Transient Mass Balance Research Articles

Residence time distribution in the chromatographic column: Applications in the separation engineering of DNA

Experimental and theoretical works were performed for the separation of large polyelectrolyte, such as DNA, in a column packed with gel particles under the influence of an electric field. Since DNA quickly orient in the field direction through the pores, this paper presents how intraparticle convection affects the residence time distribution of DNAs in the column. The concept is further illustrated with examples from solid-liquid systems, for example, from chromatography showing how the column efficiency is improved by the use of an electric field. Dimensionless transient mass balance equations were derived, taking into consideration both diffusion and electrophoretic convection. The separation criteria are theoretically studied using two different Peclet numbers in the fluid and solid phases. These criteria were experimentally verified using two different DNAs via electrophoretic mobility measurements, which showed how the separation position of the DNAs varies in the column in relation to the Peg/Pef values of an individual DNA. The residence time distribution was solved by an operator theory and the characteristic method to yield the column response.

Transport modelling of a polyelectrolyte in membrane chromatography under an electric field

Electrophoretic separations were theoretically performed for large polyelectrolytes such as DNA in the membrane chromatography. This paper deals with the study of the mass transfer involved in multi-layered membrane chromatography in the presence of an electric field. The different contributions to mass transfer were considered in order to establish a relationship providing intramembrane convection effects as a function of an electric field since a polyelectrolyte is quickly oriented through pores in the field direction. Dimensionless transient mass balance equations were derived considering diffusion and electrophoretic convection, and the mass transfer was theoretically studied using two different Peclet numbers in the fluid and solid phases, showing how the concentration profiles of polyelectrolyte inside a membrane varies in the membrane according to values of Peclet number of an individual polyelectrolyte. Governing equations were solved by an operator theory.

Indoor Air Quality Assessment in an Underground Parking Facility

This paper presents an indoor air quality (IAQ) assessment in a representative underground parking facility (UPF) using carbon monoxide and volatile organic compounds as IAQ indicators. For this purpose, concentrations of both pollutants were measured at different time intervals throughout the day. Vehicular emission factors (EFs) were estimated using the EMFAC7F1.1 model. A transient mass balance model was then applied to construct concentration profiles. The ventilation rates required to maintain pollutant concentrations within acceptable standards were estimated under maximum UPF occupancy, and a sensitivity analysis was conducted to evaluate the effect of changes in EMFAC7F1.1 input parameters on EFs and ventilation rates.

Separation of DNA by mass-transfer properties in the presence of an electric field

Experimental and theoretical works were performed for the separation of large polyelectrolyte such as DNA in the column packed with gel particles under influence of an electric field. Since DNA quickly orients through the pores in the field direction, this paper presents how intraparticle convection affects the separation of DNAs in the column. Dimensionless transient mass balance equations were derived considering diffusion and electrophoretic convection. The separation criteria are theoretically studied using two different Peclet numbers in the fluid and solid phases and these criteria were verified using two different DNAs by electrophoretic mobilities measured experimentally, showing how the separation position of DNAs varies in the column according to values of Pe f/ Pe g of individual DNA. Governing equations are solved by an operator theory and the characteristic method to yield the column response.

Indoor Air Quality Assessment in an Underground Parking Facility

This paper presents an indoor air quality (IAQ) assess ment in a representative underground parking facility (UPF) using carbon monoxide and volatile organic com pounds as IAQ indicators. For this purpose, concentra tions of both pollutants were measured at different time intervals throughout the day. Vehicular emission factors (EFs) were estimated using the EMFAC7F1.1 model. A transient mass balance model was then applied to con struct concentration profiles. The ventilation rates re quired to maintain pollutant concentrations within ac ceptable standards were estimated under maximum UPF occupancy, and a sensitivity analysis was con ducted to evaluate the effect of changes in EMFAC7F1.1 input parameters on EFs and ventilation rates.

Carbon Monoxide and Volatile Organic Compounds as Indicators of Indoor Air Quality in Underground Parking Facilities

The provision of underground parking facilities (UPFs) has become a necessity particularly in densely populated urban areas and business districts. Indoor air quality (IAQ) in such facilities is of great concern due to vehicle-induced emissions. This paper presents an IAQ assessment at six representative UPFs in Beirut using carbon monoxide and volatile organic compounds as IAQ indicators. For this purpose, concentrations of both pollutants were measured at different time intervals throughout the day. Vehicular emission factors (EFs) were estimated using the EMFAC7F1.1 model. A transient mass balance model was then applied to construct concentration profiles. The ventilation rates required to maintain pollutant concentrations within acceptable standards were estimated under maximum UPF occupancy, and a sensitivity analysis was conducted to evaluate the effect of changes in EMFAC7F1.1 input parameters on EFs and ventilation rates.

Carbon Monoxide and Volatile Organic Compounds as Indicators of Indoor Air Quality in Underground Parking Facilities

The provision of underground parking facilities (UPFs) has become a necessity particularly in densely popu lated urban areas and business districts. Indoor air quali ty (IAQ) in such facilities is of great concern due to vehi cle-induced emissions. This paper presents an IAQ as sessment at six representative UPFs in Beirut using car bon monoxide and volatile organic compounds as IAQ indicators. For this purpose, concentrations of both pol lutants were measured at different time intervals throughout the day. Vehicular emission factors (EFs) were estimated using the EMFAC7F1.1 model. A tran sient mass balance model was then applied to construct concentration profiles. The ventilation rates required to maintain pollutant concentrations within acceptable standards were estimated under maximum UPF occu pancy, and a sensitivity analysis was conducted to evalu ate the effect of changes in EMFAC7F1.1 input parame ters on EFs and ventilation rates.

Large Scale Experiment on Transport of Trichloroethylene in a Controlled Aquifer

Pollution by dense non-aqueous phase liquids (DNAPLs) represents a major threat to groundwater resources. In a real case of site contamination, the efficiency of remediation techniques is often limited by a lack of knowledge of both the extent of the pollution and the behavior of the different phases of the pollutant in the subsurface. An experiment simulating pollution of an aquifer by a chlorinated solvent (Trichloroethylene: TCE) was conducted on a large controlled experimental site called SCERES. The experiment consisted of an injection of 8.9 liters of TCE under controlled conditions at 35cm below the soil surface with an appropriate set up. The goal was to study the behavior of the three phases of the pollutant (trapped TCE phase forming the impregnation body, vapors in the vadose zone, and dissolved traces in the aquifer) in order to better comprehend the mechanisms which govern the propagation and the transfer of this type of pollution underground. The SCERES experimental data indicate that mass transfer from the saturated zone to the vadose zone is important, affecting the repartition of the vapor plume and causing a significant decrease of dissolved TCE concentrations in the groundwater. Furthermore, vertical leaching of TCE vapors due to rainfall strongly influences the degree of groundwater pollution and its lateral extent. The transient mass balance of the experiment is very satisfactory and shows that the main part of the spilled quantity is lost to the atmosphere.

Separation of ϕX HAE III DNA with electrochromatography

Experimental and theoretical works were performed for the separation of large polyelectrolytes such as DNA in the column packed with gel particles under an electric field. This paper shows how intraparticle convection effects the separation of DNAs in the column because DNAs quickly oriented through the pores in the field direction. Dimensionless transient mass balance equations were derived considering diffusion and electrophoretic convection. The separation criteria is theoretically studied using two different Peclet numbers in the fluid and solid phases and these criteria were verified uing two different DNAs by electrophoretic mobilities measured experimentally, showing how the separation position of DNAs varies in the column according to values ofPe f/Peg of individual DNA. Governing equations are simultaneously solved by operator theoretic and characteristic methods to yield the column response.

On the active surface layer in CF3+ etching of Si: Atomistic simulation and a simple mass balance model

Molecular dynamics (MD) simulations of CF3+ ion bombardment of Si predict the formation of a steady-state fluorocarbosilyl mixing layer that actively participates in the etching of the underlying Si. The active nature of this mixing layer has been characterized by computing atomic residence time distributions (RTDs) for adsorbed fluorine and carbon. The average residence time of carbon in the layers is seen to increase dramatically as ion energy increases, while that of fluorine is not sensitive to ion energy. The overall RTDs compare well with those of an ideal stirred tank. A simple “well-mixed” transient mass balance model is presented. The phenomenology of this model is based on interpretations of the MD results. The model correctly predicts the evolution of atomic concentrations in the mixing layer. Both the MD and model results shed new light on how CF3+ ions etch Si.

Finite element simulation of steady state and transient forced convection in superfluid helium

In this paper, the solution of transient mass, momentum and energy balances in superfluid helium are discussed by means of a finite element algorithm. A simple linearization procedure is used for the non-linear pseudo-diffusion term in the energy balance arising because of the unique counterflow heat transport mechanism in superfluid helium. The linearization algorithm is analyzed for accuracy order and stability. The reliability of the algorithm devised is shown in practical tests, comparing the numerical solutions with experimental data available in the literature. Copyright © 1999 John Wiley & Sons, Ltd.

Investigation and Modeling of Surface Sorption/Desorption Behavior of Volatile Organic Compounds for Indoor Air Quality Analysis

Volatile organic compound contents of 20 Kuwaiti houses were measured by pumped sampling versus time and 17 predominant compounds (namely, benzene, toluene, xylene, styrene, trimethylbenzene, butylbenzene, propylbenzene, dichloroethylene, trichloroethane, dichloropropane, dibromomethane, chloroform, tetrachloroethane, tetrachloroethylene, chlorotoluene, dichlorobenzene and fluorobenzene) have been identified. Other related factors, such as the outdoor concentrations, the surface areas of the walls and the furnishings, and the previously measured air exchange rates were incorporated in a transient mass balance model which also took into consideration the phenomenon surface sorption and the source emission. An integrated form of this model was used to analyze the experimental time-averaged concentration data. A plot of indoor concentration versus outdoor concentration for a specific VOC gave a straight line from which the removal rate by adsorption and the source emission were obtained. It was found that the data correlated satisfactorily if the surface area for sorption was estimated based not only on the surfaces of walls, ceiling and floor (an approach often used by other workers in previous studies) but also those of all components of interior, such as those of furniture. The surface sorption rate constants for each VOC under study were evaluated and they varied between 0.2 to 0.96 m hr−1 depending on the chemical nature of the VOCs. These results showed also that the removal rates of aliphatic and aromatic VOCs by adsorption increased if they were attached to halogen and alkyl groups respectively. On the other hand, relatively low removal rates were observed when unsaturated alkyl and phenyl groups were attached to a halogen.

Investigation and Modeling of Surface Sorption/Desorption Behavior of Volatile Organic Compounds for Indoor Air Quality Analysis

Volatile organic compound contents of 20 Kuwaiti houses were measured by pumped sampling versus time and 17 predominant compounds (namely, benzene, toluene, xylene, styrene, trimethylbenzene, butylbenzene, propylbenzene, dichloroethylene, trichloroethane, dichloropropane, dibromomethane, chloroform, tetrachloroethane, tetrachloroethylene, chlorotoluene, dichlorobenzene and fluorobenzene) have been identified. Other related factors, such as the outdoor concentrations, the surface areas of the walls and the furnishings, and the previously measured air exchange rates were incorporated in a transient mass balance model which also took into consideration the phenomenon surface sorption and the source emission. An integrated form of this model was used to analyze the experimental time-averaged concentration data. A plot of indoor concentration versus outdoor concentration for a specific VOC gave a straight line from which the removal rate by adsorption and the source emission were obtained. It was found that the data correlated satisfactorily if the surface area for sorption was estimated based not only on the surfaces of walls, ceiling and floor (an approach often used by other workers in previous studies) but also those of all components of interior, such as those of furniture. The surface sorption rate constants for each VOC under study were evaluated and they varied between 0.2 to 0.96 m hr−1 depending on the chemical nature of the VOCs. These results showed also that the removal rates of aliphatic and aromatic VOCs by adsorption increased if they were attached to halogen and alkyl groups respectively. On the other hand, relatively low removal rates were observed when unsaturated alkyl and phenyl groups were attached to a halogen.

Dynamic behaviour of a fixed-bed biofilm reactor: analysis of the role of the intraparticle convective flow under biofilm growth

The influence of the intraparticle convective flow on the dynamic response of a fixed-bed biofilm reactor is evaluated theoretically for a stepwise increase in substrate concentration in the feed line. Dimensionless transient mass balance equations for the substrate within the bulk liquid, biofilm and support phases are derived considering a substrate inhibition kinetic model (Haldane equation) and biofilm growth. Governing equations are simultaneously solved for phenol biodegradation by finite element collocation methods to yield the bed response, in terms of dimensionless phenol concentration at outlet against a reduced time (breakthrough curves), for two different bulk flow rates. The results reported in this study, for different intraparticle velocities, show that the major beneficial effect of the intraparticle convective flow on the production of a good effluent quality is expressed by the occurrence of later breakthrough curves, due to the improvement of intraparticle mass transfer by convection, particularly when higher diffusional limitations are present inside the biofilm and support. Thus, bioreactors must be operated under such conditions that liquid flow occurs within biofilms.

Mathematical model of sugar dehydration during storage in a laboratory silo

In the industrial processing of sugar beet, after drying and cooling, the sugar crystals are still likely to contain water. During storage in silos, the bound water on the crystal surface is changed into free water that enhances caking and the formation of lumps. Preventing caking requires a good knowledge of mass and heat transfer phenomena. To obtain this information, a laboratory silo was built and equipped out with thermohygrometers. Moreover, a model based on transient mass and heat balances was conceived which correlated well with experimental results (i.e. the temperature and humidity of air and the moisture content of sugar). Identification of certain parameters such as the mass transfer coefficient for sugar dehydration was thus possible.

Mathematical modelling of fluidized bed combustion: 1. Combustion of carbon in bubbling beds

A three-phase model for carbon combustion in fluidized beds has been developed to take into account the effects of gas backmixing and solids circulation. Inclusion of these phenomena is expected ultimately to be important in extending the fluidized bed combustion model to predict N 2O emissions. Transient mass and energy balance equations are presented in this paper for the fast bubble regime; these equations can be easily modified for the slow bubble operating regime. The system of nine first-order semilinear hyperbolic partial differential equations has been solved numerically. The transient solution permits comparison of the predictive capability of the model with published data on combustion rates, burnout times and particle temperatures obtained from batch experiments. The comparisons reveal that the agreement with data on combustion rates and burnout times is in most cases within ∼30%. In addition, these comparisons show that improvement in the predictive capabilities of the model depends on ability to predict the product CO CO 2 ratio.

Photocatalyzed Oxidation of Ethanol and Acetaldehyde in Humidified Air

Photocatalyzed oxidation of ethanol and acetaldehyde in humidified air was carried out to establish a first complete kinetic model for a photocatalyzed multispecies network. Two photocatalysts were examined in a batch, recirculation reactor, near-UV illuminated TiO2(anatase) coated (i) on the surface of a nonporous quartz glass plate and (ii) on a porous ceramic honeycomb monolith. The former contained only illuminated (active) surfaces, the latter consisted of substantial “dark” surfaces coated with a thin layer of illuminated (active) catalyst. Ethanol was photooxidized to acetaldehyde and formaldehyde intermediates, and eventually to carbon dioxide and water products. The catalyst and monolith surfaces adsorbed appreciable fractions of the trace ethanol, acetaldehyde, formaldehyde, carbon dioxide, and water present. Ethanol, acetaldehyde, and carbon dioxide adsorption isotherms were measured on both catalysts; the formaldehyde adsorption isotherms were assumed identical to those of acetaldehyde. On the fully illuminated glass plate reactor, all four species were accounted for, and closure of a transient carbon mass balance was demonstrated. Completion of a transient carbon balance on the monolith reactor required inclusion of additional reaction intermediates (acetic and formic acids), which appear to reversibly accumulate on only the dark surfaces. The ethanol and acetaldehyde photocatalyzed oxidation kinetic networks were modeled using Langmuir–Hinshelwood rate forms combined with adsorption isotherms for reactant, intermediates, and product CO2. For both the quartz plate and monolith catalysts, satisfactory kinetic models were developed to predict the entire time course of ethanol and acetaldehyde multicomponent batch conversions.

Transient Response of Multilayer Electroenzymic Biosensors

A numerical model is used to analyze the transient response of amperometric enzyme sensors. Different biosensor types are considered according to whether the enzyme is fixed on the inner or the outer surface or inside the membrane itself. The numerical simulation is performed with a finite volume method which makes it possible to easily solve the transient mass balance equations in all the juxtaposed layers of the biosensors. Phenomena induced by the transient mass transfers in the different layers are emphasized partially by analyzing the effects of the stirrin strength and the assay protocol on the shape of the current-time curves. This makes it possible to suggest theoretically significant opportunities to improve the reliability and performance of biosensors. The peak current, itch appears in some cases when stirring is decreased, offers a measure that is faster and less sensitive to the operating conditions than the steady-state value. After determination of a maximum number of physicochemical parameters by independent experiments, the model satisfactorily by experimental results performed with free enzyme and immobilized enzyme sensors

Dynamic operation of a three-phase upflow reactor for the hydrogenation of phenylacetylene

The selective catalytic hydrogenation of phenylacetylene to styrene presents an attractive and interesting process both from scientific and industrial viewpoints. The process mechanism consists of two consecutive reaction steps in which a high selectivity towards the intermediate product is desired. There have been many papers published so far, that report an increase in the selectivity of adsorption-desorption-based gas-phase processes performed on solid catalysts, due to a periodic (pulsing) operation of the reactor system. Pulsing the gas feed is believed to create more preferential adsorption conditions for the required reactant (transient conditions). This paper presents the results of a theoretical and experimental study concerning the influence of forced periodic oscillations of the gas-phase component on the performance of a three-phase reactor. The theoretical results are based on a heterogeneous mathematical reactor model considering both the transient mass balance over the reactor length as well as accumulation inside the catalyst pellets. Numerical dynamic reactor simulations with a simplified model neglecting accumulation of components within the catalyst pellet showed little improvement in conversion and selectivity when compared to results of the steady-state model. These results were supported by dynamic experiments in a bench scale reactor. It is shown that due to mass transfer limitations, the original square wave shape of the oscillation in the gas-phase concentration was almost completely faded into a smooth and rather flat oscillation at the catalyst surface. Simulations with a model in which accumulation of the relevant components inside the particles was considered showed no further improvement in performance at transient conditions, but demonstrated the impact of accumulation capacity of the pellets on the shape of oscillation present at the catalyst surface. The study is a result of direct cooperation between Politecnico di Torino, where the modelling and calculations were performed, and DSM Research. Its aim was to offer an attractive alternative to the conventional approach.

Prediction of an iron oxide concentration in the induction smelting process

Induction smelting process, ISP, is a direct smelting process in which self‐fluxed composite pellets (made of iron ore, brown coal and lime) are melted in a single phase induction furnace in conjunction with top blown oxygen. A mass transfer model to predict the concentration of iron oxide, has been developed for this process. Transient mass balance equation with complex boundary conditions has been worked out by adopting a relatively new approach known as the cell model approach. These equations have been solved numerically using finite difference technique to obtain the iron oxide concentration profile in the induction smelting process. Numerical and experimental results obtained for the iron oxide concentration show a reasonable agreement with each other.