Vapor-liquid separators Entrainment

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. 

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 ... 

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. 

The majority of the process vessels you see in your plant are gravity, vapor-liquid separators. Their main purpose is to settle out droplets of entrained liquid from the upflowing gas. Factors that affect the settling rate of these droplets are... 

The heating surface usually determines the evaporator cost and the vapor head the space requirements. The vapor—liquid separator must have enough horizontal plan area to allow the bulk of the initial entrainment to settle back against the rising flow of vapor and enough height to smooth out variations in vapor velocity and to prevent splashing directly into the vapor outlet. Separators are usually sized on the basis of the Souders-Brown expression ... 

The absence of an interphase compositional differential makes the separation of azeotropes into their constituent components impossible by conventional vapor-liquid separation processes. The azeotropic pattern may be altered by adding an external component—an entrainer—that breaks the original azeotrope while forming new ones with the feed components. The process may be designed so that the... 

Example 4 illustrates that the practical application of horizontal vapor-liquid separators is limited to the complete removal of liquid entrainment in the 50 to 100 micron range. For critical circuits where only limited residual entrainment can be tolerated, it is best to use these drums as a precleaner. Such a precleaner would then be followed by a more efficient cleaner (for instance, a vertical drum with a mesh pad). 

Remember that a vapor-liquid separator (a drum where entrainment is generated) should have a lower than a knockout drum (a drum where residual entrainment from an external source is further reduced). 

Vapor/Liquid Separator Sizing Method Uquid Entrainment Method... 

Separator Efficiencies If vapor-liquid separation is poor, the product will be carried over into the condensate of the following effect. If solids are carried up into the wire mesh of demister-type separators, a high pressure drop and loss in At result. Recovered product yield and the capacity are then reduced, and the pollution load increases. The reduction in yield wastes all the energy previously expended to produce the lost product and increases the cost of raw material. If scaling materials are entrained, fouling is accelerated. 

Entrainment, 6-7, 314, 407-411, 414 distillation trays, 6-7 vapor-liquid separation, 407-411 demisters, 409-411 centrifugal separators, 411... 

Flooding Massive entrainment of liquid caused by high-level or high velocity of the up-flowing vapor. A vapor-liquid separator (KO or BCnock Out drum) that floods can wreck a downstream compressor in a few seconds. 

For a two-phase vapor-liquid separator, both vertical and horizontal separators are used, and the selection should be made on a case-by-case basis. In a horizontal separator, with an increase in liquid level, the area of the vapor space is reduced and the possibility of liquid entrainment increases. On the other hand, the vapor-flow area remains constant in the vertical separator, and liquid entrainment is not an issue at high liquid level. Vertical separators also have the advantage of lower space requirement and easy-to-install control systems. 

The heated oil is flashed into the VPS flash zone where vapor and liquid separate. Split between distillate and bottoms depends on flash zone temperature and pressure. Separation of vapor and liquid in the flash zone is incomplete, since some lower boiliug sidestream components are present in the liquid while bottoms components are entrained in the vapor. The liquid from the flash zone is steam stripped in the bottom section of the VPS to remove the lower boiling components. 

So far, the separation of azeotropic systems has been restricted to the use of pressure shift and the use of entrainers. The third method is to use a membrane to alter the vapor-liquid equilibrium behavior. Pervaporation differs from other membrane processes in that the phase-state on one side of the membrane is different from the other side. The feed to the membrane is a liquid mixture at a high-enough pressure to maintain it in the liquid phase. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, maintaining it in the vapor phase. Dense membranes are used for pervaporation, and selectivity results from chemical affinity (see Chapter 10). Most pervaporation membranes in commercial use are hydrophyllic19. This means that they preferentially allow... 

Vapor-Liquid Gravity Separator Design Fundamentals The critical factors in the performance of a horizontal separator are the vapor residence time and the settling rate of the liquid droplets. However, two other factors enter into the design—the vapor velocity must be limited to avoid liquid entrainment, and there must be sufficient freeboard within the vessel to allow for a feed distributor. For vertical separators, the design is based on a vapor velocity that must be less than the settling velocity of the smallest droplet that is to be collected, with due allowance for turbulence and maldistribution of the feed. The vapor residence time is a function of the vapor flow rate (mass), vapor density, and volume of vapor space in the separator, based on the following ... 

Determine the effects of inadequate separation of liquid and vapor (e.g., entrainment) on the efficiency. The effects of entrainment on efficiency can be quite drastic, especially in vacuum columns, as the vapor rate in the column approaches flooding velocities. The corrected-for-entrainment efficiency can be calculated as follows ... 

---