Mechanism Of Coke Formation Research Articles

Carbonization in the tube bomb leading to needle coke: III. Carbonization properties of several coal-tar pitches

Carbonization properties of several coal-tar pitches of different origins were studied, by using tube bomb, to correlate their analytical characteristics with their carbonization properties and to find their respective optimum carbonization conditions for the production of excellent needle coke. The pitch rich in naphthenic structure produced an excellent needle coke of low CTE and uni-axially arranged flow texture under a wider range of carbonization conditions, whereas the pitches of high oxygen and alkyl contents or of very high aromaticity gave cokes of larger CTE under the standard carbonization conditions of 500 °C, 8 kg/cm 2, because of more mosaic texture or poor uni-axial orientation in the resultant cokes. However, the two kinds of poor pitches were found to produce better cokes under their respective appropriate conditions. The former one did at a slightly lower carbonization temperature of 480 °C and the latter one did under a lower carbonization pressure of 4 kg/cm 2. A lower temperature allowed the development of bulk mesophase through the moderation of carbonization reactions for the former reactive pitch. The lower pressure improved uni-axial arrangement of mesophase molecules at the solidification stage through the sufficient gas evolution of good timing for the latter highly aromatic pitch. The higher pressure of carbonization delayed the solidification to be off-timing to the gas evolution. The mechanism of needle coke formation was discussed to explain the respective optimum conditions for the pitches of different analytical characteristics.

On the mechanism of coke formation in steam cracking—conclusions from results obtained by tracer experiments

The coke formation from 68 14C-labeled hydrocarbons, each used in small amounts as a constituent of a straight-run naphtha, was systematically investigated at 810°C. Though in line with the pertinent literature the coking rates on Cr-Ni steel are much higher than on quartz or platinum, no significant differences could be found in the selectivity of coke formation on surfaces of different materials. This surprising observation indicates that surface properties obviously control only the number and not the nature of active centers responsible for coke formation on the materials from gas phase hydrocarbon species.

The modeling of the kinetics of deactivation of a commercial hydrocracking catalyst in the reaction of cumene disproportionation

The mechanism of coke formation on a commercial hydrocracking catalyst is studied. The data by R. P. L. Absil J. B. Butt, and J. B. Dranoff ( J. Catal. 85, 415, 1984) for cumene disproportionation have been analyzed using the theory and developments of J. Corella and J. M. Asua ( Ind. Eng. Chem. Process Des. Dev. 21, 55, 1982). Several coke formation mechanisms are presented and their Langmuir-Hinshelwood kinetic equations are deduced. Only the mechanisms with two active sites in the controlling step or with deactivation order 1.5 fit the data. The chemical mechanism of coke formation in this process is presented and a reasonable mechanistic explanation for the empirical deactivation kinetic equations given previously is presented.

Rigorous kinetic models for catalyst deactivation by coke deposition: Application to butene dehydrogenation

Recently developed models describing catalyst deactivation in terms of coverage of active sites and blockage of pores are extended and applied to published data on the dehydrogenation of butene on a chromia-alumina catalyst. The textural and catalytic properties of the catalyst are determined by classical physicochemical techniques. Regression of the coke versus time-on-stream curves allows the estimation of fundamental properties of the coke such as molecular mass and enables an insight into the mechanism of coke formation to be obtained. The best regression results are obtained by considering two stages in the coke formation sequence: there is a first stage of site coverage by coke precursor accompanied by instantaneous growth up to an intermediate size and a second stage of further growth at a rate of the same order of magnitude as the rate of coke formation corresponding to the first stage. Some 15% of the catalyst surface becomes inaccessible by blockage of the smallest pores. The deactivation of the dehydrogenation itself is predicted in a satisfactory way by using the parameter estimates derived from the coking data only.

The microscopy of mesophase formation and of anisotropic cokes produced from solvent‐refined coals

SUMMARYThermal processing of coal and coal products can lead to the formation of ordered carbons through the mesophase mechanism. The performance characteristics of metallurgical coke may be improved by the extent of this ordering; however, in the direct hydroliquefaction of coals the objective is to make species of low molecular weight, and coking is an undesirable retrograde reaction. This study is concerned with fundamental mechanisms of coke formation and their roles in these two processes.Most studies of mesophase formation have involved the heat treating of samples followed by quenching and examination at room temperature. This project has used hot‐stage microscopy and the production of a 16‐mm colour film to study the dynamics of mesophase growth and coalescence.Various solvent‐refined coal (SRC) and SRC fractions were carbonized in gold tubes at 450°C and 34.5 MPa, and the pyridine‐insoluble fractions (semi‐coke) examined microscopically. Greater molecular mobility, manifested by lower proportions of anisotropic carbon and larger domain sizes, was evident for SRCs which had undergone more extensive hydrogenation at the pilot plant, and for SRC fractions of lower functionality.The performance of metallurgical coke in the blast furnace can be related to its inherent carbonaceous structure. Coals which give cokes of marginal quality include low‐rank bituminous coals which form an optically isotropic coke. One method of improving the carbon ordering in these cokes is the use of a carbonizing additive. In this study SRC was used as the additive, giving improvements in the coke structure and microstructure.

Science and technology of graphite manufacture

This review summarizes current descriptions of the science and technology of graphite manufacture. The delayed coking process is outlined and mechanisms of coke formation put forward including considerations of the composition of feedstocks. As graphite manufacture uses calcined delayed cokes of which the most valuable is needle-coke, a comparison of rotary kiln, rotary hearth and vertical shaft calciners is made. Grinding and sizing of calcined coke is important as this with the binder pitch leads to baked carbon. The stages in the manufacturing process are described as mixing, forming, extrusion, moulding, baking, impregnation and graphitization. The more advanced processing technology includes isostatic hot pressing, artifact densifications, high pressure impregnation/baking and binderless graphitization. A comprehensive list of references is included.

Coke formation during cracking of hydrocarbons. I. The effect of pre‐sulphiding on coke formation on a nickel‐chromium‐iron alloy under steam cracking conditions

AbstractStudies have been carried out on the effect of pre‐sulphiding a nickel‐chromium‐iron alloy on coke formation under steam cracking conditions. Pre‐sulphiding has no great effect on the gaseous product spectrum as long as some sulphur is present, but the coking rate is dependent on the amount of sulphur pre‐deposited. Small amounts of sulphur reduce the coke formation, but the coking rate increases with increasing amounts of pre‐deposited sulphur. As the sulphur content increases still further, coke formation passes through a maximum. A tentative explanation of these observations is advanced in terms of the effect of metal sulphides on the established mechanism of coke formation. Initial sulphiding is suggested to increase grain boundary penetration by coke and to result in the accumulation of iron sulphide on the surface. Eventually this may act to seal off grain boundaries, thereby inhibiting further coke formation.

Mechanism of coke formation from liquid phase

Mechanism of coke formation from liquid phase

Parallel Reaction Mechanism for Benzene and Coke Formation in the Dehydrogenation Reaction of Cyclohexane on Alumina-Molybdena Catalysts

酸化モリブデンをアルミナに担持させた触媒体を用い,流通法でシクロヘキサンの脱水素反応を行なって,べンゼンおよびコークの生成速度の経時変化を測定した。酸化モリブデン担持量fは0.073-0.243g/g-carr.の範囲でいくつか変えた。各活性点上のコーク粒子の数をPoissonの確率分布関数で表わし,コーク沈積量と被毒されずに残っている活性点の数との関係を導いた。また活性点の作用はベンゼンおよびコークの双方の生成に関して有効であり,反応経時0における活性点の数は担持量fに比例するものとして,当該反応系の全反応機構を総合的に検討した。その結果ベンゼンの生成速度は被毒されずに残っている活性点の濃度およびシクロヘキサン分圧に比例すること,またコーク生成速度は残っている活性点濃度の二乗に比例し,シクロヘキサン分圧には無関係であることが実験的に確かめられた。ベンゼンとコークの生成に関して並列反応機構を提出することによって,経時変化を含めた速度論的なすべての実験結果を説明することができた。

Investigation of the mechanism of coke formation during thermal decomposition of asphaltenes

Investigation of the mechanism of coke formation during thermal decomposition of asphaltenes

Manufacturing Process of Petroleum Coke

Description is made on delayed coking process and fluid coking process for mass production of petroleum coke.Qualities and properties of petroleum coke (i. e. graphitizability, density, strength etc.) are strongly influenced by operating conditions (carbonization temperature, pressure, rate of heating) and constitutions of starting oils.The mechanisms of coke formation are also discussed.

PERFORMANCE OF FIXED BED REACTORS WITH CATALYST FOULING

In many chemical processes such as cracking and dehydrogenation of hydrocarbons on solid catalyst, the activity of catalyst gradually decreases as the reaction proceeds. These decreases are often caused by deposition of a carbonaceous substance (coke) which is formed by reactions parallel or consecutive to the main reaction. Fouled catalyst may be regenerated by burning off the carbonaceous product, usually with air. In this paper, the mechanisms of coke formation are studied for dehydrogenation of n-butane over alumina-chromia catalyst, and the rate of reaction and of catalyst fouling are determined. The effects of catalyst fouling on the overall efficiency of fixed bed reactors are analyzed with an analogue computer. The concept of the average activity (the fraction of effective catalyst to the total amount of catalyst) is proposed as a satisfactory index in deciding the end point of the reaction period for regeneration.

Mechanism of coke formation in the cracking of component groups in petroleum residues

Mechanism of coke formation in the cracking of component groups in petroleum residues

The mechanism of coke formation

The mechanism of coke formation