Introduction

The conversion of carbonaceous feedstocks into higher value products occurs through two major routes: (1) indirect oxidation; and (2) direct oxidation, as shown in Figure 3.1. In the indirect oxidation approach, carbonaceous feedstock is first oxidized to syngas, which can then be further converted to value-added products. With respect to syngas generation, the thermodynamics and reaction characteristics of metal oxide oxygen carriers are discussed in Chapter 2; chemical looping reactor configurations are discussed in Chapter 4; and the techno-economic analyses of several chemical looping processes are discussed in Chapter 6. In the direct oxidation approach, the carbonaceous feedstock is directly upgraded to the desired product. Conceptually, the direct route simplifies the overall process and reduces costs by removing processing steps, but no commercial-scale system exists. Early research activity on direct oxidation processes, specifically for methane, can be traced back to the 1920s, but the utilization of methane in the chemical industry has been limited due to its high molecular stability. Currently, direct methane utilization involves conversion to value-added products such as aromatics, oxygenates, olefins, and paraffins, of which the three major oxidative processes are partial oxidation to methanol, to formaldehyde, as given in Section 5.4, and to ethylene and ethane via the oxidative coupling of methane (OCM).

Volatility in petroleum prices and limited petroleum reserves have made natural gas resources progressively more attractive as an energy source. Recent discoveries of natural gas reserves, increased accessibility to shale gas, and low natural gas prices have propelled a resurgence in methane-to-chemicals research. Monetizing cheap natural gas to obtain higher value-added products would provide a critical opportunity to the petroleum and chemical industry.9 Research into one promising direct route, OCM, has exhibited peaks and troughs, as shown in Figure 3.2. To date, pilot-scale systems were constructed and tested at the Atlantic Richfield Company (ARCO) in the 1990s. In 2012, Honeywell announced plans to scale up a proof-of-concept direct methane conversion process to ethylene. More recently, in 2015 Siluria Technologies began operating a pilot-scale OCM demonstration unit to convert methane to ethylene or gasoline.

In this chapter, the direct oxidation of methane to ethylene and higher hydrocarbons through OCM is presented.