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