Prevention and Mitigation of Fouling in Heat Exchangers: Liquid Side, Gas Side, and Cleaning Strategies

 Prevention and Mitigation of Fouling

Ideally, a heat exchanger should be designed to minimize or eliminate fouling. For example, heavy-fouling liquids can be handled in a direct contact heat exchanger since heat and mass transfer takes place due to direct contact of the fluids over the "fill" or the surface in such an energy exchanger. The fill can get fouled without affecting energy transfer between the fluids in direct contact. In fluidized-bed heat exchangers, the bed motion scours away the fouling deposit. Gasketed plate-and-frame heat exchangers can easily be disassembled for cleaning. Compact heat exchangers are not suitable for fouling service unless chemical cleaning or thermal baking is possible. When designing a shell­and-tube heat exchanger, the following considerations are important in reducing or cleaning fouling. The heavy fouling fluid should be kept on the tube side for cleanability. Horizontal heat exchangers are easier to clean than vertical ones. Geometric features on the shell side should be such as to minimize or eliminate stagnant and low-velocity regions. It is easier to clean square or rotated square tube layouts mechanically on the shell side [with a minimum cleaning lane of ¼” in. (6.35 mm)] than to clean other types of tube layouts.

Some control methods are now summarized for specific types of fouling. Crystallization fouling can be controlled or prevented by preheating the stream so that crystallization does not occur. To control particulate fouling, use a filter or similar device to capture all particles greater than about 25% of the smallest gap size in the flow path. Eliminate any dead zones and low-velocity zones. Use back flushing, "puffing," or chemical cleaning, depending on the application. Chemical cleaning is probably the most effective cleaning method for chemical reaction fouling. For corrosion fouling, initial selection of corrosion-resistant material is the best remedy. For example, use proper aluminum alloy to prevent mercury corrosion in a plate-fin exchanger. Biofouling is usually easy to control with biocides, but must check compatibility with the exchanger construction materials. Chlorination aided by flow-induced removal of disintegrated biofilm is the most common mitigation technique.

General techniques to prevent or control fouling on the liquid or gas side are sum­marized briefly.

1.      Prevention and Control of Liquid-Side Fouling

Among the most frequently used techniques for control of liquid-side fouling is the online
utilization of chemical inhibitors/additives. The list of additives includes (1) dispersants
to maintain particles in suspension; (2) various compounds to prevent polymerization

and chemical reactions; (3) corrosion inhibitors or passivators to minimize corrosion; (4) chlorine and other biocide/germicides to prevent biofouling; and (5) softeners, poly-carboxylic acid and polyphosphates, to prevent crystal growth. Alkalis dissolve salts. Finally, filtration can be used as an efficient method of mechanical removal of particles. An extensive review of fouling control measures is provided by Knudsen (1998).

Mitigation of water fouling and the most recent review of the related issues are discussed extensively by Panchal and Knudsen (1998), where they suggest the following methods.

·     Chemical additives: dispersants or coagulators for particulate fouling; dispersants, crystal modifiers, and chelating agents for crystallization fouling; inhibitors or surface filming for corrosion fouling; and biocides, biodispersants, and biostats for biofouling.

·     Process adjustments: monitoring, modifications and replacements of devices, water flow reduction, and recirculation strategies.

·     Physical devices for cleaning: sponge-ball cleaning and the use of reversing-flow shuttle brushes.

·       Utilization of enhanced heat transfer surfaces and devices: It has sizable influence on fouling mitigation. The use of tube inserts (in particular, in refinery processes), such as wire mesh, oscillating wires, and rotating wires, is a standard method.

·       Various alternative devices and/or methods: magnetic fields, radio-frequency, ultraviolet and acoustic radiation, and electric pulsation. Surface treatment and fluidized-bed designs are also used.

·     The most frequently used technique for preventing water-side fouling is still the conventional water treatment. Strict guidelines have been developed for the quality of water for environmental concerns (Knudsen, 1998).

Heat transfer surface mitigation techniques can be applied either on- or offline. Online techniques (usually used for tube-side applications) include various mechanical techni­ques (flow-driven or power-driven rotating brushes, scrapers, drills, acoustic/mechanical vibration, air or steam lancing on the outside of tubes, chemical feeds, flow reversal, etc.). In some applications, flows are diverted in a bypass exchanger, and then the fouled exchanger is cleaned offline. Other offline techniques (without opening a heat exchanger) include chemical cleaning, mechanical cleaning by circulating particulate slurry, and thermal baking to melt frost/ice deposits. Offline cleaning with a heat exchanger opened or removed from the site include (1) high-pressure steam or water spray for a shell-and­tube heat exchanger, and (2) baking compact heat exchanger modules in an oven (to burn the deposits) and then rinsing. If fouling is severe, a combination of methods is required.

2.      Prevention and Reduction of Gas-Side Fouling

The standard techniques for control and/or prevention of fouling on the gas side are (1) techniques for removal of potential residues from the gas, (2) additives for the gas-side fluid, (3) surface cleaning techniques, and (4) adjusting design up front to minimize fouling. Details regarding various techniques for gas-side fouling prevention, mitigation, and accommodation are given by Marner and Suitor (1987).

Control of gas (or liquid)-side fouling should be attempted before any cleaning method is tried. The fouling control procedure should be preceded by (1) verification

of the existence of fouling, (2) identification of the feature that dominates the foulant accumulation, and (3) characterization of the deposit.

Some of the methods for mitigation of gas-side fouling are as follows:

·     Crystallization fouling can be prevented if the surface temperature is kept above the freezing of vapors from the gaseous stream; the solidification can be minimized by keeping a "high" velocity of freezable species, having some impurities in the gas stream, and decreasing the foulant concentration, if possible.

·     Particulate fouling can be minimized (1) by increasing the velocity of the gas stream if it flows parallel to the surface and decreasing the velocity if the gas flow impinges on the surface, (2) by increasing the outlet temperature of the exhaust gases from the exchanger above the melting point of the particulates, (3) by minimizing the lead content in gasoline or unburned hydrocarbons in diesel fuel, (4) by reducing the fuel—air ratio for a given combustion efficiency, and (5) by minimizing flow impact (e.g., flow over a staggered tube bank) or ensuring the narrowest dimension in the flow cross section, to three to four times the largest particle size anticipated.

·     Chemical reaction fouling can be minimized (1) by maintaining the right tempera­ture range in the exhaust gas within the exchanger, (2) by increasing or decreasing the velocity of the gaseous stream, depending on the application, (3) by reducing the oxygen concentration in the gaseous stream, (4) by replacing the coal with fuel oil and natural gas (in that order), and (5) by decreasing the fuel—air ratio.

·     Corrosion fouling is strongly dependent on the temperature of the exhaust stream in the exchanger. The outlet temperature of the exhaust gas stream from the exchan­ger should be maintained in a very narrow range: above the acid dew point [above 150°C (300°F)] for sulfuric or hydrochloric acid condensation or below 200°C (400°F) for attack by sulfur, chlorine, and hydrogen in the exhaust gas stream. Since sulfur is present in all fossil fuels and some natural gas, the dew point of sulfur must be avoided in the exchanger, which is dependent on the sulfur content in the fuel (Shah, 1985). From the electrochemical condition of the metal surface, the corrosion rate increases with velocity up to a maximum value for an active surface and no sizable effect for a passive surface. The pH value has a considerable role in the corrosion fouling rate; the corrosion rate is minimum at a pH of 11 to 12 for steel surfaces. Low oxygen concentrations in the flue gases promote the fire-side corrosion of mild steel tubes in coal-fired boilers. Stainless steel, glass, plastic, and silicon are highly resistant to low-temperature corrosion [T_gas < 260°C (500°F)], stainless steel and superalloys to medium-temperature corrosion [260°C (500°F) < T_gas < 815°C (1500°F)], and superalloys and ceramic materials to high-temperature corrosion [T_gas > 815°C (1500°F)]. Chrome alloys are suitable for high-temperature sulfur and chlorine corrosion, and molybdenum and chrome alloys protect against hydrogen corrosion.

3.      Cleaning Strategies

An important element in mitigating a fouling problem is selection of a cleaning strategy (i.e., the cleaning-cycle period). The cleaning-cycle period is delineated by the operation of an exchanger until the performance reaches the minimum value acceptable. Subsequently, the exchanger must be cleaned by one of the methods summarized in Section 1 or 2. In the case of asymptotic fouling in a given application, generally no cleaning is necessary. If fouling rate data are available, ideally the exchanger can be optimized based on the life-cycle cost, and accordingly, the cleaning schedule can be established.

The cleaning-cycle period may also be determined based on a regular maintenance schedule during process shutdowns. In any case, the functional relationship between the operation time and fouling resistance should be known at least partially. The importance of rational cleaning schedules based on such an understanding is critical when the allowable deviation from the process stream temperature is small compared to absolute values (steam power plant condensers), and (2) the cost of cleaning is a significant fraction of the operating cost.

Depending on the fouling process, the cleaning strategies for preventing maintenance are of two types: reliability-based and cost-based. There are three scenarios for reliabil­ity-based cleaning strategies: (1) maintenance restores exchanger performance (this is an idealized maintenance scheme, taking place at equal time intervals); (2) by decreasing the preventive maintenance time interval gradually (due to fixed degradation of perform­ance after each maintenance interval), the exchanger performance is restored; (3) for the preceding case of a fixed degradation of performance after each preventive maintenance time interval, if the maintenance takes place at equal time intervals, it reduces the exchanger performance. The cost-based cleaning strategy includes the costs associated with online chemical cleaning, offline cleaning, additional fuel/power consumption due to fouling, and severity of the financial penalty associated with exchanger performance due to fouling.

It should be noted that operating a heat exchanger at the critical risk level of a system or component is important in some applications, such as in a heat exchanger network in a refinery. In this situation, an acceptable level of heat exchanger overall heat transfer coefficient will primarily govern the maintenance strategy. However, in some situations, heat exchangers are not in a network, or in a critical system; here, maintaining the exchanger at a higher reliability level r (or at a lower risk level p) implies more frequent maintenance intervals, which can often result in increasing operation and maintenance costs. It is thus important to note that in situations in which the cost of operation and maintenance is an important factor, along with exchanger reliability (r = 1 — p), main­tenance decisions can be optimized by developing cost as a function of reliability (or risk level) and then searching for a minimum cost-based solution. This cost-optimized main­tenance solution will also result in an optimal level of heat exchanger reliability.

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References:- Marner and Suitor (1987), Knudsen (1998), Zubair et al. (1997),Knudsen (1998), Dusan P.