Apply ceramic coatings to extend radiant tube life in process heaters
Process tubes in refining applications are typically steel alloy (ASTM A335 P22, P5 or P9), which contain 2.25%, 5% and 9% Cr, respectively. These grades oxidize at operating temperatures, and scale will grow continuously on the surface, often reaching 2 mm in thickness in higher-temperature/high-heat-flux units. The layers of scale are very insulating and represent a significant barrier to conductive heat transfer to the process.
Process tubes in refining applications are typically steel alloy (ASTM A335 P22, P5 or P9), which contain 2.25%, 5% and 9% Cr, respectively. These grades oxidize at operating temperatures, and scale will grow continuously on the surface, often reaching 2 mm in thickness in higher-temperature/high-heat-flux units. The layers of scale are very insulating and represent a significant barrier to conductive heat transfer to the process.
The rate of heat transfer by conduction through the tube wall is calculated using Eq. 1:
(1)
To and Ti are the outer and inner tube wall temperatures, respectively, of internal diameter Di and outer diameter D0. The coefficient of thermal conductivity for the tube material is k.
When a tube is clean, the value for k is 29.08 W/m°K. When the tube becomes oxidized and only 1.6 mm of scale is formed, the value for k falls to 15.02 W/m°K, resulting from the insulation of the layers of scale.
It is obvious that the rate of heat transfer by conduction is, therefore, reduced to approximately half of the original level. This means that either too much fuel is being used to force To higher to allow for the poor conductivity, or the unit becomes a bottleneck as firing duty is maximized, unable to process sufficient feed.
A high-emissivity ceramic coating, applied to a clean, new tube or a cleaned, old tube, will prevent oxidation of the tube surface and maintain the value for k at 28.4 W/m°K. The result is the ability to run the process at design condition, with no penalty from excessive fuel use. Alternatively, and more interestingly, perhaps, it is possible to fire the unit harder and obtain significant increases in throughput.
High-emissivity coatings for refractory surfaces
The use of high-emissivity ceramic coatings, applied to the refractory surfaces in radiant sections of fired heaters and tubular reformers, has become an accepted means of improving the efficiency of radiant heat transfer. The application of the technology has led to energy savings, as well as environmental and reliability benefits. A range of ceramic coatings has been developed and used reliably for many years, for different substrate types, operating environments and targeted application benefits.
In fired heaters and tubular reformers, the thermal energy necessary to drive the endothermic processes is provided by burning a fuel/air mixture and transferring it to the process by three heat transfer mechanisms: radiation, convection and conduction. The primary means of heat transfer is radiation.
Why high emissivity? In the radiant section, much of the radiant energy from the flame/flue gas is transferred directly to the process tubes; however, a significant proportion interacts with the refractory surfaces. The mechanism of this interaction has an appreciable effect on the overall efficiency of radiant heat transfer. A major factor in determining the radiant efficiency is the emissivity of the refractory surface.
At process heater operating temperatures, new ceramic fiber linings, for example, have emissivity values of around 0.4. Insulating fire brick (IFB) and castable materials have emissivity values of around 0.6. These materials have been designed with structural considerations and insulating efficiency as the primary requirements. They tend not to handle radiation in the most efficient way. Newer coatings,a with emissivity values of above 0.9, have been designed specifically to supplement the radiation characteristics of the refractory surfaces.
It is important to understand how the emissivity property of a surface can affect the efficiency of heat transfer. Two factors must be considered. The first is the spectral distribution of the radiation absorbed/emitted from a surface, and the second is the value of the emissivity of that surface. The amount of heat, Q, radiated from a surface (area, A; temperature, T; emissivity, ε) is given by the well-known Stephan Boltzmann equation (Eq. 2):
Q = AεσT4 (2)
where σ is the Stephan Boltzmann constant.
Lobo and Evans1 and others extended the calculation with reference to fired heaters. A simplified equation would appear as shown in Eq. 3:
QR = Aσ(T14 – T24) ÷ F (3)
where F = 1 ÷ ε1 + [A1 ÷ A2][(1 ÷ ε2) – 1] for tubes of area A2, surface temperature T1 and emissivity ε2 inside an enclosure, area A1, with surface temperature T1, and emissivity ε1. The effects of maximizing the emissivity ε1 of the enclosure are obvious; a significant increase in radiant heat transfer to the tubes is observed. As stated earlier, much of the radiant heat to the tubes travels directly from the flame/flue gas, but the emissive property of the refractory surface has a profound effect.
The chart in Fig. 1 shows the energy spectra for two major components of the combustion products of natural gas—water vapor and carbon dioxide. They are compared with the spectrum of a perfect radiator, or black body, at the same temperature. The combustion products will radiate and absorb energy in the narrow wave bands shown, whereas a black body will radiate and absorb energy over a much wider wavelength range. High-emissivity surfaces radiate energy across a broad wavelength band, lessening the interference of the CO2 and H2O in the flue gas.
Fig. 1. Energy spectra of combustion products of natural gas.
When the radiation from a flame strikes a perfect radiator, all of the energy is absorbed; but most importantly, it is transformed into “black-body radiation” as the wide waveband form. As the energy is re-emitted from the surface, it can penetrate the atmosphere in the furnace, composed of the combustion products, with little being reabsorbed and taken to the stack by the draft. Therefore, it is more readily available to heat the load in the furnace.
If the surface were a poor radiator, or one having a very low emissivity value, the energy striking the surface would be reflected from the surface still in its untransformed state, therefore more readily absorbed by the furnace atmosphere. The effect is to “superheat” the furnace atmosphere, or flue gas, resulting in wasted energy lost to the stack.
The improvement in radiant heat transfer efficiency naturally leads to a reduction in flue gas temperature. This has consequences for the convective heat transfer, in both the radiant and convection sections of the fired heater. In the convection section, heat in the flue gas is used to produce steam, as well as preheating of combustion air and often process fluids. The heat transfer/absorbed duty balance should be examined closely to ensure that the balance is not adversely affected. A minor contribution comes from convective heat transfer in the radiant section, which may be characterized by Eq. 4:
Qc = hcA2(T1 – T2) (4)
Where hc, the film heat transfer coefficient, is an empirically derived factor related to the design of the radiant section and the tube configuration.
Catalytic reformer case study
The unit of this study was a 24,000-bpd (154-m3/hr), four-cell catalytic reformer. Each heater has a wicket-style radiant tube configuration. The metallurgy of the radiant tubes was ASTM A335 P9 (9% Cr, 1% Mo). The refractory lining was ceramic fiber walls and roof, with brick floors. The fuel was gas.
The initial evaluation required an infrared (IR) thermography inspection and corresponding operating data from the time of the inspection. Design specifications and general arrangement drawings provided the necessary physical and dimensional data to complete the picture.
The sample IR inspection image shown in Fig. 2 provides insight into the degree of scale formation and the surface temperature of the scale. The result of the evaluation was a recommendation that ceramic coatings should be considered for both tubes and refractory surfaces in all four cells. The calculated benefit showed that the improvement in radiant heat transfer efficiency would provide more than 4% absorbed duty improvement or firing duty reduction. The payout for this application was estimated at fewer than 12 mos, with a healthy return on investment over the anticipated life of the ceramic coatings.
Fig. 2. Infrared thermography inspection illustration before coating.
The client decided to go ahead with the ceramic coating application, and plans were put in motion.
Operational phase. Any coating application requires strict attention to surface preparation and protection of fired heater areas, where necessary. The 9% Cr process tubes were grit-blasted to NACE 1 (SSPC-SP-5) quality, with a proprietary surface profile required by the ceramic coating. To maintain the blast quality, the ambient humidity inside the heaters was maintained at or below 50% relative humidity (RH).
Refractory surfaces were prepared by light brushing on the ceramic fiber to remove any friable, recrystallized surface fibers and wire brushing to ensure that the brick surfaces were clean and contained no contamination.
A variation of high-emissivity ceramic coatingsa were applied to the process tubes, the ceramic fiber surfaces and the brick (Fig. 3). The entire job lasted approximately 15 d.
Fig. 3. Thickness check for coated process tubes.
Results of the application. IR thermography analyses from before (left) and after (right) the application are shown in Fig. 4. The heavy scale present beforehand is evident, as is the lower temperature and clean surfaces after the coating application.
Fig. 4. Comparison of IR thermography images.
As predicted in the evaluation phase, the benefit was an increase (4.27%) in efficiency, as elaborated in Table 1. This was calculated to provide a payout in 6 mos, with a total return on investment (ROI) of $13.8 MM over 8 yr.
Recommendation
The high-emissivity ceramic coatingsa have been successfully used in more than 200 catalytic reformer applications. Due to the fired heater configurations, relatively high heat flux and process outlet temperatures in this type of unit, they have proven to be excellent candidates for ceramic coating applications, which more typically see around 7% efficiency improvement, leading to even better payout and ROI results. HP
NOTE
a Cetek Ceramic Coatings
REFERENCES
- Lobo, W. E. and J. E. Evans, Trans AIChE, 35: 743, 1939.
The Author
Bacon, J. - IGS-Cetek, Akron, Ohio
John Bacon worked for 41 yr with the Foseco Group in various research and development and senior management positions, with products and services for the steel, glass, non-ferrous, oil refining, petrochemicals and power generation Industries. Since 1995, he has worked with Cetek, initially a division of that same group, from its inception to retirement in 2015. He has been retained by Cetek on a consulting basis, assisting senior management with technical, commercial and marketing aspects of the day-to-day business. In 2017, he helped steer the acquisition of Cetek by IGS. Mr. Bacon holds a BSc degree in physics from Leeds University in the UK and completed postgraduate work in ceramics, also at Leeds University, under the sponsorship of the UK Atomic Energy Authority.
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