Coke formation limitation

In single-stage units which do not produce kerosene or other critical stocks, flash zone temperatures may be as high as 750 - 775 F. The principal limitation is the point at which cracking of distillates to less valuable gas or the rate of coke formation in the furnace tubes becomes excessive. 

Deactivation of supported metal catalysts by carbon or coke formation, which has its origin in the CH4 dissociation and/or CO disproportionation, is the most serious problem hindering the application of the C02 reforming of methane. Attempts to overcome this limitation have focused on the development of improved catalysts. 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

Employing process conditions similar to those used for steam reforming of natural gas (e.g. fixed-bed reactors, temperatures in the 800-900 °C range) has been demonstrated to be inadequate for processing thermally unstable biomass liquids [29]. The most important problem is represented by coke formation, especially in the upper layer of the catalyst bed and in the reactor freeboard, that limits the operation time (e.g. 3—4 h on commercial Ni-based catalysts) and requires a long regeneration process for the catalyst (e.g. 6-8 h on commercial Ni-based catalysts). 

A differential pressure sensor monitors the pressure drop across the reactor, giving also an indication of the coke formation. The outside shell of the reactor is thermally insulated to limit heat loss. 

The aim of this study is to convert as much as the hydrogen in the fuel into hydrogen gas while decreasing CO and CH4 formation. Process parameters of fuel preparation steps have been determined considering the limitations set by the catalysts and hydrocarbons involved. Lower S/C (steam to carbon) ratios favor soot and coke formation, which is not desired in catalytic steam and autothermal reforming processes. A considerably wide S/C ratio range has been selected to see the effect on hydrogen yield and CO formation. 

The hydrogen content in the gas influences the coke formation rate [7] as well. In a recent article Hou et al. [19] showed the influence of the removal of hydrogen on coking rates in a membrane steam reformer using palladium membranes. The need of a minimum concentration of hydrogen is of special importance when operating a membrane steam reformer, because it limits the process conditions at which such a reactor can be operated. 

It is also of interest to observe that the coke laydown observed experimentally is more than an order of magnitude, less than would be predicted from equilibrium calculations. That is, the amount of coke on the catalyst per liter of methane on Figure 4 at a given temperature and steam methane ratio is about 5% of that shown on Figure 2 formed under equilibrium conditions suggesting that coke formation is rate limited. 

With zeolite catalysts it is possible to determine the coke composition, essential for the understanding of the modes of coke formation, of deactivation and of coke oxidation. As the micropores cause an easy retention of organic molecules through condensation, electronic interactions or steric blockage, the formation of coke molecules begins within these micropores. Their size is therefore limited by the size of channels, of cavities or of channel intersections. However the growth of coke molecules trapped in the cavities or at the channel intersections close to the outer surface of the crystallites leads to bulky polyaromatic molecules which overflow onto this outer surface. 

The limitation to low conversion is the major disadvantage of differential operation. This is not critical if the influence of the catalyst properties on deactivation is studied. If, on the other hand, one is interested in the mechanism and the kinetics of coke formation and in the deactivation of the main reactions, it is necessary to reach higher conversions. A solution to this problem is to combine the electrobalance with a recycle reactor. The recycle reactor is operated under complete mixing, so that the reactor is gradientless. Since in a completely mixed reactor the reactions occur at effluent conditions and not at feed conditions, a specific experimental procedure is necessary to obtain the deactivation effect of coke. 

The zeolite was pelletized, sieved, and the fraction between 0.5 and 0.71 mm diameter was retained. Scanning electron microscopy did not reveal any modification of the zeolite crystals by the pelletization. Three experiments were performed using different particle sizes, to ensure that the pelletization did not introduce transport limitations. No difference in coke formation, conversion, or selectivities was observed. 

It is now suggested that in the absence of Mo 2 as a hydrogen activating phase the thermal coke formation is substantial. A very limited amount of Mo is already sufficient to dissociate hydrogen which leads to termination of radicals, thus preventing their condensation to coke. This explains the strong drop of the coke selectivity going from zero to 0,2% Mo,... 

Note that as a first approximation the effect of hydrogen is not taken into account, which implies that the model will hold only for a limited range of hydrogen pressures. As a driving force for the reaction we use the gas-phase concentration, Cq, of the coke precursor Q, is the equilibrium constant of adsorption of Q on the catalyst surface. The rate constant for coke formation, kc, depends on the amount of coke present on the surface ... 

Catalysts with a distinct but limited extent of hydrogenation activity give rise to the lowest levels of coke formation. 

Cracking is carried out in a fluid bed process as shown in Fig. 7.9. Catalyst particles are mixed with feed and fluidized with steam up-flow in a riser reactor where the reactions occur at around 500°C. The active life of the catalyst is only a few seconds because of deactivation caused by coke formation. The deactivated catalyst particles are separated from the product in a cyclone separator and injected into a separate reactor where they are regenerated with a limited amount of injected air. The regenerated catalyst is mixed with the incoming feed which is preheated by the heat of combustion of the coke. 

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