Chemical Engineering May 2018 - 40

Technology Profile
Propylene Oxide Production
By Intratec Solutions
P
ropylene oxide (PO) is a
highly reactive commodity
compound that serves
as a starting material for
several widely used products, from
polymers and solvents to industrial
fluids. Historically, PO has been produced
by the so-called chlorohydrin
process, which is further described
in this article. Currently, new production
technologies that generate less
waste are starting to replace the
chlorohydrin production process.
The process
The following
describes a typical
chlorohydrin process for PO production
from propylene and chlorine
(Figure 1).
Hypochlorination. Initially,
gaseous polymer-grade
fresh,
propylene
and chlorine are mixed with water
to form an aqueous solution. Such
compounds react to produce a
propene-chloronium complex, an
intermediate that reacts with water
to yield hydrochloric acid and propylene
chlorohydrin (PCH) isomers. The
gaseous effluent from the reactor is
fed to a separator, which separates
a solution containing PCH from the
vent gas. The PCH solution is directed
to the epoxidation stage, while
the vent gas is passed through a
caustic scrubber and released.
Epoxidation. The PCH solution and
a caustic solution (from a chlor-alkali
unit) are fed to a saponifier, where a
dehydrochlorination
reaction
takes
place. Here, the PCH is converted
to propylene oxide, while organic impurities
are stripped from the brine.
Caustic
soda
3
2
CW
Caustic
solution
1
Propylene
Chlorine
Water
Steam
11
4
6
Acid Caustic Water
9
Treated brine
40
10
ST
DCP
N2
Heavy ends
FIGURE 1. The process diagram here shows the production of propylene oxide from propylene and chlorine
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14
5
7
Heavy ends
CW
12
13
8
Propylene oxide
Off-gas
Propylene,
chlorine
Propylene,
ethylbenzene
Propylene,
acetaldehyde
Chlorohydrination
Oxidation/
epoxidation
Oxidation/
epoxidation
Propylene oxide
Epoxidation
Oxidation/
epoxidation
Direct oxidation
FIGURE 2. There are several production pathways to arrive at propylene oxide
n Raw material n Process n Main product
The brine from the saponifier bottom
is directed to a treatment step, while
a stream containing PO from the saponifier
overhead is routed to purification
steps.
Purification. The PO-rich stream is
distilled in a first column to obtain a
crude propylene oxide stream, which
is removed from the column overhead.
A solution containing dichloropropane
(DCP) byproduct from the
column bottom is decanted, yielding
a DCP-rich stream and water. In a
second column, residual light ends
are stripped off from the crude PO
stream and burned for fuel. The column's
bottom product is fed to a third
column, which further removes impurities
from the PO. A 99.9 wt.% PO,
withdrawn from column overhead, is
condensed and routed to storage facilities.
Heavy ends from the column
bottom are burned for fuel.
The DCP-rich stream is treated with
an acid-caustic-water wash, for the
neutralization and removal of epoxidation
byproducts, and then distilled
for the removal of heavy components.
The brine stream from the epoxidation
step and the wastewater from the
first column are treated by biodegradation
and filtration. The treated brine
is returned to the chlor-alkali facility.
Light ends
Production pathways
There are two main routes for PO production
- one based on the dehydrochlorination
of propylene chlorohydrin
with a base, and the other based on
the oxidation of propylene by an organic
hydroperoxide (Figure 2).
Economic performance
The total operating cost (raw materials,
utilities, fixed costs and depreciation
costs) estimated to produce
propylene oxide was about $2,360
per ton of propylene oxide in the
second quarter of 2014. The analysis
was based on a plant constructed
in the U.S. with capacity to produce
500,000 metric ton per year of
propylene oxide.
This column is based on " Propylene
Oxide from Propylene and
Chlorine - Cost Analysis, " a report
published by Intratec. It can be
found at: www.intratec.us/analysis/
propylene-oxide-e11a.
n
Edited by Scott Jenkins
Editor's note: The content for this column is supplied
by Intratec Solutions LLC (Houston; www.intratec.us) and
edited by Chemical Engineering. The analyses and models
presented are prepared on the basis of publicly available
and non-confidential information. The content represents
the opinions of Intratec only. More information about the
methodology for preparing analysis can be found, along with
terms of use, at www.intratec.us/che.
1. Hypochlorination reactor
2. PO decanter
3. Caustic scrubber
4. Saponifier
5. Water removal column
6. DCP decanter
7. Light ends column
8. Heavy ends column
9. Acid/caustic/water washes
10. DCP column
11. Treatment unit
12. Cooling tower
13. Steam boiler
14. Air separation unit
CW Cooling water
ST Steam
N2 Nitrogen
Propylene,
hydrogen
peroxide
Propylene,
isobutane
Propylene,
oxygen

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Chemical Engineering May 2018

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