April 2024
Process Optimization
New generation slurry reactor for the hydrotreatment of heavy oils and distillates
Due to the ongoing transition to a low-carbon energy system, refiners must produce less fuel oil. Among the technologies used for the commercial-scale hydroconversion of the heavy fraction of crude oil, the use of bubble column upflow reactors with slurry catalyst (slurry technology) does not produce fuel oil.
Due to the ongoing transition to a low-carbon energy system, refiners must produce less fuel oil. Among the technologies used for the commercial-scale hydroconversion of the heavy fraction of crude oil, the use of bubble column upflow reactors with slurry catalyst (slurry technology) does not produce fuel oil. This is because, following a process through which hydrogenation involves a high chemical deasphalting of the heavy oil (~99.5%), the product slate includes exclusively gaseous and distilled hydrocarbons.
The slurry reactor, however, operates with a hydroconversion capacity limited to only 0.1 m3/hr of heavy oil per m3 of reactor volume. Low unit capacity is a critical aspect of current slurry technology that must be overcome to achieve more favorable economic results.
Despite the high degree of chemical deasphalting, the percentage of charge converted into products is limited (~94%).1 To remove the insoluble material that the hydroconversion process produces, it is necessary to purge approximately 6% of the residual reaction liquid. The insoluble material is largely made up of organic material, while the remaining part is made up of the sulphides of the metals present in the fed charge.
Purging inevitably produces losses of unconverted charge and catalyst and generates waste that must be disposed of. To overcome this second critical issue of current slurry technology, the hydroconversion process should prevent the formation of such insoluble organic material.
This article will detail how, by using an improved bubble column reactor, it is possible to carry out the slurry hydroconversion of heavy oils with a unit capacity much higher than the current values, and a drastically reduced (if not eliminated) purge.
Current slurry reactor limits.
In the hydroconversion of heavy oils using upflow-type reactors, liquid and gas in the form of bubbles rise. Consequently, both the gaseous products and the reaction liquid exit at the head of the reactor. With this type of reactor, the capacity to extract the conversion products is determined by the surface velocity of the hydrogen (H2). Since the H2 cannot be increased beyond certain limits, the unit capacity of the upflow reactor itself is consequently limited.
In bubble column reactors, H2 diffuses from the gas phase into the reaction liquid across the interface between the gas bubbles and the liquid. What diffuses into the reaction liquid, and is then incorporated into the products, is only a minimal fraction of the total quantity of H2 fed to the base of the reactor.2 This is due to a bubbling mixture generated within the reactor that has an insufficient gas-liquid contact surface area. A bubbling mixture with a low gas-liquid contact surface limits the hydrogenation capacity of the reactor and, consequently, the possibility of hydroconverting heavy oils with higher flowrates and a higher degree of conversion.
Benefits of a new slurry reactor.
To operate at a higher capacity and with a higher degree of conversion, the reactor should:
- Implement a flexible extraction of the reaction liquid that is not dependent on the surface velocity of H2.
- Implement a H2 distribution that creates bubbling mixtures with as large a gas-liquid contact surface as required.
Flexible extraction of the reaction liquid.
For the reaction liquid to be extracted without flow constraints related to the H2 flow, the liquid inside the reactor is diverted downwards to be withdrawn from the bottom. This descending liquid bubble column slurry reactor has been recently described in literature3 and allows users to choose the concentration of the products in the reaction liquid with which to operate the reactor.
The concentration of the products can therefore be lowered to eliminate the loss of unit capacity that occurs, as the height increases, when the reactor is of the upflow type. The ratio between maltene and asphaltene hydrocarbons in the reaction liquid can be lowered as needed to avoid the precipitation of the asphaltene fraction which, in upflow reactors, occurs when the height of the reactor is excessive in relation to the H2 fed.
A low concentration of products in the reaction liquid, with a consequential higher concentration of residue, can be chosen to increase the cracking capacity—with an adequate hydrogenation capacity, the reactor can hydroconvert a greater flowrate of heavy oils.
Creation of bubbling mixtures with high gas-liquid contact surface.
In a homogenous bubble flow regime, the bubble packing condition at the coalescence threshold allows maximum liquid filling of the reactor and the maximum gas-liquid contact surface. Under packing condition, a distribution of bubbles according to a tetrahedral lattice pattern is assumed. With L indicating the length of the edge of the tetrahedron, the number of bubbles per unit of volume (Nb) is given by Eq. 1:
Nb = √ (2) / L3 (1)
The contact area between the surface of the gas bubbles and the liquid, measured in m2 per m3 of gas-liquid mixture and called the gas-liquid surface unit area, is denoted by uS and is solved by the product of the number of bubbles (√(2)/L3) by the surface of the single bubble [π (db )2].
When bubbles are packed at the coalescence threshold, the maximum possible gap between two bubbles is the diameter (db) of the bubble itself.4 The diameter of the bubble is therefore equal to half of L. The expression for the gas-liquid unit surface area then becomes (Eq. 2):
uS = π /2√ (2)L (2)
In a bubble column reactor, since the bubbles are at the vertices of a tetrahedron, the length L of the edge of the tetrahedron is the geometric factor that determines the value of the gas-liquid surface unit area. When the orifices for gas distribution are arranged in a square lattice, the value of L is close to the orifice spacing. Instead, the value of L coincides with the spacing of the orifices (Os) if these are arranged according to a pattern of adjacent equilateral triangles. With such an arrangement of orifics, the gas-liquid unit surface area can be expressed as a function of Os using Eq. 3:
uS = π / 2√ (2) Os (3)
By appropriately choosing the Os of the orifices, the bubbling mixture of packed bubbles obtained will have a gas-liquid unit surface area of a pre-established value uS.
Gas distribution with a high-density of orifices per m2—thus with correspondingly reduced spacing—has been described in literature recently.5 With gas distribution methods with a high density of orifices—keeping the bubbles packed for the reasons explained above—the H2 is fed with a surface velocity that is limited in relation to the number of orifices used per m2.
The distribution of H2 with gas distribution means that have a high orifice density is preferably carried out using the aforementioned descending liquid bubble column slurry reactor. Unlike an upflow reactor, it can operate with the required extraction of reaction liquid (therefore, of products) even when, as a consequence of the use of gas distribution means with a high density of orifices, the H2 is fed to the reactor with a reduced surface velocity.
An orifice spacing of ≤ 0.1 m allows the generation of bubbling mixtures with a gas-liquid surface unit area greater than that with which hydroconversion reactors operate (ebullated catalytic bed or slurry catalyst in a homogeneous bubbly flow regime), which feed the H2 with spacing between entries > 0.125 m.
The resulting low values of the gas-liquid unit surface area, together with the low solubility of H2 in the hydrocarbons, cause the limited performance that characterize these reactors, despite the high pressures in which they operate.
The framework of the new reactor.
The slurry reactor to extract the reaction liquid without constraints and generate bubbling mixtures with a sufficiently high unit area of the gas-liquid surface will be a descending liquid bubble column slurry reactor, equipped with gas distribution means with a high orifice density: a descending liquid reactor (DLR).
THE HYDROCONVERSION OF HEAVY OILS USING A DLR
The operational flexibility achieved by the above configured reactor allows the hydroconversion of heavy oils with greater unit capacity and a higher, if not complete, conversion.
Heavy oils are converted with a high flowrate per m3 of reactor volume.
As can be seen in FIG. 1, the 50-m tall reactor (1) is equipped at the base with a means of gas distribution (2) with > 100 orifices per m2.The means of gas distribution (2) are fed through Line 3 with H2 at a surface velocity reduced in relation to the orifice density. The hydrogenation capacity of the reactor, compared to that of a conventional reactor, increases by 1.74 times.
The charge to be converted—consisting of vacuum residue (> 540°C) with a density of 1 t/m3—is fed via Line 4, above the gas distribution means (2) with a flowrate of 7,500 kg/hr (or 7,5 m3/hr) per m2 of reactor section. Through Line 5, the reaction liquid, cooled below the means of distribution (2) by heat exchange with the feed (not shown in the figure) is extracted from the bottom of the reactor with a flowrate necessary to keep the level of the bubbling liquid constant in an upper part (6) of the reactor. The reaction liquid taken from the bottom of the reactor is sent to a distillation section to extract atmospheric gasoil (9) and vacuum gasoil (11). The residual reaction liquid of the vacuum distillation (10), consisting of partially converted feed, is recycled to the reactor, above the means of gas distribution (2), via Line 12 with a flowrate of 18,000 kg/hr per m2 of reactor section: this is a predefined value to extract (5) a reaction liquid containing 25% conversion products (see below).
Under the temperature conditions used in the slurry hydroconversion of heavy oils, a gaseous flow (7) containing the unreacted H2 and the light conversion products emerges from the top of the reactor. In the flow (7), all products with a boiling point up to 170°C are present.
In a typical heavy oil hydroconversion product slate, C1- (170°C) hydrocarbons make up approximately 20%. In the present example, the C1- (170°C) hydrocarbons exiting the reactor head are equal to 20% of the feed introduced into 4, and are therefore equal to 1,500 kg/hr per m2 of the reactor section. The remaining > 170°C high-boiling conversion products, which are extracted from the bottom of the reactor as liquids, are recovered through the distillation section at a (maximum) flowrate of 7,500 – 1,500 = 6,000 kg/hr per m2 of the reactor section. Therefore, the > 170°C high-boiling conversion products are present in the reaction liquid extracted from the bottom of the reactor in a percentage (at most) equal to 100 x 6,000/(6,000 + 18,000) = 25%. In the same reaction liquid, there is a residue to be converted in a percentage of (at least) 100 x 18,000/(6,000 + 18,000) = 75%.
The high concentration of residue in the reaction liquid (1.5 times that of a conventional reactor) and the high availability of H2 (1.74 times, as stated above)—both ensured by a bubble column reactor with descending liquid and equipped with gas distribution means with a high density of orifices—bring the hydroconversion capacity to 7.5 m3/hr per m2 of the reactor section, with a reactor height of 50 m.The unit capacity therefore rises to 0.15 m3/hr per m3 from a value of 0.1 m3/hr per m3 found in conventional slurry upflow reactors. With a height of 50 m and a diameter of just over 5 m, a single reactor with a useful reaction volume of 1,000 m3 converts 22,264 bpd of vacuum residue (> 540°C).
Heavy oils are hydroconverted with little, if any, purge.
The recycling of the residual reaction liquid, Line 12 in FIG. 1, also returns insoluble material generated by the hydroconversion process to the reactor. Hydroconversion does not act on this insoluble material, as this material is dispersed as a solid in the reaction liquid. This material, of organic and inorganic nature, is left to accumulate in the reaction liquid at the maximum concentration compatible with the regular operation of the system so that it can be removed, through Line 13, with the minimum purge compared to the charge introduced into 4.
The organic fraction (the most prevalent) is made up of asphaltenes with a low H2 content. Asphaltenes, which are soluble when they have a H2 content > 6%, start to become insoluble when the percentage of H2 drops below this value.6 In the usual conditions of limited availability of H2 in the reaction liquid, the asphaltenes, upon exiting a hydroconversion stage, are strongly dehydrogenated. This is because the amount of H2 available to hydrogenate asphaltenes is only that which remains after the instantaneous saturation (with H2) of the radicals generated by cracking. In the hydroconversion of heavy oils with a H2 content of 10%–11% (due to insufficient hydrogenation), the residual reaction liquid, which is extracted at the bottom of the vacuum column (10), shows that the percentage of H2 was halved, or more than halved. This loss of H2 produces asphaltenes with a low H2 content (< 6%), which separate as solids from the reaction liquid, and, as such, are excluded from the hydroconversion process. No longer convertible, the insoluble low- H2 content asphaltenes become organic waste that must be disposed of (sometimes called coke).
The new slurry reactor’s ability to create bubbling mixtures with a gas-liquid surface of greater area carries out the hydroconversion with greater availability of H2 in the reaction liquid. This ensures the H2 content in the residual reaction liquid to prevent the formation of organic insolubles and, therefore, of organic residue.
In the absence of organic insolubles, since only metallic sulfides must be removed, reaction liquid purges limited to 1% (or less) of the fed charge are applied. Heavy oils are thus converted into products at 99% (or more), while the catalyst loss will be reduced by 6 times and limited to 10 ppm (or less) of the charge for every 1,000 ppm present in the reaction liquid.
Since no insoluble organic residue is produced, the metal sulfides can be mechanically separated from the reaction liquid without liquid purging for complete conversion of the heavy oil.
Incidentally, the elimination of organic insolubles can also be achieved with current reactors by adequately lowering the reactor's temperature, with an inevitable loss of capacity. A product yield that rises to 99% from 94% and a catalyst consumption reduced by 6 times, as well as a drastic reduction in waste to be disposed of (about 6 times), bring economic benefits that could more than compensate for a reduced capacity.
First prospects for the application of DLR slurry technology.
The DLR slurry technology, in terms of hourly capacity, degree of conversion and catalyst consumption achieved—for new slurry plants and the retrofitting of current ones—can also represent a valid solution for the conversion of ebullated catalytic bed reactors, penalized by low conversion.
Takeaways.
From a nanodispersed catalyst, such as molybdenum dihydrosulphide (HS-Mo-SH), generated from molybdenum ethylhexanoate as a precursor1, it is possible to obtain, despite what has been reported in literature7, a hydrogenation rate that increases continuously with the concentration of the catalyst. This is well beyond the concentration threshold initially found, if the amount of H2 that diffuses from the gaseous phase into the reaction liquid is made to increase similarly, for example by extending the area of the gas-liquid surface.
By operating with a high gas-liquid surface unit area and nanodispersed catalysts used at high concentrations, a DLR is capable of obtaining higher hydrogenation rates, many times those currently observed. By using a DLR, it is possible to fully exploit the high activity of nanodispersed catalysts, which has so far not been adequately captured. HP
LITERATURE CITED
- U.S. Patent No. 11,499,103 B2, “Process for the hydroconversion of heavy oil products with recycling,” Eni S.P.A., November 2022.
- Patron, L., “Heavy oil hydrocnversion using an upflow reactor variant,” H2Tech, Q2 2022.
- International patent, application number PCT/IT2023/050271.
- Shaikh, A. and M. Al-Dahhan, “A review on flow regime transition in bubble columns,” International Journal of Chemical Reactor Engineering, Vol. 5, 2007.
- International patent, application number PCT/IT2022/050288.
- Wiehe, I., “Petroleum resid conversion: Opportunities and limitations,” 2011 AIChE Spring Meeting and Global Congress on Process Safety, Chicago, Illinois, 2011.
- U.S. Patent No. 42266742.
The Author
Luigi Patron served as Chairman and CEO of the engineering company Snamprogetti—formerly a subsidiary of ENI and later merged with Saipem—which designed and built the demonstration unit used to validate the first commercially applied heavy oil slurry hydroconversion technology. After retirement, Patron has continued to work independently on a slurry technology capable of providing solutions to the challenges that the energy transition will pose to crude oil refining.
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