distillation column design

AL-ABDUL-QADER ABDUL-AZIZ AHMAD[Type the company name]

1/1/20132013

King Fahd University of Petroleum & Minerals DEPARTMENT OF CHEMICAL ENGINEERING

CHE 495 Term 131

DISTLATION COLUMN DESIGN

Author:

Ageel Buhlaigah 200872740

Group (5)

Submitted to:

Dr. Muataz Ali Atieh

DISTILLATION COLUMN DESIGN

Abstract:

In this report, the design of distillation columns was carried out. The number of stages (trays), the stage on which the feed enters, reflux ratio, diameter of both stripping and rectify sections, the operating velocity, overall efficiency of the column ,the length of the tower and the material of construction are calculated and tabulated in table 1and 2.

INTRODUCTION

A distillation column (fractionating column or fractionation column) is an essential

item used in the distillation of liquid mixtures so as to separate the mixture into

its component parts, or fractions, based on the differences in their volatilities.

Fractionating columns are used in small scale laboratory distillations as well as for

large-scale industrial distillations. Multi-stages columns are used to enhance the

separation process by increasing the contact area between the vapor and the

liquid inside the column. The concept of the separation in distillation processes is

to vaporize the more volatile component and collect it from the top of the column

in its simplest form (binary mixture).

The most commonly used method in the design of Distillation column is by

McCabe-Thiele technique which is a graphical based on vapor-liquid equilibrium

data and a mass balance. The main assumption made in the McCabe-Thiele

method is that there must be equi-molar overflow through the tower between

the feed inlet and the top tray and between the feed inlet and the bottom tray.

In designing distillation towers, several parameters should be determined such as:

Diameter of the tower. Height of the tower. Overall efficiency of the tower. Hydraulic design of the internal trays.

Figure 1: The basic equipment required for continuous distillation.

Theoretical Background:

A.1.Distillation Column Design:

For the minimum number of trays:

Using Fenske equation:

NMin=

ln(( X A

1−X A)Dist .

( X A

1−X A )Bottom)

ln (αAB)

(1)

Where: A Light key and B (the reference) is Heavy key.

For the theoretical number of stages (N):

Using Gilliland correlation:

x=( LD )−( LD )

Min

( LD )+1 (2)

For 0.01 ≤ x ≤ 0.90

N−Nmin

N+1=0.545827−0.591422 x+ 0.002743

x (3)

For the theoretical feed stage:

Using Kirkbride’s correlation:

log( N f−1N−N f )=0.260 log {BD ( zHK

z LK )[ xLK

xHK ]2} (4)

For the overall efficiency of the column, the following equation is used:

E=0.52782−0.27511× log (αμ )+0.044923×[ log (αμ )]2 (5)

Where: α is the relative volatility

μ is the viscousity of the feed

To find the diameter of the column, the following equation will be used:

Dia=[ 4×V ×MW v

π ×η× ρv× (3600 )×U op ]0.5

(6)

Where Uop is the operating vapor velocity in ft/s, and can be found by the following formula:

Uop=U flood× ( fraction ) (7)

The fraction is typically lays in between 0.65 to 0.9, and good performance at 75 % of flooding velocity.

The flooding vapor velocity based on net area of vapor flow is determined from the following relation:

U flood=C sb( σ20 )

0.2√ ρl−ρv

ρv

(8)

σ is the surface tension of liquid in dynes/cm, and C sb is tha capacity factor and a function of flow parameter FP

For 18-in tray spacing (as recommended from heuristics for tower trays):

log10C sb ,f=−1.0262−0.63513 log10 Flv−0.20097× (log10 F lv )2 (9)

Where F lvis the flow parameter and the following relation is used to find it:

FP=F lv=W L

WV(ρ v

p l)

0.5

(10)

WL and WV are the mass flow rate of liquid and vapor and densities are mass densities.

W L

W V will be calculated by the use of the following formula:

W L

WV= L

V×

MW L

MW V (11)

A.2.Sieve tray layout and tray hydraulics:

Sieve tray designs probably provide the majority of installed tray types used in the process industry and are widely accepted due to their excellent operating characteristics, low cost Investment and low maintenance requirements. As a result, sieve tray was chosen for constructing the internal stages.

Figure: A photo for a sieve tray.

The total cross sectional area of the column is:

Atotal=π (Dia )2

4 (12)

The active area can be estimated for two downcomers:

Aactive=Atotal (1−2 (1−η ) ) (13)

Where η is the fraction of the column cross-sectional area that is available for vapor flow above the tray. Typically η lies between 0.85 - 0.95 and 0.9 will be used.

Obviously, Ahole can be determined from tray layout:

Ahole= (No .of holes )(πdo

2

4) (14)

Or

Ahole=β Aactive (15)

Where β corresponds to a value of β=Ahole

Aactive. The average value of β is between 7%

and 16%, with 10% a reasonable first guess.

The vapor velocity through the holes,vo, in feet per second (ft/sec) can be calculated from

vo=V MW v

3600 ρv Ahole (16)

Where V is Ib moles/hr

The downcomer area Ad can be determined from

Ad=12r2(θ−sinθ ) (17)

Or

Ad=(1−η)A total (18)

Combining equations (13), and (14), we can solve for angle θ and length of the weir Iweir

The head of clear liquid in the downcomer, hdc, can be determined from the sum of heads that must be overcome.

hdc=hΔp , dry+hweir+hcrest+hgrad+hdu (19)

On sieve trays, the liquid gradienthgrad, across the tray is often very small and is usually ignored.

The head of liquid required to overcome the pressure drop of gas on a dry tray, hΔp ,dry, can be measured experimentally or estimated (Ludwig, 1995) from

hΔp ,dry=0.003 v oρv( ρwater

ρL) (1−β2 )/C o

2 (20)

The orifice coefficient,Co, can be determined (in inches )from the correlation of Hughmark and O’Connell (1957). This correlation can be fit by the following equation (Kessler and Wankat, 1987):

Co=0.85032−0.04231do

t tray+0.0017954 ( do

t tray )2

(21)

Where t tray is the tray thickness. The minimum value of do /t tray is 1.0. The weir height, hweir, is the actual height of the weir. The minimum weir height is 0.5 inch with 2 to 4 inches more common.

The height of the liquid crest over the weir, hcrest, can be calculated from the Francis weir equation.

hcrest=0.092Fweir ( Lg/ Iweir )2 /3 (22)

Where hcrest is in inches. In this equation Lg is the liquid flow rate in gal/min that is due to both L and e. the factor Fweir is a modification factor.

There is a frictional loss due to flow in the downcomer and under the downcomer onto the tray. This term, hdu, can be estimated from the empirical equation (Ludwig, 1997; Bolles, 1963).

hdu=0.56( Lg

449 Adu)

2

(23)

Where hdu is in inches and Adu is the flow area under the downcomer apron in ft2. The downcomer apron typically has a 1-in gap above the tray.

Adu=(gap)Iweir (24)

The condition for avoiding excessive weeping can be determined from estimation of (Kessler and Wankat, 1987) as

hΔp ,dry+hσ≥0.10392+0.25119 x−0.021675 x2 (25)

wherex=hweir+hcrest+hgrad. Equation (25) is valid for β ranging from 0.06 to 0.14.

The surface tension head hσ can be estimated from the following eqation

hσ=0.040σρLdo

(26)

Results:

Table 1: Summary for distillation column T-202

Distillation Summary Table for T-202

Diameter (m) 2.48

Height (m) 58.63

Orientation Vertical

Internals Sieve trays

Number of trays 78

Pressure (bar) 6.3

Material of Construction Carbon Steel

Table 2: Summary for distillation column T-204

Distillation Summary Table for T-204

Diameter (m) 3.1

Height (m) 20.8308

Orientation Vertical

Internals Sieve trays

Number of trays 46

Pressure (bar) 6.2

Material of Construction Carbon Steel

Table3: Distillation column calculation summary table for T-202

F Z Top Xd Bot Xb

ac 1.4588 0.0004 1.28592 0.0285 0 0eth 169.9502 0.0466 28.23158 0.6257 141.9188 0.0394ea 3.2823 0.0009 3.384 0.075 0.10806 0.00003

h2o 3466.474 0.9505 12.21398 0.2707 3454.318 0.959bu 3.2823 0.0009 0 0 3.2418 0.0009aa 2.1882 0.0006 0 0 2.1612 0.0006

3646.635 45.11549 3601.748

Density V

8.373 Uo 0.72542 Nf 41.6

Density L

824.5 Anet 4.374241

Nact 78

WL 6.92E+04 n 0.9 Nfact 51WV 95676 Di 2.48762

7L (m) 58.63

Fp 0.072844571 Vol 1.45 ψ 0.05Csp 0.273050091 Nmin 24.4613 Atot 4.83

σ 46.13 Nfmin 15.28359

Ad 0.483

Uflood 3.186187616 N 65 A_activ 3.864

Table4: Distillation column calculation summary table for T-204

F Z Top Xd Bot Xbeth 141.9188 0.0394 137.3882 0.6897 4.53056 0.0013ea 0.10806 0.00003 0 0 0.10806 0h2o 3454.318 0.959 61.752 0.31 3392.566 0.997bu 3.2418 0.0009 0.05976 0.0003 3.18204 0.0009aa 2.1612 0.0006 0 0 2.1612 0.0007

199.2 3402.548

Density V 7.34E+00 Uo 0.849 Nf 16.5Density L

828.5 Anet 6.97E+00 Nact 46

WL 6.17E+04 n 0.9 Nfact 23WV 1.56E+05 Di 3.139753 L (m) 35.208Fp 0.037141336 Vol 1.95764 ψ 0.08Csp 0.295856382 Nmin 11.0788 Atot 7.74σ 47.28 Nfmin 5.942495 Ad 0.774Uflood 3.72E+00 N 32.5 A_activ 6.192

Calculation Procedure:

For distillation column( T-202 )

Assumptions:

1. Binary mixture.

2. Constant molal overflow (CMO).

3. No heat leak.

α AB=PsatLKPsatHK = 1.45

Fortheminimumnumberoftrays:


NMin=

ln(( X A

1−X A)Dist .

( X A

1−X A )Bottom)

ln (αAB)

⇒Nmin=24.46 trays

So Nmin = 25 equilibrium stages including reboiler


where ( LD )Min

=2.5 from heuristics and ( LD )=50

Therefore,

N = 2.5 (25+1)

N=65trays

For Nfmin

Nfmin = (ln((0.075/0.2707)/(0.0009/0.9505))/ln(α AB)) = 15.28 trays


Nf = ( LD )Min

∗¿

N f=41.6 trays


Eo=0.52782−0.27511× log (αμ )+0.044923×[ log (αμ )]2

For αμ= (1.45 ) (0.07328 )=0.106

⟹ Eo=0.8386 = 83.8%

Nactual=NEo

= 650.8386

=77+1(∂ reboiler)≈78 trays

N F=N F

Eo= 41.6

0.8386=49.6+1≈51 trays

TheDiameter,entrainmentandweepingFortherectifyingsection:

L=RD=50×45.12=2256 kmol/h

V=L+D=2256+45.12=2301.12 kmol/h

W L

W V= L

V×

MW L

MW V

F lv=W L

WV(ρv

p l)

0.5

=0.0728

For18inchspacing:

C sb=10[−1.0262−0.63513× log ( 0.0728 )−0.20097× ( log (0.0728 ) )2 ]

= 0.273

Surfacetension:

σ=46.13 dynecm


0.2√ ρl−ρv

ρv

U flood=3. 186 fts

U op=U flood× (0.75 )=3.186×0.75=2.38 fts=0.72542 m

s

Dia=[ 4×V ×MW v

π ×η× ρ× (3600 )×Uop ]0.5

Dia=2.48m

Hence,columnlength:L=1.2(0.61)(N actual−2) ¿1.2×0.61×(78−2)=55.63mFrom heuristics:

Fortowersabout0.9mdia,add1.2matthetopforvapordisengagement,and1.8matbottomforliquidlevelandreboilerreturn.

⟹ Lactual=55.63+1.2+1.8=58.63m

LD

=58.632.48

=23.48

Materialofconstruction(MOC):

Since the liquid and gas mixtures are not corrosive and the column temperature under 400 C. The carbon steel is the appropriate MOC.

Figure: 3 Entrainment correlation from Fair (1963)

With75%floodingandFlv=0.0728,

ψ=0.05

ψbelow0.1.Hence,noentrainmentintherectifyingsection.

ψ= eL+e

0.05= e2256+e

e=118.7 Kmolhr

L+e=2256+118.7=2374.7 Kmolhr

The total cross sectional area of the column:

Atot=π (Dia)2

4=π ×(2.48)2

4=4.83m2

Downcomerarea:

Ad=(1−η ) A tot=(1−0.9 )×4.83=0.4 83m2

Table 3: Geometric relationship between η and Iweir/diameter.

η 0.8 0.825 0.85 0.875 0.9 0.925 0.95 0.975

lweir/Dia 0.871 0.843 0.811 0.773 0.726 0.669 0.593 0.478

Iweir

Dai=0.726

Iweir=0.726×2.48=1.8m

Activearea:

Aactive=Atot (1−2 (1−η ))

Aactive=4.83 (1−2 (1−0.9 ) )=3.864m2

Areaofthehole:

Ahole=β Aactive=0.1×3.86=0.38 6m2

VaporVelocity:

vo=V MW

3600 ρv Ahole

vo=2301.12×41.57

3600×8.373×0. 38=8.35 m

s

Chosen tray is a std. 14 gauge tray with thickness (T tray) = 0.078 in with a common hole diameter do= 3/16 inch for normal operation and clean service. Pitch Std. spacing between the holes of 3.8do = 0.1725 inches. A 2.5 in space between the edge holes and the column wall is chosen, and a space of 4 in between the edge hole and the tray weir.

Orifice coefficient:

Co=0.85032−0.04231( do

ttray )+0 .0017954 ( do

ttray )2

Co=0.85032−0.04231 (2 .4 )+0.0017954 (2 .4 )2=0 .7 59

Calculatingheadsonsievetray:

hΔ p=0.003 vo2ρv ( ρH 2O

ρl)(1−β2)/Co

2

hΔ p ,dry=0.003×8.352×8.373×( 62.351.4 7 )× 1−0.12

0.7 592 =3.6 inches

hσ=0.04 σρLdo

= 0.04×46.1351.47× (0.1725 )

=0 .207

Assume hweir=2inch

Lg=2374.7 Kmolhr

×41.57 KgKmol

× 1m3

8 24.5 Kg× 1hr

60min=1.995 m

min

Abscisa=Lg

Iweir2.5 =1.995

1.82.5 =0.449

The correction factor:

Fromfigure(10-20)inCHE306(SeparationProcessEngineering,Wankat2nded)

Fw=1

hcreast=0.092Fw( Lg

Iweir)

0.667

hcreast=0.092×1×(1.9951.8 )

2 /3

=0.0985 inch

hΔ p+hσ≥0 .10392+0 .25119 x−0.021675 x2

x=hweir+hcreast+hgrad=2+0. 0985+0=2. 0985∈¿

3.6+0. 207≥0 .10392+0.25119 (2.0985)−0.021675(2.0985)2

3.807≥0.535

From the equality above, weeping would not be a problem in the rectifying section.

For distillation column( T-204 ),

Assumptions:

4. Binary mixture.

5. Constant molal overflow (CMO).

6. No heat leak.

α AB=PsatLKPsatHK = 1.957

Fortheminimumnumberoftrays:


NMin=

ln(( X A

1−X A)Dist .

( X A

1−X A )Bottom)

ln (αAB)

⇒Nmin=11.0788 trays

So Nmin = 12 equilibrium stages including reboiler


where ( LD )Min

=2.5 from heuristics and ( LD )=20

Therefore,

N = 2.5 (12+1)

N=32.5trays

For Nfmin

Nfmin = (ln((0.075/0.2707)/(0.0009/0.9505))/ln(α AB)) = 5.9 trays


Nf = ( LD )Min

∗¿

N f=16.5 trays


Eo=0.52782−0.27511× log (αμ )+0.044923×[ log (αμ )]2

For αμ= (1.96 ) (0.07328 )=0.1388

⟹ Eo=0.731 = 73%

Nactual=NEo

= 32.50.731

=44.45+1(∂ reboiler)≈46 trays

N F=N F

Eo= 16.5

0.731=22.5+1≈23 trays

TheDiameter,entrainmentandweepingFortherectifyingsection:

L=RD=20×199.2=3984kmol /h

V=L+D=3984+199.2=4183.2 kmol /h

W L

W V= L

V×

MW L

MW V

F lv=W L

WV(ρv

p l)

0.5

=0.0 371

For18inchspacing:

C sb=10[−1.0262−0.63513× log ( 0.0371 )−0.20097× ( log (0.0371 ))2 ]

= 0.2958

Surfacetension:

σ=47.28 dynecm


0.2√ ρl−ρv

ρv

U flood=3.72 fts

U op=U flood× (0.75 )=3.72×0.75=2.38 fts=0. 849 m

s

Dia=[ 4×V ×MW v

π ×η× ρ× (3600 )×Uop]0.5

Dia=3.14m

Hence,columnlength:L=1.2(0.61)(N actual−2) ¿1.2×0.61×(46−2)=32.208mFrom heuristics:

Fortowersabout0.9mdia,add1.2matthetopforvapordisengagement,and1.8matbottomforliquidlevelandreboilerreturn.

⟹ Lactual=32.208+1.2+1.8=35.208m

LD

=35.2083.14

=10.257

Materialofconstruction(MOC):

Since the liquid and gas mixtures are not corrosive and the column temperature under 400 C. The carbon steel is the appropriate MOC.

Figure: 3 Entrainment correlation from Fair (1963)

With75%floodingandFlv=0.0371,

ψ=0.08

ψbelow0.1.Hence,noentrainmentintherectifyingsection.

ψ= eL+e

0.08= e3984+e

e=346.4 Kmolhr

L+e=3984+346.4=4330.4 Kmolhr

The total cross sectional area of the column:

Atot=π (Dia)2

4=

π ×(3.14)2

4=7.74m2

Downcomerarea:

Ad=(1−η ) A tot=(1−0.9 )×4.83=0.774 m2

Table 3: Geometric relationship between η and Iweir/diameter.

η 0.8 0.825 0.85 0.875 0.9 0.925 0.95 0.975

lweir/Dia 0.871 0.843 0.811 0.773 0.726 0.669 0.593 0.478

Iweir

Dai=0.726

Iweir=0.726×3.14=2.28m

Activearea:

Aactive=Atot (1−2 (1−η ))

Aactive=7.74 (1−2 (1−0.9 ) )=6.192m2

Areaofthehole:

Ahole=β Aactive=0.1×6.192=0.6192m2

VaporVelocity:

vo=V MW

3600 ρv Ahole

vo=4183.2×37.38

3600×7.34×0.6192=9.55 m

s

Chosen tray is a std. 14 gauge tray with thickness (T tray) = 0.078 in with a common hole diameter do= 3/16 inch for normal operation and clean service. Pitch Std. spacing between the holes of 3.8do = 0.1725 inches. A 2.5 in space between the edge holes and the column wall is chosen, and a space of 4 in between the edge hole and the tray weir.

Orifice coefficient:

Co=0.85032−0.04231( do

ttray )+0 .0017954 ( do

ttray )2

Co=0.85032−0.04231 (2 .4 )+0.0017954 (2 .4 )2=0 .7 59

Calculatingheadsonsievetray:

hΔ p=0.003 vo2ρv ( ρH 2O

ρl)(1−β2)/Co

2

hΔ p ,dry=0.003×9.552×7.34×( 62.351. 72 )× 1−0.12

0.7592 =4.16 inches

hσ=0.04 σρLdo

= 0.04×47.2851.72× (0.1725 )

=0 .212

Assume hweir=2inch

Lg=4330.4 Kmolhr

×37.38 KgKmol

× 1m3

828.5 Kg× 1hr

60min=3.26 m

min

Abscisa=Lg

Iweir2.5 = 3.26

2.282.5=0.415

The correction factor:

Fromfigure(10-20)inCHE306(SeparationProcessEngineering,Wankat2nded)

Fw=1

hcreast=0.092Fw( Lg

Iweir)

0.667

hcreast=0.092×1×(3.262.28 )

2 /3

=0.117 inch

hΔ p+hσ≥0 .10392+0 .25119 x−0.021675 x2

x=hweir+hcreast+hgrad=2+0.117+0=2.117∈¿

4.16+0.117 ≥0.10392+0 .25119 (2.117 )−0.021675(2.117)2

4.277≥0.539

From the equality above, weeping would not be a problem in the rectifying section.

Process control:

1- Control level in the reflux drum is by manipulation of distillate flow rate.

2- A flow controller has been placed on the reflux line (toensuresteadyflowofrefluxtothecolumn).

Figure : Process Control Diagram for the distillation column.

Conclusion:

To sum up, the design of distillation columns (T-202 and T-204) was carried out. Details of staged column design such as stage geometry, determination of column efficiency, calculation of column diameter, downcomer sizing, and tray layout were discussed and estimated. The dimensions of T-202 were found to be D=2.48 m,L= 58.63 m and n= 78 trays. Also, dimensions of the column T-204 were found to be D=3.14 m,L= 35.2 m and n= 46 trays. Moreover, the sieve tray specifications, weir length, vapor flooding velocity, and liquid weeping were calculated. The flooding velocity of the vapor was ensured to be in the acceptable range. Also, the weeping would not be a problem as shown.

Reference(s):

Wankat,Phillip“SeparationProcessEngineering”,PrenticeHall,2ndEd. Hysysdatabase.

distillation column design

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