Twelve salts were tested for their ability to coagulate microalgae cells in cultures of Chlorella minutissima The final aim was to develop an easy and efficient approach for harvesting microalgae biomass in dense cultures Aluminum, ferric, and zinc salts coagulated C minutissima cultures, while optimum concentration was 075 and 05 g L−1 for sulfate and chloride salts, respectively Aluminum salts were most efficient, but caused some cell lysis, which may render this approach inappropriate in some cases Ferric and zinc salts were ranked second and third, respectively, according to their culture cell-coagulation efficiency Ferric salts caused a change in the color of the cells, mainly at concentrations higher than 1 g L−1 Zinc salts were less harmful for the microalgal cells, but an additional problem was observed with cell aggregates adhering to the walls of the glass test tubes Selection of the appropriate coagulant is related to the purpose of the coagulation process

Harvesting Chlorella minutissima using cell coagulants

Economically viable microalgal biodiesel production is unrealistic and unsustainable owing to expensive harvesting or dewatering techniques. Hence, immense and meticulous exploration of harvesting process is essential to identify knowledge leads by which suitable harvesting technique could be ascertained for lucrative biodiesel production. With this in view, this review aims to collate and highlight the spectrum of harvesting techniques applied to microalgae, i.e., conventional – modern, high cost- inexpensiveness, energy efficient- energy consuming process. At the outset, global energy outlook and demand had been critically addressed, and the scientific ways to tackle or satiate the fuel demand had also been highlighted in this reveiw. This review manuscript has thrown widespread light on the physical harvesting methods namely centrifugation, sedimentation, filtration, flotation and technical advantages thereof. Due to the energy-intensive and cost barrier of physical harvesting techniques, chemical methods entailing organic, inorganic, and electroflocculation have come to limelight and in this regard, microalgae used, floc recovery and the dose of flocculants have been compared and presented in detail. Further, state of the art harvesting techniques viz., bioflocculation by microalgae/bacteria, flocculation by pH adjustment, and magnetic nanocomposite based microalgal harvesting had been critically articulated. Besides discussing the several methods, this paper has summarized the key challenges in conventional and advanced harvesting techniques and also provided the scope thereof. Hence, the key suggestions and findings given in this manuscript would positively offer a well-defined roadmap in choosing foreseeable harvesting technology for cost-effective microalgal biofuel development.

A comprehensive review on harvesting of microalgae for biodiesel – Key challenges and future directions

We consider the structure of the stable boundary layer using the concept of local scaling. In this scaling approach turbulence variables, non-dimensionalized with measurements taken at the same height, can be expressed as a function of a single parameter z/Λ, where z is the height and Λ a local Obukhov length. One of the consequences is that locally scaled variables become constant above the surface layer. This behavior is illustrated with observations of the Richardson number. With local scaling as a closure hypothesis we then formulate a model of the stable boundary layer. Its solution for steady-state conditions is given. One result we obtain is the well-known Zilitinkevich equation for the boundary-layer height. A comparison of this equation with observations results in a reasonable agreement. Also we discuss some alternative expressions for the stable boundary-layer height and compare them with observations. Another result of our model is an explicit profile for the K-coefficient as a quadratic function of height. We discuss the consequences of this expression for the dispersion of a point source emission. We find that the time scale of diffusion in this case is about 5 hours.

Some Aspects of the Turbulent Stable Boundary Layer

Direct numerical simulations of stably stratified, turbulent channel flow at low to moderate Reynolds number have been performed using large computational boxes and considering a wide range of stratification levels. For weak stratification or high Reynolds number, the turbulence is affected by buoyancy in the core of the channel, but the near-wall region differs little from the neutral case. With strong stratification, large laminar patches appear in the near-wall region and turbulent momentum and buoyancy fluxes vanish in the core of the channel. With increasing stratification, the near-wall streaks remain essentially unmodified, while large-scale global modes are damped. In the central region, internal gravity waves are dominant. In addition, there is an intermediate outer layer where the dynamics of the turbulent structures is governed by local fluxes. In this region, energy spectra collapse when using local Obukhov scaling.

Turbulence modification by stable stratification in channel flow

Publisher Summary This chapter discusses the onset and development of thermal convection in fully developed shear flows. The subject of thermal convection in shear flows is rich in results that have both basic interest and practical importance in science and engineering. Although the field has matured greatly during the part 15 years, many topics still need study. For instance, the effects of channel sidewalls on both the initial instability and the instability of longitudinal rolls need to be clarified both theoretically and experimentally. More effort should be devoted to the use of controlled disturbances in studying the onset of instability due to the convective nature of the instability. Such studies might prove useful also in suggesting how turbulent convection can be promoted or postponed in shear flows. This chapter presents an in-depth account of the basic concepts and of some of the major results that have occurred during the past 15 years. It focuses primarily on simple basic states involving a Boussinesq fluid and fully developed forced flows, mainly of the Couette or Poiseuille type. The chapter also provides some references to similar phenomena in other basic flows, for example, when the basic flow itself is due to natural (or “free”) convection.

The Onset and Development of Thermal Convection in Fully Developed Shear Flows

The longitudinal vortex and its effects on the transport processes in combined free and forced laminar convection between horizontal and inclined parallel plates

Effects of crystal growth rate and heat and mass transfer on solute distribution

Stratification effects on transport processes in the atmospheric surface layer or in the wall region are investigated by a laboratory experiment. A fully developed stratified condition was attained in turbulent flow between horizontal parallel plates kept at different temperatures. The experiment was performed both for stable and unstable conditions and provided conclusive data on stratification effects. A local gradient model is proposed, based on the concept that stratification effects may be evaluated at the same non-dimensional distance from the wall z+ (= zu*w/vw u*, vw being friction velocity and kinematic viscosity, respectively, at the wall), the effects depending only on e (e z/L; L being the Monin-Obukhov length). A comparison of the shear functions with previous field and laboratory experiments shows excellent agreement. By use of this concept, the measured intensities of velocity and temperature fluctuations near the wall are also well correlated with e.

A laboratory experiment on momentum and heat transfer in the stratified surface layer

The solubilities of three sulfate salts (Na2SO4, K2SO4, MgSO4) and two chloride salts (NaCl, KCl) with a second salt in water were measured at 313 K. The ternary systems studied were water + NaCl + KCl, water + NaCl + Na2SO4, water + KCl + K2SO4, water + Na2SO4 + K2SO4, water + K2SO4 + MgSO4, and water + Na2SO4 + MgSO4. The solubilities of two salts in the ternary systems were obtained, and the data were used to obtain the eutectic concentrations of the ternary systems. The solubility data were also predicted by the electrolyte−NRTL model, in which parameters were obtained from the binary data. The qualitative effect of the second salt on the solubility of the first salt in water is discussed.

Keisuke Fukui

Papers

The longitudinal vortex and its effects on the transport processes in combined free and forced laminar convection between horizontal and inclined parallel plates

Effects of crystal growth rate and heat and mass transfer on solute distribution

A laboratory experiment on momentum and heat transfer in the stratified surface layer

Solubility of Two Salts Containing Sulfate and Chloride Ions in Water for Ternary Systems at 313 K

Solid-liquid phase transition of binary Lennard-Jones mixtures on molecular dynamics simulations