Severity, cracking

The quantity of coproduct acetylene produced is sensitive to both the feedstock and the severity of the cracking process. Naphtha, for example, is cracked at the most severe conditions and thus produces appreciable acetylene up to 2.5 wt % of the ethylene content. On the other hand, gas oil must be processed at lower temperature to limit coking and thus produces less acetylene. Two industry trends are resulting in increased acetylene output (/) the ethylene plant capacity has more than doubled, and (2) furnace operating conditions of higher temperature and shorter residence times have increased the cracking severity. 

Significant products from a typical steam cracker are ethylene, propylene, butadiene, and pyrolysis gasoline. Typical wt % yields for butylenes from a steam cracker for different feedstocks are ethane, 0.3 propane, 1.2 50% ethane/50% propane mixture, 0.8 butane, 2.8 hill-range naphtha, 7.3 light gas oil, 4.3. A typical steam cracking plant cracks a mixture of feedstocks that results in butylenes yields of about 1% to 4%. These yields can be increased by almost 50% if cracking severity is lowered to maximize propylene production instead of ethylene. 

Instead of conversion, some producers prefer to use other identifications of severity, including coil outlet temperature, propylene to methane ratio, propylene to ethylene ratio, or cracking severity index (33). Of course, all these definitions are somewhat dependent on feed properties, and most also depend on the operating conditions. 

Figure 3-15. Component yields vs cracking severity for a typical gas oil. ...

It provides more precise control of the operating variables against the unit s constraints and, therefore, obtains incremental throughput or cracking severity. 

Finally, process control systems allow the unit to operate smoothly and safely. At the next level, an APC package (whether within the DCS framework or as a host-based multivariable control system) provides more precise control of operating variables against the unit s constraints. It will gain incremental throughput or cracking severity. 

A refiner has several options to increase LCO yield. Since it is often desirable to maintain a maximum cracking severity while maximizing light cycle oil yield, the simplest way to increase LCO yield is to reduce the gasoline end point. Gasoline end point is usually reduced... 

Distribute the feed and regenerated catalyst throughout the cross-section of the riser to ensure that all feed components are subjected to the same cracking severity... 

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

Patient age 60-75 Actinic keratoses Prior skin cancers Wrinkling throughout Makeup cakes cracks Severe acne scarring... 

Figure 2. The effects of hydrotreating on catalytic cracking performance -- low cracking severity.

These processes are specifically designed for ethylene production but they also yield C4 hydrocarbons as coproducts. The amount of C4 compounds produced depends on the feedstock, the cracking method, and cracking severity. Steam cracking of naphtha provides better yields than does catalytic cracking of gas oil. With more severe steam cracking both butenes and overall C4 productions decrease, whereas the relative amount of 1,3-butadiene increases. Overall C4 yields of 4-6% may be achieved. 

Let us look at changes in cracking severity and how they affect feed requirements, by-product production, and ethylene production costs. We have selected the once-through yield of ethylene as a convenient means of representing severity. 

One potential pitfall in using x-ray crystallography is that under x-ray irradiation the crystal may react further, or react too quickly and thus crack. Several examples of solid-state reactions have been discovered in this way, when it was found that an expected structure gave a poor refinement, leading to the realization that the crystal had been transformed on the diffractometer [60,72]. 

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