Sample heat transfer fluids to offset carbon effects on thermal plant efficiency
The long-term viability of a plant depends on maintaining continuous output and reducing production costs.
The long-term viability of a plant depends on maintaining continuous output and reducing production costs. Carbon accumulation in a heat transfer plant can lead to a reduction in plant efficiency. A number of interventions are available to counteract the buildup of carbon, including the adoption of a program of routine sampling of the heat transfer fluid (HTF), which potentially reduces the energy consumed and avoids unwanted interruptions to production that may occur through failure of component parts.
An HTF is central to manufacturing and is used as a heat carrier in the processing of food, chemicals and energy.1 The main types of HTF media include air and other gases, water and steam, mineral-based HTFs, synthetic-based HTFs, molten salts and liquid metals. The cost efficiency of any manufacturing plant is critical to its long-term survival and includes the cost of the HTF, the storage of the energy produced, the cost of the heater, and the cost of land and property for the operation.2
The efficiency of a plant also includes the HTF’s physiochemical properties, such as mechanical and chemical stability, and the compatibility between the HTF and the heater and/or storage material. However, efficiency can be measured more simply as production output vs. the energy consumed to generate the output. Therefore, revenue is equal to the production output divided by the energy consumed to produce the output.
Energy consumption can be increased by technical inefficiencies, meaning that while output is maintained, the cost of energy consumption increases. The net effect is an increase in the cost of operation and a reduction in revenue.
Maintaining continuous output and stable energy consumption are critical to a plant’s long-term cost efficiency. The condition of the HTF can negatively impact the energy consumed by a plant, as discussed in this article.
Heat transfer fluid condition
Past research has shown that the condition of an HTF is improved by regular sampling with the optimal sampling frequency reported to be once every three months for a mineral-based HTF. Once sampled, an HTF is chemically analyzed to assess and ascertain its state of thermal cracking, the degree of oxidation, the system’s safety, the extent of HTF contamination and the degree of system wear.
Fig. 1. Typical system that uses a mineral-based HTF. Note: The sources of carbon formation are highlighted.
Prior research used the tests reported in TABLE 1 and ranked the occurrence of events. Results showed that total acid number (TAN) and closed flashpoint temperature ranked as the most frequently occurring events (i.e., ≥ 1 in 3 systems). Interestingly, when HTF systems were sampled less frequently (i.e., every 2–3 years), the buildup of carbon was the second-most-common event (i.e., ≥ 1 in 3 systems). This dropped to ≤1 in 20 cases when sampled at least once per year.
Carbon occurs as a byproduct of thermal cracking and oxidation of an HTF. All HTFs will degrade over time, and this is why carbon needs to be routinely monitored. When carbon is formed, it becomes suspended in the HTF and acts as a sticky substance that will adhere to the internal surfaces of the HTF system, including the heater. FIG. 1 depicts a system that uses a mineral-based HTF. Also shown are the potential sources of carbon formation, which include:
- Adherence of the carbon to the internal pipework, including the heater, which acts as an insulator, increasing the heat needed to heat the HTF
- Adherence of carbon to internal pipework and a reduction in the diameter of the pipework
- The formation of carbon sludge that accumulates in the expansion tank and circulates around the HTF system has the potential to bake onto internal pipework.
Changes in pump efficiency
Fig. 2. The response of changes in pump speed to changes in HTF kinematic viscosity and resistance to flow.
In normal function, the speed of the pump in a thermal plant drives the flow of the HTF. The efficiency of the pump is affected by the buildup of carbon as it increases the kinematic viscosity of the HTF. Kinematic viscosity of a non-Newtonian fluid, such as an HTF, is calculated by dividing the absolute viscosity (also known as dynamic viscosity) by its mass density. The viscosity of an HTF is temperature dependent, so the reference temperature must be standardized for the results to have any meaning. In the laboratory, kinematic viscosity is normally presented at 40°C and 100°C.
Increased carbon in the HTF raises kinematic viscosity. This means the pump must consume more energy, assuming a constant temperature to pump the HTF and to overcome the increased resistance to flow presented by the higher viscosity of the HTF, as shown in the bottom left of FIG. 2. Likewise, the buildup of carbon on internal pipework reduces the internal diameter of pipes and increases the resistance to flow. For turbulent flow, there is increased resistance, and much higher pressures are required to drive HTF flow. The point at which turbulent flow occurs is referred to as the critical velocity and calculated as: (viscosity × Reynold’s number) / (2 × density x radius). Therefore, radius and viscosity contribute to the overall resistance experienced under turbulent flow conditions. For changes in internal diameter, the pump will consume more energy to achieve sufficient pressure to drive flow (FIG. 2, top right).
Fig. 3. Options for managing the buildup of carbon in an HTF system.
In real life, viscosity changes and carbon buildup will occur during the process of thermal degradation, and changes in resistance to flow will occur as a result of the increased viscosity and reduced conductivity of the pipework. When combined, energy consumption and the demand on the pump are increased (FIG. 2, bottom right).
The buildup of carbon on the heater’s internal surfaces cannot be ignored. While this does not form part of the model presented in FIG. 2, carbon buildup would lead to carbon lining the internal surfaces of pipework. As carbon is a good insulator, more energy would be needed to achieve the same operational temperature. The net effect is an increase in the energy consumed to heat the HTF.
Practical methods to reduce carbon
Monitoring carbon is important for efficient operations and lower OPEX. A number of options exist to help reduce carbon buildup. These options are outlined in FIG. 3 and include:
- Sampling—Prior research has shown the effectiveness of increased sampling, which has been associated with improved HTF condition. This is based on the association between fluid cleanliness and component life.
- Temperature—HTF manufacturers generally recommend that an HTF be sampled at least once per year when operating near its upper operating temperature. Some manufacturers recommend that this sampling be conducted twice yearly if the operating temperature is 20°C below its upper operating temperature. This scenario relates to Arrehenius’ Law, which shows that a correlation exists between the rate of a reaction and temperature, meaning the rate of a fluid’s degradation doubles for every 10°C rise in temperature.
- Dilution—The HTF in the system is partly drained and then filled with virgin HTF to dilute the existing fluid. This process effectively removes some of the carbon and other degradation byproducts.
- Filtration—Filtration effectively removes containments from the system. If contaminants are left in the HTF, then they can catalyze the degradation of the fluid. In existing systems, filters with finer pores than the strainer (FIG. 1) can be used as a temporary or permanent addition to an HTF system and enable the continuous filtration of particles. The effectiveness of this approach can be demonstrated by incorporating an assessment of fluid cleanliness (i.e., ISO 4406:1999) to quantify the number and distribution of particulates suspended in the HTF.
- Recharge—An option to drain and refill an HTF system with a virgin HTF always exists. The client can choose to use either a mineral- or synthetic-based HTF. Synthetic-based HTFs can be used at much higher temperatures and are more resistant to thermal degradation.
- Nitrogen blanket—The effect of oxygen (FIG. 1) is detrimental to an HTF at temperatures exceeding 60°C and leads to the formation of corrosive acids, carbon sludge and carbon fouling. The use of a nitrogen blanket to prevent the HTF from coming into contact with air is an appropriate countermeasure.
- Antioxidants—Oxygen can significantly increase the degradation of an HTF and damage HTF system components. Antioxidant packs or repellents are used to deplete the oxygen in the HTF.
Recommendations
Fig. 4. The effect of carbon formation on plant efficiency and revenue. In Scenario 1, carbon accumulation leads to increased kinematic viscosity and reduced pipework diameter and heater efficiency. If left unmanaged, carbon accumulation will eventually lead to reduced production output, increased energy consumption and an overall decrease in revenue. In Scenario 2, the sources of carbon are the same; however, routine sampling is incorporated. This process can be used to maintain kinematic viscosity, internal pipework diameter and heater efficiency.
The revenue from an efficient operation can be understood in terms of the ratio of the revenue gained from the production output relative to the cost of energy to produce the output, as depicted in FIG. 4. In Scenario 1, this model assumes that an HTF is not routinely managed and accepts that the accumulation of carbon will eventually lead to an increase in energy consumption. In the longer term, this could also lead to component failures and interrupt operation output, as seen in FIG. 4.
In Scenario 2, the effect of carbon is still an influencing factor, but HTF sampling has been incorporated into the model. This scenario represents an additional cost and loss of revenue, but it is a proactive approach to avoid the longer-term detrimental effects of carbon accumulation. This scenario works to maintain constant energy consumption. The net effect is that revenue will remain relatively constant as output and energy consumption are consistently maintained. Another advantage is that the cost of replacing component parts is potentially avoided as routine maintenance is used to correct increases in the levels of carbon and sustain the HTF and the HTF system. Options available for the management of carbon include the management of operating temperature, dilution of the HTF, installation of a temporary or permanent filtration unit, the complete replacement of the HTF, and strategies to manage oxidation of the HTF. HP
ACKNOWLEDGEMENTS
The author would like to acknowledge the writing support provided by Red Pharm Communications, a part of the Red Pharm Co.
LITERATURE CITED
- Wagner, W., “Heat transfer technique with organic media,” Heat Transfer Media, 2nd Ed., begellhouse, Graefelfing, Germany, 1997.
- Tian, T. and C. Y. Zhao, “A review of solar collectors and thermal energy storage in solar thermal applications,” Applied Energy, Iss. 104, 2013.
The Author
Wright, C. - Global Heat Transfer, Leeds, UK
Chris Wright is a research scientist and holds a BSc degree and a PhD from the University of Leeds in the UK. His research focuses on the use and maintenance of heat transfer fluids in manufacturing and processing, including specialist chemicals, food, pharmaceutical and solar sectors.
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