Solids flow patterns

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

Werdmann, C. C., and Werther, J., Solids Flow Pattern and Heat Transfer in an Industrial Scale Fluidized Bed Heat Exchanger, Proc. 12th Intern. Conf. on Fluid. Bed Comb., 2 985-990 (1993)... 

When one of the three draft tube velocities was increased to simulate upset conditions, stable operations were still possible. These upset conditions could also be detected by pressure drop differences among various draft tubes and downcomers when differences in draft tube velocities were large. For severe upset conditions, where some of the draft tubes become downcomers, pressure drop measurement alone could not distinguish the solids flow pattern inside the draft tubes. 

Solids Circulation Pattern. Yang et al. (1986) have shown that, based on the traversing force probe responses, three separate axial solids flow patterns can be identified. In the central core of the bed, the solid flow direction is all upward, induced primarily by the action of the jets and the rising bubbles. In the outer regions, close to the vessel walls, the solid flow is all downward. A transition zone, in which the solids move alternately upward and downward, depending on the approach and departure of the large bubbles, was detected in between these two regions. 

In circulating fluidized beds two main attrition sources, namely the riser and the return leg, may be distinguished. Although a lot of information is available about solids flow patterns and flow structures inside the circulating fluidized bed risers, no systematic investigations have been found in the open literature on the influence of riser geometry and flow conditions inside the riser on attrition. With respect to attrition occurring in the return leg, the work of Zenz and Kelleher (1980) on attrition due to free fall may be mentioned (cf. Sec. 4.3). 

Experience has shown that a concave-downward (Fig. 17-10f) gas distributor is a better arrangement than a concave-upward (Fig. 17-10-e) gas distributor, as it tends to increase the flow of gases in the outer ortion of the bed. This counteracts the normal tendency of the gas to ow into the center of the bed after it exits the gas distributor. In addition, the concave-downward type of gas distributor tends to assist the eneral solids flow pattern in the bed, which is up in the center and own near the walls. The concave-upward gas distributor tends to have a slow-moving region at the bottom near the wall. If solids are large (or if they are slightly cohesive), they can build up in this region. 

Early bubbling FBC units were designed to burn coal, and the heat released was removed by heat transfer to in-bed tubes and/or to the water-wall tubes used to enclose the furnace. These surfaces experienced high rates of metal loss through the combined effects of erosion and abrasion. Protective measures such as plasma-sprayed coatings and metal fins to disrupt the solids flow pattern were used. These were effective for only short periods before requiring replacement, and so maintenance requirements were high. 

The key to solving these problems is to design the vessel for a mass flow pattern. This involves consideration of both the hopper angle and surface finish, the effect of inserts used to introduce gas and control the solids flow pattern, and sizing the outlet valve to avoid arching and discharge rate limitations. In addition, the gas or liquid must be injected such that the solid particles are uniformly exposed to it, and flow instabilities such as fluidization in localized regions are avoided. 

As mentioned, the flow rate in a standpipe depends on the solid feed device as well as the flow control valve. In this section, we discuss the gas-solid flows in a simple standpipe system where the feed device is a mass flow hopper and the solid flow regulator is a discharge orifice [Chen et al., 1984]. As shown in Fig. 8.15, the entrance of the vertical standpipe is connected to a conical hopper feeder of half angle dilute suspension flow, and the other is a solid moving bed. In this case, the following additional assumptions are needed ... 

The solids flow rate can be controlled by nonmechanical valves such as the L-valve, as noted in Chapter 8. The L-valve has a long horizontal leg. Thus, it is convenient to characterize the pressure drop across an L-valve by two terms. One term is the pressure drop through the elbow (A/ V). This term can be described by the equations developed for the mechanical valve because the solids flow patterns between the two are similar... 

Bader, R., Findlay, J. and Knowlton, T. M. (1988). Gas-Solids Flow Pattern in a 30.5cm Diameter Circulating Fluidized Bed. In Circulating Fluidized Bed Technology II. Ed. Basu and Large. Toronto Pergamon Press. 

Obviously, bed temperature should be kept the same during scale-up as well. Bed moisture content needs to be similar. Because of the difference in gas-solids flow pattern, including bubble behavior, the time for each granulation phase may vary. 

Donsi, G., and L. Sesti Osseo. Gas Solid Flow Pattern in a Circulating Fluid Bed Operated at High Gas Velocity," in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 696-701. Somerset, Pennsylvania (1993). 

In fast fluidized beds, a proper modification to the suspension emissivity is usually needed according to the gas-solids flow pattern, such as the following correlation originally proposed by Grace (1982) ... 

It should, however, be noted that the solids movement pattern as mentioned above has been observed in beds with sufficiently large values of length to diameter ratio (L /d l). Industrial fluid beds normally operate with 1, 1 values less than or close to unity and the solids flow pattern could be entirely different. More recent experimental studies such as... 

The solids movement in a spouted bed is initiated by the interaction between the particles and the high-velocity gas jet, so that particle flow in the spout region shapes the entire solids-flow pattern. While a mutual dependence between the solids flow in the spout and in the annulus is inherent to a spouted bed, it is nevertheless convenient to discuss the flow in the spout and in the annulus separately. 

Fig. 17. Solids flow pattern in the annulus (Thorley et at, Tl). Column diameter, 24 in. air-inlet diameter, 4 in. cone angle, 60° bed depth expanded, 48 in. air flow, 210 cfm minimum spouting condition f vertical component of particle velocity (in./sec) horizontal component of particle velocity (in./sec) data in ovals give solids flow (Ib/sec) based on resultant particle velocity data in rectangles give solids flow (Ib/sec) based on particle velocity at column wall.

The speed with which intermixing of solids occurs in a spouted bed is obviously dependent on the solids flow pattern, but since intermixing is a bulk property of the bed, it is difficult to relate it quantitatively to the... 

The spout is like a riser through which particles are being transported in a dilute phase, with the added features of a decreasing gas flow and an increasing solids flow along the height. The spout voids are therefore determined by interaction between the gas and solids flow patterns. 

The theoretical analysis of Lefroy and Davidson (L2) discussed in Section IV,B is essentially a further development of the second method above it enables the momentum balance, Eqs. (43) and (44), to be solved for voidage with the aid of equations describing the gas and solids flow pattern, instead of relying on actual measurements of pressure drop and particle velocities. The system of Eqs. (41)-(53), therefore, provides a... 

Bader R, Findlay J, Knowlton, T.M, Gaa/solids flow patterns in a 30.5 -cm-diameter circulating fluidised bed, in CFB Technology II, editors Basu P, Large J.F. 123(1988)... 

Solids flow pattern For most continuous dryers, the solids are... 

Continuous fluid beds may be even more varied than batch fluid beds. The main distinction between continuous fluid beds will be according to the solids flow pattern in the dryer. The continuous fluid bed will have an inlet point for moist granular material to be dried and an outlet for the dried material. If the moist material is immediately fluidizable, it can be introduced directly onto the plate and led through the bed in a plug-flow pattern that will enhance control of product residence time and temperature control. If the moist granular material is sticky or cohesive due to surface moisture and therefore needs a certain degree of drying before fluidization, it can be handled by a backmix fluid oed, to be described later. 

Solid flow pattern Fixed Flow-regime dependent... 

In an early study Rowe and Partridge [115] (see also [116]) did show that gas fluidized beds are characterized by the formation of bubbles which rise through denser bed zones of the bed and determine a gross scale gas and solid flow pattern. 

A very complex solids flow pattern will result when solid obstacles exist in the fluidized bed. The solids recirculation pattern in a cylindrical bed with a single sphere was presented by Lin, Chen, and Chao (1985), and in a 2-D bed with a single and multiple cylinders by Ai (1991). It was demonstrated that large obstacles would not only affect the local solids velocity, but also the global solids circulation patterns. 

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