Reactor effluent that is composed of oils with various boiling temperatures is fed into the base of the fractional distillation tower where it is fractionated into selected product streams. Various product streams in the distillation tower are fractionated from
Figure 5.18 Flow diagram of distillation tower
Figure 5.18 Flow diagram of distillation tower reactor effluent due to their different boiling temperatures. The control variables are the temperature gradient in distillation tower, temperature of reboiler located at the main column, reflux ratio of product oil and pressure (vacuum or atmospheric pressure). Figure 5.18 shows two streams produced from the main column, including light product and heavy oil (HO). However, these product streams can be changed by regulation of the tower structure. Light products are taken overhead to the accumulator of light product after it is cooled in condenser, and are separated into both gas products and light oil (LO). The condensation of the product streams in the condenser may be cooled by a water stream. Gas products consisting of C5 and below, with mainly alkane components may be used as fuel gas in the pyrolysis process or sent to a gas plant, the quality of which can be continuously monitored by an on-line analyzer.
Light oil is mainly obtained in the gasoline range and has a partial reflux stream by control of the reflux ratio that controls the temperature gradient in the distillation tower and also the product quality. HO consists mainly of kerosene and diesel range compounds, which is the desired product as fuel oil with high calorific value. The boiling range of LO and HO depend on the current desired product specifications, which are obtained by control of experimental variables in the distillation tower. A portion of the main column bottom passes to the heavy oil settler, which receives the overflowing heavy oil with high viscosity generated from the main column bottom. They are returned to the melting reactor or cracking reactor, in order to crack the heavy oil again into the light oil. In many cases, the oils produced by this method are marketed without any additional processing.
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