Performance Investigation of O-Ring Vacuum Membrane Distillation Module for Water Desalination
Performance Investigation of O-Ring Vacuum Membrane Distillation Module for Water Desalination
Adnan Alhathal Alanezi,1 H. Abdallah,2 E. El-Zanati,2 Adnan Ahmad,3 and Adel O. Sharif4
1Department of Chemical Engineering Technology, College of Technological Studies, The Public Authority for Applied Education and Training (PAAET), P.O. Box 117, 44010 Sabah Al-Salem, Kuwait
2Chemical Engineering and Pilot Plant Department, Engineering Research Division, National Research Centre, Dokki, Giza, Egypt
3Department of Polymer Engineering and Technology, University of the Punjab, Quaid-e-Azam Campus, P.O. Box 54590, Lahore, Pakistan
4Qatar Environment and Energy Research Institute, HBKU, Qatar Foundation, Doha, Qatar
Received 14 July 2016; Revised 9 September 2016; Accepted 15 September 2016
Academic Editor: Rosa Maria Gomez Espinosa
Copyright © 2016 Adnan Alhathal Alanezi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
A new O-ring flat sheet membrane module design was used to investigate the performance of Vacuum Membrane Distillation (VMD) for water desalination using two commercial polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) flat sheet hydrophobic membranes. The design of the membrane module proved its applicability for achieving a high heat transfer coefficient of the order of 103 (W/m2 K) and a high Reynolds number (Re). VMD experiments were conducted to measure the heat and mass transfer coefficients within the membrane module. The effects of the process parameters, such as the feed temperature, feed flow rate, vacuum degree, and feed concentration, on the permeate flux have been investigated. The feed temperature, feed flow rate, and vacuum degree play an important role in enhancing the performance of the VMD process; therefore, optimizing all of these parameters is the best way to achieve a high permeate flux. The PTFE membrane showed better performance than the PVDF membrane in VMD desalination. The obtained water flux is relatively high compared to that reported in the literature, reaching 43.8 and 52.6 (kg/m2 h) for PVDF and PTFE, respectively. The salt rejection of NaCl was higher than 99% for both membranes.
1. Introduction
Fresh water shortages and water scarcity are major global issues, especially in the arid and semiarid regions of the world, where fresh water can be obtained through different techniques, such as the desalination of seawater. Desalination is one of the earliest methods known to man of obtaining salt-free water. In nature, it forms the source of the hydrological cycle. Desalination usually refers to the process of reducing the concentration of salt and dissolved substances in seawater or brackish water to make it palatable and suitable for consumption. In addition to salt removal, some desalination techniques also remove suspended material, organic matter, bacteria, and viruses [1–5]. Desalination has great potential for supplying fresh water for the 2.4 billion people living in coastal areas, which is equivalent to 39% of the world population. As a result, over the past 15 years, the daily water production has increased from approximately 13 million m3/day to the current 48 million m3/day in the 17,000 desalination plants operating worldwide [6]. Globally, more than 80% of the world’s desalination capacity is provided by two processes: multistage flash (MSF) and reverse osmosis (RO) [7]. However, these technologies are energy intensive, with the energy mainly supplied by fossil fuel sources, and are not linked to renewable energy sources. Among the recent technologies, membrane distillation (MD) has the advantage of performing at moderate temperatures and pressure [2, 3, 8, 9]. MD process is an emerging thermally driven membrane process and can be applied successfully in desalination [10, 11]. The MD process is economical in terms of energy because the heat source for the process can be low grade and/or alternative energy sources such as solar and geothermal energy and because energy is continuously recovered [2, 8, 9, 12]. During the MD process, a hot saline solution is brought in contact with a hydrophobic membrane, which allows water vapor to diffuse through the membrane, restricting the flow of liquid and hence dissolved salts through its pores [13, 14]. The mass transfer of water vapor through the membrane pores is facilitated by the vapor pressure difference, as well as the temperature difference between the two sides of the hydrophobic membrane, that is, the feed side and the permeate side, as shown in Figure 1 [2, 3, 8, 14–16]. MD can be divided into four configurations (Figure 2) such as (a) Direct Contact Membrane Distillation (DCMD), where the membrane is in direct contact with the cold and hot fluids, (b) Air Gap Membrane Distillation (AGMD), where an air gap is in (...truncated)