Monolithic Samples

A common inexpensive way to prepare solid electrolytes is the formation of monolithic samples. Depending on the required phases and final compounds, a large variety of preparation methods are known. These methods usually provide polycrystalline materials. 

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Fig. 17. The voltage-current relationship (a) and resistive heating curve (b) for a CFCMS monolith (sample 21-2B, 18% bum-off, 2.5-cm diameter x 7 5-cm length) [23]...

Fig. 20. Typical C02/CH4 breakthrough plot for CFCMS monolith sample 21-11 (9% bum-off) at 25° C...

Along this line, sol-gel materials based on Si02find increasing use in solid-phase assisted synthesis. The sol-gel synthesis creates well ordered porous glasses which retain a rigid and exposed surface area (300-1000 m g ) [56]. Gelation occurs after a sol is cast into a mold so that the monolithic samples can be tailored to a desired size or shape [57]. 

In summary, it can be concluded that the monolithic stirrer reactor is a convenient reactor type both for the laboratory and the production plant. It is user-friendly and can be used to compare different catalysts in the kinetically limited regime or hydrodynamic behavior in the mass transfer controlled regime. Stirrers or monolith samples can be easily exchanged and reloaded to suit the desired enzyme and/or reaction conditions. 

In a second and possibly alternative stage of the kinetic investigation, laboratory experiments are performed over the same catalyst as for the microreactor tests, but now in the form of small monolith samples with volumes of few cubic centimeter. Flow rates, as well as catalyst size, are thus typically increased about by a factor of 100 with respect to the microreactor kinetic runs. This experimental scale provides data either for intermediate validation of the intrinsic kinetics from stage one, or directly for kinetic parameter estimation if runs over catalyst powders are omitted. 

If kinetic runs over the same catalyst in powder form are available, comparing them with tests over small monolith samples at the same conditions permits also a direct experimental evaluation of the role of diffusion processes in determining the catalytic performances. 

In the laboratory experiments, DOC monolith samples (length 7.5 cm, diameter 1.4 cm) with rather thin catalyst layer coating ( 25 pm) were employed to minimize the internal diffusion effects. The samples were placed into a thermostat to suppress the formation of temperature-gradients along the channels. In the course of each experiment, the temperature of the inlet gas and the monolith sample was increased at a constant rate of /min within the range of 300-800 K. The exhaust gases at the inlet of the converter were simulated by synthetic gas mixtures with defined compositions and flow rates (cf. individual figure captions all gas mixtures contained 6% C02 and 6% H20). 

Fig. 11. Evaluation of kinetic parameters for the DOC model—CO and HC oxidation. Comparison of experimentally observed and simulated outlet concentrations in the course of the oxidation light-off for simple mixtures (a) CO, reaction Rl (b) Ci0H22, reactions R4 and R7 (cf. Table II). Lab experiments with isothermal monolith sample using synthetic gas mixtures (14% 02, 6% C02, 6% H20, N2 balance). Rate of temperature increase /min, SV = 30,000 h 1 (Kryl et al., 2005). Reprinted with permission from Ind. Eng. Chem. Res. 44, 9524, 2005 American Chemical Society.

Fig. 13. Evaluation of kinetic parameters for the DOC model—NO oxidation (reaction R5 in Table II). Comparison of measured and simulated outlet NOx concentrations in the course of temperature ramp (2K/min) for two different space velocities (SV= 50,000 and 100,000 h-1). Lab experiment with isothermal monolith sample using synthetic gas mixture (100 ppm CO, 100 ppm C3H6, 500ppm NO, 8% 02, 8% C02, 8% H20, N2 balance).

Figure 46 presents the comparison between experimental results (symbols), obtained over a different monolith sample (volume 10cm3) upon performing a TPR run, and the corresponding model predictions (solid lines) 1,020 ppm of NH3 and 960 ppm of NO were fed in a stream of 10% H20, 10% 02, balance nitrogen, with an SV = 36,000 h-1. 

Steady-state periodic heating and unsteady-state methods can be applied to measure the thermal conductivity and diffusivity of coal. Methods such as the compound bar method and calorimetry have been replaced by transient hot-wire/line heat source, and transient hot plate methods that allow very rapid and independent measurements of a and X. In fact, such methods offer the additional advantage of measuring these properties not only for monolithic samples but also for coal aggregates and powders under conditions similar to those encountered in coal utilization systems. 

Thermal conductivity increases with increasing apparent density, volatile matter, ash, and mineral matter content. Due to the high porosity of coal, thermal conductivity is also strongly dependent on the nature of gas, vapor, or fluid in the pores, even for monolithic samples (van Krevelen, 1961). Moisture has a similar effect and increases the thermal conductivity of coal since its thermal conductivity value is approximately three times higher than that of dry coal (Speight, 1994, and references cited therein). However, the thermal diffusivity of coal is practically unaffected by moisture since the /Cp value is not essentially changed by moisture. 

It should be noted that in the initiation of the cryochemical reactions observed, what is important is the very fact of formation of a new crack (or zone cut by such cracks), and not the increase in the specific internal surface of the sample. Indeed, slow heating of a monolithic sample, or of a sample... 

Scientific objectives. The three most common are determination of the available release potential (this notion aims to evaluate the total elementary content of pollutant available for leaching), study of solubilities at equilibrium (these tests which are conducted on crushed waste are carried out either at controlled pH or by addition of given quantities of acid when measuring the acid neutralization capacity, ANC), and assessment of the release dynamics (these tests are carried out on monolithic samples over long periods from one to three months or more) [33-36]. 

Monolithic Sample Area Sbet (mV ) External Area Total Pore Volume 0-10pm (cmV ) Pore Volume (cm g ) Micropore Mesopore Macropore 0-2 nm 2-50 nm 50 nm-10 pm Threshold Diameter (pm)... 

Data shown for pellets are the average of four runs on four different samples of the same catalyst reproducibility was within 8%. For monolith samples, rates for 2-3 duplicate samples agreed within 5%. 

The bulk density of materials was measured by Hg pycnometry from independent measurements of the mass and the volume of monolithic samples. The geometrical volume of the sample is determined fi om the weight difiference between a flask (calibrated volume) filled up with mercmy and the same flask filled up with the sample and mercury. As mercury is a non-wetting liquid and as no pressure is exerted, mercury does not enter in the porosity of the sample or crush it. 

In order to identify the volume variation mechanisms on the precipitated silica sample, experiments were performed at various maximum pressure below and near the point of slope change P,.. A monolithic sample of high dispersive precipitated silica was weighted and its specific volume (2.04 cm /g) was determined using mercury pycnometry. It has been submitted to mercury porosimetry until a pressure (40 MPa) just below the characteristic transition... 

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