Push-pull systems

Osmotic Pressure Controlled Oral Tablets. Alza Corp. has developed a system that is dependent on osmotic pressure developed within a tablet. The core of the tablet is the water-soluble dmg encapsulated in a hydrophobic, semipermeable membrane. Water enters the tablet through the membrane and dissolves the dmg creating a greater osmotic pressure within the tablet. The dmg solution exits at a zero-order rate through a laser drilled hole in the membrane. Should the dmg itself be unable to provide sufficient osmotic pressure to create the necessary pressure gradient, other water-soluble salts or a layer of polymer can be added to the dmg layer. The polymer swells and pushes the dmg solution through the orifice in what is known as a push-pull system (Fig. 3). The exhausted dmg unit then passes out of the body in fecal matter. 

BEOs are most often used for point sources or small line or surface sources. See Chapter 7 for descriptions of sources. BEOs are sometimes used for lines or surfaces when the source is moving along the line or on the surface. This naturally demands the exhaust to move with (or be moved with) the source movements (e.g., during painting or seam welding). They have also been used for side suction from baths and tanks-- and these exhausts are usually called rim exhausts see Rim Exhausts. However, for these sources push-pull systems (Section 10.4.3) are often more efficient. Side hoods can also be used, e.g., when molten metal is poured however, in these cases an enclosed exhaust is more efficient. 

For a push-pull system, the source is usually an open surface tank and the airflow acts as a horizontal curtain above the surface. In this case, the person could be anywhere as long as the system works as intended and the curtain is not broken. The curtain will be broken when parts or material are lifted out of or placed into the bath and the contaminants could be spread either through convection or because the supply air blows against the material or part. 

This section deals mainly with side push-pull ventilation. Center push-pull ventilation is also sometimes used, where two jets of air are blown from a central pipe towards two parallel exhaust hoods at opposite ends of the tank. Much of what vve say about side push-pull systems is equally valid to center push-pull. 

FIGURE (0.69 Schematic diagram of side push-pull system. 

A number of workers at Pennsylvania State University examined the push-pull system and found good agreement between their numerical and experimental work. The computational algorithm SIMPLER was used to solve the flow in the two-dimensional push-pull system and it was concluded that for a tank 1.8 m long, the push jet must have an initial velocity of 3.8 m s, that the exhaust flow rate per unit width should be 0.495 m s", and that the ratio of the pull to push flow rates, q /qj, must be between 8.8 and 17.8. 

More recently, in the middle 1990s, the UK s Health and Safety Executive (HSE) also reviewed the push-pull system. Hollis and Fletcher offer a comprehensive literature review on push-pull ventilation and note that the main conclusions of previous work on push-pull ventilation of tanks are that the control is primarily supplied by the inlet jet, forming a wall jet along the surface of the tank, and that the main purpose of the exhaust hood is to remove the air and contaminant contained within the push jet. 

The ACGIH " gives recommendations for the design of a push-pull system, apparently based largely on the work of NIOSH in the 1980s the nozzle should be between 3.2 mm and 6.4 mm and that the so-called momentum factor of the jet, the product of its velocity and flow rate per unit width, U,ij should be between 0.39 and 0.59 m s -. The outlet flow rate may then be calculated using the formula... 

Flow Patterns Induced by a Push-Pull System... 

Ingham. - This gives the required minimum value for the momentum ol the equivalent wall jet we must also recall the relationship shown in Fig. 10.72 to determine the required momentum of the offset jet in the push-pull system. 

The flow ratio method was first suggested for use in designing receptor hoods and then it was suggested for design of push-pull systems. The concept of the method is described as follows. 

The system with a horizontal jet (Fig. 10.92) is similar to the push-pull system used on open surface tanks (Section 10.4.3). One difference is that in this system the jet functions as an injector, whereas in push-pull systems the main function is as a curtain. 

The aminobutynones 342 contain a push-pull system with a strongly electron-withdrawing carbonyl group therefore, they show electrophilic properties. Cyclizations with their participation proceed differently from those with ynamines (91UK103 00UK642) and acetylenic ketones (73UK511). 

Using fundamentally the same concepts as those embodied in the push-pull system, ALZA scientists along with those at Merck have commercialized... 

Thiophene-1-oxide and 1 -substituted thiophenium salts present reduced aromaticity.144 A variety of aromaticity criteria were used in order to assess which of the 1,1-dioxide isomers of thiophene, thiazole, isothiazole, and thiadiazole was the most delocalized (Scheme 46).145 The relative aromaticity of those molecules is determined by the proximity of the nitrogen atoms to the sulfur, which actually accounts for its ability to participate in a push-pull system with the oxygen atoms of the sulfone moiety. The relative aromaticity decreases in the series isothiazole-1,1-dioxide (97) > thiazole-1,1 -dioxide (98) > thiophene-1-dioxide (99) then, one has the series 1,2,5 -thiadiazole-1,1 -dioxide (100) > 1, 2,4-thiadiaz-ole-1,1-dioxide (101) > 1,2,3-thiadiazole-1,1 -dioxide (102) > 1,3,4-thiadiazole-l,1-dioxide (103) in the order of decreasing aromaticity. As 1,2,5-thiadiazole-1,1-dioxide (100) was not synthesized, the approximations used extrapolations of data obtained for its 3,4-dimethyl-substituted analogue 104 (Scheme 46). 

Six-membered rings can only act as acceptors in push-pull systems in combi-... 

Aza analogs of cyclopentadiene and cyclononatetraene rings could act as acceptor parts in push-pull systems, and possibly be more powerful than their carbocyclic analogs, but no stereochemical studies of such systems seem to have been reported. The remaining group of systems with potentially aromatic acceptors, the quinone methides, have a number of counterparts in heterocyclic chemistry. 

Case 1. (90°) 8> s (0°) (Fig. 1). This is a normal push-pull system, and a donor group A has different environments in the energy minima near 0° and near 180°, provided that the acceptor groups X and Y are different. The energy required to pass across the tt barrier can be measured by monitoring the band shape of the A resonance when the preexponential lifetimes are intermediate on the NMR time scale. 

One should expect the activation entropy (AS ) to C=C rotation in Case 1 push-pull ethylenes to be negative, since the increase in polarity in the transition state should increase the order in the solvated structure. The effect should increase with increasing difference in polarity between ground and transition states, and also with increasing solvent polarity. These expectations have been completely borne out by experiments (78,140,143), as Table 22 shows. Contrary to what is generally found for conformational processes (144), AS values -20 e.u. are frequently found for C=C rotation in push-pull systems. 

The molecule under subject is a push-pull system which contains a benzene ring with either a donor group D (i.e. an electron-rich fragment) and an acceptor group A (i.e. an electron-poor fragment) which are positioned at the opposite sides of the 7i-unsaturated ring.69... 

The first isolated singlet carbenes were push-pull systems featuring both an electron-donating and an electron-withdrawing group that interact directly with the carbene center (Scheme 2). Phosphinocarbenes 6-8 are perfect in this category,... 

Electron withdrawing substituents such as aldehyde or imonium functions at the other end of the alkene or the 1,3-diene transform enamines and 1,3-dienamines into push-pull systems characteristic of dyes. Electron release (push) of the donor group shields the / , S, e,... carbons and electron withdrawal (pull) of the acceptor group (carbonyl oxygen or imonium nitrogen) deshields in the a, y,... position [343],... 

The barriers to passage of the 90° twisted state in case 1 type push-pull ethylenes have been shown to correlate roughly with Ectr of the acceptors46 and the nitro group should be a better acceptor than acyl and thioacyl groups. Consequently, push-pull systems with two nitro groups as acceptors, like 11 and 12, show practically perpendicular acceptor parts with 0 = 89.0 and 86.9°, respectively47. 

The structures of 123-126 were based on spectroscopic data, on combustion analyses, and chemical evidence. Compounds 124a-c showed a characteristic orange color attributed to the local push-pull system of conjugated double bonds and lone pairs. Compounds 125a-c showed a characteristic red color. The results of combustion analysis and spectroscopic data suggested the presence of 3-amino-17/-pyrazole-4,5-dicarbonitrile 126 as a precipitate from the reaction between 121c and 122. The structure of 126 was confirmed by comparison with authentic sample. 

Modification and functionalisation of natural polyenes, the carotenoids, is an efficient way for the molecular engineering of polyenic chains. Terminal bis-pyridinium carotenoids, termed caroviologens, represent an approach to electron conducting molecular wires (5). Fitting polyconjugated chains with an electron donor group on one end and an electron acceptor on the other end yields push-pull systems of type 1 that may be considered as polarized, unidirectional (oriented) molecular wires and also possess marked NLO properties. 

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