Chemical processes are usually nonlinear singular systems. In this study

a soft sensor using nonlinear singular state observer is established for unknown inputs and uncertain model parameters in chemical processes

which are augmented as state variables. Based on the observability of the singular system

this paper presents a simplified observability criterion under certain conditions for unknown inputs and uncertain model parameters. When the observability is satisfied

the unknown inputs and the uncertain model parameters are estimated online by the soft sensor using augmented nonlinear singular state observer. The riser reactor of fluid catalytic cracking unit is used as an example for analysis and simulation. With the catalyst circulation rate as the only unknown input without model error

one temperature sensor at the riser reactor outlet will ensure the correct estimation for the catalyst circulation rate. However

when uncertain model parameters also exist

additional temperature sensors must be used to ensure correct estimation for unknown inputs and uncertain model parameters of chemical processes.