Expansion temperatures

Kleppe, C.A. Chart for Compression and Expansion Temperatures, Chemical Engineering, Sept. 19, 1960, p. 213. 

The most common control functions in these early appliances are the control of temperature, pressure, position or distance. Mechanical sensing devices were introduced for these purposes, such as bimetal temperature switches or liquid expansion temperature switches for ovens, washing machines, dishwashers, refrigerators, etc. Electromechanical pressure switches and potentiometric level sensors have also been introduced quite early. 

The blowing agents must be suited to the polymer because the expansion temperature must be compatible with the temperatures of the various processing stages. 

Thermal expansion Temperature of maximum density decreases with increasing salinity for pure water it is at 4°C Fresh water and dilute seawater have their maximum density at temperatures above the freezing point this property plays an important part in controlling temperature distribution and vertical circulation in iakes... 

The density of chemicaUy-blown LDPE foam was altered by varying the amount of blowing agent, degree of crosslinking of the polymer, and the foam expansion temperature. A theory was proposed for the equilibrium density, based on the gas pressures in a Kelvin foam structure, and a rubber-elastic analysis of the biaxial stretching of the cell faces. 20 refs. 

The expansion temperature ranges for a number of acids at 1 4 dynes per cm. compression have been obtained by Adam with the following results. 

It will be noted that the expansion temperature for an acid increases with the length of the hydrocarbon chain (about 8° 0. per CHg), there being no difference between the odd and even numbers of the series as exists in the melting points of the crystals... 

A number of expansion temperatures, i.e. temperatures at B, together with the equilibrium pressures are given in the following table, which can be compared with the data presented on pp. 64, 65. 

Temperature, °C. Density. Coefficient of Expansion. Temperature, "C. Density. Coefficient of Expansion. 

Figure 9.8-9. Influence of the pre-expansion temperature on the particle size in the system glyceride-C02-...

The influence of the pre-expansion temperature of the equilibrated gas-saturated solution on the particle size was studied over a pressure range from 80-200 bar. The temperature is represented as the temperature-difference between the saturation temperature before expansion and the melting point of the substance. The results are shown in Figure 9.8-9. 

The mean particle size is a function of the pre-expansion temperature the particle size reduces with lower pre-expansion temperatures.The closer the saturation temperature is to the melting point of the substance, the smaller are the formed particles. The smallest particles were obtained at high pressure and low temperature, which could have been expected, owing to the higher solubility of CO2 in the melted substances and the resulting higher level of supersaturation during the expansion. 

In order to avoid agglomeration of micronized particles, and thermal degradation of nifedipine at high temperatures (175 and 185°C), the hydrophilic polymer PEG 4000 was added to nifedipine to reduce its melting point. Micronization at pre-expansion temperatures between 50 and 70°C was possible and fine powdered co-precipitates of nifedipine/PEG 4000 were obtained. Their dissolution rate was twice as high as that for pure micronized nifedipine. 

It has been shown that small changes in the pre-expansion pressure at constant pre-expansion temperature and solute concentration do not affect the precipitate characteristics (Mohamed et al., 1989a). On the other hand, for changes in the pre-expansion pressure of 69 bar, the observed precipitate morphology varied from micron-size spherical to fiber shape (Lele and Shine, 1994). However, Domingo et al. (1997) reported that changes in the pre-expansion pressure of their system proved to be inconclusive. Interestingly, Ksibi (1995) noticed a decrease in particle size with an increase in the pre-expansion pressure of about 41 bar. This was accompanied by a narrower particle size distribution. 

An additional advantageous possibility in heat transfer rockets is the use of a diabatic nozzle in which propellant heating continues during the expansion process. While difficult to achieve in practice, such heating extends the potential propellant performance beyond the limitation associated with a maximum, pre-expansion temperature. 

A change from a saturated chain to one in which there is a double bond in the middle of the chain very considerably lowers the expansion temperature e.g. oleic acid is fully expanded even at 0°, and may be estimated to have an expansion temperature somewhere about —30°, while the saturated acid, stearic, of equal chain-length, expands at 46°. The stereochemical configuration of the double bond in the chains is important a trans bond such as that in elaidic acid causes expansion to occur at a temperature only some 40° below that of the corresponding saturated acid. 

The nature of the end group also affects the temperature of expansion. For equal lengths of saturated chain there may be differences of expansion temperature of over 35° between substances with different nd groups e.g. the nitrile with a sixteen-carbon chain in addition to the CN group expands at 17°, but the alcohol expands at 54°. 

Table IV summarizes the results of measurements on expanded films. Section I shows the effect of lengthening the chain in one series section II the effect of the end group, the figures being given for a constant length of chain. In this section the figures have sometimes been obtained by interpolation from other members of the same series, using the rule for variation of expansion temperature stated above. It has been found in all cases that if one member...

A double bond in the aj8 position to the carboxyl group lowers the expansion temperature much less than in the middle of the chain (c). Hughes3 has shown that it approximately doubles the surface potential, however, presumably through the displacement of electrons near the carboxyl head rendering the a carbon more negative and the jS more positive than in the saturated chain. A double bond in the middle of the chain increases the surface potential of the molecule only slightly, but there is still a slight increase in dipole moment when a double bond is present in the middle of the chain. 

The influence of the parameters concentration, pre- and post-expansion pressure and pre- and post-expansion temperature and geometry of the nozzle on the particle size distribution wasn t studied at all. Merely Mohamed [4] examined the influence of concentration, pre- and post-expansion pressure and temperature of the system carbon dioxide - naphthalene. No particle size distribution was measured, only the size of the smallest and biggest particles were measured. For these investigations, anthracene was used as a model substance, while the solubility of anthracene in carbon dioxide [5] is approximately more then 100 times smaler than for naphthalene. 

Liquid carbon dioxide (purity 99, 95 Vol %) was undercooled (W2) to avoid cavitation in the membran pump (P). After the compression to pre-expansion pressure, the fluid is heated to the extraction temperature (W3). The supercritical fluid loaded with anthracene leaves the extractor (V = 0,6 1). With a additional heat exchanger (W4), the solution is heated to pre-expansion temperature. In the separation vessel, the supercritical solution is expanded through a nozzle. The expanded gas will be condensed (Wl) and recompressed or let off. After the experiment, the separation vessel is opened and the particles were collected. The particle size is measured by laser diffraction spectroscopy (Malvern Master Sizer X). 

To determine the influence of the internal nozzle diameter on the particle size distribution in dependence on the post-expansions pressure at constant pre-expansion temperature T = 110°C and pre-expansion pressure p = 220 bar, two nozzles with different internal nozzle diameter (d = 100 pm, d = 150 pm) were under investigation. As it is shown in figure 3, the particle size is smaller for the nozzle diameter d = 150 pm and these differences were smaller as the consistency of the measuring results. 

At constant pre-expansion pressure, the post-expansion pressure was varied from 5 bar til 60 bar. As it is shown in figure 4 at constant pre-expansion temperature T = 110 °C, there is no provable influence of the post-expansion pressure on the particle size distribution for these three pre-expansion pressures. It is not necessary to expand the supercritical solution til atmospherical pressure, the gas could be expanded to 60 bar and recompressed. 

Band broadening and temperature The five terms of Equation (24-14) can be examined in the context of the influence of temperature on flow rates, retention volumes, and diffusion coefficients to obtain an estimate of the overall influence of temperature on band broadening. Through thermal expansion, temperature also influences such factors as thickness of a liquid film and particle and column diameters, and it may also influence slightly the empirical constants in (24-14). With a liquid mobile phase, flow velocity (with the same inlet and outlet pressures) is strongly dependent on temperature. But with flow velocity u maintained constant the first term of (24-14) becomes smaller as diffusion coefficients increase in the mobile phase. For flow rates near the optimum the first term is approximately inversely proportional to The second and third terms increase in direct proportion to the diffusion coefficients in the mobile and stationary phases D and D, whereas the fourth and fifth... 

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