Equipment Design and

Costs for Separating

Homogeneous Mixtures

Dr. Syeda Sultana Razia

Department of Chemical Engineering,

Bangladesh University of Engineering and

Technology (BUET), Dhaka -1000

1. Distillation

Design Procedures for Columns with

Sieve Trays

Designation of design bases

Composition and physical properties of feed and product

Special limitations: maximum temperature and pressure drop restrictions, presence of reactive materials or toxic components etc

Selection of design variables: operating pressure, reflux ratio, feed condition

Establishment of physical equilibria data

data for binary pairs are combined with a model (Wilson, NRTL and UNIQUAC) to predict multi-component behavior; UNIFAC model is used for prediction based on functional group

Design Procedures for Columns with

Sieve Trays (cont’d)

Determination of number of equilibrium stages

Minimum number of stages from Fenske equation

Number of equilibrium stages, N as a function of and (Gilliand equation)

minN

avHK/LK

BLK

HK

DHK

LK

minln

x

x

x

xln

N

2/1

BHK/LKDHK/LKavHK/LK

minR

566.0

minmin

1R

RR175.0

1N

NN

Design Procedures for Columns with

Sieve Trays (cont’d)

Selection of column

internals

Design Procedures for Columns with

Sieve Trays (cont’d)

Determination of column diameter

5.0

v

vL

2.0

sbnf20

CV

nfn V%9050V

n

"

vn

V

mA

dnC AAA

2/1

cA4D

Design Procedures for Columns with

Sieve Trays (cont’d)

Efficiency:

Tray Spacing: 0.46 to 0.61 m (0.3 and

0.91 m are also used)

Column height

245.0

avHK/LKLo 492.0E

o

actE

NN

HH1NH sactc

sH

Design Procedures for Columns with

Sieve Trays (cont’d)

Sieve Tray Geometry

Hole dia: 0.005-0.025 m

Fractional free area: 0.06-0.16 m2

Fractional downcomer area: 0.05-0.3 m2

Pitch/hole dia ratio: 2.5-4.0

Tray spacing: 0.305-0.915 m

Weir height: 0.025-0.075 m

Design Procedures for Columns with

Random Packing

Determination of diameter

Vapor velocity is 70 to 90 % of flooding velocity

Recommended pressure drop

400 to 600 Pa/m for atmospheric and high-pressure

separation

4 to 50 Pa/m for vacuum operations

200 to 400 Pa/m for absorption and stripping

column

Design Procedures for Columns with

Random Packing (cont’d)

Heights of columns

HTU method

)NTU)(HTU(Z

ceG

"

v

AaK

mHTU

1

2

y

y

*yy

dyNTU

eL

L

ev

vLv

ak

V

ak

VHTUHTUHTU

L

mV

Design Procedures for Columns with

Random Packing (cont’d)

HETP method

1

lnHTUHETP

NHETPZ

Design Procedures for Columns with

Structured Packing

Design Procedures for Columns with

Structured Packing (cont’d)

Diameter

Height

HETP: Rule of thumb 10a

29.9HETP

p

Other Distillation Processes

Batch distillation:

Food, pharmaceuticals and biotechnolgy

industries

Rayleigh equation

2i

1i

x

x ii

i

xy

dx

F

Wln

Other Distillation Processes (cont’d)

Azeotropic distillation

Cost Estimation

Cost Estimation (cont’d)

Costs of distillation column

25 trays 50 trays

Cost Estimation (cont’d)

Costs of sieve tray

Cost Estimation (cont’d)

Cost Estimation (cont’d)

Cost Estimation (cont’d)

Cost Estimation (cont’d)

2. Absorption and Stripping

Gas Treatment with Solvent Recovery

Design Procedures

Column diameter: 70 to 90% of the

flooding velocity, Larger of the top or

bottom diameter is used

Number of equilibrium stages: Modified

Kremser equation

1A

AA1N

i

i

1N

i Solute fraction absorbed

1S

SS1N

i

i

1N

i Solute fraction stripped

VK

LA

i

i

L

VKS i

i

Design Procedures (cont’d)

Stage efficiency and column height

Overall efficiency

Column height: Tray spacing/HTU/HETP

Lo log8.572.19E

3. Membrane Separation

Selection of Membranes

Fabricated from natural and synthetic

polymers

Membrane modules

Plate and Frame ($250-400/m2)

Spiral-wound ($25-100/m2)

Hollow fiber ($10-20/m2)

Tubular ($250-400/m2)

Capillary ($25-100/m2)

Ceramic ($1000-1600/m2)

Concentration Profile across Membranes

Design Parameter

Permeance: porosity, solubility or partition

coefficient

Separation factor/selectivity

Purity and yield

Flow Patterns

4. Adsorption

Selection of Adsorbent

Activated Carbon

Molecular Sieve Zeolites

Silica gel

Activated Alumina

Basic Adsorption Cycles

Temperature Swing cycle

Cycle time: few hours

Capacity: 1 kg per 100 kg

of adsorbent

Basic Adsorption Cycles (cont’d)

Inert Purge cycle

Regeneration is done by purging inert

gas and lowering the partial pressure of

the adsorbate

Cycle times are only a few minutes

Capacity 1 to 2 kg adsorbate per 100 kg

adsorbent

Basic Adsorption Cycles (cont’d)

Pressure Swing cycle

Cycle time: few minutes

Capacity: 1 to 2 kg per

100 kg adsorbent