Catalyst maximum allowable

There are two general temperature poHcies increasing the temperature over time to compensate for loss of catalyst activity, or operating at the maximum allowable temperature. These temperature approaches tend to maximize destmction, yet may also lead to loss of product selectivity. Selectivity typically decreases with increasing temperature faster deactivation and increased costs for reactor materials, fabrication, and temperature controls. 

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... 

If a breakthrough of CO occurs from the LTS converter or a breakthrough of CO2 from the absorption system occurs, the intensely exothermic methanation reaction can reach temperatures that exceed 500°C. Controls should be installed and other measures taken to avoid these high temperatures because tlie catalyst may be damaged or the maximum allowable operating temperature of the pressure vessel may be exceeded74. 

The main variables that dictate the uniformity of catalyst irrigation are the liquid velocity, the particle diameter and the kinematic viscosity of the liquid Figure 8 gives the maximum allowable particle diameters for several reactor lengths as a function of the kinematic viscosity. 

Figure 10. Maximum allowable particle diameters as a function of the fraction catalyst (1 - b) in a diluted bed.

In a few cases it is possible to redisperse sintered materials on a catalyst The procedures involved are dependent on the nature of the system In typical examples, dispersion is increased by the formation of oxides or oxychlorides The operation involves careful control of regeneration conditions bearing in mind the nature of the catalyst, the IS, and the maximum allowable temperature under regeneration conditions... 

As time progresses, the catalyst in the HDS reactor decays because of metal (vanadium and nickel) and coke depositions. The deposition of these metals occurs nonuniformly along the length of the reactor (more deposits occur near the reactor inlet than at the reactor outlet). In normal plant operations, the catalyst activity decline is counterbalanced by a rise in feed temperature, a reduction in the amount of quench fluids fed to the reactor or both, so as to achieve the same quality product. The process is terminated upon the attainment of a maximum allowable temperature (MAT) anywhere in the reactor. The catalyst bed is then regenerated. The time required to achieve the MAT is often called the reactor cycle life. 

Aside from defining the size of distinct component particles in a mixed catalyst composite, R also defines a maximum allowable magnitude for the... 

Figure 1 shows the consumption of CH4 and O2, and the amount of the different products formed for several CH4/O2 ratios over a 6 % PbO/Si02 catalyst. For a CH4/O2 = 2 ratio, the consumption of CH4 reaches a value near or equal to the maximum amount allowed by the amount of or rgen available for the reaction. At this point all of the available or rgen is consumed (100% O2 conversion). For a CH4/O2 = 1 ratio, the consumption of CH4 does not reach the maximum allowed value (86.4 % O2 conversion). This is because with a decreased amount of methane, and a constant... 

In some cases the optimal temperature is very high at the entrance which may cause damage to the catalyst (Figure 6.23). In this case a constraint on the maximum allowable temperature is imposed on the optimization algorithm during the computation. 

The optimal temperature policy in a batch reactor, for a first order irreversible reaction was formulated by Szepe and Levenspiel (1968). The optimal situation was found to be either operating at the maximum allowable temperature, or with a rising temperature policy, Chou el al. (1967) have discussed the problem of simple optimal control policies of isothermal tubular reactors with catalyst decay. They found that the optimal policy is to maintain a constant conversion assuming that the decay is dependent on temperature. Ogunye and Ray (1968) found that, for both reversible and irreversible reactions, the simple optimal policies for the maximization of a total yield of a reactor over a period of catalyst decay were not always optimal. The optimal policy can be mixed containing both constrained and unconstrained parts as well as being purely constrained. 

The absolute optimum temperature profile gives a considerable increase in conversion for all reactors with the maximum increase in conversion of 11.58% occuring for reactor IV. Obviously, such an operation must be non-adiabatic. The constrained optimal policy that takes into consideration the maximum allowable temperature of the catalyst gives results which are very close to those of the absolute optimum (e.g. increase of conversion over operating conversion of reactor IV being 10.49%). 

Typical catalyst poisons are lead and phosphorus. Lead is present at very low levels in unleaded gasoline. Typical lead levels are 0.003 g/gal although 0.05 g/gal is the maximum allowed lead level in unleaded fuel. Lead is not believed to be a major catalyst poison at the 0.003 g/gal level. On the other hand, use of leaded fuel will poison three-way catalysts, and catalyst activity is not fully recovered upon changing back to unleaded fuel. Figure 8... 

Through the years, equipment design has adapted to developments in catalysts, in other words more severe operating conditions, particularly at very low pressures. This situation has resulted in a substantial decrease in the maximum allowable pressure drop in the furnaces, exchangers and reactors. In fact, at a given pressure, the pressure drop conditions the dimensioning of the hydrogen recycle compressor, and hence its cost. 

Very frequently non-optimal setpoint trajectories are used for controlling reactor temperatures in batch reactors [25,39,179,180]. Reactor temperatures maybe allowed to increase from ambient temperatures up to a maximum temperature value, in order to use the heat released by reaction to heat the reaction medium and save energy (reduce energy costs). The temperature increase is almost always performed linearly, because of hardware limitations and simplicity of controller programming. After reaching the maximum allowed temperature value, reactor temperature is kept constant for a certain time interval, for production of polymer material at isothermal conditions. At the end of the batch, the reaction temperature is increased in order to reduce the residual monomer content of the final resin, usually with the help of a second catalyst. Heuristic optimum temperature trajectories were also formulated for batch polymerizations of acrylamide and quaternary ammonium cationic monomers, in order to use the available heat of reaction [181]. The batch time was split into two batch periods an isothermal reaction period and an adiabatic reaction period. 

Anderson et al. [42] have proposed various cycles for the PSA-type SEWGS cycle that centres on a basic cycle that involves feeding syngas into a packed bed of sorbent/catalyst until CO2 breaks through to a maximum allowable level. The bed is then depressurised and a vacuum pulled to remove most of the CO2 from the sorbent. Once regeneration has been... 

The liquid-off duration is determined by the maximum allowable temperature of the catalyst and the time for depletion of the liquid-phase reactant (when present). 

The most typical reversible poisons of reforming catalysts, and the approximate maximum allowable in the feed (ppm wt) (4) are sulfur (0.5-1), nitrogen (0.5-1), chlorides (0.5), fluorides (0.5), water, or oxygenated hydrocarbons (2-4). The irreversible poisons most frequently found and the concentration allowed in the feed (ppb wt) are arsenic (1-2), lead (10-20), copper, silica, and phosphorus. These maximum values allowed for these impurities are approximate, since they may be very different for different catalysts. A list of many poisons of reforming catalyst and their toxicities in benzene hydrogenation on Pt/Al203 catalyst is reported in Reference (136). Sulfur is one of the most important poisons. 

In addition, the macroscopic shape of the catalyst should allow a pressure drop of maximum 2-3 bar. 

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