Chapter 17: Absorption of Carbon Dioxide

INTRODUCTION

The absorption of carbon dioxide is mandatory in closed and semi-closed circle breathing systems. The elimination of CO₂ from exhaled gases is achieved through chemical neutralization in transparent canisters containing absorbent granules. The ideal CO₂ absorbent should have high efficiency, low airflow resistance, no toxicity or reactions with inhalation anesthetics, and low cost. Effective carbon dioxide absorption prevents CO₂ rebreathing and the development of hypercapnia.

CHEMISTRY OF ABSORBENTS

There are several types of carbon dioxide absorbents used today. Each type has a different degree of efficiency for CO₂ elimination.

Soda Lime

The components of soda lime are calcium hydroxide (80%), water (15%), and two catalysts: sodium hydroxide (5%) and potassium hydroxide (<0.1%). Some types of soda lime lack potassium hydroxide. Silica is added to make the granules harder and more stable, which reduces alkaline powder formation (which could cause bronchospasm). It has a pH of 13.5. Soda lime absorbs about 19% of its weight in carbon dioxide, hence 100 g of soda lime can absorb approximately 26 L of carbon dioxide.

The ability of soda lime to absorb CO₂ is due specifically to NaOH. The neutralization of CO₂ involves a number of chemical reactions:

1. CO₂ + H₂O ⇋ H₂CO₃

2. H₂CO₃ + 2NaOH (or KOH) ⇋ Na₂CO₃ (or K₂CO₃) + 2H₂O + Heat

3. Na₂CO₃ (or K₂CO₃) + Ca(OH)₂ ⇋ CaCO₃ + 2NaOH (or KOH)

The first neutralization reaction involves the formation of carbonic acid from CO₂ and water. Then, NaOH (and to a lesser extent, KOH) acts as an activator to speed up the formation of sodium (or potassium) carbonates. Calcium hydroxide reacts with the carbonates within minutes to form calcium carbonate, an insoluble precipitate. In this neutralization reaction, additional sodium (or potassium) hydroxides are regenerated. Some carbon dioxide may also react directly with Ca(OH)₂ to form calcium carbonates, but this reaction is much slower. Soda lime is exhausted when all hydroxides have become carbonates.

Amsorb

Amsorb consists of calcium hydroxide lime (70%), water (14.5%), calcium chloride (0.7%), and two agents to improve hardness (calcium sulfate and polyvinylpyrrolidine). Amsorb has half the absorbing capacity of soda lime and costs more per unit. Calcium chloride serves as a moisture-retaining agent to allow for greater water availability. As a result, there is no need for alkali agents like NaOH or KOH. Without these strong monovalent bases, calcium hydroxide lime has fewer adverse reactions associated with the breakdown of inhalation agents (such as the formation of compound A or carbon monoxide [CO]).

Neutralization of carbon dioxide with Amsorb begins with ...