Endothermic cracking

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

Primary air flow, supplied at the bottom of the bed, is insufficient for complete combustion of the fuel. The zone below the elevation of secondary-air jets is endothermic, cracking oil to yield carbon and fuel gas species. These burned in the upper, exothermic zone. Alumina product is withdrawn from a standpipe receiving solid from the upper zone, and burn-off of carbon in this zone is sufficient to yield a product that is acceptably white. Fluidizing-gas velocity being lower in the primary combustion zone than in the secondary, density is higher. Provision of the two zones accomplishes two purposes (1) affording a sufficient solid residence time in the primary zone and (2) reducing horsepower needed for air compression. 

Endotherms and attractive reaction products have been achieved through the endothermic cracking of tar model compounds (without a-electrons) using inexpensive calcined mineral materials as the cat ysts. These results provide the basis for the continued development of tar cracking systems, which enable thermochemical processes operation with practical catalysts. The following specific conclusions can be drawn from the results of this work. 

It will be convenient to divide this section more formally than the others, according to the following scheme. Under Sec. 9.7.1 we discuss the genera form of the equations when the wall temperature is constant and illustrate this by considering an endothermic cracking reaction. Under Sec. 9,7.2 we shall consider cocurrent and countercurrent cooled reactors and use the ammonia synthesis reactor as an illustration. These correspond to the two subcases of the third type of design problem mentioned in Sec. 9.5. 

The spent catalyst mixes with air and clean catalyst at the base of the regenerator. Here the coke deposited during cracking is burned off to reactivate the catalyst and provide heat for the endothermic cracking reactions. The recirculating loop of clean catalyst provides added heat for initiation of the carbon bum. The catalyst and air flow up the regenerator riser and separate at a T-shaped head. The flue gas is further cleaned of catalyst in cyclones at the top of the regenerator. 

Thermal decomposition (TD) - It is the endothermic cracking at high temperature for methanol, CHgOHfvap) = 2Hj+CO-95 kJ/ mol (it is not used alone but adding water, i.e. SR) for methane, CH (g)=2Hj(g)+C(s)-75 kJ/mol, using a Ni-catalyst on silica, that must be regenerated with oxygen from time to time to get rid of the carbon deposited notice that no CO is involved. 

Hydrogen is made available by combining a storage cartridge with a catalytic ammonia cracker in which part of the released ammonia is used as an energy source for both the endothermic cracking of ammonia and the release of additional ammonia from the storage material (see Fig. 19.12). 

Figure P4.20 shows the Amoco Model 4 lluidizcd-bed catalytic cracking unit. Several hydrocarbon feeds (gas oil, slurry recycle, etc.) are fed into the reactor alotig with hot solid catalyst from the regenerator. The endothermic cracking reactions cool the catalyst and deposit coke on it. The catalyst from the reactor is circulated back to the regenerator, where air is added to bum off the coke. The heat from the combustion reaction heats the catalyst. 

Methanol is endothermally cracked over a catalyst to produce synthesis gas plus traces of methanol, ether, and methane. A two-stage membrane separation system extracts the hydrogen from a CO-rich fuel that fires the cracker (see Fig. 5-3). 

Coking of catalysts can be reduced by increasing the hydrogen partial pressure or by partial neutralization of the acid sites with promoters, as we have already seen. Coke that has already formed is removed by periodic regeneration of the catalyst. The deactivated catalyst is purified by controlled combustion of the carbon layer. In flui-dized-bed crackers the catalyst circulates continuously between the reactor and the regenerator, in which combustion takes place. The heat of combustion is used to maintain the catalyst at the temperature of the slightly endothermic cracking reaction. 

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