DMTM Process

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 dear 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. 

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. 

Chapter 10 examines the partial oxidation of heavier homologues of methane. It is the most rapidly developing direction in the modem studies of the DMTM process, motivated by the need to effectively process associated oil gases and heavier components of natural gas. 

The major gas-phase products of the DMTM process are carbon oxides and hydrogen, with the yield of carbon monoxide at high pressmes being several times that of carbon dioxide. The main pathways of formation of carbon dioxide in this process are apparently not directly related to carbon monoxide, since according to a number of studies, carbon dioxide is formed before carbon monoxide in the induction period [54]. With rising temperature, the yields of ethane and ethylene increase rapidly. 

The fact that formaldehyde, along with methanol, is the principal product of the DMTM process has been demonstrated in many studies, but then it is quickly removed by secondary reactions. Formaldehyde, dominating at low pressures, probably due to some specific features of the oxidation mechanism, remains present in significant quantities at high pressures as well, in both gas-phase and catalytic processes. 

In early studies of the DMTM process, the composition of the products was carefully analyzed, and a large number of various oxygen-containing and oxygen-free hydrocarbon oxidation products were separated and identified. In the later works, the focus was largely on the yield and kinetics of formation of the main products. Therefore, the scardty of information on byproducts in contemporary works seems to reflect a decline in interest in their formation in this process. However, in part, it may also be assodated with the changeover to more rigid conditions and shorter reaction times, which is less favourable for their formation. 

Below are listed the organic compounds observed in various works concerning the DMTM process ... 

FIGURE 2.14 Dependence of the CH3OH selectivity on the CH4 conversion obtained from fast-flow experiments with t, < 20 s and P > 30 atm (based on the data from Appendix I). The dashed line is the trend of experimental data, whereas the solid line represents the results of kinetic simulations of the gas-phase DMTM process at P = 100 atm and T = 420 °C [89]. (For colour version of this figure, the reader is referred to the online version of this book.)... 

Figure 2.15 displays the dependence of the methanol yield on the methane conversion, which shows a clear tendency to be limited by 2.5%. As can be seen, like in the case of the selectivity, the experimental trend line is in close agreement with the predictions of the kinetic simulations of the gas-phase DMTM process. 

There are a number of studies of the DMTM process at very high pressures. Several series of experiments in static steel reactors at pressures from 140 to 14 000 atm, oxygen concentrations of 8—9%, and initial temperature of 250—340 °C were carried out in [46]. The authors noted a marked influence of the state of the reactor surface and, tentatively attributing to it a poor reproducibility of the results (Appendix 1). Two series of experiments in a static reactor at pressures of 1700 and 3400 atm, temperatures of 270—310 °C, an oxygen concentration of 8%, and a residence time of up to 30 min were performed in [48]. For a t3qjical set of... 

Clear and convincing dependences of the main parameters of the DMTM process in a quartz flow reactor at P = 91 atm, T = 427 °C, and a residence time of fr = 35 s were presented in [98]. The experiments were performed with natural gas containing up to 4% ethane and 1% C3+ hydrocarbons. Figure 3.36 shows dependences plotted based on the averaged data from this work. As can be seen, over a wide range of oxygen concentrations, from 1.5 to 12.5%, the methane conversion increased linearly with the oxygen concentration (Fig. 3.36). 

Another set of results confirming a weak or negative dependence of the DMTM process rate on the oxygen concentration was presented in [97] for the oxidation of natural gas in a pilot plant at a constant initial temperature, the reaction time was practically independent of oxygen concentration variation within 1—3%. 

At a long (50—300 s) residence time of the reactants in a Pyrex reactor (T = 450 °C, P = 50 atm), its effect on the yield and selectivity of methanol in the DMTM process was investigated in [45]. Both the dependences obtained (Fig. 3.51) are nearly identical with the only difference that the selectivity of methanol formation after reaching maximum remains... 

An analysis of the above works on the DMTM process suggests that, despite frequent references to the design features as a cause of discrepancies between the results of different studies, it is true only for experiments with a long residence time of the reagents in the... 

Along with the well-known attempts to increase the conversion of natural gas while avoiding a deep oxidation of the target products by using two-stage schemes with intermediate conversion of natural gas into more easily convertible products, such as s)mgas, methyl halides, or bisulfate, various methods of isolation of the products have been applied. These include continuous extraction in some way or the binding of the products, as well as the separation of CH4 and O2 prior to their direct interaction, for example, on a catalytic permeable membrane, etc. Unfortunately, no feasible methods for the selective adsorption or membrane separation of methanol at temperatures close to the temperature of its formation in the DMTM process have been proposed. 

However, the subsequent transformations of these radicals are less evident. Kinetic simulations of the DMTM process in the framework of the mechanism developed by Vedeneev with co-workers [63—66], have demonstrated that an important role is played not only by the interaction of this radical with molecular products to form peroxides, the decomposition of which provides an effective branching in this reaction — no less important are the radical—radical reactions involving CH3O2. 

The set of reactions shown in Table 5.1 accounts for the formation of the main oxidation products methanol, formaldehyde, and water, but does not provide for their further transformation, since only the initial stage of the process is considered. Nevertheless, its analysis can explain the main qualitative features of the DMTM process, although its quantitative modeling requires much more complex, open-type models that would take into account the totality of homogeneous and heterogeneous elementary steps important in this range of conditions. This means that the model includes all the relevant elementary steps and, if required, it can be readily extended and that all the kinetic parameters are taken from independent databases. Thus, these parameters can and should be modified based only on the subsequent recommendations of these databases. 

MECHANISM OF THE GAS-PHASE OXIDATION OF METHANE TABLE 5.1 The Mechanism of the Initial Stage of the DMTM Process... 

Based on numerous experimental studies and kinetic analysis of the DMTM process, it is now firmly established that the partial oxidation of methane at high pressure is a degenerate branched-chain process with a very short chain length and a significant induction period. Rather intense chain branching in this reaction is provided by several parallel pathways that lead to the formation and subsequent decomposition of the CH3OOH and HOOH peroxides. A compact detailed mechanism includes 70 elementary reactions [63—66]. The reactions that play the most important role at the initial stage of the process and account for its main features are listed in Table 5.1. 

The influence of the reactor surface material and its pretreatment on the kinetics of DMTM and the yield of the products has been observed in many studies, so that this issue has received much attention. While earlier studies were performed mainly in reactors with metal surfaces (stainless steel, copper), present-day experiments are carried out in reactors in which the metal surface is more or less isolated from the reaction volume by inserts made of more or less inert material, such as quartz, Pyrex, and various forms of alumina. However, the results of many studies indicate that quartz, alumina, and other similar materials also cannot be considered as inert with respect to the reactants under conditions typical of the DMTM process. For example, according to [133], at 600 °C in the presence of silica and oxygen, up to 4.5% of the methane was converted into formaldehyde. The authors of [134] point out that Vicor glass and quartz reactor walls exhibit a significant activity in the formation of CH2O. At higher temperatures, above 620 °C, these materials were observed to catalyze the... 

A comparative study of the DMTM process at a reaction time of 50—300 s, P = 1—50 atm, and T = 375—500 °C revealed a strong dependence of the selectivity of methanol formation on the material of the reactor s irmer sinface [45]. The methanol formation selectivity obtained on the studied materials decreased in the order F rex > quartz > stainless steel. Note, however, that, with increasing pressme, the effect of the sm-face material declines, and the values of the selectivity converge (Fig. 6.5). 

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