Pipe with static mixer

Mass Tranrfer in Pipes With Static Mixers... 

A continuous bulk polymerization process with three reaction zones in series has been developed. The degree of polymerization increases from the first reactor to the third reactor. Examples of suitable reactors include continuous stirred tank reactors, stirred tower reactors, axially segregated horizontal reactors, and pipe reactors with static mixers. The continuous stirred tank reactor type is advantageous, because it allows for precise independent control of the residence time in a given reactor by adjusting the level in a given reactor. Thus, the residence time of the polymer mixtures can be independently adjusted and optimized in each of the reactors in series (8). 

Figure 7.29 shows a Sulzer type SMX static mixer where the mixing element consists of a lattice of intermeshing and interconnecting bars contained in a pipe 80 mm diameter. It is recommended for viscous materials in laminar flow. The mixer shown is used in food processing, for example mixing fresh cheese with whipped cream. 

Example 5.7 We have to design a servo-controller for a mixing system. A blue dye for making denim is injected into a stream of water. The injected dye is blended into the pipe flow with the aid of in situ static mixers. A photodetector downstream is used to monitor the dye concentration. The analog output of the detector is... 

Contacting ozone gas with water can be achieved with every kind of gas diffuser, which is made of a material resistant to ozone. Ring pipes, porous diffusers and porous membranes, injector nozzles as well as static mixers can be employed. The different types of diffusers are mainly characterized by the diameter of the bubbles produced, e. g. micro (dB = 0.01 — 0.2 mm), small (dB 1.0 mm) or big (dB - 2.5 mm) bubbles (Calderbank, 1970 Hughmark, 1967). 

The insertion of small static mixing elements (SME) is common to achieve swirls and eddies in pipe flow, albeit usually not being turbulent [71]. The flow obstacles are fairly small compared with the pipe diameter, unlike typical packings of static mixers which fully cover the diameter of the channel. Such mixing elements provide abrupt changes in surface orientation to result in flow separation and subsequent eddy production. 

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. 

A loop reactor is used for the bioconversion of methane to produce biomass used, e.g., as fish meal. This is a large-diameter pipe operated at high liquid circulation velocity with the 02/CH4 feed injected at several locations along the reactor. Cooling of the exothermic aerobic fermentation is accomplished by external heat exchangers. Static mixers are used to maintain gas dispersion in the liquid. 

Most manufacturers of static mixers have published (either in sales literature or in the technical literature) design methods for pressure drop. The pressure drop design methods, from Myers et al. (1997) for the Kenics HEM and Kenics HEV mixers are presented. The Darcy friction factor for the standard HEV mixer, Ntr = 2, L/D = 1, (with X/D = 3 downstream pipe) is presented in Figure 10.31. The friction factor is not given below Nrc = 1,000 because the HEV mixer should not be used for Nrc < 3,000. 

The enhanced heat transfer coefncient is due to the increase in radial mixing compared with the empty tube and possibly a contribution due to the fin effect associated with the presence of the metal element— particularly if there is good contact between element and tube wall. It is difflcult to relate the heat transfer characteristics of static mixers to empty pipes but low pressure drop mixers (e.g. Kenics, SM X) give increases in heat transfer coefficient of the order of 300% for an increase of pressure drop of the order of 700%. Some simple correlations have been presented for the Kenics mixer ... 

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