Agitators helical

Close-Clearance Stirrers For some pseiidoplastic fluid systems stagnant fluid may be found next to the -essel walls in parts remote from propeller or turbine impellers. In such cases, an anchor impeller maybe used (Fig, 18-6), The fluid flow is principally circular or helical (see Fig, 18-7) in the direction of rotation of the anchor. Whether substantial axial or radial fluid motion also occurs depends on the fluid iscosity and the design of the upper blade-supporting spokes. Anchor agitators are used particularly to obtain irnpro ed heat transfer in high-consistency fluids,... 

A wide variety of configurations exists for coils in agitated vessels. Correlations of data for heat transfer to helical coils have been of two forms, of which the following are representative ... 

This design is not well adapted to free-convection heat transfer outside a tube or coil therefore, for this discussion only agitation is considered using a submerged helical coil, Oldshue and Kern . 

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

Carreav. P, J.. Patterson, I., and Yap, C. Y. Can J. Chem. Eng. 54 (1976) 135. Mixing of viscoelastic fluids with helical ribbon agitators 1. Mixing time and flow pattern. 

A simple jacketed pan or kettle is very commonly used in the processing industries as a reaction vessel. In many cases, such as in nitration or sulphonation reactions, heat has to be removed or added to the mixture in order either to control the rate of reaction or to bring it to completion. The addition or removal of heat is conveniently arranged by passing steam or water through a jacket fitted to the outside of the vessel or through a helical coil fitted inside the vessel. In either case some form of agitator is used to obtain even distribution in the vessel. This may be of the anchor type for very thick pastes or a propeller or turbine if the contents are not too viscous. 

An example of a low conversion reactor would be a conventionally agitated kettle with large turbine agitators and jacket cooling. The utility of this type of reactor can be extended to intermediate conversions by the use of anchor or helical agitators to partially overcome heat transfer and mixing problems at higher viscosities. 

Helical agitators have been developed to provide good turnover and satisfactory heat transfer at viscosities as high as 5x10 cp. 

Coyle ( ) showed that the heat transfer coefficient for helical agitators remained in the region of 3-5... 

BTU/hr. sq.ft. over a wide range of viscosities and rotational speeds. This is equivalent to the thermal resistance of a fluid film equal to about 1/2 the clearance between the helical agitator and the vessel wall. This represents Reynolds numbers in the range of 10 to 10. This is the region of creeping flow where, with no inertial effects, there is little displacement of the fluid adjacent to the wall. 

The time required to pass through the high shear zone will be proportional to the volume of the vessel divided by the flow from the turbine. This will be porportional to 1/ND. With a turbine in turbulent flow, turnover is relatively rapid and all the fluid will pass through the impeller region in a relatively short period of time. The flow regime in an anchor or helically agitated vessel can be inferred from the flow studies by Smith and Peters (], 13j. These indicated... 

A recent patent (34) describes a process similar in several ways to the one above. The initial turbine agitated CSTR is eliminated. Instead, the feed flows to the helically-agitated CSTR operating at 25-35% conversion and 125-135°C. 

This patent then calls for two or more reflux cooled conical CSTR s with helical ribbon agitators operating in series in an "intermediate conversion zone" ranging from 65 to 85% conversion. The conversion in succeeding reactors in this zone should show a relative difference of 15-25%. As with the earlier patent,... 

Paddle, anchor and helical ribbon agitators (Figures 10.56a, b, c), and other special shapes, are used for more viscous fluids. 

Figure 10.56. Low-speed agitators (a) Paddle (b) Anchor (c) Helical ribbon...

Agitators with a large blade area which rotate at low speeds. These include anchors, paddles and helical screws. 

Gray (1963) investigated the mixing times of helical ribbon agitators and found the following equation to hold ... 

Propeller A propeller is a three- or four-bladed flow impeller, having helically shaped blades. The flow is primarily axial (discharge flow parallel to the agitator shaft) and is most effective in low-viscosity fluids. The marine-type propeller (Figure 3.15) is characteristically operated at relatively high speed, particularly in low-viscosity liquids (Treybal, 1980). 

On the other hand, the deposition process is also important to prepare blend samples. A mixture of homopolypeptide solutions in which they take a random coiled structure are added into a poor solvent. For polypeptides, water is a poor solvent in general. If the hydration rate is different for each polypeptides, they form their preferred secondary structures by themselves and then do not blend with each other. On the basis of this assumption, in order to make the hydration at the same time, the solution is added to alkaline water. In this review, two kinds of quieting solvents such as water and alkaline water have been used. (Methods 1-4 and Method 5). Method 1 Helical polypeptide and (3-sheet polypeptide are dissolved in DCA and agitated... 

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