Using Separations in Chemical Processing

agreementkittensSemiconductor

Nov 2, 2013 (3 years and 1 month ago)

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Using Separations in Chemical Processing

reactor

separator

1

2

3

4

6

5

raw

materials

products

r
ecycle 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

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

At an oil refinery, fractional
distillation
columns separate
hydrocarbons into separate streams, cuts or fractions

Column distillation

Flash vaporization

Flash drum for hydrocarbon vapor recovery

Water desalination plant in Cyprus

Multistage flash distillation

A steam stripping column removes
H
2
S and CO
2

to regenerate the amine

Absorption and stripping

A column filled with an amine solution
is used to absorb H
2
S and CO
2

from
“sour” natural gas

Liquid
-
liquid extraction

M
ixer
-
settlers used
for continuous, counter
-
current
liquid
-
liquid extraction of
rare
-
earth ions

L
eaching

Cyanide leaching of gold ore, Nevada

Sublimation

Sublimation of HgI
2

for use in semiconductor manufacturing,
as well as in detectors for X
-
ray and
g
-
ray imaging

Crystallization

Multiple
-
Effect Crystallizer for Sodium Sulfate
(Na
2
SO
4
) 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 value
-
added products

Examples

1.

Petroleum refining

c
rude oil



gasoline, diesel, jet fuel, fuel oil, waxes, coke, asphalt


2.

Pharmaceuticals




sub
-
ppm level of metal catalyst required for human consumption


3.

Semiconductors

SiO
2




SiCl
4




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 (Ca
2+
); others not (Hg
2+
)

Process Diagram for Ethylene hydration:

C
2
H
4

+ H
2
O



C
2
H
5
OH

Why do separations cost a lot?



unmixing
” causes reduction in entropy



t
his 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

c
olumn with trays (stages)

v
ertical drum

horizontal drum

valve

h
eat 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

liquid

vapor

t
hermal equilibrium:
T
liq

=
T
vap

m
echanical equilibrium:
P
liq

=
P
vap

chemical equilibrium:
m
liq

=
m
vap

(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 K
i

= f(T), and (b) interfacial area

(2)
c
oncentration gradient


becomes very small as equilibrium is approached, ∞ time required to achieve

Consider a single equilibrium stage

25

liquid

vapor

feed


flow rate F

T
F
, P
F

c
omposition
z
i

vapor product


flow rate V

T, P

c
omposition
y
i

liquid product


flow rate L

T, P

composition
x
i

T, P


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.,
m
i
L


=

m
i
V



m
i
F


if > 1 chemical species present, then
x
i


y
i


therefore separation has occurred


vapor
-
liquid equilibrium (VLE) limits the
amount of separation that can be achieved

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

• 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.

s
tage 1

F

V

L

stage 2

V
2

L
2

stage 2

V
3

L
3

1

1

Better Alternative:

Counter
-
current cascade

1

F

V
1

L
1

V
2

2

L
2

V
3

3

L
3

• replace by


• replace by

1

F

V
1

L
1

V
2

2

L
2

V
3

3

L
3

Variable temperature cascade

T
1

> T
2

> T
3

Variable pressure cascade

P
1

> P
2

> P
3

compressor

weir

p
erforated tray

3

2

1

F

V
1

L
1

V
2

L
2

V
3

L
3

An even better alternative

Integrate the heat exchangers:

a
llow contact between condensing
vapor and vaporizing liquid streams

downcomer

• integrated
column
is
isobaric

and
non
-
isothermal

• promotes mixing of liquid and vapor phases

vapor

liquid

1

F

V
1

L
1

V
2

2

L
2

V
3

3

L
3

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.