A wellspring at 45 km altitude

Venus is generally dry. Its cloudcities and the flying port want water. The flying city may rely on the stations to drop it here. But the cloud layer already has some water, and more hydrogen and oxygen bound up with the sulfuric acid. Let's get at it.

Zubrin was big on better living through chemistry - on Mars. Once he'd pressured enough carbon dioxide into one Earth atmo, added some catalyst like nickel (if, as he snarked, you are cheap) and heated it to 300 Centigrade (573 K): Zubrin could run hydrogen though it, and get water and methane back. This is the Paul Sabatier reaction. The reaction was exothermic too so the energy could be used elsewhere - like in splitting the water, to retrieve the hydrogen for oxygen.

Sabatier remains in active research, mainly looking for lower-temperature catalysts. For this planet I have in mind a different engineering scheme.

Venus can run Sabatier for, almost literally, the cost of a nickel.

The Kelvin hits 573 K from 21 km on down. The atmosphere here is 21 bars higher than Earth's, most of that being the carbon-dioxide we're making use of. (Yeon Joo Lee's thesis pdf). Not only that, but the methane starts undergoing pyrolysis - if you drop the methane 'waste' further down, you will get some hydrogen back, again, for free; and carbon soot. We do have to worry about electronics melting: tin, for one, liquifies at 505 K.

Plant cellulose can be returned to water and carbon soot, and perhaps phosphorous ash, simply by grinding into powder and exposing it to the outdoors at the higher pressures below.

Now, what about all that soot. This can be recycled for whatever else, maybe graphite or diamond. Or: lower the soot and some captured sulfuric acid to that 20ish km level, until the vitriol breaks back into sulfur dioxide and water (and into CO₂, which you just vent). I prefer to use carbon-dust for that because, although vitriol has its own pyrolysis: that occurs at 830 C, hotter even than the surface.

So here's a proposal for another chain in Venus' colonies.

This colony's basis rides over, or on, Aphrodite or Ishtar. Such bears enough air-conditioning to keep the liquids, liquid; and to chill whatever electronics need protecting from Venus' metal-melting environs. It also has our nickel-dust catalyst, which won't melt - and we'll take measures that it won't corrode, neither. The site can be on the surface (3 km, in Aphrodite); or tethered to cables 15 km above - which would help with the A/C at the cost of being a bit more shaky. What this tier doesn't have, is humans.

This remote-controlled reactor is in contact with an air-conditioned balloon, or series thereof, in the cloud bank - riding where the pressure (about two-bar) and temperature allow pure water to condense. The top balloons are there first to catch sulfuric acid, and to drop it to the reactor. Don't worry: we'll get to, "how".

The lower establishment periodically sucks the carbon-dioxide from outside and runs hydrogen at it - voilà, Sabatier. The methane is kept in place in this temperature and allowed to break into carbon-soot and back to hydrogen. The heated water-vapour rises back up to a balloon. It is assuredly a rising gas at this temperature and pressure.

You'll notice that we've lost some hydrogen to water. For replenishment, and for any requests for additional hydrogen, we crack the vitriol from above. The cooled acid is drained into the high-temperature soot, which then goes to break the vitriol back to sulfur-dioxide and water (and CO₂ again). Some of that water is broken down by electrolysis. The sulfur-dioxide is simply vented. The carbon-dioxide may as well be vented too. The oxygen from the electrolysis gets stored in a decidedly separate balloon - we don't want a Hindenberg down here.

Now to connect the cloud to the reactor. I suggest cables - multiple chains of cable: each with its own alternating balloons, and intermittently interlinked. Several materials support a chain over dozens of kilometers (Kevlar is the nerds' preference but... it dissolves in acid). If one link breaks, it can be fixed. They are how thermoses of chilled vitriol get down to the reactor; and how balloons of water and (where needed) methane, carbon, oxygen, and hydrogen rise back to the top. The cables can also carry fiber-optics for mutual communications. And robot avatars in case something needs fixing - we will, of course, have to yank the robots themselves down or up for repairs when done. (I'd earlier considered plain ol' tubes, but I ruled keeping those leak-free in these conditions a clear Unobtainium. I don't think fiber-optics have the same problem.)

How do we keep this vast chain airborne? I don't like the energy-cost of dragging 60 km of all this below the permanent flotilla, so we'll rule that out. And they'd be a dead weight on a cloud-city as well (although maybe we can use hydrogen balloons, especially further down, and can live with the city floating lower in the clouds). The rest'll hold for the sequel.