Carbon hollow fiber membranes for a molecular sieve with precise-cutoff ultramicropores for superior hydrogen separation.

Carbon molecular sieve (CMS) membranes with rigid and uniform pore structures are ideal candidates for high temperature- and pressure-demanded separations, such as hydrogen purification from the steam methane reforming process. Here, we report a facile and scalable method for the fabrication of cellulose-based asymmetric carbon hollow fiber membranes (CHFMs) with ultramicropores of 3-4 Å for superior H2 separation. The membrane fabrication process does not require complex pretreatments to avoid pore collapse before the carbonization of cellulose precursors. A H2/CO2 selectivity of 83.9 at 130 °C (H2/N2 selectivity of >800, H2/CH4 selectivity of >5700) demonstrates that the membrane provides a precise cutoff to discriminate between small gas molecules (H2) and larger gas molecules. In addition, the membrane exhibits superior mixed gas separation performances combined with water vapor- and high pressure-resistant stability. The present approach for the fabrication of high-performance CMS membranes derived from cellulose precursors opens a new avenue for H2-related separations.

Pilot⁻Scale Production of Carbon Hollow Fiber Membranes from Regenerated Cellulose Precursor-Part I: Optimal Conditions for Precursor Preparation.

Industrial scale production of carbon membrane is very challenging due to expensive precursor materials and a multi-step process with several variables to deal with. The optimization of these variables is essential to gain a competent carbon membrane (CM) with high performance and good mechanical properties. In this paper, a pilot scale system is reported that was developed to produce CM from regenerated cellulose precursor with the annual production capacity 700 m² of CM. The process was optimized to achieve maximum yield (>95%) of high quality precursor fibers and carbonized fibers. A dope solution of cellulose acetate (CA)/Polyvinylpyrrolidone (PVP)/N-methyl-2-pyrrolidone (NMP) and bore fluid of NMP/H2O were used in 460 spinning-sessions of the fibers using a well-known dry/wet spinning process. Optimized deacetylation of spun-CA hollow fibers (CAHF) was achieved by using 90 vol% 0.075 M NaOH aqueous solution diluted with 10 vol% isopropanol for 2.5 h at ambient temperature. Cellulose hollow fibers (CHF) dried at room temperature and under RH (80% → ambient) overnight gave maximum yield for both dried CHF, as well as carbon fibers. The gas permeation properties of carbon fibers were also high (CO2 permeability: 50⁻450 Barrer (1 Barrer = 2.736 × 10-9 m³ (STP) m/m² bar h), and CO2/CH4 selectivity acceptable (50⁻500).

Pilot⁻Scale Production of Carbon Hollow Fiber Membranes from Regenerated Cellulose Precursor-Part II: Carbonization Procedure.

The simultaneous carbonization of thousands of fibers in a horizontal furnace may result in fused fibers if carbonization residuals (tars) are not removed fast enough. The optimized purge gas flow rate and a small degree angle in the furnace position may enhance the yield of high quality carbon fibers up to 97% by removing by-products. The production process for several thousand carbon fibers in a single batch is reported. The aim was developing a pilot-scale system to produce carbon membranes. Cellulose-acetate fibers were transformed into regenerated cellulose through a de-acetylation process and the fibers were carbonized in a horizontally oriented three-zone furnace. Quartz tubes and perforated stainless steel grids were used to carbonize up to 4000 (160 cm long) fibers in a single batch. The number of fused fibers could be significantly reduced by replacing the quartz tubes with perforated grids. It was further found that improved purge gas flow distribution in the furnace positioned at a 4-degree to 6-degree angle permitted residuals to flow downward into the tar collection chamber. In total, 390 spun-batches of fibers were carbonized. Each grid contained 2000⁻4000 individual fibers and these fibers comprised four to six spun-batches of vertically dried fibers. Gas permeation properties were investigated for the carbon fibers.

Spinning Cellulose Hollow Fibers Using 1-Ethyl-3-methylimidazolium Acetate⁻Dimethylsulfoxide Co-Solvent.

The mixture of the ionic liquid 1-ethyl-3-methylimidazolium acetate (EmimAc) and dimethylsulfoxide (DMSO) was employed to dissolve microcrystalline cellulose (MCC). A 10 wt % cellulose dope solution was prepared for spinning cellulose hollow fibers (CHFs) under a mild temperature of 50 °C by a dry⁻wet spinning method. The defect-free CHFs were obtained with an average diameter and thickness of 270 and 38 µm, respectively. Both the XRD and FTIR characterization confirmed that a crystalline structure transition from cellulose I (MCC) to cellulose II (regenerated CHFs) occurred during the cellulose dissolution in ionic liquids and spinning processes. The thermogravimetric analysis (TGA) indicated that regenerated CHFs presented a similar pyrolysis behavior with deacetylated cellulose acetate during pyrolysis process. This study provided a suitable way to directly fabricate hollow fiber carbon membranes using cellulose hollow fiber precursors spun from cellulose/(EmimAc + DMSO)/H2O ternary system.

Effect of Water Interactions on Polyvinylamine at Different pHs for Membrane Gas Separation.

In our previous work, it was shown that the separation performance of the fixed-site-carrier polyvinylamine (PVAm) composite membrane increases exponentially with increasing relative humidity content in the gas. Through these efforts, it has been important to develop a greater understanding of the relationship between the water, structural, and interfacial properties of the PVAm surface. The degree of hydrophilicity of a given surface plays a crucial role in the separation performance of the membrane when exposed to a humidified gas. Therefore, in the current work, the wettability properties of PVAm at different pHs have been studied by experimental measurements and molecular dynamic simulations. It was confirmed that the intramolecular interactions are not linearly dependent on pH. As well as the H-bonding between protonated and unprotonated amine groups, the conformation polymer chain and the distribution charge density play a crucial role in the surface stability and wettability properties.

Carbon molecular sieve membranes: a promising alternative for selected industrial applications.

Carbon molecular sieve (CMS) membranes (hollow fibers) have been studied for application as possible separation units for selected industrial gas streams. Gas streams at petrochemical plants (polypropene and polyethene) and upgrading of biogas to fuel specifications have been in focus. Gases present in biogas (N(2), CO(2), H(2)O(vap), and CH(4)) and gas streams at polyolefin plants (C(2)H(4), C(3)H(6), and C(3)H(8)) have been measured; both as pure gases and in mixtures. Aging of the CMS-membranes as a function of humidity and pore blocking is discussed; likewise, possible regeneration methods when flux decrease is experienced. Transport mechanisms depending on pore size and molecular properties are also discussed. Excellent separation properties were documented for these applications, but also the need for frequent regeneration of the membrane in order to maintain permeability flux. The mixed gas experiments documented clearly the need for careful pore tailoring in order to optimize selectivity when the membranes were used for alkane-alkene separation.