Carbon dioxide conversion

Steam-Reforming Natural Gas. Natural gas is the single most common raw material for the manufacture of ammonia. A typical flow sheet for a high capacity single-train ammonia plant is iadicated ia Figure 12. The important process steps are feedstock purification, primary and secondary reforming, shift conversion, carbon dioxide removal, synthesis gas purification, ammonia synthesis, and recovery. 

The second step after secondary reforming is removing carbon monoxide, which poisons the catalyst used for ammonia synthesis. This is done in three further steps, shift conversion, carbon dioxide removal, and methanation of the remaining CO and CO2. 

The diffusion of oxygen and carbon dioxide also depends on their partial pressure gradients. Oxygen diffuses from an area of high partial pressure in the alveoli to an area of low partial pressure in the pulmonary capillary blood. Conversely, carbon dioxide diffuses down its partial pressure gradient from the pulmonary capillary blood into the alveoli. 

Description Natural gas or another hydrocarbon feedstock is compressed (if required), desulfurized, mixed with steam and then converted into synthesis gas. The reforming section comprises a prereformer (optional, but gives particular benefits when the feedstock is higher hydrocarbons or naphtha), a fired tubular reformer and a secondary reformer, where process air is added. The amount of air is adjusted to obtain an H2/N2 ratio of 3.0 as required by the ammonia synthesis reaction. The tubular steam reformer is Topsoe s proprietary side-wall-fired design. After the reforming section, the synthesis gas undergoes high- and low-temperature shift conversion, carbon dioxide removal and methanation. 

After shift conversion, carbon dioxide, residnal carbon monoxide, and snlfur compounds (only present in the synthesis gas from partial oxidation) have to be removed as they are not only a useless ballast but, more importantly, can poison the ammonia synthesis catalyst. 

The reactor temperature can reach over 900 C in the secondary reformer due to the exothermic reaction heat. The second step after secondary reforming is removing carbon monoxide, which poisons the catalyst used for ammonia synthesis. This is done in three further steps, shift conversion, carbon dioxide removal, and methanation of the remaining CO and C02. Every stage involved in ammonia synthesis is environmentally important. Any emission or mishandling would represent not only a short-term disastrous threat to human health, but also a long-term environmental problem of grave consequences. 

This process for production of synthetic ammonia by catalytic steam reforming of natural gas is a relatively clean process and presents no unique environmental problems. To assess the environmental impacts of a modem ammonia plant on air, water, and soil, each step in the ammonia synthesis namely, desulfurization, reforming, shift conversion, carbon dioxide removal, final purification, ammonia synthesis, and refrigeration should be examined. The sources of pollutants need to be identified and matched with cost-effective solutions for minimization/elimination by using the best available pollution control measure. 

In the above reactions, the oxidation process takes place in the anode electrode where the methanol is oxidized to carbon dioxide, protons, and electrons. In the reduction process, the protons combine with oxygen to form water and the electrons are transferred to produce the power. Figure 9-1 is a reaction scheme describing the probable methanol electrooxidation process (steps i-viii) within a DMFC anode [1]. Only Pt-based electrocatalysts show the necessary reactivity and stability in the acidic environment of the DMFC to be of practical use [2], This is the complete explanation of the anodic reactions at the anode electrode. The electrodes perform well due to the presence of a ruthenium catalyst added to the platinum anode (electrode). Addition of ruthenium catalyst enhances the reactivity of methanol in fuel cell at lower temperatures [3]. The ruthenium catalyst oxidizes carbon monoxide to carbon dioxide, which in return helps methanol reactivity with platinum at lower temperatures [4]. Because of this conversion, carbon dioxide is present in greater quantity around the anode electrode [5]. 

When synthesis gas is purified for the production of pure hydrogen or anunonia, carbon monoxide is generally removed by a combination of processes, including shift conversion, carbon dioxide removal, and methanation. However, when a pure CO byproduct is desired, this approach is not applicable. Three technologies that can be employed to remove and recover carbon monoxide from synthesis and other gases are adsorption (see Chapter 12) cryogenics, as discussed in the preceding section and absorption by a liquid, which is discussed next. 

Figure 28.2 illustrates a process that starts with coal or oil as a raw material. The process includes coal handling, preparation, and pulverization and partial oxidation of coal or oil to synthesis gas, followed by carbon and ash removal, carbon monoxide conversion, carbon dioxide and H2S removal, low temperature scrubbing with liquid nitrogen, and compression for ammonia synthesis. Air separation and sulfur recovery are steps unique to coal or heavy oil feedstocks. 

Conventional high and low temperature shift conversion carbon dioxide removal by a physical process methanation. 

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