TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred System: The system further comprises a slurry sample outlet provided on the connection between the outlet of the pretreatment arrangement and the inlet of the enzymatic hydrolysis reactor; and a hydrolysate sample outlet provided on the enzymatic hydrolysis reactor, where the image capturing device is arranged to capture images of cellulose particles within either a slurry sample diverted through the slurry sample outlet or a hydrolysate sample diverted through the hydrolysate sample outlet.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method, the process parameter controlled in step d) is selected from: a pH-value of the pretreatment process; a residence time of the cellulosic material in the pretreatment process; a temperature of the pretreatment process; a pressure of the pretreatment process; an amount of hydrolytic enzymes added in the enzymatic hydrolysis process; a residence time of the cellulosic material in the enzymatic hydrolysis process; a biomass composition of the lingocellulosic biomass pretreated in step a); and a degree of disintegration of the biomass prior to the pretreatment in step a). The step c) is an online analysis and the method further involves diverting a sample comprising cellulosic particles after the pretreatment process in step a) but prior to the enzymatic hydrolysis process in step b) and where the analyzing of cellulosic particles in step c) is performed on the diverted sample. The step c) is an online analysis and the method further includes diverting a sample comprising cellulosic particles from the enzymatic hydrolysis process in step b) and where the analyzing of cellulosic particles in step c) is performed on the diverted sample. The step c) includes taking a digital image in the visible range and processing the image using an algorithm such that the data set is obtained. The step c) further includes determining a size, a shape, and/or a degree of darkness of the cellulosic particles based on the obtained data set. The size and/or degree of darkness of cellulosic particles is determined in step c) and compared to a reference value and if the size is larger than the reference value and/or if the degree of darkness of the cellulose particles is lower than the reference value, at least one of the following process adjustments are performed: increase of the severity of the pretreatment process; increase of the efficiency of the enzymatic hydrolysis process; and increase a degree of disintegration of the biomass prior to the pretreatment in step a). The size and/or degree of darkness of cellulosic particles is determined in step c) and compared to a reference value and if the size is smaller than the reference value and/or if the degree of darkness of the cellulose particles is higher than the reference value, at least one of the following process adjustments are performed: decrease of the severity of the pretreatment process; decrease of the efficiency of the enzymatic hydrolysis process; and decrease a degree of disintegration of the biomass prior to the pretreatment in step a). The severity of the pretreatment process is increased by increasing a pressure, temperature and/or residence time in the pretreatment and/or by changing a pH value in the pretreatment process. The severity of the pretreatment is decreased by decreasing a pressure, temperature and/or residence time in the pretreatment and/or by changing a pH value in the pretreatment process. The efficiency of the enzymatic hydrolysis is increased by increasing the amount of hydrolytic enzyme in the enzymatic hydrolysis process and/or by increasing a residence time of the cellulosic material in the enzymatic hydrolysis process. The efficiency of the enzymatic hydrolysis is decreased by decreasing the amount of hydrolytic enzyme in the enzymatic hydrolysis process and/or by decreasing a residence time of the cellulosic material in the enzymatic hydrolysis process. EXAMPLE - No suitable example is given.