Direct contact heat exchangers (quenching columns) are considered to be the optimum types of heat exchangers for the fast pyrolysis process. In this study, the hydrodynamics and heat transfer characteristics of a bench scale quenching column are presented. These have been compared with the experimental observations on flooding phenomena which are reported when the quenching column is operated at the design gas flow rates of the fast pyrolysis reactor. The quenching column was found to operate without flooding at 10% of the design flow rate, while flooding was still present even at 50% of the design gas flow rate. Four different design configurations, which are different in terms of weirs and hole placement on the disc and donut plates, are modelled and tested under full gas flow rate conditions. All four cases show normal quenching column operation without any flooding phenomena present and a gas flow time of less than 1 s. The pressure drop across the system was considerably reduced to 15Pa in the modified configuration compared to 90Pa in the baseline model. The hydrodynamic and heat transfer characteristics are thoroughly analysed and proposed optimal design configuration for the effective quenching operation. (C) 2014 Elsevier B.V. All rights reserved.