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The residence time distribution and mixing of the gas phase in the riser of a circulating fluidized bed
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Mahmoudi, Shiva, Seville, J. P. K. (Jonathan P. K.) and Baeyens, J. (2010) The residence time distribution and mixing of the gas phase in the riser of a circulating fluidized bed. Powder Technology, Vol.203 (No.2). pp. 322-330. doi:10.1016/j.powtec.2010.05.024 ISSN 0032-5910.
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Official URL: http://dx.doi.org/10.1016/j.powtec.2010.05.024
Abstract
CFBs are increasingly used for both gas-catalytic and gas-solid reactions. The conversion is a function of the gas hydrodynamics, subject of the present research.
Available literature on the gas mixing in the riser of a CFB shows contradictory results: some investigators neglected back-mixing of gas, whereas others report a considerable amount of back-mixing in CFB risers. The present paper reports experimental findings obtained in a 0.1 m I.D. riser, for a wide range of combined superficial gas velocity (U) and solid circulation flux (G). The gas flow mode (plug vs. mixed) is strongly affected by the operating conditions, however with a dominant mode within a specific (U, G)-range. Sand was used as bed material. The superficial gas velocity was varied from 5.5 to 8.3 m/s, the solids circulation flux was between 40 and 170 kg/m(2) S. A tracer pulse response technique was used with a pulse of propane injected at the bottom and detected at the riser exit. The cumulative response curves, F(t), define (i) an average residence time (t(50)) obtained for F(t) = 0.5; and (ii) the slope of the curves (a steeper one corresponding with more pronounced plug flow) and expressed in terms of a span, sigma. These parameters (t(50) and sigma) define the gas flow mode. A quantitative comparison of experimental results with literature RTD-models is inconclusive although the occurrence of both mixed flow and plug flow is evident, and (U, G)-dependent. The experimental results are expressed in empirical design equations, and the comparison of predicted and experimental results is fair: low values of sigma determine the plug flow regimes, whereas back-mixing is more pronounced at higher value of sigma. Experiments with similar systems might favor plug flow or mixing as function of the combined (U, G)-values. The introduction of the RTD-function in reaction rate equations can improve the prediction of the gas-conversion in a riser-reactor. (C) 2010 Elsevier B.V. All rights reserved.
| Item Type: | Journal Article | ||||
|---|---|---|---|---|---|
| Subjects: | T Technology > TP Chemical technology | ||||
| Divisions: | Faculty of Science, Engineering and Medicine > Engineering > Engineering | ||||
| Journal or Publication Title: | Powder Technology | ||||
| Publisher: | Elsevier Science SA | ||||
| ISSN: | 0032-5910 | ||||
| Official Date: | 10 November 2010 | ||||
| Dates: |
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| Volume: | Vol.203 | ||||
| Number: | No.2 | ||||
| Number of Pages: | 9 | ||||
| Page Range: | pp. 322-330 | ||||
| DOI: | 10.1016/j.powtec.2010.05.024 | ||||
| Status: | Peer Reviewed | ||||
| Publication Status: | Published | ||||
| Access rights to Published version: | Restricted or Subscription Access |
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