ROOWT

Recycling of produced water during the SAGD process is important for environmental and economic reasons. However, the treatment of the wastewater comes with many challenges such as fouling, erosion and corrosion. The location where fouling happens within a wastewater treatment plant varies but in general, fouling is more pronounced as production water cools in heat exchanger. In Once Through Steam Generator (OTSG) however, fouling occurs predominantly where the heat flux is highest (stage where radiation and convection heat transfer contribute to the heat) and significant amount of steam is first generated in this region of the OTSG. One factor contributing to these fouling issues is dissolved organic matter. The concentration of dissolved organic matter in boiler feed water (BFW) is usually between 200 and 500 ppm whereas the abundance of organics in the boiler blowdown (BBD; the residual liquid phase after steam separation) often exceeds 2,000 ppm.

Elemental analysis of scale formed in OTSG shows that several contaminants contribute to the deposit – silica, hardness ions (calcium and magnesium), carbonate ions, and organic compounds (dissolved compounds or TOC; suspended hydrocarbon, or both) with the amount of carbon in the deposit is typically above 50% and often surpasses 70% by weight. This might indicate that organics play an important role in scale formation.

Previous tests with a one flow-through fouling apparatus revealed that the dominant species present in the fouling layer of the solids was organic carbon, which suggests that organic species in the process water are a major contributor to boiler tube deposit mass.

Recent initial in-house studies of the effect of heat on the composition of organics in SAGD wastewater with and without the addition of magnesium ions revealed that surface active alkyl benzenesulfonates (common in SAGD wastewater) are thermally stable and concentrate under boiler heating conditions. In addition, even though the number of heating experiments were limited there were some indications that deoxygenation of highly oxidized species is the main transformation process leading to the production of compounds with lower number of oxygen by retaining overall high DBE and carbon numbers. Some of these transformed species have likely characteristics of surface active interfacial material similar to alkyl benzenesulfonates and might cause fouling. These are some initial hints pointing to a contribution of organics to fouling issues in OTSG.

This COSIA NSERC CRD funded project aims to improve our knowledge of fouling mechanisms in WWTPs with special focus on OTSGs.