Atmospheric Diffuser

The atmospheric diffuser is a displacement-type washer which is operated under

medium consistency conditions. Its fields of application extend into the highertemperature

regions of brownstock washing and oxygen delignification, as well as

into the bleach plant. The machine can be placed either directly on top of an

upflow bleaching tower or on top of a storage tower for the washed pulp leaving

the diffuser, which is of advantage if space is limited.

Atmospheric diffusers can be built as either one-stage or two-stage units. Double

diffusers have two screen units, one above the other in mirror-image position.

Each screen has its own wash liquor supply and liquor extraction arrangement.

The two-stage units can therefore be used in countercurrent mode with a lower

first stage and an upper second stage, or alternatively just as one stage for higher

capacities [37].

Figure 5.30 shows an atmospheric diffuser with two screen units on top of a

storage tower. Pulp at around 10% consistency enters the distribution cone at the

bottom and approaches the first screen unit. As the pulp moves upward between

552 5 Pulp Washing

the concentric cylindrical screens, nozzles on the rotating nozzle arms distribute

the wash liquor while plowing through the pulp. The wash liquor displaces the

liquor from the pulp radially inward and outward into the screens where it is collected

and guided through the screen arms to the liquor extraction pipes. The

same displacement process is repeated in the upper section of the double diffuser.

When the washed pulp reaches the top, it is shoved to the outer perimeter of the

diffuser by the scraper and finally brought to the pulp outlet by the ring scraper,

still at about 10% consistency.

Fig. 5.30 Kvaerner Pulping double atmospheric diffuser [37].

While the pulp proceeds upwards between the screens, the complete screen

units are also moving upwards at a rate slightly faster than the pulp until they

reach their top position. The liquor extraction is then closed and the screens make

a quick downstroke, clearing the screen holes by the combined effect of move-

5.6 Washing Equipment 553

ment and backflushing. From the lower position the cycle starts again and is

repeated over and over. The elevating mechanism of the reciprocating screen unit

is driven by a number of hydraulic cylinders.

If installed as a brownstock washer after a continuous digester, the digester

pressure can be used to transfer the pulp through the diffuser without additional

pumping. Generally, diffusers are amongst the washers with the lowest electrical

power consumption.

Since the screen units are submerged, there is no contact of wash liquor or filtrate

with the atmosphere, and this reduces the tendency to foaming. The washing

efficiencies, expressed as E factors, are about 3.0–4.5 for the single diffuser

and 6.0–8.,0 for the double diffuser.

Дата добавления: 2015-10-21