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Design Of Distillation Design Of Distillation ColumnColumn

ByMUHAMMAD TARIQ

2005-Chem-89

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• What is distillation?

• Selection b/w Plate and Packed

column

• Selection of tray type

• Design Calculations

• Plate Hydraulic Design

• Specification Sheet

SynopsisSynopsis

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“Process in which a liquid mixture of two or more substances is separated into its component fractions of desired purity on the basis of difference in relative volatility , by the application and removal of heat”

What is Distillation?What is Distillation?

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Comparison b/w Tray & Packed Columns

Column Internals Column Internals

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The choice between these two for a given mass transfer operation is based on a detail cost analysis.

However, The decision can be made on the

basis of a qualitative analysis of relative advantages and disadvantages.

Choice b/w Tray & Packed Choice b/w Tray & Packed ColumnColumn

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• Non-foaming systems. • Liquid rates without flooding.• Periodic cleaning is easy in tray

columns• Inter stage cooling • Design information• Height of the Column

Tray & Packed Column Tray & Packed Column Contd.Contd.

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I have selected the tray column for this particular system as;

• As our system is non foaming.• Height of the column is large.• Temperature is high, i.e.

172.3oC• Diameter is greater than 0.67m

Selection Of Tray Selection Of Tray ColumnColumn

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Overview of Tray Overview of Tray ColumnColumn

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• Cross-flow contactors are the most common type of plate contactors.

• In a cross-flow plate the liquid flows across the plate and the vapour up through the plate.

• The liquid move from plate to plate via down comer.

• A certain level of liquid is maintained on the plates by weir.

Plate ContactorsPlate Contactors

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Cross Flow Plate Cross Flow Plate ContactorsContactors

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Three principal types of cross flow trays are used classified according to the method used to contact the vapour and liquid.

• Sieve Plates (Perforated Plate)• Bubble Cap Plates• Valve plates (floating cap

plates)

Plate Contactors Plate Contactors Contd.Contd.

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Sieve Plate Bubble Cap plate

Valve Plate

Plate Contactors Plate Contactors Contd.Contd.

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I’ve selected the Sieve Plates because;

• Pressure drop is lowest. • Their fundamentals are well

established.• Sieve plates are the cheapest.• They are lighter in weight.• Maintenance cost is reduced

due to the ease of cleaning

Selection Of Sieve Selection Of Sieve Plate Plate

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Overview of Sieve Tray Overview of Sieve Tray

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Adverse Vapor Flow conditions can cause;

• Weeping• Coning• Excessive Entertainment• Flooding

Factors affecting Factors affecting Distillation Column Distillation Column

OperationOperation

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Column Pressure And Type Of Column Pressure And Type Of

CondenserCondenser

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Distillation Column in Distillation Column in PFDPFD

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Simulation on HYSYSSimulation on HYSYS

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Feed F= 2990 (kg/hr)

T=161oCP=130KP

a

Bottom Product 2500 (Kg

/hr)T=220oC

P=160KPa

Top Product D=490 (Kg /hr)T=117oCP=101K

Pa

T=137o

CP=110

KPa

3.864×106 KJ/hr

3.642×106 KJ/hr

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Heavy Key Component= Maleic Anhydride (MAN)Light Key Component = Quinone

Material BalanceMaterial Balance

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Design Calculations Of Design Calculations Of Distillation ColumnDistillation Column

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Data Required For Data Required For DesignDesign

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Calculation of• Minimum Reflux Ratio (R min).• Actual Reflux Ratio (R).• Minimum number of stages(N min).• Theoretical number of stages.• Actual number of stages.• Diameter of the column.• Weeping , entrainment, etc• Total Pressure drop.• Height of the column

Designing Steps of Designing Steps of Distillation ColumnDistillation Column

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Using Underwood equation;

As feed is entering as Saturated Liquid, q = 1

= 24.8 Now, Rmin = 4.77

Minimum Reflux RatioMinimum Reflux Ratio

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There is no hard and fast rules for the selection of optimum reflux ratio, but for many systems the optimum will lie between 1.2 to 1.5 times the minimum reflux ratio;

R = 1.25(R min)

R = 6

Actual Reflux RatioActual Reflux Ratio

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The minimum No. of Stages Nmin can be find from Fenske Equation which is,

N min = 20

Minimum Number of StagesMinimum Number of Stages

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Using Gilliland-Eduljee relation;

N = 39Considering one stage achieved in

Reboiler & Condenser

N = 38

Theoretical Number of Theoretical Number of StagesStages

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Using Kirkbride relation;

& Hence,

ND = 19 NB = 19

Location of Feed PlateLocation of Feed Plate

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A quick estimate of the overall column efficiency can be obtained from the correlation given by O, Connell which is shown below,

E o = 70.92%

Overall Tray EfficiencyOverall Tray Efficiency

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As overall column efficiency is 70.92%So,

N act= 38/0.709

N act= 54

Now, Actual ND = 27

Actual NB = 27

Actual Number of Stages Actual Number of Stages

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Flow Parameter based on Rectifying Section;

FLV = 0.032Assume ,Plate Spacing = 0.5mFrom Graph we get

K1 = 0.07Now Flooding velocity;

uf = 2.59 m/sec

Determination of Determination of Flooding VelociFlooding Velocityty

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Correction factor for surface tension, σ = 56.43 dyne/cm So, unf= 3.18 m/sec

Assume ,Actual operating velocity is the 85% of

the flooding velocity, un= 2.71m/sec

Actual Operating Actual Operating VelocityVelocity

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An = 0.29m2

Taking Down-comer Area as 12% of total cross sectional area,

Total Column cross sectional Area = Net Area/0.88

Cross Sectional Area Of The Cross Sectional Area Of The ColumnColumn

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Column Diameter

D = 0.64m (based on Rectifying section)

In the similarly way, D = 0.69m (based on Stripping

section)As, Two Diameters are nearly same.

So, the Diameters will be same for both the Sections.

Diameter Of The ColumnDiameter Of The Column

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Plate Hydraulic DesignPlate Hydraulic Design

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Down-Comer Area = 0.12(Ac)

AD = .041m2

Active Area = Ac – 2AD

Aa = 0.26m2

Hole Area = 0.1(Aa) = .026m2

Take weir height = 50mm

Tray DesignTray Design

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As we have,

From Graph we get

So, Lw = 0.5m

Weir LengthWeir Length

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Using relation,

how = 23.69 mm

For minimum Crest use 70% of maximum flow

how(min) = 18.68 mmHence,

Weir Liquid CrestWeir Liquid Crest

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Using relation;

From Graph K2 = 30.56

U min = 6.43 m/sec

Actual minimum Vapor velocity through holes;

V act(min) = 17.85 m/sec

Which is higher than min. So, There will be no

Weeping CheckWeeping Check

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Weeping Phenomena Weeping Phenomena

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Support AreaSupport Area

Lw/Dc = 0.76 From graph; θ = 99o

r = 0.33m

Arc length = r×(п-θ) = .46 mAs = 2×Arc. × .05

= 0.04 m2

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From Fig.

a = .25 m

Az = 4a × 0.05

= 0.054m2

Calming Zone AreaCalming Zone Area

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Ap= Column Area – 2(Down-comer area) – Support area – Calming zone Area

= 0.16 m2

Perforated AreaPerforated Area

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Single hole Area

= 1.96× 10-5 m2 No. of holes = Total hole area / single hole area =

1330 holes

Hole Pitch;

From the graph,So,

Lp = 11.5 mm

Number Of HolesNumber Of Holes

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Graph To Find The Hole Graph To Find The Hole PitchPitch

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The pressure drop over the plates is an important design consideration, the total plate pressure drop is given below,

Total head loss per plate = hd + hr + hw + how

h d = Dry plate drop hr = Residual head

Pressure Drop CalculationPressure Drop Calculation

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We know that,

We have:

and Plate thickness / hole diameter = 1

From graph we get: Co = 0.91

Hence,h d = 30 mm-liquid

Dry Plate Pressure Drop Dry Plate Pressure Drop

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This is the head loss due to bubble formation and other losses.

This is given by

hr = 11.41 mm-liq.

Residual Head Residual Head

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Total head loss per plate = h d + hr + hw + how

ht = 109.76 mm-liq.

Pressure drop per plate = ΔPt = 925 Pa

Total Pressure drop in the column;

ΔP = No. of plates × ΔPt

= 50 KPa

Total Pressure DropTotal Pressure Drop

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Assuming that down comer is 6 mm below the weir level;

Then height of aperture thus formed is ;

hap= hw- 6

=44mmArea under apron; A ap =h ap× L w = 0.02m2

Down Comer BackupDown Comer Backup

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h dc =2.7mm

= 50+18.68+109+2.7 = 180.34mm

Down Comer BackupDown Comer Backup

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Liquid residence time in down comer is given as

t r = 4.3 sec

Residence TimeResidence Time

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Height of the distillation column can be find by the given below formula,

Where ΔH is the additional height required for the column operation.

H c = 30 m

Height of the ColumnHeight of the Column

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Specification Sheet Specification Sheet