Bubble Characterization in a Fluidized Bed with Vertical Tubes

Abstract

Vertical tubes are commonly used in industrial fluidized beds as heat exchanger tubes. In this study, the influence of vertical tube banks on the axial bubble growth in a 145-mm inner diameter (ID) fluidized bed with Geldart B particles is experimentally investigated by means of pressure fluctuation measurement (PFM) and optical probing (OP). The employed tube bank configurations differ in their tube-to-tube spacing, tube diameter, and tube arrangement (square versus triangular). PFM and OP show that immersed vertical tube banks, irrespective of their configuration, significantly reduce the axial bubble growth. The bubble size reduction is even more significant for higher gas velocities (u 0/u mf > 4.6, where u 0 is the superficial gas velocity and u mf is the minimum fluidization velocity) as well as if the tube-to-tube spacing is narrow and the tube diameter is small. The tube arrangement (square versus triangular), in turn, does not show any significant differences. Depending on the gas velocity, the ratio of the bubble diameter to the tube diameter and the tube-to-tube spacing is changed. This change invokes a different bubble flow pattern between the vertical tubes and different bubble growth mechanism, which defines the effectiveness of vertical tubes to delay slugging and makes the fluidization smoother.

Influence of vertical internals on a bubbling fluidized bed characterized by X-ray tomography

“…For the reactor with vertical internals, the equivalent bubble diameter reduces by about 16% compared with the measurement without vertical internals, which is similar to the data previously reported based on pressure fluctuations Rüdisüli et al (2012b). Bubble diameters predicted by the correlation of Horio and Nonaka are located between the measurements with and without vertical internals.…”

“…Those correlations generally provide mean values of bubble size, velocity and hold-up and are used for the development of computer models, such as the two-phase model Toomey and Johnstone (1952) and Kopyscinski et al (2011). To describe the hydrodynamic properties of bubbling fluidized bed reactors with vertical internals, none of the existing correlations seem appropriate, and reliable data is rarely found in the open literature Rüdisüli et al (2012b). Additionally, because these bubble correlations are frequently used to validate complex computational fluid dynamic models, a more quantitative and critical assessment would be of great value.…”

Methanation for Synthetic Natural Gas Production – Chemical Reaction Engineering Aspects

“…While most of the correlations have been determined for freely bubbling fluidized beds, isothermal methanation reactors comprise a dense packing of vertical heat exchanger tubes. it was further observed that both depend on the geometry of the cooling tubes packing, that is, tube diameter and tube distance, but hardly on the arrangement (quadratic versus triangular) of the tubes [189]. optical probe measurements showed that indeed, the average pierced chord length of the bubbles and rise velocity increase with increasing bed height, but that distributions exist of both bubble diameter and bubble rise velocity; see Figure 4.25.…”

“…Pressure fluctuation measurements in a cold flow model indicated [189] that, by these vertical tubes, the bubbles growth is decreased and a smoother fluidization can be observed. While most of the correlations have been determined for freely bubbling fluidized beds, isothermal methanation reactors comprise a dense packing of vertical heat exchanger tubes.…”

“…25 Kernel smoothed density of the bubble pierced chord length distribution measured by optical probes. optical probe heights are: (a) 0.243 m and (b) 0.450 m. reprinted from[189]. optical probe heights are: (a) 0.243 m and (b) 0.450 m. reprinted from[189].…”