Base Case Study

Mixer performance

According to Bennington, the specific power dissipation, e, for a high-shear mixer

lies in the range between 106 and 107 Wm–3. These two values are used to calculate

the kLa in the mixer, using Eq. (53). Considering the conditions shown in Tab.

7.20, the corresponding kLa values calculate to 0.0169 and 0.169 s–1, respectively,

with an X g value of 0.171 at the entrance of the tower. Dissolved oxygen concentration

values of 5.21.10–5 mol L–1 and 5.013.10–4 mol L–1 are achieved in the highshear

mixer, which constitute only 0.96% and 9.3% of the saturated oxygen concentration

at the base conditions, respectively. These results show that the efficiency

of the mixer in terms of oxygen dissolution is rather limited. The development

of the degree of dissolved oxygen relative to the saturated oxygen concentration

and the resulting course of kappa number degradation are calculated by taking

these two mixer performances into account and assuming kLa values in the

tower to be in the range between 0.002 and 0.01s –1, as determined by Rewatkar

and Bennington [28]. The results, which are summarized in Fig. 7.36, clearly

reveal that the efficiency of the high-shear mixer, expressed as specific power dissipation,

e, has no overall influence on the development of the dissolved oxygen

concentration throughout the retention tower, assuming that a constant kLa in the

tower not related to the kLa in the mixer.

0 20 40 60

0 1 2

Kappa number

ratio dissolved to saturated

oxygen concentration [%]

Time [min]

k

L

a (M) k

L

a (R)

0.0169 0.002

0.0169 0.01

0.1690 0.01

dissolved oxygen

saturation, %

Time [min]

Kappa number k

L

a (M) k

L

a (R)

0.0169 0.002

0.0169 0.01

0.169 0.01

0.169 infinite

experimental data Tormund&Lindstrom (2000)

Fig. 7.36 Development of the degree of dissolved oxygen relative

to the saturated oxygen concentration and the resulting

course of kappa number degradation as a function of kLa in

the mixer (M) and the reactor (R), according to the slightly

modified model from van Heiningen et al. [27]. The calculated

kappa numbers are compared to those obtained from laboratory

experiments published in the KAM 100 report [37].

The higher mixing intensity yields a noticeable increase in the dissolved oxygen

concentration only during the first 1–2 min after mixing. The assumption of a

constant kLa value in the tower independently from the kLa value in the mixer suggests

that mixing intensity has no influence on the efficiency of oxygen delignification.

The experimental results, already cited, are however in distinct contrast to

this conclusion. The (chosen) mass transfer rate in the tower, kLa (R), has a significant

influence on the extent of delignification, as depicted in Fig. 7.36 and

Tab. 7.21.

Tab. 7.21 Degree of delignification as a function of kLa (R).

Parameter kLa (R) [s–1] Lab

Дата добавления: 2015-10-21