As I am sure, you are aware that the vapor outlet line from the kettle reboiler should not be below the liquid level in the bottom of the tower. That is, the liquid level in the bottom of the tower should be below the vapor inlet line. I would check the liquid level controller on the bottom of the tower to make sure it is working and properly set.

How far below the vapor inlet nozzle the liquid level should be depends upon many considerations such as the vapor velocity entering the tower and the possible liquid level variation. 450mm to 600mm would be a good minimum distance. This would also be a good minimum figure for the vapor inlet nozzle to be below the tray or packing above the inlet. For large vapor inlets, 1 to 1.5 vapor inlet pipe diameters would not be unreasonable for these clearance distances. The above clearances are only a rule-of-thumb and the exact value is a function of inlet vapor velocity and the tower diameter. If these kinds of clearance distances are not possible, you may need to consider a vapor sparger or distributor.

The vapor entering the tower should not impinge upon liquid leaving the bottom tray downcomer, which may cause liquid entrainment to be carried up on to the bottom tray and cause the bottom tray to flood. In the case of packing, which has a low pressure drop, if the inlet vapor velocity is too high, you may get vapor maldistribution and reduce tower efficiency. Also, in the case of packing, you don't want the inlet vapor to entrain liquid back into the packed bed with will lower efficiency and even flood the bottom of the packed bed.

If the tower has worked in the past and is now not working for any apparent reason, it could be the level control system is not working as it should. If changes have been made to the tower or the throughput has been increased, one would need to suspect increased vapor velocities caused problems or maybe even the bottom liquid draw system as being too small.

Sorry I cannot give more definite help. Here are some references that may help you. You may also want to check Fractionation Research, Inc's webpage at www.fri.org for an engineering company near you that can help with your problem.

Kister, H. Z., Distillation Operation, McGraw Hill, Inc., New York, 1990, p. 434.

Kister, H. Z., Distillation Troubleshooting, John Wiley & Sons, Inc., Hoboken, New Jersey, 2006, p. 194.

Fair, J. R., "REBOILERS / A General Review of Predictive Models and Design Practices," December 7, 1992, p. 1 (prepared for The Encyclopedia of Chemical Processing and Design, J.J. McKetta, Editor).

Strigle, R. F. Jr., Random Packings and Packed Towers: Design and Applications, Gulf Publishing Company, 1987, p. 273.