I&EC
-RTS & COMMENTS Breathable gas mixture from laughing gas decomposition Polyolefin melt viscosity prediction
A new model for gas absorption into a turbulent fluid
SYNTHETIC AIR SUPPLY Mixture of O a and N2 provided to closed systems from catalytic decomposition of nitrous oxide Nitrous oxide, Nz0, has as its main claim to fame its use, now diminishing, as a general anesthetic for dentistry and surgery (for which it has become known, jocularly but with some accuracy, as laughing gas). Since NpO does not by itself support life, it is customarily mixed with oxygen for anesthetic purposes. A growing use for the gas is as the propellant fluid for aerosols, and it is this use which seems to assure the continued production of NpO, usually from the thermal decomposition of ammonium nitrate. It has long been known that nitrous oxide is capable of being decomposed by heat according to: 2 Ne0 = 2 Na
+ Oz
The decomposition begins at about 520’ C. and is complete at 940’ C. The use of suitable catalysts materially reduces the temperature at which decomposition starts, and in fact the catalytic decomposition of NzO has been adopted by researchers as a test reaction to investigate the catalytic action of oxides, especially to deduce relations between catalytic activity and semiconductivity of oxides. The detailed kinetics of the catalyzed decomposition are also of considerable interest, and center on the existence of possible rate-controlling steps, such as the desorption of oxygen from the catalyst surface. [Such a kinetic study is described by Tanaka and Ozaki in J. Catalysis 8, 307 (1767).1 However, a potentially very valuable practical use for the nitrous oxide decomposition is in the production of a nitrogen/oxygen mixture
which is breathable by man. Ever since the January 1967 Apollo incident at Cape Kennedy, in which three men died in a cabin fire fed by a pure oxygen atmosphere, alternatives to pure oxygen as the breathable gas in closed systems have been sought. Moreover, the effects on a human resulting from breathing pure oxygen for an extended period of time, such as on a long space mission, are thought to be deleterious; there is evidence that nitrogen has a significant physiological role in animal respiration. It has been found that for comfort and well-being, oxygen partial pressures in a breathable atmosphere should be between 100 and 425 mm. Hg. In air at atmospheric pressure the OZpartial pressure is 157 mm. Hg, so that as pressure is diminishedas it often is in spacecraft to minimize leakage losses from the craft to the vacuum of space-increased oxygen concentrations in the atmosphere are needed for easy breathing. Consequently it seems desirable to
provide closed “ecological” systems with a gas having the same constituents as air ( 0 2 , Nz, Con, water vapor) .in approximately the same proportions, but with increased oxygen concentration so that its partial pressure is adequate for breathing at reduced pressures. The storage of nitrogen and oxygen presents difficulties because of the high pressures needed for liquefaction, necessitating heavy-walled cylinders, and the low specific gravities of the liquids, Moreover, ratio controls are needed to meter the mixture to the closed system in the required proportions. According to a patent recently issued to the Isomet Corp. (US. 3,351,562, Nov. 7, 1967), it is possible to arrange for the catalyticdecomposition of N 2 0to provide a suitable mixture (see sketch below). The product gas contains 67% Na and 33% Oz in accordance with the decomposition reaction equation above. The catalyst claimed to be effective in the patent is copper-magnesia supported on
Procerr f m cntnlvtic dccombosition of nitrous oxide to giue brenthblc mixture of oxygen and nitrogen VOL 6 0
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I&EC R E P O R T S
Standardized Adsorbents Woelm For Column and Thin-Layer Chromatography For purification of organic solvents For Research For Manufacturing Process
Aluminas, act. I (basic, neutral, and acid) Silica Gels, Polyamides, Magnesium Silicates, Acetyl Cellulose for the resolution of racemates
asbestos fiber, and the temperatures at which the reaction is substantially complete are quoted variously as 490°, 570°, and 650" C., depending on the rate of nitrous oxide flow. Naturally the figures in the patent are merely illustrative, but since N 2 0 is thermodynamically unstable with respect to nitrogen and oxygen, many catalysts can be found to make the reaction go at these comparativeIy low temperatures. The great attraction of the Isomet process is that it calls for the storage of nitrous oxide only. NzO is commercially available as a liquid under gas pressures of about 800 psi and the liquid has a high density, which reduces the required storage space. The catalyst has to be supplied with heat to maintain a high enough temperature, but since the decomposition reaction is exothermic this external heat demand is somewhat lessened. The suggested process seems worthy of serious consideration for adoption in spacecraft and in other situations where a breathable atmosphere must be provided for humans enclosed in a sealed system.
PREDICTION OF FLOW BEHAVIOR OF POLYOLEFINS
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Considerable data may be found in the literature relating melt viscosity either to shear conditions or to temperature. I t is, however, frequently difficult or impossible to apply these data to predicting the flow behavior of the specific resin of interest to an engineer or rheologist. Moreover, the actual measurement of melt viscosity at a large number of temperatures and shear rates is normally a tedious and time-consuming task. Dr. Robert A. Mendelson, Monsanto Co., presented at the South Texas Section A.1.Ch.E. meeting, Oct. 13, 1967, an experimental basis for a generalized method for predicting the flow curves of various olefin polymers at any desired temperature from experimental data for those
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
polymers at one temperature. This prediction is accomplished by means of a shear rate-temperature superposition metod. Typical flow curves for a particular olefin were obtained by plotting log shear stress us. log shear rate at temperatures of 120°, 150°, 175", 200°, 250°, and 300 O C. These curves may be shifted along the shear rate axisLe., at constant shear stress for each point on a curve, to superimpose on a single master curve corresponding to an arbitrarily chosen reference temperature. The horizontal shift factors were calculated from
at constant 7,. I t should be noted that this equation may also be written
at constant T ~ which , is the frequent use of an approximate form of the reduced-variable viscosity equation. Dr. Mendelson has demonstrated the validity of the method for several samples by constructing master curves. In the case of the polypropylene sample, where some divergence of the data appears at the low shear end of the curve, it has been postulated that this is due to polymer degradation at long residence times and high temperature rather than to failure of the superposition concept. This appears to provide a basis for a rather general technique for predicting melt viscosity flow curve data at various temperatures. Equation 1 may be rew-ritten in terms of the shear rate at a temperature, T , as follows :
where the corresponding shear stress is the same at the true shear rates. Thus, given a set of shear stress-shear rate data at the reference temperature and a knowledge of AT as a function of temperature, it is possible to construct sets of rw- y w - ?la data at any desired temperature.
The temperature dependence of the resultant shift factor has, for each system studied, been shown to be capable of representation by an Arrhenius-,type equation where constants appear to be a function of the (generic) system, but independent of molecular weight and molecular weight distribution. Thus, a generalized tool is available using the superposition equation and the shift factor temperature dependence to predict flow curves at various temperatures for given samples of any of these polymers from a single flow curve at one temperature for the particular sample.
GAS ABSORPTION INTO A TURBULENT LIQUID Gas absorption by turbulent liquids is a process that is difficult to predict numerically, and is not easy to measure except as a gross over-all phenomenon. Models for predicting gas absorption have therefore been developed to replace the exact physical situation by a much simpler one. The value of such models depends on how generally they can be applied, how accurate they are, and the extent to which they involve arbitrary parameters that vary from system to system. Because the models that have been proposed to date retain one or more arbitrary constants, the only way in which comparison with experimental results can serve to distinguish between them is by holding everything in the equations constant except the diffusion coefficient. Unfortunately, it is difficult to vary the diffusion coefficient over a wide range, while it is easy to elaborate models sufficiently to match available experimental information. G. E. Fortescue and J. R. A. Pearson [(Chern. Eng. Sci. 22, 1163-7 (1 967) ] present a new model. This model attempts to retain most of the physical characteristics of a deforming (plane) surface such as is observed
at the interface between a turbulent liquid and a gas, and uses only such parameters as can be readily correlated with measurable kinematic qualities characteristic of the liquid flow field. All theories agree that processes close to the surface determine rates of gas absorption, and that mixing and thus mass transfer in the bulk of the fluid are rapid by comparison. With this premise, Fortescue and Pearson developed a quasisteady large-eddy model and the revelant transport equation. The success of a simple calculation scheme under both laminar and turbulent flow conditions suggests that the method may be more generally applied. In either case, all that is really required is information about the surface velocity distribution, whether it is steady or only statistically defined. For all circumstances where the Peclet number is large, it can be assumed that the component of the velocity vector parallel to the surface is a function of surface position only. Therefore, the component normal to the surface can be obtained from the equation of continuity. An advantage of this model is that with suitable refinement, it could be used in circumstances where both random turbulent eddies and steady secondary flows are significant in promoting mass transfer across a free surface. This situation often arises when vigorous stirring occurs well beneath the surface. I t requires the free surface to remain essentially flat. Cases when large surface waves rise, possibly with some breaking, often lead to rapid increases in the mass transfer coefficient. This is a nontrivial matter because, in practical circumstances where large absorption rates are required, the establishment of very large interfacial areas is of prime importance. The large-eddy approach could also be extended to cover the case of absorption with reaction by addition of a reaction term to the basic mass transfer equation in the model.
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