Question

1000 SFCM of air enters an ammonium nitrate prilling tower at 15 C and 2 atm pressure with a relative humidity of 80%. Calculate the entering water flow in lbm/hr.

Answer 1

Vapor pressure of water at 15 deg.c ( From vapor pressure data)=0.0168 atm

Relative humidity=100*( partial pressure of water vapor/ Vapor pressure of water at 15 deg.c)

Given relative humidity= 80%

Hence 0.8= partial pressure of water vapor/0.0168

Partial pressure of water vapor =0.8*0.0168=0.01344 atm

Partial pressure of dry air =2-0.01344= 1.98656 atm

Partial pressure of water vapour/ toal pressure= moles of water vapour/ Total moles of air (1)

Total Moles of air can be calculated from the flow rate of air at standard conditions which is 1000 SCFM= 1000 standard cubic feet/minute=( 1000/60) ft3/hr=16.67 ft3/hr

One lb mole of any gas at standard conditions occupies 359 ft3

Hence 16.67 ft3/ hr correspond to 16.67/359 lbmoles/hr =0.046435 lb moles/hr of total air (water vapour + dry air)

From 1

0.01344/2= moles of water vapour/moles of air = moles of water vapour/0.04643

Moles of water vapor =0.046435*0.01344/2=0.000312lb moles of water vapour/hr

Mass of water vapour =moles of water vapour* Molecular weight= 0.000312* 18 lb/hr=0.005617 lb/hr