Dense phase transport systems

Figure 16. Molerus dense phase transport system.

The use of recirculating fluid beds has caused considerable interest in dense phase vertical conveying. These units are indeed dense phase transport systems with a significant amount of recirculation taking place. 

Whatever the mechanism used to tackle the plug problem, all commercial dense phase transport systems employ a blow tank which may be with fluidizing element (Figure 8.13) or without (Figure 8.14). 

Although lots of information is available on dilute phase transport that is useful for designing such systems, transport in the dense phase is much more difficult and more sensitive to detailed properties of the specific solids. Thus, because operating experimental data on the particular materials of interest are usually needed for dense phase transport, we will limit our treatment here to the dilute phase. 

Tsuji et al. (1990) have modeled the flow of plastic pellets in the plug mode with discrete dynamics following the behavior of each particle. The use of a dash pot/spring arrangement to account for the friction was employed. Their results show remarkable agreement with the actual behavior of real systems. Figure 28 shows these flow patterns. Using models to account for turbulent gas-solid mixtures, Sinclair (1994) has developed a technique that could have promise for the dense phase transport. 

It is believed that the air velocities in a large-diameter dilute-phase system can be 50 to 100% higher than an equivalent well-designed dense-phase system. Hence, much greater wear problems are expected in the dilute-phase system, although significant advances have been made in the technology of wear-resistant materials and bends (Wypych and Arnold, 1993). Other features involved with dilute-phase transport systems include ... 

Pneumatic conveying systems can be classified on the basis of the angle of inclination of pipelines, operational modes (i.e., negative- or positive-pressure operation), and flow characteristics (i.e., dilute or dense phase transport steady or unsteady transport). A practical pneumatic conveying system is often composed of several vertical, horizontal, and inclined pipelines. Multiple flow regimes may coexist in a given operational system. 

Hydraulic design aims at the realization of an intensive heat and mass transfer. For two-phase gas-liquid or gas-solid systems, the choice is between different regimes, such as dispersed bubbly flow, slug flow, churn-turbulent flow, dense-phase transport, dilute-phase transport, etc. 

The blow tank is automatically taken through repeated cycles of filling, pressurizing and discharging. Since one third of the cycle time is used for filling the blow tank, a system required to give a mean delivery rate of 20 t/h must be able to deliver a peak rate of over 30 t/h. Dense phase transport is thus a batch operation because of the high pressures involved, whereas dilute phase transport can be continuous because of the relatively low pressures and the use of rotary valves. The dense phase system can be made to operate in semi-continuous mode by the use of two blow tanks in parallel. 

Fluidization occurs in stationary (segregation of bed and free board), turbulent (circulating fluid bed), and forced dense-phase (transport reactor) mode. All systems require hot gas cyclones either to recycle entrained particles back into the fluid bed or just to separate them from the product gas. 

Knowlton has cautioned on the difference between small diameter and large diameter systems for pressure losses. The difference between these systems is especially apparent for dense phase flow where recirculation occurs and wall friction differs considerably. Li and Kwauk (1989, 1989) have also studied the dense phase vertical transport in their analysis and approach to recirculating fluid beds. Li and Kwauk s analysis included the dynamics of a vertical pneumatic moving bed upward transport using the basic solid mechanics formulation. Some noncircular geometries were treated including experimental verification. The flows have been characterized into packed and transition flows. Accurate prediction of the discharge rates from these systems has been obtained. 

T. Ramakrishnan, K. Ramakoteswara Rao, M.A. Parameswaran, S. Sivakumar, Experimental investigation of a dense-phase pneumatic transport system, Chem. Eng. Process. 32 (1993) 141-147. 

In effect, such a multi-scale analysis resolves a macro-scale heterogeneous system into three meso- to micro-scale subsystems—dense-phase, dilute-phase and inter-phase. Thus, modeling a heterogeneous particle-fluid two-phase system is reduced to calculations for the three lower-scale subsystems, making possible the application of the much simpler theory of particulate fluidization to aggregative fluidization and the formulation of energy consumptions with respect to phases (dense, dilute and inter) and processes (transport, suspension and dissipation). 

For instance, if the solids flow rate is specified at Gs = 50 kg/(m2s), choking will take place at Ug = 3.21 m/s for system FCC/air as indicated in the figure. Throughout the entire regime spectrum, only at this unique point (l/pl, K ) can both dense-phase fluidization and dilute-phase transport coexist. At velocities higher than Upt, only dilute transport can exist, shown as Mode FD in Fig. 4 at velocities lower than l/pt, only dense-phase fluidization can take place, shown as Mode PFC in Fig. 4. The transition point at l/pt identifies the unique Mode PFC/FD on the curve of Fig. 5 for the coexistence of both modes, the relative proportion of which depends on other external conditions such as the imposed pressure APimp as reported by Weinstein et al. (1983). 

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