Rate of deactivation

Propylene, butylenes, or amylenes are combiaed with isobutane ia the presence of an acid catalyst, eg, sulfuric acid or hydrofluoric acid, at low temperatures (1—40°C) and pressures, 102—1035 kPa (1—10 atm). Sulfuric acid or hydrogen fluoride are the catalysts used commercially ia refineries. The acid is pumped through the reactor and forms an emulsion with reactants, and the emulsion is maintained at 50% acid. The rate of deactivation varies with the feed and isobutane charge rate. Butene feeds cause less acid consumption than the propylene feeds. 

These metals, when deposited on the E-cat catalyst, increase coke and gas-making tendencies of the catalyst. They cause dehydrogenation reactions, which increase hydrogen production and decrease gasoline yields. Vanadium can also destroy the zeolite activity and thus lead to lower conversion. The deleterious effects of these metals also depend on the regenerator temperature the rate of deactivation of a metal-laden catalyst increases as the regenerator temperature increases. 

In general, the rate of deactivation was much lower at the lower fresh gas feed rate than at the higher feed rates. For example, the rate of deactivation was 0.022%/mscf/lb during the period between 600 and 800 hrs stream time at the lower feed rate of 206 standard cubic feet per hour (scfh) whereas it was 0.143%/mscf/lb for the period between 100 and 400 hrs when the fresh feed rate was about 386 scfh. 

The average catalyst deactivation rate over the entire experiment was 0.0291%/mscf/lb. The rate of deactivation during the initial 462 hrs of operation at a fresh feed space velocity of about 2090/hr (216 scfh) was very low, 0.0017%/mscf/lb from 500 hrs to 841 hrs with 2990/hr space velocity, the deactivation rate increased to 0.040% /mscf/lb. Catalyst deactivation rates were calculated (Table IX) for various operating periods and fresh feed space velocities. 

Flame-Sprayed Raney Nickel Plates vs. Pellets of Precipitated Catalyst in a Packed Bed. Experiments HGR-13 and HGR-14 demonstrated that the performance of the plates sprayed with Raney nickel catalyst was significantly better than that of the precipitated nickel catalyst pellets. The sprayed plates yielded higher production of methane per pound of catalyst, longer catalyst life or lower rate of deactivation, lower CO concentration in the product gas, and lower pressure drop across the catalyst bed. 

The initial reactivities of the catalyst beds in experiments HGR-13 and HGR-14 are considered satisfactorily high however, the overall rate of deactivation of the Raney nickel catalyst bed (0.029% /mscf/lb) was... 

Another factor that may account for the relatively higher rate of deactivation with the HGR reactor system is the entrainment in the catalyst bed of oil vapors from the hot recycle gas compressor. This occurrence was evidenced by traces of heavy oil collected downstream of the HGR reactor. Such oil vapors would tend to decompose thermally... 

Improve Catalyst Life and Steadiness. Regeneration or replacement of a catalyst is expensive both in direct cost and in lost production represented by the down time. Lowering the rate of deactivation of the catalyst whether by fouling, by sintering, or any other irreversible process will improve the economics of a process. 

A model developed by Leksawasdi et al. [11,12] for the enzymatic production of PAC (P) from benzaldehyde (B) and pyruvate (A) in an aqueous phase system is based on equations given in Figure 2. The model also includes the production of by-products acetaldehyde (Q) and acetoin (R). The rate of deactivation of PDC (E) was shown to exhibit a first order dependency on benzaldehyde concentration and exposure time as well as an initial time lag [8]. Following detailed kinetic studies, the model including the equation for enzyme deactivation was shown to provide acceptable fitting of the kinetic data for the ranges 50-150 mM benzaldehyde, 60-180 mM pyruvate and 1.1-3.4 U mf PDC carboligase activity [10]. 

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. 

Stability tests of catalyst. All catalysts deactivate during their life by various causes (see Chapter 3). The aim of stability tests is to examine the cause and rate of deactivation. These experiments are usually performed at conditions similar to those planned for the commercial unit. In some cases, accelerated tests are carried out using a feedstock with an elevated level of impurities or at a temperature significantly higher than that anticipated for the full-scale reactor. A laboratory reactor used for such tests is usually a down-scaled reactor or a part of the full-scale-reactor. Standard analytical equipment is used. 

Catalyst degradation can be a dominant issue in the choice of reactor configuration, depending on the rate of deactivation. Slow deactivation can be dealt with by... 

Quantum yields and lifetimes of emission (fluorescence) as well as other principal rates of deactivation have been measured on 2-hydroxy benzophenone and 2-hydroxyphenyl benzotriazole derivatives. Polymerizable UV screening agents have been prepared and copolymerized with acrylics in order to obtain transparent films containing nonfugitive UV screening agents. Preliminary results of studies of photodegradation on these copolymers are also reported here. 

The efficiency of energy transfer (E) is the ratio of the number of energy transfer occurrences from D to A divided by the total number of excitations of a donor molecule. This is the same as the ratio of the rate of energy transfer to the total rate of deactivation of the excited donor. The rate of energy transfer between single donor and acceptor molecules is proportional to 1 /r6DA (Eq. (1.1)) this is a very... 

There are several guidelines that should be followed in order to increase the chemoselectivity of the monoadduct. Firstly, radical concentration must be low in order to suppress radical termination reactions (rate constant of activation [fcal and fca2] < < rate constant of deactivation kd t andfcd2]). Secondly, further activation of the monoadduct should be avoided ( al> >kd2). Lastly, formation of oligomers should be suppressed, indicating that the rate of deactivation (kd 2[Cu"LmX]) should be much larger than the rate of propagation ( [alkene]). Alkyl halides for copper-catalyzed ATRA are typically chosen such that if addition occurs, then the newly... 

The isolation of product is usually possible after evaporation of the solvent and extraction with hexane, ether, or toluene. Supported versions, for example on polystyrene grafted with PPh2 groups, have proved unsatisfactory because the rate of deactivation is greatly enhanced under these conditions [37]. Asymmetric versions exist, but the ee-values tend to be lower than in the Rh series [38]. With acid to neutralize the basic N lone pair, imine reduction is fast. Should it be necessary to remove the catalyst from solutions in order to isolate a strictly metal-free product, a resin containing a thiol group should prove satisfactory. A thiol group in the substrate deactivates the catalyst, however. 

Using MCH as probe molecule, we have put more emphasis on the role of metal in the distribution of RC isomers. The MCH reaction was studied on HY and Pt/HY catalysts. It was found that significant differences in initial activity, rate of deactivation, and product distribution were observed between the two catalysts. 

They found that aluminum-deficient mordenite (SiO /Al O ratio of 18), pre-calcined in a flow of dry air, is twice as active for toluene disproportionation and coke formation as compared to regular mordenite. The aluminum-deficient form also has a higher rate of deactivation. It is assumed that the formation of new, strong acid sites during the process of moderate dealumination is responsible for the observed change in activity. 

Already in the first activity tests, information on the rate of deactivation is gained because the S02 conversion is measured several times during an activity... 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

Under conditions of steady illumination, a steady state will be reached, where the rate of formation of excited molecules, R, is equal to the rate of deactivation by the intramolecular processes ... 

We now look at the intermolecular deactivation of an excited molecule by another molecule (of the same or different type), a process called quenching. Any substance that increases the rate of deactivation of an electronically-excited state is known as a quencher and is said to quench the excited state. 

Quenching is a bimolecular process that is, it involves the collision of both Si and Q molecules. Thus, in the presence of the quencher, where the rate constant is kQ and the rate of deactivation by quenching is QJ ... 

Therefore, the overall rate of deactivation of Si, QJtotai, is given by the sum of the rates of the unimolecular and bimolecular processes ... 

Now, the rate of fluorescence emission is Jf = kf [Si], and since the fluorescence quantum yield was shown in Section 3.5 to be equal to the ratio of the rate of fluorescence to the total rate of deactivation, the fluorescence quantum yields in the presence and absence of a quencher, Q( )f and < )f respectively, are ... 

The results have shown that spinning/falling basket autoclaves can be used effectively for gathering data on coal hydroliquefaction, a single contact being representative of steady state conditions. As with other types of reactors for coal liquefaction, the catalysts were deactivated to a constant activity but the rate of deactivation was much more rapid in tiie autoclaves. 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

Since reactant and product molecules are similar in structure, deactivation is caused by both A and R. With diffusional effects absent, the rate of deactivation is found to be... 

The lower acid strength of the ferrizeolites has implications also in the relative rates of deactivation (vis-h-vis the Al-analogs) in those reactions where bulky polyalkylaromatics formed within the pore system can... 

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