Apr-2014

Reformer monitoring via in-tube temperature measurement

A temperature monitoring technology applied to steam reformers in synthesis gas plants provides protection against over-firing plus detailed catalyst monitoring

OLIVER J SMITH IV, Air Products and Chemicals, Inc
BILL COTTON, Johnson Matthey

Article Summary

Large-scale steam reformers in syngas plants (including ammonia, methanol and hydrogen production) are vulnerable to over-firing, especially during plant transient conditions. This vulnerability is due to the fact that external temperature instrumentation lags during normal operation, and that human monitoring cannot be conducted on a sufficiently frequent basis. The consequence is that, over the past decades, many of the catastrophic reformer failures documented have occurred during the most common transient operation — start-ups.

Despite repeated education as to the risks of over-firing, these events continue to happen. It is considered even more important to ensure that regular and frequent visual inspections of the reformer tubes are made during critical periods associated with transients. Most reformer instrumentation is designed for monitoring and control of operation during normal operation. During plant transients, when flow rates are well below design values, reliance on standard plant instrumentation measurements (of the header or pigtail exit temperature) alone is dangerous. In low flow situations, the measured reformer gas exit temperature is not a good indication of the metal temperatures of the reformer tubes.

A relatively new temperature measurement technology, called CatTracker, is available. This technology allows in-tube temperature monitoring of the catalyst tube, enabling the reformer to be more reliably monitored and protected from over-firing. It can also give additional information during normal operation that can be used to better monitor the performance of the catalyst and optimise reformer operation.

This article reviews the benefits of CatTracker for safety and monitoring of catalyst performance that was gained after installation in a number of reformers. These installations include several hydrogen reformers operated by Air Products and Chemicals.

Temperature measurement
Previous papers1 have discussed the details of the various techniques for tube wall temperature measurement. While these benefits will not be addressed in this article, Table 1 summarises the advantages and disadvantages of the various measurement techniques for reference purposes.

Optical and gold cup pyrometers do not allow accurate temperature measurement at all points along the tube. It is therefore likely that by using this techniques alone, the maximum tube wall temperature may not be measured.

The reformer imager allows for the measurement of the tube wall temperature profile along the full length of the reformer tube and hence will allow for the measurement of the maximum tube wall temperature. A further benefit with the reformer imager is that it provides a high resolution image rather than a single point measurement. It would be impractical to take the same number of measurements as the reformer imager does using an optical pyrometer.

For other tube wall temperature measurement devices, to fully understand the tube wall temperature profile either an advanced modelling technique2 or an alternative temperature measurement system is required. One such system is the CatTracker.

CatTracker
The catalyst temperature tracking system patented by Daily Thermetrics was first introduced commercially in 2001. It is now a proven technology with over 500 installations across five continents profiling hundreds of vessels. With 25 000 sensing points it has shown superior operational service. At present there are 33 systems installed in a range of synthesis gas plants, which will increase to 81 installations this year.

The CatTracker, employing patented aerospace thermocouple technology, offers the syngas industry the most rugged yet flexible temperature probes designed to be in direct contact with the process. Its temperature sensors are engineered to withstand the harshest environments and the most strenuous temperature demands making them highly suitable for installation in a reformer.

Each probe consists of an insulated cable which can sense temperature at predetermined locations along its length. Multiple temperature sensing points are independent and isolated from one another, while at the same time ungrounded from the sheath. The system’s design eliminates any possibility of signal interference (due to the fact that there is no common leg) and provides unsurpassed reliability for the most demanding applications.

There are many potential benefits, including the following:
• Continuous on-line monitoring of the in-tube temperature profile, with the profile available in real time through the plant DCS system
• Interlocks or trips, based on process gas temperatures within the catalyst tubes, preventing potential injury to plant personnel and costly reformer damage
• Tube temperature profile measurement leading to more accurate prediction of the operating proximity of the catalyst to the carbon formation zone thus allowing operation avoiding carbon formation
• Operation of the reformer at lower steam to carbon ratios without the risk of carbon formation by using the CatTracker process gas temperatures to calibrate a reformer process simulation
• Improved understanding of catalyst parameters for an improved understanding of catalyst performance and change out scheduling. Improved understanding of the catalyst performance can be determined by using the measurements in conjunction with a detailed process simulation of the reformer
• Monitoring real reformer tube outlet temperature to determine actual pigtail temperatures leading to a reduction in pigtail failures
• Calibration of pyrometers when used in conjunction with process simulation of the reformer.

Design
A CatTracker comprises a number of sensing points (4, 9 or 11) separated by an inert material and enclosed in a metal sheath. The 11 point design is illustrated in Figure 1.