Continuous multicomponent distillation column

MCSTILL - Continuous Multicomponent Distillation Column System... 

Continuous Multicomponent Distillation Column 501 Gas Separation by Membrane Permeation 475 Transport of Heavy Metals in Water and Sediment 565 Residence Time Distribution Studies 381 Nitrification in a Fluidised Bed Reactor 547 Conversion of Nitrobenzene to Aniline 329 Non-Ideal Stirred-Tank Reactor 374 Oscillating Tank Reactor Behaviour 290 Oxidation Reaction in an Aerated Tank 250 Classic Streeter-Phelps Oxygen Sag Curves 569 Auto-Refrigerated Reactor 295 Batch Reactor of Luyben 253 Reversible Reaction with Temperature Effects 305 Reversible Reaction with Variable Heat Capacities 299 Reaction with Integrated Extraction of Inhibitory Product 280... 

Continuous multicomponent distillation simulation is illustrated by the simulation example MCSTILL, where the parametric runs facility of MADONNA provides a valuable means of assessing the effect of each parameter on the final steady state. It is thus possible to rapidly obtain the optimum steady state settings for total plate number, feed plate number and column reflux ratio via a simple use of sliders. 

Residue Curve Maps. Residue curve maps are useful for representing the infinite reflux behavior of continuous distillation columns and for getting quick estimates of the feasibiHty of carrying out a desired separation. In a heterogeneous simple distillation process, a multicomponent partially miscible Hquid mixture is vaporized ia a stiH and the vapor that is boiled off is treated as being ia phase equiHbrium with all the coexistiag Hquid phases. 

For a multicomponent liquid mixture with nc number of components, usually (nc-l) number of continuous columns will be necessary to separate all the components from the mixture. For a mixture with only 4 components and 3 distillation columns there can be 5 alternative sequences of operations to separate all the components (Figure 1.4). For a mixture with only 5 components, 4 distillation columns can be sequenced in 14 different ways. The number of alternative operations grows exponentially with the number of components in the mixture. These alternative operations do not take into account the production of off-specification materials or provision for side streams (this would further increase dramatically the number of columns and or operational sequences). 

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. 

For single separation duty, Diwekar et al. (1989) considered the multiperiod optimisation problem and for each individual mixture selected the column size (number of plates) and the optimal amounts of each fraction by maximising a profit function, with a predefined conventional reflux policy. For multicomponent mixtures, both single and multiple product options were considered. The authors used a simple model with the assumptions of equimolal overflow, constant relative volatility and negligible column holdup, then applied an extended shortcut method commonly used for continuous distillation and based on the assumption that the batch distillation column can be considered as a continuous column with changing feed (see Type II model in Chapter 4). In other words, the bottom product of one time step forms the feed of the next time step. The pseudo-continuous distillation model thus obtained was then solved using a modified Fenske-Underwood-Gilliland method (see Type II model in Chapter 4) with no plate-to-plate calculations. The... 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

Truly multicomponent solutions based on continuous distillation shortcut methods have been proposed for batch distillation. The Fenske, Underwood, and Gilliland equations or correlations are commonly used in conjunction with each other to solve continuous distillation problems as described in Section 12.3. Diwekar and Madhavan (1991) describe how these techniques may be modified for the design of batch distillation columns for variable and constant reflux cases. 

Multicomponent rectification Consider a multicomponent mixture of m-species continuously fed into a distillation column (Acrivos and Amundson, 1955 Amundson, 1966 Ramkrishna and Amundson, 1985). Let and be the compositions of the ith species on the nth plate for the liquid phase and the vapor phase, respectively. Based on constant molal overflow of liquid with a downflow rate L and a vapor upward flow rate V, the steady-state mass balance for the ith species in the rectifying section above the nth plate leads to the equation... 

Figure 10-2 Approach of the steady state solutions for the start-up period of a batch column to the solution for a corresponding continuous column. [C. D. Holland, Unsteady State Processes with Applications in Multicomponent Distillation, 1966, by courtesy Prentice-Hall, Inc.]...

To illustrate the procedure, we consider a fairly complex process sketched in Fig. 6.4, which shows the process flowsheet and the nomenclature used. In the continuous stirred-tank reactor, a multicomponent, reversible, second-order reaction occurs in the liquid phase A + B C + D. The component volatilities are such that reactant A is the most volatile, product C is the next most volatile, reactant B has intermediate volatility, and product D is the heaviest component a/ > ac > olb > OiQ. The process flowsheet consists of a reactor that is coupled with a stripping column to keep reactant. A in the system, and two distillation columns to achieve the removal of products C and D and the recovery and recycle of reactant B. 

Calculates continuous, multicomponent, multistage distillation, absorption, and stripping columns by equilibrium-stage and rate-based methods... 

This chapter introduces how continuous distillation columns work and serves as the lead to a series of nine chapters on distillation. The basic calculation procedures for binary distillation are developed in Chapter 4. Multicomponent distillation is introduced in Chapter 5. detailed conputer calculation procedures for these systems are developed in Chapter 6. and sinplified shortcut methods are covered in Chapter 7. More complex distillation operations such as extractive and azeotropic distillation are the subject of Chapter 8. Chapter 9 switches to batch distillation, which is commonly used for smaller systems. Detailed design procedures for both staged and packed columns are discussed in Chapter 10. Finally, Chapter 11 looks at the economics of distillation and methods to save energy (and money) in distillation systems. 

There are a number of phase equilibrium driven separation processes where the separation devices are such that crossflow of two bulk phases exists. Crossflow is utilized to enable continuous contacting between two immiscible phases, vapor and liquid, in an efficient fashion, as in a plate located in a distillation column. In chromatographic processes, crossflow of the solid adsorbent particles and the mobile fluid phase (liquid or gas) can lead to continuous separation of a multicomponent feed mixture introduced at one location of the mobile fluid (eluent) phase. We will illustrate first how crossflow of adsorbent particles or the adsorbent bed and the mobile fluid phase overcomes the batch nature of multicomponent separation in elution chromatography. Then we will focus on the cross-flow plate in a distillation column. 

A major feature of the three techniques of continuous chromatography in the crossflow configuration is that the feed stream is introduced only over a small section of the flow cross section of the carrier fluid. Had the feed stream been introduced throughout the flow cross section in the carrier fluid flow direction, multicomponent separation would not have heen possible feed introduction mode is important. We see in Section 8.3.2 that introduction of the feed fluid across the whole flow cross section for this fluid in a crossflow system is a common feature of separation in a plate in a distillation column which can produce only a hinary separation. 

We have seen in Section 8.1.1.3 that continuous operation of a distillation column with countercurrent flow of vapor and liquid can separate a binary mixture only. If we have a multicomponent nonazeotropic mixture as feed, then only one of the two product streams can have one species with sufficient purity the other product stream will contain aU other species in quantities reflecting their feed concentration. This stream has to be fed to another distillation column that can produce two product streams, where each product stream will have sufficient purity with respect to one of the two remaining species for a ternary feed to the first distillation column. Similarly, if the feed to the first column has four components, in general, we will need three columns to obtain four product streams, each product stream being sufficiently purified in one of the species. In general, to separate n species in the feed by distillation in simple distillation columns of the type shown in Figure 8.1.19(b), we will need n - 1) distillation columns. 

Smith-Brinkley shortcut method A quick procedure used to estimate the components in a multicomponent mixture leaving the top and bottom of a disfillation column operating with continuous feed. The procedure is applicable to any stage-wise separafion process. For a distillation column with a single feed and a total condenser, the fractional recovery of any component in the bottom product is calculated from details that include the reflux ratio, internal flows of liquid and vapour above and below the feed point (i.e., the rectifying and stripping sections), and the relative volatilities of the components. In the calculation, the reboiler counts as stage one. 

Batoh distillation is frequently used for small-volume products. One column can be used to separate a multicomponent mixture instead of requiring NC — 1 continuous colimms. The energy consumption in batch distillation is usually higher than in continuous, but with small-volume, high-value products energy costs seldom dominate the economics. 

When only one product (distillate) is produced in a continuous column using only one pass by processing a binary or a multicomponent mixture, the operation is defined as SPSS operation. In this type of operation the bottom product after the first pass is not processed further. Figure 11.3 shows the operation for a ternary mixture. Only one product (rich in component A) is obtained using one pass. 

When a number of products are produced sequentially using one pass to produce one distillate product from a binary or a multicomponent mixture in a continuous column, the operation is defined as SPSSS operation. For binary mixtures there will be a maximum of two distillate cuts, one being a specified product cut and the other being an off-specification product cut. Under such condition, of course, the final bottom product will be of specified purity. Figure 11.4 illustrates the operation for a ternary mixture. Here two passes are allowed in the same column sequentially. The first pass separates component A and the second pass separates component B. 

When a distillate product of specified purity is produced using a number of passes in a continuous column, the operation is defined as MPSSS operation. The number of distillate products depends on the number of components in the original mixture. This type of operation is used mainly to improve the recovery of a particular species in the feed mixture (see example section for further explanation). This type of operation can be applied to both binary and multicomponent mixtures. 

This process is used extensively in the laboratory and in small production units where the same equipment can serve different applications. Between each batch operation, the equipment can be cleaned as necessary. When the charge is a multicomponent mixture, batch distillation in a single column can separate all constituents, whereas continuous distillation would require several columns. 

Many of the distillations of industry involve more than two components. While the principles established for binary solutions generally apply to such distillations, new problems of design are introduced which require special consideration. An important principle to be emphasized is that a single fractionator cannot separate more than one component in reasonably pure form from a multicomponent solution, and that a total of C - 1 fractionators will be required for complete separation of a system of C components. Consider, for example, the continuous separation of a ternary solution consisting of components A, B, and C, whose relative volatilities are in that order (A most volatile). In order to obtain the three substances in substantially pure form, the following two-column scheme can be used. The first column is used to separate C as a residue from the rest of the solution. This residue is necessarily contaminated with a small amount of B and an even smaller amount of A. The distillate, which is necessarily contaminated with a small amount of C, is then fractionated in the second column to give nearly pure A and B. 

We have considered only binary distillation to this point because multicomponent systems increase complexity by orders of magnitude. Furthermore, we continue to fight the battle of too little data. In order to properly design a multicomponent column, we would need equilibrium data for the multicomponent system and enthalpy data. The latter is usually not even available for binaries. Even with all of the required data available, the column design would be an extraordinarily complicated calculation requiring a stage-by-stage determination. 

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