disc thickener optimization

[silverlilly1]

I'm working in a pulp and paper operation, and we have a set of disk thickeners bringing pulp from about one to two percent consistency up to eight or nine percent. There is very little control over the feed stock consistency. We are trying to increase consistency to at least 10%, possibly 11 or 12%, but so far any attempts to decrease rpm, increase tank level, etc, have resulted, oddly, in decreased consistency. Vacuum comes from white water removal from the tank, but has proven difficult to control. Does anyone have any suggestions as to how we can increase consistency, and maybe, control our vacuum system better?

 

[Barry1961]

Some VFD’s let you write your own PID algorithm, Hitachi is one.

I am not familiar with your particular process but I have heard with a large liquid mass that the integral gain becomes the dominant factor. It has been awhile since I had this discussion but I seem to remember it was around I-10, D-1or 2 and P-0 or 1. This was from a PHD who had retired after doing well in the chemical industry. It was a total shock to me being in automation where the ratio is P-10, D-1 and I-0 most of the time. But after some thought it did make sense.

Barry1961

 

[silverlilly1]

Unfortunately, I do not have access to these kinds of programs where I am, and my system is far more complicated than would allow me to calculate these things on my own. I'm pretty much on a testing schedule, where I can change one thing, see how it affects the outputs, then change it again, record results, change another thing, and so on.

I am looking more for what kinds of variables can I change (or combinations of such) that would affect my system in the proper direction, also what kinds of instrumentation and maintenance changes might help.

 

[HEC]

Silverlilly1,

It is a while since I have dealt with pulp, however from my vague recollections, is the variability problem more associated with variations in freeness or water draining properties of the pulp? To get a higher consistiency (drier pulp cake) try reducing the level of the feed tank or reducing the speed of the discs. This will allow a greater time under vaccume for removal or draining water from the pulp cake. My experience was with a drum filter, (it is all comming back to me now!) the balance between throughput and consistiency was masintained by the drum speed. Pulling more vaccume did not assist that much.

Regards
hutton4eng@picknowl.com.au

Mark Hutton