Control catalysts, degradation

Generally speaking, temperature control in fixed beds is difficult because heat loads vary through the bed. Also, in exothermic reactors, the temperature in the catalyst can become locally excessive. Such hot spots can cause the onset of undesired reactions or catalyst degradation. In tubular devices such as shown in Fig. 2.6a and b, the smaller the diameter of tube, the better is the temperature control. Temperature-control problems also can be overcome by using a mixture of catalyst and inert solid to effectively dilute the catalyst. Varying this mixture allows the rate of reaction in different parts of the bed to be controlled more easily. 

Separability. One of the greatest advantages of a solid catalyst is that it can be separated easily from the products of reaction. To do this successfully requires careful control of the process conditions so that exposure of the catalyst to nonreactant liquids capable of affecting or dissolving either the catalytic material or the support is prevented or rninimi2ed. Solid catalysts typically are used in axial or radial flow beds and multitubular reactors. Many successful commercial processes maintain the reactants and products in the gas phase while in contact with the catalyst to avoid catalyst degradation problems. 

In rhodium catalysed hydroformylation reactions, conversions achieved using a biphasic system were lower than those achieved in pure ionic liquid 40% in [Bmim][PF6]-scC02 99% in [Bmim][PF6] alone.However, the selectivity of linear to branched isomer was reversed and therefore these results were highly significant. This approach led to the development of a continuous-flow system for hydroformylation of alkenes, and under careful control the system could be used for several weeks without any visible sign of catalyst degradation. It should be noted that biocatalysts have also been used and recycled using biphasic ionic-liquid-carbon dioxide approaches. 

T he successful use of platinum monolithic oxidation catalysts to control automobile emissions over many thousands of miles requires an intimate understanding of the many factors which contribute to catalyst degradation. Contamination of the active catalyst by lead and phosphorus compounds present in fuel and lubricating oil is a major factor in catalyst deterioration. 

The Chemistry of Degradation in Automotive Emission Control Catalysts... 

Thermal Stability. Our initial work on catalyst degradation processes was designed to answer qualitatively several simple yet vital questions. This report is organized similarly. The question naturally arises whether platinum or palladium is best for automotive emission control. One aspect... 

The sulfur slurry that accumulates in the bottom of the settler vessel is periodically withdrawn and pumped through a plate and frame filter press where the sulfur is removed and the filtrate is returned to the process. The filtercake is then water washed and air blown to minimize catalyst losses (not shown). The wash water is returned to the system where it serves as a primary source of make-up water to replace that lost by evaporation. The water wash and air blow cycle are the determining factors in the chemical makeup requirements of the process. A small amount of catalyst concentrate is added after each filtration cycle to replace filtercake losses. It is claimed that no catalyst degradation occurs under normal operating conditions. The only other chemical make-up requirement consists of a small amount of alkaline solution for pH control. 

Many difficult process control problems have two distinguishing characteristics (i) significant interactions occur among key process variables, and (ii) inequality constraints exist for manipulated and controlled variables. The inequality constraints include upper and lower limits. For example, each manipulated flow rate has an upper limit determined by the pump and control valve characteristics. The lower limit may be zero, or a small positive value, based on safety considerations. Limits on controlled variables reflect equipment constraints (for example, metallurgical limits) and the operating objectives for the process. For example, a reactor temperature may have an upper limit to avoid undesired side reactions or catalyst degradation, and a lower limit to ensure that the reaction(s) proceed. 

Double-Absorption Plants. In the United States, newer sulfuric acid plants ate requited to limit SO2 stack emissions to 2 kg of SO2 per metric ton of 100% acid produced (4 Ib /short ton Ib = pounds mass). This is equivalent to a sulfur dioxide conversion efficiency of 99.7%. Acid plants used as pollution control devices, for example those associated with smelters, have different regulations. This high conversion efficiency is not economically achievable by single absorption plants using available catalysts, but it can be attained in double absorption plants when the catalyst is not seriously degraded. 

The acetylation reaction is stopped by the addition of water to destroy the excess anhydride, causing rapid hydrolysis of the combined sulfate acid ester (Eig. 7). This is followed by a much slower rate of hydrolysis of the acetyl ester groups. The rate of hydrolysis is controlled by temperature, catalyst concentration, and, to a lesser extent, by the amount of water. Higher temperatures and catalyst concentrations increase the rate of hydrolysis. Higher water content slightly iacreases the hydrolysis rate and helps minimize degradation (85). The amount of water also influences the ratio of primary to secondary... 

Solution Process. With the exception of fibrous triacetate, practically all cellulose acetate is manufactured by a solution process using sulfuric acid catalyst with acetic anhydride in an acetic acid solvent. An excellent description of this process is given (85). In the process (Fig. 8), cellulose (ca 400 kg) is treated with ca 1200 kg acetic anhydride in 1600 kg acetic acid solvent and 28—40 kg sulfuric acid (7—10% based on cellulose) as catalyst. During the exothermic reaction, the temperature is controlled at 40—45°C to minimize cellulose degradation. After the reaction solution becomes clear and fiber-free and the desired viscosity has been achieved, sufficient aqueous acetic acid (60—70% acid) is added to destroy the excess anhydride and provide 10—15% free water for hydrolysis. At this point, the sulfuric acid catalyst may be partially neutralized with calcium, magnesium, or sodium salts for better control of product molecular weight. 

Because homogeneous catalysis is usually carried out in the liquid phase, temperature control is relatively easy. However, the temperature must not be too high to prevent degradation of the catalyst complexes. 

---