Application of a pore network model to a biofilter treating ethanol vapor

Pore volume clogging due to biomass growth from the biodegradation of volatile organic compounds and other pollutants has significant implications for biofilter operation. As the larger pores in the biofilter narrow and the smaller pores fill, airflow through the biofilter is restricted, and headloss increases. The biomass surface area available for contaminant biodegradation is reduced, resulting in diminished removal efficiencies. As biomass clogging increases, flow channeling may occur, further reducing treatment efficiency. Biofilter designers try to overcome the effect of biomass clogging by making beds larger to reduce loading, which is costly. Better insight into the phenomena that occur during biofilter clogging is, therefore, clearly needed. In this paper the effect of biomass accumulation on the removal efficiency and pressure drop of a bench-scale biofilter treating an air stream containing ethanol vapor was investigated using a pore network model. In the model, the biofilter pore structure is described by a cubic lattice of cylindrical pores of uniform length and varying diameters following an experimentally determined pore size distribution. The model assumes that at the pore level biomass growth depends on the oxygen diffusion in the biofilm and on oxygen-limited Monod-type kinetics. Unlike prior biofilter models, this model accounts in detail for phenomena that occur at the pore level, and for the impact of the pore network structure on biofilter behavior. It accounts for the biomass growth in the biofilter and its interaction with the airflow distribution, and explains its influence on the headloss and the ethanol removal efficiency.     

Viscous stabilization of drying front: Three-dimensional pore network simulations

In this study, a recently developed pore network drying model [Metzger, T., Irawan, A. and Tsotsas, E., 2007, Isothermal drying of pore networks: influence of friction for different pore structures, Dry Technol, 25: 49–57], which accounts for liquid viscosity, is applied to three dimensions for the first time. Isothermal convective drying is simulated for a cubic network (25 × 25 × 50) with pore throats with a narrow radius distribution. The role of liquid viscosity is assessed by comparison with non-viscous drying of the same network. Simulation results are presented as phase distributions, saturation profiles and drying rate curves. In the viscous case, a stabilization of the drying front is observed. However, as the network dries out from the surface, the finite drying front gradually widens up and does not approach an asymptotic limit.     

Modeling of a bacterial and fungal biofilter applied to toluene abatement: Kinetic parameters estimation and model validation

Biofiltration has been established as a promising alternative to conventional air pollution control technologies. However, gas biofilters modeling has been less developed than experimental research due to the complexity of describing the fundamental processes and the lack of globally accepted physical, chemical and biological parameters. In addition, biofiltration modeling based on degradation activity of fungi has been rarely considered. For this reason, in this work, a dynamic model describing toluene abatement by a bacterial and fungal biofilter is developed, calibrated and validated. The mathematical model is based on detailed mass balances which include the main processes involved in the system: convection, absorption, diffusion and biodegradation. The model was calibrated and validated using experimental data obtained from two equal lab-scale biofilters packed with coconut fiber and pine leaves, respectively. Both reactors were operated under similar conditions during 100 days at an empty bed residence time of 60 s and an average inlet load of 77 g toluene m⁻³ h⁻¹. Biofilters were initially inoculated with a bacterial consortium, even though reactors were mostly colonized by fungi after 60 days of operation according to microscopic observation and reactors pH. Removal efficiency increased notably from 20% for the bacterial period to 80% for the fully developed fungal biofilters. Since kinetic parameters are strongly dependent on the biological population, semi-saturation constants for toluene and maximum growth rates were determined for bacterial and fungal operation periods. Kinetic parameters were fitted by means of an optimization routine using either outlet concentrations or removal efficiency data from the coconut fiber biofilter. A novel procedure in gas biofilters modeling was considered for checking the model calibration, by the assessment of the parameters confidence interval based on the Fisher Information Matrix (FIM). Kinetic parameters estimated in the coconut fiber reactor were validated in the pine leaves biofilter for bacterial and fungal operation. Adequate model fitting to the experimental outlet gas concentration for both bacterial and fungal operation periods was verified by using a standard statistical test.     

A pore network model for diffusion in nanoporous carbons: Validation by molecular dynamics simulation

A hybrid molecular dynamics simulation/pore network model (MD/PNM) approach is developed for predicting diffusion in nanoporous carbons. This approach is computationally fast, and related to the structure of the real material. The PNM takes into account both the geometrical (a distribution of pore sizes) and topological (the pore network connectivity) characteristics of nanoporous carbons, which are obtained by analysing adsorption data. The effective diffusion coefficient is calculated by taking the transport diffusion coefficients in single slit-shaped model pores from MD simulation and then computing the effective value over the PNM. The reliability of this approach is evaluated by comparing the results of the PNM analysis with a more rigorous, but much slower, simulation applied to a realistic model material, the virtual porous carbon (VPC). We obtain good agreement between the diffusion coefficients for the PNM and the VPC, indicating the reliability of the hybrid MD/PNM method and it can be used in industry for materials design.     

Alkylation of the toluene methyl group: A DFT study

Alkylation of the methyl group of toluene was modeled at the B3LYP/6-311++G⁷ level. In the presence of Na₂; the model basic catalyst, the methyl group is more active for the Na/H exchange than the H atoms at the aromatic ring. The PhCH₂Na molecule formed is next alkylated by ethene to produce PhC₃H₆Na. This needs a 30 kcal/mol barrier to be overcome. Finally, the Na/H exchange between PhC₃H₆Na and (unreacted) toluene molecules proceeds through a 18 kcal/mol barrier and ca. 10 kcal/mol is released. The study has confirmed the basic alkylation scheme proposed by Pines, Vesely, and Ipatieff more than 50 years ago.     

A pore network model for calculating pressure drop in packed beds of arbitrary-shaped particles

A pore network model is built to predict pressure drop in packed beds of arbitrary-shaped particles, using a method that consists of particle packing by the rigid body technique, pore network construction by the maximal sphere algorithm, and numerical calculation of fluid flow. The pore network model is firstly validated by comparing with experiments, Ergun-type equations, and particle-resolved computational fluid dynamics (CFD). The pore network model is as accurate as the particle-resolved CFD, and is remarkably two to three orders of magnitude less computationally intensive. Then, the pore network model is used to calculate the pressure drops in the beds packed with particles of different shapes and sizes, as well as using different flow media. These calculation results prove the versatility of the pore network model. This work provides an accurate yet efficient pore network model for predicting pressure drop, which should be a powerful tool for designing packed beds.     

Influence of nitrogen on the degradation of toluene in a compost‐based biofilter

Two identical laboratory‐scale bioreactors were operated simultaneously, each treating an input air flow rate of 1 m³ h^?1. The biofilters consisted of multi‐stage columns, each stage packed with a compost‐based filtering material, which was not previously inoculated. The toluene inlet concentration was fixed at 1.5 g m^?3 of air. Apart from the necessary carbon, the elements nitrogen, phosphorus, sulfur, potassium and other micro‐elements are also essential for microbial metabolism. These were distributed throughout the filter bed material by periodic ‘irrigations’ with various test nutrient solutions. The performance of each biofilter was quantified by determining its toluene removal efficiency, and elimination capacity. Nutrient solution nitrogen levels were varied from 0 to 6.0 g dm^?3, which led to elimination capacities of up to 50 g m^?3 h^?1 being obtained for a toluene inlet load of 80 g m^?3 h^?1. A theoretical analysis also confirmed that the optimum nitrogen solution concentration lays in the range 4.0–6.0 g dm^?3. Validation of the irrigation mode was achieved by watering each biofilter stage individually. Vertical stage‐by‐stage stratification of the biofilter performance was not detected, ie each filter bed section removed the same amount of pollutant, the elimination capacity per stage being about 16 g m^?3 h^?1 per section of column. © 2001 Society of Chemical Industry     

Evaluation of mercury porosimeter experiments using a network pore structure model

A square network model has been developed to interpret mercury penetration and retraction behaviour in the widely employed mercury porosimetry technique for investigating pore structure and pore size distribution. A network of arbitrary size is constructed by assembling cylindrical pore segments of equal length and pseudo-random number generation is used to assign pore diameters according to any stipulated size distribution function. Application of the simple Washburn equation then predicts movement of mercury into the network under increasing pressure (penetration) and the corresponding withdrawal under reducing pressure (retraction) The network model is superior to the classical parallel bundle model, since it implicitly produces hysteresis between penetration and retraction, predicts that mercury entrapment on retraction is a result of interconnectedness of pore segments and provides a better estimate of the intrinsic distribution of segment sizes. Comparison with porosimeter experiments on a commercial hydrodesulphurisation catalyst show that the approach can be applied to practical measurements and the model may provide an improved basis for the study of diffusion, reaction and deactivation in catalyst pellets.     

An effect of tar model compound toluene treatment with high-temperature flames

A major drawback of renewable gasification technologies is contamination of the syngas produced with “tar”, which can induce fouling in downstream equipment. The effect of continuous injection of acetylene and hydrogen high-temperature flames into the blend of gases containing a tar model compound toluene in order to decompose the latter has been studied. The experimental results indicate that treatment of the reaction mixture with the acetylene and hydrogen oxy-flames promotes reforming of toluene into H₂ and CO. The same heating values of the flames result in different ratios between H₂ and CO; this points out on a difference in mechanism of that reforming implying an interaction between toluene and combustion products which include a large specter of intermediate species (radicals). A better understanding of these mechanisms will help to obtain an optimal ratio between external oxy-flame and internal combustion regularly employed to increase the temperature of the producer gas in order to decompose volatile organics and tars in it. Utilization of oxy-flames for high-temperature clean-up of producer gas (gasification products) is very similar to the application of plasma steam tested with positive results in semi-industrial gasification units.     

A pore network study of evaporation from the surface of a drying non‐hygroscopic porous medium

The phenomena occurring at the surface of a porous medium during drying in the capillary regime are investigated by pore network simulations. The impact of the formation of wet and dry patches at the surface on the drying rate is studied. The simulations indicate an edge effect characterized by a noticeable variation of saturation in a thin layer adjacent to the porous surface. Also, the results indicate a significant nonlocal equilibrium effect at the surface. The simulation results are exploited to test Schlünder's classical model which offers a simple closure relationship between the evaporation rate and the degree of occupancy of the surface by the liquid. In addition to new insights into the surface phenomena, the results open up new prospects for improving the continuum models of the drying process. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1435–1447, 2018     

The effect of pore morphology and agarose coating on mechanical properties of tricalcium phosphate scaffolds

Three-dimensional biocompatible porous structures can be fabricated using different methods. However, the biological and mechanical behaviors of scaffolds are the center of focus in bone tissue engineering. In this study, tricalcium phosphate scaffolds with similar porosity contents but different pore morphologies were fabricated using two different techniques, namely, the replica method and the pore-forming agent method. The samples fabricated using the pore-forming agent showed more than two times higher compressive and bending strengths and more than three times higher compressive moduli. Furthermore, a thin layer of agarose coating improved the compressive and bending strength of both types of ceramic scaffolds. Subsequently, the samples’ capability to guide biomineralization was evaluated by immersion into a simulated body fluid that developed Ca-P nano-platelets formation and enhanced the compressive strength. Finally, the tetrazolium-based colorimetric (MTT) assay was used to evaluate L929 cell viability and proliferation on all the samples and confirmed that cell behavior was not affected by pore morphology or agarose coating. In summary, samples produced by the use of the pore-forming agent showed higher potential to be applied as bone scaffolds in tissue engineering applications.     

Three-dimensional ordered mesoporous cobalt oxides: Highly active catalysts for the oxidation of toluene and methanol

Three-dimensional (3D) ordered mesoporous cubic Co₃O₄ (denoted as Co-KIT6 and Co-SBA16) were fabricated adopting the KIT-6- and SBA-16-templating strategies, respectively. It is shown that Co-KIT6 and Co-SBA16 possessed large surface areas (118–121 m²/g), high oxygen adspecies concentrations, and good low-temperature reducibility. Over Co-KIT6 at space velocity = 20,000 mL/(g h), 90% toluene and methanol conversions were achieved at 180 and 139 °C, respectively. The excellent catalytic performance of Co-KIT6 and Co-SBA16 was associated with their larger surface areas, higher oxygen adspecies concentrations, better low-temperature reducibility, and 3D ordered mesoporous structure.     

A Monte Carlo pore network for the simulation of porous characteristics of functionalized silica: pore size distribution, connectivity distribution and mean tortuosities

The simulation of the random porous network of five samples of functionalized SiO₂ took place using a dual-site-bond model (DSBM) and Monte Carlo techniques for achieving the proper arrangement of the pores into the system. The simulation took place in 7×7×7 lattice. As a starting point of simulation the adsorption branch of N₂ isotherm was considered from which the pore size distribution was estimated. Then as a benchmark of simulation the N₂ desorption branch was considered whose fitting was achieved using the Monte Carlo technique for selecting the proper place of each pore into the 7×7×7 lattice. From the obtained model, it was possible to estimate the distribution of pore connectivities of each system. The mean value of those connectivity distributions tallies with the corresponding mean values estimated using the standard methodology of Seaton. In addition, the mean tortuosity of the porous network was estimated and the results were favorably compared with values of tortuosity estimated recently via the so-called corrugated-pore-structure-model (CPSM) for the same solids. The degree of functionalization of the parent SiO₂ affects both connectivity and tortuosity in a linear way. Some discrepancies observed between the results obtained via this methodology and the ones obtained using the Seaton or the CPSM model are discussed.     

A nanoscale model for characterizing the complex pore structure of biochars

The development of a novel nanoscale model that can accurately describe the reactivity of solids consisting of multiple components and having ordered and random pores is presented. Domains of multiple solid phases are distributed on a computational grid to simulate reactants with different specific reactivities and dispersions. Sub‐nanometer slit pores and larger cylindrical pores with given size distributions are also distributed on the grid in regular and random arrangements respectively. The generated solids are then eroded using rules that simulate a gas‐solid, non‐catalytic reaction occurring in the kinetic control regime. A parametric study is first carried out to demonstrate how key pore structural parameters affect the reactivity patterns. Model predictions are found to be in excellent agreement with experimental thermogravimetric data for the combustion of biochars, both when the slit and random cylindrical pores are fully accessible to the reactant and when diffusional limitations appear in the smaller slit pores. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3412–3420, 2013     

Numerical study of pore structure effect on SO2-CaO reactions☆

The effect of varying pore structures on the kinetics of SO2–CaO reactions is not fully understood in the previous studies. Combining fractal pore model, gas molecular movement model and two-stage reaction model, a new desulfurization model is established in this paper. Fractal pore model is used to simulate CaO particle and gas molecular movement model is used to simulate gas diffusion in pores. Fractal dimension is used to characterize complexity of pore structure instead of tortuosity factor. It is found that the reaction is significantly affected by pore structures. A modulus?is introduced to characterize the relationship between varying pore structures and apparent reaction parameters. And this relationship is verified by thermo-gravimetric analysis (TGA) data. Comparing to the previous models, the effect of varying pore structure on the kinetics of the reaction is described more accurately by the desulfurization model.     

A study of the pyrolysis of toluene and phenols at low pressure

Pyrolyses of o-cresol, 2,4-xylenol and toluene have been carried out at 10⁻⁶ torr.In addition to the products previously obtained in pyrolysis at atmospheric pressure and under high pressure, unexpected compounds, such as ketones from phenols and hydrogenated products from toluene, have been obtained. Possible reaction mechanisms are discussed.     

Use of enzyme tests to monitor the biomass activity of a trickling biofilter treating domestic wastewaters

Easy enzyme tests were used to monitor the biomass activity of a trickling biological aerated filter processing a domestic influent. Biofilter wash‐waters were used as biomass source. Enzyme tests (hydrolases and dehydrogenases) carried out on wash‐waters showed relationships with the process active biomass (estimated by volatile suspended solids). Differences in dehydrogenase (DHA) specific activities were observed in two sampling campaigns and were linked to process performance. The DHA activity evaluated using glucose or acetate was also related to the substrate mass applied on the biological aerated filter (kg CODt m⁻³ biolite). These results indicated that, under normal operation, DHA activity is related to quantity of substrate applied to the biofilter. Similar relationships were obtained for hydrolases. However, β‐glucuronidase, Leu‐aminopeptidase and protease, expressed specifically, were more significantly inversely related to process removal performance. This reaction was probably caused by the biomass reacting with an influent that is difficult to biodegrade. Generally speaking, these tests can be easily applied to the regular monitoring of the active biomass from a process using biological filters or simply as an indicator of the active biomass content in the process. © 2000 Society of Chemical Industry