$26.$ For a fluidized bed reactor with air density at $1\mathrm{.}2$ kg$/$m and particle density of $2600$ kg$/$m$,$ calculate the minimum fluidization velocity for particles of $150$ m diameter, then estimate the bed height at three times the minimum fluidization velocity.

$28.$ In a bubble column with a diameter of $1$ m operating at a gas flow rate of $0\mathrm{.}05$ m$/$s$,$ calculate the gas holdup for a homogeneous flow regime. Following this, estimate the average bubble diameter assuming a spherical bubble shape.