Crystallization Fouling Research Articles

A Review of Crystallization Fouling in Heat Exchangers

A vast majority of heat exchangers suffer from unwanted deposition of material on the surface, which severely inhibits their performance and thus marks one of the biggest challenges in heat transfer. Despite numerous scientific investigations, prediction and prevention of fouling remain unresolved issues in process engineering and are responsible for large economic losses and environmental damage. This review article focuses specifically on crystallization fouling, providing a comprehensive overview of the state-of-the-art of fouling in heat exchangers. The fundamentals of the topic are discussed, as the term fouling resistance is introduced along with distinct fouling behaviour, observed in laboratory and industrial environments. Insight into subsequent phases of the fouling process is provided, along with the accompanying microscale events. Furthermore, the effects of fluid composition, temperature, flow velocity, surface condition, nucleate boiling and composite fouling are comprehensively discussed. Fouling modelling is systematically reviewed, from the early work of Kern and Seaton to recently used artificial neural networks and computational fluid dynamics. Finally, the most common fouling mitigation approaches are presented, including design considerations and various on-line strategies, as well as off-line cleaning. According to our review, several topics require further study, such as the initial stage of crystal formation, the effects of ageing, the interplay of two or more fouling mechanisms and the underlying phenomena of several mitigation strategies.

Crystallization Fouling in Domestic Appliances and Systems

Formation of mineral scale, particularly calcium carbonate, has long been a concern in domestic appliances such as boilers, steamers and washing machines. In this work, the formation of inorganic deposits was studied on solid surfaces and in the bulk solution as a result of evaporation of potable water. Scale precipitation was evaluated on metallic samples for various bulk temperatures, surface roughness and surface material as a function of time. The morphology of scale crystals from both surface and solution was evaluated. The microstructure and morphology of deposits on the surface were analyzed using Scanning Electron Microscopy and X-ray diffraction. The present work has demonstrated that deposition rates increase as the evaporation process proceeds as expected. The nature of the material and the surface roughness both affect surface deposition kinetics. The complex relationship between the saturation ratio (as the bulk solution shows precipitation) and the kinetics of bulk and surface precipitation/deposition is demonstrated. The prevailing crystalline form of calcium carbonate is aragonite. However, the polymorphic composition of surface deposition and the bulk precipitate is influenced by temperature.

Analysis of Crystallization Fouling Durability of Novel Heating Elements for Electric Water Heating

New materials and technologies offer the means to develop novel electrical heating systems with improved durability against crystallization fouling. In this study, an analysis of both conventional and novel heating elements under the influence of crystallization fouling is presented. The novel heating elements are based on two different coating technologies, namely, thermal spraying and thin film sputtering. The novel heating elements reach heat fluxes exceeding 15 W/cm2 and an electrical power of approximately 1000 W. All presented heating elements operate under clean surface conditions for approximately 1000 h without any difficulties. Different properties of the presented heating elements result in different types of fouling behavior. The duration tests also demonstrate different fouling durability, which results in different lifetimes. Furthermore, it is shown that the electrical resistance of the heating conductor is adversely affected by the fouling. A comparison shows that all novel electric heating elements exhibit longer lifetimes than tubular heating elements.

Modeling of crystallization fouling on a horizontal-tube falling-film evaporator for thermal desalination

Scale formation within horizontal-tube falling-film evaporators is an important issue for thermal desalination, because of the deterioration of heat transfer performance and added maintenance costs. Predicting the fouling process is crucial for desalination plant design, operation, and maintenance. In this paper, a model is developed to predict the crystallization fouling process during seawater falling-film flow over a horizontal tube bundle. The scaling is estimated based on deposition theory, with couple heat transfer including the in-tube steam condensation, conduction through the tube wall and the scale layer, and falling-film evaporation. The spatial and temporal variations of temperature, heat transfer coefficient, and scale thickness are predicted, and the effects of variations in process parameters of steam, feed seawater, and tube properties on the scale thickness, evaporation rate, and heat transfer coefficient for falling-film evaporation are analyzed. Comparisons to existing experimental data show the scaling layer thickness falls in the general range of reported measurements, with a slight overestimation due to neglecting the delay of scaling onset under real conditions. The scale layer thickness increases dramatically as the in-tube steam pressure is increased or the seawater flow rate is decreased. In general, the scaling layer becomes thicker on the lower tubes in the tube bundle, due to the heat and mass transfer of the falling film. The effect of tube material on scaling appears mainly dependent on thermal conductivity; thus, using polymer rather than a stainless steel tube decreases scaling and evaporation rate 96% and 88%, respectively. This work has potential to guide thermal desalination plant design, material selection, and operating parameter optimization.

Correlation between the fouling of different crystal calcium carbonate and Fe2O3 corrosion on heat exchanger surface

ABSTRACT As fouling and corrosion often occur in the heat exchanger simultaneously, it is difficult to research the two problems separately. In this work, the adsorption behaviour of Fe2O3/Fe, calcite and aragonite on the corroded surface were studied by molecular dynamics and the corrosion-fouling experiment. The results prove that the parameters of the Fe(0 0 1) surface can be affected by the Fe2O3 corrosion. The interaction energy between the calcium carbonate and substrate shows that both Fe and Fe2O3 surfaces have certain adsorption effect on calcium carbonate crystallisation fouling. In addition, the adsorption effect of Fe2O3(0 0 1) on calcite during the fouling induction period is three times than that on the Fe(0 0 1) surface. Although the aragonite is prone to adhering to the non-corroded substrate, the binding ability between aragonite and substrate is weaker than that between calcite and substrate. Moreover, according to the corrosion-fouling experiment, the precipitate on the surface of the non-corroded is mainly calcite-phase calcium carbonate mingled with the acicular aragonite-phase calcium carbonate. The mass gain of fouling increases with the increasing corrosion of the substrate. The work provides evidence that the fouling nucleation of the calcium carbonate can be influenced by the corroded surface.

Thermally Conductive Polyethylene/Expanded Graphite Composites as Heat Transfer Surface: Mechanical, Thermo-Physical and Surface Behavior.

Composites of high-density polyethylene (HDPE) and expanded graphite (EG) are prepared for heat exchangers in multi-effect distillation (MED) desalination. At 50 wt.% EG loading, the thermal conductivity of HDPE was increased by 372%. Moreover, the surface wettability of the HDPE/EG composite was enhanced by corona and RF plasma treatment as demonstrated by the increase in surface free energy from 28.5 mJ/m2 for untreated HDPE/EG to 55.5 and 54.5 mJ/m2 for HDPE/EG treated by corona and RF plasma, respectively. This enhanced surface wettability was retained over a long time with only a 9% and 18% decrease in RF and corona plasma-treated samples’ surface energy after two months. The viscoelastic moduli and the complex viscosity profiles indicated that EG content dictates the optimum processing technique. At loading below 30 wt.%, the extrusion process is preferred, while above 30 wt.% loading, injection molding is preferred. The plasma treatment also improved the HDPE/EG composite overall heat transfer coefficient with an overall heat transfer coefficient of the composite reaching about 98% that of stainless steel. Moreover, the plasma-treated composite exhibited superior resistance to crystallization fouling in both CaSO4 solution and artificial seawater compared to untreated composites and stainless-steel surfaces.

Examining the effect of ionic constituents on crystallization fouling on heat transfer surfaces

The effect of the most abundant constituents in potable water on fouling of aluminium surface has been studied systematically in this work. The role of sodium, chloride, magnesium and sulphate ions and total organic carbon (TOC) on the fouling kinetics and morphology was assessed using a once-through open flow cell. The findings showed that the fouling resistance to heat transfer increases with the concentration of chloride and sodium. A complex influence of magnesium was found on the scaling process, varying between inhibition and promotion of scale formation depending on the concentration. At high concentrations of Mg2+, the formed scale layer consists of needle-like aragonite coated by a crust of magnesium deposits. The inhibitory performance of sulphate SO42− was found to be insignificant when compared with Mg2+ under similar conditions. Even though it is undesirable in potable water, inhibition efficiencies of TOC were 31.3% and 47.9% at concentrations of 2 and 4.3 mg/L respectively. The morphology observations illustrated that the presence of TOC produces a rough scale layer.

Incorporation of surface energy properties into general crystallization fouling model for heat transfer surfaces

Crystallization fouling models for heat transfer surfaces are indispensable to provide reliable prediction of fouling behavior that would, in turn, facilitate the optimal design and operation of heat exchangers. Nonetheless, most previous models exclude surface properties in terms of surface energy and its components despite many studies confirmed otherwise. In this study, a general fouling model is developed which includes surface energy and its components. Primarily, the surface properties are incorporated into the work of adhesion. Thereafter, these properties, via a force balance, are corresponded to the shear strength of deposit in terms of removal rate. The model predicts that the shear strength of CaSO4 deposit and its adhesion on various coatings would be decreased up to 66% and 91% compared to the untreated surface, respectively. Finally, the validity of the proposed model is examined against the experimental results of CaSO4 crystallization fouling which resulted in good agreement.

Investigations on crystallization fouling on PEEK films used as heat transfer surfaces: experimental results

Kinetics and quantity of crystallization fouling of calcium carbonate on polyether ether ketone (PEEK) surfaces are studied and compared with those on stainless steel. A laboratory scale set-up of a flat plate polymer film heat exchanger was used as a screening apparatus for the fouling experiments. The scaling affinity of the polymeric surfaces was found to be low compared to the stainless steel surface under equivalent conditions. Additionally, the polymer surface appears to be easier to clean due to a low adhesiveness. The low adhesiveness of crystals on polymers is attributed to their different surface properties (surface free energy and roughness) compared to stainless steel. The quantity of the deposit is also affected by the degree of supersaturation arising from higher local temperatures and overall concentrations. The deposits on the polymeric films were analyzed with respect to their appearance, distribution and thickness. Scanning electron microscopy (SEM) and optical coherence tomography (OCT) are used to determine the morphology of the deposited material and the fouling layer thickness, respectively.

Analysis of crystallization fouling in electric water heating

In the present study, the main influencing factors of crystallization fouling in electric water heating were investigated. Dependence on the saturation index and four operating parameters was analyzed, namely: liquid temperature, surface temperature, flow rate and electrical leakage current. A focus of the study was the influence of electrical leakage current on the fouling behavior and the morphology of the deposited crystal layers. The investigation shows that the aforementioned parameters influence the fouling process and crystal size growth in a different way. The experimental results were used to find a correlation between the electrical leakage current and induction time. In addition, a modeling approach is presented which allows an estimation of the fouling mass flux.

Determination of the lead oxide fouling mechanisms in lead bismuth eutectic coolant

An increased interest in the use of liquid metals for novel energy conversion systems is present today. The Accelerator Driven System (ADS) called MYRRHA under design at the Belgian Nuclear Research Centre (SCK-CEN) is an example of such an innovative system. The use of Lead-Bismuth Eutectic (LBE) as a coolant for this reactor implies that an accurate knowledge on the chemical properties of the coolant needs to be available. An important factor is the risk of coolant oxidation due to oxygen ingress in the system. Although the formation of lead oxide (PbO) is well understood, the deposition mechanism and kinetics are not yet studied. In this work the deposition mechanism of PbO on 316L stainless steel is investigated. The evolution of the dissolved oxygen concentration during thermal cycling of LBE indicates that fouling of isothermal surfaces by PbO can only proceed by particle deposition. On the other hand, the fouling of non-isothermal surfaces by PbO is dominated by crystallization fouling. A real-time measurement of the PbO deposition rate shows an asymptotic behavior of PbO crystallization fouling. By predicting the onset of PbO nucleation and subsequent growth, a kinetic model for the crystallization fouling is put forward. Quantitative agreement between deposition rate predictions and validation measurements is obtained around 673 K.

Influences of material types on the mechanisms of crystal growth in an aggregate scaling device

Oilfield fouling has been a persistent problem for many years. In view of this problem, the key idea that the crystallization fouling process is changed form preventing and controlling to accelerating was proposed in this research. The objective of this paper was thus to reveal the effects of the characteristics of material surfaces on the mechanisms of crystal growth for use in the design of an aggregated scaling device. Initially, the influence of the material type on the crystal growth was researched using dynamic simulation experiments. Second, based on the correlations between the surface free energy/roughness/contact angle and the scaling tendency, the effects of these physicochemical parameters of the surface on the crystalline growth characteristics were essentially investigated. Third, it was determined that when the material surface was covered by a lamellar fouling layer, this material might facilitate the further aggregation and deposition of crystals.

A model for the prediction of thermal resistance of calcium sulfate crystallization fouling in a liquid–solid fluidized bed heat exchanger with cylindrical particles

Calcium sulfate fouling was determined experimentally in a heat exchanger during liquid–solid fluidized bed with cylindrical particles and forced convective (without particles) heat transfer. Then a model to predict the fouling resistance during the solid–liquid fluidized bed heating system was proposed. According to the proposed model and experimental data, a correlation is presented for the prediction of fouling resistances of calcium sulfate during liquid–solid fluidized bed heating at any time. Finally, the prediction of the presented correlation for performed fouling is compared with measured data. Results show that the absolute average percent relative error between experimental data and those obtained by the proposed model vary between 0.6 and 22.8.

Measuring Local Crystallization Fouling in a Double-Pipe Heat Exchanger

Although fouling is a problem varying in space and time, sizing and assessment of a process apparatus is almost always based on one single integral fouling resistance value. Furthermore, the integral fluid dynamic behavior, e.g. the development of time-dependent pressure drop in a heat exchanger, can be influenced by local constrictions. While it is generally possible to determine the time dependency of the integral fouling behavior, local differences are not taken into consideration at present. Therefore, this paper introduces a metrological, an incremental and a segmental approach to study the local development of crystallization fouling by calcium sulfate in a countercurrent double-pipe heat exchanger. The consecutive approaches allow for thermal, volumetric, gravimetric, and optical fouling investigations, aiming to examine the axial distribution of deposit as well as local differences in the deposit morphology. All methods provided congruent results and local fouling could be described properly. An almost clean surface at the colder end of the heat exchanger and an exponential increase of deposit thickness were observed towards the hot end. Hence, the section near to the hot water inlet turned out to be a key area with regards to increasing fouling mass and structural changes of the layer.

Fouling Propensity of Modified Heat Transfer Surfaces

As much as attention that has been paid to surface treatment as an efficient, and environmentally friendly approach toward fouling mitigation, the characterization of many innovative modified surfaces has become a matter of much debate. The latter is closely associated with the intermolecular interaction energies which would profoundly influence the adhesion of precursors onto the modified surfaces. In this study, based on the extended Derjaguin, Landau, Verwey and Overbeek (DLVO) theory, a new criterion is proposed to predict the propensity of a surface when prone to crystallization fouling or biofouling. Thereafter, the proposed criterion is examined against the present experimental results as well as those from previous studies where the required information for the determination of new criterion is available. The comparison shows that deposit formation onto heat transfer surfaces decreases with increasing the new proposed fouling propensity indicator criterion. Moreover, nearly 75% of the collated crystallization and biological fouling data points are predictable with this criterion and reasons for those that are not in compliance with the proposed criterion are discussed.

Innovative non-metal heat transfer surfaces to mitigate crystallization fouling

In this study, non-metal silicon carbide (SiC) substrates were used as non-adhesive heat transfer surfaces to eliminate the negative impacts of crystallization fouling of CaSO4 solutions. The experimental results showed that the SiC substrates, to large extent, were able to mitigate scaling in terms of lower initial fouling rates by as much as four-fold to that of baseline stainless steel (SS) substrates. The superiority can be related to the unique surface characteristic of SiC substrates in terms of higher value of the electron donor component which was 8 times higher than SS surfaces. The post-fouling examination of deposit layers revealed that the layers were very porous where only few number of crystal clusters formed on the surfaces. The deposited crystal clusters, to some extent, were also non-stickable as such that they dropped off the surface completely after each dismantling from the test rig. Larger porous deposit layer on SiC substrates showed also better thermal performance compared to similar tests for the SS substrates. The fouling runs of SiC surfaces showed similar propensity for different surface temperatures, where a reduction of surface temperature by 10 °C resulted in 89% lower initial fouling rate which is noticeable for implementation in industry. The low stickability of the deposit layer on SiC surfaces would implicitly indicate much less hazardous chemical inhibitors and/or cleaning agents would be required to clean the surface from formed deposits.

Numerical simulation of CaSO4 crystallization fouling in a rectangular channel with vortex generators

Use of passive vortex generators (VGs) is an efficient heat-transfer enhancement technique, which utilizes a special type of extended surface that mainly generates vortices parallel to the flow direction to not only improve the heat transfer performance of the heat exchangers, but also prevent the formation of fouling. In order to study the growth characteristics of fouling on the rectangular channel with different VGs, three types of VGs with the same frontal area were used in the present study and the deposition process of calcium sulfate (CaSO4) fouling was studied by numerical simulation method. First, the effects of the changes in inlet velocity from 0.5 to 0.8 m/s, concentration of CaSO4 from 2.0 to 3.5 kg/m3, and surface temperature from 320 to 350 K on fouling characteristics were investigated. Then, the Performance Evaluation Criterion (PEC) was used to evaluate the heat transfer characteristics and fouling characteristics on a channel with different types of VGs. The results showed that the fouling resistance of the three types of VGs followed the same trend. The value of asymptotic fouling resistance decreased with the increase of the velocity, and increased with the increase of the concentration and surface temperature, respectively. For the rectangular channels with three different types of VGs, when the flow resistance was taken into account, the Performance Evaluation Criterion with fouling (PECf) of the half-cylinder VG was found to be the best among the three different types of VGs.

Characterization of the Evolution of Crystallization Fouling in Membranes.

Liquid-to-air membrane energy exchangers (LAMEEs) are promising in heating, ventilating, and air-conditioning applications because they are able to use semipermeable membranes to transfer heat and moisture between air and liquid desiccant streams. However, the development of crystallization fouling in membranes may pose a great risk to the long-term performance of LAMEEs. The main aim of this paper is to characterize the evolution of crystallization fouling in membranes through the use of both noninvasive and invasive methods. Noninvasive methods are used to study the development of fouling in the LAMEE by monitoring the changes in moisture flux through the membrane and overall moisture-transfer resistance of the LAMEE. On the other hand, invasive methods are implemented to characterize fouled membranes by using optical microscopy and scanning electron microscopy (SEM) to depict the morphology of crystal deposits and energy-dispersive X-ray spectroscopy (EDX) to identify the composition of the deposits. Experiments are performed by using air to dehydrate MgCl2(aq) at two operating conditions of low and high fouling rates. The results show that the moisture flux decreases and the moisture-transfer resistance increases more considerably during the test at the high fouling rate than in the test at the low fouling rate. SEM micrographs show that cake crystal deposits cover the membrane surface in the test at the high fouling rate, whereas only few crystal particles are observed on the membrane in the test at the low fouling rate. Furthermore, the crystal deposits undergo more structural changes in the tests at the high fouling rate than in the tests at the low fouling rate, possibly because of the higher moisture transfer rate through the membrane in the tests at the high fouling rate. Finally, the SEM–EDX analysis confirms that the crystal deposits primarily consist of Mg, Cl, and O elements.

Application of indirect non-invasive methods to detect the onset of crystallization fouling in a liquid-to-air membrane energy exchanger

Liquid-to-air membrane energy exchangers (LAMEEs) use semi-permeable membranes and are designed to transfer heat and moisture between air and liquid streams in heating, ventilating, and air-conditioning (HVAC) systems. However, crystallization fouling in membranes is possible in practical HVAC applications, and this may lead to severe degradation in the performance of LAMEEs.The main aim of this paper is to apply three indirect non-invasive methods to detect the onset of crystallization fouling in a LAMEE. The methods are used to detect fouling by analyzing experimental measurements of moisture flux and moisture transfer resistance. The experimental tests involve dehydrating varying concentrations of MgCl2(aq) desiccant solutions using two types of membrane with different vapor diffusion resistances (VDRs).The results confirm that two of the three methods can effectively detect the onset of crystallization fouling in the LAMEE. Thus, the methods can be used for online monitoring of operating LAMEEs in order to timely identify the initiation of fouling. The results also show that the membrane with a lower VDR produces a higher moisture flux which leads to a higher degree of fouling than the membrane with higher VDR.

Application of membrane distillation for the treatment of anaerobic membrane bioreactor effluent: An especial attention to the operating conditions

This study was carried out by applying the direct contact membrane distillation (DCMD) into the treatment of effluent from anaerobic membrane bioreactor. The treatment efficiency of DCMD was highly emphasized, which was expected to be improved through the optimization of operating conditions. Three operating conditions, including temperature difference, cross-flow velocity and membrane pore size, were considered. The relative flux (the ratio of actual flux to initial flux) increased from 0.50 to 0.98 as the operating conditions changed and that was enhanced by the increment of temperature difference and cross-flow velocity. Regarding the wastewater treatment efficiency, except for ammonia nitrogen, the interception ratio was greater than 90.0%, which even reached 99.0% for CODCr, protein and polysaccharide by optimizing operating conditions. In addition, the interception ratio of PO43--P almost reached 100.0% under any operating condition. Further study about membrane fouling was carried out, and the crystallization fouling was found to be the main fouling type.