Gas quench tower

An example of Type 1 operation is a hot gas quench tower using water as the liquid coolant. In this application sensible heat is transferred from a hot gas stream to the cooler water. As the liquid temperature increases, water is vaporized into the gas stream, which raises its humidity. Thus, mass transfer is in the opposite direction to heat transfer. 

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Pyrolysis Gas Quench Tower 12Q Light Ends Towers 105... 

The coke is taken to a quench tower, where it is cooled with a water spray or by circulating an inert gas (e.g., nitrogen), a process known as dry quenching. The coke is then screened and sent to a blast furnace or to storage. 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

A typical ethane cracker has several identical pyrolysis furnaces in which fresh ethane feed and recycled ethane are cracked with steam as a diluent. Figure 3-12 is a block diagram for ethylene from ethane. The outlet temperature is usually in the 800°C range. The furnace effluent is quenched in a heat exchanger and further cooled by direct contact in a water quench tower where steam is condensed and recycled to the pyrolysis furnace. After the cracked gas is treated to remove acid gases, hydrogen and methane are separated from the pyrolysis products in the demethanizer. The effluent is then treated to remove acetylene, and ethylene is separated from ethane and heavier in the ethylene fractionator. The bottom fraction is separated in the deethanizer into ethane and fraction. Ethane is then recycled to the pyrolysis furnace. 

Applications include reactor off-gas quenching, vacuum condensers, cooler-condensers, desuperheating and humidification. Water-cooling towers are a particular example of direct-contact heat exchange. In direct-contact cooler-condensers the condensed liquid is frequently used as the coolant, Figure 12.65. 

Effluents from the naphtha and gas oil cracking coils are at 1300°F and 1200°F, respectively. They are combined in the line just before discharge into a quench tower that operates at 5 psig and 235°F at the top. Water is sprayed into the top of this tower. The... 

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

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. 

Raw gas from the quench tower is compressed in a multistage centrifugal compressor (5) to greater than 500 psig. 

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