Holdup time measurement

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Methane is commonly used as a marker for measuring the gas holdup time (tm), which was done on a capillary column 25 m long by 0.25 mm ID by 0.25 pm film thickness. A retention time for methane of 1.76 min was obtained. Determine the average linear gas velocity (v) and the average volumetric flow rate (Fc). Explain how these values differ from the actual velocity and flows at the column inlet and outlet. 

Injector and detector temperatures were maintained at 150 and 200°C, respectively. Nitrogen carrier flow rates were measured with a Gasmet flow meter and were maintained between 22 and 24 ml min-1. Gas holdup times were measured with 20 1 injections of methane. 

Radial distributions of gas-phase characteristics were measured from the wall to the center of the column in 1/4-inch increments. For gas-liquid flows, steady-state operation was achieved in 10 minutes, whereas for gas-liquid-solid flows, measurements were not performed until one hour after flow conditions were established. At the end of each run, average gas holdup was measured by quick closure of the feed stream valve. The sampling rate for the conductivity probes was 0.5 millisecond per point, and the total sample time for each local measurement was 60 seconds. These sampling conditions are comparable to those of another investigator of gas-phase characteristics in bubble columns (11). 

There was no pressure drop across these columns, obviating the need for the James/Martin correction (16). Gas holdup times were measured with 10 fi injections of methane. Hamilton 7101NCH syringes were used to inject 2 fi volumes of pentane (neutral probe) and t—butylamine (Lewis base) injections of t-butanol (Lewis acid) were restricted to 0.5 fi in order to obtain sharp peak fronts. Three chromatograms were obtained on each column with each probe at 30 C after conditioning the column overnight at 50 C. 

A first prediction attempt of GCxGC retention using this strategy was carried out by Beens et al. [10]. Retention times in the D column were calculated from those of n-alkanes and the analyte retention index. For the analyte retention times in D, k was first obtained by interpolation for the n-alkane series at the elution temperatures, the retention times f of these compounds were calculated using the corresponding holdup times, and then t Ri for the analyte was calculated from its RI at the elution temperature using Equation (3). Differences between calculated and experimental values were observed for D retention times, although predicted elution profiles were similar to the experimental patterns. A similar approach [27] used k values measured at several temperatures to obtain a better interpolation. The accuracy of the prediction of retention times was... 

In the first simulation (model A) the liquid holdup was regarded as a constant (relative to LF =10 mL/min). In a second calculation (model B), the external holdup was input as a function of the liquid feed. From the residenee time measurements in the trickle-bed reactor, the correlation of Equation 13-30 was obtained. 

The conventional heuristic used for sizing a decanter is to specify at least 20 min of holdup time. This is larger than for simple surge drums because the small difference in liquid densities between the two liquid phases requires a large settling time. The inventory of both phases must be measured and controlled. 

Marker. A reference component that is chromatographed with the sample to aid in the measurement of holdup time or volume for the identification of sample components. 

It is not easy to determine which factors play the greatest role in obtaining good accuracy and precision. One must consider the assumptions inherent in the theory as well as the chemical, mechanical, and instrumental parameters. In general, gas chromatographic methods agree within 1-5% with other physicochemical methods. For example, Hussam and Carr (22) showed that in the measurement of vapor/liquid equilibria via headspace GC, complex thermodynamic and analytical correction factors were needed. These often came from other experimental measurements that were not necessarily accurately known. Another source of significant error can be in determination of the mass of stationary phase contained within the column (59). Other sources of error include measurement of holdup time (60), flowrate, sample mass, response factors, peak area, or baseline fidelity. 

Determine the total holdup time Thu of the tank hy dividing the volume of the tank, as measured between the minimum and maximum level control points, by the maximum flow through the control valve (Equation 7.9). It is important to note that the volume in Equation 7.9 is the volume of the tank between minimum and maximum controlled levels, not the total tank volume. 

Nonintrusive Instrumentation. Essential to quantitatively enlarging fundamental descriptions of flow patterns and flow regimes are localized nonintmsive measurements. Early investigators used time-averaged pressure traverses for holdups, and pilot tubes for velocity measurements. In the 1990s investigators use laser-Doppler and hot film anemometers, conductivity probes, and optical fibers to capture time-averaged turbulent fluctuations (39). 

The retention time of the non-adsorbing methane (ti) is the measure of the column void volume or holdup. Ethylene is adsorbed by the catalyst, hence it does not reach the detector until the available surface is saturated, at which point ethylene breaks through and is detected by the sensor (t2). The adsorbed volume of ethylene is given simply by ... 

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. 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

Foust et al. (F4) measured gas holdup in mechanically stirred gas-liquid contactors of various diameters (from 1 to 8 ft) and various liquid contents (from 5 to 2250 gal). The nominal gas velocity varied from 1 to 5 ft/min and the power input from 0.01 to 6.5 hp. The contact time (sec/ft) could be correlated by the following expression ... 

According to their measurements, the gas holdup increases with the gas velocity but the average contact time drops. This is not surprising, as will be shown. The volumetric gas flow rate is... 

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