Mixer helical elements

The mixer helical elements were of polypropylene construction Le/D = 0.833. 

The Kenics STATIC MIXER unit is a series of fixed, helical elements enclosed within a tubular housing. The fixed geometric design of the unit produces the following unique patterns of flow division and radial mixing simultaneously. 

The Kenics mixer, Figure 10.14(a), for example, consists of a succession of helical elements twisted alternately in opposite directions. In laminar flow for instance, the flow is split in two at each element so that after n elements the number of striations becomes 2". The effect of this geometrical progression is illustrated in Figure 10.14(b) and points out how effective the mixing becomes after only a few elements. The Reynolds number in a corresponding empty pipe is the major discriminant for the size of mixer, one manufacturer s recommendations being... 

Mixer 81 [M 81] Barrier-embedded Micro Mixer with Helical Elements... 

Mixer type Barrier-embedded micro mixer with helical elements... 

Figure 1.164 (a) Complete view of the barrier-embedded helical micro mixer (b) Kenics cell with a helical element only (c) Kenics cell with a helical element and two barriers [3] (by courtesy of IOP Publishing Ltd.). 

The static elements were internals of a circular T-pipe. The mixer realized had Kenics cells with helical elements twisted by 180°, giving a fully 3-D structure (see Figure 1.163) [3], Two barriers were placed on the wall of the pipe. Counterclockwise-and clockwise-rotated helical elements were alternately arranged. Four barriers were introduced in the pipe wall in each Kenics cell. For comparison, pipes with only helical elements (no barriers) and empty pipes (no internals) were also fabricated. 

M 81] [P 70] A microscopy-image analysis of the color formation due to a reactive approach reveals that the micro mixer with helical elements and barriers gives a better performance than the micro mixer with helical elements but without barriers and a reference pipe structure without either helical elements or barriers (see Figure 1.167) [3], The pipe gives the expected profile with two colorless fluid compartments on top and at bottom, separated by a colored interface. This is indicative of the absence of any swirling, secondary flow. 

There are two kinds of static mixers. One type has helical elements that twist the fluid, and another set of elements that cut the fluid, divide it, and twist it again. The twisting and cutting is continued until the production and scaleup uniformity is achieved. This is useful in viscous fluids. 

COVr for the Kenics Helical Element Mixer (HEM) is presented in Figure 10.28 for low Nrc. COVr for the HEV and the HEM are given in Figure 10.29 for high Nrc. Mathematical relationships for COVr as a function of the Reynolds number and the number of tab rows (N = Ntr) for an HEV mixer and for number of elements (N = Ne) in an HEM mixer were provided by Julian Fasano of Chemineer. 

The pressure drop is a function of the number of elements (Ne), the Reynolds number (NRe), the length to diameter ratio of each helical element (Le/D), and the void fraction (VF) of the mixer. The void fraction must be considered because some HEM mixers, especially those made of plastic, have void fractions 0.5. Figure 8 of Myers et al. (1997) gives K for the HEM as a function of Reynolds number for Le/D = 1 and 1.5. Although the void fraction is not specified in Myers et al., the void fraction for most metal element HEM mixers above 1 /4" diameter is about 0.9. The correlation for the effect of (Le/D) and VF is... 

The choice of the optimum style of in-line mixer is not nearly as certain as is the choice of the optimum style of agitator impeller. However, a firm recommendation can be made based on the experimental record and on judgment. There are several considerations that lead to the recommendation of the Kenics helical element mixer (HEM) as the most suitable mixer for handling fast consecutive reactions ... 

Figure 10.47. Yield data vs initial molar feed ratio for 3 Bourne Reaction occurring in a 1/4 inch (0.64 cm) Kenics Helical Element Mixer used on a recycle loop on 20 liter semi-batch reactor agitated with a 6BD impeller.

Pipeline Mixers used as Reactors for Fast C/C Reactions. Taylor (1996) and colleagues (1998) conducted a study to determine scale-up procedures for fast C/C reactions in pipeline mixers. They used the fourth Bourne reaction, which is the acid catalyzed hydrolysis of dimethoxypropane (DMP) to acetone and methanol. This is an extremely rapid reaction when catalyzed by HCl. The competitive reaction scheme, which is a unique one, is one in which NaOH reacts practically instantaneously with the HCl to remove the catalyst for the hydrolysis reaction. A water/ethanol solution of NaOH and DMP was fed as a main stream to a Kenics helical element mixer and an aqueous side stream containing slightly greater (abour 5% greater) than equimolar amount of NaOH was fed as the side stream. The product was analyzed by GC for methanol and acetone. For extremely rapid mixing, essentially no hydrolysis occurred however, for slow mixing, essentially all the DMP is hydrolyzed because acidic conditions cause very rapid hydrolysis of the DMP. 

Use at least a 9 element Kenics helical element mixer (HEM). 

Figure 10.48. Yield of the Dimethoxypropane Hydrolysis Reaction vs Static Mixer Residence Time for three Kenics Helical Element Mixer...

Twelve element Kenics helical element mixer with an inside diameter of 1/8 in. 

In a helical element mixer, the cross section for flow is a... 

Static mixers for viscosity ratio < 100 1 and Re > 10000, use turbulent vortex element for viscosity ratio > 100 1 and Re < 10000 use helical elements. For pipe diameter < 0.3 m, element is 1.5 X pipe diameter for pipe diameter > 0.3 m element = pipe diameter. For Re< 10 use 18 elements with the number of elements reducing to 2 as Reynolds no. increases to > 5000. 

A, Bertsch, et al. Laminar flow in static mixer with helical elements, 1998, http // www,bakker.org/cfmbook/lamstat,pdf... 

Motionless mixers. Mixing of two fluids can also be accomplished in motionless mixers with no moving parts. In such commercial devices stationary elements inside a pipe successively divide portions of the stream and then recombine these portions. In one type a short helical element divides the stream in two and rotates it 180°. The second element set at 90° to the first again divides the stream in two. For each element there are 2 divisions and recombinations, or 2" for n elements in series. For 20 elements about 10 divisions occur. 

Mixing mixing of two component materials can be done using a static (sometimes called motionless ) mixer where the materials are forced through a series of right- and left-handed helical elements at 90° with no moving parts. Dynamic mixers that use... 

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