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Absorbtion--FLUE GAS DESULFURIZATION Powered By Docstoc
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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
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 .
   The absorber may be a packed column , plate
    column , spray column , venturi scrubbers ,
    bubble column , falling films , wet scrubbers
    ,stirred tanks
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,
   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

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
      Packed columns are also used in the chemical
,petrochemical,food, pharmaceutical,paper, and aerospace
 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
   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
              STIRRED TANKS
    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

   Stirred tanks can be used in
    lime slurry carbonation,paper
    stock chlorication, regular oil
    broth aeration,penicilin
    production, citric acid
    production,and aeration of
    activated sludge.
            BUBBLE COLUMN
   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
   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
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
Increasing the liquid-to-gas ratio will increase the
potential solubility of the pollutant in the liquid.
Flooded elbow
   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.
   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.
   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
   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.
   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: -
   Advanges                    Disadvanges
   High efficiency of          Restricted by pressure
    absorption drop             Film breakup
   Low outlet temperature      Flooding
   No need after-cooling
   Low flow resistance
   Easy maintenance
  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.
   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
   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.
   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.

   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.
   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
  Gas desulfurization can be accomplish by wet, dry, or
  alkali scrubing.These methods are covered in this
    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.
    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
   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
   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.
   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.
   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 water-
    logged sludge that poses difficult and costy disposal
    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.
   A major component of this system is the slurry-
    generating 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
   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
     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.

    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.
   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
    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 evaporator-
    crystallizer 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
    2NaHSO3 ----------- NaSO3 + SO2 + H2O
   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
   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.
   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 500-
    to 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.
   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

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Lingjuan Ma Lingjuan Ma MS
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