desalination & membrane technology
THERMO-IONIC DESALINATION DRIVEN BY SOLAR OR WASTE HEAT B Sparrow, J Zoshi 0.060
Abstract A novel proof-tested desalination system is presented. It harn esses low grade heat or solar energy to provide concentrated brine as the driving force. A 1,000 Uday pilot plant has been constructed in Vancouver, Canada and is currently operating on harbour seawater with chemical-free pre-treatment. Applying the small but measurable voltage differences between concentrated brine and more dilute salt solut ions, through a series of cel ls each fitted with cation and anion membranes, has enabled seawater to be reduced to potable concentration. The electrical energy is secondary and reduced to that necessary to pump the streams through the low-pressure PVC network, of the order of 1 kWh per kl. Provided inexpensive evaporation facil ities can be provided, the process can be more economic than reverse osmosis. Thus the technology could offer a low impact desalination sol ution for dry regions in Austral ia.
Introduction The authors present a patent-pend ing and proof-test ed thermo-ionic desalinat ion process. Solar or low temperature energy is used to evaporate water from brine, producing a concentrated sol ution . There is a chemical energy d ifference between a concentrated salt solution and a dilute solution which is expressed as a voltage difference. This voltage is applied to an adjacent cell fitted with cation and anion membranes, driving sodi um ions one way and ch loride ions the opposite way, thus reduc ing the salt co ncentration in that cell. By applying a series of such cells it has proved possible t o reduce seawater to potable concentration. The bulk of the pipework and vessels can be low cost PVC. A 1,000 U day pilot plant has been constructed in Vancouver, Canada and is currently operating on harbour seawater with chemical free pre-treatment, prod ucing potable water. The plant can be tuned to various saltwater concentrations, including the potential for a zero liquid discharge that enables salt harvest ing.
Theory The important variab le in a concentration gradient system is the change in net chemical energy which may be modelled both in terms of osmotic pressu re or galvanic potential, which represents Gibbs free energy (P itzer et al. 1984). This is a summary prepared by the Editor of the paper presented by Sparrow at Ozwater10 (#035). A demonstration rig, packed in a suitcase, was operated for a number of interested delegates.
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Figure 1. Galvanic potential of NaCl (aq) referenced to 3.5% salt mass. Ion exchange processes are more suited to galvanic potential, or the voltage difference between solutions. Figure 1 shows the voltage difference between two solutions of aqueous sodium chloride: a first reference solution of 3.5% salt mass fraction representing seawater and depicted by the curve; and a second solut ion at various concentrations depicted along the x-axis. Aqueous sodium chloride data is far more abundant than data on sea salt, and sufficient to provide the general model of concentration gradient energy disclosed. The voltages in Figure 1 were calcu lated using activity coefficient data for NaCl (aq) (Hammer et al. 1972) . Figure 1 shows that -0.09 volts is established between a freshwater sol ution (y-intercept) and a solution at 3.5% salt, or 0.035 mass fraction (x-intercept). This voltage needs to be overcome to drive salt ions from a solution approaching potable concent ration to a solution at seawater concentration of 3.5% . It also shows that the chemical energy difference between a solution at 3.5% and concentrate at 18% is 0.04 volts. The desalting device combines multiple diluentconcentrate compartments in an additive fashion, analogous to batteries in series, producing an additive voltage. For example three concent rat e-di luent compartments each prod ucing 0.04 volts collectively produce a net 0.12 volts. This voltage is sufficient t o drive salt out of a product sol ution compartment at -0.09 volts. An external voltage is not necessary to move the ions, instead this voltage is produced through concentrat ion gradients. The various compartments are separated by ion bridges that are permeable to only positive or negative ions. These have been fabricated from polystyrene fil ms which have been functionalised, somewhat simi lar t o electrodialysis membranes, but more efficient. They are stacked and so t hat positive ions transfer from the concentrat e to t he diluent through a positive ion bridge while negatives transfer in the opposite direction. The product compartment completes the circuit, with positive and negative
A novel concept undergoing scale-up from pilot plant.
technical features