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     

Studies into the swelling of gelatine films using a scanning fluid dynamic gauge

Fluid dynamic gauging (FDG) has been developed to measure both the thickness and strength (cohesive or adhesive) of a range of fouling deposits, in situ and in real time. We report the development of a scanning FDG device (sFDG) with fully automated movement and deposit thickness measurement. Thickness-time profiles can be collected for several points on the surface over the course of a single experiment. sFDG is employed here to study the cleaning kinetics of model gelatine fouling layers when contacted with aqueous solutions at different pHs and temperatures, and with a commercial dishwasher formulation containing enzymes.Solution pH is an important parameter: above ～pH 10.6 the gelatine layers swell markedly, due to protonation of the amino acids proline and hydroxyproline, and subsequent charge repulsion within the gel. An unusual behaviour (discontinuity) is reported during swelling in the commercial formulation. It is proposed that hydration of the deposit and an extent of swelling (ratio of the final to the initial deposit thickness) of ～12 is necessary before the enzymes can promote deposit removal.     

Optimum cleaning cycles for heat transfer equipment undergoing fouling and ageing

Complete elimination of fouling in heat transfer equipment is rarely achieved in practice, so cleaning of fouled units is a regular task in the process industries. Algorithms for scheduling cleaning have, to date, minimized the net losses due to fouling by focusing on when and which units to clean. In contrast, this paper focuses on when and how to clean a unit, when more than one cleaning method is available.The model formulation is approached by idealizing a foulant deposit as consisting of two layers, soft (fresh) and hard (aged). The hard material is formed through the ageing of the soft material. Hard deposits are more difficult to remove and require time- and cost-intensive cleaning methods (such as mechanical cleaning). Soft deposits are removed through less time- and cost-intensive methods such as chemical cleaning by recirculation of solvents. The hard deposit usually consists of more thermally conductive material and hence, for a given thickness, has a lower thermal resistance compared to the soft deposit.This work introduces a new methodology to identify optimum cleaning cycles (OCCs) under the presence of both soft and hard deposit, when two cleaning methods (solvent and mechanical cleaning) are available. The analysis of OCCs is extended and a new concept called the ‘cleaning supercycle’ is elaborated, which can be related to the optimal time between plant shutdowns.     

Development of a novel non‐contact proximity gauge for thickness measurement of soft deposits and its application in fouling studies

This paper describes the development of a novel proximity gauging technique for soft deposits, exploiting a siphon effect. This non‐contact technique has the capability to measure the thickness of soft deposits on a surface in situ and in real time. A theoretical model of performance has been developed and its validity demonstrated by calibration experiments. Local thickness measurements of soft deposits such as whey protein deposit, supermarket butter and sticky foam on metal surfaces had an accuracy of ±20 μm for a deposit thickness of 500 to 1000 μm. The potential of this technique for on‐line monitoring of fouling and cleaning processes in liquid environments is demonstrated by preliminary studies of alkali (NaOH) cleaning of whey protein deposits from stainless steel surfaces.     

钛金属膜过滤油田污水的污染阻力研究

以阻力系列模型为基础,考察了钛金属膜过滤油田污水时膜结构和操作条件等因素对污染阻力的影响。结果表明:滤饼层阻力是主要阻力,膜孔堵塞阻力为第二关键阻力。膜孔径增大,滤饼层阻力降低,膜孔堵塞阻力增加,且二者均随压力增加而增大;随着膜面流速的增加,滤饼层阻力降低,膜孔堵塞阻力增大;在过滤操作的前15～20min,污染阻力增长迅速。实验所得结论将为确定最佳工艺条件和膜清洗方法提供可参考的依据。     

填料塔污垢分析及清洗方式的制定

在简要论述填料塔污垢危害的基础上,采用多种电子检测方法,对填料塔污垢的成分进行了精确的分析,并对结垢机理进行了说明论述。通过对填料塔污垢的结垢、溶垢实验,可知物料的成分、浓度以及填料材质对垢层厚度均有一定影响。填料塔中的高分子聚合物污垢成分复杂,结垢厚度不一,不溶于水并且不易与设备表面分离,所以清洗方式根据物理清洗与化学清洗的技术特点,选用化学清洗,为清洗效果更佳同时结合物理清洗方式进行。通过此论述可定制填料塔的清洗方式,以及对深入了解填料塔污垢结垢机理有较好的参考价值。     

Investigation of wax deposit ‘sloughing’ from paraffinic mixtures in pipe flow

Deposit ‘sloughing’ from ‘waxy’ crude oils has been described in the literature as a possible mechanism, leading to partial or complete dislodging of the deposit from the pipe wall due to changes in flow parameters. A bench‐scale flow loop apparatus was used to investigate ‘sloughing’ with prepared single‐phase ‘waxy’ mixtures of a multicomponent paraffinic wax dissolved in a multicomponent solvent. Experiments were performed to study the changes in the deposit‐layer thickness due to step increments in the ‘waxy’ mixture flow rate, the mixture temperature, and the coolant temperature. It was observed that the deposit‐layer thickness decreased with an increase in each of the three parameters; however, a complete or sudden dislodging of the deposit‐layer did not occur in any of the experiments. A steady‐state heat‐transfer model was used to predict the variation in the deposit mass or thickness due to changes in the selected parameters. In each case, the step‐wise decrease in the deposit thickness, as observed experimentally, was predicted to be caused by changes in the thermal resistance and/or thermal driving force.     

Thermo‐hydraulic analysis of refinery heat exchangers undergoing fouling

A complete, systematic approach is presented for the analysis and characterization of fouling and cleaning in refinery heat exchangers. Bringing together advanced thermo‐hydraulic dynamic models, some new formulations, and a method for dynamic analysis of plant data, it allows: extracting significant information from the data; evaluating the fouling state of the units based on thermal measurements and pressure drops, if available; identifying the range of deposit conductivity leading to realistic pressure drops, if pressure measurements are unavailable; estimating key fouling and ageing parameters; estimating the effectiveness of cleaning and surface conditions after a clean; and predicting thermal and hydraulic performance with good accuracy for other periods/exchangers operating in similar conditions. An industrial case study demonstrates the performance prediction in seamless simulations that include partial and total cleanings for over 1000 days operation. The risks of using thermal effects alone and the significant advantages of including pressure drop measurements are highlighted. © 2016 American Institute of Chemical Engineers AIChE J, 63: 984–1001, 2017     

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     

Estimation of fouling in a plate heat exchanger through the application of neural networks

Pasteurization is an important, if not the most important, process step in the packaging of milk. It is subject to alterations stemming from the variation in the temperature, pH and raw milk quality. The variability may manifest itself in changes in the formation of the deposit (fouling) in the pasteurization unit, such that there is a need for tools, both instrumentation and computational, to help in monitoring the process and keeping it on the desired course. In this paper we describe a practical procedure based on artificial neural networks (ANN) that allows prediction of the deposit thickness, the overall heat transfer coefficient and the critical time (the time that the unit has to be stopped for cleaning) for reducing the impact of fouling on such processes. The procedure determines when the cleaning operation is required once the system is under critical conditions of operations. A combination of fundamental studies and plant measurements were used for study of the operating conditions and thus evaluation of the trades‐offs between operating conditions and longer operating life span. The results are encouraging, enough to validate current operating industrial techniques. Copyright © 2005 Society of Chemical Industry     

Impact of deposit aging and surface roughness on thermal fouling: Distributed model

A kinetic model was recently proposed to describe the effect of aging on deposit thermal conductivity and the thermal performance of a shell‐and‐tube heat exchanger undergoing crude oil fouling. The model is adapted for implementation within a dynamic, distributed system with spatial and temporal distributions, relaxing several of the previous assumptions. The evolution of surface roughness is also considered, using conjectural linear and asymptotic functions. Simulations are performed for a single tube representative of a refinery exchanger. The results demonstrate the substantial effects over time of aging and roughness on heat transfer and pressure drop. Roughness effects yield apparently negative initial fouling resistances, as reported in some experimental tests. The importance of accounting for roughness dynamics in short time scale pilot plant scale tests and aging over longer time scales in industrial applications is highlighted. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Effect of Fouling Conditions and Cake Layer Structure on the Ultrasonic Cleaning of Ceramic Membranes

Abstract

Homogeneous alumina membranes fouled by polystyrene latex particles at different pH values and ionic strengths were subjected to ultrasonic cleaning. Cleaning was more effective at high and low pH than at neutral pH. At low pH values, less repulsive particle‐particle interactions resulted in the removal of millimeter‐scale aggregates and highly effective cleaning. At near‐neutral pH, stronger repulsive particle‐particle interactions caused detachment to occur as individual particles from the cake layer rather than as flocs, which was a slightly less effective cleaning mechanism. Ultrasonic cleaning of cake layers formed at high ionic strength (>0.3 M KCl) was less effective than cleaning at lower ionic strength (<0.3 M KCl). High ionic strength caused particles to coagulate in solution and deposit as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at the sub‐optimal cleaning conditions used in this study, perhaps due to particle attachment occurring within a primary energy minimum. Membrane cleaning experiments performed with particles of varying size showed that particle size was less important than the surface potential of the particles. For a given mass, particles that possessed the largest surface potential formed the thickest fouling layer, irrespective of particle size, and showed the greatest improvement in flux with ultrasonic cleaning. These results demonstrate that solution conditions influence ultrasonic cleaning of membranes primarily by modifying particle‐particle and particle‐membrane interactions as well as cake layer structure, rather than by impacting the extent or magnitude of cavitation events.     

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。     

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.     

Design of shell and tube heat exchangers considering the interaction of fouling and hydraulics

We extend a formulation of the heat exchanger design optimization with fouling modeling to consider the effect of fouling on hydraulics. A shell-side fouling model was added so the optimization can consider cases with fouling in the tube and shell sides. The focus is the design of heat exchangers with fouling behavior associated with a threshold model, like in crude preheat trains. We solve the optimization problem by using a newly proposed Set Trimming technique. We compare our results with the traditional approach of using fixed fouling factors and the previous approach of not considering the hydraulic impact of fouling. We conclude that considering the deposit thickness leads to more realistic results that are different than the ones obtained using the previous approaches. Moreover, we show that previous approaches can render designs with larger pressure drops than the maximum imposed by the constraints, as well as exchangers with higher areas.     

直接空冷凝汽器翅片管污垢层清洗的数值研究

直接空冷凝汽器翅片管作为空冷单元的重要换热组件,翅片管的清洁程度直接影响空冷单元的高效换热能力。目前对如何提高直接空冷凝汽器翅片管换热能力的研究较多,但是对于直接空冷凝汽器翅片管粘有污垢层后的清洗状况鲜见报道。对此,本文对直接空冷凝汽器蛇形翅片管污垢层清洗进行了数值模拟,分析了翅片管表面具有不同厚度的垢层时,在清洗过程中所受到的压力和剪切应力随时间的变化规律。结果表明：清洗带有不同厚度垢层的翅片管,翅片管表面垢层越薄,就越容易清洗,所需清洗时间越短;翅片管表面垢层受到的压力和剪切应力随垢层厚度的减小而减小,随清洗时间的增加而减小;经计算得出,清洗喷嘴的移动速度为1.1～1.5m/min时可更好的清洗翅片管表面的污垢。     

Numerical method for heat transfer and fouling analysis of a shell and tube heat exchanger using statistical analysis

Through proper monitoring, problems can be identified and isolated well before the economics of the process are threatened. In contrast to most conventional methods, fouling can be detected when the heat exchanger operates in transient states. Statistical analysis is used to develop a fouling growth model of a heat exchanger subjected to fouling. The statistical analysis was considered for four different types of distributions out of which the lognormal distribution was found to be most suitable. Experiments were conducted with a single pass shell and tube heat exchanger with water both as the hot and cold fluids. The results show that the proposed tool is very effective in detecting critical fouling in a heat exchanger, which can be utilized for predicting the optimal maintenance schedule. Hence, the results of this work can find application in predicting the reduction in heat transfer efficiency due to fouling in heat exchangers that are in operation and assist the exchanger operators to plan cleaning schedules.     

Application of radioactive particle tracking to indicate shed fouling in the stripper section of a fluid coker

The stripper section of a fluid‐coker consists of a system of baffles (sheds) that enhances the removal efficiency of entrained and adsorbed hydrocarbons from the fluidised coke‐particles. If the particles contain a thin liquid film layer of heavy hydrocarbons, making them excessively ‘wet’ or ‘sticky’, and if they stay in contact with sheds for too long, solid deposits are formed that lead to stripper fouling. Extensive fouling decreases stripping efficiency and liquid product yield and can shorten run‐times between shutdowns. Because of the fouling, the shape of sheds mostly changes by increasing their surfaces thickness. An early indication of that fouling and the ability to follow its development are essential for choosing optimal parameters of the process. The radioactive particle tracking (RPT) method has been tested to determine its applicability to indicate the change in the shape of internals within a fluidised bed reactor when direct observation is impossible. A single radioactive tracer‐particle has been traced in experiments lasting from 2 to 6 h. The experiments were conducted in a lab‐scale, cold‐flow fluidised bed into which a single shed with walls of different thickness was incorporated. This experimental fluidised bed provides intensive solid phase mixing that allows a single tracer‐particle to be located in any place within the reactor. By registering the frequency of the tracer‐particle appearance within a defined internal space surrounding the shed, the shape of shed was reconstructed. The conducted experiments suggest that RPT technique allows for tracking internals' fouling within a fluidised bed reactor. © 2012 Canadian Society for Chemical Engineering     

Cleaning efficiency and impact on production fluxes of oxidising disinfectants on a pes ultrafiltration membrane fouled with proteins

The cleaning step in the ultrafiltration of skim milk is not always complete before the following disinfection start, raising concerns about the effect of disinfectants on any protein remaining. For a Koch polyethersulfone (PES) UF membrane [model HFK131], the fouling in skim milk UF is exclusively due to proteins. Used in right quantity, hypochlorite well finishes the cleaning and can also be used alone as a cleaner [removing 95% of the proteinaceous fouling]. However, if the amount of hypochlorite is not sufficient to end the cleaning, this can lead to large reductions production flux [a 13% decrease on skim milk flux was observed]. Other oxidants such as PVP-iodine, hydrogen peroxide, peracetic acid and sodium perborate do not achieve hydraulic cleanliness [90% of flux recovery]. Oxidants based on hydrogen peroxide and PVP-iodine, when used on a membrane which was still fouled by protein, yielded flux recovery worse than for a single rinsing.     

考虑关键换热器备用的原油预热系统清垢周期优化

原油结垢是原油预热过程面临的一个严峻问题,它使换热器的传热系数降低,影响正常的生产过程。应对原油预热过程的结垢问题最普遍的方法就是对换热器进行周期清洁,并优化清洁周期。许多结垢严重的行业都采用换热器备用的方式来减少因换热器离线造成的热回收减小,但原油预热系统尚未考虑,因而进一步考虑了关键换热器的备用来进行原油预热系统清垢周期优化。首先选取关键换热器,再通过模拟退火算法进行清洁周期优化。应用所提出的方法对简化的原油预热系统进行了清垢周期优化,结果显示,考虑换热器备用时所得到的清垢周期经济效益更好。