Shives of hemp (Cannabis sativa) were delignified with the reaction mixture "acetic acid - hydrogen peroxide - sulfuric acid catalyst - water" at a sulfuric acid concentration of 0.45%, a liquid module of 6, and a temperature of 85°C. The influence of the concentration of hydrogen peroxide and the duration of the process on the yield, strength properties and whiteness of technical cellulose was studied. Due to its high strength characteristics, peroxide cellulose from hemp shives can be used in composition with other fibrous semi-finished products in the production of mass types of paper and cardboard products
hemp, shives, cellulose, delignification, hydrogen peroxide, peracetic acid, cellulose whiteness, cellulose strength
As a result of breeding work, varieties of industrial hemp (Cannabis sativa) were bred, in which the content of tetrahydrocannabinol and other psychoactive substances does not exceed 0.01%, and in 2011 industrial cultivation of this crop was allowed in Russia. It is expected that by 2025 the sown area will reach 20 thousand hectares, and the hemp yield will be 8.5 centners per hectare.
About 65% of the mass of hemp trusts is fibrous shives. One of the promising areas of industrial use of technical hemp is the production of pulp and paper products. From one hectare of cultivated area under hemp, you can get the same amount of cellulose as 4 - 7 hectares of forest. Also noteworthy is the high strength inherent in hemp cellulose paper.
Oxidative delignification of plant raw materials with peroxo compounds is considered as a "green" and resource-saving alternative to existing industrial methods of cellulose production. To date, the results of a large number of studies in this area have been published, including reviews [1-4]. The essence of the method lies in the processing of plant materials with an aqueous solution of hydrogen peroxide and acetic acid. In this reaction system, acetic acid undergoes catalyzed oxidation to peracetic acid, which, in its middle, oxidizes lignin, converting it into a soluble state. Sulfuric acid is used as catalysts, as well as its combinations with tungstic acid, tungstate and sodium molybdate, titanium dioxide.
We have studied the effect of the conditions of one-stage delignification ("cooking") of hemp shives by the oxidative method on the yield and properties of technical cellulose.
The raw material for the research was shives from hemp brand "Surskaya". The chemical composition is determined by conventional methods [5]: mass fraction of cellulose (Kurschner-Hoffer method) 41.2%; lignin (sulfuric acid method modified by Komarov) 23.4%; extractives (extraction in a Soxhlet apparatus with an azeotropic ethanol-toluene mixture) 4.64%; ash 1.10%.
Shives were delignified with the reaction mixture "acetic acid - hydrogen peroxide - sulfuric acid catalyst - water". Constant delignification conditions: the initial concentration of acetic acid in the cooking solution is 6 g-mol/dm3 (36%); sulfuric acid concentration 0.046 g-mol/dm3 (0.45%); liquid module 6.0; isothermal cooking temperature 85°C.
Variable cooking factors:
X1 – initial concentration of hydrogen peroxide in the cooking solution (variation interval 2 ... 4 g-mol/dm3);
X2 – cooking duration (interval of variation 135 ... 225 minutes).
The values of these factors varied according to a three-level design of the second-order experiment on the elements of a cube [6] (table 1).
At the end of cooking, a sample of the formed liquor was taken and analyzed for the content of hydrogen peroxide and peracetic acid. The pulp washed after cooking was ground in a CRA apparatus (Yokro mill) to a grinding degree of 34 ... 36°ShR. Paper casts of 75 g/m2 were made on a Rapid-Keten sheet-molding machine. The experimental results were characterized by the following output parameters:
Y1 – concentration of residual hydrogen peroxide in the liquor, %;
Y2 – concentration of residual peracetic acid in the liquor, %;
Y3 – solid residue yield (technical cellulose), %;
Y4 – grinding time in CRA up to 35°SHR, min;
Y5 – breaking length, m;
Y6 – bursting resistance, kPa;
Y7 – whiteness of castings, %;
Y8 – density of castings, g/cm3.
The results of the experiments are shown in table 1.
Table 1 - Conditions and results of the experiment
|
Mode number |
Variable factors |
Output parameters
|
|||||||||
|
– |
X1 |
X2 |
Y1 |
Y2 |
Y3 |
Y4 |
Y5 |
Y6 |
Y7 |
Y8 |
|
|
1 2 3 4 5 6 7 8 9 10 11 |
3 2 3 4 2 3 4 2 3 4 3 |
180 135 135 135 180 180 180 225 225 225 180 |
4.01 4.08 5.27 6.51 3.41 4.25 5.78 2.81 3.65 4.42 3.75 |
0.85 0.66 0.76 1.33 0.66 0.89 1.52 0.57 0.76 1.33 0.81 |
53.9 88.2 64.1 54.9 80.6 54.4 48.8 67.7 52.1 47.8 53.4 |
2 21 6 2 15 2 2 6 2 2 2 |
8500 2970 8120 9630 5050 8550 10240 6160 12000 12500 8450 |
198 71 171 251 110 220 272 123 238 362 180 |
57 45 56 66 48 57 70 51 60 74 53 |
0.651 0.368 0.534 0.705 0.405 0.651 0.783 0.498 0.594 0.717 0.651 |
|
Mathematical processing of the results was performed using the Statgraphics Centurion software package. The dependence of each of the output parameters on variable factors was approximated by polynomial second-order regression equations [8]:
Ý – b0 + b1X1 + b2X2 + b11X12 + b22X22 + b12X1X2.
The terms with an estimate of the confidence probability of the regression coefficients of less than 95% were excluded from the equation with the recalculation of the remaining coefficients. Statistically significant coefficients (threshold significance level 0.05) are shown in table 2.
Regression equations were used to graphically represent the results in the form of three-dimensional response surfaces [7].
The dependences of the concentration of residual values of hydrogen peroxide Y1 and peracetic acid Y2 in the liquor on variable factors (fiigure 1) are almost identical and predictable, they are due to the nature of the above-mentioned sequentially occurring oxidative reactions.
Table 2 – Coefficients and statistical characteristics of regression equations
|
bij coefficients and statistical characteristics
|
Output parameters |
|||||||
|
Y1 |
Y2 |
Y3 |
Y4 |
Y5 |
Y6 |
Y7 |
Y8 |
|
|
b0 b1 b2 b11 b22 b12 Determination coefficient,% Forecast standard error for Ý |
4.47 1.07 -0.02 – – –
92.5
0.332 |
1.51 -0.80 – – 0.198 –
95.9
0.076 |
246.5 -82.1 -0.37 9.08 0.074 –
98.2
2.30 |
125.3 -52.2 -0.32 5.20 – 0.083
97.5
1.32 |
-7953 12551 -110 -1586 0.409 –
97.4
586.4 |
-244 96.8 0.852 – – –
96.0
18.4 |
12.91 11.0 0.067 – – –
95.5
2.14 |
0.129 0.156 – – – –
84.3
0.0055 |
Figure 1 – Dependence of the concentration of hydrogen peroxide Y1 and peracetic acid Y2 in the liquor on the variable cooking factors
The yield of cellulose naturally decreases with an increase in the initial concentrations of active reagents in the cooking liquor and the duration of cooking (Y3, figure 2). At the same time, the grinding time is significantly and almost symbatically reduced with the yield (Y4, figure 2) - a consequence of the removal of cellulose incrustations as the shives are delignified.
Figure 2 – Dependence of the yield of cellulose Y3 and the duration of grinding Y4 on the variable cooking factors
As a result of the deepening delignification, the strength characteristics of cellulose naturally increase - resistance to tearing and punching (Y5 and Y6, Figure 3), as well as its whiteness (Y7, figure 4).
Figure 3 – Dependence of the resistance of cellulose to tearing Y5 and bursting shear Y6 on variable cooking factors
A characteristic feature of peroxide cellulose that has not yet received an explanation is the increased density of the paper sheet made from it [3]. In the experiment under discussion, this feature manifested itself most clearly (Y8, Table 1 and figure 4): a decrease in the cellulose yield from 88 to 48% was accompanied by an increase in density from 0.37 to 0.72 g/cm3.
Figure 4 – Dependence of Y7 whiteness and Y8 dressing density on variable cooking factors
Due to its high strength characteristics, hemp shives peroxide cellulose can be used in combination with other fibrous semi-finished products in the production of many mass types of paper and cardboard products.
The work was carried out within the framework of the state assignment of the Ministry of Education and Science of Russia for the implementation of the project "Technology and equipment for the chemical processing of plant raw materials biomass" by the team of the research laboratory "Deep processing of plant raw materials" (topic number FEFE-2020-0016).
1. Pen R.Z., Karetnikova N.V. Catalyzed delignification of wood with hydrogen peroxide and peroxyacids (review) // Chemistry of vegetable raw materials. 2005, № 3, P. 61-73.
2. Pen R.Z., Karetnikova N.V., Shapiro I.L.Catalyzed delignification of plant raw materials with hydrogen peroxide and peroxyacids (review) // Chemistry of vegetable raw materials. 2020, № 4, P. 329-347. (DOIhttps://doi.org/10.14258/jcprm.2020048119)
3. Pen R., Karetnikova N., Shapiro I. Peroxide cellulose. Delignification of plant raw materials with peroxo compounds. Saarbrücken (Germany), LAMBERT Academic Publishing, 2013. 245 P.
4. Kuznetsov B. N., Kuznetsova S. A., Yatsenkova O. V., Danilov V. G. Obtaining cellulose by catalytic delignification with hydrogen peroxide. Krasnoyarsk, Siberian Federal University, 2014. 146 P.
5. Obolenskaya A.V., Elnitskaya Z.P., Leonovich A.A. Laboratory workshop on the chemistry of wood and cellulose. Moscow, 1991. 320 P.
6. Penh R.Z. Planning an Experiment in Statgraphics Centurion. Krasnoyarsk, Siberian State Technological University, 2014. 293 P.
7. Pen R.Z., Pen V.R. Statistical methods of mathematical modeling, analysis and optimization of technological processes. Saint Petersburg, Lan, 2020. 308 P.
