Fouling Rate Research Articles

Experimental study on crude oil fouling in preheat exchangers at different operating conditions

Fouling is one of the leading causes of energy loss in oil industry, particularly in preheat exchangers of refineries and desalting units. For each day that an oil operation unit producing 80,000 BPD is shut down due to fouling, a significant economic loss of approximately 6,400,000 $ (assuming 80 $/bbl) will occur. Unfortunately, there is currently no global or specific solution for reducing crude oil fouling in these units. On the other hand, conventional fouling detection methods are beaded on an inaccurate pressure difference measurement. This study proposed an accurate and sensitive thermal method based on surface temperature variation for fouling detection in crude oil system which was rarely reported in the literature. Experiments were performed to investigate the impact of three key operating parameters including heat flux (51.3–72.7 kW/m2), bulk temperature (33–40 °C) and fluid velocity (0.09–0.14 m/s) on the fouling rate of crude oil. The results indicated that the applied heat flux primarily determines a direct or inverse relationship between fouling rate and fluid velocity. Increasing fluid velocity, at a lower heat flux, decreases the fouling rate up to 23 % due to particle removal by shear stress while at higher heat flux, the simultaneous positive effect of diffusion and adhesion forces increases the fouling rate up to 63 %. Increasing bulk temperature reduces the fouling tendency of crude oil but demonstrates different effects at the beginning and end of the experiments. As heat flux increases, fouling occurs more rapidly due to increas ed adhesion forces, thermal driving force, and particles diffusion.

Prediction of particulate fouling in full-scale reverse osmosis plants using the modified fouling index – ultrafiltration (MFI-UF) method

This study aims at applying and verifying the MFI-UF method to predict particulate fouling in RO plants. Two full-scale RO plants treating surface water, with average capacity of 800–2000 m3/h, were studied. Firstly, the MFI-UF of RO feed and concentrate was measured using 5–100 kDa membranes at same flux applied in the RO plants (20–26 L/m2.h). Subsequently, the particle disposition factor (Ω) was calculated to simulate particle deposition in RO cross-flow filtration. Finally, particulate fouling rates were predicted based on MFI-UF and Ω, and compared with the actual fouling rates in the plants. For plant A, the results showed that the fouling rates predicted using MFI-UF measured with 100 kDa membrane have the best agreement with the actual fouling (with 3–11 % deviation). For plant B, the fouling rates predicted based on both 10 and 100 kDa membranes agree well with the actual fouling (with 2 % and 15 % deviation, respectively). However, the fouling predicted based on 5 kDa membrane is considerably overestimated for both plants, which is attributed to the effect of the low surface porosity of 5 kDa membrane. More widespread applications of MFI-UF in full-scale RO plants are required to demonstrate the most suitable MFI-UF membranes for fouling prediction.

Performance of Anaerobic Membrane Bioreactor (AnMBR) with Sugarcane Bagasse Ash-based Ceramic Membrane treating Simulated Low-strength Municipal Wastewater: Effect of Operation Conditions.

The online version contains supplementary material available at 10.1007/s11270-023-06173-3.

Purifying surface waters contaminated with natural organic matters and bacteria using Ag/PDA-coated PES membranes

Polymeric membrane is susceptible to fouling particularly when it is used to treat surface water contaminated by natural organic matters and bacteria. Surface modification may address the issue by imparting antifouling and antibacterial properties to the membrane. In this study, a neat polyethersulfone (PES) membrane (denoted as membrane P1) was prepared by phase inversion method followed by polydopamine (PDA) coating to improve its membrane hydrophilicity. Thereafter, the physicochemical properties of the PDA-coated PES membrane (denoted as membrane P3) were further improved by in situ reduction of silver (Ag) onto its surface to impart antibacterial properties (denoted as membrane P4). All membranes were instrumentally characterized while their filtration performance against humic acid (HA) and antibacterial properties were evaluated. Results showed that membrane P1 removed 77.81% of HA and achieved a fouling recovery rate (FRR) of 65.72%. Upon Ag immobilization, the HA rejection of membrane P4 was further improved to 95.14% with FRR recorded at 92.17%. This membrane also demonstrated good stability and excellent antibacterial properties over other fabricated membranes (P1, P2 and P3) in this study. Moreover, it exhibited a lower total flux decline and higher contaminant removal performance over membrane P1 in treating surface water samples.

The effect of heat transfer characteristics of macromolecule fouling on heat exchanger surface: A dynamic simulation study

AbstractAt the city gate gas pressure reduction stations (CGSs), to prevent natural gas from forming a hydrate in the throttle valve, the natural gas is heated by the heater before reaching the pressure relief valve. Heat exchangers are an essential component of industrial processes that contribute significantly to total system energy. Since the element impacting heat exchanger performance is the fouling process, all fouling processes and models were dynamically simulated in this study. Through coding in the C++ language and simultaneous use of fluent functions, or, in other words, user‐defined function (UDF), fouling‐related models were defined for this software. The dynamic simulation was performed, and parameters such as fouling strength and layer thickness were calculated. The effects of changing operating conditions, such as gas inlet velocity, surface temperature, and fouling species concentration on fouling growth, were also evaluated. As the concentration of fouling species increased, the fouling rate also increased. The amount of supersaturation and fouling rate increased as the surface temperature increased. Due to the operational limitations of the system, to reduce the fouling rate, the gas inlet velocity should be as high as possible, and the fluid inlet temperature, surface temperature, and concentration of fouling species should be as low as possible. In this study, the required time to reach the efficiency of 70% of the heat exchanger was calculated using the modelling of this chamber, which was equivalent to 190 days. Additionally, the critical thickness of the fouling layer at this time was 3.5 cm.

Biochar Addition in Membrane Bioreactor Enables Membrane Fouling Alleviation and Nitrogen Removal Improvement for Low C/N Municipal Wastewater Treatment

Membrane bioreactors (MBRs) are frequently used to treat municipal wastewater, but membrane fouling is still the main weakness of this technology. Additionally, the low carbon-nitrogen (C/N) ratio influent has been shown to not only increase the membrane fouling, but also introduce challenges to meet the effluent discharge standard for nitrogen removal. Herein, the authors addressed the challenges by adding cost-effective biochar. The results suggested that the biochar addition can enable membrane fouling alleviation and nitrogen removal improvement. The reduced membrane fouling can be ascribed to the biochar adsorption capacity, which facilitates to form bigger flocs with carbon skeleton in biochar as a core. As a result, the biochar addition significantly altered the mixed liquor suspension with soluble microbial product (SMP) concentration reduction of approximately 14%, lower SMP protein/polysaccharide ratio from 0.28 ± 0.02 to 0.22 ± 0.03, smaller SMP molecular weight and bigger sludge particle size from 67.68 ± 6.9 μm to 113.47 ± 4.8 μm. The nitrogen removal is also dramatically improved after biochar addition, which can be due to the initial carbon source release from biochar, and formation of aerobic-anaerobic microstructures. Microbial diversity analysis results suggested more accumulation of denitrification microbes including norank_f__JG30-KF-CM45 and Plasticicumulans. Less relative abundance of Aeromonas after biochar addition suggested less extracellular polymer substance (EPS) secretion and lower membrane fouling rate.

The impact of sunlight on fouling behaviors and microbial communities in membrane bioreactors

Membrane fouling is a significant obstacle to applying membrane bioreactors (MBRs) for wastewater treatment. Here we report the impact of sunlight irradiation on membrane fouling, biopolymers, signal molecules, and microbial communities in MBRs. The degradation of signal molecules, which induce membrane biofouling, occurred through solar photolysis in batch tests. However, MBR sludge exposed to sunlight exhibited different biological behaviors creating more soluble microbial products (SMP) and signal molecules (particularly autoinducer-2). Cell lysis and deflocculation occurred when the MBR mixed liquor was exposed to sunlight. MBR fouling rates coincided with the temporal concentration profiles of SMP and signal molecules. Sunlight caused drastic changes in the MBR microbial community, stimulating the preferential growth of specific bacteria (e.g., Deinococcus Runella, Flavitalea, Glaiimonas, and Rurimicrobium). The nonmetric multidimensional scaling analysis of the MBR community structures with and without sunlight irradiation showed two distinct microbial community clusters and their reversibility. Network analysis based on Spearman's rank correlations revealed that with sunlight irradiation, fouling rates had significant positive connections with SMP proteins and the Flavitalea genus. The findings of this study demonstrate that sunlight is a considerable factor affecting membrane fouling and microbial ecology in MBRs, needing shade for fouling mitigation and sustainable operation.

New insights into membrane fouling during direct membrane filtration of municipal wastewater and fouling control with mechanical strategies

Direct membrane filtration (DMF) technology achieves energy self-sufficiency through carbon recovery and utilization from municipal wastewater. To control its severe membrane fouling and improve DMF technology, targeted research on fouling behaviour and mechanisms is essential. In this study, a DMF reactor equipped with a flat-sheet ceramic membrane was conducted under three scenarios: without control, with intermittent aeration, and with periodic backwash. This system achieved efficient carbon concentration with chemical oxygen demand below 50 mg/L in permeate. Membrane fouling was dominated by intermediate blocking and cake filtration. And reversible external resistance accounted for over 85 % of total resistance. Predominant membrane foulants were free proteins, whose deposition underlies the attachment of cells and biopolymers. Backwash decreased the fouling rate and increased fouling layer porosity by indiscriminately detaching foulants from the membrane surface. While aeration enhanced the back transport of large particles and microbial activity, causing a relatively thin and dense fouling layer containing more microorganisms and β-d-glucopyranose polysaccharides, which implies a higher biofouling potential during long-term operation. In addition, aeration combined with backwash enhanced fouling control fivefold over either one alone. Therefore, simultaneous operation of backwash and other mechanical methods that can provide shear without stimulating aerobic microbial activity is a preferred strategy for minimizing membrane fouling during DMF of municipal wastewater.

Improved quantitative evaluation of the fouling potential in spacer-filled membrane filtration channels through a biofouling index based on the relative pressure drop

In this study, a biofouling index based on the relative pressure drop is presented to quantitatively evaluate the amount of fouling in spacer-filled membrane filtration channels. The biofouling index was defined as the inverse of the time to reach a relative pressure drop of 100% and can be interpreted as a fouling rate or cleaning frequency. The index was applied to evaluate biofilm growth in membrane fouling simulators with reverse osmosis membranes and commercial feed spacers operated with different feed water nutrient concentrations and crossflow velocities. Biofilm accumulation on the membrane and feed spacer was characterized in situ using optical coherence tomography. We showed that the biofouling index is directly related to the volume of biofouling independent of the applied crossflow velocity and a suitable tool for improved quantitative comparison of the biofouling rate. Furthermore our results suggest that the pressure drop is better described as function of the velocity at the perimeter of a spacer cell instead of the average velocity in the channel. Although the biofouling index is developed for biofouling, the index may be applied to quantitatively assess mitigation strategies in spacer filled channels for a wider range of fouling types.

The optimum conditions for calcium sulfate fouling rate under subcooled flow boiling using Taguchi statistical method

A comprehensive experimental investigation as well as a statistical design of experiment method (the Taguchi method with L16 orthogonal array robust design) was performed in order to measure the heat transfer reduction and fouling rate of CaSO4 aqueous solutions. Moreover, the effects of different variables (flow rate, CaSO4 concentration, heat flux and bulk temperature) were investigated for two heat transfer mechanisms including forced convection and subcooled flow boiling. Each parameter was studied at two levels so that 16 experiments were totally conducted aiming at introducing the best set of conditions in order to resulting the fewer fouling rate of CaSO4. Based on analysis of variance (ANOVA), the percentage contributions of each factor are ranked as follows: (1) concentration (34%), (2) heat flux (24%), (3) flow rate (23%), and (4) bulk temperature (13%). It also verifies that the interaction of heat flux and flow rate has the greater impact rather than other parameters interactions with a contribution of 2.9%. This experimental result optimum conditions with a fouling rate of 2.1 × 104 m2K/kW.min was very close to the predicted value (1.99 × 104 m2K/kW.min). This demonstrates a good consistency between the experimental and the predicted values, with only a 5% deviation, and approves the effectiveness of the experimental design to attain the optimal condition.

Advanced water treatment process by simultaneous coupling granular activated carbon (GAC) and powdered carbon with ultrafiltration: Role of GAC particle shape and powdered carbon type

In current ultrafiltration systems, limited removal for small-sized contaminants and membrane fouling remain longstanding obstacles to overcome. Herein, a novel process by simultaneous coupling powered carbon (PC) and fluidized granular activated carbon (GAC) with ultrafiltration was proposed aiming to achieve high effluent quality and mitigated membrane fouling. This study conducted mechanistic explorations on the performances of different-shaped GAC particles on fouling control and PC release during fluidization, meanwhile comparing the utilizations of powdered activated carbon (PAC) and biochar in terms of their adsorption, deposition and interactions with aquatic contaminants during filtration. The results showed that the effluent COD of biochar-UF was slightly higher than PAC-UF attributed to lower specific surface area and pore volume present on biochar. Compared with PAC-UF, the biochar-UF without fluidized GAC exhibited higher fouling propensity due to more organics attached on membranes via bridging with Ca2+ released by the biochar. Concurrently, distinct morphologies were found for PAC and biochar depositions, where PAC uniformly dispersed on membranes but biochar tended to agglomerate. Interestingly, fluidized spherical GAC (RGAC) with highest particle momentum and least energy consumption appeared highly effective in reducing fouling associated with biochar, and the overall fouling rate of RGAC-biochar-UF was even lower than RGAC-PAC-UF system. More importantly, substantial amount of small-sized PC was released by two cylindrical-shaped GACs, which were determined to be around 12–16 mg/L in contrast to merely 3.4 mg/L produced from RGAC. Consequently, the RGAC-biochar-UF system achieved commensurate effluent quality but better permeability than RGAC-PAC-UF along with a 20% expenditure saved, which might be a promising water treatment system more suitable for large-scale applications.

Direct Membrane Filtration of Municipal Wastewater: Studying the Most Suitable Conditions for Minimizing Fouling Rate in Commercial Porous Membranes at Demonstration Scale.

This study aimed to evaluate the feasibility of applying a commercial porous membrane to direct filtration of municipal wastewater. The effects of membrane pore size (MF and UF), treated influent (raw wastewater and the primary settler effluent of a municipal wastewater treatment plant) and operating solids concentration (about 1 and 2.6 g L-1) were evaluated on a demonstration plant. Filtration periods of 2-8 h were achieved when using the MF membrane, while these increased to 34-69 days with the UF membrane. This wide difference was due to severe fouling when operating the MF membrane, which was dramatically reduced by the UF membrane. Use of raw wastewater and higher solids concentration showed a significant benefit in the filtration performance when using the UF module. The physical fouling control strategies tested (air sparging and backwashing) proved to be ineffective in controlling UF membrane fouling, although these strategies had a significant impact on MF membrane fouling, extending the operating period from some hours to 5-6 days. The fouling evaluation showed that a cake layer seemed to be the predominant reversible fouling mechanism during each independent filtration cycle. However, as continuous filtration advanced, a large accumulation of irreversible fouling appeared, which could have been related to intermediate/complete pore blocking in the case of the MF membrane, while it could have been produced by standard pore blocking in the case of the UF membrane. Organic matter represented more than 70% of this irreversible fouling in all the experimental conditions evaluated.

Effect of spraying polyvinyl alcohol solution on the surface of liquid film on the structure and antifouling properties of polyvinylidene membrane

Abstract Polyvinylidene fluoride (PVDF) membrane was prepared by a two-step method of surface gelation-immersion precipitation. The surface of the scraping solution film was sprayed with polyvinyl alcohol (PVA) aqueous solution to gel the film surface first, and then the liquid film was immersed in a coagulation bath for phase transformation to obtain the surface modified PVDF membrane. The effects of PVA solution with different mass fraction on the structure and properties of PVDF membrane were studied. The results showed that with the increase of spraying PVA mass fraction, the contact angle of the upper surface of PVDF membranes gradually decreased, the porosity and mean pore size of PVDF membranes gradually increased, and the crystallinity of PVDF membranes gradually decreased. The pores on upper surface of the membranes first increased and then decreased, and the finger-like macropore structure appeared inside the membranes, the water flux first increased and then decreased, and the rejection rate increased. Dynamic cycle filtration experiment of bovine serum protein (BSA) solution showed an increase in membrane flux recovery from 68.31 to 95.08% and a decrease in the irreversible fouling rate from 31.69 to 4.92%, indicating an improvement in the hydrophilicity and antifouling properties of the modified PVDF membrane.

Chemical cleaning−solvent treatment−hydrophilic modification strategy for regenerating end-of-life PVDF membrane

Membrane will inevitably reach its end of life during long-term application. The current disposal approach of replacing end-of-life (EOL) membranes by new membranes constrains the economic efficiency and sustainability of the membrane-based wastewater treatment. Herein, we conducted a proof-of-concept study of regenerating EOL polyvinylidene fluoride (PVDF) membranes from a full-scale membrane bioreactor (MBR), to achieve an economical and sustainable membrane-based wastewater treatment. A novel chemical cleaning−solvent treatment−hydrophilic modification strategy was proposed for regenerating the EOL PVDF membranes. After the regeneration, the water permeance of the EOL membrane could be recovered from 43.7 ± 8.8 to 426.0 ± 40.3 L m−2 h−1 bar−1, which was comparable with that of the new membrane. The solvent treatment slightly dissolved the membrane matrix and facilitated the detachment of recalcitrant foulants, which was responsible for the markedly recovered water permeance. The hydrophilic modification using polydopamine coating effectively improved surface hydrophilicity of membrane, endowing the regenerated membrane with better antifouling performance over the solvent treated membrane. The regenerated membrane was tested in an MBR for real municipal wastewater treatment, where the regenerated membrane possessed a comparable effluent quality, as well as a lower fouling rate over EOL, chemical cleaned, and solvent treated membranes. Compared with the conventional way of membrane replacement, the reduction in expenditure and carbon emission of membrane regeneration was ∼3.0 USD/m2-membrane/year and 0.059 kg CO2-e/membrane module, respectively. This proof-of-concept study offers a promising strategy of the EOL membrane regeneration to achieve economical and sustainable membrane-based wastewater treatment.

Effect of biomass retention time on performance and fouling of a stirred membrane photobioreactor

Co-culture of microalgae-activated sludge has the potential to purify wastewater while reduce energy demand from aeration. In this work, a mechanically stirred membrane photobioreactor (stirred-MPBR) was used to evaluate the impact of the biomass retention time (BRT) on the treatment performance and membrane fouling. Results showed that stirred-MPBR was affected by BRT during treating domestic wastewater at a flux of 16.5 L m−2 h−1. The highest productivity was attained at BRT 7d (102 mg L−1 d−1), followed by BRT 10d (86 mg L−1 d−1), BRT 5d (85 mg L−1 d−1), and BRT 3d (83 mg L−1 d−1). Statistical analysis results showed that BRT 7d had a higher COD removal rate than BRT 10d, however, there is no difference in total nitrogen removal rate. The highest TP removal occurred when the biomass operated at BRT as short as 3d. Reduced BRTs caused a change in the microalgae-activated sludge biomass fraction that encouraged nitrification activity while simultaneously contributing to a higher fouling rate. The bound protein concentrations dropped from 31.35 mg L−1 (BRT 10d) to 10.67 mg L−1 (BRT 3d), while soluble polysaccharides increased from 0.99 to 1.82 mg L−1, respectively. The concentrations of extracellular polymeric substance fractions were significantly altered, which decreased the mean floc size and contributed to the escalating fouling propensity. At the optimum BRT of 7d, the stirred-MPBR showed sufficient access to light and nutrients exchange for mutualistic interactions between the microalgae and activated sludge.

Superior methylparaben removal by anaerobic fluidized bed ceramic membrane bioreactor with PVDF tubular fluidized biocarrier: Reactor performance and microbial community

An anaerobic fluidized bed ceramic membrane bioreactor (AFCMBR) was operated with fluidized tubular biocarrier consisting of polyvinylidene difluoride (PVDF) for treating synthetic low-strength wastewater containing high dosage of methylparaben. The AFCMBR was run over 150 d to investigate the effect of hydraulic retention time (HRT) on the removal efficiency of methylparaben and any potential adverse impact of the addition of methylparaben on methane content in the biogas produced by AFCMBR. The fluidization of the PVDF biocarrier along the membrane surface played a key role in cleaning the membrane by maintaining the transmembrane pressure below 0.05 bar at 10.4 L/m2·h (LMH) of permeate flux (corresponding to 8 h of HRT). The fouling rate was also effectively reduced by conducting intermittent backwashing with the permeate produced by AFCMBR. Continuous feeding of methylparaben into the AFCMBR did not deteriorate microbial activity during the entire operational period applied in this study. Biodegradation through the biofilm formed on PVDF media was thought to be the dominant pathway to remove methylparaben. Microbial analysis of the biofilm community revealed the presence of aromatic-compound-degrading syntrophs (Syntrophorhabdus and Longilinea), propionate-oxidizing syntrophs (Syntrophobacter and Smithella), acetoclastic Methanothrix, and hydrogenotrophic methanogens (Methanobacterium, Methanolinea, and Methanoregula), indicating a syntrophic partnership for metabolizing methylparaben and propionate in the feed wastewater.

Superhydrophilic membrane with photo-Fenton self-cleaning property for effective microalgae anti-fouling

Membrane filtration is one of the effective approaches to harvest microalgae for industrial biofuel production. However, during the filtration process, microalgae cells and extracellular organic matter (EOM) will deposit on the membrane surface leading to reversible membrane fouling that can be removed by physical methods. When hydrophobic EOM is adsorbed on the membrane surface or inside pores, it will build up a gel layer, causing irreversible membrane fouling. Irreversible fouling can only be removed using chemical methods that will decrease membrane lifespan and increase operational costs. Here, we introduce a versatile superhydrophilic membrane with photo-Fenton self-cleaning property, which can prevent the reversible fouling and remove the irreversible fouling. Tannic acid (TA) and 3-aminopropyltriethoxysilane (APTES) were co-deposited on the polyvinylidene fluoride (PVDF) membrane via Schiff base and Michael addition reactions, and β-FeOOH nanorods were inlaid on the membrane surface by in situ mineralization. The water contact angle of the modified membrane is reduced from 120° to 0° Under 60 min visible light, the hydroxyl radical (·OH) generated by the photo-Fenton reaction degraded the irreversible fouling that blocked membrane pores. The irreversible fouling rates of modified membrane was reduced from 39.57% to 3.26%, compared with the original membrane. Microalgae harvesting results illustrated that the membrane has a high flux recovery rate (FRR) of 98.2%, showed excellent passive antifouling and active antifouling performance. We believe this work will spark a novel platform for optimizing energy-efficient microalgae harvesting separation membrane modules. In addition, this method of anti-fouling filtration for microorganisms can be extended to the industrial production of various bioenergy sources and will have very promising practical applications.

Experimental investigation of temperature and hydrodynamics on CaCO3 fouling during convective heat transfer and subcooled flow boiling

Deposit formation in heat exchangers is an undesirable process in many industries. Severe fouling can potentially reduce the performance of heat exchangers compromising the efficiency of operating units. This research aimed to examine experimentally the effect of fluid bulk temperature (Tb) during deposition of CaCO3 solution under convective heat transfer and subcooled flow boiling conditions. To do so, several tests were performed at a constant concentration of CaCO3 solution (0.2 g/l) and Tb values of 55–75 °C. Heat flux ranging from 13 to 83 kW/m2 and fluid flow rates of 2 to 4 l /min) were also applied to investigate experimentally fouling resistance and heat transfer coefficient of CaCO3 solution. It was found that Tb would significantly affect the mechanism of deposit formation which has been less discussed in the past. Higher Tb changed the propensity of fouling resistance curve from linear to asymptotic, and the crystal type of deposit layer from aragonite to calcite. Results also showed that at low Tb (55 °C), an increase in heat flux enhanced the fouling rate while at high Tb, an increase in heat flux reduced the fouling, due to increasing the evaporation rate of the solution. In addition, at low heat flux (13 kW/m2), an increase in the fluid flow rate led to higher fouling rate, whereas at high heat flux, increasing the fluid flow rate resulted in lower fouling rate.

Monitoring residual fouling after cleaning of multi-fiber membrane modules fiber-by-fiber using non-invasive MRI monitoring

In this study non-invasive low field magnetic resonance imaging (MRI) technology was used to monitor fouling induced changes in fiber-by-fiber hydrodynamics inside a multi-fiber hollow fiber membrane module containing 401 fibers. Using structural and velocity images the fouling evolution of these membrane modules were shown to exhibit distinct trends in fiber-by-fiber volumetric flow, with increasing fouling causing a decrease in the number of flow active fibers. This study shows that the fouling rate is not evenly distributed over the parallel fibers, which results in a broadening of the fiber to fiber flowrate distribution. During cleaning, this distribution is initially broadened further, as relatively clean fibers are cleaned more rapidly compared to clogged fibers. By tracking the volumetric flow rate of individual fibers inside the modules during the fouling-cleaning cycle it was possible to observe a fouling memory-like effect with residual fouling occurring preferentially at the outer edge of the fiber bundle during repeated fouling-cleaning cycle. These results demonstrate the ability of MRI velocity imaging to quantitatively monitor these effects which are important when testing the effectiveness of cleaning protocols due to the long term effect that residual fouling and memory-like effect may have on the operation of membrane modules.

Effects of torrefaction on ash-related issues during biomass combustion and co-combustion with coal. Part 2: Ash fouling behavior

Torrefaction is a promising pretreatment technology that allows the utilization of biomass on large scales. This work provides new knowledge regarding the effects of torrefaction on ash fouling during biomass combustion and co-combustion by correlating them to element repartitioning due to torrefaction, which has little been evaluated in the literature. Two herbaceous biomasses (rice husk and corn stalk) and one woody biomass (sawdust) were torrefied on a fixed bed at 270℃ in N2. The raw and torrefied biomass were fired and co-fired with two coals on a drop-tube furnace at 1300℃ in air. The samples were characterized by various techniques such as X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, and ion chromatography. The results show that the fuel interactions during co-combustion inhibit the formation of inside fouling while the incorporation of alkalis in the aluminosilicates promotes the growth of outside deposits. Torrefaction reduces the formation of chlorides and sulfates during combustion directly and consequently inhibits the inside deposition due to the fractional removal of Cl, S, and alkalis in biomass. Besides, the less condensation of chlorides/sulfates on the surface of the tube and the coarse particles tend to reduce the formation and growth of the outside deposits. However, the elemental repartitioning during torrefaction leads to more retention of alkalis which can aggravate the formation of mixed aluminosilicates during combustion and favors the outside deposition. Both co-combustion and biomass torrefaction have the potential to decrease the ash fouling tendency during biomass combustion considering that they reduce the inside fouling deposits which are problematic in practice. Based on the ash composition of the fuels, a modified fouling index [soluble (Na2O + MgO + K2O + CaO)] * [SiO2/(SiO2 + Al2O3)] is proposed which shows a superior linear correlation with the inside fouling rates of the various fuels investigated, indicating its feasibility for prediction of the ash fouling tendency.