Improved SOx reduction in partial burn FCC

Refinery applications of a new additive demonstrate improved SOx reduction performance in partial burn FCC operation

W. R. Grace

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Article Summary

The oil and gas industry continues to move toward more restrictive environmental laws for emission control; in parallel, many refineries are looking for increased flexibility to process more challenging feedstocks that are higher in sulphur for maximum profitability. As such, SOx emission control remains a crucial topic for refineries to ensure competitiveness in the market. Grace recognises this and has recently commercialised Emisscian, an FCC SOx reduction additive that is highly effective in both full and partial burn operating conditions. SOx additive performance in partial burn has always been considered more challenging when compared to full burn operation, but is a key topic of interest in order to allow such operations to increase processing rates of high sulphur resid feedstocks.

Emisscian is based on the Desox technology platform, and is the result of a significant capital investment made at Grace’s SOx additive manufacturing facilities, where an additional processing step was incorporated to improve the physical properties of the additive. Compared to the previous Super Desox additive, the new additive incorporates improved cerium and vanadium dispersion across the additive particle, as well as increased magnesium aluminate spinel content,1 both of which bring significant increases in SOx reduction performance. During initial trials, the new SOx additive was referred to as Super Desox CV+ to signify these improved physical features. However, the new additive technology is subsequently being branded as Emisscian to better reflect the step-out performance that was achieved in multiple back-to-back trials.

Since the commercialisation of Emisscian, many refineries have switched from competitor SOx additives to the new product due to its step-out performance improvements. Grace has detailed the improved performance with Emisscian at a major refinery in Europe operating in full burn.2 This focuses on a partial burn case study from Sonatrach Raffineria Italiana, where improved SOx additive efficiency and higher levels of SOx reduction resulted in a higher percentage of SOx reduction for a given additive usage rate.

Additive technology for SOx reduction
During the process of SOx additive development, it is important to consider that there are three key steps in the SOx reduction mechanism. The first step occurs in the regenerator, where sulphur contained in the coke on the catalyst is oxidised to about 90% SO2 and 10% SO3. In this step, cerium plays a critical role in promoting the full oxidation of SO2 to SO3 in the presence of oxygen. In the second step, SO3 is captured by a magnesium species to form magnesium sulphate in the regenerator. The third and final step takes place in the FCC reactor/stripper section, where magnesium sulphate is reduced to magnesium oxide with the release of hydrogen sulphide. Here, both the magnesium and vanadium functionalities facilitate this important additive regeneration step.

In terms of developing SOx additive technology for full and partial burn applications, Grace has performed extensive research into whether modified SOx additive technology is required for partial burn applications. Grace has prepared various SOx additive formulations that differ in the functionalities incorporated, and the ranking observed for performance under full burn conditions has always mirrored the ranking for performance under partial burn. This is a logical observation when considering the mechanism and rate-determining steps for SOx reduction in the FCC unit.3

One of the key steps for SOx reduction in both full and partial burn operations is the oxidation of SO2 to SO3, which is a prerequisite before SOx can be captured by the additive in the form of magnesium sulphate. SOx reduction additives contain cerium functionality to facilitate the reaction of SO2 to SO3, but regenerators operating under partial burn have low excess oxygen levels, which makes the oxidation reaction much more challenging. The improved cerium dispersion within Emisscian allows more oxidation of SO2 and therefore facilitates the overall SOx reduction mechanism for both full and partial burn operations, though it plays a more significant role within partial burn. Another feature of Emisscian is an improved vanadium dispersion and optimised magnesium aluminate spinel content. This facilitates additive regeneration within the SOx reduction catalytic cycle, which is often the rate determining step for full burn operations but can also be a limiting factor in partial burn. The effectiveness of the additive was first confirmed in pilot plant testing, and then in multiple commercial trials described in the following.

Pilot plant and commercial testing results
The following section discusses pilot plant testing of Emisscian as well as three recent commercial applications (two in partial burn, the other in full burn) where the new additive was evaluated. For the development of SOx additives, Grace utilises larger scale test equipment, specifically the Davison Circulating Riser (DCR) pilot plant. Such a unit provides valuable information based on continuous operation. Additionally, compared to bench scale units, the DCR pilot plant has the advantage that it mimics all of the processes present in a commercial operation and can also operate at the same hydrocarbon partial pressure as a commercial unit. Continuous catalyst regeneration in the DCR allows for the measurement of regenerator SOx and NOx emissions and testing of environmental additives, experiments which cannot be done in a batch unit.

Figure 1 shows DCR testing, comparing SOx reduction performance for SOx additives with less-dispersed vanadium and cerium versus the same SOx additives with a higher level of dispersion. The testing is initially operated without SOx additive to establish the baseline level of SOx, then at ‘time = 0 hours’ the SOx additive is introduced in a single dose to observe the initial level of SOx reduction. The subsequent period required for SOx emissions to increase back to the baseline level of SOx is used as a measurement for the additive’s effectiveness for SOx reduction. The two additives with high and low vanadium dispersion were tested at the standard level of 1 vol% oxygen. Results highlight that both additives show the same initial level of SOx reduction, but the additive with a higher level of vanadium dispersion retains the SOx reduction activity over approximately twice the period of time. When the additives with high and low cerium dispersion were tested at the standard level of 1 wt% oxygen, both additives showed a similar profile for SOx reduction. However, when tested at a lower oxygen level of 0.2 wt%, the additive with a higher level of cerium dispersion shows an improved capability for SOx reduction. The testing highlights the importance of vanadium dispersion in full burn operations, as well as the benefits of improved cerium dispersion for lower oxygen or partial burn applications. Emisscian incorporates both of these features, resulting in better SOx reduction performance for both partial and full burn applications.
Case study: SOx additive performance at Sonatrach Raffineria Italiana
Sonatrach Raffineria Italiana Srl owns the Augusta refinery in Sicily, as well as depots in Naples, Palermo and Augusta. The company belongs to the Algerian Sonatrach group, which is a government owned company created in 1963, and is one of the world’s leading oil companies. The Augusta refinery has a flexible set-up that includes an atmospheric distillation capacity of 206000 b/d with high fuels conversion capacity. At Augusta there is also a world-scale lubricants plant capable of delivering about 800000 t/y. The site is also a large asphalts producer and exporter. The Augusta refinery occupies a strategic location in the Mediterranean, close to crude oil sources in North Africa and the Caspian Sea, and is well positioned to supply developing markets in Africa and Asia.

The Augusta FCC unit uses SOx reduction additives for SOx emission control. It is a partial burn unit, and feed quality can vary a lot during normal operations, with sulphur content varying accordingly. The capability for resid processing is a crucial factor for the refinery, therefore being able to effectively control SOx emissions is an important factor in maintaining the profitability of the unit. The refinery performed an industrial trial to evaluate Emisscian performance. Figure 2 highlights the baseline SOx levels for the period prior to and during the use of Emisscian. These are the baseline SOx levels that would be obtained without SOx additive which are calculated using an established correlation based on operating conditions. It also shows the actual SOx levels obtained. The average baseline SOx levels were similar for both periods assessed, though a higher percentage of SOx reduction was achieved with Emisscian, resulting in a reduction of actual SOx by 8%.

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