Flow Chemistry | Hacker News Reader

Flow Chemistry

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25 pointsmindcrime16 comments

Flow chemistry is super useful in some situations and can actually be quite "green" as well as you avoid a lot of solvent usage.

However, it can be really changing as well. If something crashed out in your typical reaction vessel, no big deal. If it crashed out in a flow system, everything grinds to a halt and it can be a huge pain getting the blockage cleared.

Agreed. I designed supercritical CO2 flow reactors. There are a lot of advantages, and agree the main disadvantage is that a clog has the potential to cause serious damage to your system. I still think it's superior because the product consistency can be much higher when you can control exactly how long a reaction takes place (vs batch processing).

I'm kind of confused why this is an article on HN, but I used to have a startup doing microreactor based flow chemistry. It's pretty great for purely liquid based processes.

I'm kind of confused why this is an article on HN,

Somebody else mentioned "continuous flow chemistry" earlier today[1], and I was intrigued by their comment and started looking for more information. Thought other people might find this of interest as well. That's pretty much it.

[1]: https://news.ycombinator.com/item?id=22627973

Ah, gotcha. Feel free to throw out any questions, I'm an expert at this.

If you have two components, which react violently - highly exo-thermally, so that there is a whole system of handling the resulting materials of the reaction - how would you design the point of contact of input substances?

Assuming all liquid products --

Exothermic reactions are very good to handle using microreactors because the surface area to volume ratio is super high. For instance, if you take a cross section of the reactor and the inner diameter is only a few millimeters then it is quite straightforward to rapidly cool it by simply submerging the tubing in a cooling bath.

As to the point of contact, one thing I've done is to build a "coaxial reactor" wherein you have two tubes carrying fluid with one inside the other. The outer "sheath" is one reactant while the inner tube is the second reactant. At the end of the inner tube the two mix via diffusion (low reynolds number) to reduce the speed of the reaction.

If that doesn't mix well enough (in reactors at that scale mixing is largely diffusion rather than convection) I've used in-line stainless steel mixers which are basically a convoluted flow path in a little canister that has ports on both sides.

I'm not quite sure what you mean by "whole system of handling the resulting materials"; if you're talking about needing to take that into something to handle solids then the above makes slightly less sense. If it's just thermal management, my personal approach would be small diameter tubing in a water bath to rapidly quench it.

I think small reactors are much simpler to operate for highly exothermic (or endothermic!) reactions because the heat transfer is so high relative to a bigger reactor. It is also a lot safer because the total amount of chemical in the reactor at a time is a lot less.

Thank you very much, I really appreciate your response! :) Reading about flow reactions, it occured to me that liquid propellant rocket engines sort of satisfy the definition - they often have liquid - sometimes gaseous - components entering the reactor (chamber), where they react, releasing heat, and the products of reaction are passing gasdynamic nozzle with the goal of increasing flow speed. One of the problems is to make sure propellants are reacting well - and the timing is critical, as propellants need to react during millisecond-level periods while they are still in the chamber. So I wondered what methods flow chemistry can offer to rocket injector designers regarding reaching good mixing. This question is important for different scale of engines - both mammoths like Saturn-V's F-1 and tiny reaction control system engines, which these days find their ways into cubesats.

Thank you again!

1. Are there any good examples of this sort of thing that scale down to level where a DIY'er could experiment with it, and that involve things that aren't crazy dangerous to work with?

2. Is there a canonical text or other reference that you'd recommend on the subject?

The epoxy gun described below isn't that far off.

Yeah, there are a variety of books on microreactors in general. I think my recommendations would be dated.

One cool one was talking about fluidics in general including how people used to think it would be simpler to build fluidics-based computers because of the size of vacuum tubes. So they had fluidic versions of things like diodes and amplifiers.

Sorry I can't remember the names though, they were mostly good finds at the library.

Flow reactors always struck me as the distinction between chemistry and chemical engineering. Chemists mix up little batches in borosilicate glass. Chemical engineers use stainless steel pipes big enough to crawl through.

You might be surprised, though I think you're broadly right about this.

Think about it like this -- if you have a molecule flowing through a tube along with the molecules it reacts with, you can flow it at a controlled flow rate through a hot zone or catalyst bed and largely the "plug" will flow through as a unit and so the total time the mixture is at a particular condition is very consistent.

Whereas for a batch reactor, you basically have a giant cooking pot with a stirrer agitating the molecules together. But nothing is typically removing the product, so some of the product can get destroyed or be less efficiently made because the time of reaction for different molecules varies.

That said, a company I used to work for was doing batch chemistry at hundreds of liter scale reactor size. Some of these batch reactors are enormous. And they make sense for situations where you need to do something like slurry processing or super high pressures (sometimes) or are not at a big enough scale to justify building a flow system. For instance, if one 200L batch meets your production needs for a year there's absolutely no economical incentive to switch to a flow reactor at such a small production rate.

But in my experience, yes, I can make a flow reactor with yields of 99% for somewhat tricky chemistry. With a batch system I might hit 95%. That isn't a big deal for a high value product, but is a huge problem for a commodity chemical.

I don't think that's accurate, as Flow Injection Analysis is a staple in any analytical chemisty lab. They're also common in next-generation autosamplers. GC, IC, and Combustion/Oxidation Analysis, or really any type of chromatographic system also depend entirely on effluent flow.

I'd say it's more about scale than continuous versus discrete reactions. Flow injection systems in a chemistry lab are table-top sized, whereas chemical engineers deal with industrial scale.

I would say flow chemistry lays firmly in the overlap between chemistry and chemical engineering.

With flow chemistry, it’s not good enough to know just the chemical side of things. Other factors like mixing, residence time, different flow types, etc becomes much more important. That’s where a chemical engineer can really add value.

Application of certain coatings is effectively flow chemistry, like polyurethane foam where you have two components flowing into the nozzle from separate containers.

epoxies?

I haven't done chemistry since high school, but was a curious as to what a simple setup would look like. This 'gentle introduction to flow chemistry' [0] article seems to do a good job.

[0]: https://pubs.rsc.org/en/content/articlehtml/2014/mh/c4mh0005...

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