Distillation and Multiple Column Processes

Complex distillation may be defined as a multistage vapor-liquid separation process that includes one or more of the following features multiple feeds, side draws, pumparounds, and side heaters or coolers. 

Absorbers and strippers are examples of multiple feed columns. In general, in multiple feed columns, the feeds are of different compositions and/or thermal conditions and are fed to different column trays. In certain processes, multiple products are taken from the column, each from a different tray, each with a distinct composition corresponding to the draw tray fluid composition. 

Side coolers or heaters are used for redistributing the heat load along the column in order to control liquid and vapor traffic or L/V ratios in the various sections of the column. Pumparounds may be used as the means of transferring heat between the side heaters or coolers and the column. 

Multistage complex columns often perform the functions of a number of columns combined in one. In analyzing the performance of complex columns, it is helpful to consider the column as made up of column sections, single equilibrium stages, and stream splitters. 

A column section is a vertical vapor-liquid countercurrent adiabatic column section with no heaters or coolers, and whose only feeds and products are a liquid feed and a vapor product at the top, and a vapor feed and a liquid product at the bottom. The feeds to the column section are considered fixed or determined by their points of origin. Assuming the number of stages and the pressure in the column section are also fixed, this unit has no variables that can be controlled independently and therefore has zero degrees of freedom. 

---

Chapter 9 Complex Distillation and Multiple Column Processes... 

The reaction studied in Section 2.2 has two reactants and therefore offers the possibility of adjusting the compositions of the reactants in the reactor to achieve some economic or control objective. In this section we first find the cost of operating single and multiple CSTR processes to achieve a specified conversion. Then we design an alternative process consisting of a reactor and a distillation column that separates product C from the unreacted A and B in the reactor effluent and recycles them back to the reactor. 

The many examples given in this book have illustrated that dynamic simulations of distillation columns can be used to develop effective control structures for a wide variety of individual columns and multiple-column systems. However, there is another use of dynamic simulations that is very important for the safe operation when process and equipment emergencies occur. The most common example is a cooling water failure, which can lead to very rapid increases in column pressure. 

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

In catalytic distillation the temperature also varies with position in the column, and this will change the reaction rates and selectivities as well as the equilibrium compositions. Temperature variations between stages and vapor pressures of reactants and products can be exploited in designing for multiple-reaction processes to achieve a high selectivity to a desired product with essentially no unwanted products. 

Reprinted from Journal of Process Control, 6, Mujtaba, I.M. and Macchietto, S., Simultaneous optimisation of design and operation of multicomponent batch distillation column-single and multiple separation duties, 27-36, Copyright (1996), with permission from Elsevier Science. ... 

In this section we present more complex distillation column processes that go beyond the plain vanilla variety. Industry uses columns with multiple feeds, sidestreams, combinations of columns, and heat integration to improve the efficiency of the separation process. Very significant reductions in energy consumption are possible with these more complex configurations. However, they also present more challenging control problems. We briefly discuss some common control structures for these systems. 

We begin here with a very simple isomerization process that is similar to some of the simplified processes studied in Chap. 2. The process consists of a reactor, two distillation columns, and a liquid recycle stream. There are four components to consider. In subsequent chapters we look at processes with many more units, many more components, and multiple recycle streams. 

---