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Elsunni and Collier: Processing of Sugarcane Rind into Non-woven Fibers
PROCESSING OF SUGARCANE RIND INTO NON-WOVEN FIBERS Maryoud M. Elsunni
Audubon Sugar Institute, Agricultural Center Louisiana State University Baton Rouge, LA 70803
and John R. Collier Chemical Engineering Department
Louisiana State University Baton Rouge, LA 70803
ABSTRACT
This process was developed for conversion of sugarcane rind to non-woven fibers. It includes partial, directional delignification, and optional agitation and/or tumbling preceding washing and subsequent steam explosion steps. The ultimate fibers in sugarcane rind are similar to those of hard wood, 2 to 5 mm long. Therefore, for these non-woven fibers to be useful raw material for applications such as yarn spinning and geotextile mats, they should be at least 2.5 cm long. A typical sugarcane crushing process causes the length of bagasse segments to be too short; hence not suitable for this study. Instead, a cane separation process similar to that referred to as the "Tilby Process," was used to prepare the sugarcane rind. In this separation process, the cane is split longitudinally in two halves, the high sucrose content pith routed out, and the resulting rind used for this process. The typical treatment in this process is to react 0.1 N NaOH at 166°C (steam pressure 620 kPa) with the rind segments in a reactor designed to specific chemical and mechanical actions. The wet and partially delignified fiber bundles can be damaged by complete rotatory action of an impeller in the reactor; therefore either an oscillating agitation and/or tumbling of the reactor around its horizonal axis was employed. In this paper all runs included tumbling and also some feature agitation. Since the rind is encouraged to split longitudinally rather than transversely by the mechanical action, lignin is removed preferentially in the longitudinal direction. As a result bundles of fibers from the rind are successively reduced in cross section with slight reduction in length, thereby yielding fibers with the necessary length and desired cross section for conversion to other useful products.
Keywords: Fiber, Lignin, Non-woven, Rind, Steam Explosion, Sugarcane.
INTRODUCTION
The ultimate objective of this research is to develop a commercial process for production of non-woven fibers from sugarcane. In this study, the concept of the severity factor (Focher, 1991) and the modified form (Collier, 1992) were further modified to account for mixing and steam explosion effects. The accuracy of the later mathematical correlation was tested for the capability to predict the suitable reaction conditions for conversion of sugarcane rind into non- woven fibers.
Bench scale results have shown that solutions of sodium hydroxide can be used to extract fiber bundles from sugarcane rind (Agrawal, 1992). Alkaline concentration, temperature, and
Journal .American Society of Sugar Cane Technologists. Vol. 16. 1996
bundles (Collier, 1992).
In this research (Elsunni, 1993), a pilot scale batch reactor was used to further establish the optimum extraction conditions The Tilby separation process was used to prepare the cane rind segments It was found that pretreatment of rind with water, followed by 0 1 N alkaline treatment at 166 C for one hour decreases the size and flexural rigidity of the extracted fibers Oscillatory or tumbling mixing during treatment followed by steam explosion enhanced fiber separation resulting in fine, fluffy, and semi-dry fibers suitable for yarn spinning
MATERIALS AND METHODS (a) Sugarcane Sample Preparation:
Fresh and well burnt sugarcane samples were collected from the cane yard of Cinclare sugar mill in Brusly, Louisiana. Typical dimensions of a sugarcane stalk in the sample was 1.5 to 3 meters in length and 18 to 50 mm in diameter. The stalks were then cut into billets of 450 to 600 mm long in preparation for rind separation through the Tilby cane separator.
(b) Cane Separation System:
The cane separation technology developed by Tilby (1976) was actually developed not only to produce sugar and improve its quality, but also to widen the range of by-products of the sugarcane industry. The separator used in this study was a pilot scale unit (Figure 1). The sugarcane billets were hand fed to the positioning rollers, that in a larger scale unit would be rotating at a speed synchronized with the subsequent rollers These rollers guide and force a billet against a sharp knife which splits it into two longitudinal halves. Each of the two halves goes through a system of two rollers adjacent to each other at a clearance of 1.5 mm. The outermost rollers, 140 mm in diameter, were designed to grip on the outer surface of the half billets and control their forward traveling speed at 550 rpm. The inside rollers, depithing wheels, are 200 mm in diameter, have longitudinal cutting knives, and rotate in the opposite direction at twice the speed of the outer rollers. The half billets start to flatten, once caught between the two rollers, and the high speed depithing wheels start to scrape off the pith portion of the half billets.
The rind strips as obtained from the cane separator were 450 to 600 mm in length.
(c) Pilot Reactor:
The pilot reactor used in this study was operated on a batch basis, with special emphasis on the effects of mechanical action and steam explosion on the degree of fiber opening. The reactor body is made of two concentric stainless steel cylinders having inside diameter of 225 and 300 mm respectively (Figure 2). The annulus of the cylinders is connected to a steam source for heating of the inner tube contents. The bottom of the reactor has a conical shape tapering down to a 50 mm discharge line The discharge is controlled by a 50 mm stainless steel ball valve rated at 3 4 MPa The top of the reactor is flanged with a 25 mm thick stainless steel ring with 12 equally spaced 16 mm holes. The reactor cover is 25 mm thick and has a 350 mm diameter.
Temperature and pressure gauges are installed on the top cover. The reactor is equipped with an oscillatory agitator powered with a 30 watt DC motor to provide mechanical action for the reactor contents The nominal capacity of the reactor is 20 liters. The reactor was kept tumbling around its horizontal axis at 0.5 rpm in all of the lignin extraction steps.
(d) Extraction Method:
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Elsunni and Collier: Processing of Sugarcane Rind into Non-woven Fibers
In this study, the extraction method refers to the removal of controlled amounts of lignin from sugarcane rind. Extraction was carried out in three main steps; water pretreatment, alkaline treatment, and steam explosion.
Figure 1. Tilby Separator Figure 2. Batch Pilot Reactor The rind strips as obtained from the cane separator were 450 to 600 mm in length. The strips were cut into shorter segments 75 to 85 mm long, and the nodal regions, which contain shorter and hard fibrous tissues, were discarded. For water pretreatment, the rind pieces were washed to remove loose fiber, pith, and dirt, and then 0.5 kg of rind at 33% moisture were placed in the reactor with 16 liters of water. The cover, discharge, and vent valves were closed, and steam valve opened to permit flow through the reactor jacket. The reactor temperature was maintained at 138°C (saturated steam pressure 240 kPa) for 30 minutes. No mechanical mixing was applied at this stage. When the reactor was sufficiently cooled, the discharge valve was opened and the pretreatment liquor was discharged.
For the treatment phase, 16 liters of sodium hydroxide solution at a selected normality were added to the rind. The reactor was heated to 166°C (saturated steam pressure 620 kPa) and maintained pressurized at that temperature for one hour. Depending on the treatment run requirement, the appropriate oscillatory mechanical mixing was applied during the heating interval. The mixer was an oscillatory type agitator with Y-shaped vanes powered by a 30 watt variable speed DC motor. The agitator was mounted on the reactor top cover, with the mixer shaft vertical but off the center position by 65 mm (Figure 2). The mixer speed was measured on a linear scale of zero to ten, with the value ten corresponding to 100 oscillations per minute.
A speed of 8.5 (85 oscillation per minute) was used to provide gentle mixing. A maximum angle of rotation of 32° was used initially, but was later increased to 80° for better mixing effect. The higher angle of rotation was suggested after a study was conducted on the mixing pattern inside the reactor.
After treatment, the steam valve was then closed and the reactor was allowed to cool
Journal American Society of Sugar Cane Technologists. Vol. 16. 1996
they can be damaged by vigorous handling. Samples of the extracted liquor were also collected for analysis. The rind was flooded with water to wash off anv alkaline residues. For subsequent separation of the fiber bundles, the rind was either transferred to a fluidized bed dryer or subjected to steam explosion.
(e) Fluidized Bed Dryer:
The fluidized bed dryer (Figure 3) is a vertical 75 mm diameter glass cylinder of 750 mm in length. The bottom end of the cylinder is tapered to a diameter of 12 mm. The bottom end is connected to a hot air blower, whereas the top is connected by a flexible tube to a horizontal expansion chamber of 200 mm in diameter and length. The open end of the expansion chamber is covered by a fine mesh fabric, to resemble a bag house, through which air can pass but not fibers.
Figure 3. Fluidized Bed Dryer
Drying was achieved by dropping small lumps of extracted fibers into the vertical cylinder. The hot air blowing from the bottom gradually removed the moisture from the rind, causing the fibers to float up and down in small balls. The rubbing action between these balls disentangled the fiber bundles. When dry, the hot air current pneumatically conveyed the fibers through the tapered top end to the bag house, where the air passed through the fabric cover and the fibers were retained. The fibers were then collected for measurement and characterization.
(f) Steam Explosion:
For separation by steam explosion (Hilton, 1990), the treated rind was de-watered by directly blowing low pressure steam inside the reactor, with the discharge valve cracked open.
The de-watering continued for about five minutes or until water drops are no longer coming out of the reactor The discharge valve was then closed and the rind inside the reactor was pressurized to 0.83 MPa (177 °C) by direct injection of saturated steam. The steam was maintained at this pressure for four to five minutes. To insure proper heat distribution, the
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Elsunni and Collier: Processing of Sugarcane Rind into Non-woven Fibers
condensate was continuously removed from the inside of the reactor by means of a condensate trap. After the rind cooking was accomplished, the pressure was suddenly released by rapidly opening the discharge valve. The explosive decompression forced the rind out of the reactor through the discharge valve to the atmosphere. The thermomechanical effect of this sudden expansion enabled extensive fiber bundle separation (Focher, 1991).
RESULTS AND DISCUSSION
The effects of the delignification and steam explosion operative conditions on sugarcane rind were analyzed by characterizing the extracted fibers. The process variables were alkaline concentration, temperature, mechanical mixing, and treatment time. For this study, it was most suitable to keep the pretreatment and treatment temperatures at 138 and 166°C, respectively.
These temperatures are above the melting temperature of lignin (125°C), but well below its degradation temperature (195°C), and that of the cellulosic fibers (260°C) (DeLong, 1990).
Furthermore, undesirable color development should be minimized as a result of using these temperatures for pretreatment and treatment. During the steam explosion process, the rind was subjected to 0.935 MPa, 177°C saturated steam. For the runs without alkaline treatment, this higher temperature is critical in weakening the lignin/xylan cross-link, and thus separating the fiber bundles by thermomechanical decompression of the rind. Due to the short explosion time, it is less likely that the fibers develop or retain much coloring matter. The alkaline treatment time was chosen as one hour throughout the experimental runs (Collier, 1992). Thus, use or non- use of a pretreatment step, oscillatory agitation, steam explosion, and the alkaline concentration were the only parameters varied from run to run.
(a) Lignin Removal:
The ultimate fibers from the sugarcane rind are too short (2 to 4 mm) (Van Dillewijn, 1952) to be processed into yarn. A selected amount of the lignin originally in the rind should be retained in the final fiber bundles to hold and bond the ultimate fibers and make it possible to obtain fiber bundles of controlled properties.
Two experimental runs were performed to determine the total lignin content in the sugarcane rind. Both runs had identical extraction conditions, where 0.5 kg of rind (33%
moisture) were treated in the reactor using 16 liters of 1.0 N sodium hydroxide solution. To insure a high degree of lignin extraction, the reactor temperature was maintained at 166°C for seven hours. During the extraction time, mechanical action was applied by operating the oscillatory agitator at its full speed of 100 oscillations/minute, and continuously tumbling the reactor around its horizontal axis at a rate of 18 revolutions/hour. Table 1 shows the results of the extract analysis of the two experiments. The lignin values obtained were consistent with values reported in literature (18 to 24%) (Paturau, 1969).
Journal American Society of Sugar Cane Technologists. Vol. 16. 1996
Table 1. Experimental estimation of total lignin in sugarcane rind. __________________
Run Rind (gm) Lignin removed_________% Lignin __________________(0% moisture) __________(gm)______________ in dry rind
1 335 82 24
2 335 75 22
According to the characterization results obtained, it was found that treatments with sodium hydroxide solutions of 0.1 N under the reactor conditions set forth gave fairly spinnable fibers. A total of eight runs related to textile fiber preparation were performed, 0.1 N alkaline solutions was used in the first six runs and 0.4 N in the last two. Table 2 shows the amount of lignin removed in each of the treatments. In experiments 1 to 3 there was no mechanical mixing applied, whereas in 4 through 8 mixing at 85 oscillations/min was applied during the treatment time.
For the same treatment conditions, more lignin was extracted when the reactants were gently mixed, compared to extraction without agitation. Therefore, for a targeted percentage of lignin to be removed, mixing can be used to enhance the reaction and to decrease the concentration of the alkaline solution needed.
Experimental runs 7 and 8 in Table 2 were performed as part of the search for the optimum alkaline normality that was suitable for extraction of spinnable fibers In both of these two runs, mixing was applied at 85 oscillations/min. They are shown here to demonstrate the effect of increased alkaline concentration on increasing the amount of lignin that could be extracted under the same pretreatment and treatment temperature and time as in experiments 1 to 6. The amount of lignin extracted is apparently not linear with alkaline concentration, but increasing alkaline concentration from 0.1 to 0.4 aided in lignin removal This result suggests that at temperatures above the melting point of lignin and lower than its degradation temperature, the amount of lignin that can be extracted may be a weak function of the alkaline concentration.
This is consistent with literature recommendations (Focher, 1991) and industry practice (Stephenson, 1950) for extraction of cellulosic lignin with weak alkaline solutions.
Treatment time for fiber extraction was the most effective, and important, reaction variable in rind delignification (Arora, 1993), assuming that the treatment temperature was maintained above that of lignin melting point. The other two possible independent variables were alkaline concentration and mixing effects (which could be expressed in terms of agitation intensity). Mild mixing, at a rate appropriate for fiber extraction, was found to enhance the rate of delignification by about 20%.
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Elsunni and Collier: Processing of Sugarcane Rind into Non-woven Fibers
Table 2. Experimental mean values of lignin extracted.
Mixing NaOH Lignin Lignin Lignin Run (Oscillations/min) solution removed removed Removed(2)
(N) _______(Gm) % dry rind(1) %
1 0 0.1 40 12.0 52.0 .
2 0 0.1 39 11.6 50.4
3 0 0.1 45 13.4 58.3
4 85 0.1 47 14.0 60.9
5 85 0.1 50 14.9 64.8
6 85 0.1 50 14.9 64.8
7 85 0.4 56 16.7 72.6
8 85 0.4 62 18.5 80.4
(1) Basis of 335 gm of 0% moisture cane rind; using 500 gm of frozen rind containing 33% moisture.
(2) Based on 23% total lignin in dry rind.
(b) Reactor Mixer:
The effect of mixing action on lignin extraction and fiber separation was investigated.
To understand the mixing pattern inside the reactor, a transparent polycarbonate reactor was constructed from 10 liter carboys to replicate the pilot batch reactor in configuration and capacity.
Both reactors have similar dimensions and capacities; the major difference between the two reactors was that the material of construction of the replica was too weak to withstand the treatment pressure. Hence, the carboy reactor could only be operated under atmospheric pressure.
The mixing pattern in the replica was assumed to simulate the actual pattern in the pressurized batch reactor.
The polycarbonate reactor was loaded with 0.5 kg of clean, 75 mm long, pieces of sugarcane rind and 16 liters of hot water. It was sealed and the agitator started. Different mixer speeds were used and the corresponding patterns were captured and recorded on a video tape.
The following qualitative observations were made:
1. No significant mixing occurred at 70 oscillations/min, and most of the rind pieces were resting in the bottom of the reactor.
2. At 80 oscillations/min, about 50% of the rind started to float randomly, but did not appear to have a definite pattern.
3. At 90 and 100 oscillations/min, about 60 to 70% of the rind was moving in an oriented motion, from bottom, upward over the top of the mixer vanes, and then downward to the bottom. Although this was the general mixing pattern at these higher speeds, still there remained a significant amount of rind (about 20%) that was either moving at random and
Journal American Society of Sugar Cane Technologists. Vol. 16. 1996
out of the observed pattern or was at rest in the bottom of the reactor.
4. The angle of oscillation of the mixer vanes was small (32°). This limited motion of the vanes resulted in no mixing effect at most of the mixer speeds.
To improve the mixing effect, the mixer driving mechanism was modified and the angle of oscillation increased to 80°. The carboy reactor was used once more to study the effect of the modification in the rind motion. Two important pattern changes were observed when the mixer was operated at 80 to 90 oscillations/min.
The first observation was the total movement of the rind segments in a well defined pattern at the mixer speed of 85 osc/min. The segments move from the bottom towards the reactant surface where they change direction to pass over the mixer vanes and then drop towards the bottom.
The other noticeable change was the directional twisting of rind segments by the vanes sides. On their way down, the segments pass between the mixer V-shaped vanes and the reactor walls. Although most of the rind segments continue the journey directly to the bottom, about 15 to 20% exit the passage from clearances that appear momentarily between the vanes sides and the reactor walls. Because these clearances are narrow and the mode of motion is downwards, the segments exit the passage while their lengths are parallel to the vanes edges. The angular motion of the vanes forces the segments out of the passage in a swirling motion to the reactor bulk which is moving upward. This behavior may enhance delignification and directional separation of fibers.
(c) Alkaline and Rind Loading Ratio:
Important treatment parameters were the alkaline solution and rind loading ratio. During the course of this research, the mass ratio of solution to rind was kept at 32 to 1. The configuration of the reactor discharge valve and the low power of the mixer limited the quantity of rind per batch. The reactor has a 20 liter volume, with 16 liters as the working capacity, a 50 mm discharge valve diameter, and a 30 watt mixer For all of the pilot reactor experiments, the reactor was tumbled at a rate of 0.5 rpm.
At the loading ratio of 32 to 1, the mixer speed was maintained at 85 oscillations/min.
When the rind portion was doubled in two experiments (16 to 1 loading ratio), it was observed that the mixer slowed down to less than 42 oscillations/min, at the same input voltage setting, and that the agitator frequently stopped. Moreover, after the liquor was drained, the treated rind settled in the reactor conical bottom, and could not be blown out when the discharge valve was opened for steam explosion. Instead, the rind jammed in the pipe leading to the discharge valve.
A higher loading ratio could have been used if either the discharge valve had been larger in diameter and/or the mixer were more powerful.
Another factor that influenced the loading ratio was the length of the rind pieces. The segments that were subjected to treatment were 75 to 100 mm in length. After treatment, these pieces lose their hardness and rigidity and become much softer and more flexible and are entangled with each other The extent of entanglement clearly depends on the starting length of the rind segments. Longer segments require a larger upstream pipe, a larger discharge valve diameter, and a more powerful mixer.
(d) Heat Transfer and Steam Requirement:
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