Middle Distillates Production through Hydrocracking Technologies – An Attractive Route to Markets with High Transportation Fuels Demand

Introduction and Context

           Despite the trend of reduction in transportation fuels demand, these derivatives are still responsible for a great part of revenues in the downstream industry and are fundamental to sustain the economic development of the nations, especially in developing economies like India as presented in Figure 1.

Figure 1 – Transportation Fuels Demand in the Indian Market (S&P Global Platts Analytics, 2019).

Another example of a market with great dependence on transportation fuels is the Brazilian domestic market. The country is the seventh world largest crude oil derivative consumer and the major part of this consumption is related to transportation fuels leading to the country reach the third position in this category, demanding mainly middle distillates (diesel and kerosene) and, according to the Brazilian Petroleum Agency (ANP), the Brazilian demand for crude oil derivatives will rise 20% until 2026 against the current 2,4 million barrels per day.

 Minimize the consumption and the environmental impact of transportation fuels is a constant objective of researchers and some refining technologies were developed over the years aiming to improve the performance and reduce the environmental footprint. Over the years, in face of the rising pollution levels associated with technological development and the rise in petroleum consumption, environmental legislation has become increasingly severe.

           Restrictions on SOx and NOx emissions induced the necessity for higher technology development that can allow reducing the contaminant levels in the petroleum derivates, mainly sulfur and nitrogen. Normally, the concentration of contaminants increases with the density of the petroleum derivate, due to his characteristics and large consumption, Diesel was the target of the most restrictive regulations over the years. Taking as an example the Brazilian market again, over the last years the maximum sulfur content in marketable Diesel falls from 1.800 ppm to 10 ppm, this requires great capital investment from refiners to adapt the refining hardware to produce cleaner fuels.

           In these markets, hydrocracking technologies can offer an attractive way to improve the production of high quality and cleaner diesel (ULSD), despite the relatively high capital investment. 

Hydrocracking Technologies – An Overview

Despite the high investment for hydrocracking units construction, this process is what gives more flexibility to refineries to processing heavy oils, so with lower cost, on the other hand, these oils produce a high quantity of derivates with a lower added value and with restricted markets like fuel oils and asphalt. Table 1 presents the main differences between hydrotreating and hydrocracking technologies.

The hydrocracking process is normally conducted under severe reaction conditions with temperatures that vary from 300 to 480 oC and pressures between 35 to 260 bar. Due to process severity, hydrocracking units can process a large variety of feed streams, which can vary from gas oils to residues that can be converted into light and medium derivates, with high added value.

Among the feed streams normally processed in hydrocracking units are the vacuum gas oils, Light Cycle Oil (LCO), decanted oil, coke gas oils, etc. Some of these streams would be hard to process in Fluid Catalytic Cracking Units (FCC) because of the high contaminants content and the higher carbon residue, which quickly deactivates the catalyst, in the hydrocracking process the presence of hydrogen minimize these effects.

According to the catalyst applied in the process and the reaction conditions, the hydrocracking can maximize the feed stream conversion in middle derivates (Diesel and Kerosene), high-quality lubricant production (lower severity process).

Catalysts applied in hydrocracking processes can be amorphous (alumina and silica-alumina) and crystallines (zeolites) and have bifunctional characteristics, once the cracking reactions (in the acid sites) and hydrogenation (in the metals sites) occurs simultaneously. The active metals used in this process are normally Ni, Co, Mo, and W in combination with noble metals like Pt and Pd.

It’s necessary a synergic effect between the catalyst and the hydrogen because the cracking reactions are exothermic and the hydrogenation reactions are endothermic, so the reaction is conducted under high partial hydrogen pressures and the temperature is controlled in the minimum necessary to convert the feed stream. Despite these characteristics, the hydrocracking global process is exothermic and the reaction temperature control is normally made through cold hydrogen injection between the catalytic beds.

According to the feed stream quality (contaminant content), is necessary hydrotreating reactor installation upstream of the hydrocracking reactors, these reactors act like guard beds to protect the hydrocracking catalyst.

The principal contaminant of hydrocracking catalyst is nitrogen, which can be present in two forms: Ammonia and organic nitrogen.

Ammonia (NH3), produced during the hydrotreating step, has a temporary effect reducing the activity of the acid sites, mainly damaging the cracking reactions. In some cases, the increase of ammonia concentration in the catalytic bed is used as an operational variable to control the hydrocracking catalyst activity. The organic nitrogen has a permanent effect blocking the catalytic sites and leading to coke deposits on the catalyst.

As in the hydrotreating cases (HDS, HDN, etc.), the most important operational variables are temperature, hydrogen partial pressure, space velocity, and hydrogen/feed ratio.

Depending on feed stream characteristics (mainly contaminants content) and the process objective (maximize middle distillates or lubricant production) the hydrocracking units can assume different configurations.

Normally for feed streams with low nitrogen content where the objective is to produce middle distillates (diesel and kerosene), the configuration with two reaction stages without intermediate gas separation and a pre-treating hydroprocessing reactor are the most common. This configuration is shown in Figure 1.  

Figure 1 – Typical Arrangement for Two-Stage Hydrocracking Units without Intermediate Gas Separation.

As aforementioned, the disadvantage, in this case, is the high concentration of ammonia and H2S in the hydrocracking reactors, which reduces the catalyst activity. In this configuration, the hydrotreating reactor is applied to control the contaminants content in the feed aiming to protect the hydrocracking catalyst that has a relatively high cost.

The higher costly units are the plants with double stages and intermediate gas separation. These units are employed when the feed stream has high contaminant content (mainly nitrogen) and the refinery looks for the total conversion (to produce middle distillates), this configuration is presented in Figure 3. 

Figure 2 – Typical Arrangement for Two-Stage Hydrocracking Units with Intermediate Gas Separation.

In this case, the catalytic deactivation process is minimized by the reduction in the NH3 and H2S concentration in the hydrocracking reactor. It’s important to consider the feedstock quality to define the better residue upgrading technology to the refining hardware, once the hydroprocessing of residual streams presents additional challenges when compared with the treating of lighter streams, mainly due to the higher contaminants content and residual carbon (RCR) related with the high concentration of resins and asphaltenes in the bottom barrel streams.

 Among the Hydrocracking Technologies which applies fixed bed reactors, it can be highlighted the RHU technology, licensed by Shell company, Hyvahl technology developed by Axens, the Isocracking Process by Chevron Lummus Company, and the Unicracking Process, developed by UOP. These processes normally operate with low conversion rates with temperatures higher than 400 oC and pressures above 150 bar. Figure 3 presents a process flow diagram for the Unicracking Process by UOP Company.

Figure 3 – Process Flow Diagram for Unicracking? by UOP Company.

Technologies that use ebullated bed reactors and continuum catalyst replacement allow higher campaign period and higher conversion rates, among these technologies the most known are the H-Oil technology developed by Axens and the LC-Fining Process by Chevron-Lummus. These reactors operate at temperatures above of 450 oC and pressures until 250 bar. Another interesting technology is the Hycon? process, developed by Shell global solutions and applies a moving bed reactor in association with a fixed bed reactor, as presented in Figure 4.

Figure 4 – Hycon? hydrocracking Process by Shell Global Solutions.

An improvement in relation to ebullated bed technologies is the slurry phase reactors, which can achieve conversions higher than 95 %. In this case, the main available technologies are the HDH process (Hydrocracking-Distillation-Hydrotreatment), developed by PDVSA-Intevep, VEBA-Combicracking Process (VCC) developed by VEBA oil, and the EST process (Eni Slurry Technology) developed by Italian state oil company ENI.

Despite the high capital investment and the high operational cost, hydrocracking Technologies produces high-quality derivates and can make feasible the production of added value product from residues, which is extremely attractive, mainly for countries that have difficult access to light oils with low contaminants.

 In countries, with a high dependency of middle distillates like Brazil (because his dimensions and the high dependency for road transport), and India, the high-quality middle distillate production from oils with high nitrogen content, indicate that the hydrocracking technology can be a good way to reduce the external dependency of these products. It’s important to take into account that, to middle distillates purpose, less severity hydrocracking units can be applied, mainly the fixed bed alternatives that present lower capital requirements.

The Heart of the Hydrocracking Technologies – Midas Touch of Hydroprocessing Catalysts

The hydrotreating catalysts are normally composed of metal sulfides of Group VI (W and Mo) or/and Group VIII (Ni and Co) carried by an oxide like alumina, zeolite, or silica-alumina. The most employed combinations in traditional hydrotreating processes are Co/Mo (Cobalt/Molybdenum), Ni/Mo (Nickel/Molybdenum), and Ni/W (Nickel/Tungsten). The combination Co/Mo is normally applied to hydrodesulfurization reactions once presents less activity to harder reactions as hydrodenitrogenation or aromatics saturation, in these cases the catalyst selected is based on Ni/Mo combination while the Ni/W catalysts are applied to deep hydroprocessing processes where the main objective is aromatics saturation. Normally, the hydroprocessing reactors are filled with a combination of these catalysts aiming to optimize the performance and operating costs.

Some promoters can be added to the hydrotreating catalysts aiming to improve the performance in specific cases. Phosphorous is added to the Ni/Mo catalysts with the objective to improve the hydrodenitrogenation activity and the Fluor is applied to improve the catalyst performance in cracking reactions through the higher acidity in the carrier, this is a great advantage in mild hydrocracking processes.

Catalysts applied in hydrocracking processes can be amorphous (alumina and silica-alumina) and crystallines (zeolites) and have bifunctional characteristics, once the cracking reactions (in the acid sites) and hydrogenation (in the metals sites) occurs simultaneously. The active metals used in this process are normally Ni, Co, Mo, and W in combination with noble metals like Pt and Pd.

It’s necessary a synergic effect between the catalyst and the hydrogen because the cracking reactions are exothermic and the hydrogenation reactions are endothermic, so the reaction is conducted under high partial hydrogen pressures and the temperature is controlled in the minimum necessary to convert the feed stream. Despite these characteristics, the hydrocracking global process is exothermic and the reaction temperature control is normally made through cold hydrogen injection between the catalytic beds.

           To hydrocracking units, the catalyst activity is defined by the required temperature to reach the desired conversion, which is defined by Equation 1.

 Conversion (%) = [(1 – (Fraction with Above TBP in the Product)/ (Fraction with Above TBP in the Feed))] x 100  (1)  

Where TBP is the True Boiling Point, which represents the desired cut point defined by the refiner.

Deactivation of Hydroprocessing Catalysts

           The main deactivation mechanisms of hydroprocessing catalysts are:

·        Metal deposition – Related to feedstock characteristics and drag of contaminants;

·        Active phase sintering process – Related to over temperature and metal deposition;

·        Coking deposition – Related to the processing conditions, feedstock characteristics, and operating issues. Is considered the only reversible deactivation process.

The metals deposition is mainly affected by Ni, V, Pb, As, Si, Fe, and Na. Nickel and Vanadium can be present is heavier fractions of crude oil and plug the catalysts pore and act as coke precursors. Lead (Pb) and Arsenic (As) can react with the active phases (metal sulfides) leading to the sintering process and consequently reduction of active phase area, Pb is found in naphtha fractions and the Arsenic can be found in all petroleum fractions.

Contamination by silicon occurs normally due to the injection of silicon based compounds in the crude oil extraction step and in downstream processes like Delayed Coking units where are applied, anti-foaming agent. The silicon acts reducing the surface area and plugging the catalyst pore, leading to a severe activity reduction. The deactivation by sodium (Na) is similar to the silicon (Si) process, in hydrocracking processes the feed contamination by sodium is a great concern once the basic character of sodium promotes the neutralization of the acid function of the hydrocracking catalysts, leading to a drastic reduction in the conversion (Equation 1).

Coking deposition is related to condensation of high weight molecules (heavier aromatics and asphaltenes) present in heavier feeds. The coke deposition is also related to dehydrogenation, cracking, and polymerization reaction of heavier fractions, the deactivation occurs through the plugging of catalysts pores blocking the mass transfer from the hydrocarbon to the active phase, as presented in Figure 5. 

Figure 5 – Reactants and Products Flows in a Generic Porous Catalyst (GONZALEZ, 2003).

The coking deposition also reduces the active surface area and is normally followed by metals deactivation, mainly to hydroprocessing units dedicated to treating bottom barrel streams.

           The Coking deposition process is positively affected by temperature and negatively affected by hydrogen partial pressure, by this reason, hydroprocessing units dedicated to processing heavier feeds operate under higher pressures with the main objective to protect the catalysts that are responsible by a great part of the operating costs of the refiners.

           In severe hydrocracking units, can be observed an inhibition effect of the NH3 over the catalysts due to the acid function neutralization, in these cases this issue is minimized through the gas separation between the reaction stages, as presented in Figure 2.

The activity of hydrotreating catalysts is monitored through the temperature required to reach desired contaminant content (normally sulfur) in the product being the maximum temperature limited by the metallurgy limits of the material applied in the design of the hydroprocessing unit.

           The most known technology developers of hydroprocessing catalysts are Haldor Topsoe, Albemarle, ExxonMobil, UOP, Advanced Refining Technologies (ART) Company, Criterion, Shell Catalysts Company, and Chevron Lummus Global (CLG) Company. 

Conclusion

           As aforementioned, despite the trend of reduction in transportation fuels demand at a global level, some markets keep still dependent on these derivatives to sustain the local economic development, this is especially true in developing economies like Brazil and India. Refiners inserted in these markets can found in the less severe hydrocracking technologies, mainly the fixed bed alternatives, an important route to improve the yield of middle distillates, mainly diesel, to comply with the internal demand and reduce the external dependence of this derivative, improve the payments balance of the country.

References:

ZHU, F.; HOEHN, R.; THAKKAR, V.; YUH, E. Hydroprocessing for Clean Energy – Design, Operation, and Optimization. 1st ed. Wiley Press, 2017.

ROBINSON, P.R.; HSU, C.S. Handbook of Petroleum Technology. 1st ed. Springer, 2017.

GARY, J. H.; HANDWERK, G. E. Petroleum Refining – Technology and Economics.4th ed. Marcel Dekker., 2001.

GONZALEZ, G. S. Junior Engineer’s Training Course – Kinetics and Reactors. Oxiteno Company, 2003.

SPEIGHT, J.G. Heavy and Extra-Heavy Oil Upgrading Technologies. 1st ed. Elsevier Press, 2013.

Dr. Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in S?o José dos Campos, Brazil. Bachelor in Chemical Engineering from the University of Maringa (UEM), Brazil, and PhD. in Chemical Engineering from the University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design, and construction to oil and gas industry including developing and coordinating projects for operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from the Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).