A Preliminary Study on Strategies for Optimal Fluid-Bed Drying

ABSTRACT

The degradation of product quality during convective drying depends on the temperature and water concentration history of the panicles and drying time. For improving product quality in combination with acceptable operation costs, optimal control of the operation variables is required.

In this preliminary study the relevance of dynamic optimal operations for batch-wise fluid-bed drying is explored by simulation. Optimal trajectories of operation variables were calculated for a pilot-plant installation by using a model which concerned the drying history of the panicles and the product quality (inactivation of biological active components). The applied objective function was based on an economic criterion combining product quality and operation costs.

Although the advances for the chosen pilot-plant application are moderate, in future studies the potentials and relevance of dynamic optimal operations for drying will be quantified for installations on industrial scale.     

A Preliminary Study on Strategies for Optimal Fluid-Bed Drying

The degradation of product quality during convective drying depends on the temperature and water concentration history of the panicles and drying time. For improving product quality in combination with acceptable operation costs, optimal control of the operation variables is required.

In this preliminary study the relevance of dynamic optimal operations for batch-wise fluid-bed drying is explored by simulation. Optimal trajectories of operation variables were calculated for a pilot-plant installation by using a model which concerned the drying history of the panicles and the product quality (inactivation of biological active components). The applied objective function was based on an economic criterion combining product quality and operation costs.

Although the advances for the chosen pilot-plant application are moderate, in future studies the potentials and relevance of dynamic optimal operations for drying will be quantified for installations on industrial scale.     

基于小孔径PVDF内衬膜A/O-MBR膜污染分析

为研究膜生物反应器膜污染问题,在缺氧-好氧一体式膜生物反应器中,对模拟生活污水的处理效果进行考察,考察了4种不同孔径聚偏氟乙烯(PVDF)内衬膜对膜污染的影响规律。结果表明,基于小孔径PVDF膜的MBR工艺对模拟生活污水中COD、NH3-N的去除率分别达到96%、90%,出水浊度在0.35 NTU以下;在操作条件固定的情况下,随着膜孔径增加,临界通量逐渐降低;孔径越小,跨膜压差上升越缓慢,膜污染较轻。膜污染模型分析结果显示:无论何种孔径的膜,在MBR系统中,滤饼层和凝胶层污染所占的比例都最大(>80%),因此,在使用过程中减缓滤饼层及凝胶层的形成至关重要。     

卷式反渗透海水淡化系统膜清洗与更换策略优化

膜污染是导致反渗透海水淡化(seawater reverse osmosis,SWRO)系统操作成本增加和产水性能下降的重要因素。为了降低系统运行操作成本,本文针对卷式SWRO系统提出了一种新的膜清洗与更换策略优化方法。首先,根据反渗透和膜污染过程机理建立了膜污染情况下的SWRO系统性能模型;然后将生产过程中的总操作费用与膜清洗和更换规划联系起来,建立了以系统日均操作费用最低为目标、以膜清洗次数、膜清洗和更换时间等为寻优变量、以开放式方程模型为约束的优化命题,并通过联立求解等技术使得原本复杂的优化命题可快速方便地求解;在此基础上对SWRO系统进行了实例研究和分析。优化求解结果表明:(1)本优化策略可以大幅降低系统操作费用,并同时获得最佳膜清洗和更换时间以及膜清洗次数;(2)进料海水温度对最优膜清洗和更换策略影响很大,固定周期的膜更换策略并不合适。另外本优化方法还可得到不同条件下最优操作费用组成、以及最优目标下最优操作压力和操作流量曲线等信息,对优化SWRO系统运行和深入分析系统内部状态变化具有重要意义。     

Submerged Membrane Bioreactor for Vegetable Oil Wastewater Treatment

A bench‐scale submerged membrane bioreactor (SMBR) was employed to treat vegetable oil plant wastewater with complete sludge retention. Treatment performance and membrane fouling of the SMBR were investigated. The system stably removed high amounts of total organic carbon, oil, and ammonia from vegetable oil wastewater and reduced the chemical oxygen demand, demonstrating the great potential of the SMBR in removing pollutants. The membrane fouling layer was not only governed by deposition of organic substances composed of extracellular polymeric substances like proteins, polysaccharides etc., and oil substances but also by inorganic elements. Organic foulants coupled to inorganic precipitation enhanced the formation of a gel layer and triggered severe membrane fouling in the SMBR.     

Effect of intermittent back ozonation for membrane fouling reduction in microfiltration using a metal membrane

In a microfiltration system using a metal membrane for municipal raw sewage reclamation, the following research points were mainly investigated: 1) Effect of intermittent back ozonation for membrane fouling reduction, 2) effect of operational parameters for permeation flux, and 3) the estimation of optimal operational conditions using an empirical model in the case of back ozonation. Intermittent back ozonation dramatically improved the membrane fouling. The permeation flux was recovered up to 90% of initial flux with dosage of 0.27 (mgO₃/cm³/cycle). Among operational parameters, we found that ozone concentration was the most influential parameter for membrane fouling reduction. In the case of back ozonation, the optimal values of each operational parameter with respect to ozone concentration, ozone gas flow rate and injection time to satisfy 90% recovery of initial flux were 20.2 (mgO₃/L), 3.1 (L/min) and 2.1 (min), respectively. Intermittent back ozonation is believed to be an innovative and feasible technology for fouling reduction and high permeation flux in case inorganic membrane material is being used.     

工业废水电渗析过程中膜污染研究进展

综述了工业废水电渗析体系膜污染类型、性质、影响因素等的研究进展. 无机污染主要是Ca2+, Mg2+或其他高价离子在离子膜表面或内部形成的结垢现象,原理是极化或溶液过饱和形成沉淀. 有机污染是由蛋白、腐植酸、表面活性剂及大分子有机物在离子膜表面沉积或渗透到膜内部而形成,原因主要是带负电荷的有机物与阴膜荷正电基团的静电作用及带苯环有机物与基膜的亲和作用,其次是有机物分子大小与膜的网络结构的作用. 膜污染形成及其性质与施加电流、共存组分、温度、pH值和运行时间等密切相关,可造成离子膜导电性、离子交换容量、水含量和极限电流密度等减小,且有机污染对电渗析性能的影响比无机污染更严重. 由有机物凝胶层形成的膜污染可用电渗析膜污染指数定量描述.     

Study of reverse osmosis membranes fouling by inorganic salts and colloidal particles during seawater desalination

Fouling phenomenon is considered among the major reasons that cause significant increase of operating cost of desalination plants equipped with reverse osmosis (RO) membranes. This phenomenon is studied in the present work in the case of RO polyamide aromatic membranes using model seawater containing inorganic salts and colloidal compounds. Different solubility conditions of CaCO₃ and CaSO₄ were applied to study RO performances with and without colloid presence. During experiments, the membrane permeate fluxes were continuously monitored. Moreover, studies of chemical composition, structure, and morphology of the materials deposited on the membrane surface were conducted using energy dispersive microanalysis (EDS) X-ray diffraction and scanning electronic microscopy (SEM). Results show that in conditions of calcium carbonate oversaturation there is a reduction in the permeate flow of 11.2% due to fouling of the membrane by the precipitation of this compound. While in the same conditions of calcium sulphate oversaturation the reduction of the flow is 5%, so we can conclude that in conditions of oversaturation of both salts, calcium carbonate produces a greater fouling of the membrane that in its view causes greater decrease in the flow of permeate. All this based on the results of the test with both salts in oversaturated conditions. Resulting in the formation of calcite and gypsum crystals onto the membranes as XRD analyses stated. Additional presence of colloidal silica in those conditions intensifies strongly the fouling, leading until to 24.1% of permeate flux decrease.     

Surface modification of polyethersulfone hollow-fiber membranes by γ-ray irradiation

The fouling of ultrafiltration membrane is often caused by gel formation on the membrane surface. This gel layer arises due to concentration polarization or macromolecular adsorption on the membrane surface. The gel layer affects both the hydraulic permeability and the rejection properties of the membrane. In this report, the adsorption of porcine albumin and the concentration polarization effect on modified and unmodified polyethersulfone (PES) hollow-fiber membrane is studied. PES ultrafiltration hollow-fiber membranes were modified by the grafting of polyethylene glycol (PEG) polymer on the internal surface using γ-ray irradiation method. The modified hollow fibers were less susceptible to fouling than were the unmodified fiber. The performance of both modified and unmodified hollow fibers was tested as a function of feed flow rates and protein concentrations. © 1994 John Wiley & Sons, Inc.     

电阻抗成像原位在线监测超滤膜污染行为研究

基于电阻抗成像（electrical impedance tomography,EIT）的原位可视化特征,以酵母与高岭土为模型污染物,在不同错流速度下对超滤（UF）膜不同的污染过滤特性进行了研究,得到通量变化、平均电压变化、EIT图像,进行了三维图像重建,并建立了膜通量与平均电压间的相关性模型。结果表明：EIT图可直观显示膜污染界面的变化情况,酵母溶液形成的膜污染呈现空间上的不均匀分布,而高岭土溶液形成的膜污染较为均匀,混合溶液形成的污染分布居于两者之间。通过EIT信号的重建图像得到了有关污染层厚度方面的信息,结果表明,酵母形成的污染层最厚,其次为酵母及高岭土混合溶液,污染层最薄的为高岭土溶液。     

The use of reverse osmosis for the treatment of river Rhine water. Part II

The article describes an investigation into membrane fouling and flux decline in the reverse osmosis treatment of river Rhine water. Special attention is paid to technical applications and process optimization. The investigations involve the effect of a number of relevant process variables, such as the extent of the pretreatment of the raw Rhine water (rapid sand filtration, chemical clarification with various degrees of floc removal and tapwater treatment), membrane quality (tubular, cellulose acetate, cured at 75°C, 80°C and 86°C), brine velocity (75 up to 150 cm/sec), pressure (20, 30 and 40 atm.) and recovery (50%, 75% and 90%).The results show a great effect of each of the process variables on membrane fouling and flux decline. It seems preferable to pretreat the raw Rhine water by chemical clarification and sand filtration and to operate high capacity installations at 20 atm., 90% recovery and brine velocities of about 75 cm/sec. Operating at 20 atm. instead of 40 atm. seems profitable as it saves energy and installation costs, and because it presents less fouling problems and is consequently an easier operation.     

Impact of Feed Spacer Filament Spacing on Mass Transport and Fouling Propensities of RO Membrane Surfaces

Material build-up on membrane surfaces is one of the vital challenges faced by Reverse Osmosis operations, leading to many operational and maintenance issues. To date, several modeling studies are dealt with flow behavior and concentration patterns for cross-flow membrane operations. However, the relative fouling propensities of top and bottom membrane surfaces are never addressed in any study for narrow channels filled with ladder-type spacers. In the present work, fluid flow patterns through different spacer configurations are visualized using ANSYS FLUENT by varying the dimensionless filament spacing, L (ratio of top or bottom filament spacing and channel height). Results clearly indicate that the average shear stress values at the top membrane surface are always higher (3–8 times) than that at the bottom membrane surface, but the mass transfer coefficient for the two walls yielded approximately similar average values having low to moderate filament spacings of L ≤ 3 (SP22 and SP33), indicating similar fouling propensities of membrane surfaces. Further increase in filament spacing with L ≥ 4 (SP44 and SP66), the average mass transfer coefficient for the top membrane indicated a sharp decline, suggesting high fouling propensity compared to bottom membrane which is not a desirable feature. Among the four spacer arrangements studied, SP44 (with L = 4) was found to be the optimal arrangement, yielding moderate pressure drop with nearly equal/higher area weighted values of mass transfer coefficient for the two walls and would lead to lower and equal fouling tendencies for top and bottom membrane surfaces, respectively.     

Development of energy-saving spinning membrane systemand negatively charged ultrafiltration membranes for recovering oil from waste machine cutting fluid

Membranes with hydrophobic surfaces have higher tendency for protein adsorption and bacteria attachment.As a result, these membranes foul rapidly in cross-flow filtration processes. Changing the membrane surface properties can slow down the membrane fouling process. For difficult membrane separation processes like oilwater emulsion separation, changing membrane properties alone cannot slow down the membrane fouling process. The ordinary cross-flow filtration system cannot be successfully employed for this kind of separation, and the spinning membrane disc system could be considered. The conventional spinning membrane disc system however is not energy efficient due to the centrifugal force acting against the permeate flow; this reduces the effective filtration pressure during the separation operation. Efforts were undertaken to develop a group of negatively charged ultrafiltration membranes, prepared from polyacrylonitrile-vinyl acetate-sodium p-sulfophenyl methallyl ether (CP-24) with polyacrylonitrile-vinyl acetate (CP-16), to be used in an energy-saving design of spinning membrane disc separation system. Our experimental results clearly demonstrate the energy saving benefit of our design; at filtration pressure of 276 kPa and at membrane disc spinning velocity of 1,000 rpm without sacrificing the oil rejection (>98% for 1,000 ppm oil-in-water) by our membrane, the permeate velocity was increased as high as 132% by our energy-saving system over conventional spinning membrane disc separation system.     

合成塔优化操作方案

为了使合成系统特别是合成塔的安全高负荷运行,实现公司利益最大化。通过对实际生产流程上的改造,并进行一系列的优化操作,既考虑到了原材料的合理利用,又充分体现节能、降耗和全系统的高负荷运行。     

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.     

Charged membranes for ultrafiltration treatment of synthetic waste water containing peptone

Continuous ultrafiltration of synthetic waste water containing peptone was carried out by using positively and negatively charged polyacrylonitrile membranes. The filtration experiments were operated in cross-flow mode under 10 kPa of applied pressure. The filtration properties of the charged membranes were compared with those of uncharged polyacrylonitrile ultrafiltration membranes having similar molecular sieve characteristics and membrane structure to the charged ultrafiltration membranes. During the continuous filtration, the filtration rate decreased and the operation pressure increased because of the formation of a peptone gel layer on the membrane surface. It was found that, for the positively charged membrane, the decrease in the filtration rate of the charged membranes was smaller than that of the uncharged membrane. In addition, the positively charged membrane maintained the initial operation pressure during the filtration. The reduction of fouling in the positively charged membrane was discussed by analyzing the gel components on the membrane surface.     

ANALYSIS OF MULTIPLE PARTICLE TRAJECTORIES AND DEPOSITION LAYER GROWTH IN POROUS CONDUITS

In the present paper a rather general method is outlined for the prediction of particle deposition on porous surfaces. In the numerical analysis a continuous dilute stream of small spherical particles subject to a laminar flow field in a porous-walled conduit has been considered. The coupled simultaneous interactions between particle trajectories, viscous fluid flow, deposition layer formation and permeation volume flux have been simulated using a unique matching scheme which relates discrete entrance positions of particle substreams with their associated landing areas, Existing analyses consider only single particles and prescribed permeation fluxes, i.e. d_ν is either a constant or an exponentially decaying function.

The new multi-particle trajectory and deposition model was compared with published results from single particle trajectory studies and hemoglobin recovery experiments. The simulator was then employed to investigate the effects of initial operational conditions, particle layer growth, and cake layer movement on permeation flux decline and particle retention efficiency of a representative membrane separation unit.

The flexible Simula don model can be used for basic investigations of internal fluid-particle systems and can be applied to study membrane fouling in pressure-driven separation devices.     

同质增强型PMIA中空纤维膜污染及其MBR工艺处理城市生活污水

以碳酸钙悬浮液、活性污泥混合液及蛋白溶液作为过滤介质,采用标准堵塞过滤和沉积过滤数学模型预测3 h内膜污染类型,结合膜孔径分布、过滤介质粒径分布和污染阻力分布,研究同质增强型聚间苯二甲酰间苯二胺（PMIA）中空纤维超滤膜污染机理。将同质增强型PMIA中空纤维膜应用于MBR系统处理城市生活污水,监测其长期运行的出水水质;借助场发射扫描电镜和特征X射线能谱仪对比分析水洗和化学清洗效果。研究结果表明,当过滤介质为蛋白时,t-t/V线性相关系数为0.9940,膜污染符合标准堵塞模型;当过滤介质为碳酸钙悬浮液和污泥混合液时,V-t/V线性相关系数分别为0.9733、0.9994,二者较为符合沉积模型。应用于MBR中的同质增强型PMIA中空纤维膜对COD、NH₄⁺-N、TP的平均去除率分别为97.78%、96.71%、49.81%,出水水质较好。经化学清洗后膜表面元素接近于原膜,清洗效果较佳。     

ANALYSIS OF MULTIPLE PARTICLE TRAJECTORIES AND DEPOSITION LAYER GROWTH IN POROUS CONDUITS

In the present paper a rather general method is outlined for the prediction of particle deposition on porous surfaces. In the numerical analysis a continuous dilute stream of small spherical particles subject to a laminar flow field in a porous-walled conduit has been considered. The coupled simultaneous interactions between particle trajectories, viscous fluid flow, deposition layer formation and permeation volume flux have been simulated using a unique matching scheme which relates discrete entrance positions of particle substreams with their associated landing areas, Existing analyses consider only single particles and prescribed permeation fluxes, i.e. d_ν is either a constant or an exponentially decaying function.

The new multi-particle trajectory and deposition model was compared with published results from single particle trajectory studies and hemoglobin recovery experiments. The simulator was then employed to investigate the effects of initial operational conditions, particle layer growth, and cake layer movement on permeation flux decline and particle retention efficiency of a representative membrane separation unit.

The flexible Simula don model can be used for basic investigations of internal fluid-particle systems and can be applied to study membrane fouling in pressure-driven separation devices.     

Tubular Membrane Filtration with A Side Stream and its Intermittent Backwash Operation

The tubular membrane filtration system is widely applied to solid-liquid separation processes. Any improvements to the filtration module would increase separation efficiency, thus reducing operating costs. In this experiment, PMMA powder with an average particle diameter of 0.8 µm was filtered by a ceramic tubular membrane with an average pore size of 0.2 µm, and the impacts of the operating variables, such as suspension concentration, the filtration pressure, and the crossflow velocity on the permeate flux were discussed. In order to understand the increased permeate flux, the proposed module is comparable to the tubular membrane filtration module, but with an additional side stream under the same filtration mass flow rate. In addition, variations of shear force on the membrane surface are analyzed by CFD simulation, and the influence of backwash operations on the permeate flux is discussed. The results show that the side stream membrane filtration increased the shear force on the membrane surface, reduced fouling on the membrane surface, and increased the permeate flux. Furthermore, a backwash operation with a side stream flow channel could effectively clean the particles deposited in the module, thus, increasing the permeate flux.