Distillation design calculations

The proper design of distillation and absorption columns depends on knowledge of vapor—Hquid equiHbrium, as do flash calculations used to determine the physical state of streams at given conditions of temperature, pressure, and composition. Detailed treatments of vapor—Hquid equiHbria are available (6,7). 

It can be seen from the previous description that the design of both a cold-feed stabilizer and a stabilizer with reflux is a rather complex and involved procedure. Distillation computer simulations are available that can be used to optimize the design of any stabilizer if the properties of the feed stream and desired vapor pressure of the bottoms product are known. Cases should be run of both a cold-feed stabilizer and one with reflux before a selection is made. Because of the large number of calculations required, it is not advisable to use hand calculation techniques to design a distillation process. There is too much opportunity for computational eiToi. 

Continuous binary distillation is illustrated by the simulation example CON-STILL. Here the dynamic simulation example is seen as a valuable adjunct to steady state design calculations, since with MADONNA the most important column design parameters (total column plate number, feed plate location and reflux ratio) come under the direct control of the simulator as facilitated by the use of sliders. Provided that sufficient simulation time is allowed for the column conditions to reach steady state, the resultant steady state profiles of composition versus plate number are easily obtained. In this way, the effects of changes in reflux ratio or choice of the optimum plate location on the resultant steady state profiles become almost immediately apparent. 

In order to develop a method for the design of distillation units to give the desired fractionation, it is necessary, in the first instance, to develop an analytical approach which enables the necessary number of trays to be calculated. First the heat and material flows over the trays, the condenser, and the reboiler must be established. Thermodynamic data are required to establish how much mass transfer is needed to establish equilibrium between the streams leaving each tray. The required diameter of the column will be dictated by the necessity to accommodate the desired flowrates, to operate within the available drop in pressure, while at the same time effecting the desired degree of mixing of the streams on each tray. 

Norman, W. S. Trans. Inst. Chem. Eng. 23 (1945) 66. The dehydration of ethanol by azeotropic distillation. Ibid. 89. Design calculations for azeotropic dehydration columns. 

The calculations made thus far are of theoretical trays, that is, trays on which vapor-liquid equilibrium is attained for all components. Actual tray efficiencies vary widely with the kind of system, the flow rates, and the tray construction. The range can be from less than 10% to more than 100% and constitutes perhaps the greatest uncertainty in the design of distillation equipment. For hydrocarbon fractionation a commonly used efficiency is about 60%. Section 13.14 discusses this topic more fully. 

Numerous computer software programs are available today to streamline design calculations. Of course, such software can be structured only after very careful analysis is made of the chemical dynamics of a given application. Calculations are particularly complex and difficult in the instances of azeotropic and reactive distillation. Software programs are described in some detail in the Kumana. Morris, and VenkuUiramkan references listed. 

Determine the relevant vapor-pressure data. Design calculations involving vapor-liquid equilibrium (VLE), such as distillation, absorption, or stripping, are usually based on vapor-liquid equilibrium ratios, or K values. For the tth species, K, is defined as K, = y, /x, where y, is the mole fraction of that species in the vapor phase and x, is its mole fraction in the liquid phase. Sometimes the design calculations are based on relative volatility c/u], which equals K,/Kj, the subscripts i and j referring to two different species. In general, K values depend on temperature and pressure and the compositions of both phases. 

The computer program for azeotropic distillation ADP/ADPLLE makes possible not only a comparison of entrainers for a separation but also gives results of a quality required for actual design calculations. 

Assuming that one of the models is correct, the design calculations can be continued to obtain the process economic analysis. At the same time, the environmental impact can also be investigated. As our selected solvent is an ester, it s MSDS shows low human effect, which may only act as an irritant to skin, eye and respiratory, and do not have any other environmental effect. So, it can be concluded that solvent is suitable for separation of EB from PX by extractive distillation. 

Rules are given for many types of equipment—from compressors, to distillation columns, to heat exchangers, to vessels. Such guidelines are useful in preliminary design calculations and cost estimates. 

Using an entirely different approach to the modeling of multicomponent mass transfer in distillation (an approach that we describe in Chapter 14), Krishnamurthy and Taylor (1985c) found significant differences in design calculations involving nonideal systems. For an almost ideal system (a hydrocarbon mixture), pseudobinary methods were found to be essentially equivalent to a more rigorous model that accounted for diffusional interaction effects. 

In view of the large influence of interaction effects found by Toor and Burchard (1960) it is a little surprising that there have been so few design calculations reported in the literature. More experience with these models is required before definitive conclusions can be made regarding the use of complicated efficiency models in sophisticated distillation codes. The whole issue of multicomponent mass transfer models in distillation column simulation is taken up again in Chapter 14. 

There are many excellent texts that discuss the design of distillation columns using equilibrium stage calculations. Some of them were cited in Chapters 12-14. These texts provide a wealth of examples that could be used as the basis for a design using the nonequilibrium model described in Chapter 14. We adapt one such example below (Exercise 14.1) in order to indicate how this might be done. 

Surface tension data of liquids are important in many process design calculations for situations where these is a two-phase interface, e.g., two-phase flow, distillation, absorption and condensation. Surface tension can be expressed as ... 

This chapter considers the vapor-liquid equilibrium of mixtures, conditions for bubble and dew points of gaseous mixtures, isothermal equilibrium flash calculations, the design of distillation towers with valve trays, packed tower design. Smoker s equation for estimating the number of plates in a binary mixture, and finally, the computation of multi-component recovery and minimum trays in distillation columns. 

Downstream process design requires distillate and bottoms data. Calculate the flow rates and compositions of these streams for a fixed feed rate of 1000 kmol/h. 

It is required to design a distillation column to separate the stream defined below into distillate and bottoms products, according to the indicated specifications. Preliminary calculations based on material balances alone are recommended in order to define the expected products flow rates and compositions. Calculate the distillate and bottoms flow rates and compositions. 

Operating Line and "Equilibrium" Curve. Both terms are of importance for the graphical solution of a separation problem, i.e., for the graphical determination of the number of stages of a cascade. This method has been developed for the design of distillation columns by MacCabe and Thiele and should be well known. For all cases, the operating line represents the mass and material balances. In distillation, the equilibrium curve represents the thermodynamical va-por/liquid equilibrium. For an ideal binary system, the equilibrium curve can be calculated from Raoult s law and the saturation-pressure curves of the pure components of the mixture. In all other cases, however, for example, for all membrane processes, the equilibrium curve does not represent a thermodynamical equilibrium at all but will represent the separation characteristics of the module or that of the stage. 

The knowledge of the occurrence of azeotropic points in binary and higher systems is of special importance for the design of distillation processes. The number of theoretical stages of a distillation column required for the separation depends on the separation factor i.e. the ratio of the 7< -factors (7required separation factor can be calculated with the following simplified relation (Reference 1) ... 

Results of design calculation for a methanol-water distillation operation are given below. 

In another study, Grayson examined the effect of K-values on bubble-point, dew-point, equilibrium flash, distillation, and tray efficiency calculations. He noted a wide range of sensitivity of design calculations to variations in K-values. 

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