Fouling of Heat Exchangers: An Overview

A.???INTRODUCTION

Water is the most abundant liquid on Earth and plays a crucial role in sustaining life. Among its many unique properties, water is known as a "universal solvent" because it can dissolve, to varying degrees, nearly any naturally occurring substance. This solubility can be advantageous in some chemical processes, but it also presents significant challenges in contemporary society. Dissolved solids in water are considered contaminants, and when present in high concentrations, they can cause negative consequences. The operation of heat exchangers is no exception to this rule.

In virtually every process involving the heating of contaminated water, equipment fouling is one of the most significant problems encountered. The impacts of heat exchanger fouling include, but are not limited to, the following:

???????Increased capital investment: Applications with anticipated high fouling rates require the use of reduced heat transfer coefficients, resulting in larger heat exchanger areas.

???????Additional operating costs: As evaporator heat exchangers become fouled, more energy is required to maintain design efficiency. Increased fouling also results in higher energy input per volume of water processed, due to thermal inefficiencies and pressure drop.

???????Higher maintenance costs: Greater fouling rates necessitate more frequent heat exchanger cleaning. Both mechanical and chemical cleanings have associated maintenance costs.

???????Loss of production: High fouling reduces operational efficiency and increases downtime needed for heat exchanger cleaning.

???????Costs of remedial action: These include the expenses of chemical programs (e.g., scale inhibitors) to minimize the rate and extent of fouling, as well as the associated chemicals and/or equipment required to clean fouled surfaces.

To provide a deeper understanding of fouling phenomena, this paper discusses the overarching theories of heat exchanger fouling, with a focus on fouling control and scaling control agents.

B.??GENERAL FOULING MECHANISMS

Fouling mechanisms can be classified into five primary types based on the principal processes involved: precipitated salts, suspended solids, organics, corrosion, and biofouling. Figure 1 provides a general overview of the most common products of these mechanisms.

Precipitated Salts Fouling

Precipitated salts are sparingly soluble materials that precipitate out of a solution due to changes in solubility resulting from alterations in process conditions such as temperature, pressure, pH, and/or concentration. These salts can cause foreign material layers to crystallize and adhere to heat exchanger surfaces or increase the quantity of suspended solids. Precipitated salts typically consist of the following types: carbonate scales, sulfate scales, metal hydroxides, amorphous silica, and complex silica.

Carbonate Scales:

Carbonate scales, such as calcium, strontium, and barium, are alkaline scales. Their solubility decreases with increasing temperature and pH. Carbonate scaling can occur via direct deposit on heat exchanger surfaces or precipitation from the bulk solution. Organic material can chelate divalent ions such as calcium, strontium, and barium, reducing carbonate scale precipitation. Carbonate scales usually accumulate on hot heat exchanger surfaces in soft, white, chalky layers. They are relatively easy to remove via mechanical cleaning and readily dissolve in low pH cleaning solutions.

Sulfate Scales:

Sulfate scales tend to be very hard and often cannot be dissolved with chemical cleaning solutions. Rigorous mechanical cleaning or specialized techniques like soda or dry ice blasting may be required to remove them. Sulfate scale solubility varies directly with temperature, becoming more soluble as temperature increases. Pressure and pH have less of an impact on sulfate scale solubility compared to carbonates.

Metal Hydroxides:

Metal hydroxides, such as magnesium hydroxide (Mg(OH)2), iron hydroxide (FeOH2), and aluminum hydroxide (AlOH3), are insoluble at very low concentrations. Their solubility can be significantly impacted by changes in temperature and pH. Metal hydroxides tend to accumulate in low-velocity areas, are prone to surface adherence due to electrostatic attraction, and behave like flocculants.

Silica:

Silica is a common heat exchanger foulant, but its formation is more complex and less understood than other types of scales. Silica can exist in water in five forms: ionic (dissolved) silica, polymerized silica, colloidal silica, amorphous silica scale, and complex silica.

Suspended Solids Fouling

Suspended solids can accumulate in heat exchanger inlet channels or deposit due to electrostatic attraction. The presence of suspended solids can also accelerate other types of fouling, specifically precipitated salts, by providing nucleation sites for crystal growth.

Organics Fouling

Organic fouling may result from the precipitation of organic materials due to a decrease in solubility caused by changes in temperature, pressure, and/or concentration. Organic materials often exist in two types: hydrophobic and hydrophilic.

Biological Fouling

Biofouling refers to the development and deposition of organic films consisting of microorganisms and the attachment and growth of macro-organisms. Biofouling is not typically considered a primary fouling mechanism for heat exchangers due to high temperatures and, in some cases, high salt concentrations.

Corrosion Fouling.

Corrosion fouling is a type of fouling that occurs in heat exchangers when the materials used to construct the heat exchanger react with the fluids being processed, leading to the formation of corrosion products on the heat transfer surfaces. This can have a negative impact on the efficiency of the heat exchanger, as the build-up of corrosion products can impede heat transfer and increase pressure drop across the unit.?Typical corrosion products include oxides of metals such as iron, copper, and zinc. These corrosion products can form a layer on the heat transfer surfaces, reducing the effectiveness of the heat exchanger.

C.???Surface Accumulation Mechanisms

?Heat exchanger surfaces commonly encounter two types of fouling accumulation mechanisms: particulate deposition and mineral scale.

?Particulate Deposition

?Particulate deposition, or soft deposit, consists of colloidal and suspended matter, organic material, corrosion products, biological growth, and some precipitated salts like metal hydroxides and complex silica. This mechanism is characterized by the ex-situ formation of fouling agents, meaning they do not form on the heat exchanger surface but in the solution or upstream in the process. The rate of particulate deposition is controlled by four steps: particle transport to the surface, attachment, particle re-entrainment (removal), and ageing.

1. ?Transport to Surface: This includes various mechanisms contributing to foulant migration on the heat exchanger surface, such as gravity, turbulent diffusion, Brownian diffusion, electrophoresis, and thermophoresis.
2. Attachment: Foulant adhesion to the heat transfer surface may be influenced by factors like van der Waals forces, electrostatic forces, and external force fields at the wall surface.
3. Removal: When particulate deposition occurs on a surface, foulant removal takes place through spallation or erosion, usually starting as soon as the initial layer is deposited.
4. Ageing: Beginning with the initial layer deposition, ageing alters the crystal and chemical structures of the foulant. Dehydration, polymerization, and thermal stress development are the main mechanisms leading to ageing. Ageing can increase foulant strength and cause soft scale-like particulate precipitation to harden, affecting the removal process.

?Mineral Scaling

?Mineral scaling is a hard and dense type of fouling. The most common types of mineral scales are crystalline precipitates of carbonate and sulfate scales, as well as various calcium-based scales and amorphous silica formation. In contrast to particulate deposition, mineral scaling is an in-situ process where the foulant forms directly on the heat transfer surface. The formation process involves supersaturation, induction, nucleation, and finally, crystal growth.

1. ?Supersaturation: Governing the mineral scaling process, supersaturation refers to the solute concentration exceeding equilibrium solubility at a given temperature. Crystalline and amorphous formation require supersaturation, which can occur from solvent removal (evaporation or freezing), heating/cooling the solution, or changing its chemistry. Mineral scaling due to supersaturation is a primary concern for heat exchanger evaporators.
2. Induction: The induction period, defined as the time between supersaturation establishment and the first crystal detection, is influenced by factors like supersaturation level, agitation state, and impurity presence.
3. Nucleation: For mineral scaling to initiate, a solution must contain solid bodies, embryos, nuclei, or seeds that act as crystallization centers. Nucleation can be primary or secondary, and homogeneous or heterogeneous. Primary nucleation results from excess chemical potential, while secondary nucleation occurs when new nuclei form on pre-existing crystals.
4. Crystal Growth: Crystal growth involves solute transport from the solution to the crystal surface, adsorption, diffusion to a growth site, and integration into the crystal lattice. Influencing factors include supersaturation level, temperature, additives, solvent, and system hydrodynamics.

?D.???Factors Impacting Fouling Processes

?Fouling processes, including particulate deposition and mineral scaling, are influenced by various factors related to the heat transfer surface's immediate environment. Key factors governing fouling processes include:

1. ?Degree of supersaturation: The most critical parameter for determining fouling rates, fouling rates tend to be much higher when the concentration of potential fouling components is significantly above the average concentration for that specific component.
2. Fluid velocity: This factor affects both deposition and removal processes. Higher velocities enhance mass transfer of fouling species from the bulk fluid to the heat transfer surface and the intensity of removal forces, but often reduce surface adhesion efficiency. Deposition rates may increase or decrease with velocity, depending on the mechanisms controlling the process. Generally, increased fluid velocity reduces surface adhesion more than mass transfer, resulting in overall fouling reduction. High Reynolds numbers and shear stress decrease attachment/formation reactions at the deposit-fluid interface while increasing solid transport from the interface. High fluid velocities also help reduce suspended solid accumulation. Foulants tend to deposit more in low-velocity regions, particularly where velocities change rapidly.
3. Temperature: Solution and surface temperatures can impact fouling rates. Surface adhesion-controlled fouling processes involving chemical reactions (crystallization or polymerization) often experience increased fouling rates at higher surface temperatures due to increased reaction rates and reduced solubility of compounds like CaCO3. Fluid viscosity variations between the heat transfer surface and bulk fluid may also affect fouling species transport, due to gradients across the wall boundary layer. Typically, fouling rates increase as temperature rises, leading to a "baking on" effect—scaling tendencies increase, corrosion rates increase, reactions accelerate, crystal formation and polymerization occur more rapidly, and some antifoulants lose activity.
4. Impurities and suspended solids: These can initiate or significantly increase fouling. Suspended solids may either deposit as a fouling layer or act as a catalyst for the fouling process. In precipitation fouling, small particles may initiate the deposition process by seeding. High velocities (greater than 1 m/s, 3.3 ft/s) help prevent particulate fouling.

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E.??Fouling Control

To reduce fouling on heat exchanger surfaces, various approaches can be employed depending on the surface accumulation mechanisms involved.

Particulate Deposition Control

Fouling due to particulate foulants in heat exchangers can be addressed in several ways. Since the concentration of total suspended solids (TSS) is a major driving force for this type of fouling, reducing TSS in the feed water typically results in decreased fouling rates. TSS removal can be achieved through sedimentation, side-stream filtration, pretreatment by clarification (e.g., coagulation and flocculation), or filtration.

Mechanical methods to prevent particulate fouling include maximizing velocity and shear stress at substrate surfaces. These factors should be considered during process and heat exchanger design. The same applies to surface temperature, bulk temperature, and heat exchanger surface material selection. Rough surfaces, for instance, are known to enhance fouling rates. Also, lower temperature differences between the bulk solution and the heating surface reduce fouling.

Chemical control is another option for dealing with particulate deposition fouling. Dispersant agents can be used to disperse suspended matter, preventing coagulation and settling out of solution. Dispersant performance is affected by factors such as water chemistry, temperature, settling time, and particle size. Anionic polymers with low molecular weight are generally considered effective dispersant agents. Metal hydroxide precipitation can be reduced using chelating agents that block metal ion reactive sites, preventing metal hydroxide formation and deposition. Iron, manganese, copper, and zinc ions are particularly stable when forming complexes with chelates. Common chelating agents include ethylenediaminetetraacetic acid (EDTA), gluconic acid, citric acid, and polymer-based antiscalants (polyacrylic acid, acrylic acid, or maleic acid-based copolymers).

F. Scaling Control

Many control parameters discussed for particulate deposition control also apply to scaling control, including TSS concentration, water chemistry, temperature differences, velocity, shear stress, surface material, and roughness. In addition, scaling control can be achieved using one or a combination of four different approaches: recovery modification, acid feeding, softening, and additive treatment.

1. Recovery modification: In evaporation processes using heat exchangers, preventing mineral scaling can be achieved by reducing the concentration factor. This allows operation under conditions where scale-forming salt solubility is not exceeded. However, recovery modification may not always be practical due to the reduced treated water-to-concentrate ratio.
2. Acid feeding: Acid feeding is a pretreatment before heat exchanger operations, aiming to remove most feed water alkalinity. Typically, acid injection is performed to leave residual alkalinity of 15-20 ppm. At low pH, HCO3- ions are converted to carbon dioxide, reducing carbonate-based scale. Since gas on heat exchanging surfaces can lead to poor heat transfer, water treated with acid should be degasified before entering the heat exchanger through a decarbonator or deaerator. Acid injection is generally controlled using predetermined pH values or based on feed water flow. Incorrect acid dosing can cause severe corrosion problems at the heat exchanger surface.
3. Softening: Softening is a pretreatment to remove hardness ions (calcium and magnesium) from feed water. Softening methods include hot and cold process lime softening and sodium cycle cation exchange. The high solubility of sodium salts makes sodium ions in solution less problematic than hardness ions.
4. Additive treatment: Additive treatment, or threshold treatment, involves small dosage injections of antiscaling agents. These agents prevent mineral scale precipitation by adsorbing onto growing crystal nuclei (affecting nucleation) and by surface modification of those crystals (affecting crystal growth).

G.????????????????Fouling Control Agents

Fouling control agents, such as dispersion, chelating, biocides, and antiscaling agents, play a crucial role in water treatment programs to prevent particulate deposition and mineral scaling on heat exchanger surfaces. Some of the most common fouling control agents used to combat mineral scaling fall into three categories: polyphosphates, organophosphonates, and synthetic polymers. Each group can exhibit more than one fouling control mechanism, and some commonly used molecules are listed below.

1. Polyphosphates: Dating back to the late 1930s, polyphosphates have been used as threshold agents, with sodium hexametaphosphate (SHMP) found to be highly effective in preventing calcium carbonate in various industrial applications. Polyphosphates have advantages such as high solubility in water, cost-effectiveness, and low toxicity. However, their stability is influenced by factors like temperature, pH, concentration, and water chemistry. Above 140°F (60°C) or at acidic pH, they hydrolyze into orthophosphate, which can react with calcium ions and form calcium phosphate scale. As a result, the use of polyphosphates as antiscaling agents has declined in favor of organophosphonates and synthetic polymers.
2. Organophosphonates: Introduced in the 1960s, organophosphonates are a class of compounds that are less susceptible to hydrolysis and have advantages in temperatures exceeding 200°F (93°C). Phosphonates and phosphonic acids are the most commonly used organophosphonates for antiscaling purposes. However, under certain conditions (e.g., calcium hardness, pH, and temperature), phosphonates can react with calcium ions, leading to the precipitation of calcium phosphates and the formation of other calcium-based scales.
3. Synthetic polymers: Introduced around the same time as organophosphonates, synthetic polymers can exhibit a variety of functional groups along the polymer chain and have good performance at high temperatures (+200°F). Common synthetic polymers used as scale control agents include polyacrylic acid (PAA), polymethacrylic acid (PMAA), polymaleic acid (PMA), and proprietary polymer-based formulated blends.

• PAA: With only one functional group (a carboxyl group), polyacrylic acid is primarily used as a scale control agent but also exhibits dispersant properties. Its performance depends on its molecular weight, with low molecular weight PAA being the most widely used among synthetic polymers.
• PMAA: Polymethacrylic acid contains both methyl and carboxyl groups and is commonly used as a dispersant.
• PMA: Polymaleic acid contains two carboxyl groups and is used for scale and suspended matter control, but its dispersant abilities are lower than PAA's.
• Proprietary polymer-based formulated blends: In cases where high levels of calcium are present in the water, using PAA, PMAA, or PMA can result in the formation of insoluble calcium-polymer salts. To overcome this issue, proprietary blends can be used, typically composed of (or a blend of) polyelectrolytes, polycarboxylates, copolymers, and phosphonates.

H.????????????????References:

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