pressure in the mixer for transport of the pulp to or to and through alkaline reactor 70 fig4 illustrates yet another embodiment of the invention which provides relatively fine pressure tuning capability for the bleaching system by incorporating gas separator 200 the quantity of gas purged can be more accurately controlled the prior art system of fig1 has gas separation in the ozone pulp contactor 40 as a consequence of its operating characteristics and substantially all of the residual gas is removed the embodiment of fig3 takes advantage of the gas separation which tends to occur naturally in a gas liquid system this allows a portion of the gas pressure to be purged as already described but such purging is limited so that a desirable quantity of gas carries forward with the pulp to the subsequent bleaching stage in most cases even though fig1 4 show either upflow or downflow through the vessels flow may be in either direction according to conduit arrangements which are determined by desired operating conditions clearly downflow of the pulp is aided by gravity while upflow requires a driving pressure to overcome gravity accordingly the pressure regulation of the present invention provides a degree of versatility which is not normally available without the use of pumps it is also clear that due to the thermal balance in the system it may be desirable to have an additional mixer to heat the pulp usually using steam in the scheme of fig4 a pressure reduction device 190 is interposed between ozone pulp contactor 140 and gas separator 200 the pulp from the gas separator flows into mixer 160 in which the temperature is increased by heating and alkaline chemicals are blended with the.