Production of Bio-Phenols for Industrial Application: Scale-Up of the Base-Catalyzed Depolymerization of Lignin

Abstract

Objective of this study was the investigation on the up-scaling of base-catalyzed depolymerization (BCD) of lignin to pilot plant dimension. The cleavage process was carried out in dilute alkaline solution at temperatures up to 340˚C and a pressure of 25 MPa in a continuously operated tubular flow reactor with throughputs up to 20 kg/h. Investigations included the proof of the feasibility of the scale-up as well as a parameter study on the cleavage of hardwood Organosolv lignin and softwood Kraft lignin within the established pilot plant. Yields and molecular compositions of the isolated product fractions BCD-oil (liquid phenolic fraction) and BCD-oligomers (solid phenolic fraction) are similar to those described in technical lab scale, showing a good scalability. Here, BCD-oils rich in phenolic monomers such as guaiacol, catechol and/or syringol were obtained with a content of up to 13.3 wt% and 14.5 wt% from Organosolv lignin and Kraft lignin, respectively. Formation of BCDoligomers strongly depends on temperature and residence times within the reactor.

Base-Catalyzed Depolymerization of Lignin: History, Challenges and Perspectives

“…(temperature, pressure and residence time) results in higher depolymerization degree and, thus, in growing formation of monomeric and dimeric phenolic products and also of gaseous and liquid degradation products. Temperature influences not only the yield but also the molecular composition caused by the already mentioned successive reactions such as decarboxylation and demethoxylation at the substituent and side chains of the aromatic ring in the lignin molecule [8,38].…”

“…This was clearly showed in the comparison of BCD of hardwood organosolv lignin with softwood kraft lignin. Syringol and dimethoxyphenols are found almost exclusively in the BCD-oil of organosolv lignin with a significant amount in the monomer phase [8,28,30]. Other studies focused on the investigation of one type of wood but with different pretreatment methods applied.…”

“…In order to overcome this phenomenon, the reaction parameters that determine the amount of repolymerization need to be optimized and the amount of lignin in the mixture should be carefully balanced [8,12]. In addition, there is a need to scavenge and deactivate the reactive species possessing phenol, carbonyl or alkene functionalities.…”

“…Large-scale prototypes for plant equipment tested in intended industrially relevant environment close to expected performance (TRL 5-6) have been reported only by Rößiger et al so far [8]. This includes not only the cleavage but also the corresponding downstream processing for the isolation and purification of the two product fractions BCD-oil and BCD-oligomers.…”

“…It can be operated in continuous process mode allowing conversions at short residence times (≤1 h) and low char formation. The latter is as phenomena, which might occur during hydrothermal conversion of lignin feedstock, and is an effect of concurrent repolymerization processes of lignin fragments during processing under the harsh reaction conditions needed [8].…”