Crude distillation

The petrochemical processes covered in this chapter inciude those that use or produce benzene, BTX aromatics, ethyibenzene, ethyiene giycoi, mixed xyienes, oiefins, paraxyiene, poiyethyiene, xyiene isomerization, and ethyiene. The benzene process uses heated hydrogen, toiuene, and heavy aromatic feedstock to produce high-purity benzene and heavier aromatics by passing it over a fixed cataiyst bed. The BTX aromatics process passes a feedstock composed of paraffin, napthenes, and aromatics through a series of fiuidized-bed reactors. Ethyibenzene manufacturing 

Ethylene glycol is commonly used as antifreeze in automobiles. The feedstock includes pure water and refined ethylene oxide. This system combines a blending feed tank, glycol reactor, and a series of distillation columns. Olefin manufacturing includes three major processes the first converts natural gas to ethylene, propylene, or butane the second produces isobutylene and isoamy-lene from hydrocarbon feedstocks the third converts gas oil feedstocks into high-octane gasoline, distillates, and C2-C5 olefins. Plastics manufacturing employs a number of polymer processes that handle polyethylene, polypropylene, and butyl polymers. 

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There are little or no olefins in crude oil or straight run (direct from crude distillation) products but they are found in refining products, particularly in the fractions coming from conversion of heavy fractions whether or not these processes are thermal or catalytic. The first few compounds of this family are very important raw materials for the petrochemical Industry e.g., ethylene, propylene, and butenes. 

An example of the good correlation between TBP and simulated distillation is given in Figure 2.4, where it is shown that 71% of a Kuwait crude distils below 535T. 

The foremost separation process is crude distillation and in second place, if deeper conversion is envisaged, solvent extraction (deasphalting). 

Feedstocks are light vacuum distillates and/or heavy ends from crude distillation or heavy vacuum distillates from other conversion processes visbreaking, coking, hydroconversion of atmospheric and vacuum residues, as well as deasphalted oils. 

Effects. Trays can become damaged several ways. A pressure surge can cause damage. A slug of water entering a heavy hydrocarbon fractionator will produce copious amounts of vapor. The author is aware of one example where all the trays were blown out of a crude distillation column. If the bottom liquid level is allowed to reach the reboiler outlet line, the wave action can damage some bottom trays. 

Natural gas and crude distillates such as naphtha from petroleum refining are used as feedstocks to manufacture a wide range of petrochemicals that are in turn used in the manufacture of consumer goods. Basic petrochemicals are... 

In crude distillation, there are thousands of different compounds present having a virtually continuous spectrum of boiling points. It would be impractical to consider each of these compounds in describing the crude or designing the equipment to process it. Instead the crude is treated as if it were composed of a manageable number (< 50) of pseudo components. These are defined by dividing the crude distillation curve into a series of adjacent boiling cuts. 

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

An old variation of the conversion type is a catalytic combination unit. Development of this scheme was necessitated by the rising cost of refinery construction after World War II and by the great demand for capital for postwar expansion. The scheme reduced the investment and operating costs for refining equipment. The basic feature of the combination unit lies in the integration of the fractionation facilities of the reduced crude distillation and catalytic cracking sections. 

Two-Stage Crude Distillation (Atmospheric and Vacuum) The vacuum stage can be used alternately to produce heavy gas oil for catalytic cracking feed or raw lube distillate cuts for lubricating oil manufacture. 

In crude distillation, there are thousands of different compounds present having... 

Usually, product specifications for a crude distillation unit are expressed in terms of the products 15/5 or ASTM distillation curves. The prediction of a product 15/5 distillation is accomplished simply by blending the quantities of the pseudo components in the stream so as to form a true boiling point, 15/5 equivalent, distillation curve. This curve can then be converted to an ASTM type distillation using an empirical method. Figure 5 illustrates how a typical ASTM curve compares to the 15/5 curve for the same material. 

The withheld information technique is used to explore the manner in which operators select and use information in process abnormalities. A particular abnormal process event is represented in a control panel "mock-up" or a "low-fidelity" simulator, and information is withheld from the worker until it is requested. This technique has been developed by Marshall et al. (1981) and has been used to elicit the diagnostic plans used by experienced workers during various process transients in a crude distillation imit. There are three main applications of this technique ... 

FIGURE 4.6. Decision/Action Diagram for Fault Diagnosis in a Crude Distillation Plant (Duncan and Cray, 1975)... 

The crude distillate, containing the whole of the meutboi, is known, ill Japan as Torioruschi. 

The reaction mixture was removed from the vessel and distilled at a pressure of 30-60 mm, and a bath temperature of 30°C to 50°C until the methanol had all been removed. The extremely viscous tarry residue remaining in the still pot was given a very crude distillation, the distillate boiling at B2°C to 1 32°C/2 mm. In an attempt to purify this distillate by a more careful distillation, 5.3 g of a liquid distilling from 53°C to 150°C/5 mm was collected. At this point, much solid sublimate was noted not only in this distillate but in the condenser of the still. 7 g of the solid sublimate was scraped out of the condenser of the still. Recrystallization of the sublimate from ethyl acetate containing a small amount of petroleum ether gave beautiful crystals melting at 175°C to 177°C (5 g). Infrared analysis confirmed that this compound was hydroquinone (9% conversion). 

Examples of hydrodenitrogenation of two types of nitrogen compounds normally present in some light and middle crude distillates are shown as follows ... 

Fig. 9.4 Corrosion control in the overheads system of a crude distillation unit...

Cycloheptatriene containing 9% toluene is available from the Shell Chemical Company, New York. Less pure cycloheptatriene, obtained by pyrolysis of bicycloheptadiene followed by a crude distillation, has been used successfully in this preparation. The quantity of the tropilidene/toluene mixture is adjusted in accord with its purity as estimated by vapor-phase chromatography on didecyl phthalate. 

Figure 2. Phase Behaviour or Bonnie Glen Crude, Distilled Water, and Surfactant System...

Two small portions of benzene are used for rinsing the flask. The benzene is distilled by reducing the pressure gradually to about 60 mm. and heating the flask gently in an oil bath (not above 90°). After the benzene has been removed, the pressure is reduced to 5-10 mm. and the bath is heated slowly to 150-155° until no more material distils (Note 2). The crude distillate (110-126 g.) is redistilled through an efficient column, and the purified tetrahydrofurfuryl bromide is collected at 69-70°/22 mm. (61-62°/13 mm., 49-50°/4 mm.) (Note 3). The yield is 90-102 g. (53-61 per cent of the theoretical amount). 

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