Xuezhong He

Physical Properties of Ionic Liquids: Database and Evaluation

A comprehensive database on physical properties of ionic liquids (ILs), which was collected from 109 kinds of literature sources in the period from 1984 through 2004, has been presented. There are 1680 pieces of data on the physical properties for 588 available ILs, from which 276 kinds of cations and 55 kinds of anions were extracted. In terms of the collected database, the structure-property relationship was evaluated. The correlation of melting points of two most common systems, disubstituted imidazolium tetrafluoroborate and disubstituted imidazolium hexafluorophosphate, was carried out using a quantitative structure-property relationship method.

Membranes for Environmentally Friendly Energy Processes

Membrane separation systems require no or very little chemicals compared to standard unit operations. They are also easy to scale up, energy efficient, and already widely used in various gas and liquid separation processes. Different types of membranes such as common polymers, microporous organic polymers, fixed-site-carrier membranes, mixed matrix membranes, carbon membranes as well as inorganic membranes have been investigated for CO2 capture/removal and other energy processes in the last two decades. The aim of this work is to review the membrane systems applied in different energy processes, such as post-combustion, pre-combustion, oxyfuel combustion, natural gas sweetening, biogas upgrading, hydrogen production, volatile organic compounds (VOC) recovery and pressure retarded osmosis for power generation. Although different membranes could probably be used in a specific separation process, choosing a suitable membrane material will mainly depend on the membrane permeance and selectivity, process conditions (e.g., operating pressure, temperature) and the impurities in a gas stream (such as SO2, NOx, H2S, etc.). Moreover, process design and the challenges relevant to a membrane system are also being discussed to illustrate the membrane process feasibility for a specific application based on process simulation and economic cost estimation.

Fabrication of Defect-Free Cellulose Acetate Hollow Fibers by Optimization of Spinning Parameters

Spinning of cellulose acetate (CA) with the additive polyvinylpyrrolidone (PVP) in N-methyl-2-pyrrolidone (NMP) solvent under different conditions was investigated. The spinning parameters of air gap, bore fluid composition, flow rate of bore fluid, and quench bath temperature were optimized based on the orthogonal experiment design (OED) method and multivariate analysis. FTIR and scanning electron microscopy were used to characterize the membrane structure and morphology. Based on the conjoint analysis in Statistical Product and Service Solutions (SPSS) software, the importance of these parameters was identified as: air gap > bore fluid composition > flow rate of bore fluid > quench bath temperature. The optimal spinning condition with the bore fluid (water + NMP (85%)), air gap (25 mm), flow rate of bore fluid (40% of dope rate), and temperature of quench bath (50 °C) was identified to make high PVP content, symmetric cross-section and highly cross-linked CA hollow fibers. The results can be used to guide the spinning of defect-free CA hollow fiber membranes with desired structures and properties as carbon membrane precursors.

Investigation on Nanocomposite Membranes for High Pressure CO2/CH4Separation

The novel nanocomposite membranes were prepared for CO2/CH4 separation, and a good selectivity >30 at high pressure >30bar was obtained by testing a plate-and-frame module with a membrane area 110 cm2. The Joule- Thomson effect was found to have negligible influence on the temperature drop inside the membrane module due to the very high heat transfer coefficient for the membrane materials, which is different from the HYSYS simulation results. The water permeance was determined to be higher compared to CO2 permenace especially at high pressure, which indicated high water vapor content should be achieved in the feed gas to avoid the drying of the membrane and maintain high membrane separation performance in a real process. A two-stage membrane system was designed to purify CH4 from a 50% CO2/50% CH4 gas mixture, and the CH4 purity of 70% can be achieved in the 2nd stage. Process simulation using HYSYS integrated with ChemBrane indicated that a multi-stage membrane system is needed to achieve the industrial requirement on the production of sweet natural gas.

Optimization of Deacetylation Process for Regenerated Cellulose Hollow Fiber Membranes

Cellulose acetate (CA) hollow fibers were spun from a CA+ Polyvinylpyrrolidone (PVP)/N-methyl-2-pyrrolidone (NMP)/H2O dope solution and regenerated by deacetylation. The complete deacetylation time of 0.5 h was found at a high concentration (0.2 M) NaOH ethanol (96%) solution. The reaction rate of deacetylation with 0.5 M NaOH was faster in a 50% ethanol compared to a 96 vol.% ethanol. The hydrogen bond between CA and tertiary amide group of PVP was confirmed. The deacetylation parameters of NaOH concentration, reaction time, swelling time, and solution were investigated by orthogonal experimental design (OED) method. The degree of cross-linking, the residual acetyl content, and the PVP content in the deacetylated membranes were determined by FTIR analysis. The conjoint analysis in the Statistical Product and Service Solutions (SPSS) software was used to analyze the OED results, and the importance of the deacetylation parameters was sorted as Solution > Swelling time > Reaction time > Concentration. The optimal deacetylation condition of 96 vol.% ethanol solution, swelling time 24 h, the concentration of NaOH (0.075 M), and the reaction time (2 h) were identified. The regenerated cellulose hollow fibers under the optimal deacetylation condition can be further used as precursors for preparation of hollow fiber carbon membranes.

Chapter 15:Carbon Molecular Sieve Membranes for Gas Separation

Carbon molecular sieve (CMS) membranes have been studied for more than 20 years as promising and energy-efficient membranes for gas separation. Due to their high permeability and selectivity, as well as their high thermal and chemical stability under adverse and harsh conditions, CMS membranes are becoming increasingly important for separating gas mixtures with quite similar molecular kinetic diameter. Carbon molecular sieve membranes can be prepared by controlling the carbonization procedure during carbonization of a polymeric material. The effects of carbonization parameters (final temperature, heating rate, soak time and atmosphere during carbonization) on the resulting carbon membrane performance can be investigated, and the optimal condition can be found to prepare the high performance carbon membranes for a specific application. Several general techniques of scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction can be employed to characterize the membranes structure and morphology. Moreover, the pore size and pore size distribution can be estimated by gas gravimetric adsorption measurements, which will be helpful in understanding the transport mechanism through carbon membranes. The operating conditions will greatly affect the CMS membranes separation performances and need to be optimized for the application. Considering the aging problem for carbon membranes, different regeneration techniques can be used to recover the performances on-line or off-line. In order to ensure the efficiency of the carbon membrane separation process, the membrane module should be prepared as hollow fibers, and the process designed according to the given process conditions and economic considerations. Based on the experimental investigations and process simulations, the carbon membranes show high potential for selected industrial applications such as CO2–CH4 separation for biogas upgrading, H2–CH4 separation wherever relevant, CO2 capture from flue gases, air separation, petrochemical and high-temperature applications.

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.

Biomimetic material functionalized mixed matrix membranes for enhanced carbon dioxide capture

Carbonic anhydrase (CA) has been widely used in gas separation membranes because of its high affinity for CO2 molecules. In this work, a novel biomimetic material (Co-2,6-bis(2-benzimidazolyl)pyridine, CoBBP) which has a similar molecular structure to the CA enzyme but with higher stability and a lower price was successfully synthesized. The excellent thermal stability, dispersibility and high CO2 selectivity make CoBBP a promising alternative to CA. Then, a series of Pebax–CoBBP mixed matrix membranes were constructed to explore their capability for CO2/N2 separation. Compared to the pristine Pebax-1657, the Pebax–CoBBP mixed matrix membrane with the optimized 1.33 wt% CoBBP loading showed an improved CO2 permeability of 675.5 barrer and a CO2/N2 selectivity of 62, surpassing the Robeson upper bound (2008). Furthermore, the hydrogen bonds between CoBBP and polyamide chains improved the chain stiffness of the linear glassy polymer, ensuring good operational mechanical stability. In short, this work could provide a promising method to exploit alternatives to the CA enzyme and to fabricate biomimetic membranes.