Feasibility of Siloxane Removal from Biogas Using a UV

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Kinetics, Catalysis, and Reaction Engineering

Feasibility of Siloxane Removal from Biogas Using a UV Photodecomposition Technique Alireza Divsalar, Lin Sun, Matthew Dods, Hasan Divsalar, Richard W Prosser, Fokion N. Egolfopoulos, and Theodore T. Tsotsis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00710 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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Industrial & Engineering Chemistry Research

Feasibility of Siloxane Removal from Biogas Using a UV Photodecomposition Technique Alireza Divsalar1, Lin Sun1, Matthew N. Dods1, Hasan Divsalar1, Richard. W. Prosser2, Fokion N. Egolfopoulos3, Theodore T. Tsotsis*1 1Department

of Chemical Engineering and Materials Science, University of Southern California, CA 90089, USA Environmental, Inc., 1230 N Jefferson St, Suite J, Anaheim, CA, 92807, USA† 3Department of Aerospace and Mechanical Engineering, University of Southern California, CA 90089, USA 2GC

*

Corresponding Author: [email protected]; [email protected] GC Environmental was recently acquired by ES Engineering Services, LLC, 1036 W Taft Avenue, Orange, CA 92865 †

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract 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.

Keywords: Biogas, landfill gas, siloxanes, UV, Photodecomposition


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1. Introduction Because of its large methane content, biogas has been investigated in recent years as a potential alternative renewable energy source.1,2 Biogas is commonly produced from the anaerobic biodegradation of organic waste in landfills and from biological sludge in digesters in waste-water treatment plants (WWTP). Although biogas is mostly composed of methane (CH4) and carbon dioxide (CO2), together with smaller concentrations of oxygen (O2) and nitrogen (N2), it also contains a variety of trace impurities. These are known as the non-methane organic compounds (NMOC) in biogas, and include sulfided compounds and halogenated organic compounds, as well as several silicon-containing compounds known as siloxanes.3 Siloxanes are commonly found in commercial and consumer products such as detergents, shampoos, deodorants, and other cosmetics and find their way into landfills and WWTP as these products are consumed or discarded. The increased usage of siloxane-containing products in recent years has resulted in increasing siloxane concentrations in landfill gas (LFG). The presence of the NMOC in biogas creates technical challenges that presently hinder its widespread use as a renewable fuel for power generation.4,5 For example, the sulfur- and halogencontaining NMOC when combusted generate mineral acids that corrode the power generating equipment, and if released to the environment may contribute to acid rain. Siloxanes, such as hexamethyldisiloxane (L2) and octamethylcyclotetrasiloxane (D4), have been shown to generate silica (SiO2) microparticulates during combustion, which have been found to damage the equipment and also present a potent threat to the environment, if released.6-15 Thus, prior to using the biogas for power and energy production, its NMOC trace constituents must be removed to acceptable levels. For the use of biogas in renewable energy production to become more economically attractive and less harmful to the environment, cost-effective remediation techniques for its NMOC constituents, including the siloxane pollutants must, therefore, be developed. To date, considerable effort has been expended on the remediation of the sulfur- and halogencontaining NMOC, but relatively little research has focused on the removal of siloxanes from biogas,16,17 which is the primary focus of this study. Currently, the most common techniques employed commercially for remediating siloxanes in biogas, including the most frequently encountered ones such as L2 and D4, are adsorption and absorption.18-20 Adsorption is, typically, an effective technique for the removal of trace contaminants from gas waste-streams, and is commonly utilized for the clean-up of biogas as well. 3 ACS Paragon Plus Environment


However, it is not all that effective for the clean-up of siloxanes because their concentration in LFG is, typically, quite low compared to those of the other NMOC in the LFG, some of which also show a lot more affinity toward activated carbon (AC), the common adsorbent utilized, rather than the siloxanes. Thus, in practical settings the adsorption capacity of the adsorbers toward the siloxane molecules is greatly hindered by the other (especially the non-volatile) NMOC, therefore diminishing the effectiveness of the adsorption technique8,21-24 for siloxane remediation. Light siloxanes, like L2, are more difficult to deal with because they break through the adsorbent beds much quicker than do heavier siloxanes (e.g., D4), as a result necessitating frequent regeneration of the adsorbent.25-27 Additionally, adsorption processes do not alter, typically, the chemical structure of the siloxane molecules. Hence, during adsorbent regeneration, which involves the incineration of the off-gas, the siloxanes are decomposed into silica microparticles that are released into the atmosphere. Absorption is another technique that has been studied for the removal of siloxanes.28 Reactive absorption processes are capable to convert siloxanes into less volatile compounds, but they require the use of harsh absorption agents (e.g., nitric acid and/or sulfuric acid), something that significantly increases the cost and technical complexity associated with these processes.29 Physical absorption techniques involve less toxic chemicals but, on the other hand, are not all that effective for the removal of light siloxanes (e.g., L2) due to their high volatilities.30 Such volatile siloxane pollutants often require relatively high pressures to achieve high removal efficiencies.28,31 Additionally, physical absorption processes also suffer from similar disadvantages with the competing adsorption techniques: (i) the need for absorbent regeneration, which can reduce siloxane removal efficiencies over time due to the accumulation of siloxanes (or decomposition by-products) inside the absorbing medium from successive regeneration cycles; (ii) the fact that the siloxanes are released intact during the regeneration process and must be properly disposed of. In addition to adsorption/absorption, other methods have also been investigated for the removal of siloxanes from biogas. They include refrigeration,32 biofiltration,5,33 and membrane separation.34 Because they require a relatively large capital investment and have high operating costs, refrigeration processes are only economically effective for high biogas flow rates and at elevated siloxane loads. The energy costs associated with refrigeration are a major drawback,29,35 as are concerns about ice formation in humid LFG streams.8,18 A novel idea is to use refrigeration in combination with another technique, such as adsorption, as did Schweigkofler et. al.,29 who 4 ACS Paragon Plus Environment

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reported a 98% siloxane removal efficiency, but high energy costs still remain a concern even for the hybrid process. Biofiltration is an effective technique for the removal of volatile organic compounds (VOC) from gas waste-streams;36 however, its application for siloxane remediation in biogas has not proven successful, because of the low removal efficiencies attained, due to mass transfer limitations and possibly due to the low biodegradability of siloxane compounds.37,38 Membrane separation technologies are continuous processes, which provide an advantage over their adsorption/absorption counterparts (which require adsorbent/absorbent regeneration). Current membrane processes are not all that effective, however, in dealing with the dilute siloxane concentrations in biogas and exhibit significant parasitic methane losses of ~7%, which reduces their economic efficiency.39 Another idea that has been tried is to minimize siloxane concentration at the point of biogas generation.40,41 Appels et. al.42, for example, have studied the removal of siloxane compounds in waste sludge activated via peroxidation, and have demonstrated siloxane removal efficiencies of 50-85%, which are relatively moderate when compared to existing siloxane removal technologies (such an approach is, however, not feasible for LFG). In summary, conventional (adsorption/desorption) and emerging (refrigeration, biofiltration, membrane separation) techniques have not, as yet, proven effective for the removal of siloxanes in biogas. What is presented here, instead, is a simple and potentially cost-effective technique for remediating siloxane contaminants in LFG that involves the use of a UV photodecomposition reactor (PhoR). The use of such reactors for the removal of trace contaminants from gaseous wastestreams is not new. In the last decade or so several studies have demonstrated the ability of UV reactors to decompose VOC in contaminated air streams into less harmful compounds,43-57and the topic was recently reviewed by Boyjoo et. al.58 Fujimoto et. al.59, for example, have recently investigated the photocatalytic decomposition of a number of VOC (octane, isooctane, n-hexane, and cyclohexane) using both TiO2 and TiO2/Pd photocatalysts inside a UV reactor. Li et. al.60 demonstrated a synergetic effect provided by combining nanostructured TiO2 with activated carbon fiber felt composites that contributes to improved photocatalytic degradation of toluene in the presence of UV light. Zhong et. al.61 have analyzed the vacuum UV and UVC photocatalytic degradation of numerous VOC pollutants for indoor air applications. The removal of air pollutants using UV/TiO2 combined with plasma has also been investigated.62,63 UV radiation at wavelengths of ~200 nm has been shown64 to cleave the O=O bond in O2, thus ozone (O3) is likely to form under such conditions via the reaction of atomic oxygen with a neighboring O2 molecule. Zhang 5 ACS Paragon Plus Environment


and Anderson65 demonstrated that the presence of O3 in UV photodecomposition reactions of VOC can have profound effects on the quantum efficiencies of these reactions, perhaps due to the fact that O3 strongly absorbs the UV photons. Li et. al.66 have even demonstrated the potential for these UV/O3 processes to remediate hydrogen sulfide (H2S), a pollutant that is also commonly found in LFG.67 Though we are not aware of the use of UV photodecomposition to remove siloxanes from biogas, a number of past studies have demonstrated the potential for UV light to decompose silicon-containing compounds68-72 to form Si-containing solid materials. By irradiating the siloxane molecules with UV light at wavelengths less than 200 nm, it is possible to cleave the SiC bonds of the siloxane molecule to yield Si and CH3 radicals (by analyzing the UV absorption spectrum of a given siloxane molecule, one can identify the different wavelengths that are potentially optimal for the cleaving of the Si-C bonds). When oxygen is present, the UV light also helps to cleave the O=O bond yielding two oxygen radicals, which are then available to react with the Si radicals from the siloxane decomposition to form SiO2 solid materials. 64, 68, 69, 73-75 Though the UV PhoR technique is not presently utilized to remove siloxanes, LFG is an ideal environment to apply the approach to decompose and remove these NMOC. One reason for that is that LFG, in addition to CH4 and CO2, commonly contains a small concentration of O2 (1-3%), which is likely to participate in the siloxane photodecomposition reaction. In addition, because the absorption spectrum of the CH4 molecule (the main component of LFG) is located in the IR region, it is unlikely that UV radiation, when used to photodecompose the siloxanes, will decompose the CH4 molecules as well. This is important in terms of diminishing parasitic reactions that may degrade the energetic content of LFG. Thus, the key focus of this study is to investigate the application in the laboratory of a UV PhoR to decompose two model siloxanes, namely L2, a linear siloxane, and D4, a cyclic siloxane, in a LFG stream (the choice of L2 and D4 is because they are the two most common siloxanes found in biogas and LFG). Although numerous efforts have recently focused on the development of heterogeneous catalytic materials such as TiO2 composites for enhanced VOC photodegradation,44-61 a homogeneous UV PhoR is utilized in this research. This is because photocatalytic reactors are not well-suited for the application, since siloxane photodecomposition on surfaces deposits Si-containing solid films that quickly degrade photocatalytic activity.76-79 In addition, the mineral acid by-products of decomposition of the halide- and sulfur-containing 6 ACS Paragon Plus Environment

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NMOC found in LFG are likely to also be quite harmful to catalyst lifetime. In the remainder of the paper, the experimental system that is used to carry the photodecomposition experiments is first described. We then continue by describing the experimental data.

2. Experimental Figure 1 depicts the schematic of the UV photodecomposition flow reactor used in the labscale experiments (with specific dimensions added). It is a simple configuration employing a single low-pressure 18W (G18T5VH) UV-C lamp (connected to an electric ballast which acts as a transformer to control the power to the lamp – note that 18W is the nominal radiation output reported by the lamp manufacturer and is not used in the simulations in this study, which instead employ the experimentally measured values80) installed inside a quartz tube (by contrast, a multiple-lamp configuration is used during the ongoing field-testing of the technology). Lowpressure UV-C lamps made of high purity quartz emit their maximum energy output at a wavelength of 254 nm and 185 nm.

Electric Ballast

Figure 1. Schematic of the UV photo-decomposition reactor (PhoR).

Figure 2 shows a schematic of the overall experimental set-up, which is placed inside an enclosure equipped with a powerful inline suction fan to prevent potential explosion in case of any system leaks. The system consists of three key parts: (i) the gas delivery system, (ii) the PhoR (see Figure 1), and (iii) the analysis section. In the laboratory studies, a simulated LFG with two different compositions is utilized. The first type of LFG (hereinafter referred to as SLFG) consists 7 ACS Paragon Plus Environment


of 52% methane, 38% carbon dioxide, 9% nitrogen, 1% oxygen, and is purchased premixed in a high-pressure cylinder from the Matheson Company. The second type of simulated LFG (hereinafter referred to as SLFGV), purchased premixed from the Air Liquide Company, has the same composition as the SLFG, but in addition contains five model NMOC. Specifically, it contains Carbonyl Sulfide (50 ppmv), 1,3-Dichlorobenzene (5 ppmv), Dimethyl Sulfide (49.8 ppmv), Trichloroflouromethane (52.4 ppmv) and Vinyl Chloride (47.9 ppmv). The high-pressure cylinder containing the simulated LFG (SLFG or SLFGV) is connected to two stainless steel lines via a pressure regulator (that controls the gas delivery pressure), each line being connected to a separate Mass Flow Controller (MFC) that is used to set the flow of gas. The temperature of the lines downstream of the MFC is kept constant at a desired value by utilizing heating tapes and temperature controllers, and by insulating the lines well with glass wool held in place wrapped with aluminum foil.

Figure 2. Schematic of the experimental set-up.

To prepare the (simulated) LFG feed mixtures containing a pre-determined concentration of siloxanes (L2, L3 and D4 are employed as model compounds in this study because they are the most common linear (L2 and L3) and cyclic (D4) siloxanes encountered in landfill gas9,12,20, as also noted above), a syringe pump (Harvard Apparatus Phd 2000) equipped with a 500 microliter syringe (1750 TLL Hamilton syringe) is used to deliver very small and consistent flow rates of liquid siloxanes. Using a heated tee-fitting, whose temperature is controlled by a controller, the liquid 8 ACS Paragon Plus Environment

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siloxane flow is delivered into one of the two gas lines noted above carrying the heated LFG stream, where it is then vaporized and transferred into the heated gas at the tee-junction. Since only trace (ppmv-level) concentrations of siloxanes are, typically, found in real landfill gas, and there are limitations on how low of a siloxane liquid flow rate the syringe pump can deliver in a stable manner, to be able to generate the low siloxane concentrations used in the experiments the siloxane-containing LFG stream needs to be mixed and diluted with the other heated LFG stream. Typically, during the experiments the syringe pump is set at its lowest stable flow rate, and the flow rates of the two heated LFG streams are adjusted in such a manner as to generate the combined LFG stream with the desired siloxane concentration. To adjust the reactor space-time, only part of the flow of the combined LFG stream is directed to the photodecomposition reactor, while the rest is diverted to the fume-hood using a by-pass line. Teflon (PTFE) tubing is used from the mixing point onward to the reactor and for the lines to the analyzer – see below – to minimize the potential for adsorption of the siloxanes on the tubing walls. In addition, all lines to the reactor and to the analyzer are also heated with heating tapes, with their temperature being controlled by controllers to further diminish the potential for adsorption. A thermocouple was placed before the reactor inlet to measure the temperature of the mixture in the heated PTFE tubing. The temperature at the outer surface of the reactor tube was also measured at four different locations along the reactor length, with the data being used in the validation of the reactor model. 80 The feed and exit siloxane compositions are analyzed using a GC/MS (7890A Agilent GC and 5975C MS), via the use of two PTFE lines connected to the 6-port sampling valve of the GC-MS, which is equipped with a 250 µL sampling loop. For the analysis, we used an Agilent GC Column 30𝑚 × 250𝜇𝑚 × 0.25𝜇𝑚. The operating settings for the analysis were: GC temperature30 °C rising @ 15.0 °C/min, final temperature- 120 °C; GC total flow 13.2 mL/min, split ratio 1:5, purge flow 3 mL/min and vent flow 8.5 mL/min. MS quad and MS source temperatures are 150 °C and 230 °C, respectively.

3. Data Analysis For the analysis of the experimental data a detailed 3-D model of the laboratory reactor has been developed, which accounts for the flow, mass transfer, heat transfer and reaction phenomena that take place inside the system. To analyze for the UV radiation profiles in the reactor, the



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radiative transfer equation (RTE) for an absorbing, emitting and scattering medium is employed, as follows:81 𝑑 𝐼(𝑟⃗,𝑠⃗) 𝑑𝑠

+ (𝑎 + 𝜎𝑠 ) 𝐼(𝑟⃗, 𝑠⃗) = 𝑎 𝑛2

𝜎𝑇 4 𝜋

𝜎

4𝜋

+ 4𝜋𝑠 ∫0

𝐼(𝑟⃗, 𝑠⃗ ′ ) Φ (𝑠⃗ . 𝑠⃗ ′ )𝑑Ω′

(1)

In Equation 1, 𝑠, 𝑟⃗, 𝑠⃗ and 𝑠⃗ ′ are the path length, position vector, direction vector and scattering direction vector, respectively. 𝐼(𝑟⃗, 𝑠⃗) is the radiation intensity, which is a function of 𝑟⃗ 𝑎𝑛𝑑 𝑠⃗. 𝑎 and 𝜎𝑠 in the 2nd term on the left are the absorption and scattering coefficients, respectively. In the first term on the right side of the equation, n, T and 𝜎 are the refractive index, local temperature 𝑊

and the Stefan-Boltzmann constant (5.62 × 10−8 𝑚2 𝐾4 ), respectively. Finally, Φ and Ω′ are the phase function and the solid angle. To describe the siloxane decomposition reaction kinetics, a global reaction rate expression is employed, given by Equation 2 below:

−𝑟𝑆𝐿 = 𝑘0𝑆𝐿 𝑒

−𝐸𝑆𝐿 𝑅𝑇

𝑛 𝐶𝑆𝐿 𝐼(𝑥, 𝑦, 𝑧)

(2)

In Equation 2, 𝑘0𝑆𝐿 , 𝐸𝑆𝐿 and n are the pre-exponential factor, the activation energy and order of the reaction, respectively. In Equation 2, the assumption commonly utilized in the scientific literature,82,83,84 is made that the reaction rate depends linearly on the fluence rate 𝐼(𝑥, 𝑦, 𝑧), defined as the total radiant power incident from all directions at that location. Equation 2 assumes a nth power dependence on the siloxane concentration (ppmv-level), but does not explicitly account for the oxygen concentration, which is in substantial excess (1%) in the lab-scale PhoR experiments analyzed (and ~3% for the real LFG in the field-scale investigations). To solve the reactor model, the ANSYS® Fluent CFD software is utilized, which has found in recent years extensive application in the modeling of reactive flows.85-92 To analyze for the UV radiation profiles in the reactor, we employ the Discrete Ordinates (DO) component in the ANSYS® Fluent to solve the RTE. In ANSYS® Fluent, the geometry of the problem is divided into certain control volumes (cells) and the CFD software solves the transport/reaction equations (including the radiation transfer Equation 1) in each control volume. The validity of the model was tested by comparing the simulation results with experimental observations, such as the UV 10 ACS Paragon Plus Environment

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radiation intensity (W/m2) and temperature profiles and the measured reactor conversions. Further details about the model development and its numerical solution will be presented in an upcoming publication,80 where model predictions of experimental reactor temperature and UV radiation profiles are also included. Here, model simulations (via the ANSYS® Fluent software) are utilized to validate from the fitting of the experimental data the applicability of the global rate expression (Equation 2) for the siloxane photodecomposition reaction, see further discussion to follow. For that purpose, a programing language was developed that links with the ANSYS® Fluent software, and which allows one to perform the non-linear parameter fitting regression of the experimental data. The experimentally validated model employing the global reaction rate parameters reported here has been utilized to upscale the laboratory size reactor and to design a larger size unit for field-testing the process at a real landfill. The field-testing and the findings of a preliminary technical and economic evaluation (TEA) of the process will be presented in a future publication.

4. Results and Discussion We study here the photodecomposition of two different siloxanes, namely the linear siloxane L2 and the cyclic siloxane D4 that are most commonly encountered in biogas and landfill gas. The photodecomposition reaction was investigated with the siloxanes being present in trace amounts in three different carrier media, specifically air, and the two aforementioned model LFG (SLFG and SLFGV), whose compositions were noted previously. In the experiments, we have studied the effect of varying the feed siloxane concentration and the space-time on the conversion of the siloxanes. As noted in Sect. 3, a key objective of these experiments was to generate the experimental data required to determine the overall global rates of decomposition for the two tested siloxanes, and to use these rates for sizing the field-scale experimental reactor.



100 90 80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70 60 15.2 ppmv

50

35.8 ppmv

40

53.2 ppmv

30

79.9 ppmv

20 10 0 0

10

20

30

40

Space Time (sec)

Figure 3. L2 conversion vs. space-time for various L2 feed concentrations in air.

In the first series of experiments, air laced with the two model siloxanes was used as the carrier gas. The reasons for carrying out the air experiments are two-fold: First, a fundamental one to determine the differences in reactivity between the two different carrier media (i.e., air and LFG). In addition, siloxane photodecomposition studies in air are potentially of practical importance on their own right during the regeneration of active carbon (AC) beds saturated with siloxanes and other NMOC. In the studies, L2 and D4 either individually or as a mixture, were added to an air stream and then passed through the reactor to undergo UV photodecomposition. In the experiments, the impact of siloxane feed concentration and the reactor space-time (defined as the empty reactor volume divided by the volumetric feed flow rate at STP conditions) were investigated. Figure 3 shows the conversion of L2 in air as a function of the reactor space-time for various siloxane feed concentrations and an inlet feed temperature of 50 ºC. The reactor conversion increases as the space-time increases, as expected. On the other hand, the feed concentration seems to have a relatively small impact on reactor conversion, which is consistent with an overall global rate expression, which is near-linear in the L2 concentration. This is further validated by employing the PhoR model to calculate the global rate parameters, i.e., the rate constant and order of reaction, see Table 1, with the global reaction order being equal to 0.902.


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Table 1. Parameters fitting results for mixtures of individual siloxanes (L2 and D4) with air and SLFG. Siloxane/carrier

Pre-exponential factor

L2 + Air D4 + Air L2 + SLFG D4 + SLFG

1.62 x10-3 8.45 x10-4 1.95 x10-3 1.49 x10-3

Activation energy (kJ/mol) 12.05 16.75 12.29 15.25

Reaction order 0.902 0.758 0.958 0.909

Three by-products were detected at the outlet of the PhoR during the photodecomposition of L2, identified as octamethyl-cyclotetrasiloxane (D4), 1-methyl-3-(trimethylsilyl)disiloxane-1,3dione, and 3-methoxy-1,3,3-trimethyldisiloxan-1-one (for the chemical structure of these compounds and plausible routes of their formation, see the Supplementary Materials section). For the experiments with a space-time of 30 sec and L2 initial concentration of 35.8 ppmv, the integration area for by-product D4 corresponded to 0.32% of the inlet feed L2 integration area, and for the 1-methyl-3-(trimethylsilyl)disiloxane-1,3-dione and 3-methoxy-1,3,3-trimethyldisiloxan1-one integration areas corresponded to 4.37% and 0.075% of the inlet feed L2 integration area, respectively. At a space time of 6.67 sec, the D4 by-product was no longer detectable by the GCMS system, and the 1-methyl-3-(trimethylsilyl)disiloxane-1,3-dione and 3-methoxy-1,3,3trimethyldisiloxan-1-one integration areas corresponded to 1.33% and 0.22% of the inlet L2 integration area, respectively. Figure 4 shows the conversion of D4 in air as a function of the reactor space-time for various D4 feed concentrations. No additional by-products were generated during these experiments. In the case of D4, a stronger dependence of reactor conversion on feed siloxane concentration is observed, which is indicative of a further departure from first order in the global reaction kinetics. This is also validated by the calculation of the rate parameters, as shown in Table 1. When using the rate parameters in Table 1 to calculate the rate of decomposition of D4 and L2 in air in the laboratory PhoR under similar conditions (a feed temperature of 50 oC, a feed concentration of 10 ppmv, and a space-time of 20 sec), the model calculates a volume-averaged photocomposition rate in the reactor for L2 of 1.71 × 10−8 mol/l.s, while the corresponding value for D4 is 1.60 × 10−8 mol/l.s.



100 90 80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70 60 13.5 ppmv 50

19.6 ppmv

40

28.5 ppmv

30

34.5 ppmv

20 10 0 5

10

15

20

25

30

35

Space Time (sec)

Figure 4. Effect of feed concentration and space-time on D4 conversion in air.

Figure 5 shows the conversion of (L2+D4) mixtures in air as a function of the reactor space time and various siloxane concentrations (as indicated on the Figure) in air for an inlet feed temperature of 50 ºC. The goal of these experiments was to investigate the effect that the linear siloxane (L2) has on the conversion of the cyclic one (D4) and vice versa, since these two siloxanes are typically found together in landfill gas and biogas. To investigate that, we have used the singlesiloxane rate parameters in Table 1 to predict the conversions in the reactor when employing the siloxane (L2+D4) mixtures, and those are plotted against the experimental values in an isoconversion graph in Fig. 6. It is clear from the graph, that the global reaction rate orders and rate constants derived from the single-siloxane experiments do an accurate job to predict the siloxane mixture experimental conversion data, which is not all that surprising, given how dilute these mixtures are.


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L2

D4

100

100

90

90

80

Conversion (%)

80 70 60 50 40 30 7.5 ppmv 14.5 ppmv 30 ppmv

20 10 0 5

15

25

35

45

55

70 60 50 40 30 6.2 ppmv 13.4 ppmv 27.3 ppmv

20 10 0

65

5

15

25

35

45

55

65

Space Time (sec)

Space Time (sec)

Figure 5. Effect of feed concentration and space-time on the conversion of the (L2 and D4) mixture in air. The concentrations shown are those for each siloxane in the mixture.

100

100

90

90

Simulated D4 Conversion (%)

Simulated L2 Conversion (%)

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60 50 40 30 20 10 0

80 70 60 50 40 30 20 10 0

0

20

40

60

80

100

0

Measured L2 Conversion (%)

20

40

60

80

Measured D4 Conversion (%)

Figure 6. The iso-conversion graph for the siloxane mixture experiments in air.

For the experiments with the (L2 + D4) mixture in air, only one by-product was detected, namely 1-methyl-3-(trimethylsilyl)disiloxane-1,3-dione. At a reactor space time of 30 sec and feed 15 ACS Paragon Plus Environment

100


L2 and D4 concentrations of 30 ppmv and 27.3 ppmv, respectively, the integration area for this byproduct corresponded to 2.98% of the inlet feed L2 integration area, while at a space-time of 8.57 sec, the integration area for this by-product corresponded to 0.73% of the inlet L2 integration area. It is likely that the pathway of formation of the 1-methyl-3-(trimethylsilyl)disiloxane-1,3-dione is similar to the pathway (see Supplementary Materials section) for the L2/air mixture. Figure 7 shows the conversion of the L2 intermixed with SLFG as a function of the reactor space time at different trace concentrations of the siloxane at feed temperature 50 ºC. As can be seen in Figure 7, there is again not a significant dependence of conversion on siloxane concentration, which is in agreement with Figure 3 reporting the results of L2 photodecomposition in air. This was further validated by the investigation of the global reaction kinetics, which shows (see Table 1) that the calculated global reaction rate order for the L2/SLFG case is 0.958.

90 80 70

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

60 50 20 ppmv 40

40 ppmv

30

72.4 ppmv

20 10 0 0

20

40

60

80

Space Time (Sec)

Figure 7. Effect of feed concentration and space-time on the L2 conversion in SLFG.

When comparing the experimental conversions of L2 in SLFG versus those in air for the same space-times in the PhoR, those in air are higher. For example, the volume-averaged reaction rate of L2 in air in the laboratory PhoR (feed temperature of 50 oC, feed siloxane concentration 10 ppmv, and a space time of 20 sec) is calculated to be 1.71 × 10−8 mol/l.s, while it is computed to be 1.12 × 10−8 mol/l.s when L2 is mixed with SLFG. Such differences in the photodecomposition


Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rates for L2 in SLFG and air are to be expected, given the role that oxygen plays in ozone generation during UV photodecomposition processes, as previously discussed. During the studies of the L2/SLFG mixture, three by-products were detected at the outlet of the PhoR: D4, 3-methoxy-1,3,3-trimethyldisiloxan-1-one, and 1,3,3,3-tetramethyldisiloxan-1-one (for the chemical structure of these compounds and plausible routes of their formation, see the Supplementary Materials section). For the experiments under a space time 60 sec and L2 initial feed concentration of 40 ppmv, the integration area for the by-product D4 corresponded to 0.57% of the L2 integration area in the feed, and the integration areas for 3-methoxy-1,3,3trimethyldisiloxan-1-one and 1,3,3,3-tetramethyldisiloxan-1-one corresponded to 1.47% and 0.75% of the inlet L2 integration area, respectively. For the experiments at a space-time of 8.57 sec, the D4 byproduct was no longer detectable by the GC-MS instrument, and the integration areas for

3-methoxy-1,3,3-trimethyldisiloxan-1-one

and

1,3,3,3-tetramethyldisiloxan-1-one

corresponded to 0.078% and 0.5% of the inlet L2 integration area, respectively. Figure 8 shows the conversion of D4 intermixed with SLFG as a function of the reactor spacetime and for different trace siloxane concentrations. The calculated global reaction rate order and rate constant are shown in Table 1. As is the case with L2, when comparing the photodecomposition reaction rate of D4 in SLFG versus that in air, the degradation rate in air is significantly higher than that in SLFG. For example, for a D4 feed concentration of 10 ppmv, a feed temperature of 50 o

C and a space time of 20 sec, the calculated volume-averaged photocomposition rate in the

laboratory PhoR for the D4/SLFG mixture is 6.98 × 10−9 mol/l.s while that for D4 in air is 1.60 × 10−8mol/l.s. As with L2, these differences are to be expected given the role that oxygen plays in the UV photodecomposition processes. As is the case with air as a carrier gas, the photodecomposition rate of D4 in SLFG is lower than that of L2. For example, for a siloxane (L2 or D4) feed concentration of 10 ppmv, a feed temperature of 50 oC, and a reactor space-time of 20 sec, the calculated volume-averaged photocomposition rate in the laboratory PhoR for L2 is 1.12 × 10−8 mol/l.s, while that for D4 is 6.98 × 10−9mol/l.s. During the studies of the D4/SLFG mixture, there was one by-product peak detected identified as 4,4,6,6,8,8-hexamethyl-1,3,5,7,2,4,6,8-tetraoxatetrasilocan-2-one (for the chemical structure of these compounds and plausible routes of their formation, see the Supplementary Materials section), and its integration area at a space-time of 60 sec and initial D4 concentration of 25 ppmv was determined to be 0.07% of the integration area for the D4 in the feed. As the flow rate 17 ACS Paragon Plus Environment


increased, the concentration of this by-product compound decreased, and for a space time of 8.57 sec, this by-product compound was no longer detectable by the GC-MS instrument.

100 90 80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

70 60

1.6 ppmv

50

3.6 ppmv

40

5.5 ppmv

30

11 ppmv

20

13.2 ppmv

10 0 0

10

20

30

40

50

60

70

Space Time (sec)

Figure 8. Effect of feed concentration and space-time on the D4 conversion in SLFG.

Figure 9 shows the conversion of (L2+D4) mixtures in SLFG as a function of the reactor spacetime and various siloxane concentrations, as indicated in the Figure. The goal of these experiments was again to investigate the effect that the linear siloxane (L2) may have on the conversion of the cyclic one (D4), and vice versa. To investigate that, we have again used the single-siloxane rate parameters for the siloxane in SLFG in Table 1 to predict the conversions in the reactor when employing the siloxane mixtures, and those are plotted against the experimental values in an isoconversion graph in Fig. 10. It is clear, once more, that the global reaction rate orders and rate constants derived from the single siloxane experiments do an accurate job to predict the siloxane mixture experimental conversion data, which is not all that surprising again, given how dilute these mixtures are. For the (L2+D4) mixture in SLFG, only one by-product was detected: 1,3,3,3tetramethyldisiloxan-1-one. At a space-time of 60 sec with L2 and D4 concentrations of 12.6 ppmv and 12.08 ppmv, respectively, the integration area for this by-product corresponded to 1.78% of the inlet L2 integration area, and the by-product was not detected by the GC-MS system at a space time of 10 sec. It is likely, that the pathway of formation of the 1,3,3,3-tetramethyldisiloxan-1-one is similar to the pathway for the (L2 /SLFG) mixture. 18 ACS Paragon Plus Environment

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L2

100

80

70

70

60 50

2.39 ppmv

40

3.36 ppmv

30

5 ppmv

20

11.9 ppmv 12.6 ppmv

10

19 ppmv 40

60

60 50

1.46 ppmv

40

2.25 ppmv

30

4.63 ppmv

20

9.52 ppmv 12.08 ppmv

10

18.2 ppmv

0

0 20

Conversion (%)

90

80

0

D4

100

90

0

80

20

40

60

80

Space Time (sec)

Space Time (sec)

Figure 9. Effect of feed concentration and space-time on the conversion of a (L2+D4) mixture in SLFG.

100

100

90

90

Simulated D4 Conversion (%)

Simulated L2 Conversion (%)

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60 50 40 30 20 10

80 70 60 50 40 30 20 10

0

0 0

20

40

60

80

100

0

Measured L2 Conversion (%)

20

40

60

80

Measured D4 Conversion (%)

Figure 10. The iso-conversion graph for the siloxane mixture experiments in SLFG.

In order to investigate the effect that the presence of other NMOC present in LFG may have on siloxane photodecomposition, experiments were also carried out in which the siloxanes were intermixed with SLFGV as a carrier gas. As a reminder, SLFGV has the same composition as SLFG, other than the fact that it contains, in addition, five model trace NMOC that are typically 19 ACS Paragon Plus Environment

100


encountered in real LFG. Figure 11 shows the conversion of L2 and D4 mixed in SLFGV as a function of space-time for various siloxane concentrations. Comparing the experimental conversions for L2 and D4 in the two different model LFG (SLFG and SLFGV), one observes that the presence of the other NMOC has little impact on the conversion of siloxanes. This is manifested more clearly in Fig. 12, which is an iso-conversion plot in which the rate constants generated from the data of each individual siloxane in SLFG are used to calculate their conversions of their mixture in SLFGV. It is interesting to note, that as Fig. 13 indicates, the PhoR is also effective in decomposing the other model NMOC contaminants present in the SLFGV, in addition to the siloxane compounds.

100

L2

100 90

90

80

80

70

70

Conversion (%)

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 50 40

D4

60 50 40 30

30

20

20 25 ppmv 10

70 ppmv

0

16 ppmv 10

112 ppmv

0 0

20

40

60

80

0

Space Time (sec)

20

40

60

Space Time (sec)

Figure 11. L2 and D4 conversions both intermixed together with SLFGV as a function of space-time.


80

Page 21 of 31

Simulated Siloxane Conversion (%)

100 90 80 70 60 50 40 30 20 10 0 0

20

40

60

80

100

Measured Siloxane Conversion (%)

Figure 12. The iso-conversion graph for the siloxane mixture (L2+D4) experiments in SLFGV.

100 90 Vinyl Chloride Trichlorofluoromethane Dimethylsulfide 1,3-Dichlorobenzene Carbonyl Sulfide

80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

Space Time (sec)

Figure 13. Conversion in the PhoR of the other NMOC components present in the SLFGV.



Table 2: Siloxane compounds known to be present in LFG Name

MW (g/mol)

B.P. (𝐾)

Hexamethylcyclotrisiloxane (D3) Octamethylcyclotetrasiloxane (D4) Decamethylcyclopentasiloxane (D5) Dodecamethylcyclohexasiloxane(D6) Hexamethyldisiloxane (L2) Octamethyltrisiloxane (L3) Decamethyltetrasiloxane (L4) Dodecamethylpentasiloxane (L5)

222.5 297.6 370.8 445.0 162.4 236.5 310.7 384.8

407 448 483 518 373 426 467 503

Vapor Pressure at 25℃ (Pa) 1140 130 50 3 4120 520 73 9

As noted previously, L2 and D4 were investigated, because they are the most commonly found siloxanes in LFG and biogas. In our process scale-up and design studies, furthermore, these two siloxanes are considered as model surrogate compounds for all the other trace siloxanes one may encounter when processing real biogas and LFG (Table 2 shows some of the other siloxane compounds that have been reported previously as being present in these renewable fuels), L2 representing the linear and D4 the cyclic siloxane compounds. To test this hypothesis, L2 together with octamethyltrisiloxane (L3) another linear siloxane compound found in LFG (see Table 2) were intermixed with SLFG and processed in the PhoR. Comparing the molecular structures of L2 and L3, one observes that L3 has an extra 𝑂 − 𝑆𝑖 bond substituting one of the methyl groups in L2. Moreover, L3 is less volatile than L2 (as Table 2 shows, the vapor pressure of L2 is 4120 Pa, while for L3 it is 520 Pa) and, thus, it is considered less challenging to remove by the more conventional techniques like adsorption or absorption. Figure 14 shows the conversion of L2 and L3 at different initial feed concentrations and flow rates. The PhoR is as effective in decomposing the L3 as it is to decompose the L2. This is indicated further by the iso-conversion plot in Figure 15 where the rate parameters for L2 from Table 1 are also used to model the L3 photodecomposition.


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Page 23 of 31

100

100

L2

80

80

70

70

Conversion (%)

90

90

60 50 40 30 20

L3

60 50 40 30 20

11 ppmv

10

10 ppmv

10

30 ppmv

25 ppmv

0

0 0

20

40

60

80

0

20

Space Time (sec)

40

Space Time (sec)

Figure 14. Conversion of L2 and L3 both intermixed with SLFG as a function of space-time.

100

Simulated L2 and L3 Conversion (%)

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 80 70 60 50 40 30 20 10 0 0

20

40

60

80

100

Measured L2 and L3 Conversion (%)

Figure 15. The iso-conversion graph for the siloxane mixture (L2+L3) experiments in SLFG.


60

80


5. Conclusions Lab-scale experimental studies of the UV-radiation assisted photodecomposition of linear (L2 and L3) and cyclic (D4) siloxanes in both air and simulated LFG were carried out. The results showed that the lab-scale PhoR is quite effective in decomposing these siloxanes in both carrier media investigated. The linear siloxanes (L2 and L3) appear to be more amenable to photodecomposition in the presence of the UV radiation than the cyclic ones (D4), as manifested by the higher conversions for such linear siloxanes attainable (under otherwise identical conditions) in the PhoR when compared to the cyclic compounds. When comparing the photodecomposition rates of both classes of siloxanes in air to those in LFG, the former rates are substantially higher. Such differences in the photocomposition rates for siloxanes among LFG and air are to be expected, given the role that oxygen plays in ozone generation during UV photodecomposition processes. Though in some instances Si-containing by-products were identified, the observed conversion of the siloxanes into these by-products is quite low,

Feasibility of Siloxane Removal from Biogas Using a UV

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