Stripper efficiency

Many side-stream steam strippers of the type shown in Fig. 15.1 do not work very well. Operating personnel report that the stripping steam is not effective in removing imdesirable lighter components from the stripper feed. Why could this be so  

One of the main reasons for this sort of poor stripping efficiency is subcooled liquid feed to the stripper. Liquid drawn from any tower or vessel is assumed to be in equilibrium with the vapor phase in the tower or vessel. We say that the liquid is at its bubble point or boiling point. We say that the vapor in the vessel is at its dew point or saturation temperature. 

When steam is mixed with a liquid at its bubble point, the partial pressure of the vapor in contact with the liquid is reduced. The liquid then begins to boil. The lighter components of the hquid are turned into vapor and are carried out of the stripper with the steam. 

If hquid drawn from a column cools below its bubble point as a result of ambient-heat loss, we say it is subcooled. Mixing a small amount of steam with subcooled hquid will reduce the partial pressure of any vapor in contact with the hquid, but not enough to promote boiling. Eventually, as more and more steam is mixed with a subcooled hquid, it will begin to boil. But for a given amount of steam, the amount of vapor that can be boiled out of a hquid will 

Wet steam will also reduce stripping efficiency. The water in the steam will be turned into steam when it contacts the hot diesel oil in the stripper, shown in Fig. 15.1. The heat of vaporization for this water must come from the sensible heat of the diesel. This reduces the temperature of the diesel, which also reduces its vapor pressure, which then makes it more difficult to vaporize its lighter gasoline components. 

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Operation of the reactor stripper be efficient. The stripper efficiency is very important to allow the release of sulfate and the formation of H,S. 

Catalyst circulation coke is a hydrogen-rich coke from the reactor-stripper. Efficiency of catalyst stripping and catalyst pore size distribution affect the amount of hydrocarbons carried over into the regenerator. 

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

Another factor inflnencing afterburn is stripper efficiency. Hydrocarbon from the reactor stripper due to poor stripping could potentially flash off the spent catalyst and combust in the dilnte phase generating an afterburn condition. Several units have seen stripper problems result in higher afterburn. 

Process condensate from reforming operations is commonly treated by steam stripping. The stripper is operated at a sufficiently high pressure to allow the overhead stripping steam to be used as part of the reformer steam requirement (71). Contaminants removed from the process condensate are reformed to extinction, so disposal to the environment is thereby avoided. This system not only reduces atmospheric emissions, but contributes to the overall efficiency of the process by recovering condensate suitable for boiler feedwater make-up because the process is a net water consumer. 

Distillation appHcations can be characterized by the type of materials separated, such as petroleum appHcations, gas separations, electrolyte separations, etc. These appHcations have specific characteristics in terms of the way or the correlations by which the physical properties are deterrnined or estimated the special configurations of the process equipment such as having side strippers, multiple product withdrawals, and internal pump arounds the presence of reactions or two Hquid phases etc. Various distillation programs can model these special characteristics of the appHcations to varying degrees and with more or less accuracy and efficiency. 

Fig. 13. The ideal efficiency of a five-stage enricher and stripper as a function of the product or waste withdrawal rate, where Zi represents stripping section...

Tray Efficiencies in Plate Absorbers and Strippers Compn-tations of the nnmber of theoretical plates N assnme that the hqnia on each plate is completely mixed and that the vapor leaving the plate is in eqnihbrinm with the liqnid. In actnal practice a condition of complete eqnihbrinm cannot exist since interphase mass transfer reqnires a finite driving-force difference. This leads to the definition of an overall plate efficiency... 

Fig. 8.7 shows a second example (Cycle A2) of carbon dioxide removal by chemical absorption from a CCGT plant, but one in which the semi-closed concept is introduced— exhaust gas leaving the HRSG is partially recirculated. This reduces the flow rate of the gas to be treated in the removal plant, so that less steam is required in the stripper and the extra equipment to be installed is smaller and cheaper. This is also due to the better removal efficiency achievable—for equal reactants flow rate—when the volumetric fraction of CO2 in the exhaust gas is raised from the 4-6% value typical of open cycle gas turbines to about 12% achievable with semi-clo.sed operation. 

Catalyst flux is defined as catalyst circulation rate divided by the full cross-sectional area of the stripper. For efficient stripping, it is desirable to minimize the catalyst flux to reduce the carryover of hydrogen-rich hydrocarbons into the regenerator. 

Catalyst residence time in the stripper is determined by catalyst circulation rate and the amount of catalyst in the stripper. This amount usually corresponds to the quantity of the catalyst from the centerline of a normal bed level to the centerline of the lower steam distributor. A higher catalyst residence time, though it increases hydrothermal deactivation of the catalyst, will improve stripping efficiency. 

Note that (Xm - Xoul)IXin is the removal efficiency of a stripper, denoted as/ Then Equation 18.16 becomes... 

G. Bonani, P. Eberhardt, HJ. Hofmann, Th.R. Niklaus, M. Suter, H.A. Synal and W. Wolfli, Efficiency improvements with a new stripper design, Nucl. Instrum. Methods B 52, 338 344 (1990). 

Innovation drivers Alternative paint strippers have been developed as a consequence of the chlorine debate . However, neither the detailed requirements related to occupational health and safety (TRGS 612 and 212) nor the lower efficiency (cf (3)) clearly supported the penetration of the market with the alternative paint stripping systems. One barrier may be that the users have to change their work and purchasing procedures to apply the alternatives successfully. By end of the nineties the construction employers liability insurance started, based on the public media, an information campaign about the risks of DCM-containing paint strippers. 

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