Chapter 1
General Introduction

1.1 Overview of Distillation Processes

The term "distillation" refers to any process that facilitates the separation of solution components using their different volatilities. Distillation processes are categorized according to the number and nature of the components being separated as shown in Figure 1.1. At a primary level, distillation processes can be categorized as simple distillation or fractional distillation. Simple distillation utilizes a still to effect the separation of two miscible liquids or a single liquid and its nonvolatile solutes in a single vaporization-condensation process. Fractional distillation refers to the separation of two or more liquids using repeated vaporization-condensation steps in a single column.

Figure 1.1 Overview of distillation processes.

Simple distillation of a mixture of two liquids facilitates enrichment of the distillate (vaporized fraction) with the most volatile component with a corresponding enrichment of the residue with the second component. The distillate is the desired component in typical industrial applications. The degree of enrichment depends on the relative volatilities of the liquids. In some applications, the distillate is subjected to a second simple distillation step in a separate still to obtain the required separation. In simple distillation involving a liquid and its nonvolatile solutes, a high degree of separation can be achieved by prolonged boiling of the liquid. Here, the distillate is free of solutes other than trace amounts transferred by the entrainment of liquid droplets in the vapor. In many cases, distillation is carried out progressively through a series of simple distillation steps in a continuous process. Removal of the liquid from its nonvolatile solutes defines this process as a stripping operation. Furthermore, the still or series of stills in which stripping occurs is referred to as an evaporator. Fractional distillation results in a high degree of liquid-liquid separation due to repetitive distillation steps. This process is referred to as rectification when used for the separation of just two liquids. Examples highlighting the importance and widespread use of simple distillation and fractional distillation processes in society are discussed below.

The production of whisky and brandy are examples of simple distillation involving two liquids, water with a boiling point of 100 °C and ethanol with a boiling point of 78 °C. Whisky is distilled from grain mash that has been fermented to an ethanol concentration of 5-7% v/v, while brandy is distilled from wine having an ethanol concentration of 8-12% v/v. These low-alcohol solutions are boiled in a pot still to produce a distillate with an ethanol concentration of 20-35% v/v. The distillate is then subjected to simple distillation in a second pot still to produce a spirit with an ethanol concentration of about 70% v/v. The volatile organic aroma components of the base material are transferred and condensed with the vapor in both steps of the process. Finally, the spirit is subjected to maturation in accordance with product identification requirements.

Simple distillation involving a single liquid and its nonvolatile solutes is a widely used form of industrial distillation. An important example in which the distillate is the desired product is the desalination of seawater or brackish water. Water is evaporated from the salty solution for subsequent condensation and consumption. This stripping process is generally carried out using multistage flash distillation (MSF), multiple-effect distillation (MED), or vapor compression distillation (VCD). These simple distillation processes owe their success to internal energy recovery mechanisms as discussed in Section 1.5. With an estimated one billion people currently without access to safe drinking water [Blanco et al. (2009)] and a rapidly expanding global population, the role of desalination processes will continue to expand.

There are numerous examples of stripping operations in the food industry where the residue is the desired product. Water is stripped from materials such as fruit juices and dairy products to produce concentrates for cost-effective transport, storage, preservation, or spray drying. MED and VCD are typically used for such purposes with MSF confined to desalination applications. In other food industry applications, water stripping is undertaken to effect solute crystallization. Indeed, one of the largest operations of this type is the concentration of sugarcane juice for subsequent sugar recovery. In this case, MED is used to concentrate the juice to near saturation before entering a separate (pan) stage where crystallization is induced. An example of rectification is the production of industrial alcohol with an ethanol concentration in excess of 90% v/v for use in formulations such as biofuels, antiseptics, and solvents. Sugars from several different sources may be fermented to produce dilute aqueous ethanol solutions for this purpose. An upper limit of 96% v/v ethanol is dictated by the formation of an ethanol-water azeotrope from which no further separation can be achieved using conventional distillation processes. Rum and vodka are also produced by rectification using feedstocks produced by the fermentation of molasses and potatoes, respectively.

A basic requirement for rectification is a column filled with high-surface area packing material. A dilute aqueous ethanol solution for example is boiled at the base of the column to produce a vapor enriched in ethanol. The vapor cools as it rises to the point where condensation occurs on the surface of the packing material. The condensate then trickles toward the base of the column. Rising hot vapor from the increasingly higher boiling residue boils some of the returning liquid to produce a vapor that is further enriched in ethanol. The new vapor rises further up the column due to its higher volatility. This process is repeated many times as vapor ascends the column to produce the required ethanol concentration for external condensation. The vapor is removed from the top of the column while the water-enriched residue remains at the base. This process may be carried out in batch or continuous steady-state mode.

A modified fractional distillation column is used when several liquid fractions require separation. A well-known example of this is the refining of crude oil using a column containing horizontal condensation plates at different heights. The feedstock is boiled at the base of the column at a temperature at which all but the heaviest components vaporize. Controlled temperature reduction with increasing column height facilitates the condensation of different boiling point fractions on plates at different heights. In this way, the crude oil is separated into fuel oil, lubricating oil, diesel, kerosene, naptha, and gasoline in ascending order for removal through ports at the side of the column. Heavy residuals such as tar, asphalt, and waxes are collected from the base of the column while light hydrocarbon gases (C1-C4) are collected from the top of the column for external condensation. This process is operated as a continuous process.

Membrane distillation (MD) and osmotic distillation (OD) separate water from its nonvolatile solutes through vaporization and condensation and can therefore be classified as stripping distillation processes. Accordingly, desalination applications involve recovery of the distillate whereas concentrate production applications involve recovery of the residue.

In accordance with normal practice, the terminology applicable to conventional distillation processes has been changed in favor of membrane process terminology when discussing MD and OD. That is, the residue is referred to as retentate, while the distillate is referred to as permeate. In concentrate production applications, the fully concentrated retentate is referred to as concentrate.

MD and OD differ from MSF, MED, and VCD by effecting distillation through a porous, air-filled (usually) hydrophobic membrane using opposing membrane faces as evaporation and condensation surfaces. Membrane hydrophobicity excludes liquid water and its nonvolatile solutes from entering the porous structure and mixing with the permeating vapor. Furthermore, the small vapor gap afforded by the membrane reduces the resistance to vapor transfer to the point of condensation. Despite the success of MSF, MED, VCD, and other nondistillation stripping processes including freeze concentration (FC), reverse osmosis (RO), and electrodialysis (ED), MD and OD have attracted considerable attention as potential alternatives to these processes in some applications. This has been due to their special characteristics with respect to product quality, simplicity of operation, and potential energy savings.

This chapter traces the development of MD and OD from laboratory novelties to their present status as advanced stripping processes entering commercialization. Qualitative descriptions of the fundamental principles and characteristics of each process are provided and compared with those of established processes. Here, a heavy emphasis is placed on the factors that affect product quality. Before proceeding, however, it is beneficial to consider the meaning of the term "°Brix" frequently encountered in this and subsequent chapters. Strictly, the term refers to the percent by weight of pure sucrose in water at 20 °C. More generally, it has been used as a measure of the approximate sugars concentration in multisolute solutions in several industries. Indeed, °Brix...