Distillation column design liquid-vapor

The inlet temperature will be the saturation temperature of the liquid at the base of the column, and the vapor temperature will be the saturation temperature of the vapor returned to the column. The composition of these streams will be fixed by the distillation column design specification. 

Since few liquid mixtures are ideal, vapor-liquid equilibrium calculations are somewhat more complicated than for the cases in the previous section, and the phase diagrams for nonideal systems can be more structured than Figs. 10.1-1 to 10.1-6. These complications arise from the (nonlinear) composition dependence of the species activity coefficients. For example, as a result of the composition dependence of yt, the vapor-liquid equilibrium pressure in a fixed-temperature experiment will no longer be a linear function of mole fraction, so that no.nideal solutions exhibit deviations from Raoult s law. However, all the calculational methods discussed in the previous section for ideal mixtures, including distillation column design, can be used for nonideal mix-, tures, as long as the composition dependence of the activity coefficients is taken into account. 

To derive simple equations for the description of the vapor-liquid equilibrium and for the design of distillation columns, the liquid is considered as an ideal mixture, that is, the forces of attraction between molecules of the pure components equal those in the mixture. 

We proceed now to use bubble and dew point calculations in the following Examples that represent typical applications of vapor-liquid equilibrium to distillation column design. 

The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design. 

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 horizontal natural circulation systems do not use a kettle design exchanger, but rather a 1-2 (1 shell side, 2 tube-side passes) unit, with the vaporized liquid plus liquid not vaporized circulating back to a distillation column bottoms vapor space or, for example, to a separate drum where the vapor separates and flows back to the process system and where liquid recirculates back along with make-up feed to the inlet of the horizontal shell and tube reboiler. See Figures 10-96A-C. 

The design of a distillation column requires a reboiler operating at 2.23 psia (vapor space above bottom liquid). The heat duty is 1,528,600 Btu/hr. The properties of the acrylonitrile mixture have been calculated to be... 

The best designs provide for the percentage vaporization per pass to have been completed by the time the fluid mixture reaches the upper end of the tube and the mixture is leaving to enter the bottom chamber of the distillation column. In order to assist in accomplishing this, the initial reboiler elevation should be set to have the top tubesheet at the same level as the liquid in the column bottom section. A liquid-level control adjustment capability to raise or lower this bottoms level must exist to optimize the recirculation. Sometimes, the level in the bottom of the column may need to be 25-30% of the reboiler tube length above the elevation of the tubesheet. Therefore, the vapor nozzle return from the reboiler must enter at sufficient elevation to allow for this possibility. 

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

Design liquid level control systems for the base of a distillation column and for the vaporizer shown bdow. Steam flow to the vaporizer is held constant and cannot be used to control level. Liquid feed to the vaporizer can come from the column and/or from the surge tank. Liquid from the column can go to the vaporizer and/or to the surge tank. 

All the obstacles in the path of distillation progress, however, were not equipment fabrication and design problems. It was discovered very early in the running of sour crudes that the shell still corroded severely at the vapor-liquid interface line and in that portion of the shell in contact with vapors. At the same time severe corrosion in pipe stills and tube stills, along with overheating and coking, resulted in expensive equipment failures. These problems started metallurgists on a chain of developments which produced the corrosion- and heat-resistant alloys used in modern oil heaters and the alloy liners used in distillation columns. 

Reflux Rate. The optimum reflux rate for a distillation column depends on the value of energy, but is generally between 1.05 times and 1.25 times the reflux rate, which could be used with infinite trays. At this level, excess reflux is a secondary contributor to column inefficiency. However, when designing to this tolerance, correct vapor—liquid equilibrium data and adequate controls are essential. 

While the limiting phenomenon of upper limit flooding in a vertical pipe is similar to ultimate capacity in distillation, there is a distinct difference. Upper limit in a vertical pipe applies to a design where a conscious effort should be made to minimize gas-liquid contact. Carried to extremes, it would involve separate tubes for liquid flowing down and vapor going up. In contrast, ultimate capacity in a distillation column corresponds to the condition where effective mass transfer disappears due to high entrainment. One could force more vapor up through the contactor, but fractionation would be poor. 

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. 

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