Refining of Petroleum

The discovery of an exploitable amount of oil in a reservoir is only the beginning of a long chain of subsequent measures that include the extraction, transportation, refining, and distribution of its refined products� The direct combustion of crude oil would represent an enormous economic waste and environmental damage� It is through refining that the components of the petroleum are separated and modified into more uniform in composition usable groups (Figure 11�1)� This refining involves numerous and complex physical and chemical steps to transform the very large number of compounds and impurities in crude petroleum into desirable and useful product compounds� The current and future refining industry needs to address increasingly satisfactorily and economically the ever stricter emission regulations and has to process the increasingly deteriorating quality of the crude available� Accordingly, refineries employ different units, processing devices, and methods� They are increasingly of large sizes and greater complexity�

The need for a supply of fuels of the right quantity will be dictated and consistent with the current and anticipated future needs of the different power and thermal systems, so as to provide higher efficiencies, reduce costs, improve reliability, and cut down to a minimum undesirable emissions� These requirements are placing enormous technical, economical, and environmental demands on the refining sector of the energy industry�

Crude oil, as has been indicated earlier, is a complex mixture of widely varying composition of naturally occurring organic compounds, mostly hydrocarbons with relatively small amounts of sulfur, oxygen, nitrogen, and with traces of metals and minerals� Because of the wide range of properties of the components of crude petroleum, it must be suitably processed through elaborate refining processes�

Figure 11�2 shows a schematic representation of a very simple thermal distillation vertical still, where separation of the product into a few stream categories can be achieved in accordance with their effective density and through the action of buoyancy� Straight-run gasoline is the product of petroleum refining obtained through straight simple distillation without the application of

deliberate chemical conversion methods� With the strict demanding requirements imposed in recent years on the properties of gasoline, this is rarely being considered to be a satisfactory fuel�

However, the more recent refining of petroleum is made up of a number of key physical and chemical processes aimed at removing undesirable

impurities and compounds as well as the production of different desirable groups of fuels with narrow variations in their properties (Figures 11�3, 11�4, and 11�7)� This is achieved through a multitude of processes that are increasing in complexity, with carefully selected associated changes in the following key controlling variables:

• Temperature • Pressure • Composition • Concentration • Residence times • Supply or removal of heat • Recirculation of materials • Employing additives • The aid of catalysts

These processes, which include fractional distillation, cracking, reforming, isomerization, and hydrogenation, as shown in Figure 11�4, lead to the production of desirable product fuels that would include gasoline, diesel, kerosene, fuel oil, and gas turbine jet fuels� The relative conversion of the crude

oil to any of these desirable products, which depends on the quality of the crude and the refining processes employed, can be varied only to within a relatively small degree� The typical assortment of the products and their relative extent derived from refining crude petroleum are shown schematically in Figure 11�5�

In the refining distillation process, the light and more volatile fractional components are obtained at the top of the fractioning tower, whereas the heavy components migrate to the bottom� Through subsequent refining processes, including the recirculation of streams, the relative production of one group of products can be adjusted upward or downward relative to the others; however, this can be done normally only to a certain degree�

In order to meet the ever-stricter specifications of refined fuels while having to process increasingly poorer quality crudes, refining processes are becoming elaborately more complex and sophisticated� Accordingly, massive refineries that can produce refined products economically with the required properties are becoming fewer in number�

If fractional distillation alone was used to produce the gasoline needed, then a disproportionally large accumulation of fuel oil, kerosene, waxes, and so on would result� By resorting to the numerous processes of thermal and catalytic cracking, the relative conversion of crude petroleum to gasoline of the right quality is normally achieved�

The cracking process denotes the decomposition of large molecules of compounds such as hydrocarbons into less-complex compounds of lower boiling points� However, cracking involves both decomposition as well as some recombination reactions� Thus, the products of cracking will contain some relatively high-viscosity compounds with higher boiling points, which are recycled to undergo further controlled cracking, albeit under different operating conditions�

Most of the refineries also include catalytic cracker units because they can produce cat-cracked gasoline that is of superior resistance to the incidence of knock in gasoline-fueled engines to that produced by thermal cracking alone� In these units, catalysts such as silica, aluminum, and nickel oxides are used� The nature and concentrations of the products of cat-cracking will depend mainly on the type and nature of the catalyst, temperature, pressure, and residence time employed� Although the catalytic cracking operation is more expensive than thermal cracking, the increase in the output of highquality fuels makes the overall process usually economical� Figure 11�5 lists the main products of refining petroleum, while Figure 11�6 shows typical average proportions of the major refined products that are produced from a sample of crude petroleum�

The action of catalysts in general proceeds through surface reactions with their effectiveness very much dependent on the surface area of contact available for the species to react� Hence, highly porous pellets or very finely divided catalysts are used normally� However, care is needed that their reaction will not deteriorate (e�g�, be poisoned) with the presence of certain undesirable metallic compounds in the crude�

The chemical processes involved in the refining and processing of hydrocarbon compounds could include the following:

• Hydrogenation-This involves the increased concentration of hydrogen in the fuel� Often these are made in the presence of suitable catalysts�

• Dehydrogenation-Dehydrogenation reactions relate to those that remove some hydrogen from the fuel molecules, usually with the aid of catalysts to produce suitable unsaturated compounds�

• Cracking-The process of breaking up of large molecules into simpler fragments� Catalytic cracking takes place at lower temperatures but in the presence of catalysts�

• Pyrolysis-The thermal cracking of a fuel when heated in the absence of air�

• Isomerization-The process of production of isomers from compounds having the same mass and elements but with different arrangements of the atoms making up the material�

• Cyclization-The process of production of cyclic compounds� • Alkylation-The process of production of compounds of alkyl form� • Polymerization-The process of combination of two or more mole-

cules to form a single larger molecule� • Oxidation-The process of the stripping of electrons off the fuel molecule� • Reduction-The process of addition of electrons to the fuel molecule� • Chlorination-The process of addition of chlorine to the fuel molecule�

Refining involves basic categories of physical and chemical treatments of the crude� Distillation separates groups of compounds by heating the feed

initially at atmospheric pressure and later in a vacuum in special fractionation towers� This is followed by a wide range of large chemical processes of catalytic reforming, isomerization alkylation, and polymerization (Figure 11�7)� Refineries incorporate a number of consecutive operations with feedback, so as to optimize the yield of certain desirable groups of products� Figure 11�8 shows the changes in the refining capacity of petroleum in USA in relation to demand over the period from 1970 to 2004�

Catalysts, which are of prime importance in refining and are extensively used in its various stages, work by increasing the speed of the chemical reaction, causing the chemical equilibrium state to be approached more quickly� With the use of suitable catalysts, the temperature at which the reaction proceeds at an adequate rate can be also lowered� Catalysts are materials of widely different characteristics, such as metals, metal oxides, nonmetallic oxides, bases, acids, and some organic compounds� Most catalysts have a specific

effect accelerating a specific reaction� The acceleration of the reactions usually takes place at the surface of the catalyst, requiring a surface area as large as possible� Hence, catalysts are employed in the form of small pellets or honeycomb bodies, producing a very large surface area to the reactive flow�

Standards usually define a set of technical guidelines for best current practice that function as instructions to designers, manufacturers, operators, and users� On the other hand, codes are strict guidelines that must be adhered to and are usually compulsory, such as codes relating to building, emissions, electricity, and fire�

There are numerous codes and standards that govern the quality and operation of fuels as well as all technical, safety, and environmental aspects of the energy industry virtually� These standards that originate from a wide variety of sources are developed through rigorous processes which require the input and scrutiny of various knowledgeable technical committees� These committees have membership from various sources such as government, public, and technical personnel and bodies� The standards and codes are regularly reviewed and updated� There are various standardization organizations around the world, both national and international, which develop and publish standards� Examples of these are International Organization for Standardization (ISO) and the American Society of Testing Materials (ASTM)�

1� List the main controlling operational parameters that may be varied to control the reaction rates of many processes employed for the mass production of a variety of hydrocarbon fuels and other organic products in the refining and petrochemical industries�

2� Outline briefly why there has been a tendency for increasingly larger size but fewer refineries in recent years�

The components of petroleum are separated and modified through refining into more uniform in composition usable groups� This refining involves numerous and complex physical and chemical steps to transform the very large number of compounds and impurities in the crude into desirable and useful product compounds, including gasoline, diesel, kerosene, fuel oil, and gas turbine jet fuels� The major processes in refining include fractional distillation, cracking, reforming, isomerization, and hydrogenation� The relative conversion of crude oil to any of the desirable products, which depends on the quality of the crude and the refining processes employed, can be varied only to within a relatively small degree�