Breathing Gas Processing: Overview

Breathing Gas ProcessingUpdated 13.08.2017 (added chapter ‘sources’)

Living in an underwater habitat requires gas treatment; an input of oxygen, the disposal of carbon dioxide and pollutants, and one kind of ‘carrier’ or inert gas (nitrogen or helium). Industrial divers live in a pressure complex on a support vessel and all air treating is done outside the habitat on the vessel. It seems there are not too many references of air being treated inside an underwater habitat. If we are mistaken do not hesitate to comment this article accordingly.

Sources

  Commercial Diving generally takes place in very deep water. Helium is used as inert gas, unscramblers are ‘translating’ the high frequency voices to an understandable level, the breathing medium has to be heated, the diving suits are equipped with circulating warm water and expensive decompression systems are available on expensive support vessels being operated by huge crews. Some example diving manuals are available in our Digital Library.  We might get inspiration form their procedures, but since our projects are not supposed to be that deep and with a longer duration, we will be able to use more simple methods. Beside that we will not have a financial turnover as in a oil company leading to the conclusion that we will have to find systems, that would fit our demands, but would be unsuitable for commercial diving activities.

One of the best among the few sources ist Living and Working in the Sea by Ian Koblick and Jim Miller. This book from 1984 describes all underwater habitats of the 60’s and 70’s and comes up with all necessary conclusions. It is most likely that most of it is not outdated in general, though there might be developments in detail. Therefore we should use that file as a basement and double-check them with recent developments. By the way, Mart is currently translating the book to German and, most important, to the metric system. Keep in touch with our page to be updated.

Alternatives

  • a constant flow of fresh air: air is pumped from the surface/shore into the habitat while redundant air bubbles out from the entrance pool. No filtering mechanism is necessary, but the flow of air has to be undisrupted. Since the fresh air travels through long pipes there should be preventive steps to avoid odours from the pipe material and moisture. The air inside the habitat should be exchanged all 60-90 minutes completely. Aquarius habitat uses compressors in the life support buoy pumping the air into the habitat.
  • compressed air tanks: the air is stored in huge tanks to provide a constant flow of air. These tanks are generally located outside the habitat not to waste precious space, and due to difficulties and risks during refilling and handling, that might damage the integrity of the hull. The tanks can be filled via hoses from compressors on the surface or completely exchanged when empty. This system requires lots of tanks (the deeper the more) while its handling is difficult. On the other hand there is no carbon dioxide filtering necessary
  • oxygen tanks: produced carbon dioxide is filtered out and replaced by oxygen stored in tanks. This system requires oxygen-clean equipment (since oxygen promotes rapid combustion, valves should be oil-free etc.) and atmosphere measuring/blending devices.

These solutions could be used for the first stages of the habitat. But after a while we should consider air treatment inside the habitat.

Proposals

Here are some proposals for processing the atmosphere inside the habitat.

Standard Scrubbers

Gas Treatment with an Amron Scrubber
Amron Scrubber

In standard dry scrubbers polluted air is led through a canister of absorbent like sodalime (see SodaSorb by Amron) which filters out the carbon dioxide and releases clean air. The absorbent has to be replaced frequently in huge amounts.

(Details needed)

Biocoil

Aquanaut and adventurer Lloyd Godson used a biocoil for gas treatment in his BioSub project. In this system chlorella algae in a transparent tubeare supposed to convert carbon dioxide into oxygen. Though a very impressive idea the system failed to work satisfactory. See all details about the biocoil on our former article.  Still the idea deserves to be followed-up. Maybe there is a way to make it function.

Amine Gas Treating

Gas Treatment by AminesWe just discussed a system used in refineries.

A typical amine gas treating process includes an absorber unit and a regenerator unit as well as accessory equipment. In the absorber, the downflowing amine solution absorbs H2S and CO2 from the upflowing sour gas to produce a sweetened gas stream (i.e., a gas free of hydrogen sulfide and carbon dioxide) as a product and an amine solution rich in the absorbed acid gases. The resultant “rich” amine is then routed into the regenerator (a stripper with a reboiler) to produce regenerated or “lean” amine that is recycled for reuse in the absorber. The stripped overhead gas from the regenerator is concentrated H2S and CO2. (Source: Wikipedia)

Systems like this are used in nuclear submarines like in the USS Nautilus. One major concern is the case of a leaky separator that would produce a chemical aerosol that might harm the aquanauts health.

Image Source: [GFDL (http://www.gnu.org/copyleft/fdl.html) oder CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons, User Mbeychok

Gas Treatment by using Sea Water

Another alternative we are just considering is the question if it would be possible to clean air by leading it through a spray of seawater? Seawater is available and disposable without limits.

to be continued…

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6 Replies to “Breathing Gas Processing: Overview”

  1. Well, there are more points why compressed air tanks should be stored outside. First is simply space, because the internal space is precious (Have you ever spent a week in a camping trailer?). Second, if the tanks have to be moved in and out for refill, that’s a hard and time consuming work. Especially if there is an air lock in between.

    However, I do not agree with the statement that compressed air tanks add a risk to the habitat when stored inside. Technically there is no difference between a storage tank for compressed air and a SCUBA tank, except (maybe) diameter or length. If these things had the tendency to suddenly explode, divers would not carry them on their backs.

    1. Right, I will add the space and refill issue to the article. I could not find any source about the reasons of keeping storage tanks outside the habitat, but I assumed, that there is a risk during the filling process (not when it’s already filled), because of the rising temperature, the temperature difference inside and outside the tanks, handling of the tanks, and maybe problems with the valves. But I might be mistaken. On the other hand, there is no reason to keep the tanks inside the habitat.

  2. Up to now, I have not filled a tank under water. But in a dive-shop in Greece, I have witnessed a filling operation in a water pond, to cool at least the lower half of the tank during filling. Don’t think there is a temperature issue.

    I can imagine two different situations:

    a) Empty tanks are replaced by full tanks
    Suggest to bring the tanks with already attached first stage and inflator hose. There is no issue to connect/ disconnect inflator hoses under water, so we could use this to connect to the air system of the habitat.

    b) A boat with a compressor on board fills the fixed tanks that are attached to the habitat
    No big deal, since we can use a long high pressure hose or a flexible tubing between the habitat and the water surface. The flexible tube comes in 10, 12, 14, …, 20 mm diameter, sold on spools. Material is stainless steel, so no corrosion issue. When not in use, the open end will have to be plugged carefully, but to make sure no water gets into the tanks we can also put a water trap plus an adsorber (such as silica gel) in between.

    1. Exactly, we have these two alternatives. But to bring each tank to the habitat would require a diver, who is dedicated to that job only. That’s not it. But keeping one end of the high pressure hose on the surface buoy, so that the fixed tanks can be refilled from there without moving the storage tanks, would be a great solution, at least for stage 2 of the project.

      The atmosphere of the initial habitat (stage 1) could be renewed by a constant air flow from a compressor on the support vessel, whenever there are divers inside the habitat. For the beginning it’s cheaper and easier to maintain. But the HP hose from the surface for fixed storage tanks was in my mind, too.

  3. Both alternatives have their Pros and Cons. There is no rental market for surface buoys, but you can rent SCUBA tanks and 1st stages. Supporters and volunteers might want to move the air tanks while visiting the habitat.
    On the other hand, if you are planning an archaeological expedition, you do not wand to rely on luck and surface buoy is perfect.

    1. Divers, who want to enter the station, unfortunately will have to pay (we won’t get rich of it, but we do have to earn at least a bit to pay some bills). Surely we will need volunteers for many situations there, but for repeated procedures we will need our own support divers, no way out.
      Right, there are no buoys for our needs from the shelf. But I was thinking to build it by ourselves anyway, since we need a navigation light on the surface. All we have to do is to find a solution to fix the High Pressure (HP) hose on it, that can be connected to the compressor on the support vessel for the beginning. We do not need air when there is nobody in the station, right?
      In a later phase we might take the compressor inside the habitat to pump the air downwards. Then we do not need to connect to the support vessel (maybe for backup?!). In that stage we will need a solution to keep the hose neck above sea level just like a snorkel.
      The buoy has some other functions as well. We have to write an article about it.

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