OCM Reaction Product Gas and Downstream

This chapter discusses the general structure of the downstream separations for the OCM process. In the following Chapters 1.2.6 and 1.2.7 each separation step is introduced in more detail.

The composition of the reaction product gas varies depending on all the choices on catalyst, reactor, and operating conditions. Due to the limited con-version, CH4 often remains as the major component in the reactor outlet gas.

As the limiting reactant, O₂ should be completely extinguished. If a diluent gas such as N₂ or CO₂ is employed, it will also have a relatively high concentra-tion in the outlet. A signicant amount of H2O is also formed. The remainder consists of the products C₂H₄ and C₂H₆ and other by-products such as CO₂, CO, and H₂. Also, C3+components such as C₃H₆ and C₃H₈ have been identi-ed in experiments and the patents from Siluria even include their purication and monetization as by-products (Siluria Technologies, 2015). However, all the kinetics discussed in Chapter 1.2.3 exclude these components, so they are also neglected within this thesis for simplication. The order of magnitude of the concentration of C2 products in the reactor outlet gas is around 5 mol%. This means that, even with improved reaction performances, the separation task will not become trivial and must be properly addressed to develop a feasible OCM process.

The process structure considered in this thesis has been developed based on previous works and patents. The structure proposed by Salerno-Paredes, 2012 for the Natural Gas-based Oxidative Coupling of Methane (NG-OCM) has been used as a departure point. Salkuyeh and Adams II, 2015 has shown the positive eect of integrating a NG-OCM with power generation by recovering reaction heat and by combusting the un-reacted methane and light gases in

a CHP unit. The combination of NG-OCM with dry reforming of methane has been proposed and evaluated by Godini et al., 2013. Finally, the patents by Siluria Technologies, 2015 also involve several integration possibilities with power generation, methanation, steam cracking, and natural gas processing.

Biogas

O2

COMPRESSION

Waste-H2O

H2O

CO2 REMOVAL DISTILLATION

Ethylene

Figure 1.3: General process owsheet for the BG-OCM process

The structure for the BG-OCM process evaluated in this thesis is shown on Figure 1.3. The biogas rst passes through a treatment step and is then fed into the OCM reactor using its CO2 content as diluent gas. The reactor is operated in adiabatic regime with oxygen or oxygen-enriched air. The hot reaction outlet gases are used to generate High-Pressure Steam (HPS) for power generation.

The gases are then further cooled down in a direct contact heat exchanger prior to a rst compression step. The following CO₂ removal step is performed either by a standalone amine-based absorption or by a hybrid membrane-absorption process. The CO2-free gas stream is further compressed to the distillation section comprised of a demethanizer column and a C2-splitter column. The rst removes the light components, whereas the second separates the main product ethylene from ethane, which can be recycled or sold as a by-product.

The fate of the stream containing the light gases and coming out at the top of the demethanizer, i.e., Lights, is of major relevance. Given the low methane

1.2 Oxidative Coupling of Methane (OCM) conversions achievable in the OCM reaction section, most of the methane fed to the process exits in this stream. In this work, the combustion of this stream for heat and power co-generation is considered. The energetic utilization of this stream is likely the more straightforward to implement and it allows the process to be self-sucient in terms of electricity and steam consumption.

Alternative separation and integration options have been proposed or are currently under development for the OCM process and are only mentioned here to complete the state-of-the-art.

Alternative Separation Structures

The cryogenic distillation step and, specially the demethanizer step, are critical in terms of energy consumption and capital investment given the low tempera-tures involved and the high quantities of methane to be separated. Hence, new alternative processes based on gas adsorption are currently being developed. In (García et al., 2017), the synthesis steps and the granulation of zeolite 5A into molecular sieve beads is described followed by material characterizations and measurement of equilibrium isotherms using the main components of the OCM process. This is followed by realization of adsorption break-through experi-ments at the OCM mini-plant located at Technische Universität Berlin (TUB) together with the development of simulation models for the cyclic PSA separa-tion (García et al., 2019). This could substitute or at least reduce the separasepara-tion duty in the cryogenic distillation step. A simulation-based comparison of the two separation processes could assist in pointing out the advantages and disad-vantages of both in techno-economical and environmental terms. More recently, van Zandvoort et al., 2020 performed transient adsorption breakthrough ex-periments with simulated OCM gases, focusing on C₂H₄/CO₂ selectivity. The authors conclude that active carbon is, surprisingly, a very suitable material for this separation although this has not been previously highlighted in literature.

The authors have also demonstrated the importance of transient breakthrough experiments with multi-component gas mixtures for adsorption process mod-eling, highlighting that the experimental separation performances with 13X, NaY, CaX, and 5A zeolites presented worse performance than that predicted on the screening stage by applying the Ideal Adsorbed Solution Theory (IAST).

Another alternative process includes a membrane contactor using a solution of of silver nitrate (AgNO₃) (Cordi et al., 1997). The OCM reactor outlet passes to a dehydrogenation reactor (quartz tube), wherein C₂H₆ is converted into C2H4 and H2, but acetylene (C2H2) is also formed as a by-product. This is followed by a hydrogenation reactor to eliminate C₂H₂. A CO₂ trap using potassium hydroxide (KOH) and a cold trap for water removal are then applied

prior to the contactor. The contactor is operated at 298 K and a complex is formed ([Ag(C₂H₄)]⁺) facilitating the transport of ethylene (and other olens) while other components are only physically absorbed. The CH₄ and C₂H₆ are recycled to the reactor and the solution is boiled in a regeneration column to release ethylene. The lack of further development and mentions to this process, e.g., patents or publications, suggest that it is unlikely a valid option to date.

Alternative Integration Options

Further integration possibilities not considered in this thesis are shown as dashed lines in Figure 1.4. Recycling the light gases to the OCM reactor is not straightforward because next to methane, hydrogen and carbon monoxide are also present. The rst may cause undesired hydrogenation of ethylene into ethane, while both can be oxidized generating additional undesired heat release in the reactor. If air or oxygen-enriched air is used in the OCM reactor, the lights stream also contains N₂, which makes a purge stream necessary. An addi-tional separation step to recover CH4would likely make the recycle stream more costly than obtaining fresh methane from biogas or natural gas. An alternative is to apply CO and CO₂ methanation reactions to consume H₂ and concentrate CH4 in the stream. The feasibility thereof depends on the H2:CO:CO2 ratio produced in the reactor and implies additional investment cost. Finally, since the OCM process can export heat, the endothermic dry reforming of methane can be used for syngas production as a side-product. Syngas is a valuable feed-stock for several processes, e.g., Fischer-Tropsch synthesis for producing liquid fuels, but this implies more processing steps and higher capital investment.

Given the scales at which biogas is currently produced (see Chapter 1.1.6), the simple integration with a CHP unit as shown in Figure 1.3 is likely to be the preferred solution. Nevertheless, these other options should also be considered in future studies, because they enable a wider product portfolio and can be used to exibilize production according to market conditions.