5 replies to this topic

[USR]

Hello everyone,
This is a topic which has been discussed and much elaborated in older posts.I have this reboiler in my distillation tower,it uses condensate control for controlling the heat duty.The reboiler is a vertical thermosyphon STHE with steam on tube side and the process fluid on the shell side.The pressure balance line is connected from the condensate drum to the steam supply line which is quite different from what Mr Art had suggested in one of the forums:the pressure balance line must connect the condensate drum with the reboiler channel head.Does this make any difference because the channel head pressure is necessarily at the same pressure as the steam supply pressure.Also since the pressure balance line is connected quite close to steam inlet line nozzle the pressure drop effect is well taken care of.But still i have observed this reboiler giving slower response as i vary the area of tubes exposed to the steam(HP).

[Art Montemayor]

USR:

Please refer to my uploaded Excel Worksheet where I depict what I believe you are trying to describe. I wish you would have taken the time to furnish a decent sketch of what you are describing because there are several ways you can configure this.

This is a strange and weird configuration of a vertical thermosyphon reboiler. It must be a VERY SPECIFIC AND SPECIAL APPLICATION because:

• It requires that the bottoms process liquid product exiting the distillation column be VERY CLEAN AND FREE OF ANY FOULING OR SOLID COMPONENTS. Rarely is this the case.
• The thermosyphon effect from this configuration is more difficult than that when the process fluid is in the tubeside. The reason for this is that the shell side always causes a larger pressure drop for the liquid fluid.
• It is very difficult (if not impossible) to detect the actual steam condensate level inside the tubes. Therefore, the level detector-controller cannot be calibrated in the field.

However, if this configuration is required and it works well, there is no reason to disown it.
Note in my sketch that the steam vapor balance line is connected directly to the top bonnet of the reboiler – AND NOT TO THE STEAM SUPPLY LINE. The reason for this is simple:

In order to ensure that the level control is controlling an accurate reading, the pressure existing above the internal condensate level must be as much as possible in equilibrium with the pressure existing in the condensate level pot located outside the reboiler. The ensures that the condensate level in the tubes is practically the same as the level in the condensate pot.

If you take the vapor balance line and connect it to the steam supply line, you will begin to measure a dynamic (rather than a static) pressure that is influenced by the flow momentum of the incoming steam vapor, This, then, is not the same pressure as that of the static condensate in the tubes. The result is that there is a tendency to have a false level detection of the condensate in the tubes.

If you are trying to control this thermosyphon reboiler’s ability to vary its heat transfer capacity by varying the level of condensate inside the tubes (i.e., varying the net available heat transfer area), then you should EXPECT TO HAVE A RELATIVELY SLOW HEAT TRANSFER RESPONSE TO THE LEVEL SIGNAL IN THE LEVEL CONTROLLER. In my experience, this type of capacity control has an inherent slow response. That is the nature of the beast.

Attached Files

•  Thermosyphon Reboiler.xls   959.5KB   465 downloads

[USR]

Hello Mr Art,
First of all i wish you a very happy new year.I think you are right in saying that this is a weird configuration.I have always believed that a horizontal thermosyphon STHE reboiler works well with steam on tube side and the process fluid on shell side.But for a vertical thermosyphon STHE it is the other way round basically for the reasons you have cited.Reboilers do handle a lot of dirt.But if the bottom fluid is clean, is it not the corrosive action of the condensate which becomes the matter of concern?Waiting for your inputs.Thank you.

[Art Montemayor]

USR:

Thank you, and a very prosperous New Year to you also – as well as to all our members.

Corrosive steam should never be taken as a given, existing, and tolerated fluid within a normal, well-designed processing plant. In my opinion, any process engineer that admits to working and accepting corrosive steam is a defeatist and will never succeed in resolving process problems.

There is no excuse for tolerating a corrosive steam condition within your plant when we have at our disposal such tools as boiler treatment chemicals, deaerators, non-condensible traps, modern chemical analysis of boiler feed water, etc., etc.. I have never operated a plant that tolerated corrosive steam – and I have operated a LOT of processing plants, not only in the USA but in foreign plant locations (so-called “developing countries”). A corrosive steam supply is a result of shabby plant design and/or shabby boiler operations. Steam does not have to be corrosive. Steam is inherently inert and non-corrosive. Bad or shabby boiler operations by humans makes it corrosive. Therefore, if you concentrate on eliminating the cause of corrosive steam, you need not worry about it – and much less design for it.

Therefore, I would not be concerned with having corrosive steam – and would never design for having it present in my plants. In your specific reboiler configuration, it is the shell side that it prone to getting fouled (not the tubeside steam) – and consequently, you are going to have a hard time dealing with the need to clean out the shell side of a fixed tubesheet design. Chemical “boil-out” is about the only way to clean the shell side – and that is always very slow, inefficient, and costly in downtime and safety procedures. THESE facts are the main reasons why vertical thermosyphon reboilers are almost always designed for steam in the shell side.

[deltaChe]

Dear Mr. Montemayor:

Happy New Year!! (although this year first is not happy, cause there is a big flooding in Australia.)

I would like to make a commnent that the control valve in your graph is located downstream of the reboiler (on the condensate side), and the result may cause that the required valve size will be smaller than the case where the control valve is located upstream of the reboiler (on the steam side). I wonder except the saving valve size cost reason or other reson you prefer
to put the control valve in this way.

Thank you so much.

[USR]

Steven our forum has a plethora of information on this,i hope you used the search engine.
When using low pressure steam, it is preferable to use control valve on the condensate line.This is logical because if we throttle steam inlet the steam side pressure in the reboiler may drop,you could observe condensate backing up in the reboiler channel head.However with control valve in the condensate line we could have the reboiler channel at the same pressure as the steam supply pressure and it is easier to have it at a higher pressure than the condensate return system pressure,you will not observe any condensate backing up in the reboiler.As you probably know with control valve in the condensate line we actually vary the heat transfer area available in the reboiler.With this control you must be extra careful with the condensate level this condensate seal should not be blown.
You must ensure a constant steam supply pressure with the condensate control scheme(by providing a PC in the steam line).