Cold box
Cold box , or cold box, German and cold chamber is a term for process plants, which are operated at low temperatures and installed ( "Cold") in a metal box ( "box").
Typical systems that are equipped with a cold box are, for example, systems for air separation or systems for the separation of synthesis gas (into its main components hydrogen , nitrogen and carbon monoxide ). In these processes temperatures of −180 ° C and less are reached; normal system components made of carbon steel are therefore unsuitable.
Constructive
All system components, i.e. apparatus and heat exchangers , are fastened together with the piping in a steel frame that is later clad with steel plates. The inside of this box is filled with insulating material (such as perlite ) to prevent heat input from the environment. When installing the system components, a minimum distance to the wall is maintained to ensure adequate insulation. The piping is completely welded without flange connections in order to avoid leaks. Due to the temperature differences, there are numerous expansion arcs. Components that tend to get dirty, such as droplet separators , are not used, so the inside of the cold box is maintenance-free. Only the valves are designed as so-called angle valves in order to enable repairs from outside. The valve sits in the cold box wall, the pipeline is routed to the valve and back again. Pipes and equipment are normally made of aluminum , but stainless steel is also used for very high operating pressures . The cold box is very often painted white, but also in other light, white-like colors (reducing the black body effect ). Ingress of moisture from the ambient air, which freezes on the cold system parts, is prevented by continuously flushing the cold box with nitrogen. In the case of custom-made cold boxes whose operating temperature is below the condensation point of nitrogen (−196 ° C at normal pressure), the inside of the cold box must be flushed with helium , which only condenses at −269 ° C.
Process engineering features
Penetration of moisture must also be prevented during the process. Therefore, cold boxes are connected upstream of molecular sieve dryers to dry the feed gas . Usually, this also removes carbon dioxide , which can also freeze in the cold box. Introduced water or carbon dioxide blocks apparatus and pipes by ice formation. This ice can only be removed by taking the cold box out of operation and then heating it up.
Web links
- Picture of the inside of a cold box (Linde AG, manufacturer of cold boxes)
- Cold box with air separation process (Air Liquide, manufacturer of cold boxes)