Trickling-pulsing flow transition

Chou et al.16 also showed an interesting dependence of trickle-flow-to-pulsed-flow transition on the fluid properties. The decrease in the surface tension shifted the transition to lower gas and liquid superficial velocities. The shift in the transition was, however, not monotonic with the change in the ethanol concentration in water, and an external transition boundary existed for a solution... 

The transition between the trickle-flow regime and the pulse-flow regime is plotted in the Figure 5.2—4 as a function of the superficial gas velocity, uc, the liquid velocity, uL, and the total reactor pressure, P, for the water-nitrogen system [17]. This figure shows clearly that the transition depends strongly on the pressure in the reactor. 

Other flow charts did not lead to a better agreement with experimental data at different pressures. Larachi et al. [19] have suggested the use of a modified Charpentier s diagram. Based on experimental data available to date on the high-pressure trickle-pulsed transition, the extended diagram is proposed to quantify directly the effect of pressure in non-foaming systems. 

An example of a microscopic approach has been proposed by Ng [20]. Let us consider the transition from trickling-to pulsing flow as described by Ng s model. This model tries to represent what is happening locally at the place where pulsing is likely to be initiated. 

Figures 5.2-10 and 5.2-11 show some results obtained by Hasseni et al. [18] and by Wammes et al. [27] with water, 3 mm glass beads, and nitrogen at different pressures compared with the predictions of the model of Dankworth et al. At moderate pressure (20 bar) the model agrees fairly well with measurements made by both teams. At higher pressures, Dankworth s model overestimates slightly the gas mass flow-rate at the trickling/pulsing boundary measured by our team. On the other hand, it underestimates grossly the gas flow-rate measured by Wammes et al. for the same transition. The reason for this difference is not easily to explain. We suspect that it is a matter of definition of the experimental transition from trickle to pulse flow [28],...

Two sizes of glass beads (1.4 and 2 mm diameter) were used, both in the trickle-and pulsing-flow regime, as well as in the transition zone (see Table 5.2-6) at total pressures between 2 and 81 bar. 

K. A. Grosser, R.G. Carbonell and S. Sunderasan, The transition to pulsing flow in trickle beds, AIChE Journal, 34 (1988) 1850-1856. 

W.J.A. Wammes, S.J. Mechielsen and K.R. Westerterp, The transition between trickle flow and pulse flow in a cocurrent gas-liquid trickle-bed reactor at elevated pressures, Chem. Engng. Science, 45 (1990) 3149-3158. 

A model is presented to predict flow transition between trickling and pulsing flow in cocurrent downflow trickle-bed reactors. Effects of gas and liquid flow rates, particle size, and pressure on the transition are studied. Comparison of theory with published transition data from pilot-scale reactors shows good agreement. Since the analysis is independent of reactor size, calculations are extended to include large-scale columns some interesting observations concerning flow transition and liquid holdup are obtained. 

Figure 1 shows the comparison of theory for the trickling-pulsing transition with experimental results from pilot scale columns for air-water systems. Since most experimental studies did not mention the pressure inside the column, a value of 1.5 atm has been assumed. The agreement with the data of Weekman and Myers (1) is excellent and a good agreement with that of Chou et al. (8) is attained up to moderate gas flow rates. The theory would appear to be in poor agreement with Sato et al. however, in Sato s paper, porosity was not mentioned. Experimental data... 

In the upper portion of the bed, trickling flow occurs, but when the gas velocity has increased enough, it tends to blow away liquid held up between the packing particles and pulsing flow is observed. This transition criterion is given in Equation (5). 

In this paper correlations presented by Sato et al. for liquid holdup and pressure drop in trickle bed reactors were used to examine the characteristics of large-scale columns. The trickling-pulsing transition relationship given by Ng was also employed to determine the flow regime present. Some interesting phenomena were observed, specifically ... 

For cncnrrfnr ga<-1igiikLHnwnflnuj over a packed bed, various flow regimes such as trickle-flow (gas continuous), pulsed flow, spray flow, and bubble flow (liquid continuous) can be obtained, depending upon the gas and liquid flow rates, the nature and size- of packing, and the nature and properties of the liquid. The flow-regime transition is usually defined as the condition at which a slight increase in gas or liquid flow rate causes a sharp increase in the root-mean-square wall-pressure fluctuations. 

Flow regime transition between trickle flow and pulse flow... 

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