Fouling removal mechanism

EXCESSIVE FOULING If mechanical cleaning is desired, the fouling medium should be placed in the tube side. This enables the tubes to be cleaned without removing the bundle from the shell. 

In this section it has generally been assumed that in addition to deposition processes, there are removal mechanisms occurring at the same time. Such a situation may not apply to all fouling processes. In some examples the mass transfer of fouling precursors towards the surface may be the only transport mechanism removal processes may be non-existent. The general shape of the curve of fouling thickness with time (Fig. 1.1) may arise, not because of the interaction of the two mass transfer processes, but because the deposition process is suppressed, i.e. an auto retardation of the deposition process till it becomes virtually zero. Such effects might arise due to electro-chemical conditions (see Chapter 6), or solution saturation conditions near the deposit/fluid interface. 

The transport mechanisms that interact and influence the deposition and accumulation of fouling substances on the surfaces of heat exchangers are extremely complex. The complexities make it difficult to come to a full understanding of the precise mechanisms that prevail in the fouling process. It is perhaps in potential removal mechanisms that the principal difficulties arise. A search for a general explanation is frustrated by the possibilities that in some fouling processes removal does not occur whereas in others, it represents a substantial proportion of the transport processes involved. 

Because chelating agents are such important materials for removing fouling deposits and surface films from industrial equipment additional details of the removal mechanisms from steel surfaces have been worked out The mechanism of removal varies according to the chemical and physical environment of the metal to be chelated. Because of this complexity, this discussion is restricted to the removal of magnetite (Fes04) from steel surfaces. 

Gleaning. Fouling films are removed from the membrane surface by chemical and mechanical methods. Chemicals and procedures vary with the process, membrane type, system configuration, and materials of constmction. The equipment manufacturer recommends cleaning methods for specific apphcations. A system is considered clean when it has returned to >75% of its original water flux. 

When fouling occurs, even mechanical cleaning does not remove all traces of the bio film. Previously fouled and cleaned surfaces are more rapidly colonized than new surfaces. Residual biofilm materials promote colonization and reduce the lag time before significant fouling reappears. 

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

Apart from the oxygen corrosion that results in HW and LP steam heating systems where water losses occur as a result of leaking pump mechanical seals, excess BD, faulty steam traps, and other sources, a subsequent effect is the development of fouling. This effect stems from the production of corrosion debris and (high iron content) sludge that eventually settle out in the boiler. This corrosion debris, sludge, and other foulants must be periodically removed from the boiler by BD, which merely adds to the water loss, and the cycle perpetuates. 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

A bottleneck in all membrane processes, applied in practice, is fouling and scaling of the membranes. These processes cause a decrease in water flux through the membrane and a decrease in retention. Much attention is paid, especially in case of nanofiltration and hyperfiltration, to prevent fouling of the membrane by an intensive pretreatment and the regular removal of fouling and scaling layers by means of mechanical, physical or chemical treatment. 

Cleaning. Fouling films are removed from die membrane surface by chemical and mechanical methods. Dissolved fouling material may pass into the membrane pores. Reprecipitation upon rinsing must be avoided. Membrane-swelling agents, such as hypochlorites, flushout material which may be lodged in the pores. 

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