8 replies to this topic

hi,

i am designing a stripping column and have found that steam is to be used for as the stripping agent. should the steam be generated in the reboiler and fed to the column as the liquid to be stripped contains water or should there be a separate stream of steam entering the stripping column?

also what is live steam?


background:

stripper used to re-generate sulfinol from a rich sulfinol solution containing acid gases



thanks

[Zauberberg]

For such systems, usually the steam reboiler is employed - due to high solvent regeneration duty requirements. Having the reboiler means that you can recover the heating steam medium in the form of hot condensate, and recycle it back to the Steam Generators/Power Plant. This is not possible to achieve with live steam injection, which is exactly as the name says - injecting LP steam from the LP steam header/source into the tower bottoms beneath the lowest tray. This steam is permanently lost with your overhead product and it will put additional load on sour water treating unit. One fraction of such water stream could be used as solvent solution make-up water, though.

Yet, sometimes refiners have an excess of LP steam which is vented to the atmosphere; in such case, you can decide to go for live steam stripping if calculations show that you will have sufficient amounts of good quality steam all the time. In the live steam injection arrangement, you will save the money required for reboiler instalation and operation.

[Art Montemayor]

John:

Zauber has hit the nail on the head with regards to the application of direct steam stripping. I have had this application before and even in 1970, the capital cost value of a new L.P. steam-heated reboiler was well worth it when compared to the combined daily operating costs of

1) treating and processing the process fluids contaminated with the water condensate; and,
2) treating, filtering, and pumping the additional boiler feed water required as makeup for the lost water condensate.

I did my economics on a Present Value system using a Return On Investment method at the time. I applied the steam reboiler to a Furfural stripper that was recovering Furfural vapors in the overheads and producing a bottoms stream of acetic acid and a bunch of other "cats & dogs" that was subject to disposal and clean up. The condensed steam exited with the bottoms stream and caused cleanup and pumping bills. That was when natural gas was selling for $.25/MM Btu, so with the increased cost of energy now you should come up with an even more attractive reason for making the capital investment and keeping your steam system as "closed" and tight as possible.

Good Luck.

thank you for clearing that up.

i thought that using a reboiler would be the best economically. i am usure as to the feed of the steam. does the steam required for the removal of acid gases soley come from the reboiler or is there a separate inlet also? is the steam required, just acheived from the water content present in the sulfinol solution or is extra required (i would think extra would be required), as i know steam is to be used for heating the reboiler.

also if the water is boiled off the the lean sulfinol would contain the majority of mdea + sulfolne, so then wouldn't water need to be added in the absorption stage.

thank you for all your help.

[Zauberberg]

- If there is a steam-heated reboiler, that's the only source of stripping vapors: steam generated by heating the Sulfolane solvent solution.
- Majority of evaporated water contained in the solvent is returned back to the tower by applying total reflux, which is the case in all solvent regenerators. You will need some make-up water definitely, because one part of it is being continuously lost via rich solvent flash vessel (flash gas) and regenerator receiver (acid gas stream). Both these streams are water-saturated, and these are the main reasons why we are losing some amount of water in any case.

Best regards,

[Zauberberg]

P.S. If the feed gas (sour gas) is undersaturated at absorber conditions, there will be a losses of water with the overhead/treated gas as well.

so how do you ensure that the reboiler would achieve the steam flowrate required or is it simply that it would be designe so that the flowrate would be acheived under steady state conditions as typically the rich sulfinol feed would be known?

out of interest how would it work upon start up, would there just be high losses until steady state is acheived?

thank you very much Zauberberg

[astro]

...
also what is live steam?
...

The internet can offer up all sorts of help:
Live Steam


That said try getting something meaningful for "dead steam" I suppose there's another word for that that is commonly used and makes sense, i.e. condensate.


Regarding your heat balance and start up questions, if your steam source is a utility boiler, then you effectively have what you need on tap and your problem is to adequately size the steam system to supply your start up and operating loads (the loads are determined from the heat and material balance that you generate as part of your process design work). If the steam is supplied from process waste heat then you've got the joy of a more involved HMB that is grass roots chemical engineering.

If your heat source is from the process, then direct heat exchange is an option (cut out the steam middle man) and is routinely done in industry. If the heat balance falls short then there's scope for a steam reboiler to make up the deficit. In both of these situations, consideration of start up is more involved.

So, the answer depends on your process constraints and the opportunities that you have to hand. Hard to offer more with the information to hand but it sounds like you're going to have to get your hands dirty and do some calculation.

[Zauberberg]

so how do you ensure that the reboiler would achieve the steam flowrate required or is it simply that it would be designe so that the flowrate would be acheived under steady state conditions as typically the rich sulfinol feed would be known?

out of interest how would it work upon start up, would there just be high losses until steady state is acheived?

Exactly, you design the reboiler based on rich Sulfinol flow rate, inlet temperature (enthalpy), required reflux rate, and target lean solvent loading; you can add 10-20% design margin on top of that.

Sweetening units are quite simple for operation, and there is no reason to expect high solvent/water losses during startup - if you do it properly.

Best regards,