Magnesium sulfate-heptahydrate is to be dried from 25% to 5.0% moisture (on dry basis) in a co-current adiabatic rotary dryer at a rate of 8000 lb/h of dry solids. The heating air enters the dryers at 248
oF with a wet bulb temperature of 118
oF (absolute humidity =0.04 lb water vapour/lb of dry air). The solids enter the dryer at 80oF. The maximum allowable mass velocity of the inlet air is 1800 lb/ft
2.h (design air mass velocity). The heat transfer number for co-current operation can be expressed as:
Nt= ln([TAi-TSi]/[TA0-TS0])
using the number of heat transfer units Nt of 2.0,
a) Estimate the diameter of the dryer
b) estimate the length of the dryer
c) the dryer is already sized and is being used, if there is an upset causing an increase in the air flow rate by 20%, what would the temperature of air existing the dryer be approximately, assume that the drying is still under constant rate and the moisture content tin the solid existing the dryer doesn't change significantly?
d) In reality, as the air mass flowrate increases, the moisture content of the solid at exist decreases, as compared to that at the design air mass flowrate. Given a short and precise explanation for this observation.
Given data:
Specific heat of water: 1 BTU/LB.
oF
Specific heat of water vapour: 0.45 BTU/LB.
oF
Latent heat of water evaporaton at 118
oF: 1025 BTU/LB
Specific heat of dry air only: 0.24 BTU/LB.
oF
Specific heat of dry solid: 0.58 BTU/LB.
oF
Equilibrium moisture content of solid=0%
Critical total moisture content of solid (on dry basis) is 3%
CHE 318 - Midterm Fall 2014
Quick Reply
