(2019) 1:6
Omer et al. BMC Chemical Engineering
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material. All cooking conditions were kept constant (the pulping variables time to reach maximum temperature, time at maximum temperature, maximum temperature, the concentration of AQ on oven dry okra stalks and Liquor to okra stalks ratio, as shown in (Table 1) with chemical charges as NaOH were varied between 15 to 21%. Pulping was carried out in 7 l electrically heated di- gester with forced liquor circulation. Results The fibers from the two okra stalks studied (core and bark) were in the range of hardwood fibers, with short fiber length, especially the core (0.66 mm) as shown in (Table 2), with more or less moderate walls, narrow lumen and fiber width. The fiber width of bark was medium – narrow and in the hardwoods range (10-35 μ m) the bark fiber has wall thickness (6.2 μ m) could be classified as medium thick (Fig. 2). The ash content of Abelmoschus esculentus okra stalks was rather high (6.3%) as indicated in (Table 3), but typ- ical for tropical non-woody plants. The silica content was comparatively high as usual for such non-woody raw material. However the silica content was rather high (1.6%) so there is some problems during cooking. The hot water extractives from okra stalks was (4.1%), cold water (0.4) ethanol/ cyclohexane (1.1), ethanol extrac- tives (1.2%) and 1% NaOH (27.6%) were rather high due to the presence of many soluble polysaccharides and phenolic compounds. On the other hand they are an indication of easy ac- cess and degradation of the cell wall materials by weak alkali. The cellulose (Kurschner-Hoffer) was (48.5%) which meant good pulp yields at suitable alkali utilization. As Obolenskaya, [21] mentioned that the kurschner-Hoffer values are usually 4-7% higher than alfa-cellulose. The lignin content was (15.3%) which was relatively moderate. This should result in moderate cooking chemical charges and a short cooking cycle. Cellulose to lignin ratio was higher than 2 (3.2) and pre- dicted normal pulping with alkaline methods [25]. The presence of water-soluble hemicelluloses in pulp fibers increases their swelling tendency and water absorption during beating as indicated by relatively high pentosans content (19.3%). Table 1 Abelmoschus esculentus okra stalks : Pulping conditions Cooking conditions Soda-AQ1 Soda-AQ2 Soda-AQ3 Active alkali charge as NaOH % 15 18 21 Anthraquinone, % 0.1 0.1 0.1 Liquor to okra stalks ratio, 5 5 5 Maximum temperature, 0 C 165 165 165 Time to maximum temperature, min 60 60 60 Time at maximum temperature, min 120 120 120
Table 2 Abelmoschus esculentus okra stalks: fiber dimensions and morphological indices. Fibers after cooking conditions presented in Table 1 Fiber dimensions Measured value of core ±SD Measured value of bark ±SD Fiber length, mm 0.66 0.09 0.92 0.08 Fiber width, μ m 17.6 0.94 21.7 0.89 Lumen width, μ m 6.4 0.95 9.3 0.97 Wall thickness, μ m 5.6 0.41 6.2 0.22 Morphological indices Runkel index 1.8 1.3 Wall fraction 63.6 57.1 Flexibility coefficient, % 36.4 42.9 Rigidity coefficient, % 31.8 28.8
Felting power (slenderness)
37.5
42.4
Anthraquinone (AQ) is a powerful redox-catalyst in alkaline pulping especially when non-woody raw mater- ial is cooked. The results of soda-AQ cooking of okra with 15-21% NaOH showed in Table 4. Use of 0.1% Anthraquinone (AQ) enhanced the delignification in the three trials. The screened yield and rejects were indi- cated in (Fig. 3), thus screened yield increase with in- crease of chemical dose applied with the rejects decrease, during cooking with alkali charge 21% as NaOH it seemed no or negligible amount of reject whereas during 15% NaoH trial about 7% rejects with lower degree of delignification (kappa number 29.6). Good viscosity, kappa number when high chemical charge applied (21%). However at lowest alkali dose 15% NaOH, highest kappa number (29.6), and lowest viscos- ity 1010 ml g − 1. The brightness for the three cooks were more or less the same. According to Gierer [26, 27] the intermediate formation of quinonemethide is preceded by the cleavage of β -ether aryl-ether bonds. The strength properties of three trails pulps (Table 4) indicated in general the slight difference in strength
Fig. 2 Abelmoschus esculentus okra stalks, fiber structure under microscope
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