Re: [foxboro] pH control
- From: "Jones, Charles R. (Chuck)" <Chuck.Jones@xxxxxxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Mon, 15 Mar 2010 11:37:56 -0400
Robert Balmer says:
"The liquid volume changes frequently going into the vessel and
we use a lime feeder to correct the pH. "
I am assuming this is the tough part of the problem at this point.
We have one very tough to control loop that adjusts the pH of water
immediately after our water softeners. The cationic softeners are
regenerated by acid. Right after switching from an old cell to a
recently regenerated one, the residual acid in the resin beads plays
havoc with the almost completely unbuffered water. To make it tougher to
deal with, the water demand can change from 900 gpm to 3200 gpm in a
matter of seconds, as this water supplies the entire refinery.
We made the loop as "feed forward" as possible. Instead of adding base
to control the pH, we add the chemical based on a ratio of chemical flow
to the water flow. The pH loop determines the ratio. The flow control
loop is tuned to give near a immediate response. The chemical flow is
very small and this part of the loop is now quite successful. It is not
a perfect solution, but it solved the problem of response time to rapid
flow changes in the main stream.
The next part of our problem is that we have a set point close to the
steep part of the adjustment curve. To solve that part is going to
require changes to the hardware or the process and is outside of the
control loop.
I hope this helps.
Chuck Jones
Automation Technologist
Tate & Lyle Lafayette Plant
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