Soda Lime absorbs carbon dioxide from exhaled air flowing through it. The world's first anesthesia device with a re-breathing system was in 1924 and relied on a development from Dräger. At a glance, today's granules look a little like cat litter, only whiter and in the form of half-round (planoconvex) lenses, each weighing in at just 25 milligrams. The cartridge of a CLIC disposable absorber contains about 40,000 of these pellets, which in total have an internal surface area roughly equal to that of a soccer field. Only this regular form of soda lime can ensure the high efficiency and consistently reproducible performance.


Soda Lime ProductionUntil the end of the 1970s, Drager soda lime production in Lübeck was exclusively in the form of broken lime, which was always irregular in texture, despite careful comminution into defined grain sizes - and so suffered from associated inconsistencies in its properties.


Manufacturing the material was a very dusty process, and this undesirable formation of dust continued to cause problems: The sharp edges of the grains rubbed against one another during transport in containers and cartridges. In addition, formation of unwanted channels through the broken lime was a recurring problem during its use. The respiratory air would take the path of least resistance through the irregular lime, forming channels. These channels in turn reduced the contact time and therefore the useful life of the soda lime. 

 

Recipes for every application

The quality of the soda lime has a decisive effect on how the overall system functions. That is another reason why Dräger keeps the entire production chain in house.

The wide range of recipes makes sure that anesthesia devices with rebreathing systems or closed-circuit diving and breathing apparatus function optimally for their specific application and according to the latest  technology. The production sequence is basically the same for all lines and all recipes. Highly reactive and pure white lime is exclusively used for the starting material.

 

The food-grade ground lime is delivered in semi-trailers capable of transporting up to 26 tons, and compressed air is used to blow the material from the trailer tanks into the silos. Water and other chemicals are then added to form a paste. This triggers a strongly exothermic reaction, rapidly releasing enormous amounts of heat. Parallels with industrial baking "For reasons related to environmental protection and economics, the company uses a cyclic system," that has saved significant amounts of energy by utilizing the reaction heat. Calcium hydroxide is the main constituent of the paste. The most important minor component is sodium hydroxide, which serves to accelerate the desired reaction with carbon dioxide.

 

Additional chemicals, for example in the Drägersorb Free soda lime, which was developed for use in anaesthesia, prevent the formation of unwanted decomposition products, which can occur in contact with standard halogenated inhalation anaesthetics particularly in low and minimal flow anaesthesia.

 

These recipes also include an indicator to show when the CO2 has dissolved in the water of the soda lime and formed carbonic acid. The pH value changes from basic to acidic, causing the previously white - coloured indicator to turn distinctly violet. This provides a clearly visible signal indicating that it is time to change the absorber. The parallels between the production of soda lime and industrial baking, which also relies on secret recipes and process steps, should already be apparent and they are unmistakable in the next steps. After being placed in a vat the paste is granulated. The pellets of soda lime are dried and shaken out of a mold, ending the production process.

 

Making an impact

The smooth, stainless steel walls of the drying cabinets, in particular, conceal lots of refinements. Even the fittings of the doors hold a few secrets. And that is even truer of the drying process, because the company wants to achieve consistent results with minimum heat energy. Otherwise the soda lime pellets will fracture, leading to irregular shapes and an increased tendency toward dust formation during container transport. Baking oven manufacturers test their designs in a similar way, using a dough that reveals the scope for improvement if it displays uneven browning.


Soda Lime ProcessBefore the soda lime pellets go on to be packaged, samples are taken and subjected to strict checks in the in-house quality assurance department. Only after this step it can be released for filling. Disposable absorbers, changeable even during the operation thanks to the CLIC concept, have become standard in anaesthesia, ensuring even greater utilization of the absorber filling and thus reducing costs. Automatic checks are carried out at every step, from filling the absorbers to sealing and packing into cartons.

A total of six employees manufacture soda lime, using a fully automated process in a hall half the size of a soccer field with less dust to be found than you would expect to see in the back corner of a closet. The computer supported processes can be remotely monitored and controlled via the Internet.

 

The Chemistry:

Exhaled air contains around four percent carbon dioxide. Soda lime removes this CO2, so that the remaining air enriched with oxygen can be fed back into the breathing circuit.

 

The process takes place in a number of steps:

 

  • Carbon dioxide and the water contained in the soda lime react to form carbonic acid: CO2 + H2O = H2CO3
  • In the second intermediate step, the carbonic acid reacts exothermically with sodium hydroxide to form sodium carbonate and water: H2CO3 + 2 NaOH = Na2CO3 + 2 H2O + heat
  • Finally, the sodium carbonate reacts with the slaked lime to form calcium carbonate and sodium hydroxide: Na2CO3 + Ca(OH)2 = CaCO3 + 2 NaOH
  • 100 grams of soda lime can absorb up to 15 liters of carbon dioxide.
  • The soda lime utilizes the kinetic limits of these reactions to a very high degree.

 

 


 
From original publication: Plain Little Pellets. courtesy Drager Medical Ltd.
Article edited by John Sandham - Oct 2009