Chemical companies are striving for more complete and reliable process control information to tighten adherence to product specifications, reduce waste and identify areas ripe for process improvement. This is spurring a drive to build more capable and agile process instrumentation and, with it, recasting the role of instrumentation in production.
The last ten years have seen a flurry of development activity — much of it driven directly or indirectly by the New Sampling/Sensor Initiative (NeSSI), a 15-year-old effort sponsored by the Center for Process Analysis and Control (CPAC) at the University of Washington, Seattle. NeSSI was born with the mission to standardize and miniaturize sampling systems to make them less expensive to deploy and able to fit in more-space-restrictive environments. This brings the instrument systems closer to the process by minimizing distance between analyzer and process. In turn, making the instruments faster markedly improves the potential of a timely response.
Instruments don't provide control information directly, though. An interpretation step is necessary to achieve the control benefit. However, a decreasing number of skilled analysts are available to convert the data into useable information. So, software has to be made smarter. Fortunately, techniques from the world of chemometrics can add to system intelligence.
SHRINKING THE SAMPLING SYSTEM
When we decide to place an instrument on-line, there has to be a connection to the process. The total cost of ownership of any analyzer is tied to the complexity of the application and the engineering effort to plumb the system into the line. Installation and maintenance often exceed the purchase price of the instrument. Various reports, e.g., Reference 1, document that the NeSSI platform reduces lifetime costs by as much as 40%. With the miniaturization and the cost advantage in place, we can look toward putting instrumented systems in locations where previously no logistical or economic driver existed.
We can see the advantage of standardization and small size represented by NeSSI by comparing two recent installations (Figure 1). The system on the left is a conventional plumbing project designed to handle six streams and feed into a near-line gas chromatograph. On the right is an eight-stream system executed in the NeSSI manner. It's clear that the compact nature of NeSSI enables instrument placements that would be too unwieldy in larger format.
So now, where we only might have considered installing one or two simple probes (e.g., for pressure or temperature), we can look toward a more-data-rich source with which to characterize the process at this sampling point — if we take advantage of the capabilities.
We also must move beyond the simple measurements of temperature, pressure, flow and level. These parameters simply don't offer enough information content to monitor effectively the chemistry differences we expect. Optical spectroscopy allows us to differentiate analytes based on functional groups (aromatic versus aliphatic versus olefinic, etc.), which ties to many critical physical properties. Chromatography, especially gas chromatography (GC), is perfect for separating complex mixtures and characterizing their population based on molecular weight.
Remember that one of our purposes in placing the NeSSI components on-line is to improve response time. Spectroscopy already is faster than most process control systems are set to react, but GC historically has been much slower. So, we'll focus on how the interplay between NeSSI and gas chromatography is helping address this issue.
REFINING GAS CHROMATOGRAPHY
Over the last ten years, a substantial effort has focused on making on-line GC more responsive to the control world. Throughout, the goals have been to:
• speed the response time;
• reduce an on-line chromatograph's footprint; and
• better utilize computational advances to extract the information content of the data stream.
Ultimately, we want to turn a process GC into an appliance. As we work to attain this goal, some fundamental changes are needed both in our approach to hardware and the scope of the software.
With the advent of NeSSI fluid handling and a standardization of the sampling approach to the process stream, the trend clearly is toward making the instrument smaller as well. Today, a number of compact and even micro GCs are available (Figure 2).
Some portions of a gas chromatograph are amenable to shrinkage but, ultimately, there's a tradeoff between the number of applications that can be handled and the system dimensions. So, we need to adopt a Goldilocks approach — i.e., selecting an instrument that's just right. With the advent of direct on-column heating, we can eliminate the largest source of power consumption and weight, the chromatographic oven. This excises well over half the weight and 75% of the size. Also, by removing the oven from the design, we gain flexibility in analyzer placement. We often can get away with abandoning an air-conditioned near-line structure in favor of a simple sun- and-rain shield, allowing system positioning much closer to the sampling point.