Fluid catalytic cracking, or FCC, is the last step in the evolution of cat cracking processes-- also introduced in 1942, just like TCC or Thermafor Cat Cracking, during the Second World War in an effort to make high-octane number gasoline. Remember that high-octane number relates to high power as you can have higher compression ratios in the combustion engines.FCC really shows an excellent integration of the cracking reactor, an endothermic reactor, with the catalyst regenerator and exothermic reactor for very high thermal efficiency. FCC is now used universally in oil refineries throughout the world-- has replaced all the previous cat cracking processes.Now, in FCC, in the feed, that is gas oil preheated to about 300 degrees Fahrenheit-- is introduced into the reactor with steam. The riser part of the reactor where the hot catalyst particles-- as you see, the green line coming from the catalyst regenerator-- are full of dyes. The particles are full of dyes because they're smaller particles. They are full of dyes and flowing gases and vapors. So they have a huge surface area to meet the incoming feed at temperatures that are close to 1,000 degrees Fahrenheit.So cracking reactions on these very fine particles that are full of dyes and flowing with the reactants takes place in a very short space of time, something that could be measured with seconds. And the products are sent to a fractionator after going through a series of cyclones, obviously, to separate the small fluid dyes, the particles of the catalyst.In the fractionators, the products, as usual, are separated into gas, gasoline, light cycle oil, heavy cycle oil, and, finally, the heaviest fractions, decant oil.Remember that LCO is used in the US for making diesel fuel through hydrocracking and hydrogenation. And decant oil could be used as fuel oil or as feedstock for making carbon black or white coking to make needle coke for graphites, electrodes.Coming back to the reactors, the cat cracking reactor, the coked catalyst now, the end of the riser where this cracking reaction takes place, are sent through the regenerator. It's not fully coked on the surface, lost its activity. Through the red line, it's sent to the regenerator where air is introduced to burn off the coke.The temperatures in the regenerator could reach to 1,300 to 1,400 degrees Fahrenheit. You should remember that the catalysts now are much improved, as well. It may include zeolites that would take high temperatures and very controlled reactivities through pore size distribution and so forth.So the combustion products or flue gases from this catalyst regenerator could be sent to a CO boiler because the gas may contain significant amount of carbon monoxide, which could be burned to CO2 to provide additional heat or to generate additional heat.So the catalysts that are now regenerated are sent to the reactor to close the catalyst cycle through that green line, as you see, to meet the incoming feed. So our catalyst cycle is pretty much complete at this point.But note this excellent integration, thermal integration, of the catalyst regeneration, the exothermic process, with the cracking reactions where the catalysts that are heated in the regenerator are sent in a very effective manner to the reactor without much heat loss. So that is the ultimate, if you will, thermal efficiency of a process. And that's why FCC is now the universally accepted catalytic cracking process.
Numerical research: Many studies were numerically implemented to study the thermal transport of supercritical fuel but the cracking reaction was not fully considered (Li et al., 2019c; Li et al., 2020; Cheng et al., 2018; Pu et al., 2019). Fortunately, the cracking reaction was considered in some openly published literature. Hu et al. (2017) pointed out that the pressure drop could be increased due to the thermal-induced acceleration caused by the cracking reaction. Gong et al. (2019) studied the secondary reaction in the thermal transport of supercritical fuel. It was evidenced that the HTD phenomenon could be reduced by the secondary exothermic reaction. Xu and Meng (2016), Xu et al. (2018b) examined the convective heat transfer as well as the cracking reaction and surface coking in a round pipe and a ribbed pipe for supercritical fuel. It was reported that heat transfer was improved by the extra heat sink because of the thermal cracking and thermal-induced acceleration. Also, the pyrolytic chemical reaction rate was increased and the pyrolytic surface coking was decreased due to the effect of the ribs.
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The number of suckers following cutting increases as stocking density of the parent stand increases up to full site utilization. The effect of age and site index on aspen suckering is not clear (35,81).
Reaction to Competition- In both the eastern and western parts of its range, quaking aspen is classed as very intolerant of shade, a characteristic it retains throughout its life. Natural pruning is excellent, and long, clean stems are usually produced when side shade is present. However, this is a clonally variable characteristic and self-pruned and unpruned clones exist side by side in some stands (69,78). The intolerance of aspen to shade dictates an even-aged silvicultural system, that is, clearcutting, for regenerating fully stocked sucker stands and maximizing growth (19,57,75),
Recently, viruses have been detected in a few quaking aspen clones. Once trees in the clone are infected, regeneration by suckering maintains the infection, which is then impossible to eliminate except by artificially culturing virus-free tissue. The full extent and seriousness of viruses in aspen is unknown but decline of some clones has been attributed to them in both the East and the West.
Early spring frosts may kill new leaves and shoots and, when especially severe, some of the previous year's shoots. Overwinter freezing can cause frost cracks. Strong wind can uproot or break mature aspen and even moderate wind can crack the bole of trees with lopsided crowns. Hail can bruise the bark of young aspen and, in severe storms, kill entire sapling stands. Aspen suffers little from ice storms or heavy wet snow, except when in leaf. Snow creep on steep slopes can bend or break aspen suckers as tall as 1.2 in (4 ft) (28).
In aspen, the clone is the biological entity-a multistemmed individual that may be thousands of years old (46). The ability to propagate by root suckers assures genetic uniformity and adaptation to the present environment (73). Despite the great genetic variability among clones and the virtually infinite amount of genetic recombination in the billions of seed produced, the chance for expression of this recombination and further adaptive change in established seedlings is very small (6). Stands that are clearcut or destroyed by fire or windstorms may provide some microsites suitable for seedling establishment. However, seedlings are not likely to compete successfully with the faster growing root suckers that are also regenerated under such circumstances (93). 2ff7e9595c
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