NOVELTY - Removing frozen material from section(s) of multiple section cryogenic unit without shutting down the unit involves introducing flushing agent(s) at least once into the section to melt the frozen material, introducing at least once purge gas(es) to remove part of the flushing agent, and introducing at least once into the section at least one solvent that is miscible with the flushing agent.    USE - The method is used for removing frozen material from at least one section of a multiple section cryogenic unit without shutting down the unit.    ADVANTAGE - The invention keeps the plant and unit operating while thawing and removing the offending rime from the section(s) of the unit that is afflicted with cryogenic fouling, thus minimizing the adverse productivity impact on the plant and minimizing possible environmental impact that may be necessary during shutdown for thawing.    DETAILED DESCRIPTION - Removing frozen material from at least one section of a multiple section cryogenic unit without shutting down the unit involves introducing at least one flushing agent at least once into the section to melt the frozen material, introducing at least once at least one purge gas to remove part of the flushing agent, and introducing at least once into the section at least one solvent that is miscible with the flushing agent. The flushing agent is a liquid with a freezing temperature below the operating temperature of the section, and is at a temperature to melt the frozen material and remove it from the section. The purge gas has a temperature of vaporization such that it will not condense to liquid at the temperature of the section after flushing. The solvent has a freezing temperature below the operating temperature of the section.    An INDEPENDENT CLAIM is also included for an olefin production plant operating such that a hydrocarbon containing feed is thermally cracked to form at least one olefin and molecular hydrogen, and a high purity molecular hydrogen stream is separated from the cracked feed by way of a multi-sectioned cryogenic unit which from time to time experiences reduced efficiency from its normal operating efficiency due to cryogenic fouling in at least one section. The fouling is caused by the formation of frozen material in at least a portion of the section. The plant operation involves continuing the operation of the plant and the unit, discontinuing the operating of the section, melting and/or dissolving and removing the frozen material from the section by flushing the section at least once with at least one liquid flushing agent that has an a freezing temperature below the operating temperature of the section, removing the melted frozen material and flushing agent from the section and unit during the flushing, terminating the flushing leaving flushing agent in the section, introducing at least once into the section at least one purge gas to remove part of the flushing agent that remains in the section, introducing at least once into the section at least one solvent that is miscible with the flushing agent, and restarting the operation of the section such that the unit is restored to its normal operating efficiency without shutting down the operation of the entire unit and with minimal loss of olefin plant production while melting and removing the frozen material.