Pulsing flow mass transfer rate

In concurrent downward-flow trickle beds of 1 meter in height and with diameters of respectively 5, 10 and 20 cm, filled with different types of packing material, gas-continuous as well as pulsing flow was realized. Residence time distribution measurements gave information about the liquid holdup, its two composing parts the dynamic and stagnant holdup and the mass transfer rate between the two. 

The hydrodynamics control the mass transfer rate from gas to liquid and the same from liquid to the solid, often catalytic, particles. In concurrently operated columns not only the gas-continuous flow regime is used for operation as with countercurrent flow, but also the pulsing flow regime and the dispersed bubble flow regime (2). Many chemical reactors perform at the border be-... 

Combination of the empirically found correlations for these pulse properties in a model in which the parts of the bed in the gas-continuous resp. dispersed bubble flow are weighted, leads to a correlation of the mass transfer rate with predictive value. 

The pulsed reactor consists of a fixed bed of catalyst pellets through which the reacting fluid moves in pulsating flow. Mass-transfer coefficients are increased because of the pulsating velocity superimposed on the steady flow. For viscous liquids, or any fluid-solid reaction system which has a high extemal-mass-transfer resistance, pulsation may be a practical way to increase the global reaction rate. Biskis and Smith measured mass-transfer coefficients for hydrogen in a-methyl styrene in pulsed flow and found increases up to 80% over steady values. Bradford" found similar results based on data for the dissolution of beds of j9-naphthoI particles in water. 

At high gas- and liquid flow rates, so called pulsed flow will take place. This is a transient phenomenon waves of denser and less dense dispersions travel with a high rate down the column. The mass transfer rate is greatly increased, but so is the pressure drop (Specchia and Baldi, 1977). 

Relations of the rate of mass transfer between gas and liquid and the influence of the stagnant and dynamic holdup were not researched intensively, until the present work, although papers on the general subject have been presented (3-6). Lately an interesting paper about mass transfer from liquid to solid in pulsing flow was presented by Luss and co-workers (7 ). 

Ultimately at high frequencies the pulses overlap and we arrive in the dispersed bubble flow regime. Thus we consider the pulses to be zones of the bed already in the dispersed bubble flow, spaced by moving compartments that are still in the gas-continuous flow regime. This concept is very helpful in calculating mass transfer and mixing phenomena, as well as in pressure drop relations (9) where it appears that above the transition point the pressure drop can be correlated linearly with the pulse frequency. Pulses are to be considered as porous to the gas flow as is shown when we plot the pulse velocity versus the real gas flow rate, figure 5. 

The value of k a, a being the gas-liquid contact area per unit volume, k the corresponding liquid side mass transfer coefficient, is considerably higher in the pulsing than in the gas-continuous flow regime. It has been tried in the past, and partially success-full, to correlate the mass transfer data to the energy dissipation rate in the bed. We made the premise, that pulses are parts of the bed already in the dispersed bubble flow regime and therefore must accredit for an increase in the transfer rate proportional to their presence in the bed. 

Actually Sato et al. expressed their particle mass-transfer coefficients in terms of an enhancement factor representing the ratio of with two-phase flow to ks at the same liquid flow rate in single-phase flow. For pulsing and dispersed bubble flow this enhancement factor was found to be inversely proportional to liquid holdup j3, which in turn is a function of the two-phase parameter A or A (see Section IV,A,3,a). For comparison, the data for single-phase liquid flow are best represented by an equation of the same form as Eq. (115) but with a constant of 0.8. 

A number of soil chemical phenomena are characterized by rapid reaction rates that occur on millisecond and microsecond time scales. Batch and flow techniques cannot be used to measure such reaction rates. Moreover, kinetic studies that are conducted using these methods yield apparent rate coefficients and apparent rate laws since mass transfer and transport processes usually predominate. Relaxation methods enable one to measure reaction rates on millisecond and microsecond time scales and 10 determine mechanistic rate laws. In this chapter, theoretical aspects of chemical relaxation are presented. Transient relaxation methods such as temperature-jump, pressure-jump, concentration-jump, and electric field pulse techniques will be discussed and their application to the study of cation and anion adsorption/desorption phenomena, ion-exchange processes, and hydrolysis and complexation reactions will he covered. 

---