Pressure Diffuser

A brother to the atmospheric diffuser, the pressure diffuser is a displacement-type

washer which is fully enclosed and operates under pressure. It is therefore wellsuited

to operate under temperatures above 100 °C, and it can also serve as a heat

exchanger. 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.

In the Kvaerner Pulping design, pulp enters the top of the pressure diffuser at

about 10% consistency and is distributed into the circular space between the pressurized

shell and the screen hanging in the center of the machine. The screen

travels downward in a continuous, slow movement and helps the pulp on its way

to the outlet at the bottom. There, a scraper ensures the uniform discharge of the

pulp, still at about 10% consistency.

As the pulp passes down it is continuously washed by wash liquor introduced

through a series of liquor nozzles arranged at numerous levels all around the perimeter

of the shell. Baffle plates inside the shell ensure uniform circular distribution

of wash liquor at each level. The wash liquor displaces the liquor in the pulp

mat, and the displaced liquor is extracted through the screen holes, collected in

the center of the screen unit, and finally discharged from the top of the machine.

When the screen has finished its slow downward movement, it is brought back

to its upper position by a rapid upstroke. During the upstroke, the slightly conical

design of the screen generates a small backflush of extracted liquor, which cleans

the screen holes. The slow downward movement then starts again, and this cycle

is repeated.

Featuring E factors of 4.0–5.5, the pressure diffuser has the highest per-stage

washing efficiency of any washer available.

Since the pressure diffuser is hydraulically full and the washing processes occur

in the absence of air, there is no risk of foaming. From an environmental perspective,

the totally enclosed design eliminates any source of malodorous gas emissions.

554 5 Pulp Washing

Fig. 5.31 Kvaerner Pulping pressure diffuser [38].

The design and operation of the Andritz pressure diffuser are similar to those

of the Kvaerner Pulping unit in many respects. However, the fact that the pulp

flows upwards in the Andritz pressure diffuser is advantageous if the pulp feed

contains gas, as the gas leaves the diffuser with the pulp at the top without impairing

the operation.

5.6 Washing Equipment 555

5.6.5

Дата добавления: 2015-10-21