Understanding Cooling Beyond Temperature
Synthetic aromatic heat transfer fluids perform essential process cooling, extracting heat from exothermic manufacturing processes for products such as formaldehyde.
Cooling is commonly thought of as making something cool to the touch as it relates to our senses. As process engineers and others in the process cooling industry know, however, cooling means a movement of thermal energy away from the process or work of a user. Many media can be used as a carrier for this energy: air, water, glycol, ammonia, finned tubing, etc. Energy transference using solids, liquids and gases is available to us. This article takes a look at specialty fluids used to cool applications operating at higher temperatures.
Whether you measure this energy by the BTU, joule or kilowatt, cooling at high temperature poses some challenges compared to typical applications involving water towers and water/glycol mixtures. Temperature in process cooling is quite relative to the application. In this discussion, I will focus on temperatures between 350 and 750°F (177 and 400°C). Keep in mind that during cooling, a temperature can drop, be steady or even rise. The concept is to transfer thermal energy away from the process using temperature as a control point. One technology well suited to perform this task is heat transfer fluids.
Within well-known temperature ranges, water and, when needed, glycol mixes are an economic, practical solution for many cooling needs. Above 350°F (177°C), however, glycol does not have the thermal stability to survive the process for an acceptable length of time. Also, the vapor pressure of water creates mechanical issues that are expensive to manage and may be impractical to overcome. For these reasons, high temperature heat transfer fluids typically are used between 350 and 750°F.
Characteristics of Specialty Fluids for High Temperature Cooling
Heat transfer fluids formulated for high temperature use typically are pure saturated mineral oils and synthetic aromatic fluids with high thermal stability. Some of the synthetic fluids have an added benefit: a boiling point within the process cooling temperature range. Some of the formulations are a eutectic mixture of 27 percent biphenyl and 73 percent diphenyl oxide (BiP/DPO), diethylbenzene (DB) and, to a lesser extent, diphenylethane (DPE) and related mixtures. Of course, many applications and processes operate outside of the 350 and 750°F temperature range.
When discussing process cooling, high temperature cooling is not mentioned nearly as often as it might be. From an engineering standpoint, the high temperatures create significant complexity to the design process due to the effect of these temperatures on the materials of construction and use.
A boiling point in the process cooling range greatly increases the efficiency of the cooling. This is because it provides the advantage of phase-change thermal-energy absorption, the evaporative effect and vapor-phase heat transfer. A general rule of thumb is a that a fluid that reaches its boiling point has a four times increase in efficiency over liquid media that does not reach its boiling point. Another important benefit is recovery heat, which is taken from the condenser. There can be enough thermal energy to create steam for the production of electricity or to simply move heat to another area for another use.
Figure 1 shows the boiling points of these heat transfer fluids through the measurement of their vapor pressure. It also compares the high vapor pressure of water at these temperatures, which demonstrates the advantages of these chemicals when used in the right application.
Producing Formaldehyde and Process Cooling at High Temperature
The production of formaldehyde (HCHO) provides an excellent example of a process that takes advantage of this efficiency. It also demonstrates the need for a specialty chemical product that can provide fast, uniform cooling under difficult conditions.
One method to produce HCHO is the metal-oxide catalyst process, which also is known as the Formox process. (It currently is licensed by Johnson Matthey Formox.) In this process, an iron/molybdenum-oxide catalyst replaces the — previously most common but costly — silver catalyst. This reduces costs and improves production outputs when producing HCHO.
With the Formox process, the reactions occur in tubes that pass through a shell filled with circulating BiP/DPO heat transfer fluid. The reaction is highly exothermic. In ideal conditions, the process occurs at 660°F (350°C). The temperature is maintained between 480 and 660°F (250 and 350°C) in the reactor by removing excess energy from reactor tube using the heat transfer fluid. (Please see the web exclusive, “Focus on Formaldehyde (HCHO) Production,” for more information on this.) With a boiling point of 495°F (257°C), the fluid turns to vapor as part of the cooling cycle. The vapor then moves on to a condensate return, which can include a heat-recovery system.
Although BiP/DPO works effectively as a coolant in this application, it also has the thermal stability to withstand the operating temperatures. Its thermal stability also provides a safety factor in case of system upsets that might cause the tube bundle to be heated to higher-than-expected temperatures.
Thermal degradation of the fluid occurs through use and is accelerated when temperatures increase above those recommended by the manufacturer. Accelerated degradation can lead to fouling that effectively forms insulating layers on the tube bundle. This fouling can lead to even higher temperatures due to poor heat transfer and potential tube failure.
Figure 2 demonstrates specialty heat transfer fluids being analyzed for thermal stability. A widely accepted relative test called the ampule test is used for analysis. More recently, it was standardized under ASTM 6743, “Standard Test Method for Thermal Stability of Organic Heat Transfer Fluid.” During testing, different fluids are sealed in an ampule and placed side by side in an oven. The fluids under test are run through the oven multiple times at various temperatures (one temperature point for each run in the oven).
Figure 3 shows the results of a fluid rated at liquid phase up to 650°F (345°C) as compared to the BiP/DPO mixture. The thermal stability of the BiP/DPO formulation along with the vapor-phase temperature range make it uniquely suited for this particular application. Correctly matching the fluid to the process is important.
Applications that involve “hot cooling” include those in food processing, packaging, plastic and polymer extrusion, and glass production. Cooling can be as essential to high temperature processes as is heating. PC