return tray for side reboiler 2

[mariatr]

We are considering the installation of a new side reboiler in one of our distillation columns. We have a process stream that is now being cooled down in an air cooler and we could use this stream as a heat source for this side reboiler, with high energy savings.

In the column the traffic is high in both the stripping and rectifying sections. We have a very pure component as a distillate and a mixed hydrocarbon as a residue.

Feed to the column is in the 21st tray. It is a flashing feed (25% wt. vapor). Temperature profile is quite flat in the rectifying section with a sudden temperature increase just below the feed tray. So we consider the liquid in 21st tray as the optimal feed to the side reboiler.

Simulation of the column shows that 22th tray is the optimal return tray for the liquid/vapour mix exiting the side reboiler (the higher reduction in bottom reboiler duty with a lighter increase in reflux)

In your opinion, is that a feasible configuration? I believe the return to the column from a side reboiler is usually above the side-draw location. In this case, returning one tray below seems to be the optimal solution (the reduction in vapour traffic in the stripping section is almost the same as the amount of vapour generated in the side reboiler)

Thanks in advance for your answers.

 

[UmeshMathur]

You have a large amount of feed flashing off after entering the column. Also, you are proposing to draw liquid off the feed tray to the side reboiler and putting the V/L mixture back a tray below.

How is this any different from simply adding heat to the feed?

I have the following suggestion to consider: You could drop feed pressure via a valve (to lower the feed stream temperature) before entering a new feed preheater. Use the same external process stream to heat the feed. This would be a lot simpler, quicker, and cheaper than shutting down the column, cutting holes for the side-reboiler piping, and so forth.

A simple simulation should verify that this scenario is not too different from what you had contemplated originally as far as energy savings. Be careful about the effect of liquid hydrostatic head on the preheater discharge temperature, as that could hurt the heat transfer significantly. If this is serious, you may have to elevate the exchanger, thus raising cost. However, this would also have been necessary for the side-reboiler, anyway, so there should not be a net cost increase on this account.

I would also suggest consideration of (a) an isolation valve between the feed heater and the column, (b) a bypass around the exchanger when it needs to go out for service, and (c) (possibly) the proper pressure relief on the feed exchanger itself for safety, depending on service T&P etc.

 

[mariatr]

Hi

We are aware of the important differences in the simulation when we change the return tray location. If we return to the column in the same tray as the feed (21st tray), the effect is exactly the same as preheating the feed. The main problem in this scenario is that reflux increases a lot (from 20 up to 30 m3/h), so a high percent of the energy savings with the side reboiler will be released to the atmosphere through the air cooler.

However, if we return to the column one tray below (22st tray), the reflux will only experience a slight increase (from 20 to 22 m3/h) and the energy will remain in the process. The scenario where we take the side draw from 21st tray and return to 22th tray is very similar to the one taking the side draw from 22th tray and returning to the same 22th tray. The only difference is that in 21st tray the temperature is a few degrees colder and we will have a higher temperature difference with our external heat source

I don’t understand perfectly why it happens like that. My explanation is the following:

In our case, like in many other distillation columns, we have two vapor sources; the reboiler and the flashing feed. The vapor traffic in the stripping section decreases from bottom to top. We need a certain amount of vapor just below the feed tray, to be in contact and “boil” the liquid from the feed tray. If we preheat the feed we will have more flashing, and more heavy compounds will rise to the top of the column, and therefore, reflux will be higher to keep constant separation.

If we place the return from the side reboiler one tray below the feed, the vapors produced will “boil” the liquid from the feed tray and so, much less vapors will have to be produced in the bottom reboiler.

It is a very rough explanation. Does anybody see it in a different way?

Thank you.

 

[jamesbanda]

I aggree with the comments above...there are other factors you should consider.

.. i would expect the operation with a side reboiler to be more complex with returning below.. there would be some interaction.. control would be difficult..expecially since it is not common and new to your site..

.. you may be able to get a comparable reduction in reboiler duty just by increasing stages.. or a pump around..

.. are you sure of your simulation..i've been fooled many times by simulations as they often dont consider the impratical...but looking from your thorough explanation i suspect this is not the case

.. if you have a flat profile on the column what are you actially distilling in the middle is there a seperation.. is you feed point wrong..you may get more benefit from that..again you have probably tried this ..

make sure you have checked the jet flood etc for the trays can they hand the loads (you have probalby done this..

if you decided on this option.. make sure you critically review the distribution trays and the internal layouts..i've just read kisters troubleshooting book (worth a read) lots of examples of failures of this type..

hope this helps rather than hinders

J

 

[sshep]

I am curious how you used your simulation to determine the "optimum" configuration. The explanation about energy remaining in the process vs being wasted to the atmosphere as it pertains to increased reflux seems a bit dodgy. I would like to know how you set up your distillation specifications. You must be using two design specifications in your simulation rather than one. This is the difference between observing a change in seperation with product purity held constant, and actually controlling the seperation and purity. I would not proceed doing what you propose without a better understanding of what is going on. More likely you are imposing a pinch point on tower seperation in the case of heat input to the feed tray- a graphical study of the simulation is useful to determine this. Your proposal to reintroduce the heat below the feed seems directionally correct, but it was not completely clear why you needed to use the feed tray as the supply to your side reboiler instead of the simpler design of using the heavier material below the feed.

There is an exergy analysis procedure which can be used to determine intermediate heat input potential so as to reduce the reboiler requirement. The analysis is not rigorous but can provide a good starting point. If temperature difference is driving the choice of heat input location then you may consider lowering the tower pressure to give a lower temperature profile and better delta-T in your side reboiler.

In so far as increased feed preheat is concerned the impact to the tower of more feed preheat vs heat on the tray may be the same, but the ability of the feed to act as a heat sink is not the same. The amount of column traffic you indicate to be high (relative to feed?), so the tower is obviously the better sink than the feed.

A return onto the same tray means an easy recirculating thermosyphon design with no control issues. From your reflux increase we can infer that the side reboiler will vaporize a significant amount of the liquid traffic coming onto the tray- it may be hard to design anything less than a total trap out and feel confident of stability.

Anyway, it is an interesting post. A bit more detail about your analysis would be nice.

best wishes,
Sean

 

[mariatr]

Hi

We held the purity constant (weight percents of light and heavy keys in distillate and residue) as specs for the simulation (two design specifications)

The question of using the feed tray as the suply to the side reboiler is just a matter of LMTD with the external heat source. The tray below is some degrees higher and we would need either a bigger reboiler or accept a lost of duty for the side reboiler if we suply the reboiler from the tray below (our feed tray is at 135 C, the tray below at 145 C and the external heat source at 170C)

The tower pressure can not be further reduced (air cooler limitations)

What I have found in different simulations is:

-- Taking the side draw from tray "x" and returning from side reboiler to tray "x+1"

and

--Taking the side draw from tray "x+1" and returning from side reboiler to tray "x+1"


In both cases we have the same result (reflux, temperature profile,.. stay the same along the column)

But in the first case, my feed to the side reboiler is colder, and I could remove more heat from my external heat source, and increase the energy saving.

So, why it is not usually done like that? Is it a question of control/stability? Danger to run dry in the feed tray? Or is it just a mechanical problem? (some kind of special tray should be used to provide a constant (not foamy) feed to the reboiler, without vapor by-passing?

What is a total trap out that you mention in your post?

Or maybe that simulation is not correct? I have done a lot of simulations and I feel confident with hysys, but maybe the program is not precise in this issue.

Anyway, as you say, we will not install this system until we are not sure of well understanding what happens inside the column. And that´s why I started this thread.

Best Wishes

M.M.

 

[UmeshMathur]

mariatr:

As a process engineering matter, I really cannot see how changing the preheat return tray by one stage will increase the reflux by 50% in any distillation simulation that has been set up correctly. Of course, I assume you have a normal column with lots of stages above and below the feed.

The devil is always in the details, but it is impossible to comment further without reviewing your simulation input. I strongly suspect that a highly non-trivial modeling error has, somehow, crept in. A lot also depends on how you have modeled the side reboiler. If a minor change in the LMTD changes the heat duty drastically - which is the only way to explain a 50% increase in reflux duty - you very likely also need to check for an erroneous side reboiler simulation specification.

Also, if the column feed enthalpy is above that for bubble point liquid, it will be at a HIGHER temperature than the feed tray liquid (and vice versa). Therefore, in that case, you would want to flash it to column pressure to lower its temperature. Further, preheating it will then allow you to add about the same amount of heat, for a given external hot stream supply temperature, relative to a side reboiler.

In the discussion above, I am assuming that your feed tray is not horribly mislocated.

 

[EmmanuelTop]

If constant heat duty Q (for the side-reboiler) is specified in simulation, one can watch for bottom reboiler duty reduction and reflux requirements when changing reboiler return location. The overall heat-input must remain the same, while reflux will increase gradually by shifting reboiler return up to the column.

One simple error that often can be made in performing various simulations, is accounting the tray above reboiler return as an active tray, which is not the case in practice. For example, if you place your side-reboiler return in the vapor space of tray No.23, you cannot assume (in real operation) that liquid on the tray No.22 performs any mass transfer with reboiler outlet vapor. The simulator does not "see" this problem, which is one of possible explanations of so drastic decrease in reflux demand in these two cases you described.

 

[mariatr]

Why cannnot we asume mass transfer in that tray? vapor maldistribution?

For Umesh,the feed tray is well located

Thanks

 

[DickRussell]

Mariatr, in your post of 25 Sept you said:
-----------
"If we return to the column in the same tray as the feed (21st tray), the effect is exactly the same as preheating the feed. The main problem in this scenario is that reflux increases a lot (from 20 up to 30 m3/h)...

However, if we return to the column one tray below (22st tray), the reflux will only experience a slight increase (from 20 to 22 m3/h)..."
-----------

Then on 29 Sept you said:

-----------
"What I have found in different simulations is:

-- Taking the side draw from tray "x" and returning from side reboiler to tray "x+1"

and

--Taking the side draw from tray "x+1" and returning from side reboiler to tray "x+1"


In both cases we have the same result (reflux, temperature profile,.. stay the same along the column)
-----------
There seems to be some confusion as to what different results have been obtained simply by moving the return tray down one and how the amount of side-reboil heat is calculated. If the side-reboil heat is being determined by approach to the heat source temperature, then we need to know how much heat is being added in the main and side reboilers and how much is removed in the condenser.

The other thing we need is more info on the separation. You mentioned that the bottoms is a "residue." Does that imply that most of the feed goes to distillate and practically none goes out with the bottoms? If this is the case, then the stripping section is running at near total reflux (equivalent to operating line very close to equilibrium line on a binary McCabe-Thiele). This means that additional bottom reboil heat will not help the stripping; any heat added part of the way up the column just reduces the heat that must be added in the reboiler to drive the separation done above the feed.

On the other hand, if most of the feed is going to the bottoms, then the amount of boilup vapor needed to effect the degree of stripping required does not change much, and any heat added to the feed just increases the heat removed in the condenser.

In between is the grey area where you perhaps are, but we don't know, because we don't have the details. Details will shape the explanations. Otherwise we are just speculating. We'll help, because engineers tend to like puzzles, so give us more clues.

 

[mariatr]

Hi.

I think that there shouldn´t be confusion because there is no disagreement between my two posts. Maybe the statements are not very well explained, but they are correct.

To clarify the posts I´ll say that taking the side draw from tray "x" and returning to tray "x" produces an important increase in the reflux rate. You can consider "x" as the 21st. tray

Anyway, here you have more information, to clarify the results.

- Our Bottom and Distillate rates are more or less the same. So aprox. half of the feed exits the column from the top and half from the bottom

-- Feed is flashing (more than 25 wt%). It is entering the column in 21st tray. We have already a feed preheater, but when we increase the feed temperature, the reflux of the column will increase a lot. We are talking about adding 12 MMBTU/h to the feed.

--So, if we want to add the same 12 MMBTU/h to the column with a side reboiler, we should add this heat below the feed tray, in order to keep the reflux under control.

-- Those 12 MMBTU/h come from a external heat source that is now being cooled down in an air cooler. So if we add those 12 MMBTU/h to the column and our reflux stays the same, doing an energy balance is easy to see that our bottom reboiler duty will be reduced 12 MMBTU/h with important energy savings.

-- However, if adding 12 MMBTU/h to the side reboiler makes our condenser duty rise, let´s say 3MMBTU/h, our savings in the bottom reboiler duty will only be 9MMBTU/h.

-- That´s why we tried, in our simulation, to optimize the side reboiler location. The lowest the location, the best, from a reflux point of view, but the worst,regarding the LMTD with the external heat source.

-- Finally, simulation results showed that taking side draw from 21st tray and returning from side reboiler to 22nd tray keeps our reflux under control (only a slight increase) and our tray is cold enough to have a reasonable LMTD with the external source.

-- I think, although the results from simulation seem correct, we can not translate the scenario the our real column because of "physical problems" as I explained in my previous posts.

I hope this information helps to clarify the so called "puzzle". I don´t see it like a puzzle but as a challenging energy saving project.

Best wishes

M.M

 

[NghiaPP]

This is an interesting subject.
Let us forget all practical issues and concentrate about the simulation result.
Two product specifications are specified. The two variables are reboiler and condenser duty. The simulator will try to vary reboiler and condenser duty until the two given specifications are met. The other parameter can either be fixed (like side reboiler duty, feed temperature, operating pressure,…) or a result of the thermodynamic, masse and heat balance (like bottom & distillate temperature & flow rate, reboiler duty..). The separation performance of this column is fixed by the two product specifications. This separation can be achieved by different ways (e.g. with one bottom reboiler, with bottom reboiler and side reboiler, with more or less number of trays and many other options). The basis elements of the distillation are the number of equilibrium stage and amount of vapour and liquid in contact in each stage. The same separation for a given system can be achieved by increase number of equilibrium stages and less amount of vapour and liquid in contact and inverse.
When specifying some duty to a side reboiler, traffic between side reboiler and bottom reboiler will be lower, the separation performance in this section is reduced. To compensate for that the simulator increases the condenser and bottom reboiler duty a little to meet the total separation requirement. The position of side reboiler will effect on this compensation. Position close to bottom reboiler cause lower separation in a few trays, requirement for compensation in other trays will be small. Position close to the condenser, the requirement is large, so the effect of side reboiler is almost nothing.
Take the feed to side reboiler from tray X and return to tray Y is the same as: send the same amount of liquid from tray X to tray Y and then inject the same amount of heat to tray Y. Moving liquid from a tray to another tray always counteracts the separation. It is only used to get other benefit.
The optimal location of a feed tray will depend on the composition and thermal condition. For the same feed, but hotter, the optimal feed tray location will move down. The optimal feed tray can be found by making different simulation with different tray location, keeping product specifications constant. The optimal case is the case with lowest reboiler duty.
Try this option : preheater the feed with the same duty Q and fed to tray 21, compare with case: feed tray to 20, side reboiler taking feed from tray 20 and return to tray 21.

Practical issues to be concerned:
-The draw tray/feed line to reboiler need to have a certain dimension to disengage vapour from the liquid to reboiler.
-I assume that the side reboiler is a thermosiphone reboiler. How is the feed flow to reboiler controlled. Is there a risk to empty draw tray too quickly (vapour fraction in return can not be controlled). Hydraulic of the whole system must be evaluated carefully for different operation case. Return to the same tray will not cause this problem.