Coke depositions

It has been indicated by several investigators that strong acids rather than weak acids, Lewis acids rather than Bronsted acids favor coke formation, and that the presence of transition metal ions as impurities, e.g., Fe and Ni ions, accelerate the formation of coke. It was reported that coke formation on a nonacidic silica was less than one-twentieth that on acidic silica —alumina. Coke deposition became less serious as the strong acid sites of silica —alumina were weakened by NaOH treatments. Coke formation from hydrocarbons is usually less serious in the case of solid bases. It is reported that the deactivation of MgO and Li/MgO for methane coupling was due to sintering and loss of alkali.  

Deactivation of M0O3 - AI2O3 in the hydrotreatment of coal-derived oil and heavy petroleum is mainly due to the deposits of carbonaceous matter and metals coming 

The rate of coke formation usually follows a phenomenological equation of the 

In addition to coke formation catalyzed by acids, heavy coke formation is observed in the reaction of methanation, Fisher-Tropsch synthesis, and steam reforming over metallic catalysts such as Fe and Ni. °  

---

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

Coke (deposited on the catalyst) which is burned in the regenerator producing energy (electricity, steam) and the necessary heat for the reaction. Produced gases are cleansed when necessary of SOj and NO as well as particles of entrained catalyst. 

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 that do not contain potassium lose activity very quickly because of coke deposition on the surface of the catalyst. Chemical changes that occur when the catalyst is removed from the operating environment make it very difficult to determine the nature of most of the promoter elements during the reaction, but potassium is always found to be present as potassium carbonate in the used catalyst. The other promoters are claimed to increase selectivity and the operating stabiUty of the catalyst. 

Coke deposition is essentially independent of space velocity. These observations, which were developed from the study of amorphous catalysts during the early days of catalytic cracking (11), stiU characteri2e the coking of modem day 2eohte FCC catalysts over a wide range of hydrogen-transfer (H-transfer) capabihties. 

The rate of coke burning for coke deposited on a zeolite-containing catalyst has been reported to be first order with respect both to coke concentration and oxygen partial pressure (23) ... 

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 %. 

Regeneration of noble metal catalysts to remove coke deposits can successfully restore the activity, selectivity, and stabiUty performance of the original fresh catalyst (6—17). The basic steps of regeneration are carbon bum, oxidation, and reduction. Controlling each step of the regeneration procedure is important if permanent catalyst damage is to be avoided. 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

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. 

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. 

A higher steam/hydrocarhon ratio favors olefin formation. Steam reduces the partial pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke deposition in the furnace tubes. Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between 0.2-1 for ethane and approximately 1-1.2 for liquid feeds. 

Feed/catalyst injection. A well-designed injection system provides a rapid and uniform vaporization of the liquid feed. This will lower delta coke by minimizing non-catalytic coke deposition as well as reducing the deposits of heavy material on the catalyst. 

Feed injection. An improved feed injection system provides optimum atomization and distribution of the feed for rapid mixing and complete vaporization. The benefits of improved feed injection aie reduced coke deposition, reduced dry gas yield, and improved gasoline yield. 

Coke deposition in areas of high heat flux as a result of the oil charring... 

Oil, fat, grease, and hydrocarbon contamination results in Film boiling Coke deposition Contaminant binding Sludging and gunk balls... 

Hydrocarbons and carbonized or coke deposits can be removed by chromic acid. The chromic acid oxidizes the binders holding the deposits together. Use a 10 to 20% solution for 12 to 24 hours at 190 to 200 °F. Chromic acid cannot be effectively inhibited and is not suitable for cleaning copper, brass, aluminum, zinc, or cast iron because these are all rapidly attacked. 

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. 

VI. Incomplete calcination of the metal catalyst electrode, resulting to coke deposition on the catalyst-electrode surface. 

Last but not least, one should check the inertness of the auxiliary electrodes in single-pellet arrangements, both under open and closed circuit conditions and, also, via the closure of the carbon balance, the appearance of coke deposition. This is especially important in systems with a variety of products (e.g. selective oxidations), where the exact value of selectivity towards specific products is of key interest. This in turn points out the importance of the use of a good analytical system and of its careful calibration. 

TEM-EDS and XPS analyses were conducted on Co/MgO catalysts. The results of surface analyses showed that Co metal is not supported on the MgO as particles, but covers MgO surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. After the reduction of catalyst at 1173 K, both cobalt oxide and CoO-MgO solid solution are observed on the surface of catalyst. In the steam reforming of naphthalene, two types of coke deposited on the surface of catalyst are observed. These are assigned to film-like and graphite type carbon by TPO analysis. 

As illustrated in Fig. 1, the activated carbon displays the highest conversion and selectivity among all the catalysts during the initial reaction period, however, its catalytic activity continues to decrease during the reaction, which is probably caused by coke deposition in the micropores. By contrast, the reaction over the CNF composites treated in air and HN03 can reach a pseudo-steady state after about 200 min. Similiar transient state is also observed on the CNFs and the untreated composite. Table 3 collects the kinetic results after 300 min on stream over catalysts tested for the ODE, in which the activity is referred to the BET surface area. The air-treated composite gives the highest conversion and styrene selectivity at steady state. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

---