Discrete particle simulations predicting mixing behavior of solid substrate particles in a rotating drum fermenter

Towards optimized drum composting: evaluation of the radial mixing performance of a model substrate on the laboratory scale

The rotating drum composter (RDC) is one of the most widespread reactor systems for biowaste treatment, worldwide. Nevertheless, knowledge on optimum operating conditions including, e.g. fill level, turning frequency, and mixing tool configuration is sparse. This study investigated the effect of static mixing tools (SMTs) on mixing in a rotating drum at high fill levels (60-80%). The methodological approach encompassed mixing experiments in a laboratory RDC using soaked wheat grains as a model material. The temporal course of material blending was quantified in terms of the entropy of mixing using digital image analysis. Experiments without SMTs showed the evolution of unmixed cores. With a single SMT, mixing was superior even at fill levels >70% while peripheral unmixed zones persisted when overly long SMTs were used. The results of this study may help to derive optimal process conditions for RDCs operated at high fill levels.

Cohesion-driven mixing and segregation of dry granular media

Granular segregation is a common, yet still puzzling, phenomenon encountered in many natural and engineering processes. Here, we experimentally investigate the effect of particles cohesion on segregation in dry monodisperse and bidisperse systems using a rotating drum mixer. Chemical silanization, glass surface functionalization via a Silane coupling agent, is used to produce cohesive dry glass particles. The cohesive force between the particles is controlled by varying the reaction duration of the silanization process, and is measured using an in-house device specifically designed for this study. The effects of the cohesive force on flow and segregation are then explored and discussed. For monosized particulate systems, while cohesionless particles perfectly mix when tumbled, highly cohesive particles segregate. For bidisperse mixtures of particles, an adequate cohesion-tuning reduces segregation and enhances mixing. Based on these results, a simple scheme is proposed to describe the system’s mixing behaviour with important implications for the control of segregation or mixing in particulate industrial processes.

Direct and highly productive conversion of cyanobacteria Arthrospira platensis to ethanol with CaCl2 addition

BACKGROUND

The cyanobacterium Arthrospira platensis shows promise as a carbohydrate feedstock for biofuel production. The glycogen accumulated in A. platensis can be extracted by lysozyme-degrading the peptidoglycan layer of the bacterial cell walls. The extracted glycogen can be converted to ethanol through hydrolysis by amylolytic enzymes and fermentation by the yeast Saccharomyces cerevisiae. Thus, in the presence of lysozyme, a recombinant yeast expressing α-amylase and glucoamylase can convert A. platensis directly to ethanol, which would simplify the procedure for ethanol production. However, the ethanol titer and productivity in this process are lower than in ethanol production from cyanobacteria and green algae in previous reports.

RESULTS

To increase the ethanol titer, a high concentration of A. platensis biomass was employed as the carbon source for the ethanol production using a recombinant amylase-expressing yeast. The addition of lysozyme to the fermentation medium increased the ethanol titer, but not the ethanol productivity. The addition of CaCl₂ increased both the ethanol titer and productivity by causing the delamination of polysaccharide layer on the cell surface of A. platensis. In the presence of lysozyme and CaCl₂, ethanol titer, yield, and productivity improved to 48 g L^-1, 93% of theoretical yield, and 1.0 g L^-1 h^-1 from A. platensis, corresponding to 90 g L^-1 of glycogen.

CONCLUSIONS

We developed an ethanol conversion process using a recombinant amylase-expressing yeast from A. platensis with a high titer, yield, and productivity by adding both lysozyme and CaCl₂. The direct and highly productive conversion process from A. platensis via yeast fermentation could be applied to multiple industrial bulk chemicals.

Cross-flow deep fat frying and its effect on fry quality distribution and mobility

Conventional industrial frying systems are not optimised towards homogeneous product quality, which is partly related to poor oil distribution across the packed bed of fries. In this study we investigate an alternative frying system with an oil cross-flow from bottom to top through a packed bed of fries. Fluidization of rectangular fries during frying was characterised with a modified Ergun equation. Mixing was visualized by using two coloured layers of fries and quantified in terms of mixing entropy. Smaller fries mixed quickly during frying, while longer fries exhibited much less mixing, which was attributed to the higher minimum fluidization velocity and slower dehydration for longer fries. The cross-flow velocity was found an important parameter for the homogeneity of the moisture content of fries. Increased oil velocities positively affected moisture distribution due to a higher oil refresh rate. However, inducing fluidization caused the moisture distribution to become unpredictable due to bed instabilities.

Combined discrete particle and continuum model predicting solid-state fermentation in a drum fermentor

The development of mathematical models facilitates industrial (large-scale) application of solid-state fermentation (SSF). In this study, a two-phase model of a drum fermentor is developed that consists of a discrete particle model (solid phase) and a continuum model (gas phase). The continuum model describes the distribution of air in the bed injected via an aeration pipe. The discrete particle model describes the solid phase. In previous work, mixing during SSF was predicted with the discrete particle model, although mixing simulations were not carried out in the current work. Heat and mass transfer between the two phases and biomass growth were implemented in the two-phase model. Validation experiments were conducted in a 28-dm3 drum fermentor. In this fermentor, sufficient aeration was provided to control the temperatures near the optimum value for growth during the first 45-50 hours. Several simulations were also conducted for different fermentor scales. Forced aeration via a single pipe in the drum fermentors did not provide homogeneous cooling in the substrate bed. Due to large temperature gradients, biomass yield decreased severely with increasing size of the fermentor. Improvement of air distribution would be required to avoid the need for frequent mixing events, during which growth is hampered. From these results, it was concluded that the two-phase model developed is a powerful tool to investigate design and scale-up of aerated (mixed) SSF fermentors.

Numerical simulation and PEPT measurements of a 3D conical helical-blade mixer: a high potential solids mixer for solid-state fermentation

Helical-blade solids mixers have a large potential as bioreactors for solid-state fermentation (SSF). Fundamental knowledge of the flow and mixing behavior is required for robust operation of these mixers. In this study predictions of a discrete particle model were compared to experiments with colored wheat grain particles and positron emission particle tracking (PEPT) measurements. In the discrete particle model individual movements of particles were calculated from interaction forces. It was concluded that the predicted overall flow behavior matched well with the PEPT measurements. Differences between the model predictions and the experiments with wheat grains were found to be due to the assumption that substrate particles were spherical, which was in the model. Model simulations and experiments with spherical green peas confirmed this. The mixing in the helical-blade mixer could be attributed to (1) the transport of particles up and down in the interior of the mixer, and (2) dispersion or micro-mixing of particles in the top region of the mixer. It appeared that the mixing rate scaled linearly with the rotation rate of the blade, although the average particle velocity did not scale proportionally. It may be that the flow behavior changes as a function of the rotation rate (e.g., changing thickness of the top region); further study is required to confirm this. To increase the mixing performance of the mixer, a larger blade or a change in the shape of the mixer (larger top surface/volume ratio) is recommended.

Substrate aggregation due to aerial hyphae during discontinuously mixed solid-state fermentation with Aspergillus oryzae: experiments and modeling

Solid-state fermentation (SSF) is prone to process failure due to channeling caused by evaporative cooling and the formation of an interparticle mycelium network. Mixing is needed to break the mycelium network and to avoid such failure. This study presents the first attempt to quantify and predict the effect of mycelium bonds on particle mixing and vice versa. We developed a novel experimental set-up to measure the tensile strength of hyphal bonds in SSF: Aspergillus oryzae was cultivated between two wheat-dough disks and the tensile strength of the aerial mycelium was measured with a texture analyzer. Tensile strength at different incubation times was related to oxygen consumption, to allow a translation to a rotating drum with A. oryzae cultivated on wheat grain. We performed several discontinuously mixed solid-state fermentations in the drum fermentor and measured the number and size of grain-aggregates remaining after the first mixing action. We integrated data on mycelium tensile strength into a previously developed two-dimensional discrete-particle model that calculates forces acting on individual substrate particles and the resulting radial-particle movements. The discrete-particle model predicted the quantity and size of the aggregates remaining after mixing successfully. The results show that the first mixing event in SSF with A. oryzae is needed to break mycelium to avoid aggregate formation in the grain bed, and not to distribute water added to compensate for evaporation losses, or smooth out temperature gradients.

Heat and water transfer in a rotating drum containing solid substrate particles

In previous work we reported on the simulation of mixing behavior of a slowly rotating drum for solid-state fermentation (SSF) using a discrete particle model. In this investigation the discrete particle model is extended with heat and moisture transfer. Heat transfer is implemented in the model via interparticle contacts and the interparticle heat transfer coefficient is determined experimentally. The model is shown to accurately predict heat transfer and resulting temperature gradients in a mixed wheat grain bed. In addition to heat transfer, the addition and subsequent distribution of water in the substrate bed is also studied. The water is added to the bed via spray nozzles to overcome desiccation of the bed during evaporative cooling. The development of moisture profiles in the bed during spraying and mixing are studied experimentally with a water-soluble fluorescent tracer. Two processes that affect the water distribution are considered in the model: the intraparticle absorption process, and the interparticle transfer of free water. It is found that optimum distribution can be achieved when the free water present at the surface of the grains is quickly distributed in the bed, for example, by fast mixing. Alternatively, a short spraying period, followed by a period of mixing without water addition, can be applied. The discrete particle model developed is used successfully to examine the influence of process operation on the moisture distribution (e.g., fill level and rotation rate). It is concluded that the extended discrete particle model can be used as a powerful predictive tool to derive operating strategies and criteria for design and scale-up for mixed SSF and other processes with granular media.

The potential for establishment of axial temperature profiles during solid-state fermentation in rotating drum bioreactors

The mixing and heat transfer phenomena within rotating drum bioreactors (RDBs) used for solid-state fermentation processes are poorly studied. The potential for the establishment of axial temperature gradients within the substrate bed was explored using a heat transfer model. For growth of Aspergillus oryzae on wheat bran within a 24 L RDB with air at a superficial velocity of 0.0023 m s(-1) and 15% relative humidity, the model predicts an axial gradient between the air inlet and outlet of 2 degrees C during rapid growth, compared to experimental axial temperature gradients of between 1 and 4 degrees C. Undesirably high temperatures occur throughout the bed under these operating conditions, but the model predicts that good temperature control can be achieved using humid air (90% relative humidity) at superficial velocities of 1 m s(-1) for a 204 L RDB. For a 2200 L RDB, good temperature control is predicted with superficial velocities as low as 0.4 m s(-1) with the airflow being switched from 90% to 15% relative humidity whenever the temperature at the outlet end of the drum exceeds the optimal temperature for growth. This work suggests that significant axial temperature gradients can arise in those RDBs that lack provision for axial mixing. It is therefore advisable to use angled lifters within RDBs to promote axial mixing.

Three-dimensional simulation of grain mixing in three different rotating drum designs for solid-state fermentation

A previously published two-dimensional discrete particle simulation model for radial mixing behavior of various slowly rotating drums for solid-state fermentation (SSF) has been extended to a three-dimensional model that also predicts axial mixing. Radial and axial mixing characteristics were predicted for three different drum designs: (1) without baffles; (2) with straight baffles; and (3) with curved baffles. The axial mixing behavior was studied experimentally with video- and image-analysis techniques. In the drum without baffles and with curved baffles the predicted mixing behavior matched the observed behavior adequately. The predicted axial mixing behavior in the drum with straight baffles was predicted less accurately, and it appeared to be strongly dependent on particle rotation, which was in contrast to the other drum designs. In the drum with curved baffles complete mixing in the radial and axial direction was achieved much faster than in the other designs; that is, it was already achieved after three to four rotations. This drum design may therefore be very well suited to SSF. It is concluded that discrete particle simulations provide valuable detailed knowledge about particle transport processes, and this may help to understand and optimize related heat and mass transfer processes in SSF.