Vessels mixing effectiveness

In Chapter 9 it is suggested that if the batch blending time is less than one-tenth the residence time and the inlet and outlet are separated in such a way that a line drawn from the inlet to the outlet passes through the impeller, fiiUy back mixed conditions will be achieved. Even in the case of a perfectly backmixed vessel, mixing effects on selectivity must also be checked. 

Cation units usually contain a sulphonic acid resin whilst anion resins fall into the two main categories of strongly basic, with quaternary ammonium groupings and weakly basic, with tertiary amine groups. The final unit is the mixed bed in which, by a mixture of cation and anion resins in the same vessel, the effect is achieved of a multiplicity of separate cation and anion units. Resin separation is necessary for regeneration purposes. Considerable improvements in water quality are obtainable by these means. 

The mixing vessels serve as buffers between the polymerization stage operated in batch mode and the separation units operated continuously. The capacity of each mixing vessel is three polymerization batches and the minimal hold-up is 0.1 polymerization batches to ensure a sufficient mixing effect. 

There are various purposes of using a stirred vessel, and the required mixing effect depends on the purpose. For any mixing purpose, rapid and homogeneous dispersion is required. In a stirred vessel, forced convection by the rotation of an impeller occurs, that is, each element of the fluid has an individual velocity finally, turbulent flow based on the shear stress accelerates the mixing. Therefore, the shape of the impeller has a very important effect on the mixing state. However, there are various types of impeller shapes, and traditional impellers are classified into three types ... 

Mechanical agitation and gas-liquid mass transfer are very important in viscous fermentation media. Most recently, Lim and Yoo (1989) and Lee and Wang (1989) have examined mixing effects in the fermentation of Xanthan gum. Aunins-cf al. (1989) evaluated the effects of paddle geometry on power input and mass transfer in small-scale animal cell culture, 500 mL Corning spinner vessels. The results indicate that power dissipation dependency differs from literature correlations and may compromise scale-up at constant power input from these vessels. 

A certain number of corrugated layers (sandwiches) is required in the plane perpendicular to the flow axis in order to achieve the mixing effect described in section I.B, which places an upper limit on the thickness of the individual sandwiches and hence on the particle size, particularly for vessels of small cross section. It will then be appreciated that the choice of particle size is, for small vessels, partly dependent on the cross-sectional dimensions of the reactor. The lower limit on particle size is given by the necessity to ensure satisfactory convection of reactants and products within the sandwich itself. As a guideline, the particle size is generally in the range 0.5-2.5 mm. 

Vessel blend times are typically 2 sec in a 1 L vessel and 20 sec in a 20,000 L vessel for low-viscosity liquids. If the blend time is small compared to the time constants of all key reactions, then there will be no mixing effects. 

The Da is extremely useful for initial evaluation of reaction conditions under which mixing effects must be considered. The reaction rate constant of the undesired reaction, A r2, can vary over several orders of magnitude. When experimental data are not available but the class of reaction is known, the magnitude of kR2 can be estimated within two orders of magnitude or less. The mixing rate in vessels should not vary by more than two orders of magnitude. With these bracketed values, upper and lower limits on Da can be readily estimated and used as a first measure of mixing sensitivity as follows ... 

Eddies (vortices, whorls) are thus superimposed on the average flow direction, and this causes a strong mixing effect. The size of the largest eddies is of the order of the smallest dimension of the vessel (e.g., the pipe diameter). These large eddies transfer their kinetic energy to smaller ones, which transfer it to still smaller eddies, etc. With decreasing eddy size, the... 

A mixed-bed (MB) contains a uniform mixture of strong add and strong base (Type I) resins in the and OH forms, respectively. Demineraftsation occurs through simultaneous exchange of cation and anions in a single vessel. In effect, this is equivalent to... 

In standard process vessels with low viscosity fluids, the approximate values of p 0.5 s are such that p is large compared to — several orders of magnitude. For not too rapid reactions, micro-mixing is much faster than the reactions and micro-mixing effects are not very critical (Section 12.2). For viscous fluids with smaller p or very fast reactions with higher k, the value of p can be small relative to A, in which case the micro-mixing effects become important (Section 12.2). 

Paul and Treybal [1971], They found that "Power per unit volume is... incapable of correlating the widely different local conditions within a stirred vessel." Instead, a criterion involving both the intrinsic reaction rate constants and a micro-time scale to account for micro-mixing effects (see Sections 12.2 and 12.3) was developed. 

Simulation of mixing effects in reactors requires good data for reaction kinetics physical constants such as viscosity, density, and diffusivily and knowledge of the exact geometrical configuration of the vessel, including feed locations. 

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