Crude petroleum as it is produced from the field is a relatively low-value material since, in its native state, it is rarely usable directly. However, it can be refined and further processed into any number of products whose value is many times that of the original oil. The first step in any petroleum refinery is the separation of the crude into various fractions by the process of distillation. These fractions may be products in their own right or may be feedstocks for other refining or processing units.

In most refineries, this process is carried out in two stages. The oil is first heated to the maximum temperature allowable for the crude being processed and for the operation being practiced and then fed to a fractionating tower which operates at slightly above atmospheric pressure. This tower is usually called the atmospheric tower. It yields several distillate products and a bottoms product which is the residual liquid material which could not be vaporized under the conditions of temperature and pressure existing in the atmospheric tower. This bottoms liquid is then reheated to the maximum allowable temperature-usually higher than the maximum temperature allowed for the feed to the atmospheric tower—and fed to a fractionating tower which operates at subatmospheric pressure. This tower is usually called the vacuum tower. This chapter describes the atmospheric tower. The vacuum tower is discussed in Chapter 3.

Although the process of crude oil distillation has been practiced in the United States for more than 100 years, the design and operation of these units is still accomplished almost exclusively on an empirical and, in some cases, even a nearly emotional basis. This is undoubtedly due to the fact that crude oil is macje up of an almost infinite number of discrete hydrocarbons, all the way from methane to materials having 70 or more carbon atoms.Thus, the process and/or its products are usually discussed in terminology which is unique to the petroleum industry. In this introductory material, the nomenclature usually employed in crude oil distillation will be discussed before proceeding into the design methods.

Analysis of Crude Petroleum and Its Fractions

A complete component-by-component analysis of a crude oil sample is not practically realizable. For this reason, the composition of any given oil is approximated by a true boiling point distillation, commonly called TBP distillation. A thorough discussion of the equipment and procedures involved is given in Chapter 4 of Nelson (1). This method is basically a batch distillation using a large number of stages and a high reflux-to-distillate ratio so that the temperature at any point on the temperature-volumetric yield curve represents the actual (true) boiling point of the hydrocarbon material present at the volume percentage point.

TBP distillations are normally run only on crude oils and not on petroleum fractions. Instead, a rapid distillation procedure is used for analysis of petroleum products and

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