(75f) MERT Performance and Technology Update

MERT (Mixing Element Radiant Tube) is a centrifugally cast radiant tube with a helical mixing element on the tube inside diameter for use in steam pyrolysis (cracking) furnaces. There are over 300 furnace installations of MERT technology worldwide in a wide range of furnace designs and tube diameters which achieve improved efficiency and productivity. MERT technology provides high heat transfer coefficient (up to 1.5 times that of bare tubes) by increasing process fluid mixing and break down of the thermal boundary layer on the tube internal surface. The improved heat transfer and mixing is achieved by a helical element integrally bonded to the tube inside diameter. Progressively the design of this element has been optimized to reduce pressure drop, first through the use of our SLIT-MERT product and now with the latest design: X-MERT.

The enhanced thermal and flow properties imparted by X-MERT elements result in lower tube metal temperatures, lower coking rates and longer run times. Additional savings are achieved through a reduction in the number of decoke cycles. The high turbulence and mixing also improves cracking performance by reducing over-cracking conditions adjacent to the tube wall and provides a more uniform hydrocarbon temperature cross section. In order to achieve these benefits, there is some resistance to flow quantified as an increased friction factor. Compared to a standard tube of the same inside diameter, this friction raises the overall initial pressure drop in the cracking coil at the start of run. But due to the low coking rate of MERT technology, this pressure drop difference may not be significant by the end of run when compared to a bare coil. Nevertheless, any increase in coil pressure is considered detrimental to cracking yield performance, thus the development of lower friction X-MERT technology.

The heat transfer and friction characteristics of X-MERT tubes have been compared to that of standard bare tubes. This comparison has been performed using CFD (Computational Fluid Dynamics), experimental laboratory experiments, computer simulations and full-scale commercial installations. The commercial installations have confirmed the beneficial effects of X-MERT, including reduced tube metal temperature and reduced coking, leading to significantly increased run times. It has been further demonstrated that feed rate and/or severity increases can be consistently achieved without reducing the original run time, due to the high heat transfer capability of MERT. These have confirmed that the thermal efficiency of X-MERT is superior to bare tubes and while offering the lower pressure drop compared to previous generations of MERT technology.