Physical phenomena occurring in the process of physical–mechanical degumming of fiber from flax straw

Experimental details

Model tests on washing out of gels from linen fabrics

Results

The measurements of water flow in the experimental degumming device showed that at the maximum achievable flow rate of 30 dm³/min, the speed of water flowing through the tank not loaded with samples was 5.2 mm/s. Loading samples of flax straw caused over twofold increase of water speed up to 10.8 mm/s producing the Reynolds number of Re = 3255, which indicates a turbulent flow. The studies showed that at the most efficient loading of the tank the batch could amount to 12.4 kg of the material. To do this, a pipe of 14.48 mm in diameter was packed with 10 cm long straw sections and given diameter given in Table 1. Laboratory tests of the straw volume indicated that the flax stems took up 44% of the total reactor volume.

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Table 1.

Speed flow of water through the flax straw at maximum load of the reactor

	Measurement 1	Measurement 2
Number of stems	61	61
Stem thickness (mm)
– average	1.30	1.37
– minimum	0.70	0.90
– maximum	1.94	2.15
Stem mass (g)	3.5467	3.5804
Liquid flow speed (mm/s)	10.22	11.48

The study on the washing out of water soluble substances from the flax straw

Water flow rate of 20 and 30 dm³/min was applied what resulted in flow velocity of 4.3 mm/s and 6.5 mm/s. Therefore, when using water flow velocity to 4.3 mm/s, the laminar flow was achieved, Re = 1667. In the laminar flow the layers of the liquid move one on another undisturbed, with the lowest velocity occurring near the stem/water boundary (see Figure 4(a)). However, increasing the water flow velocity to 6.5 mm/s allowed for the turbulent flow at the boundary with the laminar flow, Re = 2526. Thus the mass exchange occurs mostly by stationary diffusion, while in turbulent flow (Figure 4(b)) the mass exchange occurs mainly by convection. OPEN IN VIEWER

Figure 4.

The model of laminar (a) and turbulent (b) liquid motion.

Admittedly, the flow velocity at 6.5 mm/s turbulent motion was not achieved, yet the obtained results confirmed higher efficiency of the intensified water flow through the stems even at the laminar flow. It must be noted that despite using turbulent flow in the process, at the stem/water boundary, an interface layer (near-wall layer) is formed, where a laminar flow was observed. The diffusion rate in the water laminar layer inside the pectin gel on the sample surface was at the micrometers per second range, thus even compared with the laminar flow it was three orders of magnitude lower. In other words, convective mass motion in the laboratory device takes place over 1000 times faster than the diffusive motion. This indicates that the main obstacle to washing out pectin is its diffusion through the laminar near-wall layer, thus the thinner the layer, the smaller the resistance and the thickness of the layer depends on the liquid flow velocity.

The experiment showed that forced water flow led to thinning of the near-wall layer which forms on the stem. Figures 5–9 present the results of tests on mass loss and physical–chemical parameters of the liquid recovered from wringed straw. OPEN IN VIEWER

Figure 5.

The effect of liquid parameters on straw mass loss in the degumming of fibrous plants.

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Figure 6.

The effect of liquid parameters on total water consumption and liquid volume after wringing of the straw in the degumming of fibrous plants.

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Figure 7.

The effect of liquid parameters on density and kinematic viscosity of the liquid after wringing of the straw.

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Figure 8.

The effect of liquid parameters on the content of organic substances and dry mass in the liquid after wringing of the straw.

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Figure 9.

The effect of liquid parameters on pectin content in the liquid after wringing of the straw.

The studies showed that the results of the osmotic degumming with forced liquid flow were better in comparison with conventional retting as more efficient removal of the washed out substances was achieved, which prevented the growth of micro-organisms.

However, the temperature increase reduced the dynamic viscosity of the liquid and, in consequence, proportionally, the thickness of the near-wall layer. During degumming, elevated liquid viscosity was observed directly at the interface boundary, which was linked to the transport of pectin into the solution (Figure 7). Thus, higher temperatures accelerated pectin removal from the plants. It was also observed that with higher temperatures the volume of post-wringing wastes increased. The analysis of the results (Figures 5–9) indicated a significant effect of temperature on acceleration of mass exchange in the degumming process for all tested physical–chemical parameters.

The analysis of the study showed that the model laboratory device allowed for evaluation of the mass exchange mechanism at the application of slow laminar liquid flow. Therefore, the evaluation of the degumming mechanism with convective mass motion at higher liquid flow rates was based on model studies of the hydrodynamic washing out of gels applied to linen fabrics.

Model tests on washing out gels from linen fabric

Analyses of hydrodynamic gel wax wash-out from the fabric were carried out using three gels: fruit pectin, starch and animal gelatin. Moreover, wax was applied on top of the gels to emulate the conditions prevailing in flax stem. The studies were compared with the process of the diffusive washing out of gels. The results of diffusive and convective washing out for the three tested gels are presented in Figures 10–12. OPEN IN VIEWER

Figure 10.

Diffusive washing out of the gels, (a) ŻelFix gelatin, (b) starch, (c) animal gelatin, depending on the process temperature.

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Figure 11.

Hydrodynamic washing out of ŻelFix gelatin depending on the process temperature.

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Figure 12.

Diffusive and hydrodynamic washing out of gels with the applied waxes, (a) ŻelFix gelatin, (b) starch, (c) animal gelatin, depending on the process temperature.

It was shown that the rate of washing out of particular gel increased as follows:

Animal gelatin → ŻelFix gelatin → Starch

The analysis of the results indicated that the washing out of gels depended to a large extent on the temperature of the process and to smaller extent on the velocity of water flow.

It was observed that for ŻelFix gelatin, at a temperature of 12℃, when both dynamic and hydrodynamic washing processes were applied, the complete removal of the gels took 20 min. In case of temperatures of 30 and 40℃, the washing out process was shortened by half as compared with the pure diffusive process.

Among the tested gels, animal gelatin was the easiest to remove from the fabric, at temperatures of 30 and 40℃, in both processes, i.e. diffusive and hydrodynamic. Additionally, the hydrodynamic flow accelerated washing out. At a process temperature of 12℃ neither diffusive nor hydrodynamic processes resulted in washing out of the gels. The most difficult gel to remove was starch, which in diffusive flow could not be washed out at all, while in the hydrodynamic process it was removed only partially. Only at a temperature of 40℃ was the starch gel washed out from the fabric by the hydrodynamic process, although not completely.

The model studies on the washing out of hydrocolloidal substances from linen fabrics indicate that convective mass motion might increase the efficiency of degumming of straw. These tests allowed us to conclude that a combination of diffusion and convection shortens considerably the time needed to wash out the gels.

Discussion

Even though the calculations indicate that the tank of 58 dm³ can be loaded with 12.4 kg of straw, in practice this is impossible. According to the literature data ⁷ 1 ton of air-dried flax straw takes the volume of 11–14 m³, which reduces the tank/reactor load by 60–68%.

When comparing the obtained results with traditional warm water retting, it must be noted that during warm water retting, the gas bubbles move up at several centimeters per second, and this way they cause local water motion accompanied with convection of mass. Therefore, in warm water retting the convective mass motion is also observed, yet it is random, uncontrolled and dependent on the biochemical process. This issue was not included in the study of the process and was revealed for the first time in relation to the study presented here.

In the traditional water retting, the liquid remains relatively static in relation to fibrous plant straw, with the above mentioned random and slow motion causing convection of the products of the biochemical process, i.e. solid, liquid and gaseous products. Mass exchange occurs under such conditions mostly by pure diffusion, and thus it is slow. Literature data reports that the wringing wastes are composed of: 1.5–5.0 g/dm³ of dry mass, 0.54–1.1 g/dm³ of soluble substances and 0.85–3.65 g/dm³ of suspended matter. ⁸

The above can be presented as follows: if a flax stem is thrown into water without stirring, when the liquid remains still, only soluble substances are washed out: mineral salts, pectin, retting bacteria and pigments. These components will diffuse slowly to places with lower concentration. Similarly, two layers of gases will diffuse slowly in inert conditions. Pure diffusion is a molecular phenomenon, as thermal conduction is. With the motion of substances it can occur only when the flow is laminar with parallel molecule routes. However, collisions of molecules disturb their movement in one specific direction to such an extent that the diffusion proceeds very slowly.

In the diffusion process, water molecules diffuse slowly to places with lower concentration, where smaller molecules diffuse faster than the larger ones. Mass exchange, in this case, runs bidirectionally. ⁹

Fungi in dew retting, and bacteria in warm water retting, do not diffuse, but their move inside the biodegraded mass occurs in a similar way to the mass motion diffusing into the diffused mass. The principal difference is that in case of fungal or bacterial biodegradation, mass loss is observed (generally of oligo- and polysaccharides) in the straw degraded by microorganisms. An analogy of this process to diffusion has not yet been proposed, and there are indications that it proceeds in a similar way and that Fick's law should apply to it. It is suggested that the diffusion index marked with D (see equation (3)) is named a biodegradation index B F D for fungal biodegradation and B B D for bacterial biodegradation. Both forms should include some coefficient for bacterial growth (propagation) and mycelium growth in time. This issue is a new one, not yet reported, but because of better knowledge of biochemical processes, it should be studied, not only regarding retting but also with respect to biodegradation of matter in general.

In order to accelerate the mass exchange, the forced water flow can be employed with a water pump. By pumping the liquid, the relative velocity of the liquid rises dramatically, even in the case of laminar flow. As a result, the thickness of the near-wall layer (referred to as “s”) drops, which shortens considerably the process time (proportional to the thickness of the “s” near-wall layer). ¹⁰ Its thickness can be expressed in the following equation:

δ ~ μ · l ρ · ν ,

(1)

The formula above provides an approximate evaluation of the thickness of the near-wall layer; thus the relation is measured in terms of quality.

The mass exchange occurs by convection, so that the stirred agent, either as a result of laminar or of turbulent flow, transports the exchanged component from places with higher concentration to those with lower concentration. ¹²

Thus, the degumming process is based on the mass exchange between two phases, i.e. water and flax stem. In the discussed case both phenomena are observed: water diffusion and convection.

In the diffusion process, the movement of water molecules and the dissolved substance occurs by penetration through pores in semi-permeable film that is epidermis, in accordance with the concentration gradient. The phenomenon of the solvent molecules penetrating through the semi-permeable films is called osmosis. In this process, the water moves inside the stem, while for the dissolved substances—in the opposite direction. The volume of such transfer for water molecules and dissolved substances will be higher with higher concentration gradient between the two solutions. Water, when penetrating into the cells, causes an increase of osmotic pressure because pectin molecules, which have a much higher molar mass than this of water, diffuse much slower. Intracellular pressure damages cells and leads to lengthwise breaking of the epidermis, which accelerates the transfer of dissolved components into the solution outside the stem.

The process ultimately leads to the molecules concentration balance. The cause of diffusion is random thermal movement of molecules. If in a given area there are more molecules than in its vicinity, then with the chaotic movement more molecules leave the area than enter it. The resultant amount of molecules penetrating a unit of area/surface in time is described as diffusive stream, according to the formula:

J D = n S t

(2)

According to the first Fick's law the volume of the diffusive stream is directly proportional to the concentration gradient of the diffusing substance.^12,13 In a one-dimensional case (diffusion occurs along a straight line), the concentration gradient equals quotient of difference in concentrations between two points and the distance between them.

J D = - D × dc dx

(3)

The factor of proportionality (D) in the equation (3) is referred to as the diffusion coefficient of a given substance. It depends on the type of the diffusing substance and on the medium and on the temperature.

Liquid diffusion is generally described by the Stokes–Einstein relation (for diluted solutions)

D μ kT = 1 6 π r D = 1 6 π r × T μ × k

(4)

This indicates that diffusion coefficient is inversely proportional to liquid viscosity and the size of molecules of the dissolved substance, and directly proportional to the T/µ ratio.

In fact, the presented degumming methods, i.e. dew retting and physical–mechanical degumming, are random, while the random character of the physical–mechanical process can be influenced in the case of a few factors to a large extent and express them in measurable units, i.e. temperature, viscosity and liquid flow rate near the fiber and stem.

Model tests of the hydrodynamic washing out of gels from linen fabrics were conducted in order to run qualitative evaluation of the convective process in the washing out of water soluble substances from flax straw.

All three used gels belong to hydrocolloid substances with the fruit pectin being the most similar to flax pectin. Pectin substances in flax are high-molecular compounds of polygalacturonic acid, similarly to fruit pectin. Two pectin fractions can be found in fibrous plants: fraction A, soluble in water, and insoluble fraction B. During retting, fraction A is easy to remove, as it is degraded by fungi and bacteria. The pectin B remains in the fiber and determines its cohesion. Excessive removal of pectin makes the fiber rough, dry and unpleasant to touch. However, complete removal of pectin results in disintegration of fiber bundles into elementary fibers. In 1941, Falser ¹⁴ and Ludke and Felser ¹⁵ reported that the molecular mass of flax pectin reaches 145,000 to 200,000 and is similar to that of fruit pectin. The high molecular mass fruit pectin can be extracted by boiling in water or in diluted acids at high temperature. On the other hand, pectin present in roots (beetroots) or stems (fibrous plants) is characterized by much higher adhesion to tissue walls than pectin in fruits. Despite this, the methods effective in extracting fruit pectin are not suitable for extracting pectin from fibrous plants without damaging pectin molecules. ¹⁶ To obtain high-molecular mass, i.e. non-depolymerized pectin, Falser ¹⁴ and Ludke with Felser ¹⁵ subjected flax straw to the acid action at room temperature and then treated the water soluble pectin with ammonium oxalate at 80℃. ¹⁷

These studies also showed that combining diffusion and convection shortens significantly the process of the washing out of the gels.

This can be explained by the fact that the process includes transfer from colloidal system (gel) into high-molecular solution, i.e. sol, where there is no interaction between the sol particles. The tixotropy phenomenon occurs here, which is a reversible isothermal process of the transformation of gel into sol under the influence of mechanical factors, e.g. shaking and mixing. In the process discussed here, it is caused by sprinkling liquid drops. The sprinkling of water drops causes the colloidal particles in the gel to undergo a reversible solvation. The force of the reversible solvation is lower than the force bonding again the coagulated particles, which leads to gel reformation. ¹⁸

The analysis of the washing out hydro colloids from linen fabric indicates that using convective mass motion might increase the efficiency of flax straw degumming.

Conclusion

The studies on degumming of flax straw showed that the use of forced flow of the liquid through the material enhances washing out of near-wall layer that forms on the flax stem during osmotic degumming. Using the liquid flow rate at 30 dm³/min caused a turbulent motion, Re = 2526, whereas increased loading of the tank with the straw had only a slight effect on the increase of the Reynolds number. It was also shown that the increase in the process temperature to 40℃ led to faster washing out of pectin and soluble substances from the stem, and thus to shortening of the degumming time.

The model tests on three gels indicated that the temperature was a crucial factor in the washing out of the gels. Waxes constitute a natural protective barrier in this process. It was shown that better washing out of the gels was achieved by applying diffusive penetration of water molecules into the gel at the initial stage of the process. This results in water being absorbed by water soluble and insoluble hydrocolloids, which determines their ability to bind water. Then, the use of the dynamic process significantly reduces the time of the washing out of the gels.