UNIFAC separation system

This effluent is cooled to 38°C and enters a flash-decanter vessel at 278 kPa. Three phases leave that vessel. The vapor phase (hydrogen rich) is sent to the vapor separation system. The aqueous phase (mostly water, with some methanol) is sent to the aqueous stream separation system. The organic-rich phase is sent to the organic stream separation system, which you will design. To obtain the composition of the feed to your section, use a simulator with the UNIFAC method to perform a three-phase flash for the above conditions. If the resulting organic liquid stream contains small amounts of hydrogen and water, assume they can be completely removed at no cost before your stream enters your separation section. 

The UNIQUAC equation developed by Abrams and Prausnitz is usually preferred to the NRTL equation in the computer aided design of separation processes. It is suitable for miscible and immiscible systems, and so can be used for vapour-liquid and liquid-liquid systems. As with the Wilson and NRTL equations, the equilibrium compositions for a multicomponent mixture can be predicted from experimental data for the binary pairs that comprise the mixture. Also, in the absence of experimental data for the binary pairs, the coefficients for use in the UNIQUAC equation can be predicted by a group contribution method UNIFAC, described below. 

They performed binary system experiments for each of the pairs in the ternary system. It was found that glycol can move the azeotrope point and therefore, enhance the separation process. Sheikholeslamzadeh et al. have used both the NRTL-SAC and UNIFAC models to perform phase calculations and assess the capacity of the mentioned models in the prediction of binary and ternary systems containing glycol and alcohols [20],... 

It should, however, be pointed out that in the above case, if one simply ascribes a single solubility parameter to each monomer, it is Impossible to predict an overall negative enthalpy of mixing. It has also been noted that a window of miscibility can be explained by a favorable specific interaction without recourse to a cross term. If one separates the normal dispersive forces from the specific interaction, then as a first approximation, when the solubility parameters of the two polymers are similar the unfavorable dispersive interactions are small and specific interactions yield miscibility. For a copolymer/polymer mixture the solubility parameters might be expected to match at some specific copolymer composition (32). A method of combining the features of both the specific interaction and the cross term is to use something similar to the UNIFAC group contribution system and model all the interactions, both favorable and unfavorable within the system. 

A helpful but rarely used tool for deciding whether accurate information on a given binary system is needed for the design of a certain separation is provided by sensitivity analysis. An example for such an analysis of the hexyl acetate RD process shown in Fig. 4.4 is given in Fig. 4.5. The basis of the sensitivity analysis is a given operating point of the RD column and a fully parameterized process model. In our case the NRTL model with two adjustable parameters per binary system was used to describe liquid-phase non-idealities in that model. Parameters for systems, for which no experimental data was available were estimated with UNIFAC in a first step. 

UNIFAC modelling results is an effective tool for the prediction and quantifying the effect of the metal-olefin complexation on the separation of the ternary system (1-hexene + n-hexane + [BMIMIINO3]) in the absence and presence of silver salt. 

Most important for the application of group contribution methods for the synthesis and design of separation processes is a comprehensive and reliable parameter matrix with reliable parameters. The present status of modified UNIFAC is shown in Figure 5.84. Today parameters are available for 91 main groups. In the recent years new main groups were introduced for the different types of amides, isocyanates, epoxides, anhydrides, peroxides, carbonates, various sulfur compounds, and so on. In the last year the range of applicability was even extended to systems with ionic liquids [56]. 

In most cases not binary but multicomponent systems have to be separated. Sometimes an additional component is needed as an entrainer e.g. for the separation by extractive distillation. As an example selected separation factors of 12 for the system benzene (l)-cyclohexane (2)-NMP (3) calculated using modified UNIFAC and default UNIQUAC parameters from a simulator are shown in Figures 11.8 and 11.9, respectively. As benzene and cyclohexane form an azeotrope, the main task of the entrainer NMP is to shift the separation factor between benzene and cyclohexane as far from unity as possible this means au 1 or au 1- In practice, typical entrainer concentrations of 50-80 mol% are employed to achieve satisfying separation factors. A higher entrainer concentration usually improves the separation factor. 

Figure 11.8 Calculated separation factors au atmospheric pressure for the system benzene (1)-cyclohexane (2) in the presence of NMP (3) using modified UNIFAC.

Figure 11.14 Phase equilibrium behavior of the ternary system acetone-chloroform-methanol at atmospheric pressure calculated using modified UNIFAC. (a) Tx-behavior (b) lines of constant separation factors (ofi2 = 1, 0fi3 = 1, of23 = l)l...

With the help of modified UNIFAC it should be checked how the separation factor 2 of the binary system benzene (1) and cyclohexane (2) is altered in the presence of 50 and 80mol% NMP(3) at 80 C. 

A column comprises individual separation stages in which the purification of the product is carried out by means of the effect that vapor and liquid have different compositions at equilibrium. Accordingly, the column design calls for knowledge of the phase equilibria of the systems [5]. Normally, phase equilibrium calculations are based on binary parameters describing the interactions of two different molecules. If multicomponent mixtures are considered, some of these interactions might be unknown. To obtain better simulation results, they should at least be estimated. This was the main reason for the development of the UNIFAC group contribution method twenty years ago. 

The DME-water binary system exhibits two liquid phases when the DME concentration is in the 34% to 93% range [2]. However, upon addition of 7% or more alcohol, the mixture becomes conpletely miscible over the complete range of DME concentration. In order to ensure that this non-ideal behavior is simulated correctly, it is recommended that binary vapor-liquid equilibrium (VLE) data for the three pairs of components be used in order to regress binary interaction parameters (BIPs) for a UNIQUAC/UNIFAC thermodynamics model. If VLE data for the binary pairs are not used, then UNIFAC can be used to estimate BIPs, but these should be used only as preliminary estimates. As with all non-ideal systems, there is no substitute for designing separation equipment using data regressed from actual (experimental) VLE. 

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