(545c) Design and Optimization of 3D Reactor Technologies for the Production of Light Olefins

Roughly 1.5 x 10⁸ t of ethylene and 8 x 10⁷ t of propylene are produced annually worldwide. Although some alternative technologies have emerged and/or gained interest over the last decade, the bulk of these molecules are and will continue to be produced through the steam cracking process. [1]

In this process, a hydrocarbon feedstock is heated together with steam in a tubular reactor to temperatures of up to 900 °C. To reach these high temperatures, the reactors are typically suspended in a gas-fired furnace. Due to these elevated temperatures and large scale of the process, it is one of the most energy-intensive processes in chemical industry, accounting for an annual energy consumption of about 3.0 x 10¹⁸ J, [2] which is the equivalent of approximately 500 million barrels of oil or twice the annual electricity consumption of Germany. Next to this, the process is also responsible for the emission of nearly 200 million tons of CO₂.

As the process has already been established for a long time, a lot of effort has been put into increasing the process efficiency, leading to a high level of complexity and limited room for further process improvements with conventional optimization techniques. In this respect, Djokic et al. [3] propose technological innovations to push beyond contemporary conventional steam crackers. These innovations range from alternative fuels such as oxy-fuel over material improvements such as high-emissivity coatings or advanced high temperature alloys to the enhancement of convective heat transfer from the reactor coil to the process gas by implementing 3D reactor technology.

These 3D reactor technologies not only reduce the fuel consumption and lead to longer coil lifetime by reducing the tube metal temperature (TMT), they also decrease coke formation which increases the process run length. This is mainly achieved by two methods: on one hand, the reactor wall shape can be altered to increase the heat exchanging surface. On the other hand, inserts or wall modifications can be introduced to increase the degree of mixing. This can be achieved at a large scale by radial mixing, i.e. secondary motions displacing hot fluids away from the reactor wall into the colder center of the flow, and on a smaller scale by increasing the turbulence in the near-wall region. This near-wall turbulence increase, will decrease the thermal boundary layer, which is the region in which the largest resistance to heat transfer can be located.

Even though a large number of heat transfer enhancing designs have been proposed and/or implemented in the field of heat exchangers, the number of designs suited for high temperature applications such as steam cracking is rather limited. Several reasons can be mentioned for the limited range of designs; specialized coil materials do not easily allow complex deformations or 3D patterns; turbulators can increase the risk of coke depositions clogging the reactor and the use of these designs inevitably leads to an increase in pressure drop over the reactor, which might negatively affect the yield of the desired light olefins.

Comparisons between different designs in an industrial setting are not widely available. To cope with this, flow, heat transfer and chemical kinetics are now modeled by means of Computational Fluid Dynamics, providing a unique insight into the effect of reactor design on flow behavior and heat transfer. This modelling effort is validated based on the results of an extensive experimental campaign, where different enhancement techniques are investigated with stereo-PIV, for flow field measurements, as well as heat transfer measurements, based on liquid crystal thermography (LCT). These experiments provide detailed local information and hence allow a thorough comparison with the simulated flow field and heat transfer capabilities of these designs.

The modelling methodology is used to investigate the performance of two recently developed reactor technologies namely SFT[4, 5] and SCOPE[6]. These technologies are then compared against three established reactor designs; finned[7] and ribbed[8] coils as well as coils with IHT inserts[9]. Simulation results will focus on pressure drop, tube metal temperatures, product distribution and coking rates, allowing a unique comparison of the most relevant industrial reactor designs.

[1] I. Amghizar, L. A. Vandewalle, K. M. Van Geem, and G. B. Marin, "New Trends in Olefin Production," Engineering, vol. 3, pp. 171-178, Apr 2017.

[2] T. Ren, M. K. Patel, and K. Blok, "Steam cracking and methane to olefins: Energy use, CO2 emissions and production costs," Energy, vol. 33, pp. 817-833, May 2008.

[3] M. R. Djokic, K. M. Van Geem, G. J. Heynderickx, S. Dekeukeleire, S. Vangaever, F. Baffin-Leclerc, et al., "IMPROOF: Integrated Model Guided Process Optimization of Steam Cracking Furnaces," Sustainable Design and Manufacturing 2017, vol. 68, pp. 589-600, 2017.

[4] C. M. Schietekat, M. W. M. van Goethem, K. M. Van Geem, and G. B. Marin, "Swirl flow tube reactor technology: An experimental and computational fluid dynamics study," Chemical Engineering Journal, vol. 238, pp. 56-65, Feb 2014.

[5] M. W. M. van Goethem and E. Jelsma, "Numerical and experimental study of enhanced heat transfer and pressure drop for high temperature applications," Chemical Engineering Research & Design, vol. 92, pp. 663-671, Apr 2014.

[6] Schmidt+Clemens. (2018, 31/01/2018). Strengthen the heart of your plant! SCOPE Fusion HTE - the steam cracker tubing of the future. Available: https://www.schmidt-clemens.com/products/scope/

[7] C. M. Schietekat, D. J. Van Cauwenberge, K. M. Van Geem, and G. B. Marin, "Computational Fluid Dynamics-Based Design of Finned Steam Cracking Reactors," AIChE Journal, vol. 60, pp. 794-808, Feb 2014.

[8] D. J. Van Cauwenberge, J. N. Dedeyne, K. M. Van Geem, G. B. Marin, and J. Floré, "Numerical and experimental evaluation of heat transfer in helically corrugated tubes," AIChE Journal, 2017.

[9] A. Carrillo, F. Bertola, G. Wang, and L. Zhang, "Intensified heat transfer technology–CFD analysis to explain how and why IHT increases runlength in commercial furnaces," in AIChE and EPC 2010 Spring National Meeting, San Antonio, TX, 2010.