PED- III
Packed_Bed_absorber
DESIGN OF PACKED BED ABSORBERSTEP 1: CALCULATE Yb, Yt , Xt Yb = kmol of GAS ABSORBED / kmol of inerts
(at bottom)
Yt = kmol of GAS ABSORBED / kmol of inerts
(at top)
STEP 2: CALCULATION OF FLOW RATES: From the V-L-E graph drawn, calculate (Lm/Gm) min (Lm/Gm)actual = (1.1 to 1.5 times) (Lm/Gm)min From (Lm/Gm)actual, calculate (Lm)actual {(Gm)actual is given in question or can be calculated from the question} Let us denote (Lm)actual = Lm and (Gm)actual = Gm Inert Mass Balance: Gm(Yb – Yt) = Lm(Xb – Xt)
(from this eqn. calculate Xb)
Now, calculate avg. mol. Wt (at top and bottom of gas and liquid) Calculate the following values of molar flow rates (kmol/time):Gm, b = Gm’(1+ Yb)
Gm, t= Gm’(1+ Yt)
Lm, b = Lm’(1+ Xb)
Lm, b = Lm’(1+ Xt)
From above values calculate Gb, Gt , Lb , Lt by multiplying respective avg. mol. Wts. Calculate densities, if not given in question.
STEP 3: CALCULATION OF TOWER HEIGHT: Calculate {L/G x (ρg/ρL)1/2} for top and bottom and select the greater among the two. Now, from fig 5.33(below) we get the value of
Where, G =Superficial mass flow rate of the gas kg/s.m2 ρL and ρg=liquid and gas density, kg/m3 µ = liquid viscosity, cP
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PED- III
Packed_Bed_absorber
The value of Fp {packing factor} is take given with packing substance details or see table. So, we substitute all the other values and obtain the value of Gas superficial velocity, Gf Gas flow rate of bottom is fixed so that the cross section can be calculated. For this we have to operate below the flooding limit, thus ‘G’ we choose should be 60-85% of Gf . Then, operating velocity G = 0.80Gf C.S. area of absorber, Ac = Gb or Gt /G
(metre)
{depends on step-3}
From here calculate the diameter of tower Dc. CHECK – Dc/dp > 10 ; for satisfactory design (dp – given with question)
STEP 4: DEGREE OF WETTING: We have to calculate the degree of wetting rate: Lw ={L/ (Ac ρL a)}
m3/m-s
[L Lb or Lt ; depends on step-3]
(This value must be > 0.85 ft3/ft-hr)
2
PED- III
Packed_Bed_absorber
STEP 5: PRESSURE DROP CALCULATION: ∆P = C2 x [10]C3Utl x ρg x (Utg)2 Where, ∆P = mm H2O/m packing ρg = gas density ,kg/m3 C2 and C3 = constants given in question. Utg and Utl are superficial velocities of gas and liquid respectively Utg = {G / ρg AC} Utl = {G / ρl AC}
STEP 6: TOWER HEIGHT Calculations: Z = HOG x NOG Where, Z, height of tower. HOG: height of overall gas transfer unit. NOG: number of overall gas transfer unit. HOG = HG + (mGm/Lm) HL Where, m = slope of linear equilibrium relationship.
Cornell’s Method: The generalized equation of Cornell et al, for gas phase transfer units, HG
HG = (0.029 D1.11 Z0.33 Scg0.5) / (L f1 f2 f3)0.5
{ will be given}
Dg , diffusion coefficient for mixture of gases (given) Scg = µg / (ρg Dg)
f1, f2 & f3 - given in question L = Lt / Ac We will get an expression in the form of HG = ……….. x z0.33 The generalized equation of Cornell et al, for liquid phase transfer units, HL
(
)(
)
(
)
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PED- III
Packed_Bed_absorber
Where, C: correction factor (0.48 for CO2 question) : Correlation parameter =liquid viscosity, Pa.s =Liquid density, kg/m3 DL =liquid–diffusion coefficient, m2/s {given} Z =height of packing, m We get the expression as HL = ………… x z0.15 Now substitute the values of HG and HL in the equation HOG = HG + (mGm/Lm) HL in which, Gm/Lm = {(Gm/Lm)t + (Gm/Lm)b } / 2
NOG Calculations: ∫
(
)
From the graph calculate Y* and fill in the following table: Y
Y*
1/(Y-Y*)
Plot a graph of 1/(Y – Y*) vs Y NOG = {Area under the curve from Yt to Yb}
(
)
So we have, Height of the tower, Z = HOG x NOG
= [HG + (mGm/Lm)HL] NOG
4
Packed_Bed_absorber
PED- III
5
