Carbon Dioxide Management in Confined Atmospheres
Following Last week’s excerpt from the training and ops manual on carbon dioxide, we round that off with an excerpt from the chapter covering support and allied equipment. Specifically, CO2 scrubbers, or sinks as they are also known.
Gas management is the name of the game in all things diving and hyperbaric. Operators and supervisors must have a good working knowledge of gasses and how to handle them, understand their behaviour, and have a good grasp of their interaction with human physiology. Its gas science.
Probably ranked as one the more important gasses to understand is carbon dioxide. If there is a single gas divers and supervisors understand its CO2. Owing to the fact that CO2 builds up relatively quickly being a by-product of metabolism, it is one that takes centre stage compared to other gasses when physiology is considered.
Carbon dioxide build up can be extremely dangerous and can reach lethal levels fairly quickly in a confined environment. Especially with multiple occupants adding to it with every breath. The same is true for rebreather systems where exhaled breath is recycled. Concentrations above 1% are considered serious with concentrations above 3% causing ill-health and at about 10%, it extinguishes life. It’s the reason we flush ventilate chambers regularly.
In fact, the famous buried alive question, which queries cause of death, can be answered by saying it’s the carbon dioxide that kills, not the lack of oxygen. Oxygen is depleted to life threatening level more slowly than carbon dioxide builds up to lethal levels. The rate of depletion and increase is the same for the two gasses since one is converted into the other proportionately, but lethal levels differ, in that it takes less time to reach these levels of concentration for CO2 than it does to reach low enough levels of O2 for the same end result to occur. CO2 is still lethal even in the presence of enough oxygen. Even 90% oxygen and 10% carbon dioxide will be deadly.
Hyperbaric supervisors make use of similar methodology to determine what level of CO2 scrubbing is required, or how often a chamber must be flush ventilated with fresh air. It’s a crucial skill. These topics are covered in greater detail in physiology and physics as they appear in the upcoming training and resource manual. With this in mind, many chamber complexes or standalone chambers will install carbon dioxide scrubbers.
Scrubbing simply means what it implies. It cleans out a target gas and simply scrubs it out of the mix. Much talk of scrubbed air is also common. It means it’s been cleaned, and CO2 scrubbers clean out the carbon dioxide.
Mentioned in physics and just about every news source, is the topic of carbon dioxide sinks. A sink is something that absorbs something else. In thermodynamics, heat sinks absorb and dissipate heat. In gas management they absorb a target gas. Various elements and compounds have the ability absorb specific gasses. Some ceramics are used as filter medium as well, especially in water filtration. They not only provide a scaffold for natural bacteria to grow which removes toxins from water (bio filter), but they also directly absorb toxins from the water such as ammonia, nitrates and chlorine. Carbon dioxide scrubbers work on the same basis.
An important commonality between the above filters is surface area. Gas laws dictate solubility of gas in other materials, but it all comes down to surface area. The greater the surface area the greater the rate and amount of inward solution or absorption. For this reason, absorption filter mediums are very often made up of a multitude of small beads or fragments which hugely increase surface area. This is true for charcoal filters too previously discussed. The higher surface area means they can be more effective.
Particulate or molecular filter mediums work differently in that they physically prevent the passing of target material.
CO2 scrubbers are essentially absorption filters targeting CO2, and the filter medium is most often lime based absorbers. Soda lime is usually used. Water also absorbs CO2 as well. In fact, it is a considerable absorber of CO2. The ocean is the planet’s CO2 sink. It has a massive surface area compared with the land and maintains the balance between atmospheric and oceanic carbon dioxide. Unfortunately it would take more surface area than we are able to provide inside a confined chamber to use water as a sink, accordingly soda lime beads are the preferred medium, although others are available such as: Amine Scrubbing for coal and gas fired power plants, minerals and zeolites, sodium hydroxide, lithium hydroxide, activated carbon and so on.
In diving and hyperbarics, the most common way to scrub carbon dioxide from the air in a chamber is with the use of sorbent canisters. NASA make use of sorbent canisters in the same way hyperbaric personnel do. NASA also employ other methods but historically they have favoured canisters. In fact, it was modified scrubbers that saved the crew of Apollo 13 when they modified round fittings to fit square inserts.
Scrubbers can be lung powered or fan driven. Any method of moving air through the scrubber in sufficient quantities and correct flow velocity will suffice. Provided exhaled air passes through the filter and crosses the increased surface area, the filter medium will absorb its CO2.
In chambers we use canisters filled with soda sorb type material and these are recharged at predictable intervals. Soda lime has a predictable CO2 carrying capacity and it can be calculated how long canisters will last given the amount of CO2 being produced in a closed system. Human beings produce a predictable amount of CO2 through respiration.
Soda sorb is a brand name and is made of soda lime and can be purchased in bulk and canisters are easily refilled and packed. In fact it wouldn’t be too challenging to build ones own scrubbers.
Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO₂ retention and carbon dioxide poisoning. The main components of soda lime are:
- Calcium hydroxide, Ca (OH)2 (about 75%)
- Water, H2O (about 20%)
- Sodium hydroxide, NaOH (about 3%)
- Potassium hydroxide, KOH (about 1%).
A carbon dioxide scrubber is a piece of equipment that absorbs carbon dioxide (CO2). Whether from a rebreather system, a chamber atmosphere or the actual atmosphere, the principals are the same. It is used to treat exhaust gases from industrial plants or from exhaled air in life support systems such as rebreathers or in spacecraft, submersible craft or airtight chambers.
Carbon dioxide scrubbers are also used in controlled atmosphere (CA) storage. They have also been researched for carbon capture and storage as a means of combating global warming. Although given the surface area required to scrub atmospheric CO2 it is hard to imagine workable solutions will prove better than the ocean as a CO2 sink. Additionally, the previously discussed gas laws would render them ineffective anyway since we simply cannot increase atmospheric pressure at will. And pressure is what determines the amount of a given gas can dissolve into a given liquid. The more CO2 removed from the atmosphere the more the ocean will release in the presence of a pressure differential as will also geological structures. Maintaining this balance is discussed in the article CARBON DIOXIDE AND THE HYPERBARIC ENVIRONMENT. The only way to permanently reduce atmospheric CO2 is to deplete it in the ocean and planetary geology which would compromise mammalian and plant biology, not to mention being impossible. Henry’s Law and Dalton’s Law explains why. Open a can of soda for a demonstration.
Practical instruction in the recharging and refilling of scrubbers is required. Below are a handful of images depicting canister (fan) and lung powered carbon dioxide scrubbers commonly encountered in hyperbaric chambers that have them.