海水冷却水过冷沸腾及析晶结垢实验研究

通过海水冷却水在换热面过冷沸腾下析晶结垢的动态监测实验,研究在不同的换热面热流密度、海水入口温度和流速下,析晶污垢的微观形态及对换热性能影响。研究表明:海水冷却水温度对污垢层的结构影响较大,流速则明显影响垢层的厚度;流速的改变对结垢速率影响最大,流速增加一倍,结垢速率降低61%;热流密度增大一倍,换热系数的降幅增加89%。     

35℃、1mmoL/L CaCO3溶液中黄铜和紫铜表面结垢特性

黄铜和紫铜是换热器常用材料,研究CaCO3析晶污垢在这两种金属表面的生长特性是抗垢研究的第一步。在35℃、1 mmoL/L CaCO3溶液中,通过静态反应法研究CaCO3在两种金属表面的生长特性,采用称重法获得不同时间结垢重量,采用扫描电子显微镜(SEM)获得不同时间污垢的微观形貌,得到了CaCO3析晶污垢在两种金属表面的生长特性以及pH值对污垢生长的影响。在材质方面,紫铜比黄铜更易结垢;水质上,pH值升高,促进黄铜结垢,反而抑制紫铜的结垢。     

不同金属材料与海水温度对海水结垢影响的实验研究

针对海水换热结垢现象进行了实验研究。对比了镀锌铁片、黄铜、紫铜和不锈钢4种金属在海水中的结垢特征,以及4种金属表面结垢量的变化。实验结果显示,不同金属在海水中的污垢形貌及结垢量不同,镀锌铁片的结垢量最大,紫铜表面腐蚀较为严重,结垢量最小。对海水污垢进行了XRD(X-Ray Diffraction)和EDX(Energy Dispersive XRay)物相分析,结果显示,不同金属材料表面形成的海水污垢的物相组成不同,镀锌铁片表面的污垢成分主要是锌的腐蚀产物,而不锈钢表面的污垢成分主要为氢氧化镁。比较了镀锌铁片和黄铜在80℃和60℃海水中的结垢量变化,镀锌铁片污垢随着海水温度的上升而下降,而黄铜污垢随着海水温度的上升而上升。     

35_ CaCO_3溶液中不锈钢304和316表面结垢特性

不锈钢304和316是换热器常用材料,研究CaCO3析晶污垢在这两种金属表面的生长特性是抗垢研究的第一步。在35℃0.05 g/L CaCO3溶液中,通过静态反应法研究CaCO3在两种金属表面的生长特性,采用称重法获得不同时间结垢重量,采用扫描电子显微镜(SEM)获得不同时间污垢的微观形貌。结果表明:在相同条件下,不锈钢304和316表面结垢量很接近;pH升高不仅使结垢量增多而且促进四方形文石和方解石的形成,并且文石不断重结晶形成热力学最稳定晶型(方解石);pH升高使表面产生更多晶核,并且晶体分布没有规律、交叉重叠生长。     

壳管式海水换热器污垢状况的火用评价方法研究

分析了壳管式海水换热器管程结垢后换热强度及流动压降变化对换热器火用损失的影响,提出了一种利用(火用)损失系数评价换热器污垢状况的方法.该方法比通过检测污垢热阻评价换热器污垢状况的方法更全面,更简捷.     

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。     

湿法烟气脱硫系统气-气换热器的结垢分析

国内某电厂的湿法烟气脱硫系统在连续运行过程中出现了气-气换热器结垢堵塞的问题.为查明结垢原因,对气-气换热器垢样、副产品石膏、锅炉飞灰等进行了取样分析,样品的颗粒表面形态由扫描电镜分析,晶体结构采用X射线衍射法分析,相关成分由X射线荧光光谱法测定.结果表明:结垢是由于一些饱和的石膏浆液液滴在通过除雾器时未能被有效除去而被带入气-气换热器中,随后被烟气中的飞灰包覆所致.     

换热器结垢工况下换热系数变化的分析研究

介绍了换热器污垢热阻的数学模型，包括污垢沉积模型和剥蚀模型。分析了换热器结垢工况下的换热系数的变化，重点研究了时间、流体雷诺数Re和流体—污垢界面温度Ts对换熟系数K的影响，以及在结垢诱导期内换热系数K的变化。得到了冷却水流速与污垢热阻之间的关系式，界面温度Ts与污垢热阻和换热系数之间关系的示意图，并得出了诱导期内的换热系数K大于结垢过程的其他四个阶段的结论。最后，阐述了分析结果对工程的实际指导意义。     

基于污水换热器污垢不同生长阶段的除垢试验研究

为了提高污水换热器的除垢效果,文章以管壳式污水换热器为研究对象,以沙粒作为除垢粒子,以污垢热阻变化率表征除垢效果,在利用烘干灼烧失重法、能谱分析法和微观结构分析法分析污垢成分的基础上,运用污垢热阻法进行除垢试验研究。通过试验研究了在污垢生长的诱导期、生长期和渐近期3个阶段除垢对污垢热阻值的影响。试验结果表明:管壳式污水换热器管内污垢的主要成分为含水量较高的有机物;结垢工况下,污垢热阻渐近值为0.74×10^-3(m^2·K)/W;在污垢生长的诱导期、生长期和渐近期进行除垢后,污垢热阻渐近值分别为0.4×10^-3,0.42×10^-3,0.6×10^-3(m^2·K)/W,与结垢工况相比,污垢热阻渐近值分别降低了45.9%,43.2%,18.9%,除垢工况下污垢热阻增长速率较结垢工况明显减缓。     

对管式换热器黏液形成菌生物污垢特性的实验研究

为了探讨黏液形成菌在管式换热器中的污垢特性,以某电厂循环冷却塔塔底黏泥中分离纯化后得出的黏液形成菌为研究对象,利用污垢动态模拟实验系统,采用对比实验的研究方法得到了不同入口温度、流速及体积浓度条件下黏液形成菌在不锈钢光管换热器中的污垢特性。结果表明:黏液形成菌的结垢过程存在诱导期。随着入口温度的升高,诱导期缩短,污垢热阻达到渐近值所需时间减少。在实验温度范围内,入口温度为30℃时污垢热阻渐近值最大,35℃时次之,25℃时最小;随着流速的增加,污垢热阻达到渐近值所需时间减少,污垢热阻渐近值减小;随着体积浓度的增加,诱导期会延长,结垢速率加快,污垢热阻渐近值增大。     

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.     

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.     

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.     

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.     

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.     

Calcium Sulfate Fouling-Precipitation or Particulate: A Proposed Composite Model

Though it is of great importance, the majority of predictive models tend not to incorporate water chemistry in their formulations. The ionic diffusion model which was developed for CaCO₃, is based purely on crystallization, and is one of the few models that incorporates water chemistry. This model does not provide satisfactory predictions for CaSO₄ fouling. In this article, a new model is proposed for CaSO₄ fouling which takes into account the effect of both crystallization and particulate fouling and is capable of predicting the fouling resistance during the cleaning cycle as well as the fouling cycle. A removal term is incorporated into the model, as the occurrence of particulate fouling for CaSO₄ tends to weaken its crystalline structure and makes it more prone than CaCO₃ to removal. Properties of the electrolyte were evaluated using MINTEQA2 computer code, which is approved by the U.S. Environmental Protection Agency. In this model, particulate fouling is estimated using the physical mechanism for particle transport and adherence, crystallization is estimated by ionic diffusion, and the removal term is approximated using hydrodynamics of flow and deposit properties. The inclusion of both crystallization and removal terms incorporates the effects of both water chemistry and hydrodynamics of the flow and provides a relationship which not only can predict fouling but also can predict dissolution, by change of water quality and/or stopping the operation, or removal by shear stress. The proposed model was assessed using published experimental data. The results indicate that this model provides good predictions: the slope of predicted rates as a function of the experimental rates is 1.05. The experimental results, though limited in number, suggest that crystallization is not the main or only mechanism contributing to CaSO₄ fouling. Particulate fouling seems to be a major contributor. Further experimentation is in process to confirm the degree of particulate fouling and to substantiate or to modify the model accordingly.     

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.     

A simplified model of direct-contact heat transfer in desalination system utilizing LNG cold energy

With the increasingly extensive utilization of liquefied natural gas (LNG) in China today, sustainable and effective using of LNG cold energy is becoming increasingly important. In this paper, the utilization of LNG cold energy in seawater desalination system is proposed and analyzed. In this system, the cold energy of the LNG is first transferred to a kind of refrigerant, i.e., butane, which is immiscible with water. The cold refrigerant is then directly injected into the seawater. As a result, the refrigerant droplet is continuously heated and vaporized, and in consequence some of the seawater is simultaneously frozen. The formed ice crystal contains much less salt than that in the original seawater. A simplified model of the direct-contact heat transfer in this desalination system is proposed and theoretical analyses are conducted, taking into account both energy balance and population balance. The number density distribution of two-phase bubbles, the heat transfer between the two immiscible fluids, and the temperature variation are then deduced. The influences of initial size of dispersed phase droplets, the initial temperature of continuous phase, and the volumetric heat transfer coefficient are also clarified. The calculated results are in reasonable agreement with the available experimental data of the R114/water system.     

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%.