Distillation Heavy Crudes

The distillation of crudes chosen for their yield in heavy fractions is the most common means. Bitumen is extracted from the residue from a vacuum distillation column (a few dozen mm of mercury), the latter being fed by atmospheric distillation residue. Unlike the practice of a decade ago, it is now possible to obtain all categories of bitumen, including the hard grades. 

For many years, petroleum and heavy oil were very generally defined in terms of physical properties. For example, heavy oil was considered to be a crude oil that had gravity between 10 and 20° API. For example. Cold Lake heavy crude oil (Alberta, Canada) has an API gravity equal to 12°, but extra-heavy oil (such as tar sand bitumen), which requires recovery by nonconventional and nonenhanced methods, has an API gravity in the range 5 to 10°. Residua would vary depending on the temperature at which distillation was terminated, but vacuum residua were usually in the range 2 to 8° API. 

Topping A process of removing light products from crude oil by distillation. Heavy products remain in the column. 

Asphalt Hydrocarbon material ranging in consistency from heavy liquid to a solid. Most common source is residue left after fractional distillation of crude oils used primarily for surfacing roads. Asphyxia Suffocation from lack of oxygen. Chemical asphyxia is produced by a substance, such as carbon monoxide, that combines with hemoglobin to reduce die blood s capacity to transport oxygen. Simple asphyxia is the result of exposure to a substance, such as carbon dioxide, that displaces oxygen. 

However, heavy crude oil and bitumen are extremely complex and very little direct information can be obtained by distillation. It is not possible to isolate and identify the constituents of the heavier feedstocks using analytical techniques that rely upon volatility. Other methods of identifying the chemical constituents must be employed. Such techniques include a myriad of fractionation procedures (Chapter 3) as well methods designed to draw inferences about the hydrocarbon skeletal structures and the nature of the heteroatomic functions. 

Thus, in the distillation of crude petroleum, light naphtha and gases are removed as vapor from the top of the tower, heavy naphtha, kerosene, and gas oil are removed as sidestream products, and reduced crude (atmospheric residuum) is taken from the bottom of the tower. 

Distillation is also a means by which the character, especially the sulfur content, of a residuum may be adjusted. For example, inspections of various crude oil residua (Table 7-4) show that, for any particular crude oil, the vacuum residuum is virtually always higher in sulfur than an atmospheric residuum from the same crude oil. Thus, although distillation is the usual primary means by which a crude oil is processed, it may be completely bypassed in the case of an extremely heavy crude oil in favor of whole-crude processing by any of the more suitable thermal methods. 

In order to satisfy the changing pattern of product demand, significant investments in refining conversion processes will be necessary to profitably utilize the heavy oils and residua. The most efficient and economical solution to this problem will depend to a large extent on individual refinery situations. However, the most promising technologies will likely focus on the conversion of vacuum residua and extra-heavy crude oils into useful low-boiling and middle distillate products. 

Technologies for upgrading heavy crude oils such as heavy oil, bitumen, and residua can be broadly divided into carbon rejection and hydrogen addition processes (Chapter 8). Briefly, carbon rejection processes are those processes in which a carbonaceous by-product (coke) is produced along with distillable liquid products. On the other hand, hydrogen addition processes involve reaction of the feedstock with an external source of hydrogen and result in an overall increase in H/C ratio of the products as well as a decrease in the amount of coke produced. 

Pyrolysis is the process of thermal degradation of a substance into smaller, less complex molecules. Many processes exist to thermally depolymerize tires to salable products. Almost any organic substance can be decomposed this way, including rice hulls, polyester fabric, nut shells, coal and heavy crude oil. Pyrolysis is also known as destructive distillation, thermal depolymerization, thermal cracking, coking, and carbonization. 

Describing the behavior of undefined mixtures, whether from natural or synthetic sources, often begins with the separation of these complex systems into effective pseudocomponents by distillation (1 ). Each pseudocomponent is then characterized as if it were a pure compound, and its characterization data are used in appropriate correlations. The presence of nonvolatile residuum poses a serious limitation to such methodology. For coal-derived liquids, heavy crude oils, tar sands, and shale oil, more than 50 percent of the fluid may not be distillable (JL). Since this nonvolatile residue cannot be separated using conventional techniques, new methods of separation and characterization must be developed to provide the necessary information for design and operation of plants utilizing the fossil fuels mentioned above (2). 

Distillation of crude provided the gas oils that were used in subsequent processing and also that are described in Table I. Note that the total that was distilled-off represents 73.7 wt %. Less effort was applied to the heavy gas oil (>460°C) and to the residuum, and none to the coking where this fraction might be directed for recovery of coke and lighter gas oils. 

A typical crude oil distillation process was described in Chapter 4. Design a crude oil unit for a refinery that processes a 50 50 mixture (by volume) of Saudi Light and Saudi Heavy crude oils using the cut points given in Chapter 4. 

The products obtained by refining and upgrading heavy crude oils are largely determined by market demand. The consumption of transport fuels is always such as to focus attention on the conversion of lighter [1] or heavier [2] feed stocks to appropriate distillation cuts. The processing of heavy residual oils into lighter oils is now widely practised, although the methods used may vary from refinery to refinery [3,4],... 

The nature of crude oils depends on their source. Initial separation into components is carried out by atmospheric and vacuum distillation. Heavy ends are particular boiling point cuts, which can include atmospheric gas oil (250-350°C), atmospheric residues (350°C+) vacuum gas oil (350-5S0°C) and vacuum residues (5S0°C+). The descriptions are based on boiling points and, within a particular distillation cut, various chemical species can be identified. These include saturated and unsaturated hydrocarbons, aromatic and polyaromatic hydrocarbons and inorganic atoms such as V, Ni, and S, which are associated with large organic molecules [5]. As a result of this complexity, the composition of the boiling cuts is often described in terms of their content of oils, resins and asphaltenes [6,7,8], the relative amounts of which vary depending on the cut and the source of the crude [6] Of these species, asphaltenes are particularly important in the present context since they are known to be associated with heavy coke formation [7,8]. 

Recently, the processing of heavy crude oil has been important, while the demand in Japan for middle distillates has been increasing steadily, demand for residual fuel oil has been declining. Therefore, oil refiners have been interested in converting residues to middle distillate products. 

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