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Separation processes

Separation processesSeparation of heterogeneous and homogeneous mixtures of chemical compoundsBased on different physically-chemical propertiesCommonly mixture of products (main and secondary) or mixture of product and unconverted reactants

Mixture systems:heterogeneous homogeneous

g – g g – g (gas mixture)

g – l (fog, foam) g – l (gas solved in liquid)

g – s (smoke) g – s

l – l (emulsion) l – l (mixture liquids)

l – s (slurry) l – s (solution)

s – s s – s (metal alloy)

Separation of heterogeneousmixtures

When fluid phase is dominant (g, l), separation is based on:- different density (sedimentation)- flow of fluid thru porous separator (filtration, membrane separation)- ionization of particles (electrostatic precipitation)Small amount of liquid phase- vitalization (drying)System s – s- different density, magnet. properties (sedimentation)- different solubility (crystallization)- different wettability (flotation)

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Separation of homogeneousmixtures

Mixture of gases- sorption (absorption, adsorption)- different molecule diameter (membrane separation)Mixture of miscible liquids

- liquids with different vapour preassure (destilation, rectification)- liquids with close vapour preassure (extraction)

Non-volatile solids soluted in (evaporation and crystalization)- solubility is not dependent on temperature- solubility is strongly dependent on temperature

Sedimentation (l-s, l-l, g-s, g-l, s-s)

• rl is different than rs• gravitational acceleration

• settling rate in gravity field =

• possibility to separate particles with different diameter (trough separator)

• possibility to separate particles with different density rs1> rl> rs2

• Dorr sedimentation (l-s)

Filtration (l-s, g-s)

• porous separator – filter• proper pores diameter

• cake / deep filtration• continuous / discontinuous filtration• driving force – pressure difference at front / behind filter

• artificial (pressure/vacuum f.)• hydrostatic

• resistance against filtration – viscosity of fluid, filter/f. cake pressure drop

• pressure difference – stable / variable= filtration rate - variable / stable

rotation vacuum cylinder filter, filter press, belt filter, candle filter, sand filter

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centrifugation (l-s, l-l)

• same principle as sedimentation (filtration)• centrifugal acceleration (3 orders of magnitude higher than g)

• density difference can be only 1%• higher performance of device (mass flow)• continual / discontinual• centrifuges (sedimentation/filtration/separation)

• cyclones, lamella separator

Membrane separation (l)

• analogy with filtration• selectivity on particle dimension basis (molecule mass)• „cross-flow“ / „death-end“ arrangement• retentate (rich solution) / permeate = filtrate (poor s.)• pressure processes

• microfiltration (10-0,1 µm, up to 0,2MPa) – microorganisms, colloid particles• ultrafiltration (10-3 nm, 0,1-0,5 MPa) – macromolecules, viruses• nanofiltration (3-1 nm, 0,5-3,5 MPa) – pesticides, carbohydrates• reverse osmosis (1,5-15 MPa) – all dissolved salts

• not only molecule sieve effect, but also selective interactions between particles and membrane (NF, RO)

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Membrane separation - Filtration

Electrostatic precipitation (g-s, g-l)

• principle – ionization of particles (-) in strong DC electric field, migration to (+) electrode

• negative electrode – wire in the middle • positive electrode – inner wall of precipitator• particles after discharge stay on (+) electrode (vibration)• up to 99.9% effectivity

Drying (g-l, s-l)

• flow of drying gas (little amount of vapors)• heat of vaporization• free / conventional drying – flow of heated gas• flows – parallel , counter or cross

• static, sprinkled, linear, drifting

• heatingconvectional, contact, radiant, electric, steam, ‘lyofilization’

• important – relative humidity of gas is lower than dried material• dryers: spraying, rotary cylindric, fluid

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h-xMolliérediagram

Flotation (s-s)• one of most important separ. process• for ore processing• different interfacial parameters in three-phase system

• wettability, production and carrying of bubbles• foam flotation – unwettable material form foam, wettable material

sediment• additives (changing of surface properties)• electroflotation (electrolysis of water)

Sodium Dodecylsulphatechem-physical parameters1) wettability2) density3) conductivity4) surface tension5) osmotic pressure6) eq. conductivity7) interphase tension

Evaporation, crystalisation (l-s)

• to separate nonvolatile solids solute in solvent• Increase of concentration above solubility

• dependence of solubility on temperature• evaporation (vacuum)• change of T

• particle properties• cooling rate, mixing

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Distillation

• for miscible liquids separation• vapors are rich for more volatile liquid

• vapor composition is different than liquid• ideal mixtures – zoetrope mixtures (Raoult law small differences)• non ideal – azeotrope mixtures (interaction – H-bonds)

• composition of azeotrope vapor is same as composition of liquid azeotrope• separation by distillation is not possible (can be done by pressure change,

addition of third liquid)

Rectification

• multiple distillation with counter flow

• number of theoretical stages• McCabe-Thiele method

• columns – hat, mesh, packed

p1: = + = ̇̇

q: = − =

q > 1 – feed is l at T < Tbpq = 1 – feed is l at Tbp0 < q < 1 – feed is mix of l and gq = 0 – feed is saturated vaporq < 0 – feed is superheated vapor

Rectification stages/columns

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Extraction (l-s, l-l)

• separation of liquids with close boiling point• extractor ex

• good slubility of one of the liquids l1 in ex (better than in l2)• non miscible with l2 (or partially miscible)• l1 and ex do not result to azeotrope

• separation coeficient =

• continual, discontinual, cascade• supercritic extraction (CO2, Tc=31 °C, pc=74 bar)• sepparation of ex and l2 (decanter) and destilation of l1 and ex

Absorption (g-g)

• sorption of one (or all except one) gas from the mixter in the liquid• physical and chemical• Henry law = • positive effect of low temperature and high pressure• difussion process – driving force: difference of concentration of A in

gas and liquid is• important factorem is contact area of gas and liquid• columns like for rectification

Adsorption (g-g, l-g, l-l)

• for separation of homogeneous mixtures• sorption at solids with high specific surface (hundrets of m2/g)• selectivity can be affected by process parameters and by adsorbent

type• positive effect of high pressure and lower temperature• physical ads. – low act. energy, exotermal, fast, multilayer, reversible• chemical ads. – higher act. energy, exothermal, slower, monolayer,

only on active sites, selective, irreversible• adsorbers with fixed bed, fluid adsorbers