Browsing by Author "Thomas, Navya"

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    CO2 absorption into aqueous ammonia using membrane contactors: Role of solvent chemistry and pore size on solids formation for low energy solvent regeneration
    (Elsevier, 2022-03-16) Bavarella, Salvatore; Luqmani, Benjamin A. ; Thomas, Navya; Brookes, Adam; Moore, Andrew; Vale, Peter C. J.; Pidou, Marc; McAdam, Ewan J.
    Solids formation can substanitally reduce the energy penalty for ammonia solvent regeneration in carbon capture and storage (CCS), but has been demonstrated in the literature to be difficult to control. This study examines the use of hollow fibre membrane contactors, as this indirect contact mediated between liquid and gas phases in this geometry could improve the regulation of solids formation. Under conditions comparable to existing literature, NH4HCO3 was evidenced to primarily crystallise in the gas-phase (lumen-side of the membrane) due to the high vapour pressure of ammonia, which promotes gaseous transmission from the solvent. Investigation of solvent reactivity demonstrated how equilibria dependent reactions controlled the onset of NH4HCO3 nucleation in the solvent, and limited ‘slip’ through transfomation of ammonia into its protonated form which occurs prior to the phase change. Crystallisation in the solvent was also dependent upon ammonia concentration, where sufficient supersaturation must develop to overcome the activation energy for nucleation. However, this has to be complemented with a reduction in solvent temperature to offset vapour pressure and limit the risk of gas-phase crystallisation. While changes to the solvent chemistry were sufficient to shift from gas-phase to liquid phase crystallisation, wetting was observed immediately after nucleation in the solvent. This was explained by a local region of supersaturation within the coarse membrane pores that promoted a high nucleation rate, altering the material contact angle of the membrane sufficient for solvent to breakthrough into the gas phase. Adoption of a narrower pore size membrane was shown to dissipate wetting after crystallisation in the solvent, illustrating membrane contactors as a stable platform for the sustained separation of CO2 coupled with its simultaneous transformation into a solid. Through resolving previous challenges experienced with solids formation in multiple reactor configurations, the cost benefit of using ammonia as a solvent can be realised, which is critical to enabling economically viable CCS for the transition to net zero, and can be exploited within hollow fibre membrane contactors, eliciting considerable process intensification over existing reactor designs for CCS.
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    Data supporting the publication "Transforming wastewater ammonia to carbon free energy: Integrating fuel cell technology with ammonia stripping for direct power production"
    (Cranfield University, 2022-03-08 09:18) Davey, Christopher J.; Luqmani, Benjamin A. ; Thomas, Navya; McAdam, Ewan
    Data File supporting article titled "Transforming ammonia to carbon free energy: Integrating fuel cell technology with ammonia stripping for direct power production"
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    Data supporting: 'CO2 absorption into aqueous ammonia using membrane contactors: Role of solvent chemistry and pore size on solids formation for low energy solvent regeneration'
    (Cranfield University, 2022-10-13 16:42) Bavarella, Salvatore; Luqmani, Benjamin A.; Thomas, Navya; Brookes, Adam; Moore, Andrew; Vale, Peter C. J.; Pidou, Marc; McAdam, Ewan
    Solids formation can substantially reduce the energy penalty for ammonia solvent regeneration in carbon capture and storage (CCS), but has been demonstrated in the literature to be difficult to control. This study examines the use of hollow fibre membrane contactors, as this indirect contact mediated between liquid and gas phases in this geometry could improve the regulation of solids formation. Adoption of a narrower pore size membrane was shown to dissipate wetting after crystallisation in the solvent, illustrating membrane contactors as a stable platform for the sustained separation of CO2 coupled with its simultaneous transformation into a solid. Through resolving previous challenges experienced with solids formation in multiple reactor configurations, the cost benefit of using ammonia as a solvent can be realised, which is critical to enabling economically viable CCS for the transition to net zero, and can be exploited within hollow fibre membrane contactors, eliciting considerable process intensification over existing reactor designs for CCS.
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    Data supporting: 'Downscaling reverse osmosis for single-household wastewater reuse: towards low-cost decentralised sanitation through a batch open-loop configuration'
    (Cranfield University, 2022-10-31 09:16) Thomas, Navya; McAdam, Ewan; Davey, Christopher J.
    There is a significant demand for water recycling in low-income countries. However, wastewater infrastructure is primarily decentralised, necessitating the development of affordable household-scale reclamation technology. In this study, a batch open-loop reverse osmosis (RO) system is therefore investigated as a low-cost clean water reclamation route from highly saline concentrated blackwater
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    Downscaling reverse osmosis for single-household wastewater reuse: towards low-cost decentralised sanitation through a batch open-loop configuration
    (IWA Publishing, 2022-04-15) Davey, Christopher J.; Thomas, Navya; McAdam, Ewan
    There is a significant demand for water recycling in low-income countries. However, wastewater infrastructure is primarily decentralised, necessitating the development of affordable household-scale reclamation technology. In this study, a batch open-loop reverse osmosis (RO) system is therefore investigated as a low-cost clean water reclamation route from highly saline concentrated blackwater. In a single-stage configuration, increasing feed pressure from 10 to 30 bars improved selective separation at water recovery exceeding 85%, whereas lower cross-flow velocity improved product recovery, reducing specific permeate energy demand from 21 to 4.8 kWh m−3. Rejection achieved for total phosphorous (99%), chemical oxygen demand (COD, 96%), and final pH (8.7) of the RO permeate was compliant with the ISO30500 reuse standard for discharge. However, the rejection of total nitrogen in the RO permeate was non-compliant with the reuse standard due to the transmission of low-molecular weight (MW) uncharged organic compounds. It is suggested that rejection may be improved by increasing feed pressure to rebalance selectivity but may also be controlled by reducing fluid residence time (storage) to constrain the hydrolysis of urea. The economic analysis identified that a high-pressure 1812 element cost of ∼US$30 meets the sanitation affordability index of US$0.05 capita−1 day−1. However, the unit cost of a high-pressure feed pump must be reduced to ∼US$500 to obtain an affordable system cost. These unit costs can be achieved by manufacturing 1812 elements at economies of scale, and by adopting pumping solutions that have been developed for other applications requiring high pressures and low flows. Overall, our findings suggest that RO in the batch open-loop configuration has the potential to deliver affordable and safe water production from blackwater in a decentralised (single-household) context.
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    On the role of crystal-liquid interfacial energy in determining scaling, nucleation and crystal growth in membrane distillation crystallisation
    (Elsevier, 2025-05-01) Vasilakos, Konstantinos; Thomas, Navya; Hermassi, Mehrez; Campo Moreno, Pablo; McAdam, Ewan J.
    While the interfacial energy (σ) of a solute contributes toward the excess surface free energy requirement for nucleation, its role in determining scaling, nucleation and crystal growth processes within membrane distillation has yet to be described. Highly soluble salts (low σ) are generally understood to possess a low nucleation energy, where the limited relative supersaturation (Δc/c∗) can favour a heterogeneous primary nucleation mechanism. This was indicated by scaling, which is generally presumed to occur in response to the membrane substrate lowering the critical Gibbs free energy requirement for nucleation (ΔG∗). For less soluble salts (high σ), primary nucleation was not observed until Δc/c∗ exceeded a threshold of 1. It was postulated that the excess chemical potential available was sufficient to favour homogeneous primary nucleation in the bulk solution, which mitigates scale formation on the membrane. In-situ characterisation methods also established how nucleation rate and crystal size could be directly attributed to the σ, which is compatible with the crystallisation literature on aqueous salts within a comparable range of solubilities. While crystallisation tends to be controlled by a combination of thermodynamic and kinetic processes, this study illustrates how interfacial energy (a thermodynamic quantity) can be used to anticipate nucleation and crystal growth mechanisms in membrane crystallisation.
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    Polydopamine-coated graphene oxide nanosheets embedded in sulfonated poly (ether sulfone) hybrid UF membranes with superior antifouling properties for water treatment
    (Elsevier, 2021-11-11) Kumar, Mahendra; Sreedhar, Nurshaun; Thomas, Navya; Mavukkandy, Musthafa; Ismail, Roqaya A.; Aminabhavi, Tejraj M.; Arafat, Hassan A.
    A novel high-performance hybrid ultrafiltration (UF) membrane was fabricated by blending polydopamine-coated graphene oxide (PDGO) nanosheets with sulfonated poly(ether sulfone) (SPES) via phase inversion method and tested for the removal of natural organic matter (humic acid; HA) from aqueous solution. The PDGO nanosheets were synthesized via self-polymerization of dopamine with GO nanosheets in alkaline tris-buffer solution at room temperature for 24 h and were fully characterized. Hybrid SPES membranes were prepared by incorporating 1–10 wt% of PDGO, which were further characterized by Raman spectroscopy, surface zeta potential, and field emission scanning electron microscopy to confirm membrane stability without any defects even by adding up to 10 wt%, of PDGO nanosheets. The membranes demonstrated a significant increase in hydrophilicity, water flux, and retention rate for HA (RHA). For instance, water permeability with 5 wt% PDGO (M5) (680.7 L m−2 h−1 bar−1) was ca. 1.8-folds that of the pristine SPES membrane (380.8 L m−2 h−1 bar−1), while maintaining an HA rejection (RHA) of 91.7% for a 50 ppm HA feed solution. This was accompanied by a distinct increase in surface hydrophilicity of M5, which showed a water contact angle of 27.8°, well below that of pristine SPES membrane (59.1°). The hybrid UF membranes also demonstrated a significant reduction in HA adhesion onto the membrane surface along with a superior antifouling performance for the membrane containing 10 wt% PDGO, giving irreversible fouling ratio (Rir) of only 6.9% compared to 32.7% for the pristine membrane.
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    Transforming wastewater ammonia to carbon free energy: Integrating fuel cell technology with ammonia stripping for direct power production
    (Elsevier, 2022-03-07) Davey, Christopher J.; Luqmani, Benjamin A. ; Thomas, Navya; McAdam, Ewan J.
    The transformation of ammonia from pollutant to energy rich carbon free fuel presents an opportunity for the transition of wastewater services to net zero. However, there is only limited knowledge of how the product quality of ammonia recovered from real wastewater might impact on its downstream exploitation in fuel cells. This study therefore exploited vacuum stripping to produce an aqueous ammonia concentrate from real wastewater that was then evaluated within a direct ammonia fuel cell, as a reference technology for energy generation. A 17 g L−1 aqueous ammonia product was created by vacuum stripping centrate from a full-scale anaerobic digester (2 gN L−1). The pH of the product was lower than expected due to the mild-acidification of solution by the co-transport of low MW volatile organic compounds. This reduced power density in the fuel cell, due to the incomplete deprotonation of ammonia (lowering oxidation potential at the fuel cell anode) and a decrease in [OH–] which is required for complete electrochemical conversion. We propose that improved vacuum stripping design can increase the distillate ammonia concentration and produce a more alkaline product, yielding markedly higher fuel cell power density by enhancing ammonia oxidation at the anode (through concentration and deprotonation) and reducing [OH–] mass transfer limitations. As the separation energy for ammonia is dominated by the latent heat demand of water vapour, a synergy exists between creation of a concentrated ammonia product (that improves power density) and reducing the energy demand for separation. The energy balance from this research evidences that despite the high latent heat demand for separation, the low cost of heat coupled with the power produced from ammonia yield a favourable economic return when compared to conventional biological treatment. This study also identifies that revaluing ammonia as a carbon free fuel can help reposition wastewater treatment for a zero-carbon future.
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    Utilizing Buckingham Pi theorem and multiple regression analysis in scaling up direct contact membrane distillation processes
    (Elsevier, 2022-02-05) Khafajah, Heba; Ali, Mohamed I. Hassan; Thomas, Navya; Janajreh, Isam; Arafat, Hassan A.
    Predicting the performance of a full-scale direct contact membrane distillation (DCMD) module based on experimental lab-scale results is rather difficult, since the DCMD performance is dependent on many different process parameters. Hence, there is a need for a methodology to perform DCMD system up-scaling based on lab-scale experimental results. In this study, we devise an approach to scale up the performance of DCMD systems by using the Buckingham's Pi theorem to group the DCMD process parameters into eight relevant dimensionless groups. Experimental data obtained from literature at various module dimensions were used to evaluate the developed dimensionless groups. An experimentally validated computational fluid dynamics (CFD) model was also developed and used to extend the coverage of operational parameters beyond the available experimental data. Then, two empirical dimensionless correlations were created, using multiple nonlinear regression analysis, and then validated, to enable the prediction of flux and pressure drop in DCMD systems at any scale.