lecture slides cpe 676_absorption & adsorption

8/2/2019 Lecture Slides CPE 676_Absorption & Adsorption

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Lesson objectives :

Throughout Chapter 2-9, the students are

expected to be able :

1. An ability to identify the necessary and the mostappropriate equipment for refinery and petrochemicalprocess and able to explain the safety measures of plantoperations and environment.

2. An ability to do conceptual design of major equipmentsused in refinery and petrochemical plants with the present

of data and details of the process.

INTRODUCTION


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Chapter 4 : Liquid Extraction and

Separation

This chapter covers :Overview of mass separation techniques:

- adsorption process, separation due tosettling, leaching, extraction and

Mass separation equipment:agitated vessel, scrubbers, spray tower,liquid-liquid separator.

Factors of design selection


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1. Vapor Liquid : Distillation

2. Gas Liquid (Sorbent): Absorption

3. Liquid Gas : Reverse of absorption (Stripping/ Desorption)

4. Liquid/Gas

Solid: Adsorption4. Special case of adsorption (separation btw ions in solution

with an ions in an insoluble solid phase) : Ion Exchange

5. Liquid Liquid : Liquid-liquid Extraction/ Solvent Extraction

6. (Solute from a) Solid

Fluid : Leaching/ extractione.g.leaching vegetable oils from solid soybeans

7. Soluble solute removed from a solution : Crystallization

8. Separation of molecules by a thin layer of barrier :

membrane


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Separation Operations Based on PhaseCreation or Addition

SeparationOperation

Feed Phase AddedPhase

Separation Agent(s) IndustrialExample

Partial

condensation or

vaporization

Vapor and/

or LiquidLiquid or

VaporHeat transfer (ESA) Recovery of H2

and N2 from

ammonia by

partialcondensation

Flash

vaporizationLiquid Vapor Pressure reduction Recovery of water

from sea-water

Distillation Vapor and/

or Liquid

Vapor and

liquid

Heat transfer (ESA) and

sometimes work transfer

Purification of

styrene

Extractive

distillationVapor and/

or LiquidVapor and

liquid

Liquid sorvent (MSA) and

heat transfer (ESA)

Separation of

acetone and

methanol

Reboiled

absorption

Vapor and/

or Liquid

Vapor and

liquid

Liquid absorbent (MSA)

and heat transfer (ESA)

Removal of

ethane and lower

molecular weight


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Absorption Vapor Liquid Liquid absorbent

(MSA)Separation of carbon diox-ide from combustion

products by absorption with

aqueous solutions

Stripping Liquid Vapor Stripping vapor (MSA) Stream stripping of naphtha,kerosene, and gas oil sidecuts from crude dislight

Refluxed stripping

(steam distillation)

Vapor and/

or liquid

Vapor

and

liquid

Stripping vapor (MSA)

and heat transfer (ESA)Separation of products from

delayed

Rebelled stripping liquid Vapor Heat transfer (ESA) Recovery of amineabsorbent

Azeotropic

distillation

Vapor and/

or liquid

Vapor

and

Liquid

Liquid entrainer (MSA)

and heat transfer (ESA)Separation of acetic acid

from water using n-buty1

acetate as an entrainer to

form an azeotrope with

Liquid-liquid

extraction

liquid Liquid Liquid solvent(MSA) Recovery of aromatics

Liquid-liquid

extraction

Liquid Liquid Two liquid solvents

(MSA1 and MSA2)Use of propane and cresylie

acid as solvents to

separate paraffins from

aromatics


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Drying Liquid and

often solid

Vapor Gas (MSA) and/or

heat transfer (ESA)Removal of water from

polyvinylchloride with

hot air in a fluid-bed dryer

Evaporation Liquid vapor Heat transfer (ESA) Evaporation of water from asolution of urea and

Crystallization Liquid Solid(and vapor) Heat transfer (ESA) Crystallization of p-xylenefrom a mixture with m-

xylene

Desublimation Vapor Solid Heat transfer (ESA) Recovery of phthalic anhy-dride from noncondensi-hie

gas

Leaching(liqui

d- solid

extraction)

Solid Liquid Liquid solvent Extraction of sucrose fromsugar beets with hot wa

Foam

Fractionation

Liquid Gas Gas bubbles (MSA) Recovery of detergents fromwaste solutions


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A unit operation used in the chemical industryto separate gases by washing or scrubbing agas mixture with a suitable liquid

Gas mixture is contacted with a liquid forpurposes of preferentially dissolving one ormore components of the gas phase and toprovide a solution of them in the liquid


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Applied in industry to purify process streams orrecover valuable components of the stream

Used extensively to remove toxic or noxiouscomponents (pollutants) from effluent gasstreams


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Coking

Coal Fired power plant

Natural gas production

Microelectronics manufacturing

Ground water remediation


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form complexes or chemical compoundswith the solute

chemical solvents

have only weaker interactions with the

solute

physical solvents


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The solvent should have the advantages of:

low volatility

low cost

low corrosive tendencies

high stability

low viscosity

low tendency to foam

low flammability


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Chemical solvents are usually preferred

when the solute must be:

reduced to very low levels

high selectivity is needed

solute partial pressure is low


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carbon dioxide (CO2)hydrogen sulfide (H2S)

sulfur dioxide (SO2)

acidic impurities

carbonyl sulfide (COS)

merceptans

organic sulfur compounds


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Absorbed gas,

SOLUTESOLVENT

Ammonia waterBenzene toluene Straw oilFormaldehyde water

Acetone waterHydrogen sulfide Alkaline solutions

Sulfur dioxide waterCarbon dioxide ethanolamines

Propane and butane keroseneCarbon monoxide Ammoniacal cuprous

chloride solutionSulfur dioxide

water


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Gas absorption is usually carried out invertical counter current columns as shown in

figure 1.The solvent is fed at the top of theabsorber , whereas the gas mixture entersfrom the bottom .The absorbed substance iswashed out by the solvent and leaves theabsorber at the bottom as a liquid solution .The solvent is often recovered in asubsequent stripping or desorption operation.

This second step is essentially the reverse ofabsorption and involves counter currentcontacting of the liquid loaded with soluteusing and inert gas or water vapor .


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The absorber may be a packed column ,

plate column , spray column , venturiscrubbers , bubble column , falling films ,wet scrubbers , stirred tanks


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PACKED COLUMN


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The packed column is a shell either filled withrandomly packed elements or having a regular solid

structure designed to disperse the liquid Dumped-type packing elements come in a great

variety of shapes and construction materials, whichare 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 largenumber of regularly arranged channels ,but a variety

of other geometries exists. Packing materials may beclassified as follows:


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1. rock

2. 3-coke3. 4-stonaware shapes

4. 4a-raching rings

5. 4b-berl saddle

6. 4c-sprial rings 1-wood slats

7. 2-broken

8. 4d-grid bloks

9. 5-miscalloneous material


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Rashing rings are :1. the most widely used form of tower

packing.2. cylindrical rings, of the some length as

the diameter of the cylinder and with the

walls as thin as the material will permit.3. always dumped into the tower at randomand not stacked regularly.

4. offer the best combination of low weight

per unit volume, free volume, free crosssection and total surface of any type ofpacking.


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A packed bed column contains a support plate, a

liquid distributor, and a mist eliminator. Misteliminators are used to condense any vaporizedscrubbing liquid. Support plates hold the packing inplace.

The advantages of packed columns include simple andusually relatively cheaper construction. These columnsare preferred for corrosive gases because packing, butnot plates, can be made from ceramic or plasticmaterials. Packed columns are also used in vacuumapplications because the pressure drop, especially forregularly structured packings, is usually less thenthrough plate columns.


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Usage examples

Packed columns are used mostly in airpollution control. The water soluble ethylenegas ishydrolyzed to ethylene gylcol.Packed columns are also used in the chemical,petrochemical, food, pharmaceutical, paper,

and aerospace industries.


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TRAY COLUMN

Tray columns are used ina refinery dehexanizer to

decrease the benzenecontent in the napthafeed to the process. Thisresults in lower

automobile exhaustemissions.


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STIRRED TANKS

If the absorbtion process includes a slow liquid-phase chemical reaction, or close control of theprocess is needed, stirred tanks are used.

Gas is introduced directly into the liquid andmixed by the stirred in a stirred tank.


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Usage examples

Stirred tanks can be usedin lime slurry

carbonation,paper stockchlorication, regular oilhydrogenation,fermentation broth aeration,penicilinproduction, citric acidproduction,and aerationof activated sludge.


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BUBBLE COLUMN

configuration lies basically between slurryreactors and trickle bed reactors.

The solid phase, consisting of catalyst particles,

is enclosed in fixed wire gauze wraps, which aremounted along the height of the column.

The gas phase is dispersed into the liquid phaseand 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 orcounter-current manner to the gas flow.


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The main advantages of this reactor type withrespect with the conventional slurry bubble

column are:

1.no problems for separating catalyst fromthe liquid;

2.improved conversion and selectivity dueto staging of the liquid phase;

3.no scale up problems because thehydrodynamics is dictated by the size of theopen channels of the catalytic structure.


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Usage Examples

Bubble columns can beused to purifynitroglycerin with water,in the chemical industryfor hydrogenation,oxidation, chlorination,and alkylation

Bubble columns can beused for radioactive

elements because thereare no moving parts.


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Venturi Scrubbers


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Adjustable-throat venturi scrubber with

movable plate

removing gaseous pollutants; however, they are notused when removal of gaseous pollutants is the only

concern.The high inlet gas velocities in a venturi scrubber resultin a very short contact time between the liquid and gasphases. This short contact time limits gas absorption.

venturis have a relatively open design compared to otherscrubbers, they are very useful for simultaneous gaseousand particulate pollutant removal, especially when:


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Scaling could be a problemA high concentration of dust is in the inletstreamThe dust is sticky or has a tendency to plugopenings

The gaseous contaminant is very soluble orchemically reactive with the liquid

To maximize the absorption of gases, venturis are

designed to operate at a different set of conditionsfrom those used to collect particles. The gasvelocities are lower and the liquid-to-gas ratios arehigher for absorption.


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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 theiropen construction enables them to remove mostparticles without plugging or scaling. Venturis can alsobe used to absorb pollutant gases; however, they are

not as efficient for this as are packed or plate towers. This ability is particularly desirable for cement kiln

emission reduction and for control ofemissions frombasic oxygen furnaces in the steel industry, where the

inlet gas enters the scrubber at temperatures greaterthan 350 C (660 F).

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


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Falling film

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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Advanges

High efficiency ofabsorption drop

Low outlettemperature

No need after-cooling

Low flow resistance Easy maintenance

Disadvanges

Restricted by pressure

Film breakup

Flooding

Its convincing advantages anddisadvantages is following: -


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SPRAY COLUMN

Spray columns are differential contactors. The liquidstream enters the column through one or more spray

nozzles at different heights in the column. The dropletsformed provide a large surface area for exposure tothe gas stream, with smaller droplets resulting in agreater Exchange area. The liquid and gas streams canflow counter-currently or in parallel. An optimumdroplet velocity is essential because low velocity willlead to low contact or turbulence and high velocity maycause flooding.


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A mist eliminator is used toseparate any liquid that is

entrained into the gaseousphase. Spray columns areused to absorb SO2 from

coal-fired boiler exhaustgases.


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WET SCRUBBER

Wetted packed towers are the simplest andmost commonly used approaches to gasscrubbing. The principle of this type of

scrubber is to remove contaminants from thegas stream by passing the stream through apacked structure which provides a large

wetted surface area to induce intimatecontact between the gas and the scrubbingliquor. the contaminant is absorbed into orreacted with the scrubbing liquor.


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The packing of the tower is normally a proprietary loosefill random packing designed to encourage dispersion ofthe liquid flow without tracking, to provide maximumcontact area for the 'mass transfer' interaction and tooffer minimal back pressure to the gas flow.

A demister is fitted at the top of the tower to prevententrainment 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 economicalfans. Although efficiency may be affected, a packedtower will usually function when gas or liquor flows varyfrom its original design parameters.


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Example of amine usedMonoethanolamine (MEA)Diethanolamine (DEA)Methyldiethanolamine (MDEA)Diisopropylamine (DIPA)Diglycolamine (DGA)

AMINE GAS TREATING


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Gas flow rate The flue gas flow rate will

determine the size of the absorber whichrepresents a sizeable contribution to the overallcost.Amine concentration depend whether amineunit is treating relatively low concentrations of bothH2S and CO2 or a very high percentage of CO2.Another factor involved is the relative solubility of

H2S and CO2 in the selected amine.CO2 / H2S removal The exact recovery choice isan economic trade off, a higher recovery will leadto a taller absorption column, higher energy

penalties and hence increased costs.

DESIGN CRITERIA


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Energy requirement

The energy consumption ofthe process is the sum of the thermal energy neededto regenerate the solvents and the electrical energyrequired to operate liquid pumps and the flue gas

blower or fan.Cooling requirement Cooling is needed to bringthe solvent temperatures down to temperature levelsrequired for efficient absorption. Also, the productfrom the stripper will require cooling to recover steamfrom the stripping process.

DESIGN CRITERIA


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OVERVIEW ON ADSORPTION CONCEPT

Adsorption is the selective collection andconcentration onto solid surfaces of particulartypes of molecules contained in a liquid or a gas.

By this unit operation gases or liquids of mixedsystems, even at extremely small concentrations,can be selectively captured and removed from

gaseous or liquid streams using a wide variety ofspecific materials known as adsorbents.

The material which is adsorbed onto the adsorbentis called the adsorbate.


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Two mechanisms involved :

PhysicalAdsorption

ChemicalAdsorption


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Two mechanisms involved :

Physical Adsorption

When gaseous or liquidmolecules reach thesurface of an adsorbentand remain withoutany chemical reaction.

Chemical Adsorption

When gaseous or liquidmolecules adhere to the surfaceof the adsorbent by means of achemical reaction and formationof chemical bonds.


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Physical AdsorptionThe mechanism of physisorption may be intermolecular,electrostatic or van der Waals forces, or may depend onthe physical configuration of the adsorbent such as the

pore structure of the adsorbent.

Physical adsorbents typically have large surface areas.

The properties of the material being adsorbed(molecular size, boiling point, molecular weight, andpolarity).

The properties of the surface of the adsorbent (polarity,pore size, and spacing) together serve to determine the

quality of adsorption.


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Chemical Adsorption

Heat releases of 10 to 100 kcal/g-mol are typical forchemisorption, which are much higher than the heat

release for physisorption. With chemical adsorption,regeneration is often either difficult or impossible.

Chemisorption usually occurs only at temperatures

greater than 200o

C when the activation energy isavailable to make or break chemical bonds.

COMMON TYPES OF ADSORBENTS


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Silica Gel, SiO2

Molecular Sieve

Tetrhedras Of AlO4And SiO4

Activated Carbon



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Activated Alumina

Polymeric Resins

Ion Exchange Resins

Ad ti M h i


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Adsorption Mechanism


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The amount of adsorption is limited by the:

available surface and pore volume, and depends also on the chemical natures of

the fluid and solid.

The rate of adsorption also depends on the amount of

exposed surface but, in addition, on the rate of diffusionto the external surface and through the pores of the solidfor accessing the internal surface which comprises thebulk of the surface.


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Diffusion rates depend on : temperature and differences in concentration or partial

pressures.

The smaller the particle size, the greater is theutilization of the internal surface, but also the

greater the pressure drop for flow of bulk fluidthrough a mass of the particles.

lecture slides cpe 676_absorption & adsorption

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