4. In the regeneration of an ion exchange resin, hydrochloric acid of density 1200 kg/m³ and viscosity 2×103 Pa s flows downward at 0.02 L/min through a bed of resin particles of density 2500 kg/m³ resting on a porous support in a tube 4 cm in diameter. The resin particles have cylindrical shape with a diameter of 0.1 mm and length of 0.2 mm, and form a bed of void fraction 0.5. The bed is 60 cm deep and is unrestrained at its upper surface. Calculate (a) The pressure drop across the bed. [12 marks] (b) The minimum fluidised velocity if the fluid flowing upwards. [13 marks]