Feed preheat effect

Liebert [218] studied feed preheat conditions and the effects on the energy requirements of a column. This topic is essential to the efficient design of a distillation system. 

The test conditions for this Microscale Simulation Test (MST) correspond to the low vapor contact times as applied in today s FCC riser technology. An effective feed preheat and feed dispersion is ensured, while the isothermal reactor bed is set to the dominating kinetic temperature in the riser, being approximately the feed catalyst mix temperature. The MST conditions enable the testing of high Conradson Carbon residue feedstocks. 

Each of the four catalysts was tested in the DCR under adiabatic conditions with a reactor outlet temperature of 521°C. For each catalyst a range of conversions was obtained by varying the cat/oil ratio via feed preheat temperature, so that conversion effects could be studied. 

The loss in conversion is also partly caused by lower "effective catalyst activity" in the riser as a result of increased coke blockage of the catalyst pores with coke and higher vanadium and hydrothermal deactivation of the catalyst. The negative effects of resid processing on FCC yields can be reduced by adjusting the FCC process conditions (lower feed preheat, increased catalyst make-up, increased steam dispersion and stripping) and by the use of FCC catalyst formulations more suitable to such applications. 

If reflux cannot be cut back (e.g., in an unrefluxed stripper, in azeotropic distillation, or when the packed section above the feed is close to its minimum wetting limit), boilup will need to be raised to compensate for the excess subcooling. Vapor and liquid traffic below the feed and reboiler duty will rise and effectively lower the column feed capacity. Premature flooding may result. If the lower capacity or higher reboiler duty cannot be tolerated, feed preheating (Fig. 12.5a)... 

The downside of this change is that more heat is rejected to cooling water in the condenser instead of being recovered by feed preheating. The effect on furnace firing depends on the configuration of the heat exchanger network used, which is not modeled in the simulation considered in this chapter. 

For fired heaters, the limitations could be heat flux or TWT. The former is applied to heaters with pressure drop larger than 20 psi, which is the most common. The latter is for low-pressure drop heaters. When the heater duty must be increased to handle duty much larger than design, the existing heater may be insufficient in meeting the limits. Installing a new heater could be very expensive. The most effective way to avoid this is to increase feed preheating via process heat recovery. 

Under the direction of M. Jose Cocero, the University of Valladolid operates a film-cooled SCWO reactor filled with AI2O3 balls for mixing and increasing conversion rates[38]. This reactor was used for the treatment of synthetic dyestuff wastewater. The reactor volume is 15 1 (effective volume 9 1) with operating conditions of 25 MPa, 600-700°C, 12-17 kg/h mass flow, organic content of 7-11 wt.%, and air oxidant. The reactor is energetically self-sufficient with feed preheated by the energy recovered from the reactor. 

Inject water to the riser along with the feed. This has the same effect as reducing feed preheat. Care must be taken to use only water that is free of sodium chloride. Incidentally, relatively small amounts of water or steam in the feed can sometimes accomplish wonders in improved yields by promoting dis 3ersion in the riser. 

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