Superheated water - AbsoluteAstronomy.com

Superheated water is liquid water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

under pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

at temperatures between the usual boiling point

Boiling point

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

(100 °C) and the critical temperature (374 °C). It is also known as subcritical water and pressurized hot water. Superheated water referred to in this article is stable because of an applied over pressure which raised the boiling point, or by heating in a sealed vessel with a headspace, where the liquid water is in equilibrium with vapour at the saturated vapor pressure. This is distinct from the use of the term superheating

Superheating

In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling...

to refer to water at atmospheric pressure above its normal boiling point, which has not boiled due to a lack of nucleation sites (sometimes experienced by heating liquids in a microwave).

Many of the anomalous properties of water are due to very strong hydrogen bonding. Over the superheated temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

range the extensive hydrogen bonds break down, changing the properties more than usually expected by increasing temperature alone. Water effectively becomes less polar and behaves more like an organic solvent such as methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

or ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

. Solubility of organic materials and gases increases by several orders of magnitude and the water itself can act as a solvent, reagent and catalyst in industrial and analytical applications, including extraction, chemical reactions and cleaning.

Change of properties with temperature

All materials change with temperature, but water shows changes which are much greater than would be expected from temperature considerations alone. Viscosity

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

and surface tension

Surface tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

of water drop and diffusivity

Diffusivity

Diffusivity can refer to:*Diffusivity of heat*Diffusivity of mass:** Molecular diffusivity ** Eddy diffusivity*Momentum diffusivity...

increases with increasing temperature.

Self-ionization of water

Self-ionization of water

The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...

increases with temperature, and the pKw of water at 250°C is closer to 11 than the more familiar 14 at 25°C. This means that the concentration of hydronium

Hydronium

In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...

ion (H₃O⁺) is higher, and hence the pH is lower (although the level of hydroxide (OH^-) is increased by the same amount so the water is still neutral). Specific heat capacity at constant pressure also increases with temperature, from 4.187 kJ/kg at 25°C to 8.138 kJ/kg at 350°C. The dielectric constant

Dielectric constant

The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

(relative permittivity) decreases significantly as the temperature rises, which has a significant effect on the behaviour of water at high temperatures.

Explanation of anomalous behaviour

Water is a polar

Dipole

In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

molecule, where the centers of positive and negative charge are separated. In an applied electric field, the molecules align with the field. In water, the extensive hydrogen bonded network tends to oppose this alignment, and the degree to which this occurs is measured by the relative permittivity

Permittivity

In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...

. In water, polarity shifts are rapidly transmitted through shifts in orientation of the linked hydrogen bonds, and therefore water has a high relative permittivity, about 80 at room temperature. This allows water to dissolve salts, as the attractive electric field between ions is reduced by about 80 fold. As the temperature increases, the thermal motion of the molecules disrupts the hydrogen bonding network, and therefore the relative permittivity decreases with temperature, to about 7 at the critical temperature. At 205°C the relative permittivity has fallen to 33, the same as methanol

Methanol

at room temperature. Thus from 100°C to 200°C water behaves like a water / methanol mixture. The disruption of the extended hydrogen bonding is also responsible for much of the anomalous behaviour of superheated water, as extra energy needs to be supplied to break the bonds (increased heat capacity), and the molecules move more freely (viscosity, diffusion and surface tension effects).

Organic compounds

Organic molecules

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

often show a dramatic increase in solubility in water as the temperature rises, partly because of the polarity changes described above, and also because the solubility of sparingly soluble materials tends to increase with temperature as they have a high enthalpy of solution

Enthalpy change of solution

The enthalpy of solution, enthalpy of dissolution, or heat of solution is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure resulting in infinite dilution....

. Thus materials generally considered “insoluble” can be very soluble in superheated water. The solubility of PAH

Polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...

s is increased by 5 orders of magnitude from 25°C to 225°C and naphthalene

Naphthalene

Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...

, for example, forms a 10% wt solution in water at 270°C,
and the solubility of the pesticide chloranthonil with temperature is shown in the table below.

Solubility of Chloranthonil in Water
T (°C)	Mole Fraction
50	5.41 x 10^-8
100	1.8 x 10^-6
150	6.43 x 10^-5
200	1.58 x 10^-3

Thus superheated water can be used to process many organic compounds with significant environmental benefits compared to the use of conventional organic solvents.

Salts

Despite the reduction in relative permittivity, many salts remain very soluble in superheated water until the critical point is approached. Sodium chloride

Sodium chloride

Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

, for example, dissolves at 37 wt% at 300°C
As the critical point is approached, the solubility drops markedly to a few ppm

Parts-per notation

In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...

, and salts are hardly soluble in supercritical water. Some salts do show a reduction in solubility with temperature, but this behaviour is less common.

Gases

The solubility of gases in water is usually thought to decrease with temperature, but this only occurs to a certain temperature, then solubility increases again. For nitrogen, this minimum is 74°C and for oxygen it is 94°C
Therefore gases are quite soluble in superheated water at elevated pressures. Above the critical temperature, water is completely miscible with all gasses. The increasing solubility of oxygen in particular allows superheated water to be used for the wet oxidation

Wet oxidation

Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidizer. It is referred to as "Wet Air Oxidation" when air is used...

processes

Corrosion

Superheated water can be more corrosive than water at ordinary temperatures, and at temperatures above 300°C special corrosion resistant alloys may be required, depending on the other components dissolved in the water. However, continuous use of carbon steel pipes for 20 years at 282°C has been reported without significant corrosion,
and stainless steel cells showed only slight deterioration after 40-50 uses at temperatures up to 350°C.
The degree of corrosion which can be tolerated depends on the use, and even corrosion resistant alloys can fail eventually. Corrosion of an Inconel U-tube in a heat exchanger was blamed for an accident

Nuclear and radiation accidents

A nuclear and radiation accident is defined by the International Atomic Energy Agency as "an event that has led to significant consequences to people, the environment or the facility...

at a nuclear power station

Indian Point Energy Center

Indian Point Energy Center is a three-unit nuclear power plant station located in Buchanan, New York just south of Peekskill. It sits on the east bank of the Hudson River, 38 miles north of New York City...

.
Therefore, for occasional or experimental use, ordinary grades of stainless steel are probably adequate with continuous monitoring, but for critical applications and difficult to service parts, extra care needs to be taken in materials selection.

Effect of pressure

At temperatures below 300°C water is fairly incompressible, which means that pressure has little effect on the physical properties of water, provided it is sufficient to maintain liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

state. This pressure is given by the saturated vapour pressure, and can be looked up in steam tables, or calculated here.

As a guide, the saturated vapour pressure at 121°C is 200 kPa

KPA

KPA may refer to:* Kenya Ports Authority* Kiln phosphoric acid, a dry process to produce phosphoric acid at high temperature in a kiln* Kilopascal , a unit of pressure* Known-plaintext attack, a method of cryptanalysis* Korean People's Army...

, 150 °C is 470 kPa, and 200 is 1 550 kPa. The critical point

Critical point (thermodynamics)

In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...

is 21.7 MPa at a temperature of 374 °C, above which water is supercritical rather than superheated. Above about 300 °C, water starts to behave as a near-critical liquid, and the physical properties, such as density, start to change more significantly with pressure. However, it has been reported that higher pressures increase the rate of extractions using superheated water below 300 °C, but this could be due to the effect on the substrate, particularly plant materials, rather than affecting the water properties.

Energy requirements

The energy required to heat water is significantly lower than that needed to vaporize it, for example for steam distillation
and the energy is easier to recycle using heat exchanger

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

s. The energy requirements can be calculated from steam tables. For example, to heat water from 25°C to steam at 250°C at 1 atm requires 2869 kJ/kg. To heat water at 25°C to liquid water at 250°C at 5 MPa requires only 976 kJ/kg. It is also possible to recover much of the heat (say 75%) from superheated water, and therefore the energy use for superheated water extraction is less than one sixth needed for steam distillation. This also means that the energy contained in the superheated water is insufficient to vaporise the water on decompression. In the above example, only 30% of the water would be converted to vapour on decompression from 5 MPa to atmospheric pressure.

Extraction

Extraction using superheated water tends to be fast because diffusion rates increase with temperature. Organic materials tend to increase in solubility with temperature, but not all at the same rate. For example, in extraction of essential oils from rosemary
and coriander,
the more valuable oxygenated terpenes were extracted much faster than the hydrocarbons. Therefore, extraction with superheated water can be both selective and rapid, and has been used to fractionate diesel and woodsmoke particulates.
Superheated water is being used commercially to extract starch material from marsh mallow root for skincare applications
and to remove low levels of metals from a high-temperature resistant polymer.

For analytical purposes, superheated water can replace organic solvents in many applications, for example extraction of PAH’s from soils
and can also be used on a large scale to remediate contaminated soils, by either extraction alone or extraction linked to supercritical or wet oxidation.

Reactions

Superheated water, along with supercritical water

Supercritical fluid

A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...

, has been used to oxidise hazardous material in the wet oxidation

Wet oxidation

process. Organic compounds are rapidly oxidised without the production of toxic materials sometimes produced by combustion. However, when the oxygen levels are lower, organic compounds can be quite stable in superheated water. As the concentration of hydronium

Hydronium

(H₃O⁺) and hydroxide

Hydroxide

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

(OH^-) ions are 100 times larger than in water at 25°C, superheated water can act as a stronger acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

and a stronger base

Base (chemistry)

For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

, and many different types of reaction can be carried out. An example of a selective reactions is oxidation of ethylbenzene

Ethylbenzene

Ethylbenzene is an organic compound with the formula C6H5CH2CH3. This aromatic hydrocarbon is important in the petrochemical industry as an intermediate in the production of styrene, which in turn is used for making polystyrene, a common plastic material....

to acetophenone

Acetophenone

Acetophenone is the organic compound with the formula C6H5CCH3. It is the simplest aromatic ketone. This colourless, viscous liquid is a precursor to useful resins and fragrances.-Production:Acetophenone can be obtained by a variety of methods...

, with no evidence of formation of phenylethanoic acid, or of pyrolysis

Pyrolysis

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

products.
and several different types of reaction in which water was behaving as reactant, catalyst and solvent were described by Katritzky et al.
Triglycerides can be hydrolysed to free fatty acids and glycerol

Glycerol

Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

by superheated water at 275°C,
which can be the first stage in a two stage process to make biodiesel

Biodiesel

Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol....

.

Superheated water can be used to chemically convert organic material into fuel products. This is known by several terms, including direct hydrothermal liquefaction,
and hydrous pyrolysis

Hydrous pyrolysis

Hydrous pyrolysis refers to the thermal decomposition which takes place when organic compounds are heated to high temperatures in the presence of water.Steam cracking is used in the petroleum industry to produce the lighter alkenes...

. There are a few commercial scale applications. The thermal depolymerization

Thermal depolymerization

Thermal depolymerization is a depolymerization process using hydrous pyrolysis for the reduction of complex organic materials into light crude oil. It mimics the natural geological processes thought to be involved in the production of fossil fuels...

or thermal conversion process (TCC) uses superheated water at about 250°C to convert turkey waste into a light fuel oil and is said to be able to process 200 tons of low grade waste into fuel oil a day.

The initial product from the hydrolysis reaction is de-watered and further processes by dry cracking at 500°C. The “SlurryCarb” process operated by EnerTech uses similar technology to decarboxylate wet solid biowaste, which can then be physically dewatered and used as a solid fuel called E-Fuel. The plant at Rialto

Rialto

The Rialto is and has been for many centuries the financial and commercial centre of Venice. It is an area of the San Polo sestiere of Venice, Italy, also known for its markets and for the Rialto Bridge across the Grand Canal....

is said to be able to process 683 tons of waste per day.

The HTU or Hydro Thermal Upgrading process appears similar to the first stage of the TCC process. A demonstration plant is due to start up in The Netherlands said to be capable of processing 64 tons of biomass (dry basis

Dry basis

Dry basis is a expression of the calculation in chemistry, chemical engineering and related subjects, in which the presence of water is ignored for the purposes of the calculation...

) per day into oil.

Chromatography

Reverse phased HPLC

High-performance liquid chromatography

High-performance liquid chromatography , HPLC, is a chromatographic technique that can separate a mixture of compounds and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.HPLC typically utilizes different types of stationary...

often uses methanol / water mixtures as the mobile phase. Since the polarity of water spans the same range from 25 to 205°C, then a temperature gradient can be used to effect similar separations, for example of phenols.

The use of water allows the use of the flame ionisation detector (FID), which gives mass sensitive output for nearly all organic compounds.

The maximum temperature is limited to that at which the stationary phase is stable. C18 bonded phases which are common in HPLC seem to be stable at temperatures up to 200°C, far above that of pure silica, and polymeric styrene / divinylbenzene phases offer similar temperature stability.

Water is also compatible with use of UV detector down to a wavelength of 190 nm.

External links

The International Association for the Properties of Water and Steam
Calculator for vapour pressure and enthalpy of superheated water.

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