Distillation columns phase equilibrium

In this chapter, we describe an algorithm for predicting feasible splits for continuous single-feed RD that is not limited by the number of reactions or components. The method described here uses minimal information to determine the feasibility of reactive columns phase equilibrium between the components in the mixture, a reaction rate model, and feed state specification. This is based on a bifurcation analysis of the fixed points for a co-current flash cascade model. Unstable nodes ( light species ) and stable nodes ( heavy species ) in the flash cascade model are candidate distillate and bottom products, respectively, from a RD column. Therefore, we focus our attention on those splits that are equivalent to the direct and indirect sharp splits in non-RD. One of the products in these sharp splits will be a pure component, an azeotrope, or a kinetic pinch point the other product will be in material balance with the first. 

The basis of distillation is phase equilibrium—specifically, vapor-liquid equilibrium (VLE) and in some cases vapor-liquid-liquid equilibrium (VLLE). Distillation can only effect a separation among chemical components if the compositions of the vapor and liquid phases that are in phase equilibrium with each other are different. A reasonable understanding of VLE is essential for the analysis, design, and control of distillation columns. 

Efficiency is a measure of how well the chromatographic column is performing. Another term that accounts for efficiency is the value N, which is called the number of theoretical plates. The term plates originates from the theory of distillation columns, where equilibrium between two phases occurs in each plate. For LC, a column can be thought of as a series of plates where equilibration of the analyte occurs between two phases. The greater the number of theoretical plates the more efficient the separation. 

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and hquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibriums to an extent dependent upon the efficiency of the contac ting tray. 

Availability of large digital computers has made possible rigorous solutions of equilibrium-stage models for multicomponent, multistage distillation-type columns to an exactness limited only by the accuracy of the phase equilibrium and enthalpy data utilized. Time and cost requirements for obtaining such solutions are very low compared with the cost of manual solutions. Methods are available that can accurately solve almost any type of distillation-type problem quickly and efficiently. The material presented here covers, in some... 

The novel approach finally taken was to conduct the reaction and purification steps in a reactor-distillation column in which methyl acetate could be made with no additional purification steps and with no unconverted reactant streams. Since the reaction is reversible and equilibrium-limited, high conversion of one reactant can be achieved only with a large excess of the other. However, if the reacting mixture is allowed to flash, the conversion is increased by removal of the methyl acetate from the liquid phase. With the reactants flowing countercurrently in a sequence of... 

In the manufacture of methyl ethyl ketone from butanol, the product is separated from unreacted butanol by distillation. The feed to the column consists of a mixture of methyl ethyl ketone, 2-butanol and trichloroethane. What would be a suitable phase equilibrium correlation to use in modelling this process ... 

The mechanism of transfer of solute from one phase to the second is one of molecular and eddy diffusion and the concepts of phase equilibrium, interfacial area, and surface renewal are all similar in principle to those met in distillation and absorption, even though, in liquid-liquid extraction, dispersion is effected by mechanical means including pumping and agitation, except in standard packed columns. 

The digital simulation of a distillation column is fairly straightforward. The main complication is the large number of ODEs and algebraic equations that must be solved. We will illustrate the procedure first with the simplified binary distillation column for which we developed the equations in Chap. 3 (Sec. 3.11). Equimolal overflow, constant relative volatility, and theoretical plates have been assumed. There are two ODEs per tray (a total continuity equation and a light component continuity equation) and two algebraic equations per tray (a vapor-liquid phase equilibrium relationship and a liquid-hydraulic relationship). 

The concept of plate theory was originally proposed for the performance of distillation columns (12). However, Martin and Synge (13) first applied the plate theory to partition chromatography. The theory assumes that the column is divided into a number of zones called theoretical plates. One determines the zone thickness or height equivalent to a theoretical plate (HETP) by assuming that there is perfect equilibrium between the gas and liquid phases within each plate. The resulting behavior of the plate column is calculated on the assumption that the distribution coefficient remains unaffected by the presence of other... 

Sometimes it is possible to evaluate the range of validity of measurements and correlations of physical properties, phase equilibrium behavior, mass and heat transfer efficiencies and similar factors, as well as the fluctuations in temperature, pressure, flow, etc., associated with practical control systems. Then the effects of such data on the uncertainty of sizing equipment can be estimated. For example, the mass of a distillation column that is related directly to its cost depends on at least these factors ... 

In some liquid-vapour equilibrium experiments with low iodine contents (Larousse, 2009), an unexpected decrease of the pressure with time, especially at elevated temperatures, is observed (Figure 3). A possible, though not proven, explanation would be HI decomposition with condensation of the resulting I2 in the liquid phase, just like what is anticipated to take place in the reactive distillation column. However, the observed kinetics are about 105 faster than what is expected from gas phase composition, which could be an indication of liquid phase decomposition. 

Similar patterns of behavior can be observed in reactive distillation columns or other integrated reaction separation processes with fast reversible reactions [11, 23] as illustrated in Fig. 5.3 for a pure rectifying column with a ternary mixture and a reaction of type 2B A + C taking place in the liquid phase. However, due to the reaction equilibrium the profiles consist of a single concentration front, which is clearly different from the nonreactive problem illustrated in Fig. 5.2. 

Lehtonen et al. (1998) considered polyesterification of maleic acid with propylene glycol in an experimental batch reactive distillation system. There were two side reactions in addition to the main esterification reaction. The equipment consists of a 4000 ml batch reactor with a one theoretical plate distillation column and a condenser. The reactions took place in the liquid phase of the reactor. By removing the water by distillation, the reaction equilibrium was shifted to the production of more esters. The reaction temperatures were 150-190° C and the catalyst concentrations were varied between 0.01 and 0.1 mol%. The kinetic and mass transfer parameters were estimated via the experiments. These were then used to develop a full-scale dynamic process model for the system. 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

To complete the model, it is usual to add equations that relate the compositions of the two streams leaving the stage. In the standard model of a distillation column, we assume that these two streams are in equilibrium with each other. Thus, the mole fractions in the exiting streams are related by the familiar equations of phase equilibrium ... 

The first three are intensive variables. The fourth is an extensive variable that is not considered in the usual phase rule analysis. The fifth is neither an intensive nor an extensive variable but is a siugle degree of freedom that the designer uses in specifying how often a particular element is repeated in a unit. For example, a distillation column section is composed of a series of equilibrium stages, and when the designer specifies the number of stages that the section contains. 

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-... 

If n-pentane is selected as the entrainer for an azeotropic distillation scheme, an ethanol product containing less water than that obtained in the extractive distillation method is easily obtained with entrainer-etha-nol ratios of 2.S-3.5, mole basis (10). For a ratio of 3.214, the water content of the ethanol is less than 3 ppm. Only 18 equilibrium trays are required in a column of less than 5 feet diameter. The heat loads in millions Btu/hour are about 10.7 for the reboiler and 11.3 for the condenser. A stripper is used to recover n-pentane and ethanol from the aqueous phase. The recovered n-pentane and ethanol can be recycled either to the feed or to the reflux stream of the azeotropic distillation column. 

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