12 replies to this topic

[daraj]

Hi, this might seem like a silly question, if so, excuse me. If the purpose of the reboiler is to partially convert the incoming liquid to vapor and send it back to column,(which is done usually with steam or other heating fluid) then what is stopping from use of pressure reduction to accomplish the same or substitute atleast part of the reboiler heating load? That is, you try to expand/pressurereduce the bottom liquid stream a little bit and flash it by using pressure reduction than by just pure heating thereby minimizing steam usage. Is there something conceptually wrong here? Please advise.

 

(I know that you need a temperature heat balance for the column and what I have suggested here upsets that, but still there are vacuum columns out there that try to do something similar, so I just had a thought along those lines)

[curious_cat]

If you did how'd you get that vapor stream flowing back into a column which is at a higher pressure by definition. 

[Art Montemayor]

CuriousCat is correct.  He just killed that idea before it got any further.

 

In order to be able to flash a stream of saturated liquid bottoms product at the sump of a distillation column you must have the column operating at an elevated pressure - otherwise there is no driving force to carry out the adiabatic free expansion that produces the superheated vapor.

 

Therefore, if the superheated vapor produced is at a lower pressure, how does one get it into the distillation column?  The answer would have to be a vapor compressor.  This represents capital monies and compressor driver energy - the eternal engineering tradeoff that comes back to remind us that there are no free rides and no free lunches.

[curious_cat]

 The answer would have to be a vapor compressor.  This represents capital monies and compressor driver energy - the eternal engineering tradeoff that comes back to remind us that there are no free rides and no free lunches.

 

Besides, if you re compressed to column pressure or higher, a lot (all?) of that stream would probably re-condense?

 

Not sure. I may be wrong.

[Bobby Strain]

There have been a few systems designed to do this. But they require recompression of the vapor. This is called a heat pump. Most applications use the compressor for the overhead vapor which serves as the heat source for the reboiler. These systems are used for close boiling components such as butane-isobutane.

 

Bobby

[curious_cat]

There have been a few systems designed to do this. But they require recompression of the vapor. This is called a heat pump. Most applications use the compressor for the overhead vapor which serves as the heat source for the reboiler. These systems are used for close boiling components such as butane-isobutane.

 

Bobby

 

But you can't flash & recompress the bottoms, right? They'd re-liquify? Or can you somehow explot VLE behavior to recompress to  tower pressure without liquifying. I wonder if even academically this is workable. 

 

I think not. 

[Bobby Strain]

If you visit IPSI site, you will see a patented process used for NGL recovery where this is done. For the typical bottom heat pump arrangement, the bottom liquid pressure is reduced and partially vaporized in heat exchange with the column overhead vapor. The vapor is then compressed and returned to the tower for reboil vapor. You can probably Google this for a detail description.

 

Bobby

[daraj]

thanks for your inputs.

Overhead vapor recompression is something different and Iam aware of it.

 

In general, if the column is not a vaccum column and the difference between top and bottom temp. isnt too high(< 60C or so) you can take the overhead vapor compress it so that its dew point is 10C above that of the temperature needed for heat transfer in the shared reboiler-condenser. While the overhead vapor condenses and provides heat, the liquid from bottom stage is reboiled. This is a proven concept and used industrially in a lot of cases.

 

Whereas, What I asked was a little different. I knew something was off when I asked that but couldnt immediately figure it out. Thanks for pointing it out.

The economics of compressing the reboiler vapor outlet(after pressure reduction) and sending it back to the column has to be evaluated with respect to the steam savings it can produce. If the column needs high quality steam in the reboiler maybe it will be favorable otherwise it might not be. You are looking at a reboiler+steam usage vs expander+compressor comparison

 

As Curious cat mentioned Iam not sure how to prevent re-liquefaction of a reboiler vapor when you recompress it.

 

In regular distillation columns also there is some pressure drop that happens across the reboiler, right?(say a thermosiphon type reboiler).how is this taken care of? Or is this small enough that it does not matter?

[latexman]

daraj,

 

A forced circulation calandria does exactly what you described.  The bottoms liquid is pumped to higher pressure.  It goes through a heat exchanger where the temperature is increased.  It is all sensible heat, no vaporization (yet).  Then it goes through a restriction orifice where pressure is reduced and it partially flashes into the bottoms.  Vapor goes up the column and liquid stays in bottom.

 

Part of the liquid from before or after heat exchanger can be diverted as bottoms product.

 

Maybe, this is what you had in mind?

[curious_cat]

daraj,

 

A forced circulation calandria does exactly what you described.  The bottoms liquid is pumped to higher pressure.  It goes through a heat exchanger where the temperature is increased.  It is all sensible heat, no vaporization (yet).  Then it goes through a restriction orifice where pressure is reduced and it partially flashes into the bottoms.  Vapor goes up the column and liquid stays in bottom.

 

On a conceptual level what is the fundamental advantage in doing it like this (pressurize to higher than column pressure -> add sensible heat without boiling -> flash to lower pressure but still higher than column pressure) over conventional boiling?

 

I've seen it used of course, but never paused to think why. 

[latexman]

Forced-circulation calandrias are widely applied when the process operating pressure is less than atmospheric, when fouling is expected as a result of boiling, when fouling can be reduced by controlling liquid velocity, and when the calandria is required to perform for a variety of conditions such as often required for a batch still.

 

As I recall in my experience, the biggie is reducing fouling.

 

The pump must be carefully matched to the heat exchanger and the system for robust reliability and performance.  The pump is usually a somewhat low NPSH pump.

[Bobby Strain]

We gave you a lot of information. But you never told us your application, which will determine a feasible and economic design solution. And, to my knowledge, the widest temperature difference between top and bottom is much less than 60 C. Ethylene/Ethane separation is about the widest boiling application that is economical. And you must carefull choose the operating pressure to get the optimum design.

 

Bobby

[daraj]

Hi Bobby

"We gave you a lot of information. But you never told us your application, which will determine a feasible and economic design solution"

 

I did not have any application in mind. If you noticed carefully mine was just a conceptual question and I wasnt sure whether the question was even right.

 

But am glad that it has provoked a little discussion here and some useful info being shared. Thanks to all.

 I will have to dig more around this forced circulation calendrias which latexman mentioned.