2” Sieve Plate Build

[Steve Broady]

Next up on my agenda was a couple sieve plates. I calculated that 18 1/8” holes in a 2” (47.5 mm i.d.) column would give me 5% open area. From reading here, it seems like that’s a good number, so I went with that.

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I started by marking out a 38mm circle, then laying out the 18 holes. I drilled those and a center hole for two plates, then cut them out with shears. The two plates are identical other than the outer diameter. The larger one is held at the bottom of a sight glass, the smaller one fits inside it.

I also cut weirs out of some 1” caps. They were cut 12mm tall, and have a center hole drilled as well. I made the center hole in the cap 1/4”, to fit tightly over a 1/4” SS threaded rod. The corresponding holes in the plates were made one step larger to make alignment less critical.

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I soldered the plates and caps together, using small pieces of solder laid around the joint so that they’d melt and flow in when everything was hot. When I can’t physically secure a joint before soldering, I try to make sure that I won’t have to touch it while it’s hot, even to add solder.

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For the downcomers, I cut 1” long pieces of 1/4” tubing, leaving one end pinched down by the cutter. I then drilled through the weirs and plates and pressed the pinched ends of the tubing into the holes. They fit tightly enough to be secure, and after soldering I used a pinch to open the pinched end of the tubing and further clamp the joint.

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I filed the end of the downcomers so the ends wouldn’t be blocked when I added the traps.

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I normally don’t like soldering loose joints that have to be propped up, but in this case these were the last joints to be made and I felt that everything else would stay secure. Also, I directed all the heat at the 1/2” caps to help reduce the chance of reflowing the earlier solder joints, and left everything set up until it had cooled down. As before, if you look closely you can see the pieces of solder in place ready for heat.

[Steve Broady]

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After some cleanup, here are the finished plates. I drilled 1/16” holes in the traps so that they’ll drain when not in use.

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I was originally going to simply use 1/4” SS threaded rod to support the upper plate, with nuts on each side of each plate. But I didn’t have enough, so I came up with another option. A 3” long piece of 3/16” copper tubing serves as a spacer, so that I only need one nut for the top, one for the bottom. I carefully tweaked the whole thing so the two plates are reasonably parallel just by messing between them at a few points around the edges.

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I had to use a little PTFE packing to seal the edge against the glass. It’s the same stuff I used at both ends to seal the glass against the bottom plate and against the flanges. A chopstick was the perfect tool to reach in and push the packing into the gaps, and also to keep the holes clear. I repurposed the silicone gaskets as centering rings.

[shadylane]

Next up on my agenda was a couple sieve plates. I calculated that 18 1/8” holes in a 2” (47.5 mm i.d.) column would give me 5% open area. From reading here, it seems like that’s a good number, so I went with that.


I also cut weirs out of some 1” caps. They were cut 12mm tall,

Use a shitload of small holes instead of 18 1/8" holes
What you call a weir is only going to act as a funnel and catch reflux falling from the plate above or dephleg.
This drains away the reflux and it isn't available to keep the plate loaded. The plate already has a downcomer and cap, the weir will do nothing good and is occupying space that can be used for more holes.

[Yummyrum]

I agree that the weir may well catch a lot of reflux from the above cap unless you can deliberately divert it to the sides . Even some lengths of wire might do it .
But I disagree that the weir does nothing good . From my experience a large weir creates a still area for bubbles to collect in and collapse to liquid . I found my weir plates were almost impossible to flood .

However , I hope this plate works better for you Steve than most folk that made 2” plated columns found . But then they used them as batch columns . Not sure if you intend to use this for batches or in your stripper.

[shadylane]

Having a downcomer and weir that's offset to the side of a sieve plate makes the reflux flow acrost the holes.
A well designed weir also lets the frothing and foam settle so the reflux is more liquid.
This makes the reflux heavier and able to hold back pressure.
Having a funnel in the middle of a plate the dumps into the downcomer isn't a weir.

[Steve Broady]

I cannot argue with experience, and freely admit that this is the first time I’ve ever made anything like this. I will say that through both accident and design, the liquid should fall quite close to the perimeter and thus on the plate below. The design is a combination of what I could make from parts on hand, what fit in a fairly tight space, and what I could understand from reading but never using a plate like this before.

Yummy, you’re right that I’m using these on the continuous column. In fact, I expect that they’ll run dry most of the time. They’re in there to increase copper contact, and if they work they’ll let me raise the product proof by not pulling all the liquid that falls from the heads trap at the top of the column.

[30xs]

That problem is the the vapor speed needed to keep 1/8” holes loaded probably going to be excessive for an effective 2” with close spacing.

Also your math was for 5%, but you didn’t deduct surface area that the cap is in. You are probably closer to 10-11% of useable plate surface. Anything that goes in the cap is going to go straight down the downcomer. It’s not staying on the plate for any length of time.

[Steve Broady]

Also your math was for 5%, but you didn’t deduct surface area that the cap is in. You are probably closer to 10-11% of useable plate surface.

That brings up a question. I have always assumed that the percentage of open area is relative to the column cross section, not the portion of the plate that’s loaded. If I’m wrong, I’m happy to learn.

It may well be that this whole thing is an exercise in Cunningham’s Law. And if so, I’ll take the education gladly. So far, it’s cost me nothing more than some scrap copper, a little propane, and a few hours one morning.

[shadylane]

I had to look up Cunningham’s Law

[Steve Broady]

I’m afraid I don’t have any pictures to share, but these got a trial yesterday. For the most part they ran dry as I expected because of the still design. However, when I did allow some reflux to fall they did start to load up and behave themselves. I can’t say for sure if the downcomers were doing anything at all, but I can say tell you that there was liquid on the plates and bubbles coming up through it. At least for my somewhat unusual application, they’re working as intended.

With that said, I think I might try to partially plug the weep holes in the downcomer traps. Based on what was happening with the bubble cap plate higher up, 1/16” is way too large for such low power, low flow situation.