-
10.5.2017
1
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)
-
10.5.2017
2
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
-
10.5.2017
3
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)
-
10.5.2017
4
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
-
10.5.2017
5
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
-
10.5.2017
6
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
-
10.5.2017
7
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