PURPOSE: To proceed effectively the evaporation of spent liquors and recovery of final products of high quality from spent liquors by dividing at least a vapor side of evaporator unit apparatus and condensing separately vapor contg. different quantity of methanol.

CONSTITUTION: Spent liquors 10 from a chamber for boiling are evaporated in expansion tanks 109, 111 and evaporator unit apparatus 103, 102 and 401 (final concentrator). The final concentrator is operated by new steam 30, the apparatus 102 is worked by an evaporated liq. 40 and residual part of heating surfaces of the apparatus 102 is run by expanding steam 20, 21 from the expansion tanks 109, 11, Liquids 50, 51 are turned into expanded liq. steam in an expansion tank 113, condensed in the apparatus 103 and steam 41 is condensed on the other heating surfaces of the apparatus 103. Steam 23 from an expansion tank 116 of liquid 52 is condensed on a part of heating surfaces of a reboiler 104 to operate a column 105. A concentrated liq. 57 from a concentrator 101 and methanol 12 from a surface condenser 106 are recovered respectively.