9/4/99 Separation Operations,Introduction 1
SEPARATION OPERATIONS:
INTRODUCTION
u What are separation operations?
u Typical applications of separation operations
u Physical basis for separation operations
u Basic types of separation operations
u Performance characterization of separation operations
u Choice of separation operations
9/4/99 Separation Operations,Introduction 2
WHAT ARE SEPARATION OPERATIONS?
A Separation Operation
u Separates a multicomponent input stream into two or
more output streams whose compositions differ from that
of the input stream,
OR
uSeparates a mulitphase stream into its constituent
substreams
9/4/99 Separation Operations,Introduction 3
TYPICAL APPLICATIONS
u Separation of a multicomponent stream
e Distillation
e Gas Absorption and Stripping
e Liquid-Liquid Extraction
e Fractional Crystalization
e Dialysis
e Pressure-Swing Adsorption (PSA)
u Separation of a multiphase stream
e Filtration (solid-liquid)
e Decantation (liquid-liquid)
e Electrostatic Precipitation (gas-solid)
e Knock-Out Drum (gas-liquid)
9/4/99 Separation Operations,Introduction 4
EO
Reactor
Gas-Gas
Exchanger
EO
A
b
s
o
r
b
e
r
Intercooler
CW
O2
C2H4
CH4
Purge
Cycle
Water
Cooler EO
S
t
r
i
p
p
e
r
CW
Condenser
Knock
Out
Drum
CO2
A
b
s
o
r
b
e
r
CO2
S
t
r
i
p
p
b
e
r
CO2
EO to
PurificationRecycle
Compressor
BFW
HP Steam
Steam
Drum
PROCESS EXAMPLE,ETHYLENE OXIDE PFD
9/4/99 Separation Operations,Introduction 5
PROCESS EXAMPLE,ETHYLENE OXIDE
u Process Chemistry:
C2H4 + 1/2 O2 → C2H4O (Ethylene Oxide [EO])
C2H4 + 3 O2 → 2 CO2 + 2 H2O
u Reactor Effluent
C2H4O (EO product)
C2H4 (unreacted raw material)
C2H6 (impurity in C2H4 feed)
O2 (unreacted raw material)
Ar (impurity in O2 feed)
CO2 (byproduct)
H2O (byproduct)
CH4 (diluent)
9/4/99 Separation Operations,Introduction 6
EO SEPARATION REQUIREMENTS
u Recover product EO and purify
e Recover EO from reactor effluent via gas absorption in H2O
e Recover EO from absorption H2O via stripping
e Reabsorb EO in H20 to separate from dissolved gases
e Further purify EO to satisfy downstream requirements
u Recover and remove byproduct CO2 and H2O
e Recover CO2 using acid gas absorption
e Remove via stripping
e Recover H2O along with EO in EO absorber
u Purge feed impurities (C2H6 and Ar)
9/4/99 Separation Operations,Introduction 7
PHYSICAL BASIS FOR SEPARATIONS
All separations are based on a difference in some property
of the materials being separated.
u Size [Particle sieving,sort by length or diameter,
filtration]
u Color [Sort be color]
u Physical Properties
e Vapor Pressure [Distillation,gas absorption,drying]
e Solubility [Crystallization]
e Magnetic Susceptibility [Ferrous metal separation]
e Density [Decantation,flotation,sedimentation]
9/4/99 Separation Operations,Introduction 8
BASIC TYPES OF SEPARATIONS
u Phase Creation [Equilibrium Phase Separations]
e Energy Separating Agent (ESA) [Simple distillation]
e Mass Separating Agent (MSA) [Gas absorption,liquid-liquid
extraction,extractive distillation]
u Use of a Barrier [Membrane Separations]
e Gas [Gas permeation]
e Liquid [Dialysis,reverse osmosis]
e Liquid-solid [Ultrafiltration]
u Use of a Solid [Adsorption]
e Gas or liquid adsorption
eChromatography
u Use of a field or gradient
eGravity [decantation,sedimentation,flotation]
e Electric Field [Electrostatic precipitation,electrodialysis]
9/4/99 Separation Operations,Introduction 9
SEPARATIONS PERFORMANCE CRITERIA
u Product Purity
e Must meet product specs to be saleable (product)
e Must meet regulatory specs to be disposable (byproducts)
u Fractional Recovery
e Fractional recovery of all important components must be high
enough to make process operable and economic
e Generally a trade off between equipment cost and operating
penalties (loss of product,disposal costs,etc.)
9/4/99 Separation Operations,Introduction 10
Feed:
WF
Zi
Product:
WP
Yi
Byproduct:
WB
Xi
SEPARATION SYSTEM
9/4/99 Separation Operations,Introduction 11
SEPARATOR CHARACTERIZATION
u Fractional Recovery (or Split Fraction) FRj
e Defined as the ratio of the amount of a key component j
appearing in the product stream to that in the feed stream
FRj = WD Yj / WF Xj
Note that 0 < FRj < 1
u Split Ratio SRj
Defined at the ratio of the amount of a key component j appearing
in the product stream to that in the byproduct stream
SRj = WD Yj / WB Xj
9/4/99 Separation Operations,Introduction 12
SEPARATION FACTOR DEFINITION
u Separation Factor SFjk (or Separation Power)
e Defined as the ratio of the split ratio of component j to the split
ratio of component k
e Components j and k and are chosen so that the separation
favors j in the product stream and k in the byproduct stream
SFjk = SRj / SRk or
[ SFj / (1 - SFj)]
SFjk =
[SFk / (1 - SFk)]
9/4/99 Separation Operations,Introduction 13
SEPARATION FACTOR INTERPRETATION
uBy the guideline for choosing the key components j and k
SRj > 1,and SRk < 1.
Therefore SFjk > 1.
u If SFjk = 1.,there is no separation.
u Numerical examples for separation factors
e FR = 0.9 for j and 0.1 for k,Then SF = 9 x 9 = 81
e FR = 0.95 for j and 0.05 for k,Then SF = 19 x 19 = 361
e FR = 0.99 for j and 0.01 for k,Then SF = 99 x 99 = 9801
In industrial practice,separation factors of 10,000 or more
are not uncommon.
9/4/99 Separation Operations,Introduction 14
CHOICE OF SEPARATION METHOD
u Choice is a tradeoff between cost and risk
u Separations that can be designed with a high degree of
confidence:
e Simple distillation
e Gas absorption and stripping
u Separations that can be designed to easily give high
product purities
e Distillation
e Liquid-liquid extraction
u Separations that have relatively high operating costs
eDistillation