Heating surfaces fouling

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. 

Suspended solid.s Clogs pipelines Fouls heat exchanger Surfaces Settling Filtration... 

Fouling of surfaces, resulting in decreased heat transfer efficiency... 

Recently, the use of microwave heating has also been applied successfully to coating processes. The use of internal heating surfaces is usually not practical since these would tend to clog and foul during the process. 

It is a general principle of heat exchange that low flows tend to promote fouling and fouling promotes corrosion. The corroded, fouled heat-exchanger surface retards flow and creates a vicious cycle. We will see this problem again in shell-and-tube heat exchangers, as discussed in Chap. 21. 

Scraped-surface exchangers are particularly suitable for heat transfer with crystallization, heat transfer with severe fouling of surfaces, heat transfer with solvent extraction, and heat transfer of high-viscosity fluids. They are extensively used in paraffin-wax plants and in petrochemical plants for crystallization. 

In some cases, as with pulp-mill liquors, the evaporator vapors contain constituents more volatile than water, such as methanol and sulfur compounds. Special precautions may be necessary to minimize the effects of these compounds on heat transfer, corrosion, and condensate quality. They can include removing most of the condensate countercurrent to the vapor entering an evaporator-heating element, channeling vapor and condensate flow to concentrate most of the foul constituents into the last fraction of vapor condensed (and keeping this condensate separate from the rest of the condensate), and flashing the warm evaporator feed to a lower pressure to remove much of the foul constituents in only a small amount of flash vapor. In all such cases, special care is needed to properly channel vapor flow past the heating surfaces so there is a positive flow from steam inlet to vent outlet with no pockets, where foul constituents or noncondensibles can accumulate. 

Heaters can be located in the space between the trms and the dryer housing, where they are not in direct contact with the product, and thermal efficiencies up to 3500 kj/kg (1500 Btu/lb) of water evaporated can be obtained by reheating the air within the dryer. Steam is the usual heating medium. The hi cost of heating electrically generally restricts its use to relatively small equipment. For materials which have a tendency to foul internal heating surfaces, an external heating system is employed. 

Develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger,... 

Figure 23.14 outlines a classification of fluidized bed dryers. For heat-sensitive materials, e.g., polymer pellets, use of immersed heat exchangers allows use of lower fluidizing air temperature without increasing drying time. Such exchangers can be used, however, only if the solids do not foul their surfaces. 

FIGURE 2.1. Temperature distribution across fouled heat exchanger surfaces... 

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