Fouling control

One of the major operational problems in batch (or continuous) crystallization is the fouling (or incrustation) of crystals on the heat transfer surfaces. This is because the supersaturation is normally highest near these surfaces. Once the deposition of crystals takes place on these surfaces, the accumulation of crystal deposits can increase rapidly, resulting in a severe fouling problem of the heat transfer equipment. To minimize this fouling problem, one needs to maintain sufficient agitation and a low-temperature gradient on the heat transfer surface. For batch crystallization, the 

CSD control in batch crystallization can provide a significant improvement in product quality and features and in downstream processing efficiency and economics. Two important technologies considered here include fines destruction and preparation of narrow CSD. 

Fines Destruction. In the operation of industrial crystallizers, one would usually want to avoid the fines (i.e., small crystals) since they may cause difficulties in downstream processing equipment (e.g., filtration) and affect both product quality and process economics. Excessive fines may also require a relatively long batch run time to achieve the desired final size of the product crystals. Karpinski (1981) proposed a controlled dissolution of secondary nuclei in order to improve CSD from fluidized bed crystallizers. Jones et al. (1984) first described the application of fines destruction in batch crystallization of potassium sulfate solutions. Their study demonstrated the experimental feasibility of this technology to dramatically reduce the amount of fines in the final product CSD. Their theoretical predictions, obtained from population balance models, agreed with the experimental results. 

Preparation of Narrow CSD. In batch crystallization, the withdrawal of crystal product is made only at the end of the batch 

FouUng can be controlled by altering the operating conditions (see section 3.5.3 for critical flux and pressure), chemicals addition (see inorganic fouling section 3.5.2) or by pretreatment. In this section the focus is on pretreatment. 

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Concentration polarisation fouling control Poor Good Moderate Good Very good... 

Pretreatment Requirements for RO In addition to CIP and other fouling control systems constructed within the main body of the RO frame, it is vital that all due consideration be given to providing the correct kinds of RO RW pretreatment, in order to further reduce risks of membrane fouling. Options for pretreatment include ... 

Natural microbial fouling control strategies are environmentally sensible because they have been optimized by natural selection. A sensible innovation strategy then, is to observe natural control, try to understand it, attempt an imitation, and explain the copy1. The new chlorine alternative and its industrial water treatment applications were accordingly developed, as follows. 

These data show that bromine works better than chlorine in high pH waters such as the ocean. Similarly, most industrial water is quite alkaline and therefore, a practical form of bromine is also preferred. The technical attributes of bromine antimicrobials are of value in water treatment and are apparently also worth the cost to many aquatic plants. Further observations of natural microbial fouling control systems reveal that animals also preferentially manufacture, in situ, certain bromine-based antimicrobials. 

A sensible approach to innovation in microbial fouling control technology can be simply stated Observe nature. Try to understand it. Try to imitate it. Explain the copy. There is much more to learn about natural microbial fouling control. Surely, there are many important clues still to be discovered. 

Curtrs, S.D., Silverstein, R.M. Corrosion and Fouling Control of Cooling Waters, Chemical Engineering Progress, My 1971, p. 39. 

Membrane fouling is the main cause of permeant flux decline and loss of product quality in reverse osmosis systems, so fouling control dominates reverse osmosis system design and operation. The cause and prevention of fouling depend greatly on the feed water being treated, and appropriate control procedures must be... 

P. Srijaroonrat, E. Julien and Y. Aurelle, Unstable Secondary OilAVater Emulsion Treatment using Ultrafiltration Fouling Control by Backflushing, 7. Membr. Sci. 159, 11 (1999). 

The availability of new membrane processes such as membrane contactors and catalytic membrane reactors, the progresses in membrane-fouling control and the development of new membranes with well-controlled structures and properties, are recognized as key factors for the design of alternative production systems. 

The physics of how moving water interacts with surfaces both explains why it is advantageous for organisms to attach to surfaces and provides insights into fouling, control limits, and antifouling... 

Seidel, Arza, and Menachem Elimelech, Coupling Between Chemical and Physical Interactions in Natural Organic Matter (NOM) Fouling of Nanofiltration Membranes Implications for Fouling Control," Journal of Membrane Science, 203 (2002). 

Fouling Control Moderate to Good Moderate to good... 

Microfiltiation Reliability (fouling) Cost Selectivity Better fouling control could improve membrane lifetime significandy... 

Ultrafiltration Reliability (fouling) Cost Selectivity Fouling remains the principal operational problem of ultrafiltration. Current fouling control techniques are a substantial portion of process costs... 

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

Shaalan H.F., Development of fouling control strategies pertinent to nanofiltration membranes. Euromed May 2002. 

Elaboration of effective methods of fouling control and membrane cleaning, avoiding the membrane blockage and flux decline... 

M.Y. Jaffrin, R. Ben Amar and B.B. Gupta, Membrane fouling control in cross flow filtration of wine with mineral membranes, in International Technical Conference on Membrane Separation Processes, Brighton, UK, 24-26 May 1989, Paper E2. 

In general, the major problem in heat exchanger operation is the accumulation of deposits on surfaces. To avoid taking the exchanger out of service for cleaning, on-line fouling control is often employed. The choice of method will, of course, very much depend on the fouling problem encountered. There are two basic methods control may be achieved by the use of chemical additives and by physical methods, or in some instances, with a combination of the two. 

The employment of additives for fouling control in combustion systems has a potentially excellent future, but a great deal more research work is required to put the technology on a sound footing. 

An alternative physical method for fouling control uses inserts within the tubes of a shell and tube exchanger. An example is the Spirelf system that consists of a spiral flexible metal device that is inserted and fixed into the tubes.The action of the device is to vibrate in response to the flow of fluid through the tube in which it is situated, thereby keeping the tube surface clean. The vibration is controlled to avoid erosion of the tube surface. It is claimed that the payback time is low of the order of months. 

Some physical methods of fouling control that are still in the development stage are the use of ultrasound and the circulation of polymer fibers. The latter shows excellent promise for fouling control in cooling water. 

The use of physical methods of control in cooling water systems has a major drawback in that it does not control the presence of Legionella that is demanded by legislation in many countries. It is necessary when using physical methods of fouling control to treat the water to kill bacteria before the water reaches the cooling tower or spray pond, where droplets of water are released into the atmosphere. 

Laborie, S. Cabassud, C. Durand-Bourlier, L. Laine, J.M. Fouling control by air sparging inside hollow fibre membranes—effects on energy consumption. Desalination 1998, 118, 189-196. 

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