Calcium hydroxide pretreatment

The pretreatment agents evaluated were alkaline hydrogen peroxide and lime (calcium hydroxide). Pretreatment time, temperature, and lime loading or hydrogen peroxide... 

Chang et al. [47] used calcium hydroxide as the pretreatment agent to enhance the enzymatic digestibility of switchgrass. The optimum pretreatment condi-... 

The other pretreatment agent considered is lime (calcium hydroxide) [18-27], which is an inexpensive reagent and can be easily recovered as calcium carbonate by neutralization with carbon dioxide. The calcium hydroxide can be subsequently regenerated using established lime kiln technology [26]. 

Blockage of the pores by carbonation will not occm if a significant proportion of the calcium ions fi om the calcium hydroxide leaches into the transported water, because the solubility product for calcium carbonate will not be sufficiently exceeded. Then the calcium ions present in the transported water will not contribute to the formation of interstitial calcium carbonate deposits and, consequently, the transported water gains calcium ions. Carbonation can be induced by bringing C03 ions to the pores by dry pretreatment with gaseous CO2 or a wet pretreatment with water containing a sufficient HCOJ concentration. CO2 and HCOJ are converted to in the pores of the material because of the high pH value... 

Figure 12.6 presents the process flow diagram of the catalytic production of xylitol. Three main sections are (i) pretreatment, (ii) reaction-condensation, and (iii) crystallization. The pretreatment section deals with the pH adjustment and the removal of the impurities contained in the feed stream, soluble and insoluble solids, and remnants of the hydrolysis of biomass. But the main goal is the removal of the alkalis, whose presence catalyzes the conversion of xylose to xylonic acid (Cannizzaro mechanism). The first step of the downstream processes (unit PI) involves a series of treatment by ionic resins, activated carbon column, and chromatographic separations. Then, the pH is adjusted (unit Rl) in a range of 5-6 through the neutralization of acids (mainly acetic acid) by 1M solution of calcium hydroxide (Ca(OH)2) ... 

Magnesium nitrate is prepared by dissolving magnesium oxide, hydroxide, or carbonate in nitric acid, followed by evaporation and crystallization at room temperature. Impurities such as calcium, iron, and aluminum are precipitated by pretreatment of the solution with slight excess of magnesium oxide, followed by filtration. Most magnesium nitrate is manufactured and used on site in other processes. 

Scale prevention methods include operating at low conversion and chemical pretreatment. Acid injection to convert COs to CO2 is commonly used, but cellulosic membranes require operation at pH 4 to 7 to prevent hydrolysis. Sulfuric acid is commonly used at a dosing of 0.24 mg/L while hydrochloric acid is to be avoided to minimize corrosion. Acid addition will precipitate aluminum hydroxide. Water softening upstream of the RO By using lime and sodium zeolites will precipitate calcium and magnesium hydroxides and entrap some silica. Antisealant compounds such as sodium hexametaphosphate, EDTA, and polymers are also commonly added to encapsulate potential precipitants. Oxidant addition precipitates metal oxides for particle removal (converting soluble ferrous Fe ions to insoluble ferric Fe ions). 

Peterson and Scarrah 165) reported the transesterification of rapeseed oil by methanol in the presence of alkaline earth metal oxides and alkali metal carbonates at 333-336 K. They found that although MgO was not active for the transesterification reaction, CaO showed activity, which was enhanced by the addition of MgO. In contrast, Leclercq et al. 166) showed that the methanolysis of rapeseed oil could be carried out with MgO, although its activity depends strongly on the pretreatment temperature of this oxide. Thus, with MgO pre-treated at 823 K and a methanol to oil molar ratio of 75 at methanol reflux, a conversion of 37% with 97% selectivity to methyl esters was achieved after 1 h in a batch reactor. The authors 166) showed that the order of activity was Ba(OH)2 > MgO > NaCsX zeolite >MgAl mixed oxide. With the most active catalyst (Ba(OH)2), 81% oil conversion, with 97% selectivity to methyl esters after 1 h in a batch reactor was achieved. Gryglewicz 167) also showed that the transesterification of rapeseed oil with methanol could be catalyzed effectively by basic alkaline earth metal compounds such as calcium oxide, calcium methoxide, and barium hydroxide. Barium hydroxide was the most active catalyst, giving conversions of 75% after 30 min in a batch reactor. Calcium methoxide showed an intermediate activity, and CaO was the least active catalyst nevertheless, 95% conversion could be achieved after 2.5 h in a batch reactor. MgO and Ca(OH)2 showed no catalytic activity for rapeseed oil methanolysis. However, the transesterification reaction rate could be enhanced by the use of ultrasound as well as by introduction of an appropriate co-solvent such as THF to increase methanol solubility in the phase containing the rapeseed oil. 

Effluent from the clarifier is saturated in calcium carbonate and this would precipitate on the filter media to clog the filter which is the next step of pretreatment. Consequently, the clarifier effluent flows by gravity to the serpentine recarbonator basin where carbon dioxide is added to reduce the pH to between 7.5 and 8.0. The insoluble calcium carbonate and magnesium hydroxide are converted to soluble calcium and magnesium bicarbonate in the recarbonator basin. The 5.62 MGD of effluent from this basin has a suspended solids concentration of about 2 mg/E and a TDSof about 1,100 mg/E. 

Inorganic colloids (hematite, 75 nm) did not cause irreversible flux decline. Pretreatment of the solutions using ferric chloride not only prevented flux decline under criticalfouling conditions (high calcium concentration and IHSS HA), but also influenced rejection. The latter depends on the charge of the ferric hydroxide precipitates. Cation rejection increased when positive ferric hydroxide colloids were deposited on the membrane, which the organic rjection decreased. 

Rock monazite, obtainable from South Africa, contains a few per cent of the mineral apatite (calcium phosphate) together with a little iron phosphate impurity. The apatite in particular does not respond to the sodium hydroxide breakdown procedure to the same extent as thorium and rare earth phosphates. Consequently, the oxide product after washing still contains a quantity of phosphate about equal to the weight of thorium present. This is too high for passing to the final solvent purification stage and it is therefore necessary to eliminate apatite before the alkali breakdown stage. Removal is accomplished by a pretreatment process " in which... 

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