$28.$ 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.

$29.$ For a trickle bed reactor operating with a liquid superficial velocity of $0\mathrm{.}015$ m$/$s and gas superficial velocity of $1$ m$/$s in a column of $0\mathrm{.}5$ m diameter, first calculate the Reynolds number for the liquid phase. Then, estimate the pressure drop over a $5$ m bed height.