Transcript Document

ABSORPTION
FLUE GAS DESULFURIZATION
CREATED BY
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ZELİHA SALDIR
ITIR SARI
TUĞBA BEĞENDİ
MUSTAFA ÖZGİRAY
YAKUP TURGUT
INTRODUCTION
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Absorption, or gas absorption, is a unit operation used
in the chemical industry to separate gases by washing or
scrubbing a gas mixture with a suitable liquid .
The fundamental physical principles underlying the
process of gas absorption are the solubility of the
absorbed gas and the rate of mass transfer. One or
more of the constituents of the gas mixture dissolves
or is absorbed in the liquid and can thus be removed
from the mixture. In some systems, this gaseous
constituent forms a physical solution with the liquid or
the solvent, and in other cases , it reacts with the liquid
chemically.
The purpose of such scrubbing operations may be any
of the following : gas purification (eg , removal of air
pollutants from exhausts gases or contaminants from
gases that will be further processed) , product recovery ,
or production of solutions of gases for various
purposes.
Gas absorption is usually carried out in vertical
counter current columns as shown in figure 1.The
solvent is fed at the top of the absorber , whereas the
gas mixture enters from the bottom .The absorbed
substence is washed out by the solvent and leaves the
absorber at the bottom as a liquid solution . The
solvent is often recovered in a subsequent stripping
or desorption operation . This second step is
essentially the reverse of absorption and involves
counter current contacting of the liquid loaded with
solute using and inert gas or water vapor .
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The absorber may be a packed column , plate
column , spray column , venturi scrubbers ,
bubble column , falling films , wet scrubbers
,stirred tanks
PACKED COLUMN
The packed column is a shell either filled with
randomly packed elements or having a regular solid
structure designed to disperse the liquid and bring it
Dumped-type packing elements come in a great
variety of shapes and construction materials, which
are intended to create a large internal surface but a
small pressure drop. Structured ,or arranged packings
may be made of corrugated metal or plastic sheets
providing a large number of regularly arranged
channels ,but a variety of other geometries exists.
Packing materials may be classified as follows,
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rock
3-coke
4-stonaware shapes
4a-raching rings
4b-berl saddle
4c-sprial rings 1-wood slats
2-broken
4d-grid bloks
5-miscalloneous material
Rashing rings are the most widely used form of
tower packing. They are cylindrical rings, of the
some length as the diameter of the cylinder and
with the walls as thin as the material will permit.
Rashing rings are almost always dumped into
the tower at random and not stacked regularly.
They offer the best combination of low weight
per unit volume,free volume,free cross section
and total surface of any type of packing.
A packed bed column contains a support plate, a
liquid distributor, and a mist eliminator.
Mist eliminators are used to condense any vaporized
scrubbing liquid. Support plates hold the packing in
place.
The advantages of packed columns include simple and,as
long as the tower diameter is not too large,usually relatively
cheaper construction. These columns are preferred for
corrosive gases becuase packing, but not plates, can be
made from ceramic or plastic materials. Packed columns
are also used in vacuum applications because the pressure
drop, especialli for regularly structured packings, is usually
less then through plate columns.
Usage examples
Packed columns are used mostly in air pollution control..
The water soluble ethylene gas ishydrolyzed to ethylene
gylcol.
Packed columns are also used in the chemical
,petrochemical,food, pharmaceutical,paper, and aerospace
industries.
TRAY COLUMN
Tray absorbers are used in applications where tall
columns are required, because tall, random-type packed
towers are subject to channeling and maldistribution of
the liquid streams. Plate towers can be more easily
cleaned. Plates are also preferred in applications having
large heat effects since cooling coils are more easily
installed in plate towers and liquid can be withdrawn
more easily from plates than from packings for external
cooling. Tray columns have got some disadvantage.
These are slow reaction rate processes, higher pressure
drops than packed beds and plugging and fouling may
be occur.
Tray absorbers are used in applications where tall
columns are required,because tall,random-type
packed towers are subject to channeling and
maldistribution of the liquid streams. Plate
towers can be more easily cleaned. Plates are also
pereffered in applications having large heat
effects since cooling coils are more easily
installed in plate towers and liquid can be
withdrawn more easily from plates than from
packings for external cooling.
Usage Examples
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Tray columns are used in a refinery dehexanizer to
decrease the benzene content in the naptha feed to the
process. This results in lower automobile exhaust
emissions.
STIRRED TANKS
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If the absorbtion process includes a slow liquid-phase
chemical reaction, or close control of the process is
needed, stirred tanks are used.the gas is introduced
directly into the liquid and mixed by the stirred in a
stirred tank.
Usage examples
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Stirred tanks can be used in
lime slurry carbonation,paper
stock chlorication, regular oil
hydrogenation,fermentation
broth aeration,penicilin
production, citric acid
production,and aeration of
activated sludge.
BUBBLE COLUMN
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Structured catalytic bubble columns are new, very
promising types of multiphase reactors. Their
configuration lies basically between slurry reactors and
trickle bed reactors. The solid phase, consisting of
catalyst particles, is enclosed in fixed wire gauze wraps,
which are mounted along the height of the column.
The gas phase is dispersed into the liquid phase and it
flows in the empty passages between adjacent
envelopes. The liquid phase may be operated in a batch
manner or it may also circulate in co-current or
counter-current manner to the gas flow.
The main advantages of this reactor type with respect
with the conventional slurry bubble column are:
1.no problems for separating catalyst from the liquid;
2.improved conversion and selectivity due to staging
of the liquid phase;
3.no scale up problems because the hydrodynamics is
dictated by the size of the open channels of the
catalytic structure.
Usage Examples
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Bubble columns can be used
to purify nitroglycerin with
water, in the chemical
industry for hydrogenation,
oxidation, chlorination, and
alkylation, and in the
biotechnological field for
effluent treatmet, single-cell
protein productin, animal cell
culture, and antibiotic
fermentation. Bubble
columns can be used for
radioactive elements because
there are no moving parts.
Venturi Scrubbers
Adjustable-throat venturi scrubber with
movable plate
Venturi scrubbers can be used for removing gaseous
pollutants; however, they are not used when removal of
gaseous pollutants is the only concern.
The high inlet gas velocities in a venturi scrubber result
in a very short contact time between the liquid and gas
phases. This short contact time limits gas absorption.
However, because venturis have a relatively open design
compared to other scrubbers, they are very useful for
simultaneous gaseous and particulate pollutant removal,
especially when:
•Scaling could be a problem
•A high concentration of dust is in the inlet stream
•The dust is sticky or has a tendency to plug openings
•The gaseous contaminant is very soluble or chemically
reactive with the liquid
To maximize the absorption of gases, venturis are
designed to operate at a different set of conditions from
those used to collect particles. The gas velocities are
lower and the liquid-to-gas ratios are higher for
absorption.
For a given venturi design, if the gas velocity is
decreased, then the pressure drop (resistance to flow)
will also decrease and vice versa. Therefore, by
reducing pressure drop, the gas velocity is decreased
and the corresponding residence time is increased.
Liquid-to-gas ratios for these gas absorption
applications are approximately 2.7 to 5.3 l/m3 (20 to
40 gal/1000 ft3). The reduction in gas velocity allows
for a longer contact time between phases and better
absorption.
Increasing the liquid-to-gas ratio will increase the
potential solubility of the pollutant in the liquid.
Flooded elbow
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Venturi scrubbers can have the highest particle
collection efficiencies (especially for very small
particles) of any wet scrubbing system.
They are the most widely used scrubbers because their
open construction enables them to remove most
particles without plugging or scaling. Venturis can also
be used to absorb pollutant gases; however, they are not
as efficient for this as are packed or plate towers.
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Venturi scrubbers have been designed to collect
particles at very high collection efficiencies,
sometimes exceeding 99%. The ability of
venturis to handle large inlet volumes at high
temperatures makes them very attractive to
many industries; consequently, they are used to
reduce particulate emissions in a number of
industrial applications.
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This ability is particularly desirable for cement kiln
emission reduction and for control of emissions from
basic oxygen furnaces in the steel industry, where the
inlet gas enters the scrubber at temperatures greater
than 350 °C (660 °F).
Venturis are also used to control fly ash and sulfur
dioxide emissions from industrial and utility boilers.
Falling film
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With high efficiency in absorbing HCl
(hydrochloric) gas, H2S, HF, SO2, NH3 gas,
graphite falling film absorbers comprise of
absorption liquid distributor, cooling and
absorption section and Gas-Liquid separator.
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Absorption liquid distributor is for film
forming and flow into absorption tube in
cooling and absorption section.
On request, cooling and absorption section has
two models basis of its heat transfer unit -Shell
and Tube and Block. Gas-Liquid separator is to
separate tail gas and product.
Its convincing advantages and
disadvantages is following: 
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Advanges
High efficiency of
absorption drop
Low outlet temperature
No need after-cooling
Low flow resistance
Easy maintenance
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Disadvanges
Restricted by pressure
Film breakup
Flooding
SPRAY COLUMN
Spray columns are differetal contactors. The
liquid stream enters the coloumn through one
or more spray nozzles at different heights in
the column. The droplets formed provide a
large surface area for exposure to the gas
stream,with smaller droplets resulting in a
greater Exchange area. The liquid and gas
streams can flow counter-currently or in
paralel. An optimum droplet velocity is
essential because low velocity will lead to low
contact or turbulence and high velocity may
cause flooding.
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A mist eliminator is used to
separate any liquid that is
entrained into the gaseous
phase. Spray columns are used
to absorb SO2 from coal-fired
boiler exhaust gases.
WET SCRUBBER
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Wetted packed towers are the simplest and most
commonly used approaches to gas scrubbing.
The principle of this type of scrubber is to
remove contaminants from the gas stream by
passing the stream through a packed structure
which provides a large wetted surface area to
induce intimate contact between the gas and the
scrubbing liquor. the contaminant is absorbed
into or reacted with the scrubbing liquor.
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The packing of the tower is normally a proprietary
loose fill random packing designed to encourage
dispersion of the liquid flow without tracking, to
provide maximum contact area for the 'mass transfer'
interaction and to offer minimal back pressure to the
gas flow. The reactivity between the contaminant and
the scrubbing liquor influences the system designer's
determination of gas and liquor flow and the height
and diameter of the packed bed.
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A demister is fitted at the top of the tower to prevent
entrainment of droplets of the scrubbing liquor into
the extraction system or stack.
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Wetted packed towers can be designed for very high
efficiencies with relatively low capital and running costs.
The low pressure drop associated with packed bed
scrubbers permits the use of smaller more economical
fans. Although efficiency may be affected, a packed
tower will usually function when gas or liquor flows
vary from its original design parameters.
Usage examples
Wet scrubbers are used by the food industry,such as in
cheese proessing for dust and ambient moisture
removal.
FLUE GAS DESULFURIZATION
SYSTEM
Gas desulfurization can be accomplish by wet, dry, or
alkali scrubing.These methods are covered in this
section.
THE WET FLUE-GAS DESULFURİZATİON
SYSTEM
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The wet FDG system, also called a wet scrubber, is
cammonly based on low-cost lime-limestone in the
form of an aqeous slurry.this slurry, brought into
intimate contact with the flue gas by various technique,
absorbs the SO2 in it.
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The wet scrubbing process was orriginaly developed in
the 1930s by Imperial Chemical Industries (ICI) in
England.In the modern version of the process, the flue
gas is scrubbed with a slurry that contains lime (CaO)
and limestone (CaCO3) as well as the salts calcium
sulfite (CaSO3 .2H2O)and calcium sulfate (in hydrate
form, naturel gypsum, CaSO4 .2H2O).The SO2 in the
flue gas reacts with the slurry to form additional sulfite
and sulfate salts, which are recycled with the addition
of fresh lime or limestone. The chemical reaction aren’t
known with certainty but are thougth to be;
CaO + H2O ----------- Ca(OH)2
Ca(OH)2 + CO2 --------- CaCO3 +H2O
CaCO3 + CO2 + H2O------- Ca(HCO3)2
Ca(HCO3)2 + SO2 + H2O --------CaSO3 .2H2O
+ 2CO2
CaSO3 . 2H2O + 1/2O2 ------------ CaSO4 .2H2O
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One technique employs a spray tower downstream of the
particulate-removalsystem (electrostatic precipitator or fabric
fitler). The flue gas is drawn into the spray tower by the main
steam-generator induced-draft fan where it flows in
countercurrent fashion to the limestone-slurry spray. A mist
eliminator at the upper exit of the tower removes any spray
droplets entrained by the gas. The gas may have to be slightly
reheated before it enters the stack to inprove atmospheric
dispersion.
The sprayed limestone slurry collects in the bottom of the tower
and is recirculated back to the spray nozzles by a pump. A system
of feed and bleed charges a fresh slurry, under pH control , and
discharges an equivalent amount from the circulating slurry. The
fresh slurry is prepared by mixing the lime-limestone with water
in a ‘slaker-grinder’ and stirred in a slurry tank. The bled slurry is
sent to a dewatering system, which is in the form of thickeners
and filters or centrifuges, where water is removed from the
calcium-sulfur salts. The reclaimed water is used to help make
fresh slurry.
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The wet scrubber has the advantages of high SO2
removal efficiencies, good reliability, and low flue-gas
energy requirements.In addition, it is capable of
removing from the flue gases residual particulates that
might have escaped the particulate-removal system.
A main disadvantages is the build up of scale in the spray tower
and possibilitiy of plugging. The prevention of such scale is
essential to the reliable operation of the tower. Scaling occurs
because both calcium sulfite and calcium sulfate have low water
solubility, normally around 30 percent, and can therefore form
supersaturated water solutions. A minimum liquid-to-gas ratio
must therefore be used, its value depending upon the SO2
content of the flue gas and the expected extent of sulfite
oxidation. Precipitation occurs at a finite rate, which
necessitates holding the SO2-absorbing liquar in a delay tank
after each pass. An insufficient delay time increases
supersturation and promates scalling. Another tecnique for
controlling scale is the use of seed crystals. These are calcium
sulfite and sulfate precipitate crystals, in a supersaturated
solution, that are maintained in the SO2-absorbing liquor. They
provide sites around which preferential precipitation takes place
and enhance the precipitation rate.
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Other disadvantages of the wet scrubers are the
reheating of the flue gas, a larger gas pressure drop
requiring higher fan power requirements than the dry
FGD system (below), and typicallyhigher capital and
operating costs.
The waste material from wet scrubbers is a waterlogged sludge that poses difficult and costy disposal
problems.
THE DRY FLUE-GAS
DESULFURİZATİON SYSTEM
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Like the wet scrubber, above, the dry FGD system, also called a
dry scrubber, utilizes an aqueous slıurry of lime, CaO, to capture
flue gas SO2 by forming calcium sulfites and sulfates in spray
absorbers. The slurry in the case, however, is atomized, usually
by a centrifugal atomizer, into a fine spray that promotes the
chemical absorption of SO2 and, because of the small spray
paticle size, is quickly dried bye the hot flue gases themselves to a
particulate suspension that is carried along with the desulfurized
gas stream. The reaction particulates as well those carried by the
flue gases (fly ash) are then removed, mainly by a fabric fitler,
before the gas is drawn by the induced-draft fan to the stack.
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A major component of this system is the slurrygenerating system. A ‘slaker’ meters lime and water into
an agitated tank to prepare a slaked lime slurry which, in
turn, is diluted by additional water and processed to
remove inert impurities called grits, which are disposed
of. The lime slurry is pumped to the spray absorber with
the flow controlled by the amount of SO2 in the flue
gas.
Particulates both coming in with the flue gas and
generated in the FGD are collected from the absorber
and fabric-filter hoppers and sent to a recycling silo for
disposal or for recycling of a portion of it with the
slurry (depending upon the extent of original utilization
of the reactant in the absorber). The recycled slurry is
enriched by an alkaline material, such as CaO, MgO,
K2O, or Na2O.
The main advantages of the dry system are he
dry, powdery nature of the waste material, which
poses fewer and less costly disposal problems then
the wet waste from the wet FGD system (thought
these problems are still large), and the mechanical
simplicity of the system.
The main disadvantage is that the efficiency of
SO2 removal is lower than that of the wet scrubber.
1979 NSPS (New Source Performances Standards)
regulations, which specify only 70 percent SO2
removal in new plants, have encouraged the
developmed of the system, however.
Other disadvantages are the need for careful design
optimization of the spray absorber and the slaker, and
the storng dependence of collection efficiency on
absorber outlet temperature, which neccessitates
opereting as close as is safe to the saturation temperature
that corresponds to the partial presure of the water vapor
in the gas in order to avoid condensation (below the
coresponding dew point). This poses problems with
fitler-bag performance.
SİNGLE ALKALİ SCRUBBİNG
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Clear water solutions of either sodium (usually in
the form of sodium hydroxide, NaoH, or sodium
sulfite, Na2So3) or ammonia (NH3) are excellent
absorbers or SO2. The advantages of alkali
scrubbing is tahat it avoids the scaling and plugging
problems of slurry scrubbing by using alkaline
earth. Ammonia scrubbing has the advantage that
the scrubber product, ammonium sulfate, can be
sold as a fertilizer, but the disadvantage that the
process produces troublesome fumes.
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A well-developed sodium scrubber is the Welman-Lord
SO2 recovery process, which has found use in
powerplants, refineries, sulferic acid plants, and other
industrial installations in the USA and Japan. The
process utilizes a water solution of sodium sulfite
(Na2SO3) for scrubbing and generates a concentrated
SO2 (about 90%), in effect removing the SO2 gas from
other flue gases.
The flue gas from fossil powerplants (or nonferrous
smelters) is first pretreated by cooling and removal of
particulate matter, such as by electrostatic precipitators,
prior to being sent to the absorber. In the absorber the
water solition of sodium sulfite absorbs the SO2 in the
pretreated flue gas to produce sodium bisulfite
NaHSO3 according to
SO2 + Na2SO3 + H2O ----------- 2NaHSO3
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The desulfurized gas is reheated before going to the
stack in order to improve atmospheric dispersion.
The sodium bisulfite is sent to a forced-circulation
evaporator-crystallizer via a surge tank. The evaporatorcrystallizer is the herth of the system. The surge tank
allows steady flow rates into it despite gas flow and
concentration fluctuations. Through the application of
low-pressure steam, such as from a turbine exhaust, the
sulfite is regenerated in the form of a slurry according
to
2NaHSO3 ----------- NaSO3 + SO2 + H2O
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The product SO2 may be utilized to produce
liquid SO2 or sulfuric acid, on
site or in a satellite plant, or to produce
elemental sulfur. A well-known process for
doing this is called the Claus process, which is
based on the addition of H2S according to
SO2 + 2 H2S ----------- 3S + 2 H2O
NO REMOVAL
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A process for the removal of NO, also by the addition
of H2S, is proposed. It is given by
NO + H2S ----------- S +1/2N2 + H2O
The combined removal of SO2 and NO is under study.
In both reactions, the H2S must be completely
consumed as it is a pollutant itself.
In 1977 the system was estimated to add an additional
$120/kW, or some 12 to 15 percent to the base capital
cost of a powerplant. It was said operating costs would
increase by about $60/MBtu.
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Most scrubbers in use by 1981 have been of the wet
type. There is not sufficient experience with the dry type
to establish which of the two may be selected by utilities
in the future. Presently all scrubber systems are large
and occupy a sizable area of a powerplant, have capital
costs that run in the tens of millions of dollars for 500to 1000-MW plants, and consume a sizable fraction of
the gross electrical output of these plants. They also
require a lot of maintenance , which results in the
doubling of operation and maintenance personel and
causes, consequently, larger operation and maintenance
costs. In addition, they generate huge amounts of waste
that has to be disposed of.
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There are two types of disposal of FGD wastes: wet
disposal, called ponding, and dry disposal in landfills,
which are getting scarce. In general utilities are not
always eager to build these disposal systems.
Nevertheless, some 19000 MW of FGD and sludge
disposal systems were in operation, and 26000 MW
were under construction or planned, in 1981. The
Electric Power Research Institute (EPRI) has published
the FGD Sludge Dİsposal Manual, which incorporates
the latest waste-disposal technology and regulations
and describes how to design an environmentally
acceptable waste-disposal system and the options
available for processing and disposal of the wastes.