Hi everyone, let me take this chance to introduce myself and what I'm looking to do. My name is Patrick, and I'm working this summer under the Rice University Biodiesel Initiative (RUBI, hence the username). My main project is improving on a falling film evaporator built by a group of students earlier this year.

It works something like this:

We will have a barrel of glycerin or biodiesel which we are lookign to recover methanol from. We will pump this mixture up into a sealed system where it will flow down a heated tray (~90 deg C). The methanol will evaporate out and condense on the walls of the system, from which it will drip into a spout that takes it to a separate container.

Here is a simple diagram of how it will work:


This is what it actually looks like:


Now we have quite a few improvements lined up for this bad boy. First, we will replace the plexiglass with a more durable and methanol resistant material, probably stainless steel. Another big thing is making it easier to open up in order to clean and such. Currently, the plexiglass is held on by several bolts and liberal amounts of silicone caulk. What do you guys think of laying down a strip of rubber where the cover comes in contact with the frame and then using the bolts to create a seal?

I've also thought about incorporating a "V" shape in the sealed container to better facilitate condesate collection, like so:


And assuming we make this cover by folding/cutting a single sheet of steel (which i think would be the easiest), we would have to seal up a few edges where we had to cut and rejoin the metal:


Whats a good way to seal it up?

Other improvements include a corrugated flow tray, which will serve to slow down the liquid and give it more surface area to flow over, increasing the evaporation rate. I might also add a few heat sinks on the cover, especially near the crease. We'll also think about adding a water cooling system (nothing much more complicated than cascading water over the cover).

Well, that is all for now, any insight would be appreciated. I'll keep you guys posted!

Are you going to be using solar energy to heat this? I like the idea, and I've been thinking of doing a similar thing, using a design based on the good old fashioned bush evaporator (simiar to yours).

a bit like this

A couple of things. How do you plan to use a corrugated tray? if you run it with the corrugations in the same direction of flow, the fluid will flow into the "valleys" of the corrugations, much like rain on a roof (less surface area, and thicker film) and if you run it perpendicular to the flow, you will get pooling in the valleys, again, effectively reducing your surface area. If you run cooling water over the cover, and the cover is stainless steel, you will cool the unit that you actually need to heat in order to evaporate the methanol. alternatively, if you were able to heat the underneath of the unit instead, using mirrors, then you could cool the top with water and get effective condensation. I would say that with the plexiglass cover, the solar radiation is actually heating the tray, rather than the cover, however I'm not sure how much radiation would be absorbed by the water passing over the top.

In regards to a sealant, I suggest contacting Sika and using one of their sealants to make a gasket of sorts.

http://www.sika.com/

I'd love to hear more about this as you progress.


edit: I just noticed it is electrically heated. Have you considered combining this with a solar heating arrangement? The fluid could possibly be heated prior to dispersing it on the tray?

Sorry if I wasn't more clear before. The corrugated tray will have corrugations run perpendicular to the flow of the liquid. I thought about the "pooling" effect too, but I was thinking if we made it something like so:

it wouldn't be too bad.

We've thought a little bit about using solar energy to heat up the tray, but that might be beyond the scope of this project. Your idea for using the sun to "preheat" the fluid sounds like a good idea though.

Thanks for the suggestions, keep em coming guys!

How does the energy consumption compare to a traditional pot still?

I don't have exact numbers on energy consumption, and I don't have any experience using a traditional pot still, but it should be much more energy efficient. Consider the fact that it only requires two small heating elements to heat up a thin metal tray, and a small pump with almost negligible power consumption.

Has fuel oxidation been a problem so far? Another version of the falling film evaporator discussed on BiodieselBasics has an inert gas flowing counter to the falling film then through a separate condenser to remove the methanol. They were using inert gas to prevent oxidation.

Honestly, the system has been run only twice by the students that built it, both times with only glycerin, so I can't answer your question about fuel oxidation.

have you tried slowing the flow rate?
what about increasing the surface temp to 120C? maybe more?

I really interested in how this works out because I was think about make something based on the same concept but using a 55 gallon drum. slow flow rate and single pass to remove the methanol. I was thinking about collecting the vapor outside the unit. this way the unit could be much hotter that 90C.

what kind of methanol purity ?

-dkenny

A good local sheet metal fabrication shop can form up any type of cover you need, and weld the corners, also go to a plumbing supply house and ask for beam clamps, they are small C clamps that would work well for clamping your cover on.

dkenny-

As for slowing down the flow, slowing it down too much (by reducing the angle of the tray) would result in an increase of the thin film height, slowing down evaporation. But as I said in my first post, we will see about using a corrugated tray to slow it down and increase surface area. Through research, we've found that 90C is right around the "sweet spot" that allows for maximum methanol evaporation and minimal water evaporation, 120C would result in a lot more water. And I don't think we can recover all the methanol from a single pass. During the test runs, they ran it for about 3-4 hours, so the liquid (about 5 gal of glycerin) definitely circulated a few times. We ran a sample of the recovered liquid through an NMR and found it was about 73% methanol, which is something we'll have to work on.

fabricator-

Good call with the metal shop and the plumbing store. Although we might handle a lot of the fabrication and machining ourselves, the clamps could definitely be useful.

I'll post pictures later, but we've started to take the system apart. We took off the plexiglass and found a lot of corrosion/rust on the steel frame, looks like the methanol really did some work on the frame. I'm not sure if its the metal itself, or perhaps the paint on it (not sure what kind they used) because theres this one section that looks like its bare metal and is unaffected. We've started to grind/sand off the rust and stuff, but do you think we should coat the steel with something again? Are there any coatings that are methanol resistant?

Steel should be compatible with methanol. Maybe just the paint.

My methanol comes in steel drums with no corrosion.

Okay guys, here are the pictures as promised.

This is an example of the corrosion on the frame. Pretty much the whole frame looks like this.


Now heres a small portion that looks like bare metal that is in good shape.


The fact that the one portion of exposed metal looks good, and that the rust looks like a "growth" on top of the metal makes me think that the methanol's attacking the paint. But why would the paint be rusting like that?

While sanding down the metal, we see that the rust has definitely gotten to the metal. But then how is that one piece okay?

rubi,
I was actual thinking a slower feed rate to the sheet. not changing the angle of the sheet.


I was just having another thought?
where do you heat your sheet? near the top or near the bottom? where's the sheet the hottest?
I think you might want to try to make the bottom of the sheet the hottest..as methanol is remove from glycerine the temp must be increased to get more methanol out( boiling point increases).
just a thought.

-dkenny

We can already adjust the feed rate as there is an adjustable valve on the pipe.

The sheet is heated by two heating coils mounted underneath, they're spread out pretty well so that they cover most of the plate to allow for the most uniform heating. I'm not sure that we want to heat it any higher than 90C because then we'll get too much water evaporating out.

Rubi,

Does the frame come in contact with the glycerin? That looks like it's from acid exposure not methanol exposure.

One possibility, did the students treat the glycerine with acid to reduce the foaming?

The frame doensn't come in contact with the glycerin, which is contained within the drip tray. As for the acid treatment, I'm pretty positive they didnt. They ran untreated glycerin that had been just separated from the biodiesel.

to improve purity, it sound like you need some sort of reflux action on the methanol vapors.
a reflex column mounted near the top and small
fan near the bottom blowing air in might help.
the column should be setup such that it drips back on to the sheet.

or use a traditional pot still to improve the purity after it removed from the glycerin.

-dkenny

I think I have the answer to the rust. The pigments in the paint must be reacting with the methanol condensate to make the methanol acidic.
Paint pigments often include various metal oxides.

You will want to know what pigments were used in whatever paint you use next time.