Modelling coke formation in an industrial ethane‐cracking furnace for ethylene production

Abstract

The development of a model for predicting coke formation in an industrial ethylene cracking furnace is described. Expressions for predicting the rates of catalytic and pyrolytic coke formation are developed and a differential equation is derived to predict changes in coke thickness with time and position. An expression is developed to account for a decline in the rate of catalytic coke formation with increasing thickness of the coke layer. The proposed coke model equations are used to extend a previously developed ethane pyrolysis furnace model that ignored coke. Three model parameters related to coke formation are estimated using industrial data to obtain reliable model predictions. Two of these parameters are coefficients that appear in the catalytic and pyrolytic coke formation rate expressions. The third is a characteristic‐length parameter used to reduce the rate of catalytic coke formation as the coke layer grows. The resulting dynamic model matches the industrial data well and can be used to simulate furnace operation and predict coke thickness profiles over a variety of the operating conditions, thereby helping process engineers who plan the decoking process.

Enhanced Coke Gasification Activity of the Mn1.5Cr1.5O4 Spinel Catalyst during Coking in Ethylene–Steam Mixtures

“…In the absence of species active for catalytic coke formation (e.g., Fe, Ni), the primary coking mechanism for the Mn–Cr–O catalysts under the tested conditions is the gas phase, radical coking mechanism. ,, At high reaction temperatures, ethylene and other gas phase hydrocarbons undergo radical reactions that rapidly form hydrogen and other free radical sites that react to create more dehydrogenated molecules. As these molecules further react and interact with the catalyst (or reactor wall) surface, they form more polyaromatic species that serve as potential coke precursors that continue to undergo dehydrogenation to form coke (mostly in sp 2 hybridization and low hydrogen content). ,, Adding steam to the hydrocarbon cracking feed limits the coke formation rates, as the water molecules can participate in the free radical reactions or react with the coke and/or coke precursors to form CO/CO 2 . , The oxygen from the cofed steam is also important in replenishing oxygen vacancies or regenerating active species in catalysts that can gasify coke and/or coke precursors . A crucial difference between the MnCr 2 O 4 and Mn 1.5 Cr 1.5 O 4 catalysts is that the Mn 1.5 Cr 1.5 O 4 catalyst has both Mn 2+ and Mn 3+ species in its structure, where the Mn 3+ species can react with coke and/or coke precursors and then become regenerated in the presence of steam …”

“…As these molecules further react and interact with the catalyst (or reactor wall) surface, they form more polyaromatic species that serve as potential coke precursors that continue to undergo dehydrogenation to form coke (mostly in sp 2 hybridization and low hydrogen content). 9,48,49 Adding steam to the hydrocarbon cracking feed limits the coke formation rates, as the water molecules can participate in the free radical reactions or react with the coke and/or coke precursors to form CO/CO 2 . 13,50 The oxygen from the cofed steam is also important in replenishing oxygen vacancies or regenerating active species in catalysts that can gasify coke and/or coke precursors.…”

Deactivation and Regeneration of Zeolite Catalysts Used in Pyrolysis of Plastic Wastes—A Process and Analytical Review

“…Both retention within catalyst pores and reactivity with catalytic surface have to be satisfied and sufficient to allow initiation of coke precursors [76]. As the nature of reactants and catalysts used in the process are known, the coking behavior of a reaction system is predictable [89]. Generally, precursors from short chain alkenes and dienes undergo very fast condensation reactions leading to polar products that are easily retained on the active sites of the zeolite.…”

Alumina-based Coating for Coke Reduction in Steam Crackers

“…Heavy hydrocarbons together with dehydrogenated complex carbon chains grow or are deposited on these filaments and form a layer that is well-known as coke [1]. During coke formation, the reactor cross-sectional area is reduced, increasing the pressure drop over the length of the steam cracker [2]. Consequently, the ethylene selectivity decreases [3,4], while the highly insulating coke layer blocks the heat transfer from the burners to the reactive gas.…”