Formation, coke

Due to the presence of a large excess of hydrogen, coke formation in hydroprocessing units is slow - so slow that it can be neglected in material balances for commercial units. However, coke formation is one of four major causes of catalyst deactivation. The other three are poisoning, fouling, and sintering (see Section 4.1). 

Consequently, there are significant differences in FCC unit operation when residue is added to normal feed. Conversion falls and less gasoline is produced, as shown in Table 5.2, and the catalyst-to-oil ratio must rise as coke yields increase. The coke also has a different composition relative to that produced from normal feed not only because of the higher Conradson carbon levels and high-boiling compounds, which are absoibed by the catalyst particles, but also from the dehydrogenation activity of the metal impurities, which leads to polymerization reactions and contaminant coke formation. 

The heat balance in an FCC rrrrit is corrrplex and depends on the combustion of coke in the regenerator. Coke formation on the catalyst must be carefuUy controlled when the feeds corttairrs residue, hrrprrrities such as orgartic rtickel, varta-dium corrtpotrrtds, and Cortradson carbon lead to increased coke deposihon artd this affects the rest of the rrrrit It is necessary to passivate the metals with additives arrd dilute or hydrotreat the residue. 

The cracking reaction is errdotherrrric arrd so requires an irrput of heat. This heat is provided by combrrstion of residrral coke on the catalyst in the regenerator. In a heat-balanced rrrrit, the level of coke deposihon is controlled so that the 

TABLE 5.12. Distribution of Delta Coke with Gas Oil and Residue Feeds. 

Only part of the eoke on spent catalyst is burnt in the regenerator. The difference between the amount of coke on spent and regenerated catalyst is referred to as delta coke (A eoke) and is usually expressed as a percentage. At steady state, this equates to the overall amount of eoke formed per pass. Coke yield is the percentage of feed that is converted to eoke. A usefiil measure of A coke is the coke yield divided by the corresponding catalyst to oil ratio. 

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Laboratory studies indicate that a hydrogen-toluene ratio of 5 at the reactor inlet is required to prevent excessive coke formation in the reactor. Even with a large excess of hydrogen, the toluene cannot be forced to complete conversion. The laboratory studies indicate that the selectivity (i.e., fraction of toluene reacted which is converted to benzene) is related to the conversion (i.e., fraction of toluene fed which is reacted) according to ... 

In the refinery the salts deposit in the tubes of exchangers and reduce heat transfer, while in heater tubes, hot spots are created favoring coke formation. 

Nickel catalysts are also used for steam methane reforming. Moreover, nickel catalysts containing potassium to inhibit coke formation from feedstocks such as LPG and naphtha have received wide appHcation. 

Zeolite-Based All lation. Zeohtes have the obvious advantages of being noncorrosive and environmentally benign. They have been extensively researched as catalysts for ethylbenzene synthesis. Eadier efforts were unsuccessful because the catalysts did not have sufficient selectivity and activity and were susceptible to rapid coke formation and deactivation. 

Propane [74-98-6] and heavier paraffins tend to form undesired products, although reaction conditions can be controlled to minimize coke formation (48). Propylene [115-07-1] can be used in a properly designed system (45,46) ... 

Reaction Conditions. Typical iadustrial practice of this reaction involves mixing vapor-phase propylene and vapor-phase chlorine in a static mixer, foEowed immediately by passing the admixed reactants into a reactor vessel that operates at 69—240 kPa (10—35 psig) and permits virtual complete chlorine conversion, which requires 1—4 s residence time. The overaE reactions are aE highly exothermic and as the reaction proceeds, usuaEy adiabaticaEy, the temperature rises. OptimaEy, the reaction temperature should not exceed 510°C since, above this temperature, pyrolysis of the chlorinated hydrocarbons results in decreased yield and excessive coke formation (27). 

As for oil and gas, the burner is the principal device required to successfully fire pulverized coal. The two primary types of pulverized-coal burners are circular concentric and vertical jet-nozzle array burners. Circular concentric burners are the most modem and employ swid flow to promote mixing and to improve flame stabiUty. Circular burners can be single or dual register. The latter type was designed and developed for NO reduction. Either one of these burner types can be equipped to fire any combination of the three principal fuels, ie, coal, oil and gas. However, firing pulverized coal with oil in the same burner should be restricted to short emergency periods because of possible coke formation on the pulverized-coal element (71,72). 

In single-stage units which do not produce kerosene or other critical stocks, flash zone temperatures may be as high as 750 - 775 F. The principal limitation is the point at which cracking of distillates to less valuable gas or the rate of coke formation in the furnace tubes becomes excessive. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Engelhard Corporation, The Chemistry of FCC Coke Formation, The Catalyst Report, Vol. 7, Issue 2. 

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. 

Certain catalyst properties appear to increase coke formation. Catalysts with high rare earth content tend to promote hydrogen transfer reactions. Hydrogen transfer reactions are bimolecular reactions that can produce multi-ring aromatics. 

Damaged or partially plugged feed nozzles can contribute to coke formation due to poor feed atomization. 

Damaged shed-trays in the bottom section of the main cloumn can cause coke formation due to non-uniform contact between upflowing vapors and downflowing liquid. 

Example 11.15 Coke formation is a major cause of catalyst deactivation. Decoking is accomplished by periodic oxidations in air. Consider a micro-porous catalyst that has its internal surface covered with a uniform layer of coke. Suppose that the decoking reaction is stopped short of completion. What is the distribution of residual coke under the following circumstances ... 

Interestingly, one can easily deduce an expression for the relative rate of coke formation as compared to that of methanation. The rate of initial coke formation depends on the combination probability of carbon atoms and hence is given by... 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

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