Distillation column design mixture

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. 

The sequencing of distillation columns and other types of equipment for the separation of multicomponent mixtures has received much attention in recent years. Although one separator of complex design can sometimes be devised to produce more than two produc ts, more... 

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 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. 

The correlations given in the previous sections apply to the condensation of a single component such as an essentially pure overhead product from a distillation column. The design of a condenser for a mixture of vapours is a more difficult (ask. 

Alatiqi presented (I EC Process Design Dev. 1986, Vol. 25, p. 762) the transfer functions for a 4 X 4 multivariable complex distillation column with sidestream stripper for separating a ternary mixture into three products. There are four controlled variables purities of the three product streams (jCj, x, and Xjij) and a temperature difference AT to rninirnize energy consumptiou There are four manipulated variables reflux R, heat input to the reboiler, heat input to the stripper reboiler Qg, and flow rate of feed to the stripper Lj. The 4x4 matrix of openloop transfer functions relating controlled and manipulated variables is ... 

This illustrative example is taken from the recent work on interaction of design and control by Luyben and Floudas (1994a) and considers the design of a binary distillation column which separates a saturated liquid feed mixture into distillate and bottoms products of specified purity. The objectives are the determination of the number of trays, reflux ratio, flow rates, and compositions in the distillation column that minimize the total annual cost. Figure (1.1) shows a superstructure for the binary distillation column. 

The continuous distillation columns are designed to operate for longer hours (typically 8000 hrs a year) and therefore each column (or a series of columns in case of multicomponent mixture) is dedicated to the separation of a specific mixture. 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

Another class of separation problem attacked has been that of designing the most effective thermally coupled distillation column arrangement to separate a multicomponent mixture. Sargent and Gaxninibandara (1975) present a general column superstructure which they optimize. Imbedded in the superstructure are all the alternative thermally coupled and ordinary column sequences to be considered. The optimization eliminates those portions of the superstructure which are not economic leaving, hopefully, the optimal substructure. 

The procedure known as the list-processing method allows the designer to identify feasible sequences for separating ideal or slightly nonideal zeotropic mixtures by means of simple distillation columns. Table 3.13 presents a list of more specific heuristics for sequencing, supplementary to those in Table 3.7. The first one shows... 

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