Heat-transfer medium

Steam is useful as a heat transfer medium because  

Steam under pressure can be used to heat plant to temperatures well above 100°C (the boiling point of water at atmospheric pressure). Steam gives up its heat by condensing since its latent heat of condensation is 1000 times greater than its heat capacity as gaseous steam. Steam is normally used at three different pressures (10, 15 and 40 bar) to allow for different heating requirements. 

One further aspect is that steam can be used as a source of power as well as of heat. Power can be extracted in machine drives and the residual heat in the steam exhausted at lower pressure can be utilized allowing high overall efficiency. If high pressure steam from a boiler plant is passed through a turbine/alternator set to generate electricity, then exhausted and distributed at a lower pressure to supply process heat, most of the energy in the original 

The desirable properties of a high-level heat transfer medium include low cost, non-flammability, nil toxicity, compatibility with common metals, remaining liquid at ambient temperature and most importantly, thermal stability. Materials cannot meet all of these criteria, but some useful ones are discussed below. 

Petroleum oils have a low cost and are non-toxic and non-corrosive. They are usable at operating temperatures up to 315°C, but are flammable and subject to oxidative degeneration, which can be countered by using a nitrogen blanket, but thermal cracking will still occur. 

Heat is transferred to or from process streams using other process streams or heat transfer media. In a final process design, every effort is made to exchange heat between process streams and thereby minimize the use of heat transfer media (usually referred to as utilities). Inevitably, however, some use of media, mostly cooling water, steam, and the products of combustion, is necessary. When media must be used, the heat exchangers are called utility exchangers. 

Heat transfer media are classified as coolants (heat sinks) when heat is transferred to them from process streams, and as heal sources when heat is transferred from them to process streams. Process design includes the selection of appropriate heat transfer media, data for which are listed in Table 13.1, where the media are ordered by temperature range of application. 

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The reaction is exothermic, and multitubular reactors are employed with indirect cooling of the reactor via a heat transfer medium. A number of heat transfer media have been proposed to carry out the reactor cooling, such as hot oil circuits, water, sulfur, mercury, etc. However, the favored heat transfer medium is usually a molten heat transfer salt which is a eutectic mixture of sodium-potassium nitrate-nitrite. 

Heat-transfer media Heat-transfer medium Heat-transfer oils Heat-transfer view Heat treating polyester Heat treatment Heavy crude oil Heavy-duty engines Heavy fuel oil Heavy gas oil Heavy metal Heavy metals... 

Heat-transfer media other than water, Heat pipes. 

HEAT-TRANSFER MEDIA OTHER THAN WATER... 

P. L. Gehringer, Handbook of Heat Transfer Media, Reinhold Pubhshing Co., New York, 1962. 

Fquipmentfor Systems Using Dowtherm Heat Transfer Media, Dow Chemical Co., Midland, Mich. 

Marlotherm Heat Transfer Media For a Wide Range of Temperature, Huls America, Inc., Piscataway, N.J. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

Heat Treatment and Heat-Transfer Salts. Mixtures of sodium nitrite, sodium nitrate, and potassium nitrate are used to prepare molten salt baths and heat-transfer media. One of the most widely used eutectic mixtures uses 40% NaN02, 7% NaNO, and 53% KNO [7757-79-1] to give a... 

The terphenyl—quaterphenyl heat-transfer medium (Table 4), sold as Therminol 75 heat-transfer fluid, is shipped in dmms, tank car, or tank tmck lots. Its U.S. freight classification is Heat-Transfer Media, NOIBN. The material does not requite a DOT ha2ardous material label, but does fall under the ha2ardous chemical criteria of the OSHA Ha2ards Communications Standard (19 CFR 1910.1200). 

Exothermicity. The catalytic reactions are often exothermic bond-forming reactions of small molecules that give larger molecules. Consequendy, the reactors are designed for efficient heat removal. They may be jacketed or contain coils for heat-transfer media, or the heat of reaction may be used to vaporize the products and aid in the downstream separation by distillation. 

See Antifreezes AND DEICING fluids Heat exchange technology, heat transfer media other than water Refrigeration. 

See HeAT-EXCHANGE TECHNOLOGY, HeAT-TRANSFER MEDIA OTHER TTTAN water ETHERS. 

Substitution If intensification is not possible, then an alternative is to consider using a safer material in place of a hazardous one. Thus it may be possible to replace flammaole solvents, refrigerants, and heat-transfer media by nonflammable or less flammable (high-boiling) ones, hazardous products by safer ones, and processes which use hazardous raw materials or intermediates by processes which do not. As an example of the latter, the product manufactured at Bhopal (carbatyl) was made from three raw materials. Methyl isocyanate is formed as an intermediate. It is possible to react the same raw materials in a different order so that a different and less hazardous intermediate is formed. 

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