PREVENTION OF SCALING IN MULTIPLE EFFECT EVAPORATOR

PURPOSE:To perfectly prevent scaling on heat-transmitting surfaces of an evaporator from occurring, by specifying the supply of a black liquor, in the final shell partitioned into a pair of evaporating chambers of the titled evaporator for concentrating a black liquor for recovering a digesting chemical in a kraft pulpe production plant. CONSTITUTION:The final shell 41 is partitioned into the evaporating chambers 42, 43, which are provided respectively with circulating lines 44, 45 for returning a liquid from a bottom part to a top part, and pumps 38 are provided in the lines 44, 45. A supplying line 49 extended from an adjacent shell is divided into a pair of branch parts 49a, 49b, which are connected respectively to the lines 44, 45. The black liquor fed from the low concentration side adjacent shell is divided into two portions, which are supplied respectively into the lines 44, 45. The black liquor concentrated in the chamber 42 is fed into the chamber 43 through a communicating line 50, is further concentrated, and is taken out through a line 51. In this case, since the liquor can be passed at a concentration near the solubility limit of an Na compound, both the quantity of a liquid evaporated and the quantity of the liquor supplied are extremely small as compared with the quantity of the liquor circulated, so that the mixed liquid is circulated at a concentration lower than a predetermined value. Accordingly, the Na compound separates out in the lines 44, 45 and the pump 48.