Comparison of Liquid Phase Ion Speciation in DEAB-CO2-H2O System with IPAB-CO2-H2O System Using 13C NMR Techniques

AbstractThe 13C NMR techniques were selected to investigate the ion speciation in both DEAB and IPAB solvents. Calibration equations were established to quantify the exact ion concentration of free amines and protonated amines at different absorption times. The results show that the NMR method can be used to obtain both qualitative and quantitative information of the amine-CO2-H2O system. It is also the first time to monitor the components in the new solvent (IPAB) and establish its VLE model

Energy analysis and optimization of hollow fiber membrane contactors for recovery of dissolve methane from anaerobic membrane bioreactor effluent

This work presents an energy analysis and optimization of the hollow fiber membrane contactors for the recovery of dissolved methane (CH4) in effluents of anaerobic membrane bioreactor wastewater treatment processes. The obtained CH4 could be merged with biogas for further purification or used with a micro-turbine for electricity generation to achieve an energy self-sufficient wastewater treatment process. A mathematical model considering simultaneous CH4 and carbon dioxide (CO2) desorption was used to estimate the membrane area required to remove the dissolved CH4, as well as quality of the outlet gas from the membrane contactor. Energy balance between electrical energy obtained from the recovered CH4 and energies consumed by vacuum and liquid pumps for the operation of membrane contactor were investigated and reported as a Net Electricity obtained per m3 of effluent or simply Net E. Results revealed that a combination of a high strip gas flow rate and slightly low vacuum condition closed to the atmospheric pressure can provide the highest Net E at 0.178 MJ/m3. This value is 85.37% of the total electrical energy that can generated from a 90% recovery of dissolved CH4 using an effluent saturated with a 60 vol% CH4 biogas and flow rate at 2 m3/day. The calculation was made based on the assumptions that 1) the membrane contactor is operated in a non-wetting mode where membrane properties remain constant, 2) flux decline due to the membrane fouling is not considered and 3) the energy required for membrane cleaning and other relevant activities are not factored into the energy analysis. Based on our results, to obtain a high CH4 mole fraction at the gas outlet, a low strip gas flow rate is recommended, however, the operating gas pressure needs to be lowered by applying a vacuum condition to improve the Net E. In addition, it was found that the Net E could be improved by increasing the number of membrane fibers, and lowering the liquid flow rate. The CH4 recovery efficiency could also be optimized to obtain an optimal Net E.NRF (Natl Research Foundation, S’pore)Accepted versio

Reaction kinetics of some important alkanolamines with carbon dioxide in aqueous solutions using stopped flow technique

The kinetics of the reactions between aqueous solutions of carbon dioxide and Aminomethylpropanol (AMP), Aminobutanol (AB), Aminopropanol (AP), Diethanolamine (DEA) and Ethanolamine (MEA) were investigated over a temperature range of 293-313 K and different amine concentrations using a stopped flow technique. It was found that at each temperature, MEA reacts faster than AB, which in turn, reacts faster than AMP and DEA respectively. Reactions of MEA, DEA and AB can be explained by the zwitterion mechanism. However, the reaction mechanism of the AMP, a hindered amine, is suggested to be similar to that for tertiary amines, but at a faster rate. The rate constants required to predict reaction rates of these amines with CO2 have been calculated and presented in this work.Scopu

Transport properties of CO2 and CH4 in hollow fiber membrane contactor for the recovery of biogas from anaerobic membrane bioreactor effluent

A significant amount of methane (CH4) produced from anaerobic digestions of wastewater is dissolved in liquid effluent and discharged. The recovery of dissolved CH4 is therefore essential in ensuring an enhanced energy production of the anaerobic processes, and minimizing environmental impacts of the greenhouse gas. In this work, a membrane contactor is employed as a mass transfer equipment for the CH4 recovery. A mathematical model considering simultaneous desorption of CH4 and carbon dioxide (CO2) is developed using a resistance-in-series model to calculate the overall mass transfer coefficients. The simulations were validated with experimental results obtained using an in-house fabricated hollow fiber membrane as well as a real effluent from Anaerobic Membrane Bioreactor (AnMBR) and synthetic effluent made of water saturated with biogas. Results showed that the CO2 fluxes were higher than those of CH4 fluxes due to its higher concentration in liquid phase. A decrease of liquid phase mass transfer resistance by an increase in liquid velocity significantly enhanced both CH4 and CO2 fluxes. While, an increase in gas velocity slightly affected the CH4 flux but enhanced the CO2 flux considerably. It was also found that the CO2 desorption increased the CH4 recovery rate. The desorbed CO2 helped to increase the mass transfer driving force by reducing the partial pressure of CH4 in the gas side, and enhancing the gas phase mass transfer coefficient to facilitate CH4 desorption. The increase of liquid velocity increased mole fraction of CH4 in the gas outlet but decreased the rate of CH4 recovery. On the other hand, applying vacuum conditions to decrease gas pressure enhanced the rate of CH4 recovery but lower the CH4 mole fraction in the product gas.NRF (Natl Research Foundation, S’pore)EDB (Economic Devt. Board, S’pore)Accepted versio

Kinetics of CO2 reaction with solutions of n-methyl-diethanolamine mixed with selected amino acids using stopped flow technique

Kinetics of CO2 reactions with blended solutions of N-methyldiethanolamine (MDEA) and salts of Glycine and Taurine are presented in this work. The obtained results show that the new blended solutions could be promising absorbents for CO2 capture from natural and industrial flue gases. The reaction kinetics were investigated using stopped flow technique at temperatures ranging from 293 to 313 K. The kinetics data obtained in the fast reaction regime and amine concentration in the range of 0.2 to 0.8M were analyzed using zwitterion mechanism, and the kinetic parameters associated to the reactions were determined. Individual rate constants were obtained by nonlinear regression at each temperature and the activation energy of each reaction was estimated from Arrhenius plot.Scopu

Fouling formation in membrane contactors for methane recovery from anaerobic effluents

Fouling in membrane contactors for recovery of dissolved methane (CH4) was investigated in this work. Two types of effluents from anaerobic membrane bioreactor (AnMBR) and upflow anaerobic sludge blanket (UASB) were tested under a continuous operational mode. Due to the higher fouling propensity of the UASB effluent, membrane fouling was more drastic, leading to a greater decline in the CH4 desorption flux with respect to the operational time. Also, the flux was observed to be influenced by the gas-liquid contact time and declined more severely with increasing liquid velocity. Membrane characterization revealed cake layer formation as the source of membrane fouling while foulants characterization indicated that the majority of the foulants were protein-like-substances with fluorescence spectra showing signals close to that of extracellular polymeric substances. On this basis, a mass transfer analysis was performed to understand the fouling resistance exerted by the cake layer and identify a parameter which best described the fouling mechanism. It was found that cake thickness can be used to express the change in fouling resistance in the case of the AnMBR effluent, while cake porosity was a better parameter in the case of the UASB effluent.National Research Foundation (NRF)Public Utilities Board (PUB)This research grant is supported by the Singapore National Research Foundation under its Environmental & Water Research Programme (Project Ref No: 1301-IRIS-49) and administered by Public Utilities Board, Singapore's national water agency

Polymer-fluorinated silica composite hollow fiber membranes for the recovery of biogas dissolved in anaerobic effluent

In this study, polymer-fluorinated silica composite hollow fiber membranes were fabricated and applied to a membrane contactor system for the recovery of methane dissolved in the anaerobic effluent. Such composite membranes allowed us to tailor the physical property such as porosity and mechanical strength and the surface hydrophobicity in separated processes. To develop the composite membranes, porous hollow fiber substrates were first fabricated with Matrimid®, a commercial polyimide. Subsequently, fluorinated silica particles were synthesized and anchored on the substrates via a strong covalent bonding. Due to the high porosity as well as the high hydrophobicity, our membrane showed an outstanding performance for the recovery of CH4 in the membrane contactor, such that the CH4 flux reached 2900 mg CH4/m2–h at the liquid velocity of 0.42 m/s at which the liquid phase still controlled the overall mass transfer. The composite membrane prepared in this work also showed a much better performance in the CH4 recovery than a commercial polypropylene membrane made for degasification of water. In addition, a long-term test with tap water saturated with the model biogas made up of 60:40 CH4/CO2 mixture demonstrated that our membrane can be stably operated for more than 300 h without experiencing pore wetting problem.NRF (Natl Research Foundation, S’pore)EDB (Economic Devt. Board, S’pore)Accepted versio

Membrane-based technologies for post-treatment of anaerobic effluents

Anaerobic digestion-based processes for converting wastewater into clean water and energy are attracting ever-growing industrial interest. However, apart from the microbial digestion step, current technologies require further progress from an integrated process point of view, including post-treatment steps. Anaerobic effluents normally undergo extensive post-treatment steps to meet stringent discharge standards, while valuable nutrients are rarely recovered. Additionally, a significant portion of the produced methane remains inevitably dissolved in the effluent, which is eventually released into the environment, causing economic loss and global warming concerns. To address these issues, several membrane-based technologies show significant promise. Here, we review current progress in membrane-based recovery of dissolved methane and nutrients, highlighting opportunities where membrane-based technologies can improve the post-treatment of anaerobic effluents. Lastly, we also share our perspectives for promising research directions and how to secure the competitiveness of membrane-based technologies for anaerobic wastewater treatment processes, focusing on current challenges for membrane development, biofouling mitigation strategies, and small-scale to large-scale implementation.NRF (Natl Research Foundation, S’pore)Published versio

Optimization of hydrophobic modification parameters of microporous polyvinylidene fluoride hollow-fiber membrane for biogas recovery from anaerobic membrane bioreactor effluent

Tailoring the hydrophobic properties of membranes is an important requirement in gas-liquid membrane contactor processes because the membrane can be wetted by liquid. This paper describes the hydrophobic modification of the chemical surface of a polyvinylidene fluoride hollow-fiber membrane using a commercial perfluoropolyether, Fluorolink S10. The contact angle of the modified membrane was influenced by the modification parameters, and hence parameter optimization was required to obtain a highly hydrophobic membrane. The sodium hydroxide concentration (pH 8, 10 and 12), dehydrofluorination time (30, 60 and 90 min), and chemical solution grafting time (30, 60 and 90 min) were identified as the control parameters of this modification and were optimized using the Taguchi approach. The optimized membrane was then applied to recover methane from anaerobic membrane bioreactor effluent and benchmarked with a commercial polypropylene membrane manufactured for degasification of water. The contact angle of the polyvinylidene fluoride membrane was improved by 32% after the surface modification (under the optimal modification condition indicated by the Taguchi method). The membrane mass transfer coefficient was measured as 1.53 × 10−4 m/s. Our membrane showed better methane recovery performance than the commercial polypropylene membrane and a very stable flux without pore wetting during 10 days of operation with synthetic effluent (made by saturating a 60:40 methane/carbon dioxide mixture into tap water). In addition, the membrane prepared in this study could be operated without significant fouling (7% flux drop during 8 days of operation) with real anaerobic membrane bioreactor effluent.NRF (Natl Research Foundation, S’pore

Medium optimization for biobutanol production from palm kernel Cake (PKC) hydrolysate by clostridium saccharoperbutylacetonicum N1-4

The study aims to optimize the medium composition for biobutanol production using a Palm Kernel Cake (PKC) hydrolysate by Clostridium saccharoperbutylacetonicum N1-4. Various nutrient factors affecting biobutanol production were screened using the Plackett-Burman design. These factors included: NH4 NO3 , KH2 PO4 , K2 HPO4 , MgSO4 .7H2 O, MnSO4 .7H2 O, FeSO4 .7H2 O, yeast extract, cysteine, PABA, biotin, and thiamin. The results were analyzed by an analysis of variance (ANOVA), which showed that cysteine (P=0.008), NH4 NO3 (P=0.011) dan yeast extract (P=0.036) had significant effects on biobutanol production. The established model from the ANOVA analysis had a significant value of Pmodel>F = 0.0299 with an F-value of 32.82 which explains that the factors can explain in detail the variation in the data about the average and the interpretation is true with an R2 value of 0.993. The estimated maximum biobutanol production was 10.56 g/L, whereas the optimized medium produced 15.49 g/L of biobutanol. Process optimizations with optimum concentration of cysteine, NH4 NO3, and yeast extract have produced 21.33 g/L biobutanol which is a 37.7% improvement from the non-optimized medium. The findings show that PKC hydrolysate with the addition of optimal concentrations of the three types of medium namely, cysteine (0.15 g/L), NH4 NO3 (0.50 g/L), and yeast extract (1.5 g/L) during ABE fermentation, yielded a maximum biobutanol concentration of 21.33 g/L. Therefore, the results of this study provide good indications for promoting PKC hydrolysate as a new source of novel substrates with great potential in producing high biobutanol through ABE fermentation by C. saccharoperbutylacetonicum N1-4