Removal of Fouling Deposits

Removal of Fouling Deposits Chemical removal of fouling can... 

The manual operation of water jets for on-line cleaning has been used for the removal of fouling deposits for many years, although modern soot blowing equipment is often preferred. Similarly shot cleaning, which involves the release of metal shot so that as it falls under the influence of gravity, it impacts the heat-transfer surface and removes any accumulated deposits. The technique is not widespread on account of the potential damage that could occur. 

The second reason is to increase the machine s efficiency. In most cases, fouling will increase the fuel or power required for a certain task. The deposits change the flow contours. Removal of the deposits will restore the original profiles and the efficiency. 

To test for fouling, a 24 h run of a pilot-scale microreactor for azo pigment production was performed using a diazo suspension [65]. At the end of this period, the pressure loss of the microreactor increased exponentially. Special means were developed to prevent clogging and instable operation. By partial removal of the deposits, the pressure loss was brought back to normal. 

Table 11.8 lists some of the models that have been discussed in the literature [Shalhi 1993] in chronological order. The increasing model complexity with time is illustrated by Table 11.8 with gradual recognition of the importance of hydrodynamics, mass transfer and reaction kinetics. In some of the models no account of removal is taken, and indeed in some extreme examples of chemical reaction fouling, removal of a deposit once formed is indeed negligible. 

Removal of the deposit may or may not begin right after deposition has started. The rate of removal is directly proportional to both the mass of deposit and the shear stress on the heat transfer surface, and inversely proportional to the deposit strength. The particular removal process will depend on the characteristics of the deposit and this may vary with time so dtat more than one removal process may be present during the period in which fouling is occurring. 

Automatic periodic membrane flush to remove recent surface deposits, thus reducing the risk of membrane fouling... 

If deactivation is rapid and caused by a deposition and a physical blocking of the surface this process is often termed fouling. Removal of this solid is termed regeneration. Carbon deposition during catalytic cracking is a common example of fouling... 

Figure 7.16 An illustration of the efficiency of back-pulsing in removing fouling materials from the surface of microfiltration membranes. Direct microscopic observations of Mores and Davis [9] of cellulose acetate membranes fouled with a 0.1 wt% yeast suspension. The membrane was backflushed with permeate solution at 3 psi for various times. Reprinted from J. Membr. Sci. 189, W.D. Mores and R.H. Davis, Direct Visual Observation of Yeast Deposition and Removal During Microfiltration, p. 217, Copyright 2001, with permission from Elsevier...

A common problem of this technique is the gradual decrease in permeate flux associated with membrane clogging or fouling, caused by adsorption or physical deposition of particles and/or macromolecules on membrane pores. Fouling can be minimized by prior clarification (particulate removal) of the feed solution, by the selection of operational conditions that minimize interactions between membranes and macromolecules, by the use of tangential flow, or by performing intermittent back-flushing operations. 

The most common type of fouling is the precipitation of solid deposits in a fluid on the heat transfer surfaces. A layer of calcium-based deposits forms after prolonged use on the surfaces at which boiling occurs, similar to what can be observed on the inner surface of a kettle. To avoid this potential problem, water in process plants is treated to remove its solid content. 

Process "simulation. This technique is applied to equipment that is easily fouled and for which spare parallel units are provided. The fouled equipment s isolated, drained of process fluid, and filled with the cleaning solution the process operation is then simulated, thereby cleaning the equipment. An example of this route is the removal of iron oxide and copper deposits from high-pressure steam generators, using ammoniated ethylenediaminetetraace-tic acid (EDTA) solution. 

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