Pumparounds sections, packing

Figure 7.11 Packing is often used in pumparound sections of trayed towers.

Since we do not rely on pumparounds to fractionate—but just to remove heat—good vapor-liquid distribution is not critical. A bed of 4 or 5 ft of structured packing is often, then, an excellent selection for the pumparound section of a tower. The capacity of such a bed potentially has a 30 to 40 percent advantage over trays. 

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. 

A computer simulation of a thermal cracker fractionator pumparound section based on equilibrium flash vaporization calculations shows that the heat-transfer coefficient for a theoretical separation stage was 1,600 BTU/hr/ft /°F. On this basis, the height equivalent to a theoretical stage of packing, such as the Flexipac type 4 in section 3 (see Table 8-3), is ... 

Commercial absorbers normally use countercurrent packed beds in externally cooled pumparound sections to remove the heat of absorption. This system is complicated by the tendency of HCHO solutions to produce a solid phase if the concentration is too high or the temperature too low. Thus, the 50 wt % HCHO solution must be discharged from the absorber at a temperature greater than 131°F [17]. 

Pumparound sections usually contain from 4 ft to 9 ft of packed depth. The traditional method for calculating bed depth is by use of Equation 6-20. This equation is a simplified representation of a complex group of heat and mass transfer processes. A considerable amount of industrial experience has led to the development of satisfactory empirical equations for the calculation of overall heat transfer 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). 

Frequently vacuum gas oils are condensed in packed sections. If several zones are used, there will be opportunity for better heat efficiency. Structured packing, sheet-metal type, has largely displaced trays for contact in vacuum pumparound zones because packing requires less pressure drop. Thus, flash zone pressures can be lower and less vacuum pitch can be produced. Flash zone pressures as low as 25Torr (millimeters of mercury absolute) have become routine. For heat efficiency, a corrosion-resistant de-entrainment mesh pad is often installed above each pumparound packed zone. 

Due to the high percentage of open area of the Flexigrid packing, section 6 seems relatively insensitive to changes in liquid loads. However, increases in vapor rates of as little as 2% are sufficient to flood section 6. Within 30 minutes of a small increase in vapor flow, the LCO pumparound turns black. 

For the majority of the Flexipac sections, the anticipated percentage of flood is 60% higher than for comparable valve trays. Section 4 flooded prematurely, presumably because of excessive entrainment originating in section 5. Flexigrid packing exhibited a high capacity and maintained a reasonable heat-transfer efficiency in the bottom pumparound service. 

The outlet vapor stream has much less mass than the inlet stream, due to a large percentage of vapor condensation in the section. The heat load on the packed bed is greater than the heat removed by external pumparound coolers because the vacuum gas oil condensate normally is discharged from the system at the same temperature it is withdrawn from the column. The logarithmic mean temperature difference driving force for each packed bed is determined from Equation 6-24. 

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