In literature, various works illustrated the different optimum reaction conditions for various types of biomasses. The previous study on hydrolysis of a raw biomass, such as tobacco chops, has evidenced that the best conditions must be optimized for each starting materials. Thus, it is necessary to find the best conditions every time for each investigated substrate.
In this thesis, another promising waste biomasses was studied: paper sludge, waste from paper factory. The paper sludge employed in this work was supplied from two paper industry of the territory: Lucart and SCA Packaging both from Lucca. They supplied these sludges which were completely considered such as waste materials with negative economic value. In this point of view, likewise for tobacco chops, the obtaining of a bulk chemical like LA from wastes appeared very interesting.
The first sludge tested was provided from Lucart paper mill. This material presented a very low cellulose content (only 15 wt. %) and high calcium percent (up to 25 weight percent). For the preliminary test runs (table 4.8), the best conditions already optimized for tobacco chops were adopted. Unfortunately, run L1 gave only traces of levulinic acid.
Successive runs were carried out with a higher pre-treatment temperature (run L2), and with higher acid concentration (runs L3 and L4, respectively 1.6 and 3.2 M). With these adjustments, levulinic acid yield increases were achieved and, as expected, higher pre-treatment temperature and higher acid molar concentration gave best results.
Even thought a yield increase was reached, the obtained values were very low. In run L4, carried out with high acid concentration, only 7 % of LA yield, corresponding to 10 % of theoretical value, was attained. It is important to note that the more significant yield increase has been reached when higher acid concentrations were employed. As it was demonstrated before, the biomass conversion to LA was favoured by high acid concentration. Furthermore, the extremely high calcium ions
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quantity of calcium chloride. Hence, in order to reach appreciable LA yield, it was necessary to use a higher amount of acid catalyst than for tobacco chops.
Run Feedstock (g) Water (ml) HCl (M) NaCl (M) T a (°C) t a (h) T b (°C) t b (h) YLA c (% wt.) LA (g) YLA d (%) tYLA e (%) L1 1.75 24 0.5 0.25 80 120 200 0.5 0.01 0.003 0.9 1.3 L2 1.75 24 0.5 0.25 120 120 200 0.5 0.02 0.005 1.9 2.7 L3 1.75 24 1.6 0.25 120 120 200 0.5 0.05 0.014 5.2 7.3 L4 1.75 24 3.2 0.25 120 120 200 0.5 0.06 0.019 7 10 Reaction conditions:
Substrate: paper sludge from Lucart (cellulose content 15 % w/w), Catalysts: Hydrochloric acid; Water; N2: 30 atm a: first step reaction: biomass pre-treatment
b: second step reaction: cellulose hydrolysis c: LA ponderal yield
d: LA yield Calculated on biomass cellulose content e: LA yield percent on theoretical value
Table 4.8: Lucart paper sludge hydrolysis to LA with HCl:
the effect of acid concentration and pre-treatment temperature
Thus, due to its characteristics, this paper sludge can not be conveniently involved in a future industrial application and it has been decided not to continue this study.
The paper mill SCA packaging supplied a sludge collected from the pulper of the factory. This paper sludge was a very heterogeneous raw material and it was not used as received because it contained plastic refuses, pieces of heterogeneous materials like glass, cloth and metal. The sludge was selected and not cellulosic materials were separated. Then, the material was grinded and the obtained powder was used for the conversion to levulinic acid. The cellulose content measured on this selected fraction, representing about 40 wt. % of the total sludge, was very high: 57.2 weight %. For starting, run S1 (see table 4.9) was carried out at the same conditions already optimized in the case of tobacco chops. The yield ascertained was promising: 35 % calculated on the basis of the cellulose content. Since the cellulose percent in the feedstock was more high than tobacco chops (cellulose weight percent of 57.2 % instead 25 %), run S1 was replicated with a greater water amount, in order to work with a lower biomass/water ratio in the reaction slurry.
Indeed, run S2 was carried out with 36 instead 24 ml of water. The acid quantity adopted for run S2 was increased in order to work with the same molar concentration already employed in previous run (HCl concentration 0.5 M). The resulted LA yield of run S2, reported in table 4.9, confirmed that a larger water amount was necessary to reach higher yield.
Chapter 4: Levulinic acid from waste biomasses
The cellulose intake of this run was 2.7 wt %, whereas for tobacco chops higher LA yields were achieved in more diluted conditions. So, run S3 was carried out with a minor amount of biomass, because the reactor maximum volume did not permit to further increase the water amount. Run S3 gave a better yield result: 42 %, corresponding to 58 % of the maximum theoretical yield. The achieved higher yield values confirmed that under the optimized conditions, the eventual formed humins were a very little quantity. Furthermore, the solution was orange/yellow coloured, thus suggesting low humins (dark brown coloured) presence in the reaction medium.
Run Feedstock (g) Water (ml) Acid (M) NaCl (M) Ta (°C) ta (h) Tb (°C) tb (h) YLAc (% wt.) LA (g) YLAd (%) tYLAe (%) S1 1.75 24 0.5 0.25 80 2 200 0.5 20 0.35 35 49 S2 1.75 36 0.5 0.25 80 2 200 0.5 22 0.39 39 55 S3 1 36 0.5 0.25 80 2 200 0.5 24 0.24 42 58 S4 1 36 1 0.25 80 2 200 0.5 32 0.32 56 78 S5 1 36 1 0.25 120 2 200 0.5 33 0.33 58 80 S6 1 36 1 0.25 - - 200 0.5 29.6 0.29 52 72 S7 1 36 1 - - - 200 0.5 15 0.15 27 38 S8 1 36 1f 0.25 - - 200 0.5 18 0.18 31 43 Reaction conditions:
Substrate: paper sludge from SCA Packaging (from the pulper, cellulose content 57.2 % w/w), Catalysts: Hydrochloric acid (for run S7, H2SO4 instead HCl); Solvent: Water, N2: 30 atm;
a: first step reaction: biomass pre-treatment b: second step reaction: cellulose hydrolysis c: LA ponderal yield
d: LA yield Calculated on biomass cellulose content e: LA yield percent on theoretical value
f: catalyst: Sulphuric acid
Table 4.9: SCA paper sludge hydrolysis to LA with HCl:
the effect of temperature, biomass intake, pre-treatment
Run S3 was then replicated with a greater acid concentration (run S4, table 4.9, 1 instead 0.5 M) and, as expected, a significant levulinic acid yield increase was ascertained.
When tobacco chops was employed as starting material, it was demonstrated that the pre-treatment of biomass before the hydrolysis step was decisive in order to obtain a high levulinic acid yield. Furthermore, it was shown that the pre-treatment temperature was fundamental and it was more effective if carried out at 120 °C.
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the pre-treatment at lower temperature. On the contrary to what happened for tobacco chops, the adopted temperature in the pre-hydrolysis step did not affect significantly the LA yield.
To verify the effective importance of the pre-treatment for this type of raw material, run S6 was carried out without pre-heating step (table 4.9): the reported LA yield for run S6 was lower than those achieved for the corresponding runs carried out with pre-treatment (runs S4 and S5, respectively at 80 and 120 °C). The significant difference between this run and those carried out with tobacco chops (T16, see table 4.4) is that in this last case, the LA yield was not as lower as one could expect.
In fact, as is clearly evidenced by the histogram reported in figure 4.12, the pre-treatment step efficacy is more remarkable for tobacco chops, where a significant LA yield increase was ascertained between the experiments carried out without and with this pre-heating step.
A possible explanation of these experimental evidences could be found by thinking to the different starting materials employed in this work. In fact, it has been demonstrated that the optimal hydrolysis conditions must be found every time for each substrate and it was impossible to give univocal reaction parameters.
0 10 20 30 40 50 60 no pre-treat 80 °C 120 °C pre-treatment temperature (°C) LA yi el d % tobacco chops paper sludge
Figure 4.12: Effect of pre-treatment for various biomasses
In the case of a vegetable biomass like tobacco chops, the pre-treatment step served to a partial solubilization of the hemicellulose fibres and the subsequent activation of the cellulose to the successive de-polymerization step. Probably, the cellulose in paper sludge, on the contrary, has its surface more prone to the acid attack.
The experiment S7 was carried out without NaCl in order to verify the efficacy of the presence of metal salt for paper sludge. This run was performed leaving unchanged all the other conditions
Chapter 4: Levulinic acid from waste biomasses
employed for run S6 (see table 4.9) and the reported LA yield was the half of what obtained in the reference run S6.
It is possible to conclude that, even thought the paper sludge did not require the pre-treatment step and the cellulose is more prone towards the hydrolysis, also this biomass was affected by the “salt effect”. The presence of an electrolyte, like sodium chloride, was also in this case fundamental to increase cellulose wettability and its activation to the hydrolysis.
Lastly, the catalytic study on paper sludge hydrolysis has been concluded with an experiment where sulphuric acid was employed as catalyst (run S8, table 4.9). The acid molar amount and the other reaction conditions were the same of run S6, but, with sulphuric acid, the ascertained yield was lower than which obtained adopting hydrochloric acid. It is possible to conclude that, also for this type of paper sludge, likewise for tobacco chops, HCl is the best catalyst for the reaction. In this raw material calcium content was lower than tobacco chops (calcium content 1.8 %) but the calcium sulphate precipitation can deactivate the solid substrate. Furthermore, the solution after the hydrolysis, was dark brown coloured thus indicating humins formation. Hence, the acid catalyst H2SO4 was less active and less selective towards levulinic acid and side reactions of humins
formation were favoured.