Thirty years ago, heat transfer fluid (HTF) manufacturers would sell virgin fluids at inflated prices and dispose of the used thermal fluids without regard for the potential of remediating the used fluid. Times have changed. With the global focus on the impact of chemicals on the environment, and corporate sustainability objectives, heat transfer fluid users are seeking alternatives to dumping and disposing of used thermal fluids by burning or landfilling, and replacing with virgin product.
Heat transfer fluids commonly are used in industrial applications for heating and cooling processes that require a continuous, steady and accurate heat source to ensure product quality. Therefore, selecting the best-matched fluid for your application is vital to a well-running heat transfer system. Choosing a poorly matched heat transfer fluid can lead to problems such as:
- Thermal degradation.
- A shorter service life.
- Inconsistent product quality.
- System maintenance issues such as fouling, coking and equipment damage.
Many companies make their heat transfer fluid choice based on the least expensive fluid — one whose upper temperature limits barely cover the application. In many cases, this will lead to a shorter fluid life, which may — in some cases — cost more in the long run than using the more expensive heat transfer fluid that has the appropriate higher temperature rating.
Even when the best-matched heat transfer fluid has been selected and maintained properly, it may still degrade. This is mainly due to residence time and high or vacillating temperature effects.
When degradation occurs, most thermal fluid suppliers will recommend draining the system and replacing with fresh fluid. This can be expensive, especially for high volume systems. Instead of fully replacing used or spent heat transfer fluids, users of thermal fluids can choose to reclaim their heat transfer fluid. Such an option can yield cost savings.
TABLE 1. Because there is a relatively large boiling-point difference between the high boilers and the recoverable heat transfer fluid, one must select the technology for recovering or recycling heat transfer fluid.
Degradation and Effects on System Performance
Heat transfer fluid degradation is caused mainly by three factors:
- Thermal cracking.
Fluid degradation occurs due to the conditions to which the heat transfer fluid is exposed to in the heater. The two most important operating parameters are excessive temperature and residence time. Residence time is defined as the length of time the heat transfer fluid is exposed to elevated or excessive temperatures.
There is an old rule for estimating the rate of chemical reactions. It states that the rate of reaction will double for every 18°F (10°C) rise in temperature near its upper limit. This also is a good rule of thumb for heat transfer fluid degradation. As important a topic as heat transfer fluid degradation is to a hot oil system, this topic will be discussed in detail in a future article.
FIGURE 1. Sample analysis shows the heat transfer fluid to be degraded. This sample shows what the used fluid may look like before distillation.
Reclaiming Heat Transfer Fluids
Synthetic heat transfer fluids do not completely wear out, but they do get dirty. The useful service life of any heat transfer fluid is affected by many factors, the most important of which are:
- Fluid bulk/skin temperature.
- Heater design and age.
- Air exposure.
- Whether the system operates at a steady temperature or heats up and cools down on a daily or campaign basis.
As a fluid degrades, it can develop lower boiling “lights ends” or thick, tarry “heavies” — or both. Light ends can be vented from the system through the expansion tank. By contrast, the high boilers, or heavies, accumulate in the system and fluid over time. This buildup of high boilers raises fluid viscosity, which impedes heat transfer. Eventually, high boilers form sludge and even hard carbon particulates.
The recommended industry norm is to reclaim heat transfer fluids when they reach a threshold of 10 percent high boilers. What does a heat transfer fluid user do when the fluid reaches that 10% threshold? They have four options:
- Replace the entire volume of heat transfer fluid.
- Continue operating with the current volume of heat transfer fluid until the system experiences a mandatory maintenance event. This can further damage the heat transfer fluid — and eventually, operations.
- During the next scheduled shutdown, send the used heat transfer fluid for processing (sometimes called cleaning). Such processing allows the facility to reuse the heat transfer fluid distillate when the plant is started up again after the outage.
- For those heat transfer fluid systems that are large enough, perform a bleed-and-recycle option. This allows the plant to improve the fluid condition without shutdown.
Sending the thermal fluid for processing or performing a bleed-and-recycle process are more commonly called reclaiming or recycling the heat transfer fluid. Reclaiming a heat transfer fluid typically is done through a process called distillation.
FIGURE 2. This sample shows what the used heat transfer fluid typically looks like after short-path distillation.
For process heating applications, most heat transfer fluids are high boiling. So recycling the thermal fluid via distillation is complicated by the high temperature to which such materials must be exposed during the evaporation of volatile components. The high temperatures can exceed the heat transfer fluid’s recommended film temperature, resulting in more degradation products being formed. Lower distillation temperatures can be achieved under vacuum, but the length of time to which materials are exposed to even these lower temperatures can result in thermal damage. Also, because there is a relatively large boiling-point difference between the high boilers and the recoverable heat transfer fluid, one must select the technology for recovering or recycling heat transfer fluid. Among them are short-path vacuum distillation, flash distillation, simple fractionation and vacuum fractionation (table 1).
Although there are many distillations methods available, this article shall focus on the short-path distillation method for the processing of heat transfer fluids. Among the benefits offered by short-path distillation are:
- It is suitable for a range of high temperature, synthetic heat transfer fluids.
- It can be used for smaller volume batches.
- It requires a short residence time under vacuum conditions, which ensures a high a distillate yield.
Processing a synthetic, high temperature heat transfer fluid allows the user to have the foulants of the fluid removed. In other words, the “good” distillate can be reclaimed. This allows the organization to reuse a chemical rather than dispose of it.
Heat transfer fluid degradation can lead to expensive maintenance costs and shutdowns due to fouling and loss of heat transfer capacity. By maintaining good heat transfer fluid hygiene, many of these issues are avoided, and the system efficiency is improved.
Fluid Reclaiming Process
In the past, if the heat transfer fluid was not performing up to the design specifications, the remediation options available were limited to things such as venting light ends and moisture, diluting with new fluid, or completely replacing with new fluid. Also, if degraded heat transfer fluid was replaced, there could be a disposal cost as well as a replacement cost.
As noted, an alternative to these options is reclaiming the heat transfer fluid using short-path distillation. The reclaiming process for heat transfer fluids generally follows this basic process:
- Sample analysis (see sidebar) shows the heat transfer fluid to be degraded and performing significantly different than the designed specifications. It may resemble the fluid shown in figure 1.
- The fluid is removed from the system in a safe manner and transported to a reclaiming site.
- If there are moisture or light ends present, they are removed. This raises the viscosity and flashpoint to within normal product specification.
- The remaining heat transfer fluid then is distilled, separating distillate from the high boiling/tars (residue). This generates the reclaimed heat transfer fluid similar to what is shown in figure 2.
- Yields for this process will vary depending on how degraded the heat transfer fluid feed is, but generally, it will result in 50 to 95 percent recovered heat transfer fluid distillate for return to the client on a volumetric basis. The remaining or removed volume is degraded material and undesirable impurities. On an un-degraded heat transfer fluid basis, the recoveries can be high — typically about 95 percent or better.
TABLE 2. The table outlines the savings that can be realized by reclaiming or recycling a used heat transfer fluid.
The quality and recovery of the reclaimed heat transfer fluid can vary slightly depending on the type of heat transfer fluid reclaimed and degree of degradation, composition and other contained impurities (table 2). In most cases, the reclaimed heat transfer fluid will meet virgin heat transfer fluid specifications. In limited situations, the reclaimed heat transfer fluid will be close but just shy of virgin heat transfer fluid specifications. Even in situations such as this, however, the reclaimed heat transfer fluid is improved and more pure that the degraded used heat transfer fluid.
In conclusion, reclaiming via vacuum distillation and recycling a heat transfer fluid for continued use is an economical, ecological and responsible approach to specialty chemical management. Reclaiming a heat transfer fluid can help a processor save operations and maintenance time and money while restoring fluid condition back to near new condition. Reclaiming degraded fluids can help improve heat transfer fluid system performance, address an organization’s sustainability initiatives and reduce its carbon footprint.