PETROLEUM DISTILLATION

PETROLEUM DISTILLATION

To gain an idea of how petroleum is used as a source of chemicals, we must see what happens in a petroleum refinery. Let us consider a simple refinery  in which crude oil, a sticky, viscous liquid with an unpleasant odor, is separated by distillation into various fractions.

The first most volatile fraction consists of methane and higher alkanes through C4 and is similar to natural gas. These are dissolved in the petroleum. The methane and ethane can be separated from the higher alkanes, primarily propane and the butanes.

The methane/ethane mixture is called “lean gas” from which the ethane can be separated if required. The C3/C4 mixture, called liquefied petroleum gas (LPG), may be

used as a petrochemical feedstock or a fuel.

Butane is also separated for use in gasoline for volatility control, although Government

regulations in the United States have decreased this application because the butane contributes to ozone layer destruction. Butane is used to a lesser extent as a raw material for chemicals. Some gas contains C5, C6, and even higher liquids. This mixture is called a condensate. It resembles light naphtha and can be used in similar applications.

In the past, the high cost of compressing or liquefying and shipping these refinery gases has dictated that most of them be flared. As the price of natural gas increases, however, shipping of methane in refrigerated tankers may eventually become commonplace.

Alternatively, methane may be converted to methanol, which is a useful organic chemical building block (Section 10.5) and is more easily shipped. Meanwhile, much refinery gas is still flared even in the United States.

The second fraction comprises a combination of light naphtha or straight run gasoline, and heavy naphtha, and is of particular importance to the chemical industry. The term naphtha is not well defined, but the material steam cracked  for chemicals generally distills between 70 and 200°C and contains C5–C9 hydrocarbons.

Naphtha contains aliphatics as well as cycloaliphatic materials such as cyclohexane, methylcyclopentane, and dimethylcyclopentane. Smaller amounts of C9_ compounds such as polymethylated cycloalkanes, and polynuclear compounds such as methyldecahydronaphthalene may also be present. Like the lower alkanes, naphtha may be steam cracked to low molecular weight olefins. Its conversion by a process known as catalytic reforming into benzene, toluene, and xylenes (BTX)  is, in the United States, its main chemical use. Catalytic reforming is also a source of aromatics worldwide, although in Western Europe benzene and toluene are mainly derived from

pyrolysis gasoline, an aromatics fraction that results from steam cracking of naphtha or gas oil. Light naphtha, with its octane number raised by addition of lead alkyls, was at

one time used directly as gasoline; hence its alternate name, “straight run gasoline.”

It contains a large proportion of straight-chain hydrocarbons (n-alkanes), and these resist oxidation much more than branched chain hydrocarbons (isoalkanes), some of which contain tertiary carbon atoms. Consequently, straight run gasoline has poor ignition characteristics and a low octane number of about 60. It is of little use in gasoline for modern high compression-ratio automobile engines, and its properties are even worse if it is unleaded. Isomerization of its components to branched compounds increases its octane number and thus its utility. Chemically, however, its significance is like that of naphtha, for it can be cracked to low molecular weight olefins. It does not perform well in catalytic reforming, giving large amounts of cracked products and small amounts of benzene. Benzene is no longer a welcome constituent of gasoline because of its toxicity and relatively low octane number. As noted above, international practice has differed. The United States has preferred to crack ethane and propane from natural gas, while the rest of the world has cracked naphtha. The rising price and predicted shortages of natural gas in the United States projected in the 1970s led to an increased interest in liquid feedstock cracking. The preferred liquid feedstock is naphtha, which has traditionally been preempted in the United States for gasoline manufacture. Accordingly, gas oil steam cracking was developed. This was not considered when natural gas was cheap and plentiful, because cracking of gas oil to olefins is accompanied by tar and coke formation.

The predicted price rises and shortages, however, motivated techniques for ameliorating this latter problem, and it is now possible to crack gas oil as well as naphtha. The industry has been loathe to do so because it is usually more economic to crack ethane and propane. They are easier to handle, provide fewer coproducts, and much less coke. From the 1980s onward, the switch to liquid feedstocks lost momentum for three reasons. First, US natural gas production was maintained. Although reserves are being depleted—the reserves/production ratio was down to 8.8 years in 1999— production was scarcely down from the 1973 peak. This reserves/production figure is the same as that in 1993, reflecting a number of new discoveries partly by “wildcatters” and the exploitation of unconventional gas reserves—gas from “tight” (low permeability) sands, sandstones and carbonates, coal seams, shales, and ultradeep (_15,000 ft) reservoirs. Such resources constituted an estimated 30% of natural gas supply in the lower 48 US states in 2000. Price rises stimulated drilling activity.

The number of active drilling rigs at the end of 2000 was 879, more than double the cyclical low point of 362 in April 1999 and three times the 1991 low point of 260. Second, natural gas discoveries in Canada meant that cheap natural gas could be imported and in 1999 was about 14% of consumption. Third, Saudi Arabia decided that its first cracker would crack ethane only, making LPG (propane– butane) mixtures available on the world market. Thus, as indicated in Table 2.4, the percent of gaseous feed cracked in the United States has decreased much less than predicted. The sharp rise in gas prices in 2000 (up to $10/million Btu from a typical value of $2.50) led to the closure of 30% of US crackers and was expected to lead to a switch to naphtha but, again, this did not occur. Naphtha was also expensive, and there was a shortage.  Because of availability of gas from the North Sea, the percentage of gas cracked in Western Europe doubled by 1995, but even so, the predominant feedstock in Europe will continue to be liquids. Gaseous feedstocks are not readily available even though the crackers could be modified to use them. Many crackers in the United States are flexible. If the price of propylene or butadiene goes up, more liquids can be cracked; indeed the optimum ratio of feedstocks is determined at frequent intervals by linear programming.

The situation in Japan approximates that in Western Europe except that even less gaseous feed is cracked and what is cracked comprises almost entirely butane. The liquid feed is entirely naphtha, and it is not expected that gas oil will be cracked in the foreseeable future.

Two decades ago this discussion of world sources of ethylene would have sufficed.

By the mid-1980s, however, the impact of the production of ethylene in other parts of the world, primarily the Middle East and Canada, was being felt, and these two giants have been joined by many Asian, South and Central American, and eastern European countries (Section 1.3.3). The availability of ethylene derivatives from third world sources had caused exports from Japan and Western Europe to decrease markedly by the late 1980s. By 1995, the new producers enjoyed a much larger share of the world’s ethylene derivatives export business. The United States still participates in trade of ethylene derivatives as long as it is able to crack gas or condensate. As indicated earlier, this capability was threatened in the early 2000s.

To summarize, the two reactions in Figure 2.3—steam cracking and catalytic reforming—are the basis for much of the world’s petrochemical production, valued in 2000 at about $1500 billion. The three main raw materials are ethylene, propylene, and benzene, while the C4 olefins, methane, toluene, and the xylenes, are important but to a lesser degree. Methane is an important source of the fertilizer, ammonia, as well as of organic chemicals, primarily methanol. Its most important reaction is the formation of synthesis gas, from which ammonia and methanol are made. In principle, naphtha may also be used for this (Section 10.4) although it seldom is. Referring to Figure 2.2, we see that kerosene is a fuel for jet aircraft, tractors, and for domestic heating and has some applications as a solvent. Gas oil is further refined into diesel fuel and light fuel oil of low viscosity for domestic use. Its use as feed for cracking units for olefin production has already been mentioned. Both the kerosene and gas oil fractions may be catalytically cracked to gasoline range materials. Actually, the term gas oil is applied to two types of material, both useful for catalytic cracking. One is so-called atmospheric gas oil which, as its name indicates, is produced by atmospheric pressure distillation. The other is vacuum gas oil, which results from the vacuum distillation of residual oil from the heavy fraction. It has a much higher boiling range of 430–530°C.

Residual oil (Fig. 2.2) boils above 350°C. It contains the less volatile hydrocarbons together with asphalts and other tars. Most of this is sold cheaply as a high-viscosity heavy fuel oil (bunker oil), which must be burned with the aid of special atomizers.

It is used chiefly on ships and in industrial furnaces. A proportion of the residual oil is distilled in vacuo at 0.07 bar to give, in addition to gas oil as mentioned above, fuel oil (b.p. _ 350°C), wax distillate (350 –560°C), and cylinder stock (_560°C). The cylinder stock is separated into asphalts and a hydrocarbon oil by solvent extraction with liquid propane in which asphalts are insoluble. The oil is blended with the wax distillate, and the blend is mixed with toluene and methyl ethyl ketone and cooled to _5°C to precipitate “slack wax,” which is removed by filtration. The dewaxed oils are purified by countercurrent extraction with such solvents as furfural, which remove heavy aromatics and other undesirable constituents.

The oils are then decolorized with Fuller’s Earth or bauxite and are blended to give lubricants. Part of the vacuum distillate and the “slack wax” can be further purified to give paraffin and microcrystalline waxes used for candles and the impregnation of paper. The petroleum industry is constantly trying to find methods by which the less valuable higher fractions from petroleum distillation can be turned into gasoline or petrochemicals.

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