Thermosyphon re-boiler operation 2

[branca1]

On one of our columns we can use separately either a hot water (HW) driven re-boiler, a steam driven reboiler or both together. To reduce cost we preferentially use the HW re-boiler. As this begins to foul we comission the 2.75bg steam re-boiler to operate alongside the other re-boiler. It will remain like this until the HW one fouls. During this operation the steam re-boiler will be running with its steam valve maybe 15-25% open, just supporting the HW re-boiler. Might this cause the steam re-boiler to foul more quickly (because of reduced driving force) than if it was put into service FULLY and the HW re-boiler taken off-line?
Would it be better to take the HW out of service and put the steam re-boiler on by itself.

Regards Branca

 

[Leclerc]

A good question, the theory is entirely plausible, and, given that you don't seem to have much to lose, worth a try.

Some years ago, working with polymerisables, we had reboilers (vertical shell and tube, large central tube acting as a downcomer and liquid circulating up through the smaller tubes and evaporating from the liquid surface above the top tubeplate) which fouled so regularly that installed spares were the norm: one on-line, one being cleaned. Unfortunatley, what with the heavy fouling/polymer skinning in the reboiler and the extra polymerisation in the pipework branch and valves of the spare shell, we had plenty problems.

We changed to a single thermosyphon reboiler, no installed spare or other excrescences where polymer could form, and lo and behold, the problems virtually disappeared.

I believe that the very short residence time of polymerisable liquid at the heating wall, together with the scouring effect of the very high velocities in the tubes were major contributory factors.

 

[tkdhwjd]

Not sure how you are maintaining the condensate level in the steam reboiler when it is operating with a HW reboiler.... But assuming the area won't change, the steam reboiler may foul faster or may not.

In order to make a single steam reboiler to cover all necessary duty, you will have to raise the chest pressure assuming you have no room to raise surface area. Higher skin temperature will be resulted, which will promote fouling. But since you have more flow through the exchanger, the tube fluid velocity will be higher, which means two "good" things will occur:
1. Less chance for foulants to accumulate and plug tubes
2. Higher transfer coefficient due to higher flow turbulence

In the end, you may have to do a "trial & error" reboiler sensitivity test.

 

[sshep]

I assume that the hot water is free waste heat, while the steam costs money. This is really a fairly simple economical maintenance frequency question for which changes in fouling rates may be largly irrelevant. There are two costs to trade in this problem- utilities use vs deferred maintenance.

No one in this forum can identify the optimum because the duration and cost of cleaning, steam price, reboiler duty, and other important data are unknown. It is reasonable to guess that there is an optimum shortly after steam is required, but you will need to calculate it yourself. At that point the HW exchanger should be brought down, cleaned, and put back in service as quickly as practical. Running both until the HW exchanger is practically dead is probably not the optimum because you are using full steam rate but doing no maintenance, while exposing both units to fouling instead of one.