Heat Exchangers petroleum fractions

The oil furnace process is the most common method of production today and is the source of over 95% of the total output of carbon black globally. In this process, a heavy aromatic fraction of petroleum distillate is atomized and sprayed into a furnace preheated to 1200-1900°C. The feedstock vaporizes and decomposes to form carbon black and combustion gases that are immediately cooled with a series of water sprays and heat exchangers to terminate the carbon black reaction and cool the carbon black product stream. The carbon black is separated from the combustion gases in bag filters and is conveyed for further densification either in pelletization processes or in agitator tanks (from which powdered, fluffy black is collected). 

A mixture of hydrocarbons such as petroleum does not boil at a single, sharply defined temperature. Instead, as such a mixture is heated, the compounds with lower boiling points (the most volatile) boil off first, and as the temperature increases, more and more of the material vaporizes. The existence of a boiling-point range permits components of a mixture to be separated by distillation (see discussion in Section 11.6). The earliest petroleum distillation was a simple batch process The crude oil was heated in a still, the volatile fractions were removed at the top and condensed to gasoline, and the still was cleaned for another batch. Modern petroleum refineries use much more sophisticated and efficient distillation methods, in which crude oil is added continuously and fractions of different volatility are tapped off at various points up and down the distillation column (Fig. 7.5). To save on energy costs, heat exchangers capture the heat liberated from condensation of the liquid products. 

Residence time in the laminar sub-layer will be a consideration in the design of heat exchangers to handle heat sensitive materials such as food products or petroleum fractions. The effects of temperature may be to decompose the material and produce fouling. Steps will be necessary to reduce or eliminate the problem (see Section 13.3 or Chapter 15). 

Crude oil is known to contain volatile mercury because mercury is found concentrated in the liquefied petroleum gas (LPG) and naphtha fractions of the atmospheric distillation when oil is refined. It is known conclusively that elemental mercury is at least one of the volatile mercury species in crude oil because it sometimes is found condensed in trays in refinery distillation towers and condensed in cryogenic heat exchangers that liquefy petroleum gases. What is not known is whether elemental mercury is the only volatile species. 

It is necessary to make initial assumptions relative to proposed heat removal schemes prior to commencing design calculations. Since the separations made in catalytic fractionators are relatively easy in comparison with atmospheric crude tower separations and since the number of trays available is quite high by petroleum fractionation standards, it follows that internal reflux requirements are comparatively low. Thus, it is desirable to remove as much heat as possible from the system. Maximizing heat removal is usually accomplished by process-to-process exchange at various points in the unit, and this affords substantial utility savings. A secondary benefit is that internal reflux is minimized which, in turn, minimizes the tower diameter. 

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