Thermobalance studies

Results of the small-scale laboratory thermobalance studies have shown that ... 

Thermal dehydroxylation of FeOOH has been studied both in vacuum and under various atmospheres. Kinetic studies of these transformations must be carried out under vacuum (Giovanoli Briitsch, 1974) and at a constant temperature. The temperature at which a phase transformation occurs, however, is determined by increasing the temperature of the sample in a controlled manner, i.e. by using a thermobalance (DTA or TGA method, see Ghap. 7). Mechanical and mechanochemical dehydroxylation of FeOOH at room temperature can also be achieved by grinding. 

The basic approach is to consider the problem in two parts. Firstly, the reaction of a single particle with a plentiful excess of the gaseous reactant is studied. A common technique is to suspend the particle from the arm of a thermobalance in a stream of gas at a carefully controlled temperature the course of the reaction is followed through the change in weight with time. From the results a suitable kinetic model may be developed for the progress of the reaction within a single particle. Included in this model will be a description of any mass transfer resistances associated with the reaction and of how the reaction is affected by concentration of the reactant present in the gas phase. 

Experimental. The test program to study the effects of temperature, hydrogen pressure, particle size, and heat-up rate on the rate and extent of kerogen removal from the shale was carried out with a thermobalance, as diagramed in Figure 1. The design and operation of the equipment have been described in an earlier publication (2). 

The thermal stability was studied on a Linseis thermobalance model L81 + DTA under a nitrogen flow of 20 L/h and at a heating rate of 10°C/min. 

The deactivation of a lanthanum exchanged zeolite Y catalyst for isopropyl benzene (cumene) cracking was studied using a thermobalance. The kinetics of the main reaction and the coking reaction were determined. The effects of catalyst coke content and poisoning by nitrogen compounds, quinoline, pyridine, and aniline, were evaluated. The Froment-Bischoff approach to modeling catalyst deactivation was used. 

Grunze and Hirschwald [15] developed a vacuum micro thermobalance with cold fingers and studied the reduction of ZnO by H2 and CO in the temperature range 450—1000°C. By making sure that the reaction products, Zn and C02 or H20, were removed to cold traps as fast as they formed, they were able to measure the intrinsic surface reaction rates. The reaction order, n, depended on temperature and pressure for T> 700°C and p < 1 torr, n 1 and for T< 500°C and p > 1 torr, n 0.2 [16]. 

Karge s combined TGA/GC method can be applied to catalysts which were used (and coked) before in a separate catalytic unit. It can, of course, be employed as well to catalysts on which the catalytic reaction was studied before in the same thermobalance (even though, in many instances, such microbalances are not very suitable as catalytic reactors, because part of the feed gas stream tends to bypass the sample holder or basket, on/in which the solid catalyst rests). If the latter is done, the mass of coke deposited during the catalytic reaction can be monitored in-situ via the weight gain. It has been shown [36] that such a strategy enables the experimentalist to perform systematic studies on the amount of coke formed and its h/wc ratio in dependence of time-on-stream and reaction temperature. 

In the present study, weight loss measurements of K2S04( ) were made using a thermobalance equipped with a Pt Knudsen cell. Combining these data with the known dissociation pressures for reaction [1] leads to partial pressure data for reaction [2], as shown in Figure 2. Complementary KMS data were also obtained. 

Dobner, S. Kan, G. Graff, R. A. Squires, A. M. "Modification of a Thermobalance for High Pressure Process Studies," AIChE Meeting, Los Angeles, November 16-20, 1975. 

The samples used in the study comprised various biomasses and a selection of Danish straws. The analyses of the fiiels used in the study are presented in Tables 1 and 3, and the chemical composition of ash, together with a summary of the thermobalance sintering tests, in Tables 2 and 4. The Danish wheat straw qualities were selected on the basis of growth site, fertilising and weathering conditions [9],... 

The ash sintering behaviour was determined in a thermobalance according to the method described in [8], The ash residues from the thermobalance experiments were studied by microscopy using the following classification criteria ... 

The gasification behaviour of different straws was also studied in AFB. The aim of these tests was a) to confirm laboratory findings (thermobalance tests) concerning the ash sintering behaviour, which sets limits to the maximum gasification temperature) and b) to assess the gasification behaviour and ash sintering/deposition tendency in a full-scale fluidised-bed reactor. 

Several studies about pyrolysis and gasification of krafl black liquor from cooking of wood can be found in literature, but there are scarcely data for black liquor coining from alkaline pulping of straw. The aim of this research is to provide more infoimation about the thermochemical conversion of this residue. Studies [1] about CO3 gasification kinetics of alkaline black liquor from straw have been previously carried out in thermobalance by our research group. The present work is focus on the... 

There are several hundreds of papers studying the kinetics of reactions involved in the pyrolysis step (27). Unfortunately for the gasifier modelling there is neither a unified approach nor an overall kinetic equation(s) describing the pyrolysis step for all possible biomass under all possible circumstances. Besides, most of the kinetic studies have been made in thermobalances or related apparatuses with low heating rates (around 25 "C/min), far from the high heating rates in fluidised beds of up to a claimed 1000 C/s or even 10000 C/s. Kinetic equations obtained in thermobalances would provide results very different from the ones in fluidised bed, which is the present case. 

Density, vapor pressure, heat capacity, and standard molar enthalpies of formation, fusion, vaporization, and combustion of parent TP have been determined (97JCED1037). Calculated enthalpies of formation of TP and some 6-substituted derivatives have been published (00CHE714). The thermal behavior of 5-oxo TP (106) and 7-oxo TP (29) hemihydrate has been studied in a calorimeter and a thermobalance (00JST(519)165). 

For gravimetric measurements, the powders were pressed (c.a.400mg), activated and sulfated in the same conditions as for the IR study. A McBain thermobalance was used. The temperature was increased from r.t. to 450°C (0.5°C.min ) and then kept at 450°C until a plateau was obtained for the weight. 

For the study of sulfate reduction, a large excess of H2 (Pe 13 kPa) was introduced at r.t. in the thermobalance and then the sulfated sample was heated under H2 atmosphere at increasing temperature (0.5°C.min ). 

In the water absorption study, a Setaran thermobalance B 60 and a Richard Eyraud isotherm differential microcalorimeter were used. The same experiment permits one to obtain both the sorption isotherm and the differential heat absorption values ( ). The accuracies were better than O.I mg and 10 cal/sec, respectively. The relative humidity was obtained by changing the temperature of a water/ice bath (accuracy 0.1°C) which was connected to the sample. We measured both the amount of absorbed water and the heat of absorption after a change in the relative humidity level. We can therefore define the average heat of absorption per water molecule corresponding to the molecules which have been absorbed after this change in water relative humidity. 

Water Adsorption. The activation of samples under vacuum at 450 °C and the study of water vapor adsorption at 25 °C and under 15 torr were effected by means of an Ugine-Eyraud electronic thermobalance with a continuous recording device. 

The thermal stability is denned as a general term (2) indicating the ability of a substance to maintain its properties as eaHy n Tanged as possible on heating. From a practical point of view, thermal stability needs to be considered in terms of the nvironmenr to be imposed on the material and the functions it has to perform. The thermobalance is a Useful technique for studying the ability of a substance to maintain its mass under a variety of conditions. 

The historical aspects ofTG have been discussed by Duval (3-5), Wend-Iandt (7), Keattch (23), and others (107-109). Perhaps thejirst thermobalance was that described by Nernst and Riesenfeld (I20X whp used a Nernst quartz torsion microbalance. equipped wit an electric furnace, to study the mass-loss on heating of Iceland spar. opal, zirconia, and other minerals. The Japanese Honda was apparently the first to use the term ihermobalance for an instrument he described in 1915 (9). The French school of thermo-gravimetry began with Urbain in 1912 when he modified two-pan analytical balance into a cril e thermobalance (24). This was followed by the work of Guichard (1923) (10), Vallet (1936), Chevenard (1936), Duval (1950), and many others.The first commercial thermobalance in the United States, which prompted funner work in TG. was that described by Mauer (31) in 1954,... 

The effect of air buoyancy changes on the sample holder and certain balance components has been studied (3, 4, 12, 15, 29. 37, 48. 51, 52). Most of the studies have used the Chevenard thermobalance (3, 4, 37, 51, 52), but other balances have been studied as well (12, 15, 29, 37, 48). 

Numerous other Japanese workers modified the Honda thermobalance and also developed new instruments. The results of their studies have been summarized in a monograph by Saito 19). 

The first application of a quartz crystal for use as a nonisothermal thermobalance was reported by Henderson ei al. (66). This was accomplished through the use of a minicomputer to characterize the temperature-frequency relationship for the crystal and to correct numerically the frequency-temperature-mass relationship to obtain the TG curve of the sample. One advantage of such a system is that very fast heating rates may be employed due to small sample size (t-40 jug typical) and high sensitivity. Thin films of sample exhibit rapid gas diffusion and thus permit thermal equilibrium to be maintained at heating rates of l00°C/min or greater. Applications of this thermobalance include the evaluation of thin films and coatings as well as various polymer studies of pyrolysis, flammability, and so on. 

The obvious advantage of the automated thermobalance system over existing instruments is the ability to determine the mass-loss curves of eight successive samples. Operation of the instrument is completely automatic, and once the cycle is begun the instrument does not require the attention of the operator until the eighth sample curve is completed. The instrument should find use for the routine TG examination of a large number of samples, each to be studied under identical thermal conditions. Because the system is completely automated, data reduction or control by a small digital computer could easily be accomplished (see Chapter 14.)... 

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