Using Separations in Chemical Processing
1
2
4
reactor
separator
5
3
6
raw
materials
products
recycle stream
Where are separations needed?
• Purification of reactor feeds
• Purification of products for sale
• Purification of waste for safe disposal
Separations as Unit Operations
• The specific design of the separator depends
on the chemical composition of the feed, and
the desired purity of the product
• However, the general design principals are
independent of the chemistry
Column distillation
At an oil refinery, fractional distillation columns separate
hydrocarbons into separate streams, cuts or fractions
A multi-purpose distillation column for mineral oils and chemicals
40 trays with multiple feed entry options, capacity: up to 45 mt/h
mode: vacuum, atmospheric or pressure up to 3 bar
temperature: up to 320°C
Flash vaporization
Flash drum for hydrocarbon vapor recovery
Water desalination plant in Cyprus
Multistage flash distillation
Absorption and stripping
A column filled with an amine solution
is used to absorb H2S and CO2 from
“sour” natural gas
A steam stripping column removes
H2S and CO2 to regenerate the amine
Liquid-liquid extraction
Mixer-settlers used for continuous, counter-current
liquid-liquid extraction of rare-earth ions
Leaching
Cyanide leaching of gold ore, Nevada
Sublimation
Sublimation of HgI2 for use in semiconductor manufacturing,
as well as in detectors for X-ray and g-ray imaging
Crystallization
Multiple-Effect Crystallizer for Sodium Sulfate
(Na2SO4) Refining
Crystallizer for Salt (NaCl)
Chromatography
Chromatography columns
Membrane filtration
Water Treatment Plant. Each white vessel contains seven
spiral-wound membrane units.
Why is good design important for separations?
• Separations equipment can be 50-90 % of the capital
investment in a chemical plant
• Separations can also represent 40-70 % of operating
costs
• Purity requirements depend on market tolerance
– High separations costs tolerated for high valueadded products
Examples
1. Petroleum refining
crude oil  gasoline, diesel, jet fuel, fuel oil, waxes, coke, asphalt
2. Pharmaceuticals
sub-ppm level of metal catalyst required for human consumption
3. Semiconductors
SiO2  SiCl4  Si
Metallurgical grade (97%) for alloying with steel and Al: $1/kg
Solar grade for photovoltaics (99.99 %): $80/kg
4. Water treatment
Industrial wastewater vs. potable water
Some impurities ok (Ca2+); others not (Hg2+)
Process Diagram for Ethylene hydration:
C2H4 + H2O  C2H5OH
Why do separations cost a lot?
• “unmixing” causes reduction in entropy
• this is not spontaneous
• achieve by adding an external separating agent
–
–
–
–
Energy (distillation)
Material (e.g. extraction)
Barrier (e.g. membrane)
Gradient (e.g. electrophoresis)
Table 1. Separation Unit Operations based on Phase Creation or Addition
vertical drum
horizontal drum
valve
column with trays (stages)
heat exchangers
condensor
reboilers
Table 1, cont. Separation Unit Operations Based on Phase Creation or Addition
heater
Table 1, cont. Separation Unit Operations Based on Phase Creation or Addition
Table 2. Separation Unit Operations based on a Solid Separating Agent
Table 3. Separation Unit Operations Based on the Presence of a Barrier
Table 4. Separation Unit Operations Based on an Applied Field or Gradient
Equilibrium-staged separations
• Make use of thermodynamics to achieve
spontaneous separation
• But thermodynamics also dictates the limits of
the separation
Definitions of equilibrium
vapor
thermal equilibrium: Tliq = Tvap
mechanical equilibrium: Pliq = Pvap
liquid
chemical equilibrium: mliq = mvap
(chemical potential)
Equilibrium is dynamic: molecules continue to vaporize and condense, but at
equal rates, so there is no net change in either phase.
Rate of approach to equilibrium depends on:
(1) rate of mass transfer
proportional to (a) mass transfer coefficients Ki = f(T), and (b) interfacial area
(2) concentration gradient
becomes very small as equilibrium is approached, ∞ time required to achieve
Consider a single equilibrium stage
• V and L are in equilibrium with each other; they
are streams leaving the same equilibrium stage.
• V and L are not in equilibrium with F,
i.e., miL = miV ≠ miF
vapor product
flow rate V
T, P
composition yi
T, P
feed
vapor
flow rate F
TF, PF
composition zi
liquid
liquid product
• if > 1 chemical species present, then xi ≠ yi
• therefore separation has occurred
• vapor-liquid equilibrium (VLE) limits the
amount of separation that can be achieved
flow rate L
T, P
composition xi
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Cascade of equilibrium stages
What if we need more separation than one equilibrium stage can provide?
Feed one of the two product streams (e.g., L) to another equilibrium stage
V
1
F
V2
stage 1
V3
stage 2
L
1
L2
stage 2
L3
• Creates many vapor streams with different compositions
• If we combine (mix) them, we destroy some of the separation we created
• If we discard them, our yield is low.
Better Alternative:
Counter-current cascade
F
Variable pressure cascade
P1 > P2 > P3
1
L1
V2
2
L2
V1
1
L1
compressor
• replace
• replace
V2
by
2
by
L2
V3
3
L3
F
V1
Variable temperature cascade
T1 > T2 > T3
V3
3
L3
An even better alternative
F
V1
Integrate the heat exchangers:
allow contact between condensing
vapor and vaporizing liquid streams
F
V1
1
liquid
downcomer
L1
V2
2
L2
weir
V3
perforated tray
1
V2
L1
2
V3
L2
3
vapor
3
L3
L3
• integrated column is isobaric and non-isothermal
• promotes mixing of liquid and vapor phases
Thermodynamic considerations
• Perfect separation requires an infinite number of equilibrium
stages
• The engineer specifies the number of stages required for an
acceptable degree of separation
• Equilibrium is not achieved on each stage in a finite time
theoretical stage: assume equilibrium is achieved
actual stage: equilibrium is not achieved (< 100 % efficiency)
• We always need more than the theoretical number of stages to
achieve the desired separation
• The engineer’s role is to decide how many more
General design procedure for
equilibrium-staged separations
1. Obtain relevant equilibrium data (where?)
* 2.
Determine no. of theoretical stages required
3. Determine no. of actual stages required
(requires knowledge of stage efficiency)
4. Size equipment, based on expected flow
rates F, V, L
* Focus of this course.