Mixtures mixing indices

If the component analysed represents 20 per cent of the mixture by mass and each of the samples removed contains approximately 100 particles, comment on the quality of the mixture produced and present the data in graphical form showing the variation of mixing index with time. 

In equation 2.28 Up is the lower of the two minimum fluidizing velocities of the two types of particle in the mixture and Ufo is the velocity at which mixing takes over or begins to dominate segregation. Thus, as the superficial gas velocity in the bed is increased, the mixing index increases from M = 0 at the lower minimum fluidizing velocity (m = Mp), where the bed is quiescent with no particle movement because of the absence of bubbles, to M = 0.5 when, by definition, the velocity is equal to Uto- The mixing index approaches a value of unity as the velocity increases still further (Nienow and Chiba, 1985). 

When performing scale-up tests, be sure to take enough samples to give an accurate description of the mixture state in the vessel. Furthermore, be wary of how you interpret your samples know what the mixing index means and what your confidence levels are. 

With granular solids the mixing index is based, not on conditions at zero mixing, but on the standard deviation that would be observed with a completely random, fully blended mixture. With pastes, assuming the analyses are perfectly accurate, this value is zero. With granular solids it is not zero. 

As the mixture approaches the random state the index will approach unity. Non-random mixtures will have a mixing index greater than unity. 

In most mixing systems this is the best possible mixture variance for these components. The experimentally determined value will lie between these extremes and can be used to calculate a mixing index for that particular mixture. Thus from equation (2.3)... 

The numerator on Equahon 6.2 would be an indicator of how much mixing has occurred, while the denominator would show how much mixing can occur. In practice, however, the values of s, even for a very poor mixture, lie much closer to s, than to Sq. Poole et al. (1964) suggested an alternative mixing index, that is. 

A few polymer chains chemically end labeled with a fluorescent probe (NBD) are mixed to the polymer melt (4). The sample is a drop of this mixture (optical index, 03), squeezed between a moving plate and the upper surface of a silica prism (optical index, ni > n2) which form the walls of a plane Couette cell. Two laser beams (wavelength in vacuum Xq) impinge on the prism surface at an incidence angle 0i greater than the critical angle for total internal reflection, 0c = sin (n2/ni). They both... 

Solubility of resins can be predicted in a similar way as for the solubility of polychloroprene rubbers in a solvent mixture (see Section 5.5) by means of solubility diagrams (plots of the hydrogen bonding index (y) against the solubility parameter (5). Another more simple way to determine the solubility of resins is the determination of the cloud point, the aniline and the mixed aniline points. 

Charge the OH functional urethane water dispersion prepared in Example 8 (200 g, 0.1603 eq. OH) to a suitable container. Vigorously stir the dispersion with a mechanical stirrer and slowly add the 18.8% NCO water-dispersible polyisocyanate described above (40.0 g, 0.1791 eq.). This will give an NCO-OH equivalent ratio (index) of 1.12. To this mixture, add Silwet L-77 (a flow aid from OSi Specialties) and thoroughly mix. Use a draw-down bar to make 6-mil (0.006-in.) wet-thickness films on glass plates and 5-mil (0.005-in.) films on Bonderite-treated steel panels (Bonderite is from Henkel). Condition the resultant films at 72°F and 50% RH for 2-3 weeks before evaluating. For the finished... 

Mixed solvents are generally unsatisfactory for use in the determination of polymer molecular weights owing to the likelihood of selective absorption of one of the solvent components by the polymer coil. The excess of polarizabilit f of the polymer particle (polymer plus occluded solvent) is not then equal to the difference between the polarizabilities of the polymer and the solvent mixture. For this reason the refractive increment dn/dc which would be required for calculation of K, or of i7, cannot be assumed to equal the observed change in refractive index of the medium as a whole when polymer is added to it, unless the refractive indexes of the solvent components happen to be the same. The size Vmay, however, be measured in a mixed solvent, since only the dissymmetry ratio is required for this purpose. 

Special care has to be taken if the polymer is only soluble in a solvent mixture or if a certain property, e.g., a definite value of the second virial coefficient, needs to be adjusted by adding another solvent. In this case the analysis is complicated due to the different refractive indices of the solvent components [32]. In case of a binary solvent mixture we find, that formally Equation (42) is still valid. The refractive index increment needs to be replaced by an increment accounting for a complex formation of the polymer and the solvent mixture, when one of the solvents adsorbs preferentially on the polymer. Instead of measuring the true molar mass Mw the apparent molar mass Mapp is measured. How large the difference is depends on the difference between the refractive index increments ([dn/dc) — (dn/dc)A>0. (dn/dc)fl is the increment determined in the mixed solvents in osmotic equilibrium, while (dn/dc)A0 is determined for infinite dilution of the polymer in solvent A. For clarity we omitted the fixed parameters such as temperature, T, and pressure, p. 

Figure 6.2. (a) The effects of salinity on the sensitivity of standard additions of ammonia in laboratory mixed waters ( ) and in waters from the Tamar estuary (A) expressed as percentage of response in river water. For comparison, the salt error curves reported by Loder and Gilbert [3] are also shown (... and —, respectively), (b) Contribution of reactive index and organic absorbance to the optical blacks in the Chemlab Colorimeter. = River water-seawater mixture, o = De-ionized water-seawater mixture. Source [2]... 

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