Decompression section

The Dynamit-Nobel extmsion process (252) utilizes a volatile plasticizer such as acetone which is injected into the decompression section of a two-stage screw and is uniformly dispersed in the vinyl resin containing a stabilizer. The resulting PVC foam has low density and closed cells. 

The decompression section in the screw downstream from the blister was where a liquid plasticizing additive was injected into the extruder. The large channel depth in this section was used to accept the additional volume and to control pressure. The sections downstream from the first meter were not the focus of this case study and will not be discussed further. 

The final portion of the screw has a deep channel section following a decompression section. The channel depth is constant over the last screw section. Again, the deeper channel in the final screw section will reduce the pressure generating capability of the screw. A more effective power reduction can be obtained by not only changing the channel depth, but the channel depth, helix angle, flight width, and radial clearance in an optimum fashion as discussed in Section 8.3. 

The success of compression agglomeration depends on the effective utilization and transmission ofthe applied external force and on the ability of the material to form and maintain interparticle bonds during pressure compaction (or consolidation) and decompression. Both these aspects are controlled in turn by the geometiy of the confined space, the nature of the apphed loads and the physical properties of the particulate material and of the confining walls. (See the section on Powder Mechanics and Powder Compaction.)... 

Values from high-pressure permeation tests can give useful information regarding the selecting of elastomer types to withstand potential explosive (rapid gas) decompression damage (Sections... 

Explosive Decompression in Solid Elastomer Sections—The Nature of ED Damage... 

Explosive Decompression in Solid Elastomer Sections—Predominantly O-ring and Other Seals... 

FIGURE 23.14 Explosive decompression (ED) damage in a fluoroelastomer O-ring seal section left and miscellaneous samples of a nonfluorinated oil-field elastomer right obtained using pressure vessels such as those shown in Eigure 23.15. 

Further to aspects discussed in Section 23.5.1.2, the simplest strategy, if it is practical, is slow decompression. If the pressure can be reduced sufficiently slowly, the gas which is dissolved in the elastomer can diffuse out of the mbber without buUding up sufficient internal pressure to cause damage. The rate of decompression required may be roughly calculated if the diffusion coefficient of the gas and its solubility in the elastomer are known— but in practice it will probably require a laboratory simulation. 

Metoclopramide is a dopamine antagonist that centrally inhibits stimulation of the CTZ. By improving gastric emptying, it can decompress the stomach, thereby decreasing a peripherally associated stimulation of the emetic center. Metoclopramide may precipitate ex-trapyramidal reactions and sedation. For further details, see earlier section, Drugs that Increase GI Motility. 

The reaction rate constant and the diffusivity may depend weakly on pressure (see previous section). Because the temperature dependence is much more pronounced and temperature and pressure often co-vary, the temperature effect usually overwhelms the pressure effect. Therefore, there are various cooling rate indicators, but few direct decompression rate indicators have been developed based on geochemical kinetics. Rutherford and Hill (1993) developed a method to estimate the decompression (ascent) rate based on the width of the break-dovm rim of amphibole phenocryst due to dehydration. Indirectly, decompres-... 

Each chamber was filled with agarose solution to a depth of 4 mm. After gelation, the agarose samples were exposed to 100-fsw (feet sea water) pressures (i.e., 44.5 psig) for 40 min at 21°C, and then decompressed to atmospheric pressure in accord with one of the seven different decompression schedules tested (see Section 8.1.1). Only bubbles formed in the bottom 3 mm of a given agarose sample were counted, so that the total volume of gel examined in each sample amounted to 0.27 ml. 

The amorphous phase appearing above 20 GPa at room temperature (see above) has also recently been studied by X-ray diffraction [135] and Raman scattering [132,133]. Serebryanaya et al. [135] identify the structure as a three-dimensionally polymerized Immm orthorhombic lattice, but find that compression above 40 GPa gives a truly amorphous structure. In contrast to the orthorhombic three-dimensional polymer structure discussed in the last section, the best fit here is found for (2+2) cycloaddition in two directions, with (3+3) cycloaddition in the third, and thus some relationship to the tetragonal phase. From the in situ X-ray data a bulk modulus of 530 GPa is deduced, about 20% higher than for diamond. Talyzin et al. [132, 133] find that this phase depolymerizes on decompression into linear polymer chains, unless the sample is heated to above 575 K under pressure. A strong interaction with the diamond substrate is also noted, such that only films with a thickness of several hundred nm are able to polymerize fully [ 132]. Hardness tests were also carried out on the polymerized films, which were found to be almost as hard as diamond and to show an extreme superelastic response with a 90% elastic recovery after indentation [133]. 

In the case of flash degassing, the polymer solution is first heated under pressure to above the boiling point of the volatile components and decompressed directly into the ZSK. The polymer and solvent (monomer) spontaneously separate from each other inside the ZSK and the majority of the volatile components are released via the back venting system. Depending on the pressure and the temperature, up to 90% of the solvent can be removed in this way. Efficiency depends on the temperature of the polymer solution at the feed intake, the pressure drop in the back vent, and the material properties of the feeding system. The back vent is located upstream from the polymer or polymer solution feeding port (see Fig. 10.2). In this case, there is no melt in the screw channel so that the entire screw cross-section is available for the removal of gas or vapors. 

Low-Density Amorphous Ice (LDA). Upon heating HDA to T > 115 K or very high density amorphous ice (VHDA) to T > 125 K at ambient pressure, the structurally distinct amorphous state LDA is produced. Alternatively, LDA can also be produced by decompressing HDA or VHDA in the narrow temperature range of 139-140 K to ambient pressure [153-155]. The density of this amorphous state at 77 K and 1 bar is 0.93 g/cm3 [152]. These amorphous-amorphous transitions are discussed in Sections III.C and III.D. 

Wavelet transforms (Section 3.6.2) are a hot topic, and involve fitting a spectrum or chromatogram to a series of functions based upon a basic shape called a wavelet, of which diere are several in the literature. These transforms have the advantage that, instead of storing, for example, 1024 spectral datapoints, it may be possible to retain only a few most significant wavelets and still not lose much information. This can result in both data decompression and denoising of data. 

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