Maximising yields and profits from the FCC unit
Comparative additive trials demonstrated a gain in petrochemicals output and profit from an integrated refinery’s FCC unit.
SILVIU SERBAN, CHUCK EKEOCHA, UDAYSHANKAR SINGH and BANI CIPRIANO
Grace Refining Catalyst Technologies
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The growth in demand for fuels worldwide is projected to slow down as a result of the electrification of the vehicle fleet and maturing of advanced economies. On the other hand, demand for petrochemicals is still on a growth trajectory. Integrating petrochemical production with their operations provides refineries an opportunity to profit from this growth trend and ultimately avoid risk of closure in the face of slowing demand. This trend of integration between refining and petrochemicals is well under way: in the past several years, megaprojects were announced in China, in which the yield of petrochemicals from each barrel of crude is slated to reach ~40%.1 New projects have been announced, most notably by Saudi Aramco, that seek to push this yield up further.2
At Grace, we believe the fluid catalytic cracking (FCC) unit facilitates the transition of refineries from fuels to a focus on petrochemicals. The reason for this is that the FCC unit generates the highest yield of light olefins, specifically propylene, compared to other units in the refinery. Therefore, the company’s focus is on developing new technologies that allow refiners seeking to integrate petrochemical production to maintain or even increase profitability in the face of slowing growth in fuel demand. In this article, we describe Zavanti additive, Grace’s latest ZSM-5 technology for maximum propylene FCC applications, and how the company worked closely with Saudi Aramco Total Refining and Petrochemicals (SATORP) to capture the most value from using this technology in their operations.
SATORP is one of the world’s most efficient integrated refining and petrochemicals platforms. Operational since 2014, it has one of the most complex and efficient sites, capable of converting a type of heavy, high-sulphur crude oil to high value added products. To accomplish this, the refinery relies on a broad range of processing units, including two distillate hydrocrackers, an FCC unit, and a coking unit. In early 2018, the SATORP platform’s production capacity was raised to 440000 b/d of crude oil, bringing annual output to around 22 million tonnes of refined petroleum products, including 200000 tonnes of high purity propylene.
The key objectives of the SATORP FCC unit are high conversion and high yield of light olefins, including C3= and C4=s. The feed is very light and therefore easy to crack, but it is very low in Concarbon and metals which results in reduced delta coke and regenerator temperature. Consequently, combining a catalyst that closes the heat balance with an optimal ZSM-5 additive to maximise light olefins is critical. Additionally, due to the nature of the feed and very high unit conversion, the riser outlet temperature (ROT) is depressed to below 505°C which imposes further challenges.
The benchmarking plots in Figure 1 show SATORP’s FCC unit compared with other FCC units in the EMEA region in terms of key parameters: feed specific gravity, ROT, and C3= and total C4= yields. The SATORP unit operates near the lowest in terms of feed specific gravity and ROT and near the highest in terms of C3= and C4= yields.
To achieve this performance, Grace supported SATORP to meet its objectives in two steps: (i) optimal catalyst design to help close the FCC unit’s heat balance and achieve improved reliability and utilisation, yet maximise gasoline olefins; and (ii) optimisation of ZSM-5 additive to crack gasoline olefins into valuable C3= and C4=s. Optimisation of catalyst design was the subject of a recent presentation.3 In this article we will focus on the application of Zavanti additive in the SATORP FCC unit.
Developments in ZSM-5 catalyst technology
In FCC, the ZSM-5 catalysts are also known as ZSM-5 additives as they are always used in combination with FCC catalyst formulated with Y-zeolite. Grace commercialised the first ZSM-5 catalyst, Additive O, in collaboration with Mobil in 1984. Since then the company has made advances in formulation and in the technologies used to stabilise and bind the ZSM-5 crystal in catalyst particles. This is shown in Figure 2.
Zavanti additive has the following features: physical properties required for optimum circulation in the unit, high retention of ZSM-5 activity, and maximum LPG olefin activity. In this section we describe each feature then present a comparison of activity between the three most recent generations of ZSM-5 additives from Grace. Finally, we summarise the results of trialling Zavanti in the SATORP FCC unit and estimate the value delivered.
In FCC units, the physical properties of a catalyst are critical for the smooth operation of a unit. Properties such as attrition, apparent bulk density (ABD), surface area, and particle size distribution are key to the retention, circulation, and performance of a catalyst in FCC units. When Grace developed Zavanti additive, it was important to retain the same strong physical properties (see Table 1) as OlefinsUltra additives.
The higher severity conditions typical of max propylene units pose a challenge to the stability of ZSM-5, in terms of surface area, acid site density, and activity retention. Therefore during the development of Zavanti one goal was to show high stability or activity retention with respect to OlefinsUltra additives. These improvements were achieved with the use of a new binder and changes in the processing technology. Surface area and activity retention can play a key role in the effective activity of an additive to maximise propylene yield in the unit. For two additives having equal starting activity for LPG olefins production but different activity retention with time (half-life decay), the additive with the best activity retention or longer half-life decay will have a higher effective activity in the unit.
LPG olefin activity
The activity of ZSM-5 additive can be increased either by adding more ZSM-5 to a catalyst particle or by increasing the inherent activity of the ZSM-5 crystal. Traditionally, adding more ZSM-5 to an additive particle to boost LPG olefin activity was a common approach. This has been the case for OlefinsUltra additives. This approach is not trivial since it becomes increasingly difficult to bind larger quantities of ZSM-5 in an additive particle and yet retain good physical properties (particle density and attrition resistance). This approach has limited potential towards increasing LPG olefin activity since it is very difficult to add more zeolite beyond a certain point.
Grace’s R&D programme during Zavanti’s development focused more on increasing the inherent activity of ZSM-5. The objective was to increase propylene activity by minimal to no increase in the amount of ZSM-5 in a catalyst particle. This was achieved with use of a better stabilisation and binding technology. As Figure 3 shows, the result was a significant increase in propylene activity for Zavanti compared with OlefinsUltra additives.
To show the difference in activity between Zavanti and previous generations of ZSM-5 additive technology, Grace tested against OlefinsUltra HZ and OlefinsUltra MZ additives in an Advanced Catalytic Evaluation (ACE) unit using a VGO feed.
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