Reactive carbon dioxide absorption

This reaction serves as literature-known model reaction to characterize mass transfer efficiency in microreactors [318]. As it is a very fast reaction, solely mass transfer can be analyzed. The analysis can be done simply by titration the reactants are inexpensive and not toxic. 

All other devices showed only the increasing part of such dependency that is, the highest performance was obtained at the longest residence time [318]. The best conversions of interdigital micromixers and caterpillar minimixers of 78 and 70%, respectively, still exceed notably the performance of a conventional mixing tee (1 mm inner diameter). 

Reactor type NaOH (mol/l) co2 (vol%) Molar ratio COj/NaOH Conversion co2 (%) Space-time yield (mol/m3 s) 

The mass transfer efficiency of the falling-film microreactor was determined at various carbon dioxide volume contents (0.1,1.0 and 2.0 M) [318]. The molar ratio of carbon dioxide to sodium hydroxide was constant at 0.4 for all experiments, that is, the liquid reactant was in light excess. The higher the base concentration, the higher was the conversion of carbon dioxide. For all concentrations, complete absorption was, however, achieved at different carbon dioxide contents in the gas mixture. The results show the interdependency of the carbon dioxide content, the gas flow velocity and the sodium hydroxide concentration. 

The mass transfer efficiency of the falling-film microreactor and the microbubble column was compared quantitatively according to the literature reports on conventional packed columns (see Table 4.3) [318]. The process conditions were chosen as similar as possible for the different devices. The conversion of the packed columns was 87-93% the microdevices had conversions of 45-100%. Furthermore, the space-time yield was compared. Flere, the microdevices resulted in larger values by orders of magnitude. The best results for falling-film microreactors and the microbubble columns were 84 and 816 mol/(m3 s), respectively, and are higher than conventional packed-bed reactors by about 0.8 mol/(m3 s). 

A more detailed mass-transfer study on the carbon dioxide absorption in sodium hydroxide solution was performed using a falling film microreactor, both 

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Example 11.7 Carbon dioxide is sometimes removed from natural gas by reactive absorption in a tray column. The absorbent, typically an amine, is fed to the top of the column and gas is fed at the bottom. Liquid and gas flow patterns are similar to those in a distillation column with gas rising, liquid falling, and gas-liquid contacting occurring on the trays. Develop a model for a multitray CO2 scrubber assuming that individual trays behave as two-phase, stirred tank reactors. 

Reactive absorption is probably the most widely applied type of a reactive separation process. It is used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. It is also often employed in gas purification processes, e.g., to remove carbon dioxide or hydrogen sulfide. Other interesting areas of application include olefin/paraffin separations, where reactive absorption with reversible chemical complexation appears to be a promising alternative to the cryogenic distillation (62). 

Reactive absorption is very old as a processing technique and has been used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. The Raschig process for the production of hydroxylamine, an important intermediate in classical caprolactam technologies (Stamicarbon, Inventa), is also an example of a multistep reactive absorption process. Here, water, ammonia, and carbon dioxide react together in an absorption column to give a solution of ammonium carbonate, which subsequently forms an alkaline... 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

The C02 absorption is hindered by a slow chemical reaction by which the dissolved carbon dioxide molecules are converted into the more reactive ionic species. Therefore, when gases containing H2S, NH3, and C02 contact water, the H2S and ammonia are absorbed much more rapidly than C02, and this selectivity can be accentuated by optimizing the operating conditions (23). Nevertheless, all chemical reactions are coupled by hydronium ions, and additional C02 absorption leads to the desorption of hydrogen sulfide and decreases the scrubber efficiency. 

The potential substrates should be cheap and freely available and products should not pollute the environment. Taking these prerequisites into account, water and carbon dioxide should be regarded as the best candidates for the substrates. In both cases, however, the reactive excited states cannot be reached simply by absorption of solar light. Fortunately, solar energy can be transferred by photosensitizers or photocatalysts. 

Other experiments were carried out also on the decomposition of carbon dioxide, the decomposition of hydrochloric acid gas and the oxidation of alcohol vapor. In no case was chemical action brought about by the absorption of an intense beam of infrared radiation. Furthermore, no case of chemical reactivity produced by longer infrared radiation has been established in the literature. Increases in reaction rates in early experiments can be traced to an increase in the temperature of the solution produced by the absorption of the heat rays. A few cases of activity of short infrared rays of 1 jx and less are on record in special reactions. 

In this paper first results of the solubility and extraction of low molecular weight polymers and plasticizers with supercritical carbon dioxide are presented and compared to the results of phase equilibria measurements. Additionally the formation of sub-micrometer particles during polymerization of acrylic acid and derivatives thereof in scCC are examined and compared with recent works [2], Finally there are some aspects to modification of polymers by absorption of scC02 and reactive components solved therein. 

Spray-dried potassium fluoride is known to aggregate rather rapidly (ref. 71) and absorption of carbon dioxide (0.4 to 2 %) have been claimed to prevent this phenomenon (ref. 71). However, in our hands, commercial spray-dried KF did not show any difference in structure and reactivity with calcined KF. 

It should be noted that the result of a reactivity test can be markedly affected by impurities in the water. For this reason, distilled water is used as the reference standard. The reactivity of a quicklime can also be depressed by absorption of water and carbon dioxide from the atmosphere during storage or sample preparation (see affinity for water and section 24.2). 

Some of the properties of quicklime, particularly its reactivity, can be significantly altered by the absorption of atmosphere moisture and carbon dioxide (see section 18.8). As it is impractical to handle the required quantities of material in an inert atmosphere, exposure times should be kept to a minimum and, where possible, mechanical sample preparation equipment should be used (e.g. crushers, sample dividers and pulverisers), taking care to avoid cross-contamination between samples. 

In the context of meeting customer specifications, the most pronounced effect of absorption of water and carbon dioxide is on the reactivity of the lime. Combined water contents greater than 1.0 %, coupled with the associated, but smaller, increases in combined CO2, can have a significant effect on reactivity. Such levels... 

Anderson and Vernon [27.1] reported that the rate of solution of lime increased as the mean apparent density decreased and as the particle size was reduced. For a given quicklime, reactivity correlated with mean apparent density (see Fig. 13.2), but reactivity was not the fundamental parameter. Moreover, reactivity could be affected by absorption of atmospheric water and carbon dioxide (see section 13.2). 

In the previous section, we studied the absorption of carbon dioxide in a low-pressure system in which a reactive amine was used. Now we want to consider a significantly different situation in which the absorber pressure is quite high, in fact high enough so that physical absorption in a suitable solvent is effective. 

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