Catalytic crackers

Figure 2.8 shows the essential features of a refinery catalytic cracker. This particular reaction is accompanied hy the deposition of carhon on the surface of the catalyst. The fiuidized-hed reactor allows the catalyst to he withdrawn continuously and circulated to a fiuidized regenerator, where the carhon is burnt ofi" in an air stream, allowing regenerated catalyst to he returned to the cracker. 

Figure 2.8 A fluidized-bed reactor allows the catalyst to be continuously withdrawn and regenerated as with the refinery catalytic cracker.

Isopentane Dehydrogenation. In isopentane dehydrogenation, which is used in the CIS, isopentane or a C fraction from a catalytic cracker is dehydrogenated to isoprene (6) ... 

IFP Process for 1-Butene from Ethylene. 1-Butene is widely used as a comonomer in the production of polyethylene, accounting for over 107,000 t in 1992 and 40% of the total comonomer used. About 60% of the 1-butene produced comes from steam cracking and fluid catalytic cracker effluents (10). This 1-butene is typically produced from by-product raffinate from methyl tert-huty ether production. The recovery of 1-butene from these streams is typically expensive and requires the use of large plants to be economical. Institut Francais du Petrole (IFP) has developed and patented the Alphabutol process which produces 1-butene by selectively dimerizing ethylene. 

P. K. Ladwig, T. R. Steffens, S. L. Laley, D. P. Leta, and R. D. Patel, "Resid Processing in Fluid Catalytic Crackers," Foster Wheeler Heavy Oils Conference, Orlando, Fla., June 7, 1993. 

The other significant industrial route to /-butyl alcohol is the acid cataly2ed hydration of isobutylene (24), a process no longer practiced in the United States. Raffinate 1, C-4 refinery streams containing isobutylene [115-11-7], / -butenes and saturated C-4s or C-4 fluid catalytic cracker (ECC) feedstocks (23)... 

The Snamprogetti fluidized-bed process uses a chromium catalyst in equipment that is similar to a refinery catalytic cracker (1960s cat cracker technology). The dehydrogenation reaction takes place in one vessel with active catalyst deactivated catalyst flows to a second vessel, which is used for regeneration. This process has been commercialized in Russia for over 25 years in the production of butenes, isobutylene, and isopentenes. 

The Cy and Cg paraffias comprise about 90% of the alkylate Cg accounts for over 60%. Over 70% of the commercial alkylation processes employ sulfuric acid as the catalyst. Among the butylenes, 2-butene is superior to 1-butene. The C —fraction from the catalytic crackers is considered to be a superior feedstock to the alkylation unit. 

The various sources of isobutylene are C streams from fluid catalytic crackers, olefin steam crackers, isobutane dehydrogenation units, and isobutylene produced by Arco as a coproduct with propylene oxide. Isobutylene concentrations (weight basis) are 12 to 15% from fluid catalytic crackers, 45% from olefin steam crackers, 45 to 55% from isobutane dehydrogenation, and high purity isobutylene coproduced with propylene oxide. The etherification unit should be designed for the specific feedstock that will be processed. 

Because of the thermal coupling of reactor and regenerator, any change on the reactor side creates a rapid change on the regenerator side, which, in turn, influences the reactor side, and vice versa. This dynamic interaction rapidly comes to equiUbrium, and the catalytic cracker adjusts to a new steady-state. 

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. 

Additive inhibitors have been developed to reduce the contaminant coke produced through nickel-cataly2ed reactions. These inhibitors are injected into the feed stream going to the catalytic cracker. The additive forms a nickel complex that deposits the nickel on the catalyst in a less catalyticaHy active state. The first such additive was an antimony compound developed and first used in 1976 by Phillips Petroleum. The use of the antimony additive reportedly reduced coke yields by 15% in a commercial trial (17). 

Power Recovery in Other Systems. Steam is by far the biggest opportunity for power recovery from pressure letdown, but others such as tailgas expanders in nitric acid plants (Fig. 1) and on catalytic crackers, also exist. An example of power recovery in Hquid systems, is the letdown of the high pressure, rich absorbent used for H2S/CO2 removal in NH plants. Letdown can occur in a turbine directiy coupled to the pump used to boost the lean absorbent back to the absorber pressure. 

Unlike coke produced from coal, petroleum cokes are derived from the residua of petroleum refining. Suitable feedstocks for good quality coke are thermal tars, catalytic cracker bottoms, and decant oils [17]. 

Figure 19. Moving bed catalytic crackers (A) Thermoform moving bed process (B) Houdry catalytic cracking process.

Figure 20. Catalyst pick-up system for a moving bed catalytic cracker.

Various design configurations for fluid catalytic crackers are illustrated in Figure 3. Their distinguishing features can be summarized as follows ... 

Figure 3. Various designs for fluid catalytic crackers.

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