Ammonia plants regenerators

The example CO2 capture process, shown in Figure 8 as an Aspen Plus EO model representation, is part of an ammonia plant. Designed to scrub CO2 from ammonia synthesis gas, it includes an absorber and two solution regeneration columns, one stripping the rich, C02 laden solution leaving the absorber to semilean concentration of absorbed CO2, and the other cleaning the solution even further to lean solution... 

In this case, carbon dioxide reacts reversibly in the adsorber with aqueous alkaline solutions to form a carbonate adduct (configuration 1). This adduct decomposes in the stripper upon heating. In early ammonia plants, an aqueous solution of 15-20 wt % monoethanolamine (MEA) was always standard for removing CO2. Primary alkanolamine solutions, however, require a relatively high heat of regeneration so that, nowadays, secondary and tertiary ethanol amines are mainly used. 

Iron Oxide. In this process, H2S present in the feed gas is reacted with iron oxide to form iron sulfide. The iron sulfide can be regenerated either in situ or outside in air. After oxidation of the sulfur in situ, the S02 is released to the atmosphere with some diluent gas, such as the primary reformer flue gas. Regeneration in air releases S02 slowly in the atmosphere. Emission of S02 in the atmosphere is very strictly under regulatory control. However, this is not a very efficient method since the sulfur levels acceptable for ammonia plants cannot be achieved and a second-stage desulfurizer must be provided. Disposing of the spent iron oxide also can pose a problem due to the large volumes involved. 

Figure 12.6 A typical simplified ammonia plant CO2 absorber-regenerator system.

If the gas is to be compressed andiransported by sub-sea pipe line, then it may be necessary to remove carbon dioxide. If only a few per cent is present it is cheaper to transport the unwanted gas and remove it on-shore than to remove it off-shore. If, as with one North Sea field, the amount is 20 per cent, then removal off-shore is undertaken. The absorption process uses monoethanolamine and is basically the same as that on an ammonia plant. Absorption can also be used to remove water, and the re-usable absorbent which is cycled between absorption tower and regenerator is triethylene glycol. Water removal limits corrosion damage and prevents the formation of solid hydrocarbon hydrates which can plug gas lines. 

Control of many modern ammonia plants is accomplished via gas analyzers and on-line computers to adjust conditions for optimum performance. The control strategy includes adjustments to the H2/N2 ratio, purge rate, and reformer furnace control. Heat recovery during regeneration of the catalysts also is included. 

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. 

Ammonium Ion Removal. A fixed-bed molecular-sieve ion-exchange process has been commercialized for the removal of ammonium ions from secondary wastewater treatment effluents. This application takes advantage of the superior selectivity of molecular-sieve ion exchangers for ammonium ions. The first plants employed clinoptilolite as a potentially low cost material because of its availability in natural deposits. The bed is regenerated with a lime-salt solution that can be reused after the ammonia is removed by pH adjustment and air stripping. The ammonia is subsequentiy removed from the air stream by acid scmbbing. 

Kranz MWS A flue-gas desulfurization system based on activated carbon. One carbon bed removes most of the sulfur dioxide. Ammonia is then injected for the SCR process to occur in the second bed, which also removes the residual sulfur dioxide. The carbon is regenerated off-site. Developed by Krantz Company, Germany. In 1986, three plants were operating in Germany. 

Manganese dioxide can be used to absorb the initially low concentration SO2 to produce the sulphate in a Mitsubishi Process (23). In this case the absorbing phase is itself the oxidising agent. Regeneration with ammonia and air simultaneously produces ammonium sulphate which can be directly marketed as a fertiliser, thus the calcium ion problem of the dry limestone process is replaced by a plant nutrient ion -... 

Today s coke plant gas purification processes are mostly carried out under atmospheric pressure, employing a circulated ammonia-based absorbent. The consumption of the external solvent is reduced via the use of ammonia available in the coke gas (138). An example of innovative purification processes is the ammonia hydrogen sulfide circulation scrubbing (ASCS) (Figure 17), in which the ammonia contained in the raw gas dissolves in the NH3 absorber and then the absorbent saturated with the ammonia passes through the H2S absorber to selectively absorb the H2S and HCN components from the coke gas. The next step is the thermal regeneration of the absorbent with the steam in a two-step desorption plant, whereas a part of the deaciditied water is fed back into the H2S absorber (25). 

The winning of the alkaloids is generally achieved by the usual procedures modified to suit the plant source and the properties of the bases. Pais el al. 17), when working with Panda oleosa, moistened the dried and ground plant material with 10% aqueous ammonia and 1% aqueous sodium carbonate followed by extraction with ether. The concentrated extract was shaken with aqueous amidosulfonic acid and the bases regenerated from the aqueous phase by ammonia. An ether solution of the liberated bases frequently yields crystals upon standing but these are virtually always a mixture of bases. 

Scutianine-A was obtained from a methanol extract of the ground plant. The somewhat concentrated extract was diluted with water, basified with ammonia to pH 8, and exhausted with benzene. The alkaloids were removed from the benzene extract by means of 5% aqueous citric acid. The bases, regenerated from the aqueous phase, were recovered with chloroform or methylene chloride (18). This procedure is recommended when large amounts of plant material are available. 

Relatively small amounts of ammonia are used in resin beds for regeneration of ion exchange resins at plants manufacturing com sweeteners. Minor amounts of ammonia are also consumed ... 

Adsorption Abatement uses acid-resistant molecular sieves to absorb the NOx from the tail gas. The adsorbent is periodically regenerated and the NOx recovered, converted to NOj and recycled to the plant for recovery as nitric acid. The tail gas may also be mixed with a reducing agent (usually natural gas or ammonia) and passed through a catalyst to reduce the NOx to elemental nitrogen. When natural gas is used, conditions must be carefully controlled and facilities installed to control the heat released by natural gas that reacts with both the NOx and the oxygen91. 

The (NH SC rich wash liquor passed to a regeneration plant constitutes the exit stream in Figure 1. Regenerated ammonia is the inlet stream. Regeneration is not shown in Figure 1. Neither are other sections such as pregnant solution filtration, tailings flotation and recycle treatment. 

EC02 A process for absorbing carbon dioxide from power plants using an aqueous ammonia solution. The resulting ammonium bicarbonate solution is regenerated by heating, releasing both carbon dioxide and ammonia, which are separated. The ammonia is recycled, and the carbon dioxide is stored. Developed by Powerspan, Portsmouth, NH, and planned to be piloted in Shadyside, OH, in 2007. 

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