Conventionally centrifuges are not the preferred

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Conventionally, centrifuges are not the preferred units for harvesting biomass. This is due mainly to the higher capital, operating and maintenance cost of centrifuges in comparison to filtration operations. The product of value in this case is also the filtrate and not the biomass. The operating conditions in this process are described in the table below. Table 2.15. Operating Condition of Biomass Removal Using Rotary Vacuum Filter Parameter Value Component Removed Aspergillus niger Percent Removed 100 % Cake Porosity 0.4 v/v Cake Wash Stream Water Filtration Time 60 minutes Figure 2.9. Rotary Vacuum Filter Source: Alibaba.com (Lianyungang Boyun Machinery Co., Ltd.) 2.3.6 Product Precipitation Precipitation is a procedure where the addition of an ionic solution to an ionic fermentation broth, forming insoluble particles where the desired product is
usually contaied in those particles. Recovery of citric acid from the fermented liquor directly by crystallization is not possible because it contains many unwanted materials which come from the raw material or from the autolysis of the microbial cells. The production of the above carboxylic acids leads to various unwanted byproducts, especially when produced by means of fermentation. Fermentation processes wherein carboxylic acids are excreted by the microorganisms will result in a decrease in the pH. Since such a decrease in pH can damage the micro-organism's metabolic process, it is common practice to add a 10 base in the fermentation media in order to neutralize the pH. As a result, carboxylic acid produced in the fermentation media is typically present in the form of a carboxylic acid salt. For years, the precipitation method has been employed for the recovery of citric acid.The standard method of citric acid recovery has involved precipitating the insoluble tricalcium citrate by the addition of an equivalent amount of lime to the citric acid solution. Successful operation of the precipitation depends on citric acid concentration, temperature, pH and rate of lime addition. The residual acids are generally neutralized through the addition of calcium hydroxide, sodium hydroxide, or lime and precipitated as gypsum or salts such as calcium sulfide (CaSO 4 ). Lime is by far the most economically favourable alkaline reagent to use for acid neutralization. Lime is significantly cheaper than caustic (NaOH), but is much more difficult to handle . As with magnesium hydroxide, lime is not very soluble in water. Although the reaction times of lime are substantially less than magnesium hydroxide, lime is difficult to handle because it is handled as a slurry. Ca(OH) 2 is divalent, yielding two moles of OH for every one mole of Ca(OH) 2 . When compared to caustic (NaOH), which is monovalent, twice the neutralizing power is available for a given molar volume of lime, thus contributing to the economy of lime. Lime is a slurry that will rapidly separate from solution and settle creating a sludge mass that may be very difficult to suspend. The storage tank must be constantly

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