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Development of low-cost ionic liquids based technology for CO2 separation
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-0453-0450
2019 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
CO2-separation med ny lågkostnads-teknik baserad på Joniska lösningar (Swedish)
Abstract [en]

CO2 separation plays an important role to mitigate the CO2 emissions due to burning of fossil fuels, and it is also of importance in biofuel production (e.g. biogas upgrading and bio-syngas purification and conditioning). The solvent-based absorption is the state-of-art technology for CO2 separation, where various solvents, e.g. amine solutions, Selexol (i.e. dimethyl ethers of polyethylene glycol), and propylene carbonate, have been introduced. However, these solvent-based technologies meet challenges such as high solvent degradation, high corrosion rate to equipment, high construction cost, high energy demand for solvent regeneration and high solvent make-up rate. Therefore, the development of novel solvents to overcome the challenges of the currently available solvents is essential.

Recently, ionic liquids (ILs) have gained great interest as new potential solvents for CO2 separation, mainly due to their very low vapor pressure and relatively high CO2 solubility. In addition, ILs have lower corrosive characteristic, lower degradation rate and lower energy requirement for solvent regeneration compared with the conventional organic solvents. However, the main challenge of the application of ILs is their higher viscosity than the conventional solvents, which can be solved by adding co-solvents such as water.

The overall objective of this thesis work was to develop low-cost IL based technologies for CO2 separation. To achieve this objective, the deep eutectic solvent (DES) of choline chloride (ChCl)/Urea with molar ratio 1:2 as a new type of IL was selected as an absorbent and H2O was used as co-solvent for CO2 separation from biogas. The conceptual process was developed and simulated based on Aspen Plus, and the effect of water content on the performance of ChCl/Urea for CO2 separation was evaluated. It was found that the optimal proportion of aqueous ChCl/Urea was around 50 wt% (percentage by weight) of water with the lowest energy usage and environmental effect.

The performance of aqueous ChCl/Urea was further compared with the commercial organic solvents in this thesis work. The rate-based process simulation was carried out to compare the energy usage and the cost for CO2 separation from biogas. It was found that aqueous ChCl/Urea achieved the lowest cost and energy usage compared with other commercial solvents except propylene carbonate. The performance comparison proved that CO2 solubility, selectivity and viscosity were three important parameters which can be used as criteria in the development of novel physical solvents for CO2 separation.

ILs with acetate anions normally show high CO2 solubility and selectivity, and the ILs with alkylmorpholinium as cations have low toxicity leading to lower environmental effect. Therefore, in this thesis work, a series of N-alkyl-N-methylmorpholinium-based ILs with acetate as counterpart anion were investigated, and water was added as co-solvent to adjust the viscosity. The CO2 solubility in these aqueous ILs was measured at different temperatures and pressures. It was found that the increase of alkyl chain length in the cation led to an increase of CO2 solubility of the ILs with the same anion. Aqueous N-butyl-N-methylmorpholinium acetate ([Bmmorp][OAc]) had the highest CO2 solubility, and it was selected to further carry out thermodynamic modeling and process simulation. The energy usage and the size of equipment of using aqueous [Bmmorp][OAc], aqueous ChCl/Urea, water, Selexol, and propylene carbonate for CO2 separation from biogas were compared. It was found that this novel IL mixing with water had better performance, that is, with lower energy usage and smaller size of equipment than the other solvents. This result suggests that using this aqueous [Bmmorp][OAc] has the potential to decrease the cost of CO2 separation.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Chemical Engineering Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-72567ISBN: 978-91-7790-300-0 (print)ISBN: 978-91-7790-301-7 (electronic)OAI: oai:DiVA.org:ltu-72567DiVA, id: diva2:1279077
Presentation
2019-04-05, E632, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2019-01-16 Created: 2019-01-15 Last updated: 2019-10-01Bibliographically approved
List of papers
1. Techno-economic analysis and performance comparison of aqueous deep eutectic solvent and other physical absorbents for biogas upgrading
Open this publication in new window or tab >>Techno-economic analysis and performance comparison of aqueous deep eutectic solvent and other physical absorbents for biogas upgrading
2018 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 225, p. 437-447Article in journal (Refereed) Published
Abstract [en]

Biogas has been considered as an alternative renewable energy, and CO2 removal from raw biogas (i.e. biogas upgrading) is needed for producing biomethane to be used as vehicle fuels or injected into the natural gas grid. Biogas upgrading with physical absorbents, such as water and other commercial organic solvents, is simple, efficient and with low energy requirements for regeneration. Recently, deep eutectic solvents (DESs) with nonvolatility, nonflammability and low price have been reported as promising alternatives to replace conventional physical absorbents in many research areas including biogas upgrading. However, the performances of these physical solvents including conventional physical absorbents and DES-based solvents have not been evaluated and compared with each other. In this work, the properties of 4 physical solvents (i.e. water, dimethyl ether of polyethylene glycol (DEPG), propylene carbonate (PC), and aqueous DES (AQDES)) were compared. Furthermore, a conceptual process was developed to upgrade biogas with these solvents and simulated based on Aspen Plus in order to conduct performance comparison. The simulation results of energy utilization, the amount of recirculated solvents and the diameters of absorber and desorber were analyzed and compared based on equilibrium and rate-based approaches, respectively. The simulation results based on the rate-based approach were further used to estimate the costs of biogas upgrading process with a same raw biogas capacity for comparison. Meanwhile, the specific cost of biogas upgrading process with a same size of equipment was also evaluated. The results show that the CO2 solubility, selectivity and viscosity are three more important properties, providing valuable information for developing novel physical solvents for CO2 separation. The simulation results show that the equilibrium and rate-based approaches result in different conclusions, especially when the solvent viscosity is relatively high, and the rate-based approach is preferable. Based on the simulation results from the rate-based approach, the performances of AQDES and PC are similar with a same amount of energy utilization, that is around 11% lower than water, and DEPG is inferior to water. For the case with the same gas capacity, the total annual costs of biogas upgrading process with these solvents show the following order: DEPG > AQ60wt.%DES > water > AQ50wt.%DES ≈ PC. For the case with the same size of equipment, compared to water, the total specific costs of biogas upgrading process with PC and AQ50wt.%DES decrease by about 30% and 45%, respectively, and the treated biogas capacities increase to 1.5 and 2 times, respectively. In general, both PC and AQ50wt.%DES show better performance than the other solvents. Considering that DES is an environmentally benign solvent, and the performance of DES can be greatly improved by further designing, it is more promising

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-68870 (URN)10.1016/j.apenergy.2018.04.112 (DOI)000438181000032 ()2-s2.0-85046867612 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-05-23 (andbra)

Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2019-01-15Bibliographically approved
2. Modeling, simulation and evaluation of biogas upgrading using aqueous choline chloride/urea
Open this publication in new window or tab >>Modeling, simulation and evaluation of biogas upgrading using aqueous choline chloride/urea
Show others...
2017 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 229, no 1, p. 1269-1283Article in journal (Refereed) Published
Abstract [en]

Biogas has been considered as an alternative renewable energy, and raw biogas needs to be upgraded in order to be used as vehicle fuels or injected into the natural gas grid. In this work, the conceptual process for biogas upgrading using aqueous choline chloride (ChCl)/urea (1:2 on a molar basis) was developed, simulated and evaluated based on the commercialized software Aspen Plus. Reliable thermophysical properties and phase equilibria are prerequisite for carrying out process simulation. In order to carry out the process simulation, the thermophysical properties of ChCl/Urea (1:2) and its aqueous solutions as well as the phase equilibria of gas-ChCl/Urea (1:2), ChCl/Urea (1:2)-H2O and gas-ChCl/Urea (1:2)-H2O were surveyed and evaluated. After evaluation, the consistent experimental data of these thermophysical properties were fitted to the models embedded in Aspen Plus. The properties needed but without available experimental results were predicted theoretically. The Non-Random Two-Liquid model and the Redlich-Kwong equation (NRTL-RK) model were used to describe the phase equilibria. The equilibrium approach was used for process simulation. Sensitivity analysis was conducted to determine the reasonable operating parameters. With a set of reasonable operating conditions, the effects of ChCl/Urea (1:2) content on the total energy utilization, the diameters and pressure drops of absorber and desorber as well as the environmental assessment of the process were studied. The simulation results showed that, with the addition of ChCl/Urea (1:2), the total energy utilization decreased by 16% compared to the process with pure water, and the diameters of both absorber and desorber decreased with increasing content of ChCl/Urea (1:2). The process using aqueous ChCl/Urea (1:2) was more environmentally benign than that with pure water. Therefore, aqueous ChCl/Urea (1:2) is a promising solvent for biogas upgrading.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Aqueous choline chloride/urea, Thermodynamic modeling, Process simulation, Biogas upgrading
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-63249 (URN)10.1016/j.apenergy.2017.03.059 (DOI)000449891500098 ()2-s2.0-85018938965 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-12-03 (inah)

Available from: 2017-05-05 Created: 2017-05-05 Last updated: 2019-09-13Bibliographically approved

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