Absorption Final

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Environmental En ineerin CMU2
http://en.wikipedia.org/wiki/Image:Air_.pollution_1.jpg


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Absorption, or gas absorption used to separategases by washing or scrubbing a gas mixturewith a suitable liquid.

The fundamental physical principles

solubility of the absorbed gas

rate of mass transfer.



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The purpose of such scrubbing operations may beany of the following

gas purification (eg , removal of air pollutants

from exhausts gases or contaminants fromgases that will be further processed)

product recovery

production of solutions of gases for variouspurposes.



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Environmental En ineerin CMU

Gas absorption is usually carried out in verticalcounter current columns .The solvent is fed at the topof the absorber , whereas the gas mixture enters fromthe bottom .The absorbed substance is washed out by

the solvent and leaves the absorber at the bottom as aliquid solution

The solvent is often recovered in a subsequentstripping or desorption operation . This second step is

essentially the reverse of absorption and involvescounter current contacting of the liquid loaded withsolute using and inert gas or water vapor .

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Characteristics of good solutions Gas Solubility Volatility Corrosiveness

Cost Viscosity

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Absorption system

aqueous systems

nonaqueous systems

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According to the two-film theory, for amolecule of substance A to be absorbed, itmust proceed through a series of five steps.

The molecule must1. Migrate from the bulk-gas phase to the gas film

2. Diffuse through the gas film

3. Diffuse across the interface

4. Diffuse through the liquid film

5. Mix into the bulk liquid


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Absorptions Devices


PACKED COLUMN

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The packed column is a shell either filled withrandomly packed elements or having a regular solidstructure designed to disperse the liquid and bring itDumped-type packing elements come in a great

variety of shapes and construction materials, whichare intended to create a large internal surface but asmall pressure drop. Structured ,or arranged packingsmay be made of corrugated metal or plastic sheets

providing a large number of regularly arrangedchannels ,but a variety of other geometries exists.Packing materials may be classified as follows,

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Absorptions Devices


Packing material

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Absorptions Devices


Rashing rings are the most widely used form oftower packing. They are cylindrical rings, of the somelength as the diameter of the cylinder and with the

walls as thin as the material will permit. Rashing ringsare almost always dumped into the tower at randomand not stacked regularly. They offer the bestcombination of low weight per unit volume,free

volume,free cross section and total surface of any typeof packing.

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Absorptions Devices


The advantages of packed columns

- include simple and,as long as the tower diameter is nottoo large,usually relatively cheaper construction.

- preferred for corrosive gases because packing, but notplates, can be made from ceramic or plastic materials.

- also used in vacuum applications because the pressure

drop, especially for regularly structured packings, isusually less then through plate columns.


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Absorptions Devices


Packed columns are used mostly in air pollutioncontrol.

The water soluble ethylene gas ishydrolyzed toethylene gylcol.

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

industries.

Usage examples


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Absorptions Devices


TRAY COLUMN


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Tray absorbers are used in applications where tallcolumns are required, because tall, random-type packedtowers are subject to channeling and maldistribution of

the liquid streams.Plate towers can be more easily cleaned. Platesare also preferred in applications having large heateffects since cooling coils are more easily installed in

plate towers and liquid can be withdrawn more easilyfrom plates than from packings for external cooling.


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Absorptions Devices


- slow reaction rate processes,- higher pressure drops than packed beds

- plugging and fouling may be occur

Tray columns have got some disadvantage.


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Absorptions Devices


used in a refinery dehexanizer to decrease thebenzene content in the naptha feed to the process. This

results in lower automobile exhaust emissions.


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Absorptions Devices


Stirred tanks can be used in

- lime slurry carbonation

- paper stock chlorication- regular oil hydrogenation

- fermentation broth aeration

- penicilin production

- citric acid production

- aeration of activated sludge.

Usage examples


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Absorptions Devices


BUBBLE COLUMNThe gas phase is

dispersed into the liquid

phase and it flows in theempty passages betweenadjacent envelopes. Theliquid phase may beoperated in a batch manneror it may also circulate inco-current or counter-current manner to the gasflow.


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Absorptions Devices


1.no problems for separating catalyst from the liquid2.improved conversion and selectivity due to staging of

the liquid phase3.no scale up problems because the hydrodynamics isdictated by the size of the open channels of thecatalytic structure.


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Usage Examples

Bubble columns can be used to purifynitroglycerin 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, andantibiotic fermentation. Bubble columns can be used forradioactive elements because there are no moving parts.

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Adjustable-throat venturi scrubber with movable plate

Venturi scrubbers can be used for removing gaseouspollutants; however, they are not used when removal of gaseouspollutants is the only concern.

The high inlet gas velocities in a venturi scrubber result ina very short contact time between the liquid and gas phases. Thisshort contact time limits gas absorption.

However, because venturis have a relatively open designcompared to other scrubbers, they are very useful forsimultaneous gaseous and particulate pollutant removal,especially when:

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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, venturisare designed to operate at a different set of conditions

from those used to collect particles. The gas velocitiesare lower and the liquid-to-gas ratios are higher forabsorption.


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Flooded elbow

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Venturi scrubbers can have the highest particlecollection efficiencies (especially for very smallparticles) of any wet scrubbing system.

They are the most widely used scrubbers because their

open construction enables them to remove mostparticles without plugging or scaling. Venturis can alsobe used to absorb pollutant gases; however, they are notas efficient for this as are packed or plate towers.

Venturis are also used to control fly ash and sulfurdioxide emissions from industrial and utility boilers.

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Venturi scrubbers have been designed tocollect particles at very high collection efficiencies,sometimes exceeding 99%.

The ability of venturis to handle large inletvolumes at high temperatures makes them very

attractive to many industries; consequently, theyare used to reduce particulate emissions in anumber of industrial applications.

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This ability is particularly desirable for cement kilnemission reduction and for control of emissions frombasic oxygen furnaces in the steel industry, where the inletgas enters the scrubber at temperatures greater than 350C (660 F).

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Falling film

1. With high efficiency in absorbing HCl

(hydrochloric) gas, H2S, HF, SO2, NH3 gas,graphite falling film absorbers comprise ofabsorption liquid distributor, cooling andabsorption section and Gas-Liquid separator.

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2. Absorption liquid distributor is for filmforming and flow into absorption tube in coolingand absorption section.

3. On request, cooling and absorption sectionhas two models basis of its heat transfer unit -Shell and Tube and Block. Gas-Liquid separator

is to separate tail gas and product.

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advantages and disadvantages

Advanges

High efficiency ofabsorption drop

Low outlet temperature

No need after-coolingLow flow resistance

Easy maintenance

Disadvanges

Restricted by pressureFilm breakup

Flooding

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SPRAY COLUMN

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Spray columns are differetal contactors. Theliquid stream enters the coloumn through one or morespray nozzles at different heights in the column. Thedroplets formed provide a large surface area for

exposure to the gas stream,with smaller dropletsresulting in a greater Exchange area. The liquid and gasstreams can flow counter-currently or in paralel. Anoptimum droplet velocity is essential because low

velocity will lead to low contact or turbulence and highvelocity may cause flooding.

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A mist eliminator is used to separateany liquid that is entrained into the

gaseous phase. Spray columns are used to

absorb SO2 from coal-fired boiler exhaustgases.


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The packing fill random packing to encouragedispersion of the liquid flow without tracking, toprovide maximum contact area for the 'mass transfer'interaction and to offer minimal back pressure to thegas flow.

The reactivity between the contaminant and thescrubbing liquor influences the system designer'sdetermination of gas and liquor flow and the heightand diameter of the packed bed.

A demister is fitted at the top of the tower to prevententrainment of droplets of the scrubbing liquor intothe extraction system or stack.

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Advantages very high efficiencies with relatively low capital and

running costs.


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