13 replies to this topic

[bubka2]

Hi,

We have a problem in our plant. We have a distillation column which works unter vacuum conditions (200 mbar). The reboiler used is a vertical thermosiphon reboiler operated with condensing steam. It has a control valve on the steam side which is driven by the sump level in the column.

We want to increase the load to the column. Since the sump level rises, the control valve on the steam side of the reboiler opens to increase the steam flow rate. However, the steam flow rate itself doesn't increase.

The steam trap etc. seem to be working fine. What could be the reason for this behaviour?

Thanks for your help!

[meetyourmaker]

Are you are increasing the load beyond the design load of column?
The reboiler could be fouled. What is the service?
Is there a FIC for the steam to the shellside?

[xavio]

Hi bubka2,

It is quite unusual to have the steam controlled by sump level.
A more common approach is to have the steam controlled by column temperature.
Have you check the maximum capacity of this control valve?
Is it able to provide the required steam rate for the higher loading?

Do you rise the sump level deliberately in order to increase column load?
For vertical thermosyphon reboiler operating at vacuum, too high a sump level could be undesirable.
Higher sump level means longer sensible heating zone of your reboiler.
Your problem could be temperature pinch.

[bubka2]

Yes, we're trying to increase load beyond the design load. I think, fouling can be ruled out. There's a FIC for the stream to the shellside.

I don't really know why the sump level is used for control, not the temperature. We checked the control valve, it should be able to provide more steam. We did not raise the sump level, we raised the inlet flow to the column. That raises the sump level, right? Could you please elaborate what temperature pinch means in this case and what it is caused by?

Thanks!

[meetyourmaker]

a diagram/basic process details would be appreciated

[acer_asd]

bubka2:

You check for steam drum chest pressure also. if you open the steam valve and chest pressure is increasing it means that complete condensation of steam is not taking place. So either your reboiler has fouled or your reboiler is not designed for steam flow greater than your current flow.

I hope this helps solving your problem

have a nice day

regards

[katmar]

If you want the reboiler to transfer more heat the only way it can do that is by having a higher temperature driving force (since the area and heat transfer coefficient are fixed). In order to get a higher temperature difference you have to raise the temperature on the steam side of the reboiler, which means that the pressure increases as well. So you have less pressure drop available across the steam inlet valve and this works against your aim of having more flow.

But this is all just general tendencies. The only way to properly answer your question is by going into the details. For the before and after situations you need to answer :
What is the steam pressure in the reboiler
What is the valve position and therefore Cv
What is the steam supply pressure
What are the calculated and measured steam flows

When you have all these numbers the answer will pop out like magic.

[Art Montemayor]

Katmar’s very valued response should be heeded. What he states is, in my opinion, the absolute truth.

For example, you don’t furnish us with a sketch or diagram of how your system is set up and/or connected. However, you do state “the control valve on the steam side of the reboiler opens to increase the steam flow rate”; therefore, I presume you are throttling the steam supply into the reboiler’s steam chest. I also presume that the steam is being controlled by a pre-set steam flow setting. If that is the case, then your steam chest (the reboiler shell) is not at the same pressure as your steam supply. The pressure in the chest is LOWER than the supply pressure – hence, the steam flow into the chest. Depending on the efficiency of the heat transfer, the chest’s pressure could be significantly low enough to reduce the rated heat transfer. This fact – together with Katmar’s observation about the temperature difference – is a key in limiting the heat transfer. The process is further aggravated if you have any fouling – either on the process or steam side – or the accumulation of inerts or non-condensables in the top section of the chest. I hope you are venting non-condensables.

One way to combat the above negative effect is to connect the reboiler’s steam chest directly to your steam supply pressure (assuming the chest’s MAWP can handle it) and drain the condensate formed using a level controller in an external condensate pot. This method has been described, discussed, and sketched out many times on our forums and prior threads should be easily retrieved using our SEARCH feature.

In summary, supplying us with the detailed basic data listed in Katmar’s response will greatly aid in getting to the solution.

[breizh]

Bubka2 ,
On top of what has been said , based on my experience , take account the quality of the steam ( saturated), the fact that the steam valve could be partially plugged with dirt.

Breizh

[bubka2]

Thanks a lot for the ideas and suggestions. I will have to think about all of them.

One more question. Katmar, you say that the heat transfer coefficient is fixed. I've been reading about vertical thermosiphon reboilers operated under vacuum and apparently, the heat transfer coefficient is a very strong function of the sump level. Along with all the suggested ideas, the sump level could also be the problem in our case.

Thanks again!

[raj.chengg]

The answer could be very simple or quite complex.

1) If you have already done the simulation of the tower operations you will know the range of steam control valve operating range. This will tell you what is the range that the valve can control.

2) The complex portion would be to simulate the two operating conditions and see the transient responses when the conditions change. Couls easily be done on Matlab and Aspen combination.

Kind regards

Raj

[juanmal]

Thanks a lot for the ideas and suggestions. I will have to think about all of them.

One more question. Katmar, you say that the heat transfer coefficient is fixed. I've been reading about vertical thermosiphon reboilers operated under vacuum and apparently, the heat transfer coefficient is a very strong function of the sump level. Along with all the suggested ideas, the sump level could also be the problem in our case.

Thanks again!

The operation of a vacuum reboiler is very sensitive to the increase in the level of liquid in the sump, which increases the boiling temperature of the fluid and decreases the thermal potential for heat transfer, this may be what is occurring.

Juan M

[breizh]

Hi ,
Take a look at this paper .

Breizh

[mbaig2007@me.com]

Dear Breizh , very impressive presentation by Daniel R. Lewin

can you load all his lectures on plant design in free download section

thanks.
m.n.baig