Acetaldehyde vapour pressure

To obtain the free aldehyde 25 g. of the aldehyde ammonia are dissolved in 25 c.c. of water, a cooled mixture of water (40 c.c.) and concentrated sulphuric acid (30 c.c.) is added, and the acetaldehyde liberated is distilled from the water bath through a calcium chloride U-tube (gently warmed if the external temperature is low) and through an efficient coil condenser. In order to prevent autoxida-tion of the acetaldehyde the apparatus is filled with carbon dioxide before distillation, and, since the vapour pressure of the aldehyde is high, a slow current of carbon dioxide is passed again, for a short time only, at the end of the distillation. Since acetaldehyde boils at 21° the receiver, which is attached to the condenser by means of a cork stopper, must be well cooled in an ice-salt freezing mixture. 

The vapour pressures of the main volatile compounds involved in esterification and polycondensation are summarized in Figure 2.25. Besides EG and water, these are the etherification products DEG and dioxane, together with acetaldehyde as the main volatile product of thermal PET degradation. Acetaldehyde, water and dioxane all possess a high vapour pressure and diffuse rapidly, and so will evaporate quickly under reaction conditions. EG and DEG have lower vapour pressures but will still evaporate from the reaction mixture easily. 

Figure 2.25 Vapour pressures of acetaldehyde, water, dioxane, ethylene glycol and diethylene glycol, where the data have been calculated from the database of the commercial process simulator Chemcad (Chemstations)...

For the solubility of TPA in prepolymer, no data are available and the polymer-solvent interaction parameter X of the Flory-Huggins relationship is not accurately known. No experimental data are available for the vapour pressures of dimer or trimer. The published values for the diffusion coefficient of EG in solid and molten PET vary by orders of magnitude. For the diffusion of water, acetaldehyde and DEG in polymer, no reliable data are available. It is not even agreed upon if the mutual diffusion coefficients depend on the polymer molecular weight or on the melt viscosity, and if they are linear or exponential functions of temperature. Molecular modelling, accompanied by the rapid growth of computer performance, will hopefully help to solve this problem in the near future. The mass-transfer mechanisms for by-products in solid PET are not established, and the dependency of the solid-state polycondensation rate on crystallinity is still a matter of assumptions. 

It may also be mentioned here that in specific molecular actions a particularly marked influence of like molecules upon one another is often to be observed. This is encountered in various ways in spectroscopy, in the extinction of the polarization of mercury resonance radiation with increasing vapour pressure, in the damping of fluorescence in concentrated solutions, and in various chemical reactions. As an example of the latter the decomposition of acetaldehyde (p. 70) may be quoted, where collisions between two molecules of the aldehyde are much more effective than collisions of aldehyde molecules with those of other gases. 

The basic global behaviour of a mixture of acetaldehyde vapour in O2 is illustrated by reference to the p-T ignition diagram. Fig. 5.39. Up to five regions of qualitatively different responses are characteristic [75]. At low ambient temperature and pressure, the system exhibits a steady dark reaction. Region I. This may support a measurable steady-state tem-... 

Type II Solutions of this type show large positive deviations from Raoult s law. The total vapour pressure rises to a maximum which is above the vapour pressure of either of the pure constituents. (Fig. 9.4) Examples of this type are acetaldehyde-carbon disulphide, ethanol-chloroform, water-ethanol etc. 

These materials are very easily autoxidised and often have a low autoignition temperature. It is reported that many of the less volatile liquid aldehydes will eventually inflame if left exposed to air on an absorbent surface. The mechanism is undoubtedly similar to that giving rise to easy ignition in the air-oxidation of acetaldehyde and propionaldehyde initial formation of a peroxy-acid which catalyses the further oxidation[l]. Autoignition temperatures of lower aldehydes are much reduced by pressure, but appear to depend little on oxygen content. The effect is worst in the presence of free liquid, in which initial oxidation appears to occur, possibly catalysed by iron, followed by ignition of the vapour phase [2], An acetaldehyde/rust mix exploded at room temperature on increasing the air pressure to 7 bar. 

The oxidation process is carried out in the temperature range 300— 450°C, and generally studied at atmospheric pressure. Excess air is usually applied (with some exceptions) and substantial amounts of water vapour may be added to the feed. High initial selectivities (>95%) are feasible, and, although some further oxidation (combustion) of the product is unavoidable, yields of 70—90% are reported in the patent literature. The main by-products are carbon oxides, in addition to minor amounts of acrylic acid, acetaldehyde and formaldehyde. Acrylic acid may be a main product depending on specific catalyst properties and reaction conditions, as described in more detail in Sect. 2.2.3. 

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