An Introduction to Distillation
1. Petroleum refining is the separation of petroleum into fractions and the subsequent treating of these fractions to make them into petroleum products. Most petroleum products, including kerosenes, fuel oils, lubricating oils, and waxes, are fractions of petroleum that have been treated to remove undesirable components. Other products, for example, gasolines, aromatic solvents, and even some asphalts, are totally or partly synthetic in that they have compositions that are impossible to achieve by direct separation of these materials from crude petroleum. They result from chemical processes that change the molecular nature of selected portions of crude petroleum; in other words, they are the products of refining or they are refined products.
2. Refining petroleum is a complex series of steps by which the original crude material is eventually converted into salable products with the desired qualities and, perhaps more important, in the amounts dictated by the market.
3. In fact, a refinery is essentially a group of manufacturing plants that vary in number with the variety of products produced; refinery processes must be selected and products manufactured to give a balanced operation: that is, crude oil must be converted into products according to the rate of sale of each. For example, the manufacture of products from the lower boiling portion of petroleum automatically produces a certain amount of higher boiling components. If the latter cannot be sold as, say, heavy fuel oil, they accumulate until refinery storage facilities are full. To prevent the occurrence of such a situation, the refinery must be flexible and able to change operations as needed. This usually means more processes-a cracking process to change an excess of heavy fuel oil into more gasoline with coke as the residual product or a vacuum distillation process to separate the heavy oil into lubricating oil stocks and asphalt—to accommodate the ever-changing demands of the market.
4.In addition, a complete refining installation must include the following: all necessary non-processing facilities; adequate tankage for storing crude oil, intermediate, and finished products; a dependable source of electrical power, material-handling equipment; workshops and supplies for maintaining a continuous 24 h/day, 7 day/week operation; waste disposal and water-treating equipment; and product-blending facilities.
5. In the early stages of refinery development, when illuminating and lubricating oils were the main products, distillation was the major and often only refinery process. At that time, gasoline was a minor, but more often unwanted, product. As the demand for gasoline increased, conversion processes were developed because distillation could no longer supply the necessary quantities.
6. Nevertheless, distillation has remained a major refinery process and a process to which just about every crude that enters the refinery is subjected. A multitude of separations are accomplished by distillation, but its most important and primary function in the refinery is its use for the separation of crude oil into component fractions.
7. Thus it is possible to obtain products ranging from gaseous materials taken off the top of the distillation column to a heavy residue or “bottom”, which is usually nonvolatile, with correspondingly lighter materials taken off at intermediate points. However, the majority of crude oils, and this applies to the heavier, more viscous petroleums, which are processed by distillation, are usually separated into the lighter fractions （gas, gasoline, naphtha, kerosene, and gas oil） and the bottom or, as it is more generally called, the reduced crude.
8.The reduced crude may then be processed by vacuum or steam distillation to separate the high-boiling lubricating oil fractions without the danger of decomposition, which occurs at high （>350℃, 660℉） temperatures. Indeed, atmospheric distillation may be terminated with a lower boiling fraction （“cut”） if it is thought that vacuum or steam distillation will yield a better quality product or if the process appears to be economically more favorable.