Steam cracking Primary fractionation

Steam stripping is not adequate for the bottoms purity required. More positive stripping is obtained by charging the tower bottom liquid to a heating unit known as a reboiler. In a typical reboiler, 50% of the feed is vaporized and returned to the tower below the bottom plate. A fractionating tower equipped with a steam heated reboiler is shown in Figure 4. The reboiler may also be heated by a hot oil stream, such as a pumparound reflux stream from the primary fractionator of a cracking unit, or by a fired furnace. 

The cracked gas stream is cooled and purified in the primary fractionator (3) and quench water tower (5). Waste heat is recovered by a circulating oil cycle, generating dilution steam (4) and by a water cycle (5) to provide heat to reboilers and process heaters. The cracked gas from the quench tower is compressed (6) in a 4- or 5-stage compressor and dried in gas and liquid adsorbers (8). C02 and H2S are removed in a caustic-wash system located before the final compressor stage. 

The hydrocarbon mixture at the furnace outlet is quenched rapidly in the transfer line exchangers (2) (TLE or SLE), generating high-pressure steam. In liquid crackers, cracked gas flows to a primary fractionator (3) after direct quench with oil, where fuel oil is separated from gasoline and lighter components, and then to a quench water tower (4) for water recovery (to be used as dilution steam) and heavy gasoline production (end-point control). 

Feedstock (after pre-treatment if necessary) is passed along with steam to the pyrolysis furnace. This cracks the compounds in the naphtha, producing a full range of products which are extremely complex. As with gas feedstock, heavier products are produced, but in increased volumes. After quenching a primary fractionator (not present in gas crackers) separates the heavy pyrolysis fuel oil from the cracked gases. 

Cracking of crude oil in the presence of steam superheated to 2000°C. with the formation, in addition to acetylene and ethylene, of various co-products (fuel gas, propylene, benzene, naphthalene, tars, pitches, etc.). This operation is followed by quenching and primary fractionation. 

A thermal cracking unit for waxes consists of a furnace, a primary separation column, a stabilization column and a distillation section. The feedstock-is vaporized, mixed with steam to 40 per cent weight, and enters a tubular furnace in which the residence time is a few seconds (2 to 10 s) at 500 to 600° . Once-throucb conversion is relatively low (15 to 30 per cent) to avoid side reactions. Operation is at atmospheric pressure or slightly above. Direct quench, or quench with a heat transfer fluid, generates steam. Primary fractionation allows the recycling of the unconverted part of the feedstock. 

The liquid and gaseous effluents obtained are quenched and sent to a primary fractionation column, which produces an oil at the bottom, partly used as a quenching fluid, and pitch, tars and a naphthalene-rich aromatic oil at the side withdrawals. Naphtha and tighter fractions are recovered at the top of the column. Alter compression, the different fractionation operations required on the light products are identical to those of conventional steam cracking, while the only significant difference relates to the acetylene recovery unit, which is larger. 

Cracking furnace 2 Quenching cooler 3 Steam generator 4 Primary fractionation ... 

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