mie modification of proteins. For example, Dr. Michiko Yamashita of the department of agricultural chemistry at the University of Tokyo describes the plastein reaction as "useful in improving both nutritional and functional properties of food proteins." The plastein reaction, she explains, is a reversal of the usual enzymic protein hydrolysis. When a high (30 to 50%) concentration of a protein hydrolyzate is incubated with a protease under slightly acidic conditions (pH 4 to 6), the oligopeptides are reformed into a proteinlike polypeptide mixture called plastein. It is also possible to alter the amino acid content of the plastein by adding the acid to the reaction mixture.. Yamashita and her coworkers Soichi Arai and Masao Fujimaki added L-methionine ethyl ester to a peptic hydrolyzate of soy protein and incubated the mixture with papain, under acid conditions. The result was a plastein in which the methionine was incorporated in a state of peptide bonding. Efforts to improve zein, gluten, and some single-cell proteins by incorporating lysine, tryptophan, and threonine also were successful. And, Yamashita adds, incorporation of glutamic acid into a soybean protein hydrolyzate yielded a plastein with good water solubility and good heat stability. Dr. David C. Sands and Dr. Lester Hankin of the Connecticut Agricultural Experiment Station increase the lysine content of naturally fermented foods by adding lysine-excreting bacteria to the fermentation mixture. Lysine is the limiting amino acid in many foodstuffs. Sands notes that most microorganisms synthesize no more amino acids, including lysine, than they need for their own metabolism. In fact, they usually deplete the amino acids in the culture medium, However, some mutant strains excrete significant quantities of specific amino acids. Working with cultures of lactic acid bacteria, Sands and Hankin developed methods to identify and isolate colonies of lysine-excreting mutants. They used both wild and mutant strains to ferment soybean milk to yogurt. Whereas the wild type decreased the lysine content of the milk 14%, the mutant strain increased it from 14 to 29%. In other experiments, Sands and Hankin obtained "high-lysine" mutants of the lactic acid bacteria that ferment corn silage. Lysine-excreting mutants also could be used to ferment sour cream, buttermilk, sauerkraut, and other fermented foods. And, Sands suggests, the nutritive value of beer might be improved by brewing it with a lysine-excreting yeast. Another route to better nutrition is the development of easily synthesized, low-cost chemicals that can take the place of more expensive natural products. At Allied Chemical, Dr. Thomas R. Steadman (now with Battelle Co24
C&ENSept. 1, 1975
lumbus Laboratories), Dr. Jan F. van Pappen, and Dr. Stylianos Sifniades synthesized 4-methylthiobutane-l,2diol, which they described in another Ag & Food session as a novel substitute for methionine. The chemical can be made by adding methyl mercaptan to 3-butene-l,3-diol, or by reducing ethyl 2-hydroxy-4-methylthiobutyrate with lithium aluminum hydride. In feeding tests, the new chemical was, on a weight basis, fully equivalent to methionine in supporting the growth of young rats. However, it was only 60 to 70% as effective in supporting the growth of young broiler chickens. On that basis, it's not competitive with DL-2-hydroxy-4-methylthiobutyric acid, the hydroxy analog of methionine. Thus, Steadman says, the experiments "were a scientific success but a commercial failure." Nevertheless, he adds, it was shown for the first time that a vicinal glycol can be metabolized to the related amino acid. "We suggest that similar biochemical transformations perhaps can be accomplished with other vicinal glycols." G
Donor approach used to desulfurize oil
OKAGC Increasing demands for desulfurizing petroleum feedstocks are rapidly exhausting conventional desulfurizing technology, says Gerald Doyle of Exxon Research & Engineering Co. With heavier feedstocks, in particular, severe conditions in the reactor are required that result in low selectivity (high hydrogen consumption) and rapid catalyst deactivation. Alternatives to the conventional technology, he says, are clearly called for. One possible alternative is the use of hydrogen donors other than hydrogen gas itself. This approach was described by Doyle at a symposium on progress in processing synthetic crudes and resids, sponsored jointly by the Division of Petroleum Chemistry and the Division of Industrial & Engineering Chemistry. Partially substituted organic compounds, such as tetralin, already have been used as hydrogen donors in some coal liquefaction processes. Tetralin also has been used as a hydrogen source in the thermal cracking of residual fuels. If efficient catalysts for the hydrogen donor desulfurization were available, Doyle reasons, this process could have several advantages over conventional hydrodesulfurization. One obvious advantage is the very high effective hydrogen concentration that can be obtained with tetralin. One bbl of tetralin, for example, is equiva-
lent to 1850 scf of available hydrogen. To reach this concentration with hydrogen gas would require very high pressures in the reactor. Another important advantage of hydrogen donors such as tetralin is that they are good solvents for heavy feeds. Their use would make handling the heavy feeds much simpler than at present. Although thermodynamic data are not available for many of the complex sulfur compounds that occur in heavy petroleum fractions, a comparison of the values for the direct vs. the donor hydrodesulfurization suggests that there are no thermodynamic limitations to the donor process. In fact, Doyle says, donor processes become more favorable at higher temperatures, and the direct hydrodesulfurization processes become less favorable. At Exxon, Doyle's investigation was concerned with the evaluation of a number of catalysts for hydrogen donor desulfurization but was primarily aimed at gaining a better understanding of the donor reaction itself. No attempt was made at an exhaustive evaluation of all the potential catalysts for donor desulfurization. A simple reaction, the desulfurization of thiophene with tetralin at 300° C, was chosen as a model for catalyst comparisons. Most of the catalysts were metallic compounds and the thiophene/metal ratio was about 60 for all of the catalysts used in the investigation. From the data gathered, it is apparent that all of the Group VI metal compounds tested are active catalysts and all of them showed about the same level of activity. The products of the donor desulfurization reaction with thiophene are similar to the products obtained by direct desulfurization under mild conditions. None of the aromatic rings undergo hydrogénation, but isolated double bonds do become saturated in some cases. As an aside, Doyle notes that a logical extension of the investigation is to try the same type of process for the removal of nitrogen. Nitrogen in heavy feedstocks appears to be the major source of nitrogen oxide pollutants in the atmosphere. None of the compounds in which nitrogen is the only heteroatom show any tendency toward denitrogenation with the catalysts tested. The mechanism for desulfurization of organic molecules, such as thiophene, or dibenzothiophene, in the presence of a hydrogen donor such as tetralin is not completely understood. Early data seemed to indicate that in the presence of a cobalt molybdate catalyst the desulfurization of thiophene was a multistep reaction scheme in which molecular hydrogen was not an intermediate. To test donor desulfurization with actual feedstocks, Doyle selected a Tia Juana medium resid. The donor desulfurization worked quite well, although higher temperatures were required
than for some of the pure compounds tried in the laboratory. In general, from a chemical standpoint, the donor desulfurization of both model compounds and actual heavy feeds is very effective and allows the reactions to be carried out in a straightforward manner under mild conditions. A number of different catalysts may be used including standard cobalt molybdate catalysts, which are converted to finely divided molybdenum sulfide in the reactions. Overall, the best catalyst found is cobalt molybdate. The mechanism of the reaction is in dispute, but it appears that a truly concerted transfer of hydrogen from donor to acceptor is not likely. A two-step process, first involving dehydrogenation of the donor and secondly reaction of the hydrogen with the sulfur-containing acceptor seems more reasonable, Doyle says. The hydrogen probably is not free, since it appears that direct hydrodesulfurization is not as effective as donor desulfurization under similar conditions. D
Pretreafment permits refining of shale oil
ϋ-KJIGG Raw shale oil, though of great potential value, cannot be fed directly to existing refineries without pretreatment. The specific extent to which a given shale oil must be prerefined depends on the composition of the raw shale oil and the requirements of a given refinery. However, the general requirements were explored at some length and described by Atlantic-Richfield's G. A. Myers in a symposium on progress in processing synthetic crudes and resids, sponsored jointly by the Division of Petroleum Chemistry and the Division of Industrial & Engineering Chemistry. One of the reasons raw shale oil cannot be used directly is that it contains many elements (mostly metallic) that must be reduced in concentration or eliminated before the oil goes to the refinery. Myers and his associates have identified 29 elements in a typical Green River shale oil, all of which have unusual activity because of the presence of high concentrations of oxygen and nitrogen in the oil. In addition to the metallic elements, there is also a significant concentration of finely divided solids, which complicates the prerefining of the shale oil, even if the elements are present only in trace amounts. The nitrogen content of most shale oils is particularly important because it is too high to allow effective catalytic cracking and makes catalytic reforming of the naphtha impractical. Two general approaches that have
been used in the past to upgrade raw shale oil are coking and hydrotreating. In the primary fractionation of shale oil, the finely divided solids are concentrated in the tower bottoms. These may be removed either by centrifugation or by filtration, but both processes involve high investment and operating expenses. An alternative is to feed the heavy ends to a coker where the solids are removed in a clean cake and the coker distillate is combined with overhead cuts and hydrotreated. Likewise, most of the trace elements also are concentrated in the heavy ends. They, too, are left on the coke. One exception is arsenic, which is distributed throughout the entire boiling range and must be removed before hydrotreating to avoid catalyst poisoning. Nitrogen removal can be handled by severe hydrotreating, but arsenic removal is a tougher problem. The arsenic level in shale oils at about 40 ppm is about 1000 times greater than in conventional crudes. Since the principal effect is catalyst deactivation, the arsenic must be removed before hydrotreating, particularly the high-severity hydrotreating necessary to decrease nitrogen levels. A number of dearsenation processes have been developed including some that use irradiation with ultraviolet light and treatment with metal oxides. Arco, says Myers, has developed a new process using caustic washes to reduce the arsenic level. In the process, which is now available for licensing, arsenic content is reduced from 40 ppm to 10 ppm with a hydrocarbon loss of less than 0.5%. D
ally determined by any technique depends on the previous steps taken. "Oil is defined by the method chosen," Shull notes. The gravimetric method is inherently inaccurate for low-boiling compounds, particularly in low concentration ranges. Consequently, Shull considers the instrumental approach the better one. The particular instrumental approach that best fits Texaco's needs is the solvent extraction-infrared finishing technique developed originally by Simhard. The variation of the technique developed at Texaco is essentially Simhard's with the exception that Freon-113 (1,1,2-trichloro1,2,2-trifluoroethane) is used as a solvent replacing the carbon tetrachloride formerly used. The method involves extraction of oily matter from the water with Freon-113, followed by measuring the net infrared absorbance of the extract in the region of 3400 c m - 1 to 2500 c m - 1 . The total absorbance then is compared with calibration standards to determine concentration of the oily matter. In applying the technique, there are two problem areas. First is that much of available historical data is based on gravimetric determinations. This lack makes it difficult to relate gravimetric and instrumental methods. The second problem arises when gasoline or other components with a significant aromatic portion are in the oily matter. However, Shull says that both problems have been overcome and that use of Freon-113 as a solvent along with proper calibration techniques has been developed to the point where the method can be used routinely on the 0- to 10-ppm range. D
Method analyzes oily residues in water Photoconductivity is basis of new detector
CHCAGQ
A new method to determine accurately the nature of oily matter contained in water samples has been developed by Dr. D. L. Shull and his associates at Texaco's research and technical department. Shull described his improved analytical technique for oily residues in water for the Division of Petroleum Chemistry!v The development was occasioned, in part, by quality control requirements of waste cleanup systems that operate on water with oil contents of about 10 ppm. Two general analytical techniques are now in use, Shull explains, one gravimetric and the other instrumental. Instrumental techniques can be further classified as that which does and that which doesn't require an extraction step prior to weighing. The definition of "oily matter" is somewhat dependent on the analytical technique used, and the material actu-
CHKAGG A newly discovered photoconductivity effect could form the basis of an ultrasensitive detection system, according to Dr. Alan Snelson of IIT Research Institute, Chicago. Snelson notes that the matrix isolation technique—in which atoms, molecules, radicals, or ions are "trapped" in an inert gas matrix—is now an established method for obtaining a variety of spectroscopic data. In experiments at ÏÏTRI, Snelson has observed a photoconductivity effect involving matrix-isolated sodium atoms and such electron acceptors as tetracyanoethylene (TCNE), TNT, dibenzofuran, and DDT. "There is reason to believe," he says, "that this photoconductivity effect could provide a new, extremely sensitive detection technique." In a typical experiment, alternate Sept. 1, 1975 C&EN
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