Distillation trays, binary systems

Multicomponent distillation is by far the common requirement for process plants and refineries, rather than the simpler binary systems. There are many computer programs which have been developed to aid in accurately handling the many iterative calculations required when the system involves three to possibly ten individual components. In order to properly solve a multicomponent design, there should be both heat and material balance at every theoretical tray throughout the calculation. 

Example The location of the best temperature-control tray in a distillation column is a popular subject in the process-control literature. Ideally, the best location for controlling distillate composition xa with reflux flow by using a tray temperature would be at the top of the column for a binary system. See Fig. 8.9o. This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from a steadystate standpoint because it keeps the distillate composition constant at steadystate in a constant pressure, binary system. Holding a temperature on a tray farther down in the column does not guarantee that x will be constant, particularly when feed composition changes occur. 

By assigning, for instance, binary variables to represent the existence or not of process units in a total process system, the potential matches of hot and cold streams so as to reduce the utility consumption or the existence of trays in a distillation- based separation system, the resulting total number of binary variables can grow to 1000 or more which implies that we have to deal with a large combinatorial problem (/. ., 21000 process flowsheets). 

Many industrial columns use temperatures for composition control because direct composition analyzers can be expensive and unreliable. Although temperature is uniquely related to composition only in a binary system (at known pressure), it is still often possible to use the temperatures on various trays up and down the column to maintain approximate composition control, even in multicomponent systems. Probably 75 percent of all distillation columns use temperature control of some tray to hold the composition profile in the column. This prevents the light-key (LK) impurities from dropping out the bottom and the heavy-key (HK) impurities from going overhead. 

O Connell derived his correlation from binary systems in distillation service with bubble-cap trays. Calculated values are slightly conservative for sieve and valve trays. Credit for the slight improvement in valve and sieve tray efficiency should be ignored and counted as a design margin. A separate correlation was developed for absorption services. 

The main emphasis will be upon stagewise, continuous feed distillation, schematically shown in figure 6.1. The column may contain trays or packing (as described later) to promote good vapour-liquid contact. The quantitative analysis is confined to two-component (binary) systems in trayed columns. 

Total reflux exists in a distillation column, whether a binary or multicomponent system, when all the overhead vapor from the top tray or stage is condensed and returned to the top tray. Usually a column is brought to equilibrium at total reflux for test or for a temporary plant condition which requires discontinuing feed. Rather than shut down, drain and then re-establish operating conditions later, it is usually more convenient and requires less... 

Note that these values for theoretical trays do contain corrections in overall efficiency, and hence are not the actual trays for the binary distillation column. Efficiencies generally run 50-60% for systems of this type which will yield a column of actual trays almost twice the theoretical at the operating reflux. 

Example 12.6. Let us consider a much more complex system where the advantages of frequencynlomain solution will be apparent. Rippin and Lamb showed how a frequency-domain stepping technique could be used to find the frequency response of a binary, equimolal-overflow distillation column. The column has many trays and therefore the system is of very high order. 

Once one has proposed alternative configurations for systems of separation devices to effect a desired separation, one must then design these devices so the various alternatives may be compared. For a distillation column, the first set of design decisions is to choose the number of trays, the feed tray location, and the reflux ratio at which to operate it. For a binary separation, the McCabe-Thiele diagram (or the concepts behind it) is an indispensable aid in making these decisions. 

Biddulph and Kalbassi (1988) investigated the distillation of the ternary system methanol(l)-l-propanol(2)-water(3). In separate experiments they determined the numbers of transfer units for each binary pair that makes up the ternary system. Estimate the number of transfer units for the ternary system at total reflux if the composition of the liquid leaving the tray is... 

In an ideal binary distillation column the dynamics of each tray can be described by first-order systems. Are these capacities interacting or not What general types of responses would you expect for the overhead and bottoms compositions to a step change in the feed composition ... 

The results of simulation models are strongly affected by the particular assumptions made in the plant model and by the model parameters. This is the reason why these parameters should be fitted to reliable data describing simplified systems, for example, binary mixtures. Regressing parameters to observations on the complex target system is usually not feasible, as the influence of the assumptions made on the performance of the equipment (e.g., tray efficiency in distillation) is not sufficiently isolated from the influence of the model parameters. The experimental data for the isolated subsystems are the key for a successful process model. They contain all the information about the process model, the model parameters are only a representation. If new data become available, the model can be upgraded by extending the database and perform a new regression. 

To provide an example of this phenomenon, let us consider the azeotropic distillation column shown in Figure 1.1. This is the first column in a two-column azeotropic distillation system to separate a binary mixmre of ethanol and water. Benzene is used as the light entrainer that carries the water overhead and produces a bottoms product of high-purity ethanol. The column has an organic reflux (OR) stream fed to the top tray, which comes from a downstream decanter. A recycle stream from the second column is also fed to the azeotropic column. The overhead vapor is condensed and goes to a decanter. The organic phase is refluxed back to the first column. The aqueous phase is fed to a recovery column whose bottoms product is high-purity water and whose distillate is recycled back to the azeotropic column. Note that the azeotropic column is a stripper with no condenser. Once the... 

One conld also try to estimate the quality ( inferential control ) by using a suitable algorithm that uses conventional measured values of for example temperatme, pressure, etc. A simple example is the pressure compensation of temperatme measurements on the tray of a distillation column. For binary mixtures this yields a measure of the composition, for a multi-component system it is an approximate measure of the composition. 

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