seader & henley, separation process principles
TRANSCRIPT
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Seader & Henley, Separation Process Principles 1
Separation Processes • Absorption – Solutes removed from a gas into a liquid
• Solutes removed from liquid into gas is called stripping or desorption
• Distillation – Thermal vapor-liquid separation processes (Ch 11); vapor phase generated from liquid
• Liquid-liquid extraction – Solute extracted from liquid A into an immiscible liquid B (a solvent)
• Leaching (extraction) – Solute extracted from a solid into a solvent phase (liquid, dense gas, or supercritical fluid)
• Membrane processing – Molecules separated using a dense (non-porous film) or porous physical barrier
• Filtration – Suspended solids separated from a liquid or gas phase using a porous membrane
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Methanol more volatile than water
Pm > Pw
Pm > 1 atm
Vapor-liquid equilibria... (e.g. ideal, methanol-water system)
BP diagram at const P (ideal)
dew-point
bubble-point
x = y (1 component)
x-y diagram at const P
P (= pm + pw) diagram at const T
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Ethanol more volatile γePe > γhPh
Ethanol less volatile γePe < γhPh
x = y at 58oC
Low T
High T
Vapor-liquid equilibria... (e.g. non-ideal, n-hexane-ethanol system)
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The greater the separation between the equilibrium and 45o
line, the easier the separation
Getting into separations
x = y
x-y diagram at const P
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α AB =yA / xA
yB / xB
=yA / xA
(1− yA ) /(1− xA )
α AB =PA
PB
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yA =α AB xA
1+ (α AB −1)xA
if α AB =1, yA = xA
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The greater the separation between the equilibrium and 45o
line, the easier the separation
Simple flash distillation (single stage; heated to T, phase split)
x = y
x-y diagram at const P
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F = V + LFxF = Vy + Lx∴ FxF = Vy + (F −V )x
heater separator F, xF
V, y
L, x 6
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Where liquid is stripped of A by raising vapor from reboiler Stripping section
Binary distillation of components A & B (A is more volatile, e.g. methanol (A)-water (B) system)
Where “cold” reflux liquid condenses some or the vapor Enriching section
Vapor enriched
in A
Liquid depleted of
A
Near yA = 1 @ TB,A (light boiler)
Near xB = 1 @ TB,B (high boiler)
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F = D+W (molar flow)FxF = DxD +WxWDF
=xF − xWxD − xW
, WF
=xD − xFxD − xW
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Vm+1 = Lm −WVm+1ym+1 = Lmxm -WxW
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ym+1 =LmVm+1
xm −WVm+1
xW
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Vn+1 = Ln + DVn+1yn+1 = Lnxn + DxD
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yn+1 =LnVn+1
xn −DVn+1
xD
W xW
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Approximation - Constant molal overflow
• Liquid and vapor flowrates are nearly constant in rectifying (top) and stripping (bottom + feed plate) sections – Ln=Ln+1=Ln+2… Vn=Vn+1=Vn+2… – L and V, rectifying; L and V, stripping
• ΔHv (condensing high boiler) ≈ ΔHv (vaporizing low boiler)
• Operating equations or lines are linear
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yn+1 =LnVn+1
xn −DVn+1
xD
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ym+1 =LmVm+1
xm −WVm+1
xW
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Variables
• # Plates, plate design, height of column, etc. (later)
• Cooling in condenser – Liquid returned to top of column
(reflux) • Heating in reboiler
– Vapor returned to bottom of column
• Location and conditions of feed – Cold? Hot? L or V or L-V?
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R =LnD
=Vn+1 −D
D (overhead product, L at B.P.)
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yn+1 =R
R +1xn −
1R +1
xD
Top plate (1) Total condenser
Partial condenser
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• Reboiler with saturated steam
• Condenser with cooling water
Heating and cooling requirements
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ms =Vm+1λλ s
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λ = latent heat steamλs = latent heat vapor mixtureVm +1 = vapor flowrate from reboiler (stripping section)
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mw =Vn+1 λ
(T2 −T1)c p,w
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cp,w = heat capacity cooling water(T2 −T1) = Temp change in cooling waterVn+1 = vapor flowrate into condensor
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q > 1 (sub-cooled L)
q = 1 (@ BP)
0 < q < 1 (L-V)
q = 0 (@ D.P.)
q < 0 (superheated V)
Feed conditions
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q =moles L in stripping section from feed
moles feed
q =HV (D.P.)−HF
HV (D.P.)−HL (B.P.)
q =(HV −HL ) + cp,L (TB −TF )
HV −HL
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Lm = Ln + qF (stripping)Vn =Vm + (1− q)F (rectifying)
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y =q1− q
x − 11− q
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McCabe-Thiele Method - # of ideal plates McCabe & Thiele, Industrial Engineering & Chemistry Research, 17 (1925) 605.
V=L, R→∞ (total reflux)
y=x (P=Pi at each tray)
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yn+1 =R
R +1xn −
1R +1
xD
xD ≡ design condition R ≡ design variable
Rectifying section
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Stripping section
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ym+1 =LmVm+1
xm −WVm+1
xW
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Feed conditions (feed line)
@ D.P.
@ B.P.
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yn+1 =R
R +1xn −
1R +1
xD
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ym+1 =LmVm+1
xm −WVm+1
xW
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y =q1− q
x − 11− q
xF
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Putting it all together…
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yn+1 =LnVn+1
xn −DVn+1
xD
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ym+1 =LmVm+1
xm −WVm+1
xW
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y =q1− q
x − 11− q
xF
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Stepping off stages (start at xD)
What we want in overhead product
What we want in bottoms product
(start here)
operating equilibrium
x = xF
4 stages + reboiler
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x = xW
Minimum # of plates
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αav = (αAαB )1/ 2
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Fenske equation :
Nm =
ln xD(1− xD )
(1− xW )xW
lnαav
*includes rebioler
OR
xB xD
V=L (op lines = 45o)
R→∞ (total reflux)
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Minimum reflux (occurs @ pinch point, P)
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yn+1 =R
R +1xn −
1R +1
xD€
Rm
Rm +1=xD − y
'
xD − x'
y ',x ' @ pinch point
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