Feed preheat increase

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. 

Increasing catalyst-to-oil ratio by decreasing the feed preheat temperature... 

Compare the cyclone loading with the design. If the vapor velocity into the reactor cyclones is low, consider adding supplemental steam to the riser. If the mass flow rate is high, consider increasing the feed preheat temperature to reduce catalyst circulation. 

Decrease in the feed preheat temperature and subsequent increase in the catalyst-to-oil ratio... 

Figure 1-5 shows a typical feed preheat configuration. A hydraulic limitation usually manifests itself when increasing fresh feed rate and/ or installing high efficiency feed injection nozzles. 

Maximize feed preheat and increase delta coke... 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

Up to this point, we have suggested that the weight flow of vapor up the tower is a function of the reboiler duty only. Certainly, this cannot be completely true. If we look at Fig. 4.2, it certainly seems that increasing the heat duty on the feed preheater will reduce the reboiler duty. 

Let us assume that both the reflux rate and the overhead propane product rate are constant. This means that the total heat flow into the tower is constant. Or, the sum of the reboiler duty, plus the feed preheater duty, is constant. If the steam flow to the feed preheater is increased, then it follows that the reboiler duty will fall. How does this increase in feed preheat affect the flow of vapor through the trays and the fractionation efficiency of the trays ... 

As the flow of vapor through the absorption section trays is unaffected by feed preheat, the fractionation efficiency of the trays in the upper part of the tower will not change as feed preheat is increased. On the other hand, the reduced vapor flow through the stripping section may increase or decrease fractionation efficiency—but why ... 

Heat balance—e.g., condensing steam gives up latent heat, or sensible heat, to increase the temperature in the feed preheater. 

Adiabatic with Cold-Shot Cooling. Some exothermic reactions are conducted in vessels with multiple beds of catalyst, which operate adiabatically (temperature increases through the bed). At the exit of each bed, a cold stream is mixed with the hot stream leaving the bed to bring the temperature back down to the desire inlet temperature for the downstream bed. This cold stream is typically some of the feedstream that has been bypassed around the reactor feed preheating system. 

DMA and TMA. Product ratios can be varied to maximize MMA, DMA, or TMA production. The correct selection of the N/C ratio and recycling of amines produces the desired product mix. Most of the exothermic reaction heat is recovered in feed preheating (3). The reactor products are sent to a separation system where firstly ammonia (4) is separated and recycled to the reaction system. Water from the dehydration column (6) is used in extractive distillation (5) to break the TMA azeotropes and produce pure anhydrous TMA. The product column (7) separates the water-free amines into pure anhydrous MMA and DMA. Methanol recovery (8) improves efficiency and extends catalyst life by allowing greater methanol slip exit from the converter. Addition of a methanol-recovery column to existing plants can help to increase production rates. 

Rotary kilns can also be fitted with a kiln-feed preheater see Figure 6.8, which enables a shorter kiln to be used, approximately one-half the length of a conventional rotary. The exhaust heat is used to preheat the burden entering the kiln thus increasing heat recovery and improving the thermal efficiency of the furnace. 

The above simple analysis highlights an important issue in process dynamics the influence of positive and negative feedback on system s stability. Instability can occur in recycle systems due to positive feedback when the gain is larger than unity. We may give as example the recycle of energy developed by an exothermal reaction in an adiabatic PFR for feed preheating. Instability may occur because of the exponential increase in reaction rate with the temperature when this cannot be properly controlled (Bildea Dimian, 1998). Another example is the recycle of impurities in a plant with recycles, whose inventory cannot be kept at equilibrium by the separation system (Dimian et al., 2000). 

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