Vapor-liquid separators Flashing

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

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. 

Figure 2.1 presents the simplistic basis upon which all separations are commonly made in our industry. Even membrane separations depend to a large degree upon the vapor pressure and temperature effects shown. A typical temperature dashed line shows how the temperature variance effects a vapor-liquid separation. Notice also the variance for pressure and enthalpy. Inside the phase envelope, the temperature and pressure remain constant while the enthalpy varies. This constant T and P occur in what is called the flash zone. 

Distillation with vapor product. When a partial condenser is used, the flash drum plays the role of a vapor/liquid separator. In the setup known as a stabilizer there is only vapor distillate, while the liquid is returned as reflux. The column has a pasteurization section when a gaseous stream leaves at the top, while the... 

As stated by Holmes and Chen [12], the reasons for using gas-liquid or vapor-liquid separators are to recover valuable products, improve product purity, reduce emissions, and protect downstream equipment. Gas-liquid separators are used after flashing a hot liquid across a valve. In this case the separator is called a flash drum. 

The above is a variation on the single-stage flash calculation for a vapor-liquid separation. 

The normal practice in the design of forced-convection reboilers is to calculate the heat transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporization in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapor-liquid separation vessel. 

This chapter looks first at equations governing an isothermal flash, and then shows how you can predict the thermodynamic quantities you need to solve the isothermal flash problem. The problems are all sets of algebraic equations, and you can solve these problems using Excel and MATLAB . The chapter then addresses more complicated vapor-liquid separations, but now using Aspen Plus because of its large database. 

Nomographs can be used to quickly size vertical knockout drums, oil separator drums, compressor dry drums, flash drums or any other vertical vapor-liquid separator. 

The common types of flashing feed and vapor distributors are the baffle type (Fig. 3.9a), the vapor-liquid separator type (Fig. 3.96), the gallery type (Fig. 3.9c), and the tangential entrance tjrpe (Fig. 2.2j). Some of the vapor distributors discussed in Sec. 3.12 are also sometimes used for flashing feeds, especially when liquid distribution to the section below is not critical (e.g., when it contains trays). 

Figure 3.9 Flashing feed distributors, (a) Baffle-type distributor, (b) vapor-liquid separator-type distributor (c) gallety-type distributor. (Parts a, c Gilbert K. Chen, excerpted by special permission from Chemical Engineering, March 5,1984, copyright by McGraw-HiU, Inc., New York, NY 10020 part b reprinted courtesy of Norton Company.)...

Often there are significant differences in the phases that exit from one process operation and enter another. For example, hot effluent gases from a reactor are condensed, or partially condensed, often before entering a separation operation, such as a vapor-liquid separator (e.g., a flash vessel or a distillation tower). In process synthesis, it is common to position a phase-change operation, using temperature- and/or pressure-reduction operations, such as heat exchangers and valves. 

In flash distillation, a liquid mixture at its boiling temperature (under pressure) is released into a vapor-liquid separator or a rectification column. No heat is added or removed (isenthalpic throttling, see Fig. 2-13). 

S Vapor liquid separator Expansion or flash evaporator ... 

Vapor-liquid separator The stream is led into a drum where vapor and liquid separate. The pressure P and the heat duty (adiabatic operation, Q = 0) are given thus, it is a P-h flash. 

Given the then each assumed value V/F (or L/f) has a unique solution for V". This is a variation on the single-stage flash calculation for a vapor-liquid separation. 

Consider Figure 2.3fl as the first method for yielding the desired product pattern. The vapor-liquid mixture from the furnace enters the first fla drum where the residual liquid, W, drops out. The first drum effluent vapor is cooled just enou to condense the overflash, Lg, and then enters the second drum where this vapor-liquid separation occurs. Althou not shown on the sketch, Lg combines with W to form the total residual liquid, W. The vapor is cooled again to some predetermined temperature and enters a third flash drum where D1 separates out. This process is repeated as D2, D3 and D4 are condensed and separated out. The overhead vapor from the sbcth drum is cooled to as low a temperature as is possible consistent with the avail-... 

Under the assnmption of equilibrium conditions, and knowing the composition of the fluid stream coming into the separator and the working pressure and temperature conditions, we conld apply our current knowledge of vapor/liquid/equilibrium (flash calculations) and calculate the vapor and liquid fractions at each stage. 

Examples of vapor/liquid separator are (I) amine flash drum, where majority of inlet flow is liquid with very little flashed vapor, (2) compressor suction knock out drum or ftiel gas knock out drum, where the majority of inlet flow is vapor with very little carryover liquid, (3) hot separator in hydrotreater, where considerable amount of vapor and liquid are in the inlet flow. In general, vapor/liquid separator can be either vertical or horizontal. 

The calculation of single-stage equilibrium separations in multicomponent systems is implemented by a series of FORTRAN IV subroutines described in Chapter 7. These treat bubble and dewpoint calculations, isothermal and adiabatic equilibrium flash vaporizations, and liquid-liquid equilibrium "flash" separations. The treatment of multistage separation operations, which involves many additional considerations, is not considered in this monograph. 

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

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