Introduction to Distillation: Steady State Design and Operation

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Introduction to Distillation Steady State
Designand Operation

• Distillation Course Berlin Summer 2008.
• Sigurd Skogestad. Part 1

1. Introduction
2. Steady-state design
3. Steady-state operation

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BASF Aktiengesellschaft
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1. Introduction to distillation

• King (Wiley, 1980) on distillation design
• Shinskey (McGraw-Hill, 1984) on distillation
  control
• Kister (McGraw-Hill, 1990) on distillation
  operation
• General info http//lorien.ncl.ac.uk/ming/distil/
  distil0.htm
• I.J. Halvorsen and S. Skogestad, Distillation
  Theory'', In Encyclopedia of Separation Science.
  Ian D. Wilson (Editor-in-chief), Academic Press,
  2000, pp. 1117-1134.
• S. Skogestad, Dynamics and control of
  distillation columns - A tutorial introduction.,
  Trans IChemE (UK), Vol. 75, Part A, Sept. 1997,
  539-562 (Presented at Distillation and Absorbtion
  97, Maastricht, Netherlands, 8-10 Sept. 1997).
• More see home page Sigurd Skogestad
  http//www.nt.ntnu.no/users/skoge/
• http//www.nt.ntnu.no/users/skoge/distillation
• Free steady-state distillation software with
  thermo package http//www.chemsep.org/

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I usually number the stages from the bottom (with
reboiler1), but many do It from the top
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Alternative Packed column
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Vapor-liquid equilibrium (VLE) Equilibrium line
yK(x)
Non-ideal
Difficult separation (almost az.)
Easy sep.
Ideal mixture
common low-boiling az.
less common high-boiling az.
Azeotropes (non-ideal)
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The equilibrium stage concept
Vi1 yi1
Stage i1
Material balance stage i (outin) Li xi Vi
yi Li1xi1 Vy-1yi-1
Li1 Xi1
Vi yi
Equilibrium (VLE) yi Ki(xi)
Stage i
Li xi
Vi-1 yi-1
Stage i-1

• The equlibrium stage concept is used for both
  tray and packed columns
• N no. of equilibrium stages in column
• Tray column N No.trays Tray-efficiency
• Packed columns N Height m / HETP m

Typical 0.7
Typical 0.5 m
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TOP
Simplified energy balance Vi Vi1 (constant
molar flows)
BTM
TOP
BTM
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When use distillation?

• Liquid mixtures (with difference in boiling
  point)
• Unbeatable for high-purity separations because
• Essentially same energy usage independent of
  (im)purity!
• Going from 1 to 0.0001 (1 ppm) impurity in one
  product increases energy usage only by about 1
• Number of stages increases only as log of
  impurity!
• Going from 1 to 0.001 (1 ppm) impurity in one
  product increases required number of stages only
  by factor 2
• Well suited for scale-up
• Columns with diameters over 18 m
• Examples of unlikely uses of distillation
• High-purity silicon for computers (via SiCl3
  distillation)
• Water heavy-water separation (boiling point
  difference only 1.4C)

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2. Steady-state Design

• Given separation task
• Find
• configuration (column sequence)
• no. of stages (N)
• energy usage (V)
• How to design a column in 5 minutes

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Multicomponent and binary mixtures

• We will mostly consider separation of binary
  mixtures
• Multicomponent mixtures For relatively ideal
  mixtures this is almost the same as binary - if
  we consider the pseudo-binary separation
  between the key components
• L light key component
• H heavy key component
• The remaining components are almost like
  dead-weight
• Composition The impurity of key component is
  the important

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Relative volatility, ?
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Ideal mixtureEstimate of relative volatility
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Estimate of relative volatility (2)
IDEAL VLE (constant a)

• Example. iso-pentane (L) pentane (H)
• Example. Nitrogen (L) Oxygen (H)

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Separation factor for column or column section

• Example Binary separation with purities 90
  light in top and 90 heavy in bottom
• Example Binary separation with purities 99.9
  light in top and 98 heavy in bottom

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Minimum no. of stages
Total reflux Infinite energy
O
Operating line xi1 yi (diagonal)
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Minimum no. of stages, Nmin(with infinite energy)
IDEAL MIXTURE
IDEAL VLE (constant a)

• Infinity energy ) Total reflux. Stage i
• Repeat for all N stages
• Fenskes formula for minimum no. of stages
• Assumption Constant relative volatility
• Applies also to column sections

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Minimum energy (minimum reflux)
pinch
(a) IDEAL VLE
(b) NON-IDEAL VLE
Infinite number of stages in pinch region
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Minimum energy, Vmin(with infinite no. of stages)
IDEAL MIXTURE
IDEAL VLE (constant a)

• Feed liquid (Kings formula, assuming pinch at
  feed)
• NOTE Almost independent of composition!! For
  sharp split (rLD1, rHD0), feed liquid
• Assumption Ideal mixture with constant relative
  volatility and constant molar flows.

feed vapor delete the D
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Examples design
IDEAL MIXTURE
IDEAL VLE (constant a)
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Design How many stages?

• Energy (V) vs. number of stages (N)
• Trade-off between number of stages and energy
• Actual V approaches Vmin for N approximately 2 x
  Nmin or larger, typically
• 2Nmin ? 25 Vmin
• 3Nmin ? 3 Vmin
• 4Nmin ? 0.3 Vmin

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Design How many stages?

• Conclusion Select N gt 2 Nmin (at least)
• Many stages reduce energy costs
• Many stages is good for control
• Can overfractionate (tight control is then not
  critical)
• or
• Get less interactions between top and bottom
  (because of pinch zone around feed)

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Real well-designed column
IDEAL MIXTURE
IDEAL VLE (constant a)

• Recall
• Choose N 2 Nmin
• Get V 1.25 Vmin and Q 1.25 Vmin ? Hvap
• N 3-4 Nmin gives V very close to Vmin
• Important insights
• Vmin is a good measure of energy usage Q
• Vmin is almost independent of purity
• Vmin is weakly dependent on feed comp. (feed
  liquid get vaporization term D/F zF)
• Design To improve purity (separation) Increase
  N
• N and Vmin both increase sharply as ? ? 1
• Example. Decrease ? from 2 to 1.1
• Nmin increases by a factor 7.3 ( ln
  2/ln1.1)
• Vmin increases by a factor 10 (
  (2-1)/(1.1-1))

feed liquid (0 for feed vapor)
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Feed stage location
NON-OPTIMAL
with extra stages in top Pinch above feed
stage (mixture on feed stage is heavier than
feed)
OPTIMAL

• No pinch
• or pinch on both
• sides of feed stage
• (mixture on feed stage has
• same composition as feed)

feed line (q-line) vertical for liquid
feed horizontal for vapor feed
NON-OPTIMAL
with extra stages in bottom Pinch below feed
stage (mixture on feed stage is lighter than
feed)
Note Extra stages (and pinch) is NOT a problem,
because it implies lower energy
usage. Preferably, the pinch should be on both
side of the feed.
Pinch Section of column where little
separation occurs
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Simple formula for feed stage location
(Skogestad, 1987)
IDEAL MIXTURE
IDEAL VLE (constant a)
Example. C3-splitter. zFL0.65, xDH 0.005,
xBL0.1, ?1.12.
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Example 5 min column design
IDEAL MIXTURE
IDEAL VLE (constant a)

• Design a column for separating air
• Feed 80 mol- N2 (L) and 20 O2 (H)
• Products Distillate is 99 N2 and bottoms is
  99.998 O2
• Component data
• Nitrogen Tb 77.4 K, ? Hvap5.57 kJ/mol
• Oxygen Tb 90.2 K, ? Hvap6.82 kJ/mol
• Problem 1) Estimate ?. 2) Find split D/F. 3)
  Stages Find Nmin and 4) suggest values for N and
  NF. 5) Energy usage Find Vmin/F for a) vapor
  feed and b) liquid feed.
• Given For vapor feed and sharp sep. of binary
  mixture Vmin/F 1/(?-1)

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Solution 5-min design
IDEAL MIXTURE
IDEAL VLE (constant a)
Also see paper (Theory of distillation)
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IDEAL MIXTURE
IDEAL VLE (constant a)
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IDEAL MIXTURE
IDEAL VLE (constant a)
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Column profiles

• Binary separation. Typical composition profile

Example column A (binary, 41 stages, 99
purities, ?1.5)
Typical Flat profile at column ends
xi mole fraction of light component
Here No pinch (flat profile) around feed because
we have few stages compared to required
separation
TOP
BTM
stage no.
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Binary distillation Typical column profiles
pinch below feed (have extra stages in bottom
compared to required separation)
Note here with composition on x-axis
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More linear profile with log. compositions
Proof for infinite reflux and constant relative
volatility
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Check of feed location

• It is the separation of key components that
  matters!
• Plot X ln(xL/xH) versus stage no.
• Feed is misplaced if pinch (no change in X)
  only on one side of feed stage
• Feed is OK if no pinch or pinch on both sides of
  feed
• If misplaced feed location May get better purity
  or save energy by moving it (if possible)

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Temperature profiles
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Temperature profiles
BTM
TOP
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Binary distillation Typical temperature profiles
T
Flat around feed when pinch
(turned around with T on y-axis)
Stage no. !
Flat temperature profile toward column end
(because of high purity)
LT ¼ -X
Again profile is much more linear in terms of
logarithmic temperatures
342K
Stage no. !
355K
Pinch region of little change (no separation)
because of extra stages
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Example using Chemsep

• http//www.chemsep.org/
• Written by Ross Taylor, Clarkson University
• Lite version max 50 stages and 5 components
• Lite version is free and extremely simple to use
• Example
• 25 nC4(1), 25 nC5(2), 25 nC6(3), 25 nC7(4)
• Key components C5 (L) and C6 (H)
• Relative volatility varies between 2.5 (bottom)
  and 3.5 (top)
• Assume we want about 99 of C5 in top and 99 of
  C6 in bottom
• How many stages (N) and approx. L/F?

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Shortcut analysis
IDEAL VLE (constant a)

• Nmin ln S / ln ? ln (1/(0.010.01)) / ln 3
  8.4
• (this no. does not depend on neon-keys)
• Lmin/F ¼ 1/(?-1) 1/(3-1) 0.5
• (but non-keys change this...)
• Let us try N 20 and L/F0.6
• Now run detailed stage-to-stage simulation...

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Data input... components
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... column configuration
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... thermodynamics
Correction Use Soave-RK also here
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... feed data
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TOP Specify L/F 0.6 BTM Specify B/F 0.5
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L/F 0.6 gives 99.9 recovery of keys
recovery keys 99.9
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Profiles 99.9 recovery
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Liquid phase composition99.9 recovery
TOP
light non-key (butane)
light key (pentane)
Stage
heavy non-key (heptane)
heavy key (hexane)
BTM
x
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Vapor phase composition99.9 recovery
TOP
Stage
BTM
y
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Flow profile99.9 recovery
TOP
V
L
Stage
BTM
Flows
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Temperature profile99.9 recovery
TOP
Stage
BTM
Temperature K
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Turn profile around
Temp.
TOP
BTM
Stage
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Log (xL/xH)-plot (key ratio profile) Use to
check feed location
TOP
Stage
log(xL/xH) straight line Feed placement OK
BTM
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With feed moved from stage 10 to 15
TOP
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Stage
10
log(xL/xH) has pinch above feed Too many stages
above feed
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BTM
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Relative volatility(Feed back to stage 10)
2.5
3.0
3.5
4.0
TOP
Stage
BTM
?
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McCabe-Thiele diagram99.9 recovery
TOP
yC5
BTM
xC5
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3. Steady-state operation

• The column is now given!
• Operational degrees of freedom
• Get right split cut (external flows e.g. D/F)
  !!!
• Adjust separation fractionation (internal
  flows L/V)
• Column (temperature) profiles
• Multicomponent mixtures
• ...other factors...
• Optimal operation (in a plantwide setting)

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Given feed (F) and pressure (p) 2 steady-state
degrees of freedom, e.g. L and V. Can use for
(for example) Control one composition for each
product (xD, xB)
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Operation conventional column

• 2 steady-state degrees of freedom
• External flows (product split D/F).
• Adjust by changing D/F
• Moves profile up and down
• Large effect on operation
• Internal flows (L/V).
• Increase L and V with D/F constant
• Stretches profile
• Improves separation factor S, but costs energy
  and limits capacity
• Small effect
• Why small effect? Recall design Purity
  (separation) mainly influenced by no. of stages
  (N), which is fixed during operation

SPLIT (CUT)
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Operation conventional column

• 2 steady-state degrees of freedom
• External flows (product split D/F).
• Adjust by changing D/F
• Moves profile up and down
• Large effect on operation
• Internal flows (L/V).
• Increase L and V with D/F constant
• Stretches profile
• Improves separation factor S, but costs energy
  and limits capacity
• Small effect
• Why small effect? Recall design Purity
  (separation) mainly influenced by no. of stages
  (N), which is fixed during operation

FRACTIONATION (SEPARATION)
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• Split D/F (external flows)
• Moves entire composition profile up or down.
• One product gets purer and the other less pure
• Large effect
• Internal flows (L/V)
• Stretches profile
• Both products get purer if we increase internal
  flows
• Smaller effect

Composition profiles for column A (F1). Change
in external flows ?D -0.02 with ?V0 Change
in internal flows ?V 1 with ?D0
TOP
BTM
Less pure Breakthrough of light component in
bottom
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Implication for control

• Important to get the right split (D/F)
• avoid breakthrough of light components in bottom
• avoid breakthrough of heavy components in top
• How can this be done?
• Measure feed composition (zF) and adjust D/F ¼ zF
  (feedforward control).
• 2. Keep column profile in place by measuring
  and fixing it somewhere in the column (feedback
  control)
• Simplest in practice Control temperature
• To minimize movement of profile

NO! Does not work in practice because of
uncertainty
Control temperature at most sensitive location
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Implication for control
TC
Idea The column is a tank filled with heavy
and light component
Need to adjust the split (D) to keep constant
holdups of light and heavy Simplest Profile
feedback using sensitive temperature
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Temperature profile multicomponent
Feed 25 C4 25 C5 (L) 25 C6 (H) 25 C7 20
stages D/F 0.5 Vary L/F
L/F0.6 99.9 recovery of L and H
Temp.
L/F0.3 99 recovery of L and H
STEEP PROFILE TOWARDS COLUMN ENDS BECAUSE OF
NON-KEYS
BTM
TOP
Stage
Control Use temperature about here (large
sensitivity)
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Summary. Steady-state operation of given column

• If split is wrong then one end will be too pure
  (overpurified), while the other end does not meet
  spec. (underpurified)
• Assume now split is right (e.g. control column
  profile)
• If column has too few stages, then it may
  difficult to obtain desired purities (even with
  maximum heat input) may need to give up one end
• You may try lowering the pressure, but usually
  limited effect
• You may consider moving the feed location (look
  at profile), but usually has limited effect
• Normally the only fix is to get more stages in
  your column
• If it has many stages, then you have two options
• Overpurify one or both ends Wont cost much in
  terms of energy, and makes control easier (no
  pinch in column)
• Keep specifications and save energy Get pinch in
  column

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Steady-state design and simulation of real
columns

• Commercial software Hysys, Aspen,
• Most important Use right thermodynamics (VLE).
  SRK or PR works surprisingly well for most
  mixtures (especially at high pressures and for
  gases)
• Design (given products) Use shortcut method to
  estimate required no. of stages feed location.
• Operation (given column) First get no. of stages
  in each section by matching data for composition
  and temperature profiles. Adjust holdups by
  matching with dynamic responses

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Trays vs. packings

• Packings
• Much smaller pressure drop (typically 1/10)
• Usually More stages for given column height
• - Problems with liquid distribution in larger
  columns (can use structured packings, but more
  expensive)
• Trays
• More easy to clean
• Better for large capacity columns
• Larger holdup (typically, 2 times larger)
  Advantage for control (have more time)
• - Can have inverse response in bottom of column
  (?- effect - difficult to predict)
• Overall Differences are surprisingly small
  also for control

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Conclusion steady-state distillation

• Understanding the steady-state behavior brings
  you a very long way towards understanding the
  control