Active Mixer

In contrast to the presented passive mixers, active micromixers rely on external power input to introduce perturbations within the laminar flow to accelerate mixing. A selection of different external perturbation sources are summarized in Table 4.1 and are discussed briefed here. 

SpiralTlevator Materials are moved upward by the centrally located spiral-type conveyor in a cylindrical or cone-shaped Nautamix vessel (Fig. 37c and d). Blending occurs by the downward movement at the outer walls of the vessel. The vessel serves the dual purposes of blending and storage. In these mixers the screw impeller actively agitates only a small portion of the mixture and natural circulation is used to ensure all the mixture passes through the impeller zone. In the case of Nautamix, an Archimedian screw lifts powder from the base of a conical hopper while progressing around the hopper wall. 

Paste Mixing. The active materials for both positive and negative plates are made from the identical base materials. Lead oxide, fibers, water, and a dilute solution of sulfuric acid are combined in an agitated batch mixer or reactor to form a pastelike mixture of lead sulfates, the normal, tribasic, and tetrabasic sulfates, plus PbO, water, and free lead. The positive and negative pastes differ only in additives to the base mixture. Organic expanders, barium sulfate [7727-43-7] BaSO carbon, and occasionally mineral oil are added to the negative paste. Red lead [1314-41 -6] or minium, Pb O, is sometimes added to the positive mix. The paste for both electrodes is characterized by cube weight or density, penetration, and raw plate density. 

Frieficements (83), manufactured in Belgium, are produced as a wet slurry of finely ground slag. When activators such as Pordand cement, lime, or sodium hydroxide are added in a concrete mixer, the slurry sets and hardens to produce concretes with good strength and durabiUty. 

What this shows is that, from the definition of off-bottom motion to complete uniformity, the effect of mixer power is much less than from going to on-bottom motion to off-bottom suspension. The initial increase in power causes more and more solids to be in active communication with the liquid and has a much greater mass-transfer rate than that occurring above the power level for off-bottom suspension, in which slip velocity between the particles of fluid is the major contributor (Fig. 18-23). 

Rubber blends with cure rate mismatch is a burning issue for elastomer sandwich products. For example, in a conveyor belt composite structure there is always a combination of two to three special purpose rubbers and, depending on the rubber composition, the curatives are different. Hence, those composite rubber formulations need special processing and formulation to avoid a gross dissimilarity in their cure rate. Recent research in this area indicated that the modification of one or more rubbers with the same cure sites would be a possible solution. Thus, chlorosulfonated polyethylene (CSP) rubber was modified in laboratory scale with 10 wt% of 93% active meta-phenylene bismaleimide (BMI) and 0.5 wt% of dimethyl-di-(/ r/-butyl-peroxy) hexane (catalyst). Mixing was carried out in an oil heated Banbury-type mixer at 150-160°C. The addition of a catalyst was very critical. After 2 min high-shear dispersive melt mix-... 

Elastomer-plastic blends without vulcanization were prepared either in a two roll mill or Banbury mixer. Depending on the nature of plastic and rubber the mixing temperature was changed. Usually the plastic was fed into the two roll mill or an internal mixer after preheating the mixer to a temperature above the melting temperature of the plastic phase. The plastic phase was then added and the required melt viscosity was attained by applying a mechanical shear. The rubber phase was then added and the mixture was then melt mixed for an additional 1 to 3 min when other rubber additives, such as filler, activator, and lubricants or softeners, were added. Mixing was then carried out with controlled shear rate... 

A typical feed rate is 1 to 3 ppm of active amine. Most filmer products contain between 3 and 10% amine (3-5% being the norm), so that product feed rates are around 10 to 30 ppm, dosed continuously to achieve 0.1 to 0.5 ppm amine (as ODA) in the returned condensate. However, products generally are diluted to 2 to 20% strength before application. Solutions are best prepared by using warm to hot condensate (150-160 °F/66-71 °C maximum) and agitated with a mechanical mixer until a uniform solution is reached. The solution should always be kept over 60 °F/16 °C. 

Gas holdup may be of the same magnitude in the various operations, although for bubble-columns, the presence of electrolytes or surface-active agents appears to be a condition for high gas holdup. The gas residence-time distribution resembles that of a perfect mixer in the stirred-slurry operation, and comes close to piston flow in the others. 

Major limitations in fission product decontamination will require tests with mixer-settlers. However, we anticipate from the distribution ratio measurements that Tc, Ru, and Pd will limit the overall decontamination from beta activity (other than from lanthanides). ... 

In order to test this concept a series of compounds was prepared in a 5 L Shaw Intermix (rubber internal mixer, Mark IV, Kl) with EPDM (Keltan 720 ex-DSM elastomers an amorphous EPDM containing 4.5 wt% of dicyclopentadiene and having a Mooney viscosity ML(1 +4) 125°C of 64 MU 100 phr), N550 carbon black (50 phr), diisododecyl phthalate (10 phr), stearic acid (2 phr), and l,3-bis(tert-butylperoxy-isopropyl)benzene (Perkadox 14/40 MB ex Akzo Nobel 40% active material 6 or 10 phr). A polar co-agent (15 phr) was admixed to the masterbatch on an open mill and compounds were cured for 20 min at 180°C in a rheometer (MDR2000, Alpha Technologies). The maximum torque difference obtained in the rheometer experiments was used as a measure of... 

RSS 2 was mixed with N-234 carbon black without additive or in the presence of quinonedihne, or with a peptizer (dibenzamidodiphenyl disulfide, activated with a metal chelate). In the case of the peptizer, rubber and carbon black were mixed for 30 s before the addition of carbon black. In the case of the quinone diimine, carbon black and QDI were added 15 s after addition of rubber to the mixer. 

Applicators, mixers, loaders, and others who mix, spray, or apply pesticides to crops face potential dermal and/or inhalation exposure when handling bulk quantities of the formulated active ingredients. Although the exposure periods are short and occur only a few times annually, an estimate of this exposure can be obtained by quantifying the excreted polar urinary metabolites. Atrazine is the most studied triazine for potential human exposure purposes, and, therefore, most of the reported methods address the determination of atrazine or atrazine and its metabolites in urine. To a lesser extent, methods are also reported for the analysis of atrazine in blood plasma and serum. 

When the data in Table 4 were given full evaluation, it was recommended that the 50W formulation of chlorpyrifos no longer be marketed in bags that allowed significant exposure for mixer-loaders of this product. This product was removed from the marketplace and was replaced with one in which the wettable powder (WP) is in water-dissolvable packets. Exposure data on other active ingredients have clearly demonstrated reduced exposure with this type of packaging. The other uses were deemed to present a minimal hazard to users, and only minor protective measures have been recommended to workers. 

Nigg and Stamper (1983) conducted potential exposure and actual exposure measurements during three weekdays over three consecutive weeks for applicators and mixer-loaders, one week using normal work clothes, the second week disposable coveralls in addition to their work clothes, and the third week disposable coveralls and NIOSH-approved respirators. In conjunction, urinary excretion of a metabolite of the active ingredient was... 

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