Carbonaceous residue, cracking

One area of cat cracking not fully understood is the proper determination of carbon residue of the feed and how it affects the unit s coke make. Carbon residue is defined as the carbonaceous residue formed after thermal destruction of a sample. Cat crackers are generally limited in coke burn capacity, therefore, the inclusion of residue in the feed produces more coke and forces a reduction in FCC throughput. Conventional gas oil feeds generally have a carbon residue less than 0,5 wt for feeds containing resid, the number can be as high as 15 wt lf. 

The deactivation of catalysts, especially zeolites, during cracking, hydrocracking, methanol conversion, etc, is one of the major technological and economic problems of the chemical industry (1). The interest of these materials lies not only in their high catalytic activity and selectivity but also in the possibility of regenerating them several times so that their Lifetime" is compatible with the cost of their production. Consequently, it is necessary to understand the manner and the rate of catalyst deactivation as well as the nature of the carbonaceous residues formed, commonly called coke". 

Under our experimental conditions, complete removal of the carbonaceous residues leads to the appearance of structure defects and/or short-range amorphization of the zeolitic framework, not detectable by X-ray diffraction. The coke produced during ortho-xylene cracking is the most difficult to remove and its elimination leads to pronounced modifications of the structure and of the crystallinity, doubtless because of its greater poly aromatic (or graphitic) character. 

Thus, the catalytic cracking of PET has been recently a subject of interest [26], Several acid solids (zeolites 4A and 13X and alumina) as well as metal salts (CuCh, MgCE and Zn, Sn, Mg, Mn acetates) have been tested as catalysts at 400-500°C. Copper chloride has been found as the most effective catalyst, reducing the degradation time by almost 3.5 times in regard to the thermal treatment and minimizing the amount of carbonaceous residues. 

Figure 6.11 shows the product yields for each catalyst. The products are classified into four lumps, i.e. gas (carbon number 1-4), gasoline (5-11), heavy oil (above 12), and a carbonaceous residue referred to as coke. In the figure, PE oil represents the feed oil and contains a 34% gasoline fraction. The feed oil was effectively cracked by solid acid catalysts. The gasoline yield was highest with REY zeolite. HZSM-5(65) yielded the... 

As it is both inexpensive and easy to handle, steam is a potential candidate carrier gas for waste plastic recycling in chemical plants. Furthermore, as mentioned in Section 2.1, the degradation temperatures for polyester resins are remarkably shifted to low-temperature regions, and the amount of carbonaceous residue produced in the degradation process is reduced in a steam atmosphere, as compared with that in a nitrogen atmosphere. Accordingly, the preparation of a catalyst that could demonstrate both stable activity for the catalytic cracking of PE-derived heavy oil, but that would also remain stable in a steam atmosphere, was examined [16],... 

Acidic zeolites are known for their excellent catalytic activity in cracking and isomerization of hydrocarbons (75). In the absence of metal, however, these catalysts rapidly deactivate due to the formation of carbonaceous products, usually referred to as coke. The carbonaceous residues are mainly formed via alkylaromatics and polyaromatics, which are the result of dehydrogenation, cyclization, and further alkylation processes. The coke deposits lower the catalytic activity by site poisoning and eventually also by pore blocking, which inhibits access of hydrocarbon molecules to the acid sites (286). 

Flo. 37. NMR intracrystalline self-diffusion coefficient Dm, (a) and effective self-diffusivity Dcir ( ) of methane in HZSM-5 crystals that were coked for different times by n-hexane cracking (131-133). Before loading with methane (9.2 CHa per u.c.), the coked ZSM-5 crystals were carefully outgassed at 623 K and 10 Pa. The remaining carbonaceous residues were defined as coke. Amounts of coke after different times on stream 1 h, 0.8 wt% C 2 h, 1.3 wt% C 6 h, 3.2 wt% C 16 h, 4.8 wt% C. The starting self-diffusion coefficient is 8.1 x 10" m s . ... 

Steam reforming was the primary reaction over these nickel catalysts. The presence of hydrocarbons (G2 to G5) which would indicate cracking reactions occurred to the extent of less than 10% in the reaction products. The presence of methane, which would indicate partial reforming, did not exceed 5% in the reaction products. There does not appear to be any significant difference in product selectivity for the n-hexane steam reforming reaction over nickel on the 2 quite different supports—zeolite vs. alumina. Carbonaceous residues accumulated in the case of all the nickel catalysts where reforming activity was sustained and the carbon deposition on the zeolite catalysts compared favorably with G56. 

Coke. When coal is heated with a large deficiency of air, the lighter constituents are volatilized and the heavier hydrocarbons crack, liberating hydrogen and leaving a residue of carbon. The carbonaceous residue containing the ash and a part of the sulfur of the original coal is called coke. 

Effective engineering design of the cracking reactor for waste plastic processing is very important since the carbonaceous solid residue is one of the cracking products (levels up to 10% or more) and its continuous removal from the reactor is necessary to ensure profitable running. Stirred vessel reactors which have augers in the bases to facilitate continuous char removal are presented. 

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