A Parr 4767 reactor vessel (Parr Instrument Company, Moline, Illinois), locally supplied by John Morris Scientific (Auckland, New Zealand) was found to be a suitable alternative reactor for our small scale work. The vessel was a 450 mL stainless steel cylinder with bolted split ring closure and a four-port head unit containing ports for temperature measurement, pressure relief, burst release and a spare port. Appendix A contains the manufacturer‘s data report and technical drawing for this vessel. The vessel was also supplied with a heating mantle, a reactor controller and software for logging the measured temperatures. Although this was a large improvement compared to the existing vessel, two important features were missing: The first was a means of measuring pressure in the vessel. The second was a means of releasing extraction liquor
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from the vessel while still under high temperature and pressure to avoid lignin re- precipitation on the surface of the wood fibres, which increases when liquor is slowly cooled before removal (Macfarlane, 2009). Therefore, modifications to the new vessel were required.
3.2.1.Modifications to the Parr vessel
To enable pressure to be measured, a pressure transducer was added to the vessel and a display module for the pressure readings was added to the reactor controller. A logging system compatible with the supplied Parr software was set up to enable pressure measurements to be logged throughout the extraction.
By rearrangement of the four ports, a 3 mm diameter stainless steel tube with a sintered metal filter element was added to the underside of the head unit to enable the removal of extraction liquor from the vessel. A relief valve and 3 mm stainless steel tube were also added to the top of the head unit. By opening the relief valve, the extraction liquor can flow from inside the vessel to outside due to the pressure gradient between the vessel and the outside environment. A gas inlet for applying an external gas such as Nitrogen gas was also added to the vessel. This was used to purge the vessel and ensure complete removal of the extraction liquor.
In addition to the modifications to the vessel itself, a safety casing was also built to surround the vessel during extraction runs. This casing was made out of aluminium, with a clear polycarbonate front cover. An inner wall was also fitted with a cooling water coil, to condense any vapour that might be produced if the vessel leaked. Finally, rigid stainless steel tubing was also added to the burst release disc port. This tubing was directed into a 20 L pail of cold water. This was to quench any vapours released in the case that the pressure inside the vessel exceeded the maximum pressure allowance and the burst release disc ruptured.
Figure 15 shows schematically the cross section of the modified reactor vessel. The split ring assembly, compression ring, drop band and gasket are not shown. A ¼‖ NPT safety head outlet leads to a 3000 psi rupture disc (1), which in turn is fitted with a 7 mm diameter (I.D) stainless steel tube that runs down into a 20 L pail of quench water.
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Liquid can be removed from the reactor through a 1/8‖ outlet and tube, with flow controllable by needle valve (3), when there is a pressure gradient between the inside and outside of the reactor. A ½‖ diameter, 18 mm long sintered stainless steel filter element (4) allows liquid to be filtered from the wood chips and removed from the vessel. There is also a type-J thermocouple (2) and a pressure transducer (5). The reactor can be pressurised with nitrogen, with flow controllable by needle valve (6), through a tee (7) in the pressure transducer line. Refer to the technical drawing in Appendix A for vessel dimensions and further details.
Figure 16 shows the final set-up of the vessel and associated equipment (quench water not shown).
3.2.2.Testing the Parr vessel 3.2.2.1.Gasket material
The gaskets supplied with the original Parr vessel to seal the vessel were made of virgin PTFE. Although these gaskets were able to provide a good seal during extraction runs, with a one hour holding time at 200 °C, the gaskets deformed during only a single run and were unable to be used in subsequent runs unless trimmed back to shape. The deformation was likely to be caused by creep flow of the gasket under pressure, a problem enhanced by temperatures greater than 150 °C (Parr Instrument Company, 2012). Trimming the gaskets was not considered to be a safe option and thus alternative gasket materials were investigated.
Silicone was trialled in the form of two O-rings of different diameters. However, as with the PTFE gaskets, although an adequate seal was provided, the material degraded during the extraction and could not be used in subsequent extraction trials.
Finally, gaskets made from Polyether ether ketone (PEEK) (SustaPEEK, Dotmar Universal Plastics, Palmerston North, New Zealand) were trialled. These gaskets not only provided an adequate seal during the extraction, but did not degrade in any way. In fact, the same PEEK gasket was used in three eight-hour extraction runs and did not degrade over this time. Therefore, PEEK was chosen as the gasket material for all future work.
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Figure 15. Schematic diagram (not to scale) of cross-section of modified reactor, with gasket, split ring assembly, compression ring and drop band excluded for
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Figure 16. Final set-up of the Parr extraction vessel (quench water not shown).
Gas Inlet Extraction Liquor Release Tube Reactor Controller Nitrogen Gas Bottle Safety Casing
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A final factor to consider was the performance of the extraction liquor discharge mechanism. A 70% ethanol-water mixture was heated in the vessel and released using the relief valve at 200°C. The mixture turned to vapour as it exited the product release tube. However, the liquor was not lost as vapour if it was bubbled into water. Therefore, in subsequent trials the end of the release tube was placed under water to retain the extraction liquor.
In some experiments, the extraction liquor was not completely removed from the vessel (refer to section 4.1.2.2 in the next chapter). This was found to be caused by blockage of the sintered metal filter element, possibly with lignin. Therefore, a cleaning procedure utilising acetone and a sonicator (Soniclean 160HT, Soniclean Pty. Ltd, Thebarton, Australia) was devised to clean the filter element. In this procedure, the element was placed in a small beaker and acetone was added to cover the element. The beaker was placed in the sonicating water bath and covered with weights to hold the beaker in the water. The sonicating bath was degassed before applying various (low, medium and high) frequencies of sonication to remove material from the element by solubilisation in acetone. However, while this cleaning method worked the first few times, it soon failed to completely clean the filter, ultimately leading to failure of the release mechanism to remove all liquor from the vessel at the end of the extraction. Therefore, for all remaining trials, extraction liquor was manually removed from the vessel using the procedure outlined in section 4.1.2.2.
3.2.2.3.Commissioning inspection
The modified vessel underwent a commissioning inspection by SGS Ltd to certify its safety under the intended conditions of use. The vessel passed this inspection.