By

Final Year Chem. Engg. (Roll no:11-20)

U.I.C.T,Mumbai-400019

• Same packing as used in G-L operations

• Advantage of using Packings

• Material of Packing to be used

• The Problem statement:Flow rate of organic stream= 1m3/hrImpurity in inlet organic stream=10000ppmDesired impurity in exit stream=500ppm

Continuous phase: SI units

Flow rate(Qc) 1 35.315

Viscosity (µc) 0.005 1.211

Density(ρc) 900 56.07

Diffusivity(Dc) 0.000000001 0.0000387

Concentration of impurity in feed(Cc1) 10000

Dispersed phase SI units

Flow rate(Qd) 5 176.575

Viscosity (µd) 0.001 0.2422

Density(ρd) 1000 62.3

Diffusivity(Dd) 0.000000001 0.0000387

Interfacial tension(γ) 0.025 715000

Equilibrium distribution coefficient (Cd/Cc) 5 5

Concentration of impurity at inlet(Cd1) 0 0

• We assume that the dispersed and the continuous phases are in plug flow

• We find the minimum value of Ud/Uc• Then we assume different values for Ud/Uc• Calculate hold up at flooding using

• εf={[(Ud/Uc)2+8 (Ud/Uc)]0.5-3 (Ud/Uc)}/[4*(1- Ud/Uc)]

• Assume a certain percentage of flooding hold up as operating hold up and hence calculate ε

• We find the terminal velocity of a drop using

• eUo=C(a*ρc/(e3*g*Δ ρ)) (-0.5) formula given in the book by Degallson and Laddha where C=0.637

• Then we calculate Ud using the correlation for slip velocity

• Find diameter of the column:• D=((Qc/Uc)*(4/3.142))0.5

• Drop diameter is found using • d=1.6(γ/(ρc-ρd)g) 0.5

• Size of Raschig rings were taken as 1”,0.75” and 0.5”.

• Overall Height of transfer unit:Koc.a=0.06* φ*(1- φ)/[(a*ρc/(g*e 3 *Δρ))0.5*(γ/(Δρ*g)) 0.5 *{(Sc)c 0.5 +(Sc)d 0.5 /m}]

(This corellation is for packing size greater than drop size)

• [HTU oc]plug flow = Uc/Koc.a

• [NTU oc]plug flow=(Cc1-Cc2)/(ΔC)LM

• Height of column:

Zt= [HTU oc]plug flow * [NTU oc]plug flow

• Distributor design: We take nozzle velocity = 0.5*eUo

Nozzle diameter value should be comparable with droplet diameter value. Hence, we take nozzle diameter= 6 mm.

No of orifices = Qd/( Area of nozzle*Vn)

Sample Calculations

• Overall mass balance• Qo*ρo*(10,000-500)=Qa*ρa*(50,000-0)• 1*900*9500= Qa*1000*50000• Thus

Qa= .9*9500/50000 = 0.171• Thus, the flow rate ratio (aq:org) or

velocity ratio should be > 0.171

Sample calculations- continued• Consider Raschig ring packings of size 1”.• Let the operating holdup=60% of holdup at flooding.• Let Ud/Uc=1.1.• εf = 0.3403• Operating holdup = 0.2042• Terminal velocity = 0.03191 m/s• By slip velocity relation, Ud= 0.00385 m/s• Uc= 0.0035 m/s• Column diameter: 0.318m• Drop diameter: 8 mm• Koc.a= 12.407• HTU=3.327 ft=1.01m• NTU=3.429• Column height=11.41 ft = 3.477 m• No of orifices for the distributor (orifice diameter=6 mm)= 70(approx)

Variation with Ud/Uc for 70% flooding - 1 inch raschig ring              

Ud/Uc 0.9 1.1 1.3 1.5 1.7 1.9 2

ε0.227

8 0.24 0.247 0.254 0.26 0.265 0.267

HTU OC 1.259 1.07 0.924 0.818 0.734 0.666 0.636

[(NTU)plug flow]continuous7545.

5 89201027

6 116181295

01427

31493

2

Height of column (based on plug flow),(Zt)plug4.465

5 3.65 3.1 2.699 2.392 2.15 2.047

D (column dia) m0.274

2 0.29 0.311 0.328 0.344 0.358 0.366

Variation with nominal diameter of packing for 70% flooding - for Ud/Uc=1.5        

  1 inch

0.75 inches  

0.5 inches

D (column dia) m 0.328 0.37   0.442

Height vs Flow ratio for 50%flooding

02468

10121416

0 0.5 1 1.5 2 2.5

flow ratio

Hei

ght

Height vs Flow ratio

60% flooding Height vs flow ratio

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Height of column(based on plugflow),(Zt)plug

70% flooding Height vs fow ratio

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70% flooding Heightvs fow ratio

Diameter vs flow ratio 50% flooding

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Diameter vs flow ratio50% flooding

Diameter vs flow ratio 60% flooding

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Diameter vs flow ratio60% flooding

Diameter vs flow ratio at 70% flooding

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Diameter vs flow ratioat 70% flooding

H/D vs flow ratio70% flooding

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H/D vs flow ratio at 60% flooding

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H/D vs flow ratio at 50% flooding

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Observations

• With increase in Ud/Uc, height of column decreases, diameter increases.

• The diameter of the column increases when packings of smaller nominal diameter are used.

• With increase in %flooding the total height of the column decreases.