Absorption of carbon dioxide

Carbon dioxide is absorbed in most anaesthetic breathing systems by means of a canister that contains a specific absorbing medium. This is often soda lime but may also be baralime in some hospitals. 

4% sodium hydroxide 15% bound water 81% calcium hydroxide 

20% barium hydroxide octahydrate Ba(0H)2.8H20 80% calcium hydroxide Ca(OH)2 

You may be asked to describe the chemical reaction that occurs when C02 is absorbed within the canister. The most commonly cited reaction is that between soda lime and C02  

Heat is produced at two stages and water at one. This can be seen and felt in clinical practice. Note that NaOH is reformed in the final stage and so acts only as a catalyst for the reaction. The compound that is actually consumed in both baralime and soda lime is Ca(OH)2. 

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In air conditioning (qv) of closed spaces, a wider latitude in design features can be exercised (23,24). Blowers are used to pass room or cabin air through arrays of granules or plates. Efficiencies usuaHy are 95% or better. The primary limiting factor is the decreased rate of absorption of carbon dioxide. However, an auxHiary smaH CO2 sorption canister can be used. Control of moisture entering the KO2 canister extends the life of the chemical and helps maintain the RQ at 0.82. 

Early Synthesis. Reported by Kolbe in 1859, the synthetic route for preparing the acid was by treating phenol with carbon dioxide in the presence of metallic sodium (6). During this early period, the only practical route for large quantities of sahcyhc acid was the saponification of methyl sahcylate obtained from the leaves of wintergreen or the bark of sweet bitch. The first suitable commercial synthetic process was introduced by Kolbe 15 years later in 1874 and is the route most commonly used in the 1990s. In this process, dry sodium phenate reacts with carbon dioxide under pressure at elevated (180—200°C) temperature (7). There were limitations, however not only was the reaction reversible, but the best possible yield of sahcyhc acid was 50%. An improvement by Schmitt was the control of temperature, and the separation of the reaction into two parts. At lower (120—140°C) temperatures and under pressures of 500—700 kPa (5—7 atm), the absorption of carbon dioxide forms the intermediate phenyl carbonate almost quantitatively (8,9). The sodium phenyl carbonate rearranges predominately to the ortho-isomer. sodium sahcylate (eq. 8). 

Absorption of carbon dioxide in weak alkali 3. Absorption of chlorine in water... 

Tepe, J. B. and B. F. Dodge, Absorption of Carbon Dioxide Sodium Hydroxide Solutions in a Packed Column, Trans. Amer. Inst. Chem. Engrs. 39, 255 (1943). 

Coppock and Meiklejohn (C9) determined liquid mass-transfer coefficients for the absorption of oxygen in water. The value of k, was observed to vary markedly with variations of bubble velocity, from 0.028 to 0.055 cm/sec for a velocity range from 22 to 28 cm/sec. These results appear to be in general agreement with the results obtained by Datta et al. (D2) and by Guyer and Pfister (G9) for the absorption of carbon dioxide by water. 

Grassman (G7) has proposed a simplified theoretical treatment of heat and mass transfer between two fluid phases, as, for example between a dispersed gas phase and a continuous liquid phase von Bogdandy et al. (V8) measured the rate of absorption of carbon dioxide by water and by decalin, and found that the absorption rate approximated that predicted by Grass-mann in the laminar region but was above the theoretical values in the... 

Massimilla et al. (M5) measured the rate of absorption of carbon dioxide in water from a mixture of carbon dioxide and nitrogen. Used as solid phase were silica sand particles of average equivalent diameter 0.22 mm, or glass ballotini of average equivalent diameter 0.50 and 0.80 mm. Columns of 30-and 90-mm i.d. were used, and the column height was varied from 100 to 1200 mm. 

In many applications of mass transfer the solute reacts with the medium as in the case, for example, of the absorption of carbon dioxide in an alkaline solution. The mass transfer rate then decreases in the direction of diffusion as a result of the reaction. Considering the unidirectional molecular diffusion of a component A through a distance Sy over area A. then, neglecting the effects of bulk flow, a material balance for an irreversible reaction of order n gives ... 

Anon., Thermoplastic sheets for absorption of carbon dioxide, Jpn. Kokai Tokkyo Koho 80,158,933, 1980. (CA 157990m)... 

The partial pressure ratio, Pcq2PH2 /PH2cPco> is decreased by the absorption of carbon dioxide. Thus, the equilibrium of the reaction 3.12 may be kept at a higher temperature simultaneously with the C02 absorption. The expression of the overall reaction is written as follows ... 

Tests are made on the absorption of carbon dioxide from a carbon dioxide-air mixture in a solution containing 100 kg/m3 of caustic soda, using a 250 mm diameter tower packed to a height of 3 m with 19 mm Raschig rings. 

SACHEM Inc., located in Cleburne, Texas, is a producer of high-purity bulk chemicals for companies that have high-purity requirements in their chemical processing. As stated in Workplace Scene 1.2, one of their products is tetramethylammonium hydroxide (TMAH), which is sold to semiconductor industries. The analysis of TMAH for trace anions such as chloride, nitrate, nitrite, and carbonate is critical for SACHEM s quality control laboratory. If these ions are present on the integrated circuit boards manufactured by one of their semiconductor customers, they may cause corrosion severe enough to affect the functionality and performance of the electronic devices in which the circuit boards are used. In SACHEM s quality control laboratory, ion chromatography procedures have been developed to measure the anion concentrations in TMAH. Because the concentration levels are trace levels, a clean room environment, like that described in Workplace Scene 1.2, is used. A special procedure for carbonate analysis is required so that the absorption of carbon dioxide from the atmosphere can be minimized. 

In an attempt to test the surface renewal theory of gas absorption, Danckwerts and Kennedy measured the transient rate of absorption of carbon dioxide into various solutions by means of a rotating drum which carried a film of liquid through the gas. Results so obtained were compared with those for absorption in a packed column and it was shown that exposure times of at least one second were required to give a strict comparison this was longer than could be obtained with the rotating drum. Roberts and Danckwerts therefore used a wetted-wall column to extend the times of contact up to 1.3 s. The column was carefully designed to eliminate entry and exit effects and the formation of ripples. The experimental results and conclusions are reported by Danckwerts, Kennedy, and Roberts110 who showed that they could be used, on the basis of the penetration theory model, to predict the performance of a packed column to within about 10 per cent. 

Cullen and Davidson ) studied the absorption of carbon dioxide into a laminar jet of water. When the water issued with a uniform velocity over the cross-section, the measured rate of absorption corresponded closely with the theoretical value. When the velocity profile in the water was parabolic, the measured rate was lower than the calculated value this was attributed to a hydrodynamic entry effect. 

Goodridge and Robb(14) used a laminar jet to study the rate of absorption of carbon dioxide into sodium carbonate solutions containing a number of additives including glycerol, sucrose, glucose, and arsenites. For the short times of exposure used, absorption rates into sodium carbonate solution or aqueous glycerol corresponded to those predicted on the basis of pure physical absorption. In the presence of the additives, however, the process was accelerated as the result of chemical reaction. 

The absorption of carbon dioxide, oxygen, and hydrogen in water are three examples in which most, if not all, of the resistance to transfer lies in the liquid phase. Sherwood and Hoij.oway 30-1 measured values of kLa for these systems using a tower of 500 mm diameter packed with 37 mm rings. The results were expressed in the form ... 

It is difficult to compare the performance of various spray towers since the type of spray distributor used influences the results. Data from Hixson and Scott 33 and others show that KGa varies as G70-8, and is also affected by the liquid rate. More reliable data with spray columns might be expected if the liquid were introduced in the form of individual drops through a single jet into a tube full of gas. Unfortunately the drops tend to alter in size and shape and it is not possible to get the true interfacial area very accurately. This has been investigated by Whitman et a/. 34 , who found that kG for the absorption of ammonia in water was about 0.035 kmol/s m2 (N/m2), compared with 0.00025 for the absorption of carbon dioxide in water. 

Cryder and Maloney(40) presented data on the absorption of carbon dioxide in diethanolamine solution, using a 200 mm tower filled with 20 mm rings, and some of their data are shown in Figure 12.14. The coefficient KGa is found to be independent of... 

Figure 12.13. Absorption of carbon dioxide in sodium hydroxide solution G = 0.24-0.25 kg/m2s,...

The absorption of carbon dioxide into aqueous amine solutions has been investigated by Danckwerts and McNeil(42) using a stirred cell. It was found that the reaction proceeded... 

A comprehensive review of work on the absorption of carbon dioxide by alkaline solutions has been carried out by Danckwerts and Siiarma(43) who applied results of research to the design of industrial scale equipment. Subsequently, Sahay and Sharma(44) showed that the mass transfer coefficient may be correlated with the gas and liquid rates and the gas and liquid compositions by ... 

Tepe, J. B. and Dodge, B. F. Trans. Am. Inst. Chem. Eng. 39 (1943) 255. Absorption of carbon dioxide by sodium hydroxide solutions in a packed column. 

Danckwerts, P. V. and McNeil, K. M. Trans. Inst. Chem. Eng. 45 (1967) 32. The absorption of carbon dioxide into aqueous amine solutions and the effects of catalysis. 

DANCKWERTS, P. V. and SHARMA, M. M. Chem. Engr. London No. 202 (Oct. 1966) CE244. The absorption of carbon dioxide into solutions of alkalis and amines (with some notes on hydrogen sulphide and carbonyl sulphide). 

Ayerst, R. R. and Herbert, L. S. Trans. Inst. Chem. Eng. 32 (1954) S68. A study of the absorption of carbon dioxide in ammonia solutions in agitated vessels. 

Table A2 (see the appendices) lists briefly the purification methods applicable to many of the solvents listed. It is, however, often advisable to guard solvents, once their bottles have been opened, from the absorption of moisture from the atmosphere, and in the case of basic solvents, also from the absorption of carbon dioxide. If purification is deemed to be necessary and no method is specified in table A2, then usually a method noted for a chemically similar solvent can be employed.

In many of these experiments, interfacial turbulence was the obvious visible cause of the unusual features of the rate of mass transfer. There are, however, experimental results in which no interfacial activity was observed. Brian et al. [108] have drawn attention to the severe disagreement existing between the penetration theory and data for the absorption of carbon dioxide in monoethanolamine. They have performed experiments on the absorption of C02 with simultaneous desorption of propylene in a short, wetted wall column. The desorption of propylene without absorption of C02 agrees closely with the predictions of the penetration theory. If, however, both processes take place simultaneously, the rate of desorption is greatly increased. This enhancement must be linked to a hydrodynamic effect induced by the absorption of C02 and the only one which can occur appears to be the interfacial turbulence caused by the Marangoni effect. No interfacial activity was observed because of the small scale and small intensity of the induced turbulence. 

Properties oi the alkali hydroxides.—The alkali hydroxides are brittle, white, translucent solids with a more or less crystalline fracture, and fibrous texture. Sodium hydroxide deliquesces on exposure to the air, but it goes solid again owing to the formation of the carbonate by the absorption of carbon dioxide from the air. Lithium hydroxide is a little hygroscopic. Potassium hydroxide is even more deliquescent than the sodium compound but its carbonate is also deliquescent. The hydroxides are very solnble in water, and they also dissolve in alcohol. The reported numbers for the specific gravities22 of sodium hydroxide range from l-723 to 2T30 and for potassium hydroxide, from l-958 to 2 6. The best representative sp. gr. are 2"54 for lithium hydroxide 2130 for sodium hydroxide 2 044 for potassium hydroxide 3"203 (11°) for rubidium hydroxide and 3-675 (11°) for csesium hydroxide. 

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