A Molecular Dynamics Study on Rotational Nanofluid and Its Application to Desalination

Abstract

In this work, we systematically study a rotational nanofluidic device for reverse osmosis (RO) desalination by using large scale molecular dynamics modeling and simulation. Moreover, we have compared Molecular Dynamics simulation with fluid mechanics modeling. We have found that the pressure generated by the centrifugal motion of nanofluids can counterbalance the osmosis pressure developed from the concentration gradient, and hence provide a driving force to filtrate fresh water from salt water. Molecular Dynamics modeling of two different types of designs are performed and compared. Results indicate that this novel nanofluidic device is not only able to alleviate the fouling problem significantly, but it is also capable of maintaining high membrane permeability and energy efficiency. The angular velocity of the nanofluids within the device is investigated, and the critical angular velocity needed for the fluids to overcome the osmotic pressure is derived. Meanwhile, a maximal angular velocity value is also identified to avoid Taylor-Couette instability. The MD simulation results agree well with continuum modeling results obtained from fluid hydrodynamics theory, which provides a theoretical foundation for scaling up the proposed rotational osmosis device. Successful fabrication of such rotational RO membrane centrifuge may potentially revolutionize the membrane desalination technology by providing a fundamental solution to the water resource problem.

Keywords: Graphene membrane; Molecular dynamics; Nano-porous materials; Reverse osmosis desalination; Rotational centrifuge.

Figure A1

Schematic illustration on how to scale-up a nano-porous rotating membrane to macroscale and its macroscale fabrication concept. (a) Multiscale porous membrane structure, (b) Rotating porous membrane, and (c) A rotator-generated centrifugal fluid motion of the feed solution (seawater in RO).

Figure 1

Proposed scaled up rotational nanofluidic device with nanoporous membrane wall. (a) Schematic illustration of an inner rotator generated swirling motion, (b) The coarse scale holes on the wall of the centrifuge, and (c) the fine scale pores in the graphene membrane patch covered on the coarse sale holes in the centrifuge wall.

Figure 2

Molecular structures of two types of models for MD calculations. (a) The top and bisection view of Model I, with light-blue tube represents nanoporous membrane and pink tube represents rotator. (b) The top and bisection view of Model II, with light-blue tube represents nanoporous membrane and dark-blue tube represents rotator.

Figure 3

Snapshots of Model II over six simulation times, in unit nano-second. Na${}^{+}$ and Cl${}^{-}$ ions are in blue and yellow color, other molecules are H${}_2$O with oxygen atom in red color.

Figure 4

(a) Water density profiles of model type II along the radial and longitudinal directions under $\omega =17.5$ rad/ns (Values along azimuthal direction are averaged). The trajectory of a typical Na${}^{+}$ ion (b) and Cl${}^{-}$ ion (c) which are located near the center at $t=0$ and move to the membrane wall at $t=2$ ns.

Figure 5

(a) Top-view of rotating water molecules (Oxygen in red and Hydrogen in white) with velocity vector (blue arrows) plotted on Oxygen. (b) 3D plot of velocity field of water molecules. Only velocity vectors (blue arrows) are shown. Azimuthal velocity $v_{\theta }$ along radial direction of Model I (c) and Model II (d) under different angular velocities.

Figure 6

(a) Averaged azimuthal velocities of fluid near $r=R_i$ as a function of angular velocity $\omega$ for Model I and Model II. The curve “Non-slip BC” means $v_{\theta }=R_i\omega$. (b) Lower and Upper limits of the centrifugal pressure $P_{\omega }$ derived from fluid dynamics and that obtained from MD calculations.

Figure 7

Taylor instability and vortex when $\omega =90$ rad/ns. (a) Two vortexes (sketched by black lines) are observed inside the container. The fluid is rendered as colored colloid for better 3D view. (b) Water density profile in radial-longitudinal plane. (c) Distribution of water density along radial direction. The Nano-Porous Tube (NPT) wall in the figure signifies the location of the CNT membrane wall and that its properties are held at constant over time.

Figure 8

(a) Volume of filtrated water as a function of simulation time. (b) Water flux as a function of angular velocity.

Figure 9

(a) Comparison of permeability of the current system with other RO materials. (b) Calculated energy efficiency as a function of simulation time, under $\omega =35$ rad/ns.