Expanded Reactor Engineering Calculations for the Oxidative Coupling of Methane

Abstract

The catalytic activation of CH₄ by limited amounts of O₂ produces a mixture of synthesis gas (CO, H₂) and light hydrocarbons (C₂H_x), the relative amounts of each depending on catalyst type and process conditions. Using an elementary reaction mechanism for the oxidative coupling of methane (OCM) on a La₂O₃/CeO₂ catalyst derived from the literature, this study replaces the activating O₂ with moist H₂O₂ vapor to reduce synthesis gas production while improving C₂H_x yields and selectivities. As the H₂O₂ content of the activating oxidant rises, more of the CH₄ conversion occurs in the gas phase instead of with the catalytic surface. In a packed bed reactor (PBR), the use of H₂O₂ allows the PBR “light-off” to occur using a lower feed temperature. In exchange for a small decline in CH₄ conversion, C₂H_x selectivity increases while synthesis gas production drops. In a continuous stirred tank reactor (CSTR), H₂O₂ improves C₂H_x over synthesis gas across a wider range of feed temperatures than is possible with the PBR. This suggests the CSTR will likely reduce OCM preheating requirements.

1. Introduction

The expanded use of hydraulic fracturing has resulted in the venting and flaring of large volumes of hydrocarbons, especially natural gas. A lack of local pipeline capacity results in more flaring [1] and fugitive emissions. Public sentiment is prompting government environmental regulators to force the reduction or outright banning of hydrocarbon flaring and fugitive emissions [2] to reduce climate change by global warming. Petroleum and natural gas companies are now actively promoting their efforts to reduce their methane footprints [3]. However, the engineering challenge of CH₄ conversion is considerable. Catalytic methods offer several conversion approaches.

Catalytic activation of CH₄ is generally classified as indirect or direct. Indirect activation produces synthesis gas (primarily CO and H₂) using an oxygen source by reforming (H₂O—steam; CO₂—dry) or partial oxidation (O₂). Synthesis gas can be catalytically converted to useful products such as alcohol (usually CH₃OH) or higher hydrocarbons (by Fischer–Tropsch process).

Direct activation of CH₄ uses no oxygen source. It directly breaks the very strong CH₃–H bond (4.39 × 10⁵ J/mol). For example, methane dehydroaromatization (MDA) uses a Mo/HZSM-5 zeolite [4] catalyst to form C₂H₄ and aromatics at 950–1030 K. Unfortunately, MDA is thermodynamically limited. In addition, catalyst activity drops quickly due to coke deposition.

An intermediate direct approach is the oxidative coupling of methane (OCM) that uses a very small amount of O₂ to activate the CH₄ while limiting the coke formation. The OCM catalysts are transition metal oxides on an oxide support, e.g., La₂O₃/CaO [5] and La₂O₃/CeO₂ [6]. Feed CH₄/O₂ molar ratios of 7–11 with temperatures ~840–1220 K have been studied. The products include C₂₊H_x and synthesis gas, with the distribution depending on the catalyst and temperature. More feed O₂ favors CH₄ conversion, but lowers C₂₊H_x selectivity in favor of CO_x.

Gambo et al. [7] reviewed recent advances in OCM, including the use of catalyst nanowires, and identified avenues for further research. Using proprietary nanowire catalysts, Siluria Technologies demonstrated OCM in a continuous flow large demonstration plant [8,9]. The primary goal is the production of C₂H₄ for subsequent conversion to gasoline and chemicals. Siluria envisions a flexible two-stage OCM reactor in which the first stage is a packed bed reactor (PBR) feeding CH₄, O₂, and possibly C₂H₆. The second stage feeds more C₂H₆ in an endothermic pyrolysis plug flow reactor that uses the first stage exothermicity.

Considering the potential and the constraints of OCM, the reactor type becomes important. Conventional packed beds are not economically viable for OCM [10]. Other configurations such as membrane reactors and fluidized beds should be considered. A recent study [11] compared the packed bed reactor (PBR) and continuous stirred tank reactor (CSTR) for OCM based on calculations with a detailed reaction mechanism [6]. Higher feed temperatures were required to achieve a “light off” of the PBR, while the CSTR required considerably lower feed temperatures to reach nearly comparable conversions. The CSTR—a fluidized bed in likely practice—favored synthesis gas production over C₂₊H_x as compared to the PBR.

In a difficult experimental study, Liu et al. [12] considered the rapid PBR “light off”. Using careful temperature control and real-time product measurement in a micro-reactor during OCM over La₂O₃ nanorod catalysts with a feed CH₄/O₂ = 3, Liu et al. observed that CO₂ is the dominant product at the lower temperatures (<853 K) when relative O₂ concentrations are high. The system transitions through a window (~873 K) to higher temperatures (>913 K) that favor C₂₊H_x at the lower O₂ levels. In effect, a competition between CO_x and C₂₊H_x formation occurs in this window. This suggests that reactor configuration and temperature will be critical for OCM reactor design. These observations also show that, in OCM, the production of byproduct syngas is unavoidable if C₂₊H_x is the goal.

In typical OCM, some of the limited O₂ dissociates on the catalyst surface [13]. An adsorbed •O_s then abstracts an H• from CH₄ to form the key •CH₃ gas phase radical. Oxygen atoms on the metal oxide lattice surface might directly abstract the H• from CH₄. In this case, the gas phase O₂ replenishes the resulting surface vacancy, leaving behind an adsorbed •O_s [7]. In either case, the •CH₃ radicals can combine to form C₂H₆. Further reactions form the desired C₂₊H_x products, and the undesired CO_x and coke. An alternate CH₃-H bond activator would reduce CO_x while possibly enhancing C₂₊H_x formation. Unfortunately, the desired C₂₊H_x products are more susceptible to oxidation than the reactant CH₄. This can occur via gas phase O₂ or surface oxygen species [7,13].

An alternate or supplementary oxidant that might be less aggressive toward C₂₊H_x by reducing surface oxygen species while still activating the CH₄ is the •OH gas phase radical. Gas phase hydroxyl radicals can be formed from the gas phase decomposition of co-fed H₂O₂ vapor: H₂O₂ + M = 2 •OH + M. Section 3 below summarizes experimental literature on the use of H₂O₂ for OCM that motivates this paper.

This paper is also a sequel to the Rivera et al. [11] study on OCM. It uses the same elementary reaction mechanism as that developed by Karakaya et al. [6]. The paper considers vapor phase H₂O₂ as a supplemental or alternative activating oxidizer to reduce syngas production in favor of C₂₊H_x. Both CSTR and PBR are considered.

2. Kinetic Mechanism and Computational Tool

The detailed reaction mechanism used in this study was developed by Karakaya et al. [6]. It is composed of series and parallel elementary reactions [13] in both the gas and surface phases. The gas phase portion is taken from Chen et al. [14]. The surface portion is inspired by the work of Alexiadis et al. [15].

In the current study, the OCM mechanism was employed in reactor simulations by Detchem^® [16]. This program achieves material and energy balances using the mechanism, based on required reactor input data and parameters. In this study, separate adiabatic PBR (modeled as plug flow) and CSTR (modeled as perfectly mixed) runs were conducted with the Detchem^® PBED and CSTR applications, respectively. See [11] for a listing of the governing balance equations used in each reactor simulation.

Comparative results were prepared in terms of conversions X_CH4, selectivities S_j of useful products (C₂H_x, CO, H₂) or byproducts (H₂O, CO₂), and yields Y_j:

$${\mathrm{X}}_{{\mathrm{CH}}_4}\equiv \frac{{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{in}}-{\mathrm{F}}_{{\mathrm{CH}}_4}}{{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{in}}}{ \mathrm{X}}_{{\mathrm{O}}_2}\equiv \frac{{\mathrm{FO}}_2{}_{,\mathrm{in}}-{\mathrm{F}}_{{\mathrm{O}}_2}}{{\mathrm{F}}_{{\mathrm{O}}_2,\mathrm{in}}}{ \mathrm{S}}_{\mathrm{j}}\equiv \frac{{\mathrm{n}}_{\mathrm{j}}{\mathrm{F}}_{\mathrm{j}}}{{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{in}}-{\mathrm{F}}_{{\mathrm{CH}}_4}} {\mathrm{Y}}_{\mathrm{j}}\equiv \frac{{\mathrm{n}}_{\mathrm{j}}{\mathrm{F}}_{\mathrm{j}}}{{\mathrm{F}}_{{\mathrm{CH}}_4,\mathrm{in}}}$$

(1)

where F_j = molar flow rates, F_j,in = molar rate at the reactor inlet, and n_j is the number of CH₄ moles needed to make one mole of product (byproduct). For example, for C₂H₄, n_j = 2; for H₂, n_j = 0.5.

In the prior study [11] of OCM with O₂ (no H₂O₂) as the activator, twenty cases were considered in separate CSTR and PBR calculations. The process parameters considered are summarized in

Table 1. Process parameters of this study, inspired by Karakaya et al. [6].

Low Feed Ratio “LR” CH4/O2 or CH4/H2O2 = 7			High Feed Ratio “HR” CH4/O2 or CH4/H2O2 = 11
Low Feed Processing Rate “LF” 37.1/s			High Feed Processing Rate “HF” 58.3/s
Feed Temperatures Tin (K)
843	943	1043	1143	1243

. The parametric study included the molar feed CH₄/O₂ ratio (“low” = 7, “high” = 11; LR and HR, respectively), molar feed rate (“low” = 8.984 × 10⁻⁵, “high” = 1.412 × 10⁻⁴ mole/s; called LF and HF, respectively), and feed temperature (843–1243). At each of the five feed temperatures, four cases were considered: LR_LF, LR_HF, HR_LF, and HR_HF. These conditions were inspired by published laboratory data [6]. Each reactor simulation assumed the same catalytic site density and total catalytic surface area, resulting in the same 2.42 × 10⁻⁶ total moles of sites. A processing rate can be defined as the ratio of total molar feed rate to the total number of catalyst sites. The processing rate range is 37.1–58.3 s⁻¹. These parameters motivate the present study. Larger reactors can be scaled from these conditions.

3. Alternate Activator H₂O₂

Garibyan et al. [17] studied OCM over Pb/aerosil, ZnO, and 10% Na₂O/ZnO catalysts. Pulses of H₂O₂ vapor into the CH₄/O₂ feed increased the C₂H_x yield while stabilizing catalytic activity during OCM at 1 atmosphere and 1023 K. With a 1% Au/5% La₂O₃/CaO catalyst, at 973–1073 K, Eskendirov et al. [18,19] observed that H₂O₂ increased CH₄ conversion, while enhancing C₂₊ hydrocarbon yields even up to benzene. They speculated that H₂O₂ decomposition resulted in more •OH radicals for gas phase activation of the CH₄. They even observed OCM in the presence of H₂O₂ vapor without catalyst at temperatures as low as 673 K, with considerable selectivity for C₂H_x. These studies motivate this paper.

Considering the importance of making any OCM process as “green” as possible, a potentially sustainable source of H₂O₂ uses a photo-activated TiO₂-Au-Si catalyst while feeding O₂ and liquid H₂O [20]. Spiegelman and Alvarez [21] developed a simple yet clever technology to produce a continuous vapor stream of H₂O₂ from a liquid solution of H₂O₂ in water. Subsequent drying of the vapor stream to raise the H₂O₂ concentration runs the risk of energetic decomposition, thus posing a safety risk. In a study of the decomposition of H₂O₂ vapor on various surfaces, Satterfield and Stein [22] generated H₂O₂ vapor concentrations of up to 0.23 atm in a 1 atm system. Therefore, in the remainder of these calculations, we used a conservative molar H₂O/H₂O₂ = 4 linkage in all cases where H₂O₂ was used.

The OCM process requires considerable preheating, so the H₂O₂ decomposition risk also calls into question the feed temperature for the H₂O/H₂O₂ vapor stream. Consider the decomposition: H₂O₂ + M → 2 •OH + M with a rate constant borrowed from the Chen et al. [14] reaction set for the CH₄ gas phase chemistry used in the OCM mechanism [6]. Assume a feed CH₄/H₂O₂ ratio of 11 (the HR case), with the coflowing H₂O vapor, and no O₂. A simple kinetic calculation shows that, at 1243 K, the H₂O₂ will be 100% decomposed in 10 microseconds. At 673 K, the time is a more realistic 5 s. This simple calculation suggests that preheating a combined CH₄, O₂, H₂O₂, and H₂O feed stream would be problematic, especially for a PBR. It also suggests keeping the CH₄/O₂ and H₂O/H₂O₂ vapor streams separate, with the CH₄/O₂ stream taking most or all the preheat. Separate feed streams are more easily handled with the CSTR.

Finally, the replacement of O₂ by H₂O₂ maintains the feed CH₄-to-O molar ratio, though it somewhat increases the overall H content of the feed. Consider the following overall reactions below. Though simplistic, Equations (1) and (2) show that replacing O₂ with H₂O₂ should increase the production of C₂H₄ and H₂O, while reducing CO and H₂. In addition, adiabatic reactor temperatures should be lower.

$${\mathrm{CH}}_4+0.5{ \mathrm{O}}_2=\frac{1}{3}{\mathrm{C}}_2{\mathrm{H}}_4+\frac{2}{3}{\mathrm{H}}_2\mathrm{O}+\frac{1}{3}\mathrm{CO}+\frac{2}{3}{\mathrm{H}}_2\hspace{1em}\Delta {\mathrm{H}}_{\mathrm{r}}^{\mathrm{o}}=-105.7 \mathrm{kJ}/\mathrm{mole}$$

(2)

$${\mathrm{CH}}_4+0.5{ \mathrm{H}}_2{\mathrm{O}}_2=0.5{ \mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}+0.5{ \mathrm{H}}_2\hspace{1em}\Delta {\mathrm{H}}_{\mathrm{r}}^{\mathrm{o}}=-72.7 \mathrm{kJ}/\mathrm{mole}$$

(3)

3.1. Incremental H₂O₂ Replacing O₂ to PBR at Fixed Feed Temperature

Rivera et al. [11] showed that, for the parametric range considered (Table 1), the highest CH₄ conversions for the PBR were nearly 40% for the LR cases, with little impact of flow rate, for a 1243 K feed temperature. The LR_LF case showed the highest sum_C₂H_x (sum of C₂H₆, C₂H₄, and C₂H₂) selectivities and yields. First, for the same 1243 K feed temperature, H₂O₂ was incrementally substituted for O₂. It was assumed that H₂O₂ vapor will be available at 20 mole percent with the balance of H₂O vapor. The starting point was the LR_LF (feed CH₄/O₂ = 7, feed processing rate 37.1/s) case [11] in the PBR. The PBR bed length was that used in the experiments described elsewhere [6]. This high feed temperature did ignore the H₂O₂ stability issue. The insights gained, however, will help identify the utility of H₂O₂ as a potential CH₄ activator.

Figure 1 shows that the final PBR bed temperature drops with increasing H₂O₂ content. This was attributed to the reduced overall reaction exothermicity suggested by Equations (1) and (2). However, the bed temperature peaked much earlier when H₂O₂ was used in the feed oxidant. A closer examination of the post-entrance region is revealed in Figure 2 below. The H₂O₂-containing cases showed a near-immediate rise from the 1243 K feed temperature. The very early single peak for the R = 1 (no feed O₂) case roughly corresponded to the exhaustion of the H₂O₂, which was consistent with the extremely rapid H₂O₂ decomposition described earlier at this temperature. The presence of the feed O₂ caused a second local temperature peak further downstream. These peaks (R = 0.33, 0.67 cases) and the much later single peak (R = 0) corresponded approximately to where the O₂ ran out, with no further adsorbed •O_s.

The R = 1 case (no feed O₂) showed almost no adsorbed species, suggesting that all the CH₄ conversion effectively occurred in the gas phase (i.e., non-catalytic). On the contrary, for the R = 0.33 and 0.67 cases, while H₂O₂ was still present, there was a complex parallel/series scheme ongoing with both catalytic and gas-phase reactions occurring. These observations were consistent with the claim that the CH₄ conversion is accelerated by •OH gas phase radicals produced from the H₂O₂ dissociation [18,19].

Figure 3 shows that increasing the H₂O₂ content improved the selectivity of sum_C₂H_x, while lowering both the selectivity and yield for syngas (H₂ + CO). There was a negligible impact on sum_C₂H_x yields. The reduction in syngas was due almost entirely to a reduction in CO. Finally, for these four cases from R = 0 to 1, the CH₄ conversions were: 39.3, 36.6, 35.1, and 33.8%, respectively. In all cases, the final O₂ and H₂O₂ conversions were 100%.

An expanded look at the long-post-entrance region provides more insight into the dramatic impact of substituting some of the feed O₂ with H₂O₂ vapor. Figure 4 shows the selectivities for the CO, C₂H₆, C₂H₄, and CH₄ conversions for the LR_LF case, at 1243 K feed temperature, for feed ratio cases R = 0 and R = 0.33. The curves were almost unchanged after the 0.006 m bed length was reached.

Some key points can be made here. In the R = 0 case, CO selectivity exceeded C₂H₄ before the temperature peaked (see Figure 1), but was lower than C₂H₆. After the peak temperature, consistent with experimental observations by Liu et al. [12], C₂H₄ exceeded CO. In the R = 0.33 case, the H₂O₂ (not shown) dissociated immediately upon entry. The resulting •OH radicals abstracted •H atoms from CH₄, causing a spike in C₂H₆ formation, and a quickly rising CH₄ conversion. The C₂H₆ rapidly dehydrogenated to C₂H₄. The CO peaked at approximately where the temperature peaked. Unlike the R = 0 case, both C₂ species exceeded CO prior to the temperature peak. This all occurred much faster than for the R = 0 case. The ultimate CH₄ conversion for the R = 0.33 case was only slightly lower than for the R = 0 case, while showing a higher C₂H₄ selectivity and lower CO. Liu et al. [12], for the R = 0 case, concluded that the selectivities of COx and C₂ depended on local O₂ concentration and temperature. Using H₂O₂ added the further complexity of gas phase chemistry to the surface reactions.

3.2. Use of H₂O₂ to Decrease Feed Temperature to PBR

We now discuss whether the replacement of O₂ by H₂O₂ allows for a lowering of the overall PBR feed temperature, which would be an energy and cost saving. This analysis used the R = 0 and R = 0.33 feeds with the LR_LF case, with the results shown in

Table 2. Impact of partial replacement of feed O₂ by H₂O₂ at various feed temperatures for PBR running LR_LF case.

Feed Temperature (K)	1243	1143	1043	1043	943	943
R value	0	0	0	0.33	0	0.33
Max Temp (K)	1603	1547	1065	1414	944	1345
Exit Temp (K)	1424	1423	1065	1343	944	1321
CH4 Conv (%)	39.3	33.8	0.891	26.4	0.044	21.8
sum_C2Hx Selec (%)	75.6	72.3	58.7	70.0	36.7	62.6
sum_C2Hx Yield (%)	29.7	24.4	0.522	18.5	0.016	13.6
CO + H2 Sel. (%)	52.4	51.6	24.3	54.5	10.2	57.8
CO + H2 Yield (%)	20.6	17.5	0.217	14.4	0.004	12.6

.

The partial H₂O₂ substitution for O₂ produced a respectable CH₄ conversion at the lower feed temperatures where O₂ feed alone showed no OCM activity (R = 0 for 1043 and 943 K feeds). These results at lower feed temperatures were consistent with those observed experimentally [17,18,19]. At the 843 K feed temperature, even the R = 0.33 case was poor.

3.3. Incremental H₂O₂ Replacing O₂ to CSTR

As mentioned above, the CH₄/O₂ and H₂O/H₂O₂ vapor streams would likely be fed separately into the CSTR due to safety concerns about preheating a vapor stream containing H₂O₂. For example, for the R = 0.33 and HR_HF case, to achieve an effective (hypothetical) 843 K feed temperature while holding the H₂O/H₂O₂ stream at 373 K, the CH₄/O₂ stream would be preheated to about 883 K. The HR_HF case was chosen for this CSTR analysis because it showed the best yield and selectivity of sum_C₂H_x at the lowest feed temperatures in the earlier study [11].

For the CSTR calculations, the volume was the same as the open (gas) volume of the packed bed, with the same catalyst surface area. The CSTR might be a single-phase ideal fluidized bed, or a perfectly mixed (e.g., jet-stirred) reactor with catalyst on the walls.

Substitution of all or some of the feed O₂ content with H₂O₂ had a marked impact on the CSTR performance. Figure 5 (left) shows that substituting H₂O₂ for O₂ reduced the exit temperature somewhat (~65–110 K), as might be expected from the lower exothermicity (see Equations (2) and (3)). However, Figure 5 (right) shows a complex story for the impact on CH₄ conversion. For effective feed temperatures of 843 and 943 K, switching from O₂ to H₂O₂ reduced CH₄ conversion by only ~3 percentage points. With a 1043 K feed temperature, there was little impact on conversion. At 1143 and 1243 K, switching to H₂O₂ actually increased CH₄ conversion. While literature experiments [17,18,19] used a PBR, the results here were still found to be consistent with those observations in terms of the activity of H₂O₂.

Figure 6 shows that the yield of sum_C₂H_x was notably higher than the CO + H₂ yield for all R cases, while increasing H₂O₂ had a greater impact on yields at the higher feed temperatures. These results were consistent with those revealed in Figure 3 for the PBR. Selectivities of sum_C₂H_x remained higher than CO + H₂, especially at the lower feed temperatures, as seen in Figure 7.

Equations (2) and (3) suggest that replacing O₂ by H₂O₂ will increase the production of C₂H₄ and H₂O, while reducing CO and H₂. Consider the HR_HF case, with the effective feed temperature into the CSTR of 1243 K, with the results shown in

Table 3. Impact of progressive replacement of feed O₂ by H₂O₂ at 1243 K effective feed temperatures for CSTR running HR_HF case. * H₂O values are corrected for H₂O in feed for R ≠ 0 cases.

R Value	0	0.33	0.67	1
Exit Temp (K)	1463	1421	1386	1356
CH4 Conv (%)	17.3	18.1	19.0	21.5
C2H4 Selec (%)	46.1	52.6	58.8	70.0
C2H4 Yield (%)	7.98	9.50	11.2	15.1
H2O Selec (%) *	29.1	34.3	38.5	42.1
H2O Yield (%) *	5.03	6.19	7.32	9.07
CO Selec (%)	35.1	26.7	14.9	0.10
CO Yield (%)	6.05	4.82	2.84	0.02
H2 Selec (%)	39.8	36.1	31.5	24.5
H2 Yield (%)	6.89	6.51	5.99	5.27
${\mathrm{O}}_{\mathrm{s}} \mathrm{coverage} \left(\mathrm{ppm}\right)$	24.5	15.8	9.61	0.48

. As the H₂O₂ fraction in the oxidant increased (i.e., higher R value), the CSTR exit temperature fell, but the CH₄ conversion increased. The C₂H₄ and H₂O production increased, while CO and H₂ dropped. Finally, the fraction of catalytic sites occupied by adsorbed O atoms (O_s) decreased as R increased. Since •H abstraction by •O_s is the primary catalytic step for CH₄ activation [6] by O₂, the drop in •O_s fraction was consistent with a shift from heterogeneous catalyzed to homogeneous non-catalyzed conversion pathways at higher R.

3.4. Brief Reactor Comparison Summary

A simple comparison between the PBR and CSTR for OCM with and without H₂O₂ as an activating oxidant is shown in

Table 4. Brief comparison of PBR vs. CSTR for OCM using the LR_LF case and R = 0.33 for the O₂/H₂O₂ runs.

	PFR	PFR	CSTR	CSTR
	O2	O2/H2O2	O2	O2/H2O2
Lowest practical feed temperature (K)	1143	943	843	843
CH4 conversion	34	22	23	28
Sum_C2Hx selectivity	72	63	55	80
CO + H2 selectivity	52	58	57	33

. This illustration is based on the LR_LF case (see Table 1). The results will vary somewhat for other cases, but the observations will be similar. This summary considers both the current results and those published earlier with just O₂ as an activator [11]. While the claims are based on calculations using the Karakaya et al. [6] mechanism for La₂O₃/CeO₂ catalyst, it is anticipated other OCM catalysts would give rise to similar claims.

Table 4 shows several points. The lowest practical feed temperature is the value below which there is no appreciable CH₄ conversion. All remaining values in each column correspond to those temperatures. Replacing a portion of the feed O₂ with H₂O₂ vapor allows the CSTR to achieve good CH₄ conversions at the lowest feed temperature. It also allows the PBR to run with a reduced feed temperature. Even at this low feed temperature, the CSTR has a sum_C₂H_x selectivity that exceeds the PBR at a much higher temperature. The CSTR also shows reduced syngas (CO + H₂) and improved sum_C₂H_x selectivity when using the H₂O₂.

4. Simple Layout of a “Green” OCM Plant

Although Equations (2) and (3) show that OCM via O₂ and H₂O₂ is exothermic, a future sustainable OCM plant must consider O₂ and H₂O₂ production and overall plant heat integration. Figure 8 offers a simple schematic. The OCM reactor feeds CH₄ and a combination of O₂ and vapor phase moist H₂O₂. The O₂ is produced in a solar powered air separation plant [23] that enjoys the energy and economic savings from chemical looping instead of cryogenic separation [24,25]. Aqueous H₂O₂ is produced by the solar powered catalyzed reaction of O₂ and acidic liquid H₂O [20]. Vapor phase H₂O₂ is stripped out of the liquid by the N₂ or He [21] recovered from the natural gas. Heavier-than-CH₄ saturated hydrocarbons (C₂H₆-C₅H₁₂) are separated out from the natural gas. Post-OCM reactor processing separates the CO and H₂ as synthesis gas and the desirable coupled hydrocarbons (e.g., C₂H₄ and C₂H₆). Byproduct CO₂ from Separations_2 and Pretreatment can be captured with caustic scrubbing, and subsequently sequestered.

5. Conclusions

Using an elementary reaction mechanism for the oxidative coupling of methane (OCM) on a La₂O₃/CeO₂ catalyst borrowed from the literature, this study considered the incremental replacement of the activating O₂ with moist H₂O₂ vapor. Both packed bed reactor (PBR) and continuous stirred tank reactor (CSTR) configurations were used. As the H₂O₂ content of the oxidant increased, more of the CH₄ conversion occurred in the gas phase with less assistance from the catalytic surface. Hydroxyl (•OH) radicals from rapid H₂O₂ decomposition abstracted •H atoms from CH₄ to produce •CH₃ radicals. This occurred in parallel to a similar abstraction by oxygen atoms (•O_s) adsorbed on the catalyst surface when O₂ was fed. In the PBR, H₂O₂ allowed the “light-off” temperature jump to occur using a lower feed temperature. Even though there was a slight decline in CH₄ conversion, the C₂H_x selectivity increased while synthesis gas dropped. Since significant preheat was still needed, process safety considerations might dictate that H₂O₂ vapor is better suited to the continuous stirred tank reactor (CSTR) configuration where the H₂O₂/H₂O vapor stream can be fed at lower temperatures separately from the preheated CH₄/O₂ stream. In a CSTR, H₂O₂ significantly improved C₂H_x production compared to synthesis gas over all feed temperatures studied, thus showing that OCM is possible with significantly less preheating compared to PBR. A future OCM plant can operate in a more “green” way with the use of solar-activated H₂O₂ production, and solar-powered O₂ production from chemical-looping air separation.