Steaming

As mentioned above, rapid and uniform impregnation prior to any chemical pulping

treatment is a necessary prerequisite. The importance of a thorough penetration

of liquor into wood was emphasized as early as 1922 by Miller and Swanson

[16]. It is generally agreed that an efficient air removal is the key for a successful

penetration. In the course of the technical development of the impregnation step,

three different pretreatment techniques have been investigated, though only one

has gained practical acceptance.

The first pretreatment is evacuation of the digester, but this has proved to be not

practical with commercial digesters. Numerous investigations at the laboratory

scale have shown, however, that evacuation is the most efficient process for

removing air from inside the chips.

An improved penetration can also be achieved by the replacement of air by condensable

gases. The air inside the cavities of the chips can be replaced by repeated

introduction of a gas which is soluble in the cooking liquor at elevated pressure,

alternating with relief. A pretreatment procedure, where air is replaced by gaseous

SO2 prior to introducing the cooking liquor, was developed by Montigny and

Maass [17]. Although the penetration results were comparable to those of pre-evacuation,

this process was not put into practice, mainly because it is more or less

limited to the sulfite process and the composition of the cooking liquor cannot be

determined precisely since the amount of SO2 inside the wood chips is essentially

unknown.

Finally, the steaming process at atmospheric or superatmospheric pressure to

remove interstitial air from the wood has achieved broad practical application.

Steaming is intended to ensure uniformity of moisture distribution in the chips

130 4 Chemical Pulping Processes

0 30 60 90 500 1000

no pretreatment 0.7 bar steam

2.4 bar steam 3.5 bar steam

Degree of Penetration [%]

Time of penetration [min]

Fig. 4.6 Effect of steam pressure during the pre-steaming of

chips from black spruce (Picea marina) on the rate of penetration

using distilled water at 60 °C; the steaming time was kept

constant at 10 min (according to Woods [18]).

and purge air from the inside of the chips. In numerous studies it has been

proved that pre-steaming of chips accelerates the rate of subsequent penetration

considerably. The degree of penetration is determined by both steam pressure and

steaming time. The effect of pre-steaming on the rate of penetration using distilled

water at 60 °C was studied by Woods for several steam pressures [18]. The

results of the trials with a steaming time of 10 min are shown in Fig. 4.6.

Steaming time can be reduced without impairing the efficiency of impregnation

by increasing the steam pressure. The higher degree of penetration of presteamed

chips can be traced back by a more extensive filling of the cooking liquor

(15–30% more than for non-pre-steamed chips) due to the additional space of the

air removed, and also to an enhanced outward diffusion of the air. Moreover, it

was shown that the cell wall becomes more accessible to water after pre-steaming.

Purging according to the Va-Purge process, which includes pre-steaming with

intermittent relief, has an even more pronounced effect on fiber structure [19].

When applied at equal pressures and total time, Va-purge steaming is slightly

more efficient as compared to continuous steaming only if longer intermittent

steaming phases are allowed.

Today, the pre-steaming of chips forms an integral part of the continuous kraft

cooking process [20]. Modern batch cooking technology, however, only uses slight

pre-steaming of the chips during the chip packing stage. The main objectives of

pre-steaming are to preheat the wood chips from ambient temperature to 100–

120 °C and to remove noncondensable gases (e.g., air) which are present in the

4.2 Kraft Pulping Processes 131

void volume of the chips. The heating of the wood chips is influenced by the wood

properties, the chip dimensions, the initial chip temperature, the steam temperature

and pressure, and the venting of the steam. Heating of the chip with saturated

steam takes place from all three wood dimensions. Most of the heat released

from the condensation of steam is consumed in the heating process. According to

the simulations, the heating efficiency for sapwood and heartwood pine chips is

almost the same [20]. From the results obtained, it can be concluded that the heating

of wood chips of typical dimensions with saturated steam is a rapid process,

and lasts for only a few minutes. Under industrial conditions, however, a longer

heating period may be expected because of the limited heat transfer within the

chip layer. When heating chips with saturated steam, only a small part of the

water present within the wood voids will vaporize at the moment when the whole

chip has been heated to the ultimate temperature. It has been shown that presteaming

with saturated steam results only in a slight increase in the moisture

content of both heartwood and sapwood. If the moisture content exceeds the fiber

saturation point, then pre-steaming can even lead to a reduction of the water content

[21].

The extent of air removal as a function of steaming time at atmospheric pressure

can be described by a simple exponential dependency according to Eq. (37):

N _ N 0 _ Exp __ k _ t _ _37_

where N is the amount of entrapped air.

The amount of remaining air as a function of steaming time for heartwood and

sapwood from pine is illustrated graphically in Fig. 4.7.

0 20 40 60

Heartwood Sapwood

Remaining air, % of initial

Steaming time [min]

Fig. 4.7 Effect of steaming at 105 °C on air removal from

sapwood and heartwood chips from pine (Pinus silvestris)

(according to [20]).

132 4 Chemical Pulping Processes

In order to achieve a degree of 90% air removal from heartwood, 30 min of

steaming is required, while for sapwood only 5–6 min is required.

By increasing the steaming temperature – for example, from 100 °C to 120 °C –

the efficiency of penetration increases further, most likely due to a better permeability

of the cell wall layers to gases. Hence, a higher temperature can accelerate

the dissolution of wood substances at the pit membranes [22].

In practice, complete removal of air may be difficult to achieve, even by applying

optimal steaming conditions. Some air cannot be removed because the pressure

gradient at the end of pre-steaming is insufficient to overcome the surface tension

forces at the liquid–air interface. Moreover, some air can be trapped within capillaries,

which are sealed by extractives.

Hardwood species such as poplar, elder and beech wood are more easily penetrated

after steaming as compared to spruce [21]. Spruce requires longer steaming

time to achieve acceptable degrees of penetration.

Quite recently, the longitudinal permeability and diffusivity of steam in beech

wood were estimated simultaneously by using a Wicke-Kallenbach-cell [23]. Both

parameters increase slightly with the moisture content of wood. The average values

obtained at ambient temperature (20 °C) were 8.1. 10–6 m2 s–1 for the axial

diffusivity and 2.2. 10–11 m2 for the axial permeability (according to Darcy’s relationship:

V = Kp·DP·g–1·L–1 v = [m·s–1] Kp= [m2] DP = [Nm–2] g = [Ns·m–2]L = [m]).

Дата добавления: 2015-10-21