Carbon dioxide removal systems

Methanation catalysts are not usually deactivated by thermal sintering. The principal reason for any loss of activity is poisoning. Sulfur compounds will poison methanation catalysts, but sulfur is not present unless the low temperature shift catalyst is by-passed. The poisons most likely to occur under normal operating conditions are those originating from the carbon dioxide removal system that precedes the methanator. Carry-over of a small amount of liquid into the methanator is not serious. Large volumes of liquid will have a... 

To produce pure hydrogen, the carbon dioxide must be removed. The gas passes through a carbon dioxide removal system, which contains a chemical solvent that selectively absorbs the carbon dioxide as the gas passes through the solvent.12 Heat then is added to the solvent to discharge the carbon dioxide. The regenerated solvent is returned to the system to continue the removal of carbon dioxide. 

In an ammonia plant (Figure 4.2), the synthesis gas from the reformer furnace is fed into a secondary reformer vessel, where air is added through a burner to create outlet vessel temperatures of -1,800° F (980° C). The outlet of the secondary reformer vessel is cooled in a quench steam generator and sent to a shift converter this is followed by a carbon dioxide removal system such as the one in a hydrogen plant. The purified nitrogen from the air added in the secondary reformer vessel and hydrogen synthesis gas is fed to a methanator to convert residual oxides of carbon back to methane (which is inert in the ammonia conversion) the gas is then compressed to -3,000 psia (2,070 kPa). The compressed synthesis gas is fed to an ammonia converter vessel. As the synthesis gas passes over catalyst beds, ammonia is formed. The ammonia product is then cooled and refrigerated to separate out impurities. 

Exxon Chemical Process. The Exxon Chemical process [1092], [1093] was specifically designed for the company s own site in Canada and so far not built for third parties. It uses a proprietary bottom-fired primary reformer furnace and a proprietary hot potash carbon dioxide removal system with a sterically hindered amine activator. Synthesis loop and converter are licensed by Haldor Topsoe A/S. Synthesis is carried out at 140 bar in a Topsoe S-200 converter and total energy consumption is reported to be 29 GJ/t NH3. 

In subsequent weeks, the main carbon dioxide removal system failed. Then the cooling system malfunctioned, leaking coolant into the air and forcing the shutdown of the drinking-water reclamation system due to contamination. Temperatures in the modules remained at 96°F (36°C) for weeks. 

As the month dragged on, the station suffered repeated computer failures, as well as failure of the carbon dioxide removal system and leaks of mysterious... 

A typical carbon dioxide removal system consists of an absorber where the feed gas is introduced at the bottom and the lean propylene carbonate solution is contacted with the rising gas in a countercurrent manner. The carbon dioxide content of the treated gas depends upon the initial content of C02 in the lean gas. The rich gas (containing the removed C02 and other compounds) is passed through an intermediate flash tank from where some of the low molecular hydrocarbons are recycled to the absorber. The stripped solvent is then passed through a low pressure flash tank where the carbon dioxide is flashed to the atmosphere and the lean gas is pumped back to the absorber. This process can be modified further to achieve lower C02 exit concentrations in the treated natural gas by adding strippers operating at atmospheric pressure followed by vacuum strippers. Power requirements for any of these units are very low, thus keeping the process very efficient and economical. 

Carbon dioxide removal systems can be made environmentally acceptable by properly specifying the instrumentation, water wash of the overhead vapors, efficient use of demisters, routine control of the solvent physical properties and chemical composition, and care in startup and emergency shutdown... 

The reactions proceed at 310 C (590°F). Therefore, the product gas must be heated after leaving the carbon dioxide removal system to carry out the methanation. The highly exothermic reactions further increase the temperature by 90-100°C (160-180 F), establishing a methanator outlet temperature of400°C (750 F). 

Comparative cost estimates are presented in Table 4 for ethylene oxide processes. The higher cost of ethylene feedstock for the air-based process is a reflection of lower overall yield. More ethylene is required to compensate for the quantity that is oxidized to carbon oxides. This cost advantage for the oxygen-based process is partially offset by the cost of the oxygen and the higher cost for methane ballast gas and other chemicals for the carbon dioxide removal system. 

About 10% of the ethylene feed is converted to carbon dioxide. Therefore, a carbon dioxide removal system, usually hot potassium carbonate, is used to separate carbon dioxide from the reactor effluent stream prior to recycle. 

U. Desideri and A. Paolucci, Performance modeling of a carbon dioxide removal system for power plants, Energy Comers. Manage. 40, 1899-1915 (1999). 

Figure 5.5 Carbon dioxide removal system, with continual regeneration of absorbent solution...

In order to examine the hydrocarbon contaminant problem in an air separation plant, we may refer to Figure 1, Some contaminants in the entering air will be removed in the prepurification system, in the compressor interstage separators, in the caustic scrubber or other carbon dioxide removal system, in the adsorptive driers, and in the heat exchanger circuit, or in the regenerators of a low-pressure cycle. Those contaminants which pass these points will enter the high-pressure column where they will be washed into the enriched air stream and pass to the hydrocarbon adsorbers. The adsorbers will have substantial capacity for all hydrocarbons except methane. Most of the methane and traces of other hydrocarbons will pass on to the low-pressure column. They will be scrubbed down the column to the reboiler-condenser and, in a cycle as pictured, be continuously removed with the product oxygen. 

Several revamp options are available for modification of the carbon dioxide removal section depending on the type of carbon dioxide removal process. The processes mostly used in ammonia plants are chemical absorption processes based on either hot potassium carbonate (HPC) such as Benfield, or Vetrocoke, or amine solutions such as MEA. The chemical carbon dioxide removal processes may be improved or replaced with a physical process in which the absorbent is regenerated by simply flashing off carbon dioxide. In this way the need for regeneration heat may be reduced or eliminated. A physical carbon dioxide removal system may result in energy savings of 0.01-0.35 Gcal/MT ammonia. 

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