Thermolysis under vacuum: Essential influence of the residual pressure on thermoanalytical curves and the reaction products

Abstract

This paper examines the influence of residual pressures in the range from 10-~to 5 torr on the course of thermal analysis. With the help of examples concerning in particular the thermolysis of gibbsite, AI(OH)3 , it is shown that a) the control of residual pressure is of virtually no use unless the rate of decomposition is also controlled (otherwise, the TG curves represent a composite phenomenon, which is practically unintelligible); b) the influence of residual pressure may be unexpectedly high both on the shape of the TG curves (and therefore on the apparent kinetic parameters) and on the nature (porosity, structure) of the products.The influence of the gas atmosphere on a thermal decomposition of the type solid A ~ solid B + gas is widely accepted. In 1951, Rowland and Lewis [1] in this way explained the usual DTA trace for calcite thermolysis, where the onset of decomposition may be observed as soon as 650 ~ (low partial pressure of CO~) and where "the reaction progressively becomes more vigourous until a peak is reached at about 925 ~ the temperature at which the reaction would begin if the furnace had been filled with COz at one atmosphere pressure. In 1960, Garn suggested simplification of the situation by operating under self-generated atmospheres [2], and several devices were proposed and used in order to control the overall pressure, e.g. the "cold sink", especially convenient in the case of water evolution [3], or the discontinuous and automatic gas extractor [4]. Nevertheless, "vacuum" was often considered as one pressure at which it may be worthwhile to carry out one experiment in order to complement a set of data obtained under higher pressures. Indeed, various authors, such as Eyraud et al. at the time of the first vacuum TG experiments, pointed out the "simplifying" part played by vacuum-operation, which was supposed to eliminate disturbing phenomena such as adsorption or diffusion processes [5] or secondary reactions [6] and to give rise to the simplest mechanism with higher decomposition rate [7]. Now pressures lower than I torr, which are usually referred to as "vacuum", actually cover a wide experimental range, down to 10 -~ torr, whereas the range of medium and *

CO2 multicyclic capture of pretreated/doped CaO in the Ca-looping process. Theory and experiments

“…[34]). Analysis (CRTA) [37,38]. In the CRTA pretreatment, the evolution of temperature during preheating is controlled by means of a feedback mechanism based on mass changes during the carbonation/decarbonation reactions, which are forced to occur at prefixed low rates.…”

The kinetics of solid‐state reactions toward consensus—Part I: Uncertainties, failures, and successes of conventional methods

“…Another striking illustration of the influence of the gas atmosphere on the reaction is included in Figure 2, where the specific surface area of gibbsite was measured by Rouquerol et al [37] by the BET method under experimental conditions that completely avoided any water re-adsorption. The influence of water pressure during the dehydroxylation is as follows:…”

The kinetics of solid‐state reactions toward consensus, Part 3. Searching for consistent kinetic results: SCTA vs. conventional thermal analysis

“…Previous work [19,20] has demonstrated the efficiency of CRTA to control the influence of experimental parameters such as sample mass and shape, particle size in the case of powder samples, and ambient atmosphere (or vacuum). CRTA allows both a good control of pressure in the sample surroundings and the use of reaction rates low enough to keep temperature gradients at a negligible level to avoid any heat or mass transfer problems.…”