Background
Analysis of High Recovery Brackish Water Desalination Processes using Fuel Cells (2009) was written by Rajindar Singh and details a water desalination technique using fuel cells. The author is the Technical Manager, Industrial Purified Water, Siemens Water Technologies, Colorado Springs, CO, USA.
Rajindar Singh
Singh has authored and co-authored several articles that mainly touch on water purification systems, these articles have been published on scientific journals such as Separation Science and Technology, Desalination and Environmental Progress & Sustainable Energy. He has also written several books on the same topic, these include Hybrid Membrane Systems for Water Purification (2006) and Influence of Slip Velocity at Membrane Surface on Ultrafiltration Performance (1977).
His years of experience working on water purification systems at Siemens Water Technologies make the paper a credible source for academic purposes as he writes the book from a professional point of view.
The paper was written in 2009 and this fits the widely accepted 5-year range within which scientific information remains valid. This implies that the information contained in the paper is up to date and can be used for scholarly purposes.
Rajinder targets individuals interested in water purification techniques and this is evidenced by the fact that he publishes his paper in a scientific journal dealing with separation technologies (Separation Science and Technology). While writing the paper, Singh refers to several peer reviewed journals, books and reports from well-known bodies and firms such as the United Nations and UTC respectively. All of these factors contribute to the article being a credible source of information.
Article Summary
The article is written in report format. The first part is the abstract and is followed by an introduction. Singh follows this with a design basis for the system used in the desalination process, then the results and discussion, and concludes with a section on summary and conclusions.
At the end of the paper, Singh give a list of references that assert that his research techniques were excellent, this is evident from the length and number of entries on the reference page. I chose to work on the System Design Basis section as it gives a full description of the method used for desalination process.
Water Desalination Process
A sketch of the desalination process using a fuel cell is shown below:
Fuel cells convert Hydrogen to electricity and this powers the system. Since hydrogen and oxygen are easily available, the system is designed to save energy and reduce operation costs. Hydrogen gas enters the fuel cell through an inlet on the cathode end and is mixed with a catalyst, this splits the diatomic Hydrogen molecules into electrons and protons.
These electrons then flow through the system to produce electricity. Electricity that has been produced is directed to other components of the desalination process. Protons go through a polymer electrolyte membrane to be reacted with Oxygen.
Gaseous fuel (Oxygen) enters the fuel cell on the anode end and is oxidized, then reacts with Hydrogen protons to form water as shown below.
O– + 2H+ —-> H2O
Fuel cells have an efficiency of between 40 and 50% and eliminate the costly installation of power transmission lines, besides, the energy losses due to electricity transmission through such lines is eliminated.
Feed water enters the heat exchanger and is used to cool the fuel cell. Before the desalination process begins, the following conditions must be set:
- Feed water flow rate ¼ 22.5m3 = hr
- Feed water total dissolved solids (TDS) ¼ 3613 mg = 1
- Feed water temperature ¼ 20, 25, and 30oC
- Product water recovery = 75%
- Product water flux same for each temperature sub-set
- Hydranautics CPA2 membranes (20 cm diameter elements).
Desalination of brackish water is mainly done through reverse osmosis (RO) and nanofiltration (NF). These processes operate at 75% product water recovery, this implies that 25% of RO feed water is wasted as concentrated brine. However, recovery can be increased by using a selective membrane process such as a hybrid RO-NF process.
Since the desalination process requires high amounts of energy, the incorporation of the fuel cell to the design is very important. However, areas that experience scarcity of water (the source of hydrogen and oxygen) can replace the fuel cell with photovoltaic solar cells or wind-powered systems (Singh, 2007, pp. 587).
The design process consists of a primary RO (PRO) component and a brine recovery component. The brine recovery component is the most important part as it separates salt from water hence providing purified water. The brine component can either be a NF or RO unit. Water coming from the PRO component is hard due to the high concentration of salt ions.
Therefore, low pressure NF is used to soften it before treatment commences because NF membranes have a high rejection of divalent ions and is prone to having a bad smell. At the end of the desalination exercise, it is observed that the recovery rate is increased. The design also saves on energy.
Reference
Singh, R. Analysis of High Recovery Brackish Water Desalination Processes using Fuel Cells. (2008). Separation Science and Technology, 44, 2009. 585–598.