Cracking of gas oil

Refinery Production. Refinery propylene is formed as a by-product of fluid catalytic cracking of gas oils and, to a far lesser extent, of thermal processes, eg, coking. The total amount of propylene produced depends on the mix of these processes and the specific refinery product slate. For example, in the United States, refiners have maximized gasoline production. This results in a higher level of propylene production than in Europe, where proportionally more heating oil is produced. 

The principal sources of feedstocks in the United States are the decant oils from petroleum refining operations. These are clarified heavy distillates from the catalytic cracking of gas oils. About 95% of U.S. feedstock use is decant oil. Another source of feedstock is ethylene process tars obtained as the heavy byproducts from the production of ethylene by steam cracking of alkanes, naphthas, and gas oils. There is a wide use of these feedstocks in European production. European and Asian operations also use significant quantities of coal tars, creosote oils, and anthracene oils, the distillates from the high temperature coking of coal. European feedstock sources are 50% decant oils and 50% ethylene tars and creosote oils. 

Gas oil, which is heavier than kerosene, is the raw material of choice in cracking and other refinery operations. Cracking of gas oil produces a variety of fuels for automotive, industrial, and domestic (furnace) use. 

The catalytic behaviour of steam/acid leached Y zeolites appears to be similar to that of USY zeolites. Bremer et al. (64) have reported that at equal conversions, cracking of gas oil over a DAY zeolite results in lower coke yields as compared to those obtained over REY zeolites. 

Synthetic zeolites and other molecular sieves are important products to a number of companies in the catalysis and adsorption areas and numerous applications, both emerging and well-established, are encouraging the industrial synthesis of the materials. There are currently no more than a few dozen crystalline microporous structures that are widely manufactured for commercial use, in comparison to the hundreds of structures that have been made in the laboratory. See Chapter 2 for details on zeolite structures. The highest volume zeolites manufactured are two of the earliest-discovered materials zeolite A (used extensively as ion exchangers in powdered laundry detergents) and zeolite Y (used in catalytic cracking of gas oil). 

In conclusion, we believe that cracking of gas-oil is taking place on zeolites via carbonium, carb mium ions and radicals. In the case of steam dealujninated samples, when more than 5 Al per unit cell (Si/Al<30) are present in the framework of the zeolite, the ionic mechanism is much more important than 1 he radical one. When the framev ork aluminum decreases and the number of defects increases, the radical mechanism becomes operative and eventually dominant when practically no aluminum is present in l he zeolite framework and superacid Brdnsted sites (at 3610 cm ) are not present. 

PVC production, on the other hand, is carried out by first high severity steam cracking of gas oil to produce ethylene. Vinyl chloride monomer (VCM) is then... 

Table VII. Effect of Catalyst Types in Fluid Catalytic Cracking of Gas Oils ...

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. 

Units like that of Figure 17.28(c) were employed at one time in the catalytic cracking of gas oils. The catalyst is transferred between regenerating and reacting zones with bucket elevators or air lifts. Some data for this equipment are given with the figure. 

Moscou and Mone measured the acidity distribution of various rare earth-exchanged Y zeolites and correlated the results with the performance of the zeolite in cracking of gas-oil (224). They found that steaming the zeolite at 675°-750°C resulted in a significant decrease in the concentration of strongly acidic sites (H0 < -8.2), while the intermediate... 

The pore opening in ZSM-5 is smaller than for zeolite Y and access of the complex gas oil molecules into the pores will be restricted. As a result, ZSM-5 has little effect on the primary cracking of gas oil and, when allowance is made for the slight loss in conversion arising from dilution of the active zeolite Y concentration, there is no significant change in coke, bottoms, or light gas yields. 

Figure 8. Prototypical molecular reaction pathways for the catalytic cracking of gas oil feedstocks.

The kinetic expressions derived by Antipina and Frost have general applicability to monomolecular heterogeneous catalytic reactions which occur on a uniform surface. The expression can be made to describe the cracking of synthin or decomposition of octene over silica-alumina as well as hydrogen disproportionation of gasoline and cracking of gas oils over the silica-alumina. Numerous other applications are discussed. 

Dr. J.A. Rabo led our catalyst research group from 1957 to 1961 and played a key role in the discovery of the catalyticly active ingredient used world wide in the catalytic cracking of gas oils to produce gasoline. 

Blasetti, A., Ng, S and de Lasa, H. I Catalytic Cracking of Gas Oil in a Novel FCC Pilot Plant Unit with Heat Exchange Reactor Performance, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 553-558. Somerset, Pennsylvania (1993). 

Fig. 2.10. Influence of se erit> on relative weight production and consumption of the steam cracking of gas oil and naphtha.

Furnaces with very short residence time (Short Residence Time technolog> developed b> Lummus) adapt ideally to the cracking of gas oils on account of their tube diameter, which is larger than that of standard equipment, the low partial pressure of steam, and decreased coking. 

This chapter focuses on the economics of cracking naphtha and gas oil. The cracking of liquid feedstock produces most of the world s ethylene. This is dominated by naphtha cracking, the character of which has been discussed previously. Where available, there is some cracking of gas oil. 

---