SCRUBBER DESIGN (PACKED COLUMN)
Prepared by : Checked by : Date :
### Input Data Packing type Packing size Packing MOC Gas pr. Drop / m bed Total packing height
Column Tag No. Job No. Client Project
: : : :
Stream
:
= Intallox Saddles = 25 mm = PP = 15 mmWC / m packing height = 3.2 m (including all packed beds)
Gas / Vapour Properties Gas / Air flow rate = =
1000 kg/h 0.2778 kg/s
Gas pressure at entry Gas temperature at entry Gas / Air mol weight
1.0000 atm 30.00 oC 29
= = =
Component to be scrubbed Component Name = Component flow rate = % comp. in air/gas = Molecular weight of comp. =
= 0.0035000 Ns/m2
Packing factor, F p
=
Charac. Packing Factor,Cf = Conversion factor, J =
147.1 (N/m2)/m
0 m3/h 0 m3/s
=
303.00 oK
(presumed) / (given by client) / (by process cal.)
Liquid / Scrubbing media Properties Scrubbing media = 20% NaOH Liquid flow rate, L = 77 kg/h = 0.0214 kg/s Liquid Density, L = 1100 kg/m3 Liquid Viscosity, µL
=
OR
=
HCL Vap 70 Kg/h 6 % (v/v) 36.5
HCL Vap.
Conversion : 3.5 Cp
=
0.00350000 Ns/m2
21 m-1 33 Ref. Table 6.3, Characterstics of Random packings 1.0 factor for adequate liquid distribution & irrigation across the bed
Sheet 1 of 11
Calculations TO CALCULATE COLUMN DIAMETER Since larger flow quantities are at the bottom for an absorber, the diameter will be chosen to accommodate the bottom conditions. To calculate Gas density Avg. molecular weight =
29.45 Kg / Kmol
If gas flow rate is given in kg/h
If gas flow rate is given in m3/h
Gas in =
Gas in
0.01 Kmol/s kmol = mass / mol wt = (kmol/s) x T in kelvin x 1.0 atm x 22.4 273 pr. In atm 1 = 0.234499 m3/s
= (m3/s) x
273 x pr. in atm x T in kelvin 1.0 atm
= =
1 22.4
0 Kmol/s 0 Kg/s mass = mol wt x kmol
Select vol. flow rate and mass flow rate from above, Selected mass flow rate = 0.28 Kg/s Selected vol. Flow rate = 0.23 m3/s Selected molar flow rate = 0.01 Kmol/s Therefore, gas density
=
1.1846 Kg/m3
(mass flow rate / vol. Flow rate)
To find L', G' and Tower c/s area Assuming essentially complete absorbtion, Component removed = 0.0207 Kg/s Liquid leaving = 0.0420 Kg/s L'
G
G'
L
0.5
=
Using
0.00497
G' 2 Cf µL0.1 J ( G
-L
(molar flow rate x % comp. x mol. Wt.) (Inlet liquid flow rate + comp. Removed)
0.00497
as ordinate,
Refer fig.6.34 using a gas pressure drop of
=
0.04 (from graph)
=
0.04
) gc G
Therefore, G'
G
(
L
Cf µ
--
0.1 L
) gc
G
0.5
J
=
1.6665 Kg / m2.s
Tower c/s area
=
0.1667 m2
( c/s area = mass flow rate / G' )
Tower diameter
= =
0.4607 m 500 mm
=
Corresponding c/s area
=
0.1963 m2
460.7 mm
TO ESTIMATE POWER REQUIREMENT
Sheet 2 of 11
147.1
(N/m2)/m
Efficiency of fan / blower
=
60 %
assumed / given
To calculate pressure drop Pressure drop for irrigated = packing
470.72 N/m2
(pressure drop per m packing x total ht. of packing)
For dry packing, O/L Gas flow rate, G' O/L Gas pressure Gas density, G
= 1.3095 Kg / m2.s (Gas inlet flow rate - Component removed) / c/s area = 100854.28 N/m2 (subtracting pressure drop across packing)
CD
=
Delta P
= CD
=
gas mol wt. x 273 x 22.41m3/Kmol T in kelvin = 1.1605 Kg/m3
Z
96.7
gas o/l pr. 101330
Ref. Table 6.3, Characterstics of Random packings
G' 2 G
=
142.89 N/m2
Pressure drop for packing =
613.61 N/m2
(irrigated packing + dry packing)
Pressure drop for internals = =
25 mmWC 245.17 N/m2
(packing supports and liquid distributors)
Gas velocity Inlet expansion & outlet contraction losses
= 7.5 m/s = 1.5 x Velocity heads = 42.19 N m / Kg = 49.97 N/m2 908.75 N/m2
=
1.5 x (V2 / 2g) (divide by density)
Total pressure drop
=
Fan power output
= pressure drop,N/m 2 x (gas in - component removed) Kg/s O/L gas density, Kg/m 3 = 201.35 N .m / s = 0.20 kW
Power for fan motor
= =
0.34 kW 0.45 hp
(packing + internals + losses)
(fan power output / motor efficiency)
Sheet 3 of 11
COLUMN DIAMETER / HYDRAULIC CHECK Liq.-Vap. Flow factor, F LV
= (L / V) x ( = 0.0025
V
/
Design for an initial pressure drop of From K4 v/s FLV,
)
L
15
mm H2O /m packing
K4
=
0.85
K4 at flooding
=
6.50
Trial % flooding
= ( =
(K4 / K4 at flooding) 36.1620
Gas mass flow rate, V m
=
K4 .
V
(
L
13.1 Fp (µL / = Trial column c/s area (Trial As)
Trial column dia., D
=
--
V
)
) x 100 (1/2)
)0.1
L
3.7763 kg/m2.s V / Vm
=
0.0736 m2
=
0.3060 m
D = (4/pi) x Trial As
Round off 'D' to nearest standard size Therefore, D = 0.500 m
Column C/S area, A s
=
% flooding
=
0.1963 m2
13.5472
As = (pi/4) x D2
% flooding = Trial % flooding x (Trial A s / As)
Conclusion Generally packed towers are designed for 50% -- 85% flooding. If flooding is to be reduced, (i) Select larger packing size and repeat the above steps. OR (ii) Increase the column diameter and repeat the above steps.
Sheet 4 of 11
HETP PREDICTION Norton's Correlation : ln HETP = n - 0.187 ln + 0.213 ln µ Applicable when, liquid phase surface tension > 4 dyne/cm & < 36 dyne/cm liquid viscosity > 0.08 cP & < 0.83 cP Conversion : Input Data 0.02 N/m = 18 dyne/cm Liquid-phase Surface Tension, = 20 dyne/cm Norton's Correlation Applicable Liquid Viscosity
=
3.5 cP
n
=
1.13080
ln HETP
=
0.84
HETP
= =
2.31 ft 0.7 m
Norton's Correlation NOT applicable
Calculation
For separations, less than 15 theoritical stages, a 20% design safety factor can be applied. Considering 20% safety factor, HETP =
0.85 m
For separations, requiring 15 to 25 theoritical stages, a 15% design safety factor can be applied. Considering 15% safety factor, HETP =
0.81 m
Sheet 5 of 11
Table 6.2 Constant for HETP Correlation
Ref.:: Random Packings and Packed Towers ---- Strigle
Ref. : : Chemical Engineering, Volume-6 , COULSON & RICHARDSON'S
Ref. : : Mass Transfer Operation : : Treybal
