A plant operating at high temperature for prolonged periods of time will see thermal degradation of its heat transfer fluids. One common byproduct of this process is the formation of “heavy” or long-chain hydrocarbons, which are detected by measuring the carbon content of a fluid. The accumulation of carbon in a heat transfer fluid, and thus a thermal plant, can potentially reduce a plant’s production output. A number of interventions can be used to maintain or reduce the presence of carbon in a fluid; these approaches are outlined here.
A heat transfer fluid (HTF) is central to many processes requiring heat to manufacture or produce, for example, processed foods, chemicals, or energy. All HTFs degrade over time and form a mixture of “light” and “heavy” chain hydrocarbons. In Plant Services’ May issue (“Don’t light my fire: Keep flammable vapors at bay”), I wrote about how light chain hydrocarbons can be effectively managed using a light-ends removal kit (LERK) or batch venting. We now turn our attention to managing their heavy-chain counterparts (i.e., carbon).
Carbon accumulates in a HTF as it thermally degrades at high temperature or oxidizes at high temperature in the presence of oxygen or upon exposure to air. Hence, there is a need for a thermal plant’s maintenance team to routinely measure the level of carbon in a HTF. This is critical for the health of the HTF and the plant, as the carbon formed and suspended in the HTF is “sticky” and will adhere to internal surfaces of the thermal system, including both the pipework and the heater.
Carbon buildup in a thermal plant
Carbon can lead to system fouling as it accumulates on the internal surfaces of a heater or pipework. At levels below 1% residue, carbon remains soft and can be removed from a thermal system using a suitable cleaner/flush fluid. However, when carbon residue increases above the 1% level, it begins to bake onto internal surfaces and cannot be so easily removed. At this point, the HTF will need to be replenished, but before this is done, the HTF system will need to be flushed and cleaned to remove carbon that may have already baked onto any internal surfaces.
The buildup of carbon on internal surfaces cannot be ignored, and monitoring carbon is critical for the long-term viability of a plant. This can be achieved via a number of options, including:
1. Sampling of a HTF. Research has shown that increased sampling is associated with improved HTF condition, so routine sampling is an important part of the management of carbon.
2. Operating temperature of the HTF. The manufacturers of HTFs generally recommend that a HTF be sampled at least once a year when operating near the upper operating temperature; however, if the operating temperature is 20 degrees Celsius below the upper operating temperature of the HTF, biannual sampling may be recommended. This is explained by the fact that the decomposition rate halves for every 10-degrees Celsius drop in HTF temperature.
3. Dilution of a HTF. Using this approach, an existing HTF system is partly drained and then filled with a virgin HTF, thus diluting the existing HTF and removing a percentage of the carbon in the system.
4. Filtration of a HTF. Filters with finer pores than the strainer can be used as a temporary or permanent addition to filter particles, including carbon from a HTF.
5. Recharge a HTF system. There is always an option to drain and refill a HTF system with a virgin HTF.
6. Use a nitrogen blanket. Exposure to oxygen/air is detrimental to the viability of a HTF, because above an operating temperature of 60 degrees Celsius, acids will form in a HTF, and these are potentially corrosive. Moreover, oxygenation of a HTF leads to the formation of carbon sludge, and this fouls a HTF system. A nitrogen blanket can be used to create a barrier between the HTF and oxygen/air, thus avoiding the formation of carbon sludge.
7. Antioxidant packs. Another option is the use of antioxidant packs or repellents, which deplete the oxygen in the HTF. However, their effectiveness has recently been questioned.
The seven options outlined can be used on a day-to-day basis to try to maintain carbon levels or to even reduce carbon levels in a HTF. There are easy-to-implement approaches such as the routine sampling of a HTF and the lowering of the plant’s operating temperature, and there are options that require capital expenditure, such as the installation of a fine filter. Complete replacement of a fluid can be avoided if regular HTF sampling or condition-based monitoring is initiated.