Hydrocarbon Processing - March 2021 - 47

Special Focus

Petrochemical Technology
B. CROM, D. HOOD and N. PATEL, SUEZ-Water Technologies
& Solutions, Tomball, Texas; and M. HONG, SUEZ-Water
Technologies & Solutions, Shanghai, China

A novel polymerization retarder with boosted
performance and improved handling characteristics
Styrene monomer production plants continually face a need to
slow polymerization rates in the purification section of the production process to increase monomer production rates, reduce polymerization/fouling and tar formation, and protect against equipment plugging, especially in the event of unplanned shutdowns.1,2
This protection is typically accomplished by the application of
polymerization retarders and true polymerization inhibitors, two
related chemistry classes that slow the propagation of polymers.1,2,3
The polymerization of styrene monomer begins with the
formation of styryl radicals. Under anaerobic conditions, styryl
radicals can go on to react with molecules of styrene, forming
larger radicals that can then further react with styrene molecules,
and the chain can continue until the radicals react with another
radical, terminating the reaction.4,5,6,7 Under aerobic conditions,
oxygen will react with styryl radicals at a much faster rate than
styryl radicals can react with styrene molecules, thus forming
styryl-peroxy radicals. When styryl-peroxy radicals are present,
many more propagation reaction paths exist.8,9,10,11
A true polymerization inhibitor is defined as a chemistry
that completely halts the rate of polymerization while it is present in the monomer solution. The inhibitor is consumed by the
process of inhibition, and when fully consumed, the rate of polymerization proceeds at an uninhibited rate. A polymerization
retarder, by contrast, does not halt polymerization, but rather it
only slows the polymerization rate. Generally, the retarder is not
consumed during the retardation process, or is regenerated so
that it can continue to react.3
The gold standard retarders in the styrene monomer industry
for the many years have been a series of " dinitro " compounds,
such as 2,6-dinitro-p-cresol (DNPC; CAS Registry Number
609-93-8); similar compounds in this family that have been
used as retarders throughout the years include 2,4-dinitrophenol
(DNP; CAS Registry Number 51-28-5), 2,4-dinitro-o-cresol
(DNOC; CAS Registry Number 534-52-1), and 2,4-dinitro6-sec-butylphenol (DNBP; CAS Registry Number 88-85-7).12
While the cost performance of these retarders is well established,
so is the toxicity and potential for environmental, health and
safety related events, including personnel exposures and environmental releases.13,14 Even if no unexpected events take place,
during turnaround and maintenance operations, costly decontamination procedures are required before equipment that has
been exposed to one of these dinitro compounds can be opened
for inspection, cleaning or repair.

As an alternative to these toxic dinitro compounds, several
process additive companies have developed a first generation
of environmentally friendly retarders, most of which are based
on quinone methide type chemistries.15 When these retarders
were initially introduced to the market more than 20 yr ago,
they showed equivalent performance to the incumbent dinitro
compounds at lower dosages; however, the cost was often two to
three times higher than the cost of the incumbent dinitro compounds, frequently making these environmentally friendly solutions uneconomical. With time, however, production processes
were optimized and raw material costs were controlled, allowing
these retarders to become cost competitive. Advantages, such as
lower personnel exposure risks, reduced wastewater treatment
system impacts, reduced decontamination costs before turnarounds, and reduction in nitrogen oxide (NOx ) generation potential from tar burning, can now be realized through this firstgeneration, environmentally friendly retarder.
This type of first-generation, environmentally friendly styrene retarder has proven success. One service providera that has
reported operating nine applications globally has been feeding
this type of process additive since the early 2000s. A typical example of a styrene plant's experiences with this type of retarder
is described here.
Case study. A world-scale styrene plant of a major designb was
feeding DNBP along with a true inhibitor. Due to a desire to transition away from this toxic retarder, the facility trialed a first-generation, environmentally friendly retarder. The trial was designed
to systematically switch the unit from a treatment program consisting of DNBP with a true inhibitor to a treatment program
employing a first-generation, environmentally friendly retarder
in combination with two different true inhibitors in three phases.
The first phase (denoted as " Phase 1 " in FIGS. 1-3) introduced
a new true inhibitor (Inhibitor 1) at a slightly lower dosage to
the previously used inhibitor; the second phase (denoted as
" Phase 2 " in FIGS. 1-3) introduced a second true inhibitor (Inhibitor 2) to optimize inhibitor cost performance. During this second phase, the combined dosage of the true inhibitors was lower
than the dosage of the pre-trial inhibitor dosage. Note: Due to
this plant's unique recycle stream from a downstream polystyrene unit, a spike in the polymer numbers was seen during this
phase, caused by an abnormal polymer recycle event.
The third phase (denoted as " Phase 3 " in FIGS. 1-3) involved
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Hydrocarbon Processing - March 2021

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