Simultaneous Enzymatic Cellulose Hydrolysis and Product Separation in a Radial-Flow Membrane Bioreactor.

Hydrolysis is the heart of the lignocellulose-to-bioethanol conversion process. Using enzymes to catalyze the hydrolysis represents a more environmentally friendly pathway compared to other techniques. However, for the process to be economically feasible, solving the product inhibition problem and enhancing enzyme reusability are essential. Prior research demonstrated that a flat-sheet membrane bioreactor (MBR), using an inverted dead-end filtration system, could achieve 86.7% glucose yield from purified cellulose in 6 h. In this study, the effectiveness of flat-sheet versus radial-flow MBR designs was assessed using real, complex lignocellulose biomass, namely date seeds (DSs). The tubular radial-flow MBR used here had more than a 10-fold higher membrane surface area than the flat-sheet MBR design. With simultaneous product separation using the flat-sheet inverted dead-end filtration MBR, a glucose yield of 10.8% from pretreated DSs was achieved within 8 h of reaction, which was three times higher than the yield without product separation, which was only 3.5% within the same time and under the same conditions. The superiority of the tubular radial-flow MBR to hydrolyze pretreated DSs was confirmed with a glucose yield of 60% within 8 h. The promising results obtained by the novel tubular MBR could pave the way for an economic lignocellulose-to-bioethanol process.

Revisiting the Effect of the Resistance to Gas Accumulation in Constant Volume Systems on the Membrane Time Lag.

The time-lag method is commonly used to determine membrane permeability, diffusivity and solubility in a single gas permeation experiment in a constant volume system. An unwritten assumption on which this method relies is that there is no resistance to gas accumulation in the downstream receiver of the system. However, this is not the case, even with the specially designed receiver used in this study when, in addition to tubing, the receiver utilizes an additional accumulation tank. The resistance to gas accumulation originates from a finite diffusivity (Knudsen diffusion) of gases in tubing, which are magnified by "resistance-free" accumulation tank(s). As a result of the resistance to gas accumulation, the time lag of the membrane is underestimated, which leads to an overestimation of gas diffusivity in the membrane. The experimentally predicted resistances in different configurations of the receiver, expressed by the difference in the time lag at two different receiver locations, were several times greater than the theoretically predicted values. A high molecular PPO membrane was used to demonstrate this effect. The time lags measured at different locations differed by as much as 30%. The diffusivity of nitrogen in a PPO of 4.04 × 10-12 m2/s determined at the optimum configuration of the receiver is at least 50% lower than the literature-reported values.

Incorporation of Functionalized Halloysite Nanotubes (HNTs) into Thin-Film Nanocomposite (TFN) Nanofiltration Membranes for Water Softening.

Incorporating nanoparticles (NPs) into the selective layer of thin-film composite (TFC) membranes is a common approach to improve the performance of the resulting thin-film nanocomposite (TFN) membranes. The main challenge in this approach is the leaching out of NPs during membrane operation. Halloysite nanotubes (HNTs) modified with the first generation of poly(amidoamine) (PAMAM) dendrimers (G1) have shown excellent stability in the PA layer of TFN reverse-osmosis (RO) membranes. This study explores, for the first time, using these NPs to improve the properties of TFN nanofiltration (NF) membranes. Membrane performance was evaluated in a cross-flow nanofiltration (NF) system using 3000 ppm aqueous solutions of MgCl2, Na2SO4 and NaCl, respectively, as feed at 10 bar and ambient temperature. All membranes showed high rejection of Na2SO4 (around 97-98%) and low NaCl rejection, with the corresponding water fluxes greater than 100 L m-2 h-1. The rejection of MgCl2 (ranging from 82 to 90%) was less than that for Na2SO4. However, our values are much greater than those reported in the literature for other TFN membranes. The remarkable rejection of MgCl2 is attributed to positively charged HNT-G1 nanoparticles incorporated in the selective polyamide (PA) layer of the TFN membranes.

Effect of Membrane Orientation and Concentration of Draw Solution on the Behavior of Commercial Osmotic Membrane in a Novel Dynamic Forward Osmosis Tests.

Dynamic performance tests, commonly used to characterize gas separation membranes, are not utilized to characterize osmotic membranes. This paper demonstrates the application of a novel dynamic forward osmosis test to characterize a commercial osmotic membrane. In particular, we report the effect of membrane orientation (active layer draw solution (AL-DS) vs. active layer feed solution (AL-FS)) and the draw solution concentration on the membrane's transient and steady-state behaviors. A step-change in the draw solution concentration initiated the dynamic test, and the mass and concentration of the feed and draw solutions were recorded in real-time. The progress of the experiments in two different membrane orientations is markedly different; also, the draw solution concertation has a different effect in the orientations. A positive salt time lag is observed in both orientations; however, the salt time lag in the AL-FS orientation (4.3−4.6 min) is practically independent of the draw solution concentration, but it increases from 7 to 20 min with the draw solution concertation in the AL-DS orientation. A negative water time lag, ranging from −11 to −20 min depending on the draw solution concentration, is observed in the AL-DS orientation. Still, in the AL-FS orientation, the water flux is practically constant from the experiment's onset, leading to a negligible water time lag (<1 min). The new method demonstrated in this paper can be a potent tool for characterizing osmotic membranes.

Novel Polyelectrolyte-Based Draw Solute That Overcomes the Trade-Off between Forward Osmosis Performance and Ease of Regeneration.

Forward osmosis (FO) is an emerging technology for seawater and brackish desalination, wastewater treatment, and other applications, such as food processing, power generation, and protein and pharmaceutical enrichment. However, choosing a draw solute (DS) that provides an appropriate driving force and, at the same time, is easy to recover, is challenging. In this study, water-soluble poly(styrene sulfonate) (PSS) was modified by a high-electrical-conductivity 3,4-ethylenedioxythiophene (EDOT) monomer to fabricate a novel draw solute (mPSS). FO tests with the CTA membrane in the active layer facing the feed solution (AL-FS) orientation, using a 50 mS/cm aqueous solution of synthesized solute and distilled water as a feed solution exhibited a water flux of 4.2 L h-1 m-2 and a corresponding reverse solute flux of 0.19 g h-1 m-2. The FO tests with the same membrane, using a 50 mS/cm NaCl control draw solution, yielded a lower water flux of 3.6 L h-1 m-2 and a reverse solute flux of 4.13 g h-1 m-2, which was more than one order of magnitude greater. More importantly, the synthesized draw solute was easily regenerated using a commercial ultrafiltration membrane (PS35), which showed over 96% rejection.

Monte Carlo Simulations for the Estimation of the Effective Permeability of Mixed-Matrix Membranes.

Recent years have seen the explosive development of mixed-matrix membranes (MMMs) for a myriad of applications. In gas separation, it is desired to concurrently enhance the permeability, selectivity and physicochemical properties of the membrane. To help achieving these objectives, experimental characterization and predictive models can be used synergistically. In this investigation, a Monte Carlo (MC) algorithm is proposed to rapidly and accurately estimate the relative permeability of ideal MMMs over a wide range of conditions. The difference in diffusivity coefficients between the polymer matrix and the filler particle is used to adjust the random progression of the migrating species inside each phase. The solubility coefficients of both phases at the polymer−filler interface are used to control the migration of molecules from one phase to the other in a way to achieve progressively phase equilibrium at the interface. Results for various MMMs were compared with the results obtained with the finite difference method under identical conditions, where the results from the finite difference method are used in this investigation as the benchmark method to test the accuracy of the Monte Carlo algorithm. Results were found to be very accurate (in general, <1% error) over a wide range of polymer and filler characteristics. The MC algorithm is simple and swift to implement and provides an accurate estimation of the relative permeability of ideal MMMs. The MC method can easily be extended to investigate more readily non-ideal MMMs with particle agglomeration, interfacial void, polymer-chain rigidification and/or pore blockage, and MMMs with any filler geometry.

Modeling the transport of CO2, N2, and their binary mixtures through highly permeable silicalite-1 membranes using Maxwell-Stefan equations.

Carbon dioxide (CO2) is the main contributor to global warming; therefore, research efforts aim at its capture. Membranes, in particular, zeolite membranes offer a promising approach for CO2 separation and capture. Membranes are typically characterized by their selectivity and permeance that are highly dependent on the operating conditions namely, total feed pressure and composition. Therefore, more reliable characterization parameters are required such as Maxwell- Stefan exchange diffusivities. In this work, a model based on Maxwell-Stefan equations and Extended Langmuir isotherm was developed to investigate the transport of binary mixtures of CO2 and N2 through thin silicalite-1 membranes. The exchange diffusivities, D12 and D21, of CO2 and N2 were determined at different total feed pressures and feed compositions. All gas separation tests were conducted at stage cut not exceeding 5%. The single component diffusivities of CO2 and N2 required by the model were found experimentally using the results of the respective single gas permeation tests. The results displayed that as CO2 concentration in the feed increased from 15% to 85%, the values of D12 and D21 decreased from 2.8 × 10-10 to 1.1 × 10-10 m2/s and 2.8 × 10-10 to 1.3 × 10-10 m2/s, respectively, while the N2 permeance decreased by about one order of magnitude from 2.7 × 10-7 to 2.4 × 10-8 mol/m2.s.Pa. Consequently, the exchange diffusivities showed considerably smaller dependence on the operating conditions compared to the permselectivity and permeance. Hence, they are more appropriate in describing the intrinsic transport characteristics of silicalite-1 membranes.

Gas Permeation Model of Mixed-Matrix Membranes with Embedded Impermeable Cuboid Nanoparticles.

In the packaging industry, the barrier property of packaging materials is of paramount importance. The enhancement of barrier properties of materials can be achieved by adding impermeable nanoparticles into thin polymeric films, known as mixed-matrix membranes (MMMs). Three-dimensional numerical simulations were performed to study the barrier property of these MMMs and to estimate the effective membrane gas permeability. Results show that horizontally-aligned thin cuboid nanoparticles offer far superior barrier properties than spherical nanoparticles for an identical solid volume fraction. Maxwell's model predicts very well the relative permeability of spherical and cubic nanoparticles over a wide range of the solid volume fraction. However, Maxwell's model shows an increasingly poor prediction of the relative permeability of MMM as the aspect ratio of cuboid nanoparticles tends to zero or infinity. An artificial neural network (ANN) model was developed successfully to predict the relative permeability of MMMs as a function of the relative thickness and the relative projected area of the embedded nanoparticles. However, since an ANN model does not provide an explicit form of the relation of the relative permeability with the physical characteristics of the MMM, a new model based on multivariable regression analysis is introduced to represent the relative permeability in a MMM with impermeable cuboid nanoparticles. The new model possesses a simple explicit form and can predict, very well, the relative permeability over an extensive range of the solid volume fraction and aspect ratio, compared with many existing models.

Highly permeable tubular silicalite-1 membranes for CO2 capture.

Membranes represent one of the most promising alternatives for CO2 separation and capture. Zeolites membranes, in particular, that can withstand high temperatures and pressures, offer energy efficient way to capture CO2 compared to conventional separation techniques such as amine absorption. In this work, silicalite-1/ceramic composite membranes were prepared on the inner surface of zirconium oxide and/or titanium oxide tubular supports by a pore plugging hydrothermal synthesis. Five types of supports with different pore sizes ranging from 0.14 to 1.4 µm, were studied. The synthesized membranes were characterized by scanning electron microscope (SEM), electron diffraction spectrometer (EDS), x-ray diffraction (XRD), and gas permeation with pure and mixed gas feeds. All membranes showed high concentrations of Si within the active layer of the support, suggesting successful pore-plugging of the membranes. The greater the pore size of the active layer of the support, the greater was the concentration of Si observed. In addition, large coffin-shape crystals, which are characteristics of silicalite-1, were also observed on top of each membrane. The analysis of XRD micrographs revealed that the crystals were mostly oriented with either the a- or b-axes perpendicular to the membrane surface, which is desirable from the point of view of minimizing the resistance to gas transport through the zeolite membrane. Except for the membranes synthesized using the supports with 0.14 µm pores, all membranes were very selective with CO2/N2 permselectivities up to 30 at low-pressure differentials. At the same time, the membranes were very permeable with CO2 permeance in the order of 10-6 mol m-2 Pa-1 s-1. Assuming the thickness of the selective layer to be equivalent to the thickness of the active layer of the support, all membranes fell above the revised Robeson upper-bound line for CO2/N2 separation.

Synthesis and Gas Transport Properties of Poly(2,6-dimethyl-1,4-phenylene oxide)⁻Silica Nanocomposite Membranes.

The emulsion polymerized mixed matrix (EPMM) method is a new approach to prepare nanocomposite membranes, in which inorganic nanoparticles are synthesized in situ at the interface of a dispersed aqueous phase in a continuous phase of polymer solution. In this paper, we report the synthesis and characterization of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)-based EPMM membranes, in which silica nanoparticles are synthesized by the polymerization of tetraethylorthosilicate (TEOS) in the presence of two different co-solvents, ethanol and acetone, which are soluble in both the aqueous phase and the polymer solution. The EPPM membranes prepared in the presence of acetone show greater conversions of TEOS and a different structure of the synthesized silica nanoparticles compared to the EPMM membranes prepared in the presence of ethanol. The former membranes are both more permeable and more selective for O2/N2 and CO2/CH4. Both types of EPMM membranes are more permeable than the reference PPO membranes. However, while their O2/N2 selectivity is practically unchanged, their CO2/CH4 selectivity is decreased compared to the reference PPO membranes.