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

Answer 1

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