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11-11-02

Exam 2 graded

Hand back

Go over (briefly)

Liquid-Liquid Extraction

Leading up to extractive distillation

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Exam 2 grades

66.3 average

22.7 std dev98 high

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Problem 1 Fenske, Underwood,

Gilliland calculations

FEED

I-Pentane 40 kgmol/h light key

N-Pentane 60 kgmol/h heavy key

N-Hexane 12 kgmol/h

N-Octane 12 kgmol/h

P 1.2 bar

sat'd liquid

K values:

I-Pentane 1.385 light key

N-Pentane 1.07 heavy key

N-Hexane 0.349

N-Octane 0.038

LKS 0.95

HKS 0.01

Do a preliminary column design using the Fenske,Underwood, Gilliland method (hand, not ChemCAD) for a

column separating a mixture of isopentane, n-pentane, n-hexane, and n-octane with the flowrates specified below fed

as a saturated liquid at 1.2 bar. The column is to operate at1.2 bar with a light key split of 0.95 and a heavy key split of

0.01. The light key is isopentane and the heavy key is n-pentane. Use the Molokanov version of the Gilliland

correlation and the Fenske method for determining the feedtray. Use an R/Rmin of 1.2. Use the K-values specified

below.

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Problem 1, contd

(a) Nmin

as estimated using the Fenske method

29.2

(b) R min

as estimated using the Underwood

method 8.32

(c) N for R/Rmin

= 1.2 as estimated using the

Molokanov form of the Gilliland correlation

59.9

(d) Nfeed

as estimated using the Fenske equation

36.2

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Problem 1, grading notesgrading notes:

genl

-3 answer close but cannot find explanation in .xls-1 answer close but cannot find explanation in .xls but is probably a 'carry over' from a previous error

nc over-rounding of ideal stage calcs - comment only

-6 missing answer (blank)

-4 wrong component as heavy key; check for cascade effect

(a)

-4 incorrect alpha (dumb mistake) - correct and see if other answers are correct, nc for other answers if yes

-5 above plus used LKS and HKS as mol fractions

(b)

-3 q incorrect

(d)

-2 NS instead of NF=NR

-5 Molokanov & wrong

-1 small formula error

-3 NS min okay, but goofed from there (not simple NS instead of NR mistake)

-3 used LKS & HKS instead of xB values in formula-3 Molokanov & okay

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Problem 2A stream containing 75 kgmol/h methanol and 75 kgmol/h

isopropanol is to be distilled to separate the methanol andisopropanol. The feed is available as a saturated liquid at pressures

between 1 and 12 bar. Methanol is the light key and the column is

to be designed for a light key split of 0.98; isopropanol is the heavykey and the column is to be designed for a heavy key split of 0.01.

Using ChemCADs shortcut distillation column model with theUnifac model for K-value estimation:

(a) determine the pressure at which the column must be

operated to have a distillate temperature of 110 degrees C.(b) compare the column specifications for the pressure found in

part a (N, R, and heat duties in particular) with thoseobtained for a column pressure of 1.2 bar. Explain the

differences between the column specifications at the two

pressures.

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Problem 2, contd(a) 4.8 bar 10

(b)

pressure found in (a) P=1.2 bar 8

Number of stages 37.5998 30.4205

Reflux ratio 3.5825 2.5791

Q, cond (MJ/h) -10755.873 -9302.6777

Q, reboiler (MJ/h) 10908.4785 9429.3418

Basically, the relative volatility is lower at the higher column pressure leading to

more stages, higher reflux, and higher exchanger duties.

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Problem 2 grading notesGrading notes

(a)

-5 simulation set up okay, but incorrect answer somehow

-5 kept' components from #1 or elsewhere

-3 set up okay, didn't converge to 110C pressure

-4 converged to a bottoms temp of 110C (nc in b for this error)

-2 incorrect feed specifications (should be sat'd liq at column pressure)

(nc for b if same mistake)

(b)-3 superficial discussion which misses point about relative volatility difference

common : denoted with "alpha! -3"

-6 completely superficial "discussion"

-1 wrong Q units

-2 incorrect feed specifications (should be sat'd liq at column pressure)

-3 obvious error in N or other serious problem with one case

-2 somewhat superficial discussion (whole range 0 to -6 available)

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Problem 3, answersN actual 27

D actual 3.25 feetH actual 40 feet

shell cost 675$ $/foot

bare shell cost 27,000$ $

installed shell cost 112,320$ $

Condenser area 241.3782332 ft^2bare cost 8,500$ $

installed cost 27,200$ $

Reboiler area 458.9966214 ft^2

bare cost 13,000$ $


Trays

bare trays 14,850$ $


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Problem 3 grading notesgrading notes

12 (a) -6 off a lot, unknown error

-3 somewhat off, unknown error

(range -2 to -6 available for this)

13 (b)

-1 not rounding diameter to 0.25 or 0.5 foot

-3 used wrong tray spacing

nc tray cost wrong if d wrong

-3 Cond area wrong

-3 Reb area wrong

-11 H calc only (+2 for shell H calc)

-2 H calc wrong (no or wrong multiplier for top & bottom)-2 transferred diameter wrong from (a)

-3 tray costs way off

-4 used Nideal

-2 exchanger area okay, but no costing

-3 used eff wrong

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Problem 4

1

1

12 2

5.5 0.5

N

R

= =

29 2

0.99

0.01

0

AD

BD

CD

D

x

x

x

36 3

0.17

0.82

0.01

AP

BP

CP

P

x

x

x

35 3

0

0.01

0.99

AW

BW

CW

W

x

x

x

3 18 2N =

100

0.35

0.30

0.35

sat'd liquid, 1.2 bar

AF

BF

CF

F

x

x

x

====

2

2

23 2

1.9 0.5

N

R

= =

Grading : simply +2 for each correct fill in (adds up to 26, but okay)

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Liquid-liquid extraction

Fundamentals

Kremser equations for L-L extraction

ChemCAD simulation of L-L extraction

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L-L extraction terminology

FeedF

S

R

E

Extraction

Solvent

Raffinate

Extract

Single theoretical

extraction stage

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Extraction schemes:

Crosscurrent extraction

1

2

3

S1

S2

S3

E1

E2

E3

F

R1

R2

R or R3

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Extraction schemes:

Countercurrent extraction

1

E2R1

2

E3

R2

3

SR or R3

E or E1F

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Solvent selection

Based on liquid liquid equilibrium

Want a solvent which reduces the activity of the

solute in the feed and is immiscible with themajor component of the feed stream

Handy chart assembled by L.A. Robbins (in

Handbook of Separation Techniques for

Chemical Engineers; P.A. Schweitzer, ed.;

McGraw-Hill 1979)

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Example:

Removal of acetone from a water stream

Acetone is a ketone (group 3 in table)

Groups 1 (acid, aromatic) and 6 (halogenatedhydrocarbons, not halogen saturated)

Can use ChemCAD to screen possible solvents

Look for phase separation (two liquid phasespresent) and higher activity of the solute in the

solvent phase relative to the feed phase

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Countercurrent liquid-liquid

extraction

Cautions & Caveats

Extractant selection

Preliminary design

CC5 for approximate S/Fmin

Kremser equations for number of stages

Detailed performance estimates (CC5)

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Cautions

Liquid-liquid extraction terminology and

working equations are very often in terms of

weight fractions even though the variablesx

andy are used.

Liquid-liquid extraction practice is *way*

ahead of design principles.

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Things we can make design

determinations about: Solvent choice(s)

Approximate S/Fmin values

Ntheoretical

Performance

Purity, recovery, etc. achievable

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The design problem

N

F, xf E, ye

R, xr S, ys

Usually F and all of the

composition variables

are known )or can be

estimated) leaving S and N

to be determined

Raffinate

Feed

Solvent

Extract

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Extractant selection

considerations Selectivity

Distribution coefficient

Insolubility of extraction solvent Recoverability

Density

Interfacial tension Chemical reactivity, vapor pressure, flammability

Availability of thermodynamic data for LLE

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Extraction solvent candidate

selection Use interaction table to determine possible

candidate solvents

Check that thermodynamic parameters areavailable for LLE prediction

In CC5 this means availability of UNIQUAC or

NRTL binary parameters for all three pairs

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Solvent candidate selection, cont.

With solvent candidate as component 1,

solute as component 2, and feed major

component as component 3 in a binodal plot Check for 2 phase region

Check selectivity of solvent candidate for solute

Check for relative insolubility of solventcandidate and feed major component

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Approximate S/Fmin determination

Build a CC5 simulation of the extractor using 10theoretical stages

Begin with S/F=1

If this runs, systematically reduce S/F until the extractor

either will not operate or until we meet our outlet specs;

the last working S/F value is our S/Fmin estimate; refine

this estimate until we have two significant figures

If this doesnt run, systematically increase S/F until the

extractor works; the first working S/F value is ourS/Fmin estimate; refine this estimate until we have two

significant figures

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Estimating Ntheoretical

Choose S/F = 1.2 S/Fmin

Use CC5 to estimate the slope(s) of the

equilibrium curve needed by the Kremserequations (a VLLE flash will do this nicely)

Use appropriate form of the Kremser

equation to estimate N

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Kremser equations, extraction

factor

F

S

m

=Solute free

mass flow

Solute free mass

flow

Slope of

equilcurve in

mass

fractions

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Kremser equations form 1

ln

111ln

1for

+

=

mY

X

m

Y

X

N

sr

s

f

Here X & Y are mass fractions

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Kremser equations form 2

1

1for

=

=

m

YX

mYXNs

r

sf

Here X & Y are mass fractions

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Liquid-liquid extraction example

For a stream of 20 mol% acetone in waterbeing extracted with 1,1,2-trichloroethane

at 25 deg C and 1 bar determine

S/Fmin

m at the composition for S/F = 1.2 S/Fmin

N for S/F = 1.2 S/Fmin and a mole percent of

acetone in the raffinate of 0.02% (that is a mol

fraction of 0.0002)

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Binodal plot

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Approximate S/Fmin determination

Build a CC5 simulation of the extractor using 10theoretical stages

Begin with S/F=1

If this runs, systematically reduce S/F until the extractor

either will not operate or we reach the desired outletconcentration; the last working S/F value is our S/Fminestimate; refine this estimate until we have twosignificant figures

If this doesnt run, systematically increase S/F until the

extractor works; the first working S/F value is ourS/Fmin estimate; refine this estimate until we have twosignificant figures

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1

1

2

3

4

S

F

E

R

Extractor Summary

Equip. No. 1

Name

No. of Stages: 10

1st feed stage 1

2nd feed stage 10

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Stream No. 1 2 3 4

Stream Name S F E R

Temp C 25.0000* 25.0000* 25.0000 24.9997Pres bar 1.0000* 1.0000* 1.0000 1.0000

Enth MJ/hr -1812.7 -7234.9 -4277.2 -4770.4

Vapor mole fraction 0.00000 0.00000 0.00000 0.00000

Total kmol/hr 10.0000 26.0000 19.2980 16.7020

Total kg/hr 1334.0401 676.7280 1708.4221 302.3460

Total std L m3/h 0.9179 0.7530 1.3691 0.3019

Total std V m3/h 224.14 582.75 432.54 374.35Component mole fractions

Acetone 0.000000 0.200000 0.269458 0.000000

Water 0.000000 0.800000 0.213010 0.999243

1,1,2-3Cl-Ethane 1.000000 0.000000 0.517533 0.000757

( )feedkg

solventkg971.1feedmol

solventmols385.0min

=FS

Documents

11-11-02 slides