Bulk rotation

For low-viscosity liquids, bulk rotation of the liquid can be reduced with the use of vessels having a rectangular cross-section, with the lateral insertion of the stirrer into the vessel, and with the use of baffles. When the stirring is weak, bulk rotation in cylindrical vessel can also be prevented by installing the stirrer off-center and/or at an angle to the axis, configurations resulting in an uneven mechanical stress on the stirrer shaft. 

In cylindrical vessels, the bulk rotation of the liquid is generally prevented by the installation of baffles, which are usually attached to the vessel wall by means of welded brackets (see Fig. la). When this is not possible, they are either attached to the cover (e.g., in enamel-coated vessels) or are made in the form of a basket with pressure-fitted rings, as shown in Fig. lb (e.g., glass vessels, wooden vats, etc.). Generally, fully effective baffling in a cylindrical vessel is achieved with four vertical baffles of width dT/10. However, in recent years novel approaches (e.g., horizontal baffles) have been investigated. The dead volume behind the baffles is reduced by using baffles of width dT/12 at a small clearance of dT/50 from the wall. 

Anchor mixers may be used in combination with other types of mixers, such as turbine mixers, high-shear mixers, or rotor-stator mixers, which were described in the previous subsection. Such mixers can be placed on a vertical shaft midway between the anchor shaft and blade. A secondary mixer can promote top-to-bottom motion and also limit bulk rotation of the fluid. A stationary baffle is sometimes placed between the anchor shaft and rotating blade to limit fluid rotation and enhance shear. 

A true vortex breaker is normally inserted just ahead of the vessel s liquid exit nozzle as shown in Fig. 13.1.1. This is a very important feature in the geometry at hand since the angular momentum of the incoming gas-liquid mixture will produce bulk rotation of the liquid pool. If a vortex is allowed to form, some of the incoming gas may exit out the underflow and create pump cavitation or other problems downstream. The vortex will also act as a type of fluidic choke and restrict the flow rate out the bottom liquid exit nozzle. 

In addition, the surface unit cell may be rotated with respect to the bulk cell. Such a rotated unit cell is notated as... 

Figure Al.7.5(a) shows a larger scale schematic of the Si(lOO) surface if it were to be biilk-tenninated, while figure Al.7.5(b) shows the arrangement after the dimers have been fonned. The dashed boxes outline the two-dimensional surface unit cells. The reconstructed Si(lOO) surface has a unit cell that is two times larger than the bulk unit cell in one direction and the same in the other. Thus, it has a (2 x 1) synnnetry and the surface is labelled as Si(100)-(2 x i). Note that in actuality, however, any real Si(lOO) surface is composed of a mixture of (2 X 1) and (1 x 2) domains. This is because the dimer direction rotates by 90° at each step edge.

The relation between the microscopic friction acting on a molecule during its motion in a solvent enviromnent and macroscopic bulk solvent viscosity is a key problem affecting the rates of many reactions in condensed phase. The sequence of steps leading from friction to diflfiision coefficient to viscosity is based on the general validity of the Stokes-Einstein relation and the concept of describing friction by hydrodynamic as opposed to microscopic models involving local solvent structure. In the hydrodynamic limit the effect of solvent friction on, for example, rotational relaxation times of a solute molecule is [ ]... 

It is also possible to measure microwave spectra of some more strongly bound Van der Waals complexes in a gas cell ratlier tlian a molecular beam. Indeed, tire first microwave studies on molecular clusters were of this type, on carboxylic acid dimers [jd]. The resolution tliat can be achieved is not as high as in a molecular beam, but bulk gas studies have tire advantage tliat vibrational satellites, due to pure rotational transitions in complexes witli intennolecular bending and stretching modes excited, can often be identified. The frequencies of tire vibrational satellites contain infonnation on how the vibrationally averaged stmcture changes in tire excited states, while their intensities allow tire vibrational frequencies to be estimated. 

Place in the tube sufficient organic compound to give subsequently about 0-3 g. of the silver halide, and weigh again. Now allow the small tube to slide carefully down the inclined Carius tube until it finally adopts the position shown in D (Fig. 72). If the compound readily loses halogen in the presence of nitric fumes, the Carius tube should first be rotated in an oblique position to wet the tube for about 10 cm. from the bottom the small tube, if cautiously inserted into the Carius tube, will now come to rest when it first reaches the wet portion of the tube and will thus be held above the main bulk of the acid until the tube is sealed. 

In the methacrylate homologous series, the effect of side-chain bulkiness is just the opposite. In this case, however, the pendant groups are flexible and offer less of an obstacle to free rotation than the phenyl group in polystyrene. As chain bulk increases, molecules are wedged apart by these substituents, free volume increases, and Tg decreases. 

The steric bulk of the three iodine atoms in the 2,4,6-triiodoben2ene system and the amide nature of the 1,3,5-substituents yield rotational isomers of the 5-A/-acyl-substituted 2,4,6-triiodoisophthalamides. Rotational motion in the bonds connecting the side chains and the aromatic ring is restricted. These compounds also exhibit stereoisomerism when chiral carbon atoms are present on side chains. (R,5)-3-Amino-l,2-propanediol is incorporated in the synthesis of iohexol (11) and ioversol (12) and an (3)-2-hydroxypropanoyl group is used in the synthesis of iopamidol (10). Consequendy, the resulting products contain a mixture of stereoisomers, ie, meso-isomers, or an optical isomer. 

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