Hydrocarbons continuous catalyst regeneration

This is a very slow reaction under normal reforming operation. Feed composition, temperature, H2/hydrocarbon ratio, and the pressure strongly influence the rate of coke deposition. Modern catalysts produce less coke and allow operation under more severe conditions, such as those used in the cyclic and in the continuous catalyst regeneration (OCR) reformers. The different reforming technologies are briefly described later. 

CCR platformi ng A continuous catalytic regeneration process that uses platinum as the catalyst to convert straight-chain aliphatic hydrocarbons into aromatic hydrocarbons and hydrogen. It is a widely used form of the platforming process. 

A supported cobalt/molybdate catalyst, probably based on the ones developed in the 1930s, was one of the first types to be used in modem ethylene plants. The front-end reactor was located in the compressor train after heavy hydrocarbons were removed but before sulfur removal or gas drying. The catalyst was, therefore, partly sulfided. Careful temperature control was required to limit ethylene loss. About 10% steam was added to cracked gas, which limited the temperature rise and improved selectivity. An unusual feature of operation was that a significant proportion of the acetylene was removed as a polymer. This decreased the potential temperature rise but meant that catalyst regeneration and subsequent reactivation was a routine procedure at intervals of 2-4 weeks and that a spare reactor was needed. To compensate for loss of activity the gas temperature was continuously increased throughout the operating cycle. Acetylene levels were reduced to about 10-20 ppm with 1-3% ethylene loss. Up to 50% of any butadiene present in the gas was also hydrogenated. The catalyst was replaced after 1-2 years. 

In a typical fluid catalytic cracker, catalyst particles are continuously circulated from one portion of the operation to another. Figure 9 shows a schematic flow diagram of a typical unit W. Hot gas oil feed (500 -700°F) is mixed with 1250 F catalyst at the base of the riser in which the oil and catalyst residence times (from a few seconds to 1 min.) and the ratio of catalyst to the amount of oil is controlled to obtain the desired level of conversion for the product slate demand. The products are then removed from the separator while the catalyst drops back into the stripper. In the stripper adsorbed liquid hydrocarbons are steam stripped from the catalyst particles before the catalyst particles are transferred to the regenerator. 

Catalysts can become de-activated in a number of ways during the course of the operation. A common cause of de-activation is the sintering of the particles to form a continuous matte as a result of the development of local hot spots within the body of the reactor. Another is the deposition of carbon on the surface of the particles, as in the case of the fluidised cracking of hydrocarbons in the oil industry, a process described in Volume 2, Chapter 6. In this case the spent catalyst is transported to a second fluidised bed in which the carbon coating is burned off and the regenerated catalyst is then recycled. 

In the process (Figure 8-12), the feedstock is vaporized upon contacting hot regenerated catalyst at the base of the riser and lifts the catalyst into the reactor vessel separation chamber where rapid disengagement of the hydrocarbon vapors from the catalyst is accomplished by both a special solids separator and cyclones. The bulk of the cracking reactions takes place at the moment of contact and continues as the catalyst and hydrocarbons travel up the riser. The reaction products, along with a minute amount of entrained catalyst, then flow to the fractionation column. The stripped spent catalyst, deactivated with coke, flows into the Number 1 regenerator. 

There have been attempts to use catalysts in order to reduce the maximum temperature of thermal decomposition of methane. In the 1960s, Universal Oil Products Co. developed the HYPROd process for continuous production of hydrogen by catalytic decomposition of a gaseous hydrocarbon streams.15 Methane decomposition was carried out in a fluidized bed catalytic reactor from 815 to 1093°C. Supported Ni, Fe and Co catalysts (preferably Ni/Al203) were used in the process. The coked catalyst was continuously removed from the reactor to the regeneration section where carbon was burned off by air, and the regenerated catalyst returned to the reactor. Unfortunately, the system with two fluidized beds and the solids-circulation system was too complex and expensive and could not compete with the SR process. 

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