Liquid contact time

Several manual and continuous analytical techniques are used to measure SO2 in the atmosphere. The manual techniques involve two-stage sample collection and measurement. Samples are collected by bubbling a known volume of gas through a liquid collection medium. Collection efficiency is dependent on the gas-liquid contact time, bubble size, SO2 concentration, and SO2 solubility in the collection medium. The liquid medium contains chemicals which stabilize SO2 in solution by either complexation or oxidation to a more stable form. Field samples must be handled carefully to prevent losses from exposure to high temperatures. Samples are analyzed at a central laboratory by an appropriate method. 

Controlled contact between two immiscible liquids has also been achieved by flowing one liquid along a solid support submerged in the second phase [28,29]. Several different arrangements have been used, although all are based on similar principles. For example, a wetted wall column which offered liquid-liquid contact times of 0.5-10 s was used to measure solute transfer rates [29]. 

For low viscosity liquids when the dimensionless viscosity group [7], NJ, = /LtiAcrp,) is less than 1, a droplet wiU be stable below a maximum size, defined by the critical Weber number and the gas liquid contact time. For long gas—liquid contact times, two large droplets are produced. For short contact times, many smaU droplets are produced. This tjq) of droplet breakup yields a very broad droplet size distribution. 

Dl = liquid phase diffusivity, m /s tL = liquid contact time, s ... 

The physicochemical parameters are determined in a laboratory apparatus that allows careful control of gas-liquid contact time and of the interfacial area. The recommended technique and its application to the oxidation of aqueous Na2S03 solution will now be discussed. 

The gas-liquid contact time ( , is denoted as 0. The average flux over contact time 0 = Uum is... 

Correlations proposed in the Bubble-Tray Design Manual4 may be used for the prediction of nG and n, for binary mixtures. These equations may be used to estimate the number of transfer units for multicomponent mixtures as shown by Graham et al.18 For the prediction of na, Eq. (2) on p. 27 of the Manual4 may be used for the prediction of n, the correlation given by Eq. (9) on p. 34 is available this correlation contains the liquid contact time as a parameter, which may be computed by use of Eq. (9) on p. 35 the liquid holdup may be estimated by use of Eq. (10) on p. 35. For sieve trays, which are not treated in the Manual,4 a correlation recommended by Gerster et al.15 has been presented by Smith.31... 

With relatively high outlet weir height hw, the clear liquid height h increases and vapor and liquid contact time increases. This improves distillation efficiency. However, a too high outlet weir height could affect the downcomer backup and tray capacity. 

As for the uptake of OH on H20(l), Hanson et al. (1992a) gave y > 3.5 x 10 for pure water at 275 K by the measurement using wetted wall flow tube reactor. Takami et al. (1998) obtained y = (4.2 2.8) x 10 for pure water at 293 K by the impinging flow method, and reported that the value decreases with gas-liquid contact time and increases by a factor of 2-3 for the acidic and alkaline water with pH = 1 and 11. They estimated that the accommodation coefficient a is close to unity by the simulation using the rate constant of OH in the aqueous phase and Henry s law constants, which agrees with the result of a = 0.83 (300 K) by molecular dynamics calculation by Roeselova et al. (2004). The recommended values of the lUPAC subcommittee and the NASA/JPL panel are a >0.1 and 0.02, respectively (Wallington et al. 2012 Sander et al. 2011). 

Since hydrogen sulfide is absorbed more rapidly titan carbon dioxide by aqueous alkaline solutions, partial selectivity can be attained when both gases ate present. The data of Garoo et al. (1958) indicate that selectivity is fevoted by short gas-liquid contact times and low temperatures. Commercial ajqtlications of selective absorption based on short residence time contact are described by Hohlfield (1979) and Kent and Abid (1985). 

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