Petroleum refineries/refining hydrocarbon separations

Up to this point in this book, we have looked at distillation columns that separate specific chemical components. In the refining of crude oil, mixmres of many thousands of components must be handled. These components vary from quite fight hydrocarbons (methane, ethane, propane, etc.) to very high-molecular-weight components that boil at extremely high temperatures. Petroleum refineries have units that separate (by distillation) and transform (by a variety of reactions) these mixtures. 

A refinery converts crude oil and other hydrocarbon feedstocks into useful products and raw materials for other industries (Figure 4.5). Refining involves the separation and blending, purifying and quality improvement of desired petroleum products. The primary products of a refinery are as follows ... 

Distillation is a common method for the fractionation of petroleum that is used in the laboratory as well as in refineries. The technique of distillation has been practiced for many centuries, and the stills that have been employed have taken many forms (Speight, 1999). Distillation is the first and the most fundamental step in the refining process (after the crude oil has been cleaned and any remnants of brine removed) (Bland and Davidson, 1967 Speight, 1999, and references cited therein Speight and Ozum, 2002, and references cited therein), which is often referred to as the primary refining process. Distillation involves the separation of the various hydrocarbon compounds that occur naturally in a crude oil into a number of different fractions (a fraction is often referred to as a cut). 

Oil (also referred to as petroleum) is a complex liquid mixture of organic substances, principally of hydrocarbons containing five to sixteen carbon atoms. Most crude oil, once removed from a well, is sent by pipeline to a refinery, where it is distilled to separate it into gasoline, heating oil, diesel oil, and asphalt. The use of catalysts during the refining process increases the yield of gasoline. In 2001, 25.7 billion barrels of oil were used worldwide, with estimated reserves of 1.05 trillion barrels. (One barrel contains 159 liters.)... 

Petroleum is a thick, dark liquid composed mostly of hydrocarbons whose molecules have from 5 to 25 carbon atoms. Natural gas, composed of hydrocarbons whose molecules have 1 to 4 carbon atoms, is often found associated with petroleum deposits. Because petroleum is a mixture of various compounds, in order for it to be used, it must be separated into fractions by the refining process. Oil refineries start with the mixture of compounds in the crude oil and by means of distillation and catalysts separate it into more usable fractions. Shorter-chain hydrocarbons boil at a lower temperature than longer-chain hydrocarbons, and so boiling is a way of separating the components of the mixture. Table 13.2 shows the primary uses of the fractions once they are separated. 

Different types of petroleum can be used in different ways. Jet fuel differs from the gasoline that automobiles consume, for example. Refineries separate different petroleum products by heating petroleum to the point that heavy hydrocarbon molecules separate from lighter hydrocarbons so that each product can be used for a specific purpose. Refining reduces the waste associated with using limited supplies of more expensive petroleum products in cases in which a cheaper, more plentiful type of petroleum would suffice. Thus, tar or asphalt, the dense, nearly solid hydrocarbons, can be used for road surfaces and roofing materials, waxy substances called paraffins can be used to make candles and other products, and less dense, liquid hydrocarbons can be used for engine fuels. 

Before the hydrocarbon mixture can really be of much economic value, it must be refined, freed from impurities or unwanted material. The mixture is separated into groups of hydrocarbons, and in some cases, the moleculcu structure of the hydrocarbons is changed. The refining process occurs at a plant called a refinery, which produces the refined mixtures and individual compounds that are used for gasoline and feedstock for the vast petrochemical industry. A number of processes occur at the refinery, starting with the fractional distillation of the crude petroleum. 

The fundamental separation process in refining petroleum is fractional distillation (Figure 2.26). Most crude petroleum that enters a refinery goes to distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into fractions. Each fraction contains a mixture of hydrocarbons that boils within a particular range. 

After the heavy oil has been separated from the sand, the bitumen cannot be processed in a normal oil refinery. It must be treated in an upgrader, where the hydrocarbon macromolecules are broken into smaller fragments. There are two processes commonly used. Petroleum coke can be produced, where any sulfur components remain. The more common process involves hydrogen cracking, which produces synthetic crude oil, which is then further refined in traditional oil refineries. The sulfur trapped in the organic matrix of the oil is converted into hydrogen sulfide. A traditional Claus process can then be used to produce sulfur. 

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