Maximising distillate while minimising bottoms

An FCC catalyst with reduced resistance to diffusion raises diesel output without the traditional bottoms penalty

Rive Technology, Inc

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Article Summary

The refining industry, especially in the US, has undergone significant changes during the last 5-10 years. Crude oil prices have gone from about $30/bbl to over $140/bbl, back down to $35/bbl, and have recently hovered around $100/bbl.  

The primary transportation fuels, gasoline, diesel and jet fuel, currently account for over 80% of US refinery output. Achieving these high yields requires converting heavy fuel oil into lighter products. For the last 50 years, the refining industry has relied on the fluid catalytic cracking (FCC) process as the primary conversion unit in the refinery. Over 90% of refineries in the US include an FCC unit. The FCC unit converts heavy fuel oils into more desirable lighter products such as distillate and gasoline blend stocks, alkylation feed, LPG and the less desirable light gases and coke. Until recently, most FCC units were operated to maximise gasoline production; however, that is changing because of shifting demand for transportation fuels.

Gasoline consumption peaked in 2007 and has continued to decline since then (see Figure 1).

Continuing improvements in vehicle fuel efficiency and the imposition of greenhouse gas (GHG) emission limits from light duty vehicles will continue to reduce gasoline demand. Average fuel economy requirements for  US light duty vehicles are expected to increase from 30.1 miles per gallon in 2012 to an equivalent of 35.5 mpg in 2016 and 54.5 mpg in 2025, based on required reductions in vehicle GHG emissions.1 A recent National Research Council report2 concluded that it is feasible to reduce light duty vehicle fuel consumption to 20% of 2005 levels by 2050. Distillate consumption dipped with the 2008 recession, but has been slightly increasing since then.

In addition to the changing demand structure, the relative prices of gasoline and diesel fuel in the US have also changed in recent years (see Figure 2). Until August 2004, monthly average wholesale prices for refiner gasoline had been higher than diesel prices 95% of the time. Since then, monthly average wholesale prices for refiner diesel have been higher than gasoline almost 75% of the time. Diesel prices have been higher than gasoline in all but two of the last 36 months. This is a dramatic reversal in the relative historical prices of gasoline and diesel.  
Refiners’ reaction
Refiners have been gradually shifting their operations to meet this changing demand and price structure, increasing distillate production at the expense of gasoline. The FCC unit is a major gasoline producer in the refinery. Many refiners have shifted their FCC operations to ‘distillate mode’, at least during part of the year, to help meet this changing demand structure. 

There are several operational changes that refiners can make to their FCC operations in order to increase distillate production at the expense of gasoline. These include:
• Minimising diesel fractions in the FCC feed
• Changing the cut point between gasoline and light cycle oil (LCO) products
• Reducing cracking severity by increasing feed temperature, lowering riser outlet temperature, or lowering catalyst activity
• Changing the FCC catalyst to a more distillate selective catalyst (lower zeolite/matrix ratio).

The first three items are operational changes that any refiner can implement. Removing diesel fractions from the feed eliminates the cracking of good quality distillate to lighter products. Changing the cut point between FCC product gasoline and LCO directly shifts gasoline to distillate without changing any other product yields or the overall FCC volume swell. Reducing cracking severity increases LCO yield by reducing upgrading of heavy products into transportation fuels. The lower cracking severity impacts all other yields in addition to LCO and gasoline. Lower value heavy fuel oil yield increases while alkylation feed decreases. Decreasing riser outlet temperature can also decrease gasoline octane, which may impact other refinery operations to maintain gasoline pool octane. Many refiners have implemented these operational changes; however, catalyst changes provide yet another powerful lever to improve LCO yields.

Catalytic opportunities
Catalyst changes can increase LCO yield while minimising the negative aspects of distillate mode FCC operation. ZSM-5 additives can be used to increase gasoline octane and light olefin production at reduced severity to maintain alkylation feed and gasoline octane. However, the net result is still a reduction in total transportation fuel yield. Increasing the matrix activity and reducing the zeolite content of the catalyst will also improve overall distillate selectivity, but generally is accompanied by an increase in coke selectivity.

Beyond the catalytic opportunities described above, overcoming the diffusion limitations of today’s catalysts can open the door to enhanced LCO selectivity while minimising any trade-offs. The conversion of large VGO and resid molecules to LCO, gasoline and gases is usually modelled as a series of parallel and series reactions, such as the network shown in Figure 3, as proposed by numerous researchers such as Heydari et al.3  All light products can be formed directly from the larger, heavy molecules through the parallel reactions, but the series pathways are also significant. The cracking reactions are usually considered as first order in the reactant. Series, first order reactions usually result in a maximum of the yield of the reaction intermediates as feed conversion increases. This is true of the FCC process. As conversion is increased, LCO, then gasoline, then LPG, all reach a maximum yield and then decline as conversion is increased.

Although the reactions are modelled as simple first order reactions, the rate constants are actually lumped parameters that include other physical processes such as the diffusion of the feed and product molecules into and out of the catalyst. In rapid conversion processes like FCC, these inter- and intra-particle diffusion transport rates can be the controlling steps in determining the overall rate of reaction and overall product selectivity in a series reaction network. Catalyst technologies that improve inter- and intra-particle diffusion transport rates can therefore be a key component of an LCO maximisation strategy.

Advantage of Molecular Highway technology
For the last 40 years, essentially all modern FCC catalysts have used faujasite (Y zeolite) as the primary, active cracking component in the catalyst. The catalyst particles are a physic-chemical mixture of the zeolite crystals, an active or inert binder and other components that can assist in the cracking reactions or provide another function such as metals trapping. The faujasite particles are introduced into the catalyst formulation as individual particles that are 1-5 microns in diameter, while the final catalyst particles have an average particle size of about 70-80 microns. The zeolite is a molecular sieve with most of its active sites inside the small pores of the individual crystallites. The size of these pores is similar to (or often smaller than) that of the molecules that are being converted, so molecular access to all of the interior active, catalytic sites is limited by diffusion.

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