Technical Information and FAQ’s on Akrosorb Soda Lime

What is Akrosorb Soda Lime?

In appearance, Akrosorb Soda Lime is a dry granular material. It contains mostly Calcium Hydroxide and a small amount of Sodium Hydroxide. It is manufactured in a form that is hard, to minimise dust formation, and has a structure and shape optimised to allow good contact and absorbance of gasses. Although it appears to be a dry granular material it can contain up to 19% moisture/water. It is used to absorb Carbon Dioxide for Medical, Diving, Submarine, Mine Rescue, Petrochemical and other closed circuit breathing systems as well as absorption of other acidic gasses in breathing and industrial applications. It chemically reacts with the gasses it absorbs and converts them to solid calcium salts, which cannot then be desorbed. When Akrosorb Soda Lime is used to absorb Carbon Dioxide the calcium hydroxide is converted to calcium carbonate (chalk), which can be disposed off easily and safely to landfill site.

What is the difference between the different grades of Akrosorb?

Akrosorb Soda Lime is made in a variety of different grades for specific applications. The size, water content, particle shape, particle size, color can be changed to provide products optimised for different applications. Smaller particles can provide more absorption capacity, but at the expense of increased resistance to flow for the same depth of the absorber material. Different dyes can be incorporated to indicate the extent of exhaustion capacity of a bed. In medical use, a white to violet or a pink to white colour change are the two most widely used products. Clearly there is little advantage in using an indicator dye in a system where the dye cannot be seen, such as a diving set. The water content is important, as water is necessary for the reaction to occur at the surface of the Akrosorb Soda Lime. If too little water is present, the reaction is slow and an unnecessarily large contact time is required. If too much water is present then the internal structure of the solid particles becomes flooded and capacity is reduced. This is further complicated by the fact that for carbon dioxide absorbance, the reaction actually produces more water. The shape and size of the particles effects the way the material packs together (the bulk density), the absorption capacity, the speed of absorbance, resistance to flow and the chances of dust production during handling and use. It is therefore important that the correct grade is used for the desired application.

What is the capacity of Akrosorb Soda Lime for carbon dioxide Absorption?

The carbon dioxide absorption capacity of Akrosorb Soda Lime depends on the gas and the way it is used. For each gas there is a minimum contact time necessary for reaction to occur. If the contact time is less than this minimum, then all the target gas at the inlet will not be removed before it gets to the outlet. As the bed gets used, the available contact time with unreacted Akrosorb Soda Lime will decrease until such time there is insufficient time for complete reaction. The minimum absorption capacity for the various grades is shown in the Akrosorb specification.

What can make the absorber fail prematurely?

Premature failure of an absorber can be due to one of three frequently encountered conditions. All are usually readily preventable once the user is aware of the issues.

Drying of the bed:

This occurs if the bed is allowed to dry out. If a very dry gas stream is passed through a bed of Akrosorb Soda Lime for extended periods of time then the water necessary for reaction to occur can be stripped from the bed and the reaction rate will become to slow for effective removal of CO₂. This can occur in a hospital situation if the absorber is left "open circuit" with high flows of dry gas between patients. During use in a breathing closed circuit, this is never a problem as the inlet air will be saturated with water vapour from expired breath. Drying of Bed can be prevented by avoiding long periods of high flow with dry gas streams.

Abnormally high gas flowrate:

If abnormally high gas flow rates are used, the residence time of the carbon dioxide or other target gas in the absorber will be too short for complete reaction to occur. For carbon dioxide absorption, a minimum of approx 0.5 seconds is needed in contact with the non-exhausted Akrosorb.

Channelling:

This is a condition where the gas flow through the absorber is not uniform and most of the gas follows the same path through only part of the absorber. The material in this region becomes exhausted and the gas is no longer in contact with active material. The most common causes for this are high gas flow rates through poorly designed or poorly packed absorber beds.

Can Akrosorb Soda Lime be regenerated?

Akrosorb Soda Lime cannot be regenerated as the calcium carbonate that is formed at the end of the reaction is a different form that the carbonate that started. Recycling is not commercially viable. It should also be noted that Akron Healthcare formulation involves adding other components such as an indicator which of course cannot be recycled.

Why is water so important to Akrosorb Soda Lime? Is it needed to start the reaction? Does it stop the reaction?

Water is required at the start of the reaction. One extra mole of water is produced for each mole of carbon dioxide absorbed. This means that for each 44g of carbon dioxide absorbed it produces 18 g of water – that is why the water builds up in the circuit with time. In situations where the system is allowed to saturate, the reaction will effectively stop since Akrosorb Soda Lime particles will be surrounded by a coating of water through which the carbon dioxide will only diffuse slowly. Conversely, if the water content drops below about 10%, then the reaction to absorb carbon dioxide starts to slow down and effectively stops when the water content becomes 1%.

How important is dust in Akrosorb Soda Lime?

Dust should be avoided at all times. It may contaminate the circuit and valves and hence is a major quality issue. Dust can be avoided by carefully handling and transporting Akrosorb Soda Lime with handling instructions conveyed to the logistics company. Careful handling and storage measures should be undertaken by users of Akrosorb Soda Lime in order to avoid dust formation. Dust must be avoided while filling the rebreathers/absorbers with Akrosorb.

Why is irregular shaped Akrosorb widely preferred?

The important issue here is the morphology which is the surface area to volume ratio. In theory a spherical ball gives the largest surface area to volume ratio since the distance to the middle of the granule is the same all the way around. However, the way the material packs is also important and it is found that with spherical materials, you get channeling which results in uneven flow of gas through the material. Overall, an irregular shaped material offers the optimum for both the maximisation of surface area to volume ratio and minimization of channeling effects.

Does Akrosorb Medical Grade Soda Lime interact with other components in the Anesthesia Stream?

There is a possibility of interaction with a breakdown product called Compound A to produce carbon monoxide but this is normally only present at sub clinical amounts and is not a significant risk. Compound A is also in the anesthetic Sevoflurane. Carbon monoxide may be present due to misuse e.g. by letting the air stream dry out by leaving it on over a weekend. In this respect education has failed and the best policy is to have good enforceable operating procedures to avoid the conditions that can form carbon monoxide.

The alternative is to use an absorber that minimises the risk of carbon monoxide formation by having small levels of a strong base and not containing potassium hydroxide. It is argued that Akrosorb Soda Lime meets both these criteria as it does not contain potassium hydroxide and has low levels of sodium hydroxide (around 3.5%) which results in a low level risk of carbon monoxide production.

The product is designed to meet (and exceed) these criteria. Most of the existing alternative products that claim low agent interaction have lower carbon dioxide absorption capacities.

Does the indicator dye have any adverse effect on Akrosorb Soda Lime?

It is possible that the indicator dye used can produce amine emanation but this is carefully controlled and is not a problem with Akrosorb Soda Lime. However, it has been an issue with other vendor products used in diving and submarine use. This has generally been due to poor quality dye at high levels of usage.