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Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf ·...

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Chemical unit operations 2. lectures 2 hours/week calculation seminars weekly 1 hours laboratory weekly 3 hours
Transcript
Page 1: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Chemical unit operations 2.

lectures 2 hours/week

calculation seminars weekly 1 hours

laboratory weekly 3 hours

Page 2: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Requirements

• Attendance will be checked.

• Requirements for the signature: – Attending on minimum 66% of the lectures and calculation

seminars, and on all laboratory practices is required.

– One calculation test will be scheduled during the semester. Exact dates and topics covered will be announced later. The calculation test have to be passed for signature. The grade of the calculation test counts as 25% of the final grade.

– Nine laboratory measurements will be scheduled. Each measurement have to be performed, and reports accepted.

• Students having valid signature may take a written exam during the examination period. Application for the exams is only possible via the Neptun system.

• Website of the course: http://kkft.bme.hu/oktatas/cuo2/

Please check it frequently!

Page 3: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Distillation

Edit Székely, Béla Simándi

Page 4: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Topics to be covered

• Distillation in the industry (introduction).

• Basic definitions. Vapour-liquid equilibria.

Measurement and simply calculation methods.

Ideal and non-ideal phase diagrams.

• Batch distillation.

• Flash distillation.

• Distillation columns = rectification.

Page 5: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Distillation in the industry

• Ethanol production

– Food industry

– Chemical industry

– Bioethanol

Page 6: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Distillation in the industry

• Ethanol production

• Oil industry

– fractionation

(products based on

distillation intermediates:

fuels, raw material of

chemical industry, plastics,

solvents, bitumen etc. )

Page 7: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Distillation in the industry

• Ethanol production

• Oil industry

• Waste water treatment

– Mainly waste waters with

high solvent contamination

– Waters from absorption

(purification of exhaust

gases).

Page 8: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Advantages of distillation

• Major separation process for homogeneous

liquids of two or more components.

• Separation is based on the difference in boiling

points (volatility).

• No additional chemicals are needed, but

energy requirement is relatively high.

• In a large number of separation task it is the

most economical option.

Page 9: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Boiling point

• Depends on:

– ingredients

• molar ratios, exact composition

– pressure

A liquid starts to boil if the sum of the partial pressure

of its ingredients is equal to the pressure of the

environment.

Page 10: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Vapour pressure of a pure component

• Vapour pressure is the pressure measured above a neat liquid component in equilibria (no other components in the system).

• It depends on the materials itself, and on temperature. Always increases with temperature.

• Antione equation:

TC

BAp

0lg

P0 vapour pressure (Pa; Hgmm), T temperature (°C, K), A,B,C Antoine-constants

Page 11: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Volatility

• If its boiling point is lower at a chosen pressure than the other’s one, that component is called more volatile.

• If a liquid mixture is boiling, in general, the more voletile component has a higher relative concentration in the vapour phase than in the liquid phase.

• Volatility is the ratio of the molar fraction of component j in the vapour phase over its molar fraction in the liquid phase.

j

jj x

yK

Page 12: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Laws of Raoult and Dalton

Raoult’s law

A mixture is ideal from a

practical point of view if law of

Raoult is valid for any

compositions in the examined

range of temperatures.

p0j vapour pressure of neat

component j (Pa) at the

given temperature,

xj molar fraction of component

j in the liquid phase.

Dalton’s law

Partial pressure is the share of

the given vapour of the total

pressure. This is proportional

with its molar fraction in the

vapour phase.

pj partial pressure of

component j (Pa),

yj molar fraction of component

j in vapour phase (-),

P total pressure (Pa).

Pyp jj jjj xpp 0

Page 13: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Bubble point – dew point diagrams

• These diagrams are constructed based on

experimental data obtained in special

equipment.

Page 14: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Measurement of vapour- liquid equilibrium

data

• The liquid is heated by electrical heating and kept continuous boiling. Pressure is stricktly kept constant.

• The vapour phase is circulated.

• At equilibrium samples are taken from both phases, and composition is analysed.

Page 15: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Bubble point – dew point diagrams

• These diagrams are constructed based on

experimental data obtained in special equipment.

• In case of ideal mixtures, phase diagrams can be

easily obtained by simple calculations.

• A mixture is ideal if:

– Law of Raoult is valid,

– Complete misciblity in liquid phase,

– The constituents are usually chemically similar (e.g.

linear chain hydrocarbons).

Page 16: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Bubble point – dew point curves and the

equlibrium curve

mixture of benzene and toluene at atmospheric pressure (P=1.013 bar)

P=constant

Page 17: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Bubble point – dew point curves and the

equlibrium curve

mixture of isopropil-ether–isopropanol at atmospheric pressure

P=constant

Page 18: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Bubble point – dew point curves and the

equlibrium curve

mixture of acetone-chloroform at atmospheric pressure

P=constant

Page 19: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Bubble point – dew point curves and the

equlibrium curve

mixture of ethyl acetate – water at atmospheric pressure

P=constant

Page 20: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Batch distillation

Page 21: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Batch distillation – mathematical description of the

unit oparation

Deriving the Rayleigh-equation

Set up the molar balance and component balance

equations for an infinitely short dt time.

During this dt time, a vapour of molar amount dV is

produced with a composition y (molar increase of

distillate is equal, since distillate is formed by

condensing the vapour). Molar amount of the boiling

liquid in the vessel is meanwhile decreased by dL.

Thus the molar balance is:

Page 22: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Rayleigh-equation

yVxxLLxL d)d)(d(

yVxL

xLxLxLxL

ddd

dd

yVxLxL ddd0

yLxLxL ddd0

)(dd xyLxL

L

L

xy

x dd

0

1

0

1

L

L

x

x L

L

xy

x dd

1

00

1L

L

xy

xx

x

lnd

neglect second-order differential dLdx and reorder:

substitute dL=dV

separate the variables

DVL ddd

Page 23: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Batch distillation – mathematical description

of the unit operation

Applications of the Rayleigh-equation

1

00

1L

L

xy

xx

x

lnd

where

y and x are compositions of vapour and

liquid phases in equilibrium;

index 0 refers to initial (at t0 time) 1

refers to the final (t1) values;

L is molar amount of the liquid phase

(mol).

If initial amount of the liquid (L0) and

its composition (x0) is known and the

desired purity is defined (x1), amount

of remaining liquid (L1) can be

calculated by the Rayleigh-equation.

Page 24: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Batch distillation – mathematical description of

the unit operation

Calculation of average composition in distillate

L0, L1, x0, x1 are already known.

From molar and composition balances the two

unknown can be calculated.

1010 LLDDLL

D

xLxLxxDxLxL DD

11001100

.distillate of ncompositio average theis where Dx

Page 25: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Applications of batch distillation

• Limits of separation. Subsequent distillations are

needed.

• When relatively smaller amount of feed have to

be purified batch distillation is more economic

than continuous distillation processes.

• When composition of feed changes frequently,

but distillation have to be performed in the same

unit.

Page 26: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Continuous single stage distillation / flash

distillation

• Consists of: – vaporizing a definite fraction of the liquid in such a

way, that the evolved vapor is in equilibrium with the residual liquid,

– separation the vapour and the liquid,

– condensing the vapour.

• It is a continuous, steady-state operation. – steady-state: temperature, pressure, compositions and

flow rates are constant.

• Separation performed is equivalent to one single theoretical stage.

Page 27: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Continuous single stage distillation at constant

pressure Molar flow rate of the

vapour is V. Its composition is y (molar fraction of the

more volatile component in case of binary mixtures).

The vapour is in equilibrium with the liquid product.

Molar flow rate of feed is F. Its composition if xF

(molar fraction of the more volatile

component in case of binary mixtures).

Condensate of heating steam

Heating steam. Molar flow rate of liquid product is L. Its

composition is x (molar fraction of the more

volatile component in case of binary mixtures).

Page 28: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Flash distillation by reduction of pressure

Vapour

Liquid phase feed

Cyclone. Vapour and drops of liquid are

separated by sedimentation.

Preheater. The liquid is heated below its boiling point at pressure P1. The final temperature should

be sufficiently higher than temperature of boiling at P2.

Liquid product

Pressure reducing valve

Page 29: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Flash distillation by partial condensation

Vapour product

Vapour feed

Partial condensator

Liquid product

Cyclone. Vapour and drops of liquid are

separated by sedimentation.

Page 30: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Flash distillation – mathematical description

Molar balance VLF

Component balance yVxLxF F

Express y as function of x and plot the expression in the equilibrium diagram!

Since x and y are equilibrium compositions, intersection of the two curves is the point of operation.

xV

Lx

V

Fx

V

VFx

V

F

V

xLxFy FF

F

Page 31: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Flash distillation

FxV

Fx

V

Ly

FFF

F

xxV

Fx

V

Ly

xx

if

Page 32: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Flash distillation – range of operation

Flash disitillation is one

single equilibrium stage.

Page 33: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Flash distillation - summary

• Rarely used as individual separation unit

because its limitations in purity.

• In case of non-ideal mixtures like

heteroazeotropic mixtures it might be an

option.

• Partial reboiler of a rectification column is a

flash distillation unit.

Page 34: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Distillation columns – continuous rectification

• A separation process performed in steady state

operation.

• As usual for continuous processes, it is mostly

applied at constant and high feed rates.

• Frequently used in industrial scale.

• Separation is based on vapour-liquid

equlibrium.

Page 35: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Rectification = subsequent partial condensations

and subsequent partial evaporations

Partial evaporation of liquid

Partial condensation of vapor

P=constant

Page 36: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Distillation column

Page 37: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Molar balance

molar flow rate of feed (F), of distillate

(overhead product) (D) and bottoms

product (W) (mol/s or kmol/h).

WDF

Rectification, balance equations

Component balance

Compositions: molar fraction values of

the more volatile component.

WDF xWxDxF

Page 38: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Simple calculation methods for inital

estimations

• The technique discussed is considered to be the simplest

method for analysis of binary distillation.

• In an ideal stage (plate or tray), the V phase leaving the stage is

in equilibrium with the L phase leaving the same stage.

• The problem is that of finding the number of ideal stages

required in an actual column to achieve a desired separation.

• If the number of theoretical stages can be determined, and

information on the stage efficiencies is available, the number

of actual stages can be calculated.

Page 39: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Reflux ratio:

R=L/D

Reflux ratio, operating lines

Basic assumtion: constant molal overflow.

Page 40: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Continuous fractionation column- Constant

molal overflow

Assumptions:

• The column is adiabatic (sufficient insulation, neglectable heat loss).

• Heat effects (heats of solution, heat losses to and from column, etc.) are negligible.

• Heat of evaporation is the major source of any heat effects in the column.

• Molal heats of vaporization of the components are roughly the same. For every mole of vapour is condensed, a mole of liquid vaporized.

Results:

• Liquid and vapour molar flow rates are constant in the rectifying and in the stripping sections, separatly.

Page 41: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Plates are numbered serially from the

top down. Flows leaving the plates

are numberred accordingly.

Assumption: liquid and molar flows

leaving a theoretical (ideal) plate

have equilibrium compositions.

Continuous fractionation

column – reflux ratio, operating

lines

Based on constant molal overflow:

VVVV n ...21

LLLLL n ...210

Page 42: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Consider the part of the column enclosed by control line, a component balance of rectifying section can be written as:

Rectifying operating line

Dnn xDxLyV 1

Molar balance of the rectifying section (similar to the balance equation of the condenser):

DLV

Expressing yn+1 from the component

balance and substituting into molar

balance equation we obtain:

DnDnn xLD

Dx

DL

Lx

V

Dx

V

Ly

1

Page 43: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Rectifying operating line

DnDnn xLD

Dx

DL

Lx

V

Dx

V

Ly

1

Divide by D and substitute the reflux ratio (R=L/D):

111

R

xx

R

Ry D

nn

Generalized form is an equation of a

straight line:

11

R

xx

R

Ry D

Page 44: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Operation line of the

stripping section

Balance equations can be set analogously to the lower part of the column. Control line includes the still and all plates below plate m.

V’ and L’ are the molar flow rates of vapor and liquid in the stripping section:

WVL ''

Wmm xWyVxL 1'

Page 45: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Operation line of the

stripping section

WVL ''

Wmm xWyVxL 1'

Wmm xV

Wx

V

Ly

''

'1

WxV

Wx

V

Ly

''

'

Page 46: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

With known reflux ratio operating line of the rectifying section can be draw in the VLE (equlibrium) diagram.

The condition of the feed can be deduced by the slope of the feed line or q-line. The q-line is that drawn between the intersection of the operating lines, and where the feed composition lies on the diagonal line.

Continuous fractionation

column – reflux ratio and

operating lines

WxV

Wx

V

Ly

''

'

Page 47: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Continuous fractionation column – feed

line/ q-line

, where HF is the enthalpy of the saturated vapour with the composition of xF (J/mol), hF is the enthalpy of the feed (J/mol), λF is molar heat of vaporization of a mixture with xF composition (J/mol).

F

FF hHq

Consistence and properties of feed hF q

Cold liquid (below its boiling point) known as subcooled liquid

hF < HF-λF 1 < q

Boiling liquid known as saturated liquid hF = HF -λF q = 1

Mixture of liquid and vapour, q is equal to liquid mole fraction

hF = HF–q· λF 0 < q < 1

Saturated vapour hF = HF q = 0

Overheated (superheated) vapour hF > HF q < 0

Page 48: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Continuous fractionation

column – effect of feed

condition on molar flow rates

F

FF hHq

LVFLV

Molar balance of the feed

plate:

When feed is miture of vapour and liquid, (q)

liquid fraction is added to the liquid and (1-q)

fraction to the vapour flow.

FqLL

FqVV )1(

Page 49: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Continuous fractionation column –

condition of feed

Fxq

xq

qy

1

1

1

Knowing q the q-line can be drawn.

Deduct equation of rectifying operating line from

equation of stripping operation line:

DW DxWxxLLyVV )()(

WW xWxLyVxV

Wx

V

Ly ''

''

'

DDn xDxLyVxV

Dx

V

Ly

FxFxFqyFq )(1

Fxxqyq )(1

FqLL

FqVV )1(

(-)

Page 50: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

1

0,8

0,6

0,4

0,2

0,2

x

0

0 0,4 0,6 0,8 1

xw

yw=xw

xD

yD=xD

Given:

R, q, xw, xD, xF

y 1

0,8

0,6

0,4

0,2

0.2

x

0

0 0.4 0,6 0,8 1

xF

q line

q>1

subcooled liquid

q>1

q=1

saturated liquid mixture of liquid

and vapour

0<q<1

q=0

saturated

vapour

q<0

overheated

vapour

1q

qtg

1R

Rtg

1R

xD

Rectifying

operation line

Stripping operation line

Effect of feed condition

Page 51: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

xw

yw=xw

xD

yD=xD

Given:

q, xw, xD, xF

Rmin=?

y 1

0,8

0,6

0,4

0,2

0,2

x

0

0 0,4 0,6 0,8 1

xF

q line

Minimal reflux ratio

N=∞

1q

qtg

1R

Rtg

min

min

1R

x

min

D

Rmin can be calculated from the read value

Upper operating line

Page 52: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

xD

yD=xD

Given:

xw, xD

Nmin=?

y 1

0.8

0.6

0.4

0.2

0.2

x

0

0 0.4 0.6 0.8 1 xw

yw=xw

Equilibrium curve

45°line: x=y

1

3

4

2

Nmin=4

Minimal number of theoretical plates

Page 53: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Simple calculation methods for inital

estimations

• Calculation of the minimal number of theoretical plates.

– gives us information about the difficultness of the desired separation,

– needed: desired compositions of overhead and bottoms products.

• Minimal reflux ratio: to achive the desired separation, reflux

ratio has a minimal value.

• Number of theoretical plates is determined: with McCabe –

Thiele method.

Page 54: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

R, q, xw, xD, xF

Ntheoretical=?

y 1

0,8

0,6

0,4

0,2

0,2

x

0

0 0,4 0,6 0,8 1

xF

q line

xD

yD=xD

xw

yw=xw

1R

Rtg

1q

qtg

1

3

4

2

5

6

7

Ntheoretical=7

1R

xD

McCabe-Thiele method (1925)

Page 55: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

cooling water in

cooling water out

Heating steam in

Condensate out

Feed

Bottoms product

Distillate (overhead product)

y 1

0,8

0,6

0,4

0,2

0,2

x

0

0 0,4 0,6 0,8 1

xD

yD=xD

xw

yw=xw

xF

xw

xD

1

2

3

4

5

6

7

xD y1

y2 x1

y3 x2

y4 x3

y5 x4

y6 x5

y7 x6

y8 x7

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Simple calculation methods for inital

estimations

• Calculation of the minimal number of theoretical plates.

– gives us information about the difficultness of the desired separation,

– needed: desired compositions of overhead and bottoms products.

• Minimal reflux ratio: to achive the desired separation, reflux

ratio has a minimal value.

• Number of theoretical plates: with McCabe – Thiele method.

• Calculation of column diameter.

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Load factor

• Load factor (Fload) :

• Depends on: linear velocity of vapour in the empty column (v) density of vapour (ρG).

• Density of vapour is influenced by: average molar mass (composition!), temperature (P, x), pressure. Pressure is lowest at top, highest at bottom of column.

GvF load

21

3Pa

m

kg

s

m

TR

MPG

real0 NPPP

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Plate efficiency

Operation range: horizontal part

sieve plate

bubble plate

valve plate

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Calculation of column diameter

• Selection of load factor

value,

• caluclate diameter at top and

bottom (D0),

• select a suitable value in

between.

G

Fv

load

P

TRVV aconst 0P

TRVV tconst

volumetric flow rate at

top and bottom

22

4

4

oo D

V

D

V

A

Vv

v

VDo

4

Rconst is the ideal gas constant 8.314 J/(mol*K)

Page 61: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Simple calculation methods for inital

estimations

• Calculation of the minimal number of theoretical plates.

– gives us information about the difficultness of the desired separation,

– needed: desired compositions of overhead and bottoms products.

• Minimal reflux ratio: to achive the desired separation, reflux

ratio has a minimal value.

• Number of theoretical plates: with McCabe – Thiele method.

• Calculation of column diameter.

• Calculation the height of the column.

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Height of column

• Determine (Ntheoretical).

• Read average plate efficeincy (ηplate) and

calculate the number of real plate. Round up.

• Multiply with plate height.

plate

ltheoreticareal

NN

HNH real

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63

Plate columns – topics covered

• Theoretical and real plates

• Operation

• Calculation of Nmin, Rmin McCabe – Thiele

method, plate efficiency

• Load factor

• Construction.

Page 64: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

64

Bubble-cap plate

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65

sieve plate

downcomer

weir

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67

Valve plate

Page 68: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

68

z

z

z

Operation of valve trays

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Packed columns

• Random or structured packing.

• Random packing is cheaper, but less efficient (same height less separation efficiency).

• Modern structured packing can be applied from laboratory scale to large industrial scales.

Page 70: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Determination of the heights of the column 1.

HETPNH

Sulzer Mellapak packing

HETP= Height Equivalent to a Theoretical Plate

70

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Determination of the heights of the column 2.

• Instead of theoretical plates the term transfer units

migh be also applied.

• NTU (Number of Transfer Units)

• HTU (Height of a Transfer Unit)

• To be calculated separatly for the upper and lower

sections of the column. Only the active height is

calculated.

uull NTUHTUNTUHTU H

71

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Theoretical background

HyyAaKyV y d)(d

V

yK

a

A

mxy

molar flow rate of vapour at top of column (mol/s),

mass tranfer coefficient (mol/(m2∙s)),

wetting coefficient(-),

relative surface area of packing (m2/m3),

cross section of column (m2),

virtual vapour concentration which would be in

equilibrium with the liquid concentration (-).

72

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Theoretical background – the two film theory

xyy

m

K

11

AiAi mxy )( AiAxA xxJ

)( AAiyA yyJ

AiA

x

A yymJ

AAi

y

A yyJ

)( AAyA yyKJ

)()( AiAx

AiAx yymm

mxx

AAiAiA

yx

A yyyym

J

1

73

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Calculation of the heights of the column

HyyAaKyV y d)(d

D

F

f y

yy

H

yy

y

AaK

VHH

dd

0

yy

y

AaK

VH

y

dd

AaK

VHTU

y

u

D

F

y

y

uyy

yNTU

d

AaK

VHTU

y

l

'

F

W

y

y

lyy

yNTU

d

74

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Calculation of HTU known: F, R, q, xF, xD, xW

DWF

DwF xDxWxF

DwDwF xWFxWxDxWxF

DW

DF

xx

xxFW

WFD

DRV 1

AaK

VHTU

y

u

FqVV 1'

AaK

VHTU

y

l

'

75

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Calculation of NTU

76

Page 77: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

77

Random packing

Page 78: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

78

Random packing / rings

Pall-ring Cascade ring Raschig-ring

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79

Random packing / saddles

Berl-saddle Intalox-saddle

Super intalox-saddle

Page 80: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

80

Further random packing elements

Konjugated rings Lessing-ring Snowflakes

Page 81: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

81

Further random packing elements

Envipack spheres Spiral packing

balls

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Page 83: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

83

Structured packing

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84

Structured packing types of distillation

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85

Liquid distributors

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86

Grids

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87

Locating grids

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photos: Sulzer

Assembly The column sleeve is lowered into the column with two cranes. Weight: 100 tons

Installation of the liquid distributor

88

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Economical optimum

89

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Economical optimum

Cooling and

heating

W

W

W

FqDR

FqV

VQ

11

)1

reb

D

D

DR

VQ

1

cond

90

Page 91: Chemical unit operations 2. - Címlapkkft.bme.hu/sites/default/files/distillation_notes.pdf · Chemical unit operations 2. ... mixture of acetone-chloroform at atmospheric pressure

Thank you for your attention!


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