Bioreactors for growing micro-organisms

A bioreactor for growing micro-organisms, comprising a reaction chamber for containing a reaction mixture comprising a reaction medium and micro-organisms, said reaction chamber having a first end, a second end, an inner height Hr defined by the distance between an inner surface at the first end and an inner surface at the second end, and an inner diameter Dr, and comprising

• a draft tube arranged inside the reaction chamber, having
• a first end, a second end and a side wall connecting the first end to the second end,
• an inner diameter Dd, wherein Dd is smaller than Dr,
• a height Hd defined by the distance between the first end and the second end, wherein Hd is smaller than Hr,
• at least one gas inlet,
• an inlet for the reaction mixture at the first end of the draft tube, and
• an outlet for the reaction mixture at the second end of the draft tube;
• means for generating flow of the reaction mixture within the reaction chamber;
• a first non-rotating blade structure attached to a side wall of the reaction chamber and surrounding the draft tube, wherein the first non-rotating blade structure comprises a plurality of blades fixed at, at least one of
• an angle α1 with respect to a direction defined by the height of the reaction chamber, the angle α1 being 20-40°, or
• an angle α2 with respect to a direction defined by the height of the reaction chamber, the angle α2 being 320-340°;
• at least one inlet for the reaction medium;
• and
• at least one outlet for withdrawing the reaction mixture.