Cat-Poly, Butene Dimerization, and Oligomerization technologies are some of the various names given to a family of technologies that oligomerize LPG-range olefins, typically n-butenes, into C8 olefins. The reaction occurs over a bed of solid acid catalyst at elevated temperature (>250°C). During the reaction, higher oligomers including C12 and C16 are formed, and some of the product is cracked to lighter products (C5-C7). The large reaction exotherm makes controlling the product slate difficult. The resulting product has good RON but a poor MON, often around 81, due to the olefin content.
Revamping one of these units to ExSact alkylation yields great benefits in product quality including raising the MON by 13 points to 94, eliminating olefin content, and narrowing the distillation curve.
The revamp can be accomplished with minimal capital investment in many situations. A study commissioned by a refiner to evaluate the conversion of an Oligomerization unit to ExSact alkylation revealed many opportunities for cost savings.
Feed Zone: Using a feed of MTBE raffinate, the existing unit had no feed pre-treatment operations. The ExSact catalyst was shown to function effectively without pre-treatment using the refiner’s feed.
Reaction Zone: Oligomerization reactors are typically a pair of fixed bed reactors designed for operation at high temperatures (400°C). The reactors are also divided into 4 or 5 beds with interbed cooling and olefin injection to help control the exotherm. These reactors are ideally suited for use in ExSact with minimal or no modification. A recycle pump is added around each reactor.
Separation Zone: The existing debutanizer and depropanizer were kept in their current service, while a de-ethanizer was converted for use as a reactor effluent surge vessel. An isostripper was added to the train to allow isobutane to be recycled to the reaction zone.
Regeneration Zone: An existing vapor-phase loop used for reactor recycle in the oligomerization unit was converted to serve as the catalyst regeneration loop in ExSact. No new equipment was required for the conversion.
Cost Savings
Additional capital expenditures for the revamp of an oligomerization unit are very small. In the unit studied, an existing de-isobutanizer column was available. The only new equipment required are a few pumps. In other unit designs, capital costs are estimated to be only about 10% of a grassroots ExSact plant due to the ability to re-use the reactors, distillation train, and regeneration equipment.
Further Reading
Verbitsky, D., M. Mukherjee, and J. Nehlsen, “Upgrading an olefin polymerization unit,”
Petroleum Technology Quarterly, Vol. 12, No. 4, Q3 (Jul/Aug), 2007.
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