Bleaching efficiency

In order to reduce bleaching earth consumption, alternative new multistep bleaching processes have been developed. Figure 4.8 shows the flow sheet of a two-stage counter-current bleaching with a prefiltration process. The main function of prefiltration is to remove all solid impurities as well as to adsorb most of the phosphatides and soaps. This increases bleaching efficiency in the second stage [4]. 

Active Carbon Active carbon is rarely used to remove chlorophyll compounds is used very little in canola oil bleaching. It presents greater difficulty in handling and it retains more oil than activated clays do and is much more expensive. In bleaching efficiency tests, active carbon has been shown to be somewhat more efficient than activated clay at high concentrations of chlorophyll compounds in canola oil, but less efficient at very low concentrations. Thus, active carbon is not a suitable adsorbent to achieve the removal of chlorophyll derivatives to the very low concentrations mentioned earlier. 

Figure 16. Correlation between surface area and relative bleaching efficiency (chlorophyll) for Clay A. (This figure is available in full color at http //www.mnv.interscience.wiley.com/biofp.)...

However, as shown by the curves in Figure 17, bleaching efficiency for Clay C does not attain maximum activity levels untU well after the surface area curve has peaked and started to decline. Note also that both clays achieve practically identical maxima in chlorophyll bleaching efficiencies (albeit requiring different acid dosages), but Clay C only achieves a maximum surface area of about 300 m /g, whereas Clay A achieves a maximum surface area of nearly 500 m /g. Clearly, surface area, per se, is not correlated with chlorophyll adsorption capacity. Similar curves (not shown) were obtained for carotene adsorption (12). 

It should also be noted that the presence of acidic hydrogen on the oxygen atom that is linked to the phosphorous atom is detrimental to the bleaching efficiency... 

The results obtained [46] showed that at equal applied chemical charges (1%), Lalancette s reagent and sodium hydrosulfite have the same bleaching efficiencies. 

This study on the bleaching chemistry of amineboranes with lignin model compounds has resulted in a bleaching efficiency ranking for those commercially available amineboranes [75]. The most efficient is tcrt-butylamineborane, closely followed by borane-ammonia. The third most efficient is dimethylamineborane, followed by the tertiary amines (trimethylamineborane and triethylamineborane). 

Bleaching Efficiency (ISO Brightness Gain) on Softwood TMP and pfQ of Amineboranes... 

C Daneault, C Leduc. Bleaching efficiency of formamidine sulfinic acid (FAS) in comparison to hydrosulfite, borohydride, and peroxide in one and two stages. Tappi J 78(7) 153-160, 1995. 

For bulk semiconductors at room temperature, the mechanism for the resonant nonlinearity can be described by the band-filling model [82,87]. This is shown schematically in Figure 16b for a direct gap semiconductor such as CdS. Absorption of photons across the band gap, g, generates electrons and holes which fill up the conduction and valence band, respectively, due to the Pauli exclusion principle. If one takes a snap shot of the absorption spectrum before the electrons and holes can relax, one finds that the effective band gap, , increases (Figure 166), since transitions to the filled states are forbidden. The bleaching efficiency per photon absorbed can be derived as... 

With the basic mechanism understood, the resonant nonlinearity of semiconductor clusters can now be quantitatively analyzed. Since one trapped electron-hole pair can bleach the exciton absorption of the whole cluster, the bleaching efficiency per absorbed photon of a nanocluster is the same as that of a molecule, as described by Eq. (20). For a given rp and r, the resonant third-order optical nonlinearity of a nanocluster is simply determined by the (a - ax) term. 

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