These reactors overcome one of the major limitations faced by small-scale fed batch reactors, of removing a homogeneous discharge prior to feeding. The design allows continuous mixing while withdrawing material from the bottom of the reactor. As there are no ‘draw’ pipes inserted into the reactor, it can also use a full depth offset blade mixer which is particularly effective for high solid feedstocks. The reactors can be mounted on ‘off the shelf’ racking units which are widely available