Abstract

An experimental high temperature coking atmosphere using H2 and CO process gases was used to evaluate the effects of alloy composition and oxide layer on the coking resistance of several stainless steel alloys including cast 35Cr-45Ni as well as an aluminum containing austenitic alloy. Oxidation process was carried out with steam and tracked using mass changes as a function of exposure times. Carbon deposits formed on the surface of iron samples during coking at 850°C for 100 hours in a gas mixture of 25%CO-25%H2-0.5%H2O-49.5%Ar. Materials were characterized by using scanning electron microscopy (SEM), optical microscopy and EDS after exposure to characterize the extent of attack and other microstructural changes. The results showed considerable difference in the coking behavior of the alloys evaluated and a strong influence of the pre-oxidation condition of the material.

Introduction

Applications such as power generation, chemical processing, fuel cells, and high temperature heat exchangers are exposed to high temperature aggressive gaseous environments which limit the life expectancy. Coking is the process of carbon deposition from a gas phase that is encountered in many reforming, cracking and other high temperature processes that can lead to carbon build up causing reduced process efficiency as well as corrosive attack and degradation of the alloy.

Ethylene (C2H2) is a hydrocarbon material used as a raw building block for many industrially critical materials such as polyethylene, PVC, polystyrene, ethylene glycol, and countless other products. The annual world-wide production is over 100 million tons with US production representing roughly 25%.¹ US based production is roughly 150 million pounds per day and is predicted to increase 35% by 2017.² Ethylene is commonly produced in steam crackers where gaseous feed stocks such as ethane or propane are cracked and formed into the ethylene structure. The systems for producing this raw chemical incorporate large reactors that are made of alloys that provide 1) high temperature mechanical strength and 2) chemical resistance to attack under the aggressive carbon-rich conditions of the system. Most production facilities are periodically taken off-line so that the reactor system can be “de-coked” using steam and air to “burn off” coke deposits. The periodic maintenance results in a loss of production efficiency (~2-8% of annual capacity), added costs, and thermal cycling of the system which imparts additional wear on the reactor. The formation of carbon deposits (coking) has been described in the literature 3,4,5 and is a function of several factors including the balance of hydrocarbon to steam, incorporation of process gas dopants (e.g. sulfur compounds), and the surface chemistry of the materials6 that make up the reactor. Understanding the relation between the alloy’s surface chemistry and coking resistance is a primary goal which will allow for the design of alloys with improved coking resistance.

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