Poisoning reversible

Additional Factors Influencing Decay. Numerous other factors may influence the observed change in activity of catalyst. These include pore mouth blocking by deposited solid, equilibrium, or reversible poisoning where some activity always remains, and the action of regeneration (this often leaves catalyst with an active exterior but inactive core). 

The more interesting situation occurs when the catalyst is partially and reversibly poisoned by impurities in the reactant gas. The degree of loss of catalyst activity then depends on the operating conditions. 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

Thus the mechanism formed by steps (l)-(4) can be called the simplest catalytic oscillator. [Detailed parametric analysis of model (35) was recently provided by Khibnik et al. [234]. The two-parametric plane (k2, k 4/k4) was divided into 23 regions which correspond to various types of phase portraits.] Its structure consists of the simplest catalytic trigger (8) and linear "buffer , step (4). The latter permits us to obtain in the three-dimensional phase space oscillations between two stable branches of the S-shaped kinetic characteristics z(q) for the adsorption mechanism (l)-(3). The reversible reaction (4) can be interpreted as a slow reversible poisoning (blocking) of... 

The steam requirements in an ammonia unit can be reduced by lowering the steam-to-carbon ratio to the primary reformer. However a number of drawbacks can exist downstream in the I I I S and LTS reactors. The drawbacks include By-product formation in the HTS, Pressure drop buildup in the HTS, Reversible poisoning of the LTS catalyst, and Higher CO equilibrium concentrations exiting the HTS and LTS reactors. 

The effect of reversible poisons will be dependent on the catalyst-to-oil, and therefore also of the coke selectivity of the catalyst and the heat balance of the FCC operation. 

This value was verified in a continuous laboratory reactor used to study the catalyst deactivation in long time kinetic runs[2]. On the basis of experimental observations, we recognized that the palladium catalyst is subjected to both reversible and irreversible poisoning. Water beiing responsible for reversible poisoning of the catalyst. Thus, we suggested, the following mechanism ... 

Initially G =1, then the evolution of 0 with time corresponding to the evolution of k, can be interpreted for reversible poisoning with the relation... 

As we have pointed out in relation to stability, it is only in theory that the catalyst is found intact at the end of the reaction. All catalysts age and when their activities or their selectivities have become insufficient, they must be regenerated through a treatment that will return part or all of their catalytic properties. The most common treatment is burning off of carbon, but scrubbing with suitable gases is also frequently done to desorb certain reversible poisons hydrogcnolysis of hydrocarbon compounds may be done when the catalyst permits it, as well as an injection of chemical compounds. When the treatment docs not include burning off carbon deposits, it is often called rejuvenation. 

It may be that on cobalt molybdate, olefins can adsorb on sites other than the desulfurization sites. If so, it would be in accordance with conclusions reached by Hammar (14), and would also explain the selective poisoning reported with nitrogen compounds and CO (17, 20), which are strongly adsorbed reversible poisons like H2S (2, 17). These poisons offer one of the few ways at present available of improving selectivity and so lowering hydrogen consumption when desulfurizing olefinic feeds. 

Sulfur oxides (S02 and S03) present in flue gases from upstream combustion operations adsorb onto the catalyst surface and in many cases form inactive metal sulfates. It is the presence of sulfur compounds in petroleum-based fuels that prevent the super-sensitive base metal catalysts (i.e., Cu, Ni, Co, etc.) from being used as the primary catalytic components for many environmental applications. Precious metals are inhibited by sulfur and lose some activity but usually reach a lower but steady state activity. Furthermore the precious metals are reversibly poisoned by sulfur compounds and can be regenerated simply by removing the poison from the gas stream. Heavy metals such as Pb, Hg, As, etc. alloy with precious metals and permanently deactivate them. Basic compounds such as NH3 can deactivate an acidic catalyst such as a zeolite by adsorbing and neutralizing the acid sites. 

Water is a reversible poison in that it will weakly adsorb (physically adsorb) on sites at low temperature but readily desorbs as the temperature is increased. 

Uniform (homogeneous) Reversible Poisoning Flat Plate Pellets. The rate of reaction for uniform or homogeneous reversible poisoning in flat plate pellets is given by (1 ... 

T is the time constant for poison laydown for homogeneous reversible poisoning and is given by ... 

Uniform or Homogeneous Reversible Poisoning Flat Plate Pellets. Substituting equation (10) into equation (16), the total production for uniform reversible poisoning in flat plate pellets becomes... 

Uniform Reversible Poisoning Spherical Pellets. For uniform reversible poisoning in spherical pellets an expression for the instantaneous total production is obtained by substituting equation (15) into equation (16)... 

For both pore mouth and irreversible uniform poisoning, an upper limit was found for the bed production of R, while with uniform reversible poisoning no such limit existed. 

For uniform reversible poisoning, the production increased with a decrease in the equilibrium surface of the catalyst poisoned. At long times the production rate became constant. 

Spherical and flat plate pellets give substantially equivalent conversions and production levels for uniform reversible poisoning when the Thiele moduli are put on an equivalent basis. 

Contradictory data on the kinetics of ammonia synthesis, especially in the earlier literature, in some circumstances may reflect a lack of attention to the influence of impurities in the gas. If oxygen compounds are present in the synthesis gas, reversible poisoning of the adsorbing areas, in accordance with an equilibrium depending on the temperature and the water vapor-hydrogen partial pressure ratio, must be taken into account when developing rate equations (see also Section 3.6.1.5). 

Such experimental results have been rationalized by assuming a chemical deactivation of some of the active centers and the presence of at least two types of species on the catalytic surface These two are isospecific polymerization centers which are unstable with time, and only slightly specific polymerization centers which, in turn, are stable with time. The latter appear to be preferentially and reversibly poisoned by the outside donor. 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

The theory of selective and reversible poisoning proposed by most authors can be satisfactorily applied in all those cases where a simultaneous decrease of both the atactic and isotactic productivities has been observed. Furthermore, it is in agreement with the fact that the Lewis base actually forms complexes with the catalyst, as seen in Section 5.2. Nevertheless, this explanation appears oversimplified to explain all the effects the base produces on the kinetics, as has been revealed by Spitz45,97 . Moreover, it seems completely inadequate to account for the increase of productivity of isotactic polymer which has been detected in a few cases 45,112, U8). 

In binary catalysts two types of propagation centers can be kinetically identified stereospecific CJ and non stereospecific C. The aluminum alkyl causes the formation of such centers by means of irreversible alkylation reactions of the corresponding S and SA sites. Moreover, it brings about the reversible deactivation of the propagation species, which is preferential for the non-stereospecific centers. The external base, in equilibrium and competition with the organoaluminum, would reversibly poison the non-stereospecific centers and, to a much lower degree, also the stereospecific centers. In the ternary catalysts a further stereospecific center, would be present. This center is most likely, but not necessarily, donor associated. In this case the aluminum alkyl, besides deactivating the various active centers to different... 

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