Direct Methane to Methanol

Foundations and Prospects of the Process

Vladimir Arutyunov

• Publisher: Elsevier
• Year: 2014
• ISBN:
  9780444632517
• Topic: Technology & Engineering
• Subtopic: Fossil Fuels
• Index: Technology & Engineering

Direct Methane to Methanol: Foundations and Prospects of the Process offers a state-of-the-art account of one of the most interesting and potentially commercial technologies for direct conversion of natural gas into valuable chemicals. The book thoroughly explains the complex and unusual chemistry of the process, as well as possible applications for direct methane to methanol (DMTM). It covers topics involving thermokinetics, pressure, direct oxidation of heavier alkanes, and more, and provides detailed appendices with experimental data and product yields.

This book provides all those who work in the field of gas processing and gas chemistry with the theory and experimental data to develop and apply new processes based on direct oxidation of natural gas. All those who deal with oil and natural gas production and processing will learn about this promising technology for the conversion of gas into more valuable chemicals.

• Reviews more than 350 publications on high-pressure, low-temperature oxidation of methane and other gas phase hydrocarbons
• Contains rare material available for the first time in English
• Explains the reasons of previous failure and outlines the way forward for commercial development of the conversion technology
• Presents a deep theoretical knowledge of this complex conversion process

Chapter 1

Historical Review on the DMTM

Abstract

This chapter offers a short historical review on the investigations connected with studying of DMTM from the beginning of the last century and up to now. Motivation for these investigations, typical experimental conditions, common drawbacks of some of the works, and the structure of the monograph are discussed.

Keywords

High-pressure oxidation of methane; Methanol; Oxygenates; Selectivity; Yield

As far back as the beginning of the last century, Bone revealed the principal possibility of producing valuable oxygenates by direct oxidation of methane [8–10]. However, a real interest in the problem arose in the 1920–30s in connection with the emergence of DMTM-based industrial processes [11,12]. Another stimulus was to verify some of the then theoretical concepts concerning the mechanism of the gas-phase oxidation of hydrocarbons [13]. At the beginning of the 1930s, several different groups of researchers [14–18] have almost simultaneously demonstrated the possibility of achieving a high selectivity of methanol formation in the high-pressure gas-phase oxidation of methane. This gave an impetus to subsequent efforts to increase the yield of oxygenates and to develop industrial processes for their production by the direct oxidation of methane and heavier alkanes. Along with some early patents on the catalytic oxidation of methane to oxygenates [19], there have appeared the first patents on the high-pressure gas-phase partial oxidation of methane [20,21].

In the 1930s, thorough investigations of DMTM were performed by Newitt and co-workers [16–18,22,23]. They studied gas-phase oxidation of methane and other hydrocarbons at high pressures to elucidate a number of fundamental issues concerning the role of alcohols in the oxidation of hydrocarbons. These works were first to demonstrate the possibility of achieving high yields of alcohols and aldehydes in the direct oxidation of hydrocarbons. Later, it was shown that methanol is also formed at atmospheric pressure, even during the induction period [24]. Although the selectivity of methanol formation for some of the stages of methane oxidation at atmospheric pressure attains ∼20%, the integrated selectivity does not exceed 5% [25]. Thus, high pressure is a key factor in providing significant methanol yields.

Already in the first works in the 1930s, the optimal conditions for DMTM were identified: high pressure (∼100 atm) [16,17,26], moderate temperature (400−500 °C) [27], and low oxygen concentration [15,27]. The attainable selectivity of methanol was demonstrated to be as high as 60% [16,17]. A number of kinetic features of the process were also established: the methanol selectivity decreased sharply with increasing oxygen concentration [15,27], and a high CO/CO₂ ratio in the products was observed [10]. Since then up to now, the main objective of most studies has been to determine conditions that would provide high and stable yields of the target products, so as to make DMTM competitive with other technologies.

By the middle of the last century, the partial oxidation of methane and, later, propane, butane, and mixtures thereof extracted from oil-associated, crude stabilization, and processing gases became a widespread petrochemical process in the United States [28]. The subsequent decline of this technology in the late 1950s was associated with the rapid progress of competing technologies based on the preconversion of hydrocarbons into syngas and subsequent catalytic synthesis of target products and with the development of the market of propane and butane as domestic fuel and raw material for many petrochemical processes. Another factor that contributed to this was the difficulty of extracting individual components from a complex mixture of products of the nonselective gas-phase oxidation of C₃−C₄ hydrocarbons, which were used in the then technologies. Important, in our view, is the fact that the partial oxidation technology of that period was based solely on empirical knowledge. Small scope of fundamental research and the lack of clear understanding of the mechanism became a serious obstacle to the improvement of the process.

Although studies on methane partial oxidation have never stopped and continued beyond this period, and even attempts to introduce new industrial processes have been made (see, e.g., [29]), none of them found practical implementation. At the same time, under the influence of the theory of branched-chain reactions, developed by Semenov and his co-workers [30], a new understanding of mechanism of the oxidation of hydrocarbons began to take shape. Results of more than half a century of research in the oxidation of hydrocarbons and new concepts of hydrocarbon oxidation mechanism were summarized in a fundamental monograph by Shtern [13].

In the mid-1980s, interest in the direct production of oxygenates from methane rekindled due to the rapid growth of the role of natural gas in the global energy mix, the oil crisis of the 1970s, and the acute need in a clean motor fuel in major industrial countries. Methanol was considered as a convenient raw material for production of components of ecologically clean motor fuels and even as a potential fuel, features that boosted interest in technologies of its production from nonoil rough materials. To some extent, this interest was excited by several experimental studies that reported very high yields of methanol in the DMTM process [31,32].

This interest induced a number of reviews on the subject [31,33–38]. However, these reviews primarily dealt with the catalytic oxidation of methane, reflecting the traditional focus on catalytic technologies and the lack, at that time, of a clear understanding of the real mechanism of the DMTM process. With the exception of [31], they contained no new original data or conclusions, being based predominantly on compilations of previous results.

Since then, dozens of experimental works have been published. The most evident and widespread drawbacks of some of these papers, especially in the early period, which significantly complicate their analysis, are an incomplete presentation of the experimental conditions and the practice of simultaneously changing several experimental parameters. It may be supposed that, in the absence of clear ideas about the reaction mechanism, some researches practiced “random” (rather than time-consuming systematic) search for optimal conditions of methanol production. Such approach significantly depreciates the results and, in some, cases makes their analysis and inclusion in the common pool of DMTM data practically impossible.

Another common drawback of some of the works consists in attempts to make promising technological predictions based solely on a limited set of their own experimental results, without serious analysis, comparison with available data, or treatment within the framework of theoretical concepts. However, the claimed high parameters of the process are usually poorly reproducible, if at all. In addition, discrepancies between the results of experiments performed under very close conditions in the absence of a serious analysis of the underlying reasons give rise to the skeptical attitude regarding the practical applicability of DMTM.

A vast body of interesting but contradictory data published in recent years and a new level of theoretical understanding of the DMTM mechanism motivated us to perform a new comprehensive analysis of the process. Such analysis showed that the potentialities of the DMTM process is high enough but needs serious elaboration.

According to cost assessments, made some time ago, for the DMTM process to be competitive with traditional technologies, it should provide a selectivity of methanol formation (or the sum of valuable organic products) in excess of 77% at a reasonable conversion of methane, no less than 5% [39]. However, at present, for most of the experiments that can be reproduced at the industrial level, at a methane conversion higher than 5%, the methanol selectivity S_CH3OH does not exceed 50% (Appendix I). Nevertheless, even these results open possibilities for some interesting technological applications of the processes. Although several works reported significantly higher values of S_CH3OH, up to 60% [40,41], 80% [42,43], and even >90% [44] (see Appendix I), the authors of [45] and others failed to reproduce them. Possible reasons for such significant discrepancies will be discussed below.

By now, dozens of experimental studies on the gas-phase partial oxidation of methane, as well as ethane and heavier homologues, to oxygenates has been published. Facilities with various types of reactors, including static reactors, well-stirred flow reactors, and plug flow reactors of various sizes with a diameter of 5–30 mm operating at a flow rate from a few liters to 1000 m³/h, have been used. The pressure was varied up to thousands of atmospheres. Along with the main parameters of the process, such as pressure, temperature, flow rate, and mixture composition, the effects of reactor surface material, heterogeneous catalysts, homogeneous promoters, and various physical methods of initiation of the process have been examined. The experiments were carried out using both premixed and individual supply of oxidant and hydrocarbon into the reactor, either cold or preheated. The oxidant was oxygen or air.

The most reliable experimental data on DMTM used in this analysis are summarized in Appendixes I and II. Note that many of the figures in the monograph, while plotted based on data from the cited original papers, are not necessarily present in these papers.

Since no detailed description of the experimental techniques used in the cited works will be given in the main text, we would like to note that most experiments in the 1930s, as well as experiments at very high (thousands of atmospheres) pressures [46–48] were performed in static reactors. In later studies, predominantly flow reactors have been used, which are more suited to the conditions of the practical implementation of the process. Experiments in flow reactors have been carried out over a wide pressure range (1−300 bar) and initial temperatures of 300–600 °C and above, with the reaction time ranging from a second to tens of minutes. In addition, more exotic reactors, such a cylinder of a high-compression internal combustion engine [49] or a rapid compression machine [50] were used.

Appendix I contains a summary of the experimental conditions and the results of the most informative (in our opinion) works on DMTM, an analysis of which enabled to draw ...