Method of separation

A method of separating an active hydrogen-containing compound (A) and a compound (B) that reversibly reacts with (A), comprising:

• a step of separating at least either (A) or (B) from a mixture containing (A) and (B) by distillation in a multistage distillation column;
• and
• a step of supplying the mixture to an inactive region formed within the multistage distillation column,
• wherein:
• the multistage distillation column is a plate column or a packed column;
• and
• the inactive region is one plate in the plate column which a surface contacting with the mixture comprises a material inactive to a reaction between (A) and (B), or the inactive region is one theoretical plate in a packed column packed with a packing material comprising a material inactive to a reaction between (A) and (B),
• wherein:
• (i) the material inactive to a reaction between (A) and (B) is a material in which an Fe atom content, a Ni atom content and a Ti atom content are each 10 mass % or less;
• or
• (ii) an area (X) (unit:m2) of an inner surface of the inactive region contacting with the mixture and a volume (Y) (unit:m3) of the mixture retained in the inactive region satisfy (X)/(Y)≤100,and
• wherein:
• the compound (A) contains a hydrogen atom bonded to a heteroatom or a halogen atom, and the compound (B) contains at least one group selected from the group consisting of formula (5), formula (6), formula(7) and formula (8):
• —N═C═Y4 (8)
• wherein:
• Y1, Y2, Y3 and Y4 each represents an oxygen atom,
• R1 and R2 each independently represents an organic group having 1 to 30 carbon atoms, and
• R″ represents an organic group.