Ethylene and other monomers need to be purified before they are polymerized. This purification is done using a two bed thermally regenerated adsorption unit. Type 13X is commonly used for this purification because of its high capacity for polar molecules. The problem is that 13X also adsorbs ethylene. This releases heat that initiates undesirable exothermic polymerization reactions. The purification process can be operated successfully, but it requires the use of specific processing steps. This includes the partial preloading of the freshly regenerated bed with ethylene prior to contacting the bed with the feed ethylene.
Some plants have used an adsorbent that contains molecular sieve within an activated alumina matrix. The idea being that when the high capacity molecular sieve adsorbent is dispersed within a lower capacity adsorbent that there would be over-all less ethylene adsorption resulting in less local heat rises. This lower capacity, lower heat generating, adsorbent still requires the preload step.
Regardless of which adsorbent is used, there have been problems. Solidified adsorbent masses have formed in the adsorbent bed ranging from size of basketballs to essentially all the bed having been solidified into a mass of polymer. The ceramic material used as support of the adsorbent bed has solidified into a pancake mass where only the area next to vessel wall is unaffected. There have been occurrences of serious heat formed inside the vessel as evidenced by the ceramic bed support particles being semi melted together, and being cracked and fused together with polymer. This heat is localized since no temperature increases are detected by any of the thermocouples in the adsorbent section. These solidification problems usually show up as high pressure drop across the adsorbent bed. Left uncorrected they will lead to loss of purification performance resulting in downstream reactor blockage and unscheduled plant shut-down.
If you have experienced some of these problems with your monomer purification, we can help. Successful operation can be obtained with your 13X or with a blended adsorbent.
**The above information is for customers’ reference only. Performance characteristics vary depending on specific application. please contact us by email at firstname.lastname@example.org if detailed application method for molecular sieve needed.
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