Reaction Fouling

To prevent the formation of reaction products from the interaction of the ozone-air sample with Alters, they arc intentionally not used at probe inlets (see Table 6-4). Some of the newer instruments, however, require Alters at the inlet of their sampling ports to prevent the particulate matter in the ambient air from fouling reaction-chamber cells or from clogging the gas-flow controllers. When the same type of Alter also precedes the calibration and zero gas sampling ports (which has not always been the practice), the problem is minimized to the extent that similar events occur during the calibration and sampling. 

Figure 6 reveals that the activity after five time constants is insensitive to the order of the fouling reaction with respect to the reactant concentration. For this functionality of a on f, however, the activity is quite sensitive to the magnitude of 0, Although not shown here, the activity is also sensitive to tfie dependence of a on f. 

Figure 7 shows that the magnitude of the maximum in the product concentration is insensitive to reactant concentration dependence of the fouling reaction. These curves have been developed by adjusting 0 to give a product concentration of 0.3 at five time constants. 

Eor the reaction under consideration, several combinations of series and parallel fouling schemes may be chosen, involving either one or more of Aj, A2, A3 or one or more of Aj + A2, A2 + A3, A3 + Aj, or Aj + A2 + A3. The different forms of the function/3 in Equation (CS3.2) resulting from these fouling reactions are summarized in Table CS3.2. Eurthermore,/2 and/4 are assumed to be given by... 

Fouling reactions occurring in the range of about 150 to 370 °C (300 to 700 °F) present problems, many of which are amenable to use of chemical anti-foulants. Above about 370 to 430 °C (700 to 800 °F), there is little experience to draw on in use of either film-forming or neutralizing corrosion inhibitors, or in the use of anti-foulants. 

Operating parameters such as temperature, pressure, and contact time, all of which increase fouling reaction rates, ordinarily are set by processing conditions. Additional factors are stream... 

Another factor that increases fouhng is the presence in the process streams of trace quantities of certain active metals such as iron, nickel, vanadium, and particularly copper. These metals are present because of their original occurrence in the crude streams, or from corrosion of process equipment constructed from the metals or their alloys. Surfaces of these metals are also active catalysts for fouling reactions. Here again, the interdependence of corrosion and fouling is illustrated, since metal contaminants resulting from corrosion in up-stream units may be reduced by the use of corrosion inhibitors. 

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams. 

The major portion of sait is found in residues as these streams serve as the bases for fuels, or as feeds for asphalt and petroleum coke production, the presence of salt in these products causes fouling of burners, the alteration of asphalt emulsions, and the deterioration of coke quality. Furthermore, calcium and magnesium chlorides begin to hydrolyze at 120°C. This hydrolysis occurs rapidly as the temperature increases (Figure 8.1) according to the reaction i. ... 

Isocyanide reaction. Add a few drops of chloroform to about 0 2 g. of the substance, and then 2 3 ml. of ethanolic NaOH solution. Mix well and warm gently the foul odour of isocyanide (carbylamine) is produced. Immediately the odour of isocyanide is detected, cool the tube and add carefully an excess of cone. HCl the isocyanide is thus hydrolysed to the odourless amine. 

Isocyanide reaction. Repeat Test i for chloroform (above) using 0 5 g. of iodoform instead of i ml. of chloroform the foul odour of isocyanide is produced as before. 

This second reaction leads to the small amount of branching (usually less than 5%) observed in the alcohol product. The alpha olefins produced by the first reaction represent a loss unless recovered (8). Additionally, ethylene polymerisation during chain growth creates significant fouling problems which must be addressed in the design and operation of commercial production faciUties (9). 

Because soHd acid catalyst systems offer advantages with respect to their handling and noncorrosive nature, research on the development of a commercially practical soHd acid system to replace the Hquid acids will continue. A major hurdle for soHd systems is the relatively rapid catalyst deactivation caused by fouling of the acid sites by heavy reaction intermediates and by-products. 

In plasticizer manufacture, eg, of phthalates or sebacates, uskig sulfuric or/ -toluenesulfonic acid catalysts, the temperature (140—150°C) requked for rapid reaction and high conversion may dehydrate or oxidize the alcohol and may yield a dark or foul-smelling product. Neutral titanates do not cause such side reactions. Although a temperature of 200°C is requked, esterifications can easily be forced to over 99% conversion without the formation of odors or... 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

These various reactions should be minimized to avoid plugging the catalyst and to prevent fouling of the downstream air preheaters, when these components condense from the gas at the lower temperatures. 

Moisture measurements are important in the process industries because moisture can foul products, poison reactions, damage equipment, or cause explosions. Moisture measurements include both absolute-moisture methods and relative-humidity methods. The absolute methods are those that provide a primaiy output that can be directly calibrated in terms of dew-point temperature, molar concentration, or weight concentration. Loss of weight on heating is the most familiar of these methods. The relative-humidity methods are those that provide a primaiy output that can be more direc tly calibrated in terms of percentage of saturation of moisture. 

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