Operation of the Mixers

The effect on the process of a change in operation of the mixer system (impeller, baffles, etc.) is the final measurement of performance. Thus, operations such as blending, uniform particle suspension, reaction, gas absorption, etc., may be acceptable under one physical system and not so to the same degree under a slightly modified one. The ratio per unit volume on scale-up must be determined experimentally. 

Installation of propeller-type mixers varies greatly, depending on the specific application. Top-entering mixers utilize either a clamp- or flange-type mounting. It is important that the mixer be installed so the propeller or paddle placement is at a point within the tank, vessel, or piping that assures proper mixing. Vendor recommendations found in O M manuals should be followed to ensure proper operation of the mixer. 

Although the process proved satisfactory from the chemical standpoint, practical problems emerged in that the hydraulic operation of the mixer-settler batteries was extremely poor. In effect, as soon as the aqueous solutions from the dissolution of irradiated targets were placed in contact with the organic extraction phases, a stable emulsion was formed, produced by the appearance of extensive precipitates at the aqueous solution/organic solution interface. As no chemical remedy was found to solve this problem, we attempted to adapt this type of process to extraction chromatographic techniques. 

The rate constant, k, depends on the physical nature of the materials being mixed and on the geometry and operation of the mixer. 

Figure 7d shows a schematic depiction of the active mixer chip design, consisting of a main flow channel and six pairs of orthogonal side channels [76]. Operation of the mixer chip relies on the perturbation of the main x-directional channel flow by y-directional cross-stream side-channel flows. The side-channel flows are per-turbated by pressure changes through thin membranes affected by a GERF microvalve (Fig. 7e). Square-wave electrical voltage signals (0-800 V) are applied between the electrodes to control the microvalve and, in turn, the perturbation, leading to pulsating sinusoidal cross-stream flows in the six pairs of side channels, as shown in Fig. 8f.

Both propeller and screw mixers have specific setup requirements. In the case ofpropellerIpaddle-type mixers, the primary factor is the position of the propellers or paddles within the tank or vessel. Vendor recommendations should be followed to assure proper operation of the mixer. 

U, the overall heat transfer coefficient, depends on the flnid properties, the operating parameters of the mixer, and the system configuration. It is the key parameter that is affected by the operation of the mixer. The available area for heat transfer, A, depends on the geometry of the system. Usually, the area per volume decreases on vessel scale-up. The temperature driving force, AT, depends on the operating conditions of both the process and the heating or cooling fluid. 

Air is compressed to modest pressures, typically 100 to 200 kPa ( 15-30 psig) with either a centrifugal or radial compressor, and mixed with superheated vaporized butane. Static mixers are normally employed to ensure good mixing. Butane concentrations are often limited to less than 1.7 mol 1 to stay below the lower flammable limit of butane (144). Operation of the reactor at butane concentrations below the flammable limit does not eliminate the requirement for combustion venting, and consequendy most processes use mpture disks on both the inlet and exit reactor heads. A dow diagram of the Huntsman fixed-bed maleic anhydride process is shown in Figure 1. 

In any event, the users of the mixer can issue a mechanical specification aud determine the speed, diameter of an impeller, aud power with in-house expertise. Or they may issue a processes specification which describes the engineering purpose of the mixing operation and the vendor will supply a description of the mixer process performance as well as prepare a mechanical design. 

The application of draft tubes as related to various mixing operations is showm in Figures 5-231 and 5-24A-5-241. The draft tubes are basically a tube or shell around the shaft of the mixer including the usual axial impeller, which allows a special or top-to-bottom fixed flow pattern to be set up in the fluid system. The size and location of the tube are related to both the mechanical and mixing performance characteristics as well as peculiar problems of the system. Usually they are used to ensure a mixing flow pattern that cannot or w ill not develop in the system. Weber gives the followdng points for draft tubes [23] ... 

Propagation of the above states through the modeling relationships completes the definition of the initial state of the plant. The results are shown in Fig. 14a. (Note that valves V34 through V43, which are downstream of the mixers, are considered closed, and they do not affect the synthesis of the switchover procedure.) The desired goal state is operationally defined by the following conditions ... 

OS 68] [R 19] [P 50] A numbering-up of five mini mixers, tested at the pilot stage, was used [134]. Automation of the entire process was required liquid-flow splitting to the single reactors was ... by no means trivial... [134]. The capacity of one mini reactor was 30 ml s , i.e. 1081 h . The complete setup hence should be operated close to 5001 h . The micro-reactor plant was operated at intervals as the preceding step was carried out batchwise. The operation of the micro-reactor plant started in August 1998 after a period of only about 1.5 years for development. 

With respect to smoke and fumes, measurement of SC in the mixing area showed no increase above background, and no hydrogen sulphide was detected. In the placement areas, there was some odor from the aromatics in the sulfur modifier. Pouring conditions were similar to those of an asphalt paving operation and included a similar odor. No special protective equipment was utilized or required. Operators at the mixer wore dust masks. 

---