| Recycling of metalworking fluids (MWFs) via microfiltration expands the usable life of MWFs by removing microorganism, particulate and tramp oil contamination, and has the potential to address the environmental, economic, and occupational health concerns associated with MWF use. The economic and technical feasibility of the technology has been demonstrated for synthetic MWFs. The widespread application of microfiltration recycling to semi-synthetic MWFs is limited, however, due to the lack of fundamental understanding on the transport of semi-synthetic MWF microemulsions through membranes.; It is the goal of this research to reveal quantitative relationships between semi-synthetic MWF formulations and microfiltration flux in order to produce guidelines for high productivity formulation development and microfiltration system design. In this research, a mechanistic model is first developed that captures the three dominating flux decline mechanisms (i.e., pore constriction, pore blocking, and surface film formation) during semi-synthetic MWF microfiltration, with model parameters consistent with their physical interpretation and microscopic observations. The model is then extended to include quantitative relationship between MWF formulation parameters and fouling characteristics. Specifically, pore constriction is mathematically modeled as a balance between adsorption and desorption of surfactants, and pore blocking is modeled in the context of queueing theory as a function of microemulsion particle density and coalescence activation energy. The resulted design model suggests that characteristics of the surfactant package used in semi-synthetic MWFs dominate the microfiltration performance, and a microfiltration compatible formulation favors surfactant packages that lead to highly stable microemulsions and also weak adsorption onto membrane surfaces. Following these guidelines, re-design of a commercial MWF is performed by replacing original surfactant package with anionic disulphonate and nonionic ethoxylated alcohol. Using the design model, ratios of oil:surfactant and anionic: nonionic are optimized and the optimal formulation has a flux that is 8 times higher than that of the commercial fluid. Finally, economic analysis framework of three realistic microfiltration implementation strategies, i.e., integrated system, mobile system, and centralized system, is developed. It is found that with the microfiltration compatible formulations developed, significant reduction on microfiltration system cost can be achieved. It is expected that this research will encourage the adoption of microfiltration recycling technology in the mainstream of the metalworking industry. |
