Boosting mild hydrocracking performance

Optimising its mild hydrocracking operations using new catalyst developments enabled a refinery to meet more complex targets for fuel quality.

XAVIER Enrique Ruiz MALDONADO, Haldor Topsoe A/S

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Article Summary

Due to more complex refinery targets, increasing demand for high quality fuels, and reduced refinery operating margins worldwide, many refiners are looking for opportunities to expand (either with new units or via revamps of existing units), or are considering innovative, high efficiency catalyst solutions for their current processes.

Implementation of new Inter-national Maritime Organization regulations (IMO 2020) has also pushed refineries to operate under more stringent conditions in order to produce bunker fuels with less than 0.5 wt% sulphur, while balancing the refinery yields of middle distillate and the feed streams to fluid catalytic cracking (FCC) units.

For many years, the mild hydrocracking (MHC) process has been a key conversion technology used to increase yields of middle distillates and improve downstream conversion and product quality in FCC operations. However, the continued development of new, improved catalysts for hydrotreating and hydrocracking is crucial in making it possible to comply with sulphur specifications in gasoline and ultra low sulphur diesel (ULSD). Improved hydrocracking technology and catalyst solutions are emerging all the time to help refineries overcome these challenges.

A conventional MHC unit operates at a design hydrogen partial pressure of 50-100 barg and a space velocity of 0.5-1.2 h-1 normally provides a true conversion within the 5-30% range. However, high operating temperatures are required to achieve any specific conversion target, resulting in overtreating of the heavy products and a less than ideal density for the middle distillate fractions.

Recently developed MHC technologies have become available to counterbalance this requirement as well as to minimise capital requirements and operating costs compared to a conventional MHC unit.
This article presents ways to optimise MHC operations as well as providing an overview of industrial and research experience within the field of MHC.

The article also examines the practical results obtained from an MHC revamp using Topsoe catalyst technology undertaken in 2019 by a European refinery company – Compañia Española de Petróleos S.A. (Cepsa) – and explains the significant new conversion capabilities that have been achieved with this set-up.

Process limitations in MHC
Partial conversion – also known as MHC – has been the main source of feed preparation for refinery FCC units for many years. Figure 1 illustrates the feed sulphur requirement to meet MHC diesel and FCC naphtha specifications. This also clearly shows the need for a deep hydrodesulphurisation (HDS) conversion (>99%) for a medium to high VGO sulphur feed (>2 wt%) in order to comply with the 10 wtppm ULSD specification.

Furthermore, when a refinery is targeting Euro V product specifications, the operating pressure has a huge impact on reaching the required maximum density of 845 kg/m3.
A minimum conversion of 40% and a hydrogen partial pressure above 110 barg are required to obtain a minimum specific gravity of 0.845 in a full-range diesel product (see Figure 2). If the refinery’s diesel hydrotreating capacity makes it possible, blending the MHC diesel with a low density diesel is normally feasible. However, doing so would then be at the cost of reduced kerosene (Jet A-1) output and the ability to meet specifications for the smoke point.

Technical and geographical differences abound
MHC units are all designed differently, especially in terms of space velocity and hydrogen partial pressure. The crude slate and feed quality also vary considerably. Any refinery’s ability to reach desired product specifications depends heavily on factors that include crude availability and refinery location.

The differences in operating parameters for MHC units frequently encountered on different continents are shown in Figure 3. European Refineries Group A is normally characterised by low feed sulphur and high space velocity, which makes it difficult to achieve MHC mode, while European Refineries Group B has good potential for increased conversion due to the designed-in low space velocity. North American refineries are usually configured for a low space velocity and high pressure, while aiming for low sulphur (<300 wtppm) FCC production. This enables these refineries to achieve high conversion. Refineries in Asia and Latin America usually process mid-to high sulphur vacuum gasoil (VGO >1.8 wt%), paving the way to increases in conversion despite the low space velocity design.

Topsoe catalyst and technology solutions
Conventional MHC technology targets a higher cetane index and better kerosene properties, which can only be achieved in a MHC conversion if the hydrogen partial pressure increases by 60%.
Topsoe, on the other hand, has invested significantly in developing innovative catalyst technology solutions for MHC processes. Topsoe configurations are able to maximise the diesel uplift, while still maintaining the HDS requirement at lower conversion levels and with lower hydrogen consumption than any conventional high pressure MHC technology. A typical comparison is shown in Table 1.

Topsoe provides selective, top tier catalysts that enhance catalyst stability in a wide range of operating conditions. The ratio between hydrotreating catalyst and hydrocracking catalyst is also a factor to take into consideration when selecting the most effective catalyst loading for a particular unit, because an optimised loading will determine levels of profitability in the refinery complex as a whole – and specifically the operating synergy between the MHC and FCC units.

The HDS and hydrogenation capacities of the catalyst are crucial to keep product sulphur levels low for as long as possible. When a hydrocracking catalyst is required in this process, nitrogen slippage and ammonia partial pressure are the deciding factors. High nitrogen slip hinders aromatic saturation capability, which considerably suppresses the capabilities of the cracking catalyst. An illustrative example of optimal hydrotreating vs hydrotreating/hydrocracking utilisation in a low to medium range hydrogen partial pressure is shown in Figure 4.

This is why Topsoe has launched new catalyst formulations based on the HyBRIM and HySwell technologies, including individual products such as TK-564 HyBRIM and TK-6001 HySwell to enhance catalyst activity and stability along the targeted cycle.

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