Temperature coke deposits

The coking of steam reforming catalysts is a well recognised problem which has received much attention [3,4,5,6]. At high temperatures coke deposition may result from reactions on the catalyst or in the gas phase [7], although the latter are less common in the steam reforming situation. 

High temperature alloys, such as HK alloys and HP alloys, are normally selected for the radiant tubes in the ethylene crackers. High temperature coke deposition and carburization are the main factors responsible for the failure of radiant tubes. Four techniques are commonly used to increase coke deposition resistance ... 

This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

The cracked products leave as overhead materials, and coke deposits form on the inner surface of the dmm. To provide continuous operation, two dmms are used while one dmm is on-stream, the one off-stream is being cleaned, steamed, water-cooled, and decoked in the same time interval. The temperature in the coke dmm is in the range of 415—450°C with pressures in the range of 103—621 kPa (15—90 psi). Overhead products go to the fractionator, where naphtha and heating oil fractions are recovered. The nonvolatile material is combined with preheated fresh feed and returned to the furnace. The coke dmm is usually on stream for about 24 hours before becoming filled with porous coke, after which the coke is removed hydraulically. 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

The specific rate is expected to have an Arrhenius dependence on temperature. Deactivation by coke deposition in cracking processes apparently has this kind of correlation. 

The separation of n-alkanes from a kerosene or gas oil fraction by a molecular sieve can be performed in a liquid phase or in a gas phase process. In the gas phase processes there are no problems of cleaning the loaded molecular sieve from adherent branched and cyclic hydrocarbons. However, the high reaction temperature of the gas phase processes leads to the development of coke-contaminated sieves, which have to be regenerated from time to time by a careful burning off of the coke deposits. 

The present characterization studies have motivated us to investigate in the future the optimum edeination and reduction temperatures to maximize the isomerization pathway, while keeping coke deposition and hydrogenolysis to a minimum. Our results suggest that compromises are expected to be made to achieve those goals. 

Coke builds up on the catalyst since the start up of operation. In the first weeks of operation, an amount between 5% and 8% of coke accumulates on the catalyst. The rate of deposition decreases with time on stream, a careful monitoring of temperature and of feed/H2 ratio is the basis for controlling deposition. Coke deposition primarily affects the hydrogenation reactions (and so denitrogenation), but the deposition rate determines the catalyst life. As mentioned above, deactivation is compensated by an increase in temperature (and some times in pressure, when denitrogenation has to be adjusted, as well). However, increasing severity, increases coke deposition and shorten catalyst life. 

After the cracking run is complete, the coked catalyst is regenerated by passing air saturated with water at room temperature over the catalyst at an elevated temperature (1250° F). The amount of coke deposited on the catalyst is determined by the difference in reactor weight before and after the regeneration. 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

Water and organic molecules occluded during the synthesis were removed from the intracrystalline volume as follows. The solids were slowly heated (5°C/min) in a N2 flow up to 550°C and held at this temperature for 2h. The coke deposit resulting from the non-oxidative degradation of the organics was then... 

Application of the IR method proved to be also suitable for the measurement of diffusivities in coking porous catalysts. This was deihonstrated by uptake experiments with ethylbenzene where the sorbent catalyst, H-ZSM-5, was intermittently coked in-situ via dealkylation of ethylbenzene at temperatures (465 K) somewhat higher than the sorption temperature (395 K). Coke deposition was monitored in-situ via the IR absorbance... 

During the cracking process, carbon deposits or coke build up on the spent catalyst particles. These deposits can deactivate the catalyst performance and must be removed. This is typically accomplished in two stages. First, the catalyst collected at the bottom of the reactor is steam stripped to remove residual hydrocarbon. The stripped catalyst then passes into the regenerator and is heated with air to temperatures as high as 1,100°F to 1,200°F (539.3°C to 648.9°C). At these temperatures, coke bums off of the catalyst making it ready for reuse within the FCC unit. See FIGURE 2-5. 

As already discussed, if a catalyst is deactivated via coking, it could be regenerated so that its initial catalytic activity is restored. The treatment at high temperatures in an oxygen-rich atmosphere can burn off the coke deposited and the catalyst may regain its activity. Moreover, if the reduced activity is a temporary event caused by an inhibitor, die removal of the inhibiting substance of the feed can restore the catalyst to its initial potential. 

The maximum temperature that can be used for regeneration is limited by the thermal stability of the catalyst. Figure 9 shows the temperature stability of natural and synthetic catalysts in bone-dry air. In commercial units these catalysts are regenerated at substantially lower temperature because the presence of steam reduces the thermal stability limit. The steam is produced from the combustion of the hydrogen in the coke deposit on the catalyst. 

Reduction in activity of the catalyst may be caused by polymer and coke deposition due to excessive temperature, low pressure, insufficient water in the feed, or too low a space velocity (25). The presence of diolefins, oxygen, caustic, or nitrogen bases, such as ammonia or amines in the feed, also causes loss of catalyst activity. 

---