Influence of Surface Defects and Aging of Coated Surfaces on Fouling Behavior

To avoid the negative effects caused by fouling in heat exchanger equipment, the heat exchanger surface can be modified energetically or mechanically. Thus, mechanical, chemical, and thermal stability of the coatings with respect to the fouling and cleaning conditions is crucial. The surface is typically characterized by the measurement of the contact angles of different wetting fluids to calculate surface energy and tactile roughness measurements. The influence of several cleaning and fouling cycles on surface energy and the composition of the coatings has been investigated. The experimental investigation of different cleaning methods from acid to base solution displays the influence of the interface reactions on the surface energy. Structural analysis of the plasma-activated chemical vapor deposition (PACVD) coatings show a build-in of oxygen inside the a-C:H matrix with time, resulting in higher surface energies and an increase of polar interactions. Also, structural defects of the coatings have been analyzed by a defined disturbance of the coating process or mechanical treatment of the already coated material. These defects act as a starting point for crystallization fouling due to reduced activation energy of nucleation. Depending on the interface and process conditions, defects can enhance fouling if the crystals are able to adhere on the coated surface. The results should lead to a better understanding of interface reactions, stability of coatings, and the aging of surfaces.     

Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

ABSTRACT

Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO₃ scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO₃ scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.     

影响换热面结垢诱导期的表面特性研究

换热面结垢是一个普遍存在的问题,而结垢诱导期的长短对污垢形成过程具有重要的影响,即使在相同实验条件下,不同材料换热面的结垢诱导期仍相差较大.因此本文通过对附着在换热面上的半球形污垢晶核进行受力分析,发现污垢晶核与换热面之间的附着力对其结垢诱导期长短起决定性作用,然后根据颗粒与平板间附着力模型,计算了污垢晶核与具有不同表面能的换热面间附着力,并与相应的结垢诱导期进行对比.结果表明:结垢诱导期的长短与换热面的表面能、污垢晶核与换热面间的附着力及表面粗糙度尺度有关.     

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.     

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.     

Investigation of adhesion of CaCO3 crystalline fouling on stainless steel surfaces with different roughness

Fouling adhering experiments on AISI 304 stainless steel surfaces with different roughness had been performed in boiling supersaturated calcium bicarbonate solution. The effect of surface roughness on adhesion of fouling is limited, and the adhesion of fouling does not have a simple linear relationship with the surface roughness of samples. The surface with roughness in middle is more easily induced to form “transitional interface” which connects the fouling and matrix surface. It is also found that the crystalline types of fouling are changed in the fouling process due to the variation of metallic ions in reaction solution.     

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.     

Nucleate pool boiling heat transfer characteristics of dilute Al2O3–ethyleneglycol nanofluids

Pool boiling heat transfer coefficients of dilute stabilized Al₂O₃–ethyleneglycol nanofluids as possible coolant fluid are experimentally quantified. The influence of different parameters such as heat flux, heating surface nano-roughness, concentration of nanofluids and fouling resistance on the pool boiling heat transfer coefficient of alumina nanofluids has experimentally been investigated and briefly discussed. Results demonstrated that there are two heat transfer regions with different mechanisms namely free convection and nucleate boiling heat transfer. Studies on the influence of parameter demonstrated that with increasing the heat flux, the pool boiling heat transfer coefficient of nanofluids significantly increases. In contrast, with increasing the concentration of nanofluid, due to the deposition of nanoparticles on the surface, the average roughness of the surface and the heat transfer coefficient dramatically deteriorate, while a significant increase in fouling resistance is reported. Also, studies reveal asymptotic and rectilinear behaviors of fouling resistance parameter in nucleate boiling and free convective domains.     

热交换器防垢涂层材料的研究进展

热交换器是工业中传热传质的重要设备,广泛应用于各个领域,但热交换器表面易结垢的问题严重影响了其运行效率.表面防垢涂层技术是解决热交换器表面结垢的一个重要研究方向.本文简要介绍表面涂层材料表面特性(表面能、接触角、粗糙度、耐腐蚀性)对基体表面结垢行为的影响,为揭示不同材料表面结垢行为差异提供依据;同时,对防垢涂层材料的类...     

A Critical Review of Basic Crystallography to Salt Crystallization Fouling in Heat Exchangers

Fouling formation on heat exchanger surfaces due to crystallization of inverse solubility salts is one of the fundamental problems in process industries. Despite numerous studies carried out in recent years, comprehensive understanding of crystallization fouling mechanism remains a challenge to chemical engineers. In this review, we first focus on the basic crystallography during deposition of calcium salts, paying attention to crystal structures and crystal forms, as well as nucleation and the subsequent crystal growth process. We then endeavor to relate a number of factors to fouling rate, which may be classified into three categories: solution composition, operating parameters, and heat exchanger surface characteristics. Each aspect is discussed from the crystallization viewpoint (science) and in terms of possible industrial applications (practice). Combining the basic knowledge of crystallography with the information from experimental investigations, several fouling mitigation methods have also been described that may reduce fouling. It is hoped that some of the ideas discussed here will provide possible economic and environmental benefits. Finally, we also try to throw some light on the future direction for research.     

海水换热器析晶污垢物相分析及动力学研究

海水换热器常用于沿海能源和化工领域的海水直流冷却系统和循环冷却系统。海水在热交换的过程中容易产生析晶结垢,造成换热效率下降和管路堵塞,这些污垢的组成及结垢机理是目前研究热点之一。本研究搭建一套海水结垢的可视化实验装置,对海水在换热过程中形成的析晶污垢进行了物相分析和动力学研究。通过X射线衍射(X-ray Diffraction,XRD)、扫描电镜(Scanning Electron Microscope,SEM)和X射线能谱(Energy Dispersive X-Ray,EDX)分析海水换热器析晶污垢的元素组成和晶体结构,结果显示污垢的元素组成和晶体结构与水合氢氧化镁铝(Mg_6Al_2(OH)_(18)·4.5H_2O)十分相似。基于Kern-Seaton模型,建立适用于海水析晶污垢的动力学模型,并根据实验结果对结垢过程的动力学参数进行拟合。结果表明:在不锈钢表面海水析晶结垢的表观活化能为1.54×10~4J/mol,在实验工况下海水析晶结垢是一种扩散控制过程。     

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%.     

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.     

Aging and Thermal Conditioning of Modified Heat Exchanger Surfaces—Impact on Crystallization Fouling

ABSTRACT

In many research studies diamond-like-carbon coatings are used to change the wetting behavior by varying the solids´ surface free energy of heat exchanger surfaces to mitigate crystallization fouling. For future industrial application, the stability of their specific surface properties, exposed to fluidic, thermal, and chemical stresses, determines their efficiency. Therefore, fluidic thermal and cleaning stresses applied to the coating are investigated. Cleaning procedures with acid, base, and heat treatment over multiple cycles were conducted in order to investigate the solids´ surface free energy over time and thereby the stability of the coating. From this information an optimal conditioning to set constant surface properties was derived. Furthermore, the fouling behavior of CaSO₄ on new and conditioned coatings was investigated in order to identify repeatable and favorable surface properties for fouling reduction. For all coatings the cleaning treatments and fouling experiments provided changes in the energetic surface properties, dominated by the change of polar/γ^? content. Most probably these changes originate from varying elementary composition and structure of the coating.     

Measuring Local Crystallization Fouling in a Double-Pipe Heat Exchanger

Abstract

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.     

基于格子Boltzmann方法的微通道受热表面析晶污垢模拟

针对现有的污垢析晶沉积模型不能有效模拟真实污垢生长的问题,建立了一种引入析晶沉积动力学模型的多物理场耦合数值模型。模型基于格子Boltzmann方法和有限差分方法,模拟了微通道非等温热表面上近壁面处的沉积物溶质质量浓度分布和污垢生长过程,研究了流速、壁温和沉积物溶质质量浓度对微通道热表面污垢析晶沉积的影响。结果表明：沉积初始时刻流速和壁温对近壁面沉积物溶质质量浓度分布具有不同程度的影响,随着污垢不断生长,污垢-流体界面处的析晶沉积速率减小;相比于流速,沉积物溶质质量浓度对污垢热阻的影响更为显著。     

Fouling Propensity of Modified Heat Transfer Surfaces

Abstract

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.     

Heat Exchanger Fouling in Phosphoric Acid Evaporators

Multistage shell and tube evaporators are frequently used in phosphoric acid plants to increase the concentration of dilute phosphoric acid to 52–55 wt% P₂O_5. The concentrated phosphoric acid solution is supersaturated with respect to calcium sulfate. As a result, part of the calcium sulfate in the liquor deposits on the heat exchanger tube walls. Because the thermal conductivity of these scales is very low, thin deposits can create a significant resistance to heat transfer. Therefore, regular cleaning of heat exchangers is required, frequently at shorter than biweekly intervals. As the major costs in modern phosphoric acid plants are the cost of energy, a thorough understanding of the fouling kinetics and of the effects of various operational parameters on the behavior of calcium sulfate is required to improve operation and design of the shell and tube heat exchangers, which are extensively used. In this investigation, a large set of heat exchanger data was collected from shell and tube heat exchangers of the phosphoric acid plant of the Razi Petrochemical Complex in Iran, and the fouling deposits were analyzed with respect to appearance and composition. The overall heat transfer coefficients and fouling resistances were evaluated at different times, and a kinetic model for the crystallization fouling was developed. It is shown that the crystallization rate constant obeys an Arrhenius relationship with an activation energy of 57 kJ/mol. The average absolute error of 12.4% shows that the predictions of the suggested model are in good agreement with the original plant data.     

Fouling surface finish evaluation

Fouling of a surface takes place as the result of a series of complex reactions that cause deposits to form on process surfaces. For many conditions, fouling can be reduced but not necessarily eliminated. The materials considered here are: carbon steel, stainless steel, and aluminium with typical finishes.Sample plates were placed vertically in tanks and exposed to untreated lake water for various time periods. Results are presented that compare surface roughness over time, for the materials/surfaces considered. The progressive change in surface appearance with increasing immersion times is also presented.Stainless steel samples showed a relatively small change in surface appearance for most periods of immersion, with a small increase in surface deterioration for increasing immersion times. Brite aluminum, an aluminium alloy with an anodized surface film, performs similar to stainless steels. Cold rolled carbon steel has the largest variation of surface appearance over time.This review includes observations on fouling and process surface materials/finishes. Conclusions and observations regarding the materials that are commonly used in designs when fouling may be a concern are presented here. Photographs of material frontal surfaces and transient surface roughness are given for a variety of surfaces.