Fluid dynamic characterization and operating window analysis of a pilot-scale multiphase loop reactor for biotechnological applications

Process Safety and Environmental Protection, vol. 232, pp. 239–250

Abstract

Bioprocess intensification through in situ product extraction can mitigate product inhibition and extend fermentation duration. However, conventional stirred-tank reactors often generate fine solvent dispersions and stable emulsions, which complicate subsequent phase separation. A multiphase loop reactor (MPLR), based on a modified airlift design with a concentric draft tube, offers a promising alternative by spatially separating aeration from organic-phase dispersion and thereby enabling simultaneous fermentation and liquid-liquid extraction. Building on previous laboratory-scale studies, this work establishes a pilot-scale facility dedicated to fluid-dynamic characterisation without biocatalytic conversion, with the aim of providing a basis for a projected operational window for future three-phase operation. Key hydrodynamic parameters relevant to process design and operation were determined in two-phase experiments, including gas holdup, volumetric mass transfer coefficient, mixing time, and liquid circulation velocity. Steady state conditions are reached within 15 s , as identified by a constant gas holdup. Further, it was shown that sufficient oxygen transfer, corresponding to k L a ≥ 100 h − 1 , is achieved at superficial gas velocities of 0.02 m s − 1 and above. At superficial gas velocities of 0.01 m s − 1 and above, mixing times below 30 s indicate that the reactor can be considered well-mixed. The liquid flow field was characterized from velocity distributions across the reactor cross-section, confirming the expected circulation between downcomer and riser. Based on the measured two-phase fluid dynamics, the potential feasibility of three-phase counter-current operation was assessed by relating the downward aqueous-phase velocity in the downcomer to droplet rise behavior. A critical droplet diameter was derived using the Schiller–Naumann correlation as a conservative estimate of droplet rise velocity. Additional three-phase experiments in the accessible low-aeration regime support the use of this approach as a conservative basis for preliminary design assessments. By combining this hydrodynamic constraint with the measured mass transfer and mixing characteristics, a first-order operating window for the MPLR was established. The resulting analysis provides a first-order estimate of operating conditions under which sufficient oxygen transfer and rapid mixing may coexist with model-predicted feasibility for counter-current extraction.

Authors 2

1. Maurice Bastard Aachen Institute of Fluid Process Engineering

   RWTH Aachen University

   Affiliation as printed

   Institute of Fluid Process Engineering, RWTH Aachen University, Forckenbeckstr. 51, 52074 Aachen, Germany

2. Andreas Jupke Aachen Institute of Fluid Process Engineering

   RWTH Aachen University · Forschungszentrum Jülich

   Affiliation as printed

   Institute for Bio- and Geosciences (IBG-2), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany

   Institute of Fluid Process Engineering, RWTH Aachen University, Forckenbeckstr. 51, 52074 Aachen, Germany