TECHNOLOGY Pilot-plant results in on fixed-bed alkylation process

A number of catalyst manufacturers and process licensors are working on alternative catalysts for refinery alkylation units. Haldor Topse A/S, Lyngby, Denmark, has come forward as the first such company to release results from pilot plant testing.

Haldor Topse's supported-liquid catalyst is designed to replace the liquid acids traditionally used in alkylation units (HF and H₂SO₄). The fixed-bed process uses a liquid "super acid" supported on a solid support to alkylate isobutane and isopentane with olefins.

"As a consequence of the unique interaction between the catalyst and the feed stream," says Topse's Sven Ivar Hommeltoft, "the catalyst in the reaction zone is kept essentially free of ASO (acid soluble oil), even under conditions passivating much of the catalyst." Hommeltoft and Bent Sarup reported the results of Topse's research in an unpublished paper titled, "Fixed Bed Alkylation with a Supported Liquid Catalyst in a Moveable Catalyst Zone: A Sturdy and Flexible Technology."

Topse has completed 9,000 hr of testing in its 0.5 b/d pilot unit. In this testing, various feeds were alkylated under an array of operating conditions. Pilot-plant results show that product octane is highest when alkylating n-butene at low temperatures and low olefin concentrations.

Additional bench-scale tests have shown that the process can be used at temperatures between -40 C. and 80 C., say Hommeltoft and Sarup.

Pilot tests

Topse has operated its pilot plant for several years. Operators control the temperature in the adiabatic reactor by recycling cooled effluent to the inlet. Table [14,507 bytes] shows octane numbers for alkylate produced from four feeds.

The researchers stress that octane values produced when alkylating a typical C₄ feed from a fluid catalytic cracking unit (FCCU) should be up to 3 numbers higher than those for the isobutene/1-butene mixture shown in Table 1[14,507].

During the first 2,000 hr of the 3,800 hr pilot plant run, Topse's goal was to demonstrate process stability and operability using two types of C₄-olefin feed:

Pure 1-butene (During this run, refinery MTBE raffinate was substituted for 1-butene for a brief period, with no significant change in operation or product quality. A mixture of 1-butene containing 3% isobutene, simulating MTBE raffinate, also was run.)
A mixture of 70% 1-butene and 30% isobutene (This mixture simulated a full C₄ olefin mixture from an FCCU.)

Fig. 1[35220 bytes] shows the results of operation on these feedstocks.

Unit operation was stable for the first 1,200 hr, after which time the isobutene content was increased to 30%. The remaining 1,800 hr of the 3,800-hr run were used to study the effects of operational changes.

Table 2 [14180] shows octane and yield results for the alkylation of 1-butene at varying temperatures and isobutane-to-olefin (I/O) ratios.

Pilot-plant operation also demonstrated that:

The reactor system can be stopped and restarted quickly.
The hydrocarbons can be drained from the reactor, leaving the acid on the support.
The catalyst can be left in the reactor for any length of time, under either hydrocarbons or nitrogen, without losing catalyst activity.
The process can handle large changes in flow rate.

Catalyst passivation

The same feed impurities that passivate conventional alkylation catalysts also passivate the supported-liquid catalyst.

Although the pilot tests showed that ASO formation depends on reaction conditions, passivation rates for the process were significantly lower than for sulfuric acid alkylation, according to the paper.

In addition, "When alkylating a mixture of 30% isobutene and 70% 1-butene," say Hommeltoft and Sarup, "the catalyst passivation rate was found to be approximately 50% more than when pure 1-butene was alkylated."

The passivated catalyst can be withdrawn from the unit without interrupting the process. According to the paper, "The catalyst can be recovered on site by a simple process not involving import/export of chemicals.

The only by-product is a highly unsaturated oil, most of which boils in the diesel range."

Operating conditions

Optimal operating conditions for the process will depend on the requirements of the individual refiner, says Haldor Topse.

Low-temperature operation maximizes alkylate octane value, particularly when alkylating pure n-butene.

In many cases, however, the octane increase does not justify investment in a refrigeration unit.

Phil Geren, Haldor Topse Inc., Houston, speaking at the Oil & Gas Journal International Catalyst Conference, said the process will be licensed by the end of the year.