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 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 Underwood equation for minimum reflux 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
min ln
x
x
x
xln
N
2/1
BHK/LKDHK/LKavHK/LK
minR
566.0
minmin
1175.0
1 R
RR
N
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
sbnf 20CV
nfn V%9050V
n
"v
n 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
oact E
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
i1N
i Solute fraction absorbed
1S
SS1N
i
i1N
i Solute fraction stripped
VK
LA
ii
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