CPE3003 Transport Processes Tutorial 5 AnswerS.J. Wilkinson

Calculate the heat transfer area of a boilerA reboiler is required to supply 0.1 kmol s-1 of vapour to a distillation column. Assume that the column bottom product is pure butane at a pressure of 19.25 bar at its boiling point of 112 oC. The energy for the vaporisation is steam with a condensing temperature of 140 oC. Calculate the heat transfer area required using the equation below for the shell-side heat transfer coefficient (assume nucleate boiling on the shell-side).

Data:Critical pressure of butane = 38 barMolar mass of butane = 58 kg kmol-1

Heat of vaporisation of butane = 233 kJ kg-1

Tube side heat transfer coefficient = 2350 W m-2 K-1 (assume all other resistances are negligible)

Total heat load Q = 0.1 x 58 x 233 = 1,351 kW (using data)

Guess U = 1500 W m-2 K-1 therefore area A:

Heat flux q is:

Shell side (nucleate boiling) heat transfer coefficient hNB is:

Overall heat transfer coefficient calculated from:

Use this as the new guess for second iteration:

A= 35.6 m2, q= 37.9 kWm-2, hNB = 2975 Wm-2K-1, U=1313 Wm-2K-1 close enough

Check for nucleate boilingCalculate the number of tubes of diameter = 30mm and length = 4m required in the reboiler and verify that the actual heat flux calculated above is less than the critical heat flux using the equations below (assume a bundle diameter = 0.75m)

Number of tubes:

Multiple tube correction factor:

Critical flux:

Ratio of flux found previously, q= 37.9 kWm-2 to critical flux:

So we are operating at 79% of the critical flux which is OK but still a little more than the recommended value of 70%. To reduce the flux we would need to increase the heat transfer area and decrease the overall heat transfer coefficient which we could do by somehow lowering the tube side heat transfer coefficient.