December 2017

Special Focus: Plant Design, Engineering and Construction

Convert the heaviest fraction of the barrel into valuable products

A proprietary slurry technologya represents an innovation in residue conversion and unconventional oil upgrading, and marks a step change in the treatment of the heavy end of the barrel.

A proprietary slurry technologya represents an innovation in residue conversion and unconventional oil upgrading, and marks a step change in the treatment of the heavy end of the barrel. The slurry technology can be classified as a hydrocracking process. It is characterized by the use of nano-dispersed catalysts and an original process scheme that allows almost full feedstock conversion.

The new technology converts the heaviest fraction of the barrel into valuable products—mainly transportation fuels—and has a significant impact on the economic and environmental valorization of hydrocarbon resources.

A 0.5-bpd pilot plant was built to investigate and verify the process’ kinetics and fluid dynamics, as well as to try various formulations of the unsupported catalyst to determine the best recipe. It was determined that the technology allows the efficient, environmentally sustainable exploitation of unconventional oil reserves, and increases the profitability of the refining system by a higher flexibility of feedstocks. This view is supported by the upcoming new marine fuel specifications, which will be discussed here.

FIG. 1. The reaction section of the commercial demonstration plant at the Eni Taranto refinery.
FIG. 1. The reaction section of the commercial demonstration plant at the Eni Taranto refinery.

The intermediate step before the construction of an industrial plant was the implementation at Eni’s Taranto refinery of a 1,200-bpd commercial demonstration plant (CDP) that started in 2005 (FIG. 1). The first test with a vacuum residue (VR) from Urals crude confirmed the process performance previously obtained at pilot plant scale, as well as the fluid dynamics of the in-house-designed slurry bubble column reactors and gas distributors.

The slurry reactors were almost isothermal, as demonstrated by the axial and radial temperatures profiles. Further tests used different feedstocks, such as VR from Athabasca bitumen, VR of a Middle East heavy crude oil, and visbroken tars. The Taranto refinery’s CDP processed more than 160 Mbbl of black feed with excellent results, a purge at steady-state condition set at 2 wt% or lower, and a liquid volume yield that always exceeded 100%. After a major turndown, the inspection of equipment and piping for fouling and corrosion showed good conditions. Particular attention was paid to optimizing the operating conditions to reduce CAPEX and OPEX of the slurry technology process, in view of a future industrial plant.

Improving on upgrading technologies

Converting the bottom of the barrel essentially means to break the largest hydrocarbon molecules contained in the oil and recombine the smaller molecules to maximize the fraction in the range of boiling points of interest, while increasing the hydrogen (H2)/carbon (C) ratio. Increasing the H/C ratio is possible only by rejecting C or adding H2.

Thermal cracking, coking and visbreaking are carbon-rejecting technologies that generate a lighter fraction and a heavier fraction. H2-addition technologies are more effective, as they allow an overall upgrading of the products with respect to the feedstock.

Carbon-rejection technologies are usually non-catalytic, while H2-addition technologies require solid inorganic catalysts operating under high H2 pressure.

H2 addition occurs by hydrocracking/hydrogenolysis and hydrogenation. The yields of coke and heavier products are lower in favor of liquid products. The reaction consumes substantial amounts of H2, and specific investment costs are relatively higher compared to thermal processes. During the process, other reactions take place, leading to the removal of sulfur (S), nitrogen (N) and metals, and to the saturation of aromatics and olefins. Catalytic residue hydrocracking processes are implemented in fixed-bed, moving-/ebullated-bed or slurry reactors.

In the fixed-bed reactors, a fast catalyst deactivation rate and potential channeling due to poor catalyst loading or to plugging phenomena might reduce the efficiency of the process, or result in an uneven temperature profile in the catalytic bed. Typical conversion rates are between 30% and 40%, and the choice of feedstocks is limited by metal content.

In the ebullated-bed processes, the feed conversion can range from 60%–70%, depending on feed and process parameters, leaving 30%–40% of heavy residue for use as low-value fuel oil or bunker oil. The conversion rate is limited by the stability of the bottom.

A further step toward achieving the total conversion of the bottom of the barrel to middle distillates is the slurry hydrocracking technology, which was initially developed in Germany in the 1920s, thanks to the contribution of scientists such as Friedrich Bergius, Carl Bosch, Alwin Mittasch and Matthias Pier.

FIG. 2. High-resolution transmission electron microscopy (HRTEM) of the new slurry technology catalyst, showing the dispersed lamellae (left) and a single MoS2 layer with superimposed atomic model (right). (Images by Eleanora Di Pada and Erica Montanari, Enu S.p.A.)
FIG. 2. High-resolution transmission electron microscopy (HRTEM) of the new slurry technology catalyst, showing the dispersed lamellae (left) and a single MoS2 layer with superimposed atomic model (right). (Images by Eleanora Di Pada and Erica Montanari, Enu S.p.A.)

The slurry process operates in the presence of a catalyst with sub-micronic particles. The catalyst is more stable than the conventional hydrocracking catalyst, and it can be used in the presence of much heavier feedstocks. After the reaction and the separation sections, the catalyst remains in the residue of the vacuum distillation unit with the nickel (Ni) and vanadium (V) sulfides. Total conversion is achieved by recycling the heavier unconverted fraction through the reactor, so that only this fraction remains in the reactor for a longer residence time.

The innovation of the new slurry technology consists of an oleo-soluble molybdenum octoate dissolved in the feedstock. The mixture is fed to the reactor, and H2 is supplied through a distributor located at the reactor bottom. Under these conditions, the catalyst precursor is converted to molybdenite, which is crystalline-layered molybdenum disulfide (MoS2), with an average particle size of a few nanometers (FIG. 2).

FIG. 3. Simplified process scheme.
FIG. 3. Simplified process scheme.

The simplified scheme of the process is shown in FIG. 3. The nano-sized hydrogenation catalysts and an original process scheme allow the complete feedstock conversion to valuable distillates, avoiding the production of residual byproducts, such as petcoke or heavy fuel oil.

The heart of the process is a slurry reactor in which the heavy feed is hydrocracked to lighter products in the presence of nano-sized, Mo-based catalyst.

The feedstock conversion starts with thermal breakage of the C-C bonds and generation of free radicals that are suddenly quenched via H-uptake reactions, preventing the free radical recombination that could evolve to coke formation. The presence of an active Mo-based catalyst, such as molybdenum sulfide, promotes the H2 activation reaction.

The use of unsupported slurry catalysts is particularly useful in feedstock containing high concentrations of pollutants, particularly metals and asphaltenes. Contrary to the conventional supported catalysts that are utilized in fixed- and ebullated-bed reactors, the dispersed molybdenite is not susceptible to plugging problems due to the metals and coke deposits on the porous supports.

The upgraded oil withdrawn from the slurry reactor is sent to a separation section to recover gas, naphtha, and middle and vacuum distillates. The unconverted material, as well as the dispersed catalyst, are recycled back to the reactor and blended with the fresh feed. The recycle stream is reprocessed to achieve almost total conversion, while a small purge is necessary to eliminate metals buildup.

The process ensures complete metal removal (HDM), excellent Conradson Carbon Residue and sulfur reduction (HDCCR and HDS), and good denitrogenation (HDN). Another peculiar characteristic of the slurry technology process is the production of a high-quality vacuum gasoil (VGO) with low sulfur and aromatic content that can be further converted into transportation fuels or used as new marine bunker.

The industrial plant

The first industrial application of the new slurry technology is at Eni’s Sannazzaro de’ Burgondi refinery. The unit has a design capacity of 23 Mbpd and allows the refinery to convert the bottom of the barrel into diesel and other valuable refinery streams (LPG, naphtha, jet fuel, etc.).

The complex also represents the first full-scale industrial plant in operation based on a slurry hydrocracking process. The schedule of the project began with front-end engineering in 2009, and the first oil was achieved in October 2013.

FIG. 4. A view of the complex at the Eni’s Sannazzaro refinery: the upgrading section (foreground) and the slurry section (background).
FIG. 4. A view of the complex at the Eni’s Sannazzaro refinery: the upgrading section (foreground) and the slurry section (background).

The complex (FIG. 4) represented a major investment with a budget of €1.2 B, including ancillary plants. The complex configuration incorporates the most advanced technical solutions and the operating experience achieved in more than 8 yr of continuous tests and operations in the demonstration plant. Construction methodologies were adopted with extensive use of the preassembly of large structures, foundations and even process heaters. Reactors of maximum size, in terms of internal diameter and weight, were installed to establish a reference for future industrial initiatives.

The operating period of the slurry technology plant at Sannazzaro has allowed technological fine-tuning and the upgrade of some ancillary plants for better performance. The main improvements have been in the separation of the vapor/liquid phases in the fractionation section. These small changes proved successful and allowed the plant to exceed the designed performance for a significant run period.

On December 1, 2016, the slurry technology plant at the Sannazzaro refinery suffered a severe accident, with fire at two different points, due to leaks in the H2 lines.

Based on inspections, evidence and instrumental recorded data, it is likely that the failures were due to mechanical stress that led to line leakage of a clamped flange. Most of the damage occurred at the high- and medium-pressure separators and the atmospheric column. Both slurry reactors and all other main equipment were found to be in good condition.

Refinery personnel managed the major accident well, and the fire was quickly extinguished. The effective activation of all safety and prevention measures allowed the immediate and safe evacuation of all plant workers without injuries. Local authorities certified that the concentration of pollutants in the air was not significant.

FIG. 5. Sannazzaro complex flow diagram.
FIG. 5. Sannazzaro complex flow diagram.

Despite the accident, the owner-operator is committed to reconstructing the plant. A global engineering company conducted a cold eye review and will assist the owner during the engineering and reconstruction phase. The process scheme of the original slurry technology plant will be maintained (FIG. 5).

Economics in the new scenario

A rapid reconstruction of the complex is also dictated by new specifications for marine fuels. The International Maritime Organization (IMO) recently introduced more stringent limits on the sulfur content of marine fuels: 0.1 wt% in sulfur emissions control areas (SECAs) from January 1, 2015, and 0.5 wt% outside of SECAs beginning January 1, 2020. After sulfur removal in gasoline and diesel, bunker fuel will provide a significant additional reduction of planetary sulfur dioxide (SO2) emissions.

Global marine fuel demand is expected to reach 320 MMtpy in 2020. The shift to low-sulfur marine bunker will result in demand of approximately 230 MMtpy of fuel oil with a sulfur content of 0.5 wt% or less, as the use of high-sulfur fuel oil (HSFO) will continue for ships equipped with scrubbers, and demand for liquefied natural gas (LNG) will be limited.

The supply of bunker fuels at 0.5% sulfur is now the most important challenge for the refining industry, which has two available pathways to comply with the new rules:

  • Process ultra-low-sulfur fuel without additional investment
  • Invest in conversion plants to upgrade the bottom of the barrel.

The first option will increase stress on sweet crude quotations and lead to a rise in the price differential between sweet and sour crudes over the medium and long terms. This will allow deep conversion refineries (which process heavy and medium sour crudes) to achieve record margins, while simple refineries will be at a disadvantage.

The slurry technology industrial unit can produce a slate of products, including light and medium distillates and marine bunker, that will meet 2020 specifications. No fuel oil is produced, and the reduced amount of purge can be easily supplied to cement and steel factories, to the gasification plant or for metals recovery.

FIG. 6. A comparison of economics for different upgrading technologies (Eni scenario 2017, Western Europe).
FIG. 6. A comparison of economics for different upgrading technologies (Eni scenario 2017, Western Europe).

One likely scenario for the IMO 2020 regulations is an increase in the sweet/sour crude oil differential in the medium and long terms, an upward stress on distillates-HSFO spreads and the collapse of the HSFO quotation. Very high spreads are expected for a significant number of years. These factors are ideal for highly complex refineries processing sour crudes, as they will squeeze exceptional margins from the barrel.

Delayed coking and fixed- and ebullated-bed technologies do not fit perfectly into the coming scenario, as they have important drawbacks. The coking technology conflicts with the large surplus of coke in the market and with decarbonization policies that have already been adopted or are anticipated. Meanwhile, existing and conventional hydrogenation technologies will continue to produce significant quantities of fuel oil with sulfur that is out of spec for marine use.

The introduction of a slurry technology plant in a refinery can greatly enhance efficiencies and the bottom line. Based on a simulation with a 2017 scenario, the contribution margin of such a plant is 30% higher compared to an ebullated bed, and 40% higher compared to a delayed coking system (FIG. 6). Looking to 2020 and beyond, the relative contribution margin is expected to widen in favor of this slurry technology. HP

NOTES

     a  The proprietary slurry technology is Eni Slurry Technology (EST).

The Author

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