NOVELTY - Process (P1) to wash spent ion exchange bed (12) in purification of biomass-derived pyrolysis oil (16), involves: (A) providing ion-depleted pyrolysis oil stream (18) of original oxygen content; (B) partially hydrotreating ion-depleted pyrolysis oil stream to reduce oxygen content and form partially hydrotreated pyrolysis oil stream (20) with less residual oxygen content; (C) passing at least portion of partially hydrotreated pyrolysis oil stream through spent ion exchange bed; and (D) passing water (50) through spent ion exchange bed after passing partially hydrotreated pyrolysis oil.    USE - For washing spent ion exchange bed employed in purification of biomass-derived pyrolysis oil; and for treating biomass-derived pyrolysis oil comprising metal ion and having original oxygen content (all claimed).    ADVANTAGE - The process for washing spent ion exchange bed in purification of biomass-derived pyrolysis oil enables alternative option for flushing biomass-derived pyrolysis oil from ion exchange beds that will not downgrade fuel value of biomass-derived pyrolysis oil, and still enables effective regeneration of ion exchange bed with water through use of appropriate flushing stream that is sufficiently miscible with water. In process partially hydrotreated pyrolysis oil stream is passed through spent ion exchange bed, and is used to flush biomass-derived pyrolysis oil from spent ion exchange bed. Because partially hydrotreated pyrolysis oil is only partially hydrotreated, heteroatoms including oxygen remain in partially hydrotreated pyrolysis oil, thus enable partially hydrotreated pyrolysis oil stream to be sufficiently miscible with both biomass-derived pyrolysis oil to be flushed from spent ion exchange bed and with water. The partially hydrotreated pyrolysis oil may have solubility in biomass-derived pyrolysis oil of greater than or equal to 10 g per 100 g of biomass-derived pyrolysis oil. Additionally, partially hydrotreated pyrolysis oil may have solubility in water of greater than or equal to 5 g per 100 g of water. Thus partially hydrotreated pyrolysis oil stream is effective for flushing biomass-derived pyrolysis oil from ion exchange bed in anticipation of washing with water, which would otherwise be difficult due to immiscibility between biomass-derived pyrolysis oil and water. The partially hydrotreated pyrolysis oil stream may be readily available from downstream processing of biomass-derived pyrolysis oil, thus avoids need to supply external flushing stream. Further, unlike flushing with alcohols such as ethanol, the process will not downgrade fuel value of biomass-derived pyrolysis oil upon mixing of partially hydrotreated pyrolysis oil stream (after flushing ion exchange bed) with other biomass-derived pyrolysis oil that is passed through ion exchange bed under normal course of operation.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) process (P2) for treating biomass-derived pyrolysis oil comprising a metal ion component and having an original oxygen content, involving: pyrolyzing a biomass feed in a pyrolysis reactor (58) to form a biomass-derived pyrolysis vapor stream (57); condensing at least a portion of the biomass-derived pyrolysis vapor stream in a condensing system to form a biomass-derived pyrolysis oil stream; passing the biomass-derived pyrolysis oil stream through an ion exchange bed to form an ion-depleted pyrolysis oil stream; partially hydrotreating the ion-depleted pyrolysis oil stream in a hydrotreating device (14) to reduce its oxygen content to produce a partially hydrotreated pyrolysis oil stream having a residual oxygen content that is less than the original oxygen content; passing at least a portion of the partially hydrotreated pyrolysis oil stream through the spent ion exchange bed; and passing water through the spent ion exchange bed after passing at least the portion of the partially hydrotreated pyrolysis oil stream through it; and    (2) an apparatus (10) for washing a spent ion exchange bed employed in purification of biomass-derived pyrolysis oil, comprising: an ion exchange bed for receiving the biomass-derived pyrolysis oil and for forming an ion-depleted pyrolysis oil stream; and a hydrotreating device for receiving the ion-depleted pyrolysis oil stream and for forming a partially hydrotreated pyrolysis oil stream; where the hydrotreating device is in fluid communication with the ion exchange bed through the partially hydrotreated pyrolysis oil stream for passing at least a portion of the partially hydrotreated pyrolysis oil stream through the ion exchange bed.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of apparatus and process for treating biomass-derived pyrolysis oil.    Apparatus (10)    Ion exchange bed (12)    Hydrotreating device (14)    Biomass-derived pyrolysis oil stream (16)    Ion-depleted pyrolysis oil stream (18)    Partially hydrotreated pyrolysis oil stream (20)    Hydrotreating reactor (22)    Water (50)    Biomass feed (56)    Pyrolysis vapor stream (57)    Pyrolysis reactor (58)