Abstract: A fermentation reactor with a loop-part includes a circulation pump, a degassing-tank, and a fermentation liquid outlet for withdrawing fermentation liquid from the fermentation reactor, the degassing-tank including: (i) a first outlet connecting the degassing-tank to a first leg of the loop-part and allowing fermentation liquid present in the degassing-tank to flow into the loop-part; (ii) a first inlet connecting the degassing-tank to a second leg of the loop-part, allowing fermentation liquid present in the loop-part to flow into the degassing-tank; and (iii) a vent tube for discharging effluent gasses from the degassing tank, such as CO2; wherein one or more of the following criteria applies: (a) the loop-part is provided with an anti-vortex device; (b) certain volumetric ratios pertain between volumes relative to cross sections of reactor parts; (d) ratios pertain between different cross sectional areas of reactor parts.