Review-distillation-1

Without reflux Differential distillation
Flash distillation
Continuous distillation(Rectification) Batch distillation
With reflux
3
Classification
Batch process
Define q:
IV I F L L q IV I L F
17
• Feed line
xF q y x 1 q 1 q
(21.31)
-------------feed line，q 线方程 a straight line slope –q/(1-q)
cross the diagonal at x=xF.
1
Classification
Flash distillation(equilibrium distillation) Differential distillation
Rectification
Azeotrope distillation
Extraction distillation
2
Classification
Differential distillation
Batch distillation Flash distillation
Continuous process
Rectification
4
Classification
Two components distillation
Multicomponents distillation
(1.1 ~ 2) RDm
Most of situation, optimum reflux is chosen
(1.6 ~ 1.9) RDm
31
(6) Plate efficiencies
• Overall efficiency
NT o N
32
• Murphree efficiency xn1 xn y n y n1 ML MV * * xn1 xn y n y n1 • Local efficiency
34
(8) Design of a plate column
• Types of tray • Normal operation of tray • Capacity graph:负荷性能图
35
(9) Concepts
• • • • • Relative volatility Ideal plate Constant mole overflow q Principle of rectification
The operating line of stripping section •A straight line
L L slope L B V
•Pass the point (xB,xB)
15
• flow rates
L RD D
V ( RD 1) D
L L qF V V (q 1) F
6
• Content :
(1) vapor-liquid equilibrium (2) Material balance
(3) Feed condition
(4) Number of ideal plates
(5) Determining of RD
(6) Plate efficiencies
(7) Special case for rectification
---------------Fenske equation Where αAB is average relative volatility. Nmin is the minimum number of plate without include reboiler.
24
• Fenske equation can also be used to calculate the minimum number pf plates in rectifying section or stripping section.
5
Continuous distillation with reflux for binary systems
Calculation of: (1) Output and composition (2) Number of ideal plates (3) Number of actual (efficiency)
B(1 xB ) B 100% F (1 xF )
13
• Operating lines:
y n 1 RD xD xn RD 1 RD 1
(21.19)
RD / RD 1
The operating line of the rectifying section.
16
(3) Feed condition
Five feed conditions
(A) Subcooled liquid (B) Saturated liquid(at bubble point); (C Vapor-liquid mixture; (D) Saturated vapor(at dew point); (E) Superheated vapor;
23
• For the special case of ideal mixtures, Equilibrium equation Operating line
N min
x y 1 1x
AB AB
y=x
ln[ xD 1 xB / xB 1 xD ] 1 ln AB
20
• Influence of feed condition on the operating line Fig.21.14 (p659)
21
(4) Number of ideal plates
• McCabe-Thiele method 图解法 • Lewis-Sorel method 逐板计算法 • Using Gillilad chart Note: Partial condenser acts as an ideal plate Reboiler acts as an ideal plate Optimum feed plate location
•A straight line with slope
•and intercept
x D / RD 1
•intersects the diagonal at point (xD, xD)
14
L BxB ym1 xm L B L B
(21.23)
RD D qF BxB ym1 xm ( RD 1) D (q 1) F ( RD 1) D (q 1) F
If α >> 1 If 1<α<1.1 If α = 1 easy to be separated difficult to be separated Azeotrope, can’t be separated by common distillation
9
10
(2) Material balance
Review of distillation
The function of distillation is to separate, by vaporization, a liquid mixture of miscible and volatile substances into individual components or, in some cases, into groups of components.
(21.67)
y y ' y y
' n ' en
' n 1 ' n 1
(21.68)
33
(7) Special cases of distillation
• • • • Stripping tower Rectification with direct steam injection Multiple feeds Side-stream drawoffs
22
(5) Determining of reflux ratio
• • • • • Total reflux(全回流) or infinite reflux N = Nmin D = 0, B = 0, F = 0 RD/(RD+1) = 1, xD/(RD+1) = 0 The operating lines coincide with the diagonal: y=x
18
xD
Feed line
Operating line
xD/(R+1) xB xF xD
19
(1) Superheated vapor; q<0 q/(q-1) >0 ↙ (2) Saturated vapor(at dew point); q=0 q/(q-1) =0 ← (3) Vapor-liquid mixture; 0<q<1 q/(q-1) <0 ↖ (4) Saturated liquid(at bubble point); q=1 q/(q-1) →∞ ↑ (5) Subcooled liquid. q>1 q/(q-1) >0 ↗
RDm
xD y ' y ' x'
(21.44)
(x’,y’) is the intersection of feed line and equilibrium curve.
26
d: pinch point
挟紧点
(x’, y’)
The intersection of feed line and equilibrium curve
(21.5)
B xD xF F xD xB
(21-6)
12
• Fraction of more volatile component (component A) obtained from overhead product, distillate: DxD D 100% FxF • Fraction of less volatile component (component B) obtained from bottom product:
•Overall material balance
F DB
• Component A balance
•(21.3)
FxF Dx D BxB
•(21.4)
11
• Eliminating B from these equation gives:
D xF xB F xD xB
• Eliminating D gives:
27
Invariant zone 恒浓区
28
Invariant zone
29
• Optimum reflux ratio :Determining by an economic balance between operating cost and equipment cost.
30
Usually optimum reflux ratio is
(8) Design of a plate column (9) Concepts
7
(1) vapor-liquid equilibrium
• • • • Chart : t-x-y chart x-y chart Formula :
y kx
x y 1 1x
8
Assቤተ መጻሕፍቲ ባይዱssment
36
Flash distillation and differential distillation
Flash distillation
N min, r N min, s ln[ xD 1 xF / xF 1 xD ] 1 ln AB ,r ln[ xF 1 xB / xB 1 xF ] 1 ln AB , s
25
• Minimum reflux • RD= RDm • Operating lines intersect with the equilibrium curve