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Design Of Distillation Column

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Distillation Column

A Distillation Column is used to separate a multicomponent liquid mixture into distillates and bottoms due to differences in their boiling points.

They are of following two types based upon construction.

Tray Column Packed Column

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Choice b/w Tray & Packed Column

Plate column are designed to handle wide range of liquid flow rates without flooding.

For large column heights, weight of the packed column is more than plate column.

Man holes will be provided for cleaning in tray Columns. In packed columns packing must be removed before cleaning.

When large temperature changes are involved as in the distillation operations tray column are often preferred.

Random-Packed Column generally not designed with the diameter larger than 1.5 m and diameters of commercial tray column is seldom less than 0.67m.

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Selection Of Tray Type

Sieve trays are selected due to following main reasons.

High capacity. Less pressure drop. High Efficiency . Lowest Cost per unit area than all

other types with the downcomer. Good flexibility in

operation(Turndown ratio).

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Designing Steps Of Distillation Column

Calculation of Minimum Reflux Ratio Rm. Calculation of optimum reflux ratio. Calculation of theoretical number of stages. Calculation of actual number of stages. Calculation of diameter of the column. Calculation of weeping point. Calculation of pressure drop. Calculation of the height of the column. Calculation of thickness of the shell & Head.

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FEED COMPOSITION

ComponentFeed in Kgs/hr

Feed in Fraction

Feed in kmoles

TDA (LK) 85.4 0.035 0.700

PPG (HK) 2217.6 0.909 1.109

Water 128.66 0.053 7.148

Carbon diaoxide 1.32 0.001 0.030

Unconverted foam 6 0.002 0.001

total 2438.98 1.000 8.988

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TOP PRODUCT      

ComponentTop prod. in Kgs/hr

Top product in Fraction

Top in kmoles

TDA (LK) 81.13 0.382 0.665

PPG (HK) 1 0.005 0.001

water 128.66 0.607 7.148

Carbon dioxide 1.32 0.006 0.030

total 212.11 1 7.843

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BOTTOM PRODUCT      

Component

Bottom prod. in Kgs/hr

Bottom product in Fraction

Bottom in kmoles

TDA (LK) 4.27 0.002 0.035

PPG (HK) 2216.6 0.995 1.108

Unconverted foam 6 0.003 0.001

total 2226.87 1 1.144

Ref:- www.cheric.org 10

DATACol. P (atm) 1Col. T ( C ) 190q (subcooled liquid) 1.3Average viscosity of feed (cp) 1.28V.P (atm) of TDA at 190oC 0.018volatility of TDA 0.018V.P (atm) of PPG at 190oC 0.004083volatility of PPG 0.004083Relative Volatility 4.41

Ref:- Estimation of Polymer Properties

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viscosity calculations

TDA

Temperature ( C ) 190

viscosity (cp) 0.295

PPG

Temperature ( C ) 190

viscosity (cp) 1.395

Ref:- Estimation 12

Nature Of Feed

q = 1 + Cpl(Tb – Tf) λ

q=1.3 (for subcooled liquid)

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Determination Of Reflux Ratio

Nmin = Ln [(X1/X2)D (X2/X1)B]

Ln (α12)av

By putting all values, we get

Nmin = 7

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Stage cut calculation

(α1xf1/α1-Θ)+(α1xf2/α2-Θ)=1-q

As q=1.3

So,

(α1xf1/α1-Θ)+(α2xf2/α2-Θ)= -0.3

By hit & trial we satisfy this eq. & get the value of “stage cut”

Θ=3.0

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Optimum reflux calculation

(α1xd1/α1-Θ)+(α2xd2/α2-Θ)=1+Rmin

Hence, Rmin=1.2

R =1.2*1.5

=2

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Feed stage location

Now by Eduljee relation (eq-15.4)No. of Theoritical stages=19

Tray Efficiency=Eo=0.492[µFavg(1 / 2)av ]Eo=57%No. of Actual Stages=33 By Kirkbride method,ND=23

NB=10So, feed enters the column 10 stages above

the bottom stage

Ref:- Estimation of Polymer Properties

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FLOW RATESTop Flow RatesLn=D*Rmin kmol/hr 9.371Vn=Ln+D kmol/hr 17.214Bottom Flow Rates  Lm=Ln+F kmol/hr 18.359Vm=Lm-B kmol/hr 17.214Vm m3/sec 1.817Vm kg/sec 7.23L/D 2V/D 3liquid density (kg/m3) 717.27M (g/gmole) 151.25R (atm.cm3/gmole.k) 82.06vapour density (kg/m3) 3.9809

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Mechanical Design

Vnf = CSb(/20)0.2(ρL-ρV/ρV)0.5

=0.559 m/s Vn = 0.80*Vnf

=0.447 m/s

mv = (V/D)*(D)*(M)/ρV

Vn

An= mv/Vn

= 3.72 m2

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Diameter calculations

Let 15% of the cross sectional area is occupied by the downcomer

Then, Cross sectional Area=Ac=

An/0.85

=4.38 m2

Dia of column=D= (4Ac/ π)1/2

=2.36m

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Height of Column

Tray spacing=Hs=1.5 ftAdditional height for phase

disengagement=▲H =2

ftHeight of Column=Hc= (Nact – 1) Hs +

▲H = 51 ft =15 meters

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PRESSURE DROP

Hd= 51 x (Uh)2 x ρv (Co)2 x ρL

Uh=velocity through holes =Vnf/0.1 = 5.56 m/secCo = 0.89 (from graph)So, we getHd=30.70 mm-liquidHr=Residual Head=12500/ρL=17.43 mm of liquidLength of weir=lw = 1.9 mHeight of weir=hw = .03 mHow Weir Crest= 750[mL/ρL*lw]2/3 = 23 mm-liqmL= liquid flow rateht (total pressure drop) = Hd+(Hw+ How)+Hrht = 98 mm-liq∆Pt= 9.81*10-3*(ht)*ρL =690Pa=0.15psi

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Ah total hole area i.e. 10% of Aa= 0.46 m2

Hole dia=3 mm (assumed)Area of one hole =7.06 (mm)2No. of holes= Ah/Area of one hole(m2)

=65100

Ref:- chem engg design By:coulson Ed.3 vol:6

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CHECK WEEPING

Lw liquid flow rate kg/sec=7.71how weir crest (mm of liquid)=23.708how+hw (at minimum)=53.71K2=30Uh=K2-0.90(25.4-dh)

ρv

Uhmin vapour velocity m/sec=4.23Uh Velocity through holes (m/s)=5.56

As actual vap velocity is much higher than the Uh min so there will be no weeping

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CHECK ENTRAINMENT

Un =max vol vap flow rate/net area=1.02m/sec

FLV= mL/mv(ρv/ ρL)1/2

FLV=0.08K1=0.085Uf=K1[(ρL- ρv)/ ρv]1/2

Uf=1.14 m/sec

Percent flooding 90 Fractional Entrainment ψ = 0.03 well

below 0. 1 So Satisfactory

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Check Tray spacing

hap height of apron (mm) =20Aap clearance area under

downcomer=0.038m2hdc head loss in downcomer mm-liq=13.11hb=hw+how+ht+hdc (eq-11.91)hb Backup in Downcomer mm-liq=149

hb Backup in Downcomer =0.149 m1/2(tray spacing+weir ht.)=0.2436 mAs hb<1/2(tray spacing+weir ht.) so tray

spacing is acceptable

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Check Residence Time

tr = (Ad)*(hb)*(ρL)/mL

=9.10 sec

As tr > 3 sec. so, result is satisfactory

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Identification:

Item Distillation columnNo. required 1

Tray type Sieve tray

Function: Separation of polyols from TDA for further recycling Operation: Continuous

SPECIFICATION SHEETSPECIFICATION SHEET

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No. of tray=33Tray efficiency = 57%Pressure = 101.32 KPaHeight of column = 15 mDiameter of column=2.36 m Hole size = 3 mm Pressure drop per tray=0.15psi Tray thickness = 5 mm

Active holes = 65100 Weir height = 30 mmWeir length = 1.9 m Reflux ratio = 2:1 Tray spacing =0.457m Active area = 3.07 m2 Fractional Flooding = 0.03 No Entrainment

Design DataDesign Data

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