Breathing Gas Processing: Overview

(Update 29.08.2018: Added water air membrane; oxygen by electrolysis. Dieser Artikel steht unter “Atemgas” auch auf Deutsch zur Verfügung) This article is intended to define the concept for ensuring the appropriate habitat atmosphere to be used in the Calamar Park modules.

For the basics of breathing gas we recommend the Wikipedia article “breathing gas”:

Introduction

Guide value for electricity costs

As a guideline for the costs of electricity we can use 0.25 € per kilowatt hour.

Oxygen

If no air from the surface is used, it will be necessary to mix the necessary breathing gas. There are some necessary considerations.

Oxygen demand of a human being

A human has a respiratory minute volume of 8 l / min. With heavier loads (as can occur in an underwater habitat), this can increase three to four times. For a better calculation, we assume a respiratory minute volume of 20 l / min, at which a person converts about 1 liter of oxygen (O2) into carbon dioxide (CO2).

An oxygen tank with a volume of 50 liters at 200 bars (50 x 200 = 10,000 liters) would give a person with a consumption of 1 l / min appr. 10.000 minutes, or 166.66 hours, so almost one week ( 6.9 days).

Oxygen availability

Medical oxygen costs about 0.02 €/l (source: www.medizinischer-sauerstoff.de). It is offered in tanks of up to 50 liters and bottled to a pressure of 200 bars (sometimes 300 bars). A tank of medical oxygen with a volume of 50 l under a pressure of 200 bars would cost 200 €.

Oxygen for diving purposes, e.g. Air Liquide, costs about half (see: www.tauchausfug.de or Tauchcenter Dortmund) or 0.01 € / l. This reduces the price of a 50-liters tank from 200 to 100 €.

For use in our underwater station, we must add the following parameters:

  •  Transport Habitat-Surface-Land-filling station (contractor) and back
  •  Regular maintenance of the connections (outside the habitat)
  •  Regular maintenance of the tanks and official approval
  • Usability for excursion dives via hookah, rebreather or mixed gas bottles

The handling of pure oxygen is not without risk. It should therefore be kept in mind, that all facilities are planned accordingly and the oxygen cylinders are installed on the outside of the habitat.

Carbon Dioxide

Although carbon dioxide is heavier than oxygen, the fear, that so-called CO2 nests near the ground may well be negligible, since gases would be  sufficiently mixed by regular air movements inside the habitat.

Normoxic gas mixture (Normox)

This term has two different meanings: at Trimix diving a normoxic gas mixture describes the “normal oxygen content in the mixture” meaning that it is equal to the proportion in the ambient air (thus 21%). With the normoxic trimix, it is guaranteed that the gas supply can be breathed at any depth up to 60m and, thanks to the 21% oxygen content, a safe ascent to the surface will be possible without changing the gas. Trimix mixtures used for diving beyond 60m must have a reduced oxygen content to prevent exceeding the pO2 limits. Such mixtures may be hypoxic just below or at the surface. They are therefore called “Hypoxic” trimix.

(Source: https://www.dekostop.ch/tauchen-know-how/technical-diving-mixed-gas-trimix/192-normoxic-hypoxic-trimix)

In aquanautics (saturated conditions) “a normoxic breathing mixture (from “NORmal OXygen”) is one in which the partial pressure of oxygen at a specified depth is close to, or slightly above, the normal (surface) atmospheric value of 0,21 ATA (atmospheres absolute = bar).” (Koblick and Miller, 1984). The partial pressure of oxygen therefore has a fixed value for every depth. A normoxic gas mixture eliminates the risk of oxygen poisoning under increased pressure (= greater depth), while ensuring the oxygen demand of Aquanauts.

At greater depths, the amount of oxygen remains the same, although the percentage of total gas decreases.

Alternatives

Air supply from the surface

Up to a certain depth range, it makes sense to pump air from the surface into the habitat. The required compressors for this purpose compress air to the pressure prevailing at the depth of use of the habitat, and press them through umbilicals to the habitat. According to German Wikipedia “the hygienic minimum air exchange rate is about 0.3 / h. It is a minimum for ensuring fresh air, below which odor problems, dust and microorganism pollution as well as too high radon concentrations can occur. “

As a result, the entire volume of the habitat should be completely replaced every 3.3 hours *. The necessary delivery capacity of the compressor in l/min. for an ambient pressure habitat can be calculated as follows:

or

* this value has to be verified and changed if necessary

Costs at complete air exchange by supply from the surface
Setupcompressors
replacement compressors
separate compressor for excursions by hookah
Insulated compressor room in LSB (Boye) or inside habitat
Operationregular maintenance
energy

Example UWL Aquarius: UWL Aquarius has a volume of about 77m³ (= 77,000 liters), is located at a depth of about 20m (= 3 bar) and has two compressors, each with an output of 510 l/min. or a total of 1020 l/min. This results in an exchange rate of 226 minutes or 3.77 hours. The air in the habitat could therefore be completely replaced every 3 hours and 46 minutes:

You can find an article about the air exchange rate under the following links:

de:https://de.wikipedia.org/wiki/Luftwechselrate
en: https://en.wikipedia.org/wiki/Air_changes_per_hour

Carbon dioxide scrubbing by soda lime

scrubber from Amron International


Soda lime is a granulate that binds carbon dioxide. Therefore the air in the habitat would be ventilated through a canister of soda lime to remove the carbon dioxide from the air. Of course, the missing volume has to be replenished by oxygen elsewhere, while the nitrogen circulates unchanged.

When CO2 is filtered out of the station air, the amount of gas in the station will decrease and the internal pressure will drop unless gas in the form of oxygen or air is replenished. The carbon dioxide bound in a lime absorber occupies much less space than a carbon dioxide gas.

1 liter of soda lime absorbs about 120 liters of CO2. This results in a duration of 120 minutes = 2 hours stay (with an oxygen requirement of 1l/min/person). It costs about 5.33 €/l (soda lime “Spherasorb”, June 2018). This requires in 24 hours 12 liters of soda lime for the total price of about 63.96 €. A group of six people therefore has a soda lime demand of 72 liters/day for a total price of 383.76 €/day.

Soda lime is not regenerable and must be disposed after use or transported to and from the habitat. The saturation of soda lime is indicated by an indicator that causes the soda lime to discolour. The process is exothermic, so it creates heat.

To keep the air in the habitat free of CO2, the entire interior volume of the habitat must be completely routed through the scrubber every 3.3 hours *.

(* this value must be checked and changed according to specification, see also “Air supply from the surface”)

Costs for removal of CO2 by soda lime
Setupwasher/scrubber
ventilation system
OperationEquipment Maintenance
soda lime per day/person63,96 €
energy for ventilation
container transportation
disposal of saturated soda lime
costs for oxygen supply

The functioning of soda lime is clearly explained on the following website (german):  http://www.chemieunterricht.de/dc2/tip/17_10.htm

Carbon dioxide scrubbing by Monoethanolamine (MEA)

In this system the medium for CO2 scrubbing is monoethanolamine (MEA) C2H7NO. Therefore  the habitat air is led into a chamber into which MEA is sprayed evenly. The chemical binds the CO2, while the purified air is given the same amount of oxygen before returning to the habitat.

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

The amines are regenerated by heating to about 260 °C (500 °F), during which the CO2 is released again and can later be disposed. One possibility for regeneration is the transport to the water surface, the warming by solar energy and the return to the habitat. In this case a corresponding tank would be necessary for the night. Another proposal was the use of a microwave oven.

One reason for avoiding amines is the likelihood that possibly escaping aerosols could smell stingy due to the relationship of amines with ammonia or sal ammoniac, they could be permanently damaging to health and could provide fertile soil for certain bacteria. To solve this circumstance activated carbon filter could be applied.

One topic to consider is the way in which gas/air and amines are introduced. If both components are supplied against each other, it is called countercurrent. Because the amines are swirled by the passing air, there is also a higher risk of aerosols that could potentially be harmful to health. If both the habitat gas and the amines are introduced from above, so that amines and gas flow side by side, it is called direct current. Due to the large appetite of the amines for carbon dioxide, the current direction for the CO2 uptake is irrelevant. Therefore, a DC system should be preferred; the amines are sprayed just like the air to be cleaned from above and discharged below.

An amine scrubber should be as big as a medium-sized compressed air compressor for scuba tanks.

Cellulose bound amines: It is possible to bind amines to cellulose and to direct the air through these membranes. This would require no liquid to pump in a circle and would avoid splashes and aerosols.

Climeworks: The Swiss company Climeworks builds plants that similarly filter CO2 from the atmosphere on a much larger scale. Smaller facilities, which would also be suitable for an underwater habitat, have the capacity to filter 135 kg of CO2 per day from the air and thereby develop a noise level of 70db. The regeneration takes place with hot water at over 100 °C. A demonstration plant had a capacity of about 40kg and would still be sufficient for a UW-Habitat. The company has an internet website at the following address: http://www.climeworks.com/our-technology/

Amine scrubber on the surface: One proposal was to position the amine scrubbers on the water surface in corresponding buoys. The habitat would be connected to them via lines. Regeneration would be done through solar energy.

Examples: MEA scrubbers are used in submarines such as the USS Nautilus. Since this is a nuclear-powered boat, there is also enough heat for regeneration available. Under the following link you will find various details:

https://www.ussnautilus.org/education/pdf/stemlessons/harris%20_How-Do-Submariners-Breathe-Underwater.pdf

CO2 scrubbing by sea water

One suggestion tried to mimic fish’s abilty to absorb oxygen from the water and release carbon dioxide into the water. Would therefore a system be possible to purify the CO2 enriched air by a seawater spraying process?

Such a plant should operate in countercurrent; the air is supplied from below, while the water is sprayed from above. In this way, the air passes just incoming  water before exiting. In the DC current, the air would at last pass through the already CO2 polluted water.

For this purpose, Calamar-Park developed a simulation program that calculates the necessary amount of water with a variable number of aquanauts and depth meters, amongst many other parameters. From this it is apparent that for the CO2 scrubbing of, for example, 2 persons at a depth of 20m, about 751 liters of seawater would be needed (see screenshot or corresponding article and original simulation program in the appendix (in progress)).

Supplying and injecting seawater in these quantities could be feasible, but would cause a not inconsiderable noise and take up a relatively large amount of space, so that a separate, well-insulated scrubber room would be necessary.

Example of a possible pump type: The “Flygt submersible pump Ready 4” has an energy requirement of 0.42 kW, a capacity of 240 l / min. And costs 541 €. Details can be found under the following link:

https://www.hkl-baushop.de/Produkte/Wasser-und-Pumpentechnik/Tauchpumpen/Flygt-Tauchmotorpumpe-Ready-4.html?userInput=Flygt&ignoreForCache[]=userInput&queryFromSuggest=true&ignoreForCache[]=queryFromSuggest

Costs for removal of CO2 by seawater
Setuppump 1 (standard)541€
pump 2 (standard)541€
pump 3 (redundance)541€
pump 4 (redundance)541€
washer casing500€ (?)
ventilation system3000€
insulated scrubber room
Operationregular maintenance
energy demand for pumps 5,04€/day (0,25€/0,42€) x2 x24h
energy demand for ventilation
costs for oxygen supply

Photosynthesis

Aquanaut and adventurer Lloyd Godson used a biocoil for gas treatment in his BioSub project. In this system, Chlorella algae will convert carbon dioxide into oxygen in a coil of transparent tubes. Although it was a very impressive idea, the system did not work satisfactorily. All details about the Biocoil system can be found in the corresponding previous article.

Nevertheless, the idea deserves to be pursued further. Proposals of bio-modules, which remove carbon dioxide and pollutants from the air, produce oxygen and, ideally, support the food supply.

Still to come: BIOSMHARS, Bios experiments, Orta di Nemo, Eden Project

Water-Air-Membrane

Between 1972 and 1973, scientists from the Soviet Union experimented in a project called Selena with an underwater tent whose membrane (possibly made of polytetrachlorethylene) should be permeable to carbon dioxide and oxygen. In the third part of this project series, an “artificial gill” was also tested, which was supposed to manage the gas exchange with seawater. (Miller and Koblick, 1984) Nothing is known about the results of these experiments, but it can be assumed that a successful conclusion would have led to the development of novel systems that obviously do not exist.

Oxygen by electrolysis

Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current passed through the water. ( de/en)

An electrolysis has a high energy requirement. If this amont of energy is available the generation of oxygen by this method would be very convenient. However, it requires distilled water, that can be easily produced. The electrolysis also produces hydrogen, which could be used to heat the distillation apparatus. However, this should be on the water surface and use the oxygen from the atmosphere. So electrolysis would be something if the habitat is far from everything (using solar panels), you have no compressor for the air supply from above and no oxygen in tanks.

Proposals for the habitat of Calamar-Park

(to be continued)

Prerequisites

All previous and future concepts must ensure the following aspects:

  • A trouble-free supply of oxygen
  • The removal of CO2 from the breathing air
  • The removal of harmful impurities
  • An even supply of all habitat areas

CO2 scrubber:

  • Availability of the scrubber medium
  • Energy requirement of the washing process
  • Method and energy requirement of the regeneration process
  • Noise development of technical equipment (compressors etc.)
  • Maintenance of technical equipment (on site, salvage, availability of spare parts, etc.)
  • Redundancy (bridging in case of failure of technical components)
  • Total cost of ensuring adequate breathing gas per person

Excursion dives

  • Method: Hookah (per umbilical to habitat) or by diver tank
  • Tank refilling
  • Emergency systems (for example during storms)
  • Dehumidification

Pending sections

  • Determination of the best combinations by branch-and-bound tree
  • possible combinations

Other sources

  • US Navy Diving Manual Rev. 7
  • Living and Working in the Sea (Ian Koblick and Jim Miller)
Support by Sharing

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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