Oct-2015

SOx additives for tight oils (TIA)

Many refineries use SOx additives to control flue gas SOx emissions. Worldwide there are vast differences in limits for SOx emissions.

Bart de Graaf, Rick Fisher, Martin Evans and Paul Diddams
Johnson Matthey Process Technologies

Article Summary

Whereas in the US point emissions (such as the FCC stack) are often controlled by consent decree, in Europe some refineries operate under a refinery wide ‘SOx bubble’. This means that a refinery’s total emissions need to be controlled and, for example, FCC SOx emissions can be offset by using clean fuels for burners. SOx additives have been used under very challenging conditions, and have been shown to be able to control SOx emissions down to 5 ppm.

The recent advent of tight gas and tight oils in the US has provided the country with cheap energy and fuel supply. Refiners use tight oil blended in with other feed, with some refiners now processing up to 100% of their feed. Though tight oil properties vary, they exhibit some typical characteristics: they are typically very light, easily crackable and can contain metals that are non-standard for FCC, such as iron, calcium and potassium. Challenges of FCC tight oil operations have been a topic of extensive discussion in recent years. As tight oils are very light and easily crackable they can put severe constraints on various refinery operations such as the crude unit. For this light feed, FCC conversion will be high and the wet gas compressor will be readily filled up. As a result, riser outlet temperature will often be low and is an important control parameter to maximise throughput. However, the low riser outlet temperature can have a substantial effect on the performance of SOx additives.

Although performance has increased dramatically over time, the functionalities of SOx additives have not changed substantially since the 1980s. They contain an oxidiser (usually cerium oxide) for the oxidation of SO2 to SO3. They contain an oxide matrix (most commonly magnesia doped with alumina) to capture the SO3 as sulphate. Some early generations of SOx additives consisted of just cerium oxide on an alumina support. The change from alumina to an alumina–magnesia mixed metal oxide as matrix made a step change in performance. SOx additives also contain a release agent to assist with reducing the magnesium sulphate back into magnesium oxide and H2S (and water). Three factors have contributed to the increasing activity of the of SOx additives over time: higher magnesium contents, improved physical properties and the use of an additional sulphur release agent.

What determines the effectiveness of a SOx additive? It depends on the operation in which the SOx additive is used. For units that operate at low SOx levels and slow turnover of inventory, matrix stability of the SOx additive is key. As SO2 and SO3 concentrations are low, capturing of SO3 into magnesium sulphate needs to be extremely effective. Units with low oxygen partial pressure in the regenerator typically benefit from more oxidiser on the additive, as the oxidiser can function as a solid oxygen carrier and help to disperse oxygen more efficiently through the inventory. In many units SOx release is not a constraint. But these units have one thing in common: a relatively high riser outlet temperature. At low temperatures the release functionality can become limiting. Fortunately there is an easy check that can be made to determine whether the sulphur release is a limitation: sulphur on e-cat. Typically the sulphur on e-cat hovers at around 0.05 to 0.15 wt%; when sulphur release is limited because of a relative cool riser outlet temperature, e-cat sulphur levels can increase to 1 wt% or more. In this case, much of the magnesium oxide has reacted into magnesium sulphate. As a rule of thumb, when more than 25% of all magnesia has reacted to magnesium sulphate, sulphur release is limiting.

When sulphur release is limiting, SOx additive performance can be improved by providing additional release functionality. Above a certain riser outlet temperature release is much faster than SO3 production or SO3 capturing. Below this inflection temperature, standard SOx additive performance can drop off rapidly. Providing additional additive sulphur release functionality can push this inflection point to a lower temperature, improving SOx additive performance.

 When riser outlet temperatures are sufficiently low, distillate mode additives will show a significant improvement in performance. In some cases, not all of the lost efficiency can be regained, however the performance of distillate mode SOx additives, such as Super SOxGetter-II DM, is in this area much better than regular additives, outperforming such additives typically by 20% or more.

As shale oil operation can force a reduction in riser outlet temperature, regular SOx additives will not always provide the most economical solution. Whether SOx additives are suffering from sulphate release is easy to establish. Johnson Matthey is willing to provide e-cat testing services for any refiner who may think they are suffering from this problem. Specialty SOx additives such as Super SOXGetter II-DM can reduce SOx additive usage (and improve SOx capture) and are becoming increasingly used in FCC units using shale oils.

This short case study originally appeared in PTQ's Technology In Action feature - Q4 2015 issue.

For more information: Bart.deGraaf@matthey.com