Deactivation of supported metal catalysts by carbon or coke formation is a problem of serious magnitude in steam reforming, methanation, and other important catalytic processes. Its causes are generally threefold: (1) fouling of the metal surface, (2) blockage of catalysts pores and voids, and/or (3) actual physical disintegration of the catalyst support. Since loss of catalytic activity and physical destruction of the catalyst by carbon deposits can occur rapidly (within hours or days) under unfavorable conditions, understanding and control of these effects are of major technological and economical importance.

Carbon Deposition in Steam Reforming and Methanation

The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane 

$$C_n H_m + nH_2 O \to nCO + \left( {n + \frac{m}{2}} \right)H_2 \left( { - \Delta H_{298}^0 < 0} \right) $$

(1)



$$ CO + H_2 O \rightleftarrows CO_2 + H_2 \left( { - \Delta H_{298}^0 = 41.2kJ\,mol^{ - 1} } \right) $$

(2)



$$ CO + 3H_2 \rightleftarrows CH_4 + H_2 O\left( { - \Delta H_{298}^0 = 206.2kJ\,mol^{ - 1} } \right) $$

(3)

The expression reforming is misleading since it is used also for the well-knowm process for improvement of the octane number of gasoline [1]. In the gas industry, reforming has generally been used for “the changing by heat treatment of a hydrocarbon with high heating value into a gaseous mixture of lower heating value” [2].

Catalytic Steam Reforming

Although transmission electron microscopy (TEM) has been in routine operation for over 25 years, it is somewhat surprising to find that there has only been limited use of the technique by workers in the field of catalysis. Turkevich and his co-workers [1] were among the first investigators to use the electron microscope to study catalytic substances. In general, most applications have centered around the determination of particle size distributions in supported metal catalyst systems [2]. More recently, however, it has been recognized that the technique can yield much more fundamental information on such aspects as the morphology of small particles and the nature of their interaction with a support medium. This advance is the result of the achievement of high resolution TEM (0.25 nm resolution).

In Situ Electron Microscopy Studies of Catalyst Particle Behavior

Coke formation and equipment fouling in steam cracking furnaces in the ethylene industry still remain a major operation problem. Ethylene producers have been actively seeking ways to reduce coke formation in order to achieve longer furnace run length. To date, many efforts have been made by academics and industrial personnel to understand the coking problem. Based on laboratory scale studies, various coking mechanisms have been proposed in the literature. However, these mechanisms are often reactor-related and contradictory to each other since laboratory scale reactors do not simulate the practical cracking operation in ethylene plants. Therefore, analyzing coke deposits produced in such reactors may not necessarily give results similar to those of coke produced in industrial plants. Due to this consideration, we analyzed two sets of coke sample deposits from typical locations (radiant coil, coil outlet, transfer line exchanger inlet and outlet) in steam cracking furnaces. One set of the samples was taken from an ethane cracking furnace, while the other set was from a naphtha cracking furnace. Analyses were carried out on both the process side and the metal side of the sample deposits. Scanning electron microscopy (SEM) was used for microstructure, proton induced X-ray emission spectrometry (PIXE) for elemental composition, energy dispersive X-ray spectrometry (EDX) for surface elemental composition, and micro-elemental analyzer for C and H contents. Based on our results, this paper aims to provide a closer insight into the coking problem in industrial steam crackers. In addition, the coking mechanisms proposed in the literature (pyrolytic coke, catalytic coke, droplet condensation and mechanically carried over coke) are also briefly reviewed.

Coke formation in steam crackers for ethylene production

Research has been carried out to understand the mechanisms under which coke formation occurs and to search for solutions to reduce or eliminate coke deposition. Significant efforts have been exerted over the past twenty years in developing coke-inhibiting methods. Coke inhibitors, i.e., chemical additives, or special coating of metal surfaces which suppress coke formation. Coke inhibitors/surface coating work by passivating catalytically active metal sites through chemical bonding interactions, and/or forming a thin layer to physically isolate the metal sites from coke precursors in a process stream, and/or interfering with those radical reactions leading to coke formation by blocking active radical sites on surfaces. This review paper investigates the effect of the most important methods such as new chemical additives, tube material construction, pretreatment of surface on the coke formation of pyrolysis furnaces.

J. Ducret

Papers

Mechanism of carbon formation during steamcracking of hydrocarbons