Quench boilers

Figure 2.11 show s a highly simpUhed flow sheet of such a fecility for the case of naphtha feedstock. It features a number of furnaces, quench boilers, and a highly complex fractionation train. The hydrocarbon feedstock enters the hot section of the unit through the convection zone of the furnace, where it is preheated (part 11 and is then mixed with steam that is also preheated in this zone. The hydrocarbons and water pass through the radiation zone of the furnace (part 2k w here the rapid temperature rise and pyrolysis reactions take place. At the furnace exit, in order to avoid any subsequent reaction, the... 

The products leaving the furnace radiatioa zone must be cooled as quickly as possible. This operation is designed in particular to prevent the effluent composition from changing by the formation of heavy polymerization products and the increase in the gasoline content. It is important for the transfer line between the furnace and the quench boiler to be as short as possible to avoid additional residence of the effluents at elevated temperature. The heat of the furnace is first recovered by indirect cooling in the quench boilers, and directly by the introduction in -line of a recycle using a heavy hydroca n cut called quench oil (Fig. Hi). 

To provide an example. Fig. Z16 shows a sin libed isometric view of a cell of a steam cracker with vertical tubes in the radiation zone and horizontal tubes in the convection zone. It also shows the positions of the adewaU burners, quench boilers, steam drum, and the injection device of the heavy cut which performs the supplementary direct quench. 

Despite all the precautions observed to minimize the foimalion of coke in the furnace tubes and in the quench boiler it is impossible to eliminate this completely. Fuihace operation must therefore be interrupted periodically to remove the coke this-is the decoking operation. 

In ideal conditions, a furnace operatmg on naphtha can run for 90 days without decoking. However, the run length is always shorter due to the inevitable fouling of the quench boiler. Progressive coke deposition in the pyrolysis tubes results in an increase in their metal skin temperature, connected with the growing ineffldency of heat transfer, and in an increase in the pressure drop caused by the redaction of the open cross section of the tube. 

The effluents from the different quench boilers are first collected together. They undergo supplementary in-line cooling, and are then sent to a so-called primary fractionation columj whose bottom stream, itself previously cooled, serves for the direct quench operation. This column also separates the gases at the top that are sent to the compression section, and a gasoline sidestream. The diluent steam which is condensed at various levels, is purified and re-used in a closed circuit with make-up process water. 

Naphtha steam cracking, quench boiler performwh... 

Scrubbing Solution Quenched Boiler Stack Gas — 110-115 43-46 — E E E plus fly ash, aeration... 

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