食品专业毕业论文外文翻译--制冷剂的利用发展过程

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1、外文文献： Refrigerant History The first practical refrigerating machine was built by Jacob Perkins in 1834； it used ether in a vapor-compression cycle. The first absorption machine was developed by Edmond Carr in 1850, using water and sulfuric acid. His brother, Ferdinand Carr demonstrated an ammonia/wa

2、ter refrigeration machine in 1859. A mixture called chemogene, consisting of petrol ether and naphtha, was patented as a refrigerant for vapor-compression systems in 1866. Carbon dioxide was introduced as a refrigerant in the same year. Ammonia was first used in vapor-compression systems in 1873, su

3、lfur dioxide and methyl ether in 1875, and methyl chloride in 1878. Dichloroethene (dilene) was used in Willis Carriers first centrifugal compressors, and was replaced with methylene chloride in 1926. Nearly all of the early refrigerants were flammable, toxic, or both, and some also were highly reac

4、tive. Accidents were common. The task of finding a nonflammable refrigerant with good stability was given to Thomas Midgley in 1926. He already had established himself by finding tetraethyl lead, to improve the octane rating of gasoline. With his associates Henne and McNary, Midgley observed that th

5、e refrigerants then in use comprised relatively few chemical elements, clustered in an intersecting row and column of the periodic table of elements. The element at the intersection was fluorine, known to be toxic by itself. Midgley and his collaborators felt, however, that compounds containing it s

6、hould be both nontoxic and nonflammable. Their attention was drawn to organic fluorides by an error in the literature. It showed the boiling point for tetrafluoromethane (carbon tetrafluoride) to be high compared to those for other fluorinated compounds. The correct boiling temperature later was fou

7、nd to be much lower. Nevertheless, the incorrect value was in the range sought and led to evaluation of organic fluorides as candidates. The shorthand convention, later introduced to simplify identification of the organic fluorides for a systematic search, is used today as the numbering system for r

8、efrigerants. The number designations unambiguously indicate both the chemical compositions and structures. Within three days of starting, Midgley and his collaborators had identified and synthesized dichlorodifluoromethane, now known as R-12. The first toxicity test was performed by exposing a guine

9、a pig to the new compound. Surprisingly, the animal was completely unaffected, but the guinea pig died when the test was repeated with another sample. Subsequent examination of the antimony trifluoride, used to prepare the dichlorodifluoromethane from carbon tetrachloride, showed that four of the fi

10、ve bottles available at the time contained water. This contaminant forms phosgene (COCl2) during the reaction of antimony trifluoride with carbon tetrachloride. Had the initial test used one of the other samples, the discovery of organic fluoride refrigerants might well have been delayed for years.

11、The development of fluorocarbon refrigerants was announced in April 1930. To demonstrate the safety of the new compounds, at a meeting of the American Chemical Society, Dr. Midgley inhaled R-12 and blew out a candle with it. While this demonstration was dramatic, it would be a clear violation of saf

12、e handling practices today. CFC Refrigerants Commercial chlorofluorocarbon (CFC) production began with R-12 in early 1931, R-11 in 1932, R-114 in 1933, and R-113 in 1934； the first hydrochlorofluorocarbon (HCFC) refrigerant, R-22, was produced in 1936. By 1963, these five products accounted for 98%

13、of the total production of the organic fluorine industry. Annual sales had reached 372 million pounds, half of it R-12. These chlorofluorochemicals were viewed as nearly nontoxic, nonflammable, and highly stable in addition to offering good thermodynamic properties and materials compatibility at low

14、 cost. Close to half a century passed between the introduction of CFCs and recognition of their harm to the environment when released. Specific concerns relate to their depletion of stratospheric ozone and to possible global warming by actions as greenhouse gases. Ironically, the high stability of C

15、FCs enables them to deliver ozone-depleting chlorine to the stratosphere. The same stability prolongs their atmospheric lifetimes, and thus their persistence as greenhouse gases. Ideal Refrigerants In addition to having the desired thermodynamic properties, an ideal refrigerant would be nontoxic, no

16、nflammable, completely stable inside a system, environmentally benign even with respect to decomposition products, and abundantly available or easy to manufacture. It also would be self-lubricating (or at least compatible with lubricants), compatible with other materials used to fabricate and servic

17、e refrigeration systems, easy to handle and detect, and low in cost. It would not require extreme pressures, either high or low. There are additional criteria, but no current refrigerants are ideal even based on the partial list. Furthermore, no ideal refrigerants are likely to be discovered in the

18、future. Toxicity A fundamental tenet of toxicology, attributed to Paracelsus in the 16th century, is dosis solo facitvenenum, the dose makes the poison. All substances are poisons in sufficient amounts. Toxic effects have been observed for such common substances as water, table salt, oxygen, and car

19、bon dioxide in extreme quantities. The difference between those regarded as safe and those viewed as toxic is the quantity or concentration needed to cause harm and, in some cases, the duration or repetition of exposures. Substances that pose high risks with small quantities, even with short exposur

20、es, are regarded as highly toxic. Those for which practical exposures cause no harm are viewed as safer. There are multiple reasons that toxicity concerns have surfaced with the introduction of new refrigerants. First, they are less familiar. Second, public consciousness of health hazards is growing. Manufacturer concerns with liability also