Gas phase coke

How do the amounts and types of coke deposited on the various metal surfaces vary as a function of time In the present investigation, the resulting coke was obtained during 120-min runs. In the future, shorter and longer runs are needed to determine the kinetics of coke formation and to determine whether one type of coke is a precursor for another type. Possibly both filament and needle cokes act to some extent as a filter for gas phase coke to form eventually amorphous or knobby coke in which metal-containing coke is eventually covered with metal-free coke. 

Farther investigations now in progress will clarify further the key factors that affect the formation of catalytically-formed coke and the collection or deposition of gas-phase coke or tar droplets on solid surfaces. 

The most important indusuial reaction of this kind occurs in die ironmaking blast furnace in which iron oxide ore is reduced by carbon in the form of coke. The mixture is heated by the combustion of part of the coke input in air to produce temperatures as high as 2000 K. The reduction reaction is caiTied out via the gas phase by the reaction... 

The kinetics of this reaction, which can also be regarded as an erosion reaction, shows die effects of adsorption of the reaction product in retarding the reaction rate. The path of this reaction involves the adsorption of an oxygen atom donated by a carbon dioxide molecule on die surface of the coke to leave a carbon monoxide molecule in the gas phase. 

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. 

Some deactivation processes are reversible. Deactivation by physical adsorption occurs whenever there is a gas-phase impurity that is below its critical point. It can be reversed by eliminating the impurity from the feed stream. This form of deactivation is better modeled using a site-competition model that includes the impurities—e.g., any of Equations (10.18)-(10.21)— rather than using the effectiveness factor. Water may be included in the reaction mixture so that the water-gas shift reaction will minimize the formation of coke. Off-line decoking can be... 

The "conventional" methods for the preparation of SiC and Si3N4, the high temperature reaction of fine grade sand and coke (with additions of sawdust and NaCl) in an electric furnace (the Acheson process) for the former and usually the direct nitridation of elemental silicon or the reaction of silicon tetrachloride with ammonia (in the gas phase or in solution) for the latter, do not involve soluble or fusible intermediates. For many applications of these materials this is not necessarily a disadvantage (e.g., for the application of SiC as an abrasive), but for some of the more recent desired applications soluble or fusible (i.e., proces-sable) intermediates are required. 

The cracking and the low-temperature oxidation of crude oils have been studied previously in order to simulate the thermal transformations of oil to gas and coke during enhanced oil recovery (1-6). Other authors characterize the thermal modifications of oil in the presence of a vapor phase (7)-... 

To minimize coking, the reactor may be operated at short residence times, or hydrogen may be added to the process stream to convert gas-phase carbon into methane. It is also advantageous to minimize the temperature upstream of the catalyst bed, since gas-phase carbon is less readily formed at low temperatures. 

In earlier sections of this chapter, the role that particulates play in a given environmental scenario was identified. This section will be devoted exclusively to combustion-generated particulates whose main constituent is carbon. Those carbonaceous particulates that form from gas-phase processes are generally referred to as soot, and those that develop from pyrolysis of liquid hydrocarbon fuels are generally referred to as coke or cenospheres. 

A major part of the fast coke is probably desorbed from the catalyst bed and burned in the gas phase. Even if none of the fast coke was desorbed, a calculation of the Thiele modulus tj for conditions in the plume burner and the top of the first zone of the kiln shows that rj is in the range 0.92-0.99. Thus, the fast coke can be assumed to bum without significant diffusion limitation. 

The most successful New Demet preparation, A, showed a MAT conversion of 77.1% compared to 65.2% for the untreated equilibrium catalyst. This increase in conversion was accompanied by an increase in gasoline yield from 6.8 to 52.3% and a decrease in coke yield from. 5 to. 3%. These results were achieved by subjecting the catalyst to a series of treatments after the basic gas phase reactions, involving oxidative and reductive washes, and ion exchange with NHz + and RE (see Fig. 1). Each of these treatments resulted in a successive improvement of catalyst performance. 

This reactor contains at least two solid phases, two Hquid phases, and a gas phase. The flows are largely driven by gravity caused by the density differences of the soHd and Hquid phases. Taconite and coke are admitted at the top of the reactor and O2 at the bottom. Liquid Fe and slag are withdrawn at the bottom of the reactor. The Hquid iron is either cast into ingots in molds or directly passed from the reactor through rolling mills to process it into sheets. 

As discussed previously, the rate of coke buildup can be related to the concentration of components in the gas phase [Eq. (26)]. An example for the coking of ring isomerization sites by cyclohexane intermediates is... 

The process in question involved the reaction of two materials, A and B, to produce a product C. The reaction was noncatalytic, homogeneous, and in the gas phase. It took place in a tubular reactor which could not be considered either adiabatic or isothermal. The reactor was divided into four sections, the first three of which were cooled while the fourth was adiabatic. Coking of the reactor tube introduced a time variant in the system, requiring adjustment of operating conditions and eventual shutdown for cleaning. 

Thermal dehydrochlorination of 1,2-dichloroethane188-190 272 273 takes place at temperatures above 450°C and at pressures about 25-30 atm. A gas-phase free-radical chain reaction with chlorine radical as the chain-transfer agent is operative. Careful purification of 1,2-dichloroethane is required to get high-purity vinyl chloride. Numerous byproducts and coke are produced in the process. The amount of these increases with increasing conversion and temperature. Conversion levels, therefore, are kept at about 50-60%. Vinyl chloride selectivities in the range of 93-96% are usually achieved. 

However, for some small molecules the temperature might be increased so far that the liquid substrate evaporates under the reaction conditions used. This results in a change from a gas - liquid process into a gas-phase process. This will increase the reaction rate greatly (see, e.g., the hydrogenation of cyclohexene to cyclohexane [15]). If the coke formation and the formation of side-products can be neglected, very good results can be achieved with strongly increased temperature. 

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