Retrograde viewing

In view of the interest in this field, an experimental investigation was undertaken to determine the applicability of supercritical phenomena to the separation of butadiene from C4 mixtures. In particular, the separation of 1-butene from 1,3-butadiene is a key factor in the separation process. Results of these studies are considered in light of predictions obtained from a representative equation of state in the retrograde region of the SC solvent-solute VLE envelope. 

There are several areas of interest in chemical processing that should be reviewed in this chapter. These areas include retrograde condensation, polymers, and electrolytes. In each case, the material is more extensive than can be covered in any detail in this chapter. Reference should be made to Prausnitz, Lichtenthaler, and Gomes de Acevedo. Furthermore, the principles covered in this chapter form the basis for viewing and calculating the phase equilibrium that applies. The subjects of polymers and electrolytes are so complex that they require a chapter by themselves. However, a brief statement follows describing retrograde condensation. 

The NGF model has led to the suggestion that all neurotrophic factors must be retrogradely transported (Thoenen et al., 1985) a point of view that is much more rigid than the evidence warrants. If there are ways for a factor to signal from the target in the absence of the transport of the factor itself, then it should be still considered a neurotrophic factor. 

The following experiment, at firft view, would feem to invalidate this opinion of the retrograde motions of the lymphatic veflels, in fome difeafes. 

Fig. 1.29a-d. Colon atresia, a Supine radiograph shows gaseous distension of howel. b,c Contrast enema, anteroposterior and lateral views show abnormally small colon (microcolon) with complete obstruction to retrograde flow of contrast material proximal to the middle transverse portion of colon, d Photograph at operation shows a distended colon up to the site of the atresia (arrow) and a microcolon distal to it... 

Fig. 1. (A) Cells labeled with Dil and DiA in the chick embryo neural plate and viewed immediately after iontophoretic applications. About four cells are labeled with Dil (red) and one cell with DiA (green). Bar is 10 gm. (B) After 48 h of development, the descendants of cells labeled as in (A) are still visible and their two colors distinct. Bar is 50 gm. (C) Motor neurons retrogradely labeled from their peripheral nerves in fixed tissue. One nerve was labeled with Dil, and the adjacent nerve with DiA. Bar is 40 gm. [See Color Plate)...

The solvent power of a supercritical fluid is a strong function of its density and it can be adjusted between gas and liquid values with small changes in pressure. The expansion of a saturated supercritical solution leads to loss in the solvent power, and as a consequence to solute precipitation. If the expansion is rapid (<10" sec), such as can be obtained in nozzles and capillaries, it leads to appreciable supersaturation and hence to small particles. Supersaturation can be reached also with temperature variations (a decrease of temperature in the normal region and a decrease in the retrograde region) and from a thermodynamic point of view the effects are similar. 

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