Separation technology 424105 - Åbo Akademiusers.abo.fi/rzevenho/SEP1617-OH2.pdf · 2016-10-11 · / Separation technology 424105 2. Absorption/desorption processer / processes

Separationsteknik/ Separation technology 424105

2. Absorption/desorption processer / processes

Ron ZevenhovenÅbo Akademi University

Thermal and Flow Engineering Laboratory/ Värme- och strömningsteknik

tel. 3223 ; [email protected]

2.1 Absorption / strippingof dilute mixtures

Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo

oktober 2016Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo 3/24

Absorption & stripping Absorption (or: gas absorption, gas scrubbing, gas

washing): contacting a gas mixture with a liquid (the ”absorbent” or ”solvent”) in order to selectively dissolveone (or more) components from the gas in the liquid

Stripping (or: desorption) is the opposite process: a liquid mixture is contacted with a gas in order to selectively remove one (or more) components from the liquid to the gas

Often absorbers and strippers are coupled, as to allowfor recovery (and re-use) of solvent.

The type of equipment used depends on the relative amounts of liquid and gas mass streams

4/24

CO2 stripping

Picture: SA05

Natural gas + CO2

Liquid solventfor example

alkanol amine

Natural gas

Liquid solvent+ CO2

Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo 5/24

Process example and equipment

↑ Typical absorption process for acetone

→ Industrial equipment: (a) tray tower; (b) packed column; (c) spray tower; (d) bubble colum Pictures: SH06


A performated tray column

Especially suitable if total gas and liquid mass streams are roughly the same and are more or less constant

tray

gas

gas

liquid

liquid

liquid

gasPicture: WK92

2.2 Counting equilibrium stages in x,ydiagrams



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Vapour-liquid stream relationships

(a) Mass balance → working line in x,y plot(b) Equilibrium → equilibrium line or curve in x,y plot


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Absorber/stripper x,y diagram

Continuous steady-state process in a countercurrentcascade with equilibrium stages:

absorber

stripper


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Graphical determination of the numberof equilibrium stages

(a) Absorberx1 = liquid at equilibrium

with gas y = y1

y2 = gas that exchanges mass with liquid x = x1


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Graphical determination of the numberof equilibrium stages

(b) Stripper

2.3 Wet gas drying using an absorption / stripping process



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Absorber: wet gas drying /1

Drying of wet methanewith di-ethylene glycol(DEG)

Regeneration of wetDEG with dry N2

DEG boiling point at 1 bar is 245°C;

DEG molar mass= 106 kg/kmol

Dry DEG

Wet DEG

ABSORPTION

DESORPTION

Wet CH4

Dry CH4 Dry N2

Wet N2Picture: WK92

V

V´

L


Equilibriumstages andnumbering

Balance for water for thetop section

L0x0-V1y1 = LnXn-Vn+1yn+1

if x and y are small, thenL and V are ~ constant

V(yn+1-y1) = L(xn-x0)

and for any tray from the top

V(y-y1) = L(x-x0)Picture: WK92

Åbo Akademi - kemiteknik - Värme- och strömningsteknik Biskopsgatan 8, 20500 Åbo 14


Absorber:wet gas drying /3

example:H2O in DEG at 72°C, 40 barlow concentrationHc for H2O in DEG = 0.02 MPa

gives equilibrium constantK = yH2O/xH2O = HcH2O / ptotal

= 0.02 MPa/4 MPa = 0.005

Picture: WK92

Hen

ry c

oef

fici

ents

of

seve

ralg

ases

in w

ater

and

fo

r w

ater

in D

EG


Absorber:wet gas drying /4

process data:H2O in wet gas = yN+1 = 0.001 mol/mol H2O in dry gas = y1= 2×10-4 mol/molH2O in “dry” DEG = x0 = 2×10-2 mol/mol, and L/V = 0.01 (kmol/s)/(kmol/s)Total mass balance gives:(xN – x0)· L = (yN+1- y1)· V → xN = 0.1Separation, absorption factor A = L/KV = 2

Picture: after SH06

For the first stage,

equilibrium between exit

gas and exit vapour

implies K = y1 / x1 →

x1 = y1 / K = 2×10-4 / 0.005

gives x1 = 0.04


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Working line V· (y-y1) = L· (x-x0) and equilibrium lineallow for calculation of theoretical equilibrium stages

Working line requires V/L ratio + one point (xi, yi+1)

equilibriumline y = Kx

working liney = y1 + L/V(x-x0)

x1

y1

x0 = 0.02 xn = 0.10

y1 = 2×10-4 yn+1 = 0.001

Note: Kremser equation: f = 0.2, S = 2 → N = 1.6Solvent is far from clean: xn = 5x0 More accurate A = (L/KV) ·(1-x0 /xN) = 1.6 → N = 1.95

Picture: WK92


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Reducing the amount of solvent (liquid) by 50% gives L/V = 5×10-3 → Separation factor A → A = 1

Equilibriumline

Operating line


Picture: WK92


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Reducing the amount of solvent (liquid) further to L/V = 3×10-3 → Separation factor A → A = 0.6

The separation cannot be accomplished

Equilibriumline

Operating line

Picture: WK92


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The minimum amount of liquid, or (L/V)min (requiresN = ∞ stages) can be found by crossing the operating line with the equilibrium line at outgoing gas specification yN+1.

Equilibriumline

Operating line

This givesAmin = 0.888for this caseand N → ∞

Picture: WK92


21/24

Stripper: wet gas drying /9

x0

xN

y´0

yN+1

y1

For the desorption, the wet DEG is stripped with dry nitrogen at 120°C, 0.1 MPa.

For H2O vapour in DEG under these conditions, Hc H2O in DEG = 0.2 MPa (see diagram p. 15)

This gives equilibrium constantK = yH2O/xH2O = HcH2O / ptotal = 0.2 MPa/0.1 MPa = 2

How many stages for y´0 = 0 in the dry N2, and V´/L = 2½×(V´/L)min ?

Picture: WK92

V

V´

L


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Stripper: wet gas drying /10

L/V´maximum = V´/Lminimum = 0.4 → × 2½ gives V´/L = 1 Draw up operating line and count stages: N = 2

Equilibriumline

Operating line

Note: operating line now underequilibrium line

Picture: WK92

25,1AC length

AB length1

0,1'

5,2'

4,0'

5,2''

'

minmin

11

0

max

N

L

V

L

V

L

V

xx

yy

V

L

N

N


Absorber + stripper wet gas drying /11

Finally the two units must be ”matched” to obtain a working set of separators

The relative streams of gases (the ratio V´/V) for N2 and CH4 can now be calculated:V´/V = (V´/L)×(L/V) = 0.01 = VN2/VCH4 (mol/mol)

The relative volume streams QCH4 (m3/s) and QN2 (m3/s) can be found from (ideal gas) densities ρ = p/RT (mol/m3) :

QCH4/QN2 = (pN2/pCH4)×(TCH4/TN2)×(VCH4/VN2)= 0.025×1.14×100 = 2.85 m3/m3


Sources SA05: Socolow, R.H. ”Can we bury global warming” Scientific American, July

2005, 49-55 SH06 J.D. Seader, E.J Henley ”Separation process principles” John Wiley, 2nd

edition (2006) Chapter 6 § 1 – 4 **

T68 R.E. Treybal ”Mass transfer operations” McGraw-Hill 2nd edition (1968)

WK92 J.A. Wesselingh, H.H. Kleizen ”Separation processes” (in Dutch: Scheidingsprocessen) Delft University Press (1992)

ÖS97 G. Öhman, H. Saxén ”Transportprocesser” Åbo Akademi Värmeteknik (1997) §3.6

Ö96 G. Öhman ”Massöverföring” Åbo Akademi Värmeteknik (1996) §4.2 – 4.5

Available on - line (28 Mb): http://users.abo.fi/rzevenho/MOF-GO96.pdf

Recommended: (cheap ~20 US$ reprint of Nov. 2013 of book from 1980): Separation Processes: Second Edition Paperback by C. J. King

* * See ÅA course library

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Separation technology 424105 - Åbo Akademiusers.abo.fi/rzevenho/SEP1617-OH2.pdf · 2016-10-11 · / Separation technology 424105 2. Absorption/desorption processer / processes