Distillation Tower Reboiler Problems and their solution

Assume a typical reboiler configuration as shown in below figure.

The process fluid is on the shell side of a horizontal thermosiphon exchanger with condensing steam on the tube side providing the source of heat. The common problem with thermosiphon reboilers is reduced circulation. This condition is diagnosed by determining the temperature rise across the reboiler. Insert a glass thermometer under the insulation on the inlet and outlet process piping of the reboiler.

The temperature difference obtained should be compared to the design ΔT (temperature difference). If the measured ΔT is much greater (more than 50%) than the design ΔT, circulation through the reboiler is insufficient. The two usual causes of decreased reboiler circulation are plugged shells and leaking trap out pans.

A series of experiments can be performed to determine whether the reboiler bundle needs to be cleaned to remove plugging or whether the distillation tower must be opened to repair a malfunctioning trap out pan. Following are details in diagnosing and correcting insufficient reboiler circulation.

Trap out pans

As a first step in testing for leaking trap out pans, consult above figure, if the tower has a reboiler start-up line, open it. Increase the tower bottom level to maximum. If the reboiler outlet temperature drops rapidly, this is a positive indication of a malfunctioning reboiler trap out pan.

If the tower is not equipped with a reboiler start-up line, try increasing the flow of liquid to the trap out pan. This is done by increasing the tower feed or the reflux rate. If the trap out pan is leaking, the increased liquid flow to the pan will cause the ΔT between the inlet and outlet to decrease.

Plugged reboilers

If neither of the above methods helps increase reboiler circulation, the shell-side flow areas are likely restricted. This is cross-checked by slowly closing the reboiler inlet block valve and observing the change in shell-side outlet temperature. Closing the block valve 60%–70% without causing an increase in shell-side outlet temperature is proof that there is substantial shell-side plugging.

Repair of trap out pans

The exact cause of malfunctioning trap out pans can easily be determined once the tower is out of service. Hose water into the center down comer, disconnect the tower from the reboiler inlet, and observe the flow of water.

Steam-side problems

Steam reboilers are subject to a wide variety of serious capacity problems. The most common is steam condensate backup into the channel head. This is checked by manually draining the steam condensate to a nearby sewer and observing if the reboiler duty increases or the steam inlet control valve closes.

As shown in above figure, the rust layer on the inside of the channel head of a reboiler shows clearly where the condensate level normally ran. This is a positive indication that 20% of the heat-transfer surface area of the reboiler is waterlogged and, therefore, useless. Steam condensate backup can be due to one of the following:

• Malfunctioning steam trap.

• Excessive pressure in the steam condensate collection header.

• Installation of a control valve directly on the condensate drain line.

 This is usually only a problem when the reboiler steam supply is 30 psig or less.

The problem is easily eliminated by the addition of an intervening condensate level control drum between the reboiler and the control valve, provided that downstream piping is sized to accommodate the flashed stream condensate.

Blown condensate seal

The opposite problem to steam condensate backup is blowout of uncondensed steam through the reboiler and out the condensate drain line. This phenomenon causes a loss in heat transfer entirely out of proportion to what might be expected. Literally half of a reboiler’s duty can be lost by an apparently small amount of steam blowing out the condensate drain line.

To determine if this problem exists on a reboiler, establish a definite condensate seal by restricting the condensate effluent line. If the tower bottom’s temperature increases, the reboiler has a blown condensate seal. This wastes steam and reboiler capacity. The correction is the same for condensate backup: installation of a steam condensate seal drum.

 Accumulations of non-condensable in the channel head will also retard the condensation rate of steam and lead to a loss in reboiler duty. Typically, CO2 contained in the steam supply collects below the bottom channel head-pass partition baffle. If left to accumulate, the CO2 will dissolve in the steam condensate and form corrosive carbonic acid.

The channel head cover should have a 3/4-inch plug located below the bottom pass partition baffle which is intended for venting non-condensable.

Note: Venting from the top of the channel head is useless as only steam from the header is vented.

read original by Rais Shahid