New process improves alkylate production

Researchers at the US Department of Energy's Idaho National Engineering and Environmental Laboratory (INEEL) have developed an energy-efficient process for producing alkylate - a high-octane gasoline blend that is very low in environmental pollutants such as sulphur and benzene.

Instead of using a liquid catalyst, the team generates alkylate using a solid acid catalyst to change low-octane gaseous feedstock into liquid alkylate.

Once the solid catalyst becomes coated with undesired hydrocarbons, researchers use a supercritical fluid solvent to clean and rejuvenate the catalyst, and then begin alkylate production again.

Researchers have been able to completely restore deactivated catalyst to 100 percent effectiveness, which increases the active lifespan of the catalyst about 20 times.

Currently, industry uses massive volumes of highly concentrated liquid hydrofluoric or sulphuric acid catalysts to make alkylate.

These liquid acids pose such severe handling, storage, and waste by-product environmental and safety hazards that federal and local governments have largely stopped issuing permits to build new hydrofluoric acid plants. 'As a result, this ultra-clean fuel makes up a mere 13 percent of the fuel market,' said INEEL researcher Dan Ginosar.

INEEL's industry partner Marathon Ashland Petroleum supplied the solid acid catalyst used in this research. The solid, zeolite catalyst poses no threat to refinery workers or the environment and should appeal to industry more than a liquid catalyst because it's safer and easier to handle, transport, and store.

Early on, researchers tried to produce alkylate and keep the catalyst from deactivating at the same time by introducing a solvent into the alkylation process itself. 'That's what industry really wanted,' said Ginosar, 'the ability to keep the catalyst active longer on the production line.'

But after extensive experimentation using many different solvents under a myriad of conditions, the team concluded the approach wasn't feasible. 'It just doesn't work. The yield of alkylate is actually lower than if we didn't add the solvent at all,' he said. So researchers began to investigate alternative approaches for regenerating the deactivated catalyst in the same vessel.

The solid zeolite catalyst is porous, providing a lot of surface area to catalyse the reactions that change butane/butene petroleum gases into liquid alkylate.

The catalyst surface, however, eventually becomes coated with undesirable hydrocarbons that effectively deactivate the catalyst, and are a challenge to 'clean.'

Burning the hydrocarbons off, the traditional approach for solid catalysts, turns out to be too harsh and quickly destroys the catalyst activity. Alternatively, liquid solvents take too long to soak into all of the catalyst pores, and solvents in gaseous form are not effective. Taking advantage of the unique properties of supercritical fluids turned out to be the best solution.

'Using supercritical fluids, we get the best solvent properties of liquids and the diffusivity properties of gas,' said Ginosar. When the catalyst bed is completely deactivated, researchers stop alkylation production and expose the catalyst to a continuous flow of a supercritical fluid solvent.

This process cleans the catalyst and can be conducted in the same vessel as the alkylation production, eliminating the need for refinery workers to handle or transfer the spent catalyst at all.

Once the catalyst has been regenerated, the vessel can be brought back into production. The team tried many different solvents and ultimately identified the fluids that are uniquely suited for this application.

In order to optimise their supercritical fluid catalyst regeneration technique, researchers also needed to refine alkylation production conditions. Originally, the team used a single pass of the raw petroleum feed, but found that this was only partially effective.

The reaction conditions were very harsh and ruined a portion of the catalyst bed. 'At best, we could recover only about 87 percent of the original catalyst activity,' said Ginosar.

However, when researchers made the allkylation process more gentle they achieved much better results using the supercritical fluid solvent afterwards. Researchers recirculate a constant flow of petroleum feed over the catalyst and keep the concentrations of butene, the main catalyst-fouling agent, low.

They optimised variables such as temperature and pressure within the vessel to achieve a high percentage of catalyst activity recovery.

'Using this approach, we not only restored the catalyst to 100 percent of original activity levels, we significantly extended the length of time in the catalyst, increasing the operating lifespan about 20 times,' said Ginosar.

Ginosar has achieved a week long run of the alkylation/regeneration cycle maintaining at least 90 percent recovery of catalyst activity. He is now investigating how many times the catalyst can be regenerated-a primary factor in how cost effective this new alkylation process could be in the future.

The longer the catalyst lasts, the more economical the process will be. Already the team has regenerated the catalyst 34 times at greater than 90 percent of original activity levels in a 208-hour experimental run in the laboratory.