Producing oxygen for some serious "paperwork"

Vaisala contributes to a trouble-free air separation process The Vaisala SPECTRACAP® Oxygen Transmitter OMT355 is an analyzer-type instrument that can be used in a variety of measurement applications. It has a measurement range of up to 100% O2. The transmitter is used in O2 production at the UPM-Kymmene Kaukas pulp mill in Finland. The oxygen used in pulp production can contribute to a more environmentally friendly paper production process.

The OMT355 oxygen transmitter is an optical, tunable diode laser oxygen analyzer for industrial gas measurements. Its measurement range enables its use in various gas manufacturing processes, such as in inert gas - typically nitrogen - manufacturing, or in oxygen production. At the Kaukas mill the OMT355 is used in an oxygen production facility managed by the AGA Corporation.

The AGA Corporation is part of the Linde Group, the world’s leading gas producer. AGA’s main gas products are oxygen, nitrogen and argon, which together generate half of the company’s revenue. At the Kaukas site the AGA oxygen production plant provides oxygen for the pulp mill.

More environmentally friendly paper production
The Kaukas paper mill uses bleached sulfate pulp in its coated printing paper products. Oxygen and ozone bleaching have partly substituted chlorine in pulp bleaching and have thus contributed to more environmentally friendly paper production. The oxygen bleaching process employs oxygen to remove lignin, which in the course of time shades the paper product yellow.

Oxygen is produced from air at the AGA Kaukas site. Air is a gas mixture mainly composed of nitrogen (78%), oxygen (21%) and argon (1%). These gases can be produced from plain air with air separation methods. There are various methods for air separation, but at Kaukas oxygen is produced with a specific separation method called the Pressure and Vacuum Swing Adsorption (PVSA) process. The raw materials required in the process are air and electricity. The PVSA method is especially suitable for small to medium-sized production facilities when the required oxygen purity is significantly lower than 100% O2.

Separating air with the PVSA method
The PVSA method is based on pressurizing and depressurizing a container with a selective molecular sieve. The conditions inside the process vary as oxygen is separated from air: pressure is needed to push air through the filter; vacuum is required in the regeneration of the system and removal of nitrogen from the molecular sieve.

In the PVSA process, ambient air is pressurized with an air compressor. The compressed air is then fed to the adsorbers to remove the unwanted content, which in oxygen manufacturing is mostly nitrogen. The two adsorbers, several cubic meters each, are filled with synthetic zeolite mineral acting as a molecular sieve or filter. The gas content changes when traveling through the sieve. The oxygen molecules pass through the sieve with minor restriction, while nitrogen molecules get trapped in the zeolite’s structure. In other words, oxygen is enriched and nitrogen is adsorbed.

When saturated with nitrogen, the adsorber performance is no longer sufficient. Excess nitrogen needs to be removed with filter regeneration. The regeneration is accomplished by utilizing a vacuum to draw the nitrogen molecules out of the filter media. A typical setup contains two tanks filled with zeolite. One tank is used to adsorb nitrogen while the other is being regenerated. The cycle time of the process is typically less than one minute - that is, one tank filters air for a minute followed by one-minute regeneration.

Simple installation directly into the process possible
The Vaisala OMT355 with in-line or sample cell installation options is highly insensitive to variations in process gas velocity, and slightly changing pressure within the operating pressure range only has a minor effect on the measurement. These special properties of the instrument enable simple installation directly into the process (in-line) or installation with a simple sampling system requiring minimal sample conditioning, also ensuring a fast response time for the measurement.

In PVSA processes both installation methods can be used. At the AGA Kaukas site the installation was made at a Y junction combining the two adsorbers with a very simple sample drawing system consisting of a 6 mm tube, a rotameter and a Vaisala-supplied sampling cell.

“The robustness of the OMT355 would easily have enabled direct (in-situ) installation to the process, which is far too demanding for other products, but the critical maintenance schedule at the pulp mill did not allow it this time,” says Heimo Märkälä, who is responsible for plant instrumentation and automation maintenance at AGA production sites. “During the test period it became evident that the pressure and vibration tolerances of the OMT355 are excellent.”

Durable tool for preventive maintenance
In a PVSA process, the oxygen content of the produced gas changes as the zeolite degrades or there is a malfunction in the valve system. Currently, the adsorber temperatures and pressures are monitored by the control system, but the reaction time of these measurements is slow and cannot predict mechanical failure before it is too late.

“With the OMT355 we can see potential problems before they cause serious degradation in the process performance and can use that information for preventive maintenance,” says Heimo Märkälä. “I can see potential for this product in preventive maintenance, and for product O2 measurement - it is easy to install, has excellent environmental tolerance and a quick response time at very reasonable cost.”

Author: Hannu Valo and Maria Uusimaa, Vaisala, Finland