Batch mixing of viscous fluids with helical‐ribbon agitators in 2.4 liter and 13 liter vessels has been studied for agitator speeds up to 200 RPM. Seven different agitators of different dimensions were employed in this work. Mixing times were measured using a decoloration technique and circulation times were determined by the tracer bead method. In addition, velocity profiles were obtained from streak photographs using selective illumination of the vessel and PVC powder as tracer particles. It was found that the mixing times of Newtonian fluids, which agreed with previously published data, were considerably (3 to 7 times) shorter than those of the viscoelastic fluids. The mixing time was strongly affected by the fluids' elasticity; increasing as the fluid elasticity increased. The velocity profiles were qualitatively similar for all the fluids but showed decreased axial circulation and increased circumferential flow as fluid elasticity increased. However, mixing is not only a function of the axial circulation (impeller pumping rate) but also is a function of the perturbations superimposed on the main flow. A simple, first approximation model based on the impeller geometry and flow patterns is proposed to correlate the circulation capacity and mixing time data for the various geometries studied.

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Five isothermal emulsion polymerizations of styrene in a tubular reactor were studied experimentally using an emulsifier

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It has been reported previously that the photoreactive p phase of p-formyl-trans-cinnamic acid represents an exception to the well-established correlation between structural and photochemical properties of crystalline trans-cinnamic acid derivatives. However, it is shown in this paper that the crystal structure reported previously to be the p phase of p-formyl-trans-cinnamic acid is actually the photostable y phase. In a wider context, the work reported here highlights the necessity of carrying out powder diffraction investigations in the determination of structure-property relationships for crystalline solids.[2 + 21 Photodimerization reactions in crystalline transcinnamic acid and its derivative^'-^ represent the classic example of a set of solid-state reactions which conform to the topochemical principle. 3-1 One consequence of this principle is that the course of certain solid-state reactions can be understood completely from a knowledge of the spatial arrangement of molecules in the crystal structure of the 'reactant' crystal. Crystals of trans-cinnamic acid and its derivatives can be classified as a-, por y-types according to their behaviour upon exposure to UV radiation. UV irradiation of a-type crystals produces a centrosymmetric (a-truxillic acid) dimer, whereas UV irradiation of P-type crystals produces a mirror-symmetric (p-truxinic acid) dimer. In contrast, no reaction occurs when y-type crystals are exposed to UV radiation. The single-crystal X-ray diffraction studies of Schmidt ' demonstrated well-defined correlations between crystal structure and photoreactivity in these materials, with the a-, pand y-type crystals each having a characteristic mode of molecular packing. Furthermore, it has been noted that, in all photoreactive (a-and P-type) crystals, the distance Paper 4/00969J