1.4. OBJETIVOS DE LA INVESTIGACIÓN
2.2.8. Cine de terror y horror en el perú:
Gas holdup measuring methods have been already discussed in Section 2.3.1.1. Since the determination of the individual riser (ej and downcomer (Ed) gas holdups were required, the manometric probe technique was chosen. Inverted U-tube manometers were used and were filled with the suspending fluid used in the devices. Manometer probes were connected to pressure taps located at two different axial positions either in the riser or in the downcomer of the devices for 6r and £d measurements, respectively. The pressure tappings consisted of 1 mm holes bored into the wall of the airlift device (flush with inner wall). Chisti (1989) reported that the contribution of any accelerational effects to the measured pressure was negligible. He showed a close agreement (± 10%) between the
holdups measured by the manometric method and by the volume expansion technique for a number of different reactor configurations. Other investigators have also observed similar results with the manometric method (Bello, 1981; Merchuk, 1986). The manometer probes were connected to the U-tube manometers via PVC tubing (3 mm dia.). Two manometer probes were attached to the riser section of the airlift contactors and another two probes were connected to the downcomer section. All the probes had a diameter of 1 mm and a length of 7.8 cm. The manometer probes were placed 1.50 m apart (in the riser and downcomer) in the external loop airlift contactor, while in the internal loop airlift there were situated 1.35 m (riser and downcomer) apart. In the external loop airlift, the protruding lengths of the probes into the riser and downcomer sections were 4.0 and 3.0 cm, respectively. In the internal loop airlift, the protruding lengths of the probes in the riser and downcomer were 5.0 and 1.5 cm, respectively.
It is essential that all the air bubbles are removed from the connecting tubes and the manometer before starting the gas flow rate. The presence of even one air bubble in the PVC tubing will give an erroneous reading. Hence it is important that, before and during an experiment, regular checks are made to see whether there are any air bubbles in the PVC tubing. The gas holdup can be calculated from Eqn 2.6. In this equation, dh is the difference between the liquid levels in the manometer and dz is the length between the two pressure tap points (see Fig 4.5).
A differential pressure transducer (Newport Ltd., UK) was also employed in the present study to measure gas holdup. It was connected to the airlift’s pressure tappings via PVC tubing (3 mm dia.). The transducer was only used to double check the pressure readings obtained by the manometers. There was good agreement (to within ± 10%) between the pressure transducer readings and the manometer values.
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1. A irlift re a c to r 2. G a s -liq u id d is p e rs io n 3. Inverted U -tu b e m a n o m e te r 4. R e a c to r liq u id5. D iffe re n ce in liq u id levels in th e m a n o m e te r (dh) 6. A ir 7. D iffe re n ce in h e ig h t b e tw e en th e tw o p re s s u re ta p p o in ts (dz) 8. M a n o m e te r gas p re ssu re Not to scale
Figure 4.5: The inverted U-tube m anom eter arrangem ent. Redraw n from Chisti (1989).
4.6 Liquid Velocity and Mixing Measurement Methods
From the various liquid velocity and m ixing m easurem ent system s (Section 2.3.1.2), we chose the therm al technique for determ ining the liquid circulation velocity, the m ixing tim e, the circulation tim e and the axial dispersion coefficient in the airlift contactors. N um erous investigators (D ouek and Livingston, 1994; Ford e t a l . , 1972;
H oogendoorn and Flartog, 1967) have utilised this m ethod. The m ain advantage o f this m ethod was that an unlim ited num ber o f experim ents can be perform ed with the same liquid. Since in this study relatively large scale experim ents w ere perform ed, the therm al technique proved to be a relatively inexpensive m ethod.
A reservoir tank (volum e o f 0.03 m^) w as filled w ith the liquid (sam e as in the airlift reactor) and the liquid in the tank was heated up to a preset tem perature (usually 65.0 - 68.0 °C). The liquid pulse injections were m ade possible by electronically operated solenoid valves; the injection tim e could be varied (from 0.5 to 5.0 secs). U sually we used an injection tim e o f 1.5 secs. This corresponded to a volum e o f 120-150 cm^ o f pulse (hot liquid) injection. A t any one tim e, the reservoir tank was connected to either the internal loop or the external loop airlift device (via a 0.025 m diam eter pipe).
Thermocouple probes were fixed to the airlift devices as shown in Figures 4.1 and 4.2 (for clarity, only the thermocouples in the riser section are shown). Two probes were placed in the riser section of the airlifts and another two probes were located in the downcomer section. All the probes had a diameter of 5 mm and a length of 8.3 cm. The diameter of the tip of the probe was 0.5 mm. In the external loop, the protruding lengths of the thermocouples into the riser and downcomer sections were 4.0 cm and 3.0 cm, respectively. The protruding lengths of the thermocouples in the riser and downcomer, in the internal loop, were 5.0 cm and 1.5 cm, respectively. On the internal loop airlift vessel the distance between the two thermocouple probes (both in the riser and downcomer) was 1.35 m. The thermocouple probes were placed 1.50 m apart (riser and downcomer) on the external loop airlift device. Each of the probes were connected to a liquid injection system (LIS). The LIS was attached to a chart recorder (two pen). A typical response curve, obtained with two thermocouples, is shown in Figure 4.6.
In the internal loop airlift vessel the pulse of hot liquid was injected into the riser section of the airlift (Fig 4.1), above the sparger. The hot liquid was injected at the bottom of the external loop airlift contactor, below the sparger, as shown in Fig 4.2.
The determination of the liquid circulation velocity involved injecting a pulse of hot liquid into the flowing liquid (in the airlift device) and measuring the time taken by the pulse to travel between a known distance. A knowledge of the time and the distance can lead to the calculation of the velocity of the flowing liquid in the airlift vessel. From the response curves the mixing and circulation times can also be obtained as shown in Fig 4.6. Furthermore, the experimental tracer response curve is also used to calculate the Bodenstein number and the axial dispersion coefficient, for that particular system, as outlined in Section 3.3.2.
When a pulse of hot liquid is injected into the column, we obtain on the chart recorder a trace of the circulation of the pulse (Fig 4.6). Knowing the chart speed (Ucr), the height between the two probes (either on the internal loop or the external loop), hp, and from the response curve obtaining the distance between two consecutive peaks on the chart (Xcr), the liquid velocity between the two thermocouple probes could be calculated:
1-igLire 4.6: A typical response curve from the tw o th e rm o c o u p le s in the riser section o f the
airlift device. How the liquid circulation velocity (the distance betw een two consecutive peaks on the chart, Xcr), the m ixing tim e (t„i) and the circulation tim e (tc) were procured arc shown. 1 he 90% m ixing tim e is given.