Co-current absorbers are usually used when the gas to be dissolved in the liquid is

Solution (By Examveda Team)

Co-current absorbers are generally used in cases where the gas to be dissolved in the liquid is sparingly soluble. Here's why:

Option A: Sparingly soluble → Correct. Co-current absorbers are effective for gases that are not easily soluble in the liquid. In such cases, the concentration of the gas is higher in the inlet gas stream and decreases as it moves through the absorber. The liquid and gas move in the same direction, allowing more contact time to dissolve the gas into the liquid, which works well for sparingly soluble gases.

Option B: Highly soluble → Incorrect. For highly soluble gases, other types of absorbers (such as counter-current absorbers) are preferred, as they provide a more efficient method for absorbing gases that dissolve easily into liquids. In co-current absorbers, the driving force for mass transfer decreases along the length of the absorber, which is not ideal for highly soluble gases.

Option C: A pure substance → Incorrect. While pure gases can be absorbed in co-current absorbers, the design of the system is typically optimized for sparingly soluble gases rather than pure substances. The solubility factor plays a key role in determining the type of absorber.

Option D: A mixture → Incorrect. For gas mixtures, especially when the components have different solubilities, counter-current absorption or other specialized systems are usually preferred to ensure more efficient separation and absorption. Co-current absorbers are not the best choice for handling mixtures with significantly different solubilities.

Thus, the correct answer is Option A: Sparingly soluble, as co-current absorbers are most effective when dealing with gases that are not highly soluble.