An online performance assessment method based on multi-space total projection of latent structures (MsT-PLS) was proposed for assessing operation status of chemical process by “offline modeling

online assessment” strategy. After the whole historical input data space was decomposed

interference information unrelated to quality variable in the input data space was eliminated by multi-space basis vector extraction method. The “off-line modeling”

an off-line data network classification model

with different operating performance grades was established in input multi-space related to the quality variable. During “online assessment”

assessment unit of data sliding time window was used to divide process performance into steady and transition performance grades and to match degree of similarity between online data and historical performance grades. According to relative contribution of all process variables

these process variables that decisively influence performance were identified and analyzed for degree of contribution

which provided a reference to identify causes for system degradation. Application to performance assessment of ethylene cracking process showed that the proposed assessment method can accurately assess system performance online.