Partial condensation, flash vaporization

In a two-stage system, the vapors from the compressor s first stage discharge are partially condensed and flashed in an interstage drum. The liquid hydrocarbon is pumped forward to the gas plant, either to the high pressure separator (HPS) or directly to the stripper. 

FIG, 13-2 Separation operations related to distillation, [a) Flash vaporization or partial condensation, h) Absorption, (c) Rectifier, d) Stripping, (e) Reboiled stripping, fj Reboiled absorption, (g) Refluxed stripping, (h) Extractive distillation. (1) Azeotropic distillation. 

Define the concepts of flash vaporization and partial condensation. 

Discuss the similarities and differences among the following operations listed in Table 1.1 flash vaporization, partial condensation, and evaporation. 

A vertical vessel (drum), as shown in Fig. 7.1, can be used to separate vapor from liquid following equilibrium flash vaporization or partial condensation. A reasonable estimate of the minimum drum diameter Dr, to prevent liquid carryover by entrainment can be made by using (13-7) in conjunction with the curve for 24-in. tray spacing in Fig. 13.3 and a value of Fha = 10 in (13-5). To absorb process upsets and fluctuations and otherwise facilitate control, vessel volume Vv is determined on the basis of liquid residence time t, which should be at least 5 min with the vessel half full of liquid. Thus... 

Equilibrium Flash Vaporization and Partial Condensation Graphical Multistage Calculations by the McCabe-Thiele Method Batch Distillation... 

When the mixture to be separated includes species that differ widely in their tendency to vaporize and condense, flash vaporization or partial condensation operations, (1) and (2) in Table 1.1, may be adequate to achieve the desired separation. In the former operation, liquid feed is partially vaporized by reducing the pressure (e.g., with a valve), while in the latter vapor feed is partially condensed by removing heat. In both operations, after partitioning of species by interphase mass transfer has occurred, the vapor phase is enriched with respect to the species that are most volatile, while the liquid phase is enriched with respect to the least volatile species. After this single contact, the two phases, which are of different density, are separated, generally by gravity. 

Often, the degree of species separation achieved by flash vaporization or partial condensation is inadequate because volatility differences are not sufficiently large. In that case, it may still be possible to achieve the desired... 

Figure 7.1. Continuous single-stage equilibrium separation, (a) Flash vaporization. (Adiabatic flash with valve isothermal flash without valve when Tv is specified. (6) Partial condensation. (Analogous to isothermal flash when Ty is specified.

Seawater is used to condense last-effect vapor, and a portion is then treated to prevent scaling and corrosion. Treatment usually consists of acidification to break down bicarbonates, followed by deaeration, which also removes the carbon dioxide generated. The treated seawater is then heated to successively higher temperatures by a portion of the vapor from each effect and finally is fed to the evaporating surface of the first effect. The vapor generated therein and the partially concentrated liquid are passed to the second effect, and so on until the last effect. The feed rate is adjusted relative to the steam rate so that the residual liquid from the last effect can carry away all the salts in solution, in a volume about one-third of that of the feed. Condensate formed in each effect but the first is flashed down to the following effects in sequence and constitutes the product of the evaporator. 

Stream is the liquid obtained by partially condensing V2, the vapor obtained from flashing vapor stream V3. After one vaporization and then a condensation, stream Lj will have a concentration close to that of stream V3. Thus it is appropriate to use stream Lj as an additional feed to stage 2 after pressure reduction. 

The liquids that are separated from the gas stream in the first separator may be flowed directly to a tank or may be stabilized in some fashion. As was discussed in Chapter 2 of Volume 1, these liquids contain a large percentage of methane and ethane, which will flash to gas in the tank. This lowers the partial pressure of all other components in the tank and increases their tendency to flash to vapors. The process of increasing the amount of intermediate (C3 to C5) and heavy (C + ) components in the liquid phase is called stabilization. In a gas field this process is called condensate stabilization and in an oil field it is called crude stabilization. 

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