Fouling of Equipment

Deposits that cause fouhng accumulate in equipment and piping and impede heat transfer or fluid flow, or cause product contamination. Deposits may be organic, inorganic, or a mixture of the two. Scales are crystaUine deposits that precipitate in a system (see Table 7.6). There are four principal sources of deposits water-side, fire-side, process-side, and preoperational. 

Water-side deposits are of many types. Hardness (calcium and magnesium)-based deposits and iron oxide are the most common water-side deposits and often affect boUers and cooling systems. Process and oil leaks can foul boilers and cooling systems. BiofouUng, mud, and debris are often found in cooling systems. Treatment chemicals, if not properly controlled, can add to deposits and scales. Silica can form hard, adherent deposits in boUers, steam turbines, and cooling systems. Corrosion products can add to deposits. 

Fire-side deposits can be extremely corrosive. Slags from burning oil and waste can corrode boiler equipment if they become moist. Fly ash deposits can accumulate in coal-fired boilers. Gas-fired boilers are generally clean. Some compounds that are burned in incinerators or waste heat boilers can seriously corrode or erode boiler tubes. 

Magnesium iron aluminum silicate (Mg, Fe)3(Si, Al)40io(OH)2.4H20 

Some process-side deposits are pyrophoric when exposed to air or oxygen. The most common is iron sulfide, which is likely to be found in natnral-gas and petrolenm-refining processes or when aqueous solutions of hydrogen snlfide (H2S) are dried in the absence of air. 

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One of Lewis first assignments was to make the distillation process more precise and continuous. By the early 1920s, Lewis introduced to Jersey Standard the use of vacuum stills. These were able to operate at lower temperatures that limited coking and fouling of equipment. Thus production engineers did not have to periodically clean out and repair equipment, which in turn facilitated the transformation of distillation from batch to continuous operations. 

As already noted (Chapter 3), petroleum oil often contains water, inorganic salts, snspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration). 

Low maintenance and little biological or chemical fouling of equipment... 

Water, the universal solvent, Is everywhere around us. While additives can actually Improve water quality for some uses, water impurities can cause corrosion and fouling of equipment and be a source of disease and pollution. Automated analysis is the key to solving critical water quality problems. Fast detection and correction of abnormalities are also important steps towards cutting treatment costs and keeping a plant in compliance with regulatory statutes. 

Furthermore, practical problems such as contact lens fouling, foaming of protein solutions, and fouling of equipment in the food processing industry, are direct consequences of the relatively high surface activity of proteins. In general, any process involving an interface in which contact with a protein solution occurs is likely to be influenced by protein adsorption to the interface. Thus, several reviews of protein adsorption have been published (1-5). 

Antisticking agents, to reduce fouling of equipment Waxes Mineral oils Sulfated tallow Pine oil Kerosene Stoddard solvent... 

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. 

Charcoal—sulfur processes need low ash hardwood charcoal, prepared at 400—500°C under controlled conditions. At the carbon disulfide plant site, the charcoal is calcined before use to expel water and residual hydrogen and oxygen compounds. This precalcination step minimises the undesirable formation of hydrogen sulfide and carbonyl sulfide. Although wood charcoal is preferred, other sources of carbon can be used including coal (30,31), lignite chars (32,33), and coke (34). Sulfur specifications are also important low ash content is necessary to minimise fouling of the process equipment. 

Only trace amounts of side-chain chlorinated products are formed with suitably active catalysts. It is usually desirable to remove reactive chlorides prior to fractionation in order to niinimi2e the risk of equipment corrosion. The separation of o- and -chlorotoluenes by fractionation requires a high efficiency, isomer-separation column. The small amount of y -chlorotoluene formed in the chlorination cannot be separated by fractionation and remains in the -isomer fraction. The toluene feed should be essentially free of paraffinic impurities that may produce high boiling residues that foul heat-transfer surfaces. Trace water contamination has no effect on product composition. Steel can be used as constmction material for catalyst systems containing iron. However, glass-lined equipment is usually preferred and must be used with other catalyst systems. 

Somerscales, E. F. C. and J. G. Knudsen, Fouling of Heat Tranter Equipment, McGraw Hill Publishing Co., June 1981. 

One of the reasons why it is important to remove suspended solids in water is that the particles can act as a source of food and housing for bacteria. Not only does this make microbiological control much harder but, high bacteria levels increase the fouling of distribution lines and especially heat transfer equipment that receive processed waters (for example, in one s household hot water heater). The removal of suspended contaminants enables chemical treatments to be at their primary jobs of scale and corrosion prevention and microbial control. 

The potential to recover heat from the gases of an oil- or coal-fired boiler is therefore limited to a temperature drop from 240°C to 170°C. This results in a 3 per cent saving. The average saving would be somewhat lower than this since fouling of the economizer surface is inevitable from the carbonaceous emissions of the firing equipment. 

Fouling of the reactor and other equipment is another problem specific for homogeneous catalysis. 

For some processes, though they would not be classified as batch processes, the period of continuous production will be limited by gradual changes in process conditions such as, the deactivation of catalysts or the fouling of heat-exchange surfaces. Production will be lost during the periods when the plant is shut down for catalyst renewal or equipment clean-up, and, as with batch process, there will be an optimum cycle time to give the minimum production cost. 

Chemical treater. A chemical treater is used to inject a bactericide if microorganisms could cause fouling of injection equipment and plugging of the injection reservoir. 

Batch reactors are often used for liquid phase reactions, particularly when the required production is small. They are seldom employed on a commercial scale for gas-phase reactions because the quantity of product that can be produced in reasonably sized reactors is small. Batch reactors are well suited for producing small quantities of material or for producing several different products from one piece of equipment. Consequently they find extensive use in the pharmaceutical and dyestuff industries and in the production of certain specialty chemicals where such flexibility is desired. When rapid fouling is encountered or contamination of fermentation cultures is to be avoided, batch operation is preferable to continuous processing because it facilitates the necessary cleaning and sanitation procedures. 

Factors involved in heat transfer, such as surface-to-volume ratio, agitation characteristics, mixing efficiency, fouling of heat transfer surfaces, scale of operations, and the resulting heat exchanged depend on the system under consideration (e.g., liquid-liquid transfer, liquid-gas transfer, free convection, or forced convection). Standard chemical engineering texts and reference books contain detailed discussions on heat transfer in process equipment. Only a brief summary follows ... 

Removal of naphtha and distillate fractions from the crude oil under atmospheric pressure distillation requires charge temperatures to be maintained below the cracking temperature of the crude oil components. This temperature will vary but can typically range from 750°F to 800°F (398.9°C to 426.7°C). Occasionally, even lower temperatures may be required. Above these temperatures, crude oil components can begin to thermally crack and foul processing equipment. 

Salts form scale which can foul refining equipment. Under refining conditions, salts can break down to liberate acids during processing. If the NaCl expressed salt content of crude oil is greater than 30 ppm, the crude oil must be desalted before processing. 

Higher-molecular-weight polynuclear aromatics can cause problems during processing of crude. They are known to contribute to deposit formation and fouling of refining equipment and fuel combustion furnaces. Also, some polynuclear aromatic compounds found in crude oil have been determined to be human carcinogens. 

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