Measuring physical properties is no longer a problem for most processes. Vendors are able to deliver instruments that are cheap, reliable and accurate. Unfortunately, this is not the case with the online analyzers used to measure the molecular composition of liquids and gasses. They have a well-deserved reputation for being expensive, unreliable and prone to drift and inaccuracy. The good news is that the technology is getting better, and we have some real-world examples to prove it.
Why Go Online?
Process plants have been analyzing liquids and gasses and even solids for a long time, but much of this analysis has been performed in on-site labs. Since most process plants know how to analyze offline, why should major changes be made to go online?
“Real-time viscosity measurement improves our timing, therefore, we end up with less variation,” says Danny Cox, corporate process control supervisor at polyester resin maker AOC, Collierville, Tenn. Moving online and measuring viscosity at the reactors allowed technicians to cool the reactor and stop the reactions immediately once they hit a desired viscosity.
Dare County Water of Dare County, N.C., installed online free chlorine residual analyzers at remote pumping stations. “Unlike our prior offline analysis, we now have continuous monitoring of chlorine levels, alarming if chlorination equipment malfunctions and trending capabilities to monitor the effects of dosage changes,” reports Dare County Water utilities director Ken Flatt.
Moving analysis online yields real-time measurements and ultimately faster cycle times. “In real-time applications faster cycle times are essential to permit detection of small process deviations,” according to Steve Miller, the chief technology officer at Scott’s Specialty Gases, Plumsteadville, Pa.
Scott’s application is the low-level analysis of moisture in corrosive gases for the electronics industry. The company is evaluating a Tiger Optics analyzer configured specifically for this application. “Our objectives are to determine that the instrument is able to make an accurate, repeatable and traceable measurement of moisture at and below 1 part per million in a corrosive gas matrix. Once it is proven that the measurements meet our quality standards, we will move on to investigate the potential of the analyzer as online unit,” says Miller, adding, “For five months the analyzer has survived a steady diet of corrosive gases with no noticeable detrimental effects. The main benefit that we have observed in our testing thus far is that the wet-up (response to moisture with sample or cal gas)/dry-down (with dry purge gas) cycle times are three to five times faster than with other analyzer types. We attribute this benefit to the low wetted surface area of the analyzer afforded by the compact design of the cavity ring-down spectroscopy cell.
Online Measurements Versus Lab Measurements
Challenges of Going Online
“Pressure, level, flow and temperature are physical properties where accurate measurements are considered less difficult to achieve than the process measurements made by analyzers, which must determine the molecular composition of a process stream,” says Doug Simmers, worldwide combustion product line manager for the Rosemount Analytical Gas division of Emerson Process Management. “The chemical sensor of a process analyzer is often in direct contact with the process stream to provide accurate molecular composition readings, leaving the sensor unprotected from degradation. Process analyzers also tend to drift more in their readings than physical measurement transmitters because they provide measurements in the miniscule ppm range.”
Jason Kuzmiak, the BioTector product manager at Ohmart/VEGA, adds, “Analyzers must take samples that are often corrosive, volatile, oily and/or fatty into their sensing chamber. Their complexity often relates to pre-treating the samples to make them measurable within the sensing chamber. This complexity is even more pronounced when dealing with gas analysis.”
Another significant challenge is converting the analyzer’s output signal to a format recognizable by the control system. “When we moved our reactor viscosity measurement online, we had to covert the serial output from the analyzer to an OPC format in order to tie it to the control system,” says AOC’s Cox.
For some end users, the challenges are so great that they are forced to verify online measurements in the lab continually. “For dissolved oxygen analyzers, we have experience with every type of technology, and we have not found any satisfactory for reliability and accuracy,” says Philip Daniels, engineering supervisor at the King County Wastewater Treatment Division, Seattle.
“The dissolved oxygen process is not easy to monitor on a continuing basis without a lot of maintenance and continuous equipment attention,” he adds. “We make grab samples for the lab several times a day to verify the accuracy of the meters. We then basically use the online analyzers for relative control without real concern for the accuracy, only the short-term repeatability.”