INDEX Aeration

Some fermentation broths are non-Newtonian due to the presence of microbial mycelia or fermentation products, such as polysaccharides. In some cases, a small amount of water-soluble polymer may be added to the broth to reduce stirrer power requirements, or to protect the microbes against excessive shear forces. These additives may develop non-Newtonian viscosity or even viscoelasticity of the broth, which in turn will affect the aeration characteristics of the fermentor. Viscoelastic liquids exhibit elasticity superimposed on viscosity. The elastic constant, an index of elasticity, is defined as the ratio of stress (Pa) to strain (—), while viscosity is shear stress divided by shear rate (Equation 2.4). The relaxation time (s) is viscosity (Pa s) divided by the elastic constant (Pa). 

Turbidity Many studies have been focused on developing a sterUizablc probe to measure the turbidity of the culture broth, because the turbidity can be an index of cell concentration, which is one of the most important parameters in bioprocess operations. Sterihzable laser or optical turbidity probes are now available and are being used successfully [1]. However, the accuracy ofthe measurements is insufficient for industrial applications, because the outputs from the probes are affected by some factors including the absorbance of the broth and the bubbles caused by aeration. 

Conventional stirred-tank polymeric reactors normally use turbine, propeller, blade, or anchor stirrers. Power consumption for a power-law fluid in such reactors can be expressed in a dimensionless form, Ne = Reynolds number based on the consistency coefficient for the power-law fluid. Various forms for the function f(m) in terms of the power-law index have been proposed. Unlike that for Newtonian fluid, the shear rate in the case of power-law fluid depends on the ratio dT/dt and the stirrer speed N. For anchor stirrers, the functionality g developed by Beckner and Smith (1962) is recommended. For aerated non-Newtonian fluids, the study of Hocker and Langer (1962) for turbine stirrers is recommended. For viscoelastic fluids, the works of Reher (1969) and Schummer (1970) should be useful. The mixing time for power-law fluids can also be correlated by the dimensionless relation NO = /(Reeff = Ndfpjpti ) (Tebel et aL 1986). 

Domestic or industrial hot-water heaters of galvanized steel through which hot aerated water passes continuously are not protected reliably in all types of water by nontoxic chemical additions such as silicates or polyphosphates. Adjustment of the saturation index to a more positive value, as discussed earlier, is sometimes helpful. Often, cathodic protection or use of nonferrous metals, such as copper or 70% Ni-Cu (Monel), is the best or only practical measure. 

Both the protective-film-rupture theory and the high-index-plane theory may be valid for one and the same system, depending on conditions. This has been demonstrated by the behavior of an iron wire strained in an aerated nitrate solution.When straining was done in the active region of potentials, the observed increase of anodic current was of the magnitude expected on the basis of the high-index-plane theory (within a power of 10). However, in the passive region, a 1500-2000-fold increase was observed which can be explained only in terms of the film-rupture mechanism. 

The degree of particle tensile strength was found by Yokoyama et al. (1982) to affect the floodability of a powder. The floodability index, using the various test methods for characterising bulk solids by the Carr s indices (ASTM D 6393-99 1999) was used to evaluate an intermediate condition between an aerated powder and a packed dense powder (see Section 1.8.1). 

The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of rust water. In aerated hot water systems, sodium silicate protects steel, copper, and brass. However, protection is not always rehable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. SUicates and phosphates do not afford the degree of protection provided by chromates and nitrites however, they are veiy useftd in situations where nontoxic additives are required. 

In aerated hot water systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends on the pH and composition of the water. The best procedure is to adjust the saturation index, as already described to facUitate formation of a protective sihcate-containing film. 

Respiratory index (RI4) Landfill settlements RI4 < 2.5 mg Oj/g DM Significant reduction Laboratory test on waste taken from the aerated landfill... 

The compressibility of resin and other materials is also used to determine factors such as flowability and the proper designing of hoppers, blowers, and vacuum systems. Two formulas used for this are the Hausner ratio and Carr s Compressibility Index [2]. These formulas use both the aerated and tapped bulk densities in order to provide an estimation on the compressibility, which then leads to flowability, but these formulas do not account for the variation that occurs between those two points. They are also used on a smaller scale basis, and do not provide results that eould accurately predict the bulk density or weight in a silo at a given point. 

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