Conventional distillation columns

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

To produce a high purity product two distillation columns are operated in series. The overhead stream from the first column is the feed to the second column. The overhead from the second column is the purified product. Both columns are conventional distillation columns fitted with reboilers and total condensers. The bottom products are passed to other processing units, which do not form part of this problem. The feed to the first column passes through a preheater. The condensate from the second column is passed through a product cooler. The duty for each stream is summarised below ... 

The equality constraints. The process model comprises the equality constraints. For a conventional distillation column we have the following typical relations ... 

The amount of hazardous chemicals on-site can be reduced by methods other than altering the scale of production. For example, the amount of hazardous material stored on-site can often be significantly reduced, and if not, the hazardous materials can be stored in many small containers in separate facilities rather than in a single container. Therefore, if a container fails, the size and catastrophic potential of the release are much reduced. In addition, the amount of material needed in the production process can be reduced by using specially designed equipment (such as Higee columns, which replace conventional distillation columns). 

In membrane distillation, two liquids (usually two aqueous solutions) held at different temperatures are mechanically separated by a hydrophobic membrane. Vapors are transported via the membrane from the hot solution to the cold one. The most important (potential) applications of membrane distillation are in water desalination and water decontamination (77-79). Other possible fields of application include recovery of alcohols (e.g., ethanol, 2,3-butanediol) from fermentation broths (80), concentration of oil-water emulsions (81), and removal of water from azeotropic mixtures (82). Membrane (pervaporation) units can also be coupled with conventional distillation columns, for instance, in esterifications or in production of olefins, to split the azeotrope (83,84). 

On the other hand, a pervaporation membrane can be coupled with a conventional distillation column, resulting in a hybrid membrane/distillation process (228,229). Some of the investigated applications of such hybrid pervaporation membrane/distillation systems are shown in Table 9. In hybrid pervaporation/ distillation systems, the membrane units can be installed on the overhead vapor of the distillation column, as shown in Figure 13a for the case of propylene/ propane splitting (234), or they can be installed on the feed to the distillation column,... 

The prices shown for distillation columns are misleadingly low. Most of the cost of a conventional distillation column is associated with the reboiler, condenser, pumps, reflux drum, and internal trays or packing. Equipment costs can be ratioed by the 0.6 power with size to obtain rough prices for larger or smaller sizes. 

An ideal mixture of n components requires a sequence of n - 1 conventional distillation columns (two product streams) to separate the components completely. The columns can be arranged sequentially without recycle between them. This picture changes when mixtures forming azeotropes must be separated. Nonideal systems sometimes require complex distillation arrangements involving more than n - 1 columns with recycle of material between the columns. For the analysis of such systems, we recommend the use of residue curve maps. We base the following summary on the excellent book by Doherty and Malone (1998), who pioneered the use of these techniques. 

To develop the alternative process configurations needed to separate a given feed mixture into a set of specified products, we need to know just what distillate and bottoms product compositions we can reach when using a conventional distillation column. We shall start by examining this problem for ideally behaving mixtures. We shall then look at the much harder problem in which the mixtures do not behave ideally. 

The first and third packed sections are similar to those of a conventional distillation column, and only distributor and support trays are required for the operation of the DWDC. A reboiler (Figure 5, element 9) that can be charged with the reacting mixture in the batch operation fashion was placed at the end of the third packed section. Finally, several thermocouples were implemented in the DWDC to register the temperature... 

A total column differs from a column section in that the former may have side feeds and products and coolers and heaters. Specifically, a conventional distillation column has one feed stream, an overhead (distillate) product and a bottoms product, a condenser, and a reboiler, as shown schematically in Figure 5.3. 

In conclusion, the temperature profile in conventional distillation columns is the result of both phase equilibrium relations and enthalpy balances. In narrow-boiling mixtures, the phase equilibrium effect is generally more pronounced, while in wide-boiling mixtures, the enthalpy balances are more significant. The importance of the distinction between the two effects is twofold. First, different mathematical solution algorithms are better suited for each situation, as will be discussed in Chapter 13. Second, the understanding and prediction of column performance is enhanced when the two effects are recognized. Examples 7.1 and 7.2 illustrate the two cases. 

The method is basically designed for solving conventional distillation columns with one feed, F, a distillate, V, a bottoms product, Lf, a condenser with duty g, and a reboiler with duty Qj. In the generalized model of Figure 13.1, all the feeds except one, all the products except V, and Lf, and all the duties except gj and Qf are set to zero. The method becomes numerically unstable for columns other than conventional distillation. Another drawback of the method is that it can handle limited types of performance specifications. In the outline that follows, the specifications are the distillate rate, V and the reflux rate, T,. It is assumed that the number of stages and the column pressure profile are fixed at given values. 

The modified Thiele-Geddes method was mainly designed for conventional distillation columns although it has been generalized to handle complex columns (Holland, 1963). It is limited in the types of performance specifications it can handle and could be numerically unstable, especially for wide-boiling or nonideal mixtures (Wang et al., 1980). 

DEVELOPMENT AND APPLICATION OF THE THETA METHOD OF CONVERGENCE TO CONVENTIONAL DISTILLATION COLUMNS... 

In this chapter, the 9 method is developed and applied to conventional distillation columns. In Sec. 2-1, the equations required to describe conventional distillation columns are presented. The formulation and application of the 9 method of convergence, the Kb method for computing temperatures, and the constant-composition method for solving the enthalpy balances for the total-flow rates are presented in Sec. 2-2. 

The development of the recurrence formulas is outlined in Prob. 2-3. An improved form of these expressions was recently proposed by Boston and Sullivan.1 For the special case of a conventional distillation column in which model 2 (see Fig. 2-2) for the feed plate is assumed, the procedure proposed by Boston and Sullivan (see Prob. 2-3) may be used.to reduce the above formulas to the following form... 

While the 6 method constitutes an exact solution to certain total reflux problems, it represents only an approximate solution to problems wherein operating conditions other than total reflux are employed. The 6 method for a conventional distillation column at any operating condition other than total may be represented with the aid of Fig. 2-7 as the shifting of the most recently calculated bjd profile up or down the same distance ( In 0 ) for each component as required to obtain a new set of bJdiS which are in agreement with the specified value of D. 

The calculation procedures [the 0 method, Kb method, and constant composition method] developed in Chap. 2 for conventional distillation columns are applied to complex distillation columns in Sec. 3-1. For solving problems involving systems of columns interconnected by recycle streams, a variation of the theta method, called the capital 0 method of convergence is presented in Secs. 3-2 and 3-3. For the case where the terminal flow rates are specified, the capital 0 method is used to pick a set of corrected component-flow rates which satisfy the component-material balances enclosing each column and the specified values of the terminal rates simultaneously. For the case where other specifications are made in lieu of the terminal rates, sets of corrected terminal rates which satisfy the material and energy balances enclosing each column as well as the equilibrium relationships of the terminal streams are found by use of the capital 0 method of convergence as described in Chap. 7. 

A complex distillation column is defined as one which has either more feeds introduced or streams withdrawn or a combination of these than does a conventional distillation column. To demonstrate the application of the 0 method and associated calculational procedures, the complex column shown in Fig. 3-1... 

After either a corrected set of xys or yy s has been computed as described above, a new set of temperatures may be computed by use of the Kb method in the same manner that was described in Chap. 2 for conventional distillation columns. 

The development of the expressions for the constant-composition form of the enthalpy balances is carried out in the same manner demonstrated in Chap. 2 for conventional distillation columns. For example, the energy balance enclosing the top of the column and some plate j located between plates p and / — 1 of the column shown in Fig. 3-1 can be expressed as follows... 

The capital 0 method for systems is introduced by the formulation of the equations for this method for the simple system of two conventional distillation columns shown in Fig. 3-8. In a subsequent section, the formulation is generalized for the case of any number of columns with any number of sidestreams withdrawn. 

Next, the 2N Newton-Raphson method is applied to reboiled absorbers, conventional distillation columns, and complex distillation columns, and then a procedure which makes use of the calculus of matrices for solving these equations is presented. 

Table 4-2 Specifications, independent variables, and functions for conventional distillation columns...

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