Coke deposit oxidation rates

It is desirable to operate the molecular sieve bed in the adsorption mode at the same temperature as in the desorption mode. Sista and Srivastava (16) show that temperatures in excess of 533 K are needed to desorb by vacuum C12 to C32 n-paraffins from type 5A molecular sieves at a pressure of 13 Pa (0.1 mm Hg). Only 5% of 2-C32 is removed at 636 K. Asher e al. (14) show that, whereas it is possible to remove 98% of Cg/C g n-paraffins from type 5A molecular sieves with ammonia at 589 K, only 79% removal is attained with C15/C33 n-paraffins even though the temperature is higher (658 K). Some of the retained material over a long period of exposure to high temperature gradually forms a carbonaceous deposit which reduces the adsorption capacity of the molecular sieve this coke deposit must be occasionally removed by a controlled oxidation step which eventually reduces molecular sieve life. Desorption rates increase with... 

The experimental results are presented in Figure 3. In this figure the amount of coke deposited on the catalyst has been plotted versus the volume of methane (at STP) fed into the reactor. Both absolute amounts of coke on a 5 g charge of catalyst, and percent coke by weight are reported. Since the feed flow rate of methane was maintained constant at 0.31 liters (STP)/min, the abscissa also represents time. Each point on Figure 3 represents an experimental run of approximately 12-15 hours duration including the reduction and subsequent oxidation of the catalyst. 

The H/C ratio of the coke deposits was quantified by temperature programmed oxidation (TPO) in a 1 % oxygen helium mixture. Temperature was raised to 850° C at a heating rate of 10° min 1. The calculations of the H/C ratio involved the results from the measurements of carbon dioxide production and oxygen uptake (according to Ref. [8]). Coke deposits were also characterized by thermogravimetry and transmission electron microscopy. 

An analysis of the rate of CO, CO2 and H2O evolution during TPO of industrial and laboratory coked cracking catalysts has provided information on the mechanism and energetics of coke combustion. The mechanism has been deduced from previously reported studies on amorphous carbon oxidation [8], while rate parameters have been calculated from non-linear regression simulations of the TPO spectra. The rate of water vapour formation has not been analysed due to re-adsorption expected to affect the apparent kinetics. "Soft" and "hard" coke have been identified in the spectra, and oxidation activation energies of each compared. A further outcome of this work is the proposal that coke deposition on cracking catalysts proceeds from "soft" to "hard" coke via a series of dehydrogenation or dehydration steps. 

Carbonaceous deposition during steam cracking is the net result of steady state formation and removal processes. If the measured oxidation rates in water vapour did represent the removal of the deposit in situ, then this would be an extremely rapid process over the temperature range at which deposition on radiantly heated process tubes is most significant. Thus, 1 mm thickness of deposit would be oxidised by 362 mm Hg water partial pressure in 300 h at 800°C, 33 h at 900°C and 5 h at 1000°C. If a hydrocarbon, or its decomposition products, enhanced the oxidation rate these times could be decreased. Coke removal by thermal oxidation cannot be ignored, therefore, although its extent would depend on specific plant operating conditions. 

The steam-carbon reaction is known to be catalyzed by metals, particularly transition metals (3,4.). In an effort to improve the rate of gasification, separate samples of the silica-alumina (Durabead) catalyst were impregnated with one of various metals prior to coke deposition, and the results for the subsequent steam-carbon reaction at 1500°F over these materials are shown in Figure 2 and Table IV. The effects of the deposited metal oxides can be summarized as follows ... 

Tests with steam were made using mixtures of essentially 50 steam and the remainder helium or nitrogen. Such mixtures have a steam content similar to steam-hydrocarbon mixtures used as feedstocks in many pyrolysis units. For reactors that had no coke deposits on their surface, the rate of oxidation of the surface can be calculated from the rate of hydrogen formation. When coke was present on the walls of the reactor, the coke was also oxidized by the steam as follows ... 

It has been shown [440] [518] for coke deposits on cracking catalysts that the reactivity after oxidation depends mainly on the surface area of the coke and that the rate quickly becomes diffusion limited with a risk of overheating the catalyst pellet. In practice, the bum-off of coke can easily be performed by adding a few percent of air to the steam flow at temperatures above approximately 450°C as illustrated in Figure 5.38 [388] [389]. 

Examples of the application of Eq. (3.3.17) to spherical particles include the work of Carter [9] for the oxidation of nickel, Kawasaki et al. [10] for the reduction of iron oxides, and Weisz and Goodwin [11] for the combustion of coke deposits on catalysts. In the latter two cases, the solid was initially a porous pellet when diffusion controls the overall rate, however, the relationships above may be used. We will discuss this in more detail when the system of a porous reactant solid is presented. Many studies on the oxidation of metals have been made in one-dimensional geometries [12,13]—hence the term parabolic law for the rate of progress of such oxidation reactions under conditions of diffusion control [see Eq. (3.3.14)]. Hutchins [14] has verified Eq. (3.3.16) experimentally using a cylindrical system. 

Using a "home made" aneroid calorimeter, we have measured rates of production of heat and thence rates of oxidation of Athabasca bitumen under nearly isothermal conditions in the temperature range 155-320°C. Results of these kinetic measurements, supported by chemical analyses, mass balances, and fuel-energy relationships, indicate that there are two principal classes of oxidation reactions in the specified temperature region. At temperatures much lc er than 285°C, the principal reactions of oxygen with Athabasca bitumen lead to deposition of "fuel" or coke. At temperatures much higher than 285°C, the principal oxidation reactions lead to formation of carbon oxides and water. We have fitted an overall mathematical model (related to the factorial design of the experiments) to the kinetic results, and have also developed a "two reaction chemical model". 

Catalytic superactivity of electron-deficient Pd for neopentane conversion was recently verified for Pd/NaHY (157, 170). The reaction rate was positively correlated with the proton content of the catalyst. Samples that contained all the protons generated during H2 reduction of the catalysts were two orders of magnitude more active than silica-supported Pd. Samples prepared by reduction of Pd(NH3)2+NaY displayed on intermediate activity. It was suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaY samples deactivated rapidly and coke was deposited. Two types of coke were found (by temperature-programmed oxidation), one of... 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

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