Note: Chemistry 2014 – Professional Knowledge 20
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Regarding the flow of gas-solid two-phase flow in a vertical pipe, which of the following statements is incorrect? A. When the fluid velocity is much higher than the settling velocity of the particles, the particle concentration distribution is generally uniform both axially and radially. B. When the material is properly suspended inside the pipe, the pressure drop decreases as the flow velocity decreases. C. When the gas velocity drops to a certain value, the pipe becomes clogged due to the continuous accumulation of material; this flow velocity is known as the clogging velocity. D. The minimum flow velocity required in a vertical pipe is such that the particles do not settle, thereby ensuring stable operation. --------------------------- There’s an answer online stating that D is incorrect, but I would like to ask that the description in option D matches that in version 268 from the associations. Let’s discuss it?
I think it's B; the pressure drop during the fluidized bed stage hardly changes with the gas velocity.
A is incorrect; it is uniform in the axial direction but not uniform in the radial direction. The rest are correct. Association Guide P268
The flow velocity is much greater than the particle settling velocity; the intense movement of the particles keeps the bed in a fully mixed state, resulting in uniform temperature and composition throughout the bed. A’s statement is fine.
As stated on page 268 of the association’s textbook, A is incorrect. At this point, the particle size distribution is uniform in the axial direction but uneven in the radial direction. What is discussed here is flow within pipes and pneumatic conveying, not bed layers.
It seems that A is incorrect; the exact wording can be found on pages 267-268 of the association’s book.
This post was last edited by yaojin911 on 2015-6-27 09:29. On page 268 of the association’s book, all four options include the original text: A: The exact words in the first two lines are: “The particle concentration is generally uniform in the axial direction, but it is not uniform in the radial direction.” B: In Figure 3.6.4-1, the AB segment represents the suspended state, and the pressure drop increases monotonically with the gas velocity. C: The exact words in the fourth line of the second paragraph are: “The pipe will experience blockage due to the continuous accumulation of material; at this point, the flow velocity becomes the blockage flow velocity.” D: The exact words in the second line of the third paragraph are: “The minimum flow velocity in a vertical pipe must be sufficient to prevent particles from settling, thereby ensuring stable operation.” Therefore, option A is incorrect. Regarding the debate about A, in my opinion, one can refer to Figure 1-13 on page 15 of Chen Minheng’s Principles of Chemical Engineering; even in cases where the velocity distribution is relatively uniform, the velocity profile inside the pipe still takes on an approximately trapezoidal shape. The boundary layer always exists, though its thickness is relatively small, so the velocity is not uniform in the radial direction. Feel free to discuss and point out errors!
B is incorrect; in a steady flow, it exhibits liquid properties, with uniform physical characteristics and a constant pressure drop that does not change with flow velocity. These characteristics remain intact unless the steady state is disrupted.
Choose A: the axial distribution is uniform, while the radial distribution is uneven
Could you tell me the source of the statement that \"the pressure drop remains constant and does not change with flow velocity\"?