|M.Sc Student||Kirmayer Saar|
|Subject||Planning a Catalytic Denitrification System for Drinking|
|Department||Department of Chemical Engineering||Supervisors||Professor Emeritus Moshe Sheintuch|
|Dr. Uri Naital|
The presence of nitrates in portable water is a worldwide spread problem, especially in agricultural rich areas. The main risk is in “Blue baby” syndrome and suspected carcinogenic effect of nitrates on human. Following that, the maximal nitrate concentration in drinking water was limited to 45 ppm, where the recommended concentration is only 25 ppm. There are several current technologies for purification of drinking water from nitrate, but the most promising one is the catalytic denitrification, which removes the nitrate quickly and without any residues, by converting the nitrates to gaseous nitrogen. The only setback of this reaction is the unwanted production of ammonium and NOx.
Extensive work has been conducted recently in order to find suitable catalysts for denitrification with both high activity and selectivity toward nitrogen. It was found that in order to reduce nitrate to nitrogen selectively, palladium catalyst activated with another metal e.g. copper, tin or indium, must be used. During the process the nitrate passes through several stages, including forming the more dangerous intermediate nitrite that then undergoes additional reduction toward nitrogen and unwanted ammonium. During reaction, in order to maintain constant charge, hydroxide ions are formed. Since the reaction is a very strong pH dependant, it is necessary to control the pH at the reaction course. When using large support particles (over 30 micron), interparticle pH gradient is formed, causing severe activity and selectivity reduction.
At the present research Activated Carbon Cloth (ACC) was successfully employed as a catalyst support. After a wide screening of catalysts, 0.5%Pd-0.125%Cu/ACC catalyst was chosen. The effect of key parameters like pH, initial conc., dissolved oxygen etc., was tested in a batch reactor. Activity of 8.4 [mmole NO2- /min gr-Pd] with 90% selectivity and 0.6 [mmole NO2- /min gr-Pd] with 98.1% selectivity was achieved in a batch reactor compared with 1.9 [mmole NO3- /min gr-Pd] with 99 % selectivity achieved with CGF and 2.2 [mmole NO3- /min gr-Pd] with 95 % selectivity achieved with alumina powder.
A continuous system was planned and built, based on batch results, to process 12 l/hr of 100-ppm nitrate contaminated water. The system worked for several hours, showing activity of 0.15 [mmole NO3- /min gr-Pd], with 98% selectivity. A decrease in activity occurred after about an hour of operation, but no metal presence was found in the water. All parameters returned to normal after a rinse in hydrochloric acid and clean water. When using tap water, three competitive processes are taking place: Adsorption, desorption and reaction. The activity during reaction is 0.08 [mmole NO3- /min gr-Pd], with only 60% selectivity. Those values decrease with time.