The experimental procedures to obtain the volumetric liquid-side mass transfer coefficients (kLa),
gas holdup (εG), and the Sauter-mean bubble diameter (d32) are detailed in the following sections.
4.4.1 Volumetric Liquid-Side Mass Transfer Coefficient
The Transient Physical Gas Absorption (TPGA) technique was employed to obtain kLa for the
gases into the liquid phase or slurry phase under the operating conditions used. The experimental procedure is similar to that used by Behkish et al. [166] and is described below:
1. A predetermined amount of liquid or slurry is charged into the reactor (95 liters). 2. Reactor content is heated to the desired temperature.
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3. The vent valve at the top of the liquid recirculation line is opened fully to ensure that any liquid trapped above is allowed to drain. The valve is then shut.
4. The entire system is vacuumed to remove any gases which might be dissolved in the liquid-phase. Once the pressure in the reactor reaches the vapor pressure of the liquid- phase, the vacuum is stopped.
5. A small amount of gas is charged to the reactor to ensure enough pressure (2 bar) to fill the liquid recirculation line with liquid.
6. Any gas present in the recirculation line is vented by opening the valve at the top of the liquid recirculation line until liquid reaches the top.
7. The gas is charged to the reactor to a high enough pressure (3 bar) in order to overcome the pressure drop in the liquid recirculation line and properly operate the pump.
8. The pump is turned on and the flow rate is regulated by the bypass valve and the liquid inlet valve until the desired liquid flow rate is reached.
9. The gas is then charged to the reactor from gas cylinder until the desired pressure for the experiment is reached.
10. The cooling water and drain valves for the compressor are opened.
11. Once the gas is charged, the compressor is turned on and the gas is initially recirculated only through the top of the reactor with valve AV-2 open and valve AV-1 closed. The gas velocity is adjusted to the desired flow rate, regulated by the gas bypass valve.
12. The top valve AV-2 is closed while simultaneously opening the bottom gas valve AV-1 allowing the gas to flow through the liquid or slurry. The reactor pressure is recorded as a function of time during the gas absorption in the liquid or slurry phase until thermodynamic equilibrium is reached.
The volumetric mass transfer coefficient, kLa, is then calculated from the transient part of the pressure-time data. If performing the experiment without any liquid recirculation, steps 3, 5, 6, 7 and 8 can be skipped.
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4.4.2 Gas Holdup
The manometric method was used to obtain the gas holdup. Once the system reaches thermodynamic equilibrium (i.e., steady state), the following procedure for obtaining the gas holdup is applied:
1. The differential pressure (dP) cells legs are purged of liquid or slurry and pressurized with the gas being used.
2. The hydrostatic pressure is measured at different positions along the height of the reactor by opening and closing the corresponding valves.
3. The computer collects the dP cells readings and calculates the gas holdup at any given position along the reactor.
4.4.3 Gas Bubbles Size Distribution and Sauter Mean Diameter
The Dynamic Gas Disengagement (DGD) technique was employed to obtain the gas bubbles size distribution and the Sauter mean diameter. In this procedure, once kLa and εG measurements have
been successfully completed at given temperature: 1. The dP cell legs at given positions are opened.
2. Using the pneumatically actuated valves, the inlet valve at the bottom of the reactor is closed while the valve at the top of the reactor is opened, directing the gas from the bottom to the top of the reactor, the compressor is turned off and as a result the gas retained in the liquid disengages.
3. The dP cell readings are recorded until all the gas bubbles are completely disengaged and the pressure leveled off.
4. The computer collects the dP cell readings and calculates the gas holdup. The bubble sizes are then calculated using the gas holdup versus time data.
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4.4.4 Solid Particles Distribution
Slurry samples were withdrawn from the sampling ports located on the legs of the dP cells at different heights in the reactor in order to measure the mass fraction of solids in the reactor using the liquid paraffins mixture at different heights from 0.240 m to 1.309 m. Four sampling ports were used and a total of eight samples were withdrawn for each experiment. To collect solid particles at a given concentration, the same procedure described in section 4.4.2 to obtain the gas holdup was followed; however the dP cells legs were not pressurized in order to allow the slurry to flow from the reactor through the dP cell legs and the sampling ports. After flushing the sampling line, the slurry was collected from each sampling ports. The samples were placed into pre-weighed glass jars and sealed immediately. The solids were then separated from the liquid paraffins mixture using the following procedure:
• The initial sample weight was recorded. • The samples were then gravity filtered.
• After filtering, the filter paper containing the “wet” solid particles as well as the sample jars containing some remnants of solids and liquid were placed for 3 days into a furnace with the temperature set just above the boiling point of the paraffins mixture.
Once the samples were dried, the final total weight including the dried glass jar and catalyst was recorded to determine the catalyst percentage by weight.
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