In the last decades companies aligned with the concepts and principles of sustainable development have been seeking to minimize the environmental and social impacts caused by their operations. Typically, the primary environmental concerns in the industry were related to water consumption, wastewater production, waste generation, energy consumption and mainly CO₂ emission - which is one of the causes of the greenhouse effect. However, the environmental eco-efficiency is not clearly seen when the evaluations of these eco-indicators are individually used and, therefore, it becomes necessary to implement a methodology that enables a joint assessment involving various aspects. In this paper we have developed an environmental index, called Eco-efficiency Comparison Index (ECI), applied to evaluate in real time a petrochemical facility. Throughout a study-case based on experimental data, the results have evidenced that the ECI is a useful tool for eco-efficiency analysis for process monitoring.