Self-Tuning PID Controller to implement in a PLC without having the plant model?
You could identify the process transfer function by recursive least squares (or other TF estimator) operating on the real-time process input/output signals. Matlab's System Identification Toolbox has some algorithms. RLS background can be found at:

https://math.stackexchange.com/questions/631498/simple-example-of-recursive-least-squares-rls