jheising

Fun with Spiral Prismatic Packing!

39 posts in this topic

I know I'm not the first person to build a DIY rig for making SPP (spiral prismatic packing), but I figured I'd share the one I just made entirely from supplies you can get from your local Home Depot. It was a lot of fun to make and even more fun to watch.

The system is powered by two drills who's speed is controlled via the immensely complicated ZTC (zip tie control) method. The wire is just standard copper coated steel mig welding wire (which will be replaced by stainless mig welding wire when I run it in a "production" mode).

Here is closeup of what the wound wire looks like:

post-5768-0-91959800-1410912590_thumb.jp

post-5768-0-34296500-1410912592_thumb.jp

Enjoy, and let me know if you have any questions.

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That is totally awesome...

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I LOVE IT, JUST MY TYPE OF MACHINE.

PS That tune in the background towards the end will probably go viral :unsure:

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PS That tune in the background towards the end will probably go viral :unsure:

My 8 year old is in a whistling phase... don't get me started on that :)

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so cool

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Fantastical. I bought some of this several months ago. This looks like much more fun. Do you have instructions?

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Hey Dave, watch the video, the "instructions " are all there!

The only thing you can't see in the video is the ZT controller but you should be able to figure that out. :unsure:

My motto is "if all else fails, read the instructions"

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What did you use for the wrapping form?

I'm also curious if you've measured the overall production rate. How long will it take to produce enough for your column?

Great video!

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What did you use for the wrapping form?

I'm also curious if you've measured the overall production rate. How long will it take to produce enough for your column?

Great video!

The mandrel is made out of a long Philips bit that I shaped on the grinder wheel into a tapered triangle. The taper is important in order for the wire to come off the end instead of just continuing to wind around the bit. One of the "a ha" key moments was cutting a notch into the tip that I could put the wire into to hold it while it gets started. The mig wire is extremely springy, and it was nearly impossible to get the wrap started so that it would hold on to the bit. Once you get 5-6 wraps around the mandrel then you can pull the wire out of the notch and the tension from the wraps will keep it secure.

I'll try to get a picture of it today and post.

As for the production rate, I haven't had a chance to measure it yet, but will get a better idea today when I run some stainless through it. My super rough guess would be about 1 liter per 15-20 minutes, but that might be really far off.

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You're not going to use the copper coated mig wire are you? Once the fin layer copper wears off there is a solid steel core underneath. That will rust.

Stainless or solid copper will be the best.

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Correct. I used the copper coated steel for testing and tuning (because it's easy to get and cheap) but I will only be using solid 308 stainless when I make the final batch.

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Would like to work out how much copper wire to buy to make some of these spirals.

Have you weighed a certain volume of the spirals? What diameter mig wire? 0.8 mm ?

I know the density of steel and copper are different but I am not expecting exact measurements.

Thanks.

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Pete,

I've found that 0.023" mig wire seems to work best— 0.03" has been giving me lots of trouble.

Some anecdotal data on the Internet seems to say:

Measurements for the optimal performance must be:

diameter of packing 1/10 to 1/12 diameter of column
diameter of SS wire 1/10 diameter of packing

Source: http://homedistiller.org/forum/viewtopic.php?f=17&t=16533

I have no idea if it's scientifically accurate, but it passed my initial B.S. meter, so I figured it was a good place to start.

As for copper.... I've run some copper through it, but it doesn't seem to work well because it's not springy enough. I think the key (at least as I understand it) to SPP is the "star-like" pattern you see on the images I posted above. That specific pattern is very efficient in exposing surface area and it emerges like that as it comes off the mandrel and tries to spring back a bit into its original shape. The copper seems to be too soft and doesn't spring back— it ends up looking more like a deformed/twisty spring which I don't think is going to work as well. I'm also concerned that the copper SPP might become deformed on the bottom due to the weight of packing on top of it. It's just so soft...

My dad mentioned that perhaps hard drawn copper wire might have more spring to it and be a better suited, but I'm hesitant to order any quantity of it without trying a little bit first.

Honestly I'd prefer to do everything in copper, so if anyone has any ideas of how to acquire a stiff/springy copper wire in the 0.025" thickness range, I'd love to hear about it!

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Does anyone know what the max column diameter is for running spp? I saw the "diameter of packing 1/10 to 1/12 diameter of column, diameter of SS wire 1/10 diameter of packing" thing, but does that scale to an 8" column?

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I have done a fair amount of experimentation on fractionator packing and made my own informal deduction that the key elements are the total available wetted surface area open to passing gas, per unit of column volume and the rate of mass  transfer through it (gas and liquid).

Fractionation of course takes place only at the interface between the moving liquid and the moving gas and its rate is proportional to the surface area of that interface and the velocity difference between the phases, at constant mass transfer, pressure and temperature for any and all points within the column. It is the mass transfer element which is easily overlooked when considering factors such as HETP ratings - which do not relate (at all) to throughput mass. Turn "the wick" down enough and you can make almost ANY column packing material look good, in terms of HETP alone.

When it comes to "SPP" and the like, the key elements seem to be the diameter of the central hole vs the wetted surface area of the SPP.

Calculating the surface area of the wire it is made from is largely irrelevant, because surface tension buries the wire (and the gaps between the spirals) well below the all-important exposed surface..

If you make your SPP too long, for example, there is a lower probability that the open ends of the tube will be close to a neighbouring tube's orifices, thus restricting gas mobility in the system. The system will ultimately "choke" on liquid phase rather than fractionate, for want of gas porosity. Conversely, if your SPP is "all hole and little wetted surface area" (i.e. too short), there will be little molecular migration between the phases and fractionation suffer just as badly!

It's well-understood that a working fractionator has a thermal differential (proportional to and generated by the varying gas and liquid composition) over its length. Careful monitoring of that thermal gradient with respect to throughput mass is essential to efficient fractionation management. Which is, as JHeising will tell you, CRUCIAL in the design of continuous fractionating stills (not mere continuous "stripping" stills".)


So, in answer to your question, there are far too many variables involved to be able to provide precise advice based purely on the diameter of your column and the size of your SPP's.

A test rig or in-still trail with comprehensive fractionator-length temperature monitoring at your target mass throughput rate is in order?

(Edit: must have been preoccupied with web-browsing at the time of posting - I'd typed HTTP instead of HETP, now corrected. Apologies!)

Edited by MythBuster
Typo
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Looks fun

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Very innovative technique to automate SPP production using generally accessible tools, really enjoyed the video jheising.  

SPP is not cheap, but is highly effective when used as column packing as indicated by MythBuster.  If you can make SPP yourself, then it will definitely save you a bit of money.

For anyone that does not have the time to make it yourself, we offer SPP as an upgrade option with our column stills.  Feel free to DM if you're interested.

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The packing size to column diameter ratio is a compromise between a couple of factors. In general, the larger the packing the lower the pressure drop and the lower the tendency to flood - meaning that you can put more heat into the column and get a higher throughput.

On the other hand, the smaller the packing the higher the fractionating power and the lower the HETP (Height Equivalent to a Theoretical Plate) and therefore the greater the separating power of a given height of packing. Smaller packing tends to be more expensive (per cubic foot and per pound) and it generally works out cheaper to get the extra separating power by using a larger packing and a taller column, but of course this is not always possible.

The thickness of the wire (or plate) does not play a large part in the mass transfer properties and is mainly set by the physical strength required to support the bed of packing above and for ease of fabrication.

The 1/10th rule of thumb is to avoid channelling. In the middle of the column the rings randomly interlock and touch each other and any path up or down through the packing will involve a lot of twists and turns. But against the wall there can be no interlocking and there will be continuous channels that allow the gas to bypass the packings. This lowers the efficiency of the packing but below the 1/10th ratio the impact is small.

I haven't seen any hard data indicating the best length for each piece, but most packings (eg Raschig and Pall rings) are "square" with the length close to the diameter. But Cascade Mini-Rings are popular and effective and they have a length roughly 50% of the diameter. The Cascade Mini-Rings have "tongues" that are bent into the center of the ring and this would prevent the rings from interlocking too far and I guess that for SPP having the length and diameter equal would be a good design.

PS Fantastic practical machine - totally awsome!

Edited by meerkat
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On 24/09/2014 at 10:42 PM, jheising said:

Pete,

I've found that 0.023" mig wire seems to work best— 0.03" has been giving me lots of trouble.

Some anecdotal data on the Internet seems to say:

Source: http://homedistiller.org/forum/viewtopic.php?f=17&t=16533

I have no idea if it's scientifically accurate, but it passed my initial B.S. meter, so I figured it was a good place to start.

As for copper.... I've run some copper through it, but it doesn't seem to work well because it's not springy enough. I think the key (at least as I understand it) to SPP is the "star-like" pattern you see on the images I posted above. That specific pattern is very efficient in exposing surface area and it emerges like that as it comes off the mandrel and tries to spring back a bit into its original shape. The copper seems to be too soft and doesn't spring back— it ends up looking more like a deformed/twisty spring which I don't think is going to work as well. I'm also concerned that the copper SPP might become deformed on the bottom due to the weight of packing on top of it. It's just so soft...

My dad mentioned that perhaps hard drawn copper wire might have more spring to it and be a better suited, but I'm hesitant to order any quantity of it without trying a little bit first.

Honestly I'd prefer to do everything in copper, so if anyone has any ideas of how to acquire a stiff/springy copper wire in the 0.025" thickness range, I'd love to hear about it!

A practical way to be able to use your preferred copper would be to work-harden it as part of the SPP manufacturing process. This can be achieved by the insertion of rolls (which need not be an elaborate set up!) All you need is to bend and re-bend the wire over a roller or two on it's way to the final, SPP former and it will be substantially more springy and tough to deform. Some trial and error experimentation is necessary to get the optimum condition in the Product, but if you knock up a "jury rig" to test the process you will see it can be made to work very well. We ended up using nothing more complicated than a couple of polished steel guides to force the moving wire around ina zig-zag form and thus bend and re-bend it. We found that lubrication was unnecessary - the guides lasted a long time and were cheaper to replace than to complicate!

(I am not an SPP-producer, BTW! I handled copper wire in a similar way with similar needs but in a totally different application).

Hope that helped.

 

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Great discussion here! We ended up ditching the SPP (at least for now) because for our particular need we found that raschig rings were just as effective. I'm still convinced that SPP is probably a superior packing material, but we aren't using it for quite the same purpose (without going into detail).

I had a couple of thoughts on the discussion (but please keep in mind I am NOT an expert in SPP by any means):

1) Size: I don't claim to fully understand the details behind MythBusters explanation, but it seems reasonable. The simple explanation I use is this: with any discrete random packing material (i.e. raschig rings, SPP, etc) you typically want its width/diameter to be equal to its height, so it doesn't favor one axis over another when it settles. As soon as it favors one axis over another, it ceases to become random and will either restrict vapor flow or not provide enough contact surface area.

2) @GENIO what an awesome still! You probably have more experience with SPP than I do (so take with a grain of salt) but the packing you show on your website looks more like SP and not SPP. From what I understand, the "Prismatic" part is really important to exposing more of the wire surface area. With a standard helical coil you "hide" a good amount of your surface area by covering it up with the coil directly above and below. By winding it in a polygonal shape (usually triangular) where each winding is slightly offset from the previous, you expose much more surface area. I might argue (but could be entirely wrong) that a standard helical coil (i.e. SP) without any turn spacing is probably not much more effective than a raschig ring.

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What size Raschig rings are you using?  What's the column diameter/height?  Interested as I'm working on a 8"x20' column, and have been hunting for packing.  The fact that you didn't see much difference between SPP and Raschig is actually positive, as it's much easier to source.  On the hobby side, those guys tend not to be so concerned about fouling, but with a big column, the packing is never leaving the column, and needs to be effectively cleaned in place.

 

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