Cost of fouling

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 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. 

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. 

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. 

Heat Recovery Equipment. Factors that limit heat recovery appHcations are corrosion, fouling, safety, and cost of heat-exchange surface. Most heat interchange utilizes sheU and tube-type units because of the mgged constmction, ease of mechanical cleaning, and ease of fabrication in a variety of materials. However, there is a rich assortment of other heat exchangers. Examples found in chemical plants in special appHcations include the foUowing. 

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). 

Forced circulation Viscous and solid-containing liquids can be circulated. Enables an erosion-fouling balance. Circulation rate can be controlled. Relatively expensive due to extra shell volume. Cost of pump and pumping. Leakage of material at stuffing box. 

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. 

Another advantage to the incorporation of bioprocessing aids into the Step I procedure is that the clarified extract can be used directly for subsequent purification steps even without the use of a Step II system to dewater or concentrate the process stream. These factors are especidly relevant when HPLC systems arc used in Step IB for the chromatographic procedures. Nucleic acids, pigmented organics and especially cellular debris can very quickly foul an HPLC column. This is an even more important consideration for large scale protein purification schemes where the volumes of material and costs of the operation are greatly increased (3). 

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