jkqwzsoo
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Comment · 2 years ago
Comment · 2 years ago
Keep seeing this take (just an installation issue), over and over and over again.
I think it's much worse than a design flaw.
Even very carefully-engineered systems can have flaws. Engineering flaws, once identified, can be engineered around, managed, or corrected.
I'm much less comfortable with the idea that the assembly plant for these planes could be a random-critical-failure generator based on how the employees handle a torque wrench.
Comment · 3 years ago
I'm not sure what your point was, but just to be clear the discussion of black body radiation has nothing to do with the switching frequency discussed in the article.
Comment · 3 years ago
> As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of.
There are no pores, so to speak. Polymer materials form amorphous solids with transient voids which open and close randomly due to thermal motion. They're not "pores" because they aren't permanent over long time scales. Rather, the polymer+water is modeled as a single fluid phase, the same as if you were modeling ethanol+water. The fact that the polymer is a "solid" doesn't affect the fact that it's actually a tangle of vibrating molecules just like any other mixture.
Other materials do have well defined pores, like MOFs and zeolites. In this case, the water does sorb as a liquid in the pore space, but is gated by transport between the pores in a similar manner.
This is made apparent because water does enter into polymers (even those which desalination) freely, with or without the presence of salt. It is not the case that "the pores are too small for water to cross normally". I can take a polymer that will swell with 50% of its own weight in water, and which has no "free" liquid water (as evidenced by the inability of the water in the polymer to form ice), yet make it reject >90% salt at very high pressures (>3000 psi). If you just let salt water sit on one side without pressure, salt and water will make their way through non-selectively. So it can't be that the water is being physically sieved from the ions to enter into the membrane. Rather, the pressure creates a change in the activity of water (due to the mechanical forces acting on the polymer near the low pressure/support material interface). Since the water is more soluble and more mobile in the polymer, it transports at a more rapid rate than the salt, resulting in desalination.
Comment · 3 years ago
The first thing that came to mind when I saw the abstract was that existing bipolar membrane electrodialysis processes already provide a convenient way of performing the pH swing process they are developing, but with membranes that are already produced on km^2/year scale. Companies like Neosepta or Veolia (formerly Suez formerly GE Water formerly Ionics) produce bipolar membranes for this task, and it's a rapidly growing area of interest.
A bipolar membrane (BPM) consists of a polymer membrane full of positively charged groups (the anion-exchange resin) intimately bound to a polymer membrane full of negatively charged groups (the cation-exchange resin). The interface (reminiscent of a p-n junction) is known as a bipolar junction, and acts as an electrode under a sufficiently high potential gradient. They are made out of cheap materials which have been used in ion-exchange resins and membranes since the 60s, but the bipolar membrane process is niche and hasn't been anywhere as highly developed as other electrodialysis membranes. And electrodialysis is fairly niche, and hasn't been nearly as highly developed as membranes for gas separation, desalination, or removal of particulates (ultra- and micro-filtration).
It turned out that electrodialysis is less efficient for seawater desalination than reverse osmosis (the potential drop through the product water becomes really severe if you're trying to produce drinking water from seawater), so electrodialysis was half-abandoned in comparison to RO. Oddly, Japanese companies developed a lot of ED technology to its current state, including ion-selective cation exchange membranes, for producing table salt, since Japan doesn't have the climate necessary for normal salt evaporation. The ion-selective cation resins were developed for removing Mg from seawater for table salt, but are now popular for researchers trying to do lithium separations.
Anyway, while I agree with the authors that BPMs have unresolved challenges (related to efficiency, mechanical stability, and the fact that current membranes are required to be loaded with transition metal catalyst to get a decent water splitting rate at a low overpotential), I don't know that I'm convinced that their approach is better just because they call BPMs "expensive" four times. If we wanted to adjust the pH of a lot of water, we would need, as a guess, roughly the same amount of electrode catalyst surface area, or the same amount of bipolar junction surface area. However, the bipolar junction is made out of commodity polymer resins heat laminated together, while the electrodes in this study are made out of silver and bismuth. If the bipolar membrane is loaded with a metal catalyst, the most common one is iron. I don't see the BPMs being the more costly solution for very long.
For full disclosure, I recently started doing some work on BPMs, but I think the problems associated with it are solvable, especially for applications like this (as opposed to much more challenging conditions like CO2 electrolyzers).
Comment · 3 years ago
> Sounds like a promising area to explore.
Indeed, and it has been. Google's Project Foghorn's process was (2014) was based on the same concept.
Comment · 3 years ago
This answer is, unfortunately, wrong. Or at best, incomplete. And they say the best way to get the correct answer on the internet is to post an incorrect one. In fact, the fact that this (11-year-old) paper was published to HN last night has been irking me, and I have thrice started, then abandoned, a comment explaining why GO work in this field is mostly useless, and why peoples' hopes in "low cost desalination" are moonshine based on a misunderstanding of the relevant thermodynamics.
First, this article is 11 years old. This is extremely old news. To the best of my knowledge, most of the serious research on GO has fizzled out, except as a random "might as well be pencil shavings" additive to enhance the perceived novelty of bad research. A favorite of mine: "Will Any Crap We Put into Graphene Increase Its Electrocatalytic Effect?" (2020) (https://doi.org/10.1021/acsnano.9b00184). It seems as if the field has moved on to trying to better understand how existing crosslinked polyamide desalination membranes can be better optimized for neutral solute rejection (something that graphene doesn't, and likely won't ever, but very good at separating), as mentioned towards the end of this comment.
The helium parts of your explanation are correct. However, the sections pertaining to water and the explanation of the molecular interactions as a mechanistic model which explains the different behaviors of He and H2O in the paper are wrong. In fact, water does not have strong interactions with graphene or other fullerenes. For example carbon nanotubes, water permeation is modeled as being nearly frictionless.
Water does have strong interactions with itself, true. The permeation of penetrant through a membrane is usually rationalized in terms of its permeability, which can be thought of as the product of how much material is in the membrane phase (as opposed to the external solution -- gas or liquid) and how fast the penetrant moves through the material. He doesn't interact strongly with materials, so it doesn't sorb very strongly into materials. It is also very small and doesn't form transient bonds with other atoms, so it tends to diffuse very quickly as well. Water is very condensable and tends to form stronger interactions with atoms, so it tends to sorb more and diffuse less. You can think of He as a 1-lane 100 MPH highway through a material and water as a 200-foot-wide moving sidewalk, in terms of mass conveyed per time.
However, the tendency of water to form hydrogen-bonded networks is not, strictly speaking, why the membranes in this study behaved the way they did. The actual answer is incredibly simple. The water condenses into and swells the graphene oxide, so the material is physically separated apart and allows water (and anything that water can carry with it) to penetrate through.
These membranes are made out of graphene oxide (GO), not graphene (a different material). GO is (obviously) an oxidized form of graphene. In an ideal model, you can think of GO as a flake of graphene with a bunch of oxide groups around the edge (=O, -OH, -COOH, etc.). Water permeates GO rapidly because it interacts strongly with the terminal oxide groups on the edges of the GO flake and (again, this is the key part) swells the GO flakes apart from one another considerably. The flakes are physically further apart, which allows the water to freely permeate. This mechanism, by which a condensable gas or vapor condense to form a liquid-like phase in a porous solid is known as capillary condensation.
The fact that the flakes are physically spaced further apart also allows other gases to permeate as well. As discussed in the article, the GO membranes are no longer helium-leak-tight when the helium gas is humidified. He permeates the membrane in large part by sorbing into the water which has condensed between the GO flakes (this type of sorption is described by Henry's law) and diffusing as a dissolved gas through the water channels formed through the swollen GO material. In water, the d-spacing of GO (the space between the flakes) goes from 3-5 Å (good for molecular sieving) to 1-2 nm (will let food dye pass through).
This type of separation (which is not necessarily what the authors were trying to do, admittedly) has been mechanistically and mathematically described in the literature for at least 80 years (e.g., for packed plugs of amorphous carbon studied by Barrer). Also, note that the capillary condensation effect observed here is mostly a function of the properties of the penetrant components, not of the GO itself, outside of how strongly GO interacts with the penetrants.
People have spent a lot of time trying to chemically stitch GO flakes together so that they don’t swell as much, but they haven't had much success. Several years ago, a group cast a piece of dry GO in epoxy, and showed that the films being physically constrained by the epoxy can have ion-selective flow edge-on (the membranes in this study are top-down), but this is more of a proof of physical concept than a practical implementation. Reduced GO (GO that's reduced back into just graphene, but now with more defects where the oxide residues were removed) can be used for gas separations and don't swell, but are hard to make (reducing conditions are not good for materials) and not particularly beneficial over polymeric materials. Single-pore graphene is still researched, but I think the interest in it is severely misguided, because:
Even if researchers were to succeed in making a high-flux high-rejection GO or graphene-based membrane for desalination, these properties membranes don’t address the real issues in water treatment. Instead, they are a showy material that appeals to metrics and gets the university PR photographer in the lab, rather than industrial partners.
Some huge issues off the top of my head include: 1) RO membranes for desalination aren't actually that inefficient, the energy cost of desalination is a large, but not even the major, operating expense, and efficiency gains to be had by using high-flux low-friction materials are minor (if we had a thermodynamically ideal desalinator, it would only use 2-3x less energy than existing technology, and GO/CNT/MOFs, etc., are unlikely to provide 200-300% improvements in efficiency) 2) practical membranes needs to be more fouling resistant and easier to clean to maximize productivity and efficiency over their lifespan, 3) any efficiency gains made by high-flux materials aren't really that 3) ultra-high flux materials tend to foul faster even if they can be more easily cleaned, 4) there are fluid dynamic reasons why ultra-high flux membrane materials (i.e., >10x current desalination membranes) are useless (you can look up "concentration polarization"), and 5) there's a much more critically pressing need to improve the rejection of other neutral compounds like urea, pharmaceuticals, NDMA (and other chloramine disinfection byproducts), and boron. Graphene, graphene oxide, carbon nanotube, and MOF based membranes for desalination are almost invariably focused on high salt rejection and high flux, which it turns out isn't really that hard to do. Neutral organic and inorganic molecules, however, tend to pass straight most membranes, requiring post-treatment.
To make a computer analogy, graphene-based membranes in the real-world applications are like several terabytes of ECC RAM hooked up to Babbage’s computation engines. They don't address the real needs or bottlenecks of separation processes.
Some reading for the curious (you will need to figure out how to access). The last two were written mostly by environmental engineers, the first two by chemical engineers.
Evolving beyond the thermal age of separation processes: Membranes can lead the way (2004) - https://doi.org/10.1002/aic.10330
Seven chemical separations to change the world (2016) - https://doi.org/10.1038/532435a
The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes (2016) - https://doi.org/10.1021/acs.estlett.6b00050
Water Treatment: Are Membranes the Panacea? (2020) - https://doi.org/10.1146/annurev-chembioeng-111919-091940
Comment · 3 years ago
What you’re suggesting is any crash can be attributed to the US method of scheduling takeoff and landing (since that’s the risk issue you’re responding to). That’s not true. If a plane’s engine explodes in midair, it has nothing to do with how the ATC schedules takeoff and landing.
I couldn’t find any crashes listed on Wikipedia in the past 20 years that could be attributed to two planes being directed occupy a runway at the same time.
Comment · 3 years ago
Thinking about it for a minute, I’m not sure why we need fluoropolymers for waterproof technical fabrics. Unless I’m very much mistaken, PDMS rubber, polypropylene, polyethylene, and other materials provide similar levels of water resistance, without requiring the use of fluorine-containing compounds. Most explanations I read for Gor-Tex-type materials using PTFE (e.g., [0]) reference the hydrophobicity of the material, which is (IMO) similar to explaining why cars are powered by rockets because rockets are very fast. Cars are, of course, not typically powered by rockets because it is not necessary.
PDMS rubber, polypropylene, polyethylene, and PTFE all have very high water contact angles (a measure of the strength of interaction of water and the surface) and low water uptake [1]. I work with a stretched polypropylene film (Celgard — a material that is often used as a support/spacer material in Li-ion batteries) and it’s extremely hydrophobic. I used a piece of this film to build a bubble trap ([2]), for example (bubble traps typically use PTFE membranes…). It is not optimized for water resistance, so it does wet eventually, but it’s pretty good for “not trying”. Surfaces coated with PDMS (or glass coated with short PDMS chains — i.e., silanized glass [3]) are extremely hydrophobic.
The only time I personally use PTFE (or PFA, MFA, FEP, or ETFE) is when I need materials to be resistant (including both resistance to chemical degradation as well as swelling) to strong organic solvents (like NMP, THF, etc.) or strong acids and bases (like piranha, aqua regia, or a nitrating solution). These conditions are unlikely to be encountered while cycling.
This all said, I’m not an expert on the design of Gor-Tex type materials. However, I assume it is highly related to the pore structure of the materials to prevent liquid water intrusion (the same as for membranes designed for membrane distillation). Given the similar hydrophobicity of these materials, it seems like it should be possible to produce similar results with PP, PE, etc. And this is all before introducing the ability of nanomaterials and nanopatterning (perhaps transferred with imprint lithography [4]) to produce metastable ultrahydrophobicity [5] on the surface of materials.
[0] https://www.sungodtech.com/how-much-do-you-know-about-ptfe-w..., https://outdoorguru.com/how-to-en/how-does-waterproof-and-br... [1] https://www.accudynetest.com/polytable_03.html, https://omnexus.specialchem.com/polymer-properties/propertie... [2] https://static.wixstatic.com/media/ec0ae9_aacc723ea77d46619d... [3] https://en.wikipedia.org/wiki/Silanization [4] https://pubs.acs.org/doi/10.1021/acsomega.8b03138 [5] https://en.wikipedia.org/wiki/Wetting#Cassie–Baxter_model
Comment · 3 years ago
What a coincidence, I just watched SANS ICS’s promo for their new HyperEncabulator last night. Finally, an Encabulator for the modern cybersecurity environment.
Comment · 4 years ago
You can definitely break a plastic fitting with too much tape. The tape and the plastic are deformable, and I’ve had taped PVC fittings effectively extruded by repeated installation with fresh tape (every time you install it, you need a little bit more). This effect will be amplified if you’re fitting is made out of a material more prone to creep like PVDF, PP, or Nylon. Granted, this isn’t a typical (or appropriate) use case, but it was a necessary kludge for a project I had.
You can get leaks with too much tape if it prevents you from reasonably getting the threads to seal. When this happens, you get a slow leak between the layers of the tape. I’ve seen water slowly bead out of fittings at 1000 psi due to this.
Also, too much tape can lead to contamination (the tape sticks to every piece of dust, lint, and oil in your workspace) and can make it easy to cross thread fittings (especially small plastic ones).
Comment · 4 years ago
The color isn’t necessarily related to the tape density. I always use Merco Threadmaster M66 (I think this, or the M77, is the McMaster default for high-density tape), which comes in all colors. I’ve had cheap tape come prepackaged with products that’s almost transparent and not with bothering with.
I also find it really helps to use the right tape width for the fitting you’re working with. I work mostly on small laboratory/pilot scale stuff (that needs to come apart in a few years, so no dope). I rarely use a fitting larger than 1/2”, but when I do, it’s a pain to use 1/2” tape. 1” (or 1.5”) will make a much neater and more consistent job. Similarly, I prefer 1/4” tape for 1/8” and smaller fittings.
Comment · 4 years ago
In case anyone from the US reads this, BSPP and BSPT fittings are rare and incredibly frustrating here, as our NPT (National Pipe Taper) threads are different and the selection of BSP(P/T) fittings is extremely poor in comparison.
Also, I work with NPT fittings quite a lot:
> For what it’s worth, I tightly wrap the tape 10 times round the male thread and get an enraged mountain gorilla to tighten it up.
This is a WTF NO!!! for NPT and I’ll assume a WTF NO!!! for BSPT as well. You need about 1.5 wraps of PTFE tape to seal a fitting. Any more is wasteful and asking for leaks (or damage, if you’re using plastic fittings). It helps if you use the correct tape width for the fittings (1/4”, 1/2”, and 1” for me) and develop a wrapping method that keeps the tape under tension at all time and in such a direction that threading it into the fitting doesn’t unwrap the tape.
Also, in my experience, when someone inexperienced first learns what pipe tape is, they try to apply it to everything. 20 wraps around a tapered pipe? Wrap a Swagelok fitting? Try to make a butt joint or an adapter for two pieces of plastic tubing? I’ve seen it all.
Comment · 4 years ago
You're totally missing the joke. Read the article and check back.
Comment · 4 years ago
I used this tactic to break up with WSJ. However, I think they technically reserve the right to try to collect fees if you don't cancel through their system.
Another time, I had a service continue to try to bill me monthly for 2 years. I just let it keep saying "your order is about to ship" and "there was a problem with your card". Eventually, they found a way to charge me (I'm really not sure how) and had an order go through, after which I cancelled through an option buried in their website.
I just finished some tonight, and I’m certain it’s sticky enough to hold its shape.