Distillation column configuration

Agrawal R. Synthesis of distillation column configurations for a multicomponent separation. Ind Eng Chem Res 1996 35 1059. 

This subsection describes how to generate the feasible combinatorial possibilities of distillation column configurations for separation of mixtures that do not form azeotropes. Components are named A, B, C, D,. . . and they are listed in the order of decreasing volatility (or increasing boiling temperature). We limit our considerations to splits where the most volatile (lightest) component and the least volatile (heaviest) component do not distribute between the top and bottom product. For simplicity we consider only separations where final products are relatively pure components. Systems containing simultaneously simple and complex distillation columns are considered. Simple columns are the conventional columns with one feed stream and two product streams complex columns have multiple feeds and/or multiple product streams. 

Agrawal, R. (1996). Synthesis of Distillation Column Configurations for a Multi-component Separation. Ind. Eng. Chem. Res., 35,1059-71. 

Agrawal, R. (1999). More Operable Fully Thermally Coupled Distillation Column Configurations for Multicomponent Distillation. Trans IChemE., 77, Part A, 543-53. 

Other distillation column configurations are possible [11-131. In these references, there are column configurations known as Pedyuk-type columns. It should be noted that even though the configuration in Figure 12.ltd looks like a Pedyuk-type II column, it is not, due to the presence of a reboiler and a condenser on the first column, units that are not present in the Ped5mk-type II column. 

Direct comparisons of the conventional multiunit process with the reactive column process at their economic optimum steady-state designs are given in Table 3.5 for five different kinetic cases. The results indicate that the TACs of both design configurations decrease as the value of (ATeq)366 increases. The results also show that the reactive distillation column configuration has lower capital cost and energy cost than the conventional configuration for all kinetic cases. These costs result in lower TAC for the reactive distillation columns compared to the reactor/column/recycle systems. 

Distillation columns have four or more closed loops—increasing with the number of product streams and their specifications—all of which interact with each other to some extent. Because of this interaction, there are many possible ways to pair manipulated and controlled variables through controllers and other mathematical functions with widely differing degrees of effectiveness. Columns also differ from each other, so that no single rule of configuring control loops can be apphed successfully to all. The following rules apply to the most common separations. 

A single-column distillation configuration called Flash Compact System has been proposed which is capable of delivering an equivalent high purity product. The key advantage lies in the lower capital and operating costs. The feed is heated and pre-flashed and then sent to a distillation column as two. separate vapour and liquid feeds. 

The majority of atmospheric crude oil distillation columns follow the configuration shown in Figure 11.17, which is basically the partially thermally coupled indirect sequence. 

The experimental facility is a pilot-scale distillation column connected to an industrial ABB MOD 300 distributed control system, which in turn is connected to a VAX cluster. The control system consists of a turbo node (configuration, history, console) remote I/O, and an Ethernet gateway, which allows communication with the VAX-station cluster through the network. This connection allows time-consuming and complex calculations to be performed in the VAX environment. Figure 10 shows the complete setup. 

Example 1.3. Our third example illustrates a typical control scheme for an entire simple chemical plant. Figure 1.5 gives a simple schematic sketch of the process configuration and its control system. Two liquid feeds are pumped into a reactor in which they react to form products. The reaction is exothermic, and therefore heat must be removed from the reactor. This is accomplished by adding cooling water to a jacket surrounding the reactor. Reactor elHuent is pumped through a preheater into a distillation column that splits it into two product streams. 

Given a number of input multicomponent streams which have specified amounts for each component, create a cost-optimal configuration of distillation columns, mixers, and splitters that produces a number of multicomponent products with specified composition of their components. 

To determine a minimum total annual cost configuration of non-sharp distillation columns out of the many alternatives embedded in the postulated superstructure, we define variables for the... 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Configuration for controlling the composition of both products of a distillation column without much interaction (left) and with interaction (right). 

Interaction is unavoidable between the material and energy balances in a distillation column. The severity of this interaction is a function of feed composition, product specification, and the pairing of the selected manipulated and controlled variables. It has been found that the composition controller for the component with the shorter residence time should adjust vapor flow, and the composition controller for the component with the longer residence time should adjust the liquid-to-vapor ratio, because severe interaction is likely to occur when the composition controllers of both products are configured to manipulate the energy balance of the column and thereby "fight" each other. 

In this configuration, the available separation section (tray or packed) is utilised in rectifying mode, with product cuts (recovered) and intermediate off-cut fractions (disposed of or recycled) collected as condensed distillate. A final residue bottom fraction may also be a desired product. Conventional column configuration has been discussed in section 1.4 of Chapter 1. Further details are given in later chapters. 

Column configurations (NT) and separation requirements (x D) for several cases are presented in Table 3.3. The condenser holdup was fixed to 2% of the total initial charge and the column hold up is varied as a percentage of the total initial charge to the column. The initial charge to the column (Bo) is 5 kmol with a light component mole fraction (xBo) of 0.6. Also the amount of distillate product required (ZD ) is set to 3 kmol. The column operates under constant condenser vapour load strategy (section 3.2.2) with a vapour load of 3 kmol/hr for all cases. 

Referring to Figure 2.2 for MVC column configuration, the model equations for the rectifying section are the same (except the reboiler equations) as those presented for conventional batch distillation column (Type III, IV, V in section 4.2). The model equations for the stripping section are the same (except the condenser equations) as those presented for inverted batch distillation column (Type III, IV, V in section 4.3.2). However, note that the vapour and liquid flow rates in the rectifying and stripping sections will not be same because of the introduction of the feed plate. 

Table 5.1 summarises the past work on dynamic optimisation of batch distillation using different types of column configurations. 

Table 5.6 presents the results of a set of 5 maximum profit problems considered by Kerkhof and Vissers (1978). Each problem has different column configuration (i.e. different number of plates, N), handles different feed mixture (characterised by different relative volatility, a)y has different distillate product quality (xq), has different cost values (i.e. Pr, C0) and different set up times (ts). The initial charge, B0 = 100 kmol, V= 60 kmol/hr, t,= 1 hr, and Cf= 150 /hr. All cases deal with binary mixtures of different initial composition, xB0. 

---