Oct-1997

A cheaper way of alkylating propylene and amylenes

The authors describe how propylene and amylenes can be alkylated using much less sulphuric acid catalyst

J Randall Peterson, David C Graves, Ken Kranz and David M Buckler,
Stratco Inc

Article Summary

Recent research has shown that propylene and amylenes can be alkylated more economically than previously reported. The benefits are lower sulphuric acid consumption and higher alkylate octanes. Dramatic acid savings are realised when a propylene-rich feed is alkylated in the higher acid strength reaction zones and the resulting acid is staged to butylene and/or amylene-rich reaction zones.

The improvements in amylene alkylation are achieved by keeping the cyclopentene out of the alkylation unit feed via fractionation, using a selective hydrogenation unit to remove the diolefins, and processing only amylenes in the final acid stage at a lower than normal spent acid strength. In fact, research shows that amylenes can be added to this final acid stage in quantities sufficient to produce up to 25 per cent more alkylate without increasing the makeup acid rate to the unit.

Refiners around the world are being forced to lower the Reid vapour pressure (Rvp) and olefin content of their gasoline products. The lower Rvp helps the environment by reducing evaporative hydrocarbon (HC) emissions. Lowering the olefin content of gasoline reduces the reactivity (smog-forming potential) of those emissions. Amylene alkylation helps the environment by removing the high Rvp C5 olefins from the FCC gasoline stream and alkylating them to predominantly C8 and C9 low-Rvp paraffins.

Propylene alkylation produces predominantly C7 and C8 paraffins that have a low Rvp and an excellent (low) Driveability Index (DI). Increased alkylate benefits the gasoline pool with lower Rvp, less olefins, higher octane numbers, improved DI, and more gasoline volume which, in turn, lowers sulphur, aromatics and benzene by dilution. Alkylate is well documented as a very clean burning fuel.

Although alkylate has been termed the “perfect gasoline blending component” by several automobile and environmental experts, most refiners have avoided alkylating their propylene and amylenes due to an anticipated high acid consumption. We encourage refiners to reevaluate the economics of propylene and amylene alkylation based on this new information. Alkylation unit operators generally spend their sulphuric acid at approximately 90 wt% H2SO4 in order to maintain a reasonable safety margin above minimum acid spending strengths.

If mostly amylenes are fed to the final stage, however, the spent strength can be lowered significantly and still maintain a similar safety margin. This is due to the unstable nature of the amylene alkylate intermediates in the acid phase. They react out of the acid phase to form alkylate much more readily than propylene or even butylenes. Spending at a lower strength saves considerable acid costs.

Amylene case study
Consider the recovery of C5 olefins from 45452bpd of 7.0psi Rvp FCC gasoline using a depentaniser. All cases assume that the diolefins and other contaminants (besides cyclopentene) have been removed prior to alkylation:
Case 1, the High Recovery case. This assumes that 96 per cent of the C5 olefins are recovered for alkylation, lowering the Rvp of the FCC gasoline to 2.2psi. In this case, 66 per cent of the cyclopentene in the FCC gasoline is sent to alkylation. The spending strength of the acid is assumed to be 90 wt%.

Case 2, the Low Recovery case. This assumes that 79 per cent of the C5 olefins are recovered, lowering the Rvp of the FCC gasoline to 2.7psi. Less than 10 per cent of the cyclopentene is alkylated in this case. The spending strength of the acid is assumed to be 90 wt%.

Case 3, the C4= Only case. Assumes an MTBE raffinate alkylated under typical conditions. The spending strength of the acid is assumed to be 90 wt%.

Case 4, C4=/C5=/Low Acid Strength case. Assumes that the Case 3 MTBE raffinate is alkylated at the higher acidities while the Case 2 amylene feed goes to a final acid stage running at 87 wt%. A flow diagram for Case 4 is shown in Figure 1. Case 1 produces 22 per cent more alkylate than Case 2 but the alkylate is 1.7 octane numbers lower on average, a result of more cyclopentene in the olefin feed. Also, because of the cyclopentene, the acid consumption of Case 1 is 65 per cent higher than Case 2.

Case 3 is shown to allow a comparison for Case 4. Case 4 produces 79 per cent more alkylate than Case 3 with only 48 per cent more makeup acid. The incremental alkylate in Case 4 is made from amylenes alkylated in a new reaction section operated at an acid strength of 87 wt%.
Note that the alkylate made from the butylenes has higher octane numbers than Case 3 while the amylene alkylate is slightly lower in octane than in Case 2. This is due to the higher and lower average acid acidities, respectively.

Cases 2 and 4 will require a larger depentaniser between the FCC and alky and more utilities than Case 1 because of the extra reflux required to keep more of the cyclopentene out of the alkylation unit. However, the additional capital and utilities required by the depentaniser will be offset by lower capital and utilities required in the feed-treating and alkylation units. Case study results are given in Table 1.

Propylene research
A major research effort for C3= alkylation is currently under way and it has been discovered that the historically high rates of acid consumption for propylene are avoidable if alkylated under desirable conditions and with a proper unit configuration.

Refiners who have alkylated propylene in the past have combined it with butylenes and alkylated them together. The problem with this configuration is that a significant portion of very stable propylene intermediates dilutes the acid and then exits the unit with the spent acid.
If a refiner uses this acid to alkylate butylenes and/or amylenes, the propylsulphate intermediates are converted to alkylate and “washed” out of the acid phase. The acid strength increases when this occurs. In Stratco’s laboratory pilot plants, the acid strength can increase by 4 wt% after washing with butylenes. In a commercial unit, the strength of the acid in the next acid stage is significantly higher than it would be without the washing effect.