Continuous mixing devices

In the last 20 years several attempts have been made to apply continuous mixing devices for the production of rubber compounds. The following devices have been applied with more or less success ... 

A continuous mixing device for elastomers, consisting essentially of a fluted rotor surrounded by a stator. 

Static mixers. Static mixers or motionless mixers are pressure-driven continuous mixing devices through which the melt is pumped, rotated, and divided, leading to effective mixing without the need for movable parts and mixing heads. One of the most commonly used static mixers is the twisted tape static mixer schematically shown in Fig. 3.25. 

Twin-screw extruders are continuous-mixing devices and can be classified as follows (Fig. 3.5 Tadmor and Gogos, 2006) ... 

In the tire industry which faces the most difficult mixing problems, this type of mixer is still practically the only accepted mixing device. The tire industry has accepted the fact that the batch type mixer will not be replaced by a continuous mixing device in the near future, especially since the radial ply tire came into being, because this tire requires very tough types of rubber. The same is not the case as far as the plastics industry is concerned. In fact, predictions made several years ago that the batch type mixer will disappear and be replaced by continuous mixing equipment has come true in the filled polymers industry. Some of the continuous mixing devices are discussed below. 

In principle, at least, any mixer may be coupled with any settler to provide the complete stage. There are several combinations which are especially popufar. Continuously operated devices usually, but not always, place the mixing and settling functions in separate vessels. Batch-operated devices may use the same vessel alternately for the separate functions. 

Calculation of the induction time is crucial, since gaining a stable and continuous process requires residence times in the mixer < precipitation induction times in order to prevent incrustation in the mixing device. The induction... 

Continuous flow devices have undergone careful development, and mixing chambers are very efficient. Mixing is essentially complete in about 1 ms, and half-lives as short as 1 ms may be measured. An interesting advantage of the continuous flow method, less important now than earlier, is that the analytical method need not have a fast response, since the concentrations are at steady state. Of course, the slower the detection method, the greater the volumes of reactant solutions that will be consumed. In 1923 several liters of solution were required, but now reactions can be studied with 10-100 mL. 

In much the same manner as pumps, compressors are classified as one of two general classes positive displacement or dynamic (see Figure 3-68) [23]. These two general classes of compressors are the same as that for pumps. The positive displacement class of compressors is an intermittent flow device, which is usually a reciprocating piston compressor or a rotary compressor (e.g., sliding vane, screws, etc.). The dynamic class of compressor is a continuous flow device, which is usually an axial-flow or centrifugal compressor (or mix of the two). 

Continuous operation In flow-type reactors, e.g., loop reactors, the space velocity of the reaction is determined through the installed static mixing device that is used to generate the dispersion, together with the velocity of the circiflating medium (catalyst- and substrate/product phase). Knowledge of these parameters allows one to set up a kinetic model for the investigated reaction. 

Gulf Canada Limited (20, 2T). The process developed by Gulf consists of a pumping system wfnch continuously delivers measured volumes of sulfur and asphalt to a mixing device which disperses the molten sulfur in the liquid asphalt. The composition of the emulsion is typically in the range 25-60 parts sulfur to 75-40 parts asphalt. The temperatures of the sulfur, asphalt and S/A emulsion are maintained in the ranges 121-154 C, 121-177 C, and 121-154 C, respectively. 

The instruments involved in this method are dynamic mixing devices with two pumps, and a cross-Low Lltration device with proper membranes (Figure 14.8). One pump is used to push the organic phase with a slow rate and the other is used to pump the aqueous phase with a fast rate Liposomes can be prepared by continuously pumping the organic phase and the aqueous phas together and then Low through the dialysis devices. 

Figure 4.2. Flowsheet of production of cast polyurethane elastomer articles from a prepolymer by the continuous method 1 - vessel to store prepolymer 2 - vessel to prepare prepolymer 3 - reactor for curing agent 4 -transfer pump 5 - metering pump 6 - mixing device 7- mold.

The metering pumps continuously deliver the components into mixing devices. There are many designs for mixing devices, and the choice depends on the properties of the system being processed, e.g., viscosity, gel formation time, temperature, etc. 

Continuous reactors, including simple plug-flow pipe reactors, tubular reactors containing static or other mixing devices, and jet reactors of various types, have been used to efficiently produce toxic materials for immediate consumption in downstream processing operations with little or no inventory. Some examples follow. 

If the mixing device generates a simple shear flow, as shown in Fig. 3.23, the maximum separation forces that act on the particles as they travel on their streamline occur when they are oriented in a 45° position as they continuously rotate during flow. However, if the flow field generated by the mixing device is a pure elongational flow, such as shown in Fig. 3.24, the particles will always be oriented at 0° the position of maximum force. 

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