| According to the demand of TMP process, one reactor was required to be explored in order to increase the solids holdup and residence time, at the same time, convey the gas-phase product out of the riser fast, High-density conveying fluidized bed was characterized by high solids holdup, homogenous axial and radial flow structure, no net downflow of solids and high contact efficiency in this study.These flow dynamic properties were mainly represented by solids holdup profiles, identification of flow regime and cluster dynamics in a riser, 100-150 mm in diameter and 10.06 m in height. The effects of gas-solid interaction on flow performance were investigated by a fiber-optic probe to detect solids holdup. Whether flow dynamics in a novel high-density conveying fluidized bed belongs to a new flow regime or just a transition state is controversial. In order to resolve this confusion, transient fluctuation signal processing and wavelet analysis have been used to identify the flow regimes in five specially designed flow systems, which have been proven to be effective tools to extract detailed information of flow features in different flow regimes. Furthermore, the fuzzy system identification method, which was used to process wavelet energy data of various scales obtained by wavelet analysis, was employed to make a quantitative distinction among the flow regimes. CO2 tracer and ozone tracer injection and sampling system were used to characterize the flow structure and define the contact efficiency. A flow model based on the profiles of solids holdup and CO2 tracer concentration was proposed to account for the gas-solid contact efficiency in the reactor. At the same time, the effect of ring-feeder internals and feed styles on flow behavior was studied to optimize the reactor‘s flow dynamics.The experimental results showed that flow regime in the high-density conveying fluidized bed is belonging to dense riser upflow(DRU) or DSU(dense-suspension upflow) regimes and transient region disappears in axial position. The new regime has some unique characteristics including intensive gas-solids interaction; high gas-solids contact efficiency, two peaks of probability density distribution(PDD) curves in the center of the fluidized bed, broader spectrum of solids holdup for PDD curves, a decreasing extent of gas-solids separation extent and unique radial detail wavelet energy percentage profiles. The results showed that the flow regime in the high-density conveying fluidized bed is a novel flow regime, which is different from traditional flow regimes such as bubbling, turbulent, fast fluidization(FF) and dense suspension up-flow(DSU) regimes since its overall degree of membership is 0.84 above all other flow regimes.Two theories about the contact efficiency from the respects of overall factors and reactor were proposed respectively. From the respect of CO2 tracing system, the contact efficiency in the high-density conveying fluidized bed is 2.5-5, which was much higher than that of HDCFB(high density circulating fluidized bed), whose value remains 1, which means the high-density conveying fluidized bed reactor would exhibit better performance to optimize the product distribution under the same operating conditions. Experimental results indicated that gas back-mixing in the top of the diameter-expanding part inhibit the gas-solids contact efficiency. For reactor itself, the gas-solids reaction efficiency in the bottom of the high-density conveying fluidized bed(0.25-1.25) is much higher than than in the CFB riser(0.25). However, this trend is reversed due to severe gas-mixing in the top of the high-density conveying fluidized bed riser. The study about the ring-feeder internals and feed styles showed that bi-ring feeder internal and U shape feed style are conductive to flow dynamics in thehigh-density conveying fluidized bed riser. |
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