Upgrading of Heavy Feeds Part 1 (Petroleum Refining)

Heavy feeds are characterized by low API gravity and high amounts of impurities. In general, it is known that the lower the API gravity, the higher the impurities content. Such properties make the processing of heavy feeds different from that used for light distillates, causing several problems:

• Permanent catalyst deactivation in catalytic cracking and hydrocracking processes, caused by metals deposition

• Temporary deactivation of acid catalysts, due to the presence of basic nitrogen

• Higher coke formation and lower liquid product yield, as a result of high Conradson carbon and asphaltene contents

• Products with high levels of sulfur

To reduce such problems, numerous catalytic and noncatalytic technologies are commercially available to upgrade heavy oils, which are summarized in the following sections.

Properties of Heavy Oils

Heavy oils exhibit a wide range of physical properties. Whereas properties such as viscosity, density, and boiling point may vary widely, the ultimate or elemental analysis varies over a narrow range for a large number of samples. The carbon content is relatively constant, while the hydrogen and heteroatom contents are responsible for the major differences in various heavy oils.

Heavy oils are comprised of heavy hydrocarbons and several metals, predominantly in the form of porphyrines. Heavy feeds also contain aggregates of resins and asphaltenes dissolved in the oil fraction, held together by weak physical interactions. With resins being less polar than asphaltenes but more polar than oil, equilibrium between the micelles and the surrounding oil leads to homogeneity and the stability of the colloidal system. If the amount of resin decreases, the asphaltenes coagulate, forming sediments. Asphaltenes are complex polar structures with polyaromatic character containing metals (mostly Ni and V) that cannot be defined properly according to their chemical properties, but they are usually defined according to their solubility. Thus, asphaltenes are the hydrocarbon compounds that precipitate by addition of light paraffin in the heavy oil. Asphaltenes precipitated with n- heptane have a lower H/C ratio than those precipitated with n-pentane, whereas asphaltenes obtained with n-heptane are more polar, have a greater molecular weight, and display higher N/C, O/C, and S/C ratios than those obtained with n-pentane.

Asphaltenes are constituted by condensed aromatic nuclei carrying alkyl groups, alicyclic systems, and heteroelements. Asphaltene molecules are grouped together in systems of up to five or six sheets, which are surrounded by the maltenes (all those structures different from asphaltenes that are soluble in n-heptane). The exact structure of asphaltenes is difficult to obtain, and several structures have been proposed for the asphaltenes present in various crudes. An asphaltene molecule may be 4 to 5 nm in diameter, which is too large to pass through micropores or even some mesopores in a catalyst. Metals in the asphaltene aggregates are believed to be present as organometal-lic compounds (porphyrine structure) associated with the asphaltene sheets, making the asphaltene molecule heavier than its original structure (Figure 1.13 ).

The complex nature of heavy oil fractions is the reason that refining of these feeds becomes so difficult. Therefore, an evaluation of the overall chemical and physical characteristics of petroleum feeds is mandatory to determine the processing strategy. Apart from having low API gravity (high density), high viscosity, and a high initial boiling point, heavy oils exhibit higher contents of sulfur, nitrogen, metals (Ni and V), and high-molecular-weight material (asphaltenes).

Generally, the majority of the sulfur and nitrogen species present in a crude oil is found in the heaviest fractions. These heteroatoms are removed from hydrocarbon streams in downstream refining units to produce ecologically acceptable fuels and/or to provide better quality feeds to subsequent processes: for example, feed with a low concentration of basic nitrogen is required to avoid the temporary poisoning effect on acid catalysts typically used in fluid catalytic cracking (FCC) and hydrocracking (HCR). Metals are found in most heavy oils in the form of metalloporphyrins and are concentrated exclusively in the residual fraction. The problem with metal-containing feeds is the permanent catalyst deactivation experienced in FCC, residue fluid catalytic cracking (RFCC), and HCR units. Asphaltenes are the most complex structures and cause many problems in refining operations. Known as coke precursors, they reduce catalyst cycle life and liquid yield and are the main contributors of solids formation, producing fouling in all types of equipment.

Hypothetical structure of an asphaltene molecule.

Figure 1.13. Hypothetical structure of an asphaltene molecule.

The properties of petroleum residue vary widely, depending on the crude of origin, as shown in Table 1.8. Crude oils and their respective residua have a similar composition (e.g., sulfur, metals, and asphaltene contents), and the latter represents a significant portion of a barrel of crude oil. In the case of heavy petroleum, the yield of residue may be as high as 85%. For this reason, in the near future the material at the bottom of the barrel will be the main raw material for obtaining valuable liquid products, to keep up with fuel demand.

Process Options for Upgrading Heavy Feeds

General Classification One way to establish the quality of heavy oils is by the hydrogen-to-carbon (H/C) ratio. Values of about 1.5 indicate high-quality feed, while poor-quality oils may have an H/C ratio as low as 0.8. Therefore, to improve the quality of heavy oil, its H/C ratio needs to be increased either by increasing the hydrogen content or by decreasing the carbon content. Based on this consideration, processes for upgrading of heavy oils can be classified into two groups:

TABLE 1.8. Properties of Various Atmospheric Residua (AR), 343°C+





Ni + V


of AR

Crude Oil








North Sea






Arabian Light







West Texas Sour

United States



















North Slope







Arabian Heavy






















United States






Cold Lake




















1. Hydrogen addition: hydroprocesses such as hydrotreating and hydro-cracking, hydrovisbreaking, and donor-solvent processes

2. Carbon rejection: coking, visbreaking, and other processes, such as solvent deasphalting

Both hydrogen addition and carbon rejection processes have disadvantages when applied to upgrading heavy oils. For example, removal of nitrogen, sulfur, and metals by exhaustive hydrodenitrogenation (HDN), hydrodesulfurization (HDS), and hydrodemetallization (HDM) is very expensive (excessive catalyst utilization), due to metal and carbon deposition. Noncatalytic processes yield uneconomically large amounts of coke and low liquid yield.

Processes for upgrading heavy oils are evaluated on the basis of liquid yield (i.e., naphtha, distillate, and gas oil), heteroatom removal efficiency (HDS, HDN, HDM), feedstock or residue conversion (RC), carbon mobilization (CM) and hydrogen utilization (HU), along with other process characteristics. Heteroatom removals and feedstock conversion are calculated from their corresponding amounts in feed and product:


where Ifeed and Iproduct represent the amount of impurity (sulfur, nitrogen, or metals) in the feed and product, respectively. 538°Cfeed and 538°Cfroduct are High values of CM and HU correspond to high feedstock conversion processes such as hydrocracking (hydrogen addition). Since hydrogen is added, HU can be greater than 100%. On the contrary, low CM and HU correspond to low feedstock conversion, such as coking (carbon rejection).

The focus on the downstream and upstream petroleum sectors for each country may vary depending on the quality of crude oil. Significant advances have been made in these sectors over the last few decades. The downstream sector has traditionally been in charge of petroleum refining. However, with the increasing production of heavy petroleum, the upstream sector has entered into the upgrading area to increase the value of the oil produced. Thus, nowadays, both sectors are looking for better alternatives to upgrade and refine heavy petroleum.

Heavy oil upgrading is usually carried out directly by using the residue as feed after crude distillation. There is a wide range of catalytic and noncatalytic conversion processes that can be classified into the carbon rejection and hydrogen addition processes, as presented in Table 1.9. These processes use a variety of reactor designs and configurations, such as multi-fixed-bed systems, ebullated-bed reactors, fluidized reactors, and moving-bed reactors. Examples of some of these upgrading technologies are presented in Figure 1.14 . The process technologies differ principally on the basis of the feedstock and process conditions (reactor) and catalyst used by the various licensers.

Carbon Rejection Processes The carbon rejection route is based on the removal of carbon in the form of coke with a low atomic hydrogen/carbon  the petroleum fractions in the feed and product, respectively, with a boiling point higher than 538°C (i.e., vacuum residue).

Carbon mobilization and hydrogen utilization are defined as follows:


TABLE 1.9. General Classification of Technologies for Upgrading of Heavy Petroleum Feeds

Carbon Rejection

Hydrogen Addition


Solvent deasphalting





Catalytic cracking of residue



Process alternatives for upgrading of heavy oils.

Figure 1.14. Process alternatives for upgrading of heavy oils.

Carbon rejection is an important process for residue conversion and is the most common method used commercially. In general, thermal cracking of residue is carried out at relatively moderate pressure and is often called the coking process. It is conducted at temperatures between 480 and 550°C and vapor-phase residence times of 20 or more, providing a significant degree of cracking and dehydrogenation of the feed, which makes subsequent processing more cumbersome and produces low-value by-products such as gas and coke. The coking process transfers hydrogen from the heavy molecules to the lighter molecules, resulting in the production of coke or carbon. The residue is hydrogen donors at high temperature.

The thermal conversion of heavy oil has attracted great interest in recent years, due to the decrease in middle distillate or increase in low-quality crude oil. Thermal processes produce a relatively high amount of gas, such as methane, ethene, propene, butane, and secondary products such as LPG and dry gas. Coke is a significant by-product whose formation mechanism is different from that of other products. Some of the thermal processes are coupled with catalytic processes. The catalytic pyrolysis of heavy oil may be a good option for a petrochemical refinery but not for the transportation of fuel oil.

Solvent deasphalting (SDA), described earlier, is a separation process in which the asphaltenic fraction is precipitated from the residue using a light paraffinic solvent (i.e., propane, butane, pentane, or n- heptane). The product is a low-sulfur/metal deasphalted oil (DAO) rich in paraffins that is normally used as feed for FCC and hydrocracking. The advantages of this method are the relatively low cost, the flexibility to adjust the DAO quality in a wide range, and the elimination of fouling problems in subsequent units. However, disposal of the SDA pitch (asphaltenic fraction) is still a matter of concern.

Thermal cracking processes are the most mature technologies for converting heavy feeds. They are carried out at moderate pressure in the absence of a catalyst. Coking processes (i.e., delayed coking, fluid coking, and flexicoking) are capable of eliminating the heaviest fractions from crude oils, producing coke that contains the majority of sulfur, nitrogen, and metals of the original oil. Delayed coking has been the upgrading process of choice, due to its flexibility to handle any type of feed and its ability to remove carbon and metals completely, along with partial conversion to liquids. Fluid coking and flexicok-ing are advanced processes that employ fluidized-bed technology, derived from FCC technology. Technically, fluid coking is only marginally better than delayed coking, as it offers a slightly higher liquid yield, less coke formation, and lower operating costs. Visbreaking, on the other hand, is a mild thermal decomposition process to improve the viscosity of heavy oils and residue, without significant conversion to distillates. In general, thermal processes appear to be attractive, due to low investment and operating costs; however, they suffer from the disadvantage of producing uneconomically large amounts of coke and having a low liquid yield. Additionally, liquid products require extensive posttreatment to meet the specifications of commercial fuels.

Catalytic cracking of residue (RFCC) is the only catalytic process found in this class of upgrading technologies. It is an extension of conventional FCC, which is employed for converting heavy feedstocks into high-octane gasoline blending components. RFCC exhibits better selectivity to gasoline and a lower gas yield than thermal cracking and hydroprocessing. However, the main drawback of RFCC is the need for good-quality feed (low metals content and H/C ratio) to avoid high coke production and excessive catalyst use; therefore, the application of RFCC directly to residues derived from heavy oil is not likely.

Additional details regarding these processes are given in the following sections.

Solvent Deasphalting Since asphaltenes cause many problems during various steps of petroleum refining, it is more convenient to remove them from heavy oil and make it a trouble-free feedstock. For example, if asphaltene separation is carried out before hydroprocessing, the following main problems encountered when handling heavy feeds can be avoided:

• Pipeline deposition and its plugging

• Efficiency decrease in refinery plants

• Precipitation of asphaltene due to blending of light hydrocarbon streams

• Sludge and sediment formation during storage as well as processing

• Catalyst deactivation in downstream processes

The most common method used for asphaltene precipitation is solvent deasphalting (SDA). This process uses a solvent (light paraffin such as C3, C4, C5, C6, and C7) to separate a residue into a deasphalted oil (DAO) and a pitch (asphaltene), the latter containing most of the impurities of the feedstock. The insoluble pitch will precipitate out of the mixed feedstock as asphaltene. Separation of the DAO phase and the pitch phase occurs in an extractor. The extractor is designed to separate the two phases efficiently and to minimize contaminant entrainment in the DAO phase. At a constant solvent composition and pressure, a lower extractor temperature increases the DAO yield and decreases the quality. With an increase in solvent ratio the DAO yield remains constant, improves the degree of separation of individual components, and results in the recovery of a better quality DAO. The solvent recovered under low pressure from the pitch and DAO strippers is condensed and combined with the solvent recovered under high pressure from the DAO separator, which is then recycled to the initial stage. DAO is normally used as fluid catalytic cracking or hydrocracker feed.

Solvent deasphalting is used in refineries to upgrade heavy bottoms streams to deasphalted oil that may be processed to produce transportation fuels. The process may also be used in the oil field to enhance the value of heavy crude oil before it gets to the refinery. Thus, SDA is an economically attractive and environmentally friendly process to upgrade heavy petroleum.

Gasification Gasification involves complete cracking of residue, including asphaltenes, into gaseous products. The gasification of residue is carried out at a high temperature (>1000°C) having synthesis gas (consisting primarily of hydrogen, carbon monoxide, carbon dioxide, and water), carbon black, and ash as major products. The syngas can be converted to hydrogen or used by cogen-eration facilities to provide low-cost power and steam to refineries. An integrated SDA – gasification facility is an attractive alternative for upgrading of heavy petroleum. The following are some of the benefits obtained in integrating deasphalting and gasification:

• Heavy oils can be upgraded economically.

• Capital and operating costs of both processes can be reduced.

• Higher yields of DAO are possible.

• Lower emissions are possible.

• Profit margins of a refinery can be increased.

Coking Depending on feedstock properties, coker unit design, and operating conditions, the solid product (petroleum coke or "petcoke") can be:

• Fuel-grade coke: the most common type of coker is the fuel grade, whose main objective is to maximize liquid yields and reduce low-value coke formation. This coke is used as fuel in process heaters and power generation facilities.

• Anode-grade coke: which is produced from low-sulfur and metals feeds, and is used for anodes in the aluminum industry.

• Needle-grade coke: which is produced from highly aromatic feedstocks with low asphaltenes, sulfur, and ash contents. This coke, with high strength and a low coefficient of thermal expansion, is used to manufacture large electrodes for the steel industry and the production of synthetic graphite.

The physical and chemical properties of fuel coke, anode coke, and needle coke vary substantially.

Three main coking processes are in use:

1. Delayed or retarded coking: which can produce shot coke (a type of fuel coke), sponge coke (used to produce anode coke or as a fuel coke), or needle coke. This process accounts for the majority of the coke produced in the world today.

2. Fluid coking: which produces fluid coke typically used as fuel coke.

3. Flexicoking: which produces a type of fluid coke that is gasified to generate a low – Btu synthesis gas.

1. delayed coking Delayed coking is a semicontinuous thermal cracking process used in petroleum refineries to upgrade and convert bottoms from atmospheric and vacuum distillation of crude oil into liquid and gas product streams, leaving behind a solid concentrated carbon material, petroleum coke, whose value will depend on its properties, such as sulfur or metals. The products of a delayed coker are wet gas, naphtha, light and heavy gas oils, and coke. The coke produced in the delayed coker is almost pure carbon and is utilized as fuel or, depending on its quality, in the manufacture of anodes and electrodes.

In a delayed coker the feed enters the bottom of the fractionator, where it mixes with recycle liquid condensed from the coke drum effluent. It is then pumped through the coking heater, then to one of two coke drums through a switch valve. The total number of coke drums required for a particular application depends on the quality and quantity of the feed and the coking cycle desired. A minimum of two drums is required for operation, with one drum receiving the heater effluent while the other is being decoked.

A delayed coking unit is frequently designed with the objective of maximizing the yield of liquid product and minimizing the yields of wet gas and coke. The conversion is accomplished by heating the feed material to a high temperature and introducing it into a large drum to provide soaking or residence time for the three major reactions to take place:

• Partial vaporization and mild cracking (visbreaking) of the feed as it passes through the coker’s furnace.

• Thermal cracking, the mechanism through which high-molecular-weight molecules are decomposed into smaller, lighter molecules that are fractionated into the products. The reaction is highly endothermic. The coker heaters supply the heat necessary to initiate the cracking reaction. Heater temperature and residence time are strictly controlled, so that coking in the heaters is minimized.

• Polymerization, the reaction through which small hydrocarbon molecules are combined to form a single large molecule of high molecular weight. The result of this reaction is the formation of coke. Polymerization reactions require a long reaction time and the coke drums provide the necessary residence time for these reactions to proceed to completion.

Delayed coking has been selected by many refiners as their preferred choice for upgrading the bottom of the barrel, because of the process’s inherent flexibility to handle any type of residua. The process provides essentially complete rejection of metals and carbon while providing partial conversion to liquid products (naphtha and diesel). The product selectivity of the process is based on the operating conditions, mainly pressure and temperature. This process is more expensive than SDA, although still less expensive than other thermal processes. The disadvantages of this process are the very high coke formation and low yield of liquid products. Despite these disadvantages, delayed coking is the favorite process of all refiners for residue processing. Advances in delayed coking have increased light products while decreasing coke production, lowering pressure and oil recirculation.

2. fluid coking and flexicoking Fluid coking is a continuous process that uses the fluidized-solids technique to convert residue feedstock to more valuable products. The heated coker feeds (petroleum residua) are sprayed into a fluidized bed of hot, fine coke particles which are maintained at 20 to 40psi and 500°C. The use of a fluid bed permits the coking reactions to be conducted at higher temperatures and with shorter contact times than in delayed coking. These conditions result in lower yields of coke and higher yields of liquid products. Fluid coking uses two vessels, a reactor and a burner. Coke particles are circulated between them to transfer heat to the reactor.

This heat is generated by burning a portion of the coke. The reactor contains a fluidized bed of the coke particles, which is agitated by the introduction of steam below. The residue feed is injected directly into the reactor and is distributed uniformly over the surface of the coke particles, where it cracks and vaporizes. The feed vapors are cracked while forming a liquid film on the coke particles. The particles grow by layers until they are removed and new seed coke particles are added. Coke is a product and a heat carrier. Flexicoking is an extension of fluid coking which includes the gasification of the coke produced in the fluid coking operation and produces syngas, but the temperature (1000°C) used is insufficient to burn all coke.

Both fluid coking and flexicoking are fluid-bed processes developed from fluid catalytic cracking technology. In both processes, the circulating coke carries heat from the burner back to the reactor, where the coke serves as reaction sites for the cracking of the residua into lighter products. Fluid coking can have liquid yield credits over delayed coking. The shorter residence time can yield higher quantities of liquids and less coke, but the products are lower in quality. Fluid coking is a slightly better process than delayed coking because of the advantage of a slightly improved liquid yield, and because delayed coking has a higher utilities cost and higher fuel consumption.

Visbreaking Visbreaking (viscosity reduction or breaking), a mature process that may be applied to both atmospheric residua (AR) and vacuum residua (VR) and even solvent deasphalted pitch, improves viscosity by means of its mild thermal decomposition. The thermal conversion of the residue is accomplished by heating at high temperatures in a specially designed furnace. A common operation is to visbreak residue in combination with a thermal cracker to minimize fuel oil while producing additional light distillates.

Visbreaking is a process in which a residue stream is heated in a furnace (450 to 500°C) and then cracked during a low specific residence time, to avoid coking reactions within a soaking zone under certain pressure and moderate temperature conditions. The cracked product leaves the soaking zone after the desired conversion is reached, and is then quenched with gas oil to stop the reaction and prevent coking, although increased conversion during visbreak-ing will turn to more sediment deposition. The residence time, temperature, and pressure of the furnace’ s soaking zone are controlled to optimize the thermal free-radical cracking to produce the desired products. In general, visbreaking is used to increase refinery net distillate yield. The main objectives of visbreaking are to reduce the viscosity of the feed stream and the amount of residual fuel oil produced by a refinery and to increase the proportion of middle distillates in the refinery output.

Carbon rejection processes are characterized by having lower investment and operating costs than those of hydroprocessing, but the yield of light products tends to be lower, which is not favored by refiners. Moreover, liquid products obtained from thermal processes contain S, N, and metals (e.g., V, Ni) that need further purification by hydrotreating processes such as HDS, HDN, and HDM, respectively. Thus, thermal processes and coking-based technologies suffer from the disadvantages of producing a large amount of low-value by-products and require further extensive processing of the liquid products. Therefore, the importance of thermal processes remains lower than that of catalytic processes, but due to their lower investment, these processes remain most common for residue upgrading.

Some of the advantages and disadvantages of carbon rejection processes are as follows:

• Visbreaking is the least expensive process but provides only a modest degree of residue conversion. Its applicability is constrained further by oil-quality considerations involving stability and compatibility.

• Delayed coking is relatively easy to implement, requires a moderate investment, provides a high degree of conversion, but may produce a large amount of low-value coke.

• Fluid coking is similar to delayed coking in many aspects but produces higher yields. However, the coke produced usually has a lower value, and the gas oils are somewhat more difficult to refine.

Residue Fluid Catalytic Cracking Fluid catalytic cracking is a well-established process for converting a significant portion of the heavy fractions (typically, heavy straight-run gas oil and light and heavy vacuum gas oils) of the crude barrel into a high-octane gasoline blending component. Residue fluid catalytic cracking (RFCC) is an extension of conventional FCC technology developed during the early 1980s which offers better selectivity to high gasoline and lower gas yield than that of hydroprocessing and thermal processes. The RFCC process uses reactor technology similar to that of the FCC process, in which the catalyst is in a fluidized bed at 480 to 540°C and is targeted for residual feeds greater than 4wt% Conradson carbon. Because RFCC requires better feed quality (e.g., a high H/C ratio, a low metal and asphaltene content), it makes this process less likely than hydroprocessing. The need for good feedstock quality is to avoid unreasonable high coke yield, high catalyst consumption, and unit operability. However, such feeds are high in price and limited in refineries.

To control heat balance and to recover part of the heat for steam production, RFCC process design includes two-stage regeneration: mix temperature control and catalyst cooler. The catalyst properties also play an important role in resisting metal content and carbon deposition. In this respect catalyst pore structure limits the diffusion of residue on the catalytic sites. The catalyst used for RFCC is an acidic matrix such as crystalline aluminosilicate zeolite in an inorganic matrix, which fulfills the required physical-chemical properties.

Hydrogen Addition Processes The hydrogen addition route, better known as hydroprocessing. reduces coke formation in favor of liquid products by means of a hydrocracking or hydrogenolysis mechanism. Hydroprocessing is a hydrogen addition process which increases the H/C ratio in products. It is the most attractive route for upgrading heavy crudes and residua. In general, hydroprocessing requires hydrogen to hydrogenate the oil at high pressures and temperatures in the liquid phase because such oils have a very high concentration of carbon. Asphaltene conversion is more complicated for heavy oils, since a wide range of molecular changes occur with temperature. Except for hydrovisbreaking, a mild process based on visbreaking but in a hydrogen atmosphere, hydroprocessing is carried out in the presence of a catalyst. Catalytic hydroprocessing is extremely relevant in petroleum refining for upgrading a variety of streams; ranging from straight-run naphtha to vacuum residues or even heavy and extra-heavy crude oils. When handling heavy feeds, hydroprocessing has the virtue of reducing the contents sulfur, nitrogen, metals (Ni and V), and asphaltenes, and contributing simultaneously to the production of liquid fuels by HCR. Nevertheless, processing of this type of feed presents many difficulties caused principally by enhanced metal and carbon deposition on the catalyst.

There are numerous hydroprocessing technologies for converting heavy feeds, differing mainly in catalyst type, reactor technology, and operating conditions. The catalyst system is chosen based on activity, selectivity, and cycle life and is generally composed of CoMo/NiMo alumina-supported catalysts designed for specific objectives, such as hydrodesulfurization (HDS), hydrodemetalliza-tion (HDM), hydrodenitrogenation (HDN), hydrodeasphaltenization (HDA), HCR, and Conradson carbon (CCR) removal. Commercial reactor technologies for hydroprocessing of heavy feeds can be classified according to the type of catalytic bed: fixed, moving, ebullated, and slurry. Selection of the reactor is generally a function of the quality and composition of the feedstock and desired level of conversion and impurities removal. Typically, dirty feeds can be processed effectively in ebullated-bed reactors, since the major disadvantage of fixed-bed reactors is the catalyst deactivation with time onstream. The major selection criterion between each type of reactor is based on the catalyst deactivation rate, which depends on the contents of metals and asphaltenes in the feed, as the products formed during their removal are known as catalyst deactivating species. Reaction severity [i.e., pressure, reaction temperature, hydrogen-to-oil ratio, and liquid hourly space velocity (LHSV)] also depends on the properties of the feed and the product quality desired; in general, for higher-boiling-point feeds, more severe conditions are required.

Traditionally, fixed-bed reactors were employed for processing light feeds, but they were gradually adapted for tougher feeds, such as vacuum gas oil and residues. The main disadvantage of using fixed-bed reactors for upgrading heavy feeds is the loss of catalyst activity during time- on- stream. This reduces the length of run drastically, due to the frequent shutdowns required for replacing the catalyst. However, recent advances in the field have led to the development of layered catalyst systems that extend significantly the length of run. Typically, these systems comprise a front-end HDM catalyst, a midsection catalyst with balanced HDM/HDS activity, and a tail-end highly active HDS/HCR catalyst. The front-end catalyst exhibits a high-metal-uptake capacity, and its main function is to disaggregate asphaltene molecules for metals removal, so that the downstream catalysts can operate with hydrocarbons of low metal and coke precursor content. In the midsection, there is additional metals elimination and partial HDS, whereas the tail-end catalyst provides hydroconversion. The combination of catalysts is selected according to the objectives of each situation.

Among all the possibilities available for treatment of heavy oils, hydrogen addition processes lead to high hydrogen consumption but higher liquid yields. These processes, which provide the feedstock for subsequent processes, require the use of well-designed catalysts capable of dealing with the high concentration of metals and asphaltenes present in the feedstock. Moreover, the multifunctional catalysts used for hydrocracking processes become poisoned by coke deposition and the heavy metals present in the feed, creating a hazardous waste which has to be disposed off properly and with safety. A high catalyst demetallization function is necessary because vanadium destroys the zeolitic catalyst used in the subsequent FCC process. Moreover, the concentration of nitrogen compounds must be reduced to a minimum to avoid poisoning the catalyst acid sites in this and subsequent FCC process. Although in the hydro-cracking process the amount of metals is not as critical as in FCC, the elimination of nitrogen compounds is determinant to avoid poisoning of the catalyst acid sites.

Catalyst cycle life does not represent a problem in moving- and ebullated-bed reactors. Such technologies allow for replacing spent catalyst without interrupting operation; therefore, they are adequate for handling the most problematic feeds (high contents of metals and asphaltenes). Moving-bed reactors combine fixed – bed operation in plug – flow mode with the possibility of replacement during time-on-stream portions of spent catalyst. Catalyst replacement is a batch operation, typically carried out once or twice a week. The application of these reactors is specifically in front-end residue demetal-lization to protect subsequent fixed-bed reactors for HDS and HCR. Ebullated-bed reactors represent the most advanced hydroprocessing technology, suited specifically for upgrading extra-heavy feeds, directly without any type of pre-treatment. The continuous catalyst replacement feature in these reactors allows using conventional high-activity HDT/HCR catalysts. Operation of these reactors is very flexible, hydroconversion is very efficient (up to 90 vol%), and products have low levels of sulfur, metals, and nitrogen. Nevertheless, ebullated-bed technologies suffer from considerable sediment formation and high catalyst consumption. Also, scale-up and design of such reactors is more difficult, due to the complex hydrodynamics.

Residue desulfurization processes (RDS/VRDS) are of common use to meet a variety of objectives, such as preparing feed for FCC, RFCC, coker, and HCR, and are available from major licensors (Chevron, Unocal, UOP, Shell, and Exxon). IFP’s Hyvahl-F is another process of this nature, characterized by a system of fixed-bed reactors in series in combination with a graded catalyst system employed for atmospheric and vacuum residue hydrotreating and conversion to liquid fuels. Aiming to increase run length, this process was modified by introducing a swing reactor system (SRS) in front of the conventional fixed-bed reactor train. The SRS, under the brand name Hyvahl-S, has two reactors that can alternate in operation when the catalyst in one reactor is deactivated. Other advances in this field include systems for shortening the time required to replace spent catalyst, such as Shell’s quick catalyst replacement system (QCR), which avoids opening and closing the reactor to load and unload catalyst. Examples of commercial developments using the moving-bed approach are the Hycon process developed by Shell and Chevron’s onstream catalyst replacement reactor (OCR).

Ebullated-bed reactors are employed for residue hydrocracking as well as for desulfurization and demetallization. The two major commercialized technologies that use this type of reactor are H-Oil, licensed by Axens (IFP), and LC-Fining, licensed by Chevron Lummus. The two have very similar characteristics in terms of process parameters and reactor design, but differ in some mechanical details.

In summary, all of these processes have serious drawbacks when applied individually to the conversion of heavy feeds; thermal processes alone yield large amounts of coke, while catalytic processes suffer from excessive catalyst consumption due to rapid deactivation and high-hydrogen inputs in the case of hydroprocessing. Therefore, a careful inspection of feed characteristics, desired goals, and available technologies is required to define the best refining strategy. Either way, hydroprocessing will play an essential role in this matter, as it is favorable for primary upgrading and certainly offers much better selectivity to liquid yield and substantially cleaner products than thermal processes. Optimal hydroprocessing can be achieved by proper matching of reactor technology, catalyst, and reaction severity with the properties of heavy feeds.

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