SABIC turns to Yokogawa TDLS analyzers to enhance safety function and production

SABIC turns to Yokogawa TDLS analyzers to enhance safety function and production

By Jon Moscovic, Lead Reliability Engineer, SABIC

SABIC is a global manufacturer of polymer resins, film and sheet products, special additives, and chemical intermediates. With plants in 35 countries, the company has an enormous variety of processes and plant designs to make its range of products. With so many plants, processes and products, there are frequent opportunities to make improvements with hardware and instrumentation.

A case in point is a polymerization process at SABIC’s plant in Selkirk, NY, which produces polyphenylene ether (NORYL). The plant has five reactors which are each equipped with sparging headers to inject pure oxygen into a mixture of feedstocks and toluene. The oxygen helps precipitate a two-stage polymerization reaction in preparation for the next phase.

The monomer absorbs the oxygen particularly during the first stage, but some does bubble through and accumulates in the headspace, passing out through an 8-inch vent line. Eventually the vent line reaches a scrubber where the toluene is captured. This mixture of oxygen and toluene can be highly flammable, and even self-igniting depending on the proportions, so safety and process considerations call for constant monitoring of the vent line contents.

 

A Three-Fold Measuring Function

The safety considerations of monitoring oxygen content in the vent line are very critical. As long the oxygen level remains below 35%, the mixture remains non-flammable and cannot burn without introduction of outside air. If the process allows the concentration to reach 35%, it shuts off the oxygen flow to the sparging headers. But this safety consideration is only one of the reasons the measurement is important.

Secondly, the amount of oxygen bubbling through the liquid is an indicator of what is happening in the reaction. During the first stage, a larger amount of oxygen is absorbed so the flow is high. Some oxygen should bubble through because it is desirable to have a slight overabundance to ensure as complete a reaction as possible, but it should only reach between 15 - 20% in the headspace. Any more than that simply wastes oxygen. Once the oxygen level in the headspace begins to increase, operators know the process has moved to the second stage, and the flow can be turned down to reduce unreacted oxygen. This is important as the thermodynamic characteristics of the process also change significantly once the first part is finished.

Thirdly, the oxygen concentration in the headspace is limited to protect the catalyst from being consumed too rapidly.

 

The Challenges of Consistent Measurement

Technologies to measure oxygen in a gas stream are not new, and there are countless applications in chemical manufacturing and other industries where oxygen levels need to be monitored. Combustion processes of any size invariably use some type of oxygen sensor in the flue gas stream to maintain efficiency.

SABIC’s situation proved to be harder than most typical applications due to a mix of specific conditions. For many years operators struggled while working with paramagnetic and electrochemical cell sensors. These are both very common and used in a wide variety of oxygen measuring applications, but they have some critical limitations which became apparent in this process.

Paramagnetic analyzers are sensitive to vibrations and cross-contamination from other gases. Although the application for these reactors doesn’t call for measuring trace amounts of oxygen, there are also sensitivity issues at very low concentrations. Electrochemical cells need to be replaced routinely and have sensitivity to different pressures, temperatures and cross-contamination.

Our sampling systems experienced the highest failure rate with electrochemical components including sampling lines being plugged from the process, filter element clogging, and failing pumps (Figure 1). Moreover, since an individual test during production took more than two minutes, there was always a possibility a climbing oxygen level might not be identified soon enough.

Figure 1: The sampling system was complex and prone to occasional plugging, filter element clogging and pump failures.

Paramagnetic and electrochemical cell oxygen analyzers have a three-month verification frequency, and the manufacturers recommended maintaining this regimen precisely. Although the testing does not take long, batches were delayed in some situations while performing the verification. The costs of delaying the process between batches, along with the costs of the process stopping for emergency maintenance, resulted in a significant amount of lost production. Due to these and other issues, a more robust oxygen analyzer technology had to be found and implemented.

 

Tuning in to Laser Technologies

One technology used commonly in combustion processes is tunable diode laser (TDL) spectroscopy, capable of detecting and measuring a variety of gasses, including oxygen, within many contexts. Theoretically, it has the capability to measure oxygen when mixed with toluene, but there was some concern about it being practical for this specific application.

A TDL analyzer sends a beam with a controlled wavelength range through the gas being analyzed to determine which products are present based on which specific wavelengths of light are absorbed. The problem in this case related to the duct size, because the transmitter and receiver must be a minimum distance apart to ensure adequate absorption.

The duct diameter here was less than half the normally recommended distance, so there was some concern as to whether it would deliver its full degree of accuracy, or even work at all. SABIC’s engineers felt the potential benefits to be gained were more than enough to justify installing one analyzer as a test. The performance would be easy to evaluate since the existing sensors were still fully operational and working in parallel.

After two weeks of operation, it was clear the Yokogawa TDL analyzers were performing very well (Figure 2). It was true that they were not delivering the full degree of precision they were capable of due to the short scanning distance, but the precision was high enough to satisfy the needs of the process.

Figure 2: While the duct size for this application was smaller than is usually recommended for TDL analyzers, Yokogawa’s instruments reliably provided readings with a high enough degree of accuracy for the application, while eliminating the maintenance problems associated with the earlier sensing technologies.

Once installed, the new analyzers proved very reliable and required far less validation and maintenance than the earlier technologies. The biggest problem proved to be debris carried into the duct from the process blocking the light transmission path between the transmitter and receiver. Adjustments to a nitrogen purging system and better control of the process itself minimized this effect, leading to trouble-free operation.

 

Facilitating the Safety Function

The safety function these analyzers perform is critical, but when they were being installed the decision was made not to make them part of a dedicated safety-instrumented function. These analyzers are components of the basic process control system for the unit, although since they perform a safety-related function, they are included in the layers of protection analysis. To reinforce this capability, each reactor has two redundant analyzers operating in parallel.

Each of these 10 TDL analyzers, two per each of the five reactors, has been installed for over two years now, with no failures due to the TDLA’s to date. The first two reactors were outfitted with the Yokogawa TDLS200 analyzer, while the other three were outfitted with the Yokogawa TDLS8000 models.

There have been occasional visibility blockage incidents, but these are rare after adjustments to the purge system. Overall, these TDL analyzers have supported higher levels of production, and added another layer of protection to the unit.

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