Investigation liquid holdup

Only a few investigations concerned with the measurement of gas holdup and residence-time distribution have been reported. The information regarding liquid holdup, which will be discussed in the following section, is considerably more abundant in some cases, values of gas holdup can be deduced from the reported data on liquid holdup and total voidage. 

The study of nonideal flow and liquid holdup can be done by residence time distribution (RTD) experiments (tracing techniques) or by use of correlations derived from literature. Dining this step, physical mechanisms that are sensitive to size are investigated separately from chemical (kinetic or equilibrium) studies (Trambouze, 1990). Here, the fixed bed is... 

The dynamic behavior of the coke gas purification process has been investigated systematically (139,140,145). For instance, local perturbations of the gas load and its composition have been analyzed. A significant dynamic parameter is represented by the liquid holdup. Figure 20 demonstrates the changes of the solvent composition after a decrease of the gas-flow rate from 67 m3/h to 36.4 m3/h and a simultaneous small increase in the liquid-flow rate. 

Experimental studies were carried out to derive correlations for mass transfer coefficients, reaction kinetics, liquid holdup, and pressure drop for the packing MULTIPAK (35). Suitable correlations for ROMBOPAK 6M are taken from Refs. 90 and 196. The nonideal thermodynamic behavior of the investigated multicomponent system was described by the NRTL model for activity coefficients concerning nonidealities caused by the dimerisation (see Ref. 72). 

Basically, two types of correlation for the dynamic or total liquid holdup are reported in the literature. Some investigators have correlated the liquid holdup directly to the liquid velocity nd fluid properties by either dimensional or dimensionless relations. In more recent investigations, the liquid holdup is correlated to the Lockhart-Martinelli parameter APl/APg (or an equivalent of it. as discussed in the earlier section). 

Schoenemann88 examined the liquid holdup in trickle-bed reactors used for butynediol synthesis. The data were obtained in a technical reactor 16 m high and 0.8 m in diameter and in a pilot-scale reactor 3.5 m high and 32 mm in diameter. Cylindrical silica catalyst pellets of 4 mm diameter were investigated. The results showed that the liqu jd holdup was, independent, fifboth the gas and i quid fl w rates tir the range of-2-thr ugh--V -fn h- - ) and was-influenced bv... 

Some investigators correlated their data for the liquid holdup in terms of dimensional relations. Satterfield and Way81 suggested a relation... 

Just as for the liquid holdup, the correlations for the k,aL are reported in two ways. Some investigators correlated kLaL to liquid and gas velocities by either dimensional30,34,35 or dimensionless34 correlations. The dimensional correlations assumed kLaL a Ui U . The values of r and s for various types of packings reported by various invesiigators are summarized in Table 6-9. Goto and Smith34 have correlated Sherwood numbers to the liquid-phase Reynolds and Schmidt numbers. 

The determination of the gas or liquid holdup is largely made from RTD measurements. Some investigators have measured the RTD of the gas phase, while others have studied the liquid phase. It should be noted that, in principle, the holdup of only one phase is required because the holdup for the other phase can be calculated if the total voidage is known. 

The liquid holdup has been measured by several investigators. Earlier investigations by Elgin and Weiss,18 Piret et al.,39 Jesser and Elgin,25 Shulman et al.,48 and Otake and Okada38 were in substantial agreement with each other. 

Although the methods proposed for scale control relate specifically to spray evaporation, the strippable film technique may well be applicable to other types of evaporator, and this requires further investigation. The other two methods are practicable as a result of the very low liquid holdup in spray evaporation, so that interruption of the feed leads to elimination of the holdup in a matter of seconds. The same process should be applicable to conventional evaporators of low holdup—e.g., long-tube climbing and falling film types—if entrainment steam were admitted at the same time as the feed is interrupted, and this possibility would well repay further investigation. 

Liquid holdup. This approach derives from the observation that at low flow rates (typical of many pilot-plant studies), the apparent rate of reaction is dependent upon liquid flow rate. This was investigated by Henry and Gilbert using a correlation proposed by Satterfield for the external holdup. 

R. Gronnerud, Investigation of liquid holdup, flow-resistance and heat transfer in circulation type evaporators. Part IV two-phase flow resistance in boiling refrigerants. Annexe 1972-1, Bull. Inst. Froid. 1979. 

As it can be seen fiom the equations of different investigators of the liquid holdup presented in part 3.2.1.2.1.3, the power to in the equation for calculation of varied from 1/3 to 2/3. That is, Eq. (25) must be used carefully taking into account the real power for the given packing. 

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