Optimizing Processes with On-Line Infrared Monitoring

KEY WORDS

Process control & instrumentation

Process control systems

Analyzers

Sidebars: Fuel manufacturing optimization Quality monitoring of solid-polymer products Speeding up control of gas-phase polymer reactors

No matter what process variable to be measured, the ultimate purpose of implementing on-line process analysis is to improve performance and increase plant profitability. Real-time analytical information generated about a process with these systems can be used to improve yield, better manage process performance, and limit off-spec losses. To accomplish this profitability goal, the analyzer needs to return significantly greater value than the cost to develop the application, install the system, use it daily, and maintain it.

FT-IR (Fourier Transform-Infrared) and FT-NIR (Fourier Transform-Near Infrared) are among the most efficient, fastest, and reliable process monitoring techniques available today. Having crossed over from laboratory applications, FT-IR, a proven spectroscopic technique, has now been used for process in-line, at-line, and on-line analysis for over 15 years. In some cases, the system’s archived data can also be used to meet environmental regulations. Even in harsh environments, FT-IR systems commonly register up to two years mean time to the first failure and typically offer 99.5% avail-ability. Many users report payback periods of a few months.

How the technology works

The most important difference between process FT infrared and FT near-infrared monitors and other types of IR systems is the analytical process. The Fourier Transform infrared spectrometer in an FT-IR monitor has been proven to be extremely accurate and remain stable for years even in harsh process environments.

An FT-IR system generates measurements based on a Fourier transform. Simply stated, this mathematical process decomposes a signal into a set of sinusoidal components. In an FT-IR spectrometer, this process is performed optomechanically by an interferometer. In its basic form, shown in the FT-IR diagram, the process starts when radiation from a source is directed to a partially transmitting, partially reflecting beam splitter. Half of the beam is reflected to a fixed mirror and the other half is transmitted to a moving mirror. Each mirror then reflects light to the beam splitter, where part of each beam returns to the source. What remains of the two beams combine, and as they combine, they interfere. At the point where the beams are exactly identical in length, all frequencies of light combine constructively. As the mirror moves, each frequency goes through cycles of constructive and destructive interference.

The frequency of these modulations is proportional to the frequency of light and to the speed of motion of the mirror. The interferogram a series of sinusoids and their sum. This sum is called an interferogram and is the signal received by the detector. The interferometer therefore encodes each optical frequency with a corresponding Fourier modulation frequency. The Fourier-encoded infrared signal is digitized at the detector. Finally, the digitized interferogram is inversely transformed by a computer into a spectrum.

Each mirror scan takes approximately a second and produces an entire spectrum. Scans are usually co-added to improve sensitivity, and in a sample measurement the process is repeated twice. First, a set of scans is made with no sample to develop an instrument function. Later, a new set of scans is collected with each sample present. These scans are ratioed against the instrument function to obtain the transmittance spectrum of the sample. This way of obtaining an infrared spectrum is precise, reliable, and cost effective. Other techniques also are available.

A single-process FT-IR analyzer can generate a reading every 10 sec to one minute, depending on the application, while monitoring multiple properties. Measurements can be made on up to 35 streams. Real-time measurements from FT-IR systems can be integrated directly into a plant’s control system for automatic monitoring of liquid, solid, or gas-based processes. Several application-specific systems are now offered by various vendors that can be integrated into a larger process control system.

Process gains from FT-IR

Overall, FT-IR and FT-NIR systems offer several advantages to on-line monitoring, including:

Fast response time (10-60 sec);

Multiple component monitoring of different properties;

Multiple sample-point analyses;

One-time calibration;

High reliability, and

Low cost of ownership.

A system that offers a quick payback and precise monitoring results can generally be configured from standardized components. To ensure best results, control engineers should work closely with a qualified vendor to develop a system most appropriate for their application.

For more information on Analect Instruments, visit www.controleng.com/info

Author Information

Bruce McIntosh is vice president, technology for Analect Instruments. Mr. McIntosh holds a bachelors degree in electrical engineering from Union College in Schenectady, N.Y.

Fuel manufacturing optimization

The refining industry has fallen under pressure from new sectors over the last few years. Rapid fluctuations of crude oil prices now affect how quickly operators must react to optimize production of gasoline or diesel fuel. New fuel composition and properties constraints imposed by environmental laws, such as benzene and vapor pressure limitations, are even more difficult to manage.

One of the best ways to meet these challenges is to improve monitoring of blending and unit operations. Many refineries have turned to infrared spectroscopy to provide this information, and early adopters often use traditional dispersive near-infrared spectrometers. While these instruments can measure single properties like octane number nicely, they often fail at more detailed composition measurements. Accurately tracking complex measurements requires that instrumentation be nearly drift free. Frequent recalibration to maintain accuracy failed on more complex applications such as detailed composition measurements. This was due to instrument drift problems and the required recalibration after service.

Fourier Transform analyzers avoid these problems. An analyzer calibrated for a given measurement application only needs to be recalibrated if the monitored composition is changed. After sufficient trials, FT-IR and FT-NIR units now are being used for blending analysis at many sites including the Exxon Research and Engineering Center (Florham Park, N.J.). FT-IR units installed on individual production units such as reformers or cat crackers can optimize unit production of high value products rather than just monitor the correctness of final product composition.

Quality monitoring of solid-polymer products

Near-IR analyzers have been used in quality control laboratories of polymer manufacturers for a number of years. These analyzers can measure such important product parameters as additive concentrations, co-monomer ratios, and density. Primarily dispersive near-IR measurements have been made on powders and pellets using the diffuse reflectance of the products to obtain required spectra. However, due to calibration problems this technique has not been a viable on-line option.

The unique capabilities of diffuse reflectance FT-NIR analyzers have now been extended to more product types and to on-line sample points. Large area illumination optics have allowed measurements of such previously difficult on-line samples as those in pellet, flake, crumb, and bale forms. High sensitivity detectors, cost-effective small diameter fiber-optic cables, and efficient optics allow the measurement of moving solid products in the production process. Making measurements earlier in the process prevents the production of large amounts of off-spec material.

Speeding up control of gas-phase polymer reactors

Polyethylene is made by several processes. One involves the gas-phase polymerization of ethylene with a variety of co-monomers to produce materials of specific properties. The solid polymer is carried by gas flow, then separated from the mixture. The remaining nonpolymer gas is recycled into the reactor.

Gas-phase polymerization is a very rapid process and accurate control during frequent product changeovers requires timely data. Originally, monitoring of the polymerization process was handled by process gas chromatographs making a measurement every eight minutes, significantly slower than the maximum process response to control inputs.

The primary impetus for improving process-measurement speed was to obtain better quality control and a more rapid lineout of the process after the frequent product transitions. Additionally, faster feedback was also required because this particular process has modes of operation that can result in polymerization of the entire reactor’s contents. If this happens, the reactor must be disassembled and the solidified polymer removed usually with chain saws. Faster composition feedback can dramatically reduce the probability of this disagreeable situation.

To improve process control, a dual-beam process FT-IR analyzer was installed to monitor the feed and recycle streams of one reactor. As a result, the measurement cycle was reduced from 8 min to 42 seconds.

The line graph includes a set of measurements for both the original gas chromatagraph and the new FT-IR analyzer from the same process transient. Smooth traces indicate the FT-IR predictions on four components. Stepped traces are the chromatagraph outputs for the same transient. Comparison of these traces indicates the inability of the gas chromatagraph to provide sufficient data over time to allow proper process control. After initial testing on a single process line, all of the plant’s reactors were equipped with the FT-IR analyzers.

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