Two-phase vapor-liquid separator

The two-phase separator is basically a gas-liquid separator. In case a two-liquid phase is separated as one, the mixed liquid properties are to be consid- 

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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 prediction of the performance of three-phase multistage separation processes is dependent on the ability to describe the thermodynamics of three-phase behavior. The mathematical solution of three-phase distillation columns is similar to two-phase vapor-liquid columns, the difference being in the model used to calculate the /(-values. If the A -valuc model predicts two liquid phases, two liquid profiles must be considered in the column instead of one. 

Figure 3.2c shows the familiar cigar-shaped vapor—liquid envelope found in many elementary textbooks on phase equilibria. At a fixed overall composition (denoted by x in this figure) there exists a single vapor phase at very low pressures. As the pressure is isothermally increased, the two-phase vapor-liquid envelope is intersected and a dew or liquid phase now appears. The locus of points that separates the two-phase vapor-liquid region from the one-phase vapor region is called the dew point curve. The concentration of the equilibrium vapor and liquid phases within the two-phase boundary of the vapor-liquid envelope is determined by a horizontal tie line similar to the one depicted in this figure. 

As the pressure is further increased at this fixed overall composition, the amount of the liquid phase increases while the amount of the vapor phase shrinks until only a small bubble of vapor remains. If the pressure is still further increased, the bubble of vapor finally disappears, then a single liquid phase exists. The locus of points that separates the two-phase vapor-liquid region from the one-phase liquid region is called the bubble point curve. This vapor-liquid envelope can now be inserted into the three-dimensional P-T-x diagram in figure 3.2a. 

Seafloor temperature and pressure data for venting fluids indicate a wide variability relative to the two-phase (vapor-liquid) boundary for seawater (Fig. 4). Chloride concentrations in vent fluids (Table 1) range from 35 to 1090 mmol/kg due to subcritical boiling and/or supercritical phase-separation. For saline fluids such as seawater, phase-separation can occur above the critical point with condensation of brine droplets from a less saline residual vapor. Obviously, vent fluids decompress on ascent from subsurface reactions zones, which are inferred to be in cracking fronts just above the l-4... 

In the calculation of vapor-liquid equilibria, it is necessary to calculate separately the fugacity of each component in each of the two phases. The liquid and vapor phases require different techniques in this chapter we consider calculations for the vapor phase. 

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. 

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. 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

There are several characteristics common to the describing equations of all types of multicomponent, vapor-liquid separation processes, both single- and multi-stage, that make it possible to exploit the inside-out concept in similar ways to solve them efficiently and reliably. The equations have as common members component and total mass balance, enthalpy balance, constitutive and phase equilibrium equations. In addition, all such processes require K-value or fugacity coefficient and vapor and liquid enthalpy models. In all cases the describing equations have similar forms, and depend on the primitive variables (temperature, pressure, phase rate and composition) in essentially the same ways. Before presenting the inside-out concept, it will be useful to identify two classes of conventional methods and discuss their main characteristics. 

The HDA process (Fig. 1.1) contains nine basic unit operations reactor, furnace, vapor-liquid separator, recycle compressor, two heat exchangers, and three distillation columns. Two vapor-phase reactions are considered to generate benzene, methane, and diphenyl from reactants toluene and hydrogen. 

Vapor-liquid separators often perform two functions. Their primary task is to separate the vapor phase from the liquid phase but they may also provide surge capacity. They must be sized to provide a low velocity and thus separate the liquid from the vapor. 

Supercritical solvent separation This method has been employed to recover solvent in deas-phalting. In recovering solvent from deasphalted oil by this method, the deasphalted oil laden with solvent is pumped to a pressure slightly higher than that of the critical pressure of the solvent (e.g., propane critical pressure = 42.1 Atm). At this supercritical condition, the solvent is no longer completely miscible with the deasphalted oil and two phases result, the supercritical solvent phase and the deasphalted oil phase that is devoid of much of its solvent. These two phases can be separated in a simple liquid/liquid separator. The result is solvent recovery without solvent vaporization. 

Equation (6-4) and Raoult s law apply for each of the three components in the mixture. Also, the mole fractions must add up to 1.0 in each of the two phases leaving the separator. The Mathcad program of Figure 6.3 is easily modified to include an additional component and solve for D, W, and the vapor and liquid compositions (see... 

Solvent extraction (3) Liquid-liquid extraction is used to separate water and AA. The top of the extractor is forwarded to a solvent separator. The extractor bottom Is sent to the raffinate stripper (5) to recover solvents. Crude acrylic acid (CAA) is separated from the solvents by distillation. The overhead vapor is condensed in an internal thermoplate condenser. The two-phase condensate is separated. The organic phase is recycled. The aqueous phase is sent to the raffinate stripper (5). The column bottom, mostly AA and acetic acid, is routed to the CAA separator (4). 

The part of the feed nitrogen that passes through the second plate-fin exchanger is expanded through a Joule-Thompson expansion valve and the two phase mixture enters a medium pressure vapor-liquid separator. The vapor from this vessel is mixed with the discharge from the turboexpander... 

The two main approaches used to model frictional pressure gradients in macro- and microscale two-phase gas-liquid flow are the homogeneous model and the separated flow model. The homogeneous model is based on the assumption that the two-phase fluid behaves as a pseudo single-phase fluid with pseudo-properties that are weighted relative to the vapor and Uquid flow fraction. Different ways to define the pseudo-properties, usually called mixture properties, have been proposed that are well detailed by Collier and Thome [54] and Thome [55]. 

The separated flow model (for more details, see Collier and Thome [54]) considers that the phases are artificially segregated into two steams one liquid and one vapor, and has been continuously developed since 1949 when Lockhart and Martinelli [56] published their classic paper on two-phase gas-liquid flow. The main goal in this approach is to find an empirical correlation or simplified concept to relate the two-phase friction multiplier, ( ), to the independent variables of the flow. For example, the... 

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