Temperature

Better temperature sensing and data gathering

Innovative controls, accessories, software and other techniques are enabling temperature instruments to reach in and help optimize many previously inaccessible applications. Here's how.

By Jim Montague

Not much seems to change on the temperature front. Thermodynamics are physical laws, after all, and not guidelines or suggestions. Processes get hot and cold, and they're measured by thermocouples, RTDs, thermistors and other instruments just as they've been for decades.

What's different and evolving quickly are all the components, controls, software and other enhanced methods surrounding the basic roots of temperature measurement. These devices are allowing more and better temperature sensing and data gathering in many locations that used to be inaccessible, and they're gathering and organizing that information for much better archiving, analysis and eventual process optimization than was possible before. So just like pressure, flow and other immutable process variables, temperature isn't changing, but the world around it sure is.

Smart Heat Saves

For instance, Hydro Aluminum in Årdal, Norway, recently sought to adopt low-temperature Oxyfuel burner technology to increase its cold-metal melting capacity and save energy, so it worked with Linde Gas and with Eurotherm by Schneider Electric to provide an intelligent burner control system for four aluminum furnaces in its cast house (Figure 1). In general, Oxyfuel increases thermal efficiency because its furnace gases don't contain nitrogen like air fuel, so its radiant heat transfer is more efficient, and its exhaust gas volume and heat loss are reduced.

Hydro Årdal produces more than 125,000 tons of foundry alloys per year, and Oxyfuel combustion in its four furnaces is managed by Eurotherm's Foxboro T2550 process automation controller, a safety PLC to comply with regulations and an HMI, which operate in conjunction Linde's Gas Oxyfuel burners. As expected, Hydro reports it can replace 20,000 tonnes of liquid primary aluminum by remelting cold metal. Previously, the charge mix was 8 tonnes cold and 22 tonnes liquid metal in production line, and now the mix is 13 tonnes cold and 17 tonnes liquid metal. As a result, Oxyfuel helped the two primary foundry alloy production lines at Hydro Årdal achieve a 60% increase in remelting capacity, a 50% reduction in fuel consumption, a cut in CO2 and NOX emissions, and a reduction of waste dross to less then 2%.

See also: pH and temperature measurement and control tips

"By using low-temperature Oxyfuel, we can melt 50% more cold aluminum in the same amount of time as we did previously," says Wenche Eldegard, cast house manager at Hydro Årdal. "In addition, our propane consumption and CO2 emissions have been halved, which is also very positive."

Controls Optimize Acid, Cook Biomass

Just like a chef perfecting a recipe, today's controls can surpass simply babysitting their temperature instruments and applications to gather and analyze data, and more quickly reduce variability and improve their processes and the quality of their end products.

For example, Sterling Chemicals in Texas City, Texas, produces acetic acid by reacting carbon monoxide and methanol in the presence of a catalyst, and then purifying it in an associated distillation train (Figure 2). Greater efficiency was possible with a better catalyst, but this decreased reactor temperature control stability, and the existing PID-based controls couldn't prevent temperature excursions during upsets. Also, a possible advanced control solution was hindered because the reactor couldn't be bump tested due to its carbonylation exothermic reaction.

To better control reactor temperature despite the interaction of feed, heat-reducing effluent and recycle flows, Sterling's engineers implemented a QuickStudy adaptive process controller, which was first used to create temperature/reactor disturbance variables models from off-line historical data, and was then put in simulate mode to learn how well the models could predict reactor temperature using more production data. QuickStudy is built and integrated by Adaptive Resources. Correlation between predicted and actual temperature was good, and steady-state gains from QuickStudy were similar to those in a calibrated unit model.

Next, the reactor's PID temperature controller was replaced by a QuickStudy predictive controller setup (PCS) block, which manipulated the existing boiler pressure controller. The dynamic models developed earlier were loaded, and then the Quick Study block was placed in control after a short online learning period. Improvements in temperature stability appeared within hours as QuickStudy refined its internal models, and disturbances that would normally have caused upsets were attenuated. The whole process was carried out in two days without the need for bump tests.

"QuickStudy significantly reduced variation in the acetic acid reactor temperature and increased throughput by more than 5%," says Trevor Arnold, senior process control engineer at Sterling Chemicals. Other benefits included reducing Sterling's temperature standard deviation from 3.6 to 0.8, eliminating high-temperature trips and stabilizing purification columns.

Similarly, GranBio Investimentos SA in Sao Paulo, Brazil, recently built its Bioflex 1 second-generation ethanol plant in Alagoas, Brazil, to produce inexpensive, clean-burning biofuel from readily available biomass. The facility started operations in early 2014, can produce about 82 million liters of biofuel per year, and relies on Yokogawa Electric Corp.'s Centum VP integrated production control system, csTuner control loop tuning software and Centum VP Batch software. They manage Bioflex 1's pretreatment, enzymatic hydrolysis, fermentation and distillation separation units; field instruments, including Foundation fieldbus Admag AXF magnetic flowmeters, DPharp FJA series pressure/differential pressure transmitters and control valves; and Plant Resource Manager (PRM) asset management software.

During the pretreatment process, agricultural materials and waste from energy cane, bagasse and straw must be carefully cooked at steady temperatures to break down the cellulose in their cell walls and remove polymers like hemicellulose and lignin. This is necessary before the pretreated fibers are further broken down using enzymes to obtain double glucose and then single glucose, which is then fermented by microbes to produce ethanol.

Pretreatment and fermentation settings are preconfigured in Centum VP Batch and then automatically downloaded to and executed by individual controllers, depending on which recipe the operators select based on which raw materials need to be processed. All-in-one HMIs with ergonomically designed process graphics show production trends, alarm summaries, control information and performance data for each completed batch. Also, because many of Bioflex 1's nearly 2,000 I/O points are exposed to high-temperature processes and are mounted in high, narrow or difficult-to-access locations, the plant's PRM allows their status and health to be monitored from its central control room with a Windows Explorer-like interface that enables maintenance engineers to quickly identify devices that need online diagnosis. This tool is especially valuable during plant start-ups when engineers and operators must check numerous loops and control valves.

"Brazil is a dominant player in the bio-ethanol market, and the biofuel market is steadily growing, so second-generation biofuel is attracting attention as an environmentally friendly energy source," says Manoel Carnauba, GranBio's vice president. "Our highly reliable field instruments and control system from Yokogawa also allowed us to complete the specifications for this second-generation ethanol plant at an early stage; maintain consistency with all specifications, which reduced the total cost of the control system; and complete our project on schedule and on budget."

Wireless is Cool

No surprisingly, one of the primary aids to collecting more and better data from temperature instruments is wireless networking.

"Temperature sensing and instrumentation via thermocouples for wide-range RTDs and PTC thermistors for narrower ranges is the same as always, but what's new is wireless temperature thermostats and transmitters, which also have added microprocessors that can collect histories, alarm detection, safety and other data," says Paul Gobeille, senior automation engineer at system integrator Stellar Refrigeration Services in Jacksonville, Florida. "So where you'd previously have a temperature switch for safety, now you can have temperature transmitters for your safety instrumented system (SIS), achieve high SIL performance, and comply with IEC 61511 and ANSI ISA S84 standards."

Gobeille reports that Stellar uses a variety of reliable, wirelessly networked temperature devices such as intelligent HART transmitters for cooling air and water and heating water in its design, build and integration projects. "We need to reduce field wiring and improve commissioning wherever we can, and wireless means we're not running around with ladders as much, which is a big help," he adds. "Wireless components are also easier to install, very reliable, less likely to get damaged, and most provide access to saved and stored data for sophisticated asset management programs and for performing more accurate calibrations. We estimate the wireless for temperature approaches 40% less cost in wiring and commissioning time."

Likewise, system integrator Patti Engineering in Auburn Hills, Michigan, recently helped Hill Country Bakery in San Antonio, Texas, upgrade the temperature monitoring and control system in its multi-zone, 55,000-square-foot cold-storage and distribution center, which stores ingredients and finished products that must be kept within a narrow temperature range to ensure product quality and safety. However, the bakery's old temperature monitoring system had been continuing to degrade, causing gaps in data collection, and had no tracking of corrective actions when temperature issues were discovered. Plus, the center's multiple zones had hardwired temperature probes, which required manual data collection and were prone to damage by forklifts.

Consequently, Patti implemented a new, wireless temperature monitoring and SCADA system with automated, real-time data collection, central SQL database for report generation, and the ability to monitor and receive alarm notifications via mobile devices. The integrator installed wireless temperature sensors from FreshLoc Technologies and used Web Studio software from InduSoft (www.indusoft.com) to develop the system's HMI and SCADA system.

Besides allowing the temperature probes to report remotely and via mobile devices, Patti also programmed them to communicate directly to their network gateway via Modbus TCP, which enables the Hill Country's engineers to cut out any PLCs or other controls between the sensors and the SCADA system, saving hardware costs and engineering development time. As a result, its new temperature monitoring system allowed Hill Country to establish a Hazardous Analysis and Critical Control Points (HACCP) food-safety plan that was benchmarked by the Global Food Safety Initiative (GFSI), and this enabled it to achieve Safe Quality Food (SQF) Level 2 Certification.