Separations Using Superheated Water

Superheated water is liquid water under pressure between 100°C and its critical temperature of 374°C. The rationale for using superheated water is explained. An overview of the work on its use for separation processes is given. The work that has been carried out is mostly of laboratory scale, although some pilot studies and small industrial processes are described. INTRODUCTION It is useful to define superheated water as liquid water under pressure between 100°C and 374°C to distinguish it from cold or hot water below 100°C and supercritical water above 374°C. It has the advantages of many other green solvents, being cheap and readily available, non-toxic and easily disposed of. The pressures required are much lower than for most supercritical fluids, being tens rather than hundreds of bars, making superheated water processes cheaper than supercritical fluid processes. The advantage of superheated water is that it is much less polar than water at ambient temperature and therefore more compatible with organic molecules. The use of superheated water is not new. It has been used in the food industry for cooking a little above 100°C and the final extractions of instant coffee are sometimes up to 120°C. Recrystallizations in water contained in sealed tubes above 100°C have been carried out for 100 years or more. Superheated water has also been used for waste treatment by the so-called wet-air oxidation process [1]. Chemical reactions have also been carried out in superheated water and this work has been thoroughly reviewed recently [2-4]. However, in recent years there has been a renewed interest in superheated water as a replacement for organic solvents in separations and related processes. Over 100 research papers on this subject have been published in the last 5 years. Much of this work has been restricted to the range of 100°C to 300°C. At these lower temperatures water is not highly compressible, and the pressure of the medium does not have much effect, as long as it is high enough to maintain the water in the liquid phase. This work on superheated water has been reviewed [5] and the manipulation of water properties with temperature to achieve process ends has been made the subject of a patent [6]. Most of the work that has been described is on a laboratory scale and some of it is directed towards analysis. However, some larger-scale processes are under consideration, mainly for environmental reasons. POLARITY OF SUPERHEATED WATER, THE SOLUBILITY OF ORGANIC COMPOUNDS IN IT AND ITS USE FOR PROCESSES Water changes dramatically when its temperature rises, because of the breakdown in its hydrogen-bonded structure with temperature. The high degree of association in the liquid causes its relative permittivity (more commonly called its dielectric constant) to be very high at around 80 under ambient conditions. But as the temperature rises the hydrogen bonding breaks up and the dielectric constant falls, as shown in Figure 1. By 205°C its dielectric constant has fallen so that it is equal to that for methanol (i.e. 33) at ambient temperature. Thus, between 100°C and 200°C superheated water is behaving like a water-methanol mixture. Partly because of its fall in polarity with temperature, superheated water can dissolve organic compounds to some extent, especially if they are slightly polar or polarizable like aromatic compounds. An early measurement [7] showed that naphthalene forms a 10 mass % solution in water at 270°C. The solubility of an organic compound is often many orders of magnitude higher than its solubility in water at ambient temperature for two reasons, one being the polarity change. The other is that a compound with low solubility at ambient temperature will have a high positive enthalpy of solution, and thus a large increase in solubility with temperature as shown in Table 1 for the pesticide chloranthonil. A large number of solubility and phase behaviour measurements have been made. Because of the greater solubility of some organic compounds in superheated water, this medium can be considered for the extraction and other processes, to replace conventional organic solvents. As would be expected, reactions of the compounds being processed may also occur, by hydrolysis, oxidation, etc. In some cases these reactions are to be avoided as far as possible by choice of conditions, such as temperature. In others, they are desirable and form part of the process for example by degrading and destroying the extract or by producing desired products. It might be imagined that superheated water processes were costly in energy terms. However, this is not the case because water stays as a liquid and the latent heat of evaporation is not required. As an example, we compare superheated water extraction with steam distillation. A greater mass of superheated water may be needed for a given mass of material to be extracted. However, only 505 kJ kg is required to heat liquid water from 30°C to 150°C, compared with 2550 kJ kg required to convert water at 30°C to steam at 100°C. Moreover, it is relatively easy to recycle the heat in a superheated water process by passing the water leaving the extraction cell through a heat exchanger to heat the water flowing to the cell. More than 80% of the energy can be realistically recovered in this way, as will be shown below. EXTRACTIONS AND RELATED PROCESSES ON A LABORATORY SCALE A simple example of an extraction apparatus for work on a small scale is shown schematically in Figure 2, which was designed for plant material extraction. The system is readily adapted for related work, the main consideration being the pressure required at the temperature to be studied. Water is deoxygenated, by sparging with helium for half an hour. It is then pumped at around 1 mL per minute into the extraction cell in an oven at the required temperature, through a long tube, which acts as a heat exchanger to bring the water up to temperature. On Figure 1: The dielectric constant (relative permittivity) of liquid water as a function of temperature at its vapour pressure. 100