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Technical Information and FAQ\'s on Agrisorb

What is Agrisorb?

In appearance,Agrisorb is a dry, granular material. It contains mostly calcium hydroxide (lime) 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 20% water. It is used to absorb carbon dioxide for medical, diving 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 Agrisorb is used to absorb carbon dioxide the calcium hydroxide (lime) is converted to calcium carbonate (chalk), which can be disposed of easily and safely to landfill.

What is the difference between the different grades of Agrisorb?

Agrisorb is made in a variety of different grades for specific applications. The size, water content and particle shape 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 absorber material. Different dyes can be incorporated to indicate the extent of exhaustion of 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 Agrisorb. 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 your application.

What is the capacity of Agrisorb for carbon dioxide?

It 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 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 Agrisorb will decrease until such time as there is insufficient time for complete reaction. The minimum absorption capacity for the various grades is shown in the Agrisorb 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 Agrisorb 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 circuit this is never a problem, as the inlet air will be saturated with water vapour from expired breath. Prevent 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 a minimum of approx 0.5 seconds is needed in contact with the non-exhausted Agrisorb.

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 Agrisorb be regenerated?

It needs to be pointed out that Agrisorb 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 and recycling is not commercially viable. Also note that the Akron Care formulation involves adding other components such as an indicator which of course cannot be recycled.

Why is water so important to Agrisorb? Is it needed to start the reaction? Does it stop the reaction?

Water is required at the start for the reaction and 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's why the water builds up in the circuit with time. If you have a situation where the system is allowed to saturate then the reaction will effectively stop since the Agrisorb 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% the reaction to absorb carbon dioxide starts to slow down and effectively stops when the water content becomes 1%.

How important is dust in Agrisorb?

Dust should be avoided at all times. It may contaminate the circuit and valves and hence is a major quality issue.

Why is Agrisorb irregular shaped and not spherical?

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 of importance and it is found that with spherical materials you get channeling which results in uneven flow of gas through the material. Overall a irregular shaped material offers the optimum for both the maximisation of surface area to volume ratio and minimization of channeling effects.

DOES AGRISORB MEDICAL GRADE 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 Agrisorb 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 Superia is designed to meet (and exceed) these criteria. Most of the existing alternative products (e.g. Amsorb) that claim low agent interaction are more expensive per kg and have lower carbon dioxide absorption capacities - which is the main reason for it being there.

Does the indicator dye have any adverse effect on the Agrisorb?

It is possible that the indicator dye used can produce amine emanation but this is carefully controlled and are not a problem with Agrisorb, though 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.