Other approaches and recommendations

The general type of approach, that is, the comparison of an experimental heat of immersion with the expected value per square centimeter, has been discussed and implemented by numerous authors [21,22]. It is possible, for example, to estimate sv - sl from adsorption data or from the so-called isosteric heat of adsorption (see Section XVII-12B). In many cases where approximate relative areas only are desired, as with coals or other natural products, the heat of immersion method has much to recommend it. In the case of microporous adsorbents surface areas from heats of immersion can be larger than those from adsorption studies [23], but the former are the more correct [24].  [c.576]

This approach works quite well for species at metal surfaces. It has been used extensively in recent years to ascertain information about organic thin films on metal surfaces. Of particular interest in many of these studies, and indeed the real forte of this technique, has been the deterrnination of molecular orientation on surfaces from such studies. Few other techniques are quite so useful for unambiguously ascertaining molecular orientation.  [c.288]

Methanol Production. Like hydrogen production, steam reforming of natural gas is by far the largest source of synthesis gas for methanol [67-56-1]. But alternative feedstocks in use include heavy oil and coal. In 1927, the first methanol plant in the United States to use synthesis gas generated from coal was built by Du Pont in BeUe, West Virginia. At least four other methanol plants are operating or have operated on coal in recent times. These facihties include the 165,000 t/yr plant at Eastman in Kingsport and a 250,000 t/yr plant at Leuna in Germany. The Leuna plant reportedly was closed in June 1990, but another 100,000 t/yr plant is operated by RWE-DEA at Wesseling. EinaHy, a methanol plant is operated in conjunction with ammonia production at AECI in South Africa near Johannesburg. Recendy, plans were announced for a new 100,000 t/yr coal-based methanol plant that will be built in China and will use the Texaco gasification process.  [c.165]

A more recent approach, developed by Advanced Separation Technologies (Lakeland, Florida), iavolves the placement of a number of columns on a carousel that rotates constantly at an adjustable speed. Instead of having one tall fixed column, this system has one taller column which has been broken down iato smaller columns on the rotating table. Each is coimected ia series which allows each column to be oa the adsorptioa cycle beyoad the normal breakthrough poiat typical of the larger column with ao other column ia series behiad it. The number of columns ia series duriag adsorptioa, backwash, regeaeratioa, and tinse is variable. Liquids flowing iato and out of each column change when the column reaches specific positions on the carousel as it rotates.  [c.383]

The Clayton Valley brine contrasts with other lithium-bearing brines which often contain higher magnesium content. Brine of the Great Salt Lake, for example, contains 0.006% lithium and 0.8% magnesium. Magnesium removal with lime is not feasible because of high reagent cost and the sheer volume of a magnesium precipitate. Instead, lithium chloride would need to be selectively removed from the bnne. Various ion-exchange and Hquid—Hquid extraction methods have been suggested but these methods are not used commercially. In one proposed procedure, lithium chloride is converted to lithium tetrachloroferrate [15274-95-0] and is extracted usiag a water-iasoluble organic solvent (31). Another approach involves precipitation of a lithium aluminate from the brine by adding freshly precipitated aluminum hydroxide or by adding aluminum chloride followed by pH adjustment (32). This method is also suggested for use ia recovery of lithium from geothermal brines (33).  [c.223]

The evolution in the world production of asbestos fibers since 1950 is illustrated in Table 5 (5) after a peak near 1980, production leveled off after 1985 at 4.2 4.3 X 10 t. Changes in the production of the two main producers, Canada and the former USSR, over the same period are also illustrated. These figures show a substantial decrease in the Canadian production with a concomitant increase in the former USSR production. During recent years, several other countries, namely Brazil, Zimbabwe, and China, have substantially increased their production of chrysotile. Most of China s production, as well as the limited production of many other countries, is used in local industrial appHcations. South Africa is the only country where the three main types of asbestos are produced (chrysotile, crocidoHte, and amosite), and the only significant producer of amphibole fibers.  [c.352]

Empirical force field calculations during the optimization procedure should be performed in a fashion consistent with the final application of the force field. With recent developments in the Ewald method, particularly the particle mesh Ewald (PME) approach [85], it is possible to perform simulations of biological molecules in the condensed phase with effectively no cutoff of the non-bond interactions. Traditionally, to save computational resources, no atom-atom non-bond interactions beyond a specified distance are included in the calculation the use of PME makes this simplification unnecessary (i.e., no distance-based truncation of non-bond interactions). Accordingly, all empirical calculations in the gas phase (e.g., water-model compound interactions, energy minimizations, torsional rotation surfaces) should be performed with no atom-atom truncation, and condensed phase calculations should be performed using PME. In addition, condensed phase calculations should also be used with a long-tail correction for the VDW interactions. Currently, such a correction is not present in CHARMM, although its implementation is in progress. Other considerations are the dielectric constant, which should be set to 1 for all calculations, and the 1,4 scaling factor (see legend of Eig. 1), which should also be set to 1.0 (no scaling).  [c.27]

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound.  [c.341]

Walden inversion A phenomenon discovered in 1895 by Walden. When one of the atoms or groups attached to the asymmetric carbon atom in an optically active compound is replaced by a different atom, the product is sometimes a derivative of the optical isomer of the original compound. It is thus possible to pass from one isomer to the other without the formation and separation of a racemic compound. ( + )-Malic acid, when treated with PCI5 gives (— )-chlorosuccinic acid, which may be converted to ( —)-malic acid by AgjO or back to (-f)-malic acid by KOH. Similarly, ( —)-malic acid is converted to (-l-)-chloro-succinic acid which undergoes similar changes. A Walden inversion occurs at a tetrahedral carbon atom when the entry of the reagent and the departure of the leaving group are synchronous - the so-called bimolecular nucleophilic substitution mechanism. Since the reagent must approach from the side of the molecule opposite to that of the leaving group an inversion of optical configuration results.  [c.424]

The preceding discussion has focused on the patii integral centroid picture of condensed phase quantum activated dynamics, primarily because of its strong analogy with the classical case, the PMF, etc, as well as its computational utility for realistic problems. However, several recent complementary developments must be mentioned. The first is due to Poliak, Liao and Shao 45 who have significantly extended an earlier idea 30 in which the exact Heisenberg population operator in hp(() in A3.8.13 is replaced by one for a parabolic barrier (plus some other unportant manipulations, such as symmetrization of the flux operator, that were not done in 30). The dynamical population operator tlien has an analytic fonn which in turn leads one to a purely analytic quantum transition state theory approximation to A3.8.13. This approach, which in principle can be systematically improved upon tlnough perturbation theory, has been demonstrated to be as accurate as the patli integral centroid-based fomnilae in A3.8.21 and A3.8.22 above the crossover temperature.  [c.893]

Studies of wave packet motion in excited electronic states of molecules with tliree and four atoms were conducted by Schinke, Engel and collaborators, among others, mainly in the context of photodissociation dynamics from the excited state [142. 143 and 144] (for an introduction to photodissociation dynamics, see [7], and also more recent work [145, 146, 147. 148 and 149] with references cited therein). In these studies, the dissociation dynamics is often described by a time-dependent displacement of the Gaussian wave packet in the multidimensional configuration space. As time goes on, this wave packet will occupy different manifolds (from where the molecule possibly dissociates) and this is identified with IVR. The dynamics may be described within the Gaussian wave packet method [150], and the vibrational dynamics is then of the classical IVR type (CIVR [M])- The validity of this approach depends on the dissociation rate on the one hand, and the rate of delocalization of the wave packet on the other hand. The occurrence of DIVR often receives less attention in the discussions of photodissociation dynamics mentioned above. In [148], for instance, details of the wave packet motion by means of snapshots of the probability density are missing, but a delocalization of the wave packet probably takes place, as may be concluded from inspection of figure 5 therein.  [c.1063]

The complications which occur with bifiircation, i.e. when more than one product arrangement is accessible, can be solved by various methods. Historically, the first close-coupling approaches for multiple product chaimels employed fitting procedures [ ], where the close-coupling equations are simultaneously propagated from each of the asymptotes inwards and then are fitted together at a dividing surface. This approach has been replaced in recent calculations by two methods. One is based on using absorbing potentials to turn the reactive problem into an inelastic one, as explained later. The other is to use hyperspherical coordinates for carrying out the close-coupling propagation [41, 42, 43, 44 and 45]. The hyperspherical coordinates consist of a single radius p, which is zero at the origin (when all nuclei are stuck together) and increases outwards, and a set of angles. For the collinear problem as well as the atom-diatom problem (mvolving tliree independent distances) the hyperspherical coordinates are typically just the regidar spherical coordinates. Close-coupling propagation starts at p = 0 and moves outward until a large value of p is reached. When the asymptote are reached one fits the wavefunction to have the fonn of equation (B3.4.4) and thus obtains the scattering matrix.  [c.2297]

The Rekker approach is still used with revised Z/ systems, e.g., in the software program Z/SYBYL [8]. Over recent decades various other substructure-based approaches have been developed that are mostly implemented and available as computer programs.  [c.493]

A similar approach is followed in a recent study of the Lewis-acid catalysis of a Michael addition in acetonitrile. See Fukuzumi, S. Okamoto, T. Yasui, K Suenobu, T. Itoh, S. Otera, J. Chem. Lett. 1997, 667.  [c.73]

As early as the nineteenth century many chemists built scale models to better understand molec ular structure We can gam a clearer idea about the features that affect structure and reactivity when we examine the three dimensional shape of a mole cule Several types of molecular models are shown for methane in Figure 1 6 Probably the most familiar are ball and stick models (Figure 1 6b) which direct approximately equal attention to the atoms and the bonds that connect them Framework models (Figure 1 6a) and space filling models (Figure 1 6c) rep resent opposite extremes Framework models empha size the pattern of bonds of a molecule while ignoring the sizes of the atoms Space filling models emphasize the volume occupied by individual atoms at the cost of a clear depiction of the bonds they are most useful in cases in which one wishes to examine the overall mo lecular shape and to assess how closely two non bonded atoms approach each other  [c.28]

Other nucleosides such as 2 3 dideoxyinosme (ddl) also block the action of reverse transcriptase and are often combined with AZT in drug cocktails Using a mixture of drugs makes it more difficult for a virus to develop resistance than using a single drug The most recent advance m treating HIV infections has been to simultaneously attack the virus on a second front using a protease inhibitor Recall from Section 27 10 that proteases are enzymes that catalyze the hydrolysis of proteins at specific points When HIV uses a cell s DNA to synthesize its own proteins the initial product is a long polypeptide that contains several different proteins joined together To be useful the mdi vidual proteins must be separated from the aggregate by protease catalyzed hydrolysis of peptide bonds Protease inhibitors prevent this hydrolysis and m combination with reverse transcriptase inhibitors slow the reproduction of HIV Dramatic reductions m the viral load m HIV infected patients have been achieved with this approach  [c.1180]

Hg) will not include any contribution from the finer mesopores, which are not penetrated by the mercury, nor from any micropores which are present (cf. Chapter 4). The importance of these qualifications is demonstrated by the results of a recent study by Sing and his co-workers. A series of alumina gels was prepared by passage of gaseous ammonia into solutions of aluminium nitrate nonohydrate in various alcohols. Their pore structure, as revealed by both nitrogen adsorption and by mercury porosimetry, varied markedly according to the particular alcohol and the concentration of the solution. A selection of mercury intrusion-extrusion plots is shown in Fig. 3.35 curves A, B and C show no tendency to approach a plateau even at the highest pressure, indicating incomplete filling of pores and the presence of a substantial proportion of pores of r < 35 A in curves D and E, on the other hand, the intruded volume tends to a  [c.188]

For the purposes of this discussion, stereoregulating catalysts will be taken to mean Ziegler-Natta catalysts. This is a somewhat restrictive view of the situation, since there are other catalysts-phenyl magnesium bromide is a Grignard reagent-which can produce stereoregularity the Ziegler-Natta catalysts are also used to produce polymers-unbranched polyethylene to name one—which lack stereoregularity. Ziegler-Natta catalysts are the most widely used and best-understood stereoregulating systems, so the loss of generality in this approach is not of great consequence.  [c.488]

In recent years, a wide range of synthetic strategies have been developed that have sought to control molecular and atomic interactions to produce functional materials through nanotechnology using the bottom-up principle. These strategies have tanged in approach from those in which biological processes and synthetic pathways are utilized directiy or are mimicked, to those that have no analogy in the natural world. Examples of synthetic strategies that have utilized biological processes directiy include the expression of proteins and nucleic acids that are designed for specific function [eg, molecular recognition (24), catalysis (25,26), therapeutics (27—30)] or to exhibit specific stmctural properties (eg, three-dimensional stmcture) (14,24). Biomimetic strategies include schemes for formation of functional bilayer membranes (31), thin organic films (32), and vesicles (33) from synthetic precursors, and template-directed crystallization and polymerization (34). Other nonbiological strategies include total synthesis of functional, noimatural molecules and local probe-assisted constmction of atomic and molecular stmctures (35—39).  [c.199]

Significant growth in acrylonitrile end use has come from ABS and SAN resins and adiponittile (see Acrylonitrile polymers). ABS resins are second to acryflc fibers as an outlet for acrylonitrile. These resins normally contain about 25% acrylonitrile and are characterized by thein chemical resistance, mechanical strength, and ease of manufacture. Consumption of ABS resins increased significantly in the 1980s with its growing application as a specialty performance polymer in constmction, automotive, machine, and appliance applications. Opportunities stiU exist for ABS resins to continue to replace more traditional materials for packaging, building, and automotive components. SAN resins typically contain between 25 and 30% acrylonitrile. Because of thein high clarity, they are used primarily as a substitute for glass in drinking cups and tumblers, automobile instmment panels, and instmment lenses. Together, ABS and SAN resins account for about 20% of domestic acrylonitrile consumption. The largest increase among the end uses for acrylonitrile over the past 10 years has come from adiponittile, which has grown to become the third largest outlet for acrylonitrile. It is used by Monsanto as a precursor for hexamethylenediamine (HMDA, CgH N2 [124-09-4]) and is made by a proprietary acrylonitrile electrohydrodimerization process (25). HMD A is used exclusively for the manufacture of nylon-6,6. The growth of this acrylonitrile outlet in recent years stems largely from replacement of adipic acid (C H qO [124-04-9]) with acrylonitrile in HDMA production rather than from a significant increase in nylon-6,6 demand. A non-electrochemical catalytic route has also been developed for acrylonitrile dimerization to adiponittile (26,27,80,81). This technology, if it becomes commercial, can provide additional replacement opportunity for acrylonitrile in nylon manufacture. The use of acrylonitrile for HMD A production should continue to grow at a faster rate than the other outlets for acrylonitrile, but it will not approach the size of the acryflc fiber market for acrylonitrile consumption.  [c.186]

A more recent approach to the problem of electrode coating is the electrodeless magnetic flow meter. In this design there are no electrodes in contact with the process. Large plates placed on the outside of a ceramic spool perform the same function and ate capacitively coupled to the transmitter through a high impedance amplifier. This meter has been found to provide satisfactory service at very low fluid conductivities and under coating conditions where other magnetic flow meters requited cleaning after short periods of operation.  [c.65]

Imports and Exports. The United States has long been a significant importer of vanadium slags, but imports of pentoxide were negligible until they rose quickly to 850 metric tons ia 1974, and 2000 t ia 1975 (mostly from the Repubhc of South Africa). Peatoxide imports thea decliaed to 1400 t ia 1980 with Finland being the maia and South Africa the minor suppHers. In recent years, U.S. imports of ammonium and potassium vanadates and of other vanadium compounds have been 100—200 t/yr, mainly from the U.K., Germany, and the RepubHc of South Africa.  [c.393]

Occasional side effects from pipera2ine include abdominal cramps, nausea, vomiting, and diarrhea. Rarely, patients may experience headache, vertigo, and tremors, or they may feel weak and have difficulty concentrating. Red patches can appear on the skin, sometimes with the flat, elevated, itching welts of urticaria. Pipera2ine is manufactured by Burroughs Wellcome Company in Research Triangle Park, North Carolina and Wallace Laboratories of Cranbury, New Jersey. Other manufacturers include Laboratories Clin Midy of Paris, France, Regent Laboratories Ltd. of London England, and Lennon Ltd. of Saxonwold, South Africa.  [c.246]

Precise metering of the amount of Hquid reagent needed for a test is generally done using a motor-driven syringe. Other variants, such as peristaltic pumps or pneumatic syringes, are also employed. The amount of reagent used per test ranges between 0.1 mL and 0.7 mL. In some analy2ers, a dedicated conduit links each reagent container to a dispensing no22le above the cuvette. Other instmments use a single reagent probe to aspirate the reagents from the reagent containers and dispense them into the reaction cuvette. If the latter approach is used, reagent cross-contamination by the probe has to be avoided. This is usually achieved by a thorough rinsing of the probe after each reagent dispense. Reagent-to-reagent carry-over typically has to be kept to less than 1 part per 20,000. Coating the reagent probe with an inert Hquid eliminates the need for the rinsing operation.  [c.396]

Upstream exploration has sometimes led to the discovery of the primary source of alluvial stones, namely, kimberlite "pipes." These stmctures of igneous origin are the principal source of natural diamonds, and there are over 1000 occurrences of them in the world. Only a small number contain a high enough concentration of diamonds to warrant mining. Even in successful mining operations the ratio of diamond to the gangue that has to be removed and cmshed is of the order of one part in a million or even less, of which the proportion of gem quality crystals is about 20%. The first key discovery of diamond-containing pipes was in South Africa in 1867, and these stmctures are weU-known from the pubHcity given to the Kimberley, Premier, and other mines there. More recent successful discoveries were a producing pipe in Botswana, Africa in 1967 and the Argyle alluvial deposits and pipes in Australia in 1979. Exploration continues in several countries including Canada and the United States for the source of diamonds brought south by the glacier into the United States. Methods include the tracing of certain heavy minerals upstream, drilling, the use of aerial photographs to try to find manifestations of circular pipe stmctures at the surface, and magnetic anomalies by aerial surveys. There is a pipe in Arkansas where amateurs can dig with a reasonable probability of finding a diamond (1).  [c.557]

Carbon disulfide was first prepared nearly two hundred years ago by heating sulfur with charcoal. That general approach was the only commercial route to carbon disulfide until processes for reaction of sulfur and methane or other hydrocarbons appeared in the 1950s. Significant commercial production of carbon disulfide began around 1880, primarily for agricultural and solvent appHcations. Both the physical and chemical properties of carbon disulfide are utilized in industry. Commercial uses grew rapidly from about 1929 to 1970, when the principal appHcations included manufacturing viscose rayon fibers, cellophane, carbon tetrachloride, flotation aids, mbber vulcanization accelerators, fungicides, and pesticides. Production of carbon disulfide in the United States has declined in recent years. Other chemical fibers and films, as well as environmental and toxicity considerations related to carbon tetrachloride, have had significant impact on the demand for carbon disulfide. Worldwide annual production capacity in 1991 was approximately 1.3 million tons, with actual production estimated at about one million metric tons.  [c.26]

From an early stage, as already mentioned, scholars grappled with the nature of crystals, which mostly meant naturally occurring minerals. This aspect of the history of science can be looked at from two distinct perspectives - one involves a focus on the appearance, classification and explanation of the forms of crystals (i.e., crystallography), the other, the role of mineralogy in giving birth to a proper science of the earth (i.e., geology). The first approach was taken, for instance, by Burke (1966) in an outstanding short account of the origins of crystallography, the second, in a more recent study by Laudan (1987).  [c.59]

Finite-element approaches can be supplemented by the other main methods to get comprehensive models of different aspects of a complex engineering domain. A good example of this approach is the recently established Rolls-Royce University Technology Centre at Cambridge. Here, the major manufacturing processes involved in superalloy engineering are modelled these include welding, forging, heat-treatment, thermal spraying, machining and casting. All these processes need to be optimised for best results and to reduce material wastage. As the Centre s then director, Roger Reed, has expressed it, if the behaviour of materials can be quantified and understood, then processes can be optimised using computer models . The Centre is to all intents and purposes a virtual factory. A recent example of the approach is a paper by Matan el al. (1998), in which the rates of diffusional processes in a superalloy are estimated by simulation, in order to be able to predict what heat-treatment conditions would be needed to achieve an acceptable approach to phase equilibrium at various temperatures. This kind of simulation adds to the databank of such properties as heat-transfer coefficients, friction coefficients, thermal diffusivity, etc., which are assembled by such depositories as the National Physical Laboratory in England.  [c.474]

While the curing hot melts provide some quick fixturing strength without the use of solvent, when first applied, prior to cure, the fixturing strength of the bond is nothing more than that of a crystalline wax. In recent years, new developments have made the curing hot melts a more viable option to replace solvent-borne adhesives. By using higher melting points (60-100°C) intermediate molecular weight polyesters based on azaleic acid or dodecanedioic acid, the initial bond strength has been improved [49,50]. Another approach to improving green strength involves incorporation of high melt flow thermoplastics, such as polyurethane thermoplastics [51]. In a third approach, acrylates are polymerized in the polyester polyol prior to prepolymer formation. The polymerized acrylate provides the improved fixturing strength upon application [52]. Other improvements include catalyst enhancements to lengthen the heated working life of curing hot melts [53].  [c.785]

Tusek has reported the Clemmensen reduction of acylated crowns such as shown above. This approach constitutes a useful two-step alkylation procedure, although the yields reported for this sequence are not as high as one might wish. Other examples of closely related acylation approaches can be found in recent work by Tashmukhamedova and coworkers ° ° and by Kauer °.  [c.27]

Hg) will not include any contribution from the finer mesopores, which are not penetrated by the mercury, nor from any micropores which arc present (cf. Chapter 4). The importance of these qualifications is demonstrated by the results of a recent study by Sing and his co-workers. A series of alumina gels was prepared by passage of gaseous ammonia into solutions of aluminium nitrate nonohydrate in various alcohols. Their pore structure, as revealed by both nitrogen adsorption and by mercury porosimetry, varied markedly according to the particular alcohol and the concentration of the solution. A selection of mercury intrusion-extrusion plots is shown in Fig. 3.35 curves A, B and C show no tendency to approach a plateau even at the highest pressure, indicating incomplete filling of pores and the presence of a substantial proportion of pores of r" < 35 A in curves D and E, on the other hand, the intruded volume tends to a  [c.188]

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows  [c.494]

Dimerization is markedly subject to steric hindrance, thus, whereas 3-n-propylindole dimerizes readily, neither 3-isopropyl- nor Z-tert-butyl-indole dimerizes. This failure is most probably the result of steric hindrance of approach of the electrophilic reagent to position 2 by the bulky 3-substituent in the unprotonated molecule. On the other hand, models show that approach of a nucleophilic reagent to position 2 of a 3-protonated molecule is quite open, it should, there-  [c.307]

See pages that mention the term Other approaches and recommendations : [c.1200]    [c.537]    [c.267]    [c.466]    [c.67]    [c.70]    [c.223]    [c.272]    [c.310]    [c.32]    [c.56]    [c.57]    [c.116]    [c.126]    [c.112]    [c.262]    [c.227]    [c.3]   
See chapters in:

Experiments in catalytic reaction engineering  -> Other approaches and recommendations