The aromatics plant is an important downstream petrochemical unit that produces various aromatic hydrocarbons such as xylenes, benzene, and toluene which are the basic building blocks for production of polyester and polymers resulting in different consumer goods. The heart of aromatics plant is a licensed high-performance Naphtha Reforming unit that operates at low pressure and high temperatures required to promote chemical reactions that improve aromatics production.

To achieve the required heat there are 4no. of fired heaters with arbor type coil configuration that raise the feed naphtha temperature to around 545 °C. The radiant tubes of the heaters are of 9Cr-1Mo (ASTM(1) A335 Gr P9) metallurgy with a maximum tube wall temperature of 652 °C. The tubes are prone to excessive scaling due to high temperature oxidation and scale thickness of up to 1-2mm were found in the tubes. The problem arising due to scaling affect not only the mechanical integrity of the radiant tubes but also lead to multiple operational constraints.

The scaling apart from consuming the metal at 0.25 mm/year, acts as an insulator hindering the heat transfer across the tubes, resulting in radiant heat loss and preventing rise of process fluid temperature to desired levels. Moreover, severe scaling also prevents accurate on-stream monitoring of tube skin temperatures.

Following a detailed root cause analysis, it was recommended to apply ceramic coating on the radiant tube and extend it to refractory in the furnace box to improve operational and thermal efficiency, while maintaining metallurgical integrity and reliability.

Post coating (applied in March 2017), the unit is capable to run at more than 100% at the same heater load with the assurance of long term integrity of heater tubes. The payback from the investment was within 6 months of project execution.

This case study, highlights the observations of the radiant tubes along with cost benefit analysis achieved by the application of ceramic coatings to extend tube life and improve operational efficiency.

Subject
External surfaces, Carburization, Coatings, Materials, Heaters, Tubes, Walls, Furnaces, Oxide scales, Heat, Ceramic coatings, Scaling, Oxidation
Keywords:
Naphtha Reforming, Ceramic Coating, Radiant Tubes, Oxidation
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2019
Association for Materials Protection and Performance (AMPP)
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