Estimation Method of Emission Rate and Effective Diffusion Coefficient using Micro Cell

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1 Estimation Method of Emission Rate and Effective Diffusion Coefficient using Micro Cell Kaoru TAKIGASAKI 1,2, Kazuhide ITO 2,3 1 Maeda Corporation, Japan 2 Tokyo Polytechnic University, Japan 3 Kyushu University, Japan Corresponding ktaki@jcity.maeda.co.jp SUMMARY In this report, we proposed the estimation method of building material properties by using both numerical analysis and measuring the time history of concentration in micro cell, and showed that the effective diffusion coefficient, emission rate and initial concentration in the building material is provided with a single measurement. The effective diffusion coefficient of testing building materials were estimated to be from about to [m 2 /s] in this proposed method. Although this proposed method is necessary to improve to obtain more reliable result, it is able to apply to relative comparison about VOCs emission characteristic in building materials as a screening test. INTRODUCTION In order to prevent indoor air pollution by VOCs emitted from building materials, it is necessary to select suitable building materials with lower VOCs emissions, and to confirm a VOCs concentration range in rooms in advance. Therefore, the information of building material properties, which is VOCs emission rate, effective diffusion coefficient and chemical content (initial concentration in building materials), are important. Generally, VOCs emission rate is measured by the chamber method [1][2], FLEC [3][4], ADSEC [5] etc., and the effective diffusion coefficient is measured by twin chamber method [6], cup method [6][7], mercury intrusion porosimetry (MIP) method [8]. However, it is the actual fact that the information of building material properties has not been got ready adequately from the viewpoint of effort and cost taken in testing. The overarching goal of this study is to develop the simultaneous measurement method that is able to estimate the emission rate and effective diffusion coefficient by single measurement. In this repot, we propose a estimation method by using micro cell and convenient gas monitor (in this research, we adopt the multi-gas monitor produced in INNOVA), and compare the effective diffusion coefficient by proposed method with conventional methods. OUTLINE OF PROPOSED METHOD In this study, we focused a time history of VOCs concentration in the air (gas) phase that occurred when target building material is covered with an airtight container (in this research, we call as Micro Cell). Generally, the VOCs concentration in micro cell gradually increases, and finally reaches an equilibrium concentration. In other words, VOCs emission rate form building materials gradually decreases toward zero. The profile of the time history of VOCs concentration in micro cell is determined by effective diffusion coefficient and initial concentration (level and distribution) in target building material. Further, when the initial concentration

2 in the building material is uniform, the profile of the time history of VOCs concentration that normalized to the initial concentration in the building material is determined by the effective diffusion coefficient alone. This proposed method estimates VOCs emission rate and effective diffusion coefficient by using both numerical analysis and continuous concentration measurement in micro cell. Before the measurement, we make the chart of time histories of VOCs concentration by numerical analysis that assumed various effective diffusion coefficients. And we measure the concentration in micro cell until it becomes equilibrium. Subsequently, the initial concentration in the building material is obtained from the equilibrium concentration, and the effective diffusion coefficient is estimated by overlapping the measurement result with chart. After estimating the effective diffusion coefficient, the emission rate when reference concentration is guideline value (e.g. 100 ug/m 3 for Formaldehyde in WHO) is estimated as a representative value. In this way, this proposed method is able to estimate the emission rate, effective diffusion coefficient and initial concentration in the building material with a single measurement. Outline of numerical analysis The micro cell is shown in Figure 1. The numerical analysis is carried out in one dimensional, since the micro cell is nearly rectangular (100mm 550mm 600mm h ) and free slip can be assumed for the internal wall. For the inside of the building material, the diffusion equation shown by Equation (1) is solved. t = D x c x (1) Here, C is the equivalent vapor-phase concentration of the objective chemical compound in building material, and Dc is the effective diffusion coefficient of the objective chemical compound. The flux conservation shown in Equation (2) exists on the interface between the surface of the building material and the air phase. D = D (2) c x a ws + x ws Here, Da is the molecular diffusion coefficient of the objective chemical compound in the air phase, ws is the interface between the building material and the air, plus (+) indicates the building material side and minus ( ) indicates the air phase side. For the air phase in the micro cell, the diffusion equation shown by Equation (3) is solved. The object of this analysis is only the diffusion phenomenon attributable to molecular diffusion assuming that the convection phenomenon and turbulent diffusion in the micro cell are negligible, and assuming isothermal distribution. t = D x a x (3) Equations (1) through (3) are analyzed by the finite differential method. The objective region of numerical analysis is shown in Figure 2. Concentrations are analyzed using a fixed value for the normalized initial toluene concentration in the building material and a fixed value for the molecular diffusion coefficient of tolu-

ene in the air phase, by changing the effective diffusion coefficient in the building material in steps. Calculation conditions and boundary conditions for the numerical analysis are shown in Table 1.

3 ene in the air phase, by changing the effective diffusion coefficient in the building material in steps. Calculation conditions and boundary conditions for the numerical analysis are shown in Table 1. Free slip (Adiabatic) 55mm 60mm 60mm Air Phase (Inside Micro Cell) 100mm Figure 1 External appearance of Micro Cell 2-45 mm Free slip Air-Solid boundary Solid Phase (Bilding Material) Free slip (Adiabatic) Figure 2 Analysis model Table 1 Calculation conditions and boundary conditions Mesh division Air:equally spaced mesh with a [m] interval Solid:equally spaced mesh with a [m] interval Initial concentration Air:0 [-] Solid:1.0 [-] (Normalized concentration) Chemical substance (Toluene) Air:Da= [m 2 /s] Solid:Dc= ~ [m 2 /s] Air-Solid boundary Equation (2) Time 0~3600 [sec] unsteady analysis Effective diffusion coefficient prediction chart Figure 3 shows the prediction results of toluene-equivalent TVOC concentration histories in the building material and in the micro cell, changing the effective diffusion coefficient (Dc) in the building material stepwise from [m 2 /s] to [m 2 /s]. All the values in the figure are normalized to the initial concentration in the building material. Figure 3(1) shows the relationship between the time history of the average concentration in the building material and Dc, Figure 3(2) shows the relationship between the time history of the average concentration in the micro cell and Dc, and Figure 3(3) shows the relationship between the emission rate (emission flux) and Dc. In this report, uniform distributions of initial concentration in the building materials are assumed as initial conditions. Normalized Concentration in Solid Phase[-] Normalized Concentration in Air Phase[-] 7.0E-2 6.0E-2 5.0E-2 4.0E-2 3.0E-2 2.0E-2 1.0E-2 0.0E+0 Flux [m/s] 1.0E-4 1.0E-5 1.0E-6 1.0E-7 1.0E-8 (1) Solid phase (2) Air phase (3) Emission flux Figure 3 Time history of toluene concentration for various Dc (4mm). Similarly, the charts when building materials thickness is different were made by numerical analysis

4 When Dc of the target building material is [m 2 /s], the concentration in the building material and in the micro cell become almost equilibrium at 500 [sec] from the beginning of analysis. When Dc is [m 2 /s], concentrations approach equilibrium values in 3600 [sec] from the beginning of analysis. As Dc becomes smaller, the change in concentration per unit time also becomes smaller. OUTLINE OF EXPERIMENT The building materials used in the experiments are sandwiched between stainless steel (SUS) boards and controlled at a constant temperature (301 [K]) for at least a month to rid the concentration gradient in testing building material in incubator [9]. A SUS board is placed lower surface of the testing building material and the upper surface is left open to allow one-sided diffusion (emission). The time history of concentration in the micro cell is continuously measured with a photoacoustic spectrometry method (multi gas monitor, INOVA). In order to ensure a closed experimental system, the air sampled from the micro cell is returned to the micro cell after the multi-gas monitor measurement of the concentration. The atmospheric temperature of the experiments is controlled at 301 [K] by placing the test building materials to which micro cells are attached in a incubator. Humidity is not controlled. Concentration measurements are continued until an equilibrium concentration in the micro cell is reached. The measurement system is shown in Figure 4, and the experimental conditions are shown in Table 2. Gas monitor Air sumpling Micro Cell Return air Bilding Material SUS baking Incubator Figure 4 Measurement system using Micro Cell Temperature Concentration measurement Testing building materials Aging conditions Table 2 Experimental conditions 301 [K] (28 [ ] ) (Humidity is not controlled) Multi-gas monitor (INOVA) Filter: Total Organic Carbon ref. Toluene Water compensation: ON Sampling interval: 60 sec Sampling rate: 140cc/sample (1) Plywood; 4mm (2) Medium Density Fiberboard (MDF); 4mm (3) Cushion Floor (CF); 2.5mm (4) Poly Vinyl Chloride (PVC) sheet; 2mm (5) Synthetic rubber; 2mm Testing building materials sandwiched between SUS boards were placed in a incubator at 301 [K] and 50 [%RH] for more than a month

Outline of effective diffusion coefficient measurement by Cup method In order to validate the measurement value by micro cell method proposed in this research, cup method and MIP method to measure

5 Outline of effective diffusion coefficient measurement by Cup method In order to validate the measurement value by micro cell method proposed in this research, cup method and MIP method to measure effective diffusion coefficient are carried out. The effective diffusion coefficient by cup method is estimated from the measurement result for the flux value in the condition that made the concentration difference on the top and bottom of building material. In this experiment, liquid toluene was placed in the stainless steel cup is shown in Figure 5 and the opening of the cup was covered with the building material. The cup was placed in a incubator at 301[K], and the toluene concentration on the upper surface of the building material was kept at zero by sufficient ventilation. The diffusion coefficient is calculated from Equation (4). m d Dc = A C (4) b sat Here, m is the weight change per unit time of toluene in the cup [mg/h], A b is the surface area of the building material [m 2 ], d is the thickness of the building material [m], and C sat is the saturated vapor phase concentration at the ambient temperature [mg/m 3 ]. Screw 88mm Building Material Teflon Sealing 12mm Figure 5 Detail of the Cup Outline of effective diffusion coefficient measurement by MIP method The effective diffusion coefficient by MIP method is estimated using the porosity (ε) and tortuosity factor (τ) are estimated from the measurement results for the amount of mercury that has entered the fine pores of the building material under pressurization. The effective diffusion coefficient is calculated from Equation (5) based on the parallel pore model [10]. D c = ε τ D KA D a (5) Here, Da is the molecular diffusion coefficient [m 2 /s] of chemical substances in the air, and D KA is the Knudsen diffusion coefficient [m 2 /s] calculated from Equation (6). T D = r (6) KA M Here, r is the mean fine pore diameter [m], T is the absolute temperature [K], and M is the molecular weight [kg/mol]. Testing building materials were vacuum-dried for a day as pretreatment, and an AutoPore III 9420 (Micromeritics) was used for the measurement. The range of the fine pore distribution measurement was from a radius of about to 100 [μm].

6 EXPERIMENTAL RESULTS The time histories of concentration in micro cell are shown in Figure 6. Although the concentrations gradually increase with time, the profile and range of concentrations are different with each building materials. Time histories of Normalized concentration in micro cell are shown in Figure 7. Concentration values in Figure 7 were normalized to the initial concentration in the building materials. The mean concentration histories in micro cell by numerical analysis are superposed on Figure 7. As for the early times, experimental values tend to rise slightly up than calculation values. However the profile of the concentration history almost fitted. In this experiment, the effective diffusion coefficients (toluene-equivalent TVOC) of the building materials are estimated to be from about to [m 2 /s]. TVOC concentration in Air Phase[mg/m 3 ] 160 TVOC concentration in Air Phase[mg/m 3 ] Plywood Synthetic Rubber MDF CF PVC Sheet 0 0 Figure 6 Time history of TVOC concentration in Micro Cell Normalized Concentration in Air Phase[-] 7.0E-2 6.0E-2 5.0E-2 4.0E-2 3.0E-2 2.0E-2 1.0E-2 0.0E+0 MDF Plywood Normalized Concentration in Air Phase[-] 5.0E-2 4.0E-2 3.0E-2 2.0E-2 1.0E-2 0.0E+0 CF Normalized Concentration in Air Phase[-] 4.0E-2 3.0E-2 2.0E-2 1.0E-2 0.0E+0 Synthetic Rubber PVC Sheet (1)Plywood, MDF (4mm) (2)CF (2.5mm) (3) PVC sheet, Synthetic rubber (2mm) Figure 7 Time history of normalized concentration in air phase The emission flux is estimated by reading the emission flux chart when the concentration in micro cell reaches the guideline value. In this study, since the dynamic range of the filter for hydrocarbons (Total Organic Carbon ref. Toluene) in the multi-gas monitor is wide, measured concentration values are overestimated in comparison with ordinary TVOC values. Therefore, the emission rate when the reference concentration in micro cell reached about 10 times as high as the TVOC guideline value (4 [mg/m 3 ] in this case) is estimated in this report. The emission rates when the air concentration is 4 [mg/m 3 ] are estimated to be from about 0.3 to 460 [mg/m 2 /h].

7 Results of the effective diffusion coefficient, emission rate and initial concentration in the building materials are shown in Table 3. This proposed method is able to obtain three values of the building material property with a single measurement. Table 3 Results of the effective diffusion coefficient, emission rate and initial concentration Testing Building Materials Effective Diffusion Coefficient (Dc) [m 2 /s] Emission Rate (E) [mg/m 2 /h] Initial concentration (C 0 ) [mg/m 3 ] Plywood MDF CF PVC Sheet Synthetic Rubber Figure 8 shows the effective diffusion coefficient by each measurement method. In this experiment, the effective diffusion coefficient value by MIP method was the largest, followed by cup method and then micro cell method. The effective diffusion coefficient of PVC sheet and synthetic rubber were small in comparison with other building materials. That difference is order of 10-2 in MIP method, and is order of 10-1 to 10-2 in micro cell method. Effective Diffusion Coefficient, Dc [m 2 /s] 1.0E E E-06 MIP Cup Micro Cell DISCUSSION Plywood MDF CF PVC Sheet Synthtic Rubber Figure 8 Effective diffusion coefficients by each measurement method The effective diffusion coefficient values obtained by this proposed method are evaluated smaller than those obtained by the other measurement methods. Previous studies reported that effective diffusion coefficient values from MIP method tend to be larger, since the effective diffusion coefficient values are calculated from the physical properties of the building materials alone without considering the influence of interactions with absorption and desorption in the building materials and influence of the humidity [8]. In case of experiments by cup method, the concentration in the cup is far higher than ordinary indoor concentration. Consequently the effective diffusion coefficient may be overestimated when the concentration of the pore in the building material is saturated or when the concentration gradient of the cavity in cup is not neglected. From this, the result that the effective diffusion coefficient values by micro cell method tend to be smaller than those by MIP method and cup method is considered

8 to be valid. Also, the effective diffusion coefficient of PVC sheet and synthetic rubber tends to be smaller in comparison with other building materials, in MIP method and both of micro cell method. This proposed method is able to apply to relative comparison about VOCs emission characteristic in building materials as a screening test. In this report, although the building material properties values by micro cell method are toluene-equivalent TVOC value by multi-gas monitor, those by MIP method and Cup method are toluene alone. The precision of estimated value greatly depends on the precision of the concentration history measurement. In order to improve the precision of estimated value in this proposed method, it is necessary to validate using measurement apparatus capable of highprecision analysis of single components. CONCLUSIONS (1) In this report, we proposed the estimation method of building material properties by using both numerical analysis and measuring the time history of concentration in micro cell, and showed that the effective diffusion coefficient, emission rate and initial concentration in the building material is provided with a single measurement. (2) The effective diffusion coefficient of testing building materials were estimated to be from about to [m 2 /s] in this proposed method, and were smaller than the effective diffusion coefficient values by cup method and MIP method. (3) Although this proposed method is necessary to improve to obtain more reliable result, it is able to apply to relative comparison about VOCs emission characteristic in building materials as a screening test. REFERENCES 1. R.Funaki, S.Tanabe Chemical Emission Rates from Building Materials Measured by a Small Chamber. Journal of Asian Architecture and Building Engineering, Vol.1 No.2, pp ISO Part 9: Determination of the emission of volatile organic compounds from building products and furnishing Emission test chamber method 3. P.Wolkoff et al Field and Laboratory Emission Cell: FLEC. IAQ 91, Healthy Buildings, pp ISO Part 10: Determination of the emission of volatile organic compounds from building products and furnishing Emission test cell method 5. J.Matsumoto, S.Tanabe, R.Aoki Development of Measurement Device (Adsec) for Aldehydes and VOCs Emission Rates Using a Diffusive Sampler, Indoor Air 2002, Vol.I, pp F.Haghighat, C.-S.Lee, W.S.Ghaly Measurement of diffusion coefficients of VOCs for building materials: review and development of a calculation procedure. Indoor Air 2002,Vol.12,pp S.Kirchner, J.R.Badey, H.M.Knudsen, R.Meininghaus, et al Sorption capacities and diffusion coefficients of indoorsurface materials exposed to VOCs: proposal of new test procedures. Indoor Air 99, VOl.1, pp , Y.Ataka, S.Kato, Q.Zhu. Evaluation of Effective Diffusion Coefficient in Various Building Material and Absorbents by Mercury Intrusion Porosimetry J. Environ. Eng., Architectural Institute, No.589, pp (in Japanese) 9. ISO Part 11: Determination of the emission of volatile organic compounds from building products and furnishing Sampling, storage of samples and preparation of test specimens 10. The Society of Chemical Engineers, Japan. Handbook of Chemical Engineering, 5th ed Maruzen. (in Japanese)

Proceedings: Indoor Air 2005

Proceedings: Indoor Air 2005 EVALUATION OF EFFECTIVE DIFFUSION COEFFICIENT IN VARIOUS BUILDING MATERIALS AND ABSORBENTS BY MERCURY INTRUSION POROSIMETRY J Seo 1*, S Kato 2, Y Ataka 3 and Q Zhu 4 1 The University of Tokyo, Tokyo, Japan