Which of the following are types of PCO reactors?

The correct choice highlights the variety of designs used in Photocatalytic Oxidation (PCO) reactors, which are essential in applications for air and water purification. In the context of PCO, specific reactor types are designed to maximize contact between the photocatalyst and the contaminants, enhancing the efficiency of the photocatalytic process.

Coated wall annular designs are beneficial because they create a large surface area for the photocatalyst while maintaining effective flow dynamics. Packed bed reactors provide a configuration that allows for high surface area contact through a bed of photocatalyst material. Flat bed reactors are also used in applications where a uniform flow of air or water over the catalyst is desired. Box reactors can accommodate various operational scales and configurations, providing flexibility in design.

Together, these reactor types optimize the various aspects of PCO treatment, such as light exposure, contact time with pollutants, and operational versatility. The other options primarily include designs that are not standard for PCO applications, focusing instead on geometric shapes or materials that do not constitute typical reactor configurations used in photocatalytic processes.