(3hr) Performance of a Forward Osmosis Mass Exchanger | AIChE

(3hr) Performance of a Forward Osmosis Mass Exchanger

Authors 

Saha, S. - Presenter, Indian Institute of Technology, Kharagpur
Mondal, S., Indian Institute of Technology Kharagpur
Research Interests

  • Forward Osmosis
  • Membrane filtration
  • Peclet Number
  • Recovery
  • Desalination

Abstract Text

The basic principle of the Forward Osmosis (FO) is the spontaneous movement of the solvent solution through a semipermeable membrane from the low concentration feed solution to the high concentration draw solution based on the osmotic pressure gradient emerged from the concentration difference. Unlike other pressure driven membrane separation processes like reverse osmosis, the FO system has an advantage of the solvent separation without any external pressure, thus energy consumption is less. Diminishing fresh drinking water is now a global problem due to population growth and rapid economic development. One of the possible solutions to this is the recycling of industrial and urban wastewater. FO is one of the most reliable membrane separation processes along with the microfiltration, ultrafiltration, nanofiltration, and reverse osmosis through which recovery of water is possible from wastewater. In the FO based wastewater treatment processes, generally, water as a solvent is separated from the low concentration feed stream (wastewater) to the concentrated saline stream (seawater) through highly selective membranes and the osmotic pressure difference across the interfaces of the membrane acts as the driving force. Over the past two decades, scientists found the reliability of the FO system in the sustainable treatment of industrial wastewater. FO based mass exchanger also attracts importance because of its potential applications in food processing, desalinating seawater, and power generation. There are at least two most important improvements remain in the design, research, and development of the FO application. They are: (i) optimum membrane morphology to minimize the concentration polarization; and (ii) module design and configuration. We have contributed mostly to the second area as well as given the idea of the concentration polarization across the membrane of the system which can help in performing research for the first area.

The water flux in a FO system is based on the concentration polarization (CP) across the membrane created by the concentration difference between the feed and draw solution near the membrane. The lower concentration polarization for the Forward Osmosis process provides the higher driving force to mass transfer through the membrane which leads higher recovery. The concentration polarization effect for higher Peclet Number provides extra resistance (because of the formation of the gel layer) to the mass transfer through the membrane which eventually decreases the concentration difference across the membrane leading to lower recovery. The mass exchanger systems for fluid transport are analogous to the heat transfer situation where heat exchangers are used. The modelling of the mass exchanger has been previously done by Mazlan et al. [1] where the area required for an FO membrane mass exchanger is calculated using the log-mean-osmotic-pressure-difference by making an analogy with the log-mean-temperature-difference (LMTD), which is widely accepted for heat exchangers. In the recent study by Mondal et al. [2], the membrane area of the FO system has been calculated by overall mass balance around the tubular section where the concentration polarisation across the membrane has been neglected and the salt concentration at the membrane surface has been assumed to be equal to the bulk concentration.

In this study, a numerical solution of the forward osmosis mass exchanger system has been introduced where we have considered the CP effect over the membrane surface by studying the boundary layer theory unlike the previous studies done by others. The emergence of the concentration gradient due to the selective mass transfer on the membrane surface creates the CP which increases the resistance against flux through the membrane. The concentration gradient near the membrane surface has been characterized by the boundary layer theory. The continuity, momentum, and species transport equations; along with the boundary condition representing the mass transfer across the membrane has been solved numerically. This solution obviates the need for the approximations based upon a log mean driving force for the mass transfer as well as the negligence of the concentration polarization effect on both the surfaces of the membrane by using overall mass balance around the tubular domain of the mass exchanger.

The present solution is based upon the detailed boundary layer analysis to calculate the estimated area, recovery, and the effectiveness of the mass exchanger. The numerical simulation of the convective and diffusive mass and momentum transport through the highly selective membrane from one domain to another has been done using a 2D axisymmetric non-dimensionalized model with Finite Element Method based COMSOL Multiphysics software V5.4. The numerical solution relates the operating and geometric parameters of the exchanger to the effective design size and input stream properties. We have shown in this paper that the formation of the boundary layer has created a significant CP across the membrane and the recovery of the mass exchanger system increases as the Peclet number of the system decreases (see Fig.1). It implies that the mass flux will be the highest through the membrane for Peclet number tends to zero. The thickness of the boundary layer for low Peclet number decreases the concentration gradient near the membrane surface which decreases the CP effect. It has also been shown the discrepancy between the estimates of areas using the analytical solution done by overall mass balance neglecting the concentration polarization effect and calculated assuming the log-mean-osmotic-pressure-difference and the calculated areas using the numerical solution, which shows that the formers will generate significant errors (see the Fig.2).

Finally, we conclude that unlike some recent publications [1,2] on the analytical area estimation of a FO based mass exchanger system where major theoretical assumptions have been considered, we have estimated the required membrane area by numerically solving the fundamental governing equations of continuity, momentum, and species transport. The present analysis has been shown that both approaches considered by Mazlan et al. [1] and Mondal et al. [2] either underpredicts or overpredicts the actual requirement. The overall mass balance approach deviates the actual estimates significantly for the operating conditions where the Peclet number is high. The high Peclet number generates a significant concentration polarisation effect which provides huge mass transfer resistance against the solvent transport through the selective membrane. This study will help researchers and industrialists to design large scale industrial wastewater treatment plants, high-efficiency seawater desalination plants that need accurate methods for estimating the membrane area requirements and the potential recovery for various combinations of salinity, flow rate ratios, and flow arrangements.

References:

[1] N.M. Mazlan, D. Peshev, A.G. Livingston, Energy consumption for desalination—a comparison of forward osmosis with reverse osmosis, and the potential for perfect membranes, Desalination 377 (2016) 138–151.

[2] Sourav Mondal, Robert W. Field, Jun Jie Wu, Novel approach for sizing forward osmosis membrane systems, Journal of Membrane Science, Volume 541 (2017) 321-328.