Entrainment of Air

The air content or, more generally, the presence of any gas in the pulp feed to the

washer negatively affects the washer capacity at only low levels (Fig. 5.19). The

effect is serious, and it is of utmost importance to keep the entrained air levels

low in order to maintain satisfactory washer operation and capacity [26].

20%

40%

60%

80%

100%

0 1 2 3 4 5 6

Relative washer capacity

Air content in pulp feed [vol.-%]

Fig. 5.19 Effect of the air content in the pulp feed on the capacity

of a pressure washer, as calculated from Eq. (15).

534 5 Pulp Washing

The mechanism by which air disturbs the washing process is attributed to small

bubbles which are mechanically trapped in the pulp fiber networks, thereby reducing

the number of free paths through which the liquor may flow [9,27]. Thus,

on the one hand the drainage rate is reduced, while on the other hand more

regions are cut off from liquor displacement, which leads to reduced washing efficiency.

5.4.6

Temperature

As the temperature rises, the increasing rate of diffusion leads by tendency to a

better washing efficiency.

Compared to the other parameters, the influence of temperature on washer capacity

is less pronounced. Higher temperatures bring about a moderate capacity

advantage, as can be expected from improved drainage due to lower liquor viscosity

(Fig. 5.20).

20%

40%

60%

80%

100%

40 50 60 70 80 90

Relative washer capacity

Pulp temperature [.C]

Fig. 5.20 Effect of pulp temperature on the capacity of a pressure

washer, as calculated from Eq. (15).

5.4.7

Equipment-Specific Parameters

In addition to the process washing parameters discussed above, different pieces of

equipment allow the adjustment of individual equipment-specific parameters.

These include the drum rotation speed of rotary washers and presses, the wire

speed of belt washers, the pressure on baffles or press nips, and also adjustable

vacuum or pressure on a particular machine.

5.4 Washing Parameters 535

Figures 5.21 and 5.22 illustrate graphically the influence of two equipment-specific

parameters on the capacity of a rotary drum pressure washer.

20%

40%

60%

80%

100%

1 2 3 4

Relative washer capacity

Drum rotation speed [1/min]

Fig. 5.21 Effect of drum rotation speed on the capacity of a

pressure washer, as calculated from Eq. (15).

20%

40%

60%

80%

100%

0 5 10 15 20

Relative washer capacity

Effective filtration pressure [kPa]

Fig. 5.22 Effect of effective filtration pressure on the capacity

of a pressure washer, as calculated from Eq. (15).

536 5 Pulp Washing

5.5

Washing Efficiency

5.5.1

Дата добавления: 2015-10-21