Equipment Design and
Costs for Separating
Homogeneous Mixtures
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 multicomponent 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
 Underwood equation for minimum reflux
 Minimum number of stages from Fenske equation
N min

 x LK   x HK  
 
 
ln 
 x HK  D  x LK  B 

ln  LK / HK av
LK / HK av  LK / HK D LK / HK B 1/ 2
Number of equilibrium stages, N as a function of Nmin
and R min(Gilliand equation)
  R  Rmin 0.566 
N  N min
 0.751  
 
N 1
  R  1  
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

Vnf  Csb  
 20 
0.2
 L   v 


 v 
Vn  50  90%Vnf
m"v
An 
Vn
AC  A n  Ad
1/ 2
 4A c 
D




0.5
Design Procedures for Columns with
Sieve Trays (cont’d)
 Efficiency:
Eo  0.492L LK / HK av 
0.245
 Tray Spacing: Hs 0.46 to 0.61 m (0.3 and
0.91 m are also used)
 Column height
N act
N

Eo
H c  N act  1Hs  H
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
Z  (HTU )( NTU )
m"v
HTU 
K G a e Ac
NTU 
y1
dy
y y  y*
2
Vv
VL
HTU  HTU v  HTU L 

k va e
k La e
mV

L
Design Procedures for Columns with
Random Packing (cont’d)

HETP method
 ln  
HETP  HTU 



1


Z  HETP N
Design Procedures for Columns with
Structured Packing
Design Procedures for Columns with
Structured Packing (cont’d)
 Diameter
 Height

HETP: Rule of thumb
HETP 
9.29
 10
ap
Other Distillation Processes
 Batch distillation:


Food, pharmaceuticals and biotechnolgy
industries
Rayleigh equation
W
ln

F
xi2

x i1
dx i
yi  x i
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
A iN 1  A i

N 1
Ai  1
Solute fraction absorbed
SiN 1  Si
 Solute fraction stripped
N 1
Si  1
L
Ai 
KiV
Si 
KiV
L
Design Procedures (cont’d)
 Stage efficiency and column height

Overall efficiency
Eo  19.2  57.8 log  L

Column height: Tray spacing/HTU/HETP
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