Example

https://www.researchgate.net/publication/319205739_Deep_Eutectic_Solvent_Delignification_Impact_of_Initial_Lignin

instead of the kappa, use the lignin content

for methods to analyse lignin content and ligno-cellulose see:

https://link.springer.com/article/10.1007/BF01982840

https://www.sciencedirect.com/science/article/pii/0269748387900413

https://link.springer.com/article/10.1007/BF01982840

https://www.sciencedirect.com/science/article/pii/S0960852415015485

Tazien matematical is good starting point , if really need need the the rate of delignification you need to meauure the time of to the deligification conversion amount to mearure the rate of conversion too