Chapter 4: Absorbers Rules of Thumb for Chemical Engineers, 5th Edition by Stephen Hall
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Problem Statement Calculate the height of a transfer unit, the number of transfer units, and the required packing depth for a packed-bed absorber. Inputs General Column operating pressure Temperature Excess L to G ratio Gas Stream In Name Flow Primary component Molecular weight Solute component Molecular weight Concentration (volume fraction), y1
atm F
lb/h
1.00 70.00 0.75
Mix Tank Vent 7,540.00 Air 28.96 CH3COCH3, acetone 58.08 0.01500
Gas Stream Out Desired solute concentration, y2 Liquid Stream Primary component Molecular weight Solute component Concentration (mole fraction), x2 Density Temperature Column Parameters Diameter Packing type Volume coefficient, gas KGA
0.00015
water 18.02 CH3COCH3, acetone 62.30 70.00
lb/ft3 F
ft
3.28 Intalox Saddles, ceramic
lb-mol/(h-ft3-atm)
20.00
Data Henry's Law constant Pressure Column cross sectional area (empty) Gas molar rate, G1
atm atm ft2 lb-mol/h
Calculations Equilibrium concentration, solute in liquid x1* Step 1
1.59 1.00 8.45 258.40
0.00943
Step 2
Slope of equilibrium curve, (L/G)min
Step 3
Liquid rate, L2
lb-mol/h
711.95
Step 4
Exit rate, gas, G2
lb-mol/h
254.56
1.5744
Step 5
Moles Liquid effluent concentration, x1
0.0014
Liquid effluent molar flow rate, L1
715.79
Step 6
Gas molar velocity, Gm
Step 7
Vapor equil at bottom, y1*
3.84
lb-mol/ft2-h
0.0022
Vapor equil at top, y2* Step 8
30.34 -
LM Driving Force, (y - y*)LM
0.00284
LM Driving Force, (1 - y)*LM
0.99137
Step 9
Number of gas phase units, NOG
Step 10 Step 11
Height gas phase unit, HOG Packed height, Z
Other calculations Gas density
5.23 ft ft
lb/ft3
1.53 8.00 2.44 0.0749
Superficial gas velocity, UG
ft/s
3.31
Superficial liquid velocity, UL
gpm/ft2
3.04
Gas capacity factor, CV Liquid capacity factor, CL Liquid flow rate
ft/s gpm/ft2 lb/h gpm
0.11 3.04 12,829.35 25.67
Liquid molar velocity, Lm
lb-mol/ft2-h
84.22
3.56
Result
Concentration in Gas
Absorber Performance 0.01600 0.01400 0.01200 0.01000 Operating Line Minimum L/G Line
0.00800
Equilibrium Line
0.00600 0.00400 0.00200 0
Material Balance L2
0.0010.0020.0030.0040.0050.0060.0070.0080.009 0.01 Concentration of Solute in Liquid
711.95
G2
254.56
x2
y2
-
y2*
0.0002 -
Result
G1
258.40
L1
715.79
y1
0.0150
x1
0.0014
y1*
0.0022
x1*
0.0094
Equilibrium Line x y 0.001179014 0.00188 0.002358028 0.00375 0.003537042 0.00563 0.004716056 0.00750 0.005895070 0.00938 0.007074084 0.01125 0.008253099 0.01313 0.009432113 0.01500
Operating Line x y 0 0.00015 0.001404592 0.015
Minimum Flow Line x 0.00943
Operating Line Minimum L/G Line Equilibrium Line
Minimum Flow Line y 0.00015 0.01500
Conversion Factors Pressure Units Convert to atm atm 1 bar(a) 0.986923 in Hg(g) 0.033421 in H2O(g) 0.00246 kPa(g) 0.009869 mm Hg(g) 0.001316 Pa(g) 9.87E-06 psig 0.068046 torr(g) 0.001316
Relative KGA for Various Packings Super Intal Intalox Sad Hy-Pak, me Pall Rings, Pall Rings, Maspac, pla Tellerettes, Raschig Ri
1.00 0.94 1.11 1.06 0.97 1.00 1.19 0.78
Data Solute Temperature Henry's Law constant Temperature factor Base Temperature Calculations Temperature Henry's Law constant, temp adj
C atm K
K atm
CH3COCH3, acetone 21.11 1.95000 4,600.00 298.15
294.26 1.59
Henry's Law constant, KH x ca = pg where KH = Henry's law constant at 298.15 K, atm per mole fraction ca = concentration in liquid phase pg = partial pressure in gas phase DeltaH/R = temperature dependence of KH Source: R. Sander(1999) Compilation of Henry's Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry http://www.henrys-law.org Substance NH3, ammonia HBr, hydrogen bromide HCl, hydrogen chloride H2S, hydrogen sulfide SO2, sulfur dioxide C2H5OH, ethanol CH3COOH, acetic acid CH3COCH3, acetone C3H3N, acrylonitrile Cl2, chlorine HCHO, formaldehyde Example 1
KH
DeltaH/R 0.937 77.9 0.03
4100 6100 0
553
2100
46.1
3100
0.3
6500
0.011
6300
1.95
4600
5
2800
608 0.017 1.75
2500 6800 0
Importance in Environmental Chemistry (Version 3)
MW 17 80.91 36.5 34.08 64.06 46.07 60.05 58.08 53.1 70.9 30.03 32
