Blend time

Fig. 10. Log—log plot scale-up by power per unit volume where for A, constant blend time, = 2/3 B, same vortex, y = 1/6 C, dispersion, = 0 D,...

If mixing quaHty better than 95% is desired, the blend time can be estimated using a variance decay model for ampHtude of concentration variation ... 

Manufacturing, analytical, and quaUty control procedures are thus estabhshed. Specifications for taw and in-process materials, as well as for final products per USP/NF and in-house standards are also determined. Process and formula vaUdation assures that each technological procedure in manufacture accomplishes its purpose most efficiently, eg, blending times for powdered mixtures in tableting, and that each formula ingredient is present in optimal concentrations (12). Thus, it serves to ensure process control (qv), reproducibiUty, and content uniformity. 

The particle size distribution of Rhovanil Extra Pure vanillin shows a less narrow profile than other standard mesh grades available on the market. The product shows an improved mixabiUty in blending operations, allowing shorter blending time of compounds or food mixes, and better homogeneity of vanillin content, especially in low content vanillin blends. 

Blend time tb, the time required to achieve a specified maximum standard deviation of concentration after injection of a tracer into a stirred tank, is made dimensionless by multipfying by the impeller rotational speed ... 

Dimensionless pumping number and blend time are independent of Reynolds number under fully turbulent conditions. The magnitude of concentration fluctuations from the final well-mixed value in batch mixing decays exponentially with time. 

For any given process, one takes a qualitative look at the possible role of fluid shear stresses. Then one tries to consider pathways related to fluid shear stress that may affect the process. If there are none, then this extremely complex phenomenon can be dismissed and the process design can be based on such things as uniformity, circulation time, blend time, or velocity specifications. This is often the case in the blending of miscible fluids and the suspension of sohds. 

If the blending process is between two or more fluids with relatively low viscosity such that the blending is not affected by fluid shear rates, then the difference in blend time and circulation between small and large tanks is the only factor involved. However, if the blending involves wide disparities in the density of viscosity and surface tension between the various phases, then a certain level of shear rate may be required before blending can proceed to the required degree of uniformity. 

There is the possibihty of misinterpretation of the difference between circulation time and blend time. Circulation time is primarily a function of the pumping capacity of the impeller. For axial-flow impellers, a convenient parameter, but not particularly physically accurate, is to divide the pumping capacity of the impeller by the cross-sectional area of the tank to give a superficial hquid velocity. This is sometimes used by using the total volume of flow from the impeller including entrainment of the tank to obtain a superficial hquid velociW. 

It turns out that in low-viscosity blending the acdual result does depend upon the measuring technique used to measure blend time. Two common techniques, wliich do not exhaust the possibilities in reported studies, are to use an acid-base indicator and inject an acid or base into the system that will result in a color change. One can also put a dye into the tank and measure the time for color to arrive at uniformity. Another system is to put in a conductivity probe and injecl a salt or other electrolyte into the system. With any given impeller type at constant power, the circulation time will increase with the D/T ratio of the impeller. Figure 18-18 shows that both circulation time and blend time decrease as D/T increases. The same is true for impeller speed. As impeller speed is increased with any impeller, blend time and circulation time are decreased (Fig. 18-19). 

However, when comparing different impeller types at the same power level, it turns out that impellers that have a higher pumping capacity will give decreased circulation time, but all the impellers, regardless of their pumping efficiency, give the same blend time at the... 

FIG. 18-19 Effect of impeller speed and power for the same diameter on circulation time and blend time for a particular impeller. 

For other situations in low-viscosity blending, the fluid in tanks may become stratified. There are few studies on that situation, but Oldshue (op. cit.) indicates the relationship between some of the variables. The important difference is that blend time is inversely proportional to power, not impeller flow, so that the exponents are quite different for a stratified tank. This situation occurs more frequently in the petroleum industiy, where large petroleum storage tanks oecome stratified either by filhng techniques or by temperature flucduations. 

Figure 18-21 gives some data on the circulation time of the hehcal impeller. It has oeen observed that it takes about three circulation times to get one blend time being the visual uniformity of a dye added to the material. This is a macro-scale blending definition. 

Emulsions Almost eveiy shear rate parameter affects liquid-liquid emulsion formation. Some of the efrecds are dependent upon whether the emulsion is both dispersing and coalescing in the tank, or whether there are sufficient stabilizers present to maintain the smallest droplet size produced for long periods of time. Blend time and the standard deviation of circulation times affect the length of time it takes for a particle to be exposed to the various levels of shear work and thus the time it takes to achieve the ultimate small paiTicle size desired. 

Scale-Up of Batch Mixers The prime basis of scale-up of batch mixers has been equal power per unit volume, although the most desirable practical criterion is equal blending per unit time. As size is increased, mechanical-design reqmrements may hmit the larger mixer to lower agitator speeds if so, blend times will be longer in the larger... 

Electrostatic charges may cause particles to repel each other. When continued blending may cause such charges to build up, it is important to determine the precise blending time required and not to overblend. 

D/T = 0.5 / -inch radial clearance Blend time—equal Heat transfer coefficient—equal ... 

If the calculated blending time is longer than desired, nozzle discharge flow rate can be increased or, nozzle diameter increased. Consideration of nozzle recirculation line pressure drops and pump characteristics is required to select the parameter to change. 

Technique Tracer Blend time reached when... 

Additions made to a vessel already undergoing agitation (blend times of stratified fluids ean be eonsiderably longer). 

Blend time and ehemieal produet distribution in turbulent agitated vessels ean be predieted with the aid of Computational Fluid Mixing... 

Blending of ehemieal reaetants is a eommon operation in tlie ehemieal proeess industries. Blend time predietions are usually based on empuieal eorrelations. When a eompetitive side reaetion is present, the final produet distribution is often unknown until the reaetor is built. The effeets of the position of the feed stream on the reaetion byproduets are usually unknown. Also, the seale-up of ehemieal reaetors is not straightforward. Thus, there is a need for eomprehensive, physieal models that ean be used to prediet important information like blend time and reaetion produet distribution, espeeially as they relate to seale and feed position. 

The objeetive of the following model is to investigate the extent to whieh Computational Fluid Mixing (CFM) models ean be used as a tool in the design of industrial reaetors. The eommereially available program. Fluent , is used to ealeulate the flow pattern and the transport and reaetion of ehemieal speeies in stirred tanks. The blend time predietions are eompared with a literature eonelation for blend time. The produet distribution for a pair of eompeting ehemieal reaetions is eompared with experimental data from the literature. 

V3.03. The tank diameter was T = 1 m. Furthermore, Z/T = 1, D/T = 0.33, C/T = 0.32, and rpm = 58. The flow pattern in this tank is shown in Figure 10-9. Experimental data were used as impeller boundary eonditions. Figure 10-10 shows the uniformity of the mixture as a funetion of time. The model predietions are eompared with the results of the experimental blend time eorrelation of Fasano and Penny [6]. This graph shows that for uniformity above 90% there is exeellent agreement between the model predietions and the experimental eorrelation. Figure 10-1 la shows the eoneentration field at t = 0 see. Figures 10-1 lb through 10-1 Id show the eoneentration field at t = 0,... 

The models presented correctly predict blend time and reaction product distribution. The reaction model correctly predicts the effects of scale, impeller speed, and feed location. This shows that such models can provide valuable tools for designing chemical reactors. Process problems may be avoided by using CFM early in the design stage. When designing an industrial chemical reactor it is recommended that the values of the model constants are determined on a laboratory scale. The reaction model constants can then be used to optimize the product conversion on the production scale varying agitator speed and feed position. 

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