Abstract: A method for controlling the pH of the liquid circulation in a continuous prehydrolysis apparatus where a mineral acid or some other acid stronger than sulfurous acid is used as a hydrolysis agent and the material to be hydrolyzed is transferred, entrained in the liquids of a first and a second liquid circulation separated by a liquid exchanger, from a steaming vessel to a hydrolysis vessel by means of a transfer valve. Such an amount of alkaline sulfite or ammonium sulfite is added into the liquid of the first liquid circulation as is necessary to make the liquid alkaline or at least neutral.

Abstract: Liquids which, when dilute, tend to froth during mixing or boiling are evaporated in an evaporator by feeding the entire liquid quantity to be evaporated into the evaporator, bringing only part of it in indirect heat exchange contact with a hot medium to concentrate the liquid, recycling part of the concentrated liquid to the heat exchange to prevent frothing of the liquid to be evaporated, and withdrawing the vaporized portion and the concentrated liquid from the evaporator.