Efficient production of butyric acid by Clostridium tyrobutyricum immobilized in an internal fibrous bed bioreactor (IFBB)

•IFBB can be used for butyric acid production by immobilized C. tyrobutyricum.•A titer of 45.38 g/L butyric acid was achieved by immobilized cells in IFBB.•Fed-batch mode could improve the B/A ratio to a high level of 12.75 g/g in IFBB.•Productivity of butyric acid reached ∼0.90 g/L/h by repeated-batch mode in IFBB.•IFBB system could efficiently produce butyric acid with a long-term performance.