Intersections blockage

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

In Figure 8.7, the curves determine the restriction downstream pressure at which hydrate blockages will form for a given upstream pressure and temperature. Gas A expands from 2000 psia and 110°F until it strikes the hydrate formation curve at 700 psia (and 54°F), so 700 psia represents the limit to hydrate-free expansion. Gas B expands from 1800 psia (120°F) to intersect the hydrate formation curve at a limiting pressure of 270 psia (39°F). In expansion processes while the upstream temperature and pressure are known, the discharge temperature is almost never known, but the discharge pressure is normally set by a downstream vessel or pressure drop. 

The formation of coke requires therefore the possibility for (a) reactant(s) to undergo bimolecular reactions and for the reaction products to be retained in or on the zeolite. This retention occurs either because the products are not volatile enough to be eliminated from the zeolite under the operating conditions or because their size is greater than the pore aperture (hence a steric blockage in the cavities or at channel intersections). Obviously the first mode of retention concerns not only the coke molecules deposited within the micropores but also those deposited on the outer surface of the crystallites. 

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. 

Spatioselectivity plays a significant role in the formation of the heavy secondary reaction products responsible for deactivation ( coke ). Indeed, coke formation involves various bimolecular steps (condensation, hydrogen transfer) that, as indicated above, are very sensitive to steric constraints. Therefore, the rate of coke formation will greatly depend on the size and shape of cages, channels and their intersections. However, as discussed earlier in 1.3.2, coke formation involves not only chemical steps (involving spatioselectivity) but also physical retention in the zeolite pores due to steric blockage (reverse shape selectivity), at least at high reaction temperatures (43, 44). 

A similarly strict conclusion cannot be drawn from the experimental data for chemisorbed ammonia, which give rise to a behavior intermediate between those of the two limiting cases. One should take into account, however, that due to its smaller size, an ammonium ion situated in a channel intersection does not lead to a simultaneous blockage of all four adjacent... 

Beyne and Froment [ref. 28] applied percolation theory to reaction and deactivation in the real three-dimensional ZSH-5 lattice. The structure of the catalyst enters in the equation for the reduced accessibility of active sites caused by blockage, P in (22) and this quantity is related to the percolation probability for this structure, P It is generally accepted that in zBH-5 the reactions take place at the channel intersections The probability that an intersection of channels (the origin in a network] is connected with an infinite number of open intersections is the percolation probability. It decreases as a growing number of intersections becomes blocked and drops to zero well before they are all blocked One way of relating P to the probability that an intersection is blocked, q, is Honte Carlo simulation. Based upon work by Gaunt and Sykes [ref. 29] on the percolation probability and threshold in diamond, Beyne and Froment derived a polynomial expression for P, However, the probability that a site is... 

Limitation or (4) blockage of the access of the reactant to the active sites of cavities, of channel intersections or of parts of channels in which no coke molecules are located. 

In Modes 1 and 2 the limitation or the blockage is due to chemical reasons i.e., the coke molecules are (1) reversibly or (2) quasi-irreversibly adsorbed on the acid sites (site poisoning or site coverage) and/or to steric reasons the diffusion of reactant molecules through the cavity or the channel intersection is (1) limited or (2) blocked. With these modes the toxicity of the coke molecules is low as only the sites located in the cavity or at the chaimel intersection, often only one site, are partially (Mode 1) or totally (Mode 2) deactivated. 

Four modes of deactivation can be distinguished. The first one corresponds to a limitation of the access of the reactant to the active sites of a cage or of a channel intersection in which is located a coke molecule, the second one to complete blockage of this access. This limitation or this blockage may be due to the adsorption of coke molecules on the acid sites or to steric reasons. The other two modes correspond to steric limitation (Mode 3) or blockage (Mode 4) of the access of the reactant to the active sites of cavities or of channels in which no coke molecules are located. The deactivating effects of coke molecules increase from Mode 1 to Mode 4. 

With mode b the deactivating effect is similar to that of a site coverage (provided however that only one acid site is located in the cavity or at channel intersection). Mode c can be considered as pore blockage. The deactivating effect of the coke molecules will increase progressively from mode a to mode c, provided however that all the acid sites are identical. 

Nicklin (1971) point out this instability, which can be seen in Fig. 4-8. The blower pressure curve is seen superimposed on the two-phase pressure-drop curve. If the system is working at point c, which is the intersection of the pressure drop and performance curve of the blower, and some disturbance occurs such as a small blockage or surge, the system pressure can move to point b and then to point a if the disturbance is large enough. Point a is on the steep part of the pressure-drop curve and is rather unstable and easily driven to the choked-flow condition. 

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