What Have I Learned? – Ratio Control (Part 1)

Ratio control provides coordination of multiple flows. One flow is an “independent flow” that is used to set production rate. Sometimes this flow is also termed a “wild flow” when the availability of this flow is not determined by the production unit. In a ratio control system, the process variable (PV) or set point (SP) of the independent flow (leader) is multiplied by a ratio factor and becomes the set point for the dependent flow (follower). Slide 1 in RatioControl.pdf shows two flow loops in a ratio control system.

If the flow is noisy, the SP of the independent loop may be preferred. Flow transmitter damping or signal filtering can be used to smooth out the noise but this adds a lag that reduces the ability of the flow loop to deal with pressure disturbances and valve issues. If pressure swings and valve response problems are negligible, the slowing down of the independent loop (leader) by the use of a signal filter may be useful in allowing the dependent flow loop (follower) to catch up with changes in production rate. If this is not the case, then the signal filtering is only put on the independent flow PV passed for multiplication by the ratio factor. I favor using whatever means possible to eliminate noise so the ratio control can use the PV rather than the SP of the independent flow loop to reduce the downstream errors from the transient response of this loop.

Regardless of whether the PV or SP of the independent loop is used, the measurement should have good repeatability and rangeability, the control valves should have minimal backlash and sticktion, and the controllers should be tuned so the follower can keep up with the leader to minimize the errors downstream.

Some blend tanks totalize the ingredient flows and use a tank blend controller to correct the input ratio to keep the blend composition in the tank closer to its target. The total in the tank for the independent feed is multiplied by the ratio, which is the set point for the total in the tank for the dependent feed. The actual total of dependent feed is the process variable for a tank blend controller to correct the ratio control system on the tank’s input flows. A proportional only controller may be desirable. The totalization of flows can be done on a batch or continuous basis. For a continuous blend tank, the material balance Equation 4-7f (without the reaction rate) in the Advanced Application Note “First Principle Process Gains ….” posted March 25, 2009 on this website is integrated. For this blend system, achieving a particular ratio is the final objective. For most ratio control systems, the target ratio changes with the composition, physical properties, and temperature of the input flows.

When a critical process variable loop is used to provide feedback correction of the target ratio, the independent flow multiplied by the ratio factor is called flow feedforward and the ratio factor may be called a feedforward gain. Some people reserve the term “ratio control” to the case of no feedback correction of the target ratio.

There are many examples of ratio control and its extension to flow feedforward control. A simple example is the inline control system where ingredient flows (main and additive flow) are added to a pipeline mixer as shown in slide 2. Often this pipeline mixer is simply a baffled piece of pipe called a “static mixer”. The combined stream coming out of the mixer is at the current ratio set by the inputs to the mixer. Sometimes the real intent is to provide a specific viscosity, density, percent solids, or consistency. In these cases, online measurements of these critical process variables at the exit of the static mixer are used in a loop whose output provides feedback correction of the target ratio.

Another examples of ratio control is catalyst to reactant feed ratio control as shown in slide 3. An enhancement used for this application is a correction for catalyst activity, which is particularly important when the catalyst is recovered and recycled. Property estimators based on batch conditions and completion times biased by at-line or lab analytical measurements are used to provide feedback correction of the target ratio.

Reactors typically use ratio control of reactant feeds. It is desirable to have an online analyzer to provide automatic correction of the target ratio of reactants as shown in slide 4. The independent flow may be the main reactant feed or a recycle reactant feed.

Neutralizers often use flow feedforward where the pH controller corrects the target ratio of reagent to the main flow (e.g. influent flow) when accurate flow measurements with sufficient rangeability are available. For food sweetener production it was found that the mass flow ratio control by the use of coriolis flow meters was tighter than pH control. The pH was then relegated to indication only. This was an extreme case where the feed compositions had tight specs and the set point was on the flat part of the titration curve so that the error in the pH measurement corresponded to a greater error in the ratio than what was achieved with the coriolis flow measurements.

Temperature control of heat exchangers is often improved by flow feedforward where the coolant flow is ratioed to the feed flow and corrected by the temperature loop. Feed forward control of columns has saved millions of dollars in many plants by a straightforward ratio of the reflux or distillate and/or steam flow to the feed flow and correction of the target ratio by a tray temperature control loop.

Combustion control of boilers and furnaces rely on air to fuel ratio control. In some cases, stack or combustion zone oxygen analyzers are used to correct the target ratio for the changes in mixing efficiency and heating values of waste fuels.

Have you ever wondered why so many ratios exist? Is it just convention or is there a fundamental underlying reason? Why do some users prefer feedforward summers over feedforward multipliers for target ratio correction? Why do oxygen controllers provide a correction of a calculated air flow rather than a target ratio? If waiting on the answers is going to keep you awake at night, you can call me at 512-832-3029 and I will tell you an answer that will put you to sleep. Warning from the Automation General: “Calling Greg McMillan while driving a car is hazardous to your health.”