ceramic tubular

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Superhydrophobic or Hydrophilic Porous Metallic/Ceramic Tubular Membranes for Continuous Separations of Biodiesel−Water W/O and O/W Emulsions Michael Z. Hu,*,† Brian L. Bischoff,† Marissa E. Morales-Rodriguez,† Kevin A. Gray,‡ and Brian H. Davison† †

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Prenexus Health, Inc., 1343 N. Colorado St., Gilbert, Arizona 85233, United States

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‡

ABSTRACT: This paper reports the effective surface effect of porous inorganic membranes on improving the emulsion separations (i.e., perm-selective extraction of either oil phase or aqueous phase from the emulsions). A novel tubular form of superwetting, porous, stainless steel/ceramic membranes were demonstrated for enabling continuous separations of oil−water emulsions, extracting either oil phase or aqueous phase from the flowing emulsions. The superhydrophobic membranes consist of macroporous (4 μm) stainless-steel SS434 tube with inner wall surface modified with superhydrophobicity (>150° contact angle) via perfluoro-silane grafting functionalization. The (super)hydrophilic membranes consist of 4-μm porous stainless-steel SS434 tube without/with inner wall coated with nanoporous alumina (6 nm average pore size), the surface of which was further modified with superhydrophilicity via Hydrophil-S solution deposition. With the cross-flow membrane filtration setup, the surface-engineered tubular membranes were studied for perm-selective extraction of aqueous or oil phase from oil−water emulsions (water-in-oil W/O and oil-in-water O/W) that are relevant to a real industrial biodiesel (FAME) production process. The superhydrophobic membrane (4 μm pore size) has demonstrated capability of extracting nearly 100% pure oil while rejecting water phase when either W/O or O/W emulsions were tested as feed. The superhydrophobic membrane showed distinct advantage of 2-orders-of-magnitude-higher flux extraction of oil at 100 times higher “break-through” pressures. On the other hand, the super/hydrophilic nanoporous membranes (6 nm) have shown selective water permeation (separation factor up to 35) when biodiesel-relevant W/O emulsions were tested. Finally, the membrane function is discussed from the perspective of improving industrial biodiesel processing yield by overcoming equilibrium limitations during the biodiesel formation reactions. Future work on in situ reaction−separation experiments are envisioned.

1. INTRODUCTION Membrane separation to remove a particular product composition during a catalytic reaction process is an important strategy to improve overall efficiency of the conversion process.1 Industrial producers of biodiesels are interested in improving commercial-scale plants. It is known that the catalyzed reaction of fats, oils, and greases to fatty acid methyl esters (FAME) is “equilibrium-limited”. Both the transesterification of bound glycerides (triglycerides (TG), diglycerides (DG), and monoglyceride (MG)) and the esterification of free fatty acids (FFA) are limited by the accumulation of side product glycerol and water, respectively. The overall conversion process can be described as

biodiesel (FAME). In addition, in certain processes, there is an undesirable side reaction with methanol that converts produced glycerol (or glycerin) to ∼15% of MPD. The MPD, as an impurity in the co-product glycerol, causes a value discount, as well as a yield loss and thus must be removed during the production reaction. In short, in the biodiesel (FAME) production process, multiple benefits can be envisioned by incorporating membrane separations to remove aqueous phase (glycerol + water + methanol) from the oil phase (mainly FAME): (1) reduced reactor size (and catalyst volume) due to elimination of backward reactions, (2) potential for milder process conditions and less excess methanol, inhibiting the methanol dehydration reaction, and (3) reduced MPD formation due to the lower average concentration of glycerin in the reaction zone. Integration of emulsion separation membrane technology into catalytic

glycerides (TG + DG + MG) + FFA + methanol → FAME + glycerol + water + MPD + methanol

Here, MPD refers to a combination of 2-methoxy-1,3propanediol and 3-methoxy-1,2-propanediol. Timely removal of aqueous phase (water and glycerol plus the background methanol) during the reaction favors the formation yield of the © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

October 4, 2018 December 20, 2018 January 3, 2019 January 3, 2019 DOI: 10.1021/acs.iecr.8b04888 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research

surface nature that makes them ideal choices for the separation of organic oil and aqueous streams (Figure 1). The surface of these membranes can be made either superhydrophobic (SO), superhydrophilic (SI), or some degree between these extremes. The degree of the super hydrophobicity or hydrophilicity can be tailored by applying a surface coating and selecting perfluoro silane grafting ligands appropriate for the separation of mixtures containing both aqueous and nonaqueous compounds of various polarity. So far, some proof-of-principle data have been obtained for tubular HiPAS membrane separations relevant to processing ethanol biofuels and biomass pyrolysis bio-oils (e.g., ethanol−water mixtureboth liquid solutions and vapors) and separations of bio-oils containing hydrocarbon mixtures (such as water, phenol, and toluene in upgraded pyrolysis vapors, or liquid emulsions.7,8 Most of the literature studies on oil−water emulsion separation/recovery materials, not necessarily just “membranes”, involves the surface-modified superwettable metal meshes (copper, stainless steel, etc.), porous sponges or foams, glass or ceramic fibers, polymers (porous or fibrous), organic− inorganic hybrid or composites, carbon materials (porous or fibrous), celluloses, fabrics or textiles, aerogels, and sand layers. These variety of many reported surface-modified oil−water separation materials (mostly superhydrophobic) were developed mainly for oil recovery or oily water treatment applications,9−11 and they are not suited to the above-said biodiesel processing purpose. Majority of the reported membranes are flat sheet type of surface-modified materials and their oil−water emulsion separation studies (for oil recovery) were mainly conducted by gravity-driven filtration experiments, which lack of control of the membrane separation process pressure. More specifically, limited numbers of papers on tubular porous ceramic membranes have been reported for microfiltration or nanofiltration for oily/emulsion wastewater treatment12−17 or for biodiesel purification.18 However, no prior work can be found on superhydrophobic porous ceramic/metallic tubular membranes for oil−water emulsion separations. One misused terminology“superhydrophobic microporous membranes”in the literature19 does, in fact, refer to a sheet of stainless-steel mesh with spray-coated ytterium oxide particles. In contrast to gravity-driven, crossflow pressure-driven membrane separation in a tubular membrane platform, as studied here in this paper, it is more suitable for modular applications process integration with the biodiesel production reactions of interest. In this paper, we will investigate, for the first time, the crossflow separation performance of newly developed superhydrophobic or (super)hydrophilic, microporous/nanoporous, metallic/ceramic tubular membranes on both real and surrogate W/O and O/W emulsions that are relevant to the industrial biodiesel processing. It is demonstrated that the micropore-sized (∼4 μm) superhydrophobic membranes can perm-selectively remove 100% oil phase (FAME) at high flux (up to a thousand LMH) from the O/W emulsion mixtures, while the nanopore-sized (∼6 nm) superhydrophilic membranes can selectively (>90%) extract the trace amount of dispersed aqueous phase (glycerol + water + methanol) from W/O emulsions. Discussion is also given on how the emulsion separation membranes can be further tested in real biodiesel (FAME) production condition to improve the equilibriumlimited formation yield of FAME and glycerol.

Figure 1. (Top) Schematic illustration on continuous cross-flow emulsion separation by a tubular porous membrane with inner wall surface super hydrophobicity or hydrophilicity. (Bottom) Photograph of various porous (typically 4 μm) metallic stainless-steel SS434 membrane tubes with/without inner walls coated with nanoporous (4−8 nm) alumina and functionalized with either hydrophobic or hydrophilic surface modifiers.

reactor may lead to fewer discrete process steps and potentially simpler and less-expensive process design. Besides the post-conversion removal of impurities such as glycerol,2,3 unreacted methanol, or water4 from produced biodiesel to improve the biodiesel quality ASTM D6751 standard, the biodiesel industry desires to find or create an in situ “barrier membrane” technology that would (ideally) continuously remove the product water and glycerol (i.e., the aqueous phase) from the oil phase (e.g., mainly FAME and small amount of residual FFA) of the reaction mixture to inhibit the backward reaction and MPD-forming side reactions and thus, increase conversion in a single flow-through reactor. It is believed that there is a high probability that oil−water emulsion separation membranes may serve as the barrier membrane that allows passage of the aqueous phase (water and glycerol) while inhibiting passage of the oil phase (larger, lesspolar methyl esters such as FAME and glycerides), thus overcoming equilibrium limitations in biodiesel production yield. Since 2013, Oak Ridge National Laboratory (ORNL) has developed first-generation HiPAS (High Performance Architectured Surface Selective) membranes that combines legacy inorganic (porous metallic/ceramic) tubular membrane technology with the most recently developed superwettable surface-coating nanotechnology to result in a new class of highflux, perm-selective, robust, and scalable tubular membranes.5,6 Besides the conventional pore size exclusion mechanism, the main idea for the unique HiPAS membranes is to utilize enlarged pores and achieve high selectivity by tailoring the membrane surface super hydrophobicity/hydrophilicity characteristics to improve discriminative separation of molecules based on their subtle polarity difference. ORNL’s development work on the invented membrane materials has received a 2014 R&D100 Award (in the Materials category). The tubular HiPAS membranes enable a unique tunable, highly selective B

DOI: 10.1021/acs.iecr.8b04888 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note


Figure 2. Cross-flow membrane separation experimental setup for perm-selective extraction/filtration of oil phase or aqueous phase from lumenside feed emulsions. [Legend: (1) feed tank, (2) stirring hot plate, (3) temperature controller for heating tape, (4) heating tape to be wrapped around the feed tank if needed, (5) Teflon-lined tubing, (6) peristaltic pump (maximum pressure of ∼40 psi), (7) T-shapped fitting for pressure gauge, (8) pressure gauge at the inlet end of the membrane tube assembly, (9) membrane tube-holder assembly, (10) shell-side permeate sampling port, (10′) another shell-side permeate sampling port, (11) pressure gauge at the outlet end of the membrane tube assembly, (12) back pressure regulator, and (13) temperature probe (into the emulsion) for the hot plate.]

2. EXPERIMENTAL SECTION Unless otherwise specified, all chemicals used in this work were typically ordered from commercial suppliers such as Sigma− Aldrich. The hydrophobic surface-modifier chemical 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (PDTMS) was purchased from SynQuest Laboratories, Inc. The superhydrophilic surface-modifier agent (i.e., Hydrophil-S solution) was purchased from LotusLeaf Coatings, Inc. The biodiesel (FAME), produced from corn oil, was provided by our industrial partner. The distilled and deionized water were supplied by a Millipore (Milli-Q) system in our laboratory. Real biodiesel W/O emulsions were prepared by dispersing FAME oil (56.82 wt %) into the aqueous phase solution containing water (0.78 wt %), methanol (34.14 wt %), and glycerol (4.23 wt %). W/O emulsions were prepared by dispersing a smaller volume (1 part) of aqueous phase (water, methanol, and glycerol) into the larger volume (1.55 part) of

oil phase (FAME) in a stirred feed tank. For fundamental studies (such as determination of break-through pressure), surrogate W/O or O/W emulsions were prepared by mixing/ emulsifying controlled volume of toluene (oil) with water. Analytical methods include (1) water (Karl Fischer Coulomont ASTM D6304/E1064), (2) methanol (Headspace GC (gas chromatography)), (3) glycerol (GC), and FAME − FTIR (Fourier Transform Infrared) spectroscopy were used for analyzing the industrial biodiesel (FAME) sample. In most of the cases, volume fraction of oil in water or water in oil are measured and calculated for permeate samples to see the membrane effectiveness of oil or water extraction from the emulsions. Tubular porous membranes studied in this work include the “bare” micropore-sized (typically ∼4 μm) stainless-steel (SS434) tubes, superhydrophobic porous SS434 tubes (with inner wall surface modified with 10 vol % PDTMS-hexane C


Research Note


Table 1. Summary of Separation Results on (Methanol-Glycerol-Water)/Fame Mixture Emulsions by Surface-Engineered Porous Metallic/Ceramic Tubular Membranesa Phase Volume Percentage for Permeate Sample membrane pore size

membrane surface

average fluxb (LHM)

cross-membrane pressure (psi)

oil phase

W-phasec

separation factor, SFAd

4 μm

hydrophilic superhydrophobic superhydrophilic

22.95 14.2 5.2

0.2 0.2 0.2

33%−45% >99% 33%−36%

67%−55% trace 67%−64%

2.3−3.8 53 3.3−3.8

6 nm

hydrophilic hydrophilic superhydrophilic

0.72−0.84 0.72−0.84 ∼0.12

30 30e 30

95%

35 17 35

a All experiments were conducted at room temperature, circulating 1000 rpm-stirred emulsion flow through the membrane tube by peristaltic pump at 100 rpm (about 2.7 g/s). Initial feed mixture (stirred liquid emulsion) contains FAME (oil phase) (56.82 wt %), methanol (38.17 wt %), glycerol (4.23 wt %), and water (0.78 wt %). bLHM = liters per hour per square meter of membrane surface. cW-phase refer to the polar liquid solution phase containing MeOH, glycerol, and water. Oil-phase contains mainly FAME. dSFA = (APerm/AFeed)/(BPerm/BFeed). eStirred at 400 rpm in feed tank, other conditions are the same as default.

Figure 3. W/O emulsion separation (i.e., perm-selective extraction of dispersed aqueous phase from the lumen-side emulsion) by a hydrophilic alumina (6 nm pore size)-coated SS434 membrane tube.

solution), and (super)hydrophilic nanoporous alumina (6 nm) coated SS434 tubes (with inner wall surface modified). The nanopore size distribution was determined by a customized air permeance method (∼0.064 sccm/cm2/cm Hg). As shown in Figure 2, the membrane tube (∼9-in. long and 1 cm in diameter) is assembled inside a tubular holder with two ends for feed inlet-and-outlet and shell-side port (between the inner membrane tube and the outer holder tube) is used to collect the permeate samples. The tubular membrane assembly is operated in a cross-flow filtration mode, with lumen-side emulsion flow recirculated through the inner side of the membrane tube driven by a peristaltic pump. A pressure regulator near the outlet end is used to control the pressure inside the membrane tube. Typical feed circulation flow rate is 3.5 mL/s (at 100 rpm peristaltic pump setting). The transmembrane pressure gradient can be adjusted in the

range of 0.1−50 psi, depending on the pore size or surface nature of the membrane. Pressure gauges (Labels 8 and 11 in Figure 2) are used to monitor and regulate the feed-side pressure. Permeation flux (in terms of liters per square meter per hour, LMH) is calculated based on the permeate sample liquid volume collection with time per membrane area. The feed tank shows the milky turbid W-in-O (W/O) emulsion stirred at typical 1000 rpm with a magnetic stir bar. The permselectivity (or separation factor) can be calculated by the volume fraction of oil or aqueous phase in the permeate sample, relative to the feed-side emulsion sample collected at the same time. The volume percentage for the permeate sample is determined by measuring the liquid volume of the oil-phase or the W-phase after complete phase separation by settling down overnight. D


Research Note


3. RESULTS AND DISCUSSION Surface-functionalized porous materials have been known as a major class of materials for emulsion separation and oil

Figure 4. Schematic illustration of the separation mechanism of the “hydrophilic 6 nm nanoporous alumina-coated SS434 membrane tube” on effective extraction of the dispersed aqueous W-phase from the biodiesel (FAME) processing relevant emulsion. Note: The FAME oil has light brownish color.

recovery. This work focus on the studies of surface wetting effect on ceramic membrane cross-flow platform that allows the continuous separation (or extraction) of oil or aqueous phase from an emulsion. The general emulsion separation process parameters involves the type of membrane (hydrophobic vs hydrophilic, pore size variation), ΔP (transmembrane pressure gradient), feed side cross-flow rate (at 100 rpm peristaltic pump setting), feed tank stirring condition (200, 400, 1000 rpm), temperature (typically room temperature at 22 °C), and initial feed W/O emulsion composition that typically contains around 60.78 vol % of oil phase (FAME) and 39.22 vol % of aqueous phase (methanol + glycerol + water). The tubular membranes of different pore sizes and surface properties used for this work is summarized in Table 1. The control run with the 6 nm porous alumina-SS434 membrane tube gives a baseline water permeation flux of 1.61 LMH (at 30 psi transmembrane pressure gradient, water feed flow rate of 3.5 mL/s, 1 h run time), while the 8 nm membrane tube gives 3−4.5 LMH. This clearly shows that larger pore size

Figure 6. Comparison of typical permeate samples collected from hydrophilic bare membrane (left) and superhydrophobic membrane (right).

allows higher water permeation across the porous membrane. Figure 3A illustrates the cross-flow tubular membrane separation of emulsion with a permeate sampling port. When the 6 nm membrane tube was used for separating the W/O emulsion (Figure 3), the W-selective permeation flux slightly decreased to 0.62−0.84 LMH while multiple permeate samples (0−1 h, 1−2 h, 2−3 h, 3−4 h, 4−5 h) were collected. However, in all samples, only a trace amount (less than ∼5 vol %) of O-phase in the form of small droplets exist at the bottom of the permeate sample vials (Figure 3B, permeate samples, and Figure 3C), and the selectivity for the aqueous

Figure 5. Schematic illustration of the “superhydrophobic 4-μm porous tubular membrane” on perm-selective oil extraction from oil−water emulsions: (A) W/O emulsion and (B) O/W emulsion. E


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emulsion (with 62.2 vol% for oil FAME in the initial mixture in the feed tank). The bare membrane surface was hydrophilic in nature. Because of the large pore size of the membrane, a low transmembrane pressure (∼0.2 psi) was needed to conduct the permeation experiment. The pure water permeation flux was determined to be 85.99 LMH, while the emulsion permeation flux decreased with permeate sampling time from 34.2 LMH (at 0.03 h) to 16.2 LMH (at 1.07 h). Such flux decrease may be due to oil fouling by oil doplet coalescence film formation with time on the inner wall surface of porous membrane. The collected permeate samples contained 33.36 vol% oil (at 0.27 h) and 44.48 vol% oil (at 1.07 h). This indicates that the larger pore-sized (4-μm) bare membrane has much less permselectivity for the W-phase (and also less rejection of the Ophase) than the nanoporous (6 nm) alumina-coated membrane. Instead, the oil fouling film formation on the hydrophilic 4-μm membrane surface helped better oil permselectivity, thus, better for oil extraction purpose. This is simply because once the oil film formed on the membrane surface, the oil film covered the hydrophilic membrane surface and thus assimilates the oil phase while rejecting the W-phase of the W/ O emulsion. The oil film then permeates through the pores of the membrane. The surface wetting (superhydrophobic or superhydrophilic) effect of the porous membranes were further investigated on the emulsion separation selectivity and flux. After modifying the inner wall of the bare 4-μm porous SS434 membrane tube into superhydrophilic by Hydrophil-S solution, the cross-flow permeation test has produced permeate samples containing 33.32 vol % oil (at 0.27 h) and 36.39 vol % oil (at 1.07 h). This shows that the superhydrophilic membrane surface, in relative to the intrinsic hydrophilic surface, contributed some degree toward better permeation for Wphase (or higher oil rejection) at longer permeation time (1.07 h): 36.39 vol % vs 44.48 vol % oil. When the surface of the 4-μm porous SS434 membrane tube was modified to superhydrophobic, the permeate samples were collected every 4 min over an hour, the average permeation flux were 14.3 LMH when the cross-flow separations were conducted at 0.2 psi transmembrane pressure gradient with initial feed W/O emulsion containing ∼66.9 vol % of oil

Figure 7. Blue-colored aqueous phase was used for easy observation of break-through pressure PB, below which 100% selective oilextracting permeation can be achieved. Above PB, the blue-colored aqueous phase is entrained into the permeate sample and can be visually seen.

(W) phase all reached greater than ∼95 vol %. This result indicates that the hydrophilic-natured nanoporous aluminacoated SS434 membrane has effectively rejected the bulk continuous O-phase in the emulsion while extracting the dispersed aqueous W-phase droplets from the feed W-in-O emulsion. Note that the W-phase contains methanol, glycerol, and water while the O-phase is FAME. The separation mechanism is further illustrated in Figure 4. The effect of the feed tank stir condition (200, 400, and 1000 rpm) on the emulsion separation were also studied. Stirring at 400 rpm is reaching the critical point to fully emulsify the smaller volume (1 part) of W-phase into the larger volume (1.55 part) of the O-phase (FAME) (i.e., 60.78 vol % oil content). Cross-flow permeation study shows less oil rejection at lower rpm. In other words, the aqueous W-phase droplets that were dispersed at higher rpm are easier to selectively permeate across the hydrophilic nanoporous membrane. This may be understood that smaller W-phase droplets have larger surface areas to be in contact with the hydrophilic membrane surface and, thus, helps the deoplet permeation. 1000 rpm is used as the baseline stirring condition of the feed emulsion tank for the follow-on experiments. In another experimental case, a bare 4-μm porous stainlesssteel (SS434) membrane tube was tested for the W/O

Figure 8. Determination of break-through pressure PB for W/O and O/W emulsions: (A) test with a bare SS434 (4-μm pore) membrane tube. (B) test with a superhydrophobic SS434 (4-μm pore) membrane tube. PB is the maximum operating pressure below which 100 vol% pure oil permeate can be obtained. F


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Figure 9. (A) Envision of a future in situ membrane/reactor testing system under real biodiesel production conditions. (B) Illustration of the flowthrough membrane separation role immediately after the catalytic reaction bed: extracting the water and glycerol out of the reaction to improve biodiesel (FAME) formation yield.

(FAME). The PDTMS-modified inner wall surface was verified to be superhydrophobic (>150° contact angle), because of the molecular functionalization of microscopic roughness surface of the porous membrane. This superhydrophobic membrane has demonstrated the capability of continuously permeating/ extracting almost 100% oil (FAME) from the emulsion when the transmembrane pressure is below the “breakthrough pressure” (PB), which is defined as the pressure above which the oil- phase permeation starts to entrain some W-phase across the porous membrane. In contrast to the emulsion separation mechanism of the superhydrophilic or bare 4-μm porous membranes (Figure 4), the superhydrophobic membrane allows highly perm-selective, high-flux extraction of the continuous oil (FAME) phase (Figure 5) in the emulsion while the dispersed droplets of aqueous W-phase in the emulsion are effectively repelled/rejected. Figure 6 shows the differences between a typical permeate sample (from hydrophilic/bare membrane experiment (left vial) and a typical permeate sample from superhydrophobic membrane experiment (right vial). The former permeate sample contains the bulk aqueous phase

solution (methanol + water + glycerol) with small amount of oil (FAME), while the latter permeate sample contains majority of oil (FAME) with trace amount (

ceramic tubular

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