Lecture 1

CENG 221

Lecture 1. Introduction to Separation Processes (2 h)

Learning Objectives:

(1) Importance of separation processes

(2) Popular chemical separation processes

(3) What are equilibrium separation processes

(4) Vapor-liquid equilibria

Learning Guides:

(1) Lecture handouts

(2) Chapters 1 and 2 of Textbook: Equilibrium-Staged

Separations

(3) Review thermodynamic notes on Raoult’s Law,

Antoine equation and DePriestler Chart

CENG 221

Lecture 1. Introduction to Separation Processes (2 h)

Learning Objectives:

(1) Importance of separation processes

(2) Popular chemical separation processes

(3) What are equilibrium separation processes

(4) Vapor-liquid equilibria

Learning Guides:

(1) Lecture handouts

(2) Chapters 1 and 2 of Textbook: Equilibrium-Staged

Separations

(3) Review thermodynamic notes on Raoult’s Law,

Antoine equation and DePriestler Chart

Application of Separation Processes

Raw Materials

Product 1 Product 2

Physical transformation

Chemical transformation

Examples:(1) water → distilled water(2) crude oil → gasoline(3) air → nitrogen gas

→ oxygen gas(4) coconut → cooking oil(5) limestone→ gravel

Examples:(1) water → hydrogen(2) crude oil → polyester(3) air → ethylene glycol (antifreeze)(4) coconut → medicinal oil(5) limestone→ cement

Application of Separation Processes

ReactorRaw Material

Products

Reactor

Raw Material 2

Separator 1

Raw Material 1

Separator 1

Product 2

Separator 1

Product 1

Separator 1

A → B

A + a ↔ B + b

Distilled Water

Tap Water

Filter Ion Exchange

Carbon Adsorber

FlashDistillation

UVDecontamination

BottledWater

Types of Separation Processes

(1) Distillation processes:* most popular chemical separation process,* capable of producing pure substance from mixture,* requires two phases: liquid and vapor,* uses energy to accomplish separation.

Production processes using distillation* oil production,* air separation,* distillation of wine to cognac and spirits.

http://lorien.ncl.ac.uk/ming/distil/distil0.htm


http://www.cheresources.com/packcolzz.shtml

(2) Absorption/stripping processes:* popular in environmental applications,* transfer gas component from gas to liquid or vice versa,* requires two phases: liquid and gas,* uses the different affinity gas component for gas and liquid.

Production processes using absorber* scrubbing smokestacks,* removal of ammonia from refinery,* remove CO2 from air.

http://www.robatel.com/gb/liquide_liquide.htm


(3) liquid-liquid extraction:* used for processes that requires low temperature operation,* transfer soluble component from one liquid to another,* requires two phases: two immiscible or partially miscible liquids,* uses the different affinity components for separation.

Production processes using extraction* food processing,* pharmaceutical separation,* oil purification.

http://www.cheresources.com/extraction.shtml


(4) solid-liquid extraction:* used liquid to extract component from a solid matrix,* transfer soluble component from solid to liquid,* requires two phases: solid and liquid,* uses the solubility of the component for separation.

Production processes using leaching* mining,* food processing,* pharmaceutical.

Phase Transformation

subcooledliquid

bubblepoint

dewpoint

saturatedliquid

saturatedvapor

superheatedvapor

http://www.lsbu.ac.uk/water/phase.html

Phase Diagram of WaterPhase Equilibria

xa, xb

xa + xb = 1

ya, yb

ya + yb = 1

Tv, Pv

TL, PL

At Equilibrium:TL = Tv (thermal equilibrium)PL = Pv (mechanical equilibrium)µL = µv (chemical equilibrium)

Equilibrium-Staged Operation

Ethanol + Water

Ethanol + Water

LIQUID

VAPOR

Ethanol

Water

Equilibrium-Staged Operation

Multiple Staged Unit

Ethanol + Water

Ethanol + Water

LIQUID

Ethanol

Water

Ethanol

Water

VAPOR

Ethanol

Water

Different Representation of VLE Data

(1) Ideal Solution:

Raoult’s Law:

PA = YAPT = PA*XA

YA= (PA*/PT ) XA

Antoine Equation:

LnPA* (kPa) = A - B/(T(°C) +C)

Compound A B C

Benzene 13.8594 2773.78 220.07

Ethanol 16.6758 3674.49 226.45

n-heptane 13.8587 2991.32 216.64

Toluene 14.0098 3103.01 219.79

Water 16.262 3799.89 226.35

Antoine Equation:

LnPE* (kPa) = 16.6758 – 3674.49/(T(°C) +226.45)

LnPW* (kPa) = 16.2620 – 3799.89/(T(°C) +226.35)

Use Antoine equation to determine the normal boiling point of ethanol and water ?

Use Antoine equation to determine the bubble point temperature of a solution containing 5 moles of ethanol and 5 moles of water.


Example 1: Determine the T-x-y relationship for ethanol and waterusing Antoine Equation assuming that ethanol-water forman ideal solution. Plot the resulting data. Please useambient pressure.

T(C) Pe* Pw* Xe Xw Ye Yw

Tmin 78.29185 101.3 44.19879 1 0 1 080.46185 110.315 48.2752 0.85469 0.14531 0.930752 0.06924882.63185 119.9885 52.66227 0.722419 0.277581 0.855695 0.14430584.80185 130.3574 57.37836 0.601839 0.398161 0.774474 0.22552686.97185 141.4599 62.44257 0.491758 0.508242 0.686714 0.31328689.14185 153.3356 67.87472 0.391118 0.608882 0.592027 0.40797391.31185 166.0253 73.69541 0.298978 0.701022 0.490009 0.50999193.48185 179.5714 79.92597 0.214501 0.785499 0.380239 0.61976195.65185 194.0176 86.58853 0.136941 0.863059 0.26228 0.73772

Tmax 99.99131 225.7879 101.3 2.28E-16 1 5.09E-16 1

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0 0.2 0.4 0.6 0.8 1

X,Y (ethanol)

T(C

)

What is the boiling point of pure ethanol and water?What is the bubble point temperature of a mixture containing0.25 mole fraction of ethanol? What is its dew point temperature?What is the bubble and dew point temperature of a solution containing 30 wt.% water?


(2) Tabulated data:

X(ethanol) X(water) Y(ethanol) Y(water) T(C)0 1 0 1 100

0.019 0.981 0.17 0.83 95.50.0721 0.9279 0.3891 0.6109 890.0966 0.9034 0.4375 0.5625 86.70.1238 0.8762 0.4704 0.5296 85.30.1661 0.8339 0.5089 0.4911 84.10.2377 0.7623 0.5445 0.4555 82.70.2608 0.7392 0.558 0.442 82.30.3273 0.6727 0.5826 0.4174 81.50.3965 0.6035 0.6122 0.3878 80.70.5079 0.4921 0.6564 0.3436 79.80.5198 0.4802 0.6599 0.3401 79.70.5732 0.4268 0.6841 0.3159 79.30.6763 0.3237 0.7385 0.2615 78.740.7472 0.2528 0.7815 0.2185 78.410.8943 0.1057 0.8943 0.1057 78.15

1 0 1 0 78.3

The experimental VLE data are usually obtained usingspecial stills where the temperature, pressure andconcentrations of the components in liquid and vapor phasescould be accurately determined.

X, Y (mole fraction) → x, y (mass fraction)for binary mixture (2-components):

x = X*MWA/(X *MWA + (1-X) *MWA)

** derive the equations for n-component and for x → X


(3) Graphical representation:

T-X-Y diagram

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X,Y (ethanol)

T(C

)

What is the boiling point of pure ethanol and water?What is the bubble point temperature of a mixture containing0.25 mole fraction of ethanol? What is its dew point temperature?What is the bubble and dew point temperature of a solution containing 30 wt.% water?

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x, y (ethanol)

T(C

)

T-x-y diagram

Compare the T-X-Y and T-x-y diagrams?What is the bubble point temperature of a mixture containing0.25 weight fraction of ethanol? What is its dew point temperature?What is the bubble and dew point temperature of a solution containing 30 wt.% water?


Example 2:

Determine the concentration of an alcohol solution if itsbubble point temperature is 92.5 C? What should be theethanol content of the vapor at equilibrium with this solution?

What is the bubble point temperature of 78 and 95 mol.% alcohol?

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X,Y (ethanol)

T(C

)


(3) Graphical representation:

x-y or McCabe-Thiele diagram

Determine the composition of the vapor at equilibrium withan 30, 60, 80 and 90 mol.% ethanol solution.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

X (ethanol)

Y (

eth

an

ol)


(4) Distribution coefficient (K-value):

KA = YA/XA

YA = KA XA = γA(PA*/PT ) XA

K(T,P)

T(C) Xe Ye Ke Xw Yw Kw

100 0 0 1 1 195.5 0.019 0.17 8.947368 0.981 0.83 0.846075

89 0.0721 0.3891 5.396671 0.9279 0.6109 0.65836886.7 0.0966 0.4375 4.528986 0.9034 0.5625 0.62264885.3 0.1238 0.4704 3.799677 0.8762 0.5296 0.60442884.1 0.1661 0.5089 3.063817 0.8339 0.4911 0.5889282.7 0.2377 0.5445 2.290703 0.7623 0.4555 0.59753482.3 0.2608 0.558 2.139571 0.7392 0.442 0.59794481.5 0.3273 0.5826 1.780018 0.6727 0.4174 0.62048580.7 0.3965 0.6122 1.54401 0.6035 0.3878 0.64258579.8 0.5079 0.6564 1.29238 0.4921 0.3436 0.69823279.7 0.5198 0.6599 1.269527 0.4802 0.3401 0.70824779.3 0.5732 0.6841 1.193475 0.4268 0.3159 0.740159

78.74 0.6763 0.7385 1.091971 0.3237 0.2615 0.80784778.41 0.7472 0.7815 1.045905 0.2528 0.2185 0.8643278.15 0.8943 0.8943 1 0.1057 0.1057 1

78.3 1 1 1 0 0


(4) Distribution coefficient (K-value): DePriestler Chart


Example 3:

Determine the composition of isobutane and n-hexane solutionthat boils at 50 C at a pressure of 300 kPa.

Plot T-X-Y diagram of isobutane and n-hexane solution for a P of200 kPa.


: T(C) Kic4 KC6 alpha

10 1.1 0.06 18.3333320 1.5 0.095 15.7894730 2 0.15 13.3333340 2.5 0.2 12.550 3 0.3 1060 3.8 0.4 9.570 4.6 0.54 8.51851980 5.5 0.7 7.85714390 6.5 0.9 7.222222

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

X,Y (i-C4)

T(C

)


Example 4:

Determine the boiling point temperature of n-pentane, n-heptaneand n-octane which are the main components of lighter fluid.

What is the liquid composition of the lighter fluid if the bubble point temperature is 40 C at ambient pressure.


(5) Relative Volatility (αAB)

αAB = KA/KB

YA = αAB XA/(1 + (αAB -1)XA)

T(C) Kic4 KC6 alpha

10 1.1 0.06 18.3333320 1.5 0.095 15.7894730 2 0.15 13.3333340 2.5 0.2 12.550 3 0.3 1060 3.8 0.4 9.570 4.6 0.54 8.51851980 5.5 0.7 7.85714390 6.5 0.9 7.222222


Example 5:

Plot X-Y diagram for isobutane-isopentane mixture if the relativevolatility is known to be 1.7


:

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

X (i-C4)

Y(i

-C4)

Xic4 Yic4

0 00.1 0.1588790.2 0.2982460.3 0.4214880.4 0.531250.5 0.629630.6 0.718310.7 0.7986580.8 0.8717950.9 0.93865

1 1


(6) Ponchon-Savarit Diagram:

H-x-y diagram

Dew Point and Bubble Point Temperatures

Example 6: Using the Ponchon-Savarit diagram for ethanol-water solution, please determine:(1) the bubble and dew point temperatures of a solution containing55 wt.% of ethanol.(2) the bubble temperature of 0.2 ethanol-0.8 water solution andthe composition of the bubble at equilibrium with the liquid solution.(3) the enthalpy of an ethanol solution (0.5-0.5) at 86 C.(4) composition of vapor and liquid for a 0.5-0.5 ethanol-water solutionat 86 C and the V/L ratio.(5) the enthalpy of the liquid (H) and vapor (h)



H-x-y diagram


Relationships between the Ponchon-Savarit, T-x-y and y-x Diagrams:



H-x-y diagram

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T(C

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Material Balance

Binary Equilibrium Solutions

Overall BalanceFeed = (Sat. liquid) + (Sat. Vapor)

F = L + V

Component BalanceFz = Lx + Vy

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T(C

)Material Balance

Binary Equilibrium Solutions

Overall BalanceFeed = (Sat. liquid) + (Sat. Vapor)

F = L + V

Component BalanceFz = Lx + Vy


(1) Bubble point temperature (Tb):is the temperature at which the first bubble of vapor is formed ata given pressure. The composition of the vapor is at equilibriumwith the liquid solution.

(Note: One can assume that Tb is the temperature at which thedistillation process starts)

xa, xb

xa + xb = 1

ya, yb

ya + yb = 1 Tv, Pv

TL, PL


(1) Dew point temperature (Td):is the temperature at which the first dew or condensate forms from a vapor mixture. The composition of the dew is at equilibriumwith the vapor mixture.

(Note: One can assume that Td is the temperature at which thedistillation process ends)

xa, xb

xa + xb = 1

ya, yb

ya + yb = 1 Tv, Pv

TL, PL


Distillation process occurs between bubble point and dew pointtemperatures of the mixture.

TL < Tb < T < Td < TH

Note: TL is the lowest boiling component in the solution/mixture,Tb is the bubble point temperature of the solution with composition A,Td is the dew point temperature of the mixture with composition A,TH is the highest boiling component in the solution/mixture.

xa, xb

xa + xb = 1

ya, yb

ya + yb = 1

Tv, Pv

TL, PL

Example 7: In a close container an alcohol solution containing0.4 mole fraction of ethanol was heated slowly from room tem-perature to its dew point temperature. Determine the changes inthe liquid and vapor composition of the mixture.

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X,Y (ethanol)

T(C

)


Bubble Point Temperature Calculation

How to set up a trial and error calculation for Tb:

(1) Bubble point - all is liquid, one bubble of vaporset xi = zi (where z is the solution composition)also note the pressure, P and if not given set P to ambient (1 bar)

(2) Guess an initial Temperature (T0)use the median temperature between the highest and lowestboiling components

(3) Use P and T0 to determine the Ki values from DePriestler chart

(4) Calculate yi = Ki xi

(5) Using mass balance, check whether the summation of yi = 1(6a) If not, then use the most volatile component (largest K) as

your reference and determine the next T by:(Ki)n+1 = (Ki)n/Sum(yi)using P and (Ki)n+1 find the new Tn+1

repeat (3) to (6)(6b) If yes, Tn is your bubble point

How to set up a trial and error calculation for Tb:

(1) Bubble point - all is liquid, one bubble of vaporset xi = zi (where z is the solution composition)also note the pressure, P and if not given set P to ambient (1 bar)



(4) Calculate yi = Ki xi

(5) Using mass balance, check whether the summation of yi = 1(6a) If not, then use the most volatile component (largest K) as

your reference and determine the next T by:(Ki)n+1 = (Ki)n/Sum(yi)using P and (Ki)n+1 find the new Tn+1

repeat (3) to (6)(6b) If yes, Tn is your bubble point

Example 8a: Using the dePriestler chart find the bubble point temperatures for a solution containing 0.5 mole n-pentane,0.3 mole n-heptane and 0.7 mole n-octane at pressure of 2 bars.


Dew Point Temperature Calculation

How to set up a trial and error calculation for Td:

(1) Dew point - all is vapor, one drop of liquidset yi = zi (where z is the solution composition)also note the pressure, P and if not given set P to ambient (1 bar)



(4) Calculate xi = yi /Ki

(5) Using mass balance, check whether the summation of xi = 1(6a) If not, then use the most volatile component (largest K) as

your reference and determine the next T by:(Ki)n+1 = (Ki)n Sum(xi)using P and (Ki)n+1 find the new Tn+1

repeat (3) to (6)(6b) If yes, Tn is your dew point

How to set up a trial and error calculation for Td:

(1) Dew point - all is vapor, one drop of liquidset yi = zi (where z is the solution composition)also note the pressure, P and if not given set P to ambient (1 bar)



(4) Calculate xi = yi /Ki

(5) Using mass balance, check whether the summation of xi = 1(6a) If not, then use the most volatile component (largest K) as

your reference and determine the next T by:(Ki)n+1 = (Ki)n Sum(xi)using P and (Ki)n+1 find the new Tn+1

repeat (3) to (6)(6b) If yes, Tn is your dew point

Example 8b: Using the dePriestler chart find the dew point temperatures for a solution containing 0.5 mole n-pentane, 0.3 mole n-heptane and 0.7 mole n-octane at pressure of 2 bars.


Lecture 1

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