Safe Handling of Heat-Transfer Fluids

42 www.aiche.org/cep March 2008 CEP Each heat-transfer fluid has a particular chemical makeup, and associated performance profile, that makes it more or less suitable for a given chemical processing application. The most common organic heattransfer fluids fall into two categories: • mineral oils: petroleum-based, solvent-refined fluids; and petroleum-based, severely hydrotreated/hydrocracked fluids • synthetics: polyalphaolefins (PAOs); benzene derivatives (aromatics); and polyalkylene glycols (PAGs). This article discusses the fundamental chemistry and manufacturing of these organic fluids. (Other types of heattransfer fluids are available for industrial applications, such as molten salts, organosilicones and fluorinated hydrocarbons, but these are not covered here.) It aims to help users understand the complexity of the various mineral oil and synthetic heat-transfer fluids by explaining their chemistries, properties and behaviors, and offers insights into some of the important safety considerations involved in the use of heat-transfer fluids. The bottom line is that all petroleum-derived heat-transfer fluids are not created equal. Solvent-refined fluids All mineral oils should be assessed based on their base oil classification, as defined by the American Petroleum Institute (API), and their significant chemical differences. The base oils in the API Group I category are manufactured using a solvent-based refining process (Figure 1). This typically removes 75% to 85% of the undesirable molecules that are present in the crude oil, namely sulfur compounds, nitrogen compounds and unsaturated hydrocarbons such as benzene derivatives. When aromatics and impurities are not adequately removed from crude oil, the resulting finished product will have higher volatility (i.e., lower flash, fire and auto-ignition points), lower thermal stability, and poorer response to anti-oxidant additives when compared to more highly saturated petroleum stocks. As a result, the use of solventrefined heat-transfer oils is generally limited to maximum bulk operating temperatures of about 550°F (288°C) for a typical ISO 32 or 46 viscosity-grade fluid. Companies offering API Group I heat-transfer oils often recommend the use of a higher-viscosity oil (ISO 68 viscosity grade and up) to meet the need for a higher-flashpoint fluid. However, the use of a fluid with higher viscosity may have a net negative impact on the overall efficiency of the heat transfer because of its lower overall heat-transfer coefficient.