(138c) In-Situ Generation of Radical Coke By Thermal Aging and Its Effect of Coke-Catalyst Contact

Carbonaceous deposits are produced during incomplete hydrocarbon combustion (soot in diesel engines),¹ and thermal decomposition of hydrocarbons (pyrolysis of ethane).² The amount and type of coke deposits formed depends on the operating conditions, nature of feed, and nature of the reactor surface. Carbon deposition on engines can cause loss of power and fuel economy. During pyrolysis, deposits of carbon can affect the production efficiency as it increases pressure drop in the reactor and the temperature of the reactor tube.^3,4 It also causes carburization of steel, influences flow in the reactor and ultimately leads to expensive shutdowns. Hence, a significant research is focused towards development of technologies that can aid the oxidation of such deposits. This work focuses on lab-scale in-situ generation of coke formed by radical reactions.

Contact between carbon and catalyst is important for effective catalytic activity in oxidizing carbon deposits. Neeft et al. defined two types of contact between carbon and catalysts – tight contact and loose contact.⁵ Loose contact is obtained by mixing with coke and catalyst samples with a spatula and has poor reproducibility. Tight contact mixture is achieved by grinding the two samples in a mechanical mill for extended time.

In this work, a specialized thermo-gravimetric analyzer (TGA) was used for generation of in-situ coke. This TGA has the ability to work with reactive gases (ethylene or propylene) and steam. The internal components are designed to work efficiently in presence of corrosive atmospheres. 2% hydrocarbon was fed in the TGA at high temperature to deposit coke on various catalytic supports. External surface area of support was determined to be an important factor in determining the coke deposition rate. Coke deposition was first order in the hydrocarbon concentration. Deconvolution of Raman spectra gave an insight into the chemical structure of the coke samples. In-situ generated coke was found to be more amorphous compared to the industrial coke sample obtained from a steam cracker. This result was attributed to the fact that industrial coke was subjected to a few months of high temperature exposure during the continuous operation of the steam cracker. Thermally aging the coke generated in-situ in an inert atmosphere increased the graphitic nature of coke. This hypothesis was supported by temperature programmed oxidation studies and Raman analysis. In-situ coke deposition was also observed to have improved coke-catalyst contact as compared to physical mixtures of coke and catalyst. In-situ generation of coke is more representative of the contacting conditions observed in industrial operations, than the tight contact method.

References:

1.        Stanmore, B. R., Brilhac, J. F. & Gilot, P. The oxidation of soot: A review of experiments, mechanisms and models. Carbon N. Y. 39, 2247–2268 (2001).

2.        Albright, L. F. & Marek, J. C. Coke Formation during Pyrolysis: Roles of Residence Time, Reactor Geometry, and Time of Operation. Ind. Eng. Chem. Res. 27, 743–751 (1988).

3.        Chan, K. Y. G., Inal, F. & Senkan, S. Suppression of Coke Formation in the Steam Cracking of Alkanes : Ethane and Propane. Ind. Eng. Chem. Res. 37, 901–907 (1998).

4.        Cai, H., Krzywicki, A. & Oballa, M. C. Coke formation in steam crackers for ethylene production. Chem. Eng. Process. Process Intensif. 41, 199–214 (2002).

5.        Neeft, J., Makkee, M. & Moulijn, J. Catalysts for the oxidation of soot from diesel exhaust gases. I. An exploratory study. Appl. Catal. B Environ. 8, 57–78 (1996).