Dry Building Materials Research Articles

Modeling VOCs emission/sorption with variable operating parameters and general boundary conditions.

An integrated model of VOCs emission/sorption from/on dry building materials with a general boundary condition, variable air exchange rate and inlet concentration is developed. An analytical solution is obtained by using the generalized integral transform technique. Good agreements are obtained between the present model and the experimental data. The effects of environmental conditions on the emission are investigated. The emission from two surfaces can increase the concentration of hexanal in the air and decrease the initial emission rate at x=δ with the increase in mass transfer coefficient at x=0. Periodical inlet concentration can lead to the periodic variation of materials between a source and a sink. Ventilation can keep the concentration in the air at a low level and help to decrease the concentration of hexanal in materials. The present model is capable of simulating indoor air quality due to the VOCs emission and sorption.

A numerical model predicting indoor volatile organic compound Volatile Organic Compounds emissions from multiple building materials.

There have been many studies on the model of volatile organic compound (VOC) emissions from individual dry building material and have been validated in the chamber. Actually, VOC emitted from multiple dry building materials simultaneously indoor. The concentration of VOC indoor increases and will inhibit the VOC emission of dry building materials indoor. This paper developed a new model predicting indoor VOC concentrations caused by simultaneous emissions from multiple dry building materials, with a consideration of impact from dynamic VOC concentrations on the emission rate. The model has been used to predict the VOC emissions from a combination of medium-density fiberboard (MDF) and consolidated compound floor (CCF) simultaneously. The study demonstrated a good prediction performance of the newly proposed model, against field experimental data. The study also showed that when multiple dry building materials emit pollutants in a common space, a mutual inhibition effect could be observed. Furthermore, when multiple dry building materials emit VOC simultaneously, the change of VOC concentrations in the air followed the trends of VOC emissions from building materials with higher initial concentration (C0), diffusion coefficient (Dm), and the partition coefficient (Kma).

A spatial fractional diffusion model for predicting the characteristics of VOCs emission in porous dry building material

Volatile organic compounds (VOCs) concentrations of dry building materials can significantly affect the quality of indoor air. In this paper, a spatial fractional diffusion model is established by introducing fractional Fick’s law, based on the fact that the pore structure of porous building materials has a strongly impact on VOCs diffusion. Furthermore, the areal porosity of the material is drawn into the convection mass transfer equation. The relevant parameters are estimated by a kind of optimization algorithm. The fractional nonlinear equations are tackled by the finite difference method combined with L2-algorithm. Results indicate that spatial fractional diffusion model agrees better with the experimental data than Deng and Kim’s model. The spatial fractional diffusion model is used to describe the formaldehyde concentration in the particleboard according to the experimental data. Taking formaldehyde as a common substance, the influences of physical parameters on VOCs diffusion are predicted by this model. The results of numerical simulation demonstrate that the diffusion is consistent with the actual situation.

The effects of temperature and humidity on the VOC emission rate from dry building materials

With the development of the economy and society, a wide variety of new building decoration and materials are widely used indoors. The concentration of pollutants such as formaldehyde, volatile organic compounds, benzene and ammonia in indoor air witnessed increase, causing increasingly serious indoor air pollution problems. Indoor building decoration materials are one of the main sources of indoor VOCs. In this paper, the emission law of VOCs in dry building materials with environmental conditions is studied. The environmental test cabin is used to test the formaldehyde and TVOC concentration released. The single factor variable method is used to study how different environmental conditions (temperature and relative humidity) affect the rate of formaldehyde and TVOC released from dry building materials. According to the results, increase of ambient temperature and relative humidity will promote the release of TVOC, which will affect its peak concentration and stable concentration, and the effect on formaldehyde release is more significant. This study clarified the release law of VOCs in artificial boards under different environmental conditions, which is of great significance for reducing indoor air pollutant levels and improving indoor air quality.

A fractional mass transfer model for simulating VOC emissions from porous, dry building material

Dry building material is a complex porous medium with a fractal structure. A developed fractional mass transfer model can accurately simulate anomalous diffusion in complex media, such as dry building material. In this work, a new time and space fractional diffusion model is first established to study the transport and emission behaviors of volatile organic compounds (VOCs) in a dry building material in an environmental chamber. In this model, the space fractional derivative is expressed by the weight coefficients of the upward and downward transfer probabilities, and the time fractional derivative and areal porosity of the material are introduced to describe anomalous diffusion. With the newly developed finite difference and parameter estimation methods, the present model can describe the early breakthrough and heavy-tailed phenomena well. Furthermore, the results of the present model show good agreement with the experimental data, as the error value (67.44 μg/m3) is much less than that of Deng and Kim's model (190 μg/m3). The advantages of long-term prediction are shown. Moreover, the effects of key parameters on VOC transport are revealed.

Influence of Precision of Emission Characteristic Parameters on Model Prediction Error of VOCs/Formaldehyde from Dry Building Material

Mass transfer models are useful in predicting the emissions of volatile organic compounds (VOCs) and formaldehyde from building materials in indoor environments. They are also useful for human exposure evaluation and in sustainable building design. The measurement errors in the emission characteristic parameters in these mass transfer models, i.e., the initial emittable concentration (C 0), the diffusion coefficient (D), and the partition coefficient (K), can result in errors in predicting indoor VOC and formaldehyde concentrations. These errors have not yet been quantitatively well analyzed in the literature. This paper addresses this by using modelling to assess these errors for some typical building conditions. The error in C 0, as measured in environmental chambers and applied to a reference living room in Beijing, has the largest influence on the model prediction error in indoor VOC and formaldehyde concentration, while the error in K has the least effect. A correlation between the errors in D, K, and C 0 and the error in the indoor VOC and formaldehyde concentration prediction is then derived for engineering applications. In addition, the influence of temperature on the model prediction of emissions is investigated. It shows the impact of temperature fluctuations on the prediction errors in indoor VOC and formaldehyde concentrations to be less than 7% at 23±0.5°C and less than 30% at 23±2°C.

On the determination of VOCs emission factors in dry building materials

A comprehensive modelling study was performed to determine volatile organic compounds (VOCs) emission factors in dry building materials. In order to examine the emission characteristics of VOCs in dry building materials (carpet), an experiment was conducted, and a mathematical methodology was considered together with a numerical work for a validation. Here, a new representative airside VOCs concentration was considered in a mode different from that of previous studies, which allows us to evaluate VOCs emission factors more accurately.

Application and limitations of a mass transfer VOC emission model for a dry building material

Volatile organic compound (VOC) emission from building materials into air has been quantified, characterized and modeled. Internal diffusion of VOC through a material based on Fick's law of diffusion is the basis for mass transfer modeling of diffusive emission used to estimate VOC concentrations in air over time. Current mass transfer models have been shown to appropriately estimate air VOC concentrations at approximate room temperature, while other research has shown that temperature has a profound effect on the diffusion coefficient, D, of VOC in a material. Here, a mass transfer model is operated at 23 °C and 40 °C using input parameters applicable for each temperature. The model estimates are validated against environmental test chamber data for styrene emission from a vinyl ester resin thermoset composite material. The model correlates well with the 23 °C chamber data, but underestimates chamber data by as much as 10−4 at 264 h for the 40 °C modeling. This suggests that the model requires adjustment for predicting VOC air concentrations at temperatures other than 23 °C.

Modeling volatile organic compounds sorption on dry building materials using double-exponential model

A double-exponential surface sink model for VOCs sorption on building materials is presented. Here, the diffusion of VOCs in the material is neglected and the material is viewed as a surface sink. The VOCs concentration in the air adjacent to the material surface is introduced and assumed to always maintain equilibrium with the materialphase concentration. It is assumed that the sorption can be described by mass transfer between the room air and the air adjacent to the material surface. The mass transfer coefficient is evaluated from the empirical correlation, and the equilibrium constant can be obtained by linear fitting to the experimental data. The present model is validated through experiments in small and large test chambers. The predicted results accord well with the experimental data in both the adsorption stage and desorption stage. The model avoids the ambiguity of model constants found in other surface sink models and is easy to scale up.

A Study on Dynamic Volatile Organic Compound Emission Characterization of Water-Based Paints

ABSTRACT Volatile organic compounds (VOCs) emitted from surface coatings have caused growing public concern for air quality. Even the low-emitted VOC impact from water-based paints on indoor air quality in urban areas has caused concern. This paper presents experimental data using a mathematical model to simulate dynamic VOC emissions from water-based paints that is based on mass transfer and molecular diffusion theories. A series of field-analogous experiments were carried out to continuously measure the VOCs emitted from two typical water-based paints using a gas chromatography–flame-ionization detector monitor in an artificial wind tunnel system. In the study cases, the mass flux of VOCs emitted from the water-based paints was up to 50 μg/m2sec. It was found that the time needed to completely emit VOCs from water-based paints is just hundreds of seconds. However, the order of magnitude of the VOC emission rate from water-based paints is not lower than that from some dry building materials and solvent-based paints. The experimental data were used to produce a useful semiempirical correlation to estimate the VOC emission rates for water-based paints. This correlation is valid under appropriate conditions as suggested by this work with a statistical deviation of ±7.6%. With this correlation, it seems feasible to predict the dynamic emission rates for VOCs during a painting process. This correlation is applicable for assessing the hazardous air pollutant impact on indoor air quality or for environmental risk assessment. Associated with the dynamic VOC emission characterization, the air-exchange rate effect on the VOC emission rates is also discussed. IMPLICATIONS This paper presents experimental data using a mathematical model to simulate dynamic VOC emissions from water-based paints that is based on mass transfer and molecular diffusion theories. A semiempirical correlation is suggested to estimate the VOC emission rates for water-based paints. With this correlation, it is feasible to interpret and predict the dynamic emission rates for VOCs during a painting process. The results obtained in this work will be useful in evaluating the indoor air quality impact by primary VOCs or hazardous air pollutants emitted from water-based paints.

General analytical mass transfer model for VOC emissions from multi-layer dry building materials with internal chemical reactions

A general mass transfer based model was developed for analyzing volatile organic compound (VOC) emissions from dry multi-layer building materials with two emission surfaces. This model adds to an earlier multi-layer model by considering chemical reactions within the materials. Consequently, it can be used to analyze the effect of these chemical reactions on removing VOCs, and for characterizing secondary VOC emissions from the building material. The model was validated with literature data and our experimental results. Some typical secondary emissions were analyzed using this model, and obviously differed from the primary emissions. The model is a useful tool for predicting, analyzing and “designing” the VOC emission characteristics, including secondary emissions, of building materials.

Numerical modeling of volatile organic compound emissions from multi-layer dry building materials

This paper presents a mathematical model capable of simulating VOCs emission from a multi-layer material. The analytical solutions of the concentration in the air and the emission rate are obtained by Laplace transform and are validated through experiment. The influences of parameters of multi-layer material on the emission of VOCs are investigated in detail. Results show that the inner layer may act as a sink or a source of the top layer, depending on their initial concentrations and partition coefficients. For the case of the inner layer being a source of the top layer, the top layer becomes a barrier layer, reducing the emission rate of VOCs from the source. A low diffusion coefficient and a large thickness may promote the effect of the barrier on the emission rate, which helps to maintain better air quality in an indoor environment. The present solution is a longitudinal extension of a single layer problem, while the emission from multi-source is a transverse extension of a single layer problem.

A multi-phase model for VOC emission from single-layer dry building materials

A multi-phase model for the emission of VOC from dry building materials is developed. Dry building materials are viewed as porous media. A general adsorption isotherm is used to construct the concentration equation in the porous media. The boundary conditions at the material-air interface are presented for both CFD model and one compartment model. With the use of Henry’s law for the adsorption isotherm, an analytical solution is obtained and further is validated with the comparison of the experiment performed by Yang et al. [1], yielding a relatively good agreement. The effects of the model parameters on the emission are investigated in detail. Increasing the effective diffusion coefficient and the partition coefficient tends to promote the emission and increase the peak value of the concentration in the air. The effect of the porosity depends on the degree of the dependence of the effective diffusion coefficient on the porosity. When a weak dependence exists, the increase of the porosity tends to suppress the emission and decrease the peak value of the concentration in the air. However, when a strong dependence exists, the increase of the porosity tends to promote the emission and increase the peak value of the concentration in the air.

A New Method for Determining the Initial Mobile Formaldehyde Concentrations, Partition Coefficients, and Diffusion Coefficients of Dry Building Materials

The initial mobile formaldehyde concentration, C m,o; the partition coefficient, K; and the diffusion coefficient, D, of a dry building material are key parameters to characterize formaldehyde emissions from the building material. The solvent extraction method and direct thermal desorption method can overestimate C m,o because of high temperature. A new method has been developed to determine C m,o under similar conditions to common indoor environment, together with K and D. In the proposed method, the tested materials are placed in an airtight environmental chamber for which the temperature can be controlled by a water bath, then the materials undergo a multisorption/emission process and the instantaneous formalde-hyde concentration in the chamber is recorded. The K and C m,o are determined from the equilibrium concentrations after every sorption by means of the linear least-square regression, and D is obtained by fitting the concentration at the emission stage into a mass-transfer-based model in the literature. Four kinds of wooden medium-density boards are tested. The C m,o measured using this method is the mobile formaldehyde concentration in the material, which differs significantly from the total formaldehyde concentration in the material measured by using the traditional method recommended by the Chinese standard (GB/T 17657-1999) extraction method. This means that the mobile formaldehyde takes only a small portion of the total quantity in the tested material. The K, D, and C m,o values measured using this new method are used to predict formaldehyde concentrations for sorption processes. The results agree well with experimental data. In addition, some factors influencing the accuracy are analyzed.

An improved extraction method to determine the initial emittable concentration and the partition coefficient of VOCs in dry building materials

Based on the extraction method presented by Smith et al. (2008), this paper proposes an improved method, the multi-emission/flush regression method, to simultaneously determine the initial emittable concentration and the partition coefficient. Compared to the extraction method, the proposed method has the following advantages: (1) it is unnecessary for the target volatile organic compounds (VOCs) to emit completely from the material, thus greatly reducing experimental time; (2) it provides a simpler way to obtain the partition coefficients of VOCs for tested materials and can avoid the measurement uncertainties at low VOC concentrations which often occur during the last few cycles of the extraction method; (3) it does not require grinding the building material into powders thus making this method more convenient to use. Comparisons were made between the initial emittable VOC concentrations determined by the original extraction method and the proposed method. Results show good agreements between these two methods. To further validate the proposed method, the type of static chamber developed by Wang et al. (2006) was used to conduct the experiment for a type of medium density board, and formaldehyde was selected as the target compound. Based on the initial emittable concentration and partition coefficient obtained using the proposed method, and the diffusion coefficient obtained by the mercury intruding porosimetry, the chamber formaldehyde concentration was predicted and compared with the experimental measurements. Results show that the predicted chamber VOC concentration using the measured parameters agree well with the experimental data.

Correlation between the solid/air partition coefficient and liquid molar volume for VOCs in building materials

A correlation between partition coefficients, K, of dry building material and liquid molar volume of volatile organic compounds (VOCs) in it, v l, is presented. Experiment for determining the partition coefficients of the target VOCs including benzene, toluene, ethylbenzene for two kinds of building materials was conducted by a method developed by us. The experimental data verified the correlation. Also the correlation was supported well by the experimental data in the literature. It is found that the correlation can predict the partition coefficients of VOCs with similar structure in the same building material.

Influence of temperature on formaldehyde emission parameters of dry building materials

The diffusion coefficient, D, partition coefficient, K, and the initial volatile organic compounds (VOCs) in dry building materials, are the three key parameters used to predict the VOC emissions. D and K may be strongly affected by temperature. We have developed a new and simple method, the C-history method, to measure the diffusion coefficient, D and the partition coefficient, K of formaldehyde in dry building materials at temperatures of 18, 30, 40 and 50 °C. The measured variations of the diffusion coefficients and the partition coefficients with temperature for particle board, vinyl floor, medium- and high-density board are presented. A formula relating the partition coefficient and related factors is obtained through analysis. This formula can predict the partition coefficient in principle and provide an insight for fitting experimental data, and it agrees well with the experimental results.

Dimensionless correlations to predict VOC emissions from dry building materials

Based on the most recently published mass transfer model of volatile organic compound (VOC) emissions from dry building materials, it is found that the dimensionless emission rate and total emission quantity are functions of just four dimensionless parameters, the ratio of mass transfer Biot number to partition coefficient ( Bi m/ K), the mass transfer Fourier number ( Fo m), the dimensionless air exchange rate ( Nδ 2 /D m) and the ratio of building material volume to chamber or room volume ( Aδ/ V). Through numerical analysis and data fitting, a group of dimensionless correlations for estimating the emission rate from dry building materials is obtained. The predictions of the correlations are validated against the predictions made by the mass transfer model. Using the correlations, the VOC emission rate from dry building materials can be conveniently calculated without having to solve the complicated mass transfer equations. Thus it is very simple to estimate VOC emissions for a given condition. The predictions of the correlations agree well with experimental data in the literature except in the initial few hours. Furthermore, based on the correlations, a relationship between the emission rates of a material in two different situations is deduced. With this relationship, the results for a given building material in a test chamber can be scaled to those under real conditions, if the dimensionless parameters are within the appropriate region for the correlations. The relationship also explicitly explains the impacts of air velocity, load ratio, and air exchange rate on the VOC emission rate, which determines the feasibility of assuming that the VOC emission rates in real conditions are the same as those in the test chambers.

An analytical solution for VOCs sorption on dry building materials

Based on the diffusion within the material and the mass transfer through the air boundary layer, a VOCs sorption model with an arbitrary inlet concentration is proposed. With the use of Laplace transform and convolution theorem, a method to get the analytical solution has been developed, which helps to understand the mechanism of the sorption theoretically and makes the computation convenient. Experimental results on VOCs sorption in an environmental chamber are used to validate the present model and a good agreement has been obtained. Two cases of different inlet concentrations are discussed in detail. The patterns of the concentration distribution in the air vary with the inlet concentration. The influences of model parameters have been investigated. With the increase of the diffusion coefficient and the partition coefficient, the rate of the adsorption increases. Relatively, the partition coefficient plays a more important role in the sorption than the diffusion coefficient.

An inverse technique to determine volatile organic compounds diffusion and partition coefficients in dry building material

A mass transfer theory based model describing volatile organic compounds (VOCs) diffusion in dry building material was presented. Effects of some key parameters on the model output were theoretically studied. An inverse method was developed to estimate the diffusion coefficient (D) and partition coefficient (K) of VOC/material combination by utilizing dynamic chamber emission data. The present inverse parameter estimation problem was solved with Levenberg–Marquardt method of minimization of the ordinary least-squares norm. Sensitivity analysis showed the feasibility of simultaneous estimation of D and K. The present inverse method was first validated by a theoretical case. Measurements with different error levels were used to show their effects on the accuracy of the estimates. Results indicate that the present inverse method can be used to accurately estimate both D and K with the additional information of measured VOC concentration in chamber. Then experimental data of styrene/carpet combination obtained from standard field and laboratory emission cell emission test were used to determine D and K using the validated inverse method. Reliable results were obtained.