4 replies to this topic

[Olaniyi]

Hi all
Apologies if this seems a naive question, I am simply not used to steam systems.

I have a distillation column for which condensing steam is to be used to provide reboiler duty. I am told that steam is available at 2 pressure levels, HP steam at 21 barg (thus ~ 215degC) and LP steam at 9 barg (175degC) - based on other users within the plant.

My column bottom temperature to meet my product specs (from my process simulations) is ~ 160degC. I have also read in literature that it is recommended to have a DT between the process and the steam of ~ 25degC. Thus ideally I want steam at around 185-190degC.

My question: How easy is it to get steam at an intermediate pressure level between these 2 pressures? To my mental model it is just a matter of letting down the steam to a lower pressure level (e.g. 12barg) and getting rid of any condensate. Is it this simple or is there something I am missing?

Thanks!

Ogeds

[Bobby Strain]

You need to look at your steam costs. It might be most economical overall to use the lower pressure steam.

Bobby

[Dacs]

For the question, you can provide a pressure letdown (to the desired Tsat) for your HP Steam and have a desuperheater to remove the superheat.

You can confirm it on your end but more likely than not, your steam supply has some superheat in it so you need to get rid of it for your reboiler operation.

All that said, just to reflect on what Bobby Stain said, HP steam is more expensive than LP steam so it may be economical to have a larger reboiler and having a lower OPEX in the long run. And it's more promising if your unit can supply the LP Steam without needed to import from outside. But that can't be answered without doing any economic analysis of the system.

[Pilesar]

If you are removing all condensate from the exchanger, the process temperature will be controlled by modulating the steam control valve. When you require more duty, the steam valve will open and the steam pressure in the exchanger steam chest will increase. The pressure in the steam chest will be less than the steam header pressure. Once the steam valve is all the way open, you will have no more control. Using the higher pressure steam will normally give you more top-end range for the same exchanger surface area at the expense of costlier steam supply. Consider the plant steam balance if these reboilers are very high duty. It may be that there are excessive LP steam users and already a large letdown flow from the HP steam header.

Confirm that steam superheat will not adversely affect your reboiler performance. Some superheat is acceptable, but you do not want to have to use too much of your surface area for desuperheating or get into an annoying film boiling situation. Be especially wary if the steam source is near the letdown valve from a higher pressure header.

If you have access to heat exchanger design software, you can estimate the effect of steam source on exchanger capital cost.

For a quick estimate at an early stage of the project, I would probably choose the HP steam source with intention to visit the optimization during a later project stage.

[Olaniyi]

Thanks guys, very very helpful comments!
Much appreciated

Ogeds