Absorption occurs when a solid takes up molecules into its structure. In other words, the absorption process sees the passage of one substance into or through the entirety of another channel. The process of absorption means that a substance captures and transforms energy. The absorbent distributes the material it captures throughout the whole and adsorbent only distributes it through the surface.

Hydrogen chloride has a great affinity for water and the absorption can be easily accomplished as long as a suitable method is found to contact the gas and the water. The absorption of hydrogen chloride into water generates a great deal of heat which has to be removed from the system. The theoretical maximum concentration achievable is dependent on the temperature and the partial pressure of HCI in the exhaust gas entering the plant.

The absorption systems produce medium-temperature energy by using operating high-temperature energy and low-temperature waste heat energy. It is an economical option as it reduces the heating amount by absorbing heat from the low-temperature heat sources to be utilized in mid-temperature.

Other absorption systems can use high-temperature and low-temperature energy by using waste heat energy of medium temperature. When the heat is supplied at the mid-temperature, it is converted into high temperature and low temperature, which is a very useful method for recovering industrial waste heat.