NOVELTY - Continuous process for producing hemicellulose sugars from biomass extraction liquor, comprises e.g.: (a) providing or obtaining extraction liquor containing water, hemicellulose oligomers, and dissolved or suspended lignin; (b) optionally introducing acid catalyst to extraction liquor; (c) for a first period of time, feeding extraction liquor through first hydrolysis reactor under hydrolysis reaction conditions to hydrolyze hemicellulose oligomers into hemicellulose monomers; and (d) at a first switching time, diverting extraction liquor from first reactor to second hydrolysis reactor.    USE - The continuous process is useful for producing hemicellulose sugars and hemicellulose derivatives from a biomass extraction liquor, where the hemicellulose derivatives are furfural, hydroxymethylfurfural, levulinic acid and/or formic acid (all claimed).    DETAILED DESCRIPTION - Continuous process for producing hemicellulose sugars from a biomass extraction liquor, comprises: (a) providing or obtaining a biomass extraction liquor containing water, hemicellulose oligomers, and dissolved or suspended lignin; (b) optionally introducing an acid catalyst to the biomass extraction liquor; (c) for a first period of time, feeding the biomass extraction liquor through a first hydrolysis reactor under hydrolysis reaction conditions to hydrolyze the hemicellulose oligomers into hemicellulose monomers; (d) at a first switching time, diverting the biomass extraction liquor from the first hydrolysis reactor to a second hydrolysis reactor; (e) for a second period of time, feeding the biomass extraction liquor through the second hydrolysis reactor under the hydrolysis reaction conditions; (f) during the second period of time, cleaning the first hydrolysis reactor by introducing a cleaning agent including steam, alkaline solution and/or organic solvent, where the cleaning agent is to dissolve precipitated lignin formed from the lignin under the hydrolysis reaction conditions; (g) at a second switching time, diverting the biomass extraction liquor from the second hydrolysis reactor back to the first hydrolysis reactor or to a third hydrolysis reactor for hydrolyzing the hemicellulose oligomers into hemicellulose monomers, while concurrently cleaning the second hydrolysis reactor by introducing the cleaning agent to dissolve the precipitated lignin; and (h) repeating steps (c)-(g) many times, thus continuously producing hemicellulose sugars from the biomass extraction liquor. INDEPENDENT CLAIMS are also included for:    (1) a continuous process for producing hemicellulose derivatives from a biomass extraction liquor, comprising: step (a) as above per se; (b1) introducing an acid catalyst to the biomass extraction liquor; (c1) for a first period of time, feeding the biomass extraction liquor through a first reactor under effective reaction conditions to hydrolyze the hemicellulose oligomers into hemicellulose monomers and to convert the hemicellulose monomers into at least one hemicellulose derivatives; steps (d)-(g) as above per se; and repeating steps (c1) and (d)-(g) many times, thus continuously producing hemicellulose derivatives from the biomass extraction liquor; and    (2) a system configured for continuously producing hemicellulose sugars and/or hemicellulose derivatives from a biomass extraction liquor, comprising at least a first hydrolysis reactor and a second hydrolysis reactor, where each of the hydrolysis reactors is in switchable communication with an operating feed stream of a biomass extraction liquor containing water, hemicellulose oligomers, and dissolved or suspended lignin, an optional operating acid catalyst feed stream, and a cleaning feed stream of the cleaning agent.