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Jul
12

Wireless Online Process Metrics

It is hard to imagine that in this day and age we are relying on “after the fact” process performance analysis on a gross scale. I wonder if we still use spreadsheets and costs sheets to judge a process unit and operator performance. We can see differences in setpoints at a shift change but have little idea as which shift’s operating points provide the most production and least cost of goods (COGs). If pet food and ethanol manufacturers have online process metrics to compare shift and plant performance, why don’t we see more of it in chemical plants? As COGs, energy conservation, and environmental emissions become of paramount importance, the benefits of online process metrics should be obvious. Since “seeing is believing” the use of wireless measurements moved from unit operation to unit operation can help develop and justify installations.

For example, if the utility flow and the inlet and outlet temperatures or pressures to coils, heat exchanger, and jacket are measured for each reactor, evaporator, and crystallizer, the energy use rate can be put online as an indicator of reaction, evaporation, or crystallization rate. The energy use per unit feed used or product produced can be monitored online for energy conservation. The overall heat transfer coefficient can be computed online to provide a performance indicator of fouling.

Inlet and outlet pressure measurements provide an indication of fouling (a heat exchanger specialist in my days in Monsanto’s Engineering Technology said the pressure drop through the tubes of a heat exchanger is the best indication of fouling). Pressure measurements also show when a key unit operation is being starved for cooling supply especially since cooling tower and refrigeration systems often lagged beyond production increases in the 1990s. For example, a sold out fed-batch reaction at the top of a structure was periodically starved for coolant water by less profitable batch operations. Override control cut back the feeds to prevent a temperature trip but wireless measurements could have led to a more profitable scheduling of unit operations and improvement in coolant system design. Piping system and pump problems also extend to the process side.

An example that had all of the above problems big time is the stressed out production unit in Figure 3-6 in Advanced Control Unleashed The multiple lines of reactors, evaporators, and crystallizers converging and diverging were going up and down like Yo-Yos due to frosting (formation of crystals on surfaces) and subsequent defrosting (heating to melt the crystals). The plant was operating at double its original design capacity. Controller outputs would go to their high limit due to pipelines and pumps that were too small. Bigger control valves did not help much because the available pressure drop was insufficient. De-bottlenecking projects actually reduced capacity due to lack of measurements and the process knowledge they provide.

Thermowells on columns can find the most sensitive tray location by making a simple manual change in the flow manipulated by the temperature control (distillate flow). The best tray is the one with the largest and most symmetrical change in temperature for changes in the distillate to feed ratio (greatest and most linear process sensitivity or gain) as shown in the Figure 7-6 of the ISA book Advanced Temperature Measurement and Control Figure 7-7 shows the consequence of selecting the best tray and not so good tray for closed loop temperature control (Advanced-Temperature-Distillation-Column-Control-Excerpt.pdf). Three dimensional (3-D) profiles can provide a dramatic visualization of the change in the process gain with time as feed composition changes.

3-D plots generated of cross direction and machine direction sheet thickness profiles from a meter traversing the sheet provided the knowledge for optimizing the mixer, extruder, and sheet line as studied in the ISA 2001 conference paper Constrained Multivariable Control of Plastic Sheets

3-D plots are a natural for batch operations to show composition (e.g. cell density), pH, and temperature batch profiles for a series of batches.

For more information do a search for “metrics” on this website and you will find a variety of opportunities discussed for online process metrics and plots to help operations, process technology, and process control improvement initiatives. Wireless measurements and metrics can fill in the missing information for process simulations and vice versa.

In the old days we installed thermowells, pressure connections, and orifice flanges so we could move temperature and pressure gauges around to provide coarse local indication of equipment and piping system performance. Today portable wireless measurements can provide accurate indication in the control room offering opportunities that could be subject of a book or two let alone the blogs on this website.

Since efficiencies and most of the process gains for temperature and pressure are function of flow ratios (reactant/product, fuel/product, fuel/steam, steam/product, reagent/product, and coolant/product) flow is a key measurement. While a favor coriolis mass meters for process stream since they provide an accurate mass flow and density useful for inferential measurements of composition, which ultimately what you want to know, averaging annubars and thermowells can enable the use of wireless DP and temperature measurements for computing mass flow. Just knowing the material balance would be a big step forward as discussed in the Jan-Feb 2010 InTech article Advances in Flow and Level Measurements Enhance Process Knowledge, Control

Wireless pH and conductivity offers the ability to develop inferential measurements and prove the best electrode technology as revealed in the Jan-Feb 2010 InTech WEB Exclusive article Opportunities for Smart Wireless pH, Conductivity Measurements