Distillation columns vapor distribution

The entrance of a liqmd-flashing vapor mixture into the distillation column feed location requires a specially designed distribution tray to separate the vapors from the liquid, w hich must drop onto the packing bed for that section in a uniform pattern and rate. 

The last step regards the detailed design of the reactive-distillation column and of other operational units. The hydraulic design is consolidated taking into account the optimal traffic of liquid and vapor. Additional internals are provided to ensure uniform distribution of fluids and a sharp residence-time distribution. 

Extractive distillation is used because the components are distributed differently between contacting liquid and vapor phases in equilibrium when a high-boiling nonideal component is added to the mixture. The added component is introduced in the upper part of a distillation column above the feed and remains in appreciable concentration in the liquid on all of the lower trays. It is removed from the column with one of the components being separated as the bottoms product. Although a nonideal component is also introduced for azeotropic distillation, the added com-... 

Distillation columns often consist of a series of vertically distributed stages. Vapor flows upward and liquid flows downward between adjacent stages some of the liquid fed to each stage vaporizes, and some of the vapor fed to each stage condenses. A representation of the lop section of a distillation column is shown on the next page. (See Problem 4.26 for a more realistic representation.)... 

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. 

Phase-equilibrium calculations were discussed for vapor-liquid equilibria (VLB), liquid-liquid equilibria (LLE), and solid-solid equilibria (SSE). Results from VLE calculations often take the form of K-factors and relative volatilities, especially when thermodynamic calculations serve as intermediate steps in computer-aided process-design programs. In those situations, K-factors are routinely provided to subprograms that size distillation columns and gas-liquid absorbers. Similarly, the distribution coefficients computed for LLE serve as bases for sizing solvent-extraction columns moreover, liquid-liquid distribution coefficients may be helpful in screening candidate solvents for use in an extraction. 

A distillation column is a series of stills placed one on top of another. As vaporization occurs, the lighter components of the mixture move up the tower and are distributed on the various trays. The lightest component goes out the top of the column in a vapor state and is passed over the cooling coils of a shell-and-tube condenser. As the hot vapor comes into contact with the coils, it condenses and is collected in the overhead accumulator. Part of this product is sent to storage the rest is returned to the tower as reflux. 

Because a packing support plate usually is located immediately above the gas inlet in an absorber or the reboiler return in a distillation column, this plate could be used to control vapor distribution. Obviously, vapor maldistribution can reduce column efficiency in the same way as liquid maldistribution although due to the turbulence in the vapor phase, its rate of radial cross-mixing is at least three times that of the liquid phase. The potential for vapor maldistribution increases as column diameters or operating pressures increase. Fortunately, the vapor phase tends to maintain a uniform distribution once it has been established. Thus, usually only the packing support plate immediately above the vapor inlet needs to act as a vapor distributor. This support plate should be located at least one vapor-inlet diameter plus 12-in. above the center-line of the vapor inlet nozzle. 

The mixture to be separated and analyzed may be either a gas, liquid, or a solid in some instances. All that is required is that the materials be stable, have a vapor pressure of 0.1 torr at the operating temperature and interact with the column material (either a solid adsorbent or a liquid stationary phase) and the mobile phase (carrier gas). The result of this interaction is the differing distribution of the sample components between the two phases, resulting in the separation of the sample components into zones or bands. The principle that governs the chromatographic separation is the foundation of most physical methods of separation, for example, distillation and liquid-liquid extraction. 

In the Mohil-Badger vapor-phase process, fresh and recycled benzene are vaporized and preheated to the desired temperature and fed to a multistage fixed-bed reactor. Ethylene is distributed to the individual stages. Alkylation takes place in tile vapor phase. Separately, file polyethylbenzene stream from the distillation section is mixed with benzene, vaporized and heated, and fed to the transalkylator, where polyethylbenzenes react with benzene to form additional ethylbenzene. The combined reactor effluent is distilled in the benzene column. Benzene is condensed in the overhead for recycle to the reactors. The bottoms from the benzene column are distilled in the ethylbenzene column to recover the ethylbenzene product in the overhead. The bottoms stream from the ethylbenzene column is further distilled in the polyefitylbenzene column to remove a small quantity of residue. The overhead polyethylbenzene stream is recycled to the reactor section for transalkylation to ethylbenzene. 

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