Risers, slug flow

Slug flow in feed riser Slug flow in rellux-drum inlet flow... 

Another instability mode of interest is due to the flow regime itself. For example, it is well known that the slug flow regime is periodic and that its occurrence in an adiabatic riser can drive a dynamic oscillation (Wallis and Hearsley, 1961). In a BWR system, one must guard against this type of instability in components such as steam separation standpipes. The design of the BWR steam separator complex is normally given a full-scale, out-of-core proof test to demonstrate that both static and dynamic performance are stable. 

Slug flow. When partial condensers are located below the reflux drum (Fig. 15.15e), and the velocity in the riser is too slow, vapor and liquid segregate in the riser. A head of liquid builds up and exerts back pressure against the column. Periodically, a slug of liquid breaks through and releases the back pressure. The riser then gradually fills up with liquid, and the cycle repeats itself. This causes fluctuations in column pressure and accumulator level. One troublesome case history of slug flow has been reported (70). 

If raising the flow rate or temperature in the riser improves column stability, slug flow should be suspected. A rough rule of thumb (239) suggests that slug flow is likely at velocities lower than 15 ft/s, and unlikely at velocities greater than 25 ft/s. Alternative criteria are discussed elsewhere (70, 188). 

To overcome slug flow, dual risers are often installed (188,239). The small-diameter riser is operated at low throughputs, while the larger is operated at higher throughputs. 

Figure 15.15 (Continued) Techniques for preventing operating problems in distillation condensers (d) Flushing tubes near condenser inlet to prevent process-side fouling (e) Slug flow may occur in risers of partial condensers located below the reflux drum.

Refinety Crude tower Column pressure and product gas rate sharply fluctuated during low-rate operation in this and other luiits. The condenser was a partial condenser located at ground level, with an elevated reflux drum. Problem was caused by slug flow in the riser from the condenser to the drum. Size risers to avoid sltig flow even during low-rate operations. 

If the velocity in the riser is too low, the two phases will separate. A head of liquid will build up and create back pressure against the distillation tower. Periodically, a slug of liquid will be pushed through the riser and relieve the back pressure. The riser then gradually refills with liquid. This type of slug flow causes fluctuating tower pressure. Other instances of slug flow in distillation service are thermosiphon reboiler outlets and partially vaporized feed lines. 

If one finds that increasing the rate or temperature in a riser improves the operating stability of a tower, slug flow should be suspected. To confirm this suspicion, calculate the linear velocity in the riser. A rough rule of thumb is above 25 ft/sec, slug flow is unlikely below 15 ft/sec, slug flow is likely. 

To overcome slug flow instability, some distillation columns are equipped with dual risers. A small-diameter riser is used at low throughputs. At higher rates, the larger diameter riser is put into service. 

FIGURE 18-6 Elevating the reflux drum above the condenser may cause slug flow in the riser. 

The problem is phase separation in the riser line. The vertical velocity in this line is too low to maintain a froth flow up the riser. Slugs of liquid form. Periodically the accumulating liquid is blown out by the vapor. This slug flow causes an erratic back pressure on the tower but not an erratic pressure in the drum. 

This problem only occurs when the drum is elevated above the partial condenser. The fundamental difficulty is excessive riser pipe diameter. To avoid slug flow in such a riser a mixed-phase velocity of at least 20 ft/s is needed. 

The third instability, classical choking, occurs on reducing the superficial gas velocity in the riser when slugs start to form right after a dense bed is created in the bottom section of a riser. Stable operation of gas-solids upward slug/plug flow may still be possible in... 

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