Читайте также:
|
|
The effects of chip size have been evaluated by several research groups. In a study
of kraft pulping of pine chips, Backman [57]stated that above a thickness of 1 mm
the reaction rate is at least partially controlled by the transport steps, while below
this thickness the rate is probably controlled by the rate of the chemical reactions
involved. Hartler and Ostberg identified that the Roe number remained constant
when the thickness was 3 mm or less [58], and Larocque and Maass found essentially
the same effect of chip size [59]. Both the chip size and the uniformity of
chip dimensions are very important criteria for pulp properties. In particular, chip
thickness is a critical dimension which strongly controls the extent of delignification,
the amount of rejects, and even strength development. Chip length and
width have been shown to have a minimal influence on delignification [12], with
4.2 Kraft Pulping Processes 159
thick chips showing very steep delignification gradients. Wood is overdelignified
at the surface, while chip centers are almost undelignified. Gullichsen et al. [8]
reported a kappa number gradient from 14 on the surface to more than 120 in the
chip center for a 8 mm-thick chip of a Scots pine cook with an average screened
kappa number of 23.4 [8]. The same authors also noted that only chips with a
thickness <2 mm can be uniformly delignified under conventional cooking conditions
[8]. Uniform thin chips without knots and reaction wood can be produced
either by efficient screening or by applying an innovative chipping technique.
Another approach involves the application of chip pretreatments and optimization
of impregnation conditions, aimed at improving penetration and efficient diffusion.
Recently, the effects of chip steaming and increased pressure impregnation during
the hot black liquor stage on the kappa number distribution inside handmade
pine chips (Pinus silvestris) were investigated by using reflection Fourier transmission
infra-red (FTIR) spectroscopy (equipped with a microscope which enables a
lateral resolution of an area of approximately 100. 100 lm) [60]. Two scenarios
with different impregnation conditions have been compared with regard to the
uniformity of delignification. Scenario A represents very poor impregnation conditions
involving no pre-steaming and applying only 5 bar overpressure during
the hot black liquor stage. Scenario D, a very efficient mode of impregnation,
combines intensive pre-steaming (30 min, 105 °C) with a high-pressure treatment
(9 bar overpressure) during the hot black liquor stage. The subsequent cooking
steps and conditions were identical for the two scenarios investigated (Superbatch
technology, 17% EA on wood, 40% sulfidity, 170 °C, 890 H-factor in cooking
stage). The handmade chips were cut to a length of 34 mm (longitudinal direction
in wood), a width of 14 mm (tangential direction in wood) and a thickness of
8 mm (radial direction in wood). To analyze the uniformity of delignification,
cooked heartwood chips were cut across the thickness dimensions at distances of
2 mm. Infrared spectra were measured along the chip length and along the chip
width from the middle to the edge, with steps of 2 and 1 mm, respectively. The
kappa number profiles within heartwood chips for cooking scenarios A and D are
illustrated in Fig. 4.19.
In cooking scenario A, the middle part of the 8 mm-thick chip (4 mm deep) was
clearly undercooked. There was a gradual transition along the chip length from
the edge kappa number of 40 to the undercooked regions with kappa number
over 90. In the tangential direction (along the chip width), kappa number rise
was, however, very abrupt close to the chip edge, which confirmed the limited
mass transfer in this direction (not seen).
In scenario D, with the application of pre-steaming and higher pressure profile,
the uniformity of delignification of heartwood chips was significantly improved,
though some gradient was present in the middle (4 mm deep) layer of the chip.
However, the undercooked region was much narrower than in scenario A. These
results confirmed the beneficial effect of reinforced impregnation conditions with
regard to the uniformity of delignification.
160 4 Chemical Pulping Processes
1 4 7 10 13 16
surface, scenario A surface, scenario D
4 mm deep, scenario A 4 mm deep, scenario D
Kappa number
Distance from the chip edge along the length, mm
Fig. 4.19 Delignification profiles within the pine heartwood of handmade chips [60].
An alternative method has recently been introduced to determine the alkali and
lignin concentration profiles in the free and entrapped liquor as a function of chip
thickness [61]. Eucalyptus globulus chips with dimensions of about 30. 30. 1 mm
and 30. 30. 6 mm were used as raw material for the impregnation and cooking
studies. Impregnation trials were carried out at 5 °C and 80 °C using a cooking
liquor with an effective alkali (EA) concentration of 19.3–22.6% on dry wood, a
sulfidity of 25–30%, and an initial liquor:wood ratio of 6:8. Cooking experiments
were conducted at 165 °C using only the 6-mm chips. At the end of each trial the
chips were immediately separated from the remaining free liquor and the excess
liquor at the surface was carefully removed with sorption paper. The chips were
then pressed to 350 bar for 2–3 min to release the entrapped liquor that had been
collected in a previously inertized flask and cooled in a similar ice bath. The free
and entrapped liquors were analyzed for EA and lignin concentrations according
to standard methods [62,63]. The concentration profiles for EA in both free and
entrapped liquors are illustrated for experiments with 1- and 6-mm chips at 80 °C,
and with 6-mm chips at 165 °C (Fig. 4.20). The results confirmed the remarkably
high difference between the entrapped and free liquor concentrations, especially
at the beginning of the reaction. For the thin chips of only 1 mm thickness, the
EA concentrations in both liquors were similar at about 60 min impregnation
time. However, for the 6-mm chips, even after 300 min, there was no equalization
of EA concentrations. Performing these experiments at a cooking temperature of
165 °C yielded a more significant decrease in the concentration of EA in the free
liquor, but this was not followed by a higher increase of EA concentration in the
entrapped liquor. These findings clearly indicate that higher consumption rates of
4.2 Kraft Pulping Processes 161
0 100 200 300
0,0
0,2
0,4
0,6
0,8
1,0
* normalized
EL 6 mm chips, 80 °C FL 1 mm chips, 80 °C
FL 6 mm chips, 80 °C EL 6 mm chips, 165 °C
EL 1 mm chips, 80 °C FL 6 mm chips, 165 °C
effective alkali concentration [g/l]*
Time [min]
Fig. 4.20 Effective alkali concentrations (normalized) in the
free (FL) and entrapped (EL) liquid phases versus time for
1- and 6-mm chips at 80 °C and 165 °C.
alkali occur when chemical reactions are conducted at temperatures above 80 °C.
Moreover, the results reveal that, in such a heterogeneous process, the alkali concentration
profiles in the free and entrapped liquors do not necessarily appear as
image and mirror image.
The consumption of EA does not correspond to the alkali concentration in the
bulk liquor because the alkali inside the chip is not totally consumed by the wood
components. It is clear that the alkali concentration in the entrapped liquor determines
the dissolution of wood components, and this must be considered in the
development of a heterogeneous delignification kinetic model.
The concentration profiles of dissolved kraft lignin in both entrapped and free
liquors depend heavily on the reaction temperature. At 80 °C, the concentration of
dissolved lignin in the bulk liquor remains very low, indicating only a small
degree of delignification at this temperature. Inside the chips, the concentration
rises quickly to a constant low level. When the temperature is increased to 165 °C,
the dissolved lignin concentration in the entrapped liquor reaches a maximum
after about 80 min. Continuing the cooking process leads to a decrease in dissolved
lignin concentration in the enclosed liquor due to a slow-down of the
delignification rate, and this results in an overall enhanced mass transfer of lignin
to the free liquor. The whole mass balance of lignin calculated from lignin concentration
profiles in the entrapped and free liquors and the lignin content in the
wood chips is illustrated in Fig. 4.21.
162 4 Chemical Pulping Processes
0 100 200 300
0,0
0,2
0,4
0,6
0,8
1,0
1,2
* normalized
Dissolved (80 °C) Dissolved (165 °C)
Residual (80 °C) Residual (165 °C)
Total (80 °C) Total (165 °C)
Amount of Lignin [g]*
Time [min]
Fig. 4.21 Mass balance of lignin during impregnation at 80 °C
and kraft cooking at 165 °C using Eucalyptus globulus chips
with a thickness of 6 mm.
The data shown in Fig. 4.21 confirm that at a temperature of 80 °C, which is a
typical temperature for impregnation, only negligible delignification occurs. At
165 °C, the expected pattern of residual lignin as a function of time can be observed.
The total normalized mass of lignin increases up to 10% of its initial value
being attributed to experimental errors as dissolved extractives contribute to lignin
concentration using UV detection at 280 nm.
Дата добавления: 2015-10-21; просмотров: 98 | Нарушение авторских прав
<== предыдущая страница | | | следующая страница ==> |
Parameter unit Axial Radial | | | Numerical Solution of the Diffusion Model |