3/17/05

To: *****

Aromatics Team Leader

From: ****

Process Control Team Leader

RE: Stabilizer Overhead Level Control.

*****,

ProControl Note: our client has OK’d use of this memo, in its “anonymous” version.

*****

The Aromatics Team requested that Process Control look at tuning this level. Before Joe (an operator at the Unit) worked on it, the level was swinging so badly the operators had to operate it in manual at times, to keep from pushing liquids to Unit 23, or wrecking the pump seals by running them dry.

By the time I reached the unit Joe had done a nice job of tuning the controllers. They were “well within the ditches.” After working on it some time, I got the level to behave marginally better at best, so when the “charge changed” a few days later, the Unit asked Steve (the instrument man) to tune the level again. Steve did what we considered to be an excellent job of tuning the level, as he got the level swing down to about plus or minus 5%. It was still swinging on a period of about every 14 minutes, however.

The unit was happy with this configuration, but I wanted to see if we could do better. I had recently been to a ProControl class dealing with controllers, and hoped we could use what I’d learned there to improve the loop performance. I did have help, however.

The results are shown in the graph below. I made the changes close to 10 am this morning, so we will have to wait awhile to ensure the changes continue to be beneficial. This is particularly true in light of your comments that the tuning on this controller is particularly sensitive to “charge changes.”

I would particularly draw your attention to what the improved control has done to the charge rate (light purple line) to the whole unit. As you know, the bottoms level of the stabilizer sets the charge rate to the Unit. There is considerable lag time between setting the charge, and when it ends up at the tower, but please note how settling the overhead reflux drum level has “lined out” the charge.” Lining out the charge in this way will allow you to dramatically improve the performance of the entire unit. The furnace firing, feed/effluent temperature controllers, hydrogen to hydrocarbon ratio, etc. will also “line out,” allowing us to use the decrease in oscillation to optimize performance or raise charge. And improvements in these qualities could possibly lead to less fouling, reducing your maintenance costs.

While I would like to “bask in the glow,” I unfortunately have to admit that Dr. Bartman (the instructor of the class) gave me the constants (the PID constants, K, T1 and T2) for both the steam controller and the level controller. What’s more, he developed these constants in about 45 minutes from the comfort of his office in Colorado. To put it in perspective, the great tuning job that Joe, Steve and I were unable to do over the course of days, Bob did in about 45 minutes with data I had sent him. (So, total time from Step-Test to controller constants could have been as little as ½ a day.) Also please note that we reduced total unit variation without the use of advanced process control techniques like a multivariable controller, and zero additional capital investment.

I’m writing this letter so that you will know what we did, but also in hopes that I can enlist your support in my purchase of ProControl’s Discover program, which allowed Dr. Bartman to conduct his dynamic analysis and provide this tuning. As a former member of the Aromatics Team, I am quite familiar with the area, and have brainstormed several improvements we could make based upon what we saw today. I’m confident that with the purchase of Discover we could make those improvements in a short time.

We’ll watch the controller for a while and make sure things stay lined out. But if it goes as expected, I hope you’ll help me sell the idea of a Discover purchase to others in the refinery.

Regards, ********.

cc: Bob Bartman, ProControl Inc.