Elsevier

Chemical Engineering Journal

Volume 250, 15 August 2014, Pages 230-239
Chemical Engineering Journal

Experimental and numerical investigation of hydrodynamics in raceway reactors used for algaculture

https://doi.org/10.1016/j.cej.2014.03.027Get rights and content

Highlights

  • The hydrodynamics in a raceway reactor is studied experimentally and numerically.

  • Effects of the paddlewheel speed and geometry are investigated.

  • Simulations successfully predict the flow rate and the impeller power consumption.

  • Mixing rates are related to the flow low-frequency unsteadiness.

  • Even at moderate intensities, wind has a crucial impact on the hydrodynamics.

Abstract

Raceways are nowadays the most used large-scale reactors for microalgae culture. This paper focuses on the hydrodynamics in such a reactor, and emphasizes on the effects of the paddlewheel geometry (two impeller configurations are tested). The global hydrodynamics behavior of the raceway (flow velocity, mixing time) is characterized experimentally by the tracer pulse injection method, and local velocity measurements are acquired by Pulsed Ultrasonic Doppler Velocimetry. Finally, the flow is modeled by using the sliding-mesh CFD technique, a method overcoming many limitations of the simulation approaches used in the literature. CFD simulations successfully estimate the flow rate in the reactor and the power consumption of the paddlewheel. It is shown that the mixing efficiency in the raceway reactor is closely related to the low-frequency flow unsteadiness that arises from the periodic motion of the blades. Concerning local velocities, CFD and experimental data are in good agreement at some positions in the reactor, but a significant disagreement is observed at some other locations. Taking into account the wind presence in the simulations reduces the discrepancy between the experimental and numerical results, showing that, even with a moderate intensity, wind has an important effect on the hydrodynamics in the reactor.

Introduction

Nowadays, growing concerns about global warming and energy shortage motivate the society to seek renewable and carbon neutral fuels as alternatives to fossil fuels. Eukaryotic microalgae and cyanobacteria (to which we will refer collectively by microalgae) are a potential source of various biofuels [1], [2] by virtue of their fast growth rate, and their oil yields, substantially higher then terrestrial oil crops, even exceeding 80% of the dry biomass weight under certain stress conditions [3]. Cultivation of microalgae in wastewater treatment reactors is even interesting from economical and synergic perspectives [4], [5]. Today cultured microalgae are used for pharmaceuticals, nutraceuticals, cosmetics, aquaculture and production of high value molecules (pigments, stable isotope biochemicals…) [6], [7].

Microalgae culture systems can be broadly classified into closed systems where the culture is enclosed into a translucent vessel, and open-air systems where the culture medium is directly exposed to the environment. Open-air cultures are grown outdoors and utilize sunlight. This is also the case for some closed systems, while others – (referred as photobioreactors) – are operated indoors with artificial illumination. Photobioreactors allow higher productivities [1] since the physicochemical environment of the culture is well controlled, light intensity and cycles optimized, and risks of invasion by competing microorganisms minimized. On the other hand, open-air systems are susceptible to location-related variables as weather conditions, and the daily and seasonal variations in light levels and temperature. However, most commercial systems used today are outdoor open-air reactors [7], [8]. In fact, photobioreactors are expensive to purchase, maintain and operate, difficult to scale up, and suffer from several drawbacks as overheating, bio-fouling and toxic oxygen accumulation, while open-air systems are simpler to design, cheap and easy to build and operate. Among open culture bioreactors, raceway ponds are by far the most widely used nowadays [9], [10].

A raceway is a pond consisting of long channels arranged in a racetrack closed loop, in which the algae growth medium is cycled continuously around the central dividing walls by means of a slow-moving paddlewheel (Fig. 1, Fig. 2). The pond is kept shallow, typically 15–40 cm, to ensure an adequate penetration of sunlight in the liquid.

Liquid in the raceway is mechanically circulated at about 0.15–0.4 m/s, which ensures mixing of the culture medium thanks to the turbulence of the flow. Although various devices have been proposed to circulate and mix the culture medium [9], [11], [12], paddlewheels remain by far the mostly used. In fact, they match well the pumping requirements of the ponds as they are high flow rate, low head devices [13]. Moreover, they are mechanically simple and require little maintenance, and their gentle mixing minimizes damage to flocculated or fragile algae. To minimize backflow, the paddlewheel may sit in an invagination in the pond floor, keeping a small clearance between the blades tip and the bottom of the channel: the paddlewheel will then behave more as a positive displacement pump [13].

Mixing the culture medium is necessary to avoid gradients in nutrient concentration, temperature and pH. It also prevents microalgae sedimentation, increases liquid-to-cell mass transfer rates, and enhances algal photosynthetic efficiency. In fact, mixing continuously moves the algal cells between the well-lit zone (close to the free interface) and the dark zone (bottom of the pond), what reduces photolimitation and photoinhibition [1]. In the raceway straight sections, there are no features that disturb the flow, and therefore, despite turbulent dispersion, mixing is relatively poor as shown experimentally by Mendoza et al. [10]. However, near the paddlewheel and in the bends, dispersion rates are important [10], [11]. Indeed, secondary flows – of Prandtl’s first kind – develop in the bends, what enhances macro-mixing.

Thanks to mixing, raceways productivity can be up to ten-fold higher than that of unmixed ponds [6], [14]. However, mixing is of great significance in terms of input energy costs. Chisti [15] estimated that 28% of the global energy input to the algal oil production process is for microalgae cultivation (so mainly for liquid circulation). According to Neenan et al. [16], mixing accounts for more than 69% of the utilities cost. Hence, reducing head losses in the raceway is of primary importance in order to make the industry competitive, particularly if the culture is dedicated to biofuel production. Thus, one of the greatest challenges in algaculture is to adequately mix the culture medium with minimum power consumption.

The raceway design must address in particular the problem of “dead zones” (separated flow) that develop near the middle wall downstream of bends, since they increase energy dissipation and reduce the pond capacity. Moreover, the resulting non-uniform velocity field leads to uneven cells residence time in the reactor, which is harmful to the pond productivity, especially if the raceway operates in continuous mode. Different strategies have been proposed to minimize the dead zones extent, among which installing flow deflectors, rectifiers or guide vanes, and modifying the island or the bends design [13], [17], [18]. Reducing the areas of stagnation straightens the flow, what lowers head losses in the reactor [10].

In the context of improving the hydrodynamics in bioreactors, CFD (Computational Fluid Dynamics) is a powerful low-cost tool that has already proven its efficiency with respect to photobioreactors design (see [19] for a review on the topic). In fact, CFD models the spatio-temporal hydrodynamics variations, and hence, it allows characterizing/estimating various key variables that are practically inaccessible to experimental measurements, as local shear stresses (that above a certain limit lead to shear-induced injury of algal cells) or cells individual history. For example, Perner–Nochta and Posten [20] simulated microalgae trajectories in a tubular photobioreactor to obtain light intensity fluctuations as experienced by individual cells. Pruvost et al. [21] designed an innovative reactor with CFD support. Despite these successes, it is only recently that CFD has been applied to raceway ponds, with the works of Hadiyanto et al. [17] and Liffman et al. [18]. These works focused on the evaluation of different raceway geometries in terms of flow uniformity and head losses in the reactor (however, none of the numerical results was confronted to experimental data). For simplification purposes, in both papers, the free surface was considered flat (with a slip boundary condition), and the paddlewheel was not included in the numerical domain. Liffman et al. [18] modeled the propulsive thrust effect of the paddlewheel by adding a body force term in the Reynolds equations. This approach had already been used to simulate the flow in aquaculture ponds [22], [23]. A different modeling strategy was used by Hadiyanto et al. [17]. The section of the channel surrounding the paddlewheel was removed from the numerical domain, and an inlet (downstream of the wheel position) and an outlet (upstream of the wheel position) boundary conditions were used to generate the flow. These two modeling approaches are interesting, since they permit low time-consuming calculations, allowing therefore a rapid evaluation of the performance of various raceway designs. However, both approaches have some major drawbacks, as they cannot capture several aspects of the hydrodynamics in the raceway:

  • They cannot take into account the paddlewheel geometry, a parameter that however has a crucial effect on the mixing performances in the reactor. Thus, both methods only consider the effects of the pond geometry and the liquid flow rate, and cannot be used to study the impact of the impeller or to improve its design.

  • They require experimental measurements to be able to reproduce given experimental conditions: the flow rate for the Liffman et al. [18] approach, and the paddlewheel mechanical power for the Hadiyanto et al. [17] method (so the additional term in the Reynolds equations can be calculated).

  • The Liffman et al. [18] approach fails to reproduce streamlines straightening immediately downstream of the paddlewheel, while the Hadiyanto et al. [17] method imposes a uniform flow at the inlet plane located downstream of the paddlewheel.

  • In these two studies, the steady-state equations were solved, and thus, the flow unsteadiness related to the periodic motion of the blades could not be accounted for. As it will be shown later, these low-frequency waves have a crucial impact on the mixing rate in the reactor.

  • The Liffman et al. [18] approach does not model reliably the paddlewheel effect. According to their method, the flow rate in the pond will be determined by the viscous dissipation (that must balance the paddlewheel power), while in reality, as the paddlewheel behaves as a positive displacement pump, the flow rate is only slightly affected by the viscous dissipation (viscous dissipation however determines the energy demand. The flow rate depends mainly on the geometry of the pond and the paddlewheel). In other words, this method imposes the paddlewheel power consumption, while in reality, it is a result of the paddlewheel speed set by the operator.

This paper tries to overcome these limitations through the use of a more general modeling approach. First, the hydrodynamics in a semi-industrial sized raceway pond is investigated experimentally. Two paddlewheels geometries are tested, which is novel to the authors’ knowledge. The tracer pulse injection method is used to determine the global hydrodynamics characteristics in the pond (i.e. the residence time and the degree of mixing) for different operating conditions. Results demonstrate that the low-frequency unsteadiness arising from the periodic blades movement has a major impact on the mixing process in the reactor. Local velocity measurements are then acquired at different locations in the pond, using the pulsed Doppler ultrasonic technique. Detailed local velocity measurements in a raceway have never been reported so far in the literature (although Chiaramonti et al. [11] conducted microacoustic Doppler velocimeter measurements, they only reported qualitative results in their paper). Then, the flow in the raceway is predicted through a numerical simulation. The paddlewheel rotation, the moving free surface as well as wind effects are explicitly taken into account. This is achieved through the combination of sliding-mesh and VOF (Volume of Fluid) techniques. Numerical results under various operating conditions are then confronted to experimental measurements. Power consumption, circulation time and mean velocities are accurately predicted through the simulation; local velocity profiles are more difficult to reproduce numerically. Taking into account the wind effect reduces this discrepancy between the experimental and numerical results at some measurements planes.

Section snippets

The raceway pond

The raceway investigated in this study is a semi-industrial scale reactor operated outdoors, and located at Narbonne, in the south of France. The region climate is characterized by high sunlight intensities and powerful winds. The raceway main dimensions are shown in Fig. 1. The algal culture depth was set to 40 cm during all the experiments. The pond is plastic lined to reduce head losses and eliminate water loss by percolation. The flow is circulated by a 1.75 m wide metallic paddlewheel (Fig. 2

Tracer experiments

Fig. 3 shows a comparison between experimental data (non-aligned blades configuration, paddlewheel speed of 10 rpm) and the Voncken et al. [24] model prediction. A good agreement was observed between model and tracer concentration for all the experimental conditions tested. By using the calibration parameters of the model, it is possible to extract the Peclet number (Pe) and the circulation time (Tc) for each conditions. The liquid bulk velocity and the dispersion coefficient versus the

Conclusion

The hydrodynamics in the raceway is critical to obtain high microalgae productivity, and CFD is a powerful tool of assistance in the reactor design. This paper focused on a numerical and experimental investigation of the hydrodynamics in such a pond. Two paddlewheel configurations have been tested in the present study.

Results demonstrate that the blades arrangement do not influence the generated flow rate, but however, has a crucial impact on the mixing rate. Contrary to the CFD approaches

Acknowledgment

The authors are grateful to OSEO FUI Salinalgue for its financal support.

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