Essay Example on Natural Gas: Captured Carbon Degraded and Stored in Earth's Crust

Introduction

The carbon that is captured in different deposits of fossilized vegetation and marine life is degraded and stored in the crust of the earth in the form of natural gas. Natural gas is a naturally occurring hydrocarbon gas, a mixture that contains primarily methane gas and other types of higher alkanes and other elements like carbon dioxide, hydrogen sulfide, nitrogen, and helium in small amounts (Marlin, Sarron, & Sigurbjornsson, 2019). This natural gas can be broken into carbon dioxide through a reformer. Carbon dioxide is then synthesized to produce methanol. The synthesis of methanol starts from pure sources, as well as the controlled concentrations of carbon dioxide and hydrogen. This is broken down into the reaction equations, as shown below.

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It should be noted that the several aspects that take place regarding the production of methanol from the syngas differ with the considerations of CO2 and H2. An example is the reaction to produce methanol from CO, which is based on the CO reaction with H2 directly thereby producing CH3OH. This is a highly exothermic reaction at -90.77 kJ/mol (Marlin, Sarron, & Sigurbjornsson, 2019). However, this reactor design presents numerous challenges, especially with the removal of the heat that is generated in the process. The methanol plant has always developed the main reactors to exclude the external cooling system. In such a case, the reactors that should be used in the process of methanol synthesis are mostly the ones limited to the boiling water (BWRs). This is due to the high heat profile that is typical for this reaction. This means that the complexity of the system will determine the nature of the material selection in such a case.

Synthesis Processes

In case the synthesis starts from the pure CO2, the reactors need a modification considering that there are less intense exothermic processes when compared to the syngas reaction. Another consideration is the use of a tube-cooled reactor that offers numerous advantages compared with the existing reactor configurations regarding the efficiency of production, lower cost, and its simplicity (Nieminen, Laari, & Koiranen, 2019). Besides, the choice of the tube-cooled reactors offers a better option as it is more efficient than the cold- shot reactors and the adiabatic reactors that require a series of multiple reactors thereby achieving the desired conversion rates. Another consideration in the process is improvement of the distribution of heat, with the reactor helping in the prevention of sintering of the catalyst, hence providing a longer life of the catalyst. In such a case, the process can continue for longer with little interruptions.

Purification of Crude Methanol

The refinement of crude methanol is critical regarding the considerations for the processes and the material selections. It is noted that the methanol can have multiple gas impurities and other liquid co-products that have demonstrated a lot of complications in the purification processes needed. It should be noted that the condensate that is obtained is a mixture of methanol and water. The water and methanol separation is not a complicated process. However, in cases of using crude methanol through the traditional and the CO2- based productions, it contains dissolved gases and other by-products that must be removed before using the gas. In the convectional methanol synthesis process that is developed in this case, the removal of impurities like CO, CO2, and ketones among others requires a separate fractionation column. This is an additional process to the primary distillation process that should be used in the process. In this case, massive energy is consumed in the condenser and the reboiler. The process requires a more advanced development than the simple flash operation due to the solubility of carbon dioxide in methanol.

Process Design

The production of methanol through the convectional method entails syngas that requires a catalyst for the reaction to form methanol. Mostly, material selection involves the presence of nickel, copper, and alumina-based catalyst. The synthesis of methanol requires a high temperature about 220 to 3000C (Marlin, Sarron, & Sigurbjornsson, 2019). In this regard, the materials selected in making the reaction vessels must be able to withstand that high temperature. Besides, the process requires the pressure of about 50 to 100 bar. The system design should be able to withstand high pressure. The methanol synthesis reaction requires the control at equilibrium that involves reaction of the carbon monoxide, hydrogen, and steam with the catalysts. This takes place in the presence of small amounts of carbon dioxide. The reaction equations are shown below.

The synthesis of methanol has been noted to involve a simple chemical reaction that is not as complex as the mixed alcohol processes. The requirements for the methanol synthesis of syngas require the hydrogen to carbon monoxide (H2/CO) ratio of at least 2 when utilizing alumina as a catalyst. Also, the carbon dioxide to the carbon monoxide (CO2/CO) ratio of 0.6 is required in order to avoid the deactivation of the catalyst (Liu et al. 2011). In the process, the use of nonreactive gases in low concentrations is imperative in building the synthesis loop. It should be noted that the steam reform is essential in giving the hydrogen a higher methanol reaction. However, this requires compression of the syngas by use of the centrifugal compressors.

Split Tower Arrangement

To get the required methanol final product, distillation is imperative. It should be noted that distillation requires a massive amount of energy due to the inherent inefficiency that is noted from the process of separation. In the process, there is a hefty duty that requires the condenser and the reboiler to be significantly active, and the reflux rates are essential in achieving the desired separation (Lee & Sardesai, 2005). In some of the facilities, there can be a consideration for the substantial heat that is left from the previous reactions and other processes, that are generated in the form of steam. However, this arrangement is not necessarily desirable due to the inefficiencies of the thermodynamic reactions. To facilitate efficiency, heat is minimized with a split column system that operates at a higher pressure. This may require a heat exchange to be integrated into the system with the reboiler.

Summary

The process design analysis in the unit operations demonstrates the advantages and the disadvantages in the production of methanol. The processes illustrate the need to consider the materials needed based on the nature of the process, like the exothermal process. In the conventional method, the materials must be considered for high quality based on the high temperatures and pressure. However, the analysis demonstrates the need for more cleaner and energy-efficient method compared with the convectional processes. Besides, the reaction requires a high amount of space that necessitates larger reactors. The convectional method of methanol production has numerous pros and cons that determine the selection of materials and the type of production plant.

References

Lee, S., and Sardesai, A. 2005. Liquid phase methanol and dimethyl ether synthesis from syngas. Top. Catal. 32, pp.197-207.

Liu, G., Larson, E.D., Williams, R.H., Kreutz, T.G., and Guo, X. 2011. Online Supporting Material, Making Fischer-Tropsch Fuels and Electricity from Coal and Biomass: Performance and Cost Analysis. Energy Fuels, 25, pp.415-437.

Marlin, D., Sarron, E., and Sigurbjornsson, O. 2019. Process Advantages of Direct CO2 to Methanol Synthesis. Frontiers in Chemistry, pp. 1-14.

Nieminen, H., Laari, A., and Koiranen, T. 2019. CO2 Hydrogenation to Methanol by a Liquid-Phase Process with Alcoholic Solvents: A Techno-Economic Analysis. Processes, 7(7): pp.405