A process for making small amounts of hydrogen by cracking ammonia is being cons
ID: 702934 • Letter: A
Question
A process for making small amounts of hydrogen by cracking ammonia is being considered, and residual uncracked ammonia is to be r removed from the resulting gas. Design a tray type absorption column to remove NH3 from a gas stream using water as solvent. The gas stream (flow rate = 0.7 kg/s) consists of H2 and N2 (in the molar ratio 3: 1) and 3% NH3 by volume. The ammonia will be removed by scrubbing the gas countercurrently with pure liquid water (flow rate = 6 kg/s) in a sieve-tray tower. The exiting gas stream should have 1% NH3. Assume that only NH3 can dissolve in water, and other gases (N2+H2) are inerts. Conditions at bottom of tower: Temperature, T = 303 K, and Pressure = 200 kPa.
Determine:
The number of stages (trays), tray spacing, and the diameter of the absorption column
Explanation / Answer
Inputs
General
Column operating pressure atm 1.97
Temperature F 85.70
Excess L to G ratio 0.75
Gas Stream In
Name Mix Tank Vent
Flow lb/h 5,652.00
Primary component Air
Molecular weight 28.96
Solute component NH3, ammonia
Molecular weight 17.00
Concentration (volume fraction), y1 0.03000
Gas Stream Out
Desired solute concentration, y2 0.01000
Liquid Stream
Primary component water
Molecular weight 18.02
Solute component NH3, ammonia
Concentration (mole fraction), x2 -
Density lb/ft3 62.30
Temperature F 70.00
Column Parameters
Diameter ft 3.28
Packing type Intalox Saddles, ceramic
Volume coefficient, gas KGA lb-mol/(h-ft3-atm) 20.00
Data
Henry's Law constant atm 1.17
Pressure atm 1.97
Column cross sectional area (empty) ft2 8.45
Gas molar rate, G1 lb-mol/h 199.28
Calculations
Step 1 Equilibrium concentration, solute in liquid x1* 0.02571
Step 2 Slope of equilibrium curve, (L/G)min 0.7779
Step 3 Liquid rate, L2 lb-mol/h 271.29
Step 4 Exit rate, gas, G2 lb-mol/h 195.26
Step 5 Moles 4.03
Liquid effluent concentration, x1 0.0037
Liquid effluent molar flow rate, L1 275.31
Step 6 Gas molar velocity, Gm lb-mol/ft2-h 23.33
Step 7 Vapor equil at bottom, y1* 0.0043
Vapor equil at top, y2* -
Step 8 LM Driving Force, (y - y*)LM 0.01663
LM Driving Force, (1 - y)*LM 0.98279
Step 9 Number of gas phase units, NOG 1.20
Step 10 Height gas phase unit, HOG ft 1.19
Step 11 Packed height, Z ft 1.43
Other calculations
Gas density lb/ft3 0.1433
Superficial gas velocity, UG ft/s 1.30
Superficial liquid velocity, UL gpm/ft2 1.16
Gas capacity factor, CV ft/s 0.06
Liquid capacity factor, CL gpm/ft2 1.16
Liquid flow rate lb/h 4,888.62
Liquid molar velocity, Lm lb-mol/ft2-h 32.09
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