10.2.1 Petroleum residua

A petroleum resid (residuum, pl. resids, residua) is the non-volatile residue obtained from petroleum after non-destructive distillation has removed all the volatile constituents of the feedstock. The temperature of the distillation unit is usually maintained below 350 °C (660 °F) because the rate of thermal decomposition of petroleum constituents is minimal below this temperature, and the rate of thermal decomposition of petroleum constituents is substantial above 350 °C (660 °F). This is not always the case, however, as residence time in the hot zone is also a factor.

Resids are black, viscous materials and are obtained by distillation of a crude oil under atmospheric pressure (atmospheric residuum) or under reduced pressure (vacuum residuum). They may be liquid at room temperature (generally atmospheric residua) or almost solid (generally vacuum residua) depending upon the nature of the crude oil. When a residuum is obtained from crude oil and thermal decomposition has commenced, many in the industry incorrectly refer to this product as pitch (Speight, 2014). The differences between parent petroleum and the residua are due to the relative amounts of various constituents present, which are removed or remain by virtue of their relative volatility.

The chemical composition of a residuum from is complex. Physical methods of fractionation usually indicate high proportions of asphaltene constituents and resins, even in amounts up to 50% (or higher) of the residuum (Gary et al., 2007; Hsu & Robinson, 2006; Speight, 2011a, 2014; Speight & Ozum, 2002). In addition, the presence of ash-forming metallic constituents, including such organometallic compounds, as those of vanadium and nickel, as well as other metal constituents, is also a distinguishing feature of residua and the non-volatile feedstocks. Furthermore, the deeper the cut into the crude oil, the greater the concentration of sulfur and metals in the residuum and, as a result, the greater the deterioration in physical properties of the residuum (Gary et al., 2007; Hsu & Robinson, 2006; Speight, 2011a, 2014; Speight & Ozum, 2002).

10.2.2 Heavy oil

When crude petroleum can be pumped from a reservoir as a free-flowing dark to light colored liquid, it is often referred to as conventional petroleum. Heavy oil is a type of petroleum that is different from the conventional petroleum insofar as it is more difficult to recover from the reservoir. Heavy oil has a much higher viscosity and lower American Petroleum Institute (API) gravity than conventional petroleum, and primary recovery of heavy oil requires thermal stimulation of the reservoir (Speight, 2008, 2009, 2013a, 2013b, 2014).

The definition of heavy oil is commonly (but incorrectly) based on the API gravity or viscosity. In fact, for many years, petroleum and heavy oil were very generally defined in terms of physical properties, despite a lack of scientific foundation. For example, heavy oils were considered to be those crude oils that had gravity somewhat < 20° API, with the heavy oils falling into the API gravity range 10-15°. For example, Cold Lake heavy crude oil has an API gravity equal to 12°, and extra heavy oils, such as tar sand bitumen, usually have an API gravity in the range 5-10° (Athabasca bitumen = 8° API). Residua would vary depending upon the temperature at which distillation was terminated, but usually, vacuum residua are in the range 2-8° API (Ancheyta & Speight, 2007; Speight, 2000; Speight, 2014; Speight & Ozum, 2002).

10.2.3 Extra heavy oil

Extra heavy oil is also a material that suffers from the use of arbitrary nomenclature. Extra heavy oil occurs in the solid or near-solid state and generally has mobility under reservoir conditions, possibly due to the temperature of the reservoir or deposit rather than the ambient properties of the material. In fact, the term extra heavy oil is a recently evolved term (related to viscosity) of little scientific meaning and often initiates confusion, as it is incorrectly used to refer to tar sand bitumen. While this type of oil may resemble tar sand bitumen and does not flow easily, extra heavy oil is generally recognized as having mobility in the reservoir compared to tar sand bitumen, which is typically incapable of mobility (free flow) under reservoir conditions. For example, the tar sand bitumen located in Alberta, Canada, is not mobile in the deposit and requires extreme methods of recovery to recover the bitumen, while much of the extra heavy oil located in the Orinoco belt of Venezuela requires recovery methods that are less extreme because of the mobility of the material in the reservoir (Speight, 2009, 2013a, 2013b, 2014).

10.2.4 Tar sand bitumen

Also, on occasion, referred to as native asphalt and extra heavy oil, bitumen is a naturally occurring material that is found in tar sand deposits, such as the oil sand deposits in Canada, where the permeability is low and passage of fluids through the deposit can only be achieved by prior application of fracturing techniques. Tar sand bitumen is a high-boiling material with little, if any, material boiling below 350 °C (660 °F), and the properties of this substance may resemble those of an atmospheric residuum. However, to get beyond the use of one or two properties to define tar sand bitumen, tar sands have been defined in the United States (FE-76-4) more correctly and from a functional aspect as:

Furthermore, the recovery of the bitumen depends to a large degree on the collective composition of the material, and generally, the bitumen found in tar sand deposits is an extremely viscous material that is immobile under reservoir conditions and cannot be recovered through a well by the application of secondary or enhanced recovery techniques (Speight, 2013a, 2013b, 2014). However, the term tar sand is actually a misnomer; more correctly, the name tar is usually applied to the heavy product remaining after the destructive distillation of coal or other organic matter (Speight, 2013c).

The bitumen in tar sand formations requires a high degree of thermal stimulation for recovery to the extent that some thermal decomposition may have to be induced. Current recovery operations of bitumen in tar sand formations involve the use of a mining technique, and non-mining techniques are continually being developed (Speight, 2009, 2013a, 2013b, 2014).

It is incorrect to refer to native bituminous materials as tar or pitch. Although the word tar is descriptive of the black, heavy bituminous material, those in the industry should avoid its use with respect to natural materials and to restrict the meaning to the volatile or near-volatile products produced in the destructive distillation of organic substances such as coal (Speight, 2013c). Thus, alternative names, such as bituminous sand or oil sand, are gradually finding usage, with the former name being more technically correct. The term oil sand is also used in the same way as the term tar sand, and these terms are used interchangeably throughout this text.

10.2.5 Other feedstocks

Other gasification feedstocks are variable and will depend upon the location of the refinery into which the gasifier has been integrated. Such feedstocks may arise from fossil fuel and from non-fossil fuel sources (Speight, 2008, 2011a, 2011b).

10.3.1 POX technology

POX is the most commonly used process for the gasification of heavy oils and other refinery residues, although virtually all mixtures are suitable feedstocks, regardless of volatility (Liebner, 2000). However, aside from special applications, gasification is a bottom-of-the-barrel process that converts feedstocks containing sulfur and nitrogen to a clean synthesis gas consisting mainly of hydrogen and carbon monoxide. In fact, gasification is replacing direct combustion due to environmental regulations, since ash removal and flue gas clean-up are more difficult and expensive than synthesis gas cleaning at elevated pressures.

The main advantages derived from the application of gasification in a refinery are: (1) the capability of processing low-quality, highly viscous, and heavy feedstocks, as well as emulsions (tank sludge), slurries (coke) and other liquid wastes in quench gasifiers; (2) the capability of processing high-sulfur feedstocks because of the almost complete removal of sulfur compounds in the gas treating unit downstream of the gasifier; (3) the possibility of producing hydrogen for the various conversion and upgrading processes of the refinery, with increased production of gas oil, which is a desirable product or feedstock for other refinery units; and (4) the many outlets for synthesis gas, such as hydrogen for the refinery or for export, electricity via the IGCC process, and the production of chemicals such as ammonia, methanol, acetic acid, and oxo-alcohols.

POX or POx reactions occur when a sub-stoichiometric fuel-air mixture is partially combusted in a reformer. The general reaction equation without a catalyst, called thermal partial oxidation (TPOX or TPOx), can be represented as:

${\mathrm{C}}_{\mathit{n}}{\mathrm{H}}_{\mathit{m}}+\left(2\mathit{n}+\mathit{m}\right)/2{\mathrm{O}}_2\to \mathit{n}\mathrm{CO}+\left(\mathit{m}/2\right){\mathrm{H}}_2\mathrm{O}$

The variable composition of the gasifier feedstocks prevents exact stoichiometric reactions. To produce an equation for such a reaction would only serve to mislead any potential kinetic studies.

A TPOX reactor is similar to the autothermal reactor (ATR), with the main difference being no catalyst is used. The feedstock, which may include steam, is mixed directly with oxygen by an injector that is located near the top of the reaction vessel. Both POX and reforming reactions occur in the combustion zone below the burner.

The principal advantage of the POX process is the ability of the process to accommodate a variety of feedstocks. These feedstocks can include high molecular-weight organic feedstocks, such as petroleum coke (Gunardson & Abrardo, 1999). Additionally, since emissions of nitrogen oxides (NO_x) and sulfur oxides (SO_x) are minimal, the technology can be considered to be environmentally acceptable. On the other hand, very high temperatures, ~ 1300 °C (2370 °F), are required to achieve a near complete reaction. This high reaction temperature necessitates the consumption of some of the hydrogen and a greater than stoichiometric consumption of oxygen.

10.3.2 Catalytic partial oxidation

Catalytic partial oxidation (CPOX or CPOx) technology offers possible means of improving the efficiency of synthesis gas production from heavy feedstocks. This technology has several advantages over steam reforming, especially the higher energy efficiency. In fact, the reaction is exothermic rather than endothermic, as is the case with steam reforming. Furthermore, a carbon monoxide-hydrogen ratio (CO/H₂) ratio approximately equal to 2.0, which is the ideal ratio for the Fischer-Tropsch process and methanol synthesis, is produced by this technology.

10.3.3 Steam reforming

While not truly the subject of this chapter, steam reforming, which is sometimes referred to as steam-methane reforming (SMR), deserves some consideration because of the production of synthesis gas. The steam reforming process involves passing a preheated mixture essentially composed of methane and steam through catalyst-filled tubes. As the reaction is endothermic, heat must be provided in order to effect the conversion, and the heat is provided by burners located adjacent to the tubes. The product of the process is a mixture of hydrogen, carbon monoxide, and carbon dioxide.

In order to maximize the conversion of the methane feed, both a primary and secondary reformer are generally utilized. A primary reformer is used to effect 90-92% conversion of methane. Here, the hydrocarbon feed partially reacts with steam over a nickel-alumina catalyst to produce a synthesis gas with an H₂/CO ratio of ~ 3:1. This partial reaction occurs in a fired tube furnace at 900 °C (1650 °F) and at a pressure of 220-450 psi. The unconverted methane is reacted with oxygen at the top of a secondary autothermal reformer, which contains a nickel catalyst in the lower region of the vessel.

The deposition of carbon can be an acute problem with the use of nickel-based catalysts in the primary reformer (Alstrup, 1988; Rostrup-Neilsen, 1984, 1993). Considerable research has been done with the aim of finding approaches to prevent carbon formation. A successful technique is to use a steam/carbon ratio in the feed gas that does not allow the formation of carbon. However, this method results in lowering the efficiency of the process. Another approach utilizes sulfur passivation, which led to the development of the SPARG process (Rostrup-Neilsen, 1984; Udengaard, Bak-Hansen, Hanson, & Stal, 1992). The SPARG process utilizes the principle that the reaction leading to the deposition of carbon requires a larger number of adjacent surface Ni atoms than does steam reforming. When a fraction of the surface atoms are covered by sulfur, the deposition of carbon is thus more greatly inhibited than it is in steam reforming reactions. A third approach is to use Group VIII metals that do not form carbides, such as platinum (Pt). However, due to the high cost of such metals, using them is not as economical as using nickel.

A major challenge in steam reforming development is its energy intensive nature due to the high endothermic character of the reactions. Thus, the development tends to focus on seeking higher energy efficiency. Improvements in catalysts and metallurgy require adaption to lower steam/carbon ratios and higher heat flux.

10.3.4 Autothermal reforming

In the autothermal reforming process (ATR process), the organic feedstock, steam, and sometimes carbon dioxide are mixed directly with oxygen and air in the reformer. The reformer itself comprises a refractory-lined vessel that contains the catalyst, together with an injector located at the top of the vessel. This type of reformer consists of three zones: (1) the burner in which the feedstock streams are mixed in a turbulent diffusion flame, (2) the combustion zone in which POX reactions produce a mixture of carbon monoxide and hydrogen, and (3) the catalytic zone in which the gases leaving the combustion zone reach thermodynamic equilibrium.

POX reactions occur in a region of the reactor referred to as the combustion zone. The mixture formed in this zone then flows through a catalyst bed where the actual reforming reactions occur. Heat generated in the combustion zone as the result of the POX reactions is utilized in the reforming zone, so that, in the ideal case, the ATR process can exhibit excellent heat balance. In addition, the process offers comparatively flexible operation, including short startup periods and fast load changes, as well as the potential for soot-free operation depending on the feedstock used.

10.3.5 Combined reforming

Combined reforming incorporates the combination of both steam reforming and autothermal reforming. In such a configuration, the feedstock is only partially converted under mild conditions in the first stage to synthesis gas in a relatively small steam reformer. The off-gas from the steam reformer is then sent to an oxygen-fired secondary reactor, the autothermal reforming reactor, where any hydrocarbons in the gas stream are converted to synthesis gas by POX followed by steam reforming. Another configuration requires the feedstock to be split into two streams that are then fed, in parallel, to the steam reforming and autothermal reactors.

10.4.1 Gas purification and quality

Purities in excess of 99.5% v/v of either hydrogen or carbon monoxide produced from synthesis gas can be achieved if required by the refinery. Four of the major process technologies available are:

1. Cryogenics plus methanation: This method utilizes a cryogenic process whereby carbon monoxide is liquefied in one or more steps to produce hydrogen with a purity of on the order of 98% v/v. The condensed carbon monoxide, which often contains methane, is distilled to produce a stream of pure carbon monoxide and a mixed stream of carbon monoxide and methane. The mixed carbon monoxide and methane stream can be used as fuel. The hydrogen stream is routed to a shift converter where any remaining carbon monoxide is converted to carbon dioxide and hydrogen. The carbon dioxide is removed, and any further carbon monoxide or carbon dioxide can be removed by methanation. The resulting hydrogen stream can have purity as high as 99.7% v/v.

2. Cryogenics plus pressure-swing adsorption: This method also uses the sequential liquefaction of carbon monoxide to produce hydrogen with a purity of ~ 98% v/v. Again, the carbon monoxide stream can be distilled to remove methane, until it is essentially pure. Depending on the hydrogen purity required, the hydrogen stream is then processed through multiple swings of pressure-swing adsorption cycles until the hydrogen purity is as high as 99.999% v/v.

3. Methane-wash cryogenic process: In this method, liquid carbon monoxide is absorbed into a liquid methane stream so that the resulting hydrogen stream contains ppm levels of carbon monoxide and ~ 5-8% v/v methane. As a result, the purity of the hydrogen stream produced by this process is limited to ~ 95% v/v. The liquid carbon monoxide-/methane stream can be distilled to produce pure carbon monoxide, as well as a carbon monoxide-methane stream that can be used as fuel gas.

4. COsorb process: This method utilizes copper ions (cuprous aluminum chloride, CuAlCl₄) in toluene to form a chemical complex with the carbon monoxide, thereby separating it from the product gas steam. This process can capture ~ 96% v/v of the carbon monoxide to produce a carbon monoxide stream having purity > 99% v/v. However, water, hydrogen sulfide, and other trace constituents that can poison the copper catalyst must be removed prior to introduction of the product gas into the reactor. Furthermore, a hydrogen stream of only up to 97% v/v purity is obtained. However, while the efficiency of cryogenic separation decreases with a decrease in the carbon monoxide content of the feedstock gas, the COsorb process is a more efficient process for treating feedstock gas with low carbon monoxide content.

10.4.2 Process optimization

Process optimization includes the development of technologies to facilitate cost-effective gasification of all hydrocarbonaceous feedstocks produced by a refinery, as well as coal and biomass. Thus, the use of high-pressure feed systems and the development of technologies for co-feeding mixtures to high-pressure gasifiers are necessary options. In addition, attention must be given to the use of the refinery gasifier to process waste, reduce the refinery footprint, and produce marketable products.

10.5.1 Other uses of residua

The residua (heavy feedstocks, bottoms, hydrocarbonaceous feedstocks) can be routed to other conversion units or blended to heavy industrial fuel, asphalt, or a combination of both. The heavy feedstocks typically have a relatively low economic value, and often they are of lower value than the original crude oil. Thus, most refineries convert or upgrade the low-value heavy feedstocks into more valuable low-boiling products, such as gasoline, jet fuel, and diesel fuel.

Thus upgrading heavy feedstocks creates a need for additional bottom-of-the-barrel processing, both for expansion and for yield improvement. Traditionally, this would automatically call for the addition of atmospheric distillation and/or vacuum distillation units as a starting point. However, there are alternative processing schemes for processing the vacuum or atmospheric residues in order to maximize the value of the heavier crude oils.

10.5.2 Gasification in the future refinery

Hydrogen management has become a priority for current and future refinery operations as consumption continues to rise for greater hydrotreating processes, as well as the processing of heavier and higher sulfur crude oils. In many cases, the hydrogen network is limiting refinery throughput and operating margins. The current main source for hydrogen is the SMR of refinery off-gases and natural gas, an inefficient and cost-incurring process.

As refineries continue to evolve (Furimsky, 1999; Speight, 2011a), the panacea (rather than the Pandora’s box) for a variety of feedstocks could well be the gasification refinery (Figure 10.1), which is capable of supplying the traditional refined products, while meeting much more severe specifications, and the use of petrochemical intermediates such as olefins, aromatics, hydrogen, and methanol (Figure 10.2) (Breault, 2010; Penrose, Wallace, Kasbaum, Anderson, & Preston, 1999; Phillips & Liu, 2002; Speight, 2011a, 2011b). Furthermore, in addition to the production of synthesis gas, the IGCC) can be used to raise power from feedstocks such as vacuum residua and cracked residua, and a major benefit of IGCC is that power can be produced with the lowest sulfur oxide (SO_x) and nitrogen oxide (NO_x) emissions of any liquid or solid feed power generation technology.

In fact, the future of the petroleum refining industry will primarily depend on processes for the production of improved quality products. Thus, the refinery of the future will have a gasification section devoted to the conversion of coal and biomass to Fischer-Tropsch hydrocarbons, perhaps even with rich oil shale added to the gasifier feedstock. Many refineries already have gasification capabilities, but over the next two to three decades, the trend will increase to the point where nearly all refineries recognize the need to construct a gasification section to handle residua and a variety of other feedstocks. Biomass, liquids from coal, and liquids from oil shale will increase in importance, and such feedstocks will likely be sent to refineries or processed at a remote location and then blended with refinery stocks. Above all, though, such feedstock must be compatible with refinery feedstocks and not cause fouling, which can lead to process or even refinery shutdown (Speight, 2011a, 2011b).

In the future, computer models for process unit and refinery economics and operations will also be optimized and integrated into plant operations via process computer controls. Alternate fuels for power generation will continue to push crude processing toward higher value products, such as transportation fuels and chemicals. Otherwise, the heavy crude oil and tar sand bitumen that are considered uneconomical to transport to a refinery will be partially refined at their source to facilitate transport, and there will be a new emphasis on partial or full upgrading in situ during recovery operations (Speight, 2009, 2014). In addition, alternative energy sources may become increasingly involved with petroleum, leading to the concept of alternative energy systems (Szklo & Schaeffer, 2005) in which petroleum refining is integrated with the production of energy from other energy sources.

Thus, refinery flexibility will be a key target, especially when related to the increased use of renewable energy sources. And, the industry can begin to work toward such flexibility by incorporating gasification technology into the refinery system as an equal partner in energy production.

In summary, gasification is the only technology allowing refineries to a achieve a zero residue target, as opposed conversion technologies, such as thermal cracking, coking, catalytic cracking, deasphalting, and hydroprocessing, which can only reduce the volume of bottoms, with the complication that the residue qualities generally get worse with the degree of conversion. The flexibility of gasification permits refineries to handle any kind of refinery residue, including petroleum coke and tank bottoms, as well as refinery sludge, while producing a range of value added products, electricity, steam, hydrogen, and various chemicals based on synthesis gas chemistry. The environmental performance of gasification is unmatched because no other technology for processing low-value refinery residues can come close to achieving the emission levels that result from gasification.