Basic Concepts of Distillation

Equilibrium data correlations can be extremely complex, especially when related to non-ideal multicomponent mixtures, and in order to handle such real life complex simulations, a commercial dynamic simulator with access to a physical property data-base often becomes essential. The approach in this text, is based, however, on the basic concepts of ideal behaviour, as expressed by Henry s law for gas absorption, the use of constant relative volatility values for distillation and constant distribution coeficients for solvent extraction. These have the advantage that they normally enable an explicit method of solution and avoid the more cumbersome iterative types of procedure, which would otherwise be required. Simulation examples in which more complex forms of equilibria are employed are STEAM and BUBBLE. 

In this section we discuss some basic concepts concerning distillation control degrees of freedom, basic manipulated variables, and constraints. 

Another thermally driven membrane process is membrane distillation. Here, a porous membrane separates two liquids which do not wet it. If the liquids differ in temperature, the resulting vapour pressure differencecauses vapour molecules to permeate from the high-temperature (high vapour pressure) side to the low-temperature (low vapour pressure) side. The basic concept of membrane distillation will be described below. 

The readership at this level is broad. The topic of separation processes taught at all engineering schools is inextricably linked to mass transport, and students will benefit from an early introductory treatment of mass transfer combined with the basic concepts of separation theory. There is, in fact, an accelerating trend in this direction, which aims for students to address later the more complex operations, such as multicomponent and azeotropic distillation, chromatography, and the numerical procedures to simulate these and other processes. 

Once the basic concept of material-balance control has been selected for a process, one must apply the same concept to all process steps. It is for this reason that the first step in designing column controls is to determine the material-balance control arrangement. Control in the direction of flow is the most commonly used concept (although the least desirable), and a frequently encountered arrangement is shown on Figure 1.5. Here level in the condensate receiver (also commonly called reflux drum or accumulator) sets the top product, or distillate flow, while the level in the base of the column sets the bottom product flow in other columns base level sets steam or other heat-transfer media to the reboiler, in which case the condensate receiver level sets top product flow. 

Solute equilibrium between the mobile and stationary phases is never achieved in the chromatographic column except possibly (as Giddings points out) at the maximum of a peak (1). As stated before, to circumvent this non equilibrium condition and allow a simple mathematical treatment of the chromatographic process, Martin and Synge (2) borrowed the plate concept from distillation theory and considered the column consisted of a series of theoretical plates in which equilibrium could be assumed to occur. In fact each plate represented a dwell time for the solute to achieve equilibrium at that point in the column and the process of distribution could be considered as incremental. It has been shown that employing this concept an equation for the elution curve can be easily obtained and, from that basic equation, others can be developed that describe the various properties of a chromatogram. Such equations will permit the calculation of efficiency, the calculation of the number of theoretical plates required to achieve a specific separation and among many applications, elucidate the function of the heat of absorption detector. 

In this chapter, four basic distillation control schemes were introduced along with a number of variations on each scheme. The concept was introduced that there can be more than one way to successfully control a distillation column. In some schemes, the separation power base is controlled by the ratio of steam/feed and then the distillate flow rate or reflux flow rate can be manipulated to control an MRT point in the distillation column. In other schemes, the separation power base is controlled by the ratio of reflux/feed, and the steam to the reboiler or the bottoms flow rate can be manipulated to control an MRT point in the column. Control of reflux drum level and column base level was presented as basic to all control schemes. Control of column pressure was considered to have an overriding effect on the stability of distillation column control. 

Jha continues, throughout this book, his distinguished track record of distilling complex theoretical physical concepts into an understandable technical framework that can be extended to practical applications across a wide array of modern industries. His big-picture approach, which does not compromise the basic underlying science, is particularly refreshing. This approach should help present-day students. 

---