Absorption rates partial pressure

Reaction between an absorbed solute and a reagent reduces the equilibrium partial pressure of the solute, thus increasing the rate of mass transfer. The mass-transfer coefficient hkewise is enhanced, which contributes further to increased absorption rates. Extensive theoretical analyses of these effects have been made, but rather less experimental work and design guidehnes. 

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10% ammonia-air mixture is scrubbed with water and the concentration of ammonia is reduced to 0.1%. If the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interface, derive from first principles an expression for the rate of absorption at any position in the column. At some intermediate point where the ammonia concentration in the gas phase has been reduced to 5%. the partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/nr and the transfer rate is ]0 3 kmol/m2s. What is the thickness of the hypothetical gas film if the diffusivity of ammonia in air is 0.24 cm2/s ... 

Carbon dioxide is absorbed in water from a 25 per cent mixture in nitrogen. How will its absorption rate compare with that from a mixture containing 35 per cent carbon dioxide, 40 per cent hydrogen and 25 per cent nitrogen It may be assumed that the gas-film resistance is controlling, that the partial pressure of carbon dioxide, at the gas-liquid interface is negligible and that the two-lilm theory is applicable, with the gan film thickness the same in the two cases. 

In turn, the concentration of C02 in the atmosphere depends on the mass of the biosphere and its rate of decay after death, and on the carbonic-anhydrase concentrations in the sea surface. In future predictions of the rate of increase of C02 partial pressure in the atmosphere due to burning fossil fuels, it will be important to include the interaction of the atmospheric C02 with the bio-organic reservoir and the catalyzation of its absorption into the sea by means of the action of carbonic-anhydrase dissolved in sea water, considerations which have not been taken into account in past computations. 

Sulphur dioxide is recovered from a smelter gas containing 3.5 per cent by volume of SO2, by scrubbing it with water in a countercurrent absorption tower. The gas is fed into the bottom of the tower, and in the exit gas from the top the SO2 exerts a partial pressure of 1.14 kN/m2. The water fed to the top of the tower is free from SO2, and the exit liquor from the base contains 0.001145 kmol SC /kmol water. The process takes place at 293 K, at which the vapour pressure of water is 2.3 kN/m2. The water flow rate is 0.43 kmol/s. 

Data were obtained on the rate of absorption of oxygen by yeast, rj without sulfanilamide and r2 with 20 mg/ml of added sulfanilamide. Oxygen partial pressure is in torr, the rates are microliters of oxygen per hour per mg of cells. Find the constants of the rate equations of the two cases. 

In the great majority of industrial processes the film thickness is not known, so that the rate equation of immediate use is equation 12.8 using kG. kG is known as the gas-film transfer coefficient for absorption and is a direct measure of the rate of absorption per unit area of interface with a driving force of unit partial pressure difference. 

If the gas rate is 0.95 kg/m2s of tower cross-section and the liquid rate is 0.65 kg/m2s, find the necessary height of the tower if the absorption coefficient KGa = 0.001 kmol/m3s(kN/m2) partial pressure difference. The equilibrium data are ... 

Gas-absorption rates furnish another test of the magnitude of effect (i). If the gas is pure (e.g., CO2 at a partial pressure of 1 atm.), and is absorbing into stirred liquid, then the momentum-transfer term ijiRci is always negligible thus an interfacial film can reduce 2 only through effect (i) above. The results of Davies and Kilner (1, M) show that for the absorption of CO2 into water, the limiting value of factor (i), by which the film reduces /c2, is 1.9 (i.e., 1.96/1.04). This may be compared with the figure of 2.2 quoted above for the ethylacetate-water system. 

Fig. 24. Relationship between partial pressure of oxygen and oxygen absorption rate by poly(L-lysine)-heme complex (a) and equilibrium curve for CO binding by poly(L-lysine)-heme complex (b)107,108)...

FIGURE 6 Demonstration of the relation between the function and the shape of the phase plane, (a) Phase plane contours of the function d> = (I - 0 - ft) = A. (b) Phase portrait for the special case of negligible desorption and identical absorption rates (< i = a2 = A -y, = y2 = 0). (c) Shape of the non-unique steady-state reaction rates for variations in reactant partial pressures for the case of negligible desorption. 

The reaction of tert-butyl alcohol and methanol to form MTBE is also catalyzed by heteropoly compounds (221-223). A relationship was found between the amount of pyridine sorbed in or on heteropoly compounds and tert-butyl alcohol conversion (221). The dependence of the rate on methanol partial pressure resembles that for the absorption of methanol in the bulk, suggesting pseudoliquid phase behavior (223). 

Reaction 3 is relatively slow and is regarded as the rate-determining step in the entire process. Other key reactions proceed virtually instantaneously. Both Reactions 2 and 3 are exothermic, and are dependent on the various partial pressures (operating pressure) and temperature. Lower temperatures favour the overall absorption chemistry. This factor, together with the exothermic nature of the reactions, determines the need for a cooling circuit within the column. 

Capital costs for a Sulfinol unit are lower than alkanolamine units that have the same capacity. This is because the equipment is smaller as a result of less foaming and lower circulation rates. However, certain treating applications may not be ideal for the Sulfinol process. A reclaimer may be needed to remove DIPA degradation products when C02 partial pressure is high. Also, aromatic and hydrocarbon co-absorption occurs when solvent -to-gas ratios are high260-... 

A given amount of liquid can absorb gas only up to the concentration at which the partial pressure of the gas from the solution formed by absorption is equal to its partial pressure in the gas phase. If the partial pressure of the gas from the solution is lower than in the vapour phase, absorption sets in and the rate of absorption, at a given total absorption coefficient, depends on the difference in the partial pressures in the two phases and on the size of the contact surface. From the dependence of the partial pressures of hydrogen chloride on the composition of the hydrochloric acid it follows that the vapour pressure of HC1 from diluted solutions is relatively very low. Therefore, praotically all the hydrogen chloride from the gas phase can be absorbed, as... 

As the equilibrium partial pressure of hydrogen chloride over a 35.2 per cent hydrochloric acid is only pxHci = 135 mm Hg the gas under partial pressure Phci = 228 mm Hg will be readily absorbed in the acid (the difference in pressures pHci — Pan — AB determines the absorption rate). 

While very little information is available on gas-phase backmixing in a mechanically agitated CSTR, the extent of mixing increases with the impeller speed. If the reaction depends on the partial pressure of gas, an increased backmixing decreases the driving force for the absorption rate. An increase in impeller speed also increases fcLaL, kgaL, etc. As a consequence, it is likely that an optimum impeller speed exists that gives maximum space-time yield (kmol/m3 s) and this optimum speed probably lies in the vicinity of critical impeller speed. 

Much of the modern research on the subject is due to Adeney and Becker,1 whose initial researches were concerned with the rate of absorption of air by water under gentle agitation. They begin with the assumption that,2 during the process of solution, the rate of passage, R, of gas into the liquid is proportional to the partial pressure of the gas, p, and the area, A, of the liquid exposed. Hence... 

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