Superheated Carbon Dioxide

K, have been tabulated (2). Also given are data for superheated carbon dioxide vapor from 228 to 923 K at pressures from 7 to 7,000 kPa (1—1,000 psi). A graphical presentation of heat of formation, free energy of formation, heat of vaporization, surface tension, vapor pressure, Hquid and vapor heat capacities, densities, viscosities, and thermal conductivities has been provided (3). CompressibiHty factors of carbon dioxide from 268 to 473 K and 1,400—69,000 kPa (203—10,000 psi) are available (4). 

The feedstocks to the styrene process are ethylbenzene and superheated steam, and a typical unit produces hydrogen, small amounts of light hydrocarbons and carbon dioxide as gaseous products, and a Hquid product stream containing 95% + styrene and minor amounts of toluene, benzene, and other aromatics. In an integrated plant, the benzene can be recycled to the ethylbenzene unit, while the other by-products usually are consumed as fuel for the highly endothermic process. 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

The cellulose specimen under examination is refluxed in the acid-oxidant mixture and the gases formed are swept continuously into an absorption train by a carrier stream of air free of carbon dioxide. Conrad and Scroggie20 have added a number of important improvements which apparently increase the reproducibility of results. One of their modifications is a stirrer in the reaction chamber which reduces the danger of bumping caused by superheating. The latter is undesirable since the reaction is apparently quite sensitive to the temperature. 

The desulfurized feedstock is mixed with superheated steam and charged to the hydrogen furnace. On the catalyst, the hydrocarbons are converted to hydrogen, carbon monoxide, and carbon dioxide. The furnace supplies the heat needed to maintain the reaction temperature. 

The desulfurized feedstock is then mixed with superheated steam and passed over a nickel catalyst (730 to 845°C 1350 to 1550°F 400 psi) to produce a mixture of hydrogen, carbon monoxide, and carbon dioxide as well as excess steam. The effluent gases are cooled (to about 370°C 700°F) and passed through a shift converter which promotes reaction of the carbon monoxide with stream to yield carbon dioxide and more hydrogen. The shift converter may contain two beds of catalyst with interbed cooling the combination of the two catalyst beds promotes maximum conversion of the carbon monoxide. This is essential in the event that a high-purity product is required. 

It may be pointed out that the isotherms plotted in the figure given above are based on theoretical calcula tions of Vcorresponding to different values of P obtained by using the van der Waals equation. The isotherms for carbon dioxide, obtained by Andrews experimentally, were in close resemblance with these curves, with the difference that the wavelike portion LMNOQ was replaced by a horizontal line. Since then more careful experiments have shown that small portions corresponding to curves LM and OQ can be realised in practice also. These represent supersaturated vapour and superheated liquid, respectively. 

Description The gas feedstock is compressed (if required), desulfurized (1) and process steam is added. Process steam used is a combination of steam from the process condensate stripper and superheated medium pressure steam from the header. The mixture of natural gas and steam is preheated, prereformed (2) and sent to the tubular reformer (3). The prereformer uses waste heat from the flue-gas section of the tubular reformer for the reforming reaction, thus reducing the total load on the tubular reformer. Due to high outlet temperature, exit gas from the tubular reformer has a low concentration of methane, which is an inert in the synthesis. The synthesis gas obtainable with this technology typically contains surplus hydrogen, which will be used as fuel in the reformer furnace. If C02 is available, the synthesis gas composition can be adjusted, hereby minimizing the hydrogen surplus. Carbon dioxide can preferably be added downstream of the prereformer. 

With an increased interest and awareness of the impact of society and industry on the environment, there has been a significant attempt in recent years to reduce or replace the usage of organic solvents. Much early work in this area concentrated on the application of supercritical and subcritical carbon dioxide, but in recent years superheated (or subcritical/pressurized hot) water (SHW) has become of interest for both chromatography and extraction [43,54], The earliest work was reported by GuUlemin et al. [55], who used the term thermal aqueous liquid chromatography. As well as using SHW for the separation of... 

As was discussed in Section 3.3.1, the theoretical value of the index, y, is 1.67 for a monatomic gas such as helium or argon. 1.4 for diatomic gases such as hydrogen, oxygen and nitrogen and 1.33 for a polyatomic gas such as carbon dioxide or superheated steam. The true indices will deviate somewhat from these values in practice for instance the value of 1.3 is normally used as a better approximation for superheated steam. 

One suggestion is to back extract with supercritical carbon dioxide, which is also a clean solvent. Another alternative is to do the reverse, i.e. extract the plant material first with carbon dioxide to obtain a concrete or oleoresin , which contains heavy materials, such as plant waxes. The concrete can then be treated with superheated water, as described below in the section on liquids. However, the use of supercritical carbon dioxide is probably not economically viable for most products. 

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