4. Equipment Description
4.3. Equipment Modifications
Further modifications were done to the apparatus. The new ROLSI™ seal provided an excellent seal which did not leak. The new mixing mechanism provided better radial mixing and reduced the time that was required to reach equilibrium.
Top Flange ROLSI™ Capillary Seal
The o-ring system as in the work of Tshibangu can fail because of a number of issues for example introduction of an incompatible fluid. The system failed at high pressure in the work of Nelson (2012) because he worked with corrosive systems. A dynamic o-ring sealing was used by Tshibangu. The dynamic seal creates a barrier between a stationary (Techtron cylindrical plug) and moving (capillary) surface containing the pressure in the cell. The primary and most common cause for o-ring seal failure in dynamic applications is extrusion and nibbling (Parker 2007). The extrusion and nibbling is caused by degradation and high pressures. The o-ring after extrusion and nibbling failure exhibits a chewed and chipped look (Figure 4.2) leading to debris inside the cell which might block the ROLSI™ capillary.
Frequent changing of the o-rings and unblocking of the capillary then becomes a necessity.
Figure 4.2: Extruded and nibbled o-ring (Parker 2007)
27 | P a g e A new, improved system was implemented in this work and is shown in Figure 4.3. The core of the system involves a Teflon gland packing. The thumbscrew cap was replaced with a bolted cap. As the bolt is tightened, the gland follower B and the base of the bolted cap press against the gland packing C which in turn is compressed onto the capillary and top flange forming a high-pressure seal. The base of the bolted cap is v-shaped to allow the gland packing to compress effectively. The gland follower was made from stainless steel and the gland packing from Polytetrafluoroethylene (PTFE). PTFE has no memory which means that when it is compressed, the gland packing will not return to its original shape.
Thus it is important to tighten the bolt at most 1½ times the first time and to only tighten further if a gas-tight seal is not formed. With time and continued usage the bolts can be tightened further and further until the extrusion of the PTFE becomes excessive. At this point, a new gland packing will be required.
The PTFE packing is excellent because it provides low friction, outstanding dynamic seal, which exhibits no degradation and therefore no introducing of polymer debris into the equilibrium cell.
Furthermore, PTFE exhibits exceptional chemical compatibility. This mechanism provided a superb seal which could withstand high pressures over a long duration. There were no leaks that arose from this seal throughout the experimental process.
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14.00
3.003.00
9.00 3.00 3.00
3.00 3.00 15.00 12.00
A
B
C
D
E
F
Figure 4.3: Top Flange Capillary sealing design. A-bolt, B- gland follower, C-PTFE gland packing, D-ROLSI™ capillary, E-bolt cap, G-Top flange. All dimensions in mm.
The Agitation Mechanism
Agitation of the phases within the equilibrium chamber promotes the attainment of equilibrium. In this study, the agitation mechanism utilized by Tshibangu (2010) was improved to hasten the equilibrium measurements. The magnetic stirrer bar and the horse shoe magnet utilized by Tshibangu (2010) were
29 | P a g e replaced with a magnetic stirrer including impellors (see “A” in Figure 4.4). The magnetic coupling between the driven magnet and the magnetic stirrer was vastly improved.
A B
C
D
E
F
Figure 4.4: The Agitation Mechanism. A-stirring blades, B-Stirring magnet, C- bottom flange, D- magnet, E- stainless steel disc, F- bottom disc connected to roller chain.
The agitation system is shown in Figure 4.4. The mixing mechanics are similar to those described by Ngema et al. (2014). It involves two Neodymium magnets of grade N45. Neodymium magnets have a high resistance to demagnetisation but begin to lose their strength if heated above their operating temperature (80oC for N series). Above 310oC they become completely demagnetised (www.kjmagnetics.com 2015). The magnets were coated with nickel to protect the material particularly iron from rust and corrosion. In the mixing mechanism in this work one ring magnet was placed inside the cell and the other outside in the thimble-like cavity at the bottom of the bottom flange. The magnet inside the cell had a volume of approximately 7.4 cm3 which reduced the cell volume from 60cm3 to approximately 52.6cm3. Disc F was connected to a roller chain which was connected to a Heidolph
30 | P a g e RZR 2021 mechanical rotator as used by Tshibangu (2010). The rotating speed was set on the mechanical rotator. Rotation of the discs F and E rotated the magnet D which then rotated the magnet A via the magnetic dipole-dipole interactions. In order to achieve this, the bottom flange used in the work of Tshibangu was removed and replaced with a flange exhibiting a thimble-like arrangement, as shown in Figure 4.4. The new bottom flange was made from series 306 stainless steel which is not magnetic, thus the magnet inside the cell was able to simply sit on the cell floor without being attracted to the metal. The only attraction was to magnet D via magnetic dipole-dipole interactions. This induced the mixing which speeds up the attainment of equilibrium. This fact can be explained by mass transfer.
Mass transfer occurs by molecular diffusion. This occurs because of a concentration gradient, a species flowing from a high to a low concentration region (Henley et al. 2011). The vertical blades increased the region of low concentration of the vapour component which increased the mass transfer rate. This rate is proportional to the area normal to the direction of mass transfer (Henley et al. 2011). Mass transfer occurred in the liquid phase, gas phase and at the gas-liquid interface. The mass transfer at the gas-liquid interface can be explained by two-film theory (Henley et al. 2011). It should be noted that there was molecular diffusion from the gas phase into the liquid phase as well as from the liquid phase into the gas phase. The transfer stopped when the concentrations were uniform and equilibrium was reached.
Figure 4.5 shows the process flow diagram with all the equipment and the major flows in and out of the equipment.
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C To Purge To vaccum
DV FV
E TC
SM
DS
CH
He flow to GC
A
Figure 4.5: Process Flow Diagram. A – Data acquisition unit, C – Gas cylinder, CH – Chiller, DS – Differential screw, DV – Drain valve, E – Equilibrium cell, FV- Feed valve, GC – Gas chromatograph, PP – Temperature probes, PT- Pressure transducer, SM – Mechanical stirrer motor, TC – Temperature controller. Red indicates the heat traced lines and heated blocks. Adapted from Subramoney et al.
(2013)
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