Impact of deposit ageing on thermal fouling: Lumped parameter model

The thermal and hydraulic performance of heat exchangers can be seriously impaired by the formation of fouling deposits on the heat transfer surfaces. The thermal effect of fouling can be complicated when the deposit is subject to ageing, represented here as a change in deposit thermal conductivity (but not thickness) over time. In this article, we revisit the ageing concept for crude oil fouling proposed by Nelson (Refiner Nat Gas Manufacturer. 1934;13:271–276, 292–298), using a numerical model incorporating first order kinetics to generate quantitative comparisons of different ageing rates. Results are reported for lumped parameter systems (which also simulate point measurement methods commonly used in laboratory testing) that demonstrate that ageing can have a substantial influence on the rate of heat transfer and hence on the surface temperature and rate of fouling. Rapid ageing (compared with the rate of deposition) does not pose problems, but slow ageing, or the use of constant heat fluxes in experiments, can lead to modified thermal fouling behavior. It is concluded that deposit ageing dynamics should be considered alongside deposition rate dynamics when interpreting experimental fouling data and when modeling fouling behavior in support of heat exchanger design or operation. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

污垢热阻实时预测模型校正与应用

针对热阻法污垢热阻预测模型在操作条件变化较大时存在很大的原理误差,不能用于污垢实时监测的问题,提出了一种对热阻法污垢热阻预测模型的校正方法。实验研究表明,校正模型原理误差小于3.5%。并针对热学法污垢热阻测量误差与结垢程度有关的特点,推导了测量误差计算式,提出测量误差实时跟踪的测量方法,依据误差值判断测量结果的有效性,提高对高污垢热阻阶段监测结果的可靠性。应用于煤油冷却器的实时监测,表明测量可靠。     

稠油开采污水蒸发结垢过程传热传质计算方法

蒸发器是一种根据热法脱盐原理,利用稠油热采地面系统废热驱动,实现污水脱盐软化,进而回用锅炉实现水资源循环利用的装置。但是,由于油田污水水质的特殊性,使得其极易在蒸发器表面发生结垢行为。通过对污水水质分析,得出其结垢类型主要为碳酸钙垢。建立了碳酸钙垢在强制对流传热和过冷流动沸腾情况下于换热表面结垢的传热传质模型,计算了污水在蒸发过程中析晶污垢形成的数量,得出了与实验数据符合较好的计算模型。对稠油污水在蒸发器中的结垢情况可以进行较好的预测,并对安全生产和设备高效利用有一定参考价值。     

水质参数与板式换热器结垢的关联

采用在线监测和手工分析相结合的方法,突出研究了天然循环冷却水（松花江水）中铁离子、细菌总数、pH值、溶解氧、浊度、电导率等水质参数与板式换热器内冷却水污垢热阻的关系。结果表明：板式换热器冷却水污垢热阻值比TEMA标准值小,且不存在诱导期;pH值和溶解氧影响铁腐蚀速率和细菌总数的变化,而浊度和电导率的变化又受细菌总数的影响;它们在运行初始阶段对结垢特性影响较大,而后进入平缓期。混合污垢间存在协同效应,减小溶解氧和细菌总数的含量可以起到抑制结垢的目的。     

聚偏氟乙烯超滤膜亲水改性研究进展

膜污染会缩短超滤膜的使用寿命,增加由于水力清洗、化学清洗以及膜组件更换而产生的费用。为减少运行成本,有必要对膜污染加以控制。膜污染与原水中污染物的性质和膜本身的性质密切相关。亲水性膜具有水通量高、抗污染性能好的特点,因而提高超滤膜的亲水性是提高膜的水通量和控制膜污染的重要方法之一。简要介绍了具有良好化学稳定性、耐辐射性、耐热性的聚偏氟乙烯膜的表面亲水改性和共混亲水改性的研究进展,指出通过不同的改性方式,聚偏氟乙烯膜都能够实现亲水性的增强。     

板式换热器黏液形成菌与CaCO3混合污垢实验

为了探究黏液形成菌（SFB）与CaCO₃混合污垢在板式换热器（PHEs）中的成垢规律,对不同浓度的CaCO₃与黏液形成菌混合进行了实验研究。结果表明：混合污垢中,黏液形成菌微生物污垢占成垢的主要地位。微生物污垢的热阻渐近值最大,混合污垢的热阻渐近值介于微生物污垢的热阻渐近值和CaCO₃污垢热阻渐近值之间,CaCO₃污垢热阻渐近值最小。这不同于以往得到的混合污垢之间相互促进的结论,说明CaCO₃对黏液形成菌存在抑制作用。维持其他条件不变,随着CaCO₃浓度的增大,混合污垢热阻值随之减小。反之,随着黏液形成菌浓度的增大,混合污垢热阻值随之增大。     

Fouling Mitigation in Tubular Exchangers Using Various Projectiles: Deposit Removal Mechanisms

Fouling of different types can severely compromise the thermal efficiency of heat exchangers. The removal of CaSO₄ deposits was experimentally investigated when projectiles of various types were propelled inside a heated tube. Projectiles were injected at a constant injection rate from the beginning of the fouling process until an asymptotic fouling resistance was approached. The removal rate was found to be inversely proportional to the fouling layer thickness. The removal of deposits due to exerted shear forces by projectiles decreased as the fouling layer developed due to hardening and intensification of the deposit layer. The results indicate that the asymptotic behavior is mainly due to the decrease in the deposition rate as the fouling layer hardens such that it becomes equal to the removal rate and not the other way round.     

Thermo-hydraulic channelling in parallel heat exchangers subject to fouling

The thermal and hydraulic impacts of fouling by crude oil on the tube side of shell-and-tube heat exchangers is analysed for cases where a centrifugal pump and control valve determine the throughput. For co- and counter-current heat exchangers where the fouling layer thickness does not vary markedly over the tube length a new method of calculating the mean fouling rate was derived based on (i) a ‘threshold’ fouling rate model and (ii) an estimated linear profile in film temperature along the tubes. Over-design of exchangers, using guidelines such as those provided by TEMA, is shown to be capable of exacerbating fouling problems. The new mean is also used to evaluate the operation of parallel heat exchangers, such as are often employed at the hot end of a preheat train; a phenomenon that we call ‘thermo-hydraulic channelling’ is identified. It is shown that this may substantially impair network performance but that its effect may be countered by introducing a simple retrofit option of flow measurement and control.     

电磁场对循环冷却水微生物污垢及水质参数的影响

基于污垢热阻监测实验台,考察了电磁场作用下换热器微生物污垢形成过程典型水质参数的变化,并用灰色关联分析法探讨了水质参数与污垢热阻的关联性。结果表明:在电磁场作用下,细菌数、污垢热阻、浊度较未加磁的对照组低,而p H、电导率较未加磁的对照组高。灰色关联分析显示水质参数与污垢的形成密切相关,其中,浊度对污垢热阻影响最为显著。     

Development and evaluation of vipertex enhanced heat transfer tubes for use in fouling conditions

Heat transfer enhancement is important in the development of high performance thermal systems. Some enhanced tubes that are currently on the market are vulnerable to fouling. Economic and technical problems associated with fouling in process systems have been previously discussed in literature; however, they still require additional examination. Parameters that influence fouling include: surface geometry, surface temperature, surface material/finish, fluid dynamics, flow velocity and fluid properties. Vipertex? enhanced surfaces are optimized process surfaces that increase heat transfer through a combination of factors that include: increasing fluid turbulence, secondary flow development, disruption of the thermal boundary layer and increasing the heat transfer surface area. Vipertubes? that have been exposed to a fouling environment produce more heat transfer than smooth tubes exposed to the same fouling conditions; additionally there was less total fouling over a given time period. The reduction in the rate of fouling is the result of secondary flow patterns that form as a result of the patented Vipertex surface design. These secondary flows circulate near the tube surface and clean it; slowing down the buildup of materials. Vipertex EHT series tubes enhance heat transfer (even under fouling conditions), minimize operating costs and recover more energy than smooth tubes under the same conditions. These surfaces provide an opportunity to advance the design of various heat transfer products.     

Chemical reaction fouling of organic fluids

Chemical reaction fouling of heat exchangers involves deposition that is caused by chemical reactions which occur in the fluid, or on the surface itself. The factors which influence fouling of organic fluids via autoxidation and thermal decomposition processes are described and the dependence of the rate of thermal fouling on chemical effects and process variables demonstrated. Mitigation techniques are outlined in brief.     

The fouling properties of SiO_2–CaO–P_2O_5 system in high-temperature rotary kiln phosphoric acid process

Kiln phosphoric acid (KPA) technology could produce P_2O_5 with high purity and has been applied in thermal phosphoric acid industry;however the formation of fouling in the high-temperature rotary kiln restricts the stable and long-term operation.In this paper,the reaction of phosphate ores with gaseous P_2O_5 was investigated in a high-temperature reactor,and the Ca O–SiO_2–P_2O_5 ternary phase diagram was analyzed to understand the fouling formation mechanism.The results showed that the low-melting-point products,such as Ca(PO_3)_2and Ca_2P_2O_7,are responsible for the fouling in the KPA process.In addition,a small amount of impurities,e.g.,aluminum and iron,could facilitate the generation of the low-melting-point products and cause serious fouling.Based on the high-temperature SiO_2–P_2O_5 and CaO–SiO_2–P_2O_5 phase diagram analysis,the control of Si/Ca molar ratio(e.g.,Si/Ca=2.0) was found to avoid fouling formation in the kiln.These results could provide the operation parameters of reaction temperature and feeds composition to suppress the fouling in the kiln reactor for the phosphoric acid production in industry.     

New configuration of a membrane bioreactor for effective control of membrane fouling and nutrients removal in wastewater treatment

Excess aeration to membrane surface is common for controlling membrane fouling in a submerged membrane bioreactor (MBR) system, but significant energy is consumed for excess air production. Therefore, an alternative strategy for membrane fouling control is currently needed. A new configuration of MBR was proposed in this study to control membrane fouling effectively. To reduce biosolids concentration near the membrane surface, the position of the membrane module in MBR was elevated from the bottom to the top in the reactor. This could divide the reactor to two different zones: upper and lower zone. Air was not supplied at the lower zone whereas aeration was given to the upper zone where the membrane filtration was carried out. Biosolids concentration was reduced in the upper zone because the mixed liquor was settled down to the lower zone. Membrane fouling could be lessened in the upper zone due to the reduced biosolids concentration. Therefore, to verify if this new configuration of MBR could mitigate membrane fouling, the effect of changing vertical position of the membrane module in MBR on membrane fouling was investigated. Prior to verification the effect of elevation of membrane module on membrane fouling, influence of MLSS concentration on membrane fouling was investigated first. Transmembrane pressure (TMP) increase became steep as MLSS concentration increased. And the immersed membrane module was elevated from the bottom to the top of the MBR. When the upper membrane was located in the bioreactor, less membrane fouling was observed. This could demonstrate a possibility of new MBR design to control membrane fouling. In addition, reduced dissolved oxygen level in the returned sludge to anoxic tank could increase denitrification efficiency if this configuration is directly applied to biological nutrient removal processes.     

Exploration of alternative models for the aging of fouling deposits

The effect of fouling in heat‐transfer devices (HTDs) is complicated by aging of the fouling deposits. Aging is, like deposition, often sensitive to temperature, so that heat transfer, deposition, and aging are coupled phenomena. Ishiyama et al. (AIChE J. 2010;56:531–545) presented a distributed model of the aging of deposits formed by chemical reaction fouling and illustrated its effect on thermal and hydraulic performance of a HTD operating in the turbulent flow regime. Two‐layer models, simpler than the distributed model, are explored. The deposit is considered to consist of two layers, fresh and aged; this simple picture is shown to be sufficient to interpret thermal and hydraulic aspects of deposit aging when HTDs are operated at constant heat flux (as reflecting laboratory experiments) but not in cases where the constant wall temperature approximation is more realistic. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

污水换热器流态化在线防、除垢实验

采用污水源热泵供暖空调是节能减排的有效途径,但换热器结垢问题尚未得到有效解决。本文将固液流态化除垢技术应用到污水源热泵换热器的防、除垢中,并设计了一套污水换热器流态化除垢系统。该系统以沙粒作为除垢颗粒,通过理论分析和实验验证相结合的方法,研究了固液流态化除垢的最佳流速、除垢能力、强化换热效果及减少磨损的最佳工作参数。实验结果表明;当换热管内污水流速达到0.87m/s时,直径2～3mm沙粒可流化,沙粒循环回收率达到95%,实现了污水换热器在线清洗;沙粒对壁面的碰撞力可有效去除换热管壁的软垢及铁锈,除垢效果明显;该除垢系统持续运行24h后,污水换热器传热系数提高了25.6%;沙粒体积分数为4%,流速为扬动流速,是减少磨损和保证除垢效率的最佳工作参数。     

Mechanisms underlying the effects of membrane fouling on the nanofiltration of trace organic contaminants

The influence of membrane fouling on the retention of four trace organic contaminants - namely sulfamethoxazole, ibuprofen, carbamazepine, and triclosan - by nanofiltration membranes was investigated in this study. Humic acid, alginate, bovine serum albumin, and silica colloids were selected as model foulants to simulate various organic fractions and colloidal matter that are found in secondary treated effluent and surface water. The effects of membrane fouling on the separation process was delineated by comparing retention values of clean and fouled membranes and relate them to the membrane properties (under both clean and fouled conditions) as well as physicochemical characteristics of the trace organic contaminants. Membrane fouling was dependent on the physicochemical properties of the model foulants. Initial foulant-membrane interaction could probably be a major factor governing the process of membrane fouling particularly by the organic foulants. Such membrane-foulant interaction was also a dominating factor governing the effects of membrane fouling on the membrane separation efficacy. In good agreement with our previous study (Nghiem and Hawkes, 2007 [1]), the effects of fouling on retention were found to be membrane pore size dependent. In addition, results reported here suggest that these effects could also be foulant dependent. It was probable that the influence of membrane fouling on trace organic retention could be governed by four distinctive mechanisms: modification of the membrane charge surface, pore blocking, cake enhanced concentration polarisation, and modification of the membrane hydrophobicity. The presence of the fouling layer could affect the retention behavior of charged solutes by altering the membrane surface charge density. While the effect of surface charge modification was clear for inorganic salts, it was less obvious for the negatively charged pharmaceutical species (sulfamethoxazole and ibuprofen) examined in this investigation, possibly due to the interference of the pore blocking mechanism. Evidence of the cake enhanced concentration polarisation effect was quite clear, particularly under colloidal fouling conditions. In addition, organic fouling could also interfere with the solute-membrane interaction, and therefore, exerted considerable influence on the separation process of the hydrophobic trace organic contaminant triclosan.     

空冷凝汽器灰垢费用评估与最佳清洗周期优化

空气悬浮颗粒物易积聚在空冷凝汽器（空冷器）翅片通道内形成灰垢,其低导热性严重降低了空冷器换热性能,影响空冷机组安全性和热经济性。以灰垢造成年累计损失费用最小为目标,基于产品损失和清洗维护费,提出一种空冷器灰垢最佳清洗周期优化算法,联合冗余面积费,构建灰垢费用评估计算模型,以某600 MW直冷机组参数为例,通过现场在线监测的实验数据建立灰垢热阻的降率预测模型。实例分析表明：600 MW直冷机组最佳清洗周期为28.3 d,年均清洗8次,单位容量机组年累计灰垢费用为4528.3元·MW^-1·a^-1,2014年底全国直冷机组1.43亿千瓦计,年累计灰垢费用为6.48亿元·a^-1,与当前现场年均清洗2次工况比较,最佳清洗周期优化后可节约2868.1元·MW^-1·a^-1,全国年累计节约灰垢费用4.1亿元·a^-1。     

换热器自转螺旋扭带自动阻垢技术应用研究

以内置自转塑料螺旋扭带换热器为研究对象,对换热管中有、无自转扭带时污垢的粘附速率、换热器的动态污垢热阻、管壁的磨腐速率进行了工业应用对比试验研究。研究结果表明:自转清洗扭带的防垢、除垢、清洗效果显著,扭带管的污垢平均粘附速率是普通光管的54%;动态污垢热阻比光管下降了30%;塑料扭带对管壁的磨损微乎其微。为自转螺旋塑料扭带的工业应用推广提供了依据。     

膜蒸馏脱盐中膜污染与膜润湿的研究进展

膜蒸馏作为一种脱盐的新兴技术受到广泛关注。然而,因为实际水质组分复杂,可能含有如表面活性剂、油类物质、易结垢盐和有机溶剂等污染物,导致一般疏水膜在长时间运行情况下极易发生膜污染或者膜润湿,最终造成膜通量或截留性能降低。本文首先简述了不同种类的膜污染和膜润湿的特点及形成原因,并分析了膜污染和膜润湿之间的区别和联系。对膜蒸馏过程中膜污染和膜润湿的监测和预测手段进行了简要介绍,最后针对膜蒸馏脱盐过程,重点介绍了近几年国内外预防膜污染和膜润湿的研究进展。研究者一般从污染物与疏水膜的相互作用力着手对疏水膜进行表面改性制备全疏膜和Janus复合膜,避免污染物在膜面的吸附以及抑制表面或孔道润湿。越来越多的研究人员采用致密亲水膜的渗透汽化脱盐来从根本上避免疏水膜带来的润湿。除此之外,对进料液进行预处理也能显著延迟膜的污染和润湿,如混凝/沉淀、膜过滤、煮沸、pH调控等,还可通过改变进料方式、辅助外加磁场等措施控制膜表面局部区域的流体力学状态,减少污染物的附着。适当的膜后处理措施也能恢复膜性能。最后,文章指出了解决膜蒸馏中膜污染和膜润湿的研究方向。