Innovation on a Excel spreadsheet

water innovation in the oil sands: stack temperature regression method

If there’s one perplexing problem for maintenance managers at in situ oil sands facilities, it’s the regular fouling of the once through steam generators (OTSGs) that provide steam for operations. Fouling or scaling is the formation of unwanted deposits inside metal tubing as the water heats up and evaporates. You see it inside your kettle, which develops a mineral coating over time. 

Imagine that problem on an industrial scale with large steam operations that operate at high pressure and temperatures. Fouling reduces the amount and quality of steam, which means the facility runs less efficiently and emits more greenhouse gases. If fouling reaches a certain point it can cause corrosion and localized heating leading to equipment failure. Before that can happen, the equipment is routinely shut down so it can be mechanically cleaned, avoiding health and safety issues, and keeping emissions in check.

Shutdowns also mean lost production and earnings, so managers would like to do them as sparingly as possible. But so many factors are involved that it’s hard to predict the optimal timing. Is it once every three months, or three years? And how does that change over time as water chemistry and process conditions vary, and the facility ages?

Stack temperature is key

COSIA members are searching for ways to address this recurring and expensive problem and a variety of different research is underway. Now, chemical engineer Subodh Peramanu, a Process and Technology advisor at Canadian Natural, may have an innovative tool that can help. He’s developed a novel method for predicting fouling buildup by measuring the temperature of the stack (or chimney), the point where residual heat from fuel gas combustion leaves the steam generator as flue gas. 

“We don’t understand under what conditions scaling forms and if conditions change, how it forms and when. These are not easy problems,” he says. “Understanding that mechanism will help us find mitigating solutions and we have research underway through COSIA in collaboration with several academic institutions to do just that.” In the interim, managers use a variety of tools and techniques to assess the extent of fouling, all with mixed success. “None of them work perfectly,” Peramanu says.

Most in situ oil sands facilities rely on OTSGs to drive the oil extraction process, whereby steam is forced underground to soften the bitumen so that it can be easily pumped to the surface where the oil is extracted. About 90 per cent of this water is then recycled back through the facility to produce steam.

How the tool works

Peramanu says he’s always been fascinated by complex industry issues and his stack temperature regression method for predicting OTSG scaling is an elegant, low-cost tool. It is a sophisticated mathematical model that requires only readily available process data. No additional instruments are needed and there is no capital cost for implementation. All calculations can be easily done in a spreadsheet and the trends can be used to optimize cleaning cycles. Peramanu explains how it works.

“The stack temperature is one of the key parameters to indicate how efficiently the OTSG is operating. When there is no fouling inside the tube, the heat transfer is more efficient, so less fuel gas is required to make the desired amount of steam. Fouling reduces heat transfer, so more fuel gas is required, resulting in higher stack temperatures. The difference between the measured stack temperature and the clean tube stack temperature indicates fouling inside the tube.”

The method relies on a baseline stack temperature for clean tubes, which is not available once scaling starts to form. But that value can be calculated by regressing stack temperatures, i.e. looking backwards through mathematical formulas that consider variables, such as feed water flow rate, feed water temperature, fuel gas flow rate, air flow rate and steam quality etc., to estimate stack temperatures before fouling began.

“There’s a lot of mathematics behind it, but the spreadsheet tool is easy to use, and you don’t have to install any specialized equipment,” Peramanu says. After a year of intense development, initial testing has proved its worth and the tool will be evaluated further at Canadian Natural’s Primrose and Jackfish operations in 2022.

Subodh Peramanu has been involved with COSIA since the beginning (2012) and currently represents Canadian Natural at the COSIA In-situ WEPA steering committee. He also represents Canadian Natural on COSIA’s Steam Generation Working Group where member companies share and discuss their experiences to find solutions to improve water management and reduce greenhouse gas emissions from in situ facilities. The group also supports multi-year OTSG projects with University of Calgary, University of Alberta and SAIT to better understand fouling, corrosion and erosion in OTSG tubes to develop mitigating solutions.