Hold Up

Because the gas always flows at a velocity greater than that of the liquid, the in situ volumetric fraction of liquid at any point in a pipeline will be greater than the input volume fraction of liquid furthermore it will progressively change along the length o the pipe as a result of expansion of the gas. 

There have been several experimental studies of two-phase flow in which the holdup has been measured, either directly or indirectly. The direct method of measmement involves suddenly isolating a section of the pipe by means of quick-acting valves and then determining the quantity of liquid napped. Such methods are cumbersome and are subject to errors arising from flie fact that the valves cannot operate instantaneously. Typical of the indirect methods is that in which the pipe cross-section is scanned by y-rays and the holdmp is determined from the extent of their attenuation.  

Lockhart and MARXiNELLfl expressed hold-up in terms of a parameter X, characteristic of the relative flowrates of liquid and gas, defined as  

Equation 5,2 is found to hold well for non-Newtonian shear-thinning suspensions as well, provided that the liquid flow is turbulent. However, for laminar flow of the liquid, equation 5.2 considerably overpredicts the liquid hold-up 6. The extent of overprediciion increases as the degree of shear-thinning increases and as the liquid Reynolds number becomes progressively less. A modified parameter X has therefore been defined 

in summary, liquid hold-up can be calculated using equation 5.2 for  

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Pipelines are cleaned and inspected using pigs . Pigs usually have a steel body fitted with rubber cups and brushes or scrapers to remove wax and rust deposits on the pipe wall, as the pig is pumped along the pipe. Sometimes spherical pigs are used for product separation or controlling liquid hold up. In field lines handling untreated crude may have to be insulated to prevent wax formation. 

Hold-up of column. The hold-up of liquid should be reduced to a minimum compatible with scrubbing effectiveness and an adequate column capacity. The ratio of charge of the still to the hold-up of the... 

There is increasing evidence that the ionisation of the organic indicators of the same type, and previously thought to behave similarly, depends to some degree on their specific structures, thereby diminishing the generality of the derived scales of acidity. In the present case, the assumption that nitric acid behaves like organic indicators must be open to doubt. However, the and /fp scales are so different, and the correspondence of the acidity-dependence of nitration with so much better than with Hg, that the effectiveness of the nitronium ion is firmly established. The relationship between rates of nitration and was subsequently shown to hold up to about 82 % sulphuric acid for nitrobenzene, />-chloronitrobenzene, phenyltrimethylammonium ion, and p-tolyltrimethylammonium ion, and for various other compounds. ... 

During the final stage of the distillation, water from the bath was poured along the column in order to minimize the hold-up. Most of the product passed over between... 

Note 2. In order to minimize the hold-up. a partial vacuum (ea. 100 mmHg) may be applied during the last stage of the distillation the fraction obtained in this way can be redistilled at normal pressure in a small distillation apparatus. 

The specific surface, a, is also relatively insensitive to the duid dynamics, especially in low viscosity broths. On the other hand, it is quite sensitive to the composition of the duid, especially to the presence of substances which inhibit coalescence. In the presence of coalescence inhibitors, the Sauter mean bubble size, is significantly smaller (24), and, especially in stirred bioreactors, bubbles very easily circulate with the broth. This leads to a large hold-up, ie, increased volume fraction of gas phase, 8. Sp, and a are all related... 

Each equation is independent of impeller type. As pointed out eadier, the absolute kpi values vary considerably from Hquid to Hquid. However, similar relationships have been found for other fluids, including fermentation broths, and also for hold-up, 8. Therefore, loss of power reduces the abiHty of the Rushton turbines to transfer oxygen from the air to the broth. 

Vinyl, alkyd, and styrene—butadiene latexes are used as film formers in most architectural coatings. Because alkyd resins (qv) require organic solvents, their use has decreased substantially for architectural coatings, but is stiU holding up in industrial appHcations, where their greater durability justifies the added expense (see Latex technology). 

Equipment. The standard equipment in magnesium foundries consists of large stationary brick-lined reverbatory furnaces which can hold up to 10—15 t of molten magnesium. Reverbatory furnaces are furnaces in which heat is suppHed by burning a fuel in a space between the reactants and the... 

The analytical research and development (R D) unit is often responsible for the preparation and vahdation of test methods. The R D lab is not faced with the same pressures for rapid analysis as the QC unit, where pending results often hold up production. In addition, R D often assigns personnel to specific instmments or techniques, whereas QC generally requires technicians to perform varied analyses. This leads to an expertise on the part of analytical chemists and technicians which is difficult to duphcate in QC. Therefore the R D test method should be mgged enough to withstand the different environment of the QC lab and stiU provide vahd results. 

In a modification the conversion process, the jarosite residue is hydrothermaHy decomposed to hematite by autoclaving at 220—250°C. This solubilizes zinc and other metal values and the hematite has a potential for iron recovery. Hematite stockpiles are less of a problem than jarosite because hematite is denser and holds up less of the soluble metals. 

Foamed or Cellular CeUular plastics such as polyurethane and polystyrene do not hold up or perform well in the ciyogenic temperature range because of permeation of the cell strnc tnre by water vapor, which in turn increases the heat-transfer rate. CeUular glass holds up better and is less permeable. 

The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated. 

The final acceptance test would probably be made by measuring two of the three quantities and using the appropriate equation. For a completely valid efficiency test the effect of hold-up (D) and loss ( ) must also be taken into account. 

Lockhart and Martinelli used pipes of one inch or less in diameter in their test work, achieving an accuracy of about -l-/-50%. Predictions are on the high side for certain two-phase flow regimes and low for others. The same -l-/-50% accuracy will hold up to about four inches in diameter. Other investigators have studied pipes to ten inches in diameter and specific systems however, no better, generalized correlation has been found.The way... 

If oil buffered seals are used on the compressors, the seal leakage toward the process side of the compressor must be carefully measured, as it is (and should be) a small value. While five gallons per day doesn t sound too small, in a four-hour run, this is less than two pints, making the hold-up time at the inner seal chamber and in the lines to the drain pots a significant value. This makes exact measurement quite difficult. 

For electrieal applieations, dielectrie strength of the tape is critieal beeause it provides a good indication of how the tape will hold up against strong eleetrieal... 

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