Fouling, catalyst

Diffusion Poisons. This phenomenon is closely akin to catalyst fouling. Blockage of pore mouths prevents full use of the interior surface area of the pellets Entrained dust par ticles or materials that can react on the catalyst to yield a solid residue give rise to this type of poisoning. 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

Impure Feeds - Impure feeds cause Claus and Stretford plant problems (3,16). The most troublesome impurities are NH3, HCN, and hydrocarbons. These can cause catalyst fouling, plugging, chemical losses, and unstable operations. Remedies include catalytic conversion of impurities, use of special burners (16), and the use of different AGR processes to reduce the amounts of these compounds in acid gases. 

Adequate control of the chemistry in the front end furnace can significantly effect the lifetime and efficiency of the downstream catalyst beds in a sulfur plant. Inadequate removal of Ce+ hydrocarbons from the acid gas feed can result in catalyst fouling by polymeric materials formed under furnace conditions. Toluenes, ethylbenzenes and xylenes have been shown to be particularly troublesome in this regard. Oxygen breakthrough into the catalyst beds can also shorten the effective lifetime of the Alumina catalyst by sulfation i.e. 

Wetting of the external catalyst surface is desired as to delay catalyst fouling. 

The best way to alternatively manufacture MIPK and MIK in the same equipment is the combination of a homogeneous and a heterogeneous catalytic step. No catalyst fouling occurs, MEK recovery is easy and cheap and plant flexibility is high. 

Figure 8. Effect of catalyst fouling on product composition with catalyst E hydrocracking of California gas oil...

At moderate severity, the hydrotreating catalysts foul, as shown in Figure 3 by the rise in nitrogen content of the products versus time. In commercial operation, the catalyst temperature would be raised and the product nitrogen content would be held constant. Commercial run lengths appear feasible, even at moderate severity. 

Stability of catalysts not affected by drying temperatures, but is affected by sulfiding. Sulfided catalyst fouled by different mechanism.81 ... 

The support has an internal pore structure (i.e., pore volume and pore size distribution) that facilitates transport of reactants (products) into (out of) the particle. Low pore volume and small pores limit the accessibility of the internal surface because of increased diffusion resistance. Diffusion of products outward also is decreased, and this may cause product degradation or catalyst fouling within the catalyst particle. As discussed in Sec. 7, the effectiveness factor Tj is the ratio of the actual reaction rate to the rate in the absence of any diffusion limitations. When the rate of reaction greatly exceeds the rate of diffusion, the effectiveness factor is low and the internal volume of the catalyst pellet is not utilized for catalysis. In such cases, expensive catalytic metals are best placed as a shell around the pellet. The rate of diffusion may be increased by optimizing the pore structure to provide larger pores (or macropores) that transport the reactants (products) into (out of) the pellet and smaller pores (micropores) that provide the internal surface area needed for effective catalyst dispersion. Micropores typically have volume-averaged diameters of 50 to... 

Catalyst Fouling Coke deposits Metal deposits... 

This process is conducted in the liquid phase using fixed beds of skeletal copper catalysts and temperatures from 300 to 400 K. Higher temperatures lead to catalyst fouling by polymerization of the product acrylamide. This deactivation can be reversed by washing the catalyst with caustic soda solution. This regeneration is a positive advantage of skeletal copper over other forms of copper catalysts used in this industrial process. 

The coke formation leads to catalyst fouling. This is solved in the UOP Process by continuously removing a portion of the catalyst and passing this to a separate regenerator. After regeneration by combustion of the coke in air, the catalyst is sent back to the main reactor. In concept this is similar to fluid-cat cracking of refinery stocks. The process layout is illustrated in the Figure 11.7. 

Catalyst Poisoning How do external poisons affect catalyst behaviour in time. Catalyst Fouling How does formation of coke and/or metal deposits affect catalyst behaviour. 

Catalyst Aging Catalyst Poisoning Catalyst Fouling... 

Figure 3 shows calculated catalyst deactivation results caused by a loss of active surface area without a change in the effective diffusion coefficient. Catalyst fouling is proportional to a loss of active surface area if diffusion does not control the reaction rate, so with a high effectiveness factor. On the other hand, a loss of active surface area is compensated to a certain extent in the apparent reaction rate when the effectiveness factor is small, because the effectiveness factor increases with the loss of surface activity. 

Figure 4 shows the typical trends of catalyst fouling with metal- and coke deposition on the catalyst. Both cause a change in the effective diffusivity and a loss of surface activity. It indicates that the initial fouling brought about by coke laydown on the catalyst is accompanied by a loss of surface activity. The ultimate catalyst life is determined by metal deposition which decreases the effective diffusivity of reactants into catalyst pores. 

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