Lube-Tech
The second improvement is in cooling of the reacted grease.
2. An alternative method of cooling grease Grease is a semi-solid mixture of soap and oil. It requires reaction heating up to 200°C – 230°C. Grease cooling or quenching refers to the cooling of grease after the reaction process is completed. Quenching impacts the quality and the yield of grease. A grease that is cooled quickly creates harder soap structures and subsequently a higher yield as more oil is needed to thin it to the desired consistency (grade). Slow cooling results in a softer grease and subsequently lower yield. For quenching to be effective about 80 to 100°C of temperature drop would be needed within a short period of time after the reaction.
Current grease manufacturers rely on cooling towers and chillers to cool the grease in jacketed vessels. Cooling water is pumped into the jacket of a vessel to cool its inside walls. Scrape surface mixing blades wipe the wall surface of the vessel. Grease is a poor conductor of heat and scraping the walls of the vessel replaces only thin layers of grease that are in touch with wall surfaces. Also, vessels that have scrape surface mixers, require heavy mix arms, gear reducers and high-powered motors to operate.
The use of water for cooling in industrial processes is an old concept that has been in use for centuries. Considered abundant and inexpensive, water as a cooling medium has become ubiquitous across many industries. Sophisticated compressors, heat exchangers, antifreeze, anti-rust and corrosion chemicals have been developed in support of water-based cooling systems.
Research has shown that relative to many solids, water is not an effective absorber of heat. For example, a 3” diameter steel ball would weigh 1.6Kg and would take up about 20% as much space as 1.6Kg of water (Figure 2).
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.146 page 3
Figure 2: Volume comparisons for a 3” diameter steel ball and equal mass of grease
The steel ball can absorb heat from a heat source much faster than the same mass of water. In contrast, water would retain the latent heat for a longer period than the steel ball would and would further take longer to cool. This concept eliminates the need for cooling water and jacketed vessels with scrape surface mixing mechanisms. It relies on solid re-useable metal objects for cooling grease from the reaction temperature to its desired quenched temperatures which is around 60°C-80°C. Simply explained, a predetermined number of steel balls whose aggregate mass equals 25% to 50% of the mass of the hot grease are introduced into the grease at carefully timed intervals. The exposure time of each ball to the grease is timed to allow full saturation of heat. During the exposure time a volume of the grease that is in contact with the ball is cooled to the desired temperature while the ball is heated to its maximum heat absorption capacity. A special wiper design wipes the grease off each ball at the end of its exposure time and the ball is transferred into a holding vessel to allow for heat exchange and heat removal. Accompanying components include an auger, indexing valves for proper timing of the release of each ball into the hot grease and its removal from the grease (Figure 3). In the northern United States, where the grease manufacturing plant is located, for about 4 months of the year, ambient temperatures are near or below freezing. Steel balls that occupy small volumes can be cooled outside before application to the grease. This in effect is the use of solar energy for heating in reverse as cooling, because no compressor energy is needed to generate the cool air used for cooling the balls before exposure to the grease.
LUBE MAGAZINE NO.175 JUNE 2023 31
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