The axial dispersion characteristics in two (gas-liquid, liquid-solid) and three (gas-liquid-solid) phase fluidized beds have been studied in a 14.5cm I.D. column.
A wide variety of liquids and three different solid particles (1.7, 3.0 and 6.0 mm glass beads) were employed. Air was used as the gas phase throughout the study.
The effects of liquid velocity (2-13cm/sec), gas velocity (0-12cm/sec), liquid viscosity (1-27cp), surface tension (38.5-76 dyne/cm) and particle size on the axial mixing of liquid phase have been examined.
Liquid phase axial dispersion coefficients were measured using a pulse tracer technique and individual phase holdups were measured using a pressure-profile technique.
In gas-liquid system, liquid phase axial dispersion coefficient increased with increasing gas and liquid velocities and gas phase holdup increased with increasing gas flow rate. However, gas holdup was nearly independent from liquid flow rate.
In liquid-solid systems, the liquid mixing intensity increased with liquid flow rate in the region of smaller liquid holdup, however, above the liquid holdup of 0.65, somewhat decreasing trend of the mixing intensity was observed.
In three phase systems, the axial dispersion coefficient increased with increasing gas flow rate and decreasing trend was observed with particle size.
The effect of liquid velocity on the dispersion coefficient depended on the gas flow rate and it was more pronounced with increase of gas flow rate. Thus, the mixing intensity increased with the ratio of gas to liquid flow rate.
Liquid viscosity generally reduced the dispersion intensity at high gas flow rates, however at small gas flow rates, the reverse trend was observed.
Considerable bubble disintegrating phenomenon was observed when the liquid surface tension decreased and the dispersion magnitude increased with increasing surface tension.
Liquid phase axial dispersion in terms of Peclet number has been correlated with Froude numbers of liquid and gas phases, Weber and liquid phase Reynolds numbers.
$Pe,P=(\frac{v_l d_p}{D_z}) = 0.0097(\frac{U_l^2}{g d_p})^{0.048}( \frac{U_g^2}{g d_p})^{-0.899}(\frac{ρ_1 d_p U_l}{\mu})^{0.022}(\frac{U_g^2 ρ_s ^dp}{\sigma} )^{0.757}$
이상(기체-액체, 액체-고체) 및 삼상(기체-액체-고체) 유동층의 액상축방향 분산특성을 직경 14.5cm 의 유동층 반응기에서 연구하였다.
여러 종류의 액상물질과 직경 1.7, 3.0 및 6.0 mm의 유리구가 고상물질로 채택되었으며 기상으로는 공기가 사용되었다.
액유량(2-13 cm/s), 기유량(0-12cm/s), 액점도(1-27cp), 표면장력(38.5-76dynes/cm)과 입자크기가 액상 축방향혼합에 미치는 영향을 연구 하였다.
액상축방향 분산계수는 pulse tracer technique을 사용하여 측정되었고, 상 체유량은 pressure profile technique을 통하여 결정하였다.
기-액계에서의 액상 축방향 분산계수는 기체와 액체속도에 따라 증가하였고, 기상체유량은 기상유량의 증가에 따라 증가하였으나 액유량에는 거의 독립적이었다.
고-액계에서는, 액상혼합도는 낮은 액상체유량의 영역에서 액유량에 따라 증가하였으나, 액상체유량이 0.65 이상일 경우에는 혼합도의 감소 현상이 관찰되었다.
삼상유동층의 축방향 분산계수는 기유량에 따라 증가하였고 입자크기에 따라 감소하는 현상을 볼 수 있었다.
분산계수에 대한 액유량의 영향은 기유량에 의존 하였고 기유량의 증가에 따라 커감을 볼 수 있었으며, 기체대 액체유량비의 증가에 따라 혼합도는 증가하였다.
액점도는 높은 기유량에서는 혼합도를 감소시켰으나 낮은 기유량에서는 오히려 증가시키는 현상을 볼수 있었다.
심각한 기포분쇄 현상이 표면장력의 감소에 따라 관측 되었으며, 표면장력의 증가에 따라 혼합도는 증가하였다.
기상 및 액상의 Froude 수, Reynolds 수 및 Weber수로써 액상축방향 분산에 관한 다음과 같은 경험식을 얻을 수 있었다.
$Pe,1=(\frac{v_l d_p}{D_z}) = 0.0097(\frac{U_l^2}{g\cdot d_p})^{0.048}( \frac{U_g^2}{g\cdot d_p})^{-0.899}(\frac{ρ_1\cdot d_p\cdot U_l}{\mu})^{0.022}\cdot(\frac{U_g^2\cdot ρ_s\cdot^d_p}{\sigma} )^{0.757}$
