Vegetable oil degumming with polyimide and polyvinylidenefluoride ultrafiltration membranes

The removal of phospholipids (‘degumming’) is the first step in the process of refining crude vegetable oil. Membrane separation has been often proposed as an alternative to the conventional procedure (water degumming). Until now, the instability of polymeric membranes in organic solvents has been the major obstacle in applying this technique. In this work, a local synthesized polyvinylidenefluoride (PVDF) and a commercial polyimide (PI) membrane were evaluated for their flux and rejection properties during degumming of soybean oil in a laboratory‐scale cross‐flow ultrafiltration cell. Degumming experiments were done at different temperatures and feed flows, keeping constant both the feed concentration and the transmembrane pressure. PVDF and PI membranes gave selectivity values and permeate color that did not differ significantly from each other. Retention coefficients larger than 98% were obtained in all cases. In every experiment, a decline in permeate flux with time occurred at the beginning of the degumming process. By increasing the feed rate, a higher permeate flux was obtained. The results show that the PVDF membrane had a higher productivity than the PI one. Copyright © 2003 Society of Chemical Industry     

Plasma Polymerization Modified Polyvinylidene Fluoride (PVDF) Membrane Development and Characterization for Degumming of Soybean Oil

Unmodified and surface‐modified polyvinylidene fluoride (PVDF) membranes were tested for their ability to degum soybean crude oil and crude oil miscellas. The membrane was modified with 1,1,1,3,3,3‐hexafluoro‐2‐propanol or hexamethyldisiloxane (HMDSO) by radio‐frequency plasma polymerization at 10–100 W glow discharge power and 1–30 min contact time. The membranes were characterized by contact angle measurements, attenuated total reflectance Fourier transform infrared spectroscopy, atomic force microscopy, and scanning electron microscopy. Modification of the PVDF membrane with HMDSO at 60 W power for 5 min increased the interfacial free energy between water and solid surface from 30 ± 2 to 64 ± 2 mJ/m². This membrane was tested for permeate flux and phospholipid rejection with crude oil and different concentrations of miscella. Although formation of the polymer film on the membrane tended to decrease membrane pore size, the modified membrane had an oil flux as good as the unmodified membrane did. In addition, the modified‐membrane improved the phospholipid rejection and removed 76 % of the phospholipids from the crude oil and 81–90 % of the phospholipids from crude oil miscellas.     

Evaluation of phosphoglycolipid elimination from rice bran oil by a nonporous membrane using NMR spectroscopy

The presence of phosphoglycolipids (PGL) in degummed rice bran oil (RBO) affects the color in the subsequent refining process, posing problems in producing a final product of acceptable quality. A nonporous membrane could achieve near complete removal of phospholipids even under hexane‐diluted conditions, including PGL present in RBO. PGL from glycolipid fractions isolated from various membrane process stream samples, analyzed by ¹H, ¹³C and ³¹P NMR spectroscopic techniques, indicated the absence of PGL in the permeate, besides identifying them to be oleates/linoleates of phosphatidic acid. Hence, the efficacy of a nonporous membrane in the removal of PGL suggests its application for effective degumming of crude RBO.     

Membrane degumming of crude soybean and rapeseed oils

Membrane separation in edible oil processing is a potential area for energy savings. However, technical and operating-cost-related barriers have impeded the successful application of membrane separation in food processing. Studies were undertaken with soybean and rapeseed oils in a magnetically stirred flat-membrane batch cell with two types of composite polymeric membranes at 3 MPa pressure and at a constant temperature of 40°C. The membranes were NTGS-1100 and NTGS-2100, and used silicon as the active layer and polysulfone and polyimide as support layers, respectively. The membrane selectively rejected phospholipids, the content being less than 240 mg/kg in the permeate without any pretreatment or dilution of crude oil with organic solvent. Long-term studies up to 97 days with soybean oil at two different pressures, 2 and 3 MPa, showed that the rejection of phosphatides was above 96% in most permeates. The permeate flux remained nearly constant but must be improved.     

Effect of additives concentration on the surface properties and performance of PVDF ultrafiltration membranes for refinery produced wastewater treatment

The aim of this study was to investigate the effect of pore-forming hydrophilic additives on the porous asymmetric polyvinylideneflouride (PVDF) ultrafiltration (UF) membrane morphology and transport properties for refinery produced wastewater treatment. PVDF ultrafiltration membranes were prepared via a phase inversion method by dispersing lithium chloride monohydrate (LiCl·H₂O) and titanium dioxide (TiO₂) nanoparticles in the spinning dope. The morphological and performance tests were conducted on PVDF ultrafiltration membranes prepared from a different additive content. The top surface and cross-sectional area of the membranes were observed using a field emission scanning electron microscope (FESEM) and energy dispersive X-ray (EDX) analysis. The surface wettability of porous membranes was determined by the measurement of a contact angle. The mean pore size and surface porosity were calculated based on the permeate flux. The results indicated that the PVDF/LiCl/TiO₂ membranes with lower TiO₂ nanoparticles loading possessed smaller mean pore size, more apertures inside the membrane with enhanced membrane hydrophilicity. LiCl·H₂O has been employed particularly to reduce the thermodynamic miscibility of dope which resulted in increasing the rate of liquid–liquid demixing process. The maximum flux and rejection of refinery wastewater using PVDF ultrafiltration membrane achieved were 82.50 L/m² h and 98.83% respectively at 1.95 wt.% TiO₂ concentration.     

基于DOTP脱色的膜分离-树脂吸附集成技术研究

以聚偏氟乙烯中空纤维膜(PVDF)、阴离子交换树脂作为膜分离-树脂吸附集成技术的分离材料应用于对苯二甲酸二异辛酯(DOTP)粗酯的脱色,采用紫外可见光谱(UV-VIS)和薄层色谱扫描仪(TLCS)测试方法分析膜分离-树脂吸附集成工艺的脱色效果和机理。结果表明:PVDF中空纤维膜在30℃操作温度下有较高脱色率,主要截留掉DOTP粗酯中的胶质和大颗粒物质,阴离子交换树脂可进一步脱去DOTP粗酯中的小分子有色物质和残留颗粒状物质。DOTP粗酯经过膜分离-树脂吸附集成技术脱色处理,DOTP颜色到达产品标准要求。     

白炭黑填充PDMS/PVDF复合膜的纳滤分离性能及传质特性

将纳米级白炭黑填充于PDMS制备了白炭黑填充PDMS/PVDF复合膜,采用红外(FT-IR)、热失重(TGA)和接触角(CA)等方法对填充复合膜进行了分析和表征,并采用纳滤的方法系统研究了复合膜对大豆油/己烷混合油的分离性能。结果表明,白炭黑填充能有效促进PDMS的交联,提高PDMS的疏水性、热失重温度以及对溶剂的稳定性;白炭黑填充量增加使复合膜渗透通量降低,但截留率从96%提高到98%;随溶液浓度增加,渗透通量和截留率同时降低;随温度的升高,渗透通量上升,截留率降低。大豆油和己烷在膜中的传质特性可用不完全的溶解-扩散模型描述,溶液渗透压实验值与计算值符合较好。     

A novel approach to process crude oil membrane concentrate using a centrifuge

The present work discusses an alternative process to handle crude oil membrane concentrate during a degumming process. In this process, the membrane concentrate, which typically consists of 15–30% phospholipids (PL) by weight of oil, is first stripped of hexane and then centrifuged to produce two phases—supernatant (PL<0.6%) and lecithin concentrate (PL>62%). The main advantages of this method are limited oil loss, potential lecithin by-product, and a supposedly simpler process. In this work, we first show that the phase behavior of an oil-PL-hexane system can be exploited to identify the various steps of the process. The steps include membrane degumming, hexane evaporation, and centrifugation. Although much knowledge already exists on these unit operations for miscella degumming, it is the combination and sequence of these steps that is proposed here. Since the novelty of this process lies in using a centrifuge after the membrane separations, we focus on this step. Here, we evaluate the dependence of hexane removal, moisture, temperature, hexane amount, residence time, centrifuge g-force, and nonhydratable PL on the phase separations.     

Membrane degumming of crude rice bran oil: Pilot plant study

The procedure for the classical chemical refining of vegetable oils consists of degumming, alkali neutralization, bleaching, and deodorization. Conventional refining of rice bran oil using alkali gives oil of acceptable quality, but the refining losses are very high. A critical work has been carried out to study the application of membrane technology in the pretreatment of crude rice bran oil. Oils intended for physical refining should have a low phosphorus content, and this is not readily achievable by the conventional acid/water degumming process. The application of membrane technology for the pretreatment of rice bran oil has been investigated. The process has already been successfully applied to other vegetable oils. Ceramic membranes, which are important from the commercial point of view, were examined for this purpose. The results showed that the membrane‐filtered oils met the requirements of physical refining, with a substantial reduction in color. It was observed that most of the waxy material was also rejected. Experiments were carried out to establish the relationship between permeate flux and rejection with membrane pore size, trans‐membrane pressure and micellar solute concentration.     

不同交联剂对PDMS/PVDF纳滤膜溶剂回收性能的影响

以聚二甲基硅氧烷(PDMS)为分离层材料,聚偏氟乙烯(PVDF)超滤膜为底膜,采用正硅酸乙酯(TEOS)、苯基三甲氧基硅烷(PTMOS)、辛基三甲氧基硅烷(OTMOS)、γ-氨基丙基三乙氧基硅烷(APTEOS)4种不同的交联剂对PDMS进行交联,制备了PDMS/PVDF纳滤膜。采用接触角、红外谱图、扫描电镜等对膜的物理和化学结构进行了分析和表征。以大豆油/己烷混合油为实验体系,考察了压力和料液浓度对纳滤膜分离性能的影响。结果表明,纳滤膜的通量随压力线性增长,截留率初始随压力上升较快,随后增幅减慢而趋于稳定。随料液浓度的增加,纳滤膜的通量和截留率都有较大幅度的下降。相比较而言,以TEOS为交联剂所制得的纳滤膜分离性能最佳。大豆油/己烷混合油体系同水溶液体系的渗透特性类似,其渗透压可用van't Hoff方程计算。     

Evaluation of surfactant-aided degumming of vegetable oils by membrane technology

The first step in the process of vegetable oil refining is degumming, in which phospholipids and mucilaginous gums are removed that otherwise result in a low-grade oil. A membrane process is remarkably simple yet potentially offers many advantages in degumming. Studies were conducted on surfactant-aided membrane degumming with soybean and rapeseed oils in a magnetically stirred flat membrane batch cell with different types of microfiltration membranes. The reduction of phospholipids in soybean oil was in the range of 85.8–92.8% during the membrane process. The phosphorus content of membrane permeates of soybean oil was in the range of 20–58 mg/kg. Crude rapeseed oil contained higher amount of nonhydratable phospholipids and hence resulted in lower reduction in phospholipids, in the range of 66.4–83.2%. Addition of hydratable phospholipids could improve the efficiency of degumming in the membrane process without using any electrolyte, resulting in improvement of quality as well as quantity of the phospholipids.     

Comparison of Methods for Separating Polar Lipids from Oat Oil

Polar lipids of crude oat oil were investigated. Oat oil was separated from oats by extraction with isopropanol. Polar lipids were fractionated from crude oil by supercritical CO₂-extraction, by ultrafiltration in hexane and by water degumming. The polar fraction from CO₂-extraction had the highest phospholipid and lowest tocopherol content. The polar fractions made by different methods possessed similar antioxidative properties. However, the polar lipids from oats were more powerful antioxidants than those made from soybean or rapeseed oil.     

Membrane processing of crude vegetable oils: Pilot plant scale remoyal of solvent from oil miscellas

The recovery of solvents used in the extraction step of edible oil processing is required for economical, environmental, and safety considerations. The miscella (mixture of extracted oil and solvent) exits the extractor at 70 to 75 wt% solvent content. Currently, the solvent is recovered by distillation. This paper reports the results of a study on separation of vegetable oils from commercial extraction solvents using various types of Reverse Osmosis (RO) and Ultrafiltration (UF) membranes. Solvent permeation rates and separation performances of various RO and UF membranes were determined by using ethanol, isopropyl alcohol and hexane as the solvents. One membrane exhibited a flux of 200 GFD (ethanol) with 1% oil remaining in the permeate. However, hexane rapidly deteriorated all but one of the membranes tested. The membrane that was compatible with hexane had a low flux and unacceptably low oil retention. Industrial-scale membranes were also evaluated in pilot plant trials. A hexane separation was attempted with a hollow-fiber membrane unit, and it was noted that the pores of the fibers swelled almost closed. Some of the commercially available membranes selectively removed solvent (ethanol or isopropanol) from the edible oil miscellas with reasonable flow rates. The research reported has shown that membranes manufactured from polyamide were the least affected by hexane. Fluxes achieved during solvent-oil separations were increased by increases in either temperature or pressure and decreased by increases in oil concentration in the feed. The processing temperature affected the percentage of oil in solution in either ethanol or isopropanol as well as the viscosity of the feed. Both of these factors in turn influenced the flux achieved. Approximately 2 trillion Btu/yr could be saved using a hybrid membrane system to recover solvents used in the extraction step of crude oil production. Studies to date report marginal success. The development of hexane-resistant membranes may make this application viable.     

PVDF/PEK-C共混膜的研究

实验以相转化法制备了PVDF/PEK-C共混膜,考察了共混体系的相容性及PEK-C的添加量对共混膜超滤性能、抗污染性能、稳定性等的影响。研究结果表明PVDF/PEK-C属于部分相容体系;添加适量PEK-C能够改善PVDF膜的超滤性能和抗污染能力;PVDF/PEK-C共混膜和纯PVDF膜具有相似的化学稳定性,与纯PVDF膜相比,共混膜具有较好的耐酸性,纯PVDF膜的耐氧化性则要优于共混膜。     

抗污染膜在电镀废水回用中的应用

用抗污染超滤膜聚偏氟乙烯(PVDF)合金膜、高压纳滤膜(聚芳香酰胺膜)处理含Cr6+,Ni2+,Cu2+重金属离子的电镀废水.抗污染PVDF合金超滤膜在经过三次电镀废水处理后,膜通量由4.2 L/h降为3.6 L/h,透过液的透光率大于96.3%,可去除大部分有机物.超滤透过液经过纳滤处理后,纳滤透过量由1.8 L/h降低为1.7 L/h,并且在1.5 h后基本保持不变,对Cr6+,Ni2+,Cu2+的去除率达97%以上.电镀废水经抗污染超滤膜-高压纳滤组合工艺处理,可达电镀清洗水标准,回用率达到85%.     

Synthesis and characterization of low content of different SiO2 materials composite poly (vinylidene fluoride) ultrafiltration membranes

A comparison of the morphology and performance of virgin poly (vinylidene fluoride) (PVDF) ultrafiltration (UF) membrane, and PVDF-composite membranes with low content of two different SiO₂ (N-SiO₂ and M-SiO₂ particles) was carried out. Cross-sectional area and surface morphology of the membranes were observed by scanning electron microscopy and atomic force microscopy. Surface hydrophilicity of the porous membranes was determined through the measurement of a contact angle. Performance tests were conducted on the composite membranes through water flux and bovine serum albumin (BSA) retention. Average pore size and surface porosity were calculated based on the permeate flux. Thermal stability and mechanical stability were determined by thermogravimetric analysis and tensile stress tests. The results indicate that N-SiO₂/PVDF (P-N) membranes possessed larger average pore size and porosity, which led to higher water flux and a slight decline in BSA retention. On the other hand, M-SiO₂/PVDF (P-M) membranes had better mechanical stability and anti-fouling performance with enhanced membrane hydrophilicity and decreased membrane surface roughness. Both of the P-N and P-M membranes exhibited typical asymmetric morphology and improved thermal stability.     

Effect of PVP Molecular Weights on the Properties of PVDF-TiO2 Composite Membrane for Oily Wastewater Treatment Process

Polyvinylidene fluoride (PVDF) hollow fiber ultrafiltration membranes consisted of TiO₂ and different molecular weight (M_w) of polyvinylpyrrolidone (PVP) (i.e., 10, 24, 40, and 360 kDa) were prepared to treat synthesized oily wastewater. The membrane performances were characterized in terms of pure water flux, permeate flux, and oil rejection while their morphological properties were studied using SEM, AFM, and tensile tester. Results show that the PVDF-TiO₂ composite membrane prepared from PVP40k was the best performing membrane owing to its promising water flux (72.2 L/m².h) coupled with good rejection of oil (94%) when tested with 250 ppm oily solution under submerged condition. It is also found that with increasing PVP M_w, the membrane tended to exhibit higher PVP and protein rejection, greater mechanical strength, smaller porosity, and a smoother surface layer. Regarding the effect of pH, the permeate flux of the PVDF-PVP40k membrane was reported to increase with increasing pH from 4 to 7, followed by decrease when the pH was further increased to 10. Increasing oil concentration in the feed solution was reported to negatively affect the water flux of PVDF-PVP40k membrane, owing to the formation of thicker oil layer on the membrane surface which increased water transport resistance. A simple backflushing process on the other hand could retrieve approximately 60% of the membrane original flux without affecting the oil separation efficiency. Based on the findings, the PVDF-TiO₂ membrane prepared from PVP40k can be potentially considered for oily wastewater treatment process due to its good combination of permeability and selectivity and reasonably high water recovery rate.     

Fouling-resistant Composite Membranes for Separation of Oil-in-water Microemulsions

Fouling-resistant ceramic-supported polymer composite membranes were developed for removal of oil-in-water （O/W） mieroemulsions. The composite membranes were featured with an asymmetric three-layer structure, i.e., a porous ceramic membrane substrate, a polyvinylidene fluoride （PVDF） ultrafiltration sub-layer, and a polyamide/polyvinyl alcohol （PVA） composite thin top-layer. The PVDF polymer was east onto the tubular porous ceramic membranes with an immersion precipitation method, and the polyamide/PVA composite thin top-layer was fabricated with an inteffaeial polymerization method. The effects of the sub-layer composition and the recipe in the inteffaeial polymerization for fabricating the top-layer on the structure and performance of composite membranes were systematically investigated. The prepared composite membranes showed a good performance for treating the O/W microemulsions with a mean diameter of about 2.41μm. At the operating pressure of 0.4MPa, the hydraulic permeability remained steadily about 190L·m^-2·h^-1, the oil concentration in the permeate was less than 1.6mg·L^-1, and the oil rejection coefficient was always higher than 98.5% throughout the operation from the beginning.     

硅橡胶膜用于植物油/己烷胶团中溶剂回收的研究

Traditional solvent recovery in the extraction step of edible oil processing is distillation, which consumes large amounts of energy. If the distillation is replaced by membrane process, the energy consumption can be reduced greatly. In this work, two kinds of membrane, PDMS (polydimethylsiloxane) composite membrane and Zeolite filled PDMS membrane were prepared, in which asymmetric microporous PVDF (polyvinylidenefluoride) membrane prepared with phase inversion method was functioned as the microporous supporting layer in the flat-plate composite membrane. The different function compositions of the PDMS/PVDF com-posite membranes were characterized by reflection Fourier transform infrared (FTIR) spectroscopy. The surface and section of PDMS/PVDF composite membranes were investigated by scanning electron microscope (SEM). The PDMS NF (nanofiltration) membranes were then applied in the recovery of hexane from soybean oil/hexane miscellas (1︰3, mass ratio). The effects of pres-sure (0.5-1.5 MPa), cross-linking temperature and PDMS layer thickness on membrane performances were investigated. The results indicated that both two kinds of NF membranes were promising for solvent recovery, and zeolite filled in PDMS NF membrane could enhance the separation performance.     

Polyimide Ultrafiltration Membranes with High Thermal Stability and Chemical Durability

Abstract

Asymmetric ultrafiltration membranes based on poly[(4,4′-oxydiphenylene)pyromelliteimide] were produced by wet technique from prepolymer casting solution, followed by solid-phase conversion of the prepolymer membranes into polyimide insoluble form at 200°C. It was demonstrated that by adding benzimidazole to the casting solution and filling of prepolymer membrane pores with inert high-boiling oil prior to thermal treatment allow us to prepare asymmetric porous polyimide membranes. The main characteristics of the membranes obtained (permeability coefficients and molecular weight cut-off) match those typical to ultrafiltration membranes. It was found that the developed asymmetric ultrafiltration polyimide membranes have excellent thermal and chemical resistance. The membranes retain rigidity above Tg (360°C) and are chemically stable at temperatures up to 400°C. The developed membranes are resistant against swelling and dissolving in aggressive and organic media including amide solvents.