦ frequently containing organic sulfur compounds as hereinbefore
described.
Referring now to FIG. 2, the hot inlet gas stream
Si is typically at a temperature of about 300-500F and a dew
I point of about 150-1850F. The gas stream may have previously
¦ been treated for preliminary particle removal by conventional
methods discussed hereina;ter. The stream Si is directed by
means of a washed fan 100 at a velocity of about 50 fps into a
venturi 101. The gas s subjected to a liquid spray quench 102
l prior to and/or simultaneously with reaching the venturi throat
¦ 103. A plug 104 having an essentially diamond-shaped cross-
section may be inserted in the venturi throat and has been found
to improve the efficiency of recovery. Venturi 101 is operated
at a lower pressure drop of the gas therethrough than more
l conventional venturis heretofore employed to remove particulates.
¦ The pressure drop of the gas therethrough is less than 20 and
preferably less than about 10 inches of water. In particular,
the use of a venturi with a diamond-shaped plug as shown has
been found to facilitate the removal of intermediate-sized
l particles larger than about 0.8 microns at this stage of the
¦ process, and such particles drop out of the gas stream either
by action of gravity or by impinging contact with the spray
formed in the venturi throat 103. The captured particles form
a slurry in the quench liquor, or, if soluble, dissolve therein.
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¦ The tur~ulen~ gas stream S, coolcd but still at a
¦ temperaturc above 150F and moisturizcd to near saturation
l by the action of the liquid quench, is next channeled through
¦ ¦ a set of baffles 105 wllich are continuously washed by a wash
I l liquor from nozzles 106. The wash liquor is drained to the
i ¦ bottom of the apparatus where the solids may be separated by
conventional means such as screen or settling tank means or
left to form a slurry. The ~ash l:iquor is combined with the
liquor from the venturi in sump 108 and is recirculated by
pump 109.
Emerging from the baffle system, the gas is substan-
tially saturated with water vapor at a temperature of at least
about 150F to 212F and nucleation of sub-micron particles
occurs. It should be noted that the increase in turbulence and
saturation of the gas within the enclosure defined by venturi
101, baffles 105 and the walls of housing 107 occurs under
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substantially adiabatic or iso~nt-halphic conditions. No
significant heat is added to or withdrawn from the gas, the heat
of the gas being employed to vapo~ize the small amount of
moisture required and the vaporization cooling the gas by
lowering its dry bulb temperature. Under equilibrium operation,
with recirculating quench and wash liquor, the temperature of
the liquor and gas will be near the wet bulb temperature of
the incoming gas.
The gas together with the entrained, nucleated particlec
is then passed in an essentially horizontal path through scrubber
bed 111, packed with any suitable packing material, preferably
the packing material disclosed in U.S. Patent No. 2,867,425,
also described in U.S. Patent No. 3,324,630, and available
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commcrci~lly under thc tradcmark "Tellerettes", more fully
described ilereinafter, wllere it i5 brought into crossflow
contact with the scrubbing liquor which is continuously sprayed
into scrubbing section 111 by nozzles 112, 113 and 11~. Al-
though FIG. 2 shows a single scrubbing section with three sets
of nozæles, the number of sections, the size of the sections,
and the number of nozzles per section is not critical and may
be varied to suit individual process requirements. The gas is
l then passed through a second packed section 115 wllich is washed
with recirculating wash liquid and makeup water from nozzles 116
to remove any entrained liquor containing TRS and solids. The
sections shown in FIG. 2 are inclined at an angle of about 8-13
from the vertical in the direction in which the gas is moving.
Such a construction is not critical but helps to pr~vent maldis-
tribution of the liquor in the packing and thus insures full use
of the packed section. The scrubbing liquor and washing liquid
from sections 111 and 115, respectively, together with particu-
lates, are drained to the bottom of the respective sections
through packing support gratings which are of such size that the
packing is supported whi~e the liquid and suspended particulates
pass through and into collection sumps 108 and 117 respectively.
Pumps 118 and 119 are used to recirculate the scrubbing liquor
and washing liquid respectively. If desired, a single collection
sump below the packed sections and venturi can replace sumps
108 and 117 and the liquor collected in the single sump can be
recirculated by one or more pumps. Where two sumps are employed
as shown, they can be separated by an overflow weir 120 whereby
excess recirculating liquid, including fresh makeup water, can
flow into sump 108. By this means, the concentration of salts
and solids in the wash liquid in sump 117 can be maintained at
a lower concentration than in the liquor in sump 108.
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To rcplace lic~lids lost with thc gas and withdrawn
¦ with slipstream 121, and to mailltain the desired concentration
of carbon and alkali during use, all as more fully explained
hereinafter, fresh makcup water is supplied at 122, concentrated
caustic is added at 123, and carbon slurry is added at 124.
Also as more fully explained hereinafter, activated carbon in
the liquor slurry in sump 108 is aerated tnrough submerged
nozzles 125 within the sump and fed at 126 through a compressor
(not shown).
Advantageously, after leaving the scrubbing section
115, the gas stream is passed through an open drainage zone 127
to allow drippage of entrained water droplets followed by a
demisting chamber 128. The demisting chamber is packed with
¦ any suitable packing material, preferably the same material
¦ used to pack the scrubbers. A subsequent demisting chamber
may also be employed. The treated gas SO from the second
I enclosure defined by baffles 105 and the walls of housing 107,
¦ is substantially free of particulates larger than about 0.1
¦ micron.
¦ As shown in FIG. 2, a single pump 109 can be used
¦ to recirculate liquor for the baffle sprays 106 and the venturi
quench 102. As a further pre-treatment, prior to the venturi
and baffles, the gas stream can optionally be passed through
l washed fan 100 for additional increases in humidity and
turbulence and to improve the wetting of the particulates.
The fan can be washed with a portion of one of the
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iOti7680
¦ rccycled aqueous liquids, for example, the makeup water from
¦ pump 119 as shown in FIG. 2.
¦ FIG. 3 is a partially cut-away perspective view of
¦ a ground level installation similar to FIG. 2 and wherein like
I parts have like numbers. Pumps 109 and 118 have been rearranged
l to pumps 130 and 131. This figure illustrates that apparatus
¦ according to this invention can be combined in a single compact
housing. FIGS. 4 and 5 are described hereinafter.
l DESCRIPTION OF THE PREFERRED EM~ODIMENrr
¦ General Description
In general, the present invention comprises the
¦ following steps:
1) In a preliminary step, the hot particulate-laden
gas containing a mixture of sulfur oxides, hydrogen sulfide,
and organic sulfur compounds is treated by conventional means
for the removal of particles larger than about 5 microns.
Such means are well known in the art and include a cyclone
separator, a spray tower, a venturi, an electrostatic preci-
pitator, and a tray column, either alone or in combination.
For example, the combination of a cyclone separator for the
preliminary removal of particles and a crossflow scrubbing
apparatus for the removal of very small particles is illustrated
in U.S. Patent No. 3,324,630. This step is optional sinee
the subsequent steps set forth below will remove large as well
as relatively small particles. However, if a significant quantity
of partieles larger than about 10 microns are prescnt, the
preliminary separation step will be more eeonomieal.
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2) The hot gas, preferably containing only particles
smaller than 10 microns in size together with various sulfur
contaminants, is next subjected to a liquid quench immediately
prior to or simultaneous with its passage through the low energy
venturi. This treatment cools (although maintaining the
temperature above 150F) and moisturi~es the gas to a point
approaching saturation conditions and introduces additional
turbulence in the gas. A wetted inlet fan can also be employed
prior to the venturi.
3) The gas, is next passed through a liquor-washed
baffle system. This further cools (although still maintaining
the saturation temperature above 150F) and moisturizes the gas
to substantial saturation and also creates additional mixing in
the gas stream~
4. On leaving the baffle system the substantially
saturated gas is at a temperature above about 150 F and these
conditions have promoted rapid nucleation among particles down
to an initial size of about 0.1 microns or less.
5. The gas stream is next passed through one or a
plurality of packed scrubbing beds in crossflow contact with a
scrubbing liquor. The preferred scrubbing liquor for gas
streams which contain TR~ in addition to acid gases comprises
an aqueous, alkaline suspension or slurry of activated carbon
as more fully described hereinafter.
6. The gas stream is recovered from the scrubber
unit essentially free of entrained particulate matter larger
than about 0.30 microns and essentially free of sulfur compounds.
The gas may then be exhausted to the atmosphere or further
treated as follows.
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7) Optionally, before discharge to the at sphere, the hot, moist
gas stream may be further cooled to remove additional particulates and/or
passed through a washing scrub~er and a drainage zone followed by a
demister. These latter steps comprise a fairly simple and economical means
for rem~ving entrained droplets of liquid from the gas stream before
exhausting it to the atmosphere. A suitable demister is preferably a unit
packed with the same packing material as the scrubber but is not washed
with any liquid.
Although the nucleatiQn mechanism for fine particulates is not
thoroughly understood, it is believed to involve condensation of moisture
on the fine particles and their agglameratiQn by collision with and bonding
to other such particles, thereby increasing their effective size. Fine
particulates are also thought to have a surfaoe electrostatic ch æge by
virtue of their high surface to mass ratio. Such charges are believed to
assist in the nucleation pro oess.
,
Adiabatic or isoenthalpic nucleation as herein disclosed is a
function of essentially three variables, the moisture content of the gas,
the turbulence of the gas, and the temperature of the gas. Thus it has
been found that adiabatic nucleation is not effective below about 150F
saturatian temperature and that higher gas saturation temperatures compensate,
in part, for a lesser degree of turbulen oe in the gas and vi oe versa.
An increase in turbulen oe in the incoming gas, to a Reynolds number of
at least 3000, and preferably of at least 10,000 or more at the time of
cooling to saturation is ne oe ssary. With higher saturation gas te~peratures,
either the venturi or baffles, or both, can in some applications be omitted,
although both are preferred. Thus, where the incoming gas has a saturatian
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temperature of about 190F to 212F, the venturi can be omitted.
~t close to 212F saturation tclnperature, both the venturi and
baffles can be omitted and a series of water jets employed. For
a given set of operating condi~ions, the turbulence of the gas
can be varied experimentally to optimize results. While it is
technically feasible to raise the saturation temperature of an
incoming gas stream to a point requiring minimum turbulence, the
cost of doing so is ordinarily prohibitive. Turbulence, however,
can be increased comparatively inexpensively.
The packing elements or units that operate most satis-
factorily in the process and apparatus of this invention are
disclosed in applicant's Patent Nos. 2,867,425 and 3,324,630 and
areavailable commercially under the trademark "Tellerettes".
"Tellerettes" provide a filamentous packing having little con-
tinuous extensive surface and about 80-85% free volume therein;
the packing consisting of randomly arranged, interlocked tower
packing units, the units being made up of approximately circular,
integrally connected filament portions having their axes
approximately tangent to a circle at approximately evenly spaced
points therearound, the number of such spaced approximately
circular portions being from 6 to 12 and the diameter of such
circle being approximately equal to the diameter of one of such
approximately circular filament portions plus the diameter of
a smaller circle whose circumference is not less than the cross-
sectional dimension of the filament portion in the direction of
its axis times the number of such filament portions and not
greater than the circumference of one of such approximately
circular filament portions. Such packing units are hereinafter
referred to in the description and claims as "toroidal elements"
which terms are to be understood as incorporating therein the
above description by reference.
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rrhe preferred scn~bbing liquor for gas streams which oontain
TRS in addi~ion to acid gases is aIkaline aqueous slurry of activated carbcn
having a particle size range preferably in the range of 0.05-10 microns and
a pH of about 8-13, more preferably 8-9.5, ~nd most preferably about 9.0-9.3.
The alkaline material in the scrubbing liquid may be soluble sodium or
potassium salt such as sodium hydroxide, sodium carbonate, or the like or a
relatively insoluble alkaline earth metal salt sudh as lime or calcium
carbonate in slurry form. Sodium hydroxide is pxeferred.
The removal of S02 and TRS by the scrubbing liquor is based on
sorption and chemical reaction with hydroxide and oxygen. S02 is converted
to sulfates and TRS to oxidized sulfur compounds. H2S for example is
oonverted at least in part to Na2S203. Such compounds are not w latile and
can be recirculated in the scrubbing liquor as dissolved or suspended salts.
In addition to the oxidized materials, the scrubbed particulates, principally -
carbonates and sulfates of sodium, recirculate with the scrubbing liquor.
Maximum recirculation of scrubbing liquor is an important part
of the present invention for reasons of cost and efficiency. With prior art
processes the highest solids or non-volatile content, i.e. the oontent of
materials which are essentially non-volatile at 212 F, that can be
recirculated is about 15% by weight. With the pxesent pro oe ss, however, the
non-vDlatile content may be as high as 25% and is preferably in the range
of 20-25% by weight. The crossflcw scrubber of this invention is stable
at such high oontent.
Crossflow scrubbing has other important advantages in t~e
present invention. m e ratio of scrubbing liquor to gas flow rates can be
varied along the depth of the packing, i.e. in the
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direction of g.lS Elow, as can the size of the packing clemcnts.
Also differellt liquors o~ diferellt comyosition or conccntration
can be employod and recirculated. Preferably, higher flow rates
of the samc scrubbing liquor are cmployed in ups~ream portions
i of the packing where thc SO2 and TRS concentrations in the gas
¦ are highest. Thus the ratio of alkali (and oxygen) to SO2 and
I T~S (and acid particulates such as Na~lSO4) ConcentratiOnS in
¦ ¦ the gas can he varied with the depth of packing. For example,
¦ in FIG. 2, the valves controlling nozzles 112, 113 and 114 can
¦ be adjusted to provide a high flow rate through nozzle 112, a
¦ lower rate through nozzle 113, and still a lower rate through
¦ nozzle 114. Under some conditions it has been found that, based
¦ on the same total flow rate, such a distribution of scrubbing
¦ liquor will be more efficient than an even distribution.
¦ Similarly, it is sometimes desirable to employ larger packing
elements, e.g. 2 inch toroidal elements, in upstream portions
of the packing and smaller elements, e.g. 1 inch toroidal elementc
in downstream portions.
Sufficient alkali and carbon are required for efficient
reaction and removal of contaminants but excess should be avoided
for economy and to limit corrosion. Alkaline pH is necessary
but the pH should be below about 9.5, and preferably 9.3, to
avoid reaction with CO2. With well-oxygenated, activated carbon,
a carbon content between about 0.03% and 0.20% by weight is
suitable and about 0.05~ to 0.15~ is preferred. These values
are lower, for a given removal efficiency, with the present
invention than with prior processes because the scrubbing liquor
f low in the crossflow scrubber is laminar over the packing,
rather than rbole=t. With laminar f1ow it is believed that the~
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suspended carbotl migrates to the surface of the flowing liquor
and concentrates in the most active portion of the scrubber
liquor, that is, tlle portion in contact with the gas. Below
l about 200 PPM of TRS in the gas, it has been found that a bulk
¦ concentration of carbon in the weight range of about 0.03~ to
0.07%is suff_cient and above 200 PPM TRS, a range of about
0.0~ to 0.15%is sufficient. Thus a carbon concentration range
between about 0.0~ to about 0.20~ by weight is preferred, the
l particular value selected being a function of operating condition
¦ and TRS inlet concentration in the gas.
To maintain the non-volatile concentration in the
recirculated scrubbing liquor, a slipstream of liquor is bled
off and returned for processing to the material balance of the
l pulp process. The high non-volatile concentration in the slip-
¦ stream permitted by this invention is advantageous because a
minimum of carbon and unreacted alkali are thereby withdrawn with
the slipstream and less heat is required to remove water for
concentrating the salts recovered in the slipstream. Fregh
makeup water and fresh alkali and carbon are added as required
to maintain pH and carbon concentration in the scrubbing liquor.
For the reasons given above, the consumption of alkali
and carbon in the present invention are low, generally in the
range of 0.3 to 0.6 pounds carbon and about 9 to 25 pounds of
alkali, measured as NaOH, per ton of air dried pulp processed,
depending on the specific process conditions and control, and
the type of wood being pulped. These relatively low values are
important since such consumption is estimated to constitute the
largest single item of cost in operating the process, including
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amor~ization of e(luii>mcnt. Properly opcrated, it is estimated
that thc economic valuc of rccovered salts returned to the pulp-
ing process can exceed the total cost o~ operating the flue gas
l treating urocess of this invention.
¦ The present invention also has a low cost for power
and heat since the nucleation step requires low power and
essentially ro heat, while the scrubbing step preferably i5
operated without significant cooling of either the gas or
l scrubbing liquor, except incidentally in withdrawing of slip-
l stream and adding of makeup materials. Crossflow scrubbing
also has an inherently lo~ pressure drop for the gas such that
the entire process can be operated with a gas pressure drop
below about 30 inches of water, and typically less. Thus the
l entire process is substantially adiabatic throughout and, so
lS ¦ operated, can reduce the particulates in the exhaust gas to about
0.03gr/sdcf. If further reduction is desired, the gas ean be
l exposed to a eooling liquid, either the serubbing liquor itself
¦ as shown in U.S. Patent No. 3,324,630, or fresh makeup water
as deseribed herein, in either the whole of the paeking of-the
¦ serubber, a portion thereof, or a separate paeked seetion. ~y
sueh eooling, where desired, partieulates ean be further redueed
to about 0.01 gr/sdef.
The eross seetional area of the paeked serubber is
l ehosen to aeeommodate the flow rate of gas to be treated and the
¦ depth of paeking, with respeet to the direetion of flow of the
¦ gas, is ehosen to provide the required removal of eontaminants
¦ to the extent desired, greater depth providing inereased
¦ removal within the limits of the proeess. The required depth
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can be provided in continuous or separated scctions. Scrubbing
liquor flow rates are chosell to maintain laminar liquid flow
over the surface of the packing, and can be varied along the
l depth of packing as described.
¦ The following examples further illustrate the present
invention.
EX~MPLE I
A series of tests were performed in an integrated
l recovery apparatus as illustrated in FIG. 2 with flue gases
¦ from a Kraft recovery pro~ess as illustrated in FIG. 1. Gas and
process operating conditions are given in TABLE 1. The pressure
i drop of the gas in the venturi was in the range between 4 and 10
inches of water, and in the total scrubber between 7 and 13
l inches of water. The depth of the packing was about 5 feet
¦ and the scrubbing liquor flow rate was varied along the depth
to provide greater flow upstream of the gas than downstream.
The system was found to be capable of a 2:1 turndown,
¦ providing desirable flexibility of operation, and was relatively
l insensitive to variations in liquid and gas flow rates. During
¦ testing, including operation 24 hours per day 7 days per week,
no solids build up, no increase in pressure drop, and no adverse
conditions such as undue foaming were observed.
'
~' : - ,
-
lQ~7~8V ~ Jc 20
ur r
_ _ .,
Gas Conditions Inlet Outlet
! Gas Flow 235,000 200,000
I (acfm)
l _ .
Temp. 300 163
Concj Sx 50-150 5-10
! Conc. Particulates 1.5 0.02-0.06
(gr/sdcf)
TRS 600 3-5
! (PPM)
1 - SOx is used to denote mixed sulfur oxides, predominantly SO2.
Scrubbing Liquid Inlet Outlet
Liquid Flow 3760 3713
j (gpm)
.
Temp. 167 167
I Venturi quench liquid - 2200 gpm at 163F
i Baffle wash liquid - 700 gpm
Make-up water - 50 gpm
Make-up NaOH - 200-1000 lbs./hr.
Make-up carbon - 5-15 lbs./hr. I
Air for oxygenation - approx. 1500 cfm
Recycle liquid: 20 gpm
22% solids
0.1% carbon
pl~ 9.3
acfm - actual cubic feet per minute
PPM - parts per million
gr/scdf - grains per standard dry cubic foot of gas
gpm - gallons per minute
TRS - total reduced sulfur
. _
2~
~. ~
. .
: . . ,: . . .
~ ' ' `; ' ' ' .
- . .
~ 7~80
p~9~ 21
EYAMPLE II
The purpose of this example was to compare thc process
of this invention with that taught by U.S. Patent No. 3,701,824,
and in particular, to compare the efficiency of a turbulent
contactor with the crossflow scrubbing process of this invention
at low levels of TRS emissions. The data for this example were
obtained from tests at TRS levels of about 10-100 PPM using two
crossflow scrubbers and one turbùlent contactor having the
Ifollowing characteristics:
¦ TABLE 2
Recovery Unit ~PCarbon Slurry - wt.-%
Crossflow Scrubber~ 10 0.03-0.06
Turbulent Contactor16 0.5
The results of these tests were plotted on the basis
of efficiency (on a logarithmic scale) against TRS concentration
as shown in FIG. 4 wherein the solid curve represents the cross-
flow data and the broken curve the turbulent contactor data.
These tests demonstrate the superior efficiency of the crossflow
¦scrubber in removal of TRS emissions despite a ten-fold reduction
¦in the concentration of carbon in the slurry. Furthermore, these
¦ data show that the crossflow scrubbers operated at about a
30~ less pressure drop, therefore requiring less power than the
turbulent contactor.
EXAMPLE III
The purpose of this example is to demonstrate the
variation of caustic consumption and thermal requirements at
varying concentrations of dissolved solids ~non-volatiles)
in the recycle scrubbing liquor in the stable crossflow scrubber
of this invention. The data
21
.. .. . . .
.
- ,
. -. , - . .. . ~
- . ~ . .
...... ; . ..
.
: - , .
.
Il p~g(~ 22
lOf~7~0
was obtained from a h;cJh emissioll boiler having the following
characteristics:
OperatincJ Level - 600 TPD
TRS - 500 PPM av.
Particulate - 1.5 gr/sdcf
Gas Flow - 200,000 acfm 160~F Sat.
The results are shown in Table 3 below:
TABLE 3
Recycle Liquor - Slip- Unreacted Thermal Load
Dissolved Solids streamNaOH loss For Conc. to
(% Concentration) rate(lb/ton 50% Solids
(GPM)* of pulp) (BTU/Hr.)
90.5 2642.8 x 106
47.6 13.721.5 x 106 ,
27.0 7.811.1 x 106
, 20 19.1 5.57.2 x 106
14.3 4.14.8 x 106
* Required to maintain solids concentration
~ 22
~. .. ,. :
- , .
~ 7~80 ~9~ 22~
~XAMPLI. IV
The followin~ tests were conducted to compare the
process characteristics of particulate removal carried out
according to the present invention with those of the process
according to U.S. Patent No. 3,324,630. An apparatus with two
scrubber units in series was employed. Run #l represents a
scrubbing process essentially according to the aforementioned
patent. Run #2 and Run #3 represent adiabatic processes
according to the present invention. By "adiabatic" it is
meant that little or no heat is added to or removed from the
gas stream during the treatment process. The results a e shown
in Table 4 below.
In Run #1, the gas was not treated prior to the scrub-
bers to increase its turbulence and increase its humidity
substantially to saturation above about 150F. As a result it
was found that removal of particulates to the desired level of
less than 0.8 gr/sdcf required the use of cool scrubbing liquor
of about 57F. Contact with the gas stream resulted in heating
! the scrubbing liquor to about 110F. 8ased on a scrubbing
¦ liquor flow rate of 150 gallons per minute, this means that
about 4 million BTU/hr. were removed from the gas stream.
Because it is environmentally objectionable to discard hot,
salt-contaminated liquid wastes, it is often necessary to recir-
l culate a major portion of the used scrubbing liquor which
¦ requires an expensive cooling operation.
By contrast, Runs #2 and #3 were conducted under
l substantially adiabatic or isoenthalpic conditions. The heat
¦ added to the gas stream by the venturi quench and that removed
~1 ~22~ l
r~
.. . . . . .
. . .
. ~ ~
10~i7~i80 l~alJe :!2D
by the slipstream (line 121 in FIG. 2) is almost negligible in
comparison to the overall heat content of the gas stream.
Because the gas stream was treated prior to the scrubbers to
increase its turbulence and increase its humidity substantially
to saturation, nucleation of particulates occurred prior to the
scrubbers and it was not necessary to use scrubbing liquor which
was cooler than the gas in order to obtain the desired reduction
in particulates. Thus, the scrubbing liuqor was recirculated
at substantially constant temperature thereby eliminating the
expensive cooling operation.
This example also demonstrates the use of a higher
scrubbing liquor flow rate in an upstream scrubber section and
a lower flow rate in a downstream section. For certain appli-
cations it has been foand that such an arrangement is more
efficient than an even distribution of scrubbing liquor across
the depth of the packing.
~ ~226
: .
: , . .
.
qo22C
067~80
~1
li :' .
l¦ TAI3L~ 4
l . .. _ .__
Gas ~oncLitions ~i~un i,ll~u~ 2 Run ~3
Inlet Flow (acfm.) 6062 N.A. N.A.
~utlet Flow (acfm.) 3302 ~669 3630
Inlet Temp. (F.) 174 260 267
Venturi Outlet (F. ) no venturi 161 158
Final ~utl~t (F.) 60 155 157
. . _ .... .., . ... . ........ _~ ~
.
i Liquid ~onditions
..... __ . . .. . . . _ . _ _ ..
Venturi Quench (gpm.) no venturi50 50
Scrubber ~1
Scrubbing Liquid (gpm.) 75 140 140
, Inlet (F.~ 57 155 157
Outlet (F.) - 110 155 157
i Scrubber #2 .
i Scru~bing Liquid (gpm.) 75 50 50
. Inlet (F.) 57 lS5 157
; Outlet ~F.) 110 155 157
! ~
N.A. - Uata not available
acfm. - actual cubic eet per minute
gpln. - gallon-~ per nlinUtQ
I .
' -22 e
ll -
:
: :
:~
:: :
:: , ' ?
.'
10~7~10
~`XA~IPLE V
This e~ample demon~trates the effectivencss of this
invention in particulate removal, thc variation of e~Eective-
ness with gas saturation temperature, and the criticality
of a gas temperature above about 150F.
A series of tests were conducted at different gas
saturation temperatures between 155F to 172F, without cooling
the recycled scrubbing liquid, and with parti~ulate loading
ranging from 0.17 to 0.54 gr/sdcf. The recovery boiler
effluent was pre-treated with an electrostatic precipitator
to remove larger particles prior to entering the scrubbing
unit. These results are plotted in FIG. 5. The smoothed
curve indicates a particulate emission ranging from 0.050
gr/sdcf at an operating temperature of 155F to 0.024 gr/sdcf
at an operating temperature of 172F, well within the proposed
1977 standard of 0.08 gr/sdcf.
E~PLE VI
Tests similar to EXAMPLE V were conducted with a recovery
boiler effluent gas pre-treated in a direct contact evaporator.
Duct thermal loss prevented conducting tests at adiabatic
temperatures above 162F. However, with inlet loadings ranging
from 0.8 to 3.0 gr/sdcf and with the scrubber system operating
at 16 to 19 inches of water, particulate emissions were reduced
to 0.11 gr/sdcf. The particles from the evaporation were
found to have hydrophobic coatings; therefore, to accelerate
the initial wetting of these particles, additional turbulence
was induced in the gas prior to the scrubber. With added
turbulence prior to scrubbing, particulate emissions were
reduced to the order of 0.03 to 0.04 gr/sdcf, again well within
proposed 1977 standards.
- 23 -
kam:jvb
.
.
.
