Parametric study of aluminium alloy fouling in marine environment using RSM technique

ABSTRACT

Fouling is known to be a prominent industrial problem which is greatly affected by parameters like temperature, time and flowrate. The aim of this work was to simulate the fouling process in AA 6061 by developing a model through Minitab 16 and evaluate the models. Box Behnken design of Response surface methodology was applied for modelling and optimization of fouling propensity in artificial sea water. A response surface model was obtained and Analysis of Variance was performed to test the significance of the model. Fouling propensity was found in terms of weight gain. An experimental rig consisting of a recirculating loop mimicking the industrial conditions of fouling was used in the study. SEM analysis shows uneven deposits on the metal surface. Sharp, irregular deposits pressed deep into the metal was observed. The finding of this work would enable us to evaluate the individual and interactive effects of the parameters.     

600MW直接空冷机组凝汽器冷却管束污垢热阻变工况特性研究

以600 MW直接空冷机组为例,限定污垢热阻取值在0～0.001 m2.K/W范围内,对管内、外污垢热阻建立数学模型,通过编程计算得到直接空冷机组冷端特性曲线,分析污垢热阻对排汽压力的影响,供发电厂运行参考。     

A 7 year long measurement period investigating the correlation of corrosion,deposit and fuel in a biomass fired circulated fluidized bed boiler

The present investigation involves a unique, 7 year (2001–2007) long study of corrosion and deposits on superheater tubes in a biomass fired circulated fluidized bed boiler. These measurements are correlated against the different fuels used over this period. In the earlier years, the boiler was run with a mixture of different biomass fuels and peat. In later years, recycled wood was introduced into the fuel mix. The deposit growth rate approximately doubled when the recycled wood content of the fuel was increased to 10–20%. Small amounts of chlorine and zinc were found both in the recycled wood and in the deposit layer. These elements together with alkali metals from the biomass, have the potential to form sticky compounds that increase the deposit growth rate. The corrosion rate of the superheater tubes varied over the study period. A number of possible explanations for this phenomenon are discussed.     

缩放管传热与污垢特性的实验研究

采用对比实验的方法,对两种缩放管及其对应光管的流动阻力、传热特性和污垢特性进行了研究。实验结果表明,缩放管的性能与其结构尺寸有关,两种缩放管的流动阻力系数明显高于光管。在加速结垢实验中,缩放管表现出良好的抗垢性能,具有结垢诱导期长、污垢热阻渐近值小的优点,而且缩放管平均传热系数都优于光管,在结垢前后都有良好的强化传热性能。     

Fouling characteristics of an ultrafiltration membrane used in drinking water treatment

The fouling characteristics of ultrafiltration membranes used in drinking water were investigated when used alone and when used in an integrated biofilter-membrane system to treat a humic-acid laden solution. Membrane strands from sacrificial modules operating in parallel with bench-scale modules were analysed from both systems (with and without pretreatment). Chemical and microbiological analyses were performed on these strands together with different process streams along the treatment train. Microscopic observations performed on the sacrificial membrane strands revealed that most of the fouling material was organic in nature with high numbers of viable microorganisms. When comparing their fouling characteristics, a positive effect from the biofilter was observed on the performance of the membrane with pretreatment, decreasing in general the amount of material deposited and reducing the fouling rate. Membranes were tested at two different permeate fluxes; this variable did not have an effect on the overall amount of material deposited, but it significantly impacted the membrane fouling rate.     

Full-scale simulation of seawater reverse osmosis desalination processes for boron removal: Effect of membrane fouling

The objective of this study is to further develop previously reported mechanistic predictive model that simulates boron removal in full-scale seawater reverse osmosis (RO) desalination processes to take into account the effect of membrane fouling. Decrease of boron removal and reduction in water production rate by membrane fouling due to enhanced concentration polarization were simulated as a decrease in solute mass transfer coefficient in boundary layer on membrane surface. Various design and operating options under fouling condition were examined including single- versus double-pass configurations, different number of RO elements per vessel, use of RO membranes with enhanced boron rejection, and pH adjustment. These options were quantitatively compared by normalizing the performance of the system in terms of E_min, the minimum energy costs per product water. Simulation results suggested that most viable options to enhance boron rejection among those tested in this study include: i) minimizing fouling, ii) exchanging the existing SWRO elements to boron-specific ones, and iii) increasing pH in the second pass. The model developed in this study is expected to help design and optimization of the RO processes to achieve the target boron removal at target water recovery under realistic conditions where membrane fouling occurs during operation.     

Relating reverse and forward solute diffusion to membrane fouling in osmotically driven membrane processes

Osmotically driven membrane processes, such as forward osmosis (FO) and pressure retarded osmosis (PRO), are attracting increasing interest in research and applications in environment and energy related fields. In this study, we systematically investigated the alginate fouling on an osmotic membrane during FO operation using four types of draw solutions (NaCl, MgCl₂, CaCl₂ and Ca(NO₃)₂) to elucidate the relationships between reverse (from draw solution to feed solution) and forward (from feed solution to draw solution) solute diffusion, and membrane fouling. At the same water flux level (achieved by adjusting the draw solution concentration), the greatest reverse solute diffusion rate was observed for NaCl draw solution, followed by Ca(NO₃)₂ draw solution, and then CaCl₂ draw solution and MgCl₂ draw solution, the order of which was consistent with that of their solute permeability coefficients. Moreover, the reverse solute diffusion of draw solute (especially divalent cation) can change the feed solution chemistry and thus enhance membrane fouling by alginate, the extent of which is related to the rate of the reverse draw solute diffusion and its ability to interact with the foulant. The extent of fouling for the four types of draw solution followed an order of Ca(NO₃)₂ > CaCl₂ >> MgCl₂ > NaCl. On the other hand, the rate of forward diffusion of feed solute (e.g., Na⁺) was in turn promoted under severe membrane fouling in active layer facing draw solution orientation, which may be attributed to the fouling enhanced concentration polarization (pore clogging enhanced ICP and cake enhanced concentration polarization). The enhanced concentration polarization can lead to additional water flux reduction and is an important mechanism governing the water flux behavior during FO membrane fouling. Findings have significant implications for the draw solution selection and membrane fouling control in osmotically driven membrane processes.     

Clarification of passion fruit juice by microfiltration: Analyses of operating parameters,study of membrane fouling and juice quality

In this study, the performance of two membranes were compared – tubular ceramic and hollow fiber poly(imide) – under transmembrane pressure of 0.5 and 1 bar, for the clarification of passion fruit pulp pre-treated by centrifugation and enzymatic treatment at the concentrations of 150 and 300 ppm. Nutritional and sensorial qualities of the clarified juice obtained were evaluated. Thus, it was possible to observe that the most adequate condition for the clarification of passion fruit pulp was with enzymatic treatment at 150 ppm and its posterior microfiltration at the ceramic tubular membrane of 0.3 μm with transmembrane pressure of 0.5 bar. The fouling mechanism was identified by estimation of model parameters according to a nonlinear regression by Bayesian inference. Analysis of the fouling mechanism results revealed that hollow fiber membrane is controlled by a cake filtration mechanism, and internal pore blocking fouling mechanism controls ceramic tubular membrane.     

Carbon nanotube-polytetrafluoroethylene nanocomposite coating for milk fouling reduction in plate heat exchanger

Cleaning of milk fouling on plate heat exchanger surface causes intermittent downtime to milk processing industry. Fouling occurrence during milk pasteurization potentially reduces the heat transfer rate of plate heat exchanger (PHE), leading to serious energy deficiency. In this study, the development of milk surface foulant was successfully inhibited by superhydrophobic nanoparticulate coating of PHE surfaces. Full dispersion of multiwalled carbon nanotubes (MWCNTs) in hydrophobic polytetrafluoroethylene (PTFE) matrix was completed using ultrasonication and high temperature annealing techniques. Nanomorphology and surface structure of PTFE and CNT-PTFE films on stainless steel surface were interpreted by field emission scanning electron microscope (FESEM). To validate the hydrophobicity of the developed CNT-PTFE nanocomposite coated surface, a water contact angle (WCA) using a simplified sessile drop method, correspondent surface energy, foulant mass, and energy efficiency after milk pasteurization were estimated and compared with stainless steel 316 and polytetrafluoroethylene (PTFE) coated surfaces. Static contact angles for the control, PTFE-coated and CNT-PTFE surfaces increased from 71.2 to 119.6 and 141.1°, respectively. By application of CNT-PTFE nanocomposite coating, the estimated surface energy of heat exchanger surface decreased by 97.0% of its original value. After continuous milk pasteurization for 5 h, the mass of foulant on CNT-PTFE coated heat exchanger surface was 70.3% less than that of the uncoated surface; thus, total energy consumption of test PHE unit also dropped down by 10.2%. Developed CNT-PTFE nanocomposite coated surface will potentially reduce the risk of under-processed milk and decrease the frequency of the routine cleaning-in-place (CIP) program.     

Elucidating enzyme-based cleaning of protein soils (gelatine and egg yolk) using a scanning fluid dynamic gauge

The enzyme-based cleaning of two model protein soils was investigated using a scanning fluid dynamic gauge (sFDG). The sFDG device allows data to be collected from more than one sample or location during a single experiment and therefore makes a range of comparative studies feasible. The sFDG was modified to allow the forces imposed on the surface to be controlled during a test. Gelatine films on stainless steel swelled in the presence of alkali at 20 °C but were not removed. Enzymes from a commercial dishwasher product interrupted swelling when the mean water volume fraction of the film reached ∼0.9 and promoted removal. The enzyme effectiveness decreased over time. Egg yolk deposits (spray dried on mica) were studied in a protease/buffer solution at 40 °C. The deposits swelled on contact with alkali, and removal started after ∼40 min. Some flow over the deposit was required to achieve complete cleaning, but the time taken to clean exhibited a weak dependence on the shear stress imposed by the flow for shear stresses above 10 Pa. The cleaning behaviour was strongly influenced by the nature of the deposit. Baking the deposit at 150 °C reduced the rate and extent of swelling as well as the rate of removal, and could result in the formation of a residual film that exhibited yield stress characteristics.