Liquid-vapor separators

Examples of Vapor-Liquid Separation Calculations Conducted with Subroutine FLASH... 

Kettle-type reboilers, evaporators, etc., are often U-tube exchangers with enlarged shell sec tions for vapor-liquid separation. The U-tube bundle replaces the floating-heat bundle of Fig. 11-36. ... 

Vapor-Liquid Separation This design problem may be important for a number of reasons. The most important is usually prevention of entrainment because of value or product lost, pollution, contamination of the condensed vapor, or fouling or corrosion of the surfaces on which the vapor is condensed. Vapor-liquid separation in the vapor head may also oe important when spray forms deposits on the w ls, when vortices increase head requirements of circulating pumps, and when shoiT circuiting allows vapor or unflashed liquid to be carried back to the circulating pump ana heating element. 

In a submerged-tube FC evaporator, all heat is imparted as sensible heat, resulting in a temperature rise of the circulating hquor that reduces the overall temperature difference available for heat transfer. Temperature rise, tube proportions, tube velocity, and head requirements on the circulating pump all influence the selec tion of circulation rate. Head requirements are frequently difficult to estimate since they consist not only of the usual friction, entrance and contraction, and elevation losses when the return to the flash chamber is above the liquid level but also of increased friction losses due to flashing in the return line and vortex losses in the flash chamber. Circulation is sometimes limited by vapor in the pump suction hne. This may be drawn in as a result of inadequate vapor-liquid separation or may come from vortices near the pump suction connection to the body or may be formed in the line itself by short circuiting from heater outlet to pump inlet of liquor that has not flashed completely to equilibrium at the pressure in the vapor head. 

Sometimes, alternative single- or multiple-stage vapor-liquid separation operations, of the types shown in Fig. 13-7, may be more suitable than distillatiou for the specified task. 

Types of Equipment The three most commonly used types of equipment for handling emergency relief device effluents are blowdown drums (also called knockout drums or catch tanks), cyclone vapor-liquid separators, and quench tanks (also called passive scruh-hers). These are described as follows. 

Horizontal Blowdown Drum/Catch Tank This type of drum, shown in Fig. 26-16, combines both the vapor-liquid separation and holdup functions in one vessel. Horizontal drums are commonly used where space is plentiful, such as in petroleum refineries and petrochemical plants. The two-phase mixture usually enters at one end and the vapor exits at the other end. For two-phase streams with very high vapor flow rates, inlets may be provided at each end, with the vapor outlet at the center of the drum, thus minimizing vapor velocities at the inlet and aiding vapor-hquid separation. 

Cyclone Separator with Separate Catch Tank This type of blowdown system, shown in Fig. 26-17 and 26-18, is frequently used in chemical plants where plot pan space is hmited. The cyclone performs the vapor-liquid separation, while the catch tank accumulates the hquid from the cyclone. This arrangement allows location of the cyclone knockout drum close to the reactor so that the length of the relief device discharge hne can be minimized. The cyclone nas internals, vital to its proper operation, which will be discussed in the following sections. 

For vapor/liquid separators, this is usually expressed in terms of maximum velocity which is related to the difference in liquid and vapor densities. The standard equation is... 

Figure 1. Design vapor velocity factor for vertical vapor-liquid separators at 85% of flooding.

For rough sizing check of vapor/liquid separators and accumulators, see the Fluor method in Chapter 8, Separators/Accumulators—Vapor/Liquid calculation method. 

Examples chosen for this category include the operations of vapor/liquid separation, heat transfer, and fluid flow. The Underwood method of estimating the... 

Vapor-liquid separators (drums) are used to separate a liquid from a vapor-liquid stream with a minimum of liquid carryover. The separator size is determined by the vapor velocity which depends on the entrainment method used. The working equation is ... 

The principal components of the cut are butene-1, butene-2, isobutylene and butadiene-1,3. Methyl, ethyl, and vinyl acetylenes, butane and butadiene-1,2 are present in small quantities. Butadiene is recovered from the C4 fraction by extraction with cuprous ammonium acetate (CAA) solution, or by extractive distillation with aqueous acetonitrile (ACN). The former process is a liquid-liquid separation, and the latter a vapor-liquid separation. Both take advantage of differences in structure and reactivity of the various C4 components to bring about the desired separation. 

Since much of the vented material will be liquid, separators such as knockout pots or tangential entry separators can provide disengagement and possible recovery. Figure 5 is a typical vapor-liquid separator design found to be effective for these applications. Inlet design superficial vapor velocity is about 100 ft/sec, with sufficient volume provided to accumulate the entire reactor liquid contents. The lip on the outlet vapor line and the horizontal plate to separate the accumulated liquid are important features to prevent re-entrainment. 

For vapor/liquid separators, there is often a liquid residence (holdup) time required for process surge. Tables 10-1, 10-2, and 10-3 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 allows time for any entrained water to settle. 

Q in Figures 4.6a and 4.6b, at equilibrium, two-liquid phases are formed at Points P and R. The line PR is the tie line. The analysis for vapor-liquid separation in Equations 4.56 to 4.59 also applies to a liquid-liquid separation. Thus, in Figures 4.6a and 4.6b, the relative amounts of the two-liquid phases formed from Point Q at P and R follows the Lever Rule given by Equation 4.65. 

Finally, if vaporization of a liquid is required for applications other than distillation, then the same principles and methods can be applied. The one distinctive difference is that with all designs, with the exception of kettle designs, a vapor-liquid separation device will be required at the vaporizer outlet, as illustrated in Figure 15.15. 

The introduction of vapor-liquid separators (economizers) acts to reduce the power requirements. For example, if a vapor-liquid separator is introduced into the cycle in Figure 24.36a, as in Figure 24.30, vapor from the first (Level 1) throttle is not passed to the low-pressure... 

Scheiman, A.D. Horizontal Vapor Liquid Separators, Hydrocarbon Processing and Petroleum Refiner, May 1964, p. 155. 

Knockout drums are sometimes called catch tanks or blowdown drums. As illustrated in Figure 8-12, this horizontal knockout drum system serves as a vapor-liquid separator as well as... 

Can be combined with secondary vapor-liquid separator such as vane impingement type. 

The diameter must be sufficient to provide good vapor-liquid separation. 

In normal process service, the superficial vapor velocity at the inlet of tangential-entry vapor-liquid separators is limited to about 120 to 150 ft/s. Higher velocities may lead to... 

Physical insight and experience in numerical analysis are important in selecting which variables to tear. For example, Figure 15.10 illustrates an equilibrium vapor-liquid separator for which the combined material and equilibrium equations give the relation... 

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