Catalysts from coking

It was initially hoped that running the reaction under oxidative dehydrogenation conditions would lead to higher conversions. Studies with molybdate-pillared hydrotalcites under more optimized conditions showed that the presence of oxygen in the feed simply kept the catalyst from coking up as fast as if oxygen were not included. Unfortunately, even with the presence of oxygen in the feed, molybdate-pillared hydrotalcites were found to lose approximately 40% of their activity after 100-150 hours on stream. 

SoHd by-products include sludge from wastewater treatment, spent catalyst, and coke from the EDC pyrolysis process. These need to be disposed of in an environmentally sound manner, eg, by sludge digestion, incineration, landfill, etc. 

Thus the amount of heat that must be produced by burning coke ia the regenerator is set by the heat balance requirements and not directly set by the coke-making tendencies of the catalyst used ia the catalytic cracker or by the coking tendencies of the feed. Indirectly, these tendencies may cause the cracker operator to change some of the heat-balance elements, such as the amount of heat removed by a catalyst cooler or the amount put iato the system with the feed, which would then change the amount of heat needed from coke burning. 

Aside from the above reforming reactions, a small amount of feed components are converted to polymeric hydrogen deficient products which deposit on the catalyst as "coke." A coke buildup results in activity and selectivity loss which ultimately requires catalyst regeneration. In semi-regenerative operation, the coking rate is maintained at a low level to provide cycles of at least three to six months. In cyclic units, coking conditions are inherently much more severe so that frequent regenerations are required. 

Heat to raise the coke on the catalyst from the reactor temperature to the regenerator dense phase temperature... 

Using the operating data from the case study. Example 5-5 shows heat balance calculations around the stripper-regenerator. The results are used to determine the catalyst circulation rate and the delta coke. Delta coke is the difference between coke on the spent catalyst and coke on the regenerated catalyst. 

In a cat cracker, a portion of the feed, mostly from secondary cracking and polymerization reactions, is deposited on the catalyst as coke. Coke formation is a necessary byproduct of the FCC operation the heat released from burning coke in the regenerator supplies the heat for the reaction. 

Feed residue coke is the small portion of the (non-residue) feed that is directly deposited on the catalyst. This coke comes from the very heavy fraction of the feed and its yield is predicted by the Conradson or Ramsbottom carbon tests. 

Catalyst circulation coke is a hydrogen-rich coke from the reactor-stripper. Efficiency of catalyst stripping and catalyst pore size distribution affect the amount of hydrocarbons carried over into the regenerator. 

In most of today s FCC operations, the desired reactions take place in the riser. In recent years, a number of refiners have modified the FCC unit to eliminate, or severely reduce, post-riser cracking. Quick separation of catalyst from the hydrocarbon vapors at the end of the riser is extremely important in increasing the yield of the desired product. The post-riser reactions produce more gas and coke versus less gasoline and distillate. Presently, there are a number of commercially proven riser disengaging systems offered by the FCC licenser designed to minimize the post-riser cracking of the hydrocarbon vapors. 

The major disadvantage of the alkylation process is that acid is consumed in considerable quantities (up to 100 kg of acid per ton of product). Hence, solid acids have been explored extensively as alternatives. In particular, solid super acids such sulfated zirconia SO/ IZr02) show excellent activities for alkylation, but only for a short time, because the catalyst suffers from coke deposition due to oligomerization of alkenes. These catalysts are also extremely sensitive to water. 

Other wastes that are typical of a refinery include (1) waste oils, process chemicals, and still resides (2) nonspecification chemicals and/or products (3) waste alkali (sodium hydroxide) (4) waste oil sludge (from interceptors, tanks, and lagoons) and (5) solid wastes (cartons, rags, catalysts, and coke). 

The response of the kiln to a 20% increase in coke on the catalyst from the reactor is shown in Fig. 27 for no control and for schemes (a) and (b). The simpler scheme (b) is clearly superior to scheme (a) although steady-state considerations predicted that scheme (a) would be the better strategy. The fluctuations in the total air rate of scheme (a) that maintains a constant amount of oxygen to the kiln causes this difference and outweighs the effects of the fluctuations in oxygen amounts present in scheme (b). This comparison showed that control strategies designed... 

The FCC process involves at least four types of reactions (1) thermal decomposition (2) primary catalytic reactions at the catalyst surface (3) secondary catalytic reactions between the primary products and (4) removal of polymerization products from further reactions by adsorption onto the surface of the catalyst as coke. This last reaction is the key to catalytic cracking because it permits decomposition reactions to move closer to completion than is possible in simple thermal cracking. 

From Figure 4.6 it can be seen that the coke yields showed different behaviors for the two types of catalysts. For the Type B catalysts the coke yield was almost unaffected by variations in the ZSA/MSA ratio. For the Type A catalysts, however, the coke yield decreased when the ZSA/MSA ratio increased, which means that more naphtha selective cracking gave decreased coke yield. This is also snp-ported by the coke yield as a fnnction of the zeolite snrface area, see Fignre 4.6b. By comparing catalyst A-1 with catalyst A-3 is it possible to see that the coke... 

The zeolite to matrix surface area ratio can be used for optimization of catalysts for catalytic cracking of atmospheric residues. For North Sea long residues this ratio should be as large as possible, but the ratio should not exceed an upper limit. For the main catalyst type (A) used in this investigation the upper limit of the ZSA/ MSA ratio was around 3.5. There is also a lower limit for the matrix surface area. If the matrix surface area is lower than this limit, the catalyst will not be able to crack all the heavy components in the residue feed, and the coke on the matrix will increase dramatically. This will prevent the catalyst from working properly. Different type of catalysts must be optimized individually, as well as different type of long residues. 

Distributor nozzles may become plugged from refractory, slumped or defluid-ized catalyst, or coke. The steam rate, as reflected by the distributor pressure drop, must be maintained to minimize the possibility of plugging. 

Table 10.2 presents the total coke yields and the nonvaporized hydrocarbons produced over a spent catalyst obtained with different feedstocks. The catalyst used was deactivated for 20 hours, 30 ReDox cycles, and 50% steam. When the 100% vacuum gas oil (VGO) is replaced with a mixture of 5%w DMO-VGO and/or 30%w DMO-VGO an increase of 30% and 120% in the coke yields was observed. While the spent catalyst from VGO cracking does not have adsorbed hydrocarbons, the mixture with DM0 does, becoming almost 1% for the mixture with 30%w DM0. The SARA (saturates, aromatics, resins, and asphaltenes) analysis of these hydrocarbons showed a high concentration of asphaltenes. 

Hydroseparator The efllnent from the oxidizer column next flows to a hydroseparator in which any lime grit, catalyst, or coke lines are physically removed. The solids are sent to a horizontal vacnnm belt filter, and the overhead liquor from this separator will be further treated to reclaim most of the process liquor and to produce a filter cake for disposal. 

Ce02-supported noble-metal catalysts such as Pt, Pd and Rh are of interest because of their importance in the so-called three-way converter catalysts (TWC), designed to reduce emissions of CO, NOx and uncombusted hydrocarbons in the environment and to purify vehicle-exhaust emissions. Such catalysts are also of current interest in steam reforming of methane and other hydrocarbons. Conventional practical catalysts for steam reforming consist of nickel supported on a ceramic carrier with a low surface area and are used at high temperatures of 900 C. This catalyst suffers from coke formation which suppresses the intrinsic catalyst activity. Promoters such as Mo are added to suppress coke formation. Recently, Inui etal(l991) have developed a novel Ni-based composite... 

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