ANS:B - Centrifugal extractor

When dealing with a system with very low density differences, such as in the case of some liquid-liquid extraction processes, the most suitable extraction method is typically a packed extraction tower. Here's why:

1. Mixer-settler extractor: Involves mixing the two phases to promote mass transfer and then allowing them to settle. However, with very low density differences, the settling process may be slow and inefficient, leading to poor phase separation.
2. Centrifugal extractor: Utilizes centrifugal force to separate the phases. While this method can be effective for phase separation, it may not be as efficient for systems with very low density differences, as the centrifugal force generated may not be sufficient to overcome the weak interfacial forces between the phases.
3. Pulsed extractor: Operates by creating pulsating flow conditions to enhance mass transfer between the phases. While this method can be effective in some cases, its suitability for systems with very low density differences may depend on the specific design and operating conditions.
4. Packed extraction tower: Involves passing the two phases through a packed bed, where intimate contact between the phases promotes mass transfer. This method is often more suitable for systems with very low density differences because it allows for longer contact times and more efficient mass transfer compared to other extraction methods.