Cocurrent absorption

F. Absorption, cocurrent downward flow, random packings, Reiss correlation Air-oxygen-water results correlated by k i/i = 0.lZEi5. Extended to other systems. = pressure loss in two-phase flow = lbf/ft2 ft AL [E] Based on oxygen transfer from water to air 77°F. Liquid film resistance controls. (Dwater 77°F = 2.4 x 10 5). Equation is dimensional. Data was for thin-walled polyethylene Raschig rings. Correlation also fit data for spheres. Fit 25%. See [122] for graph. k La = s"1 Dl = cm/s El = ft, lb Us ft3 Vi = superficial liquid velocity, ft/s [122] [130] p. 217... 

F. Absorption, cocurrent downward flow, random packings, Reiss correlation... 

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

Packed column Countercurrent, cocurrent Differential Liquid and/or gas Absorption, rectification, stripping, humidification, dehi i midi ficati o n... 

Line mixer Cocurrent Differential Liquid or gas Absorption, stripping... 

Gas/Liquid Mass Transfer This topic has been widely investigated for gas absorption in packed beds, usually countercurrent. One correlation for cocurrent flow in catalyst beds is by Sato et al. (First Pacific Chemical Engineering Congre.s.s, Pergamon, 1972, p. 187) ... 

By far the major portion of the available gas-absorption data have been obtained for countercurrent flow, which is the normal mode of operation for packed-bed absorbers. Special mention may be made of the results of Dodds et al. (D6), who examined mass transfer by the absorption of gas in liquid under cocurrent downward flow at flow rates higher than those corresponding to the flooding point for countercurrent operation. 

Mass transfer controlled by diffusion in the gas phase (ammonia in water) has been studied by Anderson et al. (A5) for horizontal annular flow. In spite of the obvious analogy of this case with countercurrent wetted-wall towers, gas velocities in the cocurrent case exceed these used in any reported wetted-wall-tower investigations. In cocurrent annular flow, smooth liquid films free of ripples are not attainable, and entrainment and deposition of liquid droplets presents an additional transfer mechanism. By measuring solute concentrations of liquid in the film and in entrained drops, as well as flow rates, and by assuming absorption equilibrium between droplets and gas, Anderson et al. were able to separate the two contributing mechanisms of transfer. The agreement of their entrainment values (based on the assumption of transfer equilibrium in the droplets) with those of Wicks and Dukler (W2) was taken as supporting evidence for this supposition. 

Fig. 13. Typical result for absorption of pure carbon dioxide in water in horizontal cocurrent flow for two liquid rates. Tubing 0.0575 ft. inside diameter 7.7 ft. long test section 15°C. outlet pressure 1 atm. abs.

Brauer (B15, B16, B17) has pointed out that information on the frequency of the waves, regarded as surface disturbances, may be of considerable importance in calculating the rates of heat and mass transfer through the wavy film interface, and, in fact, Konobeev et al. (K20, K21) have shown that the rate of absorption of C02 by a water film in wavy cocurrent flow can be correlated in terms of the length and amplitude of the surface waves over the range of small liquid flow rates investigated. 

Konobeev ei al. (K20), 1957 Study of C02 absorption in water films in upward and downward cocurrent flow VKe = 5-200, gas velocities 11.6-39 m./sec. It is suggested that only wavelength and amplitude of interfacial waves affect the rate of mass transfer. Values of wavelength and amplitude measured and compared with previous theories. 

Collins (C14), 1958 Film thicknesses and COi absorption by water film in cocurrent flow with gas stream in vertical tube, 2.05 X 36 in. Thicknesses by light-absorption technique. 

Konobeev et al. (K21), 1961 Experimental study of C02 absorption by water film, with upward and downward cocurrent gas/film flow, inside tubes 1.05-1.66 cm. i.d., 20-87 cm. long. Gas velocities 6-86 m./sec. IVko = 5-105. Length and amplitude of surface ripples and local film thicknesses measured. Rate of mass transfer stated to be function of wave characteristics only. 

Many chemical and biological systems include multistage processes rather than only continuous contact ones. The most common multistage systems are absorption and distillation columns. Most of these systems involve more than one phase and they therefore fall under the category of heterogeneous multistage systems. Multistage systems can be cocurrent or countercurrent. 

In contrast to continuous packed bed columns, each stage, whether cocurrent or countercurrent, can be considered to be at equilibrium for many multi-phase mass-transfer processes such as distillation, absorption, extraction etc. Such stages are usually called ideal stages . 

Many chemical and biological processes are multistage. Multistage processes include absorption towers, distillation columns, and batteries of continuous stirred tank reactors (CSTRs). These processes may be either cocurrent or countercurrent. The steady state of a multistage process is usually described by a set of linear equations that can be treated via matrices. On the other hand, the unsteady-state dynamic behavior of a multistage process is usually described by a set of ordinary differential equations that gives rise to a matrix differential equation. 

Spray chamber Cocurrent, cross-flow, countercurrent Differential Gas Absorption, stripping, humidification, dehumidification... 

Gaylord and Miranda [Chem. Eng. Prog., 53,139M (1957)] using a multitube cocurrent-flow falling-film hydrochloric acid absorber for hydrogen chloride absorption found... 

Reactive absorption can be realized in a variety of equipment types, e.g., in him absorbers, plate columns, packed units, or bubble columns. This process is characterized by independent how of both phases, which is different from distillation and permits both cocurrent (downflow and uphow) and countercurrent regimes. 

Kenig EY, Kholpanov LP, Katysheva LI, Markish IH, Malyusov VA. Calculation of two-phase nonisothermal absorption in a liquid film in downward cocurrent flow. Theor Found Chem Eng 1985 19 97-102. 

Falling-film absorbers. These are usually vertical heat exchangers with the cooling medium in the shell and the absorption taking place in the tubes. The solvent flows downward, while the gas may enter either at the bottom (countercurrent flow) or at the top (cocurrent flow). 

Availability consumptions in section 3 (absorption) are inherent in the formation of sulphuric acid from SO3 gases. These cannot be eliminated completely by any process route but part of consumption can be transferred to cooling uater by hot cocurrent absorption system. 

In this system acid at 80°C and gases at 200°C enter cocurrently and leave the intermediate absorption touer at 110°C. This helps in increasing temperature of uarm uater from acid coolers to 75°C thus reducing availability consumption in acid coolers by 0.315 million KJ per hour. 

The major availability consumptions are in combustion furnace, uaste heat boiler and absorption sections. It is not possible to eliminate these consumptions in sulphuric acid plant for the process system being folloued presently. Houever marginal improvements can be achieved by high temperature combustion system, use of gas turbines, generation of high pressure superheated steam and cocurrent absorption system. 

The two bubble class model is applied here to the absorption of CO2 in NaOH, which conforms to a fast pseudo-first order reaction under certain operating conditions (15). In the data reported by Schumpe et al. ( 7 ), COo was absorbed during cocurrent flow in NaOH solution in a 0.102 m diameter bubble column. The gas phase consisted of approximately 10 vol % of CO2 in N2. The gas velocities ranged from 0.025 to 0.15 m/s. Since the churn turbulent regime prevailed for gas velocities greater than approximately 0.07 m/s, only the data in the range 0.07 m/s to 0.15 m/s were considered. 

Gas-liquid-solid catalytic (GLSC) reactors are usually run with cocurrent flow of both gas and liquid. Gas-liquid reaction in an absorption tower (GSLI) is often operated under countercurrent-flow conditions. 

The reported study on gas-liquid interphase mass transfer for upward cocurrent gas-liquid flow is fairly extensive. Mashelkar and Sharma19 examined the gas-liquid mass-transfer coefficient (both gas side and liquid side) and effective interfacial area for cocurrent upflow through 6.6-, 10-, and 20-cm columns packed with a variety of packings. The absorption of carbon dioxide in a variety of electrolytic and ronelectrolytic solutions was measured. The results showed that the introduction of gas at high nozzle velocities (>20,000 cm s ) resulted in a substantial increase in the overall mass-transfer coefficient. Packed bubble-columns gave some improvement in the mass-transfer characteristics over those in an unpacked bubble-column, particularly at lower superficial gas velocities. The value of the effective interfacial area decreased very significantly when there was a substantial decrease in the superficial gas velocity as the gas traversed the column. The volumetric gas-liquid mass-transfer coefficient increased with the superficial gas velocity. 

Saada25 measured the gas liquid mass-transfer coefficients for absorption of carbon dioxide into NaOH solutions for cocurrent upflow. Goto et al.8 evaluated the liquid-gas mass-transfer coefficients for the desorption of oxygen from water into nitrogen in a 2.58-cm-i.d. glass tube packed with CuO-ZnO particles... 

Nishikawa et al.92 measured /cr. i. in a three-phase cocurrent gas-liquid-flow spouted-bed column. Absorption rates of oxygen into copper-catalyzed sodium sulfite solution were measured. The data indicated that the k,a, increased with both gas and liquid flow rates. As shown by Ostergaard and Fosbol,103 while kLaL increased with particle size, its values in the case of 1.01-mm particles were lower than the ones in the absence of solids and otherwise identical conditions. The effect of particle size on kLaL obtained by these investigations is shown in... 

In summary, the above studies show that in a three-phase fluidized bed, large absorption rates require large particle sizes, smaller beds, and larger gas velocities. There are no data available on the effect of fluid properties on the volumetric mass-transfer coefficient in three-phase fluidized beds with cocurrent gas-liquid upflow). 

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