distillation 08
TRANSCRIPT
KAT / Distillation 1
• Two-component distillation
• Multi-component distillation• Short-cut methods
• Rigorous methods– Efficiency– Column design– Batch distillation– Packed column for distillation
DISTILLATION
KAT / Distillation 2
Two-component distillation
KAT / Distillation 3
Two-component distillation
• Short-cut methods based on:– Constant relative volatilities– Equal molar heat of vaporisation– Negligible heat of mixture
• Methods:– Graphical (McCabe-Thiele and H-x diagram)– Numerical (Lewis-Sorel Method)
• Key parameters:– Number of theoretical plates at total reflux
• Minimum number of plates– Minimum reflux ratio, Rm
KAT / Distillation 4
Multicomponent distillation
KAT / Distillation 5
Multi-component distillation
• Several columns or one column with side-streams• Complex calculations• Operation in a column influence the operation in
other columns• Feedback
KAT / Distillation 6
Specify Operation. Degree of FreedomDegrees of freedom = number of variables - number
of design relationships
Column
• C+2 are always specified– Feed composition C– Feed rate 1– Feed enthalpy 1 – Feed Temperature 1 – Pressure 1
⎥⎦⎤
⎢⎣⎡
+=⎥⎦⎤
⎢⎣⎡
6 )C( feed incomponents of Number
freedomof Degrees
KAT / Distillation 7
Specify Operation. Grades of Freedom
• Specify required separation, e.g. – purity of one or more products – recovery of one or more components
• Four variables remain to specify, e.g.,– Flow rate for Distillate / Bottom product – Reflux ratio– Number of plates– Concentrations (two)
• Do not over-specify
KAT / Distillation 8
Multi-component methods Rating methods
Existing column determine the separation• Input Data:
– Number of plates– Feed location– Reflux ratio– Distillate rate / feed rate
• Output Data:– Distillate composition– Bottom products composition– (Concentration and temperature profiles)
KAT / Distillation 9
Multi-component methodsDesign methods
Given separation dimensioning the column• Input Data:
– Distillate composition– Optimum feed stage– Bottom products composition– Design: use the minimum reflux ratio (Rm)
• Output Data:– Number of plates – Feed location– Reflux ratio,– Distillate flowrate
KAT / Distillation 10
Short-cut methods/ Multi-component• Empirical relationships, assumptions:
– (Saturated reflux, constant relative volatility, non-azeotropic mixture). Simple calculations
• Rating method:• S-B (Smith-Brinkley). Analytical solution for
separation (Perry)
• Design method:• FUEM (Fenske-Underwood-Erbar-Maddox) • FUG (Fenske-Underwood-Gilliland)
KAT / Distillation 11
FUEM/FUG method
• Step 1 Define key components:– Light key component = lightest component in the
bottom product– Heavy key components = heaviest component in
the distillate
HKLK
KAT / Distillation 12
FUEM/FUG method• Step 2 Specify
– Separation of the light and heavy key component– Reflux Ratio (actual Reflux) /(minimum Reflux)– Feed location
• Step 3 Estimate the minimum number of ideal plates, n - Total Reflux
Fenske’s equ. [ ]AvHK,LK
BLK
HK
DHK
LK
log
xx
xxlog
1nα
⎥⎦
⎤⎢⎣
⎡⎟⎠
⎞⎜⎝
⎛⎟⎠
⎞⎜⎝
⎛
=+
KAT / Distillation 13
• Step 4 Estimate minimum reflux ratio, Rm
Underwood’s method
– θ are the root of equ. CR2-11.114
q = Fraction of liquid at boiling point in the feedαHK< θ< αLK
FUEM/FUG method
∑ θ−α⋅α
=+j HK,j
jdHK,j
mx
1R
∑ θ−α⋅α
=−j HK,j
jfHK,j x
q1
KAT / Distillation 14
• Step 5 Estimate number of required ideal plates, n:
– Erbar-Maddox diagram
– Gillilands diagram (CR2 Fig 11.42)
Relationship between number of ideal stages, N and reflux ratio, R where Nm and Rm are parameters
FUEM/FUG method
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Erbar-Maddox diagram
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Gillilands diagramRm = Minimum reflux ratioNm = Minimum number of ideal stage, (n+1)
Line-line scale Log-log scale
KAT / Distillation 17
Rigorous methods
• For each plate:– Mass balance for each components– Energy balance– Vapour-Liquid Equilibrium– Other (e.g., mixtures properties)
• Nonlinear equation system
KAT / Distillation 18
Rigorous methods
• Different methods solve equations system in different ways.
• High accuracy • Other methods (Perry, 7th Ed, 13-39)
– Matrix method: Naphtali and Sandholm
KAT / Distillation 19
Efficiency
• Usually less than 1.0• Vapour-liquid equilibrium is not reach in
each plate. Poor contact between the phases:• Too large vapor flow rate• Big bubbles• Low liquid depth on the plate• Poor flow distribution on the plate. Stagnant
liquid
KAT / Distillation 20
Efficiency• Efficiency
– Overall column efficiency– Murphree plate or Local efficiency
not applicable for multi-component distillation
• Overall column efficiency
traysactual ofNumber plates ideal ofNumber
efficiencycolumn Overall =
⎥⎥⎦
⎤
⎢⎢⎣
⎡
KAT / Distillation 21
Overall column efficiency• Overall column efficiency:
– Decreases with viscosity and relative volatility(surface tension)
– May decrease slightly with vapor flow and increases with liquid flow
– Dependent on geometry of the tray – Increases with pressure
• Empirical relationships.– Efficiency vs. (viscosity) • (relative volatility)
(O´Connell based on hydrocarbon – systems)– Efficiency vs. vapour flow rate (F-factor)
KAT / Distillation 22
Overall column efficiency
Make sure that the plate operate properlywithout weeping or flooding
Misoperation such as excessive foaming, entrainment, weeping etc. lowers the plate efficiency
KAT / Distillation 23
Quick estimation of overall effciency
KAT / Distillation 24
Typical data for efficiency
KAT / Distillation 25
Azeotropic / extractive distillation
• Cases– Separation of components is difficult– Azeotropic mixture
• Azeotropic distillation– Add en new component (entrainer) to form an
azeotrop• Extractive distillation
– Add en new component (extractive agent) that modify the relative volatility
KAT / Distillation 26
Azeotropic Distillation
KAT / Distillation 27
Extractive Distillation
KAT / Distillation 28
Batch distillation
– Unequal feed– Small amount– Several fractions,
high purity– The process can
be tracked
KAT / Distillation 29
Batch distillation• Methods of operation
– Constant reflux ratio• Distillate purity decreases with time
– Constant distillate composition• Reflux ratio must increase continuously
• Calculations (Operation varies with time)– Short-cut methods:
• Based i Fenske-Underwood-Gilliland (FUG)– Rigorous methods:
• Transient differential equation for each plate
KAT / Distillation 30
Constant Reflux Ratio
KAT / Distillation 31
Constant distillate composition
KAT / Distillation 32
Criteria to choose types of trays
• General aspects– Vapor flow capacity– Liquid flow capacity– Flexibility– Pressure drop– Cost
• Operating range (CR2 Fig.11.54) for stableoperation – Operating limits
KAT / Distillation 33
Operating range - Performance diagram
Limitedrange of vapor and liquidflow rates
KAT / Distillation 34
Types of trays
KAT / Distillation 35
Bottnar
• Bubble Cap Trays– Expensive – high pressure drop – can handle
very low liquid flow – can handle very lowvapor flow – large operating range
• Sieve trays– Simple construction – cheap – low pressure
drop – smaller operating range• Valve Trays
– Rather cheap – low pressure drop – largeoperating range and flexible
KAT / Distillation 36
Design of colomn for distillation
• Diameter determined from:– Upper limit of vapor velocity
• Liquid entrainment• Pressure drop – flooding
• Number of actual plates determined by:– Separation – Efficiency
• Plate spacing determined from criterion:– Extent of entrainment (Medstänkning) – Pressure drop
KAT / Distillation 37
Distillation in Packed Columns
• when the separation is easy• unsuitable for low liquid reflux
Packed bed height based on• HETP (Height Equivalent of a Theoretical Plate)
stages ideal ofNumber packing ofHeight
HETP⎥⎥⎦
⎤
⎢⎢⎣
⎡⎥⎥⎦
⎤
⎢⎢⎣
⎡
=
KAT / Distillation 38
HETP
packingsfor ConstantsC and C,Cheight PackedZ
diameterColumn d vapour theof velocity Mass'G
ραμZcd'GCHETP
321
C
L
L1/33C2C
1
====
⋅⋅⋅=
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
⇒⇒drop Pressure
HigherHETPLower
PackningSmaller
KAT / Distillation 39
HETP for Full-scale Plant
Type of packing, application HETP, (m)
25 mm diam. packing 0.46
38 mm diam. packing 0.66
50 mm diam. packing 0.90
Absorption duty 1.5 - 1.8
Small columns (d< 0.6 m) Column diameter
Vacuum columns Values above + 0.1m
KAT / Distillation 40
HTU (Height transfer unit)
• Number of transfer units
ZH1Z
'Ga'k
yydyN
G
Gy
y iG
t
b
==−
= ∫
ZH1Z
'La'k
xxdxN
L
Lx
x iL
t
b
==−
= ∫
KAT / Distillation 41
HTU (Height of a Transfer Unit)
• Height, Z
– A low value of HG or HL corresponds to an efficient column
• Height of an overall transfer unit
LLGG NHZNHZ ⋅=⋅=
aKLH
aKGH '
l
'
OL'g
'
OG ⋅=
⋅=
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