Residence time catalytic cracking

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

It is important to separate catalyst and vapors as soon as they enter the reactor. Otherwise, the extended contact time of the vapors with the catalyst in the reactor housing will allow for non-selective catalytic recracking of some of the desirable products. The extended residence time also promotes thermal cracking of the desirable products. 

Catalytic coke is a byproduct of the cracking of FCC feed to lighter products. Its yield is a function of conversion, catalyst type, and hydrocarbon/catalyst residence time in the reactor. 

Post-riser hydrocarbon residence time leads to thermal cracking and non-selective catalytic reactions. These reactions lead to degradation of valuable products, producing dry-gas and coke at the expense of... 

The CFB catalytic cracking reactor plays an important role in the petroleum industry because of its better gas-solids contact and narrow residence time distribution, but its non-uniform radial flow structure and the extensive backmixing of gas and solids lead to a lower conversion rate and poorer selectivity to desired intermediate products [14]. 

This type of reactor aims to challenge fast fluidization with its 1 to 10 second gas residence time as the prime reactor for the catalytic cracking of petroleum. The claim is that the higher cracking temperature and shorter residence time will give a very different—and better—distribution of reaction products. 

However, the use of short residence times with the catalyst placed in a more optimal position to insure better contact with the feedstock has resulted in the millisecond catalytic cracking (MSCC) unit that is used to process residua (Hydrocarbon Processing, 1998). The unit is flexible in terms of feedstock changes and the improved metals tolerance of the process allows the unit to handle a wide range of feedstocks. 

MSCC process a short-residence time process (millisecond catalytic cracking) in which the catalyst is placed in a more optimal position to insure better contact with the feedstock. 

Non-stationary operations have found large scale industrial application. An important classical example is catalytic cracking, where oil is exposed with a short residence time to a rapidly deactivating zeolitic catalyst, which is regenerated in a second step by removal of deposited coke. A novel non-stationary process is selective butane oxidation over a regenerable oxidation catalyst (see Chapter 2). Undoubtedly we will see more examples of this type of process, in which the proper catalytic step and the regeneration of the catalytic sites occur in different compartments under different conditions. A nice application involves... 

NEXCC [NEste Catalytic Cracking] catalytic cracking process developed by Neste Oil, Finland, from 1998. Novel engineering features permit the very short residence time of 0.7 to 2.2 sec. 

Fuel Gas yield Thermal vs. Catalytic Cracking. A higher ratio of fuel gas/iso-butane was found in the case of the MR, which is to be expected with a relatively low catalyst volume fraction and longer residence time of the hydrocarbons in the reactor (see Table I). With increasing catalyst fraction (or conversion) the ratio decreases due to the formation of more iso-butane. A reduction in contact time will also result in a lower ratio, because fewer thermal cracking reactions will take place. 

In quite a different application, a novel approach for producing olefins via a hydrocarbon-steam cracking process, without the use of a catalyst, was demonstrated to benefit from the use of a honeycomb monolithic catalytic reactor [28]. A typical problem associated with cracking processes of this type is maintaining the appropriate combination of heat transfer and residence time, which, if not balanced, will lead to either poor conversion... 

The conversion and the coke yield as a function of residence time with different CTOs are shown in figure 4a and 4b, respectively. The conversion in the absence of catalysts is a measure for the amount of thermal cracking. At a residence time of 4 s it is at maximum 6wt%. The catalytic conversion after 0.15 s is remarkably high, over 30 wt%. The conversion increases proportionally with residence time up to a conversion of ca. 75 wt%, then levels off and reaches a maximum value of 90-95 wt% after 3-5 s. The coke yield (figure 4b) is hardly influenced by residence time, except for a minor increase at a CTO of 6. Clearly, almost all the coke has been generated within 0.15 s. 

The conversion of a hydrowax feedstock due to thermal cracking at a residence time of 4 s was at maximum 6 wt%. The product yields obtained catalytically could all be described by the same function of conversion, regardless the reactor length or the CTO used. Up to a conversion of 95 wt% no gasoline over-cracking has been observed. 

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