Distillation 08

KAT / Distillation 1

• Two-component distillation

• Multi-component distillation• Short-cut methods

• Rigorous methods– Efficiency– Column design– Batch distillation– Packed column for distillation

DISTILLATION


Two-component distillation


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


Multicomponent distillation


Multi-component distillation

• Several columns or one column with side-streams• Complex calculations• Operation in a column influence the operation in

other columns• Feedback


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


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


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)


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


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)


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


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

⎥⎦

⎤⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛

=+


• 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


• 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


Erbar-Maddox diagram


Gillilands diagramRm = Minimum reflux ratioNm = Minimum number of ideal stage, (n+1)

Line-line scale Log-log scale


Rigorous methods

• For each plate:– Mass balance for each components– Energy balance– Vapour-Liquid Equilibrium– Other (e.g., mixtures properties)

• Nonlinear equation system


Rigorous methods

• Different methods solve equations system in different ways.

• High accuracy • Other methods (Perry, 7th Ed, 13-39)

– Matrix method: Naphtali and Sandholm


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


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 =

⎥⎥⎦

⎤

⎢⎢⎣

⎡


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)


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


Quick estimation of overall effciency


Typical data for efficiency


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


Azeotropic Distillation


Extractive Distillation


Batch distillation

– Unequal feed– Small amount– Several fractions,

high purity– The process can

be tracked


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


Constant Reflux Ratio


Constant distillate composition


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


Operating range - Performance diagram

Limitedrange of vapor and liquidflow rates


Types of trays


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


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


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⎥⎥⎦

⎤

⎢⎢⎣

⎡⎥⎥⎦

⎤

⎢⎢⎣

⎡

=


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


HETP for Full-scale Plant

Type of packing, application HETP, (m)

25 mm diam. packing 0.46



Absorption duty 1.5 - 1.8

Small columns (d< 0.6 m) Column diameter

Vacuum columns Values above + 0.1m


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

==−

= ∫


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 ⋅=

⋅=


KAT / Distillation 43 KAT / Distillation 44



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Distillation 08