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Brian: If the process is one like we have here in Ann Arbor, Mich., it is because near the beginning of the process, you have the biomass and food (waste termed as biological oxygen demand, or BOD) so the biomass (bugs, if you will) is consuming the waste as fast as it can, using up the oxygen present. As the waste stream continues to the lower side we are still supplying oxygen (air blowers) but as the food (BOD) is depleted, the dissolved oxygen (DO) increases. If we do not see this increase, then we do not know for sure that the BOD has been consumed. And if we have a higher DO in the first pass, then we have to worry if the bugs are able to do their jobs (did the return activated sludge (RAS) pump fail? Have the bugs been killed by some unknown toxin dumped into the waste stream?
Greg: Whether you are talking about bioreactors for waste treatment or pharmaceutical production, the uniformity of the dissolved oxygen concentration depends upon the relative locations of the injection of air and feed of food and the degree and type of mixing. Most of the transfer of oxygen occurs from rising bubbles rather than from the overhead vapor space. If you also take into account that the additional liquid head increases the partial pressure oxygen, a change in concentration of a factor of five or more is possible. The gradient can even be greater for the manufacture of new, more complex proteins for drugs because the agitation is intentionally gentle to avoid breaking apart the animal cells, which don’t have cell membranes like the bacterial cells used for older products.
Stan: Yet just a point measurement is often made by the installation of a single or dual set of dissolved oxygen electrodes.
Greg: An experienced instrument engineer can help ensure the bioreactor has a series of nozzles to establish a profile and even resurrect the dissolved carbon dioxide electrode, which would tell the other half of the story for animal cell cultures.
Stan: Unfortunately it may be hard to find one. Our alma mater went from several hundred instrument engineers in 1960s to a handful. In fact, the exact number is unknown because they are laying low. If you go to a local ISA meeting you will see 20 vendors, five retirees, two students, and one user. The ratio of Power Point slides to users at these meetings is upwards of 50 to 1.
Greg: The instrument engineer is an endangered species. The best place to view them is at a retirement resort controlling the pH of their pool. However, if you want to see one under the age of 50, your best bet is a “User Group Meeting” where you can lure them in for a closer look with free software, although free pizza has been reported to work as well. They are all but extinct at corporate headquarters and, at the plants, they outnumbered by 10 to 1 by process engineers. The process engineers end up trying to buy instruments, making the plants vulnerable to the deal with best hype or lowest price. The classic example of this is a supplier of manual on-off valves slapping on a cheap positioner and pushing the “Sloppy Joe” package as a control valve.
Stan: A real instrument engineer can select and size a real control valve from a real control valve manufacturer. Now it mostly left up to whoever has the attention of the process engineer.
Greg: You use to be able spot a real instrument engineer by the number of catalogs. Now you need to look for the number of screens and software packages on his desk.
Stan: Why is the instrument engineer a rare find? The answers may be in the standard dialog on the causes of endangerment.
Habitat Destruction: Our planet is continually changing, causing habitats to be altered (industry is going offshore). When changes occur at a fast pace, there is no time for individual species to react and adjust to new circumstances (instrument engineers are too busy doing their job to sell their value to make existing processes more competitive).
Introduction of Exotic Species: Exotic species (programmers) are interlopers. These species were introduced into new environments by way of human activities (computerization). These interlopers may be viewed by native species (instrument engineers) as foreign elements. They may cause no obvious problems and may eventually be considered as natural but they may seriously disrupt delicate ecological balances (between instrument engineers and management).
Overexploitation: A species that faces overexploitation is one that may become extinct due the rate in which it is being used (bizarre project schedules and budgets). A moratorium on the hunting season (by cutback-crazy management) is being sought. Adaptation to a changing environment can be achieved (by the introduction of new software to measure and document process performance as a function of control system performance).
Greg: Is your job headed for extinction? Here are the top ten signs.
This Month's Puzzler:
You Can’t Drop a Control Valve on Him
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