The Cost of Fouling

In Chapter 1 some of the factors that contribute to the cost of fouling were mentioned. It is the purpose of this chapter to give more detail in respect of these costs. 

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Thackery, P.A., 1979, The cost of fouling in heat exchanger plant, in Fouling -Science or Art Inst. Corrosion Science and Technology and Inst. Chem. Engineers, Guildford. 

The target composition of the undesirable species in each MSA is assigned by the designer based on the specific circumstances of the application. The nature of such circumstances may be physical (e.g., maximum solubility of the pollutant in the MSA), technical (e.g., to avoid excessive corrosion, viscosity or fouling), environmental (e.g., to comply with environmental regulations), safety (e.g., to stay away from flammability limits), or economic (e.g., to optimize the cost of subsequent regeneration of the MSA). 

Corrosion with the formation of insoluble corrosion products may be unacceptable where heat-transfer equipment is concerned. Fouling by corrosion products has to be allowed for when sizing the equipment and the extra cost of using resistant material may not be as great as the increased cost of a larger exchanger in the less resistant material plus the cost of downtime to clean fouled surfaces. 

Most waterside problems develop insidiously. Over time, scale and other types of deposit are gradually formed on internal heat transfer surfaces, which gradually raises the cost of providing heat energy. Some types of deposition can be very difficult and costly to remove. Corrosion wastes away the fabric of the plant (sometimes very quickly) and may produce an unexpected and untimely boiler plant shutdown, with a consequential loss of space heating, electricity, or process manufacturing capability. Likewise, fouling reduces the size of waterways and increases boiler operational problems. 

Forced circulation reboilers may be either horizontal or vertical. Since the feed liquid is at its bubblepoint, adequate NPSH must be assured for the pump if it is a centrifugal type. Linear velocities in the tubes of 15-20 ft/sec usually are adequate. The main disadvantages are the costs of pump and power, and possibly severe maintenance. This mode of operation is a last resort with viscous or fouling materials, or when the fraction vaporized must be kept low. 

Flux. The film model (Equation 6.6) illustrates that increasing flux has an exponential effect on CP. If we accept that fouling is a consequence of CP the impact of excessive flux is obvious. As a result high flux membranes tend to be short lived and foul unless improved fluid management is able to enhance k. Selection of the appropriate flux and crossflow velocity is a trade-offbetween capital and operating costs (see cost of fouling below). 

In a sold-out market, a DuPont intermediates process was operating at 56% of peak capacity. The major cause of the rate limitation was identified as poor decanter operation. The decanter recovered a valuable catalyst, and the poor operation was caused by fouling from catalyst solids. Returning the process to high utility required a 20-day shutdown. During the shutdown, the vessel was pumped out and cleaned by water washing. The solids and hydrolyzed catalyst were then drummed and incinerated. A waste stream analysis identified three cost factors the volume of wastewater that had to be treated, the cost of the lost catalyst, and the incineration cost. 

Impurities affecting the catalyst must be removed. Evaluate the cost of an extra purification system for feeds, as well as the cost of recycling harmful impurities, including equipment fouling and maintenance. 

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