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.     

Amelioration of ultrafiltration process by lime treatment: Case of landfill leachate

Nowadays membrane ultrafiltration process is generally used in effluents treatment. However, at industrial level, this process has major limitations such as important membrane fouling. In this study, a pre-treatment with lime upstream ultrafiltration was envisaged. The effluent considered is landfill leachate. The final waste storage centres produce leachates resulting from the percolation of waters through the hidden waste mass. These effluents strongly charged in mineral and organic matters, must be treated before their release into natural environment. Concerning the cleaning up, results have shown that the selectivity of membranes has an important influence on elimination of pollution from organic source. As expected, any action of retention on salts and heavy metals has been shown. During the filtration of raw leachate, the fouling of membranes turns to be very important and does not allow reaching satisfactory productivity no matter the cut-off limit. There seems to be no impact from hydrodynamics on velocity circulation higher than 4 m s^− 1; this shows the existence of a dense and adhesive deposit on the membrane. The pre-treatment by lime allows (i) to precipitate carbonates under calcium carbonates form and ii) to eliminate by co-precipitation humic acids that are responsible for irreversible membrane fouling. Industrially, the implementation of the pre-treatment may allow reducing the costs of an ultrafiltration unit at about 50% in terms of investment and from 5 to 30% for operating costs.     

Critical fouling conditions induced by colloidal surface interaction: from causes to consequences

Critical fouling conditions (CFC) are defined as the process operating conditions leading to the formation of multilayer irreversible fouling at the membrane surface. This irreversible fouling is the result of a phase transition in the accumulated matter from a dispersed phase (concentration polarisation) to a condensed phase (deposit or gel formation): the spinodal decomposition. Properties of concentrated colloid dispersions and their related phase transitions are integrated into a classical filtration mass balance via colloidal osmotic pressure, II. This then allows us to define CFC for both cross-flow and dead-end filtration. These CFC are expressed in terms of critical pairs of operating conditions: the set permeate flux/boundary layer thickness (directly linked to cross-flow velocity) in cross flow and the critical set permeate flux/filtered volume in dead end.     

Characterization of Irreversible Fouling after Ultrafiltration of Thermomechanical Pulp Mill Process Water

Large volumes of wastewater with dissolved wood components are treated in wastewater treatment plants at thermomechanical pulp mills. It has been shown previously that hemicelluloses in these wastewater streams can be recovered by membrane filtration. A serious obstacle when treating lignocellulose process streams is fouling of the membranes. Fouling not only increases operating costs but also reduces the operating time of the membrane plant. When optimizing the membrane cleaning method, it is important to know which compounds cause the fouling. In this work fouling of an ultrafiltration membrane was studied. The fouling propensity of untreated process water and microfiltrated process water was compared. Fouled membranes were analyzed using scanning electron microscopy and attenuated total reflection Fourier transform infrared spectrometry. Acid hydrolysis of membranes exposed to untreated process water and microfiltration permeate revealed that 508 mg/m² and 37 mg/m² of polysaccharides, respectively, remained on the membranes even after alkaline cleaning.     

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。     

膜生物反应器膜污染的研究进展

从膜的结构性质、反应器操作条件、处理液微生物性质三个方面介绍了膜生物反应器膜污染机理研究的进展,总结了优化膜生物反应器设计、调节膜生物反应器操作条件、在线超声控制、化学方法等膜污染控制的常用方法,对未来膜污染研究进行了展望。     

Dead-end filtration of natural organic matter: experimental evidence of critical conditions

The development of membrane technology has reached a state whereby operational optimisation is becoming the major issue to both researchers and industrial operators. A key focus is towards sustainable operation where fouling is limited and cleaning is greatly reduced. The paper presents an example of such an approach for the filtration of natural organic matter (NOM) in a dead-end ultrafiltration cell. Sustainable operation has been assessed in relation to the application of a cessation period followed by a gentle rinse. The work demonstrates the existence of a critical filtered volume below which the mass accumulated at the membrane's surface is reversible and above which significant fouling occurs. Further, appropriate selection of operating conditions (filtered volume and applied pressure) makes it possible to avoid the formation of an irreversible fouling layer.     

Treatment of Waste Water from Wet Lime (Stone) Flue Gas Desulfurization Plants with Aid of Crossflow Microfiltration

Abstract

A treatment method has been developed to remove heavy metals such as As, Cd, Cr, Cu, Hg, Ni, Pb, Se and Zn very efficiently from waste water from wet lime(stone)-gypsum flue gas desulfurization plants (FGD).

This method has been based on coprecipitation of heavy metal hydroxides and sulfides followed by crossflow microfiltration as a post-treatment. The experiments were carried out on pilot plant scale with actual waste water from a FGD plant downstream of a coal fired boiler. The effect of membrane fouling as a function of several process parameters was investigated.

From SEM-EDS analysis it was found that the fouling layer on the membrane surface mainly consists of Mg-, Al-, Si-, Fe-compounds and gypsum. Membrane fouling could be decreased by periodical backwashing and by increasing the liquid velocity in the membranes. Moreover, a computer controlled start up procedure of the crossflow microfiltration unit was developed to minimize the effect of membrane fouling and to increase the operating time before chemical cleaning of the membranes is necessary. The developed chemical cleaning was found to be very successful, because it was possible to achieve the initial permeate flux after each experiment.     

麦芽糖醇纳滤浓缩过程中膜的污染和清洗再生

采用纳滤方法对麦芽糖醇进行提纯和浓缩,可以节约大量的能耗。文中系统研究了该纳滤过程中操作条件对膜污染的影响,并对膜的清洗和再生效果进行了考察。实验结果表明,膜的污染程度随操作压差和原料液质量分数的增大而加剧,随循环流量和操作温度的升高而减轻。另外,采用常压、循环流量为100—120 L/h,40℃左右的去离子水冲洗30 m in,可达到满意的效果,再将被污染的膜浸泡36 h,基本能使膜纯水通量恢复到使用前的状态。     

Intermittent ultrasound-assisted ceramic membrane fouling control in ultrafiltration

In this study, remediation of ceramic membrane fouling by an in-line intermittent ultrasound system was investigated. A piezoelectric ultrasonic transducer was integrated into a membrane unit that provided ultrafiltration (UF) of a diluted skim milk solution containing 0.10 wt% of protein. The effects of ultrasound at varied frequencies (20, 28, and 40 kHz) and power intensities (1.44, 2.88, and 5.76 W/cm²) under continuous operation and intermittent mode at various intervals (0.50, 1.0, 1.5, 2.0, 2.5, and 3.0 minutes) on membrane fouling were studied. The quality and flow rate of the permeate stream were monitored for the evaluation of the UF process performance. Optimal conditions of continuous ultrasound were found at 28 kHz and 2.88 W/cm². Moreover, at optimal ultrasonic conditions, the optimal intermittent time was found at 0.50 minute. At optimal ultrasonic conditions, the permeate amount increased by 79.8% and 94.2% for 0.50 minute intermittent ultrasound and continuous ultrasound, respectively, as compared with that of the UF process without ultrasound. Also, intermittent ultrasound induced better fouling control at a lower protein concentration of 0.05% by weight. The cleaning effect of ultrasound could be attributed to the cavitation bubbles generated by the rarefaction and pressure cycles of the applied ultrasound.     

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。     

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     

大化肥设备的化学清洗

论述了水冷器、换热器、锅炉和尿素高压设备结垢类型及形成原因介绍了停车化学清洗和不停车化学清洗的几种工艺流程事实证明，用化学清洗方法清除结垢和提高换热效率是一种非常有效的方法     

Treatment of Oily Wastewaters Using the Microfiltration Process: Effect of Operating Parameters and Membrane Fouling Study

This work focuses on the treatment of oily wastewater using the cross-flow microfiltration (MF) process to determine the effect of different operating parameters such as transmembrane pressure (TMP) and cross-flow velocity on the separation performance and to study the mechanism of membrane fouling during microfiltration of oil in water emulsions. In this regard, the permeation flux and oil rejection of a polyvinylidene fluoride (PVDF) membrane in a flat-frame MF module for separation of 3000 ppm oil/water emulsions were measured. The results indicated that the permeate flux increased by an enhancement in both TMP and cross-flow velocity, while the oil rejection decreased. The analysis of variance (ANOVA) showed that the individual effect of TMP and cross-flow velocity is more important than the interactional effect of these operating parameters on the permeate flux and oil rejection. The results of fouling modeling revealed that the membrane fouling mechanism was affected by the applied TMP. The cake filtration model dominates the fouling mechanism at lower operating pressures. The fouling mechanism was changed from the cake formation to intermediate pore blocking and then to standard pore blocking as the TMP varied from 1 to 3 bar. Finally, a five-step procedure was used for cleaning the oil/water fouled membranes.     

反渗透膜有机污染的研究进展

介绍了反渗透进水中一些主要有机污染物质的污染机理以及操作条件、膜自身特性、溶液化学性质对于污染过程的影响,还综述了预处理技术、清洗方法以及通过膜的表面改性等措施来控制反渗透膜的有机污染,最后指出了目前研究中需关注的一些问题。     

Characterising the cleaning mechanisms of yeast and the implications for Cleaning In Place (CIP)

Deposition of yeast inside brewery process plant is a serious industrial problem. Investigation of the cleaning of beer fermenter deposits revealed two types of fouling; yeast foam (type A) and yeast film (type B). Rheological characterisation indicated both deposits could be mimicked in lab scale fouling experiments using yeast slurry aged for different times. Water and chemical rinsing of these deposits on a lab scale flow cell revealed three distinct cleaning phases: (i) hydration and swelling, (ii) removal in the flow by dissolution and in patches and (iii) no further removal. At 30 and 50 °C water rinsing at the flow velocities investigated could remove up to 85% of the deposit. At a water rinsing temperature of 70 °C, less deposit could be removed overall. Rheological studies indicated that increasing the temperature of the deposit generated a more elastic deposit which may decrease cleanability. Chemical cleaning using 2 wt% Advantis 210 (a NaOH base cleaning agent) eventually gave a visually clean surface at all flow velocities and temperatures. Chemical cleaning at 70 °C gave the shortest cleaning times for all flow velocities, but comparable cleaning times were observed when rinsing at 30 and 50 °C, suggesting that an increase in temperature from 30 to 50 °C might not decrease the cleaning time.     

Fouling characterisation of a PVDF membrane

Several studies have shown the importance of a biofilm on the membrane surface and some have identified EPS as the components responsible for membrane fouling. This study, performed within the framework of the European AMEDEUS project, investigates the membrane deposit, its composition and origin, using two industrial modules provided by A3 Water Solutions, which operated at pilot-scale for one year with typical biological operating conditions (SRT = 28 days, MLSS = 10 g/l). One of the modules was subjected to an intensive cleaning, using hydrogen peroxide, before being removed. Membrane sample deposits were taken on each module: differences were found between the fouled and the cleaned membrane in particular with regards to the organic fraction of the deposit, suggesting some changes in the biofilm after cleaning. The composition of the membrane deposit was also compared to the sludge supernatant and the permeate characteristics. Results showed the retention of biopolymers by the filtration step and their presence in the membrane deposit. However, all the biopolymers retained by the membrane between two maintenance cleanings were not found in the membrane deposit. They could have been removed by physical cleanings, linked to some compounds or not detected. Biopolymers probably play a role in membrane fouling but chemical and physical phenomena prevented the understanding of the fouling mechanisms.     

Optimisation of ultrafiltration of a highly viscous protein solution using spiral-wound modules

The ultrafiltration process of highly viscous protein process water with spiral-wound modules was optimised by analysing the fouling and developing a strategy to reduce it. It was shown that the flux reduction during filtration is mainly caused by the adsorption of proteins on the membrane and not by the high osmotic pressure. In laboratory experiments, the concept of the critical flux was proved to reduce fouling. Additionally, the reduction of the effect of concentration polarisation by spacers at laboratory and pilot scale was evaluated. While at laboratory scale the spacers influenced the mass transfer, the effect in spiral-wound modules was low. However, pilot plant experiments showed that operating at low pressures to reduce the fouling and avoid local fouling required a spacer with a low pressure drop along the module. A cleaning strategy including a hygienic evaluation was tested. An enzymatic cleaning followed by a caustic cleaning step gave sufficient results. This investigation has also shown that the concentrated protein solution as well as the produced water in the permeate are hygienically safe, and, in principle, the latter could be reused somewhere else within the production process.     

MODELING AND OPTIMIZATION OF MEMBRANE CHEMICAL CLEANING BY ARTIFICIAL NEURAL NETWORK,FUZZY LOGIC,AND GENETIC ALGORITHM

Robust artificial neural network (ANN) and fuzzy logic (FL) models were derived for chemical cleaning of microfiltration membranes fouled by milk under a wide range of operating conditions. The accuracies of the models were compared with multiple linear regressions (MLR). The developed models are useful tools for predicting the performance of chemical cleaning. The effects of different operating conditions on cleaning performance were elucidated using the ANN developed model. Moreover, optimum cleaning condition was determined by genetic algorithm and ANN model. The current research demonstrated that fuzzy logic and an artificial neural network can quantitatively capture cumulative effects of a range of operating conditions on flux recovery and resistance removal during a cleaning process.