Here are my thoughts, no particular order:
1. I'm not sure I would call your dilemma "autocorrelation". Seems to me more like you have a covariate or noise that can affect treatments. Can you measure the "condition or cleanliness" of the mixer in the lab? Or can you measure the conditions of the continuous process?
2. Can you take advantage of any split-plot designs to handle the noise and desired restrictions on randomization? Randomizing won't let you assign the variation due to noise.
3. I'm not sure cleaning the process or mixer before every treatment is actually a good idea. The design space is not really representative of reality (as they seldom or never clean or stop the continuous process in reality). This would be an inappropriate inference space and might even induce unusual noise or factor effects.
4. Replication is, of course, another good option, though it may be difficult to keep the conditions "constant" within the block. Can you purposely change the conditions? If so then you might be able to exaggerate the effect between blocks and let the random variation of the condition occur within blocks.
5. I might also suggest sampling (component of variation) studies to determine the consistency of the mixer or continuous process and possibly even get an idea of the magnitude of the "conditions" on the response variables. This would give some good clues for deciding how to manage the condition during experimentation.
"All models are wrong, some are useful" G.E.P. Box