Maintenance Mixer

Special safety constraints apply to equipment selection, design, and operation in nuclear reprocessing (269). Equipment should be reHable and capable of remote control and operation for long periods with minimal maintenance. Pulsed columns and remotely operated mixer—settlers are commonly used (270). The control of criticaHty and extensive monitoring of contamination levels must be included in the process design. 

When abrasive solid particles must be suspended, maintenance costs for the submerged shaft seal of a side-entering propeller may become high enough to make this type of mixer an uneconomical choice. 

Two maintenance workers were replacing part of a ribbon in a large ribbon mixer. The main switch was left energized the mixer was stopped with one of three start-stop buttons. 

Dynamic mixers are a class of secondary mixers where part of the mixing device is aiiowed to move reiative to the barrei and screw surfaces or has an active channei positioned in the barrei waii. In generai, these mixers provide improved mixing over traditionai secondary mixers, but they are more compiicated and costiy to produce, require higher maintenance ieveis, and tend to have higher ieveis of wear. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

Special attention should be paid to proper batching technique and maintenance of fins in the mixers to assure formation of a proper air-void system. 

Motionless mixers are compact, thus requiring a small site and a lower capital expenditure (CAPEX). Inherent safety is improved due to a smaller reacting inventory. In addition, since there are no moving parts, sealing problems are reduced and maintenance is minimized. 

Maintenance of a stable emulsion in the continuous metering system s total product flow is critical for an efficient margarine production and is achieved through the use of specially designed static in-line mixers. These are installed in the main pipelines downstream of the metering system as can be seen from Figure 27. 

The ideal mixer should rapidly produce a complete blend with as gentle as possible a mixing action to avoid damage. It should be easily cleaned and discharged, be dust-tight, and require low maintenance and low power consumption. 

Their advantage compared with stirrers (the dynamic mixers) are utilization in a wide viscosity range, narrow residence time spectrum, easy insertion in the existing pipe system, often no extra space requirements, negligible maintenance and minimal wear, low investment and low operating costs [416],... 

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