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Nov-2019

Gas analysis in a hot climate (TIA)

Servomex has developed a customised combustion monitoring solution to meet the specific challenges faced by a global petrochemicals producer.

Jo Calvert
Servomex
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Article Summary
As part of its ethylene glycol manufacturing process, the company uses ethylene cracker plants in Kuwait to produce ethylene from natural gas. The furnaces within these plants require effective gas analysis solutions to ensure efficient combustion control.

The key to a well-controlled combustion process is an optimised air-to-fuel ratio. High excess air leads to inefficient burning and costly fuel consumption. However, low oxygen, fuel rich conditions may lead to a potentially dangerous explosion.

To maintain an optimised air-to-fuel ratio that is both safe and fuel efficient, plant operators use gas analysers to monitor oxygen levels, and often add a carbon monoxide or combustibles analyser to maximise efficiency.

Chemical plants around the world generate very hot conditions. When the natural climate of Kuwait is factored into the equation, the analysers are typically operating in an environment with an ambient temperature around 70-80°C (158-176°F).

The usual nominal ambient temperature for a combustion gas analyser is around 50-60°C (122-140°F). The internal temperature of the analyser is typically around 10-15°C (18-27°F) above ambient, so in this case the inside of the analyser was reaching temperatures close to 100°C (212°F).

Under hotter conditions, the components of these analysers will age prematurely so the device will fail and will need early repair or replacement. In addition to the costs incurred by increased maintenance or the requirement to install a new analyser, a failing analyser will be less accurate, affecting combustion efficiency and increasing fuel costs. Given the significant amount of fuel required to keep cracker furnaces at around 1200°C (2192°F), just a small drop in efficiency can be very costly.

Heat damage to analysers is a common issue for plant operators in regions with high background temperatures. While it is possible for manufacturers to build gas analysis equipment that operates normally in ambient conditions of 80°C (176°F) or more, they are much more expensive than analysers developed for more temperate environments.

Servomex’s solution was to supply the plant with modified Servotough FluegasExact 2700 analysers for this application. This combustion analyser uses zirconium oxide sensing for fast response, reliable oxygen measurements. It also has a thick film calorimeter for accurate combustibles measurements, providing a complete combustion control solution in a single instrument.

The operating range for the FluegasExact 2700 is from -20 to +70°C (-4 to +158°F), so it is already able to cope with a variety of ambient temperatures. In addition, its integral sampling system is custom designed for operation in some of the hottest and most challenging combustion environments.

Even with these advantages, it was clear to Servomex that further modifications were required to overcome the issue of higher ambient temperatures. So each of the analysers was fitted with a Vortec cooler to provide a continuous supply of cool air and purge the analyser.  This flow of air through the analyser lowers the internal temperature by preventing a build-up of hot gases. This maintains a ‘normal’ temperature within the device, allowing a normal lifespan for the electronic components.

Different molecules move at different speeds and directions depending on their mass and temperature, leading to the creation of a hot end and a cold end within the cooler. This technique ensures that instrument air can provide a regular blast of cool air to the analyser, keeping it at a lower temperature than ambient. While this was a simple solution in principle, it required expert applications knowledge and engineering expertise to ensure the air flow was correct for the process conditions at the Kuwait site.

Twelve FluegasExact 2700 systems were delivered, with installation and commissioning carried out by Servomex engineers. They have operated successfully for 4-5 years, showing none of the ill effects normally experienced by analysers in hot environments.

This short case study originally appeared in PTQ's Technology In Action feature - Q4 2019 issue.

For more information: info@servomex.com
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