Air containing 5 mol% NH3 at a total flow rate of 40 lbmol/hr enters a packed co
ID: 527473 • Letter: A
Question
Air containing 5 mol% NH3 at a total flow rate of 40 lbmol/hr enters a packed column operating at 25 degree C and 1 atm, where 90% of the ammonia is scrubbed by 3000 lb/hr of water flowing countercurrently. Use the generalized chart with packing factor FP to estimate the superficial gas flooding velocity (ft/s), the column inside diameter (in) for operation at 70% of flooding, and the pressure drop per foot of packing (in H_2 O/ft) for one-inch ceramic Raschig rings (F_p = 179 ft^2/ft^3). b) Calculate the number of transfer units and the length of the tower (ft) required. The following equilibrium data are available for the ammonia-water system at 25 degree C and 1 atm.Explanation / Answer
Ans:- a) Given molar mass of ammonia NH3 = 17
molar mass of air = 32
Average molecular weight = 0.9x17+0.1x32
= 18.5g
gas flow rate = 40 x 18.5/3000
= 0.2466kg
Density of the gas = total pressure x Average molecular weight / RT
= 101.3 x 18.5 /8.314x298
=0.756kg/m3
volumetric flow rate = gas flow rate / density
= 0.2466/0.756
= 0.3275m3/s
Now total NH3 absorbed = 40x0.1x0.9x17
= 61.2kg/hr
liquid flow rate = 40x18+61.2/3000
= 0.2604kg/s
density of the liquid = 1000kg/m3
flow parameter = volumetric flow rate/ gas flow rate (density of gas/liquid flow rate)0.5
= 0.3275/0.2466(0.756/0.2604)0.5
= 2.24
Now n = exp(flow parameter)
=exp(2.24)
=9.39
viscocity of the liquid= 0.84x10-3 , here liquid = water
Packing factor Fp = 179ft2/ft3
superficial gas flooding velocity V= (n/Fp x viscocity)1/2
= (9.94/ 179x0.84x10-3 )1/2
= 0.41m/s
From the generalised chart pressure drop P = (Fp)0.7
= (179)0.7
= 16054Pa
column diameter D = 4 x volumetric flow rate / 0.7xVxflow parameter)
= 4x 0.3275/ 0.7x0.41
= 4.67m
Ans b) Let A be the amount
A = liquid flow rate / flow parameter x gas flow rate
= 0.2604/2.24x0.2466
= 0.471
Total xNH3 = 0.02+0.04+0.06+0.07+0.082 =0.272
Total yNH3 =0.025+0.08+0.021+0.26+0.40
= 0.786
Number of transit unit = 1/1-Aln[flowparameter(XNH3 ) -YNH3] / 1-1/A (1-(flow parameter XNH3) -YNH3]
=1/1-0.471ln(2.24x0.272 -0.786)/1-1/0.47(1-2.24x0.272-0.786)
= 5.236
length of the tower = height of the system/ number of transit unit
= 1/5.236
=19.09ft
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