Carbon Dioxide on ISS

    In October, a new (fundamentally) air regeneration system was installed on the ISS, which would double the air cycle closure .

    Sabatier reactor, top and bottom view

    However, if everything will be fine with technological progress, then this option will not last long - and we are waiting for a rollback to the system of the previous type. But if progress will slip, the new system will become the gold standard for decades.

    Short content: What is stuffiness: low oxygen? - NASA and US Navy CO 2 standards - How much CO 2 does a person emit? - I generation of air regeneration systems - II generation - III generation - Prospects for systems with full regeneration - Comparison table

    What is stuffiness?

    Everyone knows that oxygen is needed for breathing. Many believe that the stuffiness in the room comes because the room breathed out some of the oxygen; and airing is necessary so that a new one comes from the street.

    In fact, it is not.

    The average person consumes oxygen ~ 1 kg / day (or ~ 1/2 g / minute).

    In the middle room (3x5x2.6 = 40), under normal conditions (O 2 content 0.28 kg / m 3 ), breathe oxygen to a level as low as high in the mountains, one person should breathe for a week.

    In reality, as it is easy to see, it will not be possible to bury in a room for a week. If a person closes hermetically in the bedroom, he will hardly spend one night. After a few hours, sleep will become restless, there will be an increasing sense of stuffiness. Day in such a room will be torture - not allegorically, but in the most literal sense. A person will be physically very bad.

    It's not about oxygen, but carbon dioxide, which a person exhales in return.

    How much CO 2 does a person emit?

    In the fresh air, the content of CO 2 ~ 0.04% (0.5 g / m 3 ).

    With an increase in the content to 0.7% and further, it is more difficult to ignore the stuffiness. This is not just psychological discomfort, but also noticeable physiological changes (from 1%): an increase in the frequency and depth of breathing, an increase in pressure, heart rate, increased sweating; the number of errors in difficult work increases, the headache begins, the maximum concentration becomes unattainable (from 2%). In civilian research do not experiment with content above 2.5%.

    It is clear that by consuming 1 kg of O 2 , the human will exhale about 1.4 kg of CO 2 .

    Why not exactly? Is the lungs not a kind of catalyst?
    При поглощении из воздуха 1 молекулы O2, разве не выделяется ровно 1 молекула CO2?

    С точки зрения биологичеких механизмов, это не обязательно так. В эритроцитах эти процессы разделены. Одна система захватывает кислород, другая выбрасывает углекислоту.

    И в реальности молекул кислорода захватывается больше, чем выделяется углекислоты.
    Проще всего это понять, если обратить внимание на жиры (в еде нашего модельного человека). По составу их можно огрубленно считать как CH2.

    Кроме 1 молекулы кислорода, чтобы окислить атом углерода, нам понадобится еще один дополнительный атом кислорода, чтобы окислить водород. В целом кислорода будет потреблено в полтора раза больше, чем выдохнуто углекислого газа.

    Однако для углеводов и белков это соотношение близко к 1:1, поэтому далее, для простоты, рассматривается «катализаторное» приближение дыхания.

    In our walled room, with a capacity of 40m 3 , with initially perfectly fresh air, in 20 minutes a person will double the “natural” CO 2 content . Overnight 20+ times - up to 1%. Per day up to 3%.

    NASA and US Navy CO 2 Standards

    In mortal life, such walled places where the window is not open, but you have to work for many days in a row, are submarines.

    Submariners are much more than astronauts. And their work is no less difficult and responsible. So there is a large and high-quality statistics.

    When developing space regeneration systems, they are guided by this experience, but the norms for astronauts put more humane ones, NASA decided to take a 1/3 multiplier for long periods:

    Permissible concentration of CO <sub><small> 2 </ small></ sub> depending on the time of stay.

    That is 0.8%.

    However, in reality, NASA is trying to keep the level on the ISS not higher than 0.5%. The fact is that even at this level, individual astronauts are beginning to experience discomfort, - psychologists at the MCC notice that people's behavior noticeably changes, even if they themselves do not complain.

    And there is a need: how to maintain low CO 2 in the air ?

    0th generation - blowing

    Historically, this is the first solution, because the simplest.

    First, even on the umbilical cord, Leonov

    There is just a gradual blowing of the spacesuit atmosphere with oxygen. Carbon dioxide released during breathing is released into the vacuum - along with the rest of the mixture. Where there is still a lot of oxygen that could breathe.

    It is clear that as a full-time system, this existed only at the very beginning of astronautics.
    Now this system is used only as a duplicate system in space suits. That is, in the event of a malfunction of the main system (see below, the next generation), or as an emergency extension in time, when the main system is already exhausted, and the astronaut did not have time to return. The estimated time of such a backup system in a modern spacesuit is half an hour.

    To make it clear how inefficient such a system is: over the course of half an hour it will be spent on blowing 1.2 kg of oxygen, of which a person will assimilate 15-20 grams. Efficiency less than 2%.

    I generation - the famous "checkers" for air

    This regeneration system became main almost immediately - and it remained such a decade.

    It was used by both the first man on the moon and the last people on the shuttles (although by then on the ISS, and before that on the Mira, and even on the Skylab, the next generation was already used as a standard version, see below).

    The air chases through a closed loop, without dumping out. The oxygen loss is compensated by the fact that oxygen is mixed in from cylinders (or, later, from water electrolysis), and lithium hydroxide tanks are used to remove CO 2 :

    2LiOH + CO 2 → Li 2 CO 3 + H 2 O

    Carbon dioxide is bound to lithium carbonate. Formally, in this reaction, water is released, which could (theoretically) try to extract, and decompose into hydrogen and oxygen, which is used again.

    In reality - after using the checker, with all its contents, it goes into the trash. Because of its compactness, such a system is used as a regular system in all modern spacesuits and delivery vehicles (Soyuz, future American ones). Because of its simplicity and reliability, such a system is considered as a spare / supplementary one on the ISS - if the standard system fails; if there are too many people at the station and the main system fails.

    When the shuttle was still flying to the ISS, each of which had a whole mob, and they all spent more time at the station than the calculated flight of the shuttle - the two full-time ISS systems (Russian and American) were not enough, they constantly “burned” their checkers on the shuttle, and then another significant portion of the stock of checkers on the ISS. Then, on the cargo ships, they added new ones.

    Modern American checker contains 3 kg of LiOH,


    Russian 5 kg.

    With checkers, ideally, much less is irreplaceably lost: carbon dioxide collected by checkers; checkers themselves. (And, if you produce oxygen from water, the hydrogen released from the water, it also goes overboard.)

    At the same time, the biggest waste in weight is the checkers themselves. And you can somehow without spending drafts?

    II generation - regular mode of the ISS

    If it is very rough, then this is an improved cat tray with a filler.

    We have a substance that is well impregnated with a gas - but not by any, but depending on the diameter of the molecule. Carbon dioxide is captured, there is almost no nitrogen and oxygen. That is, we have before us the so-called "molecular sieve". Since the time of Skylab, this is zeolite .

    So that the zeolite does not get wet (at the station is normal humidity, each person exhales a liter of water per day), first the air is dried. Cools down. And served in the chamber with zeolite.

    On the example of the new system

    There are two such cameras (in the American system), or three (in Russian). For a while, one of the chambers absorbs carbon dioxide, then the air flow switches to the second. At this time, vacuum is injected first, and the zeolite is heated. Carbon dioxide comes out of it. This is one cycle. Now we can again use the first chamber for air purification, and put the second one on weathering in a vacuum.

    Ideally, you only take carbon dioxide from the atmosphere of the ISS. This is your irreparable loss (you send this gas overboard), but the adsorbents themselves are used many times, unlike cat trays or systems on checkers. (And of course, you continue to throw hydrogen overboard, as a by-product of electrolysis when you receive oxygen.)

    Question: And if throwing carbon dioxide overboard was a pity? He is two-thirds more than oxygen!

    Generation 2.5 - experimental, unsuccessful

    The system was tried to be developed for Mira, but nothing good came of it.

    On the one hand, we must pay tribute to the courage of Soviet engineers. If the system were to work, it would be a complete closure of the oxygen cycle.

    On the other hand, it is impossible not to remember the classic one - “My dear, are you going to burst?” Perhaps, if the efforts were directed towards a less ambitious task (the Americans from the very beginning did all the work for such a less ambitious task, although they had far more resources ), then the Soviet engineers would have perfectly decided it, and the 3rd generation systems would have been successfully used for thirty years.

    What is the idea. To convert carbon dioxide into oxygen, the so-called Bosch reaction can be used: carbon dioxide is mixed with hydrogen, and at high temperature, carbon dioxide is first reduced to carbon monoxide, and then carbon disintegrates to atomic carbon on the catalyst. Water (steam) is obtained, and carbon in the form of sediments:

    CO 2 + 2H 2 → C + 2H 2 O

    The main difficulty is already visible from the description: the reaction takes place on a catalyst that is covered with a bloom of graphite. And what to do?

    First of all, cleaning is difficult and expensive (costly in the cosmic sense: additional equipment is required, and consumables, and the cost of useful mass for this turn out to be greater than the gain in stored oxygen).

    Secondly, these cleanings should be very frequent - if there are three crews, then 1 kg of graphite should settle on the catalyst per day.

    3rd generation - fresh

    From the very beginning, the Americans decided not to do Bosch’s reaction, but Sabatier’s reaction. It is often called the Sabatier reactor, since the reaction requires not only high temperature, but also increased pressure.

    The reaction goes on the catalyst, hydrogen is added to carbon dioxide, that is, the reagents are similar to the Bosch reaction, but the output of the reaction is different:

    CO 2 + 4H 2 → CH 4 + 2H 2 O

    water and methane.

    The technological advantage of Sabatier to Bosch is that all products are gaseous and it is easy to continue working with them. In the variant that has now been delivered to the ISS, methane is simply emitted to the outside (as in systems of the 2nd generation carbon dioxide is emitted to the outside).
    But there is a minus. Let us recall where the new oxygen is taken from the station. Decomposition of water.
    Oxygen is used, and hydrogen (in systems of the 2nd generation) is simply thrown overboard. Now we can (and should! Take the hydrogen for the reaction from somewhere) to use this hydrogen, sending it to the Sabatier reactor.

    And then the nuance. In water, there are 2 hydrogen atoms per 1 oxygen atom. And in the Sabatier reaction, 4 oxygen atoms must fall on 1 oxygen atom (2 goes to replace the bond of oxygen with carbon, and another 2 hydrogen is molded to this separated oxygen, forming water).

    Thus, if you rely only on the electrolysis of water and the Sabatier reactor, the oxygen cycle can be closed only by 50%. Half of the CO 2 can be recycled, and for the remainder, there is no hydrogen anymore.

    Block diagram after adding the reactor Sabatier

    (If you just shot down at this moment, do not worry. Even the drafters of the first press release on the ESA website did not immediately realize what was happening, and at first drew the wrong block diagrams and cut down all of the catalyst nedoeffektivnost.)

    In reality, of course, until it turns out not theoretical 50%, but less, about 40%. At the beginning of the article, only the Sabatier reactor itself is shown, an element of innovation — in the block laugh it is around the green arrow.

    The whole system is much larger, just like the one that the Americans had had before. For the full scientific stand, half a ton.

    Gerst portrays as if a ton of iron had dumped him.

    Prospects IV generation - the development of III generation?

    The question immediately arises: why not use additional hydrogen? Delivered to the ISS in addition to the water we allow for electrolysis?

    Indeed. Consider the part of CO 2 that has to be thrown into a vacuum. For every 12 mass of carbon we lose 32 mass of oxygen. And if we add the missing hydrogen to the reactor, and we bind carbon in CH 4 , then the oxygen will remain at the station, and in the exhaust we will lose only 4 mass of hydrogen. The weight gain is 32: 4 = 8 times. 1 kg of hydrogen would save as much as 8 kg of oxygen!

    The problem is that hydrogen is not water. This can be used to transport water conventional containers. For simplicity, we put on the container 1/10 of the weight of the delivered water.

    In the case of hydrogen, though compressed, even liquefied, it’s just the opposite: the ratio of the mass of the container to the mass of hydrogen contained in it will be ~ 10/1.

    We cannot deliver to the ISS just a kilogram of hydrogen. We still have to lift 10 kilograms of his container.

    Not to mention the need to solve safety problems along the way: when hydrogen is stored, there is a regular leak at the valves (if delivered as gas), and a similar dump at the containers (if liquefied), due to the need to maintain a low temperature inside. In addition to the danger, these leaks still make it impossible to store in reserve for a long time. Bleed hydrogen must either be used immediately or irretrievably lost.

    The result is that it will be easier (and more economical) to deliver to the ISS not additional hydrogen for the Sabatier reactor, but additional water for electrolysis. And work in a half closed loop, throwing off extra carbon dioxide into a vacuum.

    Prospects of the fourth generation - another development from the second generation

    While it was about closing the system only for oxygen. Carbon was considered as a useless element, inevitably entering the system (through people's breathing) from food. We did not take into account the mass consumption of food that is constantly introduced into the air regeneration cycle.

    And what if you still try to save carbon? What if oxygen is extracted from carbon dioxide, binding carbon not to methane, but to carbohydrates?

    Carbohydrates, if you look only at the number of components of chemical elements, it is approximately equal mixture of carbon and water.

    Recall the atomic masses of the participants: hydrogen - 1, carbon - 12, oxygen - 16.

    Let's compare the effectiveness of the considered methods of carbon sequestration, in terms of the mass of substance discharged into the vacuum (which must be lifted from the ground to the station!):

    1. When all CH 4 is dumped overboard (and hydrogen is sent there from electrolysis), we lose two water molecules per carbon atom, that is, 1 mass of carbon 3 mass of water.
    2. During the Sabatier reaction (due to the lack of hydrogen), we lose by the water molecule for each carbon atom, that is, for 1 mass of carbon, 1.5 mass of water
    3. When converting to carbohydrates, we spend on the water molecule for each carbon atom, that is, for 1 mass of carbon, 1.5 mass of water.

    As you can see, the cycle electrolysis + Sabatier efficiency is the same as the cycle electrolysis + carbohydrates.

    But! In the reaction of Sabatier, we drop this substance from the station, lose it irrevocably. And carbohydrates - you can try to make them suitable for food?

    Astronauts should have in their food not only carbohydrates (for simplicity, 400 grams), but also fats (100 grams) and proteins (100 grams). Because of this, to close the cycle on oxygen and nutrition, making only carbon dioxide from carbon dioxide, will not work. But to replace at least the carbohydrate part of the products? This is 2/3 if by dry composition!

    Then the final balance will change:

    - on the one hand, we reduce the waste of water by 3 times compared to the cycle through Sabatier (from 560 grams to 165, this is for binding in carbohydrates of that carbon, which came from eaten proteins and fats, its 110 grams; theoretically, even these 165 grams water can not be discharged, but sugar can be stored on board, but it will simply not be in demand for the cycle, there will be an accumulation of pure carbohydrates)

    - plus, the food consumption (by dry composition) becomes 400 grams per person per day (we closed the cycle of food for carbohydrates).

    In total, a gain of ~ 700 grams per person per day.

    What to expect

    To sum up: NASA, ESA see the prospect in returning to the previous regeneration system (through adsorbers without Sabatier reactor) - only now, when unloading the adsorbent, use not laboratory vacuum, but open. Closed vacuum chambers from which carbon dioxide is pumped out and stored to direct it to the carbohydrate production.
    And there remains a mere trifle: how to convert carbon dioxide into carbohydrates?

    1. You can try to do it purely chemically. But it is difficult. If it were simple, we would have long since taken sugar and bio-feed not from the plantations, but from extensions to power plants.
    2. You can try to do it biologically, through photosynthesis - but not everything is smooth here either.

    Housekeeping note: how many indoor plants do you need so that you can never air out?
    Иногда простые люди, любители домашних растений, вдруг задаются таким вопросом. А доброжелатели предлагают им простыни сбивчивых расчетов — которые начинаются с перемножения справочных данных: средний объем вдоха, среднее число вдохов в минуту, доля углекислого газа в выдыхаемом воздухе… На самом деле, пригодный для практического ума ответ очень прост. И напрочь лишает энтузиазма. Это должна быть плантация такого размера, чтобы вы ежедневно собирали с нее ~800 граммов сухих листьев (на одного человека, в зависимости от того, сколько он ест). А если этих человек шесть или десять? И плантацию надо соорудить не на заднем дворе, а на орбите? А потом разогнать ее к Марсу, а у Марса затормозить, и как-то спустить вниз; и снова поднять, чтобы лететь обратно к Земле?

    Here, the plans of the agencies either really diverged, or the management decided to play a healthy competition, and not to put all the eggs in one basket. (This is always a good option, and especially when there is experience with the competition of Sabatier and Bosch reactions. One took off, the other did not.)

    NASA announces contests with millions of prizes in which it offers to produce sugar in purely chemical ways.

    The ESA promises to raise an algae tank to the ISS as early as next year, and feed them with carbon dioxide, which is now excessive for the Sabatier reactor.

    And if with carbohydrates from carbon dioxide nothing will come of it?

    You can also try to make carbohydrates and hydrogen from methane and water. NASA chose carbon dioxide as a starting point to catch two birds with one stone: this solution can be useful not only for closing the regeneration cycle in flight, but also for the active growth of matter on the planet - in the atmosphere of Mars, where there is CO 2 . And here, on Earth, it would be useful.

    But most likely, the Sabatier reactor circuit will remain the most efficient - and, given the real speed of progress in manned space technology, by decades.

    comparison table
    generationCO 2 removal methodloss of 1
    (/ person / day)
    as a regular systemas spare
    / add.
    blowing50 kg O 2 compressed 2space suits
    ILiOH Checkers1.1 kg of water
    1.5 kg of checkers
    delivery ships (Soyuz, future American,
    last shuttles)
    IImolecular sieve
    evaporation to vacuum
    1.1 kg of waterISS
    (formerly Mir,
    Skylab - for the first time)
    IIImolecular sieve,
    decomposition to graphite
    0 3(experimental,
    for "Peace"
    IIImolecular sieve
    50% in CH 4
    0.6 kg of waterISS (amer. Segment)
    ?molecular sieve,
    in sugar
    0.2 kg of water 4 / increment of 0.2 kg of water 5

    1 Ideally.
    2 Excluding tare.
    3 Excluding consumables for cleaning / replacing the catalyst.
    4 When combined with the carbohydrate food cycle; 5 wherein the food becomes shorted cycle of carbohydrates, i.e. there massozatrat less than 0.4 kg (dry weight carbohydrates in the food) is greater in the air cycle water loss (if considered separately from the total loss ballansa) - is formally and can be interpreted as an increase in the useful mass (if we compare it with the situation when carbohydrates enter the food cycle from the ground).

    What has not been stated above, but it is useful to understand for completeness

    In addition to the losses during the regeneration of the breathing cycle (in modern systems, this is reduced, as can be seen, to some water loss), there is a mass loss in other cycles associated with people.

    First of all, it is a toilet cycle. Even when systems trying to close this cycle are used to the maximum on the ISS, the effectiveness of these systems is limited: from a few kilograms of water and food, ~ 80% of water can be returned to the cycle. That is, the loss of ~ 1 kg per person per day. (This is not counting the container in which the food goes. Not all of it is sublimated in tea bags. There are also ordinary cans.)

    Thus, it is senseless to make absolutely fantastic efforts, trying to further improve the regeneration system for CO 2 , until the losses in toilet cycle.

    Therefore, the real goal that NASA has set itself is to bring the system from the current 40% to 75%.

    But even if it turns out both of these cycles to close to zero, or almost to zero, and that's not all. It will not mean at all that a person can fully work in a closed cycle in oxygen and water.

    Each exit to the open space is the inevitable loss of water. It is used to cool the spacesuit. Although at first glance it may seem that the spacesuit itself, and the “backpack” of the life support system are completely covered with thermal insulation - no. The lower end of the "backpack", which ends under the astronaut's ass, is not covered. This is a radiator cooler, and in the radiator pores, where water is supplied to the external circuit - for evaporation. In one exit, depending on the duration, ~ 1-2 kg per exit participant is lost.

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