Abrupt exit

Figure 10.11. Effects of gas and solids flow rates on the axial profile of the cross-sectional averaged voidage in a riser with an abrupt exit (from Brereton and Grace, 1993b) (a) Abrupt exit geometry (b) Voidage profiles.

Inasmuch as heat transfer depends on the hydrodynamic features of fast fluidization, if the fast fluidized bed is equipped with an abrupt exit, the axial distribution of solids concentration will have a C-shaped curve (Jin et al., 1988 Bai et al., 1992 Glicksman et al., 1991. See Chapter 3, Section III.F.l). The heat transfer coefficient will consequently increase in the region near the exit, as reported by Wu et al. (1987). 

Brereton et al. characterized riser gas mixing with a pseudo-axial dispersion coefficient [96]. They concluded that axial dispersion increases with total pressure drop across the riser (approximately proportional to the suspension density). In addition, they found the smooth exit configuration increased axial dispersion coefficients compared to an abrupt exit. They attributed this phenomenon to a more uniform solids distribution in the case of the abrupt exit which, in turn, corresponds to a decreased irregularity in the upward and downward solids movement primarily responsible for the axial mixing of the gas. The experimental values, reported as DyU L, ranged between 0.01 and 0.18 (D = 6,600 to 119,000 cm /s). 

Gas dispersion is influenced by the entrance and exit structure of the riser, as well as its length and diameter. Brereton et al. (1988) showed that Dge tends to be higher for an abrupt exit than for a smooth one at a given C7g and Gj. On the other hand, Dgg from the smooth exit was higher than from the abrupt exit for a given apparent suspension density. 

Figure 5. A figure illustrating the impact of exit effects in circulating fluidised bed systems. The lines show density profiles for identical conditions of gas velocity (7.1 m/s) and circulation rate (73 kg/mh) for an abrupt exit (circles) and a smooth exit (triangles). The solids are returned 1.98 m above the gas distributor causing the "nose" in the profile, (Brereton, 1987).

Units such as the Studsvik Energy or Ahlstrom license systems do not possess external heat transfer surfaces. In these cases the cost of the unit can be minimised by reducing the external recycle of solids. This is best served by design of a very abrupt exit (or even an internal separator) to maximise internal refluxing of particles. This requires a different exit design than the previous case, with no conflicting demands other than erosion tendencies of the increased internally refluxing stream. 

Figure 7. Diagram showing how similar total pressure drops in smooth and abrupt exit columns result in different profile shapes. The smooth exit unit has a choked dense phase at the base which may be a zone of increased gas backmixing.

Abrupt Exit Promotes vigorous internal mixing and high uniform suspension densities at low external solids circulation rates. Ideal for minimising recycle loop sizes and maximising temperature unifomity and heat transfer in certain CFB combustion systems. 

These devices are employed for fine powders (40-400 im) and also coarser particles. In the Franken or Vandenhoek inertial separators [13,14] (Fig. 4a) particle-air mixture enters from the top and falls down in the inlet pipe. The air turns abruptly to the outlet pipe inclined by about 45-50° and carries the fine particles. Coarse fraction proceeds straight down because of high inertia towards the discharge pipe. On its way this material undergoes repeated separation in a secondary air flow carrying fines into a curvilinear chamber. Then they exit via the outlet pipe with blades mounted for better air distribution. These classifiers, manufactured by Buell Co., are used for separation of pulverized limestone at cut size of 150 pra with feeds from a few pounds to 600 t/h [15], In the Buell s variant of this device [14] blades have different angles to increase the efficiency. Such devices were used for separation of 15-1000 pm particles. 

It is more compact and is commonly used in large flow applications. The tube consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube. This section, called the inlet shoulder, is followed by the converging inlet cone and a diverging exit cone. The two cones are separated by a slot or gap between the two cones. 

If a nonreactive tracer were being continuously and steadily injected into the stream and then abruptly turned off, R t) would represent its relative concentration in the reactor effluent. The frequency function f t) defines the fraction of exiting material that had residence times between t and t -I- dt in the reactor. It is given by... 

An illustrative example of the previous considerations may be given for polyethylene melts. It is admitted that low density polyethylene (LDPE) melts develop rapid vortex growth in an abrupt contraction, and that high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) melts do not. However, in exit flows, all these polyethylene melts can swell notably, and, for many years, there has been no clear understanding about differences in entry and exit flows of these polymer melts. 

This paper reports experiments and numerical simulations related to a linear low-density polye ylene (LLDPE) and a low-density polyethylene (LDPE), in a significant number of axisymmetric and planar mixed flows. Converging and abrupt contraction geometries involving short and long dies were considered as well as extrudate swell flows occurring at the exit of the ducts under investigation. 

Breakthrough curve The concentration profile observed at the column exit as a response of the column to an abrupt change in the composition of the mobile phase stream pumped at the inlet. 

The flow expansion process at the exit of the tube may be more gradual with the addition of a nozzle than that observed in the current simulations where the tube opens abruptly into the ambient atmosphere. This observation is consistent... 

---