Caprolactam production requires tight oxygen control to prevent explosive conditions developing in reactor vessels. For a Chinese manufacturer, the slow response and high maintenance of their O2 analyzers was unacceptable.
Changing to GPro® 500 in situ TDL sensors has reduced measurement time from 20 seconds to 2, and almost eliminated analyzer maintenance.
Rapidly growing demand for essential monomer Caprolactam (CPL) is an important organic compound monomer. Approximately 70 % of caprolactam produced worldwide is used in the production of nylon 6 fibers for textiles, carpets, and heavy duty tires. With economic growth in Asia is driving CPL demand, the caprolactam market was USD 8,500 million in 2012 and is expected to reach USD 13,800 million by 2019.
90 % of caprolactam is produced through the ammoximation of cyclohexanone to cyclohexanone oxime with ammonia and hydrogen peroxide (H2O2). This is the preferred process due to mild reaction conditions, the production of fewer by-products, shorter process time, and less environmental risk.
Explosion risks must be mitigated
The ammoximation process has been consistently improved to increase product quality and yield, but the process still relies on large quantities of H2O2 which are fed constantly to the ammoximation reaction vessel. During the process H2O2 can undergo an undesired decomposition side-reaction, which not only consumes valuable precursor but also produces oxygen, leading to an explosion risk. In order to ensure stable and safe production, typically, oxygen content must be kept below 1 %. If the concentration exceeds safe limits, an interlock program should be started to stop the reactor feed and introduce an inert gas to dilute the oxygen content.
Paramagnetic analyzers are commonly used in ammoximation reactors to monitor oxygen levels. After extraction, process gas must pass through sample conditioning to prevent incorrect measurement due to interference from background gases, or total failure of the analyzer if the measurement cell comes into contact with moisture in the process. Sample conditioning systems require regular maintenance to sustain operation.
Major CPL producer searches for fast oxygen measurement
One of China’s largest CPL manufacturers was frustrated with the high level of maintenance required on their ammoximation reactors’ sampling and conditioning systems, and they were regularly faced with either stopping production while maintenance was carried out, or running production without oxygen measurement. Further, even when the analyzer was operating correctly, slow response time was a constant cause of concern.
Production managers at the facility began looking for an alternative oxygen measurement technology that would require little maintenance and would respond rapidly to O2 level changes.
Their search focused on tunable diode laser (TDL) analyzers as they have a very low maintenance requirement, do not suffer from interference from background gases, and are installed directly in reaction vessels, so response time is very fast and sample extraction and conditioning is not required. However, they usually require the installation of two flanges: one for the TDL’s laser source and the other for the receiver that analyzes the laser light that has passed through the reactor headspace. This necessitates very precise alignment of the two units, and realignment after calibration.
METTLER TOLEDO approached the facility production managers with a TDL solution that would meet all their requirements and be easy to install.
Rapid oxygen measurement directly in the reactor
The GPro 500 oxygen analyzer requires only one flange for installation as both laser source and receiver are housed in a single unit. A probe attached to the GPro 500 that extends into the reaction vessel, contains a corner cube at its end. This ’prism’ directs the laser beam back to the receiver, so loss of alignment is never a concern.
Tests showed that oxygen measurements from the GPro 500 were as accurate as from one of the installed paramagnetic analyzers. And unlike the 20 second response time of the paramagnetic units, the GPro 500 responded to O2 changes in only 2 seconds, so process safety would always be assured.
In addition, as the GPro 500 has no moving parts, there is very little that can go wrong with it.
Further testing convinced the plant managers that the METTLER TOLEDO solution would meet all their needs and five GPro 500s were installed in the facility’s ammoximation reactors. In the 12 months since, all the GPro 500s have operated exactly as required – ensuring rapid detection of any oxygen upsets.
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