After the extensive research work on lignin oxidation in batch mode, the contin­uous process is the natural next step in view of an eventual industrial application. An experimental pilot set up was built for lignin oxidation in continuous operating mode at the LSRE [119, 120]: a schematic diagram of the operational pilot installation is shown in Fig. 12.11. The bubble column reactor has a capacity of 8 l and is possible to fill it with modules of structured packing to improve the overall mass transfer performance of the system. This setup allows the operation with different flow conditions, for example in semi-batch mode (closed to gas or liquid), and is designed to work in very strong alkaline media (pH of 14), temperatures up to 170°C and pressures up to 15 bar. More details on the reactor set up can be found in the literature [119, 120, 122].

In a typical run, the alkaline solution of kraft lignin (60 g/l in NaOH 2 M) is fed to the reactor at 1-2.5 l/h, the temperature is selected (e. g. 403 K) and N2 is used to pressurize the system. The oxidation is initiated when the operating tempera­ture, pressure, and flow rates stabilized. Two different reactor configurations were tested: structured packed bubble column reactor (SPBCR) and bubble column reactor (with no internals, BCR).

The steady state condition was attained at approximately 6 h of operation for both types of reactor configuration. BCR was tested for O2 flow rates (QO2) of 1 L and 2 LNTP/min, reaching to the vanillin yields of 0.56 and 0.67 g/l, respectively. The higher yield of vanillin for higher QO2 is probably due to the higher pO2 which improved the oxygen solubility in the liquid phase. However, the continuous oper­ation led to about 25-30% of the maximum of vanillin concentration produced in the

Feed Deposit

Fig. 12.11 Layout of the experimental set-up of continuous lignin oxidation with O2 in alkaline medium. The design and construction were performed at LSRE within the PhD work of Daniel Araujo (advisor Professor Alirio Rodrigues) [119] (image was a courtesy of Dr. Daniel Araujo, FEUP, Portugal) batch process. In the SPBCR the hydrodynamics environment (dispersion coeffi­cient, phase hold up or even heat transfer coefficients) is quite different from the simple bubble column reactor. The oxidations performed at QO2 = 2,000 mlNTP/ min and feed rates of 2.0 l/h and 1.0 l/h lead, at steady state, to the final yields in vanillin of 0.73 and 0.89 g/l, respectively. The improvement on the vanillin content in the SPBCR configuration is due to an increase in the mass transfer of oxygen. The mass transfer coefficient was 35% higher for SPBCR than for BCR [119]. However, in both cases, the insufficient mass transfer of oxygen from the gas phase to the liquid phase was stated as the main reason for the low conversion.

To improve the performance of the continuous reactor and reach the production levels obtained in batch mode, the influence of some operating conditions was studied using a model developed by Araujo [119]. In this approach, pure oxygen in the gas feed was considered, decreasing simultaneously the residence time to avoid vanillin oxidation. From the SPBCR it was possible a vanillin concentration in the exit stream of 1.8 g/l, which is 85% of the maximum levels of vanillin concen­tration obtained in the batch reactor. Besides the low rates of oxygen transfer, the amount of vanillin produced is probably also limited by the type of lignin used.