Equipments Regarding Absorption

7/29/2019 Equipments Regarding Absorption

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EQUIPMENTS REGARDING ABSORPTIONAbsorption, 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 theconstituents 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. The

solvent is fed at the top of the absorber; whereas the gas mixture enters from the

bottom .The absorbed substance 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, or stirred tanks.

PACKED COLUMN

The packed column is a shell either filled with

randomly packed elements or having a regular solid structuredesigned 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


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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 misteliminator. 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 because packing, but not plates, can be made from

ceramic or plastic materials. Packed columns are also used in vacuum applications

because the pressure drop, especially for regularly structured packings, is usually less

then through plate columns.

Packed columns are used mostly in air pollution control, for instance, the water

soluble ethylene gas is hydrolyzed to ethylene glycol.Packed columns are also used in the chemical, petrochemical, food, pharmaceutical,

paper, and aerospace industries.


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TRAY COLUMN

Tray absorbers are used in applications

where tall columns are required, because tall,

random-type packed towers are subject to

channeling and misdistribution of the liquid

streams. Plate towers can be more easilycleaned. 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 misdistribution 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 are used in a refinery dehexanizer to decrease the benzene content in

the naphtha feed to the process. This result in lower automobile exhaust emissions.

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.

Stirred tanks can be used in lime slurrycarbonation, paper stock chlorication, regular oil

hydrogenation, fermentation broth aeration,

penicilin production, citric acid production,and

aeration of activated sludge.


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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.

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 treatment, single-cell protein productin,

animal cell culture, and antibiotic fermentation.

Bubble columns can be used for radioactive elements

because there are no moving parts.


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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, becauseventuris 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 liquidTo 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 timebetween phases and better absorption.

Increasing the liquid-to-gas ratio will increase

the potential solubility of the pollutant in the

liquid.

FOLDED 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.


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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.

This ability is particularly desirable for cement kiln emission reduction and forcontrol 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 are the following:

Advantages: High efficiency of absorption drop Low outlet temperature; No need after-

cooling; Low flow resistance; Easy maintenance

Disadvantages: High efficiency of absorption drop; Low outlet temperature; No need

after-cooling; Low flow resistance; Easy maintenance

SPRAY COLUMNSpray columns are differential contactors. The liquid

stream enters the column 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

parallel. 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 isentrained into the gaseous phase. Spray columns are used to

absorb SO2 from coal-fired boiler exhaust gases.


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WET SCRUBBERS

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 intimatecontact 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 maybe affected, a packed tower will usually

function when gas or liquor flows vary

from its original design parameters.

Wet scrubbers are used by the food

industry, such as in cheese proessing for

dust and ambient moisture removal.

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Equipments Regarding Absorption