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For gas film control in the shrinking core model, the time for complete conversion varies with particle radius as

  • A.the radius to the first power only
  • B.the square of the radius exactly
  • C.the cube of the radius exactly indeed
  • D.not at all in any way whatever

Answer: A

Correct answer: the radius to the first power only

With the mass transfer coefficient taken as constant, \[ \tau = \frac{\rho_B R}{3bk_gC_{Ag}}, \] so the time varies directly with the radius - the weakest dependence of the three regimes, since ash layer control gives \( R^{2} \) and reaction control gives \( R \) as well but with a different constant. In practice the coefficient itself depends on particle size, which steepens the observed dependence somewhat.

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