Design of Spray column with
Chemical Reaction
Problem
To reduce the impurity (A) level in an organic
liquid from 10000ppm to 500ppm using an
aqueous solvent containing a solute (B) which
reacts with A following the rate equation,
rate = Kmn*[A]m * [B]n
Given data
•
•
•
•
•
•
•
Organic phase flow rate = 1m3/hr
ρaq =1000 kg/m3
ρorg = 900 kg/m3
µaq = 1 mPas
µorg = 5 mPas
σ = 0.025 N/m
Distribution coefficient = 5
Co-relations used
• u0 =
• dp =
• kd = (0.00375*u0)/(1+µd/µc)
• kc = 0.725* (dp*u0*ρ/µc)^-0.43*(µc/ρcD)^-0.58*(1-Φd)
Approach
• Case 1- The rate of chemical reaction is very
slow as compared to rate of mass transfer
• Case 2- The rate of mass transfer is
comparable to the rate of chemical reaction
• Case 3- The rate of chemical reaction is very
high as compared to rate of mass transfer
Case 1
• As the rate of chemical reaction is very low as
compared to rate of mass transfer the
concentration profile will be as below
Case 2
• When the rate of chemical reaction is
comparable to the rate of mass transfer
Case 3
• As the rate of chemical reaction is very high
the concentration of A in aqueous phase will
be zero
Assumption
• The dispersed phase mass transfer co-efficient
is very high as compared to continuous phase
mass transfer co-efficient
• Hence, resistance to mass transfer on the
dispersed side is neglected
Calculations
• Calculate minimum flow rate for aqueous
liquid from given data
• Assume flow rate of aqueous liquid higher
than the minimum value calculated
• Calculate u0
• Calculate dp
• Calculate Φf
• Assume Φd value ranging from 0.1 times Φf to
0.9 times Φf increasing it by 0.1 every time
• Hence calculate vc and vd
• From the values of vc and Qc we get the
column diameter.
• Then we calculate a = 6 Φd/d
• Then we find the no of orifices in the
distributor
• Then we find the values of kd and kc
• Then we find the relative rate of reaction as
regards to the rate of mass transfer
• Based on the relative values, we can find the
volume of the contactor and the height of the
contactor
Effect of water flow rate on volume
V vs vd
12
10
8
4
5
8
6
Poly. (4)
Poly. (5)
4
Poly. (8)
2
0
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
When the phases are interchanged
• When water is the continuous phase, due to
Morongani effect we consider only regimes 3
and 4
• Calculations similar to the previous case
• Continuous phase mass transfer is the
controlling mechanism
Effect of %flooding on Volume
0.014
0.012
0.01
1
0.008
2
3
Poly. (1)
0.006
Poly. (2)
Poly. (3)
0.004
0.002
0
0
10
20
30
40
50
60
70
80
90
100
Effect of %flooding on Height
0.09
0.08
0.07
0.06
1
0.05
2
Poly. (1)
0.04
Poly. (2)
0.03
0.02
0.01
0
0
10
20
30
40
50
60
70
80
90
100