The Refinery Fccu Unit

The VGO feed from storage or the vacuum distillation unit flows to feed surge drum V-107 and is heated to about 445°F (see Figure 4-3). The preheated feed is mixed with a small amount of dispersion steam and enters FCCU reactor V-101, where a stream of hot catalyst coming from regenerator V-102, at 1360°F, contacts the feed. The catalyst/oil ratio is about 5.4. The catalyst, oil, and steam mixture flows up through the reactor tube, where cracking reactions occur during its short residence at a temperature of approximately 990° F. At the end of the reactor, the cracked oil/catalyst mixture is separated by cyclones. The reactor effluent passes through one or more cyclones to separate any entrained catalyst particles; next any entrained hydrocarbons are stripped from the catalyst by medium pressure (MP) steam. The effluent flows out of the reactor to

FRESH CATALYST EQUILIBRIUM

FRESH CATALYST EQUILIBRIUM

Atmospheric Pressure Distillation
Figure 4-3. Fluid catalytic cracking unit. LCO=light cycle oil.

distillation column C-101, where the cracked effluent is separated into different products. About 5% of the feed is converted to carbon, which is deposited on the catalyst.

After steam stripping, the hot, separated catalyst is transported to FCCU regenerator V-102 by a stream of air. In the regenerator, the carbon deposited on the catalyst is burned off in a stream of hot air. Air for combustion is preheated to 435°F. Due to combustion of carbon, the catalyst temperature rises to about 1340°F, and the carbon on the catalyst is burned off. Flue gases pass through a number of cyclones to reduce particulate emission, produced from the attrition of the catalyst particles. The hot, regenerated catalyst flows back to FCCU reactor V-101 to continue the cycle. The hot flue gases, at 1340°F, generated by combustion of coke on the catalyst, are sent to a power recovery turbine. The power generated is used in an air blower to supply air to the regenerator.

Makeup catalyst is added to the regenerator to compensate for the loss of catalyst due to particle attrition and emitted to atmosphere.

The FCCU reactor effluent is sent to fractionation column C-101, very similar to a crude distillation column, with side strippers for the side cuts. The column has about 45 trays. The feed is introduced at the bottom of the column. The main cuts are;

• Vapors from the overhead reflux drum, which are sent to the gas concentration unit. The liquid, the light gasoline, is also sent to the gas concentration unit. Part of this stream provides the reflux to the fractionating column.

The broad cuts of heavy gasoline, distillate, and light cycle oil are drawn from the main column to their respective steam strippers, with six plates in each, and stripped light ends are returned to the column. The stripped side cuts are withdrawn as product.

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