Conventional contactors are typically very tall vessels, often the tallest at any processing facility, filled with packing material.
The liquid solvent is sprayed on the packing material at the top of the tower and flows down through the packing while gas flows upward. This is called a counterflow design.
The packing slows the downward flow of liquid. It creates a large surface area either through a convoluted structured packing or through a pile of shaped pieces (often rings, saddles or other shapes that allow the gas to pass up through). This is known as random packing.
The height of the tower -- up to 200 feet (60 metres) or more -- is fixed by the contact surface and time required to achieve the level of carbon dioxide scrubbing required.
The allowable back-pressure determines the diameter.
Over the past several decades there have been improvements in packing materials provided by major suppliers, though these have not substantially impacted the size and cost of absorbers.
More recently contactor towers have been built from membrane lined concrete rather than more traditional steel vessels.