Ziegler-Natta catalysts are primarily used in polypropylene
(PP) production. These catalysts are very sensitive to various
poisons and their activity varies according to the nature and
level of catalyst poisons. Among others, typical propylene
polymerization catalyst poisons include acetonitrile, arsine,
carbon dioxide, carbon monoxide (CO), carbonyl sulfide,
cyclopentadiene, ethylene oxide, oxygen, palladium, phosphine,
moisture, methanol and propylene oxide.
Catalyst poisons usually are present as impurities within
feedstreams like propylene, ethylene and hydrogen. Each of the
poisons has a varying degree of influence on the catalyst
activity. Though their general behavior is known, it is always
difficult to quantify the losses due to individual poisons
within commercial-size plants. Among the difficulties, accurate
measurement of trace impurities in ppb levels remains the
biggest challenge. Experience shows that there are great
difficulties in offline sampling as well as online sampling.
Changes in process conditions during polymerization is also
another factor. It is not always possible to normalize catalyst
productivities against changing process parameters.
CO is one of the strongest poisons for Ziegler-Natta
catalysts. If not treated properly at the source unit, CO is
present with the propylene feed as a contaminant and it can
reduce catalyst activity drastically (Fig. 1
and Fig. 2). As per the published literature,
a concentration of about 6 ppb (wt%) CO reduces catalyst
activity by approximately 5%.
1. Catalyst activity reduction due to
2. Relative poison strength.
Few PP process technologies specify feed specification as low
as 20 ppb (wt%) in propylene feed to achieve the guaranteed
figures of catalyst activity. If the feed-treating unit is not
designed carefully to knock down the CO levels to the safe
limit, the facilitys bottom line will shrink due to
excessive catalyst usage and cost.
This study from a commercially operating unit shows the
importance of using a proper guard against possible feed
contaminants acting as catalyst poisons. The unit has two
different primary sources of propylene supply. Source A has CO
treatment beds (typically, a sulfur removal bed followed by a
CO removal bed) while Source B doesnt have any guard beds
to protect against CO (Fig. 3). Normally both
sources feed propylene to a PP unit. In this case, one of the
sources is under a shutdown and an alternative Source C is used
to meet the propylene demand.
3. Propylene source configuration.
Fig. 4 shows the effect on catalyst activity
when Source A (without CO guard beds) goes offline and then
comes online again. The daily average catalyst activity is
plotted for an entire month when the polymerization unit is
running at the same residence time and at the same
H2 concentration (same polymer grade) in a gas-phase
Fig. 4 reveals that catalyst activity is
approximately 25% lower when untreated propylene feed Source A
is online. This translates into an additional cost of
approximately $5 million/yr due to excessive catalyst
4. Effect of untreated propylene
on catalyst activity.
Fig. 5 illustrates the effect that varying CO
levels in propylene feed Source A have on catalyst activity for
a period of nine days on a real-time basis. The figure shows
that the CO level is increasing while catalyst activity is
dropping. A loss of approximately 45% catalyst activity is seen
when CO levels go up from approximately 90 ppb to approximately
5. Effect of CO in propylene on catalyst
Fig. 6 portrays a relationship in a commercial
reactor between levels of CO in a propylene feed vs. catalyst
activity when other process parameters are kept constant. A
simple extrapolation yields a further activity rise by 10% to
13% if there is no CO contamination in the propylene
6. Relationship between CO and
activity in a commercial reactor.
Since the source of the CO contamination was known and the
average CO level was confirmed by the online CO analyzer, it
was easier to calculate lost revenue. Various options were
evaluated against the cost and feasibility of carrying out the
In Fig. 7, it was proposed to partially
fill the existing dryer on Source B with CO-removal catalysts.
This proposal would save on potential capital and construction costs associated with
installing a new bed, associated piping and required plot
space. All operational aspects and regeneration were carefully
evaluated and documented accordingly.
7. Present and proposed scheme for
using existing bed with two types of
The end result was a new mixed bed with a molecular sieve and
CO-removal catalysts. After commissioning, an immediate
increase was observed in catalyst activity.
As shown in Fig. 8, CO at the outlet of the
bed decreased from approximately 120 ppb to below 20 ppb. A
quick analysis based on the two month operation revealed a
possible return on investment in less than a year.
8. Catalyst activity before and
mixed bed commissioning.
Keep it clean
To realize full activity from polymerization catalysts, feed
streams need to be as clean as possible from all contaminants
and catalyst poisons. A very low level of even a single
catalyst poison can reduce catalyst activity drastically. In
line with the keep it clean philosophy, the feed
treatment unit must be carefully selected to treat all possible
contaminants. The money invested can give a quick return and
the payback period can be completed in as little as six to nine
Hanif Poorkar is a senior process
engineer for Tasnee in Al-Jubail, Saudi Arabia, where
he has been employed for more than eight years. He
has mainly worked with different polypropylene
process technologies throughout his 16 year career.
He holds a degree in petrochemical engineering
from Dr. Babasaheb Ambedkar Technological University
in Lonere, India, and an MBA degree
from Karnataka University in India.
Hamad Al-Shbrain is the polymer
process engineering manager for Tasnee in Al-Jubail,
Saudi Arabia. He is a chemical engineering graduate
with more than 16 years of experience.
Saad Al-Harbi is an operations
manager at Tasnee in Al-Jubail, Saudi Arabia. He is a
chemical engineering graduate and has worked for
Tasnee for more than 11 years.
Abdullah Al-Saeed is a chemical
engineering graduate from King Saud University in
Saudi Arabia. He currently works as an operations
manager for Tasnee in Al-Jubail, Saudi Arabia.