Batch distillation - altering reflux ratio and subsequent batches

[GibbsFE]

Hi all,

We have a small (3m) packed column used to purify a chemical. The standard process takes a forecut near total reflux to remove light impurities, then we increase the collection rate to collect the heart after most of the impurities are removed.

I have implemented a one-time change to the hearts collection rate for a single batch, increasing it to reduce cycle-time of the batch. The batch is analyzed, and it passes our quality criteria easily.
However, when subsequent batches are run with the same raw material with the standard procedure with less product takeoff, the next 2-3 batches fail for high impurities of a particular component.
Why would the separation be worse in subsequent batches? The only thing I can think of would be the column packing getting dried out with less reflux coming down the column and it takes a few "normal" runs for the packing to previous wettability. It could be that the change has nothing to do with what we're seeing, but this is two times now that changing one run has affected a few subsequent batches... I'm stumped.

Any ideas?

 

[StoneCold]

Are you dumping the new batches in on top of the bottoms of the first batch? That would make the separation harder. Are you waiting long enough to get the front cut done before going to the heart cut?
How close are the batches together? What I am getting at is the start up time for batch one and batch two the same? So then the time to get the light ends off is the same?
Lots of variables in batch distillation.

Regards
StoneCold

 

[GibbsFE]

StoneCold,

New batches are charged on top of previous bottoms for x runs, then the reboiler is emptied and the waste discarded. Front cuts are standard, y amount is taken from each batch independent of time (though the run time doesn't deviate by more than 5%).

This method seems to work fine until the hearts is tweaked. We use the above method and product quality is consistent for dozens of batches. Everything else being the same (assuming I'm not overlooking a variable that has changed), a slightly faster collection of the heart messes up subsequent batches. I don't know why that would mess up subsequent batches given the same parent raw material.

Given that, my specific question is:

1) Have I overlooked a variable that has actually changed when I believe it hasn't. Or to put it another way, would changing the collection rate of the hearts fraction upset the column in some way (i.e. we're operating under the minimum reflux ratio) that would affect the efficiency of the column for subsequent batches.

One theory is that increasing the hearts collection rate (i.e. less time) is drying or flooding out certain areas in the packing, and it takes a few "normal" runs to get back to normal.

Also, I'm not sure what you mean by "startup times" and "how close together are the batches"

 

[StoneCold]

GibbsFE
What I was thinking is that your batches might be tied together a little bit due to residual heat in the column and reboiler from the previous run. On the first run it might take a while for the column to heat up and liquid to start coming back as external reflux. If on the second run you are dumping new material into the reboiler and the column has been pre-heated you get to the point where you are making external reflux almost immediately. So the batches are tied together by residual heat. If the batches are far enough apart that the column cools down to about ambient then when you start it up the packing should be wet by just the internal reflux occuring as the column heats up.

Operating Point
My inital assumption was that you were running the column at close to flooding and the only way to increase the take off rate was to decrease the reflux ratio.
If that is not the case why not increase the heat in the reboiler and leave the reflux ratio the same to get more product per hr?

I have to ask is the 3 meter column description the diameter or the height? Three meters seem like a very big diameter batch column. It is pretty big for a continuous column outside of a refinery or petrochemical complex.

Does the column have a reflux drum or is it a drumless design? Reflux drums can be problems on batch columns by slowing down your composition change as you switch from front cut to product cut.

Are you able to accurately determine the split between product takeoff and reflux or is it more of a guess? Could the problem be that you are not actually getting the same reflux ratio on each batch?
Many small columns use a timer method or a three way ball to split the flows and the accuracy can be a problem from batch to batch.

My main thought is that your reflux ratio is now too low. It works on the first batch but once you start accumulating bottoms you go off spec.

Regards
StoneCold

 

[GibbsFE]

Thanks for your thoughts, StoneCold. 3m is the height. The diameter is 75mm. Our batches aren't all that close together, for safety reasons we let process cool down before a new batch is charged or the reboiler emptied.

We are limited in how much heat we can add to the reboiler. I do have more wiggle room in the cooling capacity (total reflux condenser). Perhaps I could increase the condenser temperature keeping the heat the same, to increase the total reflux rate. Traditionally, we just set it as low as possible, but higher than where any freezing could occur.

But you are right in that the reflux sometimes varies among batches. We control to a certain pressure (vapor rate), so if the batches vary in composition the vapor rate at a certain pressure won't be identical.

Our reflux is collected on a small plate which overflows into a small container that channels the reflux into the packing. We takeoff product from the plate with a metering valve, like you say with a stopwatch. The forecut is easily repeatable as we run near total reflux, the the hearts is trickier and does indeed vary between batches. Our theory is that we remove most of our impurities, which are light, in the forecut and we aren't as sensitive to slight variations in the faster hearts collection. It could very well be that the column has already been optimized by my predecessors and I'm just going too far. Or that certain raw material lots cannot be run faster. It's complex when it's a multicomponent distillation and we're concerned about contaminants in the PPB range, with no way to analytically determine the acutal compounds, just their individual constituents (thus no VLE data).

 

[StoneCold]

GibbsFE
That is a small column. So you are not going to get much off of it per minute. We have a small column that sounds very similar (100mm) We try to run it with a 10:1 reflux ratio and we get a trickle off of it. Maybe 50cc/min. I think that you are running too low a reflux ratio to keep the separation when you recycle the bottoms material. You could run two batches and drum up the bottoms material after the first one and see if your method works. Then combine all the bottoms in the third run and see if you have problems. My guess is that you will. With such a tiny column you don't have much choice but to go slow.

Regards
StoneCold