A joint study was completed comparing the relative ability of several chemicals to remove dissolved oxygen from water. Dissolved oxygen in water can cause corrosion in industrial processes such as steam generators. The study compared seven chemicals at different conditions in order to determine their relative oxygen removal ability.
Dissolved oxygen is one of the major corrosion-causing factors in water-based industrial processes. Much of the damage to steam generators and boilers is caused by oxygen-induced corrosion, necessitating that oxygen be eliminated from steam generating systems' feed water. Oxygen corrosion can manifest itself in a wide range of phenomena including general attack, pitting, water-line corrosion, crevice corrosion and under-deposit corrosion. While much of the dissolved oxygen can be removed by mechanical means, oxygen scavengers are generally required to complete the removal. Oxygen scavengers are chemicals that are added to water to reduce oxygen-based corrosion. As their name implies, they are compounds that react with dissolved oxygen in aqueous solutions. Hydrazine is the most widely used chemical for this application because of its cost, volatility and ability to form magnetite on steel surfaces. Alternate oxygen scavenging molecules have been identified over the last 20 years and are now also being used in boiler systems.
In order to compare the oxygen scavenging ability and relative rates of various products, Arch Chemicals, Inc. and the Environmental Research Institute at the University of Connecticut conducted a joint study. Oxygen scavenging tests were performed in order to compare three 35% hydrazine solutions (Scav-OX@, Scav-Ox II@, and Scav-Ox PluS@)1 produced by Arch Chemicals, Inc. with several alternate products. The tests were performed at 25QC and at 45°C.
Goal
The primary goals of the tests were to measure the rates of dissolved oxygen (DO) removal by nine selected chemical oxygen scavengers at the temperature 25QC. and at 45°C to understand the temperature influence on the DO degradation rate.
The oxygen-saturated water solution was prepared by aerating (with air ULTRA zero grade) 20 L of deionized (01) water in a Nalgene polypropylene container under isothermal conditions (Le., 25QC or 45QC) by placing the container in an incubator. The pH of the solution was adjusted to 9.5 by adding carbonate buffers (Le., 18.04 g sodium carbonate and .40.32 g sodium bicarbonate) that resulted in a total ionic strength of 0.05 M. The resulting solution had an initial DO concentration of 8 mg/I at 25°C and 5.8 mgll at 45°C prior to use.
Test Procedure
The rates of oxygen removal at two temperatures (25QC and 45QC) were measured with a Yellow Springs Instruments (YSI) oxygen meter (Model 51 B). The oxygen-sensitive membrane electrode (YSI 5905 BOD Probe) of the YSI DO meter was mounted in the neck of a 300 ml biological oxygen demand (BOD) bottle. In a typical run, a BOD bottle was filled with oxygen-saturated solution and placed on a stir plate (Nuova II, Thermolyne) that was placed in an incubator set at the desired temperature. The electrode then was inserted, causing overflow in order to eliminate any air spaces. While stirring, an appropriate volume of oxygen-scavenger solution was immediately injected into the bottle and the recorder was started. Usually, the volume ratio of scavenger solution to the oxygen¿saturated solution was 8:500. For several very fast reactions, the ratio was adjusted to 2:500 or even 0.5:500 in order to capture enough readings for data analysis. The 25QC tests were run in duplicate for all nine chemicals to ascertain the reproducibility of the experimental data. The tes
