Test chamber kinetics

Through this step and based on experimental evidence we try to develop the appropriate model to describe the test chamber kinetics. As was anticipated in the introduction of this Chapter, from a conceptual point of view, two broad categories of models can be developed empirical-statistical and physical-based mass transfer models. It should be emphasized that, in several cases, even the fundamentally based mass transfer models are indistinguishable from the empirical ones. This happens because the mass transfer models are generally very complex in both the physical concept involved and the mathematical treatment required. This often leads the modelers to introduce approximations, making the mass transfer models not completely distinguishable from some empirical models in terms of both functional formulations and descriptive capabilities. Considering the current status of models which have been developed to describe VOC emissions (and/or sink processes), we could define the mass transfer models as hybrid-empirical models. 

In our case study, the experimental observations (i.e. concentration versus time data) were used as input to the conceptualization phase of a mathematical model with two sink compartments (the so-called two-sink model). Following the above discussion, this model (schematically represented in Fig. 2.3-1) can be eonsidered as a hybrid-empirical model. Conceptually, this model describes the test chamber kinetics of a VOC for the three types of experiments which have been carried out. The adsorption-desorption kinetics is described by the rate constants k, k, k. Given that the conceptualization... 

However, the two-sink model as well as other existing adsorption (sink) models do not seem to be able to describe the strong asymmetry between the adsorption/desorption of VOCs on/from indoor surface materials (the desorption process is much slower than the adsorption process). Diffusion combined with internal adsorption is assumed to be capable of explaining the observed asymmetry. Diffusion mechanisms have been considered to play a role in interactions of VOCs with indoor sinks. Dunn and Chen (1993) proposed and tested three unified, diffusion-limited mathematical models to account for such interactions. The phrase unified relates to the ability of the model to predict both the ad/absorption and desorption phases. This is a very important aspect of modeling test chamber kinetics because in actual applications of chamber studies to indoor air quality (lAQ), we will never be able to predict when we will be in an accumulation or decay phase, so that the same model must apply to both. Development of such models is underway by different research groups. An excellent reference, in which the theoretical bases of most of the recently developed sorption models are reviewed, is the paper by Axley and Lorenzetti (1993). The authors proposed four generic families of models formulated as mass transport modules that can be combined with existing lAQ models. These models include processes such as equilibrium adsorption, boundary layer diffusion, porous adsorbent diffusion transport, and conveetion-diffusion transport. In their paper, the authors present applications of these models and propose criteria for selection of models that are based on the boundary layer/conduction heat transfer problem. 

For many years test chambers and cells belong to the most important tools for the simulation of indoor related conditions and for the evaluation of emission rates. It can be assumed that their application will become even more important in the near future. Moreover, powerful kinetic models are now available that help to understand emission characteristics of sources. The current trend to develop devices en miniature brings us to the borderline between emission and content analysis. It will be interesting to see new chamber designs and intelligent applications for indoor related studies. 

For the calibration and evaluation of kinetic response of pH electrodes, a Milton-Roy sapphire and Hastalloy pump (capable of 6,000 psi) were built into the pressure line in order to feed acid, base, or buffered solutions through the test vessel by way of stainless capillary tubing of 0.030-in. i.d. Outflow from this system can be controlled by means of two Hoke Micro-Metering valves which, when mounted in series, can provide an "engineered leak with an outfall that can be matched by the pump to allow system flows from 0.6 cm3/min to 16 cm3/min. Such a pumpable system allows fresh reference solution to be continuously added to the system while maintaining constant pressure. This avoids a possible pH drift caused by reactions between the walls of the test chamber and the reference solution. This system also allows... 

In the following, a real case study will be given to practically demonstrate how the chamber kinetics of VOCs can be modelled by applying mathematical theory to handle experimental data resulting from experiments performed in test chambers. Through this case study, the different steps of model building and model validation processes will be clearly identified and thoroughly discussed. 

Physieal-meehanieal eharacteristies of fibrous materials were determined with PM-3-1 tensile testing machine according to TU 25.061065-72. Kinetics of UV-aging was studied with Feutron 1001 environmental test chamber (Germany). Irradiation of samples was carried out with a 375 W high pressure Hg-lamp, at a distance of 30 cm. 

Figure 2.39 shows the rjs = f (yO, functions obtained for a sample of OBUA at different Xr relaxation time variations. The latter was determined as the time between the moment of preliminary sample deformation (e.g., the end of measurement in the previous experiment) and the beginning of the Ps registration in the current experiment. The sample stayed in the test chamber. The Xr variable is formally similar to the Xex value in the experiments on curing kinetics of equilibrium swollen rubber films in oligomer, considered in the previous paragraph, however, Xr represents a different level of the system approach to thermodynamic equilibrium than Xex. 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

Pollutants emitted by various sources entered an air parcel moving with the wind in the model proposed by Eschenroeder and Martinez. Finite-difference solutions to the species-mass-balance equations described the pollutant chemical kinetics and the upward spread through a series of vertical cells. The initial chemical mechanism consisted of 7 species participating in 13 reactions based on sm< -chamber observations. Atmospheric dispersion data from the literature were introduced to provide vertical-diffusion coefficients. Initial validity tests were conducted for a static air mass over central Los Angeles on October 23, 1968, and during an episode late in 1%8 while a special mobile laboratory was set up by Scott Research Laboratories. Curves were plotted to illustrate sensitivity to rate and emission values, and the feasibility of this prediction technique was demonstrated. Some problems of the future were ultimately identified by this work, and the method developed has been applied to several environmental impact studies (see, for example, Wayne et al. ). 

Once a chemical submodel has been developed, it must be tested extensively prior to its application in comprehensive computer models of an air basin or region. This is done by testing the chemical submodel predictions against the results of environmental chamber experiments. While agreement with the chamber experiments is necessary to have some confidence in the model, such agreement is not sufficient to confirm that the chemistry is indeed correct and applicable to real-world air masses. Some of the uncertainties include those introduced by condensing the organic reactions, uncertainties in kinetics and mechanisms of key reactions (e.g., of aromatics), and how to take into account chamber-specific effects such as the unknown radical source. 

Salthammer, T. (1996) Calculation of kinetic parameters from chamber tests using nonlinear regression. Atmospheric Environment, 30,161-71. 

The ISO (International Standards Organization) suspended sample test (3d), now adopted as MCC-1 by the U.S. Dept, of Energy Materials Characterization Center,(7) and the Sanders static cell test (8,9)(3f) make it possible to control the critical variables needed for kinetics analysis. Addition of tubing to and from the chambers in 3d and 3f is straightforward and enables corrosion to be studied as a function of flow rate. 

Important facts Baertschi et al. have presented include the importance of container shape, the effect of sample closure on testing, the effect of source selection on kinetics, chamber mapping and their effect the results obtained (272). 

After the operator has selected the desired method menu of the relevant samples and has started the instrument, all subsequent steps are fully automated. Since 1987 it is also possible to effect a direct identification of the sample so that there are no longer any problems in respect of a dialogue with a central EDP system. The samples are taken from the sample vessel by means of disposable single use pipette tips that are used for one sample only and exchanged via a computer-monitored pipetting unit. This method excludes the possibility of a carry-over between samples. In accordance with the preset conditions, the required slides are automatically moved to the sample dosage unit (see Fig. 23). Samples of 11 pi serum or plasma will be sufficient for kinetic measurements (enzymes), 10 pi of sample for all other tests. As soon as application of the sample has been completed, the slide is moved to the appropriate incubation chamber by means of the slide rotor (see Fig. 23). The chemical reactions take place in these chambers. This is followed by measurement either by reflectometer (end point or kinetic) or a potentiometric measurement unit. 

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