The role of osmotic agent in water flux enhancement during osmotic membrane distillation (OMD) for treatment of highly saline brines

Highlights

• The effect of osmotic agent on the performance in treatment of brine using OMD was investigated.

• Water flux of OMD for concentrating hypersaline brines was significantly higher than MD.

• Mathmatical models were applied to reveal the effect of temperature and concentration polarization on permeate flux.

• OMD is more cost and energy efficient compared to conventional MD processes.

Abstract

Brine concentration is of critical importance in achieving zero liquid discharge in modern industrial processes. The effect of osmotic agent (OA) on the system performance in the treatment of different concentrations of brine using osmotic membrane distillation (OMD) was investigated. Significant improvement in water flux for concentrating the hypersaline brine was achieved compared to conventional membrane distillation processes. By altering the hydrodynamic conditions and using different combinations of feed streams and osmotic agents, a mathematical approach was applied, for the first time, to reveal the contribution of temperature and concentration polarization on flux enhancement in treating highly saline brines. This improvement was found to be caused mainly by the temperature polarization on the OA side of membrane, with less contribution from the decrease in vapor pressure of the bulk of the OA. This work illustrates that OMD is a promising process for efficient concentration of highly saline streams such as those found in the chlor-alkali industry. During the concentration of the spent brine from the electrolyte cell (NaCl concentration from 180 g/L to 310 g/L), OMD was shown to be the more cost and energy efficient compared to conventional MD processes.

Introduction

Zero liquid discharge (ZLD) is regarded as a sustainable solution for mitigating the environmental impacts caused by the disposal of highly saline effluents from industrial processes [[1], [2], [3]]. Brine concentration is the critical step in ZLD systems where typically membrane separation processes such as reverse osmosis (RO) and electrodialysis (ED) are employed to recover pure water and to concentrate brine streams, respectively. These processes have drawn significant attention owing to their low cost, high efficiency and reliability. However, due to the increased osmotic pressure at high brine concentrations, the retentate from the most advanced RO systems (i.e., dish tubular reverse osmosis (DTRO)) can only reach a maximum salt concentration of ~100 g/L. For ED systems, the highest concentration possibly reached range from 150 g/L to 200 g/L, as further concentration is compromised by water permeation through the ion exchange membranes due to the significant osmotic pressure difference between the diluted and the concentrate streams [4,5].

Membrane distillation (MD) has been proved to be the only membrane process that has the ability to overcome this limitation, producing saturated salt solutions so that solid salts could be precipitated directly by crystallization [6]. MD is a thermally driven separation process, in which only vapor molecules are able to transport through the membrane under a vapor pressure difference across the membrane [7]. As this process can be operated at ambient temperatures, low grade thermal energy such as waste heat from power plants and renewable energy can be used [8]. It has been used for treatment of seawater reverse osmosis concentrate [9], recovery of lithium chloride from aqueous solutions [10], and zero liquid discharge desalination combined with crystallization [11]. Highly concentrated brines are also needed in some industrial processes, such as in the chlor-alkali plants where the depleted brine (or anolyte) generated from the membrane-electrolysis cell, commonly known as spent brine, needs to be re-concentrated before it is re-processed.

During the concentration process, water permeability across the MD membrane decreases with increasing feed concentration. The permeate flux during the concentration of potassium chloride (KCl), sodium chloride (NaCl) and magnesium chloride (MgCl₂) solutions was reported to decrease by 44.4%, 59.6% and 86.8%, respectively, as the salt concentration increased from 2.0 M to 4.0 M [12]. This can be explained by the reduction in water activity in the concentrated solution which decreases the driving force across the membrane at constant operating temperatures. On the other hand, if a solution with low water activity was pumped to the permeate side of the MD membrane to decrease its vapor pressure, water flux of the concentration process could be further improved. This is referred to as osmotic membrane distillation (OMD) as it combines direct contact membrane distillation (DCMD) and osmotic distillation (OD) into one single process [13]. As expected, OMD exhibits higher water flux compared to DCMD or OD alone, driven by both temperature and concentration gradients across the membrane.

In OMD, the water flux depends on the water vapor pressure difference between feed side and the permeate side of the membrane, and can be expressed in Eq. (1) below [14]. The permeate side is also referred to as the osmotic agent (OA) side in the literature.

$$J=K_m\times \left(P_{f,m}-P_{p,m}\right)$$

where J is water flux (L/m²·h, or LMH), K_m is the membrane mass transfer coefficient (L/m²·h·kPa) that can be related to membrane properties, and P_f.m and P_p,m are the water vapor pressure (kPa) at the feed and permeate side of the membrane surface, respectively. However, due to the occurrence of temperature and concentration polarization phenomena, it is difficult to measure and calculate these values at the membrane surface. Hence, the overall mass transfer coefficient (K) and water vapor pressures of the bulk feed and permeate streams can be used instead:

$$J=K\times \left(P_{f,b}-P_{p,b}\right)$$

The overall mass transfer coefficient K is expressed as [15] (Fig. 1):

$$K={\left(\frac{1}{K_f}+\frac{1}{K_m}+\frac{1}{K_p}\right)}^{-1}$$

where $\frac{1}{K_f},\frac{1}{K_m}$, and $\frac{1}{K_p}$ represent the resistance of the boundary layer of the feed side, the membrane, and the boundary layer of the permeate side, respectively.

The vapor pressure of an aqueous salt solution can be estimated by [12]:

$$P\left(T\right)=P^{\ast }\left(T\right)\bullet \alpha \left(T\right)$$

where α(T) is the water activity of the salt solution, P^∗(T) is the saturated vapor pressure of pure water at temperature T, which can be calculated bythe Antoine equation:

$$P^{\ast }\left(T\right)=\mathit{exp}\left(23.1964-\frac{3816.44}{\left(T-46.13\right)}\right)$$

where P^⁎ is in Pa and T is the temperature in K.

Temperature polarization is caused by the temperature difference between the bulk and the membrane surface on both feed and permeate sides, whereas concentration polarization occurs due to the accumulation of solutes adjacent to the feed-membrane interface and liquid water adjacent to the membrane-permeate interface [16]. Total polarization effect (PE) is the reduction in the driving force due to both concentration and temperature polarization effects on the feed and the osmotic agent side, which is given by [17]:

$$\mathrm{PE}={\mathit{PE}}_f+{\mathit{PE}}_p=\left({\alpha }_{f,b}\ast P_{f,b}^{\ast }-{\alpha }_{f,m}\ast P_{f,\mathrm{m}}^{\ast }\right)+\left({\alpha }_{p,m}\ast P_{p,m}^{\ast }-{\alpha }_{p,b}\ast P_{p,b}^{\ast }\right)$$

The improvement in separation efficiency using OMD in comparison to direct osmotic distillation (OD) for the concentration of olive mill wastewaters was reported by E-labbassi et al. [13]. Babu et al. [17] also studied systematically the effect of osmotic solution (CaCl₂) concentration, cross flow velocity and feed concentration on the polarization effect and the permeate flux during OMD of pineapple juice. To date, the influence of the type of osmotic solution on system performance in brine concentration using OMD has not been studied and the associated polarization effects has not been fully elucidated. Therefore, osmotic agents (i.e., NaCl, K₂CO₃, and MgCl₂) of different physical properties are used in this work, to explore their potential in enhancing water flux and salt rejection in the applications of OMD in concentrating feed streams of different salt concentrations. The role of temperature and concentration polarization in these processes is also examined by increasing the cross flow velocity. The minimal thermal energy requirement for brine concentration by OMD with different osmotic agent concentrations is also compared with conventional MD processes.