After having thoroughly investigated the adequate conditions for strainG. thermoglucosi-dasius PB94A and after the procedure for the flax treatment process had been improved, it was necessary to find the best equipment for the fiber treatment. Several possibilities were tested in laboratory-scale before selecting the system to be used in the 200 L capacity pilot plant. The beaker scale treatment worked well for up to 50 g of fibers. With larger quantities of fibers it was necessary to introduce aeration, otherwise the system became anaerobic. Also the fibers tended to entangle, thus a way to maintain the fibers parallel was needed.
3.6.1 Beaker scale trials
The fibers were placed loosely in beakers and were floating in the liquid phase. They were coiled because they did not fit in parallel (see Fig. 3.29). In Section 3.2.3 the fiber treatment in beaker scale was described.
Figure 3.29: Beaker treatment of flax fibers usingG. thermoglucosidasius PB94A.
3.6.2 Fiber treatment in a packed bed reactor
Another concept tested was to apply pressure over the fibers to improve the mass transfer.
A 5 L jacketed vessel of 14.5 cm diameter by 30 cm height was tested (see Figs. 3.30 and 3.31). The total volume of the system including the tubing and pump was 6.5 L. The
vessel was fed through a 1 cm diameter tubing that distributed its flow among 120 outlet orifices. The ratio of the outlet to inlet area was 4.8. A strong pump had to be used to overcome this difference. The fibers were rolled tightly around a stainless steel cage and positioned inside the reactor. The liquid was continuously recirculated from the distribution tubing at center of the reactor outward through the fibers.
Figure 3.30: Schematic representation of the packed bed reactor used for flax fiber treatment.
Five trials were made in this reactor and it was consistently found that despite long treatment times of 5 to 10 days and high bacterial counts, the obtained fibers had a poor quality. The resolution was reduced from 3.33 to 2.48 and the fineness from 17.08 to 9.16 dtex. No lyase activity was detected.
Figure 3.31: System setup for the packed bed reactor used for flax fiber treatment. The recirculation pump is on the back of the picture; the packed bed reactor has a jacket connected to a water bath located on the right.
The fiber packed bed reactor showed channeling and the fiber treatment was not uniform. This problem has also been described for unretted flax bobbins, when channeling occurred if the bobbins were not tight enough when bleached [61].
Another disadvantage of the packed bed reactor concept was the low fiber to liquid ratio of about 1:30 and that it was troublesome to roll the fibers around the net. This would make it highly impractical in view of an industrial process.
3.6.3 Floating fibers prototype in bench scale
The best quality fibers obtained so far were those from the beaker scale trials (Section 3.2.3), where the fibers were allowed to float freely in the liquid phase and a gentle rotary motion was applied. This gave the idea to test the same concept for the pilot plant.
The scheme of this concept can be seen in Fig. 3.32. The experiments were made in a Julabo water bath SW23, which has a total capacity of 20 L, a recirculation pump, a temperature control and a shaking mechanism (Fig. 3.34). This water bath had the advantage that the area for oxygen exchange was larger than that of the beakers and therefore the anaerobiosis problems were reduced.
Figure 3.32: Reactor concept of free-floating fibers in a water bath.
To increase the capacity and effectiveness of the concept, the fibers were compart-mentalized by a grid, which acted as a physical barrier to avoid fibers to float altogether to the surface and at the same time leave some free space between the bulk of them to facilitate mass and heat transfer. Here, the long parallel fibers were stacked in stainless steel trays (Fig. 3.33), and were subjected to a gentle motion. The process was carried out in the water bath and the solutions required for the process were transferred in and out as needed. The shaking velocity of the trays was adjusted to permit the fibers to remain parallel.
With the objective of testing the reusability of the bacterial culture of G. thermoglu-cosidasius PB94A, five batches were made using green fibers (Fl¨uh04 type) in the water bath.
This experiment was also accompanied by cup plat tests to detect cellulase activity.
Figure 3.35 shows a plate with four wells. At the central well water was used as negative
Figure 3.33: Trays filled with flax fibers used in the free-floating fibers prototype.
Figure 3.34: Water bath with parallel fibers used for testing the concept of free-floating flax fibers.
control and in the 3 wells around it, the supernatant of the first three batches was placed.
No halo was formed for any sample, therefore no cellulase activity was present.
Figure 3.35: Cup plate test for detecting cellulolytic activity in the flax treatment in the “floating fibers” prototype. No halo was formed around any sample, therefore no cellulolytic activity was present.
The fiber quality was similar for the five batches made, yielding good fiber resolution, between 1.65 and 2.86 as well as fiber fineness within the range of 6.75 to 12.05 dtex.
Figure 3.36 shows the fineness and resolution value for all the five samples. The fiber tenacity was 39.68± 2.88 cN/tex, which complies with the requirements. The visual appreciation of the fibers was positive for all the experiments. This was the main reason
0
Figure 3.36: Fiber resolution () and fineness () for the five consecutive experiments performed at the “floating fibers” prototype.
why this concept was chosen to build the pilot plant.