Recycle-flow mixing

The most general form of the recycle flow mixing system is illustrated in Fig. 13. 

For simplicity, in this section we have again largely restricted our attention to first-order reactions. General design charts of the form of Fig. 14 can, in principle, be constructed for any recycle flow-mixing system and any form of kinetics. 

It might be expected that all recycle flow-mixing systems would give a performance the same as that of an equal size CSTR for infinitely large recycle ratios. This is not necessarily so and, recently, considerable attention has been focused on various forms of Gj (s) and G2(s), in Fig. 13, which exhibit surprising behaviour in this respect. References 50—54 are concerned with this problem. 

Recycle-flow Mixing Based on Eddy Formation... 

Gibilaro, L. G. The Recycle Flow Mixing Model. Chem. Eng. Science 26 (1971) 299. 

The following systems represent differing combinations of ideal plug-flow, mixing, dead space, flow recycle and flow by-pass. 

In the recycle flow pressurization system (Figure 27.10), a portion (15-50%) of the clarified effluent from the flotation chamber is recycled, pressurized, and semisaturated with air in the air dissolving tube. The recycled flow is mixed with the unpressurized main influent stream just before admission to the flotation chamber, with the result that the air bubbles come out of aqueous phase in contact with suspended particulate matter at the inlet compartment of the flotation chamber. The system is usually employed in applications where preliminary chemical addition and flocculation are necessary and ahead of flotation. It eliminates the problems with shearing the flocculated particles since only the clarified effluent passes through the pressurizing pump and the friction valve. It should be noted, however, that the increased hydraulic flow on the flotation chamber due to the flow recirculation must be taken into account in the flotation chamber design. 

Gibilaro [49] has considered a recycle model of the form of eqn. (60) where Gj (s) and G2(s) are general series combinations of PFR and equal size CSTR reactors and he gives sixteen references to published work involving more restricted forms of Gj (s) and G2 is). With an infinite choice over the forms of G (s) and G2(s) and the magnitude of R, the recycle model is seen to be the most flexible of all flow-mixing models. The performance of each specific form of Gj (s) as a potential reactor must be investigated individually in practice, the model is often reduced to a pure PFR element... 

One might intuitively expect that infinite recycle rates associated with a system as described by eqn. (61) would produce a completely well-mixed volume with concentration independent of location. This is indeed so and under these conditions, the performance tends to that of an equal sized CSTR. At the other extreme, when R is zero, PFR performance pertains. Fractional conversions at intermediate values of R may be determined from Fig. 14. The specific form of recycle model considered is thus seen to be continuously flexible in describing flow mixing between the PFR and CSTR extremes just as was the tanks-in-series model. The mean and variance of this model are given by eqns. (62) and (63) and these may be used for moments matching purposes of the type illustrated in Example 6. 

What can you tell about the influencing resistances for the porous catalyst from the data of Table P18.27 obtained in a recycle type mixed flow reactor. In all runs the leaving stream has the same composition, and conditions are isothermal throughout. 

For pure gas use high recycle approaching mixed flow. 

Recycle flow, where a portion of the fluid leaving the vessel or leaving a flow region is recirculated and returned to mix with fresh fluid. 

Aiba (A3), Fox and Gex (F8), Kramers, Baars and Knoll (K15), Metzner and Taylor (MIO), Norwood and Metzner (N3), Van de Vusse (V5) and Wood et al. (W12) have studied flow patterns and mixing times. In addition, Brothman et al. (B22), Gutoff (G9), Sinclair (S16) and Weber (W3) analyzed flow in a stirred tank in terms of the recycle flow model of Fig. 23F. This model corresponds to the draft-tube reactor, and with sufficiently large recycle rate the performance prediction of this model approximates backmix flow. 

Recycle-flow Coanda-effect Mixing Based on Taylor Dispersion Most Relevant Citations... 

Figure 1.186 Schematic of the recycle-flow micro mixer with five mixing elements [56] (by courtesy of Transducer Research Foundation).

Figure 1.188 Microscope images of the color formation due to a reactive characterization of mixing in the five mixing elements of the recycle-flow micro mixer (Re = 28 150 pm). Phenolphthalein and NaOH solutions were mixed [56] (by courtesy ofTransducer Research Foundation).

Example 4 Computational order for styrene process. A styrene process flow diagram is shown in Fig. 4-5. From this flow sheet it is apparent that there are two recycle streams in this simple process the unreacted ethylberizene is recycled and mixed with the fresh feed and the reactor effluent is recycled back to the heat exchanger. 

Reactors in which the solid phase is perfectly mixed on a macro scale, such as a stirred tank slurry reactor and the riser reactor with recycle of both phases, are particularly useful for fast catalyst deactivation processes. Notice that the residence time of both phases can be varied independently by introducing an extra recycle flow of... 

Pressurization could be carried out on the entire feed stream (full-flow pressure flotation) or a fraction of the feed stream while the remainder is introduced directly without aeration into the flotation tank (split-flow pressure flotation). The spht-flow system offers a cost saving over the full-flow units, since only a portion of the influent needs to be pressurized. In both cases, however, if the sohd particles in the feed stream are flocculated before introducing to the flotation tank, the high shear during pressurization, aeration, and pressure release can destroy the floes. Also, if the particle loading in the feed stream is high, both systems are susceptible to block e of the air release devices. To minimize these problems, recycle-flow pressure flotation is often practiced (Fig. 19-71). In this process, the feed stream, flocculated or otherwise, is introduced directly into the process vessel, and part of the clarified effluent is pressurized, aerated, and recycled to the flotation tank in which it is mixed with the flocculated feed. The air bubbles are released as they attach to the floes and float to the tank surface. The recycle-flow devices are found to offer the highest unit capacities. 

The construction of a combined model starts with one image (created, supposed or seeded) where it is accepted that the flow into the device is composed of distinct zones which are coupled in series or parallel and where we have various patterns of flow flow zones with perfect mixing, flow zones with plug flow, zones with stagnant fluid (dead flow). We can complete this flow image by showing that we can have some by-pass connections, some recycled flow and some slip flow situations in the device. 

The spreadsheet shows an initially assumed value of the recycle flow rate (ha) of 100.0 mol/s (Cell E13) and an assumed value of the mixing point outlet temperature of 50°C (Cell D8). The value of hi will be varied until the calculated recycle flow rate (/i4c) in Cell J13 equals the assumed value, which it now does not. (The actual calculation will be done by finding the value of 4a that drives the value of ha - ha in Cell 117 to zero.) Once the flow rates are correct, the mixing point temperature will be varied to determine the value that drives A// = S out out (in Cell D4) to zero for the adiabatic mixer. 

The flow-mix mode of operation allows a fast reaction (i.e. the reaction is rapid in comparison with the time constant of the instrument). Here, the reaction is initiated inside the calorimetric cell by the mixing of the pre-equilibrated reagents as they enter the cell. Figure 1. The effluent is not recycled but can be retained for further analysis. 

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