Cyclone inlets

Table 7. Dust Loadings and Cyclone Inlet Velocities above Which Erosion Is Excessive ...

The cyclones are typically designed with diameters of 100—160 cm for ease of maintenance. Cyclone inlet velocities are usually restricted to 18—21 m /s in the first stage and to 20—26 m/s in the second stage to achieve satisfactory pressure drop and erosion characteristics (62). The number of sets of two-stage cyclones thus depends on the total gas flow. Finding room to house all the necessary cyclones within the regenerator frequently requires considerable ingenuity (62). 

Reentrainment is generally reduced by lower inlet gas velocities. Calvert (R-12) reviewed the hterature on predicting the onset of entrainment and found that of Chien and Ibele (ASME Pap. 62-WA170) to be the most reliable. Calvert applies their correlation to a liquid Reynolds number on the wall of the cyclone, Nrcl = 4QilhjVi, where is the volumetric liquid flow rate, cmVs hj is the cyclone inlet height, cm and Vi is the Idnematic liquid viscosity, cmVs. He finds that the onset of entrainmeut occurs at a cyclone inlet gas velocity V i, m/s, in accordance with the relationship in = 6.516 — 0.2865 lu A Re,L ... 

As with dust cyclones, no reliable pressure-drop equations exist (see Sec. 17), although many have been published. A part of the problem is that there is no standard cyclone geometry. Calvert (R-12) experimentally obtained AP = 0.000513 J Q /hiWi) 2.8hiWi/dl), where AP is in cm of water Pg is the gas density, g/cm is the gas volumetric flow rate, cmVs hj and Wj are cyclone inlet height and width respectively, cm and is the gas outlet diameter, cm. This equation is in the same form as that proposed by Shepherd and Lapple [Ind. Eng. Chem, 31, 1246 (1940)] but gives only 37 percent as much pressure drop. 

Average cyclone inlet velocity, based on area m/s ft/s ft/s... 

FIG. 17-42 Effect of cyclone inlet loading on pressure drop. Coutiesy of PSRI, Chicago.)... 

The sharpness of separation of the mineral from the gangue is dependent on (1) the stability of the suspension, which is influenced by the size of the medium (2) the specific gravity of the medium (3) the cleanliness of the medium (4) the cone angle (5) the size and ratios of the internal openings in the cyclone (inlet, apex, and vortex) and (6) the pressure at which the pulp is introduced into the cyclone. A 20° cone angle is the most common. Cyclone diameter will be determined by the separation to be made as well as by the capacity required. The 0.5- and 0.6-m (20- and 24-in) cyclones are most common in coal plants, whereas multiple cones of 0.25- or 0.3-m (10- or 12-in) diameter are used in higher-gravity separations. 

The distance above the catalyst bed in which the flue gas velocity has stabilized is refened to as the transport disengaging height (TDH). At this distance, there is no further gravitation of catalyst. The center-line of the first-stage cyclone inlets should be at TDH or higher otherwise, excessive catalyst entrainment will cause extreme catalyst losses. 

Specific surface area, sq ft/cu ft Acceleration due to gravity, 32.2 ft/s or 9.8 m/s Area of segment of a circle, sq ft Cro.ss-sectional flow area, sq ft Cross-sectional area at bottom of vessel occupied by continuous aqueous phase, sq ft Cyclone inlet area = WjH,- for cyclone wdtb rectangular inlet, sq ft... 

Cyclone inlet V elocity head, in. water Height of a segment of a circle, ft Height of rectangular cyclone inlet duct, ft Height of dispersion band, ft Width of interface, ft... 

V,- = Cyclone inlet velocity, average, based on area a) ft/sec... 

Fev = Friction loss (inlet-velocity heads) A pev = Pressure drop through the cyclone (inlet-velocity heads)... 

These devices are generally designed to meet specific pressure-drop limitations. For ordinary installations operating at approximately atmospheric pressure, fan limitations dictate a maximum allowable pressure drop corresponding to a cyclone inlet velocity in the range of 20 to 70 feet per second. Consequently, cyclones are usually designed for an inlet velocity of 50 feet per second. 

There are two regions in the regenerator the dense phase and the dilute phase. At velocities common in the regenerator, 2 ft/sec to 4 fl/sec (0.6 to 1.2 m/sec), the bulk of catalyst particles are in the dense bed immediately above the air distributor. The dilute phase is the region above the dense phase up to the cyclone inlet, and has a substantially lower catalyst concentration. 

Penetration of the gas outlet tube into each cyclone should be at least 80% of the cyclone inlet duct height. 

Dipleg is the part of a cyclone separator that provides a barometric seal between the cyclone inlet and the cyclone solid outlet. 

Ab = Cross-sectional area at bottom of vessel occupied by continuous aqueous phase, sq ft Ac = Cyclone inlet area = WjHc for cyclone wiLh rectangular inlet, sq ft... 

Rosin et al. (1932) derived a simple expression for cyclone efficiency for a fixed cyclone diameter based on a force balance on the particle at the cyclone inlet, i.e.,... 

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

In the spiral or tangential cyclone inlet duct, the particles are accelerated to a velocity which is related to the inlet gas velocity Ue. Neglecting as a first approximation the slip between solids and gas, the kinetic energy of the solids which is transported per unit time into the cyclone is approximately given by... 

More recently, Reppenhagen and Werther (1997) have conducted a comprehensive study ofthe attrition mechanism in a cyclone. Their experimental set-up is shown in Fig. 16. The 0.09 m ID cyclone is operated in the suction mode. The solids (spent FCC, surface mean diameter 105 microns) are introduced via a vibrating feeder into the cyclone inlet tube which allows an independent variation of cyclone inlet velocity Ue and solids loading ll... 

According to the model described above the cyclone attrition rates Ra c obtained under steady-state conditions have been plotted against the square of the cyclone inlet velocity in Fig 18. Although the number of experiments is not large the general relationship between Rac and Ue predicted by Eq. (19) is confirmed. 

Figure 15. Changes in the size distribution of FCC catalyst due to attrition in a 5" cyclone, (inlet velocity 35ft/s). (Zenz andKelleher, 1980.)...

The cyclone inlet velocity is thus shown to be the most significant factor influencing the solids loss due to attrition in the cyclone. 

Cyclones. According to the model presented above, Eq. (24), a minimum loss rate due to cyclone attrition requires to avoid both high inlet velocities Ue and high solids mass fluxes mc m at the cyclone inlet. The latter requirement can be fulfilled by locating the cyclone inlet above the transport disengaging height (TDH) (Kunii and Levenspiel, 1991). In addition, an enlargement of the freeboard section will reduce the amount of particles that are entrained and thus the mass flux, mc in. 

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