L. Patron, Contributing Author, Milan, Italy
One of the prevailing factors of the energy transition toward a low-carbon energy system is that the decarbonization of transport will lead to the contraction of the main markets in which crude oil refining products are sold.
Hydroconversion technologies must continue to progress to allow the reduction of fuel oil as well as the reduction of heavy distillates that are now used as precursors of transport fuels. These can possiblly be replaced with light distillates that can be used for petrochemicals—demand for these products is expected to grow.1
To obtain a higher fraction of light distillates from the hydroconversion of heavy oils—preferably the production of (atmospheric) light distillates only—hydroconversion systems with a much higher hydrogenation capacity than the systems now is use will be required.
Regardless of the type of catalyst used (supported or slurried), even using an excess of catalyst and reactors of great height, it has been found that only ~10% of the hydrogen (H2) fed to the base of a hydroconversion reactor is incorporated into the conversion products. This clearly indicates that in a hydroconversion process, due to the temperatures used and the limited diffusion of H2 from the gaseous phase to the reaction liquid, hydrogenation constitutes the slow stage. Once the pressure and temperature of the reactor have been set, the hydrogenation rate can only be increased by increasing the gas-liquid contact surface through which the H2 reaches the reaction liquid.
In the reactors now is use, most of the H2 feed (which does not diffuse into the reaction liquid) does not act on the chemical processes of the hydroconversion treatment. For the purposes of hydrogenation, the H2 is unnecessarily recycled at high surface velocity in the reactor. To expand the gas-liquid contact surface, and therefore the hydrogenation capacity, it is necessary to redesign the way in which H2 is distributed to the reactor in hydroconversion systems now in use.
Features of heavy oil hydroconversion systems. Hydroconversion systems using ebullated catalytic bed reactors constantly operate with a H2-deficient reaction liquid that limits the degree of conversion and the hydrogenation capacity.2 This condition is due to the H2 distribution mode and the type of process used (i.e., open-cycle without recycling) and cannot be overcome.
A different hydroconversion technology that uses a slurry catalyst in a bubble column reactor has had recent and multiple confirmations on a commercial scale and achieves a complete chemical conversion. It injects H2 directly into the reaction liquid and operates in a closed liquid circuit with recycling, and can be transformed into a version with increased hydrogenation capacity.
The reasons it is necessary to operate in a closed liquid circuit with recycling is clarified in the next section.
Upflow reactor variant with expandable gas-liquid contact surface. In a bubble column reactor, the fluid dynamic condition in which the gas bubbles—at the threshold of coalescence, they are packed with a gap between two bubbles equal to the diameter of a bubble—is known as bubble packing. Assuming that the bubbles all have the same diameter, the fluid dynamic condition corresponding to the packing of the bubbles implies a theoretically calculated gas holdup value equal to 0.296 ≃ 0.3, regardless of the diameter of the bubbles. Experimentally, therefore, with the natural distribution of the diameter of the bubbles, the bubble packing is found at a gas holdup value of about 0.299, substantially equal to the theoretical one.3
The bubble packing condition is the most favorable for a heavy oil hydroconversion process, as it allows maximum liquid filling of the reactor while the gas-liquid unit surface (m2/m3 of reactor volume, measurement of the gas-liquid contact surface area) is at its maximum.4 Incidentally, bubble packing is the fluid dynamic condition in which the upflow reactor usually operates in a slurry hydroconversion system.
In a liquid-gas mixture, under bubble packing conditions, the gas-liquid unit surface is determined solely by the average diameter of the bubbles and is inversely proportional to it. By maintaining the packing condition of the bubbles, the progressive reduction of the diameter of the bubbles continuously expands the gas-liquid unit surface, with the hydrogenation rate reaching its maximum when the rise speed of the bubbles in the reactor (due to the reduction of their diameter) is lowered until it is equal to that of the liquid.
In an upflow reactor equipped with a means of H2 distribution with a large number of orifices per m2 (e.g., 100 or more), if the surface velocity with which the H2 is fed is reduced by a certain extent, depending on the density of the orifices, the bubble packing condition is preserved while smaller diameter bubbles are generated. Without the reduction of the surface velocity of the fed H2, the coalescence of the bubbles would be induced with the corresponding undesired increase in their diameter. For example, by using an upflow reactor equipped with H2 distribution means with 400 orifices/m2 rather than 50/m2 as in a conventional upflow reactor, and by suitably lowering the surface velocity of the H2, the packing of the bubbles is preserved while the bubbles generated have a diameter halved compared to the bubbles of a conventional upflow reactor, as roughly shown in FIG. 1.
The variant of the upflow reactor equipped with a H2 distribution means with 400 orifices/m2 allows the generation of a liquid-gas mixture that has a gas-liquid unit surface that is double to that of a conventional reactor, with the doubling of the hydrogenation rate. More details are described in the literature.5
Paradoxically, to increase the hydrogenation capacity of an upflow reactor, the surface velocity (i.e., flowrate) with which the H2 is fed to the reactor must be lowered.
However, this is precisely what is required so the H2 distributor, despite having a high density of orifices to produce small diameter bubbles, continues to generate a liquid-gas mixture in which the packing of the bubbles is preserved.
The required lowering of the surface velocity of H2 can lead to a flowrate reduction of 50% or more. Such a lowering of the H2 flowrate is feasible only when the extraction of the conversion products from the reactor does not take place exclusively in the vapor phase, but is completed by distillation of the reaction liquid downstream of the reactor—this is the case with slurry hydroconversion systems that operate in a closed-liquid circuit with recycling. In hydroconversion systems that use ebullated catalytic bed reactors in an open-liquid circuit without recycling, the extraction of the conversion products that is carried out exclusively in the vapor phase prevents limiting the surface velocity of H2, which is essential to expand the area of gas-liquid interface. The extraction of the conversion products from the reactor exclusively in the vapor phase is incompatible with a reaction mixture with a high gas-liquid contact surface.
H2 needed to hydroconvert heavy oils into light hydrocarbons. In an example case, in the slurry hydroconversion of a vacuum residue with a H2 content of 10.5%, a set of conversion products (product slate) is obtained in which the average weight content of H2 typically rises to 14%. This corresponds to an amount of H2 incorporated in the products equal to 3.5% of the converted charge.
Reference is now made to a typical product slate consisting of:
If the heavy fraction [350°C–540°C (662°F–1,004°F)] with a H2 content of 12.5% were to be converted into the light fraction [350°C (662°F)] that, on average, has a H2 content of 15.2%, the amount of H2 incorporated into the products would rise from 3.5% (10.5% to 14%) to 4.7 (10.5% to 15.2%), an increase of 1.34x. For product slates with a further increased H2 content (e.g., 16%) to bring the H2 content of atmospheric distillates close to that of paraffinic hydrocarbons, the hydrogenation capacity of the slurry reactor must be increased in proportion of the greater quantity of H2 incorporated in the product slate, which will rise from 3.5% (10.5% to 14%) to 5.5% (10.5% to 16%) of the converted charge, an increase of 1.57x. The variant of the upflow reactor illustrated in FIG. 1, which has a double hydrogenation rate compared to a conventional reactor, will be able to supply all the H2 required.
The slurry catalyst bubble column reactor to convert heavy oils to light hydrocarbons. The use of a slurry-type catalyst, characterized by a limited molecular cracking activity and, therefore, with a limited consumption of H2 for the saturation of radicals, allows more H2 to be diverted to the hydrogenation of unsaturated structures.
The combination of a bubble column reactor with the required gas-liquid unit surface with the use of a slurry catalyst allows the complete conversion of heavy oils into [350°C (662°F)] hydrocarbons, or lighter, suitable for petrochemicals. However, this combination alone is not enough. For the reasons described below, a specific process variant must also be adopted.
Process requirements. Since the upflow reactor with a high gas-liquid unit surface operates with a (sometimes considerably) lower H2 surface velocity compared to the usual values—thus reducing the extraction of the conversion products in the vapor phase—the extraction of the same must be completed by extracting the liquids from the reactor.
Furthermore, given the asphaltenic nature of heavy oils, to avoid the separation of asphaltenes that would otherwise make hydroconversion impractical, it is necessary to adequately reduce the concentration of light hydrocarbons in the reaction liquid. By extracting the conversion products as liquids from the reactor, the concentration of light hydrocarbons can be lowered, as necessary.
Process variant for extracting the conversion products as liquids from the reactor. The conversion products in the liquid state are extracted by feeding an adequate flowrate of a heavy mixture consisting of feedstock and atmospheric residue to the reactor. Increasing the flowrate of this heavy mixture correspondingly increases the amount of conversion products withdrawn as liquids from the reactor and transferred to the distillation section for recovery. Further details are described in the literature.5
Slurry hydroconversion technology version with high hydrogenation capacity. Implementing the variant of the upflow reactor (FIG. 1), which allows the gas-liquid unit surface to be expanded, and the process variant, which allows the conversion products to be extracted from the reactor as liquids, transform the current slurry hydroconversion technology into the high hydrogenation capacity version.
This high hydrogenation capacity version converts heavy oils, including crude oils, entirely to [350°C (662°F)] hydrocarbons, or lighter, producing products with crescent H2 content by gearing up the reactor's hydrogenation capacity. The new version of the slurry hydroconversion technology allows the refinery to redesign the product slate with the flexibility that the shrinking transport fuels market requires.
Since this new version of hydroconversion technology does not require simultaneous production of transport fuels, it also allows crude oil to be used exclusively for petrochemicals, with the crude converted at the petrochemical site or at the wellhead. This solution will allow the development of the petrochemical sector, regardless of how the transport fuels market evolves.
Applications and achievable results. Hydroconversion of heavy oils entirely to [350°C (662°F)] hydrocarbons, or lighter, can be implemented in existing plants, based on slurry technology, through the limited adaptations describe above. In addition to allowing the replacement of heavy distillates with light distillates as a consequence of the greater quantity of H2 in the reaction liquid, the improved slurry technology version also brings advantages in terms of denitrification capacity, desulfurization capacity and reduced catalyst consumption of following the complete chemical conversion that is achieved.
Considering the market prospects mentioned above, the high hydrogenation capacity version of the slurry hydrogenation technology can make it profitable to transform the current open-liquid circuit systems, using ebullated catalytic bed reactors, into closed-liquid circuit systems, using slurry bubble column reactors.
For new installations, the high hydrogenation capacity version of the slurry hydroconversion technology, which reduces the recycling gas rate and does not require the vacuum section and the subsequent hydrocracking section, will allow for significant reductions in the cost of the plant. HP
LITERATURE CITED
Luigi Patron served as Chairman and CEO of the engineering company Snamprogetti between 1997 and 2005. Snamprogetti, formerly a subsidiary of ENI and later merged with Saipem, designed and built the demonstration unit used to validate the first commercially applied heavy oil slurry hydroconversion technology. Mr. Patron has since continued working on hydroconversion technologies independently.