Holdup requirements

With a depth of 5 ft, total volume of the pond would amount to a 45.5-h holdup, which is more than the 24-h holdup required to maintain a fairly constant discharge temperature throughout the day. 

The aforementioned correlations for the gas holdup require knowledge of N. Hydrodynamics in two- and three-phase reactors has been widely investigated as discussed in Sections 7A.3 and 7A.4, respectively. Various investigators have reported reliable correlations for over a wide range of geometric configurations and operating... 

Vertical vapor bodies may be sized using a value of 0.2 for F in Equation 14.2. Horizontal vapor bodies should be sized with a target value of F = 0.15 based on the surface area at the gas/liquid interface. Recommended vertical flash tank dimensions are shown in Figure 14-1. Liquid holdup requirements may control the size of flash tanks. In such cases, the tank should be oversized with respect to the gas flow in order to acheve practical vessel dimensions. A height-to-diameter ratio of 2 to 5 is usually economical. 

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Product Quality Considerations of product quahty may require low holdup time and low-temperature operation to avoid thermal degradation. The low holdup time eliminates some types of evaporators, and some types are also eliminated because of poor heat-transfer charac teristics at low temperature. Product quality may also dic tate special materials of construction to avoid met hc contamination or a catalytic effect on decomposition of the product. Corrosion may also influence evaporator selection, since the advantages of evaporators having high heat-transfer coefficients are more apparent when expensive materials of construction are indicated. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high hquid and vapor velocities used, the frequent presence of sohds in suspension, and the necessary concentration differences. 

In order to cool to the equilibrium temperature, a pond of infinite size would be required for warm water. An approach of 1.7 to 2.2°C (3 to 4°F) is the lowest practicable in a pond of reasonable size. For a pond having more than a 24-h holdup, the leaving-water temperature will vary from the average by plus or minus 1.1°C (2°F) for a 0.9-m (5-ft) depth and 1.7°C (3°F) variation for a 0.9-m (3-ft) depth. 

A long heating cycle is necessaiy because the size of the solid objects or permissible heating temperature requires a long holdup for internal diffusion of heat or moisture. This case may apply when the cycle will exceed 12 to 24 h. 

High recovery of a volatile component by a batch operation is required. Liquid holdup is much lower in a packed column. 

Sepn. Purif., 3, 19 (1989)] takes holdup into account and applies to random as well as structured packings. It is somewhat cumbersome to use and requires three constants for each packing type and size. Such constants have been evaluated, however, For a number of commonly used packings. A more recent pressure drop and holdup model, suitable for extension to the flood point, has been pubhshed by Rocha et al. [Jnd. Eng. Chem. Research, 35, 1660 (1996)]. This model takes into account variations in surface texturing of the different brands of packing. 

Data are not currently available on the dispersion with the newer fluidfoil impellers, but they are often used in industrial mixer-settler systems to maintain dispersion when additional resonance time holdup is required, after an initial dispersion is made by a radial- or axial-flow turbine. 

The heat of reaction requires cooling water at the rate of 10 to 40 L/(1,000 L holdup)(h). Vessels under about 500 L (17.6 fF) are provided with jackets, larger ones with coils. For a 55,000-L vessel, 50 to 70 m" maybe taken as average. 

This means that parts must be ordered in advance for the turnaround and other work must be planned so that the whole operation may proceed smoothly and without holdups that could have been foreseen. This usually means close collaboration with the manufacturer or consultant and the OEM (or specialty service shop) so that handling facilities, service men, parts, cleaning facilities, inspection facilities, chrome plating and/or metaliz-ing facilities, balancing facilities, and some cases even heat treatment facilities, are available and will be open for production at the proper time required. This is the planning, which must be done in detail before the shutdown with sufficient lead-time available in order to have replacement parts available at the job site. 

The accumulator holdup will often be the limiting item setting time required for samples to be representative of the column operating conditions. One changeout or turnover of the accumulator may be insufficient time, if test conditions are much different from conditions prior to the test. The following example will serve to show this. 

For vaporAiquid separators there is often a liquid residence (holdup) time required for process surge. Tables 1, 2, and 3 give various rules of thumb for approximate work. The vessel design method in this chapter under the Vapor/Liquid Calculation Method heading blends the required liquid surge with the required vapor space to obtain the total separator volume. Finally, a check is made to see if the provided liquid surge allow s time for any entrained water to settle. 

Assign a length-to-diameter ratio of 5, and size a tank to accommodate the required holdup time. 

An LH(CO)A is provided at a low level, actuating a cut-off valve in the closed drain header. Holdup below this LH(CO)A is the closed drainage requirement. 

The holdup between the two LH(CO)As must equal the liquid diversion requirement. 

Lower holdup of liquid Less than other packings, may require special attention to operationtil controls. 

Gas holdup Vm is the volume of carrier gas that passes through the column to elute an unretained substance, such as argon or methane. The time required is tm. 

Clark and Vermeulen (C8) later reported an extensive experimental study of power requirements in agitated gas-liquid systems. They correlated their data in dimensionless form as a function of fractional gas holdup, Weber number, and a geometrical factor. Their correlation is shown in Fig. 5. 

As a first approximation, suppose that the concentration of oxygen in the gas phase changes instantly from 20.9% oxygen to 100% oxygen. Then a j will change instantly from 0.219 to 1.05mol/m, and the gas-phase balance is not required. The parameter k , = 0.1 s was specified in Example 11.1 so the only unknown parameter is the liquid holdup, VijV. A typical value for a mechanically agitated tank is 0.9. The liquid-phase balance becomes... 

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