1. Introduction
Many countries worldwide suffer from freshwater shortages due to vast population growth and lack of natural water resources. Reports have indicated an increase of 2% in freshwater demand with almost a doubling of population growth rates [
1]. Therefore, clean water is a critical international problem, which should be addressed comprehensively in more studies. Hence, desalination systems as an alternative water resource have been developed extensively to satisfy the current demand and overcome the shortage.
Membrane distillation (MD) is a thermal based technology, which can be used for the desalination of saline feed. Based on the vapor pressure gradient created across the hydrophobic membrane, clean water can be extracted [
2]. A temperature difference across a hydrophobic membrane in MD systems creates partial vapor difference as a driving force. It leads to water molecule evaporation at the hot side, transporting across the membrane in the vapor phase, and finally condensing at the cold side [
3]. Membrane distillation is a non-isothermal process, and operation can be conducted at atmospheric pressure and produce highly pure water [
4]. Hence, MD can be used at industrial sites where low heat and high salinity water are available. Consequently, membrane distillation technology is considered in the present study.
Various types of membrane distillation have been studied recently. Based on the method of withdrawing vapor from the hot side of the membrane, MD can be classified as direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD), and vacuum membrane distillation (VMD) [
5]. Among these configurations, the DCMD configuration has attracted a lot of attention, considering its simple design and higher pure water production rate [
6]. In contrast, the VMD system has the benefit of higher energy efficiency compared to the DCMD design. It is noteworthy that although the energy efficiency of DCMD is lower than that of VMD systems, the ability to achieve higher permeate flux results in ignoring the lack of sufficient energy efficiency. Therefore, the present study has focused on the DCMD method [
7,
8].
Different studies have investigated various processing parameters that affect the performance of the DCMD system [
9,
10]. Manawi et al. [
11] have reported the feed flow rate and temperature effect on permeate flux of a single spacer and single-stage DCMD process with experimental and analytical studies. It was concluded that increases of both feed flow rate and temperature provide higher permeate flux. However, the effect of salinity and the number of stages were not addressed in their study. Khalifa et al. [
12] considered the effect of feed temperature on permeate flux with single spacer and single-stage DCMD. Both experimental and analytical models showed that an increase in feed temperature leads to an increase of permeate flux in single spacer and single DCMD. Additionally, the effect of salinity, feed flow rate, and the number of stages was not considered in their study. Therefore, a comprehensive study that considers the influence of all inlet parameters on permeate flux in bispacer and multi-stage DCMD is needed.
The spacer in DCMD has been used to support the membrane as well as increase the feed stream turbulence. As an increase in turbulence leads to permeate flux enhancement, the effect of the spacer on DCMD performance is unavoidable. Hence, some studies address the influence of spacers on DCMD operation. Zhang et al. [
13] presented an analytical model, which considers the characteristics of the spacer in single-stage DCMD. It is worth noting that the local Nusselt number was used in the presented spacer model, which was calculated from the turbulent flow. It was revealed that the error between the presented model and experimental was less than 10% in single spacer DCMD with respect to feed temperature. Moreover, the effect of flow rate and salinity on error was not reported in their study. The experimental investigation of heat transfer on single stage and single spacer DCMD flow channels with and without a spacer was studied by Linh Ve et al. [
14]. Different kinds of spacers were proposed considering materials including plastic, stainless steel, fiberglass, and aluminum with woven and non-woven structures. The analytical model was conducted for the single non-woven plastic spacer. Although the proposed model was beneficial for predicting the single spacer DCMD, the effect of a bispacer was not considered. Further study was reported to analyze the effect of feed inlet temperature and feed concentration on mass transfer in single spacer and single-stage DCMD for proposed spacers [
15]. It was concluded that mass transfer characteristics were affected by polymer-based spacers significantly. Although the effect of the spacer and its material and geometry were studied previously, using bispacer multi-staged DCMD was not addressed.
DCMD technology can be used both in single and multi-stage configurations called MDCMD. It has been shown that an increase in membrane surface area leads to an increase in thermal efficiency at a constant flow rate [
16]. Hence, as none of the previous studies addressed the effect of bispacer MDCMD, it is considered in the present study.
The purposes of spacer usage in DCMD systems include (1) increasing mechanical stability, and (2) increasing turbulence. Hence, two spacers can be proposed in DCMD to emphasize each purpose separately. Here, using a combination of two spacers with different geometrical properties, an innovative bispacer multi-staged DCMD module was designed. Spacer 1 plays a significant role in the mechanical strength of the membrane in MDCMD, while spacer 2 was used to increase the turbulence of hot side streams. Mechanical stability improvement and turbulence enhancement of hot side streams facilitate the higher pressure and flowrate working operation. Furthermore, the temperature difference reduction between bulk fluid and membrane surface by increasing turbulence reduces the temperature polarization effect. Therefore, the permeate flux and thermal efficiency are increased compared to conventional MDCMD systems.
The MDCMD module under countercurrent flow is presented to study the effect of the proposed bispacer configuration. This process is applicable for both large-scale and footprint desalination industrial sites. Thus, the objective of the present study is to investigate the effect of main feed parameters such as feed temperature, flow rate, salinity, and the number of stages on permeate flux, thermal efficiency and GOR of bispacer MDCMD analytical models in conditions with and without spacers.
In the present study, the analytical approach is presented in
Section 2. Heat transfers of flow through three layers, including feed stream to the membrane boundary layer, the membrane, and membrane surface to the permeate stream were considered using conservation of energy correlations. The nonlinear model was solved using the iterative numerical method by MATLAB. The mass transfer model is presented considering the effect of bispacer properties. A counterflow MDCMD setup is presented in
Section 3 to investigate the performance of the presented bispacer. The effect of feed temperature, flow rate, salinity, and the number of stages on the presented bispacer MDCMD are studied in
Section 4. The efficiency of the proposed MDCMD is estimated using thermal efficiency, gained output ratio (GOR), temperature polarization coefficient, (TPC), and specific thermal energy consumption (STEC) parameters.
3. Materials and Methods
Low water production is one of the main challenges of DCMD process. To enhance the system performance, an innovative design of the DCMD module was developed. The modular or multi-staged design makes it possible to increase the heat flux by putting the plates together. Therefore, multi-staged direct contact membrane distillation (MDCMD) was considered in this study. The schematic of MDCMD of the present study is shown in
Figure 2A. Noteworthily, the parallel (single-effect) and counter-current flow design membrane modules were considered in the present study. The fluid flow schematic of this consideration is shown in
Figure 2B [
30].
As demonstrated in
Figure 2A, different size spacers were put together. Since the spacers have different mesh sizes and filament diameters, implementing bispacer design in the MDCMD system improves mechanical stability, which assists in obtaining better working operation conditions such as higher feed flowrate.
As shown in
Figure 2B, due to the counter-current design of the MDCMD system, the hot and cold side inlets were in opposite directions. The hot feed inlet stream and cold permeate stream enter through the down-side inlet and top-side inlet, respectively. The hot and cold streams flow inside the module, in parallel, through every other one by using an O-ring gasket, which seals the opposite inlet streams, as shown in
Figure 2C. It has been shown in previous studies that this flow configuration effectively uses the temperature driving force and demonstrates higher permeate flux [
16]. The details of the membrane and membrane module are presented in the following.
3.1. Membrane and Membrane Module
The design and construction of the membrane module was performed in Iran (Taha Ghaleb Toos Company, Mashhad, Iran), as shown in
Figure 3. Tests were conducted using a commercial poly-tetra-fluoroethylene (PTFE) membrane with 190.19 cm
2 effective area for one-staged and 950.95 cm
2 for five-staged by Membrane Solution (80% average porosity, 180-μm thickness, 0.22-μm average pore size, and approximate 100° contact angle). The membrane was inserted between two symmetrical polyethylene (HD-PE) plates. Hence, two channels of 2 mm gap at both sides were created, which were sealed by silicone and rubber cords, as shown in parts A and B of
Figure 3. The schematic and dimension of assembled plates are shown in
Figure 3C.
The membrane is supported by a plastic net spacer. In addition, the net spacer promotes turbulence in the feed and permeate channels. In the present study, two kinds of spacers with large (spacer 1) and small (spacer 2) network layouts were used, as shown in parts A and B of
Figure 4. Spacer 1 was placed in the channel plates to prevent excessive pressure drop in the process while creating turbulence in the channels. Spacer 2 was in contact with the membrane to provide good physical strength for the membrane. In addition, this spacer increases the Reynolds number and makes the boundary smaller; hence, the mean flux will be increased. Further details of both presented spacers are provided in
Table 3.
3.2. Experimental Procedure and Set Up
The experiments were conducted in the IWET laboratory at the Center of Innovative Technology (CIT) center. Considering the experimental setup of
Figure 5, the main module was responsible for mass transfer and wastewater treatment. The electrical heating element (3 KW) was used to provide the desired temperature of feed solution. Pure water was cooled by an air chiller and maintained at a temperature of 302 ± 0.5 K. The temperature and pressure of the input and output flows of the module were measured by temperature and pressure sensors. The flowmeters were used to maintain the required flow rate of both hot and cold streams. A conductivity/TDS sensor (EZDO 7200 with an accuracy of ±2 ppm; GONDO electronic, Taipei City, Taiwan) was used to measure the conductivity of the feed and permeate solutions at the outlet of the module to detect the leaks or pore wetting of the membrane. The pumps were capable of applying pressure up to 2 bar. Two storage tanks with a level meter were used to store feed and permeate. A make-up water tank was used to keep the salt concentration constant in the hot tank. The details of experimental setup is provided in
Table 4.
4. Results and Discussion
A comprehensive experimental and analytical study on the effective feed side variables of the bispacer MDCMD system were considered in the current investigation. The feed side is always more effective than the permeate stream on the permeate flux, because it is the source of vaporization and controls the permeation process. In this section, the feed temperature (Tb,f) was 313.15 K to 343.15 K, and the influence of different flow rates (125 to 435 L/h) was examined. The feed salinity was varied 0.5 to 3.5%. Additionally, the effect of stages on MDCMD performance was investigated by considering 1 to 5 stages. Note that all experiments were repeated two times, and the mean of results was considered in the study. The duration of the conducted experiment in each test was 90 min to ensure the steady-state condition was obtained.
4.1. Effect of Feed Temperature
The feed temperature’s effect on the permeate flux was investigated experimentally and analytically (without and with spacers conditions), as shown in
Figure 6. Other parameters were kept constant, including a flow rate of 200 L/h, salinity of 1.3%, and permeate temperature of 302.15 K. It should be noted that the salt removal percentage in all experiments was more than 99.99%, which shows the high capability of the current MDCMD system in removing salt from saline effluents.
Considering
Figure 6, as the feed temperature increases, the mean permeate flux will increase. The permeate flux increases exponentially, which is in agreement with the Antonine Equation (19). The increase in the permeate flux with respect to feeding temperature is due to two phenomena. Firstly, an increase in temperature leads to an increase in partial vapor pressure, which is a driving force of the mass transfer mechanism. Therefore, the permeate flux is increased. Secondly, increasing the temperature enhances the kinetic energy of the molecules. As the kinetic energy increases, the liquid molecules convert to the vapor phase more quickly. Hence, the amount of permeate flux is increased by the temperature increment.
Considering
Figure 6, the experimental permeate flux (Exp.) is higher than the analytical without spacer (B). To overcome this shortcoming, the analytical study considering the spacer (A) is presented in
Figure 6, which is near to experimental results, which indicates the reduction of temperature difference between the membrane surface and the bulk. Therefore, more permeate flux can be provided. A previous study confirmed this finding [
31].
4.2. Effect of Feed Flow Rate
The experimental and analytical results of permeate flux versus different feed flow rate are provided in
Figure 7. The other conditions, including salinity, feed temperature, and permeate temperature, were 1.3%, 333.15 K, and 302.15 K, respectively, and remained constant during the process.
As shown in
Figure 7, the increase of the feed flow rate leads to an increase of the permeate flux. Two phenomena are involved in permeate flux enhancement. Firstly, the Reynolds number of the feed stream increases with an increase of the feed flow rate. Hence, the turbulence of flow increases and affects temperature and concentration polarization. Secondly, increasing the feed flow rate leads to an increase of the average bulk temperature of the hot side, which provides a higher membrane temperature of the feed side. Therefore, more permeate flux was obtained by feed flow rate increment, because of both increasing flow turbulence and increasing feed temperature.
4.3. Effect of Feed Salinity
One of the feed parameters that affects permeate flux is salinity. To study this parameter, seven levels of salinity were tested, namely 0.5 to 3.5%. The salt solution was prepared with NaCl ≥99% purity. Feed and permeate flow rates were 200 L/h and 100 L/h, respectively. The feed and permeate bulks temperatures were considered at 333.15 K and 302.15 K, respectively. The mean permeate flux versus different salinity is shown in
Figure 8.
As shown in
Figure 8, an increase of feed salinity or concentration leads to a decrease of the permeate flux. Two main phenomena cause this result as follows: (1) The presence of NaCl molecules in water causes the formation of hydrogen bonds with water molecules, which increases the boiling point. Hence, more kinetic energy is needed to overcome the molecular bonds. Therefore, to convert the liquid phase into the vapor phase, the input energy should be increased. (2) The presence of salt ions in the aqueous phase creates additional resistance to the passage of vapor molecules through the membrane. Therefore, high feed salinity provides low mean flux.
4.4. Effect of Number of Stages
The operability of the current MDCMD was investigated in the previous sections. The effect of the number of stages is investigated in the current section. In order to provide the same conditions in all tests, the feed and permeate velocities were increased to provide the same Reynolds number in the flow channels of all samples. The permeate temperatures and salt concentrations constantly remained 302.15 K and 1.3% in all experiments, respectively. The results of the experimental for five stages with respect to feed temperature are provided in
Figure 9.
Considering
Figure 9, as the stages of MDCMD increase, daily water production will increase. By increasing the number of mass transfer plates, the treatment capacity increases linearly (maintaining the conditions constant in the channels). This result, which is in agreement with single spacer MDCMD [
19], indicates the ability to increase the capacity of the MD method in the designed module.
4.5. The Efficiency
To investigate the operability and sustainability of the presented bispacer MDCMD, different output efficiency parameters, including thermal efficiency (η), gained output ratio (GOR), and temperature polarization coefficient (TPC), were studied in this section.
Considering salinity of 1.3%, a permeate temperature of 302.15 K and flow rate of 200 L/h, the thermal efficiency or
η,
GOR, and
TPC of the presented MDCMD with respect to variation of feed temperature are provided in
Figure 10.
As shown in
Figure 10A, it was revealed that with the feed temperature increment, an increase in the thermal efficiency of the bispacer MDCMD was obtained. By increasing the feed temperature, both water evaporation inside pores and as a result permeate flux and conduction heat losses increase. However, the increasing slope of evaporation heat transfer is significantly higher than conduction heat transfer. The portion of the evaporation and the conduction heat transfer contribution to thermal efficiency is shown in
Figure 10B and Equation (22). By increasing the feed temperature, the magnitude of the numerator has dramatically enhanced compared to conduction heat transfer, and as a result, the thermal efficiency increases. Therefore, the maximum thermal efficiency of 79% is achieved with the feed temperature of 338.15 K.
In addition, an increase of feed temperature leads to an increase of the GOR. When feed temperature increases, the partial pressure difference increases. Hence, a larger mass flux will be provided. Considering Equation (23), the mass flux is proportional to GOR. Thus, with a feed temperature of 338.15 K, the maximum GOR is provided.
Furthermore, the increase of feed temperature results in a decrease of temperature polarization coefficient (
TPC), as shown in
Figure 10. The feed temperature or bulk feed temperature is related to
TPC inversely based on Equation (24). As a result, the maximum
TPC of 80% is achieved when the feed temperature is 313.15 K.