Valmet is in the forefront in the development that has led to today’s horizontal tube reactors for prehydrolysis of non-woods. Based on more than 50 years of experience in non-wood pulping and from installations worldwide, the Valmet tube digester is recognized as the most reli­able and efficient on the market.

The primary application for tube digesters have been processing of agricultural residues such as bagasse and wheat straw or other annual plants for the production of fibers for papermaking.

The horizontal tube reactor has been adapted to conditions applied in prehydrolysis. Prehydrolysis is sometimes called hydro-thermal treatment or just pre-treatment since it is a treatment step before enzymatic hydrolysis. Prehydrolysis is typically performed at acid conditions, high temperature and pressure up to 25 bar.

An internal screw transports the biomass through the reactor, which gives the biomass a well-controlled retention time. Direct steam for heating is added through the headers along the reactor. Depending on the feed rate and retention time, several tube reactors can be connected in series.

Valmet being the leader in continuous pulping of wood also has extensive experience of continuous vertical digesters for cooking of various annual plants. For higher biomass feed capacities vertical reactors become more cost effective. The discharge from the reactor can be hot or cold depending on the requirements in subsequent steps. With hot blow steam explosion takes place which means disintegration of the biomass into small particles that gives excellent access for the enzymes in a subsequent saccharification step.

Cold blow, or dilution discharge, is applied when the objective is to separate the sugars dissolved in the liquid phase during the prehydrolysis from the remaining solid biomass. It gives the possibility to treat the sugars dissolved in the prehydrolysis, mainly from hemicelluloses, separately. Separation of the liquid from the biomass is done on fiber washing equipment, for instance, TwinRoll presses or screw presses.