to denote cooking fluid that has been drawn from the digester 2 after a bulk delignification that is equivalent to at least 40 of the total bulk delignification has taken place or after at least 50 of the total reduction in kappa value has taken place however the withdrawal must take place after a minimum of 30 minutes of cooking in order for the fluid to be characterised as black liquor one skilled in the arts will realise that the location of the withdrawal will vary depending on the particular method of cooking and the cooking conditions that are associated with the method and can thus be constituted by a withdrawal at the beginning the centre or the end of the digester in a concurrent flow zone or a countercurrent flow zone or as a withdrawal between an upper concurrent flow zone and a subsequent countercurrent flow zone it is also possible to use more than one withdrawal the transfer system 4 comprises when considered in the direction of flow of the chips a final concurrent flow mixing zone z 2 in the impregnation vessel 1 with a retention time t 2 for the chips in this mixing zone that constitutes a maximum of 25 of the retention time timp of the chips in the impregnation vessel such that t 2 lt 0 25 t imp a transfer line 6 between the outlet 5 of the impregnation vessel and the inlet 3 of the digester possibly also comprising a high pressure feeder 8 see fig2 at a location after the outlet 5.