Process for fractionation and purification of synthetic lubricants using liquefied CO2 + propane as solvent

ALDANA PIZZANO; BELEN RODRIGUEZ REARTES; PABLO E. HEGEL

Buenos Aires

Conferencia; WCCE11; 2023

AAIQ

Carbon capture, storage and utilization could help reduce emissions from the use of non-renewable carbon-containing fuels. To this end, different technologies are being developed and tested by several energy companies worldwide. In this work we study the purification of synthetic oil lubricants using liquefied CO2 + propane mixtures as solvent. These liquefied gases have shown interesting properties for extraction of viscous fluids regarding, environmental aspects, high selectivity, and reduced flammability properties [1], while taking advantage of CO2. In the industry, lubricants have strict composition regulations regarding the presence of residual monomers and oligomers that alter the product viscosity index. The removal of these non-desired minor compounds from the oily lubricants by classical vacuum distillation is difficult and expensive due to the high temperatures governing the process. Organic solvents, like phenol, are typically used for lubricant purification in liquid/liquid extraction processes. These solvents are toxic and non-environmentally safe, so they should be replaced for benign options in modern processes. Moreover, the production processes of biolubricants, which are replacing mineral oils, must use renewable solvents with a low environmental impact and CO2 can play an important role [2].To evaluate the technical feasibility of CO2 + propane as liquefied solvent mixtures for the purification of lubricant oils, we consider a synthetic oil mixture prepared from a Silicone oil (MW: ≈8000) and Decamethylcyclopentasiloxane (D5, MW: ≈371) as a model lubricant. The liquid-liquid-vapor phase behavior of the multicomponent system is studied in a high-pressure view cell to set preliminary operating conditions. The effect of the different variables governing the separation process such as solvent composition (CO2/propane: 60/40 to 90/10) and temperature (T: 298 K to 323 K) are studied. Then, an analytical method is used to evaluate the composition of the system under equilibrium and to determine the distribution coefficient of D5 in the liquid phases. Experimental and modeling results are pointing out that liquefied gases with propane concentrations between 10 and 26 wt.% are suitable to remove the monomers from the raw lubricant oils with good selectivity and extraction yields. A conceptual engineering design of the liquid-liquid fractionation process shows that liquefied gases with 85 wt.% CO2 enable the removal of 99 % of the monomers present in the lubricant oil with an oil recovery greater than 90 % at the end of the purification process.