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%.
| Fig. 1. Catalyst activity reduction due to |
| Fig. 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.
| Fig. 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 reactor.
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 consumption.
| Fig. 4. Effect of untreated propylene source |
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 250 ppb.
| Fig. 5. Effect of CO in propylene on catalyst activity.|
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 feed.
| Fig. 6. Relationship between CO and catalyst |
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 change.
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.
| Fig. 7. Present and proposed scheme for |
using existing bed with two types of material.
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.
| Fig. 8. Catalyst activity before and after |
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 months. HP
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.