Removal of carbon dioxide

In the majority of cases thermal cleavage of carbon-carbon bonds consists of decarboxylation of a carboxylic acid RCOOH the tenacity with which the carboxyl group is retained varies within wide limits. Aliphatic acids can normally be decarboxylated only under rather extreme conditions, and the same applies to simple aromatic carboxylic acids unless the attachment of the carboxyl group is weakened by, e.g., ortho- or /rara-hydroxyl groups or by a hetero-ring atom (at a suitable distance from the carboxyl group). On the other hand, many carboxylic acids are known that lose carbon dioxide at or relatively little above room temperature either spontaneously or under the influence of acidic or basic catalysts. In most cases, the decarboxylation occurs by a polar mechanism, in an SE reaction  

In such heterolytic decarboxylations the electron pair binding the carboxyl-carbon atom to the organic group R thus remains attached to the group R, with the consequence that the electronic nature of R and, in particular, of its a- or -carbon atom may contribute appreciably to the conditions required for the reaction. Three possibilities can be distinguished in respect of the contribution of R to cleavage of a carboxyl group  

The group R has almost no influence on the ease of fission of the acid, which is then difficult to decarboxylate. 

Decarboxylation is relatively easy and smooth this occurs when the electron density at the carbon atom in the -position to the carboxyl group is relatively low so that the electron affinity of the group R is high (type 1). 

Heterolysis is also made easier when a cationoid centre can be formed at the carbon atom in the / -position to the carboxyl group, as such a centre can take up the electron pair binding the carboxyl group, thus forming a double bond (type 2). 

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Benfield process Removal of carbon dioxide from fuel gases, such as those obtained by gasifying coal in the Lurgi process, by countercurrent scrubbing of the gases by hot potassium carbonate solution. 

Breweries. Air conditioning and the extensive use of refrigeration are necessary to provide controEed temperature in wort cooling, fermentation, storage, and final packaging of the finished beer. Sanitation and removal of carbon dioxide are important aspects of this appHcation (see... 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Anhydrous lithium hydroxide [1310-65-2], LiOH, is obtained by heating the monohydrate above 100°C. The salt melts at 462°C. Anhydrous lithium hydroxide is an extremely efficient absorbent for carbon dioxide (qv). The porous stmcture of the salt allows complete conversion to the carbonate with no efficiency loss in the absorption process. Thus LiOH has an important role in the removal of carbon dioxide from enclosed breathing areas such as on submarines or space vehicles. About 750 g of lithium hydroxide is required to absorb the carbon dioxide produced by an individual in a day. 

Hot potassium carbonate processes are intended for the removal of carbon dioxide, or the co-removal of hydrogen sulfide and carbon dioxide. As a result of the regeneration chemistry, these hot-pot processes are not suitable for the removal of hydrogen sulfide without significant carbon dioxide also in the untreated gas stream. 

Except as an index of respiration, carbon dioxide is seldom considered in fermentations but plays important roles. Its participation in carbonate equilibria affects pH removal of carbon dioxide by photosynthesis can force the pH above 10 in dense, well-illuminated algal cultures. Several biochemical reactions involve carbon dioxide, so their kinetics and equilibrium concentrations are dependent on gas concentrations, and metabolic rates of associated reactions may also change. Attempts to increase oxygen transfer rates by elevating pressure to get more driving force sometimes encounter poor process performance that might oe attributed to excessive dissolved carbon dioxide. 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

Ammonia production from natural gas includes the following processes desulfurization of the feedstock primary and secondary reforming carbon monoxide shift conversion and removal of carbon dioxide, which can be used for urea manufacture methanation and ammonia synthesis. Catalysts used in the process may include cobalt, molybdenum, nickel, iron oxide/chromium oxide, copper oxide/zinc oxide, and iron. 

Spector, N. A., and B. F Dodge, Removal of Carbon Dioxide from Atmospheric Air, Trans. Amer. Inst. Chem. Engrs. 42, 827 (1946). 

Because of the role these algae play in the oceans biological productivity and their impacts on climate due to the removal of carbon dioxide, satellite sensors have been employed to measure the chlorophyll a contents in oceans, lakes, and seas to indicate the distribution and abundance of biomass production in these water bodies. Detection is set at the specific reflectance and absorption wavelengths of the light from the upper layer of the ocean where photosynthesis occurs. 

Cathodic reduction is the most promising approach to the removal of carbon dioxide from a closed atmosphere. Methods developed so far provide for electrode materials, electrolytes, and electrolysis conditions where CO2 can be reduced to hquid organic products of low molecular weight such as formic acid. More complex systems are required to regenerate foodstuffs from the rejects of human vital activities during... 

Blood flow to most tissues in the body is determined by the metabolic needs of those tissues. Metabolically active tissues require enhanced delivery of oxygen and nutrients as well as enhanced removal of carbon dioxide and waste products. In general, as the metabolic activity of a tissue increases, its blood flow increases. An important feature of the circulatory system is that each tissue has the intrinsic ability to control its own local blood flow in proportion to its metabolic needs. 

CATACARB [Catalyzed removal of carbon dioxide] A process for removing carbon dioxide and hydrogen sulfide from gas streams by absorption in hot potassium carbonate solution containing a proprietary catalyst. Developed and licensed by Eickmeyer and Associates, KS, based on work at the U.S. Bureau of Mines in the 1950s. More than a hundred plants were operating in 1997. See also Benfield, Carsol, Hi-pure, Giammarco-Vetrocoke. 

At a pressure of 30 bar and with excess steam the fractional conversion of methane in the reformer is reasonably satisfactory. The high pressure of 30 bar will favour the removal of carbon dioxide, following the shift reaction CO + H2O CO2 + H2, and reduce the cost of compressing the purified hydrogen to a value, typically in the range 50-200 bar, required for ammonia synthesis. 

In the determination of constitution it is often necessary to remove carboxyl groups (such as are formed by oxidation, for example) and so to break down the molecule. The simplest method of doing this, namely, removal of carbon dioxide by distillation of a salt with soda-lime ... 

This compound on bromination, followed by hydrolysis of the ester group with hydrobromic acid and removal of carbon dioxide, gave a-brom-S-chlor-y-valerolactone, from which by treatment with ammonia 7-oxyproline was obtained —... 

The excessive amount of bicarbonate in the blood means that blood has a much greater capacity to neutralize acids. Many acids accumulate in the blood during strenuous activity, for example lactic acid. Excretion of bicarbonate through the kidneys and the removal of carbon dioxide through respiration also regulate the carbonic acid/ bicarbonate blood buffer. 

The hemicelluloses also include the polyuronides, or polyuronic acids, for instance a polymer of a hexuronic acid such as galacturonic acid. There is a possible generic link between the polyuronides and the pentosans since the latter might be produced as the result of the decarboxylation of hexuronic acids. The possibility of transforming hexuronic acids into pentosans by the removal of carbon dioxide... 

It should be appreciated that this example is one of the few very simple practical cases of a packed column reactor. The removal of carbon dioxide by reaction is a step in many important industrial processes, e.g. the nitrification of natural gas and of hydrogen in the manufacture of ammonia. Most of these processes0 " use as the liquid absorbent a solution which can be regenerated, for example solutions of amines or potassium carbonate0", and the design of these columns is distinctly more complicated. 

Carbon Dioxide Condensation, Sulfurous Compound Absorption. Carbon dioxide is condensed by cooling the gas from its dew point to about -55°C the condensate is contaminated with sulfurous compounds. Substantial amounts of carbon dioxide can be condensed if the dew point is relatively warm. A synthesis gas at 1000 psia with 30 mol % carbon dioxide has a dew point of about -30°C, and approximately 65% of the carbon dioxide condenses to a liquid in cooling to -55°C. Removal of carbon dioxide by condensation reduces the amount which must be removed subsequently by absorption. Condensation is preferred over absorption because it is more reversible and hence is more energy efficient and less capital intensive. 

Carbon dioxide removal by slurry absorption is attractive down to about -75°C, a temperature easily achieved by slurry regeneration to slightly above one atmosphere carbon dioxide pressure. For example, with a -75°C exit gas temperature, slurry absorption reduces the carbon dioxide content of a 1000 psia synthesis gas from about 13 to about 4 mole percent, a 70% reduction in carbon dioxide content. The exact level to which carbon dioxide can be removed from a treated gas by slurry absorption also depends on the solubility of solid carbon dioxide in the treated gas the solubility of solid carbon dioxide in synthesis gas (3H2 CO) is illustrated in Figure 10 for several synthesis gas pressures. Fine removal of carbon dioxide to lower levels is accomplished by conventional absorption into a slip stream of the slurry solvent which is regenerated to meet particular product gas carbon dioxide specifications. 

Removal of carbon dioxide is the only membrane-based natural gas separation process currently practiced on a large scale—more than 200 plants have been installed, some very large. Most were installed by Grace (now Kvaerner-GMS), Separex (UOP) and Cynara and all use cellulose acetate membranes in hollow fiber or spiral-wound module form. More recently, hollow fiber polyaramide (Medal) membranes have been introduced because of their higher selectivity. 

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