Siloxane Removal Research Articles

Superior single- and multi-component siloxane removal from water using a faulted silica DON zeolite adsorbent

Pure porous silica zeolite adsorbents with framework surface faults, in the form of siloxy groups, are selective toward single- or multi-component linear siloxanes for removal from water.

Electrospun Al-MOF fibers as D4 Siloxane adsorbent: Synthesis, environmental impacts, and adsorption behavior

Here, we unveil the great potential of metal-organic framework (MOF) composite fibers produced by the electrospinning method to remove D4 siloxane from gaseous solutions. The fibers are based on polyacrylonitrile (PAN) and a microporous aluminum-based MOF known as DUT-4. The electrospinning configuration and DUT-4:PAN ratio in the precursor solutions play an important role in the textural properties of the fibers. Characterizations of morphology and textural properties demonstrated that the best dispersion on the fiber surface was achieved using coaxial electrospinning with a relationship of 1:1.4 of DUT-4:PAN. The Al-MOF fiber composite was evaluated as a D4 siloxane adsorbent, reaching over 85% of the D4 siloxane uptake capacity of individual DUT-4 crystals but with faster adsorption kinetics (effective diffusion coefficient 3.31 times higher). In addition, the synthesis of the composite showed a lower environmental impact and better thermal stability than that observed for DUT-4. This work shows the novel DUT-4 fibers have an outstanding potential for siloxane removal from biogas streams.

Techno-economic and sustainability analysis of siloxane removal from landfill gas used for electricity generation

A technoeconomic analysis (TEA) and life cycle assessment (LCA) was conducted on the use of landfill gas (LFG) for electricity generation using an internal combustion engine. This study provides insights that can guide LFG waste to energy (WTE) operators on decisions concerning installation of contaminant removal from LFG for electricity generation. Four scenarios were analyzed; the first (Scenario 1) was a facility with a single siloxane removal unit (SREU) sized for 6 months of continuous use, the second (Scenario 2) was a facility with parallel SREUs sized for one month of use, the third (Scenario 3) was a facility with no SREU, and the fourth was a facility that flared all LFG captured. The TEA revealed that the chiller cost was over 50% the total purchase cost of the LFG pre-treatment system. When the complete LFG to electricity process was analyzed, the internal combustion engine had the highest percentage of total capital investment and the total annual cost. For the base case, it became economically beneficial to install a SREU at facilities with LFG flowrates greater than ∼2000 m3/h. Sensitivity analysis showed that at a base case of 1700 m3/h, LFG (50% CH4), and 50 mg/m3 D4, the net income of facilities in Scenarios 1 to 3 became positive at an electricity sales price greater than 5.5 cents/kWh. LCA revealed that Scenario 2 had the greatest CO2 emission reduction. Scenario 3 is observed to save less CO2 emissions as biogas flowrate increases due to frequent engine shutdowns. Although there are differences in the global warming potential (GWP 100) for Scenarios 1 to 3, with Scenario 2 being the best and Scenario 3 being the worst, the differences are very small. For this reason, economics alone are sufficient in decision making.

Environmental assessment of metal-organic framework DUT-4 synthesis and its application for siloxane removal

In this study, the effect of different solvents in metal-organic framework (MOF) synthesis was evaluated in a technical study and an environmental study. The technical study considered the octamethylcyclotetrasiloxane (D4) adsorption on MOF DUT-4 for biogas purification, while the environmental study was implemented using the Life Cycle Assessment (LCA) methodology. In addition, the obtained DUT-4 adsorbents were characterized by X-ray diffraction, N2 adsorption-desorption isotherms, solid-state 27Al magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and Fourier transform infrared spectroscopy techniques. The results indicated that technically similar materials are obtained through solvothermal synthesis with similar D4 siloxane adsorption capacities (9.8 mg/g). Conversely, hydrothermal synthesis led to a low D4 siloxane adsorption capacity (1.8 mg/g). The environmental study demonstrated that the solvothermal synthesis had the highest contribution to the environmental damage due to the use of toxic solvents in the synthesis, cleaning, and solvent exchange stages (environmental score factor up to 244 mPt). In addition, a techno-environmental impact factor was introduced to identify the most suitable synthesis route of DUT-4. The overall results pointed out that solvothermal synthesis is the best scenario for DUT-4 production due to its low environmental impact/adsorption capacity ratio (11.44). This study is a step toward a more holistic approach integrating technical and environmental criteria in MOF synthesis for adsorption applications.

Adsorptive purification of volatile methyl siloxanes in a digester biogas stream

AbstractBACKGROUNDSiloxanes, one of the various impurity contaminants contained in biogas, comprise a group of volatile organic silicon compounds that pose a major obstacle to biogas application as a consequence of their potential to severely damage energy conversion equipment. Therefore, volatile methyl siloxanes (VMSs) need to be effectively removed from biogas before application.RESULTSThe concentration profile of octamethlytrisiloxane (L3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in a biogas were determined. Then in order to investigate the adsorption characteristics of these three siloxane compounds, two commercial granular activated carbons (ACs) were tested for the removal of L3, D4 and D5 in a dynamic adsorption column using biogas streams. The observed concentrations of L3, D4 and D5 in the wastewater treatment plant biogas were 0.62 ± 0.20, 3.07 ± 0.66 and 4.67 ± 1.02 mg m−3, respectively. The best adsorption capacities of L3, D4 and D5 were calculated as 4, 22 and 107 mg g−1, respectively, in real biogas.CONCLUSIONThe adsorption capacity of AC2 was found to be higher than AC1 adsorbents for each siloxane compounds studied. Although the elemental and physical structure of the ACs were similar, and AC1's surface area was higher than AC2, AC2 was more effective in siloxane removal due to high mesoporosity causing excess Van der Waals interaction. The relatively higher mesoporous volume and lower rate of mesh size of AC2 resulted in higher adsorption of siloxane compounds. The order of the breaking points was determined as L3 < D4 < D5 in both AC types. © 2021 Society of Chemical Industry (SCI).

Hexamethyldisiloxane removal from biogas using reduced graphene-oxide aerogels as adsorbents

Volatile methyl siloxane is a trace impurity in biogas produced by anaerobic bacteria. It decreases the energy utilization of biogas and thus needs to be removed. In this study, six reduced graphene-oxide aerogels (rGOAs) were prepared from industrial-grade multilayer graphene oxide by a hydrothermo-chemical reduction method in the presence of various amine additives, and the use of the rGOAs as adsorbents for hexamethyldisiloxane (L2) removal was investigated. The developed rGOAs were characterized via the Brunner-Emmet-Teller technique, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results revealed that the different pKa values of the amine molecules led to an increase in the rGOA contact angle, the specific surface area and the total pore volume in the following sequence: 0-rGOA < E-rGOA < A-rGOA < D-rGOA < T-rGOA < U-rGOA, representing the rGOAs without amine and those with ethylenediamine, aniline, diethylenetriamine, triethylenetetramine, and urea, respectively. Also, the adsorptive performance of the rGOAs materials for L2 was enhanced in this sequence. The adsorption capacities of rGOAs were significantly related to their textural properties and hydrophobicity. Moreover, U-rGOA exhibited the highest specific surface area (167.9 m2 g−1), total pore volume (0.737 cm3 g−1), and contact angle (146.8°), and its theory breakthrough adsorption capacity for L2 at 0 °C was 112.4 mg g−1. A lower bed-temperature, carrier gas flow rate, and inlet concentration were beneficial for the L2 adsorption by the U-rGOA. The spent U-rGOA adsorbent could be readily regenerated for reuse by heating at 80 °C. The results suggest that the rGOA materials have good application potential as adsorbents for methyl siloxane removal.

Facile fabrication of iron-modified biochar as a renewable adsorbent for efficient siloxane (L2) removal

The volatile methylsiloxanes (VMS) in biogas significantly hinders the application. Thus it is vital to identify new materials for achieving the effective removal of VMS from biogas. In this study, iron-modified biochar (FeMBC) materials have been prepared via an impregnation and calcination approach using biochar and Fe (III)-ethanol solution for effective removal of hexamethyldisiloxane (L2). The developed FeMBC materials have been characterized by N2-adsorption/desorption, FTIR, SEM-EDS, XRD and XPS. The results revealed that the as-prepared FeMBC materials possessed a well-developed microporous structure. Among these materials, FeMBC-3 exhibited the highest specific surface area (1068.43 m2 g−1) and micropore volume (0.51 cm3 g−1), leading to an adsorption capacity of 356.4 mg g−1 using 83.82 mg L−1 of L2 with dry N2 as the carrier gas. It was concluded that L2 adsorption capacities were strongly related to the texture of FeMBC materials, especially the pore volume and surface area of micropores. In addition, the influence of temperature and inlet concentration was investigated using dynamic adsorption experiments, and the data were fitted by applying Yoon-Nelson model, Langmuir-Freundlich thermodynamic model and kinetics models, obtaining the thermodynamic and kinetic parameters. The results revealed that the adsorption of L2 on FeMBC-3 was a spontaneous and exothermic process. Finally, regeneration of FeMBC-3 could be swiftly achieved by heating at 100 °C, with the adsorption capacity for L2 remaining stable in the subsequent continuous dynamic adsorption experiments. Thus, the as-prepared FeMBC material represents a promising siloxane adsorbent for biogas up-gradation.

Siloxane adsorption by porous silica synthesized from residual sand of wastewater treatment

Wastewater from municipal sewage may be an interesting source of biogas, in which case siloxanes are commonly present above the spec limit to be used in power engines. On the other hand, residual sand from Wastewater Treatment (WWT) Plants is also rich in inorganic matter, which can be a suitable feedstock for the synthesis of porous inorganic adsorbents. This work describes the synthesis of an alternative adsorbent for siloxane adsorption using residual sand from a WWT plant. The ultimate goal was to find a substitute for commercial activated carbons or silicas currently employed in siloxane removal from biogas. The residual sand was subject to thermal and acid treatment, which successfully increased its silicon content while developing adequate internal porosity. The textural characteristics and the chemical composition of the new adsorbent were similar to those found in commercial silica gels. Liquid phase adsorption isotherms and uptake rate curves were measured for siloxanes Octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). The siloxane uptakes and adsorption kinetics were comparable and even superior to those found in commercial silicas, especially at low adsorbate concentrations. These results demonstrate the potential of residual sand from WWT as a suitable raw material to produce a good adsorbent for the removal of cyclic siloxanes from biogas.

Biogas Upgrading Approaches with Special Focus on Siloxane Removal—A Review

Biogas, a product of anaerobic digestion process that consists mainly of methane and carbon dioxide is a suitable alternative fuel if unwanted impurities are removed as they have a negative impact on the equipment. The most significant technologically troublesome trace compounds that must be removed are siloxanes since they are converted into silica on gas surface engines and turbines resulting in equipment damage. The quality of the gas is certainly improved by reducing the amount of impurities and the end use determines the extent of biogas cleaning needed. The major aim of this study was to compile information that can assist researchers or even designers in selecting a suitable technology to remove siloxanes. Siloxane removal definitely can be achieved using different methods and the effectiveness of each method relies on careful consideration of the characteristics of both biogas and siloxane, as well as the technological aspects of the method. Herein, we review on different cleaning techniques for siloxanes in raw biogas, the negative effects they have, their levels and technologies to reduce their concentrations. This review also incorporates the sources of the siloxanes, the progress to date on their removal and possible ways of regenerating adsorbents. The reviewed literature suggests that biogas upgrading technology should be promoted and encouraged especially in siloxane removal as it has detrimental effects on engines. The parameters and effectiveness of adsorption processes are discussed, and individual adsorbents are compared.

Siloxane removal for biogas purification by low cost mineral adsorbent

The potential use of raw and expanded perlite as low-cost adsorbents for biogas purification has been investigated. The thermal expansion of perlite causes a reduction in the density of silanol groups from 2515.43 to 653.75 OH/nm2; in contrast, the specific surface area of perlite increased two-fold due to the thermal expansion. To determine the equilibrium adsorption capacity and the adsorption kinetics batch experiments were conducted. The adsorption capacities are in the following order: activated carbon (6.8 mg/g) > silica gel (6.6 mg/g) > expanded perlite (5.8 mg/g) > raw perlite (5.6 mg/g) when compared at the same experimental conditions. The equilibrium adsorption data showed that perlite can be used to reduce the octamethylcyclotetrasiloxane concentration below 28 mg/m3, as recommended by leading manufacturers. The adsorption kinetics of octamethylcyclotetrasiloxane onto raw and expanded perlite followed the Linear-Driving Force model suggesting that the mass transfer is the rate-controlling step. In addition to its low cost, expanded perlite has the advantage of requiring lower desorption temperature (200 °C) for regeneration in comparison to the reported values for activated carbon (>400 °C) and fast desorption kinetics (20 min), which could contribute to a cleaner production of biogas.

Desiloxanation process of biogas using an amorphous iron hydroxide-based adsorbent: A comparison between laboratory and field-scale experiments

Amorphous iron hydroxide-based material was selected as a suitable adsorbent for the siloxanes from biogas and landfill gas. The adsorptive characteristics of the desiloxanation (DeSiloxs) process were studied through a lab-scale experiment using a simulated gas containing a mixture of siloxanes (D4, and D5) and a field experiment on an actual environment scale. The iron hydroxide-based adsorbents exhibited high activity in adsorption of siloxanes during lab-scale experiments, where the obtained adsorption capacity was about 26.3 % with N2 as a carrier gas. However, from an applicability point of view, the effectiveness of the siloxane removal process was significantly affected by the space velocity during the in-situ test in anaerobic digestion gas plant using wastewater sludge. In the experimental case of various space velocities (50, 100, 300 h−1), a very high removal efficiency (100 %) was obtained.

Zeolite-templated carbons as effective sorbents to remove methylsiloxanes and derivatives: A computational screening

Though widely used in our daily lives, volatile methylsiloxanes and derivatives are emerging contaminants and becoming a high-priority environment and public health concern. Developing effective sorbent materials can remove siloxanes in a cost-effective manner. Herein, by means of Grand Canonical Monte Carlo (GCMC) simulations, we evaluated the potentials of the recently proposed 68 stable zeolite-templated carbons (ZTCs) (PNAS2018,115, E8116-E8124) for the removal of four linear methylsiloxanes and derivatives as well as two cyclic methylsiloxanes by the calculated average loading and average adsorption energy values. Four ZTCs, namely ISV, FAU1, FAU3, and H8326836, were identified with the top 50% adsorption performance toward all the six targeted contaminants, which outperform activated carbons. Further first principles computations revealed that steric hindrance, electrostatic interactions (further enhanced by charge transfer), and CH-π interactions account for the outstanding adsorption performance of these ZTCs. This work provides a quick procedure to computationally screen promising ZTCs for siloxane removal, and help guide future experimental and theoretical investigations.

The regulation of micro/mesoporous silica gel by polyethylene imine for enhancing the siloxane removal

In this study, different micro/mesoporous silica gel (MMSG) materials were prepared by mixed sol-gel process of polyethylene imine (PEI) and silicate. The developed materials were used to explore the optimal conditions for hexamethyldisiloxane (L2) removal in dynamic adsorption experiments. MMSG prepared using the conditions of pH = 3 and mPEI = 1.0 g was observed to be the optimum adsorbent for L2, which demonstrated an adsorption capacity of 367.1 ± 9.5 mg·g−1 using 83.82 mg·L−1 of L2 with dry N2 as the carrier gas. In addition, the influence of adsorption temperature and L2 inlet concentration were also investigated using dynamic adsorption experiments. Characterization of the MMSG material was carried out using several physical and chemical techniques, such as XRD, SEM and N2-BET. The results concluded that the L2 adsorption capacities were strongly related to the textural development of MMSG, especially the pore volume and surface area of micro/mesopores. Finally, MMSG regeneration could be easily achieved via heating at 100 °C, and its adsorption capacity for L2 was observed to be stable in the subsequent continuous dynamic adsorption experiments. These results indicated that the developed MMSG material represents a promising siloxane removal adsorbent for biogas up-gradation.

Biogas purification through membrane bioreactors: Experimental study on siloxane separation and biodegradation

Sewage biogas valorization to different energy applications is hampered by the presence of volatile methyl siloxanes. Despite the high operating costs, adsorption onto activated carbon is the most implemented technology for siloxane removal from biogas. In order to purify biogas sustainably, the current work explores the diffusion of siloxanes (octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane) together with other biogas impurities (limonene, toluene and hexane) through polydimethylsiloxane membranes. Abiotic tests revealed transport efficiencies above 75% towards a clean air stream for most compounds, although the transport of the most hydrophobic pollutants was challenged when water was circulated through the shell side of the membrane. Moreover, the performance of a hollow-fiber membrane bioreactor, inoculated with anerobic active sludge, was evaluated towards biogas purification in anoxic conditions. Toluene and limonene were successfully degraded, hexane’s removal efficiency was positively correlated with the residence time, and siloxanes removal was achieved up to 21%. CO2 was detected in the outlet gas as the mineralization product as well as some byproducts from the degradation of limonene and siloxanes. The presence of 1% of O2 in the gas, as a strategy to substitute NO3−, efficiently supported high removal for volatile organic compounds and moderate for siloxanes, which would ultimately reduce the operating costs of the technology.

Biogas Purification: A Comparison of Adsorption Performance in D4 Siloxane Removal Between Commercial Activated Carbons and Waste Wood-Derived Char Using Isotherm Equations

Biogas production from organic waste could be an option to reduce landfill and pollutant emissions into air, water, and soil. These fuels contain several trace compounds that are crucial for highly efficient energy generators or gas injection into the grid. The ability of adsorbents to physically remove such adsorbates was investigated using adsorption isotherms at a constant temperature. We experimentally modelled isotherms for siloxane removal. Siloxanes were considered due to their high impact on energy generators performance even at low concentrations. Octamethylcyclotetrasiloxane was selected as a model compound and was tested using commercially available carbon and char derived from waste materials. The results show that recyclable material can be used in an energy production site and that char must be activated to improve its removal performance. The adsorption capacity is a function of specific surface area and porous volume rather than the elemental composition. The most common adsorption isotherms were employed to find the most appropriate isotherm to estimate the adsorption capacity and to compare the sorbents. The Dubinin-Radushkevich isotherm coupled with the Langmuir isotherm was found to be the best for estimating the adsorption capacity.

Field Testing of a UV Photodecomposition Reactor for Siloxane Removal from Landfill Gas

We present in this paper the results of the field testing at a California Landfill of a UV photodecomposition reactor (PhoR) used for the removal of siloxane impurities from landfill gas (LFG). Prior to its field testing, the PhoR technology was tested in the laboratory with simulated LFG and was shown to be capable of completely removing the trace siloxane compounds and to convert them into silica particulates. The key objective of the field test was to validate the ability of the PhoR system to treat real LFG in a practical setting. The field-scale PhoR again proved quite efficient in attaining complete siloxane removal at different concentrations in the real landfill environment. These promising findings have led us to propose a scaled-up, commercial size PhoR system, competitive to conventional adsorption systems that can be practically applied in existing landfill plants to obtain 99+% siloxanes removal rates without associated secondary emissions.

Managing siloxanes in biogas-to-energy facilities: Economic comparison of pre- vs post-combustion practices

Siloxanes present in small concentrations in biogas interfere with the operation of biogas-to-energy facilities. During biogas combustion, siloxanes form white deposits on engine components (engine heads, spark plugs, valves) in crystals or amorphous forms depending on the temperature. The purpose of this study was to evaluate the economic feasibility of biogas-to-energy systems for managing the operational challenges due to siloxanes in biogas. The facility maintenance cost data were compiled by a survey of biogas-to-energy facilities in the United States. Economic analyses were performed to compare the operational costs due to increased maintenance for removing the white deposits forming on the engine components and the installation of a pretreatment system (carbon adsorption) to remove siloxanes prior to combustion. Numerical analyses showed that for the facilities with operating capacities less than 1300 m3/h (750 scfm), the costs for installation and operation of the carbon adsorption system exceeded the maintenance costs for removal of deposits from the engine components. The maintenance costs correlated well with the reported maintenance needs which were between 120 and 800 man hours per year. On the basis of siloxane removal costs alone, it is not economically feasible to install a carbon adsorption system for siloxane removal prior to combustion for small facilities processing less than 1300 m3/h (750 scfm) of biogas. However, using a process for siloxane removal prior to gas engines (e.g., carbon adsorption) would be improve the overall performance of the gas engines and reduce maintenance need at all facilities.

A UV photodecomposition reactor for siloxane removal from biogas: Modeling aspects

The goal of this research was to develop a model for a UV photodecomposition reactor (PhoR), which is used for the removal of trace siloxane impurities from biogas. The model properly accounts for the phenomena taking place inside the reactor, including fluid flow, mass and heat transfer, the transmission of UV radiation, and the photodecomposition reaction. Lab-scale experimental data were used to experimentally validate the model, and to also provide a “proof-of-concept” of the proposed PhoR technology prior to its field-testing at a real landfill site. The model was then utilized in the design of the experimental system used in the field-testing component of this study. Two reactor configurations were modeled: In the first configuration (termed PhoRI), the UV lamps are not in direct contact with the biogas, whereas in the second one (PhoRII), the lamps are placed in direct contact with the biogas stream. The PhoRII configuration is substantially more efficient for siloxane degradation, but concerns exist about the long-term robustness of the UV lamps in such an environment. The same model also presently finds use for further process design and scale-up, and in the preliminary technical and economic analysis (TEA) of the PhoR technology.

Feasibility of Siloxane Removal from Biogas Using an Ultraviolet Photodecomposition Technique

The goal of this study is to assess the technical feasibility of remediating siloxane contaminants in biogas via a photochemical process. Specifically, we studied in the laboratory a process that involves the use of an ultraviolet (UV) photodecomposition reactor (PhoR) to convert siloxane trace impurities, commonly found in biogas produced in water treatment plants and landfills, into silica particulates. These can then be effectively removed from the reactor effluent with the use of a downstream filter. High siloxane conversions were obtained, which demonstrates the effectiveness of the technique. The proposed technology is presently being field-tested in a California landfill.

Proton transfer reaction mass spectrometry for the gas cleaning using commercial and waste-derived materials: Focus on the siloxane removal for SOFC applications

The research efforts for SOFC plants fed by biogas moved from the prototypal research to the feasibility of pilot plants up to achieve industrial size plants. Siloxanes among the other trace compounds contained in biogas appear to be the most detrimental for the fuel cell performance. Siloxanes are difficult to be detected and monitored continuously in the gas matrix. A direct injection mass spectrometry technique (PTRMS) was adopted for the monitoring of siloxane removal. Commercial and waste derived sorbent materials are experimentally tested for the removal of siloxanes. Waste derived material was selected to implement the circular recovery purposes. A simple parametric investigation study was developed. It was considered the influence of gas velocity and sulphur compounds, as co-vapors. Physical and chemical characteristics were correlated to the adsorption capacity. Results show three separated groups. Group I shows the best performance in terms of siloxane removal. There is a direct and strong relation between active surface area and microporous volume with the adsorption capacity. This direct correlation is not verified for some elements such as Fe and S, while it is respected for Cu and K. Higher performance are registered for not all the commercial carbons. In fact, the physical structure and impregnating agents are crucial for the siloxane removal.