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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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OVERVIEW ON ADSORPTION CONCEPT
Two mechanisms involved :
PhysicalAdsorption
ChemicalAdsorption
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OVERVIEW ON ADSORPTION CONCEPT
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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OVERVIEW ON ADSORPTION CONCEPT
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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OVERVIEW ON ADSORPTION CONCEPT
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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COMMON TYPES OF ADSORBENTS
Silica Gel, SiO2
Molecular Sieve
Tetrhedras Of AlO4And SiO4
Activated Carbon
COMMON TYPES OF ADSORBENTS
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COMMON TYPES OF ADSORBENTS
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.