I am back from vacation. I am still feeling fine from a nice break from the heat of a book deadline and Austin’s record temperatures. I was up north in Minnesota and Wisconsin where it was 25 degrees cooler. I happened across an exhibit of Cray computers in the Museum of Science and Technology in Chippewa Falls, the home of Cray Research, Inc. Samuel Cray attributed part of the company’s success to a motto of “taking our jobs seriously but not taking ourselves seriously.” Hopefully my Control Talk column is an example of this motto by combining a humorous look at ourselves with technical straight talk. A compilation of the column’s comics was featured in the July issue of Control magazine in the online section “Out of Control Cartoons“.
Then there are the outbursts of craziness designed to loosen us up such as The Funnier Side of Retirement for Engineers and People of the Technical Persuasion, which just won the ISA Raymond D Molloy Award as the best selling book in 2008. Since humor is derived from exaggeration of commonly recognizable traits, please don’t buy this book if you want a detailed analytical realistic treatise. For this you can get any one of a dozen or more guides to retirement. If you like bizarre humor, this book may offer some laughs.
The following list of articles and associated papers in 2007 – 2008 are totally serious except for an occasional top ten list.
“Improve Control Loop Performance“, Chemical Processing, Oct, 2007
(1) Nearly all control loops eventually affect the process by the manipulation of a flow via a control valve. Control loop performance depends upon valve performance.
(2) Valve specifications do not require a valve actually move in response to a change in signal. When valve performance has been considered, response time and rangeability are frequently the criteria. The real issues are valve resolution (sticktion) and deadband (backlash). If a properly selected and sized valve-actuator assembly has good resolution and sticktion, the valve will generally have good rangeability and response.
(3) Using a “state of the art” digital positioner can eliminate the positioner sensitivity problems prevalent in positioners for the last 50+ years but the positioner can be lying about valve performance if the feedback measurement is actuator shaft rather than ball or disk position in a rotary valve. Putting a digital positioner on a valve designed for on-off service and tight shutoff by a piping manufacturer is like putting makeup on a pig. On the other hand, putting a digital positioner on a valve designed by throttling service by a control valve manufacturer may be the best thing you can do for your loop.
(4) For pH control, the resolution of the control valve can determine the number of stages of neutralization needed.
“Virtual Control of Real pH“, Control, Nov, 2007
“Advances in pH Modeling and Control“, ISA 54th International Instrumentation Symposium, Pensacola, May, 2008
An online virtual plant can be adapted to match the actual plant by the simple innovative use of an integrated model predictive control (MPC). In this neutralization system, the influent acid concentration was quickly adapted to match the ratio of reagent to influent flow in the virtual plant to the actual plant. The virtual plant demonstrated of ability of model predictive control to replace fuzzy logic control for reagent optimization. An improvement in the kicker algorithm provided immediate savings of more than $100K per year in reagent cost.
“PAT Tools for Accelerated Process Development and Design“, Bioprocess International, Process Design Supplement, Mar, 2008.
Pharmaceutical Manufacturing, June, 2008
Most process and control system improvements in bioreactors are set by biochemists and biochemical engineers in the research. A virtual plant running 500 times real time can complete a bioreactor batch in 15 minutes that would take several weeks in the lab or pilot plant. Virtual experimentation can accelerate process development and design. The integration of advanced control tools in the virtual plant can demonstrate the effectiveness of substrate and batch profile control. The results can justify additional online analytical measurements. The fast playback of virtual and actual plant batches in a minute or two offers incredible opportunities for online analysis via integrated data analytics and adaptive control tools. The potential benefits are faster commercialization, higher yields, and real time release.
“Unlocking the Secret Profiles of Batch Reactors“, Control, July, 2008
The purpose of a batch reactor is to manufacture a product of a particular composition. The progression of the batch to the desired end point (the batch composition profile) is the most important indicator of batch performance. However, batch reactors rarely have any measurement of this profile. For chemical reactors, the main measurements indicative of the hidden profile of real interest are pressure, temperature, and feed flows. Multivariate statistical techniques such as Projection to Latent Structures (PLS) may be able to predict end points but the composition profile still remains a secret. If actual or inferential measurements of the profile are available, model predictive control can maximize the slope of the profile and hence the progression of the batch. The result is a faster batch for a given end point or a higher end point for a given cycle time. Also, the variability in batch profiles is transferred to feeds resulting in more repeatable batch profiles.
There is a misconception that biological processes are not as highly automated as chemical processes. Bioreactors generally have more control loops than a typical chemical reactor. Cell cultures have temperature, pressure, air flow, oxygen flow, inert flow, carbon dioxide flow, sodium bicarbonate flow, substrate flow, nutrient flow, pH, and dissolved oxygen control. Major advances in at-line composition measurements, such as the Nova Bioprofile Flex Analyzer combined with an auto sampler can provide measurements of substrates, nutrients, byproducts and cells every 4 to 12 hours depending upon the application. The Fogal Dielectric Spectroscopy probe can provide a measurement of the integrity of the cell membrane (cell viability). When combined with a turbidity measurement of cell density, the Fogale probe offers an online indication of live and dead cell concentrations.
One of the obstacles of online composition control is the time delay from the sample cycle time. The time in between samples for at-line analyzers can vary from an hour to a day. Fortunately, an unexpected side benefit of the enhanced wireless PID (developed to handle the larger and more variable time delays of wireless measurements) is exceptional control using measurements from at-line analyzers. The wireless enhanced PID has been shown to provide tight and stable control using at-line analyzers in specific studies for glucose control and in generic studies for continuous and batch processes. The results are documented in slides 29-34 of Interphex2009_Advances_In_Bioreactor_Modeling_and_Control.pdf. See the May 11, 2009 entry “What have I Learned – Cost and Source of Oscillations (Part 4)” for more details.
The new control algorithms (max slope MPC setting the enhanced wireless PID) coupled with new at-line and online analytical measurements will make bioreactor profile control common place leaving chemical reactor control even further behind. Are we going to let this happen?
Next week we conclude with the 2009 articles that include results of wireless control in a bioreactor with a disposable liner called a “Single Use Bioreactor” (SUB).