Biochemical Engineering Journal 48 (2010) 195–203 Contents lists available at ScienceDirect Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej A model-based investigation of the potential advantages of multi-layer packed beds in solid-state fermentation David Alexander Mitchell a,∗ , Lara Elize Nascimento Cunha a , Alex Vinicius Lopes Machado a , Luiz Fernando de Lima Luz Jr. b , Nadia Krieger c a Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Paraná, Cx.P. 19046 Centro Politécnico, Curitiba 81531-980, Paraná, Brazil b Departamento de Engenharia Química, Universidade Federal do Paraná, Cx.P. 19011 Centro Politécnico, Curitiba 81531-980, Paraná, Brazil c Departamento de Química, Universidade Federal do Paraná, Cx.P. 19081 Centro Politécnico, Curitiba 81531-980, Paraná, Brazil article info Article history: Received 20 July 2009 Received in revised form 14 September 2009 Accepted 9 October 2009 Keywords: Bioreactor systems Solid-state Heat transfer Mass transfer Packed-bed bioreactors Mathematical modeling abstract We use a mathematical model, based on the N-tanks-in-series approach, to evaluate the potential advan- tages that can be obtained in solid-state fermentation processes by operating packed-bed bioreactors as multi-layer beds. We explore classical operation, in which air is blown unidirectionally through a sub- strate bed that remains immobile, with two strategies that involve movement of the layers. The first strategy involves batch operation, in which the positions of the layers are changed at 1 h intervals, in a cycling motion. The second strategy involves continuous plug-flow of the layers, with the regular addi- tion of new layers at the air outlet and removal of spent layers at the air inlet. Under the conditions of the simulation, the rate of metabolic heat generation during the steady state of the continuous plug-flow process is only 60% of the peak value predicted for classical operation. As a result, the maximum bed temperature in the continuous plug-flow process is 4.5 ◦ C lower than that predicted for classical opera- tion. We conclude that the operation of multi-layer packed beds in the continuous plug-flow mode can improve bioreactor performance significantly. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Various different types of bioreactors can be used in solid- state fermentation (SSF) processes, including packed beds, rotating drums and forcefully aerated agitated beds [1]. If the process microorganism is sensitive to mixing, then packed beds are the most appropriate, since the substrate bed remains static during the entire process. However, due to the unidirectional air flow within packed beds, it is impossible to prevent a rise in the bed temperature between the air inlet and the air outlet [2]. In fact, the temperature in some parts of the bed can reach such high val- ues that it slows growth or causes death, leading to low product levels [3]. As a result, the main consideration in scaling-up packed- bed bioreactors is to avoid high temperatures being obtained at the outlet end of the bioreactor [4,5]. A strategy for minimizing axial temperature gradients in packed-bed bioreactors is suggested by the work of Lu et al. [6], who divided the bed into layers, creating a “multi-layer packed- bed bioreactor”, which is represented schematically in Fig. 1a. This division led to improved heat and mass transfer in comparison with ∗ Corresponding author. Tel.: +55 41 33611536; fax: +55 41 32662042. E-mail address: davidmitchell@ufpr.br (D.A. Mitchell). a bioreactor in which the same total amount of substrate was placed on a single perforated base plate. A patent request for a multi- layer packed-bed bioreactor with cooling plates located between the layers was deposited a year later by Lüth and Eiben [7]. How- ever, despite its potential advantages, the multi-layer packed-bed design has only been used in one other investigation [8]. Impor- tantly, despite the fact that the order of the layers can be changed during the process, this possibility has not been previously inves- tigated. The ability to change the order of the layers within a multi- layer packed-bed bioreactor means that new operating strategies can be tried. For example, in an attempt to distribute the exposure to high temperatures over all the substrate in the bioreactor, the layers could be moved, at frequent intervals, in the cycling pattern shown in Fig. 1b. Each layer would be periodically exposed to high temperature (when it were near the air outlet) but it would also be exposed to lower temperatures (when it were near the air inlet or in the middle region of the bioreactor). Further, it is possible to operate the multi-layer packed bed as a continuous plug-flow bioreactor, in the manner shown in Fig. 1c. In this mode of operation, new layers would be introduced at the top of the bioreactor at regular intervals; when a new layer were added, the other layers would all move down one position, and the layer at the bottom would be removed. 1369-703X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2009.10.008