Desuperheating section

Temperature difference correction for desuperheating section only see Figure 10-30 for 1 shell pass, 2 or more tube passes. 

Superheated feeds from chemical reactors or petroleum cracking units are a special case, often requiring a baffle-tray desuperheating section in the tower. 

F, no fired preheater is necessary. On the other hand, it is often necessary to add a desuperheating section to reduce the mostly vaporized feed mix to a temperature that will allow distillation without rapid coke formation. 

Because of the low individual coefficient on the gas side, the overall coefficient in the desuperheater section is small, and the area of the heating surface in that section is large in comparison with the amount of heat removed. This situation should be avoided in practice. Superheat can be eliminated more economically by injection of a spray of liquid directly into the superheated vapor, since small drops evaporate very rapidly, cooling the vapor to the saturation temperature. The desuperheating section is thereby eliminated, and condensation occurs with high coefficients. 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

Carbohydrazide itself is of very low volatility, but it decomposes at relatively low temperatures to produce volatile carbon dioxide and ammonia. In theory, the combined corrosive effects of these two materials should be negated in the condensate system, but in practice, this is not always so and both steel and copper corrosion transport problems may develop, primarily as the result of corrosion-enhancement reactions resulting from oxygen in-leakage. It is presumed, therefore, that (similar to hydrazine) some deliberate after-desuperheating line addition of CHZ is necessary if post-boiler section corrosion is to be avoided. 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

As shown in Figure 8—2, the reactor vapor enters the fractionator at 980°F, while the vapor leaving the slurry oil pumparound has been cooled to 630 F. For a 50,000-B/SD FCCU charge, the weight of reaction vapor in the vapor feed to the fractionator is about 700,000 Ib/hr. This amount of vapor must be cooled or desuperheated in the slurry pumparound section. 

The slurry pumparound section has a heat-transfer coefficient significantly lower than the other packed pumparound sections and lower than reported coefficients for trays. The duty in the section is, however, devoted largely to desuperheating the cracked vapors from 940 F to 700 F, whereas the major component of duty in the other pumparound sections is condensation. It is likely that heat transfer coefficients would be higher for condensation as opposed to desuperheating vapors. 

Determine which section initiates flooding Desuperheating reactor vapor Bubble effect in slurry P/A Reactor vapor line quench... 

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