10 replies to this topic

[bezuid2324]

Hi all

Can someone help me by explaining the exact proses of varaible pressure control in a vertical thermosyphon reboiler?

At the moment we have two vertical thermosyphons connected to a condensate drum, and connected to a distillation column. The condensate level in the thermosyphons are controlled by the level in the condensate drum. Thus, we are using the variable area control system in this process.

We are thinking of switching to a variable pressure control system. If we change to the varible pressure system:

a) Will the condensate level in the thermosyphon stay constant?
b) How would this system work?

Also, if possible, could you help me with the pros and cons of both types of control systems?

Thank you so much for your help

[Art Montemayor]

Bezuid2324:

I’ve never seen, read, or heard about controlling the capacity of a reboiler by varying the steam pressure fed to it. That doesn’t mean it doesn’t exist or isn’t any good. My opinion is that any control based on varying the steam pressure is going to be slow, and more complex. On the other hand, I am very familiar with varying heat transfer by varying the heat transfer area – raising and lowering the steam condensate level in the steam chest. (you fail to state WHAT pressure you are proposing to vary, and I have to assume it is the steam chest pressure)

The driving force that makes the reboiler work is the temperature difference between the heated fluid and the saturated steam in the steam chest. If you review a table listing of saturated steam temperature and corresponding pressure you will find that it takes a lot of pressure to make a significant increase in temperature. This means that your reboiler must be rated to take the highest possible steam pressure available to it. Additionally, the effect of changing the driving force (the temperature difference) is going to be slow because every time you change the steam chest temperature you are having to change the temperature of the reboiler as well (all the metal and insulting components) so that you have to wait for that to happen in order to have really steady state heat transfer. This is not a requirement of varying the heat transfer area. Varying the area has an immediate effect on the rate of heat transfer and the result is better control.

Additionally, you fail to explain exactly how you plan to establish a constant, varying steam pressure in the steam chest. Bear in mind that you are dealing with a system that is changing phases – steam is condensing into a liquid – and if you are controlling the steam pressure with a steam throttling control valve, then the steam chest is going to be at a lower pressure. This lower pressure depends on the heat transfer rate taking place in the reboiler according to the equation, Q = U* A* LMTD. The steam pressure – and its temperature – will be in accordance with that equation. Many engineers out in industry fail to realize that every time they control steam into a heat exchanger, the effective steam temperature is NOT the saturated temperature of the steam upstream of the control valve. It is lower because the steam is being adiabatically expanded to a lower pressure (and sometimes actually superheated a little bit). This all works, in my opinion, to make reboiler control via steam chest pressure control more complex and slower.

If you want to discuss level control on a reboiler, please heed my questions and requests on your prior thread that you did not respond to.

[Bobby Strain]

You didn't comment, but I presume that there is some temperature control on a lower tray that is cascaded to the level control. The purpose of this is to control the bottom composition. If this is the case, you will get much better control if you control the condensate outlet from the drum directly with the temperature controller. The level will settle where it will provide the necessary heat transfer area. If you want to vary the steam pressure in the reboiler, then you will need a control valve in the steam inlet line.

Bobby

[bezuid2324]

As I have found out so far. There is two ways to control reboilers. One is the variable area control system, and the other is controling the steam entering the reboiler by the means of a control valve in the steam inlet line. The current system is using the first system. I have to get the pros and cons regarding both of these systems.

As I understand it, when you change the steam flow towards the reboilers, you change the pressure in the reboilers shell (chest pressure) where the steam flows. Thus changing Hvap and thus increase or decrease the amount of liquid vapourised in the tubes.

[Bobby Strain]

Any time you control the condendate, you get a system that is slow to reduce area. Steam flow control is much faster. But, you don't necessarily want a fast response. Even if you contol steam flow, it's always possible that condensate will build in the reboiler to provide sufficient pressure for the condensate to flow into the condensate system. It's probably more important to control the proper variables that you have available. You probably don't have enough experience to make this choice. You might get one of Kister's books to help. Liberman is Art's favorite, but for distillation design and control, no one is more qualified than Mr. Kister.

Good Luck,
Bobby

[S.AHMAD]

Let me try to answer your questions:
a) Will the condensate level in the thermosyphon stay constant?
b) How would this system work?

My answers:
a) Will the condensate level in the thermosyphon stay constant?
Yes as long as the condensate level control is working fine. My advice is to set the level set point as low as possible this is to avoid interference of the level in the heat duty. e.g. when steam flow increases (namely set P increases), condensate flowrate is also increases this will reduce the heat transfer area availability and hence reduces the heat duty which is contradict the purpose of increasing the steam flowrate.

b) How would this system work?
PRC works the same principles as FRC. That is when the P set pressure is increased, the valve will open more (lower delta P) thus steam flowrate increases and when P is set lower, valve is pinched down and this reduces the steam flowrate.

Both control schemes work well and proven in the industry.

[bezuid2324]

Thanks S.AHMAD that helped alot.

Can anyone help me with the pro's and con's regarding these two systems.

[Bobby Strain]

You won't find either to be satisfactory if you don't have the right temperature sensor location. So, best to go back to square one.

Bobby

[bezuid2324]

WE have the correct temperature sensor location!!!

[Bobby Strain]

And, on what basis have you determined that the location is the appropriate one?

Bobby

[Pilesar]

Controlling steam inlet to the reboiler gives a faster dynamic response to setpoint changes. Modulating the inlet valve changes the steam immediately. On the contrary, modulating a condensate valve affects the steam supply indirectly and the response is dependent upon where the level of condensate is. For example, if the condensate level is at 90% and you change it to 80%, then you doubled your surface area. But if the condensate level is at 20% and you change it to 10% then the surface area increases only by 12%. Usually reboilers do not have to respond quickly, but having different response dynamics can lead to control challenges.
Things to watch for with steam inlet control: Increasing steam chest pressure also increases the dew point of your steam -- sometimes you can run into a film boiling situation where increased steam pressure no longer increases heat transfer. On the other side, as the exchanger is turned down, you may get to the reduced pressure where the condensate level builds because of the back pressure of the condensate system. Then, the uncontrolled condensate level results in slow or erratic response. Low chest pressure can especially be a problem when you have much excess surface area in the exchanger or the exchanger is new and not yet fouled. With low pressure steam as heating medium, you may need to put in a condensate pot and repressure the condensate to get it into your condensate header.