Coking Resistance of Specialized Coil Materials during Steam Cracking of Sulfur-Free Naphtha

Abstract

The reactor material strongly affects coke formation during steam cracking of hydrocarbons. Therefore, in the past decade several specialized reactor materials have been developed that have proven to be efficient in reducing coke formation for ethane steam cracking. However, their beneficial anticoking properties are questioned when heavier feedstocks such as naphtha are cracked. Therefore, the effect of the composition of the reactor material has been investigated for ethane and naphtha cracking in an electrobalance setup under industrially relevant conditions. A significant reduction of coke formation is obtained for specialized alloys compared to typical Fe–Cr–Ni heat resistant steels when a sulfur-free naphtha is cracked. A thin layer of alumina on the surface along with manganese chromite provides the highest resistance to coking, as was demonstrated by the SEM and EDX analyses. The decrease in coking rate translates in a run length increase of 50% for a typical naphtha furnace equipped with reactors made out of an Al-enhanced alloy instead of typically applied heat resistant steel.

Effect of selective oxidation and sulphur/phosphorus‐containing compounds on coking behaviour during light naphtha thermal cracking

“…Numerous investigations have been conducted to reduce coke formation, including altering the inner surface compositions on the inner surface of the coils, adding compounds, and increasing steam dilution ratio of feedstock …”

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

“…Maintaining a desirable conversion and high yield of ethylene requires that more heat be provided via combustion as the coke layer becomes thicker. The resulting higher furnace temperature increases coil surface temperatures, which may reach or exceed the maximum permissible value imposed by the metallurgy of the coil . Note that coke formation is also associated with the formation of local hot spots that can damage furnace coils .…”

“…The resulting higher furnace temperature increases coil surface temperatures, which may reach or exceed the maximum permissible value imposed by the metallurgy of the coil . Note that coke formation is also associated with the formation of local hot spots that can damage furnace coils . Furthermore, the coke layer on the inside of the coils reduces the cross sectional area, which increases pressure drop through the coils if a constant ethylene production rate is maintained .…”

“…Unfortunately, lower temperatures in the furnace are associated with lower conversion of ethane to ethylene. The amount of coke formed on the coils also depends on the feed‐gas composition and the properties of the alloy used in the reactor coil . The objective of the current article is to develop a mathematical model to predict coke formation rates in an industrial ethane cracking furnace.…”

“…Maintaining a desirable conversion and high yield of ethylene requires more heat to be provided via combustion as the coke layer becomes thicker. The resulting higher furnace temperatures increase the outer coil wall temperature, which must be kept below a permissible maximum value, determined by the coil metallurgy . Furthermore, the coke layer on the inside of the coil reduces the cross sectional area available for gas flow, which increases the pressure drop through the coils .…”