Crystallization Fouling Research Articles

Adhesion of single crystals on modified surfaces in crystallization fouling

In crystallization fouling it has been observed that during a certain initial phase the fouling is formed by a non-uniform layer consisting of a population of single crystals. These single crystals are frequently formed by inverse soluble salts such as CaCO3. During heterogeneous nucleation and heterogeneous growth an interfacial area between the crystal and the heat transfer surface occurs. The development of this interfacial area is the reason for the adhesion of each single crystal and of all individual crystals, once a uniform layer has been built up. The emerging interfacial area is intrinsic to the heterogeneous nucleation of crystals and can be explained by the thermodynamic principle of the minimum of the Gibbs free energy. In this study CaCO3 crystals were grown heterogeneously on untreated and on modified surfaces inside a flow channel. An untreated stainless steel (AISI 304) surface was used as a reference. Following surface modifications were investigated: enameled and electropolished stainless steel as well as diamond-like-carbon based coatings on stainless steel substrate. The adhesion was measured through a novel measurement technique using a micromanipulator to shear off single crystals from the substrate which was fixed to a spring table inside a SEM.

Crystallization fouling of CaCO3 – Analysis of experimental thermal resistance and its uncertainty

Crystallization fouling occurs when dissolved salts precipitate from an aqueous solution. In the case of inversely soluble salts, like calcium carbonate (CaCO3), this may lead to crystal growth on heated walls. Crystallization may also take place in the bulk solution either via homogeneous nucleation or heterogeneous nucleation on suspended material.In this paper, surface crystallization of CaCO3 and crystallization in the bulk fluid and its effect on the fouling rate on a heated wall are studied. The fouling experiments are done in a laboratory scale set-up of a flat plate heat exchanger. Accuracy of the results is analyzed by uncertainty analysis. SEM and XRD are used to determine the morphology and the composition of the deposited material.The uncertainty analysis shows that the bias and precision uncertainties in the measured wall temperature are the largest source of uncertainty in the experiments. The total uncertainty in the fouling resistance in the studied case was found to be ±13.5% at the 95% confidence level, which is considered to be acceptable.Surface crystallization rate is found to be controlled by the wall temperature indicating that the surface integration dominates the fouling process. The flow velocity affects the fouling rate especially at high wall temperature by decreasing the fouling rate with increasing flow velocity. Crystallization to the bulk fluid is found to enhance significantly the fouling rate on the surface when compared to a case in which fouling is due to crystal growth on the surface.

Influences of bubble formation on different types of heat exchanger fouling

In this paper, a systematic comparison is performed to investigate fouling of suspended particles under forced convective and subcooled flow boiling heat transfer. For this purpose, two different types of fouling are separately considered: crystallization fouling of dissolved CaSO4 particles in water and particulate fouling of suspended Al2O3 particles in n–heptane. The effect of hydraulic parameters such as fluid velocity and also bubble generation under subcooled flow boiling are studied. Results of the experiments demonstrate that creation of boiling condition in the heat exchanger has opposite influence in these two types of fouling. It means that bubble generation on the heat transfer surface promotes scale formation under crystallization fouling. This is due to the fact that increased bubble generation creates higher supersaturation beneath the vapor bubble, therefore, increasing the crystal concentration in the boundary layer. On the other hand, boiling condition inhibits the scale formation under particulate fouling because the suspended particles are repelled from the boundary layer by the strong turbulences created by the swarm of bubbles.

The impact of crystallization fouling on a microscale heat exchanger

Due to their very high surface-to-volume ratios micro heat exchangers provide possible advantages in heat and mass transfer operations. Besides the potential of process intensification and low investment the process stability of these micro structured devices is a required criterion for industrial applications. Micro structured devices are very sensitive to unwanted deposition such as crystallization of inverse soluble salts on the surface (fouling). In general, fouling results in (i) an increase of the pressure drop, (ii) a decrease of the heat exchanger performance, (iii) a maldistribution of the flow in the micro structures and (iv) a change of the residence time behavior. This paper presents investigations about the impact of crystallization fouling on the heat transfer performance of a micro heat exchanger. Fouling experiments with calcium carbonate (CaCO3) were analyzed regarding thermal and fluid dynamic behavior. The observed fouling developed heterogeneously from a supersaturated solution in micro channels and caused a decrease of the heat transfer performance and a strong increase of the pressure drop. The extracted fouling resistances Rf were in the range of 10−5–10−3m2KW−1. In general, the fouling behavior in microscale is comparable to that in macroscale.

Fouling and fouling mitigation on heated metal surfaces

An apparatus was built to study heat transfer fouling on different heat exchanger pipe surfaces by visually observing the progressive fouling deposition under the same solution conditions. Test pipes were centrally located in a cylindrical tank with a concentric vertical agitator to give constant and uniform flow conditions near the pipe surface. Pipes with either smooth or roughened surfaces provided quantitative data on the progressive build-up and the composition of the deposits. The calcium sulphate deposition on four different metal surfaces (copper, aluminium, brass, and stainless steel) was investigated. The results show that fouling increases with time but at a decreasing rate over set intervals. The deposition also increases with the increasing thermal conductivity of the metal, or the total surface energy. Chemical reaction fouling along with particulate and crystallisation fouling occurred on reactive surfaces when corrosive chemicals were used, and this was compared with crystallisation-only fouling on non-reactive surfaces. Bleached Kraft softwood pulp fibres at various concentrations were added to the fouling solution to study their affects on fouling on the hydraulically smooth pipes. Fouling was retarded with only a low fibre concentration and reduced further as fibre concentration was increased.

The use of turbulence generators to mitigate crystallization fouling under cross flow conditions

Different types of turbulence generators were used to mitigate crystallization fouling of a water-cooled pipe at 5°C in a cross flow of hot sodium sulphate (Na2SO4) solution. The Reynolds number of the hot solution (based on pipe diameter, RehD) was varied from 250 to 485 at a constant temperature, Th=40°C. The Reynolds number of the cold water within the pipe was 25,000. Four types of turbulence generators were investigated, namely, punch plates of different blockage area, wire mesh of 50% blockage area, delta wings, and an identical pipe. These were placed upstream from the cold pipe, which was aligned normal to the hot cross flow. The surface temperature of the pipe was measured at four positions for all conditions. It was found that the turbulence generators increases the heat transfer coefficient and decreases the fouling resistance appreciably. For example, the punch plate with blockage area of 80% yielded an 85% reduction in fouling resistance at higher Reynolds numbers. However, the extent of the benefit depends on the shape of the turbulence generator, solution Reynolds number, and blockage area. For example, no appreciable effect was detected for the case where upstream pipe was used for the turbulence generator. Considerable differences in local surface temperature at various positions were noticed. The positions deduced to be subjected to the highest turbulence intensity were found to exhibit the highest local temperatures and thus lower fouling resistance.

The effects of temperature and hydrodynamics on the crystallization fouling under cross flow conditions

Crystallization fouling experiments were performed in a channel for a cross flow of hot saturated sodium sulphate (Na2SO4) solution over a pipe containing cold water. The heat transfer coefficient and fouling thermal resistance were determined from measurements of local surface temperature and the thickness of the crystalline fouling layer. A systematic assessment of the asymptotic variation of these parameters with time was obtained for a range of hot salt solution Reynolds number based on pipe diameter (RehD) of 165–485, salt solution bulk temperature of 30 °C–50 °C, and cold water (at 5 °C) Reynolds number in the pipe (Rec) of 7500–25000. It was found that the temperature of the salt solution has a large effect on the fouling rate. Increasing the pipe surface temperature decreases the crystallization fouling rate. Increasing the hot solution Reynolds number, via the velocity, decreases the fouling thermal resistance, while increasing the cold water Reynolds number, via the velocity, decreases the fouling thermal resistance, but at the same time it increases the fouling layer thickness. The crystallization process that applies here was found to be under activation control (i.e. chemical kinetic control) so that the mass transport of salt plays only a minor role.

Induction Time in Crystallization Fouling on Heat Transfer Surfaces

AbstractThis work deals with the induction period in crystallization fouling on heat transfer surfaces. While the crystal growth period can be calculated, the influencing effects in the initial stage of crystallization fouling are not fully understood and quantified so far. To identify the factors influencing the induction time, experiments with calcium sulfate were performed on modified surfaces. As surface parameters the free surface energy, roughness and topography were determined by drop shape analysis and/or AFM. The visual information by microscopic studies with a SEM shows different behavior in the initial stage of crystallization on modified surfaces at constant process conditions. These microscopic findings could be verified with induction times of fouling experiments, leading to a prediction of the induction time based on surface parameters and supersaturation of the salt solution.

Study on Adsorption of CaCO<sub>3</sub> in Metal Surface Based on Electrochemical Techniques

The study of calcium carbonate adsorption is important significance to understand the fouling mechanism that is a major component of heat exchanger fouling. The influence on the adsorption of CaCO3 crystallization fouling was researched in different material, roughness and supersaturation by the electrochemical impedance-time method. The results show that the material is not the essential influence on CaCO3 absorption by analyzing the CaCO3 adsorption layer data. The optimum conditions of maximum adsorption CaCO3 quality: the roughness of the electrode polished by the sandpaper of 360 mesh, the supersaturation of 0.291g / L CaCO3 solution. The experimental results of supersaturation are concordant with the roughness experiments. When the supersaturation is the middle value, the maximum adhesive force can reach the maximum. So the regulation of calcium carbonate supersaturation and roughness of heattransfer equipment can prevent the fouling. This study is a powerful detecting method to research the influence of the roughness on the adsorption of CaCO3 crystallization fouling.

Density Functional Theory Studies on the Formation of CaCO3 Depositions on Cristobalite, Diamond, and Titanium Carbide Surfaces

Fouling caused by inversely soluble salts, like CaCO3, is a general problem on heat transfer surfaces. Carbonate depositions are typically cleanable with acids, but costs of energy losses, operation, and maintenance are significant. In this study, formation of CaCO3 depositions was investigated on cristobalite, diamond, and titanium carbide surfaces. The aim of the study was to clarify the detailed mechanisms of crystallization fouling during the initiation on crystalline phases existing in materials used as coatings (SiOx, TiCN, diamond-like carbon [DLC]), and to compare the results to the fouling mechanism of stainless steel (Cr2O3). In experimental studies of fouling, detailed mechanisms and description of sterical and electrostatic factors of surfaces are often very much simplified. In this work, molecular modeling was used to describe surface structures and to investigate the effect of process fluid (water) on the structures. The adsorption of water can be molecular or dissociative. During the dissociative adsorption, hydroxylated surface structures are formed. The existence of hydroxyl groups on the surfaces has an effect on the fouling mechanism. First, the dissociation probability of water on different surfaces was determined according to the adsorption mechanism and energies, and then the attachment of CaCO3 onto optimized and hydroxylated surfaces was investigated. As a result, the formation mechanism with detailed intermediate steps of CaCO3 deposition was obtained. The fouling takes place via hydrogen carbonate intermediates, but the final deposition structure was found to vary between surfaces.

10.2478/s11814-009-0315-3

Heat exchangers are an important part of industrial processes as they handle a major portion of total energy consumption. Fouling could have serious impact on their performance and hence affect the economic performance of the process plant. The aim of this work was to simulate the crystallization fouling process in a heat exchanger by developing a C++ program and adopting UDF functions through Fluent software; and hence evaluate all the given models and consequently implement the model which would best suit our particular case. The finding of this work would enable us to evaluate the thickness and fouling rate in the heat exchangers. Furthermore, the effect of pulsating flow on the crystallization fouling of calcium sulfate (CaSO4) in the heat exchanger was also investigated, and the effect of operation of different amplitude of the oscillations (10–70) and frequencies (1.59–12.73 Hz) on the fouling of this compound was studied.

Experimental observation of surface morphology effect on crystallization fouling in plate heat exchangers

In this study, stainless steel test plates with different surface roughness and textures, which are used as the heat transfer surface of a plate heat exchanger, are tested individually in calcium carbonate fouling experiments. The present experimental results clearly indicate a strong correlation between the surface roughness and the amount of crystallization fouling deposit. Through detailed image analysis, four stages of the formation of crystallization fouling are identified, and the impact of the surface morphology on the extent of crystallization fouling is described qualitatively.

Roughness and constriction effects on heat transfer in crystallization fouling

This contribution addresses apparent negative fouling resistances for crystallization fouling. Two effects contribute to this phenomenon; surface roughness enhances heat transfer in the roughness controlled phase. In the crystal growth phase, surface roughness as well as the constriction of flow cross section due to the fouling layer build-up is taken into consideration. Fouling experiments were carried out in a double pipe heat exchanger with a supersaturated aqueous CaSO4 solution at a Reynolds number of 17,500 corresponding to a flow velocity of 0.65ms−1. The measured pressure drop between inlet and outlet allowed the calculation of the integral friction factor for the current surface roughness. With the given friction factor it was possible to estimate the actual heat transfer coefficient of the inner tube. Accounting for the increase in heat transfer caused by surface roughness in the roughness controlled phase, the fouling resistance was recalculated. In the subsequent growth phase flow acceleration due to constriction effects is considered in addition to the roughness effect. Overall the integral fouling resistance and consequently the deposit thickness are underestimated by a factor of up to 2.5 when simply using heat balance. With the proposed approach apparent negative fouling resistances can be eliminated quantitatively.

Crystallization fouling of finned tubes during pool boiling: effect of fin density

Bubble characteristics such as density, size, frequency and motion are key factors that contribute to the superiority of nucleate pool boiling over other modes of heat transfer. Nevertheless, if heat transfer occurs in an environment prone to fouling, the very same parameters may lead to accelerated deposit formation due to concentration effects beneath the growing bubbles. This has led to the widely accepted design recommendation to maintain the heat transfer surface temperature below the boiling point if fouling may occur, e.g., in seawater desalination. The present paper aims at investigating the formation of deposits on finned tubes during nucleate pool boiling of CaSO4 solutions. The test finned tubes are low finned tubes with fin densities of 19 and 26 fins/in. made from Cu–Ni. The fouling experiments were carried out at atmospheric pressure for different heat fluxes ranging from 100 to 300 kW/m2 and a CaSO4 concentration of 1.6 g/L. For the sake of comparison, similar runs were performed with smooth stainless steel tubes. The results show that: (1) the fouling resistance decreases with increasing fin density, (2) fouling on the finned tubes was reduced with increasing nucleate boiling activity and (3) if any fouling layer occurred on the finned tubes it could be removed easily.

Influence of sintering on deposit formation during pool boiling of calcium sulphate solutions

The objective of this work is to determine the influence of sintering effects on crystalline deposits formed during pool boiling of CaSO 4 solutions. Fouling experiments have been performed with plain tubes where the surface temperature of the heating element was varied to investigate its effect on boiling and crystallisation. Samples of the deposit layers were removed at the end of each fouling run for subsequent analysis using the scanning electron microscope. It was found that sintering of the fouling layer occurs if the temperature of the fouling layer is above a minimum sintering temperature, and that the fouling resistance reaches an asymptotic value if sintering takes place. Operating heat exchangers under pool boiling at a temperature above the minimum sintering temperature of the fouling layer accelerates the asymptotic behaviour and leads to a thinner and more compact deposit, compared to non-sintering conditions where the fouling layer thickness and thermal resistance continuously increase. Crystallisation fouling during pool boiling mainly occurs at sites where the steam bubbles grow, i.e. large pores in the deposit known as steam chimneys. However, as sintering takes place, the size and the number of pores are reduced, leading to reduced number and size of bubbles, and consequently to a declining fouling rate. For pool boiling heat transfer, the sintering-related change in fouling layer from a porous structure to a fully sintered structure is the main cause for the asymptotic behaviour.

Prediction of thickness and fouling rate in pulsating flow heat exchangers, using FLUENT simulator

Heat exchangers are an important part of industrial processes as they handle a major portion of total energy consumption. Fouling could have serious impact on their performance and hence affect the economic performance of the process plant. The aim of this work was to simulate the crystallization fouling process in a heat exchanger by de- veloping a C ++ program and adopting UDF functions through Fluent software; and hence evaluate all the given models and consequently implement the model which would best suit our particular case. The finding of this work would enable us to evaluate the thickness and fouling rate in the heat exchangers. Furthermore, the effect of pulsating flow on the crystallization fouling of calcium sulfate (CaSO4) in the heat exchanger was also investigated, and the effect of operation of different amplitude of the oscillations (10-70) and frequencies (1.59-12.73 Hz) on the fouling of this compound was studied.

Mitigation of ice crystallization fouling in stationary and circulating liquid–solid fluidized bed heat exchangers

Liquid–solid fluidized bed heat exchangers are attractive ice crystallizers since they are able to mitigate ice crystallization fouling and exhibit high heat transfer coefficients. Experiments show that the fouling removal ability of stationary fluidized beds increases with decreasing bed voidage (95–80%) and increasing particle size (2–4 mm). The removal of ice crystallization fouling appears to be more effective in circulating fluidized beds, especially at high circulation rates. Fouling removal is realized by both particle–wall collisions and pressure fronts induced by particle–particle collisions. A comparison between ice crystallization experiments and impact characteristics shows that the removal rate is proportional to the impulse exerted on the wall. A model based on these phenomena is discussed and predicts the transition temperature difference for ice crystallization fouling in both stationary and circulating fluidized beds with an average absolute error of 9.2%.

Extending the Induction Period of Crystallization Fouling Through Surface Coating

To minimize the negative effects of scale formation in heat exchangers, new anti-fouling strategies are focusing on the modification of heat transfer surfaces. These modifications should lead to tailor-made surfaces for different technical applications. The aim of this surface modification is the extension of the induction period to minimize the negative effects of fouling and maximize the endurance of the heat exchanger. To achieve this, different surface coatings on stainless steel were investigated with respect to fouling tendency. The effects of flow velocity with respect to Reynolds number on the induction time of CaSO4 crystallization fouling were tested in different test units. Diamond-like carbon (DLC) coatings extend the induction time at every measured flow velocity. At higher Reynolds numbers, the effect of different surface crystallization due to energetic modification is reduced because of the dominating effect of the low adhesive surface. Thus the induction time can be extended by the factor of 2 for low fluid velocities (DLC or SICON®) and by more than 14 for higher Reynolds numbers (DLC and SICON®). The combination of limited nucleation spots due to electro-chemical treatment of the substrate before coating can give a tailor-made surface with maximum induction time for crystallization fouling.

Experimental Investigation of Crystallization Fouling on Grooved Stainless Steel Surfaces During Convective Heat Transfer

The beneficial aspects of enhanced or extended heat transfer surfaces may be offset if operated under fouling conditions. In this article, preliminary experimental results for crystallization fouling of CaSO4 solutions onto surfaces with different structures are reported. Flat stainless steel plates (50 mm × 59 mm) with “V”-shaped grooves on the side of fluid flow were used as heat transfer surfaces. Experiments were carried out under both clean and fouling conditions to discern how the same surface structures perform under such circumstances. In addition, the impact of both the direction of grooves with respect to fluid flow (crossed, longitudinal, and mixed flow grooves) and the groove dimensions has also been investigated. Fouling trends are discussed in terms of induction time and fouling rate. Significant differences have been found for the various flow conditions.

Effect of the microstructure on the properties of Ni–P deposits on heat transfer surface

In the present work Ni–P deposits with various microstructures were prepared on the low carbon steel substrate by adjusting the electroless plating processing parameters. The results of anti-fouling experiments show that Ni–P deposits can greatly inhibit the adhesion of crystallization fouling in comparison with uncoated copper, stainless steel and carbon steel surfaces. The further experiments indicated that the amount of nanocrystalline phase in the Ni–P deposit affects the surface free energy, as well as changing the anti-fouling and corrosion-resistant properties of the deposit. When the phase structure of the Ni–P deposit is almost amorphous phase, the Ni–P deposit exhibits the best anti-fouling and corrosion-resistant properties. The amorphous Ni–P deposit exhibits high corrosion resistance which is attributed to its extreme structure homogeneity without preferential corrosion paths like grain boundaries or other structural defects, and the higher corrosion resistance could ensure this deposit is not easy to form “transitional interface” connecting fouling and matrix.