Catalyst continued zeolite cracking

As the number of important catalyst families increased in the post World War II decades, researchers began to specialize in areas such as ammonia catalysts, reforming and cracking of petroleum fractions, Ziegler-Natta catalysts, zeolites, homogeneous catalysis, and use of enzymes as industrial catalysts. Creating a unified discipline of catalysis from all these fields continues to be challenge today as it was in the past. 

Cracking of n-heptane was carried out on catalysts USY-1 and U1F-25 in a continuous flow, fixed bed reactor (15), at 450 aC and atmospheric pressure. In all experiments, 0.223 g of zeolite catalyst, and 8.625 g of n-heptane were used. With each catalyst the reaction was performed at 75, 150 and 375 seconds of time on stream. The catalyst was regenerated "in situ" after each experiment by passing flow of air at 5209C during 4 hours, and liquids were analyzed by GC by means of a Porapak-Q silica and a S-30 columns respectively. 

Conversion of n-butane into isobutene over theta-1 and ferrierite zeolites was studied in a continuous flow microreactor at 530°C and 100% n-butane as a feed. The zeolites were used as catalysts in the H- and Ga-forms. Insertion of Ga into the zeolites resulted in improved isobutene selectivities due (i) to an increase in the dehydrogenation activities and (ii) to a decrease in the cracking activities of the catalysts. The highest selectivities to isobutene (-27%) and butenes (-70%) were obtained with the Ga-theta-1 catalyst at n-butane conversions around 10%. These selectivities decreased with increasing conversion due to olefin aromatisation, which was enhanced considerably by the Ga species present in the catalysts. 

At present more than 100 zeolitic structures (both natural and synthetic) have been reported and their number grows annually as new structures are continuously being discovered which opens up a wide range of possible applications [61, 62]. However, from a practical viewpoint, only a few zeolites are used as industrial catalysts such as Y, ZSM-5, Beta and mordenite (Table 3.1), mainly due to the cost and difficulties inherent to their preparation [60]. When zeolites are applied for the catalytic cracking of polymers, their microporous structure causes important diffusional and steric hindrances for the access of the bulky plastic molecules to the internal acid sites [5, 24]. 

Fluid catalytic cracking (FCC) (Fig. 13.5) was first introduced in 1942 and uses a fluidized bed of catalyst with continuous feedstock flow. The catalyst is usually a synthetic alumina or zeolite used as a catalyst. Compared to thermal cracking, the catalytic cracking process (1) uses a lower temperature, (2) uses a lower pressure, (3) is more flexible, (4) and the reaction mechanism is controlled by the catalysts. Feedstocks for catalytic cracking include straight-run gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil, and vacuum residuum. Coke inevitably builds up on the catalyst over time and the issue can be circumvented by continuous replacement of the catalyst or the feedstock pretreated before it is used by deasphalting (removes coke precursors), demetallation (removes nickel and vanadium and prevents catalyst deactivation), or by feedstock hydrotreating (that also prevents excessive coke formation). 

Catalysts used in this process are of two types. Special acid-washed clays of particle diameters in the 200-400- xm range are used in fluidized bed versions of cracking [19]. In these units the catalyst is kept suspended or fluffed up on an upward moving stream of hot gas oil vapors, which ensures continuous exposure of all catalyst faces to the raw material and provides continuous turnover of catalyst. Synthetic catalysts are prepared from a mixture of 85-90% silica and 10-15% alumina, or from synthetic crystalline zeolites (molecular sieves) [19]. They are either used in a small particle size suitable for use in a fluidized bed, or can be formed into 3- to 4-mm diameter pellets appropriate for crackers, which use a moving bed for catalyst cycling. 

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