Jet mixing in tanks

The closest standard speed is 16.5 rpm (Table 6-2). This is much higher than the 4 rpm required and will result in a higher-than-necessary power consumption. Decrease the impeller diameter to D/T = 0.9, keeping everything else the same. The new N = 7 rpm is much closer to the smallest available speed of 16.5 rpm (0.275 rps). 

The next step is to calculate the Reynolds number using the apparent viscosity and the Metzner-Otto equation. For helical ribbon impellers, kg = 30  

This is far into the laminar regime check the RexL- From eq. (9-46), Rctl = 35, so the helical ribbon is a good choice. Because a helical ribbon impeller was selected, caverns are not a concern in this application. The power draw will be 

The closest standard motor size is 75 hp, and the next largest is 100 hp (Table 6-2). A slight further reduction in the impeller diameter to 0.88T reduces the power draw to 62 hp, which is a better match for the motor size. The blend time is still well below the requirement of 10 min. 

Jet mixers are commonly used in large storage tanks, where the contents must be homogenized, but the required blend time can be on the order of hours rather 

---

The available experimental studies of jet mixing in tanks are summarized briefly below. Van de Vusse layered liquids of different density in a 36.0 m-diameter storage tank and measured the mixing time from the start of the recirculation pump to the time when the densities of samples drawn from the tank were within a few percent of the expected mean tank density. He correlated his results at an Re of about 1.5 x 10 by... 

For blending in large tanks, several nozzles may be used. The operating characteristics of such systems are treated in detail in Chapter 9 together with jet mixing in tubular devices. 

Fig. 25. Flow patterns in jet mixed tanks where represents 2ones that are poody mixed (a) side entry and (b) axial.

Figure 5-36B. Illustration of jet mixing for biending of oiis by circulation within the tank. Oii from the top is drawn down and entrains the oii in the bottom of the tank through the eductor nozzle (jet). By permission, Ketema, Schutte and Koerting Div.

Dimoplon, W. (1978) Hyd. Proc. 57 (May) 221. What process engineers need to know about compressors. Fischer, R. (1965) Chem. Eng., NYT2 (Sept. 13th) 179. Agitated evaporators, Part 2, equipment and economics. Fossett, H. and Prosser, L. E. (1949) Proc. Inst. Mech. Eng. 160, 224. The application of free jets to the mixing of fluids in tanks. 

Gladki H. Power dissipation, thrust force and average shear stress in the mixing tank with a free jet agitator. In Tatterson GB, Calabrese RV, Penny WR, eds. Industrial Mixing Fundamentals with Applications. New York American Institute of Chemical Engineering, 1995 146-149. 

The scale-up of tanks mixed by free jets is still more limited by lack of data. In general, the mixing time can be decreased by increases in DjUj (product of jet diameter and velocity), while increases in tank volume will increase the mixing time the specific relations are indicated by Eqs. (21) or (24). The work of Fox and Gex shows that if Dju,) is constant, the mixing time increases as the square root of the batch volume in a given tank, the mixing time varies as (DjU )-B/6 or DjUj) 2, depending on the jet Reynolds number. 

Fossett, H. and Prosser, L. E. (1949) Proc Inst Mech Eng. 160, 224. The application of free jets to the mixing of fluids in tanks. 

---