P
opcorn polymer fouling is thought to be
exclusive to butadiene plants, although there
are reports of isolated cases of popcorn
polymer fouling events in light ends sections of
ethylene plants.
1,2,3
Surprisingly, several of these cases have
occurred in areas where butadiene (BD) bulk concentration
is less than 70%. These observations challenge the
conventional view, i.e. there is no risk of popcorn in areas
with bulk BD concentrations <70%. This article presents the
results of a global study that highlight the relevance of
popcorn fouling in steam cracking, along with a discussion
of methods to identify industrial samples as popcorn.
Proliferous polymerisation
Butadiene polymer fouling may manifest itself in several
different forms, commonly described as sheet, rubber,
glassy, amber or popcorn polymer. The diversity in the
physical appearance of polybutadiene based foulant is
largely due to the various isomers of polybutadiene
(Figure 1). Popcorn polymer is unique among the polymer
morphologies because of its rate of formation, high
degree of cross linking, ability to damage equipment, and
unpredictable pyrophoric nature.
Popcorn polymer is formed via proliferous
polymerisation, an abnormal reaction encountered in free
radical polymerisation systems.
4,5
The initiation of
proliferous polymerisation is closely linked with the
presence of peroxides and corrosion byproducts,
particularly haematite (Fe
2
O
3
). Once an active popcorn
seed is present, no outside catalytic influence is required
for growth; popcorn polymer propagates when fed with
butadiene monomer either in the gas phase or liquid
phase. Consequently, once popcorn fouling does occur,
the olefins producer is at greater risk for subsequent
events due to residual active seeds that may remain.
Reports of severe popcorn fouling events often reveal
that mild popcorn accumulation had been found
previously.
1
Proliferous polymerisation propagates at a constantly
accelerating rate via a phenomenon of seed generation;
parent popcorn seeds produce subsequent generations of
popcorn seeds. Propagating seeds are subject to high
internal strain caused by the polymerisation of reactive
monomers that have diffused into the core. The internal
strain imparts distinct morphological features which can
be used to identify samples as popcorn via microscopic
techniques.
6
Recently, the unusual growth behaviour is
being exploited to detect popcorn growth online via a
non-invasive acoustic sensor.
7
Popcorn polymer is not exclusive to butadiene.
Several conjugated dienes can undergo proliferous
polymerisation, including styrene, isoprene, piperlyene and
methacrylates.
8-10
However, some monomers require the
presence of a cross linking agent. Resultant polymer
formed via proliferous polymerisation occupies
significantly more volume relative to conventional linear
polymerisation.
3
On an industrial scale, the force exerted
by popcorn polymer growth on its surroundings is enough
to damage metallurgy of piping or vessels, posing a serious
safety and environmental hazard.
4
Popcorn samples recovered from butadiene and
ethylene plants commonly contain a combination of
popcorn and glassy polymer. There is a relationship
between the activity of popcorn and its physical
appearance. In laboratory studies, Miller et al. investigated
consecutive generations of polybutadiene popcorn seeds
exposed to butadiene vapour by measuring growth
activity and morphology.
10,11
Initially, the activity of
popcorn increased from generation to generation until a
maximum growth rate was reached during the generations
8 - 12. The morphology of these active generations had a
distinct popcorn like structure. Subsequent generations
had a lower growth activity; these samples had
considerable amount of a dense, glassy like structure. In
summary, propagation of popcorn polymer ultimately
yields lower activity glassy polymer.
Studies in Dow Benelux B.V., ‘Dow’, were
conducted in liquid butadiene (Figure 2).
12
The findings
were consistent with the studies in butadiene vapour. The
growth rate peaks at the 7
th
generation, and up until the
12
th
generation the growth rate trend is described by a 4
th
order polynomial. Generation 22 and generation 30
samples were stored for 8 and 10 weeks, respectively in
water purged with nitrogen before additional experiments
were carried out. In both cases, the storage resulted in a
decrease of growth activity more marked than the trend.
Oxidation and age have an impact on the activity and
appearance. Aged and oxidised popcorn is less active than
fresh oxidised popcorn, and oxidised popcorn turns to a
yellow colour (Figure 1).
13
Popcorn observations in
steam crackers
Recent learning from incidents (LFIs)
1,2
inspired the
execution of a non-exhaustive global survey to elucidate
the frequency of popcorn formation in light ends.
3,14
Popcorn fouling events in debutaniser overhead
condensers of gas (ethane/propane) crackers with >70%
bulk butadiene concentration were not included since this
section is known to be at risk for popcorn polymer fouling;
chemical protection in this area is an industry standard
practice. The survey discovered 34 suspected popcorn
occurances of varying degrees of severity in light ends
locations with <70% bulk butadiene over a 17 year time
span. The results, summarised in Figure 3, include 19
observations in depropanisers (trays, pressure relief values,
reboilers and gaskets) and six observations in top section
of debutanisers (overhead condenser, pressure relief value
and trays). The most surprising results were popcorn in
deethaniser reboilers (four observations) and bottom
section of debutanisers (five observations). These locations
are particularly astonishing given that the bulk butadiene
concentrations are very low. The deethaniser popcorn
events were severe and damaged the reboiler tube
bundles. In the case of the debutaniser bottom examples,
the samples contained copolymers of C5 monomers, such
as isoprene and piperylene. The popcorn formed between
downcomer pieces in the vapour spaces or the reboiler.
Porous gasket material increases the risk of popcorn
fouling. This material can absorb monomer, providing
increased residence time for reactions to occur.
Popcorn fouling in light ends towers has one thing in
common: vapour space is more vulnerable, e.g. under sides
of trays, above the liquid level in horizontal reboilers,
between downcomer metal pieces, or pressure relief
values. Additionally, it was found that popcorn seeds
could fall into the liquid phase or be distributed
throughout the unit, and continue to propagate.
Jessica M.
Hancock and
Debby Rossana,
Nalco Champion,
an Ecolab
Company, USA
,
and Stefanus
J. Korf, Dow
Benelux B.V.,
The Netherlands,
present a global
survey of popcorn
observations during
ethylene production
and include a
method to diagnose
industrial samples.
POPCORN
IN LIGHT ENDS
25
December
2014
HYDROCARBON
ENGINEERING
