Jan-2012

Catalyst additives reduce rare earth costs

Catalyst additive technologies help to control rare earth costs in fresh FCC catalyst additions and in SOx capture

Ray Fletcher
Intercat (Johnson Matthey)

Article Summary

Rare earth is an integral component of today’s FCC catalyst. Rare earth stabilises the zeolite, adding substantial activity, is sometimes used as a vanadium trap and is also present as an oxidising agent within SOx additives. Rare earth is therefore an essential element in today’s FCC catalyst technologies, supplying increased activity, improved gasoline yields and oxidation function in SOx additives. Rare earth has also passed through a price surge, increasing over 1500% in the last 18 months.

In an attempt to reduce catalyst costs, most FCC catalyst suppliers have begun offering very low or zero rare earth formulations. Reducing rare earth drops a catalyst’s intrinsic activity, potentially leading to large increases in catalyst addition rates and potentially offsetting these savings in rare earth costs. An alternative solution for many refiners is to use a rare earth-free metal trap. One such technology is Cat-Aid, which has been shown 
to trap vanadium and absorb feedstock nitrogen. The net result produces an increase in conversion, along with reduced catalyst addition rates and/or rare earth concentrations. Additionally, Intercat has 
successfully commercialised new SOx-reducing additive technology containing 50% less cerium. This additive has exceeded expectations in every commercial trial.

This article provides guidelines for the FCC operator, enabling the most effective application of these two technologies. Best available technology related to the injection of these additives into the circulating inventory will be discussed.

Reducing FCC catalyst costs
Rare earth levels on fresh catalyst prior to the recent price escalation had risen to an average value of approximately 2.5 wt%. This increase in concentration was due to the strong positive impact of rare earth on both catalyst stability and activity. The result of this increased concentration has been an overall reduction in fresh catalyst addition rates, observed industry wide.

The rare earth effect is 
strongest within units processing heavy feedstocks. These feedstocks typically produce higher delta coke, resulting in more severe hydrothermal conditions within the regenerator. Rare earth-stabilised zeolites better withstand the high-severity conditions observed within the regenerator. These feedstocks also typically contain higher concentrations of vanadium and nitrogen, which have the effect of reducing catalyst activity. Rare earth provides additional activity to counteract the loss in activity imparted by these poisons.

With the recent rare earth price escalations, refiners have been driven to reduce rare earth levels on their fresh catalyst in order to contain operating costs. The net result of this change is a reduction in catalyst stability and intrinsic activity. These two effects require refiners to increase their fresh catalyst addition rate. These increased additions offset the reduction in fresh catalyst pricing.

Intercat has commercialised a rare earth-free metal trap, Cat-Aid, capable of passivating vanadium and absorbing 
nitrogen. Vanadium passivation limits the formation of vanadic acid and its subsequent attack on the zeolite within the catalyst particle. Vanadic acid attack results in dealumination of the zeolite crystal, leading to activity loss. Cat-Aid effectively limits the negative impact of vanadium in the circulating catalyst inventory.

Cat-Aid has the additional benefit of absorbing nitrogen present in the feedstock (see Figures 1 and 2). Nitrogen is a temporary poison that 
neutralises FCC catalyst acid sites, thereby suppressing 
activity. These adsorbed nitrogen-bearing molecules are then oxidised in the regenerator, thereby uncovering the previously inactive acid site. These acid sites are then subjected to further nitrogen poisoning during subsequent feed contact in the riser mixing zone. Cat-Aid has been observed to absorb nitrogen in every commercial 
application. The net result of nitrogen absorption by the metal trap particle is to retain base catalyst activity, which leads to increased conversion and reduced catalyst additions.

One refiner using Cat-Aid recently took the opportunity to reduce the rare earth level on their fresh catalyst (see Figures 3 and 4). The rare earth level was reduced by 1.2 wt%, which led to a 2.0 wt% loss in MAT activity. However, conversion on the unit was constant. Additionally, the refiner was able to reduce their fresh catalyst addition rate by 17%. The additive cost was fully compensated by the combined effect of the reduced rare earth level plus the reduced catalyst addition rate. As an added benefit, the refiner was able to lower the SOx-reducing additive injection rate due to the inherent capability of Cat-Aid to absorb SOx in the regenerator.

Cat-Aid is most effective in FCC units processing heavy feedstocks. Two simple guidelines enable the refiner to determine whether it will be effective in their specific operation. It performs best in units sensitive to nitrogen poisoning. It is recommended that the process engineer plot conversion versus nitrogen. The unit will respond well to Cat-Aid injection if the slope of this curve is greater than or equal to 1.0 wt% conversion loss per 200 ppm basic nitrogen. Second, the vanadium passivation functionality of this catalyst will be most effective when the concentration of vanadium on equilibrium catalyst is greater than 2000 ppm.

Controlling SOx-reducing additives costs
High-activity SOx-reducing additives are tri-functional 
catalysts. These functionalities include an oxidation step to convert SO2 to SO3, a chemisorption step of SO3 onto surface oxide sites, followed by a reduction in the chemisorbed SO3 to H2S in the riser and reactor stripper. Cerium oxide is the catalyst that oxidises SO2 to SO3.

It is important to understand how cerium oxide functions as both an oxidant and oxygen carrier in SOx-reducing additives.1 The mixture of two oxidation states, Ce(III) and Ce(IV), creates defect sites in the crystal structure where oxygen ions are missing (oxygen vacancies). These vacancies are filled up in the regenerator, allowing cerium oxide to act as a monatomic oxygen sponge. Monatomic oxygen is more reactive than molecular oxygen. Cerium oxide is therefore a very effective catalyst for oxidation reactions.

Reducing the additive cost by simply dropping the cerium oxide concentration works up to a certain point. However, exceeding this point results in loss of SOx-reducing activity, leading to substantially increased additive injection rates. As an example, current-generation zero rare earth SOx additives have shown themselves to be unsuitable for use where high levels of SOx reduction are required.