Agitation variables

When proeessing is eontrolled by heat transfer variables, a log mean temperature differenee (ATLj. j.p) and heat transfer surfaee area will predominate over the agitation variables. Provided it is suffieient to give a homogeneous proeess fluid temperature, inereased agitation ean only reduee the inside film resistanee, whieh is one of a number of resistanees that determines the overall heat transfer eoeffieient. 

The best values of the parameter Cj are 1.51, 1.36, and 2.01 for no mass transfer, d and c direetion of transfer respectively. The product af is considered as the agitation variable in the equation, since the fit could not be improved if a and / were treated separately. The average absolute value of the relative deviation in the predicted values of d 2 from the experimental points is 16.3%. Even in packed columns, the separation can be substantially improved by pulsing of the continuous phase resulting from greater shear forces that reduce the drop size and increase the interfacial area [1, Chapter 8]. 

Johnson et al. (J3) suggest the use of the hydrogenation reaction of a-methylstyrene with a suspended palladium-alumina catalyst as an alternative test system to establish the effect of agitation variables on liquid-phase mass-transfer coefficients. They found the over-all hydrogen transfer coefficient to vary in a complex manner with agitator speed, and to increase with the 0.6 power of the superficial gas velocity up to a point beyond which the transfer showed no further change with gas velocity. 

In the normal program, the physical properties of the materials do not change with the scale of the operation. In a given vessel then, the major agitation variables are the agitator speed N and the impeller diameter D. In the application of dimensionless groups to agitated systems, the characteristic velocity term has usually been ND (proportional... 

Finally, it should be noted that external variables like gas rate, volume-ratio of different phases, rate of throughput with continuous processes, etc., may need to be considered in a particular process. Other processing variables, like temperature, reactant ratios, etc., may not be directly connected with the nature of the agitation, but will often have major significance in determining the performance of the process and may thus interact with some of the agitation variables. 

When consecutive or parallel reactions are carried out between a gas and a liquid, the concentration gradients near the interface may influence the selectivity as well as the overall rate of reaction. For chlorination or partial oxidation of hydrocarbons, several workers have reported that the yield of intermediate products was influenced by agitation variables [6,7] and was less than predicted from the kinetic constants. Rigorous analysis of multiple reactions is complex, but film theory can be used to show when mass transfer effects are likely to change the selectivity [8]. 

These values are for pure turbulent liquids only, but the equations show that, assuming d does not change across the 2.5 mm value, is independent of agitation variables and therefore,... 

Polyoxymethylene is obtained as a finely divided soHd. The bulk density of the product, which is very important for ease of handling in subsequent manufacturing steps, is influenced by many reaction variables, including solvent type, polymerisation temperature, and agitation. 

Some of the important parameters in the Bnchamp process are the physical state of the iron, the amount of water used, the amount and type of acid used, agitation efficiency, reaction temperature, and the use of various catalysts or additives. When these variables are properly controlled, the amine can be obtained in high yields while controlling the color and physical characteristics of the iron oxide pigment which is produced. 

The constant may depend on process variables such as temperature, rate of agitation or circulation, presence of impurities, and other variables. If sufficient data are available, such quantities may be separated from the constant by adding more terms ia a power-law correlation. The term is specific to the Operating equipment and generally is not transferrable from one equipment scale to another. The system-specific constants i and j are obtainable from experimental data and may be used ia scaleup, although j may vary considerably with mixing conditions. Illustration of the use of data from a commercial crystallizer to obtain the kinetic parameters i, andy is available (61). 

External Conditions The principal external variables involved in any drying study are temperature, humidity, air flow, state of subdivision of the solid, agitation of the solid, method of supporting the solid, and contact between hot surfaces and wet solid. Alf these variables will not necessarily occur in one problem. 

Sfo is the Stokes number based on initial nuclei diameter do [Adetayo et al.. Powder Tech., 82, 37 (1995)]. Extent (/cf), depends logarithmically on binder viscosity and inversely on agitation velocity. Maximum granule size depends hnearly on these variables. Also, (/cf ), has been observed to depend hnearly on liquid loading y. Therefore, the maximum granule size depends exponentially on liquid loading. Fig. 20-73 illustrates this normahzation of extent (/cf), for the drum granulation of hmestone and fertilizers. 

Agitators and mixers (variable density) Blowers, exhausts and fans (over 75 kW) Rotary compressors and pumps (other than centrifugal)... 

Similarity concepts use physical and mathematical relations between variables to compare the expected performance of mixing/agitation in different sized systems [33]. This is usually only a part answer to the scale-up problem. 

For steam jacketed, agitated closed reactor ketdes, the overall U usually will range from 40-60 Btu/hr (ft ) ( F). Of course, the significant variables are the degree or type of internal wall turbulence and the viscosity and thermal characteristics of the internal fluid. For water or other liquid cooling in the reactor jacket, the U value usually ranges from 20-30. 

The rate of a chemical reaction is influenced by pressure, temperature, concentration of reactants, kinetic factors such as agitation, and the presence of a catalyst. Since the viability of a plant depends not only on reaction efficiencies but also on the capital cost factor and the cost of maintenance, it may be more economic to alter a process variable in order that a less expensive material of construction can be used. The flexibility which the process designer has in this respect depends on how sensitive the reaction efficiency is to a change in the variable of concern to the materials engineer. 

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